Defective Hyphal induction of a Candida albicans phosphatidylinositol 3-phosphate 5-kinase null mutant on solid media does not lead to decreased virulence

A novel phosphoinositide kinase Fab1 regulates biosynthesis of pathogenic aflatoxin in Aspergillus flavus

Aspergillus flavus (A. flavus) is one of the most important model environmental fungi which can produce a potent toxin and carcinogen known as aflatoxin. Aflatoxin contamination causes massive agricultural economic loss and a critical human health issue each year. Although a functional vacuole has been highlighted for its fundamental importance in fungal virulence, the molecular mechanisms of the vacuole in regulating the virulence of A. flavus remain largely unknown. Here, we identified a novel vacuole-related protein in A. flavus, the ortholog of phosphatidylinositol-3-phosphate-5-kinase (Fab1) in Saccharomyces cerevisiae. This kinase was located at the vacuolar membrane, and loss of fab1 function was found to affect the growth, conidia and sclerotial development, cellular acidification and metal ion homeostasis, aflatoxin production and pathogenicity of A. flavus. Further functional analysis revealed that Fab1 was required to maintain the vacuole size and cell morphology. Additional quantitative proteomic analysis suggested that Fab1 was likely to play an important role in maintaining vacuolar/cellular homeostasis, with vacuolar dysregulation upon fab1 deletion leading to impaired aflatoxin synthesis in this fungus. Together, these results provide insight into the molecular mechanisms by which this pathogen produces aflatoxin and mediates its pathogenicity, and may facilitate dissection of the vacuole-mediated regulatory network in A. flavus.

Vacuolar H+-ATPase: An Essential Multitasking Enzyme in Physiology and Pathophysiology

Vacuolar H+-ATPases (V-ATPases) are large multisubunit proton pumps that are required for housekeeping acidification of membrane-bound compartments in eukaryotic cells. Mammalian V-ATPases are composed of 13 different subunits. Their housekeeping functions include acidifying endosomes, lysosomes, phagosomes, compartments for uncoupling receptors and ligands, autophagosomes, and elements of the Golgi apparatus. Specialized cells, including osteoclasts, intercalated cells in the kidney and pancreatic beta cells, contain both the housekeeping V-ATPases and an additional subset of V-ATPases, which plays a cell type specific role. The specialized V-ATPases are typically marked by the inclusion of cell type specific isoforms of one or more of the subunits. Three human diseases caused by mutations of isoforms of subunits have been identified. Cancer cells utilize V-ATPases in unusual ways; characterization of V-ATPases may lead to new therapeutic modalities for the treatment of cancer. Two accessory proteins to the V-ATPase have been identified that regulate the proton pump. One is the (pro)renin receptor and data is emerging that indicates that V-ATPase may be intimately linked to renin/angiotensin signaling both systemically and locally. In summary, V-ATPases play vital housekeeping roles in eukaryotic cells. Specialized versions of the pump are required by specific organ systems and are involved in diseases.

Phosphatidylinositol 3,5-bisphosphate: low abundance, high significance

Recent studies of the low abundant signaling lipid, phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2 ), reveal an intriguingly diverse list of downstream pathways, the intertwined relationship between PI(3,5)P2 and PI5P, as well as links to neurodegenerative diseases. Derived from the structural lipid phosphatidylinositol, PI(3,5)P2 is dynamically generated on multiple cellular compartments where interactions with an increasing list of effectors regulate many cellular pathways. A complex of proteins that includes Fab1/PIKfyve, Vac14, and Fig4/Sac3 mediates the biosynthesis of PI(3,5)P2 , and mutations that disrupt complex function and/or formation cause profound consequences in cells. Surprisingly, mutations in this pathway are linked with neurological diseases, including Charcot-Marie-Tooth syndrome and amyotrophic lateral sclerosis. Future studies of PI(3,5)P2 and PI5P are likely to expand the roles of these lipids in regulation of cellular functions, as well as provide new approaches for treatment of some neurological diseases.

