Crosstalk between the calcineurin and cell wall integrity pathways prevents chitin overexpression in Candida albicans.

Alessandra Da Silva Dantas,Louise A Walker,Megan D Lenardon,Filomena Nogueira,Neil A R Gow,Matt Edmondson,Alexandra C Brand,Keunsook K Lee

doi:10.1242/jcs.258889

Alessandra Da Silva Dantas, Louise A Walker + Show 6 more

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https://doi.org/10.1242/jcs.258889

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Abstract

ABSTRACTEchinocandins such as caspofungin are frontline antifungal drugs that compromise β-1,3 glucan synthesis in the cell wall. Recent reports have shown that fungal cells can resist killing by caspofungin by upregulation of chitin synthesis, thereby sustaining cell wall integrity (CWI). When echinocandins are removed, the chitin content of cells quickly returns to basal levels, suggesting that there is a fitness cost associated with having elevated levels of chitin in the cell wall. We show here that simultaneous activation of the calcineurin and CWI pathways generates a subpopulation of Candida albicans yeast cells that have supra-normal chitin levels interspersed throughout the inner and outer cell wall, and that these cells are non-viable, perhaps due to loss of wall elasticity required for cell expansion and growth. Mutations in the Ca2+-calcineurin pathway prevented the formation of these non-viable supra-high chitin cells by negatively regulating chitin synthesis driven by the CWI pathway. The Ca2+-calcineurin pathway may therefore act as an attenuator that prevents the overproduction of chitin by coordinating both chitin upregulation and negative regulation of the CWI signaling pathway. This article has an associated First Person interview with the first author of the paper.

Highlights

Echinocandins are a major class of frontline antifungal drugs that result in damage to the essential cell wall polysaccharide β-1,3 glucan, thereby exhibiting fungicidal effects against Candida species, Aspergillus fumigatus and other fungal pathogens (Walker et al, 2010; Mroczyńska and Brillowska-Dąbrowska, 2020)
We observed that treatment of C. albicans with CaCl2 and Calcofluor White (CFW; referred to hereafter as CaCl2+CFW) or with caspofungin induced substantial increases in chitin content relative to untreated controls, as assessed visually by the intensity of CFW fluorescence (Fig. 1A; 0 h versus 1-12 h time points)
While high-chitin mother cells remained growth arrested or died in the presence of cell wall stressors, daughter cells were able to replicate and filament when the stressor was removed (Fig. S2, Movies 1 and 2). These observations suggest that CaCl2+CFW treatment increased chitin to levels that were tolerable and did not significantly affect viability, whereas the very high chitin levels seen in caspofungin-treated cells were harmful, causing growth arrest, loss of viability and cell death

Summary

Introduction

Echinocandins are a major class of frontline antifungal drugs that result in damage to the essential cell wall polysaccharide β-1,3 glucan, thereby exhibiting fungicidal effects against Candida species, Aspergillus fumigatus and other fungal pathogens (Walker et al, 2010; Mroczyńska and Brillowska-Dąbrowska, 2020). The structure of chitin makes it one of the strongest biomaterials in nature It is a homopolymer of β-1,4 N-acetyl-glucosamine that exists as non-elastic, antiparallel-folded, linear hydrogen-bonded chains that form robust microfibrils in the cell wall (Gow and Gooday, 1983; Gow et al, 1987; Lenardon et al, 2010). Fungi can strengthen their cell wall when β-1,3 glucan is damaged by elevating the chitin content to levels that compensate for glucan damage and maintain cell viability (Lee et al, 2012, 2018; Yang et al, 2017; Ries et al, 2017). It is important to understand the mechanism by which echinocandin exposure leads to modifications of the cell wall

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