Smooth Muscle Myosin Phosphatase Subunit Isoforms

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DESCRIPTION (provided by applicant): Myosin phosphatase (MP) is the primary effector of smooth muscle relaxation and a key target of signaling pathways that regulate vessel tone. MP is a hetero-trimer composed of catalytic (PP1c), targeting/regulatory (MYPT1) and 21 kD (M21) subunits. Our long term goals are to understand the regulated expression of MP/MYPT1 isoforms in relation to vascular function in development and disease. We have shown that isoforms of MYPT1 generated by alternative splicing of a 31 nt exon (E23) are tissue-specific, developmentally regulated, evolutionarily conserved and modulate in disease. E23 is spliced in smooth muscle tissues with fast (phasic) or intermediate contractile properties, e.g. portal vein and mesenteric resistance arteries, and skipped in the slow (tonic) smooth muscle of the large arteries and veins. A switch from E23 skipping to splicing occurs in the perinatal period in tissues that acquire a fast phenotype. In disease models of altered blood flow/pressure, the PV and mesenteric resistance arteries shift to E23 skipping as part of a generalized shift towards the slow phenotype. E23 skipping codes for a MYPT1 C-terminus leucine zipper (LZ) motif required for cGMP-dependent protein kinase (cGK1) dimerization and activation of MP, resulting in calcium de-sensitization of force production. Inclusion of the 31 nt exon codes for the MYPT1 LZ- isoform. We have shown in a number of models that tissues that express the E23-included/LZ- isoform are less sensitive to NO/cGMP-mediated relaxation, suggesting that the regulated splicing of E23 serves as a way for vascular smooth muscle to fine tune its sensitivity to NO/cGMP signaling. This renewal focuses on the regulation of MYPT1 E23 splicing. We propose to test the hypothesis that Transformer2b is a novel regulator of E23 splicing/vascular smooth muscle phenotypic specification in development and modulation in disease, and the corollary that Tra2b can be used as a novel nodal point in this line of investigation. The Transformer splicing factors were originally identified as master regulators of sexually dimorphic traits in the fly. The role of the vertebrate homologues in phenotypic specification is unknown. In the previous funding period we showed 1) a strong and evolutionarily conserved correlation between Tra2b expression and E23 splicing in developmental and disease models 2) Tra2b binds E23 and trans-activates its splicing from a mini-gene construct. The following aims are proposed: Aim 1: Define the role of Tra2b cis-element in the regulation of MYPT1 E23 splicing through the generation of mutation/deletion/chimeric constructs, measurements of binding affinity, and assay for binding in vivo. Aim 2: Test the role of Tra2b in vivo with a) LacZ targeting the Tra2b locus as a reporter for the expression of Tra2b in different vascular beds throughout development b) Cre-lox mediated conditional inactivation of TRa2b in VSM (loss-of-function) c) forced expression of Tra2b in the mesenteric arteries in the disease models (gain-of-function/rescue). Aim 3: Tra2b as a novel nodal point for the dissection of VSM diversification. An ~500 nt ultra-conserved sequence (UCS) in the first intron of Tra2b will be tested for its ability to drive tissue-specific transcription in fast smooth muscle in a transgenic assay. The UCS will be dissected through bio-informatics, deletion/mutation, and candidate factor (T3,NFAT) approaches. These studies will provide novel insights regarding the role of Tra2b, and novel links between transcriptional and splicing controls, in the generation and modulation of vascular smooth muscle contractile phenotypic diversity.