Rapid structural changes and acidification of guard cell vacuoles during stomatal closure require phosphatidylinositol 3,5-bisphosphate

Rapid stomatal closure is essential for water conservation in plants and is thus critical for survival under water deficiency. To close stomata rapidly, guard cells reduce their volume by converting a large central vacuole into a highly convoluted structure. However, the molecular mechanisms underlying this change are poorly understood. In this study, we used pH-indicator dyes to demonstrate that vacuolar convolution is accompanied by acidification of the vacuole in fava bean (Vicia faba) guard cells during abscisic acid (ABA)-induced stomatal closure. Vacuolar acidification is necessary for the rapid stomatal closure induced by ABA, since a double mutant of the vacuolar H(+)-ATPase vha-a2 vha-a3 and vacuolar H(+)-PPase mutant vhp1 showed delayed stomatal closure. Furthermore, we provide evidence for the critical role of phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P2] in changes in pH and morphology of the vacuole. Single and double Arabidopsis thaliana null mutants of phosphatidylinositol 3-phosphate 5-kinases (PI3P5Ks) exhibited slow stomatal closure upon ABA treatment compared with the wild type. Moreover, an inhibitor of PI3P5K reduced vacuolar acidification and convolution and delayed stomatal closure in response to ABA. Taken together, these results suggest that rapid ABA-induced stomatal closure requires PtdIns(3,5)P2, which is essential for vacuolar acidification and convolution.

Deletion of vacuolar proton-translocating ATPase V(o)a isoforms clarifies the role of vacuolar pH as a determinant of virulence-associated traits in Candida albicans

Vacuolar proton-translocating ATPase (V-ATPase) is a central regulator of cellular pH homeostasis, and inactivation of all V-ATPase function has been shown to prevent infectivity in Candida albicans. V-ATPase subunit a of the Vo domain (Voa) is present as two fungal isoforms: Stv1p (Golgi) and Vph1p (vacuole). To delineate the individual contribution of Stv1p and Vph1p to C. albicans physiology, we created stv1Δ/Δ and vph1Δ/Δ mutants and compared them to the corresponding reintegrant strains (stv1Δ/ΔR and vph1Δ/ΔR). V-ATPase activity, vacuolar physiology, and in vitro virulence-related phenotypes were unaffected in the stv1Δ/Δ mutant. The vph1Δ/Δ mutant exhibited defective V1Vo assembly and a 90% reduction in concanamycin A-sensitive ATPase activity and proton transport in purified vacuolar membranes, suggesting that the Vph1p isoform is essential for vacuolar V-ATPase activity in C. albicans. The vph1Δ/Δ cells also had abnormal endocytosis and vacuolar morphology and an alkalinized vacuolar lumen (pHvph1Δ/Δ = 6.8 versus pHvph1Δ/ΔR = 5.8) in both yeast cells and hyphae. Secreted protease and lipase activities were significantly reduced, and M199-induced filamentation was impaired in the vph1Δ/Δ mutant. However, the vph1Δ/Δ cells remained competent for filamentation induced by Spider media and YPD, 10% FCS, and biofilm formation and macrophage killing were unaffected in vitro. These studies suggest that different virulence mechanisms differentially rely on acidified vacuoles and that the loss of both vacuolar (Vph1p) and non-vacuolar (Stv1p) V-ATPase activity is necessary to affect in vitro virulence-related phenotypes. As a determinant of C. albicans pathogenesis, vacuolar pH alone may prove less critical than originally assumed.

Vacuole inheritance regulates cell size and branching frequency of Candida albicans hyphae

Hyphal growth of Candida albicans is characterized by asymmetric cell divisions in which the subapical mother cell inherits most of the vacuolar space and becomes cell cycle arrested in G1, while the apical daughter cell acquires most of the cell cytoplasm and progresses through G1 into the next mitotic cell cycle. Consequently, branch formation in hyphal compartments is delayed until sufficient cytoplasm is synthesized to execute the G1 ‘START’ function. To test the hypothesis that this mode of vacuole inheritance determines cell cycle progression and therefore the branching of hyphae, eight tetracycline-regulated conditional mutants were constructed that were affected at different stages of the vacuole inheritance pathway. Under repressing conditions, vac7, vac8 and fab1 mutants generated mycelial compartments with more symmetrically distributed vacuoles and increased branching frequencies. Repression of VAC1, VAM2 and VAM3 resulted in sparsely branched hyphae, with large vacuoles and enlarged hyphal compartments. Therefore, during hyphal growth of C. albicans the cell cycle, growth and branch formation can be uncoupled, resulting in the investment of cytoplasm to support hyphal extension at the expense of hyphal branching.

