The Type 2 Inositol (1,4,5)-Trisphosphate (InsP3) Receptor Determines the Sensitivity of InsP3-induced Ca2+ Release to ATP in Pancreatic Acinar Cells

Abstract

Binding of secretagogues to G␣ q -coupled receptors on pancreatic acinar cells results in phospholipase C-induced production of inositol (1,4,5)-trisphosphate (InsP 3 ) 3 and a subsequent elevation of intracellular Ca 2ϩ (1). These intracellular Ca 2ϩ signals are tightly regulated both spatially and temporally and serve to initiate zymogen granule exocytosis and primary pancreatic fluid secretion (1-6). Acinar cell Ca 2ϩ signals result from the release of Ca 2ϩ from intracellular stores followed closely by Ca 2ϩ entry across the plasma membrane. Ca 2ϩ release is initiated by the binding of InsP 3 to InsP 3 receptors (InsP 3 R) localized primarily to the apical "trigger zone" of the acinar cell (7-9). Regulation of InsP 3 -induced Ca 2ϩ release (IICR) is thought to be central to defining the spatial and temporal aspects of the Ca 2ϩ signals in pancreatic acinar cells (10, 11).All three isoforms of InsP 3 R (InsP 3 R1, InsP 3 R2, and InsP 3 R3) are expressed in pancreatic acinar cells (7, 9, 12), although InsP 3 R2 and InsP 3 R3 constitute the predominant isoforms expressed (13). The importance of these particular isoforms to exocrine gland function was recently demonstrated using knock-out mice. Genetic ablation of both InsP 3 R2 and InsP 3 R3 resulted in severely attenuated exocrine gland function and death soon after weaning (12). The observed phenotypes were linked to a dramatic reduction in secretagogue-induced Ca 2ϩ signals in acinar cells of the pancreas and salivary glands (12). Given that these isoforms are central to stimulus-secretion coupling, modulation of Ca 2ϩ release from InsP 3 R2 and InsP 3 R3 by cellular factors likely impacts exocrine gland physiology and/or pathophysiology.InsP 3 R can be modulated by multiple mechanisms in addition to InsP 3 binding. Modulatory factors include cytosolic Ca 2ϩ levels, the phosphorylation state of the receptors, ATP binding, and protein-protein interactions (14 -17). Furthermore, because of primary sequence variability, the three InsP 3 R isoforms are differentially susceptible to many of these regulatory inputs. For example, InsP 3 R2 has a higher affinity for InsP 3 , and cells predominantly expressing this isoform exhibit a higher functional sensitivity for InsP 3 (18). InsP 3 R1 is more susceptible than the other isoforms to phosphorylation by protein kinase A (19). In addition, proteins such as ERp44 (20) and AKAP9 (21) preferentially bind to this isoform. The three isoforms are also differentially modulated by ATP binding, with InsP 3 R1 and InsP 3 R2 regulated by lower ATP concentrations than InsP 3 R3 (22-24). Additional functional heterogeneity is * This work was supported, in whole or in part, by National Institutes of Health Training Grant T32-DE07202 from NIDCR (to M. J. B.), National Institutes of Health Grants RO1-DK054568 and RO1-DE016999 (to D. I. Y.), and a National Institutes of Health grant (to J. C). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "ad...

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