All three IP3 receptor isoforms generate Ca2+ puffs that display similar characteristics

Inositol 1,4,5-trisphosphate (IP3) evokes Ca2+ release through IP3 receptors (IP(3)Rs) to generate both local Ca2+ puffs arising from concerted openings of clustered IP(3)Rs and cell-wide Ca2+ waves. Imaging Ca2+ puffs with single-channel resolution yields information on the localization and properties of native IP(3)Rs in intact cells, but interpretation has been complicated because cells express varying proportions of three structurally and functionally distinct isoforms of IP(3)Rs. Here, we used TIRF and light-sheet microscopy to image Ca2+ puffs in HEK-293 cell lines generated by CRISPR-Cas9 technology to express exclusively IP3R type 1, 2, or 3. Photorelease of the IP3 analog i-IP3 in all three cell lines evoked puffs with largely similar mean amplitudes, temporal characteristics, and spatial extents. Moreover, the single-channel Ca2+ flux was similar among isoforms, indicating that clusters of different IP3R isoforms contain comparable numbers of active channels. Our results show that all three IP3R isoforms cluster to generate local Ca2+ puffs and, contrary to findings of divergent properties from in vitro electrophysiological studies, display similar conductances and gating kinetics in intact cells.