Candida albicans cell type switches and functional plasticity in the mammalian host
. Author manuscript; available in PMC: 2018 May 17.
Published in final edited form as: Nat Rev Microbiol. 2016 Nov 21;15(2):96–108. doi:
Abstract
Candida albicans is a ubiquitous commensal of the mammalian microbiome as well as the most prevalent fungal pathogen of humans. A cell type transition between C. albicans yeast and hyphal morphologies was thought to underlie much of the variation in virulence in different host tissues. However, novel yeast-like cell morphotypes, including opaquea/α, gray, and GUT cell types, were recently reported that exhibit pronounced differences in vitro and in animal models of commensalism and disease. In this Review, we explore the characteristics of the classic cell types yeast, hyphae, pseudohyphae and chlamydospores as well as the newly identified yeast-like morphotypes. We highlight emerging knowledge about the associations of these different morphotypes with different host niches, virulence potential as well as the environmental cues and signalling pathways involved in the morphological transitions.
———
First described ~150 years ago, Candida albicans is now recognized as the most prominent fungal commensal and pathogen of humans. As a commensal, C. albicans colonizes the gastrointestinal tract
, mouth
, skin
,
, and female reproductive tract
,
of at least 70% of healthy adults.
Human hosts are usually colonized in infancy
and longitudinal molecular typing studies indicate that strains persist clonally for many years, with little evidence for strain replacement.
These observations, coupled with the failure to identify an environmental reservoir, suggest that C. albicans is exquisitely adapted to healthy mammalian hosts. However, benign commensal colonization can become pathogenic if hosts develop immune deficits, epithelial damage, or microbial dysbiosis (
).
Ironically, the pool of vulnerable patients has increased with the availability of modern medical treatments such as antibiotics, cancer chemotherapy, and solid organ transplantation, and Candida species now rank as the 3rd or 4th most common cause of invasive bloodstream infections in hospitals in the United States.
,
,
In this context, it is notable that fundamental questions regarding the mechanisms by which C. albicans thrives during its commensal and pathogenic lifestyles remain to be answered. For example, how is commensal colonization first established, and how does C. albicans persist for extended periods despite host immunity and bacterial competition? What controls the transition from commensalism to pathogenesis in vulnerable hosts? How does C. albicans succeed in the wide diversity of niches it encounters as a commensal and a pathogen? Some insights into these questions have been provided by a series of reports that link newly described C. albicans cell types to niche-specific functional adaptations.
,
,
Text Box 1. Candida albicans occupies many niches in health and disease.
The ability of C. albicans to thrive on and in human tissues cannot be overstated. As a commensal, C. albicans colonizes mucocutaneous surfaces of the mouth, skin, female reproductive tract and gastrointestinal tract of most healthy humans.
In addition, patients with specific risk factors are vulnerable to C. albicans disease syndromes involving virtually any organ.
,
,
For example, wet diapers and athletic socks are associated with superficial C. albicans skin and nail infections. Prosthetic devices put patients at risk for C. albicans biofilm formation and infection of surrounding tissues. Patients with AIDS and others with defective T cell immunity frequently suffer from oral thrush and invasive esophagitis. Gastrointestinal surgery can be complicated by leakage of gut commensals, putting patients at risk for postoperative infections such as intra-abdominal abscess. Prematurity is a strong risk factor for Candida meningitis. Antibiotic treatment is the most common risk factor for a broad range of candidiasis syndromes, presumably because antibiotics deplete bacterial competitors. Moreover, rarer defects in cell-mediated immunity, owing to hematologic malignancy, organ transplant, or cytotoxic chemotherapy, confer the highest risk for invasive disease. Immunocompromised patients as well as immunocompetent patients with multiple risk factors (for example, hospitalized individuals, treatment with antibiotics, catheters, surgery, and other invasive procedures) are highly vulnerable to bloodstream candidiasis, which carries a mortality of ~40%
and creates the opportunity for secondary infections of the eye (Candida endophthalmitis), heart (Candida endocarditis), bone (Candida osteomyelitis), liver and spleen (hepatosplenic candidiasis), and many other tissues.
The fungal kingdom is characterized by vast morphological plasticity. Fungi range in scale from the micron-sized microsporidia family of obligate intracellular pathogens
to Armillaria ostoyae, a tree pathogen whose 9.6 km2 mycelial clone in Northern Oregon is considered the world’s largest living organism.
Furthermore, many species undergo morphological transformations in response to specific environmental cues. For example, ‘thermally dimorphic’ fungal pathogens propagate as multicellular, branching, filamentous structures known as mycelia in environmental niches such as soil, and transition into unicellular, budding yeasts (or spherules, in the case of Coccidioides immitis) within warm-blooded hosts.
,
,
Given that the entire known lifecycle of C. albicans occurs in mammalian hosts, one might expect less morphological plasticity from this species; however, the opposite is true, and nine distinct cell shapes have already been described. In this Review, we discuss the characteristics of the classic cell types yeasts, hyphae, pseudohyphae and chlamydospores as well as yeast-like morphotypes, including opaquea/α, gray and GUT cells. We highlight emerging knowledge about the associations of these different morphotypes with different host niches and propensities towards virulence vs. commensalism. Finally, we discuss the environmental cues, signalling pathways, and transcriptional regulatory circuits that control the morphological transitions.
Classic cell types
Yeasts, hyphae, pseudohyphae and chlamydospores were the first C. albicans cell types to be described. They differ in morphology, mode of division, occurrence and virulence potential.
Yeasts, hyphae, pseudohyphae and chlamydospores
Among the four classic C. albicans cells types, yeasts and hyphae are the best characterized (
and
; reviewed in
,
,
), whereas pseudohyphae and chlamydospores are less well understood (
and
; reviewed in
,
,
,
). Standard yeasts, also known as ‘white’ cells, have a round-to-oval cell morphology, similar to that of Saccharomyces cerevisiae. Yeasts reproduce by budding, and nuclear division occurs at the junction between mother and daughter cells. Because progeny cells detach completely from their mothers after cytokinesis, yeasts are considered to be unicellular (reviewed in
; see also
). By contrast, hyphal cells are thin, tube-shaped cells that resemble segments of a garden hose (
). Nuclear division occurs within hyphal daughter cells, followed by migration of one progeny nucleus back into the mother cells. Hyphal cells remain firmly attached end-to-end following cytokinesis, such that iterative rounds of cell division produce multicellular, sparsely branched, filamentous structures called mycelia. Ellipsoid-shaped pseudohyphal cells share features of both yeasts and hyphae (
), and there remains some controversy over whether they represent a bona fide terminal cell type or an intermediate between these other, better characterized cell types.
Unlike for yeasts and hyphae, there are no known in vitro conditions to induce pure, stable populations of pseudohyphae. Like hyphae, pseudohyphal cells remain attached following cytokinesis and generate mycelia after multiple rounds of cell division. As in yeasts, nuclear division in pseudohyphae occurs at mother-daughter junctions; in contrast to hyphae, these junctions are demarcated by visible indentations. Finally, chlamydospores are large, spherical, thick-walled cells observed in vitro under certain harsh conditions, such as starvation and hypoxia
(
; reviewed in
). Chlamydospores are generated by suspensor cells, which are cells at the distal ends of mycelial filaments. Nuclear division occurs within the suspensor cell parent, followed by migration of a progeny nucleus to the nascent chlamydospore, which remains attached to its mother.
Table 1.
Features of Candida albicans cell types
MTL Locus
GenotypeCell shapeUnicellular
versus
multicellularSpecial
morphological
featuresIn vitro inducing
signalsSpecial functionsHost interactionsYeast (white (a/α))a/αRound-to-ovalUnicellularN/ADefault cell shape under most in vitro conditionsBiofilm formation (conventional)Virulence (bloodstream model); commensalism (mouth, skin, vagina and gastrointestinal tract)Hypha
a/αTubeMulticellularN/A37°C, N-acetylglucosamine, serum, immersion in agar, hypoxia, hypercarbia and alkaline pHThigmotropism; biofilm formation (conventional)Induced endocytosis; active penetration of host epithelial cells; virulence (mouth, vagina and bloodstream models)Pseudohypha
a/αElongated ellipsoidMulticellularIndented cell-cell junctionsHypha-inducing cues
Biofilm formation (conventional)Virulence (mouth, vagina and bloodstream)Chlamydosporea/αRound-to-ovalMulticellular
Thick cell wallNutrient scarcity, hypoxiaUnknownUnknownWhite(a) and white(α)a/Δ, a/a and α/Δ, α/αRound-to-ovalUnicellularN/A37°C, glucose and alkaline pHBiofilm formation (sexual)UnknownOpaque(a) and opaque(α)a/Δ, a/a and α/Δ, α/αEllipsoidUnicellularSurface pimplesN-acetylglucosamine, hypercarbia and acidic pHMatingHigh fitness in a neonatal mouse skin colonization modelOpaque(a/α)a/αEllipsoidUnicellularSurface pimplesNutrient scarcity, N-acetylglucosamine and hypercarbiaUnknownHigh fitness in a neonatal mouse skin colonization modelGray(a/α)a/αEllipsoidUnicellularSmallest cell typeNutrient abundanceUnknownHigh fitness in an ex vivo tongue infection modelGUTa/αEllipsoidUnicellularN/AUnknownUnknownHigh fitness in a mouse gastrointestinal commensalism modelGUT, gastrointestinally induced transition; MTL, mating-type-like.
*
Please note that a and α cells form hyphae and pseudohyphae under certain environmental conditions, but these cell types have not been well characterized.
**
Pseudohyphae arise as a subpopulation under most hypha-inducing conditions.
***
Chlamydospores are produced by the terminal cells of hyphae and pseudohyphae under nutrient-poor and oxygen-depleted conditions.
Figure 1. C. albicans Cell Type Transitions.
A. C. albicans transitions reversibly among yeast (also known as whitea/α), hypha, and pseudohypha cell types under different environmental conditions. B. Chlamydospores are generated by terminal (suspensor) cells of mycelia (multicellular hyphae or pseudohyphae) under adverse growth conditions. C and D. In mucocutaneous infection models, such as oropharyngeal candidiasis, yeasts are associated with commensalism (C), whereas the filamentous forms (hyphae and pseudohyphae) are associated with tissue invasion and damage (D). E. Yeasts, hyphae and pseudohyphae all seem to have roles in disseminated disease, for example in abscesses within host internal organs. F. MTLa (“a”) or MTLα (“α”) cells can undergo an epigenetic switch between whitea and opaquea phenotypes. Whitea cells have the same appearance as typical whitea/α yeasts, while opaquea cells are elongated and have “pimple” structures on their cell surface. G. Mating in C. albicans requires three events: Loss of one allele of MTL (MTLα or MTLa) to generate a whitea phase a or α strain; an epigenetic switch from whitea to opaquea; and pheromone sensing by opaquea cells of opposite mating type, which triggers sexual filament production and mating.
