Cloning of the alpha 1 subunit of a voltage-dependent calcium channel expressed in pancreatic beta cells.

Abstract

The isoforms of the a, subunits of voltagedependent Ca2+ channels expressed in human pancreatic islets were identified by using a pair of degenerate oligonucleotide primers and the polymerase chain reaction (PCR) to amplify mRNAs encoding a, subunit-like sequences. The sequences of the PCR products indicate that islets express the heart-type a, subunit as well as a second isoform whose complete sequence has not been previously reported. The sequences of cloned cDNAs encoding the human fl-cell, or neuroendocrine-type, a, subunit indicate that it is composed of 2181 amino acids. It shares 68%, 64%, and 41% identity with the sequences of the a, subunits of rabbit heart, skeletal muscle, and brain, respectively, and is predicted to have a similar structure including four homologous domains composed of six membranespanning segments each. RNA blotting studies indicate that the f-cell-type a, subunit is also expressed in brain as well as in the insulin-producing cell lines RINm5F and flTC-3; however, it could not be detected by RNA blotting in a third cell line, HIT-T15. In situ hybridization studies revealed expression of fl-cell-type a, subunit mRNA in fl cells of rat pancreatic islets, implying that this protein may play a role in the regulation of insulin secretion.

Tissue-specific expression of calcium channels

“…cDNA of the third gene (CaCh3) was isolated from neural and endocrine tissues and represents a neuroendocrinespecific L-type calcium channel (Williams et al 1992a, Seino et al 1992, whereas the gene products of the fourth and fifth genes (CaCh4 and CaCh5) have been found exclusively in neural tissues. Calcium channels transiently expressed from cRNA of CaCh4 induce high-voltage-activated calcium currents that are insensitive to nifedipine and o-conotoxin but are inhibited by a mixture of toxins from the funnel web spider that characterize this channel as a P-type calcium channel (Mori et al 1991).…”

Functional Interaction of Auxiliary Subunits and Synaptic Proteins With CaV1.3 May Impart Hair Cell Ca2+Current Properties

“…In contrast to other Ca 2ϩ channels, wherein the heterologous expression of the pore-forming ␣ subunit closely mirrors the native channel current, the biophysical properties of the expressed Ca V 1.3 are quite distinct from typical hair cell Ca 2ϩ currents (Bell et al 2001;Koschak et al 2001;Mikami et al 1989;Seino et al 1992;Williams et al 1992;Xu and Lipscombe 2001). Although it has been suggested that splice variants of Ca V 1.3␣ 1 channel in the chick basilar papilla may suffice to confer the unique properties of the DHP-sensitive Ca 2ϩ currents in hair cells (Kollmar et al 1997b), in other systems, there is ample evidence to suggest that co-assembly of auxiliary ␤ and ␣ 2 ␦ subunits with the ␣ 1 subunit may contribute to the properties of the native Ca 2ϩ current.…”

Voltage-dependent K + channels in pancreatic beta cells: Role, regulation and potential as therapeutic targets

“…Transcripts encoding L-type Ca 2+ channels (Fig. 2) thought to be largely responsible for Ca 2+ influx in beta cells were identified in the early 1990's [30,31,32], however there is still some controversy regarding the role of other Ca 2+ channels as N-type, P/Q-type and T-type channels have also been detected in insulin-secreting cells [33]. Clinically, some cases of PHHI, particularly those resulting from K ATP channel defects which are therefore not responsive to diazoxide, could be treated with VDCC antagonists [34,35,36].…”