Novel mRNA isoforms of the sodium channels Na v1.2, Na v1.3 and Na v1.7 encode predicted two-domain, truncated proteins

Abstract

The expression of voltage-gated sodium channels is regulated at multiple levels, and in this study we addressed the potential for alternative splicing of the Na v1.2, Na v1.3, Na v1.6 and Na v1.7 mRNAs. We isolated novel mRNA isoforms of Na v1.2 and Na v1.3 from adult mouse and rat dorsal root ganglia (DRG), Na v1.3 and Na v1.7 from adult mouse brain, and Na v1.7 from neonatal rat brain. These alternatively spliced isoforms introduce an additional exon (Na v1.2 exon 17A and topologically equivalent Na v1.7 exon 16A) or exon pair (Na v1.3 exons 17A and 17B) that contain an in-frame stop codon and result in predicted two-domain, truncated proteins. The mouse and rat orthologous exon sequences are highly conserved (94–100% identities), as are the paralogous Na v1.2 and Na v1.3 exons (93% identity in mouse) to which the Na v1.7 exon has only 60% identity. Previously, Na v1.3 mRNA has been shown to be upregulated in rat DRG following peripheral nerve injury, unlike the downregulation of all other sodium channel transcripts. Here we show that the expression of Na v1.3 mRNA containing exons 17A and 17B is unchanged in mouse following peripheral nerve injury (axotomy), whereas total Na v1.3 mRNA expression is upregulated by 33% ( P=0.003), suggesting differential regulation of the alternatively spliced transcripts. The alternatively spliced rodent exon sequences are highly conserved in both the human and chicken genomes, with 77–89% and 72–76% identities to mouse, respectively. The widespread conservation of these sequences strongly suggests an additional level of regulation in the expression of these channels, that is also tissue-specific.