Hyphal elongation is regulated independently of cell cycle in Candida albicans

The CRH family coding for cell wall glycosylphosphatidylinositol proteins with a predicted transglycosidase domain affects cell wall organization and virulence of Candida albicans

In Candida albicans UTR2 (CSF4), CRH11, and CRH12 are members of a gene family (the CRH family) that encode glycosylphosphatidylinositol-dependent cell wall proteins with putative transglycosidase activity. Deletion of genes of this family resulted in additive sensitivity to compounds interfering with normal cell wall formation (Congo red, calcofluor white, SDS, and high Ca(2+) concentrations), suggesting that these genes contribute to cell wall organization. A triple mutant lacking UTR2, CRH11, and CRH12 produced a defective cell wall, as inferred from increased sensitivity to cell wall-degrading enzymes, decreased ability of protoplasts to regenerate a new wall, constitutive activation of Mkc1p, the mitogen-activated protein kinase of the cell wall integrity pathway, and an increased chitin content of the cell wall. Importantly, this was accompanied by a decrease in alkali-insoluble 1,3-beta-glucan but not total glucan content, suggesting that formation of the linkage between 1,3-beta-glucan and chitin might be affected. In support of this idea, localization of a Utr2p-GFP fusion protein largely coincided with areas of chitin incorporation in C. albicans. As UTR2 and CRH11 expression is regulated by calcineurin, a serine/threonine protein phosphatase involved in tolerance to antifungal drugs, cell wall morphogenesis, and virulence, this points to a possible relationship between calcineurin and the CRH family. Deletion of UTR2, CRH11, and CRH12 resulted in only a partial overlap with calcineurin-dependent phenotypes, suggesting that calcineurin has additional targets. Interestingly, cells deleted for UTR2, CRH11, and CRH12 were, like a calcineurin mutant, avirulent in a mouse model of systemic infection but retained the capacity to colonize target organs (kidneys) as the wild type. In conclusion, this work establishes the role of UTR2, CRH11, and CRH12 in cell wall organization and integrity.

Morphogenesis and cell cycle progression in Candida albicans

Candida albicans, an opportunistic human pathogen, displays three modes of growth: yeast, pseudohyphae and true hyphae, all of which differ both in morphology and in aspects of cell cycle progression. In particular, in hyphal cells, polarized growth becomes uncoupled from other cell cycle events. Yeast or pseudohyphae that undergo a cell cycle delay also exhibit polarized growth, independent of cell cycle progression. The Spitzenkörper, an organelle composed of vesicles associated with hyphal tips, directs continuous hyphal elongation in filamentous fungal species and also in C. albicans hyphae. A polarisome mediates cell cycle dependent growth in yeast and pseudohyphae. Regulation of morphogenesis and cell cycle progression is dependent upon specific cyclins, all of which affect morphogenesis and some of which function specifically in yeast or hyphal cells. Future work will probably focus on the cell cycle checkpoints involved in connecting morphogenesis to cell cycle progression.

Bistable expression of WOR1, a master regulator of white-opaque switching in Candida albicans

Candida albicans, a commensal organism and a pathogen of humans, can switch stochastically between a white phase and an opaque phase without an intermediate phase. The white and opaque phases have distinct cell shapes and gene expression programs. Once switched, each phase is stable for many cell divisions. White-opaque switching is under a1-alpha2 repression and therefore only happens in a or alpha cells. Mechanisms that control the switching are unknown. Here, we identify Wor1 (white-opaque regulator 1) as a master regulator of white-opaque switching. The deletion of WOR1 blocks opaque cell formation. The ectopic expression of WOR1 converts all cells to stable opaque cells in a or alpha cells. In addition, the ectopic expression of WOR1 in a/alpha cells is sufficient to induce opaque cell formation. Importantly, WOR1 expression displays an all-or-none pattern. It is undetectable in white cells, and it is highly expressed in opaque cells. The ectopic expression of Wor1 induces the transcription of WOR1 from the WOR1 locus, which correlates with the switch to opaque phase. We present genetic evidence for feedback regulation of WOR1 transcription. The feedback regulation explains the bistable and stochastic nature of white-opaque switching.

Repression of CDC28 reduces the expression of the morphology-related transcription factors, Efg1p, Nrg1p, Rbf1p, Rim101p, Fkh2p and Tec1p and induces cell elongation in Candida albicans

The ability of the human fungal pathogen Candida albicans to transit its cell shape is important for its pathogenicity. To obtain additional evidence that the cell cycle of C. albicans is associated with its morphology, we generated and characterized a conditional mutant of C. albicans CDC28, a cyclin-dependent kinase. In the constructed strain, the expression of CDC28 was regulated by the MET3 promoter and could be repressed in the presence of methionine and cysteine. Cdc28p-depleted cells demonstrated highly polarized growth and wider filaments than serum-induced hyphae. Hyphae-specific genes, HWP1, RBT4 and ECE1, were activated in the elongated filaments caused by the Cdc28p depletion. Furthermore, the protein expression levels of the transcription factors involved in morphological transition, Efg1p, Nrg1p, Rbf1p, Rim101p, Fkh2p and Tec1p, decreased under conditions that repress CDC28 expression. Taken together, these data indicate that repression of CDC28 affected the protein levels of the morphology-related transcription factors, the regulation of hyphae-specific genes and cell shape in C. albicans.

