Steryl Glycosides in Fungal Pathogenesis: An Understudied Immunomodulatory Adjuvant

Ergosterol distribution controls surface structure formation and fungal pathogenicity

Ergosterol, the major sterol in fungal membranes, is critical for defining membrane fluidity and regulating cellular processes. Although ergosterol synthesis has been well defined in model yeast, little is known about sterol organization in the context of fungal pathogenesis. We identified a retrograde sterol transporter, Ysp2, in the opportunistic fungal pathogen Cryptococcus neoformans. We found that the lack of Ysp2 under host-mimicking conditions leads to abnormal accumulation of ergosterol at the plasma membrane, invagination of the plasma membrane, and malformation of the cell wall, which can be functionally rescued by inhibiting ergosterol synthesis with the antifungal drug fluconazole. We also observed that cells lacking Ysp2 mislocalize the cell surface protein Pma1 and have abnormally thin and permeable capsules. As a result of perturbed ergosterol distribution and its consequences, ysp2∆ cells cannot survive in physiologically relevant environments such as host phagocytes and are dramatically attenuated in virulence. These findings expand our knowledge of cryptococcal biology and underscore the importance of sterol homeostasis in fungal pathogenesis. IMPORTANCE Cryptococcus neoformans is an opportunistic fungal pathogen that kills over 100,000 people worldwide each year. Only three drugs are available to treat cryptococcosis, and these are variously limited by toxicity, availability, cost, and resistance. Ergosterol is the most abundant sterol in fungi and a key component in modulating membrane behavior. Two of the drugs used for cryptococcal infection, amphotericin B and fluconazole, target this lipid and its synthesis, highlighting its importance as a therapeutic target. We discovered a cryptococcal ergosterol transporter, Ysp2, and demonstrated its key roles in multiple aspects of cryptococcal biology and pathogenesis. These studies demonstrate the role of ergosterol homeostasis in C. neoformans virulence, deepen our understanding of a pathway with proven therapeutic importance, and open a new area of study.

Targeting Sterylglucosidase A to Treat Aspergillus fumigatus Infections

Invasive fungal infections are a leading cause of death in immunocompromised patients. Current therapies have several limitations, and innovative antifungal agents are critically needed. Previously, we identified the fungus-specific enzyme sterylglucosidase as essential for pathogenesis and virulence of Cryptococcus neoformans and Aspergillus fumigatus (Af) in murine models of mycoses. Here, we developed Af sterylglucosidase A (SglA) as a therapeutic target. We identified two selective inhibitors of SglA with distinct chemical scaffolds that bind in the active site of SglA. Both inhibitors induce sterylglucoside accumulation and delay filamentation in Af and increase survival in a murine model of pulmonary aspergillosis. Structure-activity relationship (SAR) studies identified a more potent derivative that enhances both in vitro phenotypes and in vivo survival. These findings support sterylglucosidase inhibition as a promising antifungal approach with broad-spectrum potential. IMPORTANCE Invasive fungal infections are a leading cause of death in immunocompromised patients. Aspergillus fumigatus is a fungus ubiquitously found in the environment that, upon inhalation, causes both acute and chronic illnesses in at-risk individuals. A. fumigatus is recognized as one of the critical fungal pathogens for which a substantive treatment breakthrough is urgently needed. Here, we studied a fungus-specific enzyme, sterylglucosidase A (SglA), as a therapeutic target. We identified selective inhibitors of SglA that induce accumulation of sterylglucosides and delay filamentation in A. fumigatus and increase survival in a murine model of pulmonary aspergillosis. We determined the structure of SglA, predicted the binding poses of these inhibitors through docking analysis, and identified a more efficacious derivative with a limited SAR study. These results open several exciting avenues for the research and development of a new class of antifungal agents targeting sterylglucosidases.

Assays and Utilization of Enzymes Involved in Glycolipid Metabolism in Bacteria and Fungi

Microbial glycosphingolipid (GSL)-degrading enzymes with unique specificity are useful tools for GSL research. On the other hand, some microbial glycolipids, not only GSLs but also steryl glucosides, are closely related to pathogenicity, and, thus, the metabolism of microbial glycolipids is attracting attention as a target for antibiotics. This chapter describes the assays and utilization of microbial enzymes useful for glycolipid research and those involved in pathogenicity or host immune reactions.

A Fungal Sterylglucosidase at the Intersection of Virulence, Host Immunity, and Therapeutic Development

Human fungal infections (mycoses) cause significant morbidity and mortality in high-risk populations. Contemporary antifungal therapies rely heavily on three classes of antifungal drugs, and to date, no fungal vaccine is in clinical use for invasive mycosis. A major gap in knowledge related to fungal vaccine development is identifying lasting mechanisms of protective immunity in immunocompromised individuals. Recent studies in Cryptococcus neoformans and now Aspergillus fumigatus have identified a fungal sterylglucosidase essential for pathogenesis and virulence in murine models of mycoses. Fungal strains deficient in this sterylglucosidase can surprisingly also induce substantial immune-mediated protection against subsequent challenge with wild-type strains in multiple immunocompromised murine models of mycoses. Here, I discuss the implications and future directions of these exciting and impactful results.

