Transcript profiles of Candida albicans cortical actin patch mutants reflect their cellular defects: contribution of the Hog1p and Mkc1p signaling pathways

The EH domain-containing protein, EdeA, is involved in endocytosis, cell wall integrity, and pathogenicity in Aspergillus fumigatus

Endocytosis has been extensively studied in yeasts, where it plays crucial roles in growth, signaling regulation, and cell-surface receptor internalization. However, the biological functions of endocytosis in pathogenic filamentous fungi remain largely unexplored. In this study, we aimed to functionally characterize the roles of EdeA, an ortholog of the Saccharomyces cerevisiae endocytic protein Ede1, in Aspergillus fumigatus. EdeA was observed to be distributed as patches on the plasma membrane and concentrated in the subapical collar of hyphae, a localization characteristic of endocytic proteins. Loss of edeA caused defective hyphal polarity, reduced conidial production, and fewer sites of endocytosis initiations than that of the parental wild type. Notably, the edeA null mutant exhibited increased sensitivity to cell wall-disrupting agents, indicating a role for EdeA in maintaining cell wall integrity in A. fumigatus. This observation was further supported by the evidence showing that the thickness of the cell wall in the ΔedeA mutant increased, accompanied by abnormal activation of MpkA, a key component in the cell wall integrity pathway. Additionally, the ΔedeA mutant displayed increased pathogenicity in the Galleria mellonella wax moth infection model, possibly due to alterations in cell wall morphology. Site-directed mutagenesis identified the conserved residue E348 within the third EH (Eps15 homology) domain of EdeA as crucial for its subcellular localization and functions. In conclusion, our results highlight the involvement of EdeA in endocytosis, hyphal polarity, cell wall integrity, and pathogenicity in A. fumigatus.

IMPORTANCE

Aspergillus fumigatus is a significant human pathogenic fungus known to cause invasive aspergillosis, a disease with a high mortality rate. Understanding the basic principles of A. fumigatus pathogenicity is crucial for developing effective strategies against this pathogen. Previous research has underscored the importance of endocytosis in the infection capacity of pathogenic yeasts; however, its biological function in pathogenic mold remains largely unexplored. Our characterization of EdeA in A. fumigatus sheds light on the role of endocytosis in the development, stress response, and pathogenicity of pathogenic molds. These findings suggest that the components of the endocytosis process may serve as potential targets for antifungal therapy.

Candida albicans END3 Mediates Endocytosis and Has Subsequent Roles in Cell Wall Integrity, Morphological Switching, and Tissue Invasion

The role of endocytosis in Candida albicans secretion, filamentation, and virulence remains poorly understood, despite its importance as a fundamental component of intracellular trafficking. Given that secretory mutants display defects in endocytosis, we have focused our attention on endocytic mutants to understand the interconnection between endocytosis and other secretory pathways. Using a reverse-genetic approach based upon CRISPR-Cas9 mediated gene deletion, we studied the functions of the gene END3, which plays a key role in clathrin-based endocytosis. In the end3Δ/Δ null mutant, clathrin-mediated endocytosis was substantially reduced. While in vitro growth, cell morphology, and vacuoles appeared normal, the mutant was impaired in actin patch formation, filamentous growth, biofilm formation, cell wall integrity, and extracellular protease secretion. In addition, susceptibility to various antifungal agents was altered. Consistent with the inability to form hyphae, in an in vitro keratinocyte infection model, the null mutant displayed reduced damage of mammalian adhesion zippers and host cell death. Thus, C. albicans END3 has a role in efficient endocytosis that is required for cell wall integrity, protein secretion, hyphal formation, and virulence-related processes. These findings suggest that impaired endocytosis subsequently affects other secretory pathways, providing evidence of the interconnection between these processes. IMPORTANCE Candida albicans is a fungal commensal organism that can cause serious opportunistic infections in immunocompromised patients leading to substantial complications and mortality. A better understanding of the microbe's biology to develop more effective therapeutic and diagnostic tools is required as invasive candidiasis is a problem of continued clinical importance. This study focuses on endocytosis, an important but incompletely understood cellular mechanism needed to uptake nutrients and communicate with a cell's environment. In this study, we have assessed the role of endocytosis in cell wall integrity, biofilm formation, and tissue invasion in C. albicans. These findings will improve our understanding of cellular mechanisms underlying endocytosis and will inform us of the interconnection with other intracellular transport processes.

