Hsp90 interaction networks in fungi-tools and techniques

Aspergillus fumigatus escape mechanisms from its harsh survival environments

Aspergillus fumigatus is a ubiquitous pathogenic mold and causes several diseases, including mycotoxicosis, allergic reactions, and systemic diseases (invasive aspergillosis), with high mortality rates. In its ecological niche, the fungus has evolved and mastered many reply strategies to resist and survive against negative threats, including harsh environmental stress and deficiency of essential nutrients from natural environments, immunity responses and drug treatments in host, and competition from symbiotic microorganisms. Hence, treating A. fumigatus infection is a growing challenge. In this review, we summarized A. fumigatus reply strategies and escape mechanisms and clarified the main competitive or symbiotic relationships between A. fumigatus, viruses, bacteria, or fungi in host microecology. Additionally, we discussed the contemporary drug repertoire used to treat A. fumigatus and the latest evidence of potential resistance mechanisms. This review provides valuable knowledge which will stimulate further investigations and clinical applications for treating and preventing A. fumigatus infections. KEY POINTS: • Harsh living environment was a great challenge for A. fumigatus survival. • A. fumigatus has evolved multiple strategies to escape host immune responses. • A. fumigatus withstands antifungal drugs via intrinsic escape mechanisms.

Candida glabrata maintains two Hap1 homologs, Zcf27 and Zcf4, for distinct roles in ergosterol gene regulation to mediate sterol homeostasis under azole and hypoxic conditions

Candida glabrata exhibits innate resistance to azole antifungal drugs but also has the propensity to rapidly develop clinical drug resistance. Azole drugs, which target Erg11, is one of the three major classes of antifungals used to treat Candida infections. Despite their widespread use, the mechanism controlling azole-induced ERG gene expression and drug resistance in C. glabrata has primarily revolved around Upc2 and/or Pdr1. In this study, we determined the function of two zinc cluster transcription factors, Zcf27 and Zcf4, as direct but distinct regulators of ERG genes. Our phylogenetic analysis revealed C. glabrata Zcf27 and Zcf4 as the closest homologs to Saccharomyces cerevisiae Hap1. Hap1 is a known zinc cluster transcription factor in S. cerevisiae in controlling ERG gene expression under aerobic and hypoxic conditions. Interestingly, when we deleted HAP1 or ZCF27 in either S. cerevisiae or C. glabrata, respectively, both deletion strains showed altered susceptibility to azole drugs, whereas the strain deleted for ZCF4 did not exhibit azole susceptibility. We also determined that the increased azole susceptibility in a zcf27Δ strain is attributed to decreased azole-induced expression of ERG genes, resulting in decreased levels of total ergosterol. Surprisingly, Zcf4 protein expression is barely detected under aerobic conditions but is specifically induced under hypoxic conditions. However, under hypoxic conditions, Zcf4 but not Zcf27 was directly required for the repression of ERG genes. This study provides the first demonstration that Zcf27 and Zcf4 have evolved to serve distinct roles allowing C. glabrata to adapt to specific host and environmental conditions.

Strain variation in gene expression impact of hyphal cyclin Hgc1 in Candida albicans

Formation of hyphae is a key virulence trait of the fungal pathogen Candida albicans. Hypha morphogenesis depends upon the cyclin Hgc1, which acts together with cyclin-dependent protein kinase Cdc28 to phosphorylate effectors that drive polarized growth. Hgc1 has also been implicated in gene regulation through its effects on 2 transcription factors, Efg1 and Ume6. Here, we report RNA-sequencing (RNA-seq) analysis of 2 pairs of hgc1Δ/Δ mutants and their respective wild-type strains, which lie in 2 different genetic backgrounds. We find that hgc1Δ/Δ mutations alter expression of 271 genes in both genetic backgrounds and 266 of those genes respond consistently with regard to up- or down-regulation. Consistency is similar to what has been observed with efg1Δ/Δ mutations and greater than observed with nrg1Δ/Δ mutations in these 2 backgrounds. The gene expression response includes genes under Efg1 control, as expected from prior studies. Hgc1-responsive genes also include ergosterol biosynthetic genes and bud neck-related genes, which may reflect interactions between Hgc1 and additional transcription factors as well as effects of Hgc1 on cellular length-to-width ratios.

