Molecular diversity and functional characterization of voltage-dependent calcium channels (CACN4) expressed in pancreatic beta-cells.

Abstract

Dihydropyridine-sensitive voltage-dependent calcium channels (VDCC) play a crucial role in insulin secretion. We recently have cloned a human alpha 1-subunit of the VDCC expressed in pancreatic beta-cells, designated CACN4. In this study we have isolated complementary DNAs encoding two forms of rat CACN4 (rCACN4A and rCACN4B) from a rat insulinoma RINm5F complementary DNA library. Rat CACN4A is a protein of 2203 amino acids and is the rat homolog of human CACN4, whereas rCACN4B lacks 535 amino acids in the carboxyl-terminal region, probably due to alternative splicing. We have found two additional variations, one in the intracellular loop between repeats I and II and the other in the extracellular region between the third and fourth segments of repeat IV. Reverse transcriptase-polymerase chain reaction analysis of rat pancreatic islet messenger RNA reveals that these variants are present in pancreatic islets. In addition, whole-cell voltage-clamp recordings of Chinese hamster ovary cells stably expressing the alpha 1-subunit (rCACN4A or rCACN4B) with or without the calcium channel beta 2-subunit show that coexpression of rCACN4A with the beta 2-subunit or rCACN4B with the beta 2-subunit elicits L-type VDCC currents, whereas expression of the alpha 1-subunit alone does not, indicating that CACN4 can associate functionally with the beta 2-subunit and that the beta-subunit is essential for functional expression of CACN4. These results suggest that there are various subtypes of CACN4 expressed in pancreatic beta-cells, and that both rCACN4A and rCACN4B can function as VDCC. Furthermore, the present study suggests that the expression of the beta-subunit as well as the alpha 1-subunit may participate in the regulation of insulin secretion.

Receptors couple to L‐type calcium channels via distinct Go proteins in rat neuroendocrine cell lines

“…Most probably, of the various L-type calcium channels present, the á1D isoforms were affected by galanin, since ù_CTX GVIA decreased the portion of currents inhibited by the hormone. In accordance with our results, á1C and á1D subunits have previously been detected in endocrine pancreas (Seino et al 1992;Pollo, Lovallo, Biancardi, Sher, Socci & Carbone, 1993;Ihara et al 1995) and have also been found in a hamster insulinoma (HIT) cell line (Boyd et al 1991;Yaney et al 1992). In RINm5F cells, galanin-induced inhibition of ICa was similar to that induced by many other hormones.…”

Suppression of Pdx-1 perturbs proinsulin processing, insulin secretion and GLP-1 signalling in INS-1 cells

“…VAMP-2, Rab3A, SNAP25A and synaptotagmin I are implicated in insulin exocytosis [44][45][46]. Voltage-dependent Ca 2+ channel Ca v 1.3 (α 1D ) subunit is involved in mediating beta cell insulin secretion in response to rising concentrations of glucose [47]. All these proteins are required for normal glucose-stimulated insulin secretion.…”

Fatty acid metabolism and insulin secretion in pancreatic beta cells

“…There is ample evidence that L-type VDCC play a pivotal role in beta-cell secretion, although the precise subunit composition of the channels involved and their metabolic modulation are not known [17,128]. This goal has been complicated by the fact that all types of VDCC, including the two L-type classes α1c and α1d and four different β subunits are expressed in pancreatic beta cells [46,47,129]. The regulated release of insulin from the beta cell seems to utilize a mechanism that is highly conserved across many cell types including neuroendocrine cells [130].…”