Ca ²⁺ signals initiate at immobile IP ₃ receptors adjacent to ER-plasma membrane junctions

Abstract

IP ₃ receptors (IP ₃Rs) release Ca ²⁺ from the ER when they bind IP ₃ and Ca ²⁺. The spatial organization of IP ₃Rs determines both the propagation of Ca ²⁺ signals between IP ₃Rs and the selective regulation of cellular responses. Here we use gene editing to fluorescently tag endogenous IP ₃Rs, and super-resolution microscopy to determine the geography of IP ₃Rs and Ca ²⁺ signals within living cells. We show that native IP ₃Rs cluster within ER membranes. Most IP ₃R clusters are mobile, moved by diffusion and microtubule motors. Ca ²⁺ signals are generated by a small population of immobile IP ₃Rs. These IP ₃Rs are licensed to respond, but they do not readily mix with mobile IP ₃Rs. The licensed IP ₃Rs reside alongside ER-plasma membrane junctions where STIM1, which regulates store-operated Ca ²⁺ entry, accumulates after depletion of Ca ²⁺ stores. IP ₃Rs tethered close to ER-plasma membrane junctions are licensed to respond and optimally placed to be activated by endogenous IP ₃ and to regulate Ca ²⁺ entry.

Abstract

IP ₃ receptors mediate Ca ²⁺ release from the endoplasmic reticulum. Here the authors show that only a small fraction of IP ₃ receptors initiate Ca ²⁺ signals; these immobile IP ₃ receptors adjacent to the plasma membrane are optimally placed to control STIM1-dependent Ca ²⁺ entry.