The vesicle transport protein Vac1p is required for virulence of Candida albicans

The putative vesicle transport protein Vac1p of the human pathogenic yeast Candida albicans plays an important role in virulence. To determine the cellular functions of Vac1p, a null mutant was generated by sequential disruption of both alleles. The vac1 null mutant strain showed defective endosomal vesicle transport, demonstrating a role of Vac1p in protein transport to the vacuole. Vac1p also contributes to resistance to metal ions, as the null mutant strain was hypersensitive to Cu(2+), Zn(2+) and Ni(2+). In addition, the loss of Vac1p affected several virulence factors of C. albicans. In particular, the vac1 null mutant strain showed defective hyphal growth, even when hyphal formation was induced via different pathways. Furthermore, Vac1p affects chlamydospore formation, adherence to human vaginal epithelial cells, and the secretion of aspartyl proteinases (Saps). Avirulence in a mouse model of systemic infection of the vac1 null mutant strongly suggests that Vac1p of C. albicans is essential for pathogenicity. In summary, the Vac1p protein is required for several cellular pathways, in particular those that control virulence and pathogenicity. Consequently, Vac1p is a novel and interesting target for antifungal drugs.

Fab1p and AP-1 are required for trafficking of endogenously ubiquitylated cargoes to the vacuole lumen in S. cerevisiae

In S. cerevisiae synthesis of phosphatidylinositol (3,5)-bisphosphate [PtdIns(3,5)P2] by Fab1p is required for several cellular events, including an as yet undefined step in the ubiquitin-dependent trafficking of some integral membrane proteins from the trans-Golgi network to the vacuole lumen. AP-1 is a heterotetrameric clathrin adaptor protein complex that binds cargo proteins and clathrin coats, and regulates bi-directional protein trafficking between the trans-Golgi network and the endocytic/secretory pathway. Like fab1Δ cells, AP-1 complex component mutants have lost the ability to traffic ubiquitylated cargoes to the vacuole lumen – the first demonstration that AP-1 is required for this process. Deletion mutants of AP-1 complex components are compromised in their ability to synthesize PtdIns(3,5)P2, indicating that AP-1 is required for correct in vivo activation of Fab1p. Furthermore, wild-type protein sorting can be restored in AP-1 mutants by overexpression of Fab1p, implying that the protein-sorting defect in these cells is as a result of disruption of PtdIns(3,5)P2 synthesis. Finally, we show that Fab1p and Vac14p, an activator of Fab1p, are also required for another AP-1-dependent process: chitin-ring deposition in chs6Δ cells. Our data imply that AP-1 is required for some Fab1p and PtdIns(3,5)P2-dependent processes.

Alternative Candida albicans lifestyles: growth on surfaces

Candida albicans, an opportunistic fungal pathogen, causes a wide variety of human diseases such as oral thrush and disseminated candidiasis. Many aspects of C. albicans physiology have been studied during liquid growth, but in its natural environment, the gastrointestinal tract of a mammalian host, the organism associates with surfaces. Growth on a surface triggers several behaviors, such as biofilm formation, invasion, and thigmotropism, that are important for infection. Recent discoveries have identified factors that regulate these behaviors and revealed the importance of these behaviors for pathogenesis.

