Region-specific proteolysis differentially modulates type 2 and type 3 inositol 1,4,5-trisphosphate receptor activity in models of acute pancreatitis

Liwei Wang,Larry E Wagner,Kamil J Alzayady,David I Yule

doi:10.1074/jbc.ra118.003421

Liwei Wang, Larry E Wagner + Show 2 more

Open Access

https://doi.org/10.1074/jbc.ra118.003421

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Abstract

Fine-tuning of the activity of inositol 1,4,5-trisphosphate receptors (IP3R) by a diverse array of regulatory inputs results in intracellular Ca2+ signals with distinct characteristics. These events allow the activation of specific downstream effectors. We reported previously that region-specific proteolysis represents a novel regulatory event for type 1 IP3R (R1). Specifically, caspase-fragmented R1 display a marked increase in single-channel open probability. More importantly, the distinct characteristics of the Ca2+ signals elicited via fragmented R1 can activate alternate downstream effectors. In this report, we expand these studies to investigate whether all IP3R subtypes are regulated by proteolysis. We now show that type 2 and type 3 IP3R (R2 and R3, respectively) are proteolytically cleaved in rodent models of acute pancreatitis. Surprisingly, fragmented IP3R retained tetrameric architecture, remained embedded in endoplasmic reticulum membranes and were not functionally disabled. Proteolysis was associated with a marked attenuation of the frequency of Ca2+ signals in pancreatic lobules. Consistent with these data, expression of DNAs encoding complementary R2 and R3 peptides mimicking fragmented receptors at particular sites, resulted in a significant decrease in the frequency of agonist-stimulated Ca2+ oscillations. Further, proteolysis of R2 resulted in a marked decrease in single-channel open probability. Taken together, proteolytic fragmentation modulates R2 and R3 activity in a region-specific manner, and this event may contribute to the altered Ca2+ signals in pancreatic acinar cells during acute pancreatitis.

Highlights

Fine-tuning of the activity of inositol 1,4,5-trisphosphate receptors (IP3R) by a diverse array of regulatory inputs results in intracellular Ca2؉ signals with distinct characteristics
This study represents a comprehensive investigation of a novel modification of R2 and R3 and its consequences in pancreatic acinar cells (PAC) in the context of two rodent models of acute pancreatitis
A previous study, in which PAC were isolated by enzymatic digestion following induction of experimental pancreatitis, reported a significant decrease in the ability of PAC from the experimental pancreatitis animals to display sustained oscillatory Ca2ϩ signals

Summary

Introduction

Fine-tuning of the activity of inositol 1,4,5-trisphosphate receptors (IP3R) by a diverse array of regulatory inputs results in intracellular Ca2؉ signals with distinct characteristics. These events allow the activation of specific downstream effectors. Proteolytic fragmentation modulates R2 and R3 activity in a region-specific manner, and this event may contribute to the altered Ca2؉ signals in pancreatic acinar cells during acute pancreatitis. Our laboratory has demonstrated that region-specific proteolytic fragmentation dramatically alters R1 activity and thereby allows the receptor to potentially activate alternative downstream effectors [12] Whether this is a regulatory event specific to R1 or a general form of regulation relevant to all isoforms of IP3R requires further investigation. Ca2ϩ transients and amylase secretion in response to secretagogue stimulation were reported to be completely abolished in pancreatic acinar cells isolated from R2 and R3 double knockouts [13]

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