This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Biswas, S. Articles by Datta, A. Search for Related Content PubMed PubMed Citation Articles by Biswas, S. Articles by Datta, A.

 Previous Article  |  Next Article 

Microbiology and Molecular Biology Reviews, June 2007, p. 348-376, Vol. 71, No. 2
1092-2172/07/$08.00+0     doi:10.1128/MMBR.00009-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Environmental Sensing and Signal Transduction Pathways Regulating Morphopathogenic Determinants of Candida albicans

Subhrajit Biswas,^1,, Patrick Van Dijck,^2,3, and Asis Datta^1*

National Centre for Plant Genome Research, New Delhi 110 067, India,¹ Department of Molecular Microbiology, VIB,² Laboratory of Molecular Cell Biology, Katholieke Universiteit Leuven, Kasteelpark Arenberg 31, B-3001 Leuven-Heverlee, Flanders, Belgium³

SUMMARY

INTRODUCTION

DIMORPHISM, AN IMPORTANT VIRULENCE FACTOR

GENETIC MANIPULATION WITH C. ALBICANS

ENVIRONMENTAL SENSING PATHWAYS REQUIRED FOR MORPHOGENESIS AND PATHOGENESIS

Amino Acid Availability Regulates Morphogenesis of C. albicans

Amino acid sensing by Csy1.

Amino acid sensing (and transport) by the general amino acid permease Gap1.

Amino acid sensing by Gpr1.

Ammonium sensing by Mep2.

Role of Gcn4 in nitrogen-regulated morphogenesis in C. albicans.

Signal Transduction, Quorum Sensing, and the MAPK Cascade Module

cAMP-PKA Pathway

Upstream components of the cAMP-PKA pathway.

Ras1, the master hyphal regulator.

Cyr1, Srv2, and Pde2.

Adenylate cyclase and CO2 sensing.

PKA.

Efg1 and Efh1, Major Transcriptional Regulators

Efg1 is a downstream component in the cAMP-PKA pathway.

Roles of Efg1 and Efh1 in morphogenesis in C. albicans.

Efg1 and embedded growth.

Efg1 and phenotype switching.

Efg1 and cell wall dynamics.

Convergent Regulation of Cph1 and Efg1: Involvement of Tec1 and Cph2

Other Positive Regulators: Cdc5, G1 Cyclins, Int1, Mcm1, and Fkh2

Transcriptional Repressor Tup1

Tup1 repression with Nrg1, Mig1, and Rfg1.

Other Negative Regulators: Rap1, Rbf1, and Rad6

Other MAPK Pathways Involved in Morphogenesis

The HOG MAPK pathway: response to oxidative stress.

Role of the PKC pathway in C. albicans morphology.

GlcNAc Catabolic Pathway of C. albicans

ß-N-Acetylglucosaminidase and virulence.

pH Regulation in C. albicans

Genes involved in pH regulation.

Rim101-dependent and -independent pathways.

Participation of endocytic machinery in pH signaling.

DOWNSTREAM TARGETS OF THE DIFFERENT ENVIRONMENTAL SENSING PATHWAYS INVOLVED IN MORPHOGENESIS

Adhesins

Role of Hwp1 in morphogenesis and pathogenicity.

ALS family and adhesion.

Extracellular Hydrolytic Enzymes

SAP gene expression in the yeast-to-hypha transition.

Phospholipase B.

FUTURE CHALLENGES

FINAL OUTLOOK

ACKNOWLEDGMENTS

REFERENCES

Top Next References

INTRODUCTION

Top Previous Next References

 
Opportunistic fungal pathogens, such as Candida albicans, are found in the normal gastrointestinal flora and the oral mucosa of most healthy humans. However, in immunocompromised patients, bloodstream infections often cause death, despite the use of antifungal therapies (152). The underlying molecular mechanisms for survival inside the human body and adaptation to various environments are probably distinct but overlapping. Dietary factors, such as an excess of or deficiency in certain nutrients, may alter the endogenous microbial flora. Mechanical factors, such as trauma or occlusive injury, can also alter the microenvironment, deplete the system of "friendly bacteria," and enable the pathogenic fungus to take over. Immunocompromised or immunosuppressed persons, including AIDS patients, neonates, and transplant recipients, are also particularly susceptible to fungal infections.

The most common systemic fungal infection is candidiasis, which accounts for well over half of these invasive mycoses. A single species, C. albicans, causes the majority of these infections. Its success stems in part from its capacity to live as a benign commensal in a variety of body locations, most notably the oral cavity, genitalia, and gastrointestinal tract (272). C. albicans expresses various traits critical for existence on mucosal surfaces, where a constant but dynamic interplay occurs between innate and acquired host defense mechanisms. The pathogenic Candida species also establish well-developed biofilms, which occur easily on various implants and are resistant to antifungal agents (76). The nature of disease resulting from tissue invasion by this organism is complex and depends on a variety of physical and physiological conditions in the host and on specific C. albicans traits. The capacity of C. albicans to rapidly acquire resistance to antifungal drugs, such as amphotericin B, flucytosine, and a series of azoles, means that continued development of new antifungals remains an important focus for clinicians and pharmaceutical companies.

An important feature of C. albicans, relevant to its pathogenesis, is its ability to switch between different morphological forms. C. albicans can grow in a single-celled, budding yeast form (blastospore) or in a filamentous form (including both pseudohyphae and true hyphae) (31). A crucial component of this versatility is the ability to survive as a commensal in several anatomically distinct sites, each with its own specific set of environmental pressures. Thus, C. albicans must be able to adapt its growth to a range of physiological extremes. To achieve adaptability, the fungus has evolved sophisticated mechanisms of sensing and responding to environmental cues by activating developmental switches that result in coordinated changes in cell physiology, morphology, and adherence. Progress in understanding many aspects of the biology of C. albicans has been hindered by the inability to carry out simple, large-scale genetic screens because of the diploid nature of this organism. A major breakthrough in assessing the contribution of specific genes to morphogenesis and virulence occurred with the development of transformation protocols and methods of deleting both alleles of a gene sequentially. Also, whole-genome microarray analysis has now become an important tool for probing signal transduction pathways during morphogenesis in C. albicans (102).

Additional interest in the molecular mechanisms of C. albicans morphopathogenic determinants originated from the necessity of identifying new drug targets due to increased drug resistance in clinical isolates. There is hope that recently developed techniques of manipulating C. albicans and the sequencing of its whole genome will lead to a thorough understanding of its virulence and biology, thus offering the possibility of a knowledge-based approach to the development of novel antifungal agents. A major strategy for determining virulence genes as molecular targets for antifungal drugs and vaccines is to identify a specific biochemical or structural target unique to C. albicans (or to fungi in general) in an attempt to specifically and selectively disrupt them and determine their effects on virulence.