Yeast-to-hyphal transition triggers formin-dependent Golgi localization to the growing tip in Candida albicans

Rapid and long-distance secretion of membrane components is critical for hyphal formation in filamentous fungi, but the mechanisms responsible for polarized trafficking are not well understood. Here, we demonstrate that in Candida albicans, the majority of the Golgi complex is redistributed to the distal region during hyphal formation. Randomly distributed Golgi puncta in yeast cells cluster toward the growing tip during hyphal formation, remain associated with the distal portion of the filament during its extension, and are almost absent from the cell body. This restricted Golgi localization pattern is distinct from other organelles, including the endoplasmic reticulum, vacuole and mitochondria, which remain distributed throughout the cell body and hypha. Hyphal-induced positioning of the Golgi and the maintenance of its structural integrity requires actin cytoskeleton, but not microtubules. Absence of the formin Bni1 causes a hyphal-specific dispersal of the Golgi into a haze of finely dispersed vesicles with a sedimentation density no different from that of normal Golgi. These results demonstrate the existence of a hyphal-specific, Bni1-dependent cue for Golgi integrity and positioning at the distal portion of the hyphal tip, and suggest that filamentous fungi have evolved a novel strategy for polarized secretion, involving a redistribution of the Golgi to the growing tip.

Rad52 depletion in Candida albicans triggers both the DNA-damage checkpoint and filamentation accompanied by but independent of expression of hypha-specific genes

We have analysed the effect of RAD52 deletion in several aspects of the cell biology of Candida albicans. Cultures of rad52Delta strains exhibited slow growth and contained abundant cells with a filamentous morphology. Filamentation with polarization of actin patches was accompanied by the induction of the hypha-specific genes (HSG) ECE1, HWP1 and HGC1. However, filament formation occurred in the absence of the transcription factors Efg1 and Cph1, even though disruption of EFG1 prevented expression of HSG. Therefore, expression of HSG genes accompanies but is dispensable for rad52Delta filamentation. However, deletion of adenylate cyclase severely impaired filamentation, this effect being largely reverted by the addition of exogenous cAMP. Filaments resembled elongated pseudohyphae, but some of them looked like true hyphae. Following depletion of Rad52, many cells arrested at the G2/M phase of the cell cycle with a single nucleus suggesting the early induction of the DNA-damage checkpoint. Filaments formed later, preferentially from G2/M cells. The filamentation process was accompanied by the uncoupling of several landmark events of the cell cycle and was partially dependent on the action of the cell cycle modulator Swe1. Hyphae were still induced by serum, but a large number of rad52 cells myceliated in G2/M.

The Cdc14p phosphatase affects late cell-cycle events and morphogenesis inCandida albicans

We have characterized the CDC14 gene, which encodes a dual-specificity protein phosphatase in Candida albicans, and demonstrated that its deletion results in defects in cell separation, mitotic exit and morphogenesis. The C. albicans cdc14Δ mutants formed large aggregates of cells that resembled those found in ace2-null strains. In cdc14Δ cells, expression of Ace2p target genes was reduced and Ace2p did not accumulate specifically in daughter nuclei. Taken together, these results imply that Cdc14p is required for the activation and daughter-specific nuclear accumulation of Ace2p. Consistent with a role in cell separation, Cdc14p was targeted to the septum region during the M-G1 transition in yeast-form cells. Interestingly, hypha-inducing signals abolished the translocation of Cdc14p to the division plate, and this regulation depended on the cyclin Hgc1p, since hgc1Δ mutants were able to accumulate Cdc14p in the septum region of the germ tubes. In addition to its role in cytokinesis, Cdc14p regulated mitotic exit, since synchronous cultures of cdc14Δ cells exhibited a severe delay in the destruction of the mitotic cyclin Clb2p. Finally, deletion of CDC14 resulted in decreased invasion of solid agar medium and impaired true hyphal growth.

Microtubules in Candida albicans hyphae drive nuclear dynamics and connect cell cycle progression to morphogenesis

Candida albicans is an opportunistic fungal pathogen whose virulence is related to its ability to switch between yeast, pseudohyphal, and true-hyphal morphologies. To ask how long-distance nuclear migration occurs in C. albicans hyphae, we identified the fundamental properties of nuclear movements and microtubule dynamics using time-lapse microscopy. In hyphae, nuclei migrate to, and divide across, the presumptive site of septation, which forms 10 to 15 microm distal to the basal cell. The mother nucleus returns to the basal cell, while the daughter nucleus reiterates the process. We used time-lapse microscopy to identify the mechanisms by which C. albicans nuclei move over long distances and are coordinated with hyphal morphology. We followed nuclear migration and spindle dynamics, as well as the time and position of septum specification, defined it as the presumptum, and established a chronology of nuclear, spindle, and morphological events. Analysis of microtubule dynamics revealed that premitotic forward nuclear migration is due to the repetitive sliding of astral microtubules along the cell cortex but that postmitotic forward and reverse nuclear migrations are due primarily to spindle elongation. Free microtubules exhibit cell cycle regulation; they are present during interphase and disappear at the time of spindle assembly. Finally, a growth defect in strains expressing Tub2-green fluorescent protein revealed a connection between hyphal elongation and the nuclear cell cycle that is coordinated by hyphal length and/or volume.

A putative dual-specific protein phosphatase encoded by YVH1 controls growth, filamentation and virulence in Candida albicans

In response to stimulants, such as serum, the yeast cells of the opportunistic fungal pathogen Candida albicans form germ tubes, which develop into hyphae. Yvh1p, one of the 29 protein phosphatases encoded in the C. albicans genome, has 45% identity with the dual-specific phosphatase Yvh1p of the model yeast Saccharomyces cerevisiae. In this study, Yvh1p expression was not observed during the initial step of germ tube formation, although Yvh1p was expressed constitutively in cell cycle progression of yeast or hyphal cells. In an attempt to analyse the function of Yvh1p phosphatase, the complete ORFs of both alleles were deleted by replacement with hph200-URA3-hph200 and ARG4. Although YVH1 has nine single-nucleotide polymorphisms in its coding sequence, both YVH1 alleles were able to complement the YVH1 gene disruptant. The vegetative growth of Deltayvh1 was significantly slower than the wild-type. The hyphal growth of Deltayvh1 on agar, or in a liquid medium, was also slower than the wild-type because of the delay in nuclear division and septum formation, although germ tube formation was similar between the wild-type and the disruptant. Despite the slow hyphal growth, the expression of several hypha-specific genes in Deltayvh1 was not delayed or repressed compared with that of the wild-type. Infection studies using mouse models revealed that the virulence of Deltayvh1 was less than that of the wild-type. Thus, YVH1 contributes to normal vegetative yeast or hyphal cell cycle progression and pathogenicity, but not to germ tube formation.

