Defects in inositol 1,4,5-trisphosphate receptor expression, Ca ²⁺ signaling, and insulin secretion in the anx 7(+/−) knockout mouse

Abstract

The mammalian anx7 gene codes for a Ca 2؉ -activated GTPase, which supports Ca 2؉ ͞GTP-dependent secretion events and Ca 2؉ channel activities in vitro and in vivo. To test whether anx7 might be involved in Ca 2؉ signaling in secreting pancreatic ␤ cells, we knocked out the anx7 gene in the mouse and tested the insulinsecretory properties of the ␤ cells. The nullizygous anx7 (؊͞؊) phenotype is lethal at embryonic day 10 because of cerebral hemorrhage. However, the heterozygous anx7 (؉͞؊) mouse, although expressing only low levels of ANX7 protein, is viable and fertile. The anx7 (؉͞؊) phenotype is associated with a substantial defect in insulin secretion, although the insulin content of the islets, is 8-to 10-fold higher in the mutants than in the normal littermate control. We infer from electrophysiological studies that both glucose-stimulated secretion and voltage-dependent Ca 2؉ channel functions are normal. However, electrooptical recordings indicate that the (؉͞؊) mutation has caused a change in the ability of inositol 1,4,5-trisphosphate (IP3)-generating agonists to release intracellular calcium. The principle molecular consequence of lower anx7 expression is a profound reduction in IP3 receptor expression and function in pancreatic islets. The profound increase in islets, ␤ cell number, and size may be a means of compensating for less efficient insulin secretion by individual defective pancreatic ␤ cells. This is a direct demonstration of a connection between glucoseactivated insulin secretion and Ca 2؉ signaling through IP3-sensitive Ca 2؉ stores.

Localization of Three Types of the Inositol 1,4,5-Trisphosphate Receptor/Ca2+ Channel in the Secretory Granules and Coupling with the Ca2+ Storage Proteins Chromogranins A and B

“…DISCUSSION Since the demonstration that the secretory granules of adrenal medullary chromaffin cells release Ca 2ϩ in response to IP 3 (9), the IP 3 -sensitive Ca 2ϩ store role of secretory granules has also been shown in other secretory cells (10,11). The presence of IP 3 R on the secretory granule membrane has been shown in adrenal chromaffin cells (16,20) and pancreatic ␤-cells (19). Further, the IP 3 R was demonstrated to co-localize with chromogranin A in the secretory granules of bovine adrenal medullary chromaffin cells (20).…”

“…Since then the existence of type 3 IP 3 R (IP 3 R-3) in the insulincontaining secretory granules of pancreatic ␤-cells had been reported (17) although this study was questioned later due to potential cross-interaction of the IP 3 R-3-specific antibody used with insulin crystals found in the secretory granules (18). Nevertheless, the existence of IP 3 R/Ca 2ϩ channels in the secretory granules of insulin-containing pancreatic ␤-cells (19) and adrenal medullary chromaffin cells (20) has been confirmed. However the identity of IP 3 R/Ca 2ϩ channel types present in the secretory granules is unknown.…”

ANX7, a candidate tumor suppressor gene for prostate cancer

“…1-6) is located on human chromosome 10q21, where potential tumor suppressor genes (TSGs) have been hypothesized to exist for prostate and other cancers (5,(7)(8)(9)(10)(11)(12)(13)(14)(15). However, the specific relevance of the ANX7 gene for cancer only became apparent after we created a knockout for this gene in the mouse (16). Although the homozygous Anx7(Ϫ͞Ϫ) deletion is embryonically lethal, the phenotype of the Anx7(ϩ͞Ϫ) heterozygote includes calcium signaling deficits and growth defects such as gigantism, and selective organomegaly.…”

Global Structural Changes in Annexin 12

“…Their high degree of conservation, the lethality of some ANX knockout mice (2), and the emerging class of ANX-related diseases (annexinopathies) indicate that ANXs play important roles in vivo (3). Many membrane-related functions have been proposed for the different ANXs, including roles in membrane trafficking and fusion, ion channel formation, and cell adhesion (1).…”