There are three isoforms of the inositol 1,4,5-trisphosphate receptor (InsP 3 R), each of which has a distinct effect on Ca 2؉ signaling. However, it is not known whether each isoform similarly plays a distinct role in the activation of Ca 2؉ -mediated events. To investigate this question, we examined the effects of each InsP 3 R isoform on transmission of Ca 2؉ 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 InsP 3 R isoforms. ATP-induced cytosolic Ca 2؉ signaling patterns were altered, regardless of which isoform was silenced, but in a different fashion depending on the isoform. ATP also induced Ca 2؉ signals in mitochondria, which were inhibited more effectively by silencing the type III InsP 3 R 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 InsP 3 R. These findings provide evidence that the type III isoform of the InsP 3 R plays a special role in induction of apoptosis by preferentially transmitting Ca 2؉ signals into mitochondria.Cytosolic Ca 2ϩ (Ca i 2ϩ ) 3 is a versatile second messenger that can simultaneously regulate multiple processes within an individual cell (1). This complex regulatory action of Ca 2ϩ is thought to result in part from the varied spatial and temporal patterns of Ca 2ϩ signals that can occur (1). These signaling patterns in turn are thought to result from special properties of the inositol-1,4,5-trisphosphate (InsP 3 ) receptor (InsP 3 R), which is the principal intracellular Ca 2ϩ release channel in most types of cells (2). The InsP 3 R is gated by InsP 3 and is localized to the endoplasmic reticulum (3) and to a lesser extent the nucleus (4, 5). There are three isoforms of the InsP 3 R, each of which has a different affinity for InsP 3 (6) and distinct functional properties at the single channel level (7,8). Each InsP 3 R isoform also has distinct effects on Ca 2ϩ signaling patterns in intact cells (9, 10). Although some cells and tissues express a single or predominant isoform of the receptor, most cells instead express two or all three InsP 3 R isoforms (9, 11-13). The presence of multiple isoforms within an individual cell suggests that Ca 2ϩ released from each isoform might mediate distinct cellular events. Apoptosis is one Ca 2ϩ -mediated event that may be influenced differently by each InsP 3 R isoform. Morphologically, apoptosis is characterized by membrane blebbing, chromatin condensation, DNA fragmentation, and eventually the formation of apoptotic bodies, which are phagocytosed by neighboring cells (14). Studies at the molecular level suggest that the InsP 3 R plays an important role in the development of apoptosis (15-17). Initial evidence suggested that Ca 2ϩ -dependent apoptotic death was ...

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