Cell cycle arrest during S or M phase generates polarized growth via distinct signals in Candida albicans

Treatments that perturb DNA synthesis or mitosis will activate checkpoints that prevent cell cycle progression and cell proliferation. In yeast-form cells of the fungal pathogen Candida albicans, exposure to hydroxyurea (HU) or shutting off expression of the polo-like kinase CaCDC5 blocked nuclear division and spindle elongation, but activated a highly polarized growth mode. We have used transcription profiling both to characterize the initiation and progression of this polar growth pattern and to determine how cell elongation may be linked to the cell cycle in C. albicans. Different gene expression patterns during early stages of cell elongation support the concept that CaCdc5p-depleted and HU-exposed cells were blocked at different stages of the cell cycle, and suggest that different signals may generate the common polarized growth phenotype. Consistent with this, BUB2 expression was modulated in CaCdc5p-depleted cells, and absence of BUB2 prevented the maintenance of cell polarization, resulting in multibudded, pseudohyphal cells with constrictions. In contrast, HU-induced filaments did not modulate or require BUB2, but were dependent on the GTPase Ras1p. However, at later stages of cell elongation, transcription profiles were more similar, and comparisons with serum-induced hyphae revealed that the cell cycle-arrested filaments expressed several targets of the hyphal signalling pathways. Thus, arresting the yeast cell cycle in S or M phase generates a polarized growth pattern through different mechanisms in C. albicans, and maintenance of the polar growth mode can ultimately lead to the expression of hyphal-associated cell wall and virulence-related factors, in the absence of any external stimuli.

Candida albicans hyphae have a Spitzenkörper that is distinct from the polarisome found in yeast and pseudohyphae

Fungi grow with a variety of morphologies: oval yeast cells, chains of elongated cells called pseudohyphae and long, narrow, tube-like filaments called hyphae. In filamentous fungi, hyphal growth is strongly polarised to the tip and is mediated by the Spitzenkörper, which acts as a supply centre to concentrate the delivery of secretory vesicles to the tip. In the budding yeast Saccharomyces cerevisiae, polarised growth is mediated by the polarisome, a surface cap of proteins that nucleates the formation of actin cables delivering secretory vesicles to the growing tip. The human fungal pathogen, Candida albicans, can grow in all three morphological forms. Here we show the presence of a Spitzenkörper at the tip of C. albicans hyphae as a ball-like localisation of secretory vesicles, together with the formin Bni1 and Mlc1, an ortholog of an S. cerevisiae myosin regulatory light chain. In contrast, in C. albicans yeast cells, pseudohyphae and hyphae Spa2 and Bud6, orthologs of S. cerevisiae polarisome components, as well as the master morphology regulator Cdc42, localise predominantly, but not exclusively, to a surface cap resembling the polarisome of S. cerevisiae yeast cells. A small amount of Cdc42 also localises to the Spitzenkörper. Furthermore, we show differences in the genetic and cytoskeletal requirements, and cell cycle dynamics of polarity determinants in yeast, pseudohyphae and hyphae. These results, together with the cytological differences between the cell types, suggest that the Spitzenkörper and polarisome are distinct structures, that the polarisome and Spitzenkörper coexist in hyphae, and that polarised growth in hyphae is driven by a fundamentally different mechanism to that in yeast and pseudohyphae.

The formin family protein CaBni1p has a role in cell polarity control during both yeast and hyphal growth in Candida albicans

Formins are conserved eukaryotic proteins playing key roles in regulating cell polarity. We have characterized the roles of a formin CaBni1p in the polymorphic fungus Candida albicans. CaBni1p localized persistently to hyphal tips during hyphal growth but to distinct growth sites at different cell cycle stages during yeast growth. Cabni1Δ yeast cells exhibited several morphological defects, such as round and enlarged cells, widened bud necks and a random budding pattern. Although Cabni1Δ cells could still undergo yeast-hypha growth switch, the hyphae were markedly swollen. Cabni1Δ also showed defects in spindle and cytoplasmic microtubule orientation and positioning. Coincidentally, the spindle orientation protein CaKar9p in Cabni1Δ yeast cells appeared as multiple random cortical spots, in contrast to the single spot at the bud tip of many wild-type cells. Interestingly, several defects manifested in Cabni1Δ yeast cells were partially corrected during hyphal growth. We found that the second formin CaBnr1p was recruited to hyphal tips, while it localized only to the bud neck during yeast growth. This behavior of CaBnr1p may play a key role in correcting Cabni1Δ defects during hyphal growth. Cabni1Δ exhibited reduced virulence in mice. These results indicate that the formins play an important role in Candida albicans polarized growth and CaBni1p's function is required for virulence.

Cyclin Cln3p links G1 progression to hyphal and pseudohyphal development in Candida albicans

G1 cyclins coordinate environmental conditions with growth and differentiation in many organisms. In the pathogen Candida albicans, differentiation of hyphae is induced by environmental cues but in a cell cycle-independent manner. Intriguingly, repressing the G1 cyclin Cln3p under yeast growth conditions caused yeast cells to arrest in G1, increase in size, and then develop into hyphae and pseudohyphae, which subsequently resumed the cell cycle. Differentiation was dependent on Efg1p, Cph1p, and Ras1p, but absence of Ras1p was also synthetically lethal with repression of CLN3. In contrast, repressing CLN3 in environment-induced hyphae did not inhibit growth or the cell cycle, suggesting that yeast and hyphal cell cycles may be regulated differently. Therefore, absence of a G1 cyclin can activate developmental pathways in C. albicans and uncouple differentiation from the normal environmental controls. The data suggest that the G1 phase of the cell cycle may therefore play a critical role in regulating hyphal and pseudohyphal development in C. albicans.

