TRPs-NCX2 Axis Jointly Preserves Pancreatic Beta Cell Susceptibility/Sensitivity to Elevated Glucose.

Background: For appropriate glucose sensing, pancreatic beta-cells essentially establish a constant mitochondria-directed endoplasmic reticulum (ER) Ca2+ leak via phosphorylated presenilin-1 (PS1). In the current work, we investigated how cells maintain the ER Ca2+ content during this continuous Ca2+ leak. Methods: Ca2+ imaging for ER, cytosolic and subplasmalemmal Ca2+, high resolution respirometry, and ELISA for insulin secretion were used. Results: A novel ER Ca2+ refilling mechanism driven by reverse-mode NCX2, which is fueled by local Na+ influxes through TRPC3 and TRPV4 channels that are independent of STIM-ORAI1 was found in pancreatic rodent and human beta cell lines. Disrupting this TRPs-NCX2 axis reduces subplasmalemmal Ca2+ levels, abolishes glucose-induced cytosolic Ca2+ oscillations, suppresses glucose-triggered elevation in mitochondrial energetics, and impairs first-phase insulin secretion. Conclusions: We identified a novel, ORAI1-independent "TRPs-NCX2" relay that couples Na+ influx via TRPC3 and TRPV4 to NCX2 reverse mode to maintain ER Ca2+ stores during physiological basal ER Ca2+ leakage, which is fundamental to beta-cell responsiveness to elevated glucose.