Virulence in yeasts, hyphae and pseudohyphae
Yeasts, hyphae, and pseudohyphae can either propagate stably as the same cell type or give rise to other cell types in a process known as morphogenesis (
), depending on cues from the local environment (see below). Morphogenesis has long been a central focus of C. albicans research because of links between each of these cell types and important host-fungal interactions. Traditionally, the filamentous forms (hyphae and pseudohyphae) were considered pathogenic, whereas yeasts were primarily viewed as commensals. Hyphae are intrinsically invasive on solid media and hyphal tip cells exhibit thigmotropism, or the unusual ability to ‘track’ along substrate surface irregularities
(reviewed in
). Moreover, hyphae express numerous cell type-specific virulence factors such as adhesins (for example, Hwp1, Als3, Als10, Fav2 and Pga55), tissue-degrading enzymes (for example, Sap4, Sap5 and Sap6), antioxidant defense proteins (for example, Sod5), and even a recently described cytolytic peptide toxin (Ece1).
,
,
,
,
The increased virulence potential of hyphae compared to other cell types has been conclusively shown in superficial candidiasis models, such as models of oropharyngeal
,
and vulvovaginal
infection (
). For example, hyphae, but not yeasts, induce their endocytic uptake by cultured human oral epithelial cells via a specific interaction between the hyphal adhesin, Als3, and host E-cadherin; internalized hyphae then proceed to damage the host cells.
Hyphae can also actively penetrate into oral epithelial cells, possibly via physical pressure and secreted enzymes.
,
Thus, in a reconstituted model of human oral epithelial tissue, invading hyphae trigger multiple pro-inflammatory host signaling pathways, whereas yeasts, which merely colonize the surface of the tissue without causing damage, produce a more muted inflammatory response.
However, the simple dichotomy between virulent hyphae vs. commensal yeasts does not account for observations of disseminated candidiasis, where both cell types seem to contribute to disease. For example, yeasts, hyphae and pseudohyphae are all present in infected tissues that were recovered from human patients and animals with disseminated candidiasis (
).
,
,
Moreover, C. albicans mutants trapped as either yeasts or filaments are both defective in bloodstream infection models, suggesting that the ability to interconvert among different cell types is required for virulence (see, for example, REFS
,
,
). Traditionally, yeasts, being smaller and unicellular, were hypothesized to disseminate through the bloodstream, whereas hyphae, being naturally invasive, were thought to escape the vasculature, penetrate into internal organs, and damage the host. Therefore, it came as a surprise when a study using a tetracycline-regulatable strain that can be propagated indefinitely as either yeasts or hyphae showed that yeast-locked C. albicans is as capable of egress from blood vessels, penetration into internal organs, and propagation within host tissues as a wild-type strain that can transition into hyphae.
Nevertheless, unlike wild-type C. albicans, the yeast-locked strain failed to kill its host, supporting previous observations that the yeast-to-hypha transition is required for virulence in disseminated infections. Similar to the requirement for all three cell types for virulence in the bloodstream infection model, they are also required for biofilm formation (
; reviewed in
,
), a C. albicans attribute of substantial clinical importance. Together, these observations in localized vs. disseminated infection models support a central role for yeast-hypha-pseudohypha morphogenesis in C. albicans-host interactions, but also suggest that yeasts may have different roles in different host niches. In contrast to the other cell types, chlamydospores, which are readily induced in vitro
,
, have rarely been observed in clinical specimens
or animal models of disease
, and their biological role remains undefined.
Text Box 2. Yeasts, hyphae, and pseudohyphae are required for biofilm formation.
Biofilms are communities of microorganisms that often form on solid surfaces in the environment or within mammalian hosts. Medical device-associated biofilms are of enormous clinical importance because of their high prevalence and intrinsic resistance to antibiotics and the mammalian immune system. C. albicans MTLa/MTLα whitea/α cells form conventional biofilms in a stereotyped fashion (reviewed in
,
, see the figure, part a). Biofilms are initiated when whitea/α phase yeasts attach to a solid substrate. Yeasts proliferate to form microcolonies, followed by the appearance and proliferation of hyphae and pseudohyphae, which constitute the bulk of the mature biofilm, together with an extracellular matrix composed of proteins, polysaccharides, and nucleic acids. Biofilm dispersion is thought to occur when whitea/α yeasts detach from a mature biofilm only to reattach at a second site. MTLa and MTLα whitea cells have recently been shown to form sexual biofilms (see the figure, part b). These biofilms differ from conventional biofilms by multiple criteria, including increased permeability, decreased resistance to antibiotics and host immune cells, and promotion of chemotropism between opaqueaMTLa and MTLα cells.
,
,
It has been proposed that a primary function of whitea cell biofilms is to facilitate mating between sexually competent MTLa and MTLα opaquea cells.
Yeast-like morphotypes
In addition to standard ‘white’, round-to-oval yeast morphology, described above, C. albicans transitions into several more elongated yeast-like cell types (opaque, gray, and GUT) that exhibit distinct in vitro properties and interactions with the host. Moreover, a minority of white and opaque cells that have lost genetic material at the Mating Type-Like Locus (MTL) exhibit further alterations in their propensities for mating, filamentation, virulence, commensalism, and/or biofilm formation, as described below and in
. The different types of white and opaque cells are not generally distinguished by genotype in the C. albicans literature. To clarify cell identity in this Review, however, we will introduce the convention of appending a superscript ‘a/α’ to white or opaque cells with the standard genotype of MTLa/MTLα. A superscript ‘α’ will designate cells containing only the MTLα allele, whereas a superscript ‘a’ will be used either as a general term for cells containing a single allele of the MTL (MTLa or MTLα) or as a specific term for ones containing only the MTLa allele.
Whitea and opaquea cells
Whitea and opaquea cells were first described in a particular C. albicans clinical isolate, WO-1, based on in vitro observations of rare but heritable changes in cell and colony morphology (
and
).
Whitea WO-1 yeasts have an identical appearance to standard whitea/α yeasts, described above, and form similar, creamy white, shiny, domed colonies on solid media. On glucose-containing media maintained at room temperature, however, whitea colonies occasionally (~1/10,000 cell divisions) give rise to slower growing sectors of opaquea cells. Opaquea colony sectors appear slightly darker, matte and flattened compared to whitea colonies. For undetermined reasons, opaquea cells also take up a dye, phloxine B, which allows for rapid visualization of opaqueα colonies and colony sectors that are stained bright pink on media containing this dye. Microscopically, opaquea cells are elongated compared to whitea cells and ~3 times larger (by volume), with more pronounced vacuoles.
Additional opaquea-specific features include cell surface ‘pimples’ (that is, protuberances with an unknown biological role that are detected by scanning electron microscopy),
relative resistance to phagocytosis by host macrophages and neutrophils,
,
sensitivity to distinct filamentation-inducing cues,
,
and changes in the expression of >1000 genes, including genes important for mating and respiration.
,
,
Similarly to yeast-hypha-pseudohypha morphogenesis (
), switching between the whitea and opaquea phenotypes is highly sensitive to environmental conditions: N-acetylglucosamine, ≥5% CO2, and acidic pH all favor switching to the opaquea state,
,
,
whereas glucose, low CO2 levels, alkaline pH, and mammalian body temperature promote the reverse switch back to the whitea state.
Figure 2. C. albicans Signaling and Morphogenesis.
Numerous host signals and fungal signaling pathways have been implicated in the regulation of C. albicans cell shape. Based largely on in vitro analysis of wild-type C. albicans and specific gene deletion mutants, the signals and pathways depicted in this figure have been demonstrated to control the (whitea/α ) yeast-to-hypha transition and, in some cases, the whitea-to-opaquea switch and mating. The PKA pathway (teal blue) incorporates signals via the GTPase Ras1 (gray) and Ras1-independent inputs resulting in the synthesis of cAMP from ATP by the adenylyl cyclase Cyr1 and cAMP-mediated activation of the two catalytic subunits (Tpk1 and Tpk2) of the PKA complex. Once activated, the PKA complex phosphorylates the downstream transcription factor Efg1, eliciting a potent effect on both filamentation and white-to-opaquea switching. The Cek1 Mitogen-Activated Protein Kinase pathway (MAPK, navy blue) initiates a kinase signaling cascade in response to embedded growth (light blue), cell wall damage (navy blue), osmotic damage (beige), and low nitrogen (gray), ultimately phosphorylating the transcription factor Cph1 to induce filamentation. In opaquea cells, mating pheromone (red) signals through the same upstream MAPK signaling cascade but leads to the additional phosphorylation of the MAPK Cek2 and activation of mating genes. The Hog1 MAPK pathway (gray blue) recognizes osmotic and oxidative stress through either the Sln1 two-component protein or the Sho1 adaptor protein and leads to phosphorylation of the MAPK Hog1. Activated Hog1 can inhibit both Cek1- and Brg1-mediated filamentation. The RIM101 pathway (green) senses alkaline pH via two putative receptors (Dfg16 and Rim21) that initiate a proteolytic signaling cascade that results in C-terminal cleavage of the transcription factor Rim101 by the protease Rim13 and activation of Efg1 and filamentation-specific genes. The Ofd1 pathway (pink) and Tor1 pathway (light green) respond to low oxygen and starvation, respectively, to regulate filamentation through the transcription factors, Brg1 and Ume6. References for signalling pathways are provided in the main text, and those for transcription factors (dark rectangles) are listed here:
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
. Note that additional transcription factors and some instances of regulation via Ume6 have been omitted for visual clarity.
Morphogenesis and mating type
The functional significance of the whitea -to-opaquea switch was revealed with the discovery that C. albicans opaquea cells are specialized for mating.
Fungal mating has been well described in the model yeast, S. cerevisiae, in which haploid cells are the sexually-competent cell type (reviewed in
). Haploid ‘a’ cells express the MATa allele of the Mating Type Locus, whereas haploid ‘α’ cells express the MATα allele. MATa and MATα encode different transcription factors that activate key mating genes in the respective haploid cell types. When a and α cells occur in proximity, pheromones secreted by mating partners of opposite mating type induce mutual cell cycle arrest, production of polarized mating projections, and cell and nuclear fusion to produce diploid a/α cells. Wild S. cerevisiae exists in the diploid form except under nutrient starvation conditions, which triggers meiosis and the formation of hardy haploid spores. These spores germinate when nutrients become available, and the mating cycle resumes.
In contrast to S. cerevisiae, C. albicans has never been observed to undergo meiosis or sporulation and was long considered to be an asexual species. However, in 2000, two groups reported low frequency mating between C. albicansa and α cells.
,
Most C. albicans strains carry single copies of two different alleles of the Mating Type-Like Locus, MTLa and MTLα, one apiece on two copies of Chromosome 5; these MTL alleles are orthologous to S. cerevisiae MATa and MATα.
Researchers generated ‘a’ and ‘α’ cells by deleting MATα or MATa from a/α strains via targeted gene disruption
or selection for loss of one copy of Chromosome 5.
Remarkably, mixtures of these engineered a and α cells in vitro
or in a mouse bloodstream infection model
produced a small number of tetraploid cells containing markers of both parental strains. One group subsequently determined the relationship between allelism at MTL, opaquea or opaqueα cell formation, and mating: unlike typical a/α cells, a and α cells (including the natural α strain, WO-1) can switch to the opaquea or opaqueα states, respectively, and opaquea and opaqueα cells are the mating-competent cell types in C. albicans (
).
