Tra2β Protein Is Required for Tissue-specific Splicing of a Smooth Muscle Myosin Phosphatase Targeting Subunit Alternative Exon

Alternative splicing of the smooth muscle myosin phosphatase targeting subunit (Mypt1) exon 23 (E23) is tissue-specific and developmentally regulated and, thus, an attractive model for the study of smooth muscle phenotypic specification. We have proposed that Tra2 beta functions as a tissue-specific activator of Mypt1 E23 splicing on the basis of concordant expression patterns and Tra2 beta activation of Mypt1 E23 mini-gene splicing in vitro. In this study we examined the relationship between Tra2 beta and Mypt1 E23 splicing in vivo in the mouse. Tra2 beta was 2- to 5-fold more abundant in phasic smooth muscle tissues, such as the portal vein, small intestine, and small mesenteric artery, in which Mypt1 E23 is predominately included as compared with the tonic smooth muscle tissues, such as the aorta and inferior vena cava, in which Mypt1 E23 is predominately skipped. Tra2 beta was up-regulated in the small intestine postnatally, concordant with a switch to Mypt1 E23 splicing. Targeting of Tra2 beta in smooth muscle cells using SM22 alpha-Cre caused a substantial reduction in Mypt1 E23 inclusion specifically in the intestinal smooth muscle of heterozygotes, indicating sensitivity to Tra2 beta gene dosage. The switch to the Mypt1 E23 skipped isoform coding for the C-terminal leucine zipper motif caused increased sensitivity of the muscle to the relaxant effects of 8-Br-cyclic guanosine monophosphate (cGMP). We conclude that Tra2 beta is necessary for the tissue-specific splicing of Mypt1 E23 in the phasic intestinal smooth muscle. Tra2 beta, by regulating the splicing of Mypt1 E23, sets the sensitivity of smooth muscle to cGMP-mediated relaxation.