The mitotic cyclins Clb2p and Clb4p affect morphogenesis in Candida albicans

The ability of Candida albicans to switch cellular morphologies is crucial for its ability to cause infection. Because the cell cycle machinery participates in Saccharomyces cerevisiae filamentous growth, we characterized in detail the two C. albicans B-type cyclins, CLB2 and CLB4, to better understand the molecular mechanisms that underlie the C. albicans morphogenic switch. Both Clb2p and Clb4p levels are cell cycle regulated, peaking at G2/M and declining before mitotic exit. On hyphal induction, the accumulation of the G1 cyclin Cln1p was prolonged, whereas the accumulation of both Clb proteins was delayed when compared with yeast form cells, indicating that CLB2 and CLB4 are differentially regulated in the two morphologies and that the dynamics of cyclin appearance differs between yeast and hyphal forms of growth. Clb2p-depleted cells were inviable and arrested with hyper-elongated projections containing two nuclei, suggesting that Clb2p is not required for entry into mitosis. Unlike Clb2p-depleted cells, Clb4p-depleted cells were viable and formed constitutive pseudohyphae. Clb proteins lacking destruction box domains blocked cell cycle progression resulting in the formation of long projections, indicating that both Clb2p and Clb4p must be degraded before mitotic exit. In addition, overexpression of either B-type cyclin reduced the extent of filamentous growth. Taken together, these data indicate that Clb2p and Clb4p regulate C. albicans morphogenesis by negatively regulating polarized growth.

Role for the SCFCDC4 ubiquitin ligase in Candida albicans morphogenesis

The ability of Candida albicans, a major fungal pathogen, to switch between a yeast form, and a hyphal (mold) form is recognized as being important for the ability of the organism to invade the host and cause disease. We found that a C. albicans mutant deleted for CaCDC4, a homologue of the Saccharomyces cerevisiae F-box protein component of the SCF(CDC4) ubiquitin ligase, is viable and displays constitutive filamentous, mostly hyphal, growth. The phenotype of the Cacdc4-/- mutant suggests that ubiquitin-mediated protein degradation is involved in the regulation of the dimorphic switch of C. albicans and that one or more regulators of the yeast-to-mold switch are among the substrates of SCF(CaCDC4). Epistasis analysis indicates that the Cacdc4-/- phenotype is largely independent of the filamentation-inducing transcription factors Efg1 and Cph1. We identify C. albicans Far1 and Sol1, homologues of the S. cerevisiae SCF(CDC4) substrates Far1 and Sic1, and show that Sol1 is a substrate of C. albicans Cdc4. Neither protein is essential for the hyphal phenotype of the Cacdc4-/- mutant. However, ectopic expression and deletion of SOL1 indicate a role for this gene in C. albicans morphogenesis.

Polarized hyphal growth in Candida albicans requires the Wiskott-Aldrich Syndrome protein homolog Wal1p

The yeast-to-hypha transition is a key feature in the cell biology of the dimorphic human fungal pathogen Candida albicans. Reorganization of the actin cytoskeleton is required for this dimorphic switch in Candida. We show that C. albicans WAL1 mutants with both copies of the Wiskott-Aldrich syndrome protein (WASP) homolog deleted do not form hyphae under all inducing conditions tested. Growth of the wild-type and wal1 mutant strains was monitored by in vivo time-lapse microscopy both during yeast-like growth and under hypha-inducing conditions. Isotropic bud growth produced round wal1 cells and unusual mother cell growth. Defects in the organization of the actin cytoskeleton resulted in the random localization of actin patches. Furthermore, wal1 cells exhibited defects in the endocytosis of the lipophilic dye FM4-64, contained increased numbers of vacuoles compared to the wild type, and showed defects in bud site selection. Under hypha-inducing conditions wal1 cells were able to initiate polarized morphogenesis, which, however, resulted in the formation of pseudohyphal cells. Green fluorescent protein (GFP)-tagged Wal1p showed patch-like localization in emerging daughter cells during the yeast growth phase and at the hyphal tips under hypha-inducing conditions. Wal1p-GFP localization largely overlapped with that of actin. Our results demonstrate that Wal1p is required for the organization of the actin cytoskeleton and hyphal morphogenesis in C. albicans as well as for endocytosis and vacuole morphology.

Transcriptional profiling of the early stages of germination in by real-time RT-PCR

By using real-time RT-PCR, we profiled the expression of CGR1, CaMSI3, EFG1, NRG1, and TUP1 in Candida albicans strains JCM9061 and CAI4 under several conditions, including induction of morphological transition, heat shock, and treatment with calcium inhibitors. Expression of CaMSI3 changed under these growth conditions except during heat shock. CGR1 expression increased during the early stages of hyphal growth in JCM9061, while expression was strain-dependent during heat shock. Both EFG1 and NRG1 were similarly expressed under hypha-inducing conditions and heat shock. Expression of TUP1 was slightly different from the expression of EFG1 or NRG1.

Candida albicans protein kinase CaHsl1p regulates cell elongation and virulence

Saccharomyces cerevisiae Hsl1p is a Ser/Thr protein kinase that regulates cell morphology. We identified Candida albicans CaHSL1 and analysed its function in C. albicans. Cells lacking CaHsl1p exhibited filamentous growth under yeast growth conditions with the filaments elongating more quickly than did those of the wild type under hyphal growth conditions, suggesting that it plays a role in the suppression of cell elongation. Green fluorescent protein-tagged CaHsl1p colocalized with a septin complex to the bud neck during yeast growth or to a potent septation site during hyphal growth, as expected from the localization in S. cerevisiae. However, the localization of the septin complex did not change in DeltaCahsl1, suggesting that CaHsl1p does not participate in septin organization. CaHsl1p was expressed in a cell cycle-dependent manner and, except for the G1 phase, phosphorylated throughout the cell cycle. In DeltaCahsl1 cells, the phosphorylation of a possible CaHsl1p target CaSwe1p decreased, while that of CaCdc28p at tyrosine18 increased. Either an extra copy of the tyrosine18-mutated CaCdc28p or deletion of CaSWE1 suppressed the cell elongation phenotype caused by CaHSL1 deletion. Furthermore, DeltaCahsl1 exhibited reduced virulence in the mouse systemic candidiasis model. Thus, the CaHsl1p-CaSwe1p-CaCdc28p pathway appears important in the cell elongation of both the yeast and hyphal forms and to the virulence of C. albicans.