The molecular mechanism preventing the whitea-to-opaquea switch in a/α cells is mediated by direct transcriptional repression of genes required for the switch by a1/α2, which is a heterodimeric transcription factor encoded by the combination of MTLa with MTLα.
,
,
,
More recently, another study reported that whitea and whiteα cells may also play a role in mating via formation of specialized ‘sexual’ biofilms that constrain mating-competent opaquea and opaqueα cells in space (
).
,
Despite these advancements in our understanding of the relationship between MTL genotype, whitea-to-opaquea switching and mating competency, the larger contribution of sex to C. albicans biology remains uncertain. Analysis of C. albicans population structures has revealed a primarily clonal mode of reproduction, with little evidence for sexual recombination among naturally circulating strains.
,
The rarity of sexual recombination is consistent with the observation that more than 90% of clinical isolates are heterozygous at the MTL locus and therefore incapable of switching or mating.
,
Similarly, it remains unknown why C. albicans, along with its close relatives, C. dubliniensis and C. tropicalis, introduced a baroque requirement for a whitea-to-opaquea phenotypic switch into its mating program, given that S. cerevisiae and the vast majority of fungi mate efficiently without such a system. Some insights into the latter question are suggested by the recent discovery of three additional cell morphologies with some features of opaquea cells in the MTLa/α genetic background, discussed below.
Opaquea/α, gray and GUT cells
The opaquea/α, gray, and GUT cell types exhibit physical similarities to opaquea cells, but are functionally and genotypically distinct.
Opaquea/α and gray cells
A recent study discovered opaquea/α cells in a screen of 94 C. albicans clinical isolates for morphological responses to opaquea-inducing signals.
This group had previously shown that exposure of whitea cells to 1% N-acetylglucosamide (as a sole carbon source) and 5% CO2 induces 100% full-colony switching to the opaquea phenotype.
Using the same conditions, they found that ~1/3 of their a/α isolates developed opaquea-like (that is, bright pink-staining with phloxine B) colony sectors. Moreover, opaque cells recovered from pink sectors were elongated, contained cell surface pimples, and expressed several opaquea-specific genes (
and
), like traditional opaquea cells. However, unlike opaquea cells, opaquea/α cells were incapable of mating.
The same group subsequently discovered additional a/α isolates that switch among whitea/α, opaquea/α, and a novel ‘gray’ phenotype (
and
).
Gray cells are smaller than conventional yeasts, lack pimples, stain only moderately with phloxine B and mate with very low efficiency.
In strains that are capable of whitea/α-opaquea/α-gray switching, the transition to gray cell morphology is induced by exposure to nutrient-rich growth medium (YEPD), whereas exposure to nutrient-poor medium (Lee’s), N-acetylglucosamine, and elevated CO2 favour the opaquea/α phenotype.
Figure 3. Opaquea/α, gray, and GUT cells.
A) Certain MTLa/α strains switch reversibly between standard whitea/α (round-to-oval) morphology and opaquea/α morphology (elongated, with cell surface pimples). A subset of these strains can also switch to a third, gray (small, elongated, no pimples) morphology. B) Several C. albicans morphotypes exhibit enhanced fitness in specific host niches. MTLa/α hyphae and pseudohyphae exhibit superior virulence in localized oral infection models, whereas whitea/α yeasts, hyphae and pseudohyphae are all required for virulence in disseminated infections. MTL heterozygous opaquea/α and MTL homozygous opaquea cells have both been reported to have superior fitness in colonizing skin, whereas MTLa/α gray cells are the fastest proliferating cell type in an ex vivo tongue infection model. Finally, MTLa/α GUT cells outcompete other cell types in the mammalian gastrointestinal tract, with a relative fitness of GUT≫whitea/α ≫opaquea. C) GUT cells thrive within the digestive tract and rapidly revert to the whitea/α phenotype upon exit from the host, when signals required to maintain the GUT phenotype are removed. Thus, passage of MTLa/α white cells through the mammalian gastrointestinal tract is required for the whitea/α -to-GUT switch.
Interestingly, initial studies in mammalian infection models suggest that opaquea/α, gray, and opaquea cells may have increased fitness on host epithelial surfaces (
).
,
,
For example, opaquea/α and opaquea cells have each been reported to colonize skin more effectively than isogenic whitea/α or whitea strains in a neonatal mouse skin infection model.
,
Likewise, in an ex vivo tongue infection model, gray cells have the fastest doubling time, followed by opaquea/α cells, with whitea/α cells proliferating most slowly.
By contrast, whitea/α cells are consistently most virulent in mouse bloodstream infection models.
,
,
,
The mechanisms underlying these functional differences have not yet been defined but, as described below, cell type-specific differences in metabolism and/or enzyme secretion appear likely to play a role.
,
,
,
GUT cells
C. albicans “GUT” (gastrointestinally induced transition) cells were discovered by means of a genetic screen for fungal mediators of commensalism within the mammalian digestive tract.
Pools of a/α gene deletion mutants were competed in a mouse model of persistent gastrointestinal colonization, in which the host remains healthy despite high levels of commensally growing C. albicans, and the fitness of each fungal strain was calculated as a ratio of the relative abundance in mouse feces to that in the infecting inoculum. Two mutants affecting a pair of mutually inhibitory transcription factors emerged because of their striking and opposite effects on commensal fitness: the efg1 knockout mutant was hyperfit, outcompeting all other mutants and wild-type C. albicans, whereas wor1 was strongly attenuated in this model. Consistent with a positive role for Wor1 in promoting commensal fitness, it was shown that expression of the WOR1 gene was induced 10,000-fold when wild-type yeasts were propagated within the host digestive tract compared to standard laboratory conditions. Furthermore, forced expression of WOR1 (WOR1OE) via a strong, heterologous promoter WOR1OE) induced a hypercompetitive phenotype. Unexpectedly, after ~10 days of exposure to the mammalian model, a subset of the WOR1OE yeasts recovered from animals exhibited altered cell and colony morphology. Moreover, these GUT cells rapidly dominated the recovered yeast population for the remainder of the 25-day time course. Similar to opaquea cells (and opaquea/α cells), GUT cells are elongated relative to isogenic whitea/α cells and generate darker, flattened colonies that stain (weakly) with phloxine B (
and
;
and Gianetti and Noble, unpublished data). Intriguingly, the initial appearance of the GUT phenotype coincided with a sharp gain in fitness of the WOR1OE strain, suggesting that the two phenotypes might be linked. Indeed, when GUT cells are introduced into naive animals, they are immediately hypercompetitive, unlike whitea/α isolates of the same strain.
After demonstrating that GUT cells lack the class features of whitea/α and opaquea cells, it was hypothesized that this novel cell type might be specialized for commensalism within the mammalian digestive tract. In support of this hypothesis, it was shown that GUT cells are substantially more fit than both whitea/α and opaquea cells in the gastrointestinal commensalism model, with a relative fitness of GUT≫whitea/α≫opaquea. This fitness advantage seems to be specific to gastrointestinal commensalism, as GUT cells proliferate more slowly than whitea/α cells under standard laboratory conditions and are less virulent in a mouse bloodstream infection model. Furthermore, unlike opaquea cells, GUT cells lack surface pimples and are unable to mate. Taken together, these data support a model in which signals from the mammalian gastrointestinal tract induce C. albicans yeasts to express WOR1 and switch from whitea/α to GUT (
). Whereas GUT cells thrive within the digestive tract, wild-type C. albicans strains rapidly revert to the whitea/α phenotype upon exit from animals, when signals required to maintain the GUT phenotype are removed. Thus, the detection of GUT cells outside of the host reflected the serendipitous use of a WOR1OE strain, as continuous expression of Wor1 presumably stabilizes the phenotype. Future investigations will be required to define the host signals and fungal machinery that affect the whitea/α-to-GUT switch.
Fitness and metabolism of yeast morphotypes
Comparative transcriptomics of whitea/α
,
, opaquea/α
, gray
, GUT
, whitea
,
and opaquea
,
yeasts has revealed metabolic differences that may help to account for the functional differences among these cell types. The clearest case can be made for GUT cells, which, compared to white cells, exhibit general downregulation of pathways for utilization of glucose and iron uptake, with concomitant upregulation of pathways for utilization of N-acetylglucosamine and short chain fatty acids. Thus, GUT cell metabolism seems to be optimized for nutrients available in the distal mammalian digestive tract, the niche in which it thrives as a commensal.
By contrast, opaquea/α and opaquea cells upregulate pathways involved in oxidative respiration (for example, the Kreb’s cycle),
,
,
whereas white a/α and whitea cells upregulate fermentation pathways (for example, glucose uptake and, to varying degrees, glycolysis).
,
,
The transcriptome of gray cells shows differences in metabolic gene expression that are harder to categorize.
The functional importance of these cell type-specific metabolic signatures will hopefully be rationalized once the natural host niches of each cell type are identified.
Regulation of morphogenesis
Morphogenesis depends on environmental cues such as temperature and nutrient availability that signal through multiple pathways (
) and activate a variety of transcriptional regulatory circuits. Most of these pathways were initially characterized with respect to the yeast-to-hypha transition by whitea/α cells; however, several of these same signaling pathways also control discrete behaviors by additional cell types. The evolution of such elaborate systems to regulate morphogenesis speaks to the central importance of morphogenesis in C. albicans biology.
Environmental cues and their signaling pathways
On the basis of in vitro studies, various signals (mammalian body temperature, serum, N-acetylglucosamine (GlcNAc), low nitrogen, CO2, peptidoglycan, and amino acids) have been shown to activate the fungal cAMP-PKA signaling pathway.
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
In whitea/α cells, cAMP-mediated signaling through the protein kinase A (PKA) complex activates transcription factors that promote the expression of hypha-specific genes and filamentation.
Alternatively, in whitea cells, PKA activation by GlcNAc or CO2 promotes a switch to the opaquea phenotype
,
,
,
Similarly, the Cek1 MAP kinase pathway can promote either filamentation or mating in different cell types. In whitea/α cells, nitrogen starvation or growth in an embedded matrix such as agar activates Cek1 to promote filamentation.
,
,
,
,
,
,
In MTL homozygous opaquea cells, Cek1 activation by mating pheromones triggers the expression of genes required for mating.
,
,
,
,
In addition, various forms of cell stress (oxidative, osmotic, cell wall damage) affect filamentation indirectly via the Hog1 signaling pathway, which inhibits Cek1 and activates a transcriptional inhibitor of filamentation.
,
,
,
,
,
,
,
,
,
,
In whitea/α cells exposed to alkaline pH, the RIM101 pH sensing pathway proteolytically activates the Rim101 transcription factor, leading to activation of hypha-specific genes and filamention.
,
,
,
Additional, less well-described pathways negatively regulate filamentation in response to low oxygen levels and starvation, respectively.