In this review, we focus on recent advances in the environmental sensing and signal transduction pathways that mediate the morphogenesis and pathogenesis of C. albicans. Where possible, we compare the pathways of C. albicans with the analogous pathways/genes in Saccharomyces cerevisiae.

DIMORPHISM, AN IMPORTANT VIRULENCE FACTOR

Top Previous Next References

 
The terms "dimorphism" and "dimorphic fungus" (i.e., existing in two morphological forms) are commonly accepted in reference to C. albicans. Strictly speaking, however, this fungus has the ability to adopt a spectrum of morphologies; thus, C. albicans can be considered a "polymorphic" or "pleomorphic" organism (71, 289). The production of germ tubes results in conversion to a filamentous growth phase or hypha, also called the mycelial form. The formation of pseudohyphae occurs by polarized cell division when yeast cells growing by budding have elongated without detaching from adjacent cells. Under certain nonoptimal growth conditions, C. albicans can undergo the formation of chlamydospores, which are round, retractile spores with a thick cell wall. These morphological transitions often represent a response of the fungus to changing environmental conditions and may permit adaptation to a different biological niche. The transition from a commensal to pathogenic lifestyle may also involve changes in environmental conditions and dispersion within the human host. Although progress has been achieved in recent years, the molecular mechanisms governing these morphogenetic conversions are still not fully understood, partly due to the difficulty of genetic manipulations with C. albicans (164), an issue we address briefly below.

GENETIC MANIPULATION WITH C. ALBICANS

Top Previous Next References

 
Sequencing of the genome of C. albicans has recently been completed. C. albicans has a diploid genome consisting of eight pairs of chromosomes that can be separated by pulsed-field gel electrophoresis. With a size of 16 Mb, the haploid genome is slightly larger than that of the model yeast, S. cerevisiae. More information on the C. albicans genome can be found at www.candidagenome.org (68). Many genes are conserved between S. cerevisiae and C. albicans, and it is based on this similarity that the mechanisms of many biological processes in C. albicans have been discovered. As highlighted throughout this review, in many cases, although the specific components of relevant signaling pathways are conserved, molecular mechanisms and environmental signals have often diverged, most likely because of the coevolution of C. albicans and its human host.

C. albicans poses special problems for scientists interested in studying gene function because it is diploid and because CUG is translated into a serine instead of a leucine (164). To analyze the function of a gene, one must disrupt each of the two alleles by transformation. Although methods of transformation (spheroplast-polyethylene glycol, lithium acetate, and electroporation) are patterned after those used with S. cerevisiae, the transformation efficiency is poor. For the study of gene function in C. albicans, a number of disruption protocols and selectable markers are commonly utilized (300). The most widely used marker is the URA3 gene, which encodes orotidine-5'-phosphate decarboxylase and confers uracil prototrophy. However, this marker must be used with caution; URA3 expression levels can affect virulence and are susceptible to chromosome position effects, thereby complicating the analysis of strains constructed with URA3 as a selectable marker (62, 261, 281, 292). To avoid the use of the URA3 marker, various other auxotrophic markers, as well as dominant markers, have been developed and are currently used (206, 224, 241, 263). To study the functions of essential genes, the C. albicans MET3 promoter, which is regulated by the level of methionine and/or cysteine in the medium, or the tetracycline on/off system can be used. These systems allow conditional expression so that the consequences of depletion of a gene product may be investigated (57, 231, 254). In order to study the expression of certain genes at either the RNA level or the protein level, a number of C. albicans-optimized reporter constructs, or tags, have been generated (32). The development of these molecular tools has greatly accelerated the elucidation of morphogenesis and pathogenesis in C. albicans.

ENVIRONMENTAL SENSING PATHWAYS REQUIRED FOR MORPHOGENESIS AND PATHOGENESIS

Top Previous Next References

 
All organisms, from bacteria and yeast to higher eukaryotes, respond to changes that occur in the environment. In C. albicans, the yeast-to-hypha transition is triggered by various environmental cues, such as serum, N-acetylglucosamine (GlcNAc), neutral pH, high temperature, starvation, CO₂, and adherence. In recent years, receptors/sensors that may mediate environmental responses have been identified and partially characterized. Many in vivo and in vitro experiments point to a prominent role for amino acids as nitrogen sources and as ligands for membrane receptors involved in the regulation of cellular morphology and virulence (summarized in Fig. 1 and Table 1). In the following sections, we highlight work aimed at identifying receptors, ligands, and signaling pathways involved in the sensing of environmental signals in C. albicans.

View larger version (37K): [in this window] [in a new window]   FIG. 1. Regulation of dimorphism in C. albicans by multiple signaling pathways. The Cph1-mediated MAPK pathway and the Efg1-mediated cAMP pathway are well-characterized signaling pathways in dimorphic regulation. In C. albicans, Ras1 is an important regulator of hyphal development and likely functions upstream of both pathways. In the cAMP-PKA pathway, two catalytic subunits or isoforms of PKA, Tpk1 and Tpk2, have differential effects on hyphal morphogenesis under different hypha-inducing conditions. The MAPK cascade includes Cst20 (PAK), Hst7 (MAPKK), Cek1 (MAPK), and the downstream transcription factor Cph1, which is a homolog of the S. cerevisiae transcription factor Ste12. Transcription of the hyphal regulator TEC1 is regulated by Efg1 and Cph2. Rim101 or Czf1 may function through Efg1 or act in parallel with Efg1. Tup1 is the negative regulator of the hyphal transition. Tup1, recruited by Rfg1, Nrg1, or Mig1, and Rbf1 are also implicated in dimorphic transitions. GlcNAc-inducible hexokinase, Hxk1, plays a negative role in hyphal development under certain conditions. Cell wall proteins (HWP1 and ECE1, etc., which are involved in adherence) are also regulated by Efg1. Transcription factors are shown in rectangular boxes.

