Candida albicans VPS4 contributes differentially to epithelial and mucosal pathogenesis

The GARP complex is required for filamentation in Candida albicans

Candida albicans is an opportunistic fungal pathogen that causes superficial infections in immunocompetent individuals, as well as life-threatening systemic disease in immunocompromised patients. A key virulence trait of this pathogen is its ability to transition between yeast and filamentous morphologies. A functional genomic screen to identify novel regulators of filamentation previously revealed VPS53 as being important for morphogenesis. Vps53 belongs to the Golgi-associated retrograde protein (GARP) complex, which mediates retrograde trafficking from the endosome to the trans-Golgi network. Here, we explored the role of the entire GARP complex in regulating morphogenesis. Deletion of any of the four genes encoding GARP complex subunits severely impaired filamentation in response to diverse filament-inducing cues, including upon internalization by macrophages. Genetic pathway analysis revealed that while hyperactivation of protein kinase A (PKA) signaling is insufficient to drive filamentation in GARP complex mutants, these strains are capable of filamentation upon overexpression of transcriptional activators or upon deletion of transcriptional repressors of hyphal morphogenesis. Finally, compromise of the GARP complex induced lipotoxicity, and pharmacological inhibition of sphingolipid biosynthesis phenocopied genetic compromise of the GARP complex by impairing filamentation. Together, this work identifies the GARP complex as an important mediator of filamentation in response to multiple inducing cues, maps genetic circuitry important for filamentation upon compromise of GARP function, and supports a model whereby GARP deficiency impairs lipid homeostasis, which is important for supporting filamentous growth in C. albicans.

The Importance of Vacuolar Ion Homeostasis and Trafficking in Hyphal Development and Virulence in Candida albicans

The vacuole of Candida albicans plays a significant role in many processes including homeostasis control, cellular trafficking, dimorphic switching, and stress tolerance. Thus, understanding the factors affecting vacuole function is important for the identification of new drug targets needed in response to the world’s increasing levels of invasive infections and the growing issue of fungal drug resistance. Past studies have shown that vacuolar proton-translocating ATPases (V-ATPases) play a central role in pH homeostasis and filamentation. Vacuolar protein sorting components (VPS) regulate V-ATPases assembly and at the same time affect hyphal development. As well, vacuolar calcium exchange systems like Yvc1 and Pmc1 maintain cytosolic calcium levels while being affected by V-ATPases function. All these proteins play a role in the virulence and pathogenesis of C. albicans. This review highlights the relationships among V-ATPases, VPS, and vacuolar calcium exchange proteins while summarizing their importance in C. albicans infections.

Caenorhabditis elegans as an Infection Model for Pathogenic Mold and Dimorphic Fungi: Applications and Challenges

The threat burden from pathogenic fungi is universal and increasing with alarming high mortality and morbidity rates from invasive fungal infections. Understanding the virulence factors of these fungi, screening effective antifungal agents and exploring appropriate treatment approaches in in vivo modeling organisms are vital research projects for controlling mycoses. Caenorhabditis elegans has been proven to be a valuable tool in studies of most clinically relevant dimorphic fungi, helping to identify a number of virulence factors and immune-regulators and screen effective antifungal agents without cytotoxic effects. However, little has been achieved and reported with regard to pathogenic filamentous fungi (molds) in the nematode model. In this review, we have summarized the enormous breakthrough of applying a C. elegans infection model for dimorphic fungi studies and the very few reports for filamentous fungi. We have also identified and discussed the challenges in C. elegans-mold modeling applications as well as the possible approaches to conquer these challenges from our practical knowledge in C. elegans-Aspergillus fumigatus model.

Candida albicans ENT2 Contributes to Efficient Endocytosis, Cell Wall Integrity, Filamentation, and Virulence

