Tissue-Specific Alternative Splicing of Tak1 Is Conserved in Deuterostomes

Alternative splicing allows organisms to rapidly modulate protein functions to physiological changes and therefore represents a highly versatile adaptive process. We investigated the conservation of the evolutionary history of the "Fox" family of RNA-binding splicing factors (RBFOX) as well as the conservation of regulated alternative splicing of the genes they control. We found that the RBFOX proteins are conserved in all metazoans examined. In humans, Fox proteins control muscle-specific alternative splicing of many genes but despite the conservation of splicing factors, conservation of regulation of alternative splicing has never been demonstrated between man and nonvertebrate species. Therefore, we studied 40 known Fox-regulated human exons and found that 22 had a tissue-specific splicing pattern in muscle and heart. Of these, 11 were spliced in the same tissue-specific manner in mouse tissues and 4 were tissue-specifically spliced in muscle and heart of the frog Xenopus laevis. The inclusion of two of these alternative exons was also downregulated during tadpole development. Of the 40 in the starting set, the most conserved alternative splicing event was in the transforming growth factor (TGF) beta-activated kinase Tak1 (MAP3K7) as this was also muscle specific in urochordates and in Ambulacraria, the most ancient deuterostome clade. We found exclusion of the muscle-specific exon of Tak1 was itself under control of TGF beta in cell culture and consistently that TGF beta caused an upregulation of Fox2 (RBFOX2) expression. The alternative exon, which codes for an in-frame 27 amino acids between the kinase and known regulatory domain of TAK1, contains conserved features in all organisms including potential phosphorylation sites and likely has an important conserved function in TGF beta signaling and development. This study establishes that deuterostomes share a remarkable conserved physiological process that involves a splicing factor and expression of tissue-specific isoforms of a target gene that expedites a highly conserved signaling pathway.