Critical role for CaFEN1 and CaFEN12 of Candida albicans in cell wall integrity and biofilm formation

Specificity of Membrane-Associated J-Domain Protein, Caj1, in Amphotericin B Tolerance in Budding Yeast

Hsp70:J-domain protein (JDP) machineries play pivotal roles in maintaining cellular proteostasis and governing various aspects of fungal physiology. While Hsp70 is known for its involvement in conferring tolerance to diverse antifungal drugs, the specific contribution of JDPs remains unclear. In this study, we examined the sensitivity of cytosolic JDP deletion strains of budding yeast to amphotericin B (AmB), a polyene antifungal agent widely utilized in fungal disease treatment due to its ability to disrupt the fungal plasma membrane (PM). Deleting Caj1, a PM-associated class II JDP, heightened susceptibility to AmB, and the protection conferred by Caj1 against AmB necessitated both its N-terminal J-domain and C-terminal lipid binding domain. Moreover, Caj1 deficiency compromised PM integrity as evidenced by increased phosphate efflux and exacerbated AmB sensitivity, particularly at elevated temperatures. Notably, phytosphingosine (PHS) addition as well as overexpression of PMP3, a positive PM integrity regulator, significantly rescued AmB sensitivity of caj1Δ cells. Our results align with the notion that Caj1 associates with the PM and cooperates with Hsp70 to regulate PM proteostasis, thereby influencing PM integrity in budding yeast. Loss of Caj1 function at the PM compromises PM protein quality control, thereby rendering yeast cells more susceptible to AmB.

Antifungal drug resistance in Candida: a special emphasis on amphotericin B

Invasive fungal infections in humans caused by several Candida species, increased considerably in immunocompromised or critically ill patients, resulting in substantial morbidity and mortality. Candida albicans is the most prevalent species, although the frequency of these organisms varies greatly according to geographic region. Infections with C. albicans and non-albicans Candida species have become more common, especially in the past 20 years, as a result of aging, immunosuppressive medication use, endocrine disorders, malnourishment, extended use of medical equipment, and an increase in immunogenic diseases. Despite C. albicans being the species most frequently associated with human infections, C. glabrata, C. parapsilosis, C. tropicalis, and C. krusei also have been identified. Several antifungal drugs with different modes of action are approved for use in clinical settings to treat fungal infections. However, due to the common eukaryotic structure of humans and fungi, only a limited number of antifungal drugs are available for therapeutic use. Furthermore, drug resistance in Candida species has emerged as a result of the growing use of currently available antifungal drugs against fungal infections. Amphotericin B (AmB), a polyene class of antifungal drugs, is mainly used for the treatment of serious systemic fungal infections. AmB interacts with fungal plasma membrane ergosterol, triggering cellular ion leakage via pore formation, or extracting the ergosterol from the plasma membrane inducing cellular death. AmB resistance is primarily caused by changes in the content or structure of ergosterol. This review summarizes the antifungal drug resistance exhibited by Candida species, with a special focus on AmB.

Anti-Biofilm Activity of Cannabidiol against Candida albicans

Candida albicans is a common fungal pathogen in humans. Biofilm formation is an important virulence factor of C. albicans infections. We investigated the ability of the plant-derived cannabidiol (CBD) to inhibit the formation and removal of fungal biofilms. Further, we evaluated its mode of action. Our findings demonstrate that CBD exerts pronounced time-dependent inhibitory effects on biofilm formation as well as disruption of mature biofilm at a concentration range below minimal inhibitory and fungicidal concentrations. CBD acts at several levels. It modifies the architecture of fungal biofilm by reducing its thickness and exopolysaccharide (EPS) production accompanied by downregulation of genes involved in EPS synthesis. It alters the fungal morphology that correlated with upregulation of yeast-associated genes and downregulation of hyphae-specific genes. Importantly, it represses the expression of C. albicans virulence-associated genes. In addition, CBD increases ROS production, reduces the intracellular ATP levels, induces mitochondrial membrane hyperpolarization, modifies the cell wall, and increases the plasma membrane permeability. In conclusion, we propose that CBD exerts its activity towards C. albicans biofilm through a multi-target mode of action, which differs from common antimycotic agents, and thus can be explored for further development as an alternative treatment against fungal infections.

Sphingolipids: Regulators of azole drug resistance and fungal pathogenicity

In recent years, the role of sphingolipids in pathogenic fungi, in terms of pathogenicity and resistance to azole drugs, has been a rapidly growing field. This review describes evidence about the roles of sphingolipids in azole resistance and fungal virulence. Sphingolipids can serve as signaling molecules that contribute to azole resistance through modulation of the expression of drug efflux pumps. They also contribute to azole resistance by participating in various microbial pathways such as the unfolded protein response (UPR), pH-responsive Rim pathway, and pleiotropic drug resistance (PDR) pathway. In addition, sphingolipid signaling and eisosomes also coordinately regulate sphingolipid biosynthesis in response to azole-induced membrane stress. Sphingolipids are important for fungal virulence, playing roles during growth in hosts under stressful conditions, maintenance of cell wall integrity, biofilm formation, and production of various virulence factors. Finally, we discuss the possibility of exploiting fungal sphingolipids for the development of new therapeutic strategies to treat infections caused by pathogenic fungi.