Amino acid sensing (and transport) by the general amino acid permease Gap1. In S. cerevisiae, the Gap1 protein functions not only as an amino acid transporter but also as an amino acid sensor for rapid activation of a fermentable growth medium-induced signaling pathway that controls protein kinase A (PKA) targets (83, 140). Receptors with dual roles as sensors have been dubbed "transceptors" (124). Our group has discovered that the C. albicans Gap1 transporter, which is a functional homolog of the budding yeast gene, may also be a transceptor important for initiating signal transduction pathways resulting in morphogenesis and virulence. GAP1 was isolated in a screen to identify and characterize genes that could be involved in the regulation of morphogenesis and virulence induced by GlcNAc, an important hyphal induction regulator of C. albicans (35) (see below). A null mutant of GAP1 has a defect in filamentation under nitrogen starvation conditions, as well as upon the addition of GlcNAc (35). Addition of serum, however, still resulted in the induction of filamentation in the gap1 mutant, suggesting that Gap1 is not required for serum-induced hyphal formation. As in S. cerevisiae, the expression of GAP1 is strongly repressed in medium containing ammonium as a good nitrogen source. How exactly amino acid transport (and/or sensing) results in hyphal formation remains to be determined. One interesting link is the fact that GlcNAc induces GAP1 expression at the yeast-to-germ-tube transition. This induction depends on Cph1, the transcription factor that is the downstream target of the mitogen-activated protein kinase (MAPK) pathway (see below). This may indicate that Gap1-mediated filamentation is regulated by this pathway.

Amino acid sensing by Gpr1. In S. cerevisiae, ScGpr1 is a G protein-coupled receptor that functions upstream of the cyclic AMP (cAMP)-PKA pathway and promotes a rapid cAMP increase after the addition of glucose or sucrose (156). In contrast, C. albicans Gpr1 is not required for the glucose-induced increase in cAMP (193) but seems to directly or indirectly sense methionine (193). Addition of methionine to wild-type cells results in a rapid internalization of Gpr1, reminiscent of ligand-induced internalization, a typical feature of GPCR systems in higher eukaryotic cells. Gpr1 is also required for the methionine-induced yeast-to-hypha transition (193). Further research is required to determine the mechanism by which Gpr1 activates the PKA pathway and what the role of methionine is in this mechanism. Interestingly, a role for methionine in morphogenesis is not limited to C. albicans. Recently, Heitman and colleagues discovered that the addition of methionine results in the internalization of a G protein-coupled receptor (Gpr4) in Cryptococcus neoformans and that a gpr4 null mutant is still responsive to glucose (323). We discuss the Gpr1 receptor in more detail in "cAMP-PKA Pathway," below.

Ammonium sensing by Mep2. Ammonium transport in C. albicans is mediated by Mep1 and Mep2 (33). Recently it was shown that Mep2, but not Mep1, also functions as a receptor for the induction of filamentous growth under nitrogen starvation conditions, placing Mep2 in the transceptor family of proteins (33, 124). More detailed analysis has shown that, as with the budding yeast protein ScGap1, the C terminus of Mep2 is required for proper sensing but not for transport (33). Signaling by Mep2 functions through both the MAPK and the cAMP-PKA pathways and is Ras1 dependent (see below for more information on the Ras pathway). Under high ammonium conditions, when expression of Mep2 is repressed, filamentation is blocked. A role for Mep2 in polarized morphogenesis is supported by work with both budding yeast and Ustilago maydis. In S. cerevisiae, ScMep2 is required for pseudohyphal induction (186). The Mep2 homolog of U. maydis (Ump1) is able to complement an S. cerevisiae mep1 mep2 mep3 strain both for ammonium transport and for pseudohyphal induction (269). Interestingly, mutation of a putative PKA phosphorylation site in either Mep2 or Ump1 specifically blocked pseudohyphal induction, whereas transport capacity was not affected. This result suggests that phosphorylation by PKA is required for Mep2 to be able to exert its effect on pseudohyphal induction.

Role of Gcn4 in nitrogen-regulated morphogenesis in C. albicans. Nitrogen availability has profound significance in fungal biology. In response to nitrogen limitation, fungi initiate morphological changes, sexual and asexual sporulation, and expression of virulence determinants (200). For instance, haploid MAT cells of C. neoformans, which typically grow as yeast, develop hyphae and fruiting bodies in nitrogen-deficient media; these processes are inhibited by ammonia (318). In S. cerevisiae, starvation for a single amino acid stimulates the expression of genes for all amino acid biosynthetic pathways in a phenomenon termed general amino acid control (GCN response) (122). Nitrogen catabolite repression, the regulation of gene expression in relation to nitrogen availability, has been demonstrated in numerous fungi (200). Though somewhat a misnomer (192), this global control mechanism prevents expression of the many genes required to utilize various secondary nitrogen sources when an adequate supply of preferred nitrogen sources is available, typically ammonium or glutamine (200). Nitrogen regulation is controlled by global positive (Gln3 and Gat1) and negative (Dal80 and Deh1) transcription factors as well as by pathway-specific transcription factors.

Tripathi et al. reported that C. albicans can respond to amino acid starvation in at least two ways: by stimulating cellular morphogenesis and by a GCN-like response (302). Both responses are dependent on Gcn4, a functional homolog of the S. cerevisiae transcription factor ScGcn4. Hence, Gcn4 plays a central role in coordinating morphogenetic and metabolic responses to amino acid starvation in C. albicans (302). In S. cerevisiae, ScGcn4 is mainly regulated at the translational level by the eIF-2 kinase ScGcn2 (122). In C. albicans, however, Gcn4 appears to be regulated mainly at the transcriptional level, with only a small contribution for Gcn2 (301). These differences between C. albicans and S. cerevisiae fit with the fact that ScGcn4 has not been implicated in pseudohyphal development (101), and transcript profiling of the GCN response has not revealed any obvious link with pseudohyphal development in S. cerevisiae (214). These results suggest a divergence in the cellular roles of Gcn4 between S. cerevisiae and C. albicans and underscore the importance of amino acids in the regulation of morphogenesis in C. albicans. In budding yeast, Gcn4 is targeted for degradation by the Cdc4 ubiquitin ligase complex, and deletion of CDC4 in C. albicans results in constitutive hyphal growth (8). This observation suggests that Cdc4-mediated protein degradation may be involved in the regulation of the dimorphic switch. Possible key targets of Cdc4 include Sol1, a homolog of ScSic1, a cyclin-dependent kinase inhibitor, and transcription factor Tec1 (63, 257), which are all targets of ScCdc4. So far, however, none of the candidates alone account for the hyperfilamentation phenotype of the cdc4 mutant.