,
Notably, the depicted pathways fail to account for certain observations in the host, such as the finding that whitea/α yeasts appear to predominate in the mammalian GI tract
, despite relatively high concentrations of GlcNAc and CO2 in this niche, which would be expected to trigger filamentation or the whitea-to-opaquea switch based on in vitro evidence. Such discrepancies suggest that additional signaling pathways and/or crosstalk among existing pathways remain to be discovered. The multiplicity and complexity of the known signaling pathways suggest a model in which C. albicans continuously surveils the mammalian host, integrating a variety of signaling inputs to generate adaptive responses to the local environment.
Transcriptional regulation of morphogenesis
Remarkably, every morphological transition described in this article is regulated to some extent by the transcription factor, Efg1 (
).
,
,
,
,
,
The roles of Efg1 in C. albicans morphogenesis were deduced from phenotypes of EFG1 mutants, with different cell shapes correlating with high or low levels of EFG1 expression. In similar studies, Wor1 was shown to oppose Efg1 in the control of the whitea-opaquea, whitea/α-gray-opaquea/α, and whitea/α-GUT transitions (
).
,
,
,
,
,
Efg1 and Wor1 are fungal-specific transcription factors whose orthologs regulate diverse morphological transitions in different fungal species.
,
,
,
,
,
In C. albicans, Efg1 and Wor1 have been demonstrated to bind to each other’s promoters, where they are thought to mediate mutual transcriptional repression.
,
Figure 4. The Efg1 and Wor1 transcription factors have central roles in C. albicans morphological plasticity.
Genetic studies have revealed roles for the transcription factors Efg1 and Wor1 in numerous morphological transitions. In each example below, arrows indicate activation, bars represent inhibition, and dashed lines indicate the direction of Efg1-promoted activity (with the exception of yeast-hypha morphogenesis (A), where Efg1 can support either transition, depending on environmental context). A) Efg1 promotes whitea/α cells to undergo the yeast-to-hypha transition upon exposure to host serum, N-acetylglucosamine, high CO2, nutrient depletion, and/or iron depletion;
however, under agar-embedded conditions, Efg1 promotes the reverse transition from hypha-to-yeast.
B) Efg1 promotes chlamydospore production by whitea/α hyphal suspensor cells under nutrient poor, oxygen-depleted conditions.
,
C) Efg1 promotes
and Wor1 opposes
,
,
the opaquea-to-whitea switch, which is also controlled by additional transcription factors in a complex regulatory circuit.
,
,
,
,
,
Under conditions in which neither cell type is favoured (glucose-containing medium maintained at 25°C), switching may occur in either direction on an infrequent, stochastic basis, depending on changes in the ratio of Efg1 to Wor1 protein in individual cells.
,
Under conditions that strongly favour whitea (glucose-containing medium at 37°C) or opaquea (N-acetylglucosamine-containing medium in ≥5% CO2) cells, switching occurs in one direction across the entire cell population.
,
,
,
D) Efg1 promotes the gray-to-whitea/α switch and Wor1 promotes the gray-to-opaquea/α switch.
,
Gray cells are favored under nutrient-rich conditions, whereas opaquea/α cells are favored under nutrient-limited conditions in the presence of N-acetylglucosamine and elevated CO2.
E) Efg1 promotes and Wor1 opposes the GUT-to-whitea/α switch.
Whitea/α cells are favored under all tested conditions except for within the mammalian gastrointestinal tract.
Regulation of C. albicans cell shape is more than a simple function of Efg1 and Wor1 levels, however, because each factor’s activity is influenced by genotype at MTL and local environmental cues. For example, upon exposure to host serum, N-acetylglucosamine, high CO2, nutrient depletion, and/or iron depletion, Efg1 promotes a/α whitea/α cells to undergo the yeast-to-hypha transition;
however, under agar-embedded conditions, Efg1 promotes the reverse transition from hypha-to-yeast (
).
In another case, under low oxygen, nutrient-depleted conditions, Efg1 promotes a/α hyphae and pseudohyphae to generate chlamydospores (
).
In contrast, upon exposure to glucose and low CO2, Efg1 promotes a and α opaquea cells to switch to the whitea state (
)
and certain a/α clinical isolates to undergo opaque-to-whitea/α or gray-to-whitea/α switches (
).
,
Finally, Efg1 promotes a/α GUT-to-whitea/α cells to switch to the whitea/α state in all tested environments other than the mammalian intestinal tract (
).
A key unanswered question is how the genotype of the MTL locus and signals associated with different host niches contribute to discrete morphological outcomes. This challenge is compounded by the fact that cues such as N-acetylglucosamine and CO2 promote distinct morphological switches in different contexts, as described above. Arguably the best characterized switch, in terms of its transcriptional regulation, is the whitea-opaquea switch of MTLa and α cells (
).
,
,
,
,
,
,
,
,
Importantly, this switch is controlled by numerous transcription factors in addition to the ‘master regulators’ Efg1 and Wor1,
,
,
,
,
,
,
,
,
which together form an interlocking circuit of positive and negative feedback loops.
,
Under conditions in which pro-whitea and pro-opaquea environmental cues are balanced (for example, glucose-containing medium maintained at 25°C), switching between whitea and opaquea cell types occurs infrequently and stochastically in single cells, and both whitea-to-opaquea and opaquea-to-whitea switching occurs.
In this setting, cell morphology is thought to be programmed by the predominance of either Efg1 or Wor1 protein, such that switching might be triggered by events such as unequal distribution of Efg1 or Wor1 protein between a mother and daughter cell.
,
In contrast, under environmental conditions that heavily favour the whitea (glucose-containing medium maintained in room air at 37°C) or opaquea (N-acetylglucosamine-containing medium in ≥5% CO2) state, switching occurs in a concerted fashion across the entire cell population.
,
,
,
In these settings, it seems likely that potent environmental cues reinforce or inhibit transcriptional regulatory components in addition to Efg1 and Wor1 to produce a specific outcome. Broadly speaking, Efg1 and Wor1 can be envisioned as central hubs that link morphological switch-specific transcription factors and signaling molecules to a large variety of potential morphological outcomes.
Conclusions
Even within the morphologically diverse fungal kingdom, C. albicans stands out for its remarkable plasticity. Not only does it shift between single-celled yeasts and mycelial forms, similar to dimorphic fungi, but this continuously host-associated species also switches among at least six distinct yeast-like morphotypes. Recent observations of GUT, opaquea/α and gray cells have provided fresh insights into the utility of morphological plasticity by showcasing C. albicans cell types that seem to be optimized for specific host niches, possibly via multiple mechanisms including metabolic polarization. Whereas hyphae and pseudohyphae predominate in most virulence models, with whitea/α yeasts also having an essential role in disseminated (bloodstream) infections, the newly described elongated yeasts may be more specialized for commensalism. For example, GUT cells were identified on the basis of their superior fitness in an intestinal commensalism model.
Meanwhile, opaquea/α and opaquea cells have been reported to outperform other cell types at skin colonization.
,
More extensive, direct comparisons of all cell types in additional animal models will be required to validate and extend these initial observations. If true, niche-specific morphologic specialization would offer a potential rationale for the introduction of the whitea-to-opaquea switch into the C. albicans mating program. That is, switching to the opaquea cell type could have less to do with the mechanics of mating per se than in optimizing the fitness of mating-competent cells in the host niche in which mating occurs (a matter of continued speculation in the field).
Clearly, much remains to be learned of the signals, fungal signaling pathways, and transcriptional regulatory networks that control C. albicans morphogenesis. In addition, it is unclear whether the transitions between commensal and pathogenic cell types are regulated and potentially subject to pharmaceutical intervention. Given that C. albicans is the most common fungal commensal-pathogen of humans, the answers to these questions will have important implications for human health and disease.
Key Points.
Candida albicans is a ubiquitous fungal component of the mammalian gut and skin microbiota
C. albicans can infect most human tissues and causes superficial and disseminated disease syndromes in both healthy and immunocompromised hosts
C. albicans shares with other fungi the ability to change shape in different environments. At least 9 different cell morphologies have been documented in this species
Different C. albicans cell types vary in their ability to colonize the host or cause disease, as well as to inhabit different host niches. Metabolic differences appear to account for some of the differences in fitness.
C. albicans has introduced an unusual cell type switch into its mating program
Researchers have identified numerous environmental (host) signals that trigger C. albicans morphological transitions in vitro
C. albicans signaling pathways transmit and integrate environmental information and induce morphological changes via fungal-specific transcription factors
Acknowledgments
We are grateful to Hiten D. Madhani for helpful comments on the review. The Noble Laboratory is supported by grant R01AI108992 from the US National Institutes of Health, an Investigators in the Pathogenesis of Infectious Disease award from the Burroughs Wellcome Fund, and a Scholar in the Biomedical Sciences award from the Pew Charitable trust. In addition, B.A.G. is supported by grant 1144247 from the US National Science Foundation, and J.W. is supported by grant T32AI060537 from the US National Institutes of Health.
Glossary
BuddingForm of asexual reproduction by yeast cells, in which a new cell develops as a focal outgrowth of the mother cell, followed by detachment once growth is complete.
CytokinesisDivision of the cytoplasm between mother and daughter cells after mitosis (or meiosis) is complete.
Dimorphic fungiSet of human fungal pathogens that grow as mycelia in the environment but as yeasts (or spherules, in the case of Coccidioides immitus) in mammalian hosts. These pathogens include Blastomyces dermatitidis, Coccidioides immitis, Histoplasma capsulatum, Paracoccidiodes brasiliensis, Penicillium marneffei, and Sporothrix schenckii.
MeiosisA type of cell division that produces four daughter cells, each with one-half of the DNA content of the mother. Used to generate sexually competent cells such as a and α cells in S. cerevisiae.
MyceliaMulticellular, filamentous structures produced by repeated rounds of cell division by hyphal or pseudohyphal cells.
Suspensor cellsTerminal cells in mycelial networks that give rise to chlamydospores under nutrient poor and oxygen-depleted conditions.
Biographies
Suzanne M. Noble is an Associate Professor of Microbiology & Immunology and a member of the Division of Infectious Diseases at the University of California, San Francisco, CA. Her research focuses on mechanisms of virulence and commensalism in the human fungal pathogen, Candida albicans.
Brittany A. Gianetti is a Ph.D. candidate in the laboratory of Suzanne Noble at the University of California, San Francisco, USA. She received her B.S. degree from Virginia Polytechnic Institute and State University, Blacksburg, VA.
Jessica Witchley is a Ph.D. candidate in the laboratory of Suzanne Noble at the University of California, San Francisco, USA. She received her B.S. degree from Massachusetts Institute of Technology, Cambridge, MA.
Footnotes
Competing interest statement
There is NO Competing Interest.