The distinct morphogenic states of Candida albicans

The human fungal pathogen, Candida albicans can grow in at least three different morphologies: yeast, pseudohyphae and hyphae. Further morphological forms exist during colony switching, for example, opaque phase cells are oblong, rather than the oval shape of yeast cells. Pseudohyphae and hyphae are both elongated and sometimes there has been little attempt to distinguish between them, as both are "filamentous forms" of the fungus. We review here the differences between them that suggest that they are distinct morphological states. We argue that studies on "filamentous forms" should always include a formal analysis to determine whether the cells are hyphae or pseudohyphae and we suggest some simple experimental criteria that can be applied to achieve this.

Candida morphogenesis and host–pathogen interactions

The human fungal pathogen Candida albicans has many morphological forms. Recent advances in genomics and cell biology are providing an improved understanding of the molecular regulation of cell shape, and providing insights into the relationships between morphogenesis and virulence. This understanding may improve our ability to develop strategies to combat Candida infections.

The serine/threonine protein phosphatase SIT4 modulates yeast-to-hypha morphogenesis and virulence in Candida albicans

SIT4 encodes the multifunctional catalytic subunit of a type 2A-related protein phosphatase of Saccharomyces cerevisiae and has been implicated in cell cycle regulation and nitrogen sensing. We have identified the Candida albicans homologue of SIT4, and we show that its disruption caused a significant reduction in general growth rate, in hyphal outgrowth and in virulence in a mouse infection model. These phenotypes were reversed by the reintroduction of the wild-type SIT4 gene. We used glass DNA microarrays to measure the transcriptional profiles of 6287 open reading frames in sit4 cells undergoing the yeast-to-hypha transition induced by serum. Although differential expression of many of the hyphae-specific genes was not affected by the SIT4 deletion, the transcription of two new hyphae-induced genes, XOG1 and YNR67, was entirely reliant upon Sit4p. Both genes represent glucanases, indicating that SIT4 may play a role in controlling cell wall biogenesis. Furthermore, sit4 cells exhibited a reduced heat shock response to treatment with serum/37 degrees C, suggesting that SIT4 acts to co-ordinate the stress response signals during morphological switching. Finally, sit4 cells displayed reduced transcript levels for the genes encoding the Hog1p MAP kinase and several modulators of protein biosynthesis. Sit4p thus plays important roles during hyphal growth in Candida albicans through the regulation of cell wall biogenesis, osmosensing and protein translation.

Inhibitors of Civ1 kinase belonging to 6-aminoaromatic-2-cyclohexyldiamino purine series as potent anti-fungal compounds

There is today a blatant need for new antifungal agents, because of the recent increase in life-threatening infections involving an ever-greater number of fungal strains. Fungi make extensive use of kinases in the regulation of essential processes, in particular the cell cycle. Most fungal kinases, however, are shared with higher eukaryotes. Only the kinases which have no human homologs, such as the histidine kinases, can be used as targets for antifungal drugs design. This review describes efforts directed towards the discovery of drugs active against a novel target, the atypical cell cycle kinase, Civ1.

Hgc1, a novel hypha-specific G1 cyclin-related protein regulates Candida albicans hyphal morphogenesis

The human fungal pathogen Candida albicans switches from yeast to hyphal growth when exposed to serum or phagocytosed. However, the importance of this morphological switch for virulence remains highly controversial due to the lack of a mutant that affects hyphal morphogenesis only. Although many genes specifically expressed in hyphal cells have been identified and shown to encode virulence factors, none is required for hyphal morphogenesis. Here we report the first hypha-specific gene identified, HGC1, which is essential for hyphal morphogenesis. Deletion of HGC1 abolished hyphal growth in all laboratory conditions tested and in the kidneys of systemically infected mice with markedly reduced virulence. HGC1 expression is co-regulated with other virulence genes such as HWP1 by the cAMP/protein kinase A signaling pathway and transcriptional repressor Tup1/Nrg1. Hgc1 is a G1 cyclin-related protein and co-precipitated with the cyclin-dependent kinase (Cdk) CaCdc28. It has recently emerged that cyclin/Cdk complexes promote other forms of polarized cell growth such as tumor cell migration and neurite outgrowth. C. albicans seems to have adapted a conserved strategy to control specifically hyphal morphogenesis.

In Candida albicans, the Nim1 kinases Gin4 and Hsl1 negatively regulate pseudohypha formation and Gin4 also controls septin organization

In the development of hyphal germ tubes of Candida albicans, a band of septin forms at the base of the germ tube (basal septin band). Later, a septin ring forms, which organizes the first septum within the germ tube (septin ring). We have investigated the role of the Nim1 kinases, Gin4 and Hsl1, in the formation of these septin structures. We show that during germ tube formation, Gin4 is required for the organization of the septin ring but not the basal septin band. Hsl1 is not required for the formation of either septin rings or basal bands. Unexpectedly, we found that both gin4Δ and hsl1Δ mutants form pseudohyphae constitutively, in a fashion that in the case of gin4Δ, is partly independent of Swe1. 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 may regulate the developmental switch from pseudohyphae to hyphae.

