Characterization of the mechanisms behind the alternative splicing of the mutually exclusive exons 18N and 18A in the sodium channel gene SCN8A and mutually exclusive exons 5N and 5A in the soldium channel gene SCN9A

Voltage-gated sodium channels are the primary molecules responsible for the rising phase of action potentials in electrically excitable cells. There are 10 distinct sodium channel isoforms Nav 1.1-1.8 (SCNIA-SCN5A and SCN8A-SCNIIA) and the majority of these undergo tissue and developmentally regulated alternative splicing. Two such examples are those of the SCN8A (Nav 1.6) and SCN9A (Nav 1.7) genes. SCN8A gene contains two mutually exclusive exons, 18N and 18A. Transcripts with exon 18N have a conserved inframe stop codon that predicts the synthesis of a truncated, non functional sodium channel. This protein is expressed in fetal brain and non-neuronal tissues. Once the exon 18A is included, the resulted protein will be a functional channel, that is expressed in adult neurons ofCNS and PNS. The SCN9A exon 5N is preferentially expressed in the PNS and CNS of adult tissues and significant usage of exon 5A was found only in DRG. These two isoforms differ in one amino acid in the S3 domain I (exons 5A and 5N). This change of one amino acid induced a small shift of activation to more hyperpolarized potentials forexon SA compared with exon SN. Analysis of SeNSA pre-mRNA splicing supports a model in which exon 18A exclusion in non-neuronal tissue is regulated primarily by the presence in the cell types of several hnRNPs proteins that function through an exonic splicing silencer (ESS) found in this exon together with the absence of neuron specific Fox-I protein. In neuronal cells the absence of these hnRNPs together with the presence of neuron specific Fox-l cause the exon to be included. The SeNSA exon 18N is included innon neuronal cells due to the SR proteins that function through an exonic splicing enhancer(ESE) found in this exon. In neuronal cells the lower levels of these SR proteins cause the exon 18N to be skipped. This type of control of mutually exclusive splicing through the proteome make-up of a cell type would appear to be influential in the temporal and tissue specific splicing of SeN8A, another member of the voltage gated sodium channels and may indeed represent a more general mechanism.