Surface Architecture Influences the Rigidity of Candida albicans Cells

Atomic force microscopy (AFM) was used to investigate the morphology and rigidity of the opportunistic pathogenic yeast, Candida albicans ATCC 10231, during its attachment to surfaces of three levels of nanoscale surface roughness. Non-polished titanium (npTi), polished titanium (pTi), and glass with respective average surface roughness (Sa) values of 389 nm, 14 nm, and 2 nm, kurtosis (Skur) values of 4, 16, and 4, and skewness (Sskw) values of 1, 4, and 1 were used as representative examples of each type of nanoarchitecture. Thus, npTi and glass surfaces exhibited similar Sskw and Skur values but highly disparate Sa. C. albicans cells that had attached to the pTi surfaces exhibited a twofold increase in rigidity of 364 kPa compared to those yeast cells attached to the surfaces of npTi (164 kPa) and glass (185 kPa). The increased rigidity of the C. albicans cells on pTi was accompanied by a distinct round morphology, condensed F-actin distribution, lack of cortical actin patches, and the negligible production of cell-associated polymeric substances; however, an elevated production of loose extracellular polymeric substances (EPS) was observed. The differences in the physical response of C. albicans cells attached to the three surfaces suggested that the surface nanoarchitecture (characterized by skewness and kurtosis), rather than average surface roughness, could directly influence the rigidity of the C. albicans cells. This work contributes to the next-generation design of antifungal surfaces by exploiting surface architecture to control the extent of biofilm formation undertaken by yeast pathogens and highlights the importance of performing a detailed surface roughness characterization in order to identify and discriminate between the surface characteristics that may influence the extent of cell attachment and the subsequent behavior of the attached cells.

Functions of Sphingolipids in Pathogenesis During Host-Pathogen Interactions

Sphingolipids are a class of membrane lipids that serve as vital structural and signaling bioactive molecules in organisms ranging from yeast to animals. Recent studies have emphasized the importance of sphingolipids as signaling molecules in the development and pathogenicity of microbial pathogens including bacteria, fungi, and viruses. In particular, sphingolipids play key roles in regulating the delicate balance between microbes and hosts during microbial pathogenesis. Some pathogens, such as bacteria and viruses, harness host sphingolipids to promote development and infection, whereas sphingolipids from both the host and pathogen are involved in fungus-host interactions. Moreover, a regulatory role for sphingolipids has been described, but their effects on host physiology and metabolism remain to be elucidated. Here, we summarize the current state of knowledge about the roles of sphingolipids in pathogenesis and interactions with host factors, including how sphingolipids modify pathogen and host metabolism with a focus on pathogenesis regulators and relevant metabolic enzymes. In addition, we discuss emerging perspectives on targeting sphingolipids that function in host-microbe interactions as new therapeutic strategies for infectious diseases.

The Role of Secretory Pathways in Candida albicans Pathogenesis

Candida albicans is a fungus that is a commensal organism and a member of the normal human microbiota. It has the ability to transition into an opportunistic invasive pathogen. Attributes that support pathogenesis include secretion of virulence-associated proteins, hyphal formation, and biofilm formation. These processes are supported by secretion, as defined in the broad context of membrane trafficking. In this review, we examine the role of secretory pathways in Candida virulence, with a focus on the model opportunistic fungal pathogen, Candida albicans.

Recent advances in understanding Candida albicans hyphal growth

Morphological changes are critical for the virulence of a range of plant and human fungal pathogens. Candida albicans is a major human fungal pathogen whose ability to switch between different morphological states is associated with its adaptability and pathogenicity. In particular, C. albicans can switch from an oval yeast form to a filamentous hyphal form, which is characteristic of filamentous fungi. What mechanisms underlie hyphal growth and how are they affected by environmental stimuli from the host or resident microbiota? These questions are the focus of intensive research, as understanding C. albicans hyphal growth has broad implications for cell biological and medical research.