References
1.Hoffmann C, et al. Archaea and fungi of the human gut microbiome: correlations with diet and bacterial residents. PLoS One. 2013;8:e66019. doi: 10.1371/journal.pone.0066019. [
] [
] [
] [
]
2.Ghannoum MA, et al. Characterization of the oral fungal microbiome (mycobiome) in healthy individuals. PLoS Pathog. 2010;6:e1000713. doi: 10.1371/journal.ppat.1000713. [
] [
] [
] [
]
3.Findley K, et al. Topographic diversity of fungal and bacterial communities in human skin. Nature. 2013;498:367–370. doi: 10.1038/nature12171. [
] [
] [
] [
]
4.Oyeka CA, Ugwu LO. Fungal flora of human toe webs. Mycoses. 2002;45:488–491. doi: 10.1046/j.1439-0507.2002.00796.x. [
] [
] [
]
5.Drell T, et al. Characterization of the vaginal micro- and mycobiome in asymptomatic reproductive-age Estonian women. PLoS One. 2013;8:e54379. doi: 10.1371/journal.pone.0054379. [
] [
] [
] [
]
6.Merenstein D, et al. Colonization by Candida species of the oral and vaginal mucosa in HIV-infected and noninfected women. AIDS Res Hum Retroviruses. 2013;29:30–34. doi: 10.1089/aid.2012.0269. [
] [
] [
] [
]
7.Odds FC. Candida and Candidosis, a Review and Bibliography. 2. W.B. Saunders; London: 1988. [
]
8.Russell C, Lay KM. Natural history of Candida species and yeasts in the oral cavities of infants. Arch Oral Biol. 1973;18:957–962. doi: 10.1016/0003-9969(73)90176-3. [
] [
] [
]
9.Odds FC, et al. Candida albicans strain maintenance, replacement, and microvariation demonstrated by multilocus sequence typing. J Clin Microbiol. 2006;44:3647–3658. doi: 10.1128/JCM.00934-06. [
] [
] [
] [
]
10.Perlroth J, Choi B, Spellberg B. Nosocomial fungal infections: epidemiology, diagnosis, and treatment. Med Mycol. 2007;45:321–346. doi: 10.1080/13693780701218689. [
] [
] [
]
11.Edmond MB, et al. Nosocomial bloodstream infections in United States hospitals: a three-year analysis. Clin Infect Dis. 1999;29:239–244. doi: 10.1086/520192. [
] [
] [
]
12.Pfaller MA, Diekema DJ. Epidemiology of invasive candidiasis: a persistent public health problem. Clin Microbiol Rev. 2007;20:133–163. doi: 10.1128/CMR.00029-06. [
] [
] [
] [
]
13.Wisplinghoff H, et al. Nosocomial bloodstream infections in US hospitals: analysis of 24,179 cases from a prospective nationwide surveillance study. Clin Infect Dis. 2004;39:309–317. doi: 10.1086/421946. [
] [
] [
]
14.Xie J, et al. White-opaque switching in natural MTLa/alpha isolates of Candida albicans: evolutionary implications for roles in host adaptation, pathogenesis, and sex. PLoS Biol. 2013;11:e1001525. doi: 10.1371/journal.pbio.1001525. This paper identifies opaquea/α cells and describes their fitness in a neonatal skin colonization model. [
] [
] [
] [
]
15.Tao L, et al. Discovery of a “white-gray-opaque” tristable phenotypic switching system in candida albicans: roles of non-genetic diversity in host adaptation. PLoS Biol. 2014;12:e1001830. doi: 10.1371/journal.pbio.1001830. This paper identifies gray cells and describes their fitness in an ex vivo tongue infection model. [
] [
] [
] [
]
16.Pande K, Chen C, Noble SM. Passage through the mammalian gut triggers a phenotypic switch that promotes Candida albicans commensalism. Nat Genet. 2013 doi: 10.1038/ng.2710. This article identifies GUT cells and describes their fitness in the mammalian digestive tract. [
] [
] [
] [
]
17.Didier ES. Microsporidiosis: an emerging and opportunistic infection in humans and animals. Acta Trop. 2005;94:61–76. doi: 10.1016/j.actatropica.2005.01.010. [
] [
] [
]
18.Ferguson BA, Dreisbach TA, Parks CG, Filip GM, Schmitt CL. Coarse-scale population structure of pathogenic Armillaria species in a mixed-conifer forest in the Blue Mountains of northeast Oregon. Canadian Journal of Forest Research. :33. [
]
19.Edwards JA, et al. Histoplasma yeast and mycelial transcriptomes reveal pathogenic-phase and lineage-specific gene expression profiles. BMC Genomics. 2013;14:695. doi: 10.1186/1471-2164-14-695. [
] [
] [
] [
]
20.Nemecek JC, Wuthrich M, Klein BS. Global control of dimorphism and virulence in fungi. Science. 2006;312:583–588. doi: 10.1126/science.1124105. [
] [
] [
]
21.Beyhan S, Gutierrez M, Voorhies M, Sil A. A temperature-responsive network links cell shape and virulence traits in a primary fungal pathogen. PLoS Biol. 2013;11:e1001614. doi: 10.1371/journal.pbio.1001614. [
] [
] [
] [
]
22.Sudbery P, Gow N, Berman J. The distinct morphogenic states of Candida albicans. Trends Microbiol. 2004;12:317–324. doi: 10.1016/j.tim.2004.05.008. [
] [
] [
]
23.Thompson DS, Carlisle PL, Kadosh D. Coevolution of morphology and virulence in Candida species. Eukaryot Cell. 2011;10:1173–1182. doi: 10.1128/EC.05085-11. [
] [
] [
] [
]
24.Sudbery PE. Growth of Candida albicans hyphae. Nat Rev Microbiol. 2011;9:737–748. doi: 10.1038/nrmicro2636. [
] [
] [
]
25.Staib P, Morschhauser J. Chlamydospore formation in Candida albicans and Candida dubliniensis--an enigmatic developmental programme. Mycoses. 2007;50:1–12. doi: 10.1111/j.1439-0507.2006.01308.x. [
] [
] [
]
26.Warenda AJ, Konopka JB. Septin function in Candida albicans morphogenesis. Mol Biol Cell. 2002;13:2732–2746. doi: 10.1091/mbc.E02-01-0013. [
] [
] [
] [
]
27.Carlisle PL, et al. Expression levels of a filament-specific transcriptional regulator are sufficient to determine Candida albicans morphology and virulence. Proc Natl Acad Sci U S A. 2009;106:599–604. doi: 10.1073/pnas.0804061106. [
] [
] [
] [
]
28.Jansons VK, Nickerson WJ. Induction, morphogenesis, and germination of the chlamydospore of Candida albicans. J Bacteriol. 1970;104:910–921. doi: 10.1128/jb.104.2.910-921.1970. [
] [
] [
] [
]
29.Whiteway M, Bachewich C. Morphogenesis in Candida albicans. Annu Rev Microbiol. 2007;61:529–553. doi: 10.1146/annurev.micro.61.080706.093341. [
] [
] [
] [
]
30.Martin SW, Douglas LM, Konopka JB. Cell cycle dynamics and quorum sensing in Candida albicans chlamydospores are distinct from budding and hyphal growth. Eukaryot Cell. 2005;4:1191–1202. doi: 10.1128/EC.4.7.1191-1202.2005. [
] [
] [
] [
]
31.Thomson DD, et al. Contact-induced apical asymmetry drives the thigmotropic responses of Candida albicans hyphae. Cell Microbiol. 2015;17:342–354. doi: 10.1111/cmi.12369. [
] [
] [
] [
]
32.Brand A, Gow NA. Mechanisms of hypha orientation of fungi. Curr Opin Microbiol. 2009;12:350–357. doi: 10.1016/j.mib.2009.05.007. [
] [
] [
] [
]
33.Kadosh D, Johnson AD. Induction of the Candida albicans filamentous growth program by relief of transcriptional repression: a genome-wide analysis. Mol Biol Cell. 2005;16:2903–2912. doi: 10.1091/mbc.E05-01-0073. [
] [
] [
] [
]
34.Lane S, Birse C, Zhou S, Matson R, Liu H. DNA array studies demonstrate convergent regulation of virulence factors by Cph1, Cph2, and Efg1 in Candida albicans. J Biol Chem. 2001;276:48988–48996. doi: 10.1074/jbc.M104484200. [
] [
] [
]
35.Nantel A, et al. Transcription profiling of Candida albicans cells undergoing the yeast-to-hyphal transition. Mol Biol Cell. 2002;13:3452–3465. doi: 10.1091/mbc.E02-05-0272. [
] [
] [
] [
]
36.Moyes DL, et al. Candidalysin is a fungal peptide toxin critical for mucosal infection. Nature. 2016;532:64–68. doi: 10.1038/nature17625. [
] [
] [
] [
]
37.Moyes DL, et al. A biphasic innate immune MAPK response discriminates between the yeast and hyphal forms of Candida albicans in epithelial cells. Cell Host Microbe. 2010;8:225–235. doi: 10.1016/j.chom.2010.08.002. This paper determines differences between whitea/α yeasts and hyphae in a reconstituted human oral epithelial infection model. [
] [
] [
] [
]
38.Phan QT, et al. Als3 is a Candida albicans invasin that binds to cadherins and induces endocytosis by host cells. PLoS Biol. 2007;5:e64. doi: 10.1371/journal.pbio.0050064. [
] [
] [
] [
]
39.Peters BM, et al. Fungal morphogenetic pathways are required for the hallmark inflammatory response during Candida albicans vaginitis. Infect Immun. 2014;82:532–543. doi: 10.1128/IAI.01417-13. [
] [
] [
] [
]
40.Dalle F, et al. Cellular interactions of Candida albicans with human oral epithelial cells and enterocytes. Cell Microbiol. 2010;12:248–271. doi: 10.1111/j.1462-5822.2009.01394.x. [
] [
] [
]
41.Wachtler B, et al. Candida albicans-epithelial interactions: dissecting the roles of active penetration, induced endocytosis and host factors on the infection process. PLoS One. 2012;7:e36952. doi: 10.1371/journal.pone.0036952. [
] [
] [
] [
]
42.Di Carlo P, et al. Surgical pathology and the diagnosis of invasive visceral yeast infection: two case reports and literature review. World J Emerg Surg. 2013;8:38. doi: 10.1186/1749-7922-8-38. [
] [
] [
] [
]
43.Chin VK, et al. Multi-step pathogenesis and induction of local immune response by systemic Candida albicans infection in an intravenous challenge mouse model. Int J Mol Sci. 2014;15:14848–14867. doi: 10.3390/ijms150814848. [
] [
] [
] [
]
44.Gupta KL. Fungal infections and the kidney. Indian Journal of Nephrology. 2001;11:147–154. [
]
45.Braun BR, Johnson AD. Control of filament formation in Candida albicans by the transcriptional repressor TUP1. Science. 1997;277:105–109. doi: 10.1126/science.277.5322.105. [