Heat shock protein 90 C-terminal inhibitor PNSA promotes anticancer immunology of CD8+ T cells

Penisuloxazin A (PNSA), a new compound from the fungus, is a novel C-terminal Hsp90 inhibitor reported by us before. It has been reported to possess antitumor activity and suppresses metastasis of breast cancer cells. However, the influence of PNSA on T cells is not fully understood. Here, we found that PNSA was much less toxic to lymphocytes than to tumor cells and it had no significant effect on populations of CD3+, CD4+ and CD8+ T lymphocytes. We discovered that PNSA directly enhanced the killing capacities of the CD8+ T and CD3+CD25- to CT26 cells, but not that of CD3+ cells due to the increase of Treg cells. What's more, PNSA pretreated tumor cells increase the sensitivity to CD8+ T cells mainly through the degradation of client protein of Hsp90 and declination of PD-L1 expression. Eventually, PNSA enhanced the killing ability of CD8+ and CD3+ T cells by simultaneously acting on lymphocytes and cancer cells. In vivo experiments, PNSA exhibited inhibition effects in the colon adenocarcinoma with increase of CD8 T cell infiltration in tumor tissues. All these results indicate that the novel Hsp90 C-terminal inhibitor-PNSA can promote lytic T cell immunological function to improve anticancer effect of PNSA, which provides a better foundation for anticancer drug development of PNSA in future.

Insights and Perspectives on the Role of Proteostasis and Heat Shock Proteins in Fungal Infections

Fungi are a diverse group of eukaryotic organisms that infect humans, animals, and plants. To successfully colonize their hosts, pathogenic fungi must continuously adapt to the host's unique environment, e.g., changes in temperature, pH, and nutrient availability. Appropriate protein folding, assembly, and degradation are essential for maintaining cellular homeostasis and survival under stressful conditions. Therefore, the regulation of proteostasis is crucial for fungal pathogenesis. The heat shock response (HSR) is one of the most important cellular mechanisms for maintaining proteostasis. It is activated by various stresses and regulates the activity of heat shock proteins (HSPs). As molecular chaperones, HSPs participate in the proteostatic network to control cellular protein levels by affecting their conformation, location, and degradation. In recent years, a growing body of evidence has highlighted the crucial yet understudied role of stress response circuits in fungal infections. This review explores the role of protein homeostasis and HSPs in fungal pathogenicity, including their contributions to virulence and host-pathogen interactions, as well as the concerted effects between HSPs and the main proteostasis circuits in the cell. Furthermore, we discuss perspectives in the field and the potential for targeting the components of these circuits to develop novel antifungal therapies.

Hsp90: From Cellular to Organismal Proteostasis

Assuring a healthy proteome is indispensable for survival and organismal health. Proteome disbalance and the loss of the proteostasis buffer are hallmarks of various diseases. The essential molecular chaperone Hsp90 is a regulator of the heat shock response via HSF1 and a stabilizer of a plethora of signaling proteins. In this review, we summarize the role of Hsp90 in the cellular and organismal regulation of proteome maintenance.

A Preliminary in vitro and in vivo Evaluation of the Effect and Action Mechanism of 17-AAG Combined With Azoles Against Azole-Resistant Candida spp

Invasive candidiasis is the primary reason for the increased cases of mortality in a medical environment. The resistance spectra of Candida species to antifungal drugs have gradually expanded. Particularly, the resistance spectra of Candida auris are the most prominent. Hsp90 plays a protective role in the stress response of fungi and facilitates their virulence. In contrast, Hsp90 inhibitors can improve the resistance of fungi to antifungal drugs by regulating the heat resistance of Hsp90, which destroys the integrity of the fungal cell walls. Hsp90 inhibitors thus offer a great potential to reduce or address fungal drug resistance. The drugs tested for the resistance include itraconazole, voriconazole, posaconazole, fluconazole, and 17-AAG. A total of 20 clinical strains of Candida were investigated. The broth microdilution checkerboard technique, as adapted from the CLSI M27-A4 method, was applied in this study. We found that 17-AAG alone exerted limited antifungal activity against all tested strains. The MIC range of 17-AAG was 8 to >32 μg/ml. A synergy was observed among 17-AAG and itraconazole, voriconazole, and posaconazole against 10 (50%), 7 (35%), and 13 (65%) of all isolates, respectively. Moreover, the synergy between 17-AAG and fluconazole was observed against 5 (50%) strains of azole-resistant Candida. However, no antagonism was recorded overall. Our result adequately verifies the influence of 17-AAG on the formation of Candida spp. biofilm. Moreover, we determined that with the use of rhodamine 6G to detect drug efflux and that of dihydrorhodamine-123 to detect intracellular reactive oxygen species (ROS), treatment with 17-AAG combined with azole drugs could inhibit the efflux pump of fungi and promote the accumulation of ROS in the fungal cells, thereby inducing fungal cell apoptosis. Thus, the mechanism of 17-AAG combined with azoles can kill fungi. Our results thus provide a new idea to further explore drugs against drug-resistant Candida spp.