Phosphatidylinositol 3,5-bisphosphate: metabolism and cellular functions

Polyphosphoinositides (PPIn) are low-abundance membrane phospholipids that each bind to a distinctive set of effector proteins and, thereby, regulate a characteristic suite of cellular processes. Major functions of phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)] are in membrane and protein trafficking, and in pH control in the endosome-lysosome axis. Recently identified PtdIns(3,5)P(2) effectors include a family of novel beta-propeller proteins, for which we propose the name PROPPINs [for beta-propeller(s) that binds PPIn], and possibly proteins of the epsin and CHMP (charged multi-vesicular body proteins) families. All eukaryotes, with the exception of some pathogenic protists and microsporidians, possess proteins needed for the formation, metabolism and functions of PtdIns(3,5)P(2). The importance of PtdIns(3,5)P(2) for normal cell function is underscored by recent evidence for its involvement in mammalian cell responses to insulin and for PtdIns(3,5)P(2) dysfunction in the human genetic conditions X-linked myotubular myopathy, Type-4B Charcot-Marie-Tooth disease and fleck corneal dystrophy.

Functional analysis of CaIPT1, a sphingolipid biosynthetic gene involved in multidrug resistance and morphogenesis of Candida albicans

In the present study we describe the isolation and functional analysis of a sphingolipid biosynthetic gene, IPT1, of Candida albicans. The functional consequence of the disruption of both alleles of IPT1 was confirmed by mass analysis of its sphingolipid composition. The disruption of both alleles or a single allele of IPT1 did not lead to any change in growth phenotype or total sphingolipid, ergosterol, or phospholipid content of the mutant cells. The loss of mannosyl diinositol diphosphoceramide [M(IP)(2)C] in the ipt1 disruptant, however, resulted in increased sensitivity to drugs like 4-nitroquinoline oxide, terbinafine, o-phenanthroline, fluconazole, itraconazole, and ketoconazole. The increase in drug susceptibilities of ipt1 cells was linked to an altered sphingolipid composition, which appeared to be due to the impaired functionality of Cdr1p, a major drug efflux pump of C. albicans that belongs to the ATP binding cassette superfamily. Our confocal and Western blotting results demonstrated that surface localization of green fluorescent protein-tagged Cdr1p was affected in ipt1 disruptant cells. Poor surface localization of Cdr1p resulted in an impaired ability to efflux fluconazole and rhodamine 6G. The effect of mannosyl inositol phosphoceramide accumulation in the ipt1 mutant and the absence of M(IP)(2)C from the ipt1 mutant on the efflux of drug substrates was very selective. The efflux of methotrexate, a specific substrate of CaMdr1p, another major efflux pump of major facilitator superfamily, remained unaffected in ipt1 mutant cells. Interestingly, changes in sphingolipid composition affected the ability of mutant cells to form proper hyphae in various media. Taken together, our results demonstrate that an altered composition of sphingolipid, which is among the major constituents of membrane rafts, affects the drug susceptibilities and morphogenesis of C. albicans.

The putative vacuolar ATPase subunit Vma7p of Candida albicans is involved in vacuole acidification, hyphal development and virulence

The vacuolar H+-ATPase (V-ATPase) component Vma7p of the human-pathogenic yeast Candida albicans regulates hyphal growth induced by serum and Spider medium and is essential for virulence. In order to characterize the functions of the putative V-ATPase subunit Vma7p of C. albicans, null mutants were generated. The resulting mutants showed reduced vacuole acidification, which correlated with defective growth at alkaline pH. In addition, defects in degradation of intravacuolar putative endosomal structures were observed. vma7 null mutants were sensitive towards the presence of metal ions. It is concluded that the sequestration of toxic ions in the vacuole via a H+ gradient generated by the V-ATPase is affected. The vma7 null mutant strains were avirulent in a mouse model of systemic candidiasis. In addition, C. albicans vma7 null mutants and the null mutant strain of the Vma7p-interacting phosphatidylinositol 3-kinase Vps34p showed similar phenotypes. In summary, the V-ATPase subunit Vma7p is involved in vacuolar ion transport and this transport is required for hyphal growth and virulence of C. albicans.