Plasma membrane organization promotes virulence of the human fungal pathogen Candida albicans

Candida albicans is a human fungal pathogen capable of causing lethal systemic infections. The plasma membrane plays key roles in virulence because it not only functions as a protective barrier, it also mediates dynamic functions including secretion of virulence factors, cell wall synthesis, invasive hyphal morphogenesis, endocytosis, and nutrient uptake. Consistent with this functional complexity, the plasma membrane is composed of a wide array of lipids and proteins. These components are organized into distinct domains that will be the topic of this review. Some of the plasma membrane domains that will be described are known to act as scaffolds or barriers to diffusion, such as MCC/eisosomes, septins, and sites of contact with the endoplasmic reticulum. Other zones mediate dynamic processes, including secretion, endocytosis, and a special region at hyphal tips that facilitates rapid growth. The highly organized architecture of the plasma membrane facilitates the coordination of diverse functions and promotes the pathogenesis of C. albicans.

Anti-Candida albicans biofilm effect of novel heterocyclic compounds

OBJECTIVES

The aims of this study were to develop new anti-biofilm drugs, examine their activity against Candida albicans biofilm and investigate their structure-activity relationship and mechanism of action.

METHODS

A series of thiazolidinedione and succinimide derivatives were synthesized and their ability to inhibit C. albicans biofilm formation and destroy pre-formed biofilm was tested. The biofilms' structure, metabolic activity and viability were determined by XTT assay and propidium iodide and SYTO 9 live/dead stains combined with confocal microscopic analysis. The effect of the most active compounds on cell morphology, sterol distribution and cell wall morphology and composition was then determined by specific fluorescent stains and transmission electron microscopy.

RESULTS

Most of the compounds were active at sub-MICs. Elongation of the aliphatic side chain resulted in reduced anti-biofilm activity and the sulphur atom contributed to biofilm killing, indicating a structure-activity relationship. The compounds differed in their effects on biofilm viability, yeast-to-hyphal form transition, hyphal morphology, cell wall morphology and composition, and sterol distribution. The most effective anti-biofilm compounds were the thiazolidinedione S8H and the succinimide NA8.

CONCLUSIONS

We developed novel anti-biofilm agents that both inhibited and destroyed C. albicans biofilm. With some further development, these agents might be suitable for therapeutic purposes.

An Interspecies Regulatory Network Inferred from Simultaneous RNA-seq of Candida albicans Invading Innate Immune Cells

The ability to adapt to diverse micro-environmental challenges encountered within a host is of pivotal importance to the opportunistic fungal pathogen Candida albicans. We have quantified C. albicans and M. musculus gene expression dynamics during phagocytosis by dendritic cells in a genome-wide, time-resolved analysis using simultaneous RNA-seq. A robust network inference map was generated from this dataset using NetGenerator, predicting novel interactions between the host and the pathogen. We experimentally verified predicted interdependent sub-networks comprising Hap3 in C. albicans, and Ptx3 and Mta2 in M. musculus. Remarkably, binding of recombinant Ptx3 to the C. albicans cell wall was found to regulate the expression of fungal Hap3 target genes as predicted by the network inference model. Pre-incubation of C. albicans with recombinant Ptx3 significantly altered the expression of Mta2 target cytokines such as IL-2 and IL-4 in a Hap3-dependent manner, further suggesting a role for Mta2 in host-pathogen interplay as predicted in the network inference model. We propose an integrated model for the functionality of these sub-networks during fungal invasion of immune cells, according to which binding of Ptx3 to the C. albicans cell wall induces remodeling via fungal Hap3 target genes, thereby altering the immune response to the pathogen. We show the applicability of network inference to predict interactions between host-pathogen pairs, demonstrating the usefulness of this systems biology approach to decipher mechanisms of microbial pathogenesis.

Motor protein Myo5p is required to maintain the regulatory circuit controlling WOR1 expression in Candida albicans

The Candida albicans MYO5 gene encodes myosin I, a protein required for the formation of germ tubes and true hyphae. Because the polarized growth of opaque-phase cells in response to pheromone results in mating projections that can resemble germ tubes, we examined the role of Myo5p in this process. We localized green fluorescent protein (GFP)-tagged Myo5p in opaque-phase cells of C. albicans during both bud and shmoo formation. In vegetatively growing opaque cells, Myo5p is found at sites of bud emergence and bud growth, while in pheromone-stimulated cells, Myo5p localizes at the growing tips of shmoos. Intriguingly, cells homozygous for MTLa in which the MYO5 gene was deleted failed to switch efficiently from the white phase to the opaque phase, although ectopic expression of WOR1 from the MET3 promoter can convert myo5 mutants into mating-competent opaque cells. However, when WOR1 expression was shut off, the myo5-defective cells rapidly lost both their opaque phenotype and mating competence, suggesting that Myo5p is involved in the maintenance of the opaque state. When MYO5 is expressed conditionally in opaque cells, the opaque phenotype, as well as the mating ability of the cells, becomes unstable under repressive conditions, and quantitative real-time PCR demonstrated that the shutoff of MYO5 expression correlates with a dramatic reduction in WOR1 expression. It appears that while myosin I is not directly required for mating in C. albicans, it is involved in WOR1 expression and the white-opaque transition and thus is indirectly implicated in mating.

