ORAI1α and ORAI1β have differential sensitivity to extracellular pH

Reduction in extracellular pH is common in pathophysiological conditions including neurological disorders, inflammation, and cancer. Like extracellular acidification, dysregulated calcium homeostasis is linked to various pathophysiological states including cancer. Changes in pH and calcium can theoretically be linked since the function of several calcium transporters is affected by pH changes. One such calcium channel is ORAI1, where changes in extracellular pH modulate gating.

Upon depletion of the endoplasmic reticulum (ER) calcium store, coupling of stromal interacting molecule 1 (STIM1) with ORAI1 facilitates calcium influx and refills the ER calcium store, a process known as Store-Operated Calcium Entry (SOCE). ORAI1 expression has been reported to be increased in several cancer types including basal breast cancer. The ORAI1 protein is expressed as two variants, ORAI1α (long) and ORAI1β (short). The differential roles of ORAI1α and ORAI1β are still not fully understood. Previous studies have shown that ORAI1 gating properties are affected by both intracellular and extracellular pH changes, but none have investigated whether ORAI1α and ORAI1β variants differ in sensitivity to pH changes.

In this study, we used cell-based calcium flux assays to compare ORAI1α- and ORAI1β-mediated calcium influx at different extracellular pHs. We developed a mathematical model of intracellular calcium dynamics incorporating SOCE to quantify variations in calcium influx. Fitting this model to our data revealed that at pH 6.8, ORAI1α is associated with more sustained calcium influx compared to ORAI1β. Point mutations in the N-terminus of ORAI1α identified potential domains responsible for the differential activation of ORAI1 variants at pH 6.8.