Iron alters the cell wall composition and intracellular lactate to affect Candida albicans susceptibility to antifungals and host immune response

Fungal pathogen Candida albicans has a complex cell wall consisting of an outer layer of mannans and an inner layer of β-glucans and chitin. The fungal cell wall is the primary target for antifungals and is recognized by host immune cells. Environmental conditions such as carbon sources, pH, temperature, and oxygen tension can modulate the fungal cell wall architecture. Cellular signaling pathways, including the mitogen-activated protein kinase (MAPK) pathways, are responsible for sensing environmental cues and mediating cell wall alterations. Although iron has recently been shown to affect β-1,3-glucan exposure on the cell wall, we report here that iron changes the composition of all major C. albicans cell wall components. Specifically, high iron decreased the levels of mannans (including phosphomannans) and chitin; and increased β-1,3-glucan levels. These changes increased the resistance of C. albicans to cell wall-perturbing antifungals. Moreover, high iron cells exhibited adequate mitochondrial functioning; leading to a reduction in accumulation of lactate that signals through the transcription factor Crz1 to induce β-1,3-glucan masking in C. albicans We show here that iron-induced changes in β-1,3-glucan exposure are lactate-dependent; and high iron causes β-1,3-glucan exposure by preventing lactate-induced, Crz1-mediated inhibition of activation of the fungal MAPK Cek1. Furthermore, despite exhibiting enhanced antifungal resistance, high iron C. albicans cells had reduced survival upon phagocytosis by macrophages. Our results underscore the role of iron as an environmental signal in multiple signaling pathways that alter cell wall architecture in C. albicans, thereby affecting its survival upon exposure to antifungals and host immune response.

Chemical-genetic interaction landscape of mono-(2-ethylhexyl)-phthalate using chemogenomic profiling in yeast

Integration of chemical-genetic interaction data with biological functions provides a mechanistic understanding of how toxic compounds affect cells. Mono-(2-ethylhexyl)-phthalate (MEHP) is an active metabolite of di-(2-ethylhexyl)-phthalate (DEHP), a commonly used plasticizer. MEHP adversely affects human health causing hepatotoxicity and reproductive toxicity. How MEHP affects cellular physiology is not fully understood. We utilized a genome-wide competitive fitness-based assay called 'chemogenomic profiling' to determine the genetic interaction map of MEHP in Saccharomyces cerevisiae. Gene Ontology enrichment analysis of 218 genes that provide resistance to MEHP indicated that MEHP affects seven cellular processes namely: (1) cellular amino acid biosynthetic process, (2) sterol biosynthetic process, (3) cellular transport, (4) transcriptional and translational regulation, (5) protein glycosylation, (6) cytokinesis and cell morphogenesis and (7) ionic homeostasis. We show that MEHP protects yeast cells from membrane perturbing agents such as amphotericin B, dihydrosphingosine and phytosphingosine. Moreover, we also demonstrate that MEHP compromises the integrity of the yeast plasma membrane and cell wall. Our work provides a basis for further investigation of MEHP toxicity in humans.

Linking Cellular Morphogenesis with Antifungal Treatment and Susceptibility in Candida Pathogens

Fungal infections are a growing public health concern, and an increasingly important cause of human mortality, with Candida species being amongst the most frequently encountered of these opportunistic fungal pathogens. Several Candida species are polymorphic, and able to transition between distinct morphological states, including yeast, hyphal, and pseudohyphal forms. While not all Candida pathogens are polymorphic, the ability to undergo morphogenesis is linked with the virulence of many of these pathogens. There are also many connections between Candida morphogenesis and antifungal drug treatment and susceptibility. Here, we review how Candida morphogenesis-a key virulence trait-is linked with antifungal drugs and antifungal drug resistance. We highlight how antifungal therapeutics are able to modulate morphogenesis in both sensitive and drug-resistant Candida strains, the shared signaling pathways that mediate both morphogenesis and the cellular response to antifungal drugs and drug resistance, and the connection between Candida morphology, drug resistance, and biofilm growth. We further review the development of anti-virulence drugs, and targeting Candida morphogenesis as a novel therapeutic strategy to target fungal pathogens. Together, this review highlights important connections between fungal morphogenesis, virulence, and susceptibility to antifungals.