Endocytosis-mediated vacuolar accumulation of the human ApoE apolipoprotein-derived ApoEdpL-W antimicrobial peptide contributes to its antifungal activity in Candida albicans

The 18-amino-acid cationic, tryptophan-rich ApoEdpL-W peptide derived from human ApoE apolipoprotein was shown to have antifungal activity against pathogenic yeasts of the Candida genus (except C. glabrata). ApoEdpL-W was active against planktonic cells and early-stage biofilms but less active against mature biofilms, possibly because of its affinity for extracellular matrix beta-glucans. Moreover, ApoEdpL-W absorbed to medically relevant materials partially prevented the formation of biofilms on these materials. The exposure of C. albicans cells to sublethal doses of ApoEdpL-W triggered a transcriptional response reminiscent of that associated with the inactivation of the MYO5 gene required for endocytosis as well as the upregulation of amino acid transporter genes. A fluorescent derivative of ApoEdpL-W accumulated at the cytoplasmic membrane and subsequently was translocated to the vacuole. Strikingly, the inactivation of MYO5 or addition of latrunculin, an inhibitor of endocytosis, prevented the vacuolar accumulation of fluorescein-labeled ApoEdpL-W and reduced the antifungal activity of ApoEdpL-W. This, together with the insensitivity of ApoEdpL-W to alterations in membrane fluidity and high salt, suggested that the ApoEdpL-W mode of action was dependent upon vacuolar targeting and differed significantly from that of other antifungal peptides, such as Histatin-5 and Magainin 2.

Lipid signalling in pathogenic fungi

In recent years, the study of lipid signalling networks has significantly increased. Although best studied in mammalian cells, lipid signalling is now appreciated also in microbial cells, particularly in yeasts and moulds. For instance, microbial sphingolipids and their metabolizing enzymes play a key role in the regulation of fungal pathogenicity, especially in Cryptococcus neoformans, through the modulation of different microbial pathways and virulence factors. Another example is the quorum sensing molecule (QSM) farnesol. In fact, this QSM is involved not only in mycelial growth and biofilm formation of Candida albicans, but also in many stress related responses. In moulds, such as Aspergillus fumigatus, QSM and sphingolipids are important for maintaining cell wall integrity and virulence. Finally, fungal cells make oxylipins to increase their virulence attributes and to counteract the host immune defences. In this review, we discuss these aspects in details.

Salivary histatin 5 internalization by translocation, but not endocytosis, is required for fungicidal activity in Candida albicans

Salivary histatin 5 (Hst 5) is a cationic salivary protein with high fungicidal activity against Candida albicans. Binding to the cell wall followed by intracellular translocation is required for killing; however, specific binding components and critical toxic events are not understood. In this study, laminarin (β-1,3-glucan) but not sialic acid, mannan or pustulan mediated Hst 5 binding to C. albicans, and was disassociated by 100 mM NaCl. Time-lapse confocal microscopy revealed a dose-dependent rate of cytosolic uptake of Hst 5 that invariably preceded propidium iodide (PI) entry, demonstrating that translocation itself does not disrupt membrane integrity. Cell toxicity was manifest by vacuolar expansion followed by PI entrance; however, loss of endocytotic vacuolar trafficking of Hst 5 did not reduce killing. Extracellular NaCl (100 mM), but not sorbitol, prevented vacuolar expansion and PI entry in cells already containing cytosolic Hst 5, thus showing a critical role for ionic balance in Hst 5 toxicity. Hst 5 uptake, but not cell wall binding, was blocked by pretreatment with azide or carbonyl cyanide m-chlorophenylhydrazone; however, 10% of de-energized cells had membrane disruption. Thus, Hst 5 is capable of heterogeneous intracellular entry routes, but only direct cytosolic translocation causes cell death as a result of ionic efflux.