] [
] [
]
46.Lo HJ, et al. Nonfilamentous C. albicans mutants are avirulent. Cell. 1997;90:939–949. doi: 10.1016/s0092-8674(00)80358-x. [
] [
] [
]
47.Murad AM, et al. NRG1 represses yeast-hypha morphogenesis and hypha-specific gene expression in Candida albicans. Embo J. 2001;20:4742–4752. doi: 10.1093/emboj/20.17.4742. [
] [
] [
] [
]
48.Saville SP, Lazzell AL, Monteagudo C, Lopez-Ribot JL. Engineered control of cell morphology in vivo reveals distinct roles for yeast and filamentous forms of Candida albicans during infection. Eukaryot Cell. 2003;2:1053–1060. doi: 10.1128/EC.2.5.1053-1060.2003. The paper dissects the roles of whitea/α yeasts vs. hyphae in disseminated infections, using a doxycycline-regulatable strain that can be forced into either morphology. [
] [
] [
] [
]
49.Desai JV, Mitchell AP. Candida albicans Biofilm Development and Its Genetic Control. Microbiol Spectr. 2015;3 doi: 10.1128/microbiolspec.MB-0005-2014. [
] [
] [
] [
]
50.Ramage G, Mowat E, Jones B, Williams C, Lopez-Ribot J. Our current understanding of fungal biofilms. Crit Rev Microbiol. 2009;35:340–355. doi: 10.3109/10408410903241436. [
] [
] [
]
51.Alicia ZS, Blanca OS, Mariana GH, Magdalena CC, Alexandro B. Rapid production of Candida albicans chlamydospores in liquid media under various incubation conditions. Nihon Ishinkin Gakkai Zasshi. 2006;47:231–234. doi: 10.3314/jjmm.47.231. [
] [
] [
]
52.Citiulo F, Moran GP, Coleman DC, Sullivan DJ. Purification and germination of Candida albicans and Candida dubliniensis chlamydospores cultured in liquid media. FEMS Yeast Res. 2009;9:1051–1060. doi: 10.1111/j.1567-1364.2009.00533.x. [
] [
] [
]
53.Chabasse D, Bouchara JP, de Gentile L, Chennebault JM. Candida albicans chlamydospores observed in vivo in a patient with AIDS. Ann Biol Clin (Paris) 1988;46:817–818. [
] [
]
54.Cole GT, Seshan KR, Phaneuf M, Lynn KT. Chlamydospore-like cells of Candida albicans in the gastrointestinal tract of infected, immunocompromised mice. Can J Microbiol. 1991;37:637–646. doi: 10.1139/m91-108. [
] [
] [
]
55.Slutsky B, et al. “White-opaque transition”: a second high-frequency switching system in Candida albicans. J Bacteriol. 1987;169:189–197. doi: 10.1128/jb.169.1.189-197.1987. This paper identifies opaquea cells. [
] [
] [
] [
]
56.Douglas LJ. Candida biofilms and their role in infection. Trends Microbiol. 2003;11 doi: 10.1016/s0966-842x(02)00002-1. [
] [
] [
]
57.Anderson J, Mihalik R, Soll DR. Ultrastructure and antigenicity of the unique cell wall pimple of the Candida opaque phenotype. J Bacteriol. 1990;172:224–235. doi: 10.1128/jb.172.1.224-235.1990. [
] [
] [
] [
]
58.Lohse MB, Johnson AD. Differential phagocytosis of white versus opaque Candida albicans by Drosophila and mouse phagocytes. PLoS One. 2008;3:e1473. doi: 10.1371/journal.pone.0001473. [
] [
] [
] [
]
59.Sasse C, Hasenberg M, Weyler M, Gunzer M, Morschhauser J. White-opaque switching of Candida albicans allows immune evasion in an environment-dependent fashion. Eukaryot Cell. 2013;12:50–58. doi: 10.1128/EC.00266-12. [
] [
] [
] [
]
60.Si H, Hernday AD, Hirakawa MP, Johnson AD, Bennett RJ. Candida albicans white and opaque cells undergo distinct programs of filamentous growth. PLoS Pathog. 2013;9:e1003210. doi: 10.1371/journal.ppat.1003210. [
] [
] [
] [
]
61.Guan G, et al. Bcr1 plays a central role in the regulation of opaque cell filamentation in Candida albicans. Molecular microbiology. 2013;89:732–750. doi: 10.1111/mmi.12310. [
] [
] [
] [
]
62.Tuch BB, et al. The transcriptomes of two heritable cell types illuminate the circuit governing their differentiation. PLoS Genet. 2011;6:e1001070. doi: 10.1371/journal.pgen.1001070. [
] [
] [
] [
]
63.Tsong AE, Miller MG, Raisner RM, Johnson AD. Evolution of a combinatorial transcriptional circuit: a case study in yeasts. Cell. 2003;115:389–399. doi: 10.1016/s0092-8674(03)00885-7. [
] [
] [
]
64.Lan CY, et al. Metabolic specialization associated with phenotypic switching in Candidaalbicans. Proc Natl Acad Sci U S A. 2002;99:14907–14912. doi: 10.1073/pnas.232566499. [
] [
] [
] [
]
65.Huang G, Srikantha T, Sahni N, Yi S, Soll DR. CO(2) regulates white-to-opaque switching in Candida albicans. Curr Biol. 2009;19:330–334. doi: 10.1016/j.cub.2009.01.018. [
] [
] [
] [
]
66.Huang G, et al. N-acetylglucosamine induces white to opaque switching, a mating prerequisite in Candida albicans. PLoS Pathog. 2010;6:e1000806. doi: 10.1371/journal.ppat.1000806. [
] [
] [
] [
]
67.Sun Y, et al. pH regulates white-opaque switching and sexual mating in Candida albicans. Eukaryot Cell. 2015 doi: 10.1128/EC.00123-15. [
] [
] [
] [
]
68.Miller MG, Johnson AD. White-opaque switching in Candida albicans is controlled by mating-type locus homeodomain proteins and allows efficient mating. Cell. 2002;110:293–302. doi: 10.1016/s0092-8674(02)00837-1. This study establishes links between the opaquea phenotype, MTL genotype and competency for mating. [
] [
] [
]
69.Madhani H. From a to alpha: Yeast as a Model for Cellular Differentiation. Cold Spring Harbor Laboratory Press; Cold Spring Harbor, New York: 2007. [
]
70.Hull CM, Raisner RM, Johnson AD. Evidence for mating of the “asexual” yeast Candida albicans in a mammalian host. Science. 2000;289:307–310. doi: 10.1126/science.289.5477.307. [
] [
] [
]
71.Magee BB, Magee PT. Induction of mating in Candida albicans by construction of MTLa and MTLalpha strains. Science. 2000;289:310–313. doi: 10.1126/science.289.5477.310. [
] [
] [
]
72.Hull CM, Johnson AD. Identification of a mating type-like locus in the asexual pathogenic yeast Candida albicans. Science. 1999;285:1271–1275. doi: 10.1126/science.285.5431.1271. [
] [
] [
]
73.Huang G, et al. Bistable expression of WOR1, a master regulator of white-opaque switching in Candida albicans. Proc Natl Acad Sci U S A. 2006;103:12813–12818. doi: 10.1073/pnas.0605270103. [
] [
] [
] [
]
74.Srikantha T, et al. TOS9 regulates white-opaque switching in Candida albicans. Eukaryot Cell. 2006;5:1674–1687. doi: 10.1128/EC.00252-06. [
] [
] [
] [
]
75.Zordan RE, Galgoczy DJ, Johnson AD. Epigenetic properties of white-opaque switching in Candida albicans are based on a self-sustaining transcriptional feedback loop. Proc Natl Acad Sci U S A. 2006;103:12807–12812. doi: 10.1073/pnas.0605138103. [
] [
] [
] [
]
76.Daniels KJ, Park YN, Srikantha T, Pujol C, Soll DR. Impact of environmental conditions on the form and function of Candida albicans biofilms. Eukaryot Cell. 2013;12:1389–1402. doi: 10.1128/EC.00127-13. [
] [
] [
] [
]
77.Park YN, Daniels KJ, Pujol C, Srikantha T, Soll DR. Candida albicans forms a specialized “sexual” as well as “pathogenic” biofilm. Eukaryot Cell. 2013;12:1120–1131. doi: 10.1128/EC.00112-13. [
] [
] [
] [
]
78.Bougnoux ME, et al. Mating is rare within as well as between clades of the human pathogen Candida albicans. Fungal Genet Biol. 2008;45:221–231. doi: 10.1016/j.fgb.2007.10.008. [
] [
] [
] [
]
79.Graser Y, et al. Molecular markers reveal that population structure of the human pathogen Candida albicans exhibits both clonality and recombination. Proc Natl Acad Sci U S A. 1996;93:12473–12477. doi: 10.1073/pnas.93.22.12473. [
] [
] [
] [
]
80.Legrand M, et al. Homozygosity at the MTL locus in clinical strains of Candida albicans: karyotypic rearrangements and tetraploid formation. Mol Microbiol. 2004;52:1451–1462. doi: 10.1111/j.1365-2958.2004.04068.x. [
] [
] [
]
81.Lockhart SR, et al. In Candida albicans, white-opaque switchers are homozygous for mating type. Genetics. 2002;162:737–745. doi: 10.1093/genetics/162.2.737. [
] [
] [
] [
]
82.Kvaal C, et al. Misexpression of the opaque-phase-specific gene PEP1 (SAP1) in the white phase of Candida albicans confers increased virulence in a mouse model of cutaneous infection. Infect Immun. 1999;67:6652–6662. doi: 10.1128/iai.67.12.6652-6662.1999. [
] [
] [
] [
]
83.Lockhart SR, Wu W, Radke JB, Zhao R, Soll DR. Increased virulence and competitive advantage of a/alpha over a/a or alpha/alpha offspring conserves the mating system of Candida albicans. Genetics. 2005;169:1883–1890. doi: 10.1534/genetics.104.038737. [
] [
] [
] [
]
84.Shapiro RS, et al. Hsp90 orchestrates temperature-dependent Candida albicans morphogenesis via Ras1-PKA signaling. Curr Biol. 2009;19:621–629. doi: 10.1016/j.cub.2009.03.017. [
] [
] [
] [
]
85.Biswas K, Morschhauser J. The Mep2p ammonium permease controls nitrogen starvation-induced filamentous growth in Candida albicans. Molecular microbiology. 2005;56:649–669. doi: 10.1111/j.1365-2958.2005.04576.x. [
] [
] [
]
86.Castilla R, Passeron S, Cantore ML. N-acetyl-D-glucosamine induces germination in Candida albicans through a mechanism sensitive to inhibitors of cAMP-dependent protein kinase. Cell Signal. 1998;10:713–719. doi: 10.1016/s0898-6568(98)00015-1. [
] [
] [
]
87.Feng Q, Summers E, Guo B, Fink G. Ras signaling is required for serum-induced hyphal differentiation in Candida albicans. Journal of bacteriology. 1999;181:6339–6346. doi: 10.1128/jb.181.20.6339-6346.1999. [
] [
] [
] [
]
