Chaperone Networks in Fungal Pathogens of Humans

Abstract

The heat shock proteins (HSPs) function as chaperones to facilitate proper folding and modification of proteins and are of particular importance when organisms are subjected to unfavourable conditions. The human fungal pathogens are subjected to such conditions within the context of infection as they are exposed to human body temperature as well as the host immune response. Herein, the roles of the major classes of HSPs are briefly reviewed and their known contributions in human fungal pathogens are described with a focus on Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus. The Hsp90s and Hsp70s in human fungal pathogens broadly contribute to thermotolerance, morphological changes required for virulence, and tolerance to antifungal drugs. There are also examples of J domain co-chaperones and small HSPs influencing the elaboration of virulence factors in human fungal pathogens. However, there are diverse members in these groups of chaperones and there is still much to be uncovered about their contributions to pathogenesis. These HSPs do not act in isolation, but rather they form a network with one another. Interactions between chaperones define their specific roles and enhance their protein folding capabilities. Recent efforts to characterize these HSP networks in human fungal pathogens have revealed that there are unique interactions relevant to these pathogens, particularly under stress conditions. The chaperone networks in the fungal pathogens are also emerging as key coordinators of pathogenesis and antifungal drug tolerance, suggesting that their disruption is a promising strategy for the development of antifungal therapy.

Keywords: antifungal drug tolerance; chaperones; fungal pathogens; heat shock proteins; thermotolerance.

Figure 1

The major roles of heat shock proteins (HSPs) in fungal pathogens. The HSPs facilitate the acquisition of thermotolerance and allow human fungal pathogens to grow at human body temperature as well as to survive after heat shock at elevated temperatures. Several of these chaperones are required for morphological changes including the yeast to hyphal transition and conidiation. The HSPs, and in particular the Hsp90s, also facilitate antifungal drug tolerance in the major fungal pathogens suggesting that inhibitors would potentiate the activities of existing antifungals. Finally, in mammalian and insect models of infection when HSPs were pharmacologically inhibited or upon inoculation with deletion mutants lacking a gene for an HSP, they were often attenuated for virulence (indicated by the blue line in the hypothetical survival curve) compared to wild type or untreated strains (red line).

Figure 2

Schematic diagrams of the major classes of chaperones. (A) An Hsp90 dimer schematic indicating the ATP binding N-terminal domain (NTD) and the C-terminal domain (CTD) which is required for homodimerization. (B) An Hsp70 schematic indicating the nucleotide binding domain (NBD) and the substrate binding domain (SBD) including the lid of the SBD in an open conformation. (C) A schematic of an Hsp100/ClpB chaperone showing the stacked hexameric rings through which peptide strands are pulled to unfold them. (D) A schematic of a CCT/TRiC complex showing the stacked oligomeric rings which form a protected environment for protein folding in the lumen when ATP is bound and the chaperonin adopts a closed conformation.

Figure 3

Network analyses of known and predicted interactions between chaperones in (A) S. cerevisiae, (B) C. albicans, (C) C. neoformans, and (D) A. fumigatus. STRING network analyses (https://string-db.org/, accessed on 5 January 2021) displaying the interactions between members of the Hsp100, Hsp90, Hsp70, JDP, sHsp, and chaperonin (Hsp60 and T-complex) families in S. cerevisiae and three of the major human fungal pathogens. Only experimentally determined interactions are shown including those inferred from interactions between putative homologs in other species. There are few known interactions with Hsp100s, JDPs, and sHsps even in S. cerevisiae. Furthermore, there are fewer characterized interactions within the chaperone networks of the human fungal pathogens compared to S. cerevisiae. Gene names are given where known and gene identification numbers from FungiDB (https://fungidb.org/fungidb/app, accessed on 5 January 2021) are provided otherwise (from Candida albicans SC5314, Cryptococcus neoformans var. neoformans JEC21, and Aspergillus fumigatus Af293).