Septation and cytokinesis in fungi

Cytokinesis is the ultimate step of a cell cycle resulting in the generation of two progeny. Failure of correct cell division may be lethal for both, mother and daughter cells, and thus such a process must be tightly regulated with other events of the cell cycle. Differing solutions to the same problem have been developed in bacteria and plants while cytokinesis in animal and fungal cells is highly similar and requires a contractile ring containing actomyosin. Cytokinesis in fungi can be viewed as a three-stage process: (i) selection of a division site, (ii) orderly assembly of protein complexes, and finally (iii) dynamic events that lead to a constriction of the contractile ring and septum construction. Elaborate mechanisms known as the Mitotic Exit Network (MEN) and the Septation Initiation Network (SIN) have evolved to link these events, particularly the final steps of cytokinesis, with nuclear division. The purpose of this review was to discuss the latest developments in the fungal field and to describe the central known players required for key steps on the road to cell division. Differences in the cytokinesis of yeast-like fungi that result in complete cell separation in contrast to septation which leads to the compartmentalization of fungal hyphae are highlighted.

Maximal polar growth potential depends on the polarisome component AgSpa2 in the filamentous fungus Ashbya gossypii

We used actin staining and videomicroscopy to analyze the development from a spore to a young mycelium in the filamentous ascomycete Ashbya gossypii. The development starts with an initial isotropic growth phase followed by the emergence of germ tubes. The initial tip growth speed of 6-10 microm/h increases during early stages of development. This increase is transiently interrupted in response to the establishment of lateral branches or septa. The hyphal tip growth speed finally reaches a maximum of up to 200 micro/h, and the tips of these mature hyphae have the ability to split into two equally fast-growing hyphae. A search for A. gossypii homologs of polarisome components of the yeast Saccharomyces cerevisiae revealed a remarkable size difference between Spa2p of both organisms, with AgSpa2p being double as long as ScSpa2p due to an extended internal domain. AgSpa2 colocalizes with sites of polarized actin. Using time-lapse videomicroscopy, we show that AgSpa2p-GFP polarization is established at sites of branch initiation and then permanently maintained at hyphal tips. Polarization at sites of septation is transient. During apical branching the existing AgSpa2p-GFP polarization is symmetrically divided. To investigate the function of AgSpa2p, we generated two AgSPA2 mutants, a partial deletion of the internal domain alone, and a complete deletion. The mutations had an impact on the maximal hyphal tip growth speed, on the hyphal diameter, and on the branching pattern. We suggest that AgSpa2p is required for the determination of the area of growth at the hyphal tip and that the extended internal domain plays an important role in this process.

CaSPA2 is important for polarity establishment and maintenance in Candida albicans

Saccharomyces cerevisiae Spa2p is a component of polarisome that controls cell polarity. Here, we have characterized the role of its homologue, CaSpa2p, in the polarized growth in Candida albicans. During yeast growth, GFP-tagged CaSpa2p localized to distinct growth sites in a cell cycle-dependent manner, while during hyphal growth it persistently localized to hyphal tips throughout the cell cycle. Persistent tip localization of the protein was also observed in Catup1Delta and Canrg1Delta, mutants constitutive for filamentous growth. Caspa2Delta exhibited defects in polarity establishment and maintenance, such as random budding and failure to confine growth to a small surface area leading to round cells with wide, elongated bud necks and markedly thicker hyphae. It was also defective in nuclear positioning, presumably a result of defective interactions between cytoplasmic microtubules with certain polarity determinants. The highly conserved SHD-I and SHD-V domains were found to be important and responsible for different aspects of CaSpa2p function. Caspa2Delta exhibited no virulence in the mouse systemic candidiasis model. Because of the existence of distinct growth forms and the easy control of the switch between them in vitro, C. albicans may serve as a useful model in cell polarity research.

Asynchronous cell cycle and asymmetric vacuolar inheritance in true hyphae of Candida albicans

Candida albicans forms unconstricted hyphae in serum-containing medium that are divided into discrete compartments. Time-lapse photomicroscopy, flow cytometry, and a novel three-dimensional imaging system were used to demonstrate that the kinetics and cell cycle events accompanying hyphal development were correlated with dynamic changes in vacuole morphology and the pattern of vacuole inheritance. Apical cells of hyphae underwent continuous extension before and after the first cytokinesis event. However, the resulting mother cell and sub-apical compartments did not immediately reenter the cell cycle and instead underwent cell cycle arrest before reentering the cycle. Vacuole was inherited asymmetrically at cytokinesis so that the distal, arrested compartments inherited most vacuole and the growing apical cell inherited most cytoplasm. Hydroxyurea release experiments demonstrated that the arrested, vacuolated hyphal compartments were in the G(1) phase of the cycle. The period of cell cycle arrest was decreased by the provision of assimilatable forms of nitrogen, suggesting that the hyphal cell cycle is regulated by nitrogen limitation that results in sup-apical cell cycle arrest. This pattern of growth is distinct from that of the synchronous, symmetrical development of pseudohyphae of C. albicans and other yeast species. These observations suggest that the cellular vacuole space correlates with alterations in the cell cycles of different cell types and that the total organelle space may influence size-regulated functions and hence the timing of the eukaryotic cell cycle.

Depletion of a polo-like kinase in Candida albicans activates cyclase-dependent hyphal-like growth

Morphogenesis in the fungal pathogen Candida albicans is an important virulence-determining factor, as a dimorphic switch between yeast and hyphal growth forms can increase pathogenesis. We identified CaCDC5, a cell cycle regulatory polo-like kinase (PLK) in C. albicans and demonstrate that shutting off its expression induced cell cycle defects and dramatic changes in morphology. Cells lacking CaCdc5p were blocked early in nuclear division with very short spindles and unseparated chromatin. GFP-tagged CaCdc5p localized to unseparated spindle pole bodies, the spindle, and chromatin, consistent with a role in spindle elongation at an earlier point in the cell cycle than that described for the homologue Cdc5p in yeast. Strikingly, the cell cycle defects were accompanied by the formation of hyphal-like filaments under yeast growth conditions. Filament growth was determinate, as the filaments started to die after 24 h. The filaments resembled serum-induced hyphae with respect to morphology, organization of cytoplasmic microtubules, localization of nuclei, and expression of hyphal-specific components. Filament formation required CaCDC35, but not EFG1 or CPH1. Similar defects in spindle elongation and a corresponding induction of filaments occurred when yeast cells were exposed to hydroxyurea. Because CaCdc5p does not appear to act as a direct repressor of hyphal growth, the data suggest that a target of CaCdc5p function is associated with hyphal-like development. Thus, an internal, cell cycle-related cue can activate hyphal regulatory networks in Candida.

