Clustering of IP3 receptors by IP3 retunes their regulation by IP3 and Ca2+

摘要

The versatility of Ca²⁺ signals derives from their spatio-temporal organization^,. For Ca²⁺ signals initiated by inositol trisphosphate (IP3) this requires local interactions between IP3 receptors (IP3R)^, mediated by their rapid stimulation and slower inhibition by cytosolic Ca²⁺. This allows hierarchical recruitment of Ca²⁺ release events as the IP3 concentration increases. Single IP3R respond first, then clustered IP3R open together giving a local Ca²⁺ puff, and as puffs become more frequent they ignite regenerative Ca²⁺ waves^,-. We demonstrate, using nuclear patch-clamp recording, that IP3R are initially randomly distributed with an estimated separation of ~1 μm. Low concentrations of IP3 cause IP3R to aggregate rapidly and reversibly into small clusters of ~4 closely associated IP3R. At resting cytosolic [Ca²⁺], clustered IP3R open independently, but with lower open probability (Po), shorter open time, and lesser IP3 sensitivity than lone IP3R. Increasing cytosolic [Ca²⁺] reverses the inhibition caused by clustering, IP3R gating becomes coupled, and the duration of multiple openings is prolonged. Clustering both exposes IP3R to local Ca²⁺ rises and increases the effects of Ca²⁺. Dynamic regulation of clustering by IP₃ tunes IP₃R sensitivity to IP₃ and Ca²⁺, facilitating hierarchical recruitment of the elementary events that underlie all IP₃-evoked Ca²⁺ signals^,.