Other elements of the Cek1 pathway have small but varied effects on virulence. cst20 mutant strains have a modest virulence defect in a mouse model of systemic candidiasis (171). However, hst7 and cph1 mutant strains are able to cause lethal infection in mice at rates comparable to those of wild-type strains (171, 182). In addition to these components, a MAPK phosphatase, Cpp1, that regulates filamentous growth in C. albicans has been identified (69, 256). Disruption of both alleles of the CPP1 gene derepresses hyphal production and results in a hyperfilamentous phenotype. This hyperfilamentation is suppressed by deletion of the MAPK Cek1 (69), indicating that Cpp1 functions primarily in this MAPK pathway. cpp1 mutant strains are also reduced for virulence in both systemic and localized models of candidiasis (69, 114).

Activation of the MAPK pathway can occur through the Cdc42 small GTPase. Cdc42 binds with high affinity to the Cst20 kinase, as well as to a second PAK kinase, Cla4 (172, 287). Cdc42 and its exchange factor, Cdc24, are both required for hyphal growth (22, 305, 306). Certain point mutants of Cdc42 do not affect its mitotic functions but strongly affect morphogenesis upon serum stimulation. Consistent with this phenotype, the expression of hypha-specific genes is also reduced or transient in these mutants (23, 306). Addition of serum results in a transient increase in CDC24 expression, and this increased expression depends on Tec1, the second transcription factor (the homolog of ScTec1) downstream of the Cek1 MAPK module (23). Cdc24 is then recruited to the tip of the hypha. The regulatory relationship between Tec1 and Cdc24 suggests a positive feedback loop that contributes to the level of active Cdc42 at the tip of the germ tube.

Inputs into the Cek1 MAPK pathway may also occur by quorum sensing. Quorum-sensing molecules allow bacteria to monitor their growth and to control cell density-dependent phenomena. Recently, similar regulatory molecules, tyrosol and farnesol, were identified in C. albicans, and they are involved in morphogenesis. Studies of the morphological transition from a filamentous to a budding yeast form in C. albicans revealed excretion of an autoregulatory substance, 3,7,11-trimethyl-2,6,10-dodecatrienoate, or farnesol, into the medium. Farnesol inhibits filamentous growth and may be involved in developmental signaling (226). Whereas farnesol is a compound that prevents hyphal formation (it is produced during high-density growth), tyrosol stimulates the growth of Candida cells and, under the proper conditions, hyphal formation (61, 125). Recent work shows that farnesol may function through the Cek1 MAPK pathway to inhibit morphogenesis, as the addition of farnesol represses the expression of CPH1 and HST7 (253). How farnesol is sensed and how the signal is transmitted to this MAPK pathway remains to be determined. Interestingly, C. albicans mutants lacking the histidine kinase Chk1 (see below) are refractory to the inhibitory effect of farnesol both in cell suspension and during the formation of a biofilm (158). This indicates a role of two-component signal transduction proteins in quorum sensing and as upstream components of the Cek1 MAPK pathway.

Upstream components of the cAMP-PKA pathway. In S. cerevisiae, the cAMP-PKA pathway is activated by a G protein-coupled receptor system consisting of the G protein-coupled receptor ScGpr1 and the G protein ScGpa2. ScGpr1 was identified in two-hybrid screens with the G protein ScGpa2 as bait (156, 324) and in a screen for mutants deficient in glucose-induced loss of heat resistance, a property controlled by the cAMP-PKA pathway (156). Apart from the receptor, ScGpa2 also interacts with ScGbp1/ScKrh2 and ScGbp2/ScKrh1, two proteins that appear to act as Gß-mimicking subunits, based on structural resemblance with classical Gß proteins (24, 117). Recently, two different molecular mechanisms by which these kelch repeat proteins function were described (116, 233). It seems that activated ScGpa2 relieves the inhibition imposed by the kelch repeat proteins on PKA, thereby bypassing adenylate cyclase for direct regulation of PKA. It is also clear that both ScKrh1 and ScKrh2 may bind different components of the pathway, thereby functioning as scaffolding proteins. Another interaction partner of ScGpa2 is ScGpg1, which has predicted structural properties typical of a G-like subunit, although ScGpg1 lacks apparent target sequences for posttranslational modifications, which are typical of G subunits (117). The activity of ScGpa2 is also controlled by the RGS protein ScRgs2 (309). Hence, a G protein-coupled receptor (GPCR) system composed of ScGpr1, ScGpa2, and ScRgs2 has been proposed to act as a glucose-sensing system for control of the cAMP pathway (296, 310). Recently we obtained evidence that ScGpr1 is a sensor for sucrose and glucose and that mannose acts as an antagonist (174). This GPCR system is required for pseudohyphal and invasive growth induction, and ScGPR1 or ScGPA2 mutants, deficient in this morphogenesis, can be suppressed by the addition of cAMP (187, 188, 293).

Genes similar to ScGPA2 and ScGPR1 have been identified in C. albicans, but their precise functions remain unclear. Genetic evidence suggests that GPA2 may function upstream of both the Cek1 MAPK pathway (249) and the cAMP-PKA pathway (193, 204). Deletion of GPA2 or GPR1 produces defects in hyphal formation and morphogenesis in C. albicans, which are reversed by exogenous addition of (db)cAMP or by overexpression of downstream components in the pathway. As expected, epistasis analysis revealed that Gpa2 (the G protein) acts downstream of Gpr1 (the receptor) in the same signaling pathway, and a two-hybrid assay indicated that the carboxy terminus of Gpr1 interacts with Gpa2. Moreover, expression levels of HWP1 and ECE1, which are cAMP-dependent hypha-specific genes, are reduced in both mutants (204). Interestingly, the morphogenesis defect is present only when cells are cultured on solid media. A possible link between Gpa2 and the CEK1 MAPK pathway was recently solidified by the discovery that Gpa2 is involved in mating. Thus, Gpa2 seems to integrate the nutrient-sensing pathway with the pheromone response MAPK pathway, providing an explanation for why the function of the latter pathway strongly depends on nutritional conditions (27).

Conflicting data regarding the possible ligand for Gpr1 exist. Miwa et al. (204) showed that, as in S. cerevisiae, Gpr1 is required for the glucose-induced cAMP signal, but we found that the gpr1 strain showed the same glucose-induced increase in cAMP as did a wild-type strain (193). However, deletion of either CDC25 or RAS1 affected the glucose- and serum-induced cAMP signal, consistent with mediation of the glucose-induced cAMP increase in C. albicans by the Cdc25-Ras1 branch of the glucose response pathway and probably not via the Gpr1-Gpa2 branch (193). Addition of serum results in a rapid response, similar to that seen with glucose, supporting a recent finding that glucose is the main factor in serum affecting Candida morphogenesis (135). Two important gaps remain in our understanding of the upstream components of the cAMP-PKA pathway in C. albicans: (i) the identity of the receptor activating the Cdc25 guanine nucleotide exchange protein and (ii) the nature of the ligand for Gpr1. No data from budding yeast or C. albicans regarding the Cdc25 receptor are available, but, as mentioned above, preliminary investigations aimed at identifying the possible ligand for Gpr1 seem to point to amino acids, specifically methionine, although glucose or other sugars cannot be excluded (193, 194).

