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 IP(3)Rs (IP3 receptors) and their regulation by Ca2+ are key determinants of these spatially organized Ca2+ signals, but neither is adequately understood. IP(3)Rs 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 IP(3)R1 or IP(3)R3, we have recently shown that low concentrations of IP3 cause IP(3)Rs to aggregate rapidly and reversibly into small clusters of approximately four IP(3)Rs. At resting cytosolic Ca2+ concentrations, clustered IP(3)Rs open independently, but with lower open probability, shorter open duration and lesser IP3-sensitivity than lone IP(3)Rs. This inhibitory influence of clustering on IP3R is reversed when the [Ca2+](i) (cytosolic free Ca2+ concentration) increases. The gating of clustered IP(3)Rs 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 IP(3)Rs 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.