Vaccination with Live or Heat-Killed Aspergillus fumigatus ΔsglA Conidia Fully Protects Immunocompromised Mice from Invasive Aspergillosis

Aspergillus fumigatus causes invasive aspergillosis (IA) in immunocompromised patients, resulting in high mortality rates. Currently, no vaccine formulations to promote immune protection in at-risk individuals have been developed. In this work, we deleted the sterylglucosidase-encoding gene, sglA, in Aspergillus fumigatus and investigated its role in fungal virulence and host vaccine protection. The ΔsglA mutant accumulated sterylglucosides (SGs), newly studied immunomodulatory glycolipids, and exhibited reduced hyphal growth and altered compositions of cell wall polysaccharides. Interestingly, the ΔsglA mutant was avirulent in two murine models of IA and was fully eliminated from the lungs. Both corticosteroid-induced immunosuppressed and cyclophosphamide-induced leukopenic mice vaccinated with live or heat-killed ΔsglA conidia were fully protected against a lethal wild-type A. fumigatus challenge. These results highlight the potential of SG-accumulating strains as safe and promising vaccine formulations against invasive fungal infections. IMPORTANCE Infections by Aspergillus fumigatus occur by the inhalation of environmental fungal spores called conidia. We found that live mutant conidia accumulating glycolipids named sterylglucosides are not able to cause disease when injected into the lung. Interestingly, these animals are now protected against a secondary challenge with live wild-type conidia. Remarkably, protection against a secondary challenge persists even with vaccination with heat-killed mutant conidia. These results will significantly advance the field of the research and development of a safe fungal vaccine for protection against the environmental fungus A. fumigatus.

Three Models of Vaccination Strategies Against Cryptococcosis in Immunocompromised Hosts Using Heat-Killed Cryptococcus neoformans Δsgl1

Vaccines are one of the greatest medical accomplishments to date, yet no fungal vaccines are currently available in humans mainly because opportunistic mycoses generally occur during immunodeficiencies necessary for vaccine protection. In previous studies, a live, attenuated Cryptococcus neoformans Δsgl1 mutant accumulating sterylglucosides was found to be avirulent and protected mice from a subsequent lethal infection even in absence of CD4⁺ T cells, a condition most associated with cryptococcosis (e.g., HIV). Here, we tested three strategies of vaccination against cryptococcosis. First, in our preventative model, protection was achieved even after a 3-fold increase of the vaccination window. Second, because live C. neoformans Δsgl1-vaccinated mice challenged more than once with WT strain had a significant decrease in lung fungal burden, we tested C. neoformans Δsgl1 as an immunotherapeutic. We found that therapeutic administrations of HK C. neoformans Δsgl1 post WT challenge significantly improves the lung fungal burden. Similarly, therapeutic administration of HK C. neoformans Δsgl1 post WT challenge resulted in 100% or 70% survival depending on the time of vaccine administration, suggesting that HK Δsgl1 is a robust immunotherapeutic option. Third, we investigated a novel model of vaccination in preventing reactivation from lung granuloma using C. neoformans Δgcs1. Remarkably, we show that administration of HK Δsgl1 prevents mice from reactivating Δgcs1 upon inducing immunosuppression with corticosteroids or by depleting CD4⁺ T cells. Our results suggest that HK Δsgl1 represents a clinically relevant, efficacious vaccine that confers robust host protection in three models of vaccination against cryptococcosis even during CD4-deficiency.

Structure and inhibition of Cryptococcus neoformans sterylglucosidase to develop antifungal agents

Pathogenic fungi exhibit a heavy burden on medical care and new therapies are needed. Here, we develop the fungal specific enzyme sterylglucosidase 1 (Sgl1) as a therapeutic target. Sgl1 converts the immunomodulatory glycolipid ergosterol 3β-D-glucoside to ergosterol and glucose. Previously, we found that genetic deletion of Sgl1 in the pathogenic fungus Cryptococcus neoformans (Cn) results in ergosterol 3β-D-glucoside accumulation, renders Cn non-pathogenic, and immunizes mice against secondary infections by wild-type Cn, even in condition of CD4+ T cell deficiency. Here, we disclose two distinct chemical classes that inhibit Sgl1 function in vitro and in Cn cells. Pharmacological inhibition of Sgl1 phenocopies a growth defect of the Cn Δsgl1 mutant and prevents dissemination of wild-type Cn to the brain in a mouse model of infection. Crystal structures of Sgl1 alone and with inhibitors explain Sgl1’s substrate specificity and enable the rational design of antifungal agents targeting Sgl1.

Sterylglucosidase 1 (Sgl1) is a virulence factor in Cryptococcus neoformans that modulates fungal pathogenesis and host response. Here, the authors characterize Sgl1 structurally, identify Sgl1 inhibitors, and demonstrate Sgl1 inhibition has efficacy in mouse models of infection.