The concentration of glucose in the medium is a very important parameter for the hyphal response. Most research groups use normal yeast extract-peptone-dextrose or synthetic complete medium containing 2% glucose. Under these conditions, a wild-type strain displays smooth colonies. However, in the presence of more physiologically relevant concentrations (around 0.1% glucose in the blood), and when there is methionine present in the medium, a wild-type strain rapidly starts to form hyphae. This methionine-dependent, low glucose concentration-induced hyphal production is completely absent in the gpr1 mutant (194), as well as in an hgt12 mutant (189). Hgt12 is homologous to S. cerevisiae Snf3, which is the high-affinity glucose sensor required for the expression of glucose transporter genes. The pathway that is activated by Hgt12 in C. albicans remains to be investigated, but the Hgt12 receptor is required for hyphal development during macrophage infection. Hgt12 does not have the long C-terminal tail that is typical of the yeast Snf3 and Rgt2 glucose sensors. Recently, conflicting data regarding the role of Hgt12 as a sensor have been presented. Brown and colleagues constructed independent Hgt12 mutants and were unable to reproduce the growth or filamentation abnormalities. They showed that Hgt4, another homolog of the yeast Snf3 glucose sensor that does contain a long C-terminal tail, might be the sensor regulating the expression of Hgt12, which, according to these authors, is a normal glucose transporter (48).

Ras1, the master hyphal regulator. Mutants of the single Ras homolog, Ras1, in C. albicans are viable but have a severe defect in hyphal growth in response to serum and other inducing conditions (92). In addition, while a dominant-negative Ras1 mutation [Ras1(A16)] causes a defect in filamentation, a dominant-active Ras1 mutation [Ras1(V13)] enhances the formation of hyphae (92). As mentioned above, the morphogenesis defect of a ras1 mutant can be suppressed by exogenous cAMP, and ras1 mutants are defective in cAMP induction in response to glucose and serum stimulation. Both phenotypes are shared with mutants lacking the guanine nucleotide exchange factor Cdc25 (193), suggesting that the Cdc25-Ras1 pathway functions upstream of the cAMP-PKA pathway. This is supported by a recent study in which a clear interaction between Ras1 and adenylate cyclase, Cyr1, is demonstrated (90). However, as with the situation in S. cerevisiae, Ras1 also appears to function upstream of the Cek1 MAPK pathway (Fig. 1), as the morphogenesis defect of a ras1 mutant can also be suppressed by overexpressing components of the filament-inducing MAPK cascade. Other evidence for a role in both pathways is that ras1 mutant strains exhibit a filamentation defect that is similar to that of the efg1 cph1 mutant strain (see below).

Cyr1, Srv2, and Pde2. C. albicans has a single gene homologous to the S. cerevisiae adenylate cyclase gene (CYR1/CDC35). Like Ras1, the cyclase is not essential for growth in C. albicans but is required for hyphal development (243). Also as in budding yeast, genes encoding an adenylate cyclase-associated protein (SRV2) and two phosphodiesterases (PDE1 and PDE2) are present in the C. albicans genome. The Srv2 protein regulates adenylate cyclase activity and is required for wild-type germ tube formation and for virulence in a mouse model of systemic infection; it is the homolog of the S. cerevisiae Cap1 protein. Like other mutants in the pathway, srv2 mutants have defects in morphogenesis, but they can be rescued by the addition of cAMP (13). Interestingly, the basal level of cAMP is higher in the srv2 mutant than in the wild type (61.8 ± 6.5 versus 45.3 ± 4.6 pmol/mg protein), indicating a possible negative-feedback inhibition.

To characterize more fully the effects of hyperactivation of the cAMP pathway, two groups disrupted the PDE2 gene, which encodes the high-affinity phosphodiesterase (PDEase) (16, 143). An advantage of studying cAMP hyperactivation by disrupting PDE2 rather than overexpressing TPK1 and TPK2, which encode the catalytic subunits of PKA, is that the pde2 mutant displays phenotypes that result solely from an increase in the cAMP level, thereby preempting any unpredictable effects of inequitable overexpression of one cAMP signaling component relative to the others. ScPDE1 and ScPDE2 encode the low- and high-affinity cAMP PDEases, respectively, in S. cerevisiae (190, 221, 252). Budding yeast Scpde1 and Scpde2 mutants exhibit sensitivity to heat shock and nutrient starvation (190), and Scpde2 mutants also have cell wall-related phenotypes, such as lysis upon hypo-osmotic shock, suggesting a role for ScPde2 (and/or cAMP) in the maintenance of cell wall integrity in S. cerevisiae (299). Although C. albicans also has a low-affinity PDEase encoded by PDE1 (127), so far only PDE2 has been studied in detail. As with the situation in budding yeast, pde2 mutants show cell wall- and membrane-related phenotypes, such as increased sensitivity to membrane-perturbing agents such as sodium dodecyl sulfate or antifungals such as amphotericin B and a strong reduction in the thickness of the cell wall, caused mainly by alterations in the ergosterol and glucan composition (144). PDE2 mutants are also sensitive to nutrient deprivation and defective in entry into stationary phase and are avirulent in a mouse model of systemic candidiasis (16). Under conditions in which wild-type strains form smooth colonies composed of budding yeasts on agar media, the homozygous pde2 mutant displayed a wrinkled colony phenotype. The wrinkled colonies exhibited mixtures of elongated yeasts and filamentous forms (pseudohyphae and true hyphae). Germ tube formation by the homozygous pde2 mutant was accelerated in liquid media compared to wild-type strains. Thus, it is clear that Pde2 is required to regulate the level of cAMP in the cells and that disruption of this enzyme results in a constitutive activation of the PKA pathway.

