Dissection of Ire1 functions reveals stress response mechanisms uniquely evolved in Candida glabrata.

Taiga Miyazaki,Yohsuke Nagayoshi,Shigeru Kohno,Hironobu Nakayama,Hiroshi Kakeya,Scott G Filler

doi:10.1371/journal.ppat.1003160

Taiga Miyazaki, Yohsuke Nagayoshi + Show 4 more

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https://doi.org/10.1371/journal.ppat.1003160

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Abstract

Proper protein folding in the endoplasmic reticulum (ER) is vital in all eukaryotes. When misfolded proteins accumulate in the ER lumen, the transmembrane kinase/endoribonuclease Ire1 initiates splicing of HAC1 mRNA to generate the bZIP transcription factor Hac1, which subsequently activates its target genes to increase the protein-folding capacity of the ER. This cellular machinery, called the unfolded protein response (UPR), is believed to be an evolutionarily conserved mechanism in eukaryotes. In this study, we comprehensively characterized mutant phenotypes of IRE1 and other related genes in the human fungal pathogen Candida glabrata. Unexpectedly, Ire1 was required for the ER stress response independently of Hac1 in this fungus. C. glabrata Ire1 did not cleave mRNAs encoding Hac1 and other bZIP transcription factors identified in the C. glabrata genome. Microarray analysis revealed that the transcriptional response to ER stress is not mediated by Ire1, but instead is dependent largely on calcineurin signaling and partially on the Slt2 MAPK pathway. The loss of Ire1 alone did not confer increased antifungal susceptibility in C. glabrata contrary to UPR-defective mutants in other fungi. Taken together, our results suggest that the canonical Ire1-Hac1 UPR is not conserved in C. glabrata. It is known in metazoans that active Ire1 nonspecifically cleaves and degrades a subset of ER-localized mRNAs to reduce the ER load. Intriguingly, this cellular response could occur in an Ire1 nuclease-dependent fashion in C. glabrata. We also uncovered the attenuated virulence of the C. glabrata Δire1 mutant in a mouse model of disseminated candidiasis. This study has unveiled the unique evolution of ER stress response mechanisms in C. glabrata.

Highlights

In eukaryotic cells, the majority of secretory and transmembrane proteins are folded and modified in the lumen of the endoplasmic reticulum (ER)
It has been believed that the unfolded protein response (UPR) mediated by the Ire1-Hac1 linear pathway is evolutionarily conserved in most eukaryotic species, but surprisingly, we found that C. glabrata Ire1 plays a role in the ER stress response in a Hac1-independent manner, despite the presence of an apparent HAC1 ortholog
Other representative Candida species exhibited higher tolerance to these agents than S. cerevisiae and C. neoformans with intriguing exceptions: Candida krusei was highly susceptible to TM, but not to DTT, while Candida tropicalis was hypersusceptible to DTT, but not to TM

Summary

Introduction

The majority of secretory and transmembrane proteins are folded and modified in the lumen of the endoplasmic reticulum (ER). Contingent on proper folding, they are either transported to the Golgi apparatus or degraded [1] Impairment of these vital cellular machines can be caused by various factors, such as chemical compounds and mutations in genes involved in ER quality control, resulting in the accumulation of unfolded or misfolded proteins in the ER, collectively called ER stress [2,3]. In humans, ER stress is implicated in the pathology of various diseases including metabolic disease, inflammation, neurodegenerative disorders, and cancer [4,5] It has been revealed in several pathogenic fungi that ER quality control is important for antifungal resistance and virulence [6,7,8,9,10,11]

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