Cryptococcus neoformans Δsgl1 Vaccination Requires Either CD4+ or CD8+ T Cells for Complete Host Protection

Cryptococcus neoformans is a fungal pathogen causing life-threatening meningoencephalitis in susceptible individuals. Fungal vaccine development has been hampered by the fact that cryptococcosis occurs during immunodeficiency. We previously reported that a C. neoformans mutant (Δsgl1) accumulating sterylglucosides (SGs) is avirulent and provides complete protection to WT challenge, even under CD4+ T cell depletion, an immunodeficient condition commonly associated with cryptococcosis. We found high levels of SGs in the lungs post-immunization with Δsgl1 that decreased upon fungal clearance. Th1 cytokines increased whereas Th2 cytokines concurrently decreased, coinciding with a large recruitment of leukocytes to the lungs. Depletion of B or CD8+ T cells did not affect either Δsgl1 clearance or protection from WT challenge. Although CD4+ T cell depletion affected clearance, mice were still protected indicating that clearance of the mutant was not necessary for host protection. Protection was lost only when both CD4+ and CD8+ T cells were depleted, highlighting a previously unexplored role of fungal-derived SGs as an immunoadjuvant for host protection against cryptococcosis.

C4-monomethylsterol β-glucoside and its synthase in Aurantiochytrium limacinum mh0186

Thraustochytrids, unicellular marine protists, synthesize polyunsaturated fatty acids (PUFAs) and PUFA-containing phospholipids; however, little is known about their glycolipids and their associated metabolism. Here, we report two glycolipids (GL-A, B) and their synthases in Aurantiochytrium limacinum mh0186. Two glycolipids were purified from A. limacinum mh0186, and they were determined by gas chromatography, mass spectrometry and two-dimensional nuclear magnetic resonance to be 3-O-β-D-glucopyranosyl-stigmasta-5,7,22-triene (GL-A) and 3-O-β-D-glucopyranosyl-4α-methyl-stigmasta-7,22-diene (GL-B), both of which are sterol β-glucosides (β-SGs); the structure of GL-B has not been reported thus far. Seven candidate genes responsible for the synthesis of these β-SGs were extracted from the draft genome database of A. limacinum using the yeast sterol β-glucosyltransferase (SGT; EC 2.4.1.173) sequence as a query. Expression analysis using Saccharomyces cerevisiae revealed that two gene products (AlSGT-1 and 2) catalyze the transfer of glucose from UDP-glucose to sterols, generating sterylglucosides (SGs). Compared to AlSGT-1, AlSGT-2 exhibited wide specificity for sterols and used C4-monomethylsterol to synthesize GL-B. The disruption of alsgt-2 but not alsgt-1 in strain mh0186 resulted in a decrease in total SG and almost complete loss of GL-B, indicating that AlSGT-2 is responsible for the synthesis of β-SGs in A. limacinum mh0186, especially GL-B, which possesses a unique sterol structure.

Structure, metabolism and biological functions of steryl glycosides in mammals

Steryl glycosides (SGs) are sterols glycosylated at their 3β-hydroxy group. They are widely distributed in plants, algae, and fungi, but are relatively rare in bacteria and animals. Glycosylation of sterols, resulting in important components of the cell membrane SGs, alters their biophysical properties and confers resistance against stress by freezing or heat shock to cells. Besides, many biological functions in animals have been suggested from the observations of SG administration. Recently, cholesteryl glucosides synthesized via the transglycosidation by glucocerebrosidases (GBAs) were found in the central nervous system of animals. Identification of patients with congenital mutations in GBA genes or availability of respective animal models will enable investigation of the function of such endogenously synthesized cholesteryl glycosides by genetic approaches. In addition, mechanisms of the host immune responses against pathogenic bacterial SGs have partially been resolved. This review is focused on the biological functions of SGs in mammals taking into consideration their therapeutic applications in the future.

Animal Models of Cryptococcus neoformans in Identifying Immune Parameters Associated With Primary Infection and Reactivation of Latent Infection

Cryptococcus species are environmental fungal pathogens and the causative agents of cryptococcosis. Infection occurs upon inhalation of infectious particles, which proliferate in the lung causing a primary infection. From this primary lung infection, fungal cells can eventually disseminate to other organs, particularly the brain, causing lethal meningoencephalitis. However, in most cases, the primary infection resolves with the formation of a lung granuloma. Upon severe immunodeficiency, dormant cryptococcal cells will start proliferating in the lung granuloma and eventually will disseminate to the brain. Many investigators have sought to study the protective host immune response to this pathogen in search of host parameters that keep the proliferation of cryptococcal cells under control. The majority of the work assimilates research carried out using the primary infection animal model, mainly because a reactivation model has been available only very recently. This review will focus on anti-cryptococcal immunity in both the primary and reactivation models. An understanding of the differences in host immunity between the primary and reactivation models will help to define the key host parameters that control the infections and are important for the research and development of new therapeutic and vaccine strategies against cryptococcosis.