Adenylate cyclase and CO₂ sensing. C. albicans is present in various parts of the body where the CO₂ concentration is more than 150-fold higher (5%) than in atmospheric air (0.033%). In other words, cells growing on the skin face much lower CO₂ concentrations than do cells present in the intestine or blood. The presence of 5% CO₂ strongly induces pseudohyphal development and invasion of the underlying agar (151), a response that requires the catalytic domain of adenylate cyclase (Cyr1/Cdc35) but not Ras1. In many cases, cellular effects observed with CO₂ can be mediated via its hydrated form, bicarbonate. C. albicans NCE103 encodes a carbonic anhydrase that greatly accelerates bicarbonate formation. Nce103 is required for C. albicans cells to grow in air but not under high CO₂ concentrations (>0.5%). Nce103 is also required for tissue damage under atmospheric conditions but not under high CO₂ conditions. Whether there is indeed a specific CO₂ sensor protein on the cell membrane (e.g., through aquaporin water channels as in plant and mammalian cells) remains to be established. Klengel et al. have already excluded C. albicans Aqy1 as a possible candidate (151). A similar CO₂-sensing pathway in Cryptococcus neoformans has also been described (14), and a review of this topic was recently published (15).

PKA. Growth and cellular differentiation of eukaryotic cells depends to a large extent on the activity of cAMP-dependent protein kinases (PKA). PKAs are structurally conserved, consisting of two catalytic subunits that are inactivated by the binding of a homodimer of regulatory subunits. External cues elevate intracellular levels of cAMP, whose binding to the regulatory subunits liberates and thereby activates the catalytic subunits. A comparison of PKA subunit isoforms in C. albicans and S. cerevisiae reveals several significant differences. (i) In S. cerevisiae, PKA is encoded by three paralogues (ScTPK1 to ScTPK3) (298), whereas only two paralogues (TPK1 and TPK2) are present in the genome of C. albicans (66). (ii) In C. albicans, both isoforms have a positive, stimulatory function on the formation of true hyphae, whereas in S. cerevisiae, ScTpk2 has a positive function and ScTpk1 and ScTpk3 have negative functions in pseudohyphal development (229).

Although both C. albicans Tpk isoforms act positively to regulate hyphal formation, they have various phenotypic effects. For example, tpk1 mutants are defective in hyphal formation on solid media but are less affected in liquid media (276) (Fig. 1). In contrast, hyphal formation in tpk2 mutants is partially affected on solid media but is blocked in liquid medium. Because Tpk1 and Tpk2 mainly differ in their N-terminal domains (80 to 90 amino acids), hybrid Tpk proteins, with exchanged N-terminal domains, were tested by Bockmuhl et al. (37). Tests of hybrid proteins suggested that the catalytic domain mediates Tpk protein specificities in filamentation, whereas agar invasion is mediated by the N-terminal domain of Tpk2.

The regulatory subunits of PKA in S. cerevisiae and C. albicans are encoded by ScBCY1 and BCY1, respectively (58, 298). Cantore's group examined the morphogenetic behavior of C. albicans yeast cells lacking the PKA regulatory subunit and generated a bcy1 tpk2 double mutant strain, because a homozygous bcy1 mutant in a wild-type genetic background could not be obtained. In the bcy1 tpk2 mutant, PKA activity (due to the presence of the TPK1 gene) was cAMP independent, indicating that the cells harbored an unregulated phosphotransferase activity. This mutant has constitutive protein kinase activity and displays a defective germinative phenotype in GlcNAc and in serum-containing media. In addition, a Tpk1-green fluorescent protein (GFP) fusion is dispersed throughout the cell in the bcy1 tpk2 double mutant, while it is normally predominantly nuclear in wild-type cells. These genetic studies, together with biochemical evidence, suggest that C. albicans Bcy1 may tether the PKA catalytic subunit to the nucleus and thereby perform a pivotal role in regulating the enzymatic activity and availability of PKA in response to growth phase-related nutritional requirements (58).

Efg1 is a downstream component in the cAMP-PKA pathway. Several lines of evidence suggest that Efg1 functions downstream of PKA. Overexpression of TPK2 is unable to suppress the mutant phenotype of efg1, whereas overexpression of EFG1 can bypass the filamentation defect in a tpk2 mutant (276). The suppression activity of Efg1 depends on a threonine residue at position 206, a potential phosphorylation site for PKA (36). Induction of morphogenesis after the addition of serum, a process known to be regulated by the PKA pathway, requires Efg1. When EFG1 transcript levels were examined in transformants overexpressing TPK1 or TPK2, a reduction (about fourfold) of EFG1 mRNA was observed (294). This finding suggests that Efg1 is down-regulated by its immediate upstream regulator, PKA, but other observations point to autoregulation of EFG1 gene expression. Overexpression of EFG1 increases the levels of a smaller 2.2-kb transcript produced by the locus, while the levels of the endogenous 3.2-kb major EFG1 transcript decline dramatically (294) (Fig. 2). Serial deletion analysis of the promoter region revealed that the TATA box region is required for EFG1 autoregulation. Binding of Efg1 to the EFG1 transcriptional initiation region by chromatin immunoprecipitation was also shown. In summary, it appears that the levels and/or activities of Efg1, Tpk1, and Tpk2 are able to down-regulate the major EFG1 promoter. In addition, Efh1, the homolog of Efg1, also affects Efg1 down-regulation, indicating that at least four proteins can contribute to EFG1 expression. Negative autoregulation and PKA-mediated down-regulation are probably mediated though the Sin3-Rpd3 (279)-containing histone deacetylase complex (Fig. 2).

View larger version (49K): [in this window] [in a new window]   FIG. 2. (Top) White-opaque switching in C. albicans. Shown are scanning electron micrographs at a magnification of x1,000 and bright-field images of white and opaque cells. (Reprinted, with permission, from the Annual Review of Microbiology [26], volume 59, © 2005 by Annual Reviews.) (Bottom) Model of morphogenetic regulation by Efg1. Under hypha-inducing conditions (e.g., serum, GlcNAc), Efg1 is induced and activated; under microaerophilic conditions, however, it is repressed. The activated Efg1 (by PKA isoforms Tpk1 and Tpk2) initiates hyphal formation by inducing genes involved in hyphal formation and/or repressing genes directing the yeast form. Efg1 also induces the cell wall proteins (HWP1, HWP2, and RBE1) that are involved in adherence. The phase-specific genes (at the white-to-opaque-phase transition period) are also induced by Efg1. In parallel, Efg1, in conjunction with the Sin3-Rpd3 deacetylase complex, silences chromatin and thereby down-regulates EFG1 promoter activity (294). The 3.2-kb major transcript of EFG1 is expressed in the white phase, and the less-abundant 2.2-kb transcript is expressed in opaque cells. The Hda1-Rpd3 deacetylase complex regulates the white-to-opaque-phase transition as well as EFG1 down-regulation (279).