The phosphatidylinositol 3-kinase Vps34p of the human pathogenic yeast Candida albicans is a multifunctional protein that interacts with the putative vacuolar H+ -ATPase subunit Vma7p

The phosphatidylinositol 3-kinase Vps34p of Candida albicans participates in protein transport and in virulence. In order to characterize the functional link between these two activities we searched for proteins interacting with C. albicans Vps34p and demonstrate physical interaction of Vps34p with the subunit of the vacuolar H+ -ATPase Vma7p. The interaction initially observed in a yeast two-hybrid system was confirmed in vitro with recombinant proteins. Functional assays show that the Vps34p protein is necessary for vacuolar acidification and growth at alkaline pH. In addition, the vps34 null mutant of C. albicans shows defective autophagocytosis. The relevance of these functions for virulence of C. albicans is discussed.

Generation and functional in vivo characterization of a lipid kinase defective phosphatidylinositol 3-kinase Vps34p of Candida albicans

The phosphatidylinositol (PI) 3-kinase Vps34p ofCandida albicanshas lipid kinase and autophosphorylation activity and is involved in virulence and vesicular protein transport. In order to characterize the roles of lipid kinase activity, a chimeric Vps34 protein was created which lacks lipid kinase but retains autophosphorylation activity. To this end, six amino acids within the putative lipid-binding site of Vps34p were replaced by the homologous region of the PI 3-kinase-likeC. albicansTor protein. The resulting chimeric Vps34T protein was recombinantly expressed inEscherichia coliand shown to lack lipid kinase activity. The corresponding chimericVPS34TORgene was inserted into the genome ofC. albicans, and this lipid-kinase-defective strain had a distinctive phenotype compared to those of the wild-type strain SC5314 and thevps34null mutant. The lipid-kinase-defective strain was non-virulent, and showed altered hyphal growth, reduced adherence, as well as defective vacuole morphology and endosomal vesicle transport. These results demonstrate an important role for the lipid kinase activity of Vps34p in virulence and vesicular protein transport. On the other hand, the lipid-kinase-defective strain and thevps34null mutant differ in their temperature- and osmotic-stress response. This indicates a possible role for activities different from the lipid kinase function of Vps34p.

Role of Candida albicans polymorphism in interactions with oral epithelial cells

BACKGROUND

Candida albicans is a polymorphic organism which undergoes morphologic transition between yeast, pseudohyphal and hyphal forms. The ability of C. albicans to change from yeast to filamentous types is a major virulence determinant of this organism. However, the exact role of hyphal transformation in establishing oral mucosal infection is still poorly understood.

METHODS

In this study we used mutants with defects in filamentation, as well as oral strains, which differ in their capacity to form true hyphae, to examine the role of hyphal transformation in the interactions of C. albicans with oral epithelial cells in vitro. These interactions included the ability of these strains to adhere to and injure epithelial cells, as well as their ability to trigger a proinflammatory cytokine response.

RESULTS

We found that strains SC5314 and ATCC28366 formed true hyphae on epithelial cells, whereas strain ATCC32077 and the tup1/tup1 mutant formed only pseudohyphae. Double mutant efg1/efg1cph1/cph1 grew exclusively as blastospores. We also found that yeast and pseudohyphal strains showed reduced adherence capacity to oral keratinocytes and caused minimal cell damage. Moreover, we showed that both yeast and pseudohyphal forms have a strongly attenuated proinflammatory phenotype, since they failed to induce significant interleukin (IL)-1alpha and IL-8 responses by oral epithelial cells.

CONCLUSIONS

Germination of C. albicans into true hyphae is particularly important in the interactions with oral epithelial cells in vitro.

Strategies for the identification of virulence determinants in human pathogenic fungi

The incidence of fungal infections is increasing in different countries. The current available therapy of these infections does not satisfy all requirements in terms of specificity and therapeutic index, a fact that has stimulated the scientific community to identify fungal virulence determinants. Several pathogenic fungi are opportunistic and, therefore, identification of virulence genes is difficult, given their close relationship with host cells. In recent years, the development of genetic tools in several pathogenic fungi has enabled the development of genetic strategies for their identification. These include several strategies based on the phenotypic analysis of strains or environmental conditions in which the expression of the putative gene(s) is either altered or deleted; and this is accomplished through the development of in vitro or in vivo systems. In the near future, this research will produce a better picture of fungal pathogenesis and therefore define novel promising targets in antifungal therapy.