Forward genetics in Candida albicans that reveals the Arp2/3 complex is required for hyphal formation, but not endocytosis

Candida albicans is a diploid fungal pathogen lacking a defined complete sexual cycle, and thus has been refractory to standard forward genetic analysis. Instead, transcription profiling and reverse genetic strategies based on Saccharomyces cerevisiae have typically been used to link genes to functions. To overcome restrictions inherent in such indirect approaches, we have investigated a forward genetic mutagenesis strategy based on the UAU1 technology. We screened 4700 random insertion mutants for defects in hyphal development and linked two new genes (ARP2 and VPS52) to hyphal growth. Deleting ARP2 abolished hyphal formation, generated round and swollen yeast phase cells, disrupted cortical actin patches and blocked virulence in mice. The mutants also showed a global lack of induction of hyphae-specific genes upon the yeast-to-hyphae switch. Surprisingly, both arp2 Delta/Delta and arp2 Delta/Delta arp3 Delta/Delta mutants were still able to endocytose FM4-64 and Lucifer Yellow, although as shown by time-lapse movies internalization of FM4-64 was somewhat delayed in mutant cells. Thus the non-essential role of the Arp2/3 complex discovered by forward genetic screening in C. albicans showed that uptake of membrane components from the plasma membrane to vacuolar structures is not dependent on this actin nucleating machinery.

SLA2 mutations cause SWE1-mediated cell cycle phenotypes in Candida albicans and Saccharomyces cerevisiae

The early endocytic patch protein Sla2 is important for morphogenesis and growth rates in Saccharomyces cerevisiae and Candida albicans, but the mechanism that connects these processes is not clear. Here we report that growth defects in cells lacking CaSLA2 or ScSLA2 are associated with a cell cycle delay that is influenced by Swe1, a morphogenesis checkpoint kinase. To establish how Swe1 monitors Sla2 function, we compared actin organization and cell cycle dynamics in strains lacking other components of early endocytic patches (Sla1 and Abp1) with those in strains lacking Sla2. Only sla2 strains had defects in actin cables, a known trigger of the morphogenesis checkpoint, yet all three strains exhibited Swe1-dependent phenotypes. Thus, Swe1 appears to monitor actin patch in addition to actin cable function. Furthermore, Swe1 contributed to virulence in a mouse model of disseminated candidiasis, implying a role for the morphogenesis checkpoint during the pathogenesis of C. albicans infections.

Aneuploid chromosomes are highly unstable during DNA transformation of Candida albicans

Candida albicans strains tolerate aneuploidy, historically detected as karyotype alterations by pulsed-field gel electrophoresis and more recently revealed by array comparative genome hybridization, which provides a comprehensive and detailed description of gene copy number. Here, we first retrospectively analyzed 411 expression array experiments to predict the frequency of aneuploidy in different strains. As expected, significant levels of aneuploidy were seen in strains exposed to stress conditions, including UV light and/or sorbose treatment, as well as in strains that are resistant to antifungal drugs. More surprisingly, strains that underwent transformation with DNA displayed the highest frequency of chromosome copy number changes, with strains that were initially aneuploid exhibiting approximately 3-fold more copy number changes than strains that were initially diploid. We then prospectively analyzed the effect of lithium acetate (LiOAc) transformation protocols on the stability of trisomic chromosomes. Consistent with the retrospective analysis, the proportion of karyotype changes was highly elevated in strains carrying aneuploid chromosomes. We then tested the hypothesis that stresses conferred by heat and/or LiOAc exposure promote chromosome number changes during DNA transformation procedures. Indeed, a short pulse of very high temperature caused frequent gains and losses of multiple chromosomes or chromosome segments. Furthermore, milder heat exposure over longer periods caused increased levels of loss of heterozygosity. Nonetheless, aneuploid chromosomes were also unstable when strains were transformed by electroporation, which does not include a heat shock step. Thus, aneuploid strains are particularly prone to undergo changes in chromosome number during the stresses of DNA transformation protocols.