Efg1 and embedded growth. As we have just described, efg1 mutants have a drastic block in true hyphal formation under most standard induction conditions, but considerable filamentation occurs in certain environments (46, 276). A limited supply of oxygen, as occurs under a coverslip during induction of chlamydospores, allows wild-type cells to form filaments, which is enhanced in efg1 mutants (275). Similarly, growth of wild-type colonies embedded in agar stimulates filamentation, which still occurs in homozygous efg1 cph1 mutants (111, 242). Thus, an Efg1-independent pathway of filamentation that is operative under microaerophilic/embedded conditions appears to exist in C. albicans. In wild-type strains, the filaments produced under microaerophilic conditions are mostly pseudohyphae; however, they are mostly true hyphae in efg1 mutants (276). Interestingly, the alternative filamentation pathway is not only independent of Efg1 but may be repressed by it to a certain degree. The enhanced filamentation in efg1 mutants does not depend on the Cph1 MAPK, because a homozygous efg1 cph1 strain is as hyperfilamentous as the homozygous efg1 mutant (276). It is possible that agar embedding generates microaerophilic conditions, which activate the same Efg1-independent pathway of morphogenesis under both conditions. The putative transcription factor Czf1 is probably an important element of the alternative pathway of filamentation in C. albicans (47). The central portion of Czf1 contains four clusters of glutamine residues, and the C terminus contains a cysteine-rich region similar to zinc finger elements. Overexpression of CZF1 stimulates filamentous growth but only under embedded conditions and in certain media lacking glucose. The expression of Czf1 is strongly up-regulated in a vps34 mutant; as a result, a vps34 mutant is derepressed in hyphal formation under microaerophilic conditions (150). Vps34 is phosphatidylinositol-3-kinase, which influences vesicular intracellular transport, filamentous growth, and virulence. The exact role of Vps34 in Czf1-mediated morphogenesis is not yet clear. Homozygous czf1 null mutants filament normally under standard induction conditions, but they are defective in hyphal development when embedded in agar. This defective phenotype occurs only during embedding in certain media, such as complex medium containing sucrose or galactose as carbon sources at 25°C, but not at 37°C, or in media containing strong inducers, including serum and GlcNAc. These characteristics suggest that factors other than Czf1 contribute to filamentation under embedded conditions. The defective phenotype of a czf1 mutant is exacerbated by the presence of a cph1 mutation, which by itself shows defects in the types of media used for monitoring the czf1 phenotype. Thus, although the cph1 mutant phenotype does not appear to be specific for embedded conditions, it worsens the filamentation defects caused by the czf1 mutation. Hyperfilamentation of efg1 single and efg1 cph double mutants suggests that Efg1 is a negative modulator of the Czf1 pathway under microaerophilic/embedded conditions. However, recent data show that this hyperfilamentation phenotype of efg1 mutants may be caused by a Czf1-independent pathway, as efg1 mutants do not express CZF1. Chromatin immunoprecipitation analysis has further indicated that Efg1 and Czf1 interact with the promoter of CZF1. This indicates that, like EFG1, CZF1 is autoregulated (312).

Other aspects of C. albicans morphogenesis are also dependent on Efg1. Chlamydospore formation requires Efg1 protein in PKA-dependent regulation, as T206 is required (275). Also, recent studies revealed the involvement of EFG1 in normal biofilm formation (237), as well as in normal filamentation under oxygen limitation conditions (260), which are probably similar to the microaerophilic conditions. Under these conditions, Efg1 seems to function as a repressor of filamentation. In contrast to efg1, an efh1 mutant does not give any clear phenotype except in an efg1 mutant background. In this case, an efh1 null mutant is hyperfilamentous under embedded or hypoxic conditions, indicating the cooperation of Efg1 and Efh1 in suppression of an alternative morphogenetic signaling pathway (81).

Efg1 and phenotype switching. As noted briefly above, one form of phenotypic switching is the white-opaque transition, first described by Soll and colleagues in 1987 (268). C. albicans colonies of some strains (e.g., WO-1) can switch from the normal size, shape, and white color with high frequency (274) to larger, flatter, and gray colonies. White cells are similar in shape, size, and budding pattern to cells of common laboratory strains. Opaque cells, however, are bean shaped and exhibit three times the volume and twice the mass of white cells (Fig. 2) (268). Transcription of EFG1 is regulated in a unique fashion during the white-opaque transition (280); the more-abundant 3.2-kb transcription is expressed in the white phase, while the less-abundant 2.2-kb transcription is detected in the opaque phase (Fig. 2). Experiments using deletion or overexpression of EFG1 show that Efg1 functions downstream of the switch event in the regulation of a subset of white-phase-specific genes involved in the generation of the round white cell shape (278). Furthermore, a detailed promoter analysis suggests that the upstream region of EFG1 contains overlapping promoters for the expression of white-phase-specific and opaque-phase-specific transcripts (280). Overlapping promoters are also seen for the and ß mRNAs of the EFG1 homolog StuA in A. nidulans (320). It should be noted that EFG1 is not the only phase-regulated gene expressed as an alternative-molecular-weight transcript during the white-opaque transition in C. albicans. The deacetylase HOS3 is also transcribed as a 2.5-kb transcript in the white phase and as a less-abundant, lower-molecular-weight 2.3-kb transcript in the opaque phase (279). Recently, a key transcription factor required to establish and maintain the opaque growth phase was described by different groups. Like Efg1, this transcription factor, Wor1 (Tos9), binds to its own regulatory region, thereby activating its own expression. It is believed that high levels of Wor1 regulate the epigenetic inheritance of the opaque phase of growth (60, 131, 277).

Efg1 and cell wall dynamics. Recently, several genes encoding cell wall components were shown to be regulated by Efg1, implying cell wall regulation in C. albicans morphogenesis. These genes include the cell wall mannoprotein Hwp1 (42, 262), the glycosylphosphatidylinositol (GPI)-anchored cell wall protein HYR1 (17), and ALS1, which encodes a cell surface glycoprotein (95). In order to analyze cell wall dynamics and the regulatory function of Cph1 and Efg1 in the transcriptional control of cell wall genes in a systematic manner, Sohn et al. used a DNA microarray to assay transcriptional profiles from wild-type cells and cph1, efg1, and cph1 efg1 double-mutant strains cultured under various yeast- or hypha-inducing conditions (271). Overall, their data demonstrate that Efg1 plays a major role in the regulation of cell wall genes analyzed under both yeast- and hypha-inducing conditions, while Cph1 plays a minor role. During induction of filamentation, many hypha-specific genes that are important for adhesion and virulence are expressed in wild-type cells (in an Efg1-dependent manner). Among those genes are the hyphal wall proteins HWP1 and HWP2. Concomitant with the up-regulation of hyphal wall proteins, the expression of yeast-specific genes such as YWP1 is down-regulated during the yeast-to-hypha transition. Thus, the reduced virulence of the efg1 mutant likely reflects both a lack of expression of hypha-specific genes and a change in the overall organization of the cell wall (155, 182). Microarray analysis has further demonstrated that apart from yeast- or hypha-specific genes, Efg1 is also important for the expression of metabolism genes, inducing glycolytic genes and repressing genes essential for oxidative metabolism (81). Finally, Erg3, which is involved in drug resistance, is also down-regulated by Efg1, and efg1 mutants are more resistant to antifungal agents (183).