A screen in Saccharomyces cerevisiae identified CaMCM1, an essential gene in Candida albicans crucial for morphogenesis

Morphogenesis in Saccharomyces cerevisiae and the pathogenic yeast Candida albicans is governed in part by the same molecular circuits. In S. cerevisiae, FLO11/MUC1 expression has been shown to be modulated by multiple signalling pathways required for pseudohyphal development. We have established a screen in S. cerevisiae to identify regulators of fungal development in C. albicans based on FLO11::lacZ expression as a reporter. This screen identified both known components of the mitogen-activated protein kinase (MAPK) cascade and the cAMP cascade that are important for hyphal development in C. albicans, as well as genes not yet known to be involved in morphogenesis. The Candida homologue of MCM1 is one of the novel factors identified in this screen as being important for morphogenesis. CaMcm1p levels do not vary significantly in different cell types and respond to an autoregulatory feedback mechanism, arguing that CaMcm1p activity is regulated by post-translational modifications. Both overexpression and repression of this essential gene led to the induction of hyphae. Moreover, we found that the expression of HWP1, a hyphae-specific gene, was induced by repression of CaMCM1. The changes in morphology and HWP1 expression were not the result of a change in expression levels of NRG1 or TUP1, known repressors of hyphal development. Thus, CaMcm1p is a component of a hitherto unknown regulatory mechanism of hyphal growth.

A conserved mitogen-activated protein kinase pathway is required for mating in Candida albicans

Candida albicans had been thought to lack a mating process until the recent discovery of a mating type-like locus and mating between MTLa and MTL(alpha) strains. To elucidate the molecular mechanisms that regulate mating in C. albicans, we examined the function of Cph1 and its upstream mitogen-activated protein (MAP) kinase pathway in mating, as they are homologues of the pheromone-responsive MAP kinase pathway in Saccharomyces cerevisiae. We found that overexpressing CPH1 in MTLa, but not in MTLa/alpha strains, induced the transcription of orthologues of S. cerevisiae pheromone-induced genes and also increased mating efficiency. Furthermore, cph1 and hst7 mutants were completely defective in mating, and cst20 and cek1 mutants showed reduced mating efficiency, as in S. cerevisiae. The partial mating defect in cek1 results from the presence of a functionally redundant MAP kinase, Cek2. CEK2 complemented the mating defect of a fus3 kss1 mutant of S. cerevisiae and was expressed only in MTLa or MTL(alpha), but not in MTLa/alpha cell types. Moreover, a cek1 cek2 double mutant was completely defective in mating. Our data suggest that the conserved MAP kinase pathway regulates mating in C. albicans. We also observed that C. albicans mating efficiency was greatly affected by medium composition, indicating the potential involvement of nutrient-sensing pathways in mating in addition to the MAP kinase pathway.

Hyphal tip-associated localization of Cdc42 is F-actin dependent in Candida albicans

The rho-type GTPase Cdc42 is important for the establishment and maintenance of eukaryotic cell polarity. To examine whether Cdc42 is regulated during the yeast-to-hypha transition in Candida albicans, we constructed a green fluorescence protein (GFP)-Cdc42 fusion under the ACT1 promoter and observed its localization in live C. albicans cells. As in Saccharomyces cerevisiae, GFP-Cdc42 was observed around the entire periphery of the cell. In yeast-form cells of C. albicans, it clustered to the tips and sides of small buds as well as to the mother-daughter neck region of large-budded cells. Upon hyphal induction, GFP-Cdc42 clustered to the site of hyphal evagination and remained at the tips of the hyphae. This temporal and spatial localization of Cdc42 suggests that its activity is regulated during the yeast-to-hypha transition. In addition to the accumulation at the hyphal tip, GFP-Cdc42 was also seen as a band within the hyphal tube in cells that had undergone nuclear separation. With the F-actin-assembly inhibitor latrunculin A, we found that GFP-Cdc42 accumulation at the bud site in yeast-form cells is F-actin independent, whereas GFP-Cdc42 accumulation at the hyphal tip requires F-actin. Furthermore, disruption of the F-actin cytoskeleton impaired the transcriptional induction of hypha-specific genes. Therefore, hypha formation resembles mating in Saccharomyces cerevisiae in that both require F-actin for GFP-Cdc42 localization and efficient signaling.

Candida Albicans: a molecular revolution built on lessons from budding yeast

Candida albicans is an opportunistic fungal pathogen that is found in the normal gastrointestinal flora of most healthy humans. However, in immunocompromised patients, blood-stream infections often cause death, despite the use of anti-fungal therapies. The recent completion of the C. albicans genome sequence, the availability of whole-genome microarrays and the development of tools for rapid molecular-genetic manipulations of the C. albicans genome are generating an explosion of information about the intriguing biology of this pathogen and about its mechanisms of virulence. They also reveal the extent of similarities and differences between C. albicans and its benign relative, Saccharomyces cerevisiae.