Widespread occurrence of chromosomal aneuploidy following the routine production of Candida albicans mutants

It has come to our attention that approximately 35% of >100 published microarray datasets, where transcript levels were compared between two different strains, exhibit some form of chromosome-specific bias. While some of these arose from the use of strains whose aneuploidies were not known at the time, in a worrisome number of cases the recombinant strains have acquired additional aneuploidies that were not initially present in the parental strain. The aneuploidies often affected a different chromosome than the one harboring the insertion site. The affected strains originated from either CAI-4, RM1000, BWP17 or SN95 and were produced through a variety of strategies. These observations suggest that aneuploidies frequently occur during the production of recombinant strains and have an effect on global transcript profiles outside of the afflicted chromosome(s), thus raising the possibility of unintended phenotypic consequences. Thus, we propose that all Candida albicans mutants and strains should be tested for aneuploidy before being used in further studies. To this end, we describe a new rapid testing method, based on a multiplex quantitative PCR assay, that produces eight bands of distinct sizes from either the left or right arms of each C. albicans chromosome.

BAR domain proteins Rvs161 and Rvs167 contribute to Candida albicans endocytosis, morphogenesis, and virulence

The Candida albicans plasma membrane plays critical roles in growth and virulence and as a target for antifungal drugs. Three C. albicans genes that encode Bin-Amphiphysin-Rvs homology domain proteins were mutated to define their roles in plasma membrane function. The deletion of RVS161 and RVS167, but not RVS162, caused strong defects. The rvs161Delta mutant was more defective in endocytosis and morphogenesis than rvs167Delta, but both were strongly defective in polarizing actin patches. Other plasma membrane constituents were still properly localized, including a filipin-stained domain at the hyphal tips. An analysis of growth under different in vitro conditions showed that the rvs161Delta and rvs167Delta mutants grew less invasively in agar and also suggested that they have defects in cell wall synthesis and Rim101 pathway signaling. These mutants were also more resistant to the antimicrobial peptide histatin 5 but showed essentially normal responses to the drugs caspofungin and amphotericin. Surprisingly, the rvs161Delta mutant was more sensitive to fluconazole, whereas the rvs167Delta mutant was more resistant, indicating that these mutations cause overlapping but distinct effects on cells. The rvs161Delta and rvs167Delta mutants both showed greatly reduced virulence in mice. However, the mutants were capable of growing to high levels in kidneys. Histological analyses of infected kidneys revealed that these rvsDelta mutants grew in a large fungal mass that was walled off by leukocytes, rather than forming disseminated microabscesses as seen for the wild type. The diminished virulence is likely due to a combination of the morphogenesis defects that reduce invasive growth and altered cell wall construction that exposes proinflammatory components to the host immune system.

The Candida albicans phosphatase Inp51p interacts with the EH domain protein Irs4p, regulates phosphatidylinositol-4,5-bisphosphate levels and influences hyphal formation, the cell integrity pathway and virulence

We previously identified Candida albicans Irs4p as an epidermal growth factor substrate 15 homology (EH) domain-containing protein that is reactive with antibodies in the sera of patients with candidiasis and contributes to cell wall integrity, hyphal formation and virulence. In this study, we use a yeast two-hybrid method and co-immunoprecipitation to show that Irs4p physically interacts with the phosphatase Inp51p. Disruption of the Inp51p Asn-Pro-Phe (NPF) motif eliminates the interaction, suggesting that this motif is targeted by Irs4p. Both inp51 and irs4 null mutants exhibit significantly increased levels of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)] without changes in levels of other phosphoinositides. Like the irs4 mutant, the inp51 mutant demonstrates increased susceptibility to cell wall-active agents, impaired hyphal formation and abnormal chitin distribution along hyphal walls during growth within solid agar. Moreover, the inp51 and irs4 mutants overactivate the cell wall integrity pathway as measured by Mkc1p phosphorylation. As anticipated, mortality due to disseminated candidiasis is significantly attenuated among mice infected with the inp51 mutant, and tissue burdens and inflammation within the kidneys are reduced. Hyphal formation and chitin distribution in vivo are also impaired, consistent with observations of embedded growth in vitro. All phenotypes exhibited by the inp51 and irs4 mutants are rescued by complementation with the respective genes. In conclusion, our findings suggest that Irs4p binds and activates Inp51p to negatively regulate PI(4,5)P(2) levels and the cell integrity pathway, and that PI(4,5)P(2) homeostasis is important for coordinating cell wall integrity, hyphal growth and virulence under conditions of cell wall stress.

