CRAC channel activity in C. elegans is mediated by Orai1 and STIM1 homologues and is essential for ovulation and fertility

Abstract

The Ca²⁺ release-activated Ca²⁺ (CRAC) channel is a plasma membrane Ca²⁺ entry pathway activated by endoplasmic reticulum (ER) Ca²⁺ store depletion. STIM1 proteins function as ER Ca²⁺ sensors and regulate CRAC channel activation. Recent studies have demonstrated that CRAC channels are encoded by the human Orai1 gene and a homologous Drosophila gene. C. elegans intestinal cells express a store-operated Ca²⁺ channel (SOCC) regulated by STIM-1. We cloned a full-length C. elegans cDNA that encodes a 293 amino acid protein, ORAI-1, homologous to human and Drosophila Orai1 proteins. ORAI-1 GFP reporters are co-expressed with STIM-1 in the gonad and intestine. Inositol 1,4,5-trisphosphate (IP₃)-dependent Ca²⁺ signalling regulates C. elegans gonad function, fertility and rhythmic posterior body wall muscle contraction (pBoc) required for defecation. RNA interference (RNAi) silencing of orai-1 expression phenocopies stim-1 knockdown and causes sterility and prevents intestinal cell SOCC activation, but has no effect on pBoc or intestinal Ca²⁺ signalling. Orai-1 RNAi suppresses pBoc defects induced by intestinal expression of a STIM-1 Ca²⁺-binding mutant, indicating that the proteins function in a common pathway. Co-expression of stim-1 and orai-1 cDNAs in HEK293 cells induces large inwardly rectifying cation currents activated by ER Ca²⁺ depletion. The properties of this current recapitulate those of the native SOCC current. We conclude that C. elegans expresses bona fide CRAC channels that require the function of Orai1- and STIM1-related proteins. CRAC channels thus arose very early in animal evolution. In C. elegans, CRAC channels do not play obligate roles in all IP₃-dependent signalling processes and ER Ca²⁺ homeostasis. Instead, we suggest that CRAC channels carry out highly specialized and cell-specific signalling roles and that they may function as a failsafe mechanism to prevent Ca²⁺ store depletion under pathophysiological and stress conditions.