The protein kinase Ire1 impacts pathogenicity of Candida albicans by regulating homeostatic adaptation to endoplasmic reticulum stress.

Shabnam Sircaik,Jesús Pla,Elvira Román,David R Andes,Priyanka Bapat,Neil A R Gow,Keunsook K Lee,Clarissa J Nobile,Sneh Lata Panwar

doi:10.1111/cmi.13307

Abstract

The unfolded protein response (UPR), crucial for the maintenance of endoplasmic reticulum (ER) homeostasis, is tied to the regulation of multiple cellular processes in pathogenic fungi. Here, we show that Candida albicans relies on an ER‐resident protein, inositol‐requiring enzyme 1 (Ire1) for sensing ER stress and activating the UPR. Compromised Ire1 function impacts cellular processes that are dependent on functional secretory homeostasis, as inferred from transcriptional profiling. Concordantly, an Ire1‐mutant strain exhibits pleiotropic roles in ER stress response, antifungal tolerance, cell wall regulation and virulence‐related traits. Hac1 is the downstream target of C. albicans Ire1 as it initiates the unconventional splicing of the 19 bp intron from HAC1 mRNA during tunicamycin‐induced ER stress. Ire1 also activates the UPR in response to perturbations in cell wall integrity and cell membrane homeostasis in a manner that does not necessitate the splicing of HAC1 mRNA. Furthermore, the Ire1‐mutant strain is severely defective in hyphal morphogenesis and biofilm formation as well as in establishing a successful infection in vivo. Together, these findings demonstrate that C. albicans Ire1 functions to regulate traits that are essential for virulence and suggest its importance in responding to multiple stresses, thus integrating various stress signals to maintain ER homeostasis.

Highlights

Eukaryotic cells have evolved sophisticated mechanisms to ensure proper folding of proteins, especially the secretory and transmembrane proteins, through processes that largely occur in the endoplasmic reticulum (ER)
Several of the proteins encoded by the top 50 upregulated genes are those that respond to oxidative, pH or nutrition stress (SOD3, RBR1, AOX2 and SOD5; Figure 2a). These findings indicate that the upregulation of stress responsive genes occurs in order to compensate for ER stress generated by the compromised expression of inositol-requiring enzyme 1 (Ire1) in C. albicans
We demonstrate the requirement of C. albicans Ire1 in circumventing ER stress induced by multiple stressors and its influence on several important virulence traits (Figure 6)

Summary

| INTRODUCTION

Eukaryotic cells have evolved sophisticated mechanisms to ensure proper folding of proteins, especially the secretory and transmembrane proteins, through processes that largely occur in the endoplasmic reticulum (ER). Despite the conservation of the Ire1-Hac1-mediated UPR pathway in most eukaryotic species, Candida glabrata Ire functions independent of Hac to counter ER stress (Miyazaki, Nakayama, Nagayoshi, Kakeya, & Kohno, 2013). This indicates that Ire1-dependent stress responsive pathways have diversified significantly in C. glabrata, compared to other fungi. During ER stress, the C. albicans HAC1 mRNA undergoes an unconventional splicing event to remove a 19 bp intron, similar to Hac homologues in other fungi This splicing event generates a functional Hac that activates UPR target genes to mount an ER stress response in C. albicans (Wimalasena et al, 2008). We show that C. albicans relies on Ire1-mediated stress responses for its pathogenesis, expanding the link between Ire1-dependent stress responsive pathways and fungal pathogenesis to C. albicans

| RESULTS

| DISCUSSION

Findings

| EXPERIMENTAL PROCEDURES