Dual regulation of IP3 receptors by IP3 and PIP2 controls the transition from local to global Ca2+ signals

Abstract

The spatial organization of inositol 1,4,5-trisphosphate (IP₃)-evoked Ca²⁺ signals underlies their versatility. Low stimulus intensities evoke Ca²⁺ puffs, localized Ca²⁺ signals arising from a few IP₃ receptors (IP₃Rs) within a cluster tethered beneath the plasma membrane. More intense stimulation evokes global Ca²⁺ signals. Ca²⁺ signals propagate regeneratively as the Ca²⁺ released stimulates more IP₃Rs. How is this potentially explosive mechanism constrained to allow local Ca²⁺ signaling? We developed methods that allow IP₃ produced after G-protein coupled receptor (GPCR) activation to be intercepted and replaced by flash photolysis of a caged analog of IP₃. We find that phosphatidylinositol 4,5-bisphosphate (PIP₂) primes IP₃Rs to respond by partially occupying their IP₃-binding sites. As GPCRs stimulate IP₃ formation, they also deplete PIP₂, relieving the priming stimulus. Loss of PIP₂ resets IP₃R sensitivity and delays the transition from local to global Ca²⁺ signals. Dual regulation of IP₃Rs by PIP₂ and IP₃ through GPCRs controls the transition from local to global Ca²⁺ signals.