Cloning of a sodium channel alpha subunit from rabbit Schwann cells.

Abstract

Overlapping cDNA clones spanning the entire coding region of a Na-channel a subunit were isolated from cultured Schwann cells from rabbits. The coding region predicts a polypeptide (Nas) of 1984 amino acids exhibiting several features characteristic of Na-channel a subunits isolated from other tissues. Sequence comparisons showed that the Na, a subunit resembles most the family of Na channels isolated from brain ('80% amino acid identity) and is least similar (-55% amino acid identity) to the atypical Na channel expressed in human heart and the partial rat cDNA, NaG. As for the brain II and III isoforms, two variants of Na1 exist that appear to arise by alternative splicing. Voltage-gated Na channels, which underlie generation of the action potential of excitable cells, are composed of a large a subunit with four homologous internal repeats (each with six membrane-spanning segments) and up to two smaller ,B subunits (1, 2). Different mammalian a-subunit isoforms have been isolated from brain [brain I, II, and III (3-6) and NaCh6 (7)] and muscle (8-10). The amino acid sequences of each a subunit exhibit a high degree of identity, suggesting that they are members of the same multigene family. Another Nachannel a-subunit isoform (isolated from adult human heart and fetal skeletal muscle; ref. 11) shows only '50% amino acid identity with the previously cloned isoforms and may be a member of a distinct subfamily of Na-channel genes. The partial sequence of a Na-channel a subunit from rat astrocytes (12) suggests it is another member of this atypical subfamily.Ritchie and colleagues (13)(14)(15) showed that Schwann cells express voltage-gated Na channels whose functional properties are similar to those of Na channels expressed in excitable issues (16,17

Expression of Alternatively Spliced Sodium Channel α-Subunit Genes

“…Inspection of genomic sequence of human, mouse and rat SCN9A suggested it has a gene structure similar to SCN8A. Specifically, the genome sequences from all three species encode potentially mutually exclusive exon 5 sequences, and evidence for alternative splicing of the rabbit paralog was deduced from cDNA sequences (24). Similarly, conservation of exon 11 alternative splice donor sites was found in human, mouse and rat genomic sequences, and evidence for alternative splicing was suggested by comparison of human, rat, and rabbit cDNA sequences (12).…”

Function and role of voltage-gated sodium channel NaV1.7 expressed in aortic smooth muscle cells

“…At present, 10 ␣-subunit (SCN1-6, 8-11A) and 4 ␤-subunit (SCN␤ [1][2][3][4] ) genes have been identified with isoform expression dependent on cell types (24). I Na has also been identified in cultured smooth muscle cells such as coronary arterial (3), pulmonary arterial (25), and bronchial (17) smooth muscle cells.…”

“…PCR products were size fractionated on 2% agarose gels and visualized under UV light. Primers were chosen based on the sequence of human SCN1-6A and 8-9A and rabbit SCN9A (5Ј-TCCTGGAAGCTCCACTGAA-3Ј and 5Ј-CAAA-TCATCATCCTTGTCTCC-3Ј) as previously reported (2,17). Total RNA of human fetal brain, skeletal muscle, heart, aorta (Toyobo), and rabbit spinal cord was used as positive and native controls.…”

Structure and distribution of a broadly expressed atypical sodium channel

“…Many different mammalian α-subunit cDNAs have been cloned [2][3][4][5][6][7][8]. Several distinctive structural properties of the VGSC a-subunits suggest that they are members of the same multigene family.…”