88.Klengel T, et al. Fungal adenylyl cyclase integrates CO2 sensing with cAMP signaling and virulence. Curr Biol. 2005;15:2021–2026. doi: 10.1016/j.cub.2005.10.040. [
] [
] [
] [
]
89.Xu XL, et al. Bacterial peptidoglycan triggers Candida albicans hyphal growth by directly activating the adenylyl cyclase Cyr1p. Cell host & microbe. 2008;4:28–39. doi: 10.1016/j.chom.2008.05.014. [
] [
] [
]
90.Maidan MM, et al. The G protein-coupled receptor Gpr1 and the Galpha protein Gpa2 act through the cAMP-protein kinase A pathway to induce morphogenesis in Candida albicans. Mol Biol Cell. 2005;16:1971–1986. doi: 10.1091/mbc.E04-09-0780. [
] [
] [
] [
]
91.Hall RA, et al. The quorum-sensing molecules farnesol/homoserine lactone and dodecanol operate via distinct modes of action in Candida albicans. Eukaryotic cell. 2011;10:1034–1042. doi: 10.1128/EC.05060-11. [
] [
] [
] [
]
92.Leberer E, et al. Ras links cellular morphogenesis to virulence by regulation of the MAP kinase and cAMP signalling pathways in the pathogenic fungus Candida albicans. Molecular microbiology. 2001;42:673–687. doi: 10.1046/j.1365-2958.2001.02672.x. [
] [
] [
]
93.Fang HM, Wang Y. RA domain-mediated interaction of Cdc35 with Ras1 is essential for increasing cellular cAMP level for Candida albicans hyphal development. Molecular microbiology. 2006;61:484–496. doi: 10.1111/j.1365-2958.2006.05248.x. [
] [
] [
]
94.Rocha CR, et al. Signaling through adenylyl cyclase is essential for hyphal growth and virulence in the pathogenic fungus Candida albicans. Mol Biol Cell. 2001;12:3631–3643. doi: 10.1091/mbc.12.11.3631. [
] [
] [
] [
]
95.Zou H, Fang HM, Zhu Y, Wang Y. Candida albicans Cyr1, Cap1 and G-actin form a sensor/effector apparatus for activating cAMP synthesis in hyphal growth. Molecular microbiology. 2010;75:579–591. doi: 10.1111/j.1365-2958.2009.06980.x. [
] [
] [
]
96.Hoyer LL, et al. A Candida albicans cyclic nucleotide phosphodiesterase: cloning and expression in Saccharomyces cerevisiae and biochemical characterization of the recombinant enzyme. Microbiology. 1994;140( Pt 7):1533–1542. doi: 10.1099/13500872-140-7-1533. [
] [
] [
]
97.Bockmuhl DP, Krishnamurthy S, Gerads M, Sonneborn A, Ernst JF. Distinct and redundant roles of the two protein kinase A isoforms Tpk1p and Tpk2p in morphogenesis and growth of Candida albicans. Molecular microbiology. 2001;42:1243–1257. doi: 10.1046/j.1365-2958.2001.02688.x. [
] [
] [
]
98.Sonneborn A, et al. Protein kinase A encoded by TPK2 regulates dimorphism of Candida albicans. Molecular microbiology. 2000;35:386–396. doi: 10.1046/j.1365-2958.2000.01705.x. [
] [
] [
]
99.Goldberg D, Marbach I, Gross E, Levitzki A, Simchen G. A Candida albicans homolog of CDC25 is functional in Saccharomyces cerevisiae. Eur J Biochem. 1993;213:195–204. doi: 10.1111/j.1432-1033.1993.tb17748.x. [
] [
] [
]
100.Enloe B, Diamond A, Mitchell AP. A single-transformation gene function test in diploid Candida albicans. Journal of bacteriology. 2000;182:5730–5736. doi: 10.1128/jb.182.20.5730-5736.2000. [
] [
] [
] [
]
101.Hope H, Schmauch C, Arkowitz RA, Bassilana M. The Candida albicans ELMO homologue functions together with Rac1 and Dck1, upstream of the MAP Kinase Cek1, in invasive filamentous growth. Molecular microbiology. 2010;76:1572–1590. doi: 10.1111/j.1365-2958.2010.07186.x. [
] [
] [
]
102.Csank C, et al. Roles of the Candida albicans mitogen-activated protein kinase homolog, Cek1p, in hyphal development and systemic candidiasis. Infect Immun. 1998;66:2713–2721. doi: 10.1128/iai.66.6.2713-2721.1998. [
] [
] [
] [
]
103.Leberer E, et al. Signal transduction through homologs of the Ste20p and Ste7p protein kinases can trigger hyphal formation in the pathogenic fungus Candida albicans. Proc Natl Acad Sci U S A. 1996;93:13217–13222. doi: 10.1073/pnas.93.23.13217. [
] [
] [
] [
]
104.Csank C, et al. Derepressed hyphal growth and reduced virulence in a VH1 family-related protein phosphatase mutant of the human pathogen Candida albicans. Mol Biol Cell. 1997;8:2539–2551. doi: 10.1091/mbc.8.12.2539. [
] [
] [
] [
]
105.Kohler JR, Fink GR. Candida albicans strains heterozygous and homozygous for mutations in mitogen-activated protein kinase signaling components have defects in hyphal development. Proc Natl Acad Sci U S A. 1996;93:13223–13228. doi: 10.1073/pnas.93.23.13223. [
] [
] [
] [
]
106.Shapiro RS, Robbins N, Cowen LE. Regulatory circuitry governing fungal development, drug resistance, and disease. Microbiol Mol Biol Rev. 2011;75:213–267. doi: 10.1128/MMBR.00045-10. [
] [
] [
] [
]
107.Magee BB, Legrand M, Alarco AM, Raymond M, Magee PT. Many of the genes required for mating in Saccharomyces cerevisiae are also required for mating in Candida albicans. Mol Microbiol. 2002;46:1345–1351. doi: 10.1046/j.1365-2958.2002.03263.x. [
] [
] [
]
108.Bennett RJ, Uhl MA, Miller MG, Johnson AD. Identification and characterization of a Candida albicans mating pheromone. Molecular and cellular biology. 2003;23:8189–8201. doi: 10.1128/MCB.23.22.8189-8201.2003. [
] [
] [
] [
]
109.Chen J, Wang Q, Chen JY. CEK2, a Novel MAPK from Candida albicans Complement the Mating Defect of fus3/kss1 Mutant. Sheng Wu Hua Xue Yu Sheng Wu Wu Li Xue Bao (Shanghai) 2000;32:299–304. [
] [
]
110.Whiteway M, Dignard D, Thomas DY. Dominant negative selection of heterologous genes: isolation of Candida albicans genes that interfere with Saccharomyces cerevisiae mating factor-induced cell cycle arrest. Proc Natl Acad Sci U S A. 1992;89:9410–9414. doi: 10.1073/pnas.89.20.9410. [
] [
] [
] [
]
111.Chen J, Chen J, Lane S, Liu H. A conserved mitogen-activated protein kinase pathway is required for mating in Candida albicans. Mol Microbiol. 2002;46:1335–1344. doi: 10.1046/j.1365-2958.2002.03249.x. [
] [
] [
]
112.Herrero de Dios C, Roman E, Diez C, Alonso-Monge R, Pla J. The transmembrane protein Opy2 mediates activation of the Cek1 MAP kinase in Candida albicans. Fungal Genet Biol. 2013;50:21–32. doi: 10.1016/j.fgb.2012.11.001. [
] [
] [
]
113.Roman E, Cottier F, Ernst JF, Pla J. Msb2 signaling mucin controls activation of Cek1 mitogen-activated protein kinase in Candida albicans. Eukaryot Cell. 2009;8:1235–1249. doi: 10.1128/EC.00081-09. [
] [
] [
] [
]
114.Monge RA, Roman E, Nombela C, Pla J. The MAP kinase signal transduction network in Candida albicans. Microbiology. 2006;152:905–912. doi: 10.1099/mic.0.28616-0. [
] [
] [
]
115.Raitt DC, Posas F, Saito H. Yeast Cdc42 GTPase and Ste20 PAK-like kinase regulate Sho1-dependent activation of the Hog1 MAPK pathway. EMBO J. 2000;19:4623–4631. doi: 10.1093/emboj/19.17.4623. [
] [
] [
] [
]
116.Ushinsky SC, et al. CDC42 is required for polarized growth in human pathogen Candida albicans. Eukaryot Cell. 2002;1:95–104. doi: 10.1128/EC.1.1.95-104.2002. [
] [
] [
] [
]
117.Roman E, Nombela C, Pla J. The Sho1 adaptor protein links oxidative stress to morphogenesis and cell wall biosynthesis in the fungal pathogen Candida albicans. Mol Cell Biol. 2005;25:10611–10627. doi: 10.1128/MCB.25.23.10611-10627.2005. [
] [
] [
] [
]
118.Posas F, Saito H. Osmotic activation of the HOG MAPK pathway via Ste11p MAPKKK: scaffold role of Pbs2p MAPKK. Science. 1997;276:1702–1705. doi: 10.1126/science.276.5319.1702. [
] [
] [
]
119.Smith DA, Nicholls S, Morgan BA, Brown AJ, Quinn J. A conserved stress-activated protein kinase regulates a core stress response in the human pathogen Candida albicans. Mol Biol Cell. 2004;15:4179–4190. doi: 10.1091/mbc.E04-03-0181. [
] [
] [
] [
]
120.Alonso-Monge R, et al. Role of the mitogen-activated protein kinase Hog1p in morphogenesis and virulence of Candida albicans. J Bacteriol. 1999;181:3058–3068. doi: 10.1128/jb.181.10.3058-3068.1999. [
] [
] [
] [
]
121.Arana DM, Nombela C, Alonso-Monge R, Pla J. The Pbs2 MAP kinase kinase is essential for the oxidative-stress response in the fungal pathogen Candida albicans. Microbiology. 2005;151:1033–1049. doi: 10.1099/mic.0.27723-0. [
] [
] [
]
122.Calera JA, Zhao XJ, Calderone R. Defective hyphal development and avirulence caused by a deletion of the SSK1 response regulator gene in Candida albicans. Infect Immun. 2000;68:518–525. doi: 10.1128/iai.68.2.518-525.2000. [
] [
] [
] [
]
123.Gomez-Raja J, Davis DA. The beta-arrestin-like protein Rim8 is hyperphosphorylated and complexes with Rim21 and Rim101 to promote adaptation to neutral-alkaline pH. Eukaryotic cell. 2012;11:683–693. doi: 10.1128/EC.05211-11. [
] [
] [
] [
]
124.Wolf JM, Johnson DJ, Chmielewski D, Davis DA. The Candida albicans ESCRT pathway makes Rim101-dependent and -independent contributions to pathogenesis. Eukaryotic cell. 2010;9:1203–1215. doi: 10.1128/EC.00056-10. [
] [
] [
] [
]
125.Li M, Martin SJ, Bruno VM, Mitchell AP, Davis DA. Candida albicans Rim13p, a protease required for Rim101p processing at acidic and alkaline pHs. Eukaryotic cell. 2004;3:741–751. doi: 10.1128/EC.3.3.741-751.2004. [
] [
] [
] [
]
126.Davis D, Wilson RB, Mitchell AP. RIM101-dependent and-independent pathways govern pH responses in Candida albicans. Mol Cell Biol. 2000;20:971–978. doi: 10.1128/mcb.20.3.971-978.2000. [
] [
] [
] [
]
