Clustering of IP3 receptors by IP3 retunes their regulation by IP3 and Ca2+.Nature 458, 655–659. doi: 10.1038/nature07763

The versatility of Ca2+ signals derives from their spatio-temporal organization1,2. For Ca2+ signals initiated by inositol trisphosphate (IP3) this requires local interactions between IP3 receptors (IP3R)3,4 mediated by their rapid stimulation and slower inhibition4 by cytosolic Ca2+. This allows hierarchical recruitment of Ca2+ release events as the IP3 concentration increases5. Single IP3R respond first, then clustered IP3R open together giving a local Ca2+ puff, and as puffs become more frequent they ignite regenerative Ca2+ waves1,5-9. We demonstrate, using nuclear patch-clamp recording10, 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+ signals3,5. IP3-activated currents recorded from patches excised from the outer nuclear envelope of DT40 cells10 expressing rat IP3R3 are entirely due to IP3R3 (Fig. 1). With 10 μM IP3 in the pipette solution (PS) the single channel open probability (Po) was 0.44 ± 0.05 (n = 6) and the mean open time (τo) was 11.9 ± 1.6 ms. The distribution of closed times (τc) had two components (Fig. 1d). Recordings in the on-nucleus configuration confirmed these results (not shown). The results are consistent with the gating scheme shown in Figure 1d (se