The antimicrobial peptide histatin-5 causes a spatially restricted disruption on the Candida albicans surface, allowing rapid entry of the peptide into the cytoplasm

Antimicrobial peptides play an important role in host defense against microbial pathogens. Their high cationic charge and strong amphipathic structure allow them to bind to the anionic microbial cell membrane and disrupt the membrane bilayer by forming pores or channels. In contrast to the classical pore-forming peptides, studies on histatin-5 (Hst-5) have suggested that the peptide is transported into the cytoplasm of Candida albicans in a non-lytic manner, and cytoplasmic Hst-5 exerts its candicidal activities on various intracellular targets, consistent with its weak amphipathic structure. To understand how Hst-5 is internalized, we investigated the localization of FITC-conjugated Hst-5. We find that Hst-5 is internalized into the vacuole through receptor-mediated endocytosis at low extracellular Hst-5 concentrations, whereas under higher physiological concentrations, Hst-5 is translocated into the cytoplasm through a mechanism that requires a high cationic charge on Hst-5. At intermediate concentrations, two cell populations with distinct Hst-5 localizations were observed. By cell sorting, we show that cells with vacuolar localization of Hst-5 survived, while none of the cells with cytoplasmic Hst-5 formed colonies. Surprisingly, extracellular Hst-5, upon cell surface binding, induces a perturbation on the cell surface, as visualized by an immediate and rapid internalization of Hst-5 and propidium iodide or rhodamine B into the cytoplasm from the site using time-lapse microscopy, and a concurrent rapid expansion of the vacuole. Thus, the formation of a spatially restricted site in the plasma membrane causes the initial injury to C. albicans and offers a mechanism for its internalization into the cytoplasm. Our study suggests that, unlike classical channel-forming antimicrobial peptides, action of Hst-5 requires an energized membrane and causes localized disruptions on the plasma membrane of the yeast. This mechanism of cell membrane disruption may provide species-specific killing with minimal damage to microflora and the host and may be used by many other antimicrobial peptides.

Role of actin cytoskeletal dynamics in activation of the cyclic AMP pathway and HWP1 gene expression in Candida albicans

Changes in gene expression during reversible bud-hypha transitions of the opportunistic fungal pathogen Candida albicans permit adaptation to environmental conditions that are critical for proliferation in host tissues. Our previous work has shown that the hypha-specific adhesin gene HWP1 is up-regulated by the cyclic AMP (cAMP) signaling pathway. However, little is known about the potential influences of determinants of cell morphology on HWP1 gene expression. We found that blocking hypha formation with cytochalasin A, which destabilizes actin filaments, and with latrunculin A, which sequesters actin monomers, led to a loss of HWP1 gene expression. In contrast, high levels of HWP1 gene expression were observed when the F-actin stabilizer jasplakinolide was used to block hypha formation, suggesting that HWP1 expression could be regulated by actin structures. Mutants defective in formin-mediated nucleation of F-actin were reduced in HWP1 gene expression, providing genetic support for the importance of actin structures. Kinetic experiments with wild-type and actin-deficient cells revealed two distinct phases of HWP1 gene expression, with a slow, actin-independent phase preceding a fast, actin-dependent phase. Low levels of HWP1 gene expression that appeared to be independent of stabilized actin and cAMP signaling were detected using indirect immunofluorescence. A connection between actin structures and the cAMP signaling pathway was shown using hyper- and hypomorphic cAMP mutants, providing a possible mechanism for up-regulation of HWP1 gene expression by stabilized actin. The results reveal a new role for F-actin as a regulatory agent of hypha-specific gene expression at the bud-hypha transition.

Morphogenesis in Candida albicans

Candida albicans is termed a dimorphic fungus because it proliferates in either a yeast form or a hyphal form. The switch between these forms is the result of a complex interplay of external and internal factors and is coordinated in part by polarity-regulating proteins that are conserved among eukaryotic cells. However, yeast and hyphal cells are not the only morphological states of C. albicans. The opaque form required for mating, the pseudohyphal cell, and the chlamydospore represent distinct cell types that form in response to specific genetic or environmental conditions. In addition, hyperextended buds can form as a result of various cell cycle-related stresses. Recent studies are beginning to shed light on some of the molecular controls regulating the various morphogenetic forms of this fascinating human pathogen.