Dynamic clustering of IP₃ receptors by IP₃

The versatility of Ca²⁺ as an intracellular messenger stems largely from the impressive, but complex, spatiotemporal organization of the Ca²⁺ signals. For example, the latter when initiated by IP₃ (inositol 1,4,5-trisphosphate) in many cells manifest hierarchical recruitment of elementary Ca²⁺ release events (‘blips’ and then ‘puffs’) en route to global regenerative Ca²⁺ waves as the cellular IP₃ concentration rises. The spacing of IP₃Rs (IP₃ receptors) and their regulation by Ca²⁺ are key determinants of these spatially organized Ca²⁺ signals, but neither is adequately understood. IP₃Rs have been proposed to be pre-assembled into clusters, but their composition, geometry and whether clustering affects IP₃R behaviour are unknown. Using patch-clamp recording from the outer nuclear envelope of DT40 cells expressing rat IP₃R1 or IP₃R3, we have recently shown that low concentrations of IP₃ cause IP₃Rs to aggregate rapidly and reversibly into small clusters of approximately four IP₃Rs. At resting cytosolic Ca²⁺ concentrations, clustered IP₃Rs open independently, but with lower open probability, shorter open duration and lesser IP₃-sensitivity than lone IP₃Rs. This inhibitory influence of clustering on IP₃R is reversed when the [Ca²⁺]_i (cytosolic free Ca²⁺ concentration) increases. The gating of clustered IP₃Rs exposed to increased [Ca²⁺]_i is coupled: they are more likely to open and close together, and their simultaneous openings are prolonged. Dynamic clustering of IP₃Rs by IP₃ thus exposes them to local Ca²⁺ rises and increases their propensity for a CICR (Ca²⁺-induced Ca²⁺ rise), thereby facilitating hierarchical recruitment of the elementary events that underlie all IP₃-evoked Ca²⁺ signals.