The Type III Inositol 1,4,5-Trisphosphate Receptor Preferentially Transmits Apoptotic Ca2+ Signals into Mitochondria

There are three isoforms of the inositol 1,4,5- trisphosphate receptor (InsP3R), each of which has a distinct effect on Ca2+ signaling. However, it is not known whether each isoform similarly plays a distinct role in the activation of Ca2+-mediated events. To investigate this question, we examined the effects of each InsP3R isoform on transmission of Ca2+ signals to mitochondria and induction of apoptosis. Each isoform was selectively silenced using isoform-specific small interfering RNA in Chinese hamster ovary cells, which express all three InsP3R isoforms. ATP-induced cytosolic Ca2+ signaling patterns were altered, regardless of which isoform was silenced, but in a different fashion depending on the isoform. ATP also induced Ca2+ signals in mitochondria, which were inhibited more effectively by silencing the type III InsP3R than by silencing either the type I or type II isoform. The type III isoform also co-localized most strongly with mitochondria. When apoptosis was induced by activation of either the extrinsic or intrinsic apoptotic pathway, induction was reduced most effectively by silencing the type III InsP3R. These findings provide evidence that the type III isoform of the InsP3R plays a special role in induction of apoptosis by preferentially transmitting Ca2+ signals into mitochondria.