The study of the C. albicans yeast-to-hypha transition has been informed by studies performed with budding yeast. Very recently, the group of Snyder performed a detailed analysis of the transcription factor network involved in the yeast-to-pseudohypha transition in S. cerevisiae. Predicted binding sites for two transcription factor-encoding genes, ScPHD1 and ScMGA, appear upstream of most genes involved in pseudohyphal growth, including Flo8 (38). Overexpression of either ScPHD1 or ScMGA1 induces pseudohyphal growth, even under noninducing conditions. This indicates that ScPhd1 and ScMga1 are the master regulators in this system. ScPhd1 is the homolog of C. albicans Efg1, and ScMga1 is homologous to orf19.3969, which has not yet been characterized in C. albicans (44). A common theme for regulation of gene expression during pseudohyphal development is the cooperative binding of transcription factors (probably in preformed complexes, as described above) to target promoters. Cooperative binding between ScTec1 and ScSte12 is well established (64, 191), and ScFlo8 and ScMss1 cooperate to activate ScFLO11 or ScSTA1 expression (149, 307). Other cooperating pairs include ScMga1 and ScFlo8 (38) and the C. albicans APSES proteins Efg1 and Efh1 (81). Recently the C. albicans FLO8 gene was characterized (56). As in S. cerevisiae, Flo8 is required for hyphal development and for hypha-specific gene expression. Cooperative binding of two different transcription factors may also occur in C. albicans, as Flo8 and Efg1 interact with each other and Flo8 controls a subset of the target genes of Efg1 (56).

Cyclin-dependent protein kinases (Cdks) regulate major cell cycle transitions in eukaryotic cells. In S. cerevisiae, three G₁ cyclins, ScCln1, ScCln2, and ScCln3, activate the Cdc28 Cdk to promote G₁-phase progression (30). C. albicans contains homologs of the S. cerevisiae G₁ cyclins, including Ccn1 (= ScCln1), Hgc1, and Cln3 (www.candidagenome.org). Deletion of CCN1 results in the inability to maintain hyphal growth under certain conditions (185), while deletion of HGC1 prevents hyphal growth under all hypha-inducing conditions (332). Ccn1 and Hgc1 are not essential for progression through the cell cycle (they have a slow-growth phenotype), suggesting that these G₁ cyclin homologs in C. albicans have evolved important roles in hyphal morphogenesis as opposed to cell cycle progression. The induction of three known hypha-specific genes, HYR1, ECE1, and HWP1, in ccn1 mutant cells in response to serum is slightly reduced (about 50%) in comparison to that in wild-type cells. On the other hand, in liquid amino acid-based Lee's medium, the ccn1 strain shows a profound defect in the transcriptional activation of all three hypha-specific genes. Thus, Ccn1 may coordinately regulate hyphal development with signal transduction pathways in response to various environmental cues (185). Cln3, however, is essential for the C. albicans yeast cell cycle. When unbudded cells are depleted for Cln3, they increase in size but do not bud (12, 59). Eventually, however, these enlarged cells spontaneously form true hyphae. Cln3 seems to be less important for the hyphal cell cycle, although morphological abnormalities are observed in Cln3-depleted cells when they are treated with hypha-inducing signals such as serum (59). Apart from the G₁ cyclins, C. albicans also contains two B-type cyclins, Clb2 and Clb4, that are important for morphogenesis but affect polarized growth negatively (28).

Other kinases have also been investigated for their role in pseudohyphal development. The Nim1 kinases, Gin4 and Hsl1, are important for the formation of septin structures, and both gin4 and hsl1 mutants form pseudohyphae constitutively (319). In S. cerevisiae, ScHsl1 regulates the tyrosine phosphorylation of the cyclin-dependent protein kinase Cdc28 by deactivating the protein kinase ScSwe1, which regulates the G₂/M transition (295). Interestingly, in C. albicans, the Hsl1-Swe1-Cdc28 pathway appears to be important for cell elongation of both the yeast and hyphal forms and for virulence (304). Gin4-depleted pseudohyphae are unable to form hyphae when challenged with serum, but this can be overcome by ectopic expression of Gin4 from the MET3 promoter. Thus, Gin4 plays an important role in regulating the developmental switch from pseudohyphae to hyphae (Fig. 3) (319).

View larger version (36K): [in this window] [in a new window]   FIG. 3. Multiple environmental conditions activate the kinase cascade, resulting in coordination of the stress response with morphogenesis. Arrows indicate activation; lines with bars indicate inhibition. See the text for details.

Finally, several cell cycle-regulated transcription factors, such as the MADS box and forkhead transcription factors, are also required for morphogenesis. The C. albicans homolog of the MADS box transcription factor MCM1 was identified in a screen for genes that could activate pFLO11::lacZ expression in S. cerevisiae (245). Either overexpression or repression of MCM1 induces hyphae in C. albicans, indicating that correct timing of expression is important for normal morphogenesis.

In S. cerevisiae, two forkhead transcription factors, ScFKH1 and ScFKH2, regulate the expression of genes whose transcription peaks in early M phase (163, 333), including mitotic cyclins. In addition, Scfkh1 Scfkh2 mutants display constitutive pseudohyphal growth. C. albicans has only one forkhead homolog, FKH2. Cells lacking this gene also form constitutive pseudohyphae under all yeast and hyphal growth conditions tested. fkh2 mutants exhibit reduced expression of hypha-specific mRNAs in the presence of serum, while under yeast growth conditions expression of several genes encoding proteins likely to be important for cell wall separation is reduced. Together, these results imply that Fkh2 is required for morphogenesis of true hyphae as well as of yeast cells. Cph1 and Efg1 are not required for pseudohyphal morphology of fkh2 mutants, implying that Fkh2 acts in a parallel or downstream pathway. Cells lacking Fkh2 do not damage human epithelial and endothelial cells in vitro, suggesting that Fkh2 contributes to C. albicans virulence (29).