Perspectives in Pharmacology Structure-Functional Diversity of Human L-Type Ca Channel: Perspectives for New Pharmacological Targets

The L-type Ca channels mediate depolarization-induced influx of Ca into a wide variety of cells and thus play a central role in triggering cardiac and smooth muscle contraction. Because of this role, clinically important classes of 1,4-dihydropyridine, phenylalkylamine, and benzothiazepine Ca channel blockers were developed as powerful medicines to treat hypertension and angina pectoris. Molecular cloning studies revealed that the channel is subject to extensive structure-functional variability due to alternative splicing. In this review, we will focus on a potentially important role of genetically driven variability of Ca channels in expression regulation and mutations, Ca -induced inactivation, and modulation of sensitivity to Ca channel blockers with the perspective for new pharmacological targets. The L-type Ca channel is a ubiquitously expressed voltage-gated ion channel supporting inward current of Ca ions that play a central role as an intracellular second messenger in many processes ranging from gene expression to cardiac and smooth muscle contraction. A number of L-type Ca channel blockers have been developed for the treatment of cardiovascular disorders. 1,4-Dihydropyridines, which have proven to be the most potent inhibitors of the channel, were used for its biochemical identification in rabbit skeletal muscle T-tubules as a protein complex composed of the poreforming 1S subunit and auxiliary 2 , , and subunits. Two dihydropyridine-sensitive genetic variants of 1S have been identified and cloned from heart ( 1C) and pancreatic beta cells ( 1D), with the cardiac 1C isoform being expressed in the vast majority of eukaryotic cells. Activity of the 1C channel is highly regulated by membrane potential, other calcium channel auxiliary subunits ( 2 , ), as well as by feedback dependence on permeating Ca , which is mediated by calmodulin. The genetic regulation of the 1C Ca 2 channel subunit occurs through species-specific variability and largely tissue-specific alternative splicing. In this review, we briefly discuss genomic organization of the 1C subunit gene and then focus attention on alternative splicing that generates functionally distinct Ca channel isoforms as potentially new pharmacological targets. Genomic Organization The 1C L-type Ca 2 channel is subject to complex genetic regulation that gives rise to a variety of channel subtypes. Both genomic variability and alternative splicing of the primary transcript appear to contribute to this complexity. The human 1C subunit gene (CACNL1A1) is composed of 53 identified exons (Fig. 1) (Soldatov, 1994) and is located in the distal region of chromosome 12p13 (Sun et al., 1992; Powers et al., 1992). There are two other 1 subunits ( 1D and 1S) of the L-type Ca channel subfamily that are encoded by different genes located in different chromosomes. The exact size of the 1C subunit gene remains to be determined. Based on the assumption that 10 kb is a rough size approximation of unknown introns, the gene was estimated to span more than 150 kb of the human genome (Soldatov, 1994). However, the high-resolution visual mapping of stretched DNA by fluorescent in situ hybridization (termed fiber-FISH) has shown that the distance from exon 10 to the 3 -end is in fact 170.4 kb (Liu et al., 1998). Thus the total length of the human CACNL1A1 gene is at least 70 kb greater than earlier estimates. In addition, we have found a repetitive element of three paired exon 45/46-related sequences in the 1C subunit gene (Soldatov et al., 1998b). Two of them, clones g6-20 and g12-5, were found by the fiberABBREVIATIONS: kb, kilobase(s); FISH, fluorescent in situ hybridization; DHP, dihydropyridine; CaM, calmodulin; nt, nucleotide(s). 0022-3565/02/3003-724–728 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 300, No. 3 U.S. Government work not protected by U.S. copyright 900063/967377 JPET 300:724–728, 2002 Printed in U.S.A. 724 at A PE T Jornals on M ay 7, 2017 jpet.asjournals.org D ow nladed from