The role of the Sigma1 receptor (s1R) in Store-operated calcium entry (SOCE) microdomains

The Sigma1 receptor (s1R) is an endoplasmic reticulum (ER) chaperone targeted to mitochondrion associated ER membranes (MAMs). The upregulation of s1R is associated with various cancers, whereas the decreased expression and mutant forms of s1R are associated with neurodegenerative disorders. The underlying cellular mechanism by which s1R exerts such a versatile impact remains elusive. At MAMs, s1R is reported to regulate inter-organelle Ca2+ levels. Following translocation to ER-plasma membrane junctions, s1R is reported to interact with various ion channels, including the key regulators of store-operated Ca2+ entry (SOCE). I have investigated the role of s1R in the regulation of SOCE within a population of cells and within the SOCE microdomains of single cells. In a population of cells, overexpression of s1R profoundly inhibited both Ca2+ store content and SOCE. Although these measurements of global [Ca2+]cyt provide essential information about the functional role of s1R in the regulation of SOCE, they do not provide detailed information about the effects of s1R on temporal and spatial aspects of SOCE within the microdomain of the ER-plasma membrane junction. Recently developed, the G-GECO1.2-Orai1 construct encodes for a fully functional Orai1 protein with a genetically encoded fluorescent Ca2+ reporter protein. The Orai1 channel is a key regulator of SOCE. GECO1.2-Orai1 allows for the monitoring of Orai1 channel activity in SOCE microdomains. Within the SOCE microdomains, overexpression of s1R reduced the frequency of highly-activated G-GECO1.2-Orai1 clusters, which resulted in cumulatively low SOCE. Overexpression of s1RE102Q, an ALS-causing mutant, failed to inhibit SOCE. In situ, s1R was shown to interact with another key regulator of SOCE, STIM1 and its less-studied homolog STIM2. Overexpression of s1R resulted in the inhibition of STIM2.2-mediated SOCE triggered by the ER Ca2+ store depletion. s1R failed to inhibit STIM2.2-mediated SOCE during basal conditions, suggesting that regulation of SOCE by s1R requires a greater drop in ER [Ca2+] than occurs under basal conditions