Epsins play a pivotal role in the formation of endocytic vesicles and potentially provide a linkage between endocytic and other trafficking pathways. We identified a Candida albicans epsin, ENT2, that bears homology to the Saccharomyces cerevisiae early endocytosis genes ENT1 and ENT2 and studied its functions by a reverse genetic approach utilizing CRISPR-Cas9-mediated gene deletion. The C. albicans ent2Δ/Δ null mutant displayed cell wall defects and altered antifungal drug sensitivity. To define the role of C. albicans ENT2 in endocytosis, we performed assays with the lipophilic dye FM4-64 that revealed greatly reduced uptake in the ent2Δ/Δ mutant. Next, we showed that the C. albicans ent2Δ/Δ mutant was unable to form hyphae and biofilms. Assays for virulence properties in an in vitro keratinocyte infection model demonstrated reduced damage of mammalian adhesion zippers and host cell death from the ent2Δ/Δ mutant. We conclude that C. albicans ENT2 has a role in efficient endocytosis, a process that is required for maintaining cell wall integrity, hyphal formation, and virulence-defining traits. IMPORTANCE The opportunistic fungal pathogen Candida albicans is an important cause of invasive infections in hospitalized patients and a source of considerable morbidity and mortality. Despite its clinical importance, we still need to improve our ability to diagnose and treat this common pathogen. In order to support these advancements, a greater understanding of the biology of C. albicans is needed. In these studies, we are focused on the fundamental biological process of endocytosis, of which little is directly known in C. albicans. In addition to studying the function of a key gene in this process, we are examining the role of endocytosis in the virulence-related processes of filamentation, biofilm formation, and tissue invasion. These studies will provide greater insight into the role of endocytosis in causing invasive fungal infections.

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.

The vaginal mycobiome: A contemporary perspective on fungi in women's health and diseases

Most of what is known about fungi in the human vagina has come from culture-based studies and phenotypic characterization of single organisms. Though valuable, these approaches have masked the complexity of fungal communities within the vagina. The vaginal mycobiome has become an emerging field of study as genomics tools are increasingly employed and we begin to appreciate the role these fungal communities play in human health and disease. Though vastly outnumbered by its bacterial counterparts, fungi are important constituents of the vaginal ecosystem in many healthy women. Candida albicans, an opportunistic fungal pathogen, colonizes 20% of women without causing any overt symptoms, yet it is one of the leading causes of infectious vaginitis. Understanding its mechanisms of commensalism and patho-genesis are both essential to developing more effective therapies. Describing the interactions between Candida, bacteria (such as Lactobacillus spp.) and other fungi in the vagina is funda-mental to our characterization of the vaginal mycobiome.

Adaptation of Candida albicans to commensalism in the gut

Candida albicans is a common resident of the oral cavity, GI tract and vagina in healthy humans where it establishes a commensal relationship with the host. Colonization of the gut, which is an important niche for the microbe, may lead to systemic dissemination and disease upon alteration of host defences. Understanding the mechanisms responsible for the adaptation of C. albicans to the gut is therefore important for the design of new ways of combating fungal diseases. In this review we discuss the available models to study commensalism of this yeast, the main mechanisms controlling the establishment of the fungus, such as microbiota, mucus layer and antimicrobial peptides, and the gene regulatory circuits that ensure its survival in this niche.

ERG2 and ERG24 Are Required for Normal Vacuolar Physiology as Well as Candida albicans Pathogenicity in a Murine Model of Disseminated but Not Vaginal Candidiasis

Several important classes of antifungal agents, including the azoles, act by blocking ergosterol biosynthesis. It was recently reported that the azoles cause massive disruption of the fungal vacuole in the prevalent human pathogen Candida albicans. This is significant because normal vacuolar function is required to support C. albicans pathogenicity. This study examined the impact of the morpholine antifungals, which inhibit later steps of ergosterol biosynthesis, on C. albicans vacuolar integrity. It was found that overexpression of either the ERG2 or ERG24 gene, encoding C-8 sterol isomerase or C-14 sterol reductase, respectively, suppressed C. albicans sensitivity to the morpholines. In addition, both erg2Δ/Δ and erg24Δ/Δ mutants were hypersensitive to the morpholines. These data are consistent with the antifungal activity of the morpholines depending upon the simultaneous inhibition of both Erg2p and Erg24p. The vacuoles within both erg2Δ/Δ and erg24Δ/Δ C. albicans strains exhibited an aberrant morphology and accumulated large quantities of the weak base quinacrine, indicating enhanced vacuolar acidification compared with that of control strains. Both erg mutants exhibited significant defects in polarized hyphal growth and were avirulent in a mouse model of disseminated candidiasis. Surprisingly, in a mouse model of vaginal candidiasis, both mutants colonized mice at high levels and induced a pathogenic response similar to that with the controls. Thus, while targeting Erg2p or Erg24p alone could provide a potentially efficacious therapy for disseminated candidiasis, it may not be an effective strategy to treat vaginal infections. The potential value of drugs targeting these enzymes as adjunctive therapies is discussed.