A forkhead transcription factor is important for true hyphal as well as yeast morphogenesis in Candida albicans

Candida albicans is an important pathogen of immunocompromised patients which grows with true hyphal, pseudohyphal, and yeast morphologies. The dynamics of cell cycle progression are markedly different in true hyphal relative to pseudohyphal and yeast cells, including nuclear movement and septin ring positioning. In Saccharomyces cerevisiae, two forkhead transcription factors (ScFKH1 and ScFKH2) regulate the expression of B-cyclin genes. In both S. cerevisiae and Schizosaccharomyces pombe, forkhead transcription factors also influence morphogenesis. To explore the molecular mechanisms that connect C. albicans morphogenesis with cell cycle progression, we analyzed CaFKH2, the single homolog of S. cerevisiae FKH1/FKH2. C. albicans cells lacking CaFkh2p formed constitutive pseudohyphae under all yeast and hyphal growth conditions tested. Under hyphal growth conditions levels of hyphae-specific mRNAs were reduced, and under yeast growth conditions levels of several genes encoding proteins likely to be important for cell wall separation were reduced. Together these results imply that Fkh2p is required for the morphogenesis of true hyphal as well as yeast cells. Efglp and Cphlp, two transcription factors that contribute to C. albicans hyphal growth, were not required for the pseudohyphal morphology of fkh2 mutants, implying that Fkh2p acts in pathways downstream of and/or parallel to Efglp and Cphlp. In addition, cells lacking Fkh2p were unable to damage human epithelial or endothelial cells in vitro, suggesting that Fkh2p contributes to C. albicans virulence.

Co-regulation of pathogenesis with dimorphism and phenotypic switching in Candida albicans, a commensal and a pathogen

Candida albicans, a common fungal pathogen of humans, can colonize in many diverse environments of the host and convert between a harmless commensal and a pathogen. Recent advances indicate that C. albicans uses a common set of conserved pathways to regulate dimorphism, mating and phenotypic switching. Major pathways known to regulate dimorphism include a mitogen-activated protein (MAP) kinase pathway through Cph1, the cAMP-dependent protein kinase pathway via Efg1, and Tup1-mediated repression through Rfg1 and Nrg1. The Cph1-mediated MAP kinase pathway is critical for the mating process, while all three pathways are implicated in the regulation of white-opaque switching. All these developmental pathways regulate the expression of hypha-specific and/or phase-specific genes. A high proportion of hypha-specific genes and phase-specific genes encode proteins that contribute directly or indirectly to pathogenesis and virulence of C. albicans. Therefore, virulence genes are co-regulated with cell morphogenesis. This supports a previous notion that the unique aspects of C. albicans commensalism and pathogenesis may lie in the developmental programs of dimorphism and phenotypic switching.

Characterization and functional analysis of the siderophore-iron transporter CaArn1p in Candida albicans

Siderophores are small organic compounds with high affinity for ferric iron. Microorganisms commonly acquire iron via siderophore secretion and uptake. Here we report the characterization of the siderophore transporter CaArn1p in the fungal pathogen Candida albicans. Deletion of CaARN1 reduced the ability of C. albicans to use iron bound to the hydroxamate-type siderophore ferrichrome and abolished it when two high-affinity iron permease genes (CaFTR1 and CaFTR2) were also deleted, indicating a role of CaArn1p as well as the permeases in ferrichrome-iron uptake. Caarn1Delta (but not Caftr1DeltaCaftr2Delta) assimilated iron from another hydroxamate-type siderophore, ferrioxamine B, suggesting that iron uptake from this compound depends on the permeases, but not on CaArn1p. Northern blot analysis revealed that the transcription repressor CaTup1p repressed CaARN1 expression under iron-replete conditions via the DNA-binding protein Rfg1p. Green fluorescent protein-tagged CaArn1p was observed predominantly in the plasma membrane, with some in the cytoplasm as distinct spots. The number of these spots increased with the increase in ferrichrome concentration, suggesting that CaArn1p internalization might be a mechanism for ferrichrome-iron uptake or for recycling the transporter. Caarn1Delta did not show reduced virulence when injected into the blood stream of mice, implying that CaArn1p is not required for iron uptake along this route of infection.

Septin function in Candida albicans morphogenesis

The septin proteins function in the formation of septa, mating projections, and spores in Saccharomyces cerevisiae, as well as in cell division and other processes in animal cells. Candida albicans septins were examined in this study for their roles in morphogenesis of this multimorphic, opportunistically pathogenic fungus, which can range from round budding yeast to elongated hyphae. C. albicans green fluorescent protein labeled septin proteins localized to a tight ring at the bud and pseudohyphae necks and as a more diffuse array in emerging germ tubes of hyphae. Deletion analysis demonstrated that the C. albicans homologs of the S. cerevisiae CDC3 and CDC12 septins are essential for viability. In contrast, the C. albicans cdc10Delta and cdc11Delta mutants were viable but displayed conditional defects in cytokinesis, localization of cell wall chitin, and bud morphology. The mutant phenotypes were not identical, however, indicating that these septins carry out distinct functions. The viable septin mutants could be stimulated to undergo hyphal morphogenesis but formed hyphae with abnormal curvature, and they differed from wild type in the selection of sites for subsequent rounds of hyphal formation. The cdc11Delta mutants were also defective for invasive growth when embedded in agar. These results further extend the known roles of the septins by demonstrating that they are essential for the proper morphogenesis of C. albicans during both budding and filamentous growth.

Spindle assembly checkpoint component CaMad2p is indispensable for Candida albicans survival and virulence in mice

Here, we report an indispensable role for spindle assembly checkpoint (SAC) component CaMad2p in the survival and virulence of Candida albicans in mice. We hypothesized that cell cycle checkpoint functions, especially those monitoring the integrity of DNA and chromosome segregation, might be required for the pathogen to repair damage caused by host defence. To test this idea, we created SAC-defective mutants by deleting the CaMAD2 gene that encodes a key component of the SAC pathway. The CaMAD2 mutant appears normal in morphology, growth rate and growth mode switch in unperturbed conditions. However, it quickly loses viability when treated with nocodazole, which causes disassembly of mitotic spindles. The mutant also exhibits increased frequency of chromosome loss. The virulence of the mutant is greatly reduced in mice, presumably because of the inability of the mutant cells to stop the cell cycle when the host defence damages cellular components important for chromosome segregation. Supporting this hypothesis, unlike the wild-type cells that can proliferate within and eventually grow out of macrophages, most of the CaMAD2 null mutant cells are unable to survive. This study suggests that SAC is required for survival of C. albicans in the host and could thus be targeted for anti-C. albicans therapies.