Blocking β-1,6-glucan synthesis by deleting KRE6 and SKN1 attenuates the virulence of Candida albicans

β-1,6-glucan is an important component of the fungal cell wall. The β-1,6-glucan synthase gene KRE6 was thought to be essential in the fungal pathogen Candida albicans because it could not be deleted in previous efforts. Also, the role of its homolog SKN1 was unclear because its deletion caused no defects. Here, we report the construction and characterization of kre6Δ/Δ, skn1Δ/Δ and kre6Δ/Δ skn1Δ/Δ mutants in C. albicans. While deleting KRE6 or SKN1 had no obvious phenotypic consequence, deleting both caused slow growth, cell separation failure, cell wall abnormalities, diminished hyphal growth, poor biofilm formation and loss of virulence in mice. Furthermore, the GPI-linked cell surface proteins Hwp1 and the invasin Als3 or Ssa1 were not detected in kre6Δ/Δ skn1Δ/Δ mutant. In GMM medium, RT-qPCR and western blotting revealed a constitutive expression of KRE6 and growth conditions-associated activation of SKN1. Like many hypha-specific genes, SKN1 is repressed by Nrg1, but its activation does not involve the transcription factor Efg1. Dysregulation of SKN1 reduces C. albicans ability to damage epithelial and endothelial cells and attenuates its virulence. Given the vital role of β-1,6-glucan synthesis in C. albicans physiology and virulence, Kre6 and Skn1 are worthy targets for developing antifungal agents.

The Significance of Lipids to Biofilm Formation in Candida albicans: An Emerging Perspective

Candida albicans, the dimorphic opportunistic human fungal pathogen, is capable of forming highly drug-resistant biofilms in the human host. Formation of biofilm is a multistep and multiregulatory process involving various adaptive mechanisms. The ability of cells in a biofilm to alter membrane lipid composition is one such adaptation crucial for biofilm development in C. albicans. Lipids modulate mixed species biofilm formation in vivo and inherent antifungal resistance associated with these organized communities. Cells in C. albicans biofilms display phase-dependent changes in phospholipid classes and in levels of lipid raft formation. Systematic studies with genetically modified strains in which the membrane phospholipid composition can be manipulated are limited in C. albicans. In this review, we summarize the knowledge accumulated on the impact that alterations in phospholipids may have on the biofilm forming ability of C. albicans in the human host. This review may provide the requisite impetus to analyze lipids from a therapeutic standpoint in managing C. albicans biofilms.

An insight into new strategies to combat antifungal drug resistance

Invasive fungal infections especially in immunocompromised patients represent a dominating cause of mortality. The most commonly used antifungal agents can be divided into three broad categories, including triazoles, echinocandins and polyenes. Antifungal resistance is on the increase, posing a growing threat to the stewardship of immunocompromised patients with fungal infections. The paucity of currently available antifungals leads to the rapid emergence of drug resistance and thus aggravates the refractoriness of invasive fungal infections. Therefore, deep exploration into mechanisms of drug resistance and search for new antifungal targets are required. This review highlights the therapeutic strategies targeting Hsp90, calcineurin, trehalose biosynthesis and sphingolipids biosynthesis, in an attempt to provide clinical evidence for overcoming drug resistance and to form the rationale for combination therapy of conventional antifungals and agents with novel mechanisms of action. What’s more, this review also gives a concise introduction of three new-fashioned antifungals, including carboxymethyl chitosan, silver nanoparticles and chromogranin A-N46.

Candida albicans gains azole resistance by altering sphingolipid composition

Fungal infections by drug-resistant Candida albicans pose a global public health threat. However, the pathogen’s diploid genome greatly hinders genome-wide investigations of resistance mechanisms. Here, we develop an efficient piggyBac transposon-mediated mutagenesis system using stable haploid C. albicans to conduct genome-wide genetic screens. We find that null mutants in either gene FEN1 or FEN12 (encoding enzymes for the synthesis of very-long-chain fatty acids as precursors of sphingolipids) exhibit resistance to fluconazole, a first-line antifungal drug. Mass-spectrometry analyses demonstrate changes in cellular sphingolipid composition in both mutants, including substantially increased levels of several mannosylinositolphosphoceramides with shorter fatty-acid chains. Treatment with fluconazole induces similar changes in wild-type cells, suggesting a natural response mechanism. Furthermore, the resistance relies on a robust upregulation of sphingolipid biosynthesis genes. Our results shed light into the mechanisms underlying azole resistance, and the new transposon-mediated mutagenesis system should facilitate future genome-wide studies of C. albicans.

The fungal pathogen Candida albicans is diploid, which hinders genome-wide studies. Here, Gao et al. present a piggyBac transposon-mediated mutagenesis system using stable haploid C. albicans strains, and use it to identify genes and mechanisms underlying azole resistance.