127.Lu Y, Su C, Solis NV, Filler SG, Liu H. Synergistic regulation of hyphal elongation by hypoxia, CO(2), and nutrient conditions controls the virulence of Candida albicans. Cell Host Microbe. 2013;14:499–509. doi: 10.1016/j.chom.2013.10.008. [
] [
] [
] [
]
128.Bastidas RJ, Heitman J, Cardenas ME. The protein kinase Tor1 regulates adhesin gene expression in Candida albicans. PLoS Pathog. 2009;5:e1000294. doi: 10.1371/journal.ppat.1000294. [
] [
] [
] [
]
129.White SJ, et al. Self-regulation of Candida albicans population size during GI colonization. PLoS Pathog. 2007;3:e184. doi: 10.1371/journal.ppat.0030184. [
] [
] [
] [
]
130.Stoldt VR, Sonneborn A, Leuker CE, Ernst JF. Efg1p, an essential regulator of morphogenesis of the human pathogen Candida albicans, is a member of a conserved class of bHLH proteins regulating morphogenetic processes in fungi. EMBO J. 1997;16:1982–1991. doi: 10.1093/emboj/16.8.1982. [
] [
] [
] [
]
131.Sonneborn A, Tebarth B, Ernst JF. Control of white-opaque phenotypic switching in Candida albicans by the Efg1p morphogenetic regulator. Infect Immun. 1999;67:4655–4660. doi: 10.1128/iai.67.9.4655-4660.1999. [
] [
] [
] [
]
132.Sonneborn A, Bockmuhl DP, Ernst JF. Chlamydospore formation in Candida albicans requires the Efg1p morphogenetic regulator. Infect Immun. 1999;67:5514–5517. doi: 10.1128/iai.67.10.5514-5517.1999. [
] [
] [
] [
]
133.Pan X, Heitman J. Sok2 regulates yeast pseudohyphal differentiation via a transcription factor cascade that regulates cell-cell adhesion. Mol Cell Biol. 2000;20:8364–8372. doi: 10.1128/mcb.20.22.8364-8372.2000. [
] [
] [
] [
]
134.Dutton JR, Johns S, Miller BL. StuAp is a sequence-specific transcription factor that regulates developmental complexity in Aspergillus nidulans. EMBO J. 1997;16:5710–5721. doi: 10.1093/emboj/16.18.5710. [
] [
] [
] [
]
135.Aramayo R, Peleg Y, Addison R, Metzenberg R. Asm-1+, a Neurospora crassa gene related to transcriptional regulators of fungal development. Genetics. 1996;144:991–1003. doi: 10.1093/genetics/144.3.991. [
] [
] [
] [
]
136.Michielse CB, et al. The Botrytis cinerea Reg1 protein, a putative transcriptional regulator, is required for pathogenicity, conidiogenesis, and the production of secondary metabolites. Mol Plant Microbe Interact. 2011;24:1074–1085. doi: 10.1094/MPMI-01-11-0007. [
] [
] [
]
137.Jonkers W, Dong Y, Broz K, Kistler HC. The Wor1-like protein Fgp1 regulates pathogenicity, toxin synthesis and reproduction in the phytopathogenic fungus Fusarium graminearum. PLoS Pathog. 2012;8:e1002724. doi: 10.1371/journal.ppat.1002724. [
] [
] [
] [
]
138.Nguyen VQ, Sil A. Temperature-induced switch to the pathogenic yeast form of Histoplasma capsulatum requires Ryp1, a conserved transcriptional regulator. Proceedings of the National Academy of Sciences of the United States of America. 2008;105:4880–4885. doi: 10.1073/pnas.0710448105. [
] [
] [
] [
]
139.Zordan RE, Miller MG, Galgoczy DJ, Tuch BB, Johnson AD. Interlocking transcriptional feedback loops control white-opaque switching in Candida albicans. PLoS Biol. 2007;5:e256. doi: 10.1371/journal.pbio.0050256. This paper describes a genetic analysis of the regulatory circuit controlling the whitea-opaquea switch. [
] [
] [
] [
]
140.Lassak T, et al. Target specificity of the Candida albicans Efg1 regulator. Mol Microbiol. 2011;82:602–618. doi: 10.1111/j.1365-2958.2011.07837.x. [
] [
] [
]
141.Wang H, et al. Candida albicans Zcf37, a zinc finger protein, is required for stabilization of the white state. FEBS Lett. 2011;585:797–802. doi: 10.1016/j.febslet.2011.02.005. [
] [
] [
]
142.Lohse MB, et al. Identification and characterization of a previously undescribed family of sequence-specific DNA-binding domains. Proc Natl Acad Sci U S A. 2013;110:7660–7665. doi: 10.1073/pnas.1221734110. [
] [
] [
] [
]
143.Hernday AD, et al. Ssn6 Defines a New Level of Regulation of White-Opaque Switching in Candida albicans and Is Required For the Stochasticity of the Switch. MBio. 2016;7:e01565–01515. doi: 10.1128/mBio.01565-15. [
] [
] [
] [
]
144.Lohse MB, et al. Systematic Genetic Screen for Transcriptional Regulators of the Candida albicans White-Opaque Switch. Genetics. 2016 doi: 10.1534/genetics.116.190645. [
] [
] [
] [
]
145.Lohse MB, Johnson AD. Identification and Characterization of Wor4, a New Transcriptional Regulator of White-Opaque Switching. G3 (Bethesda) 2016;6:721–729. doi: 10.1534/g3.115.024885. [
] [
] [
] [
]
146.Hernday AD, et al. Structure of the transcriptional network controlling white-opaque switching in Candida albicans. Mol Microbiol. 2013;90:22–35. doi: 10.1111/mmi.12329. [
] [
] [
] [
]
147.Edwards JA. Candida Species. In: Bennett JE, Dolin R, Blaser MJ, editors. Principles and Practice of Infectious Diseases. Saunders; 2014. pp. 2879–2894. [
]
148.Nobile CJ, Johnson AD. Candida albicans Biofilms and Human Disease. Annu Rev Microbiol. 2015;69:71–92. doi: 10.1146/annurev-micro-091014-104330. [
] [
] [
] [
]
149.Soll DR, Daniels KJ. Plasticity of Candida albicans Biofilms. Microbiol Mol Biol Rev. 2016;80:565–595. doi: 10.1128/MMBR.00068-15. [
] [
] [
] [
]
150.Banerjee M, et al. UME6, a novel filament-specific regulator of Candida albicans hyphal extension and virulence. Mol Biol Cell. 2008;19:1354–1365. doi: 10.1091/mbc.E07-11-1110. [
] [
] [
] [
]
151.Zeidler U, et al. UME6 is a crucial downstream target of other transcriptional regulators of true hyphal development in Candida albicans. FEMS Yeast Res. 2009;9:126–142. doi: 10.1111/j.1567-1364.2008.00459.x. [
] [
] [
]
152.Brown DH, Jr, Giusani AD, Chen X, Kumamoto CA. Filamentous growth of Candida albicans in response to physical environmental cues and its regulation by the unique CZF1 gene. Mol Microbiol. 1999;34:651–662. doi: 10.1046/j.1365-2958.1999.01619.x. [
] [
] [
]
153.Lu Y, Su C, Liu H. A GATA transcription factor recruits Hda1 in response to reduced Tor1 signaling to establish a hyphal chromatin state in Candida albicans. PLoS Pathog. 2012;8:e1002663. doi: 10.1371/journal.ppat.1002663. [
] [
] [
] [
]
154.Su C, Lu Y, Liu H. Reduced TOR signaling sustains hyphal development in Candida albicans by lowering Hog1 basal activity. Mol Biol Cell. 2013;24:385–397. doi: 10.1091/mbc.E12-06-0477. [
] [
] [
] [
]
155.Braun BR, Kadosh D, Johnson AD. NRG1, a repressor of filamentous growth in C.albicans, is down-regulated during filament induction. Embo J. 2001;20:4753–4761. doi: 10.1093/emboj/20.17.4753. [
] [
] [
] [
]
156.Kadosh D, Johnson AD. Rfg1, a protein related to the Saccharomyces cerevisiae hypoxic regulator Rox1, controls filamentous growth and virulence in Candida albicans. Mol Cell Biol. 2001;21:2496–2505. doi: 10.1128/MCB.21.7.2496-2505.2001. [
] [
] [
] [
]
157.Khalaf RA, Zitomer RS. The DNA binding protein Rfg1 is a repressor of filamentation in Candida albicans. Genetics. 2001;157:1503–1512. doi: 10.1093/genetics/157.4.1503. [
] [
] [
] [
]
158.Ishii N, Yamamoto M, Yoshihara F, Arisawa M, Aoki Y. Biochemical and genetic characterization of Rbf1p, a putative transcription factor of Candida albicans. Microbiology. 1997;143( Pt 2):429–435. doi: 10.1099/00221287-143-2-429. [
] [
] [
]
159.Sahni N, et al. Genes selectively up-regulated by pheromone in white cells are involved in biofilm formation in Candida albicans. PLoS Pathog. 2009;5:e1000601. doi: 10.1371/journal.ppat.1000601. [
] [
] [
] [
]
160.Zhao R, et al. Unique aspects of gene expression during Candida albicans mating and possible G(1) dependency. Eukaryot Cell. 2005;4:1175–1190. doi: 10.1128/EC.4.7.1175-1190.2005. [
] [
] [
] [
]
161.Srikantha T, et al. The sea pansy Renilla reniformis luciferase serves as a sensitive bioluminescent reporter for differential gene expression in Candida albicans. J Bacteriol. 1996;178:121–129. doi: 10.1128/jb.178.1.121-129.1996. [
] [
] [
] [
]
162.Lane S, Zhou S, Pan T, Dai Q, Liu H. The basic helix-loop-helix transcription factor Cph2 regulates hyphal development in Candida albicans partly via TEC1. Mol Cell Biol. 2001;21:6418–6428. doi: 10.1128/MCB.21.19.6418-6428.2001. [
] [
] [
] [
]
163.Nie X, Liu X, Wang H, Chen J. Deletion of EFG1 promotes Candida albicans opaque formation responding to pH via Rim101. Acta Biochim Biophys Sin (Shanghai) 2010;42:735–744. doi: 10.1093/abbs/gmq076. [
] [
] [
]
164.Bockmuhl DP, Ernst JF. A potential phosphorylation site for an A-type kinase in the Efg1 regulator protein contributes to hyphal morphogenesis of Candida albicans. Genetics. 2001;157:1523–1530. doi: 10.1093/genetics/157.4.1523. [
] [
] [
] [
]
165.Leng P, Lee PR, Wu H, Brown AJ. Efg1, a morphogenetic regulator in Candida albicans, is a sequence-specific DNA binding protein. J Bacteriol. 2001;183:4090–4093. doi: 10.1128/JB.183.13.4090-4093.2001. [
] [
] [
] [
]
166.Tebarth B, et al. Adaptation of the Efg1p morphogenetic pathway in Candida albicans by negative autoregulation and PKA-dependent repression of the EFG1 gene. J Mol Biol. 2003;329:949–962. doi: 10.1016/s0022-2836(03)00505-9. [
] [
] [
]
167.Doedt T, et al. APSES proteins regulate morphogenesis and metabolism in Candida albicans. Mol Biol Cell. 2004;15:3167–3180. doi: 10.1091/10.1091/mbc.E03-11-0782. [
] [
] [
] [
]