Infectious Diseases Society of America.

Isogenic strain construction and gene mapping in Candida albicans.

The polymorphic fungus Candida albicans is a member of the normal human microbiome. In most individuals, C. albicans resides as a lifelong, harmless commensal. Under certain circumstances, however, C. albicans can cause infections that range from superficial infections of the skin to life-threatening systemic infections. Several factors and activities have been identified which contribute to the pathogenic potential of this fungus. Among them are molecules which mediate adhesion to and invasion into host cells, the secretion of hydrolases, the yeast-to-hypha transition, contact sensing and thigmotropism, biofilm formation, phenotypic switching and a range of fitness attributes. Our understanding of when and how these mechanisms and factors contribute to infection has significantly increased during the last years. In addition, novel virulence mechanisms have recently been discovered. In this review we present an update on our current understanding of the pathogenicity mechanisms of this important human pathogen.

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Candida albicans pathogenicity mechanisms

The fungus Candida albicans is often a benign member of the mucosal flora; however, it commonly causes mucosal disease with substantial morbidity and in vulnerable patients it causes life-threatening bloodstream infections. A striking feature of its biology is its ability to grow in yeast, pseudohyphal and hyphal forms. The hyphal form has an important role in causing disease by invading epithelial cells and causing tissue damage. This Review describes our current understanding of the network of signal transduction pathways that monitors environmental cues to activate a programme of hypha-specific gene transcription, and the molecular processes that drive the highly polarized growth of hyphae.

Growth of Candida albicans hyphae

The recent rate of emergence of pathogenic fungi that are resistant to the limited number of commonly used antifungal agents is unprecedented. The azoles, for example, are used not only for human and animal health care and crop protection but also in antifouling coatings and timber preservation. The ubiquity and multiple uses of azoles have hastened the independent evolution of resistance in many environments. One consequence is an increasing risk in human health care from naturally occurring opportunistic fungal pathogens that have acquired resistance to this broad class of chemicals. To avoid a global collapse in our ability to control fungal infections and to avoid critical failures in medicine and food security, we must improve our stewardship of extant chemicals, promote new antifungal discovery, and leverage emerging technologies for alternative solutions.

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Worldwide emergence of resistance to antifungal drugs challenges human health and food security

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Regulatory Circuitry Governing Fungal Development, Drug Resistance, and Disease

Candida albicans is the leading fungal pathogen of humans, causing life-threatening disease in immunocompromised individuals. Treatment of candidiasis is hampered by the limited number of antifungal drugs whose efficacy is compromised by host toxicity, fungistatic activity, and the emergence of drug resistance. We previously established that the molecular chaperone Hsp90, which regulates the form and function of diverse client proteins, potentiates resistance to the azoles in C. albicans and in the model yeast Saccharomyces cerevisiae. Genetic studies in S. cerevisiae revealed that Hsp90's role in azole resistance is to enable crucial cellular responses to the membrane stress exerted by azoles via the client protein calcineurin. Here, we demonstrate that Hsp90 governs cellular circuitry required for resistance to the only new class of antifungals to reach the clinic in decades, the echinocandins, which inhibit biosynthesis of a critical component of the fungal cell wall. Pharmacological or genetic impairment of Hsp90 function reduced tolerance of C. albicans laboratory strains and resistance of clinical isolates to the echinocandins and created a fungicidal combination. Compromising calcineurin function phenocopied compromising Hsp90 function. We established that calcineurin is an Hsp90 client protein in C. albicans: reciprocal co-immunoprecipitation validated physical interaction; Hsp90 inhibition blocked calcineurin activation; and calcineurin levels were depleted upon genetic reduction of Hsp90. The downstream effector of calcineurin, Crz1, played a partial role in mediating calcineurin-dependent stress responses activated by echinocandins. Hsp90's role in echinocandin resistance has therapeutic potential given that genetic compromise of C. albicans HSP90 expression enhanced the efficacy of an echinocandin in a murine model of disseminated candidiasis. Our results identify the first Hsp90 client protein in C. albicans, establish an entirely new role for Hsp90 in mediating resistance to echinocandins, and demonstrate that targeting Hsp90 provides a promising therapeutic strategy for the treatment of life-threatening fungal disease.

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Hsp90 governs echinocandin resistance in the pathogenic yeast Candida albicans via calcineurin.

Antifungal drug resistance: evolution, mechanisms and impact.

The fungal pathogens Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus have transitioned from a rare curiosity to a leading cause of human mortality. The management of infections caused by these organisms is intimately dependent on the efficacy of antifungal agents; however, fungi that are resistant to these treatments are regularly isolated in the clinic, impeding our ability to control infections. Given the significant impact fungal pathogens have on human health, it is imperative to understand the molecular mechanisms that govern antifungal drug resistance. This review describes our current knowledge of the mechanisms by which antifungal drug resistance evolves in experimental populations and clinical settings. We explore current antifungal treatment options and discuss promising strategies to impede the evolution of drug resistance. By tackling antifungal drug resistance as an evolutionary problem, there is potential to improve the utility of current treatments and accelerate the development of novel therapeutic strategies.

Molecular Evolution of Antifungal Drug Resistance.

Fungal biofilms are a major cause of human mortality and are recalcitrant to most treatments due to intrinsic drug resistance. These complex communities of multiple cell types form on indwelling medical devices and their eradication often requires surgical removal of infected devices. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. We previously established that in the leading human fungal pathogen, Candida albicans, Hsp90 enables the emergence and maintenance of drug resistance in planktonic conditions by stabilizing the protein phosphatase calcineurin and MAPK Mkc1. Hsp90 also regulates temperature-dependent C. albicans morphogenesis through repression of cAMP-PKA signalling. Here we demonstrate that genetic depletion of Hsp90 reduced C. albicans biofilm growth and maturation in vitro and impaired dispersal of biofilm cells. Further, compromising Hsp90 function in vitro abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal drugs, the azoles. Depletion of Hsp90 led to reduction of calcineurin and Mkc1 in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 levels led to a marked decrease in matrix glucan levels, providing a compelling mechanism through which Hsp90 might regulate biofilm azole resistance. Impairment of Hsp90 function genetically or pharmacologically transformed fluconazole from ineffectual to highly effective in eradicating biofilms in a rat venous catheter infection model. Finally, inhibition of Hsp90 reduced resistance of biofilms of the most lethal mould, Aspergillus fumigatus, to the newest class of antifungals to reach the clinic, the echinocandins. Thus, we establish a novel mechanism regulating biofilm drug resistance and dispersion and that targeting Hsp90 provides a much-needed strategy for improving clinical outcome in the treatment of biofilm infections.

https://europepmc.org/articles/PMC3169563?pdf=render

Nicole Robbins

Papers

Regulatory Circuitry Governing Fungal Development, Drug Resistance, and Disease

Hsp90 governs echinocandin resistance in the pathogenic yeast Candida albicans via calcineurin.

Antifungal drug resistance: evolution, mechanisms and impact.

Molecular Evolution of Antifungal Drug Resistance.

Hsp90 governs dispersion and drug resistance of fungal biofilms.