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Investigating the mechanism by which the natural product compound puupehenone potentiates the antifungal drug caspofungin

ABSTRACT: The cell wall-targeting echinocandin antifungals, although potent and well-tolerated, are inadequate in treating fungal infections due to their narrow spectrum of activity and their propensity to induce pathogen resistance. A promising strategy to overcome these drawbacks is to combine echinocandins with a molecule that improves their activity and also disrupts drug adaptation pathways. In this study, we show that puupehenone (PUUP), a marine sponge-derived sesquiterpene quinone potentiates the echinocandin drug, caspofungin (CAS) in CAS-resistant fungal pathogens. We have conducted RNA-Seq analysis, followed by genetic and molecular studies, to elucidate PUUP’s CAS-potentiating mechanism. We found that the combination of CAS and PUUP blocked the induction of CAS-responding genes required for the adaptation to cell wall stress through the cell wall integrity (CWI) pathway. Further analysis showed that PUUP inhibited the activation of Slt2 (Mpk1), the terminal MAP kinase in this pathway. We also found that PUUP induced heat shock response genes and inhibited the activity of heat shock protein 90 (Hsp90). Molecular docking studies predicted that PUUP occupies a binding site on Hsp90 required for the interaction between Hsp90 and its co-chaperone Cdc37. Thus, we show that PUUP potentiates CAS activity by a previously undescribed mechanism which involves disruption of Hsp90 activity and the CWI pathway. Given the requirement of the Hsp90-Cdc37 complex in Slt2 activation, we suggest that inhibitors of this complex would disrupt the CWI pathway and synergize with echinocandins. Therefore, the identification of PUUP’s CAS-potentiating mechanism has important implications in the development of new antifungal combination therapies.

ORGANISM(S): Saccharomyces cerevisiae

PROVIDER: GSE140563 | GEO | 2019/12/04

REPOSITORIES: GEO

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Project description:Aspergillus species are a leading cause of invasive fungal infections. As resistance to first-line therapy involving triazole antifungal agents is rising, second-line therapy with echinocandins is expanding. Resistance to echinocandins is well-established to result from amino acid substitutions in the echinocandin drug target β-(1,3)-D-glucan synthase encoded by the fks1 gene. Recently, we identified several high MIC clinical isolates of A. fumigatus from patients failing echinocandin therapy that did not contain any mutation in the fks1 gene, indicating that echinocandin resistance in these isolates results from an undefined mechanism. To explore possible new mechanisms of resistance, we used a lab-derived strain, RG101, with a nearly identical susceptibility phenotype, as a model system. This strain does not contain fks1 mutations but showed prominent resistance to the echinocandin class drug caspofungin (CAS), while remaining sensitive to other echinocandins, azoles and polyenes. Glucan synthase isolated from RG101 was fully sensitive to drug. Yet, exposure to CAS during its growth yielded a modified enzyme that was insensitive (4-log orders) in kinetic inhibition assays to CAS, as well as other echinocandins. This induction of cross-resistance by CAS was also observed in clinical isolates. To determine the nature of a presumptive posttranslational modification (PTM) of the enzyme, we analyzed whole enzyme PTMs, including the known hot-spot regions, for methylation, acetylation and phosphorylation. While we did not identify any PTMs linked to resistance, analysis of the lipid microenvironment of CAS-induced resistant enzyme revealed a prominent increase in the abundance of dihydrosphingosine (DhSph) and phytosphingosine (PhSph). Exogenous addition of DhSph and PhSph to sensitive enzyme in in vitro kinetic inhibition assays recapitulated the CAS insensitivity of the cellular-derived enzyme. To further examine induction of drug-induced resistance, we used an in vitro assay to demonstrate that CAS, but not other echinocandin class drugs, prominently induced the production of mitochondrial-derived Reactive Oxygen Species (ROS) in A. fumigatus. RNASeq evaluation of whole cells confirmed a ROS signature in cells treated with CAS. Dampening the formation of ROS by antimycin A or thiourea eliminated the induction of drug resistance by CAS. We conclude that CAS-induced formation of ROS promotes a cellular stress response that alters the composition of plasma membrane lipids surrounding glucan synthase, changing its enzymatic properties to make it insensitive to echinocandins. This stress-induced response constitutes a novel mechanism of echinocandin resistance in Aspergillus, with implications for drug resistance and/or tolerance mechanisms in other fungal pathogens.

Project description:The Neurospora crassa GUL-1 is part of the COT-1 pathway, which plays key roles in regulating polar hyphal growth and cell wall remodeling. We show that GUL-1 is a bona fide mRNA binding protein (RBP) that can associate with 828 “core” mRNA species. When cell wall integrity (CWI) is challenged, the repertoire of associated RNA species increases up to 2628 mRNAs (including its own transcript). GUL-1 can bind mRNAs of genes related to translation, cell wall remodeling, circadian clock, endoplasmatic reticulum (ER), as well as CWI and osmoregulatory response MAPK pathway components. GUL-1 interacts with over 100 different proteins, including stress granule proteins, components of the translational and cell wall remodeling machinery, as well as ER components and those of the MAPK, COT-1 and STRIPAK complexes. Several additional RBPs were also shown to physically interact with GUL-1. We demonstrate that under stress conditions GUL-1 can localize to the ER and that GUL-1 affects the CWI pathway as is evident via altered phosphorylation levels of MAK-1, interaction with mak-1 transcript and involvement in the expression level of the transcription factor adv-1. Taken together, we suggest that the RBP GUL-1 functions in multiple cellular processes, including the regulation of cell wall remodeling. The latter is mechanistically concerted with the MAK-1 pathway in a stress-related manner. Methods: In this study we show that GUL-1 is a bona fide mRNA binding protein (RBP) that associates with different mRNAs, including its own transcript, as determined by RNA antisense purification and RNA immunoprecipitation experiments. Results: We demonstrate that GUL-1 may physically interact with multiple proteins included translation processes, cell wall remodeling and cell wall integrity (CWI). We show that GUL-1 affects the CWI pathway as is evident via altered phosphorylation levels of MAK-1, interaction with mak-1 transcript and involvement in the expression level of the transcription factor adv-1. Conclusions:We suggest that the RBP GUL-1 functions in the regulation of cell wall remodeling in concert with the MAK-1 pathway in a stress-related manner.

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Investigating the mechanism by which the natural product compound puupehenone potentiates the antifungal drug caspofungin

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