THE SM-22 PROMOTER DIRECTS TISSUE-SPECIFIC EXPRESSION IN ARTERIAL BUTNOT IN VENOUS OR VISCERAL SMOOTH-MUSCLE CELLS IN TRANSGENIC MICE

The transcriptional signals underlying smooth muscle differentiation a re currently unknown. We report here the complete sequence and charact erization of the single mouse gene for the smooth muscle-specific prot ein SM 22 and the transcriptional activity of its promoter in cultured smooth muscle cells in vitro and in transgenic mice, In the transgeni c animals, promoter constructs ranging in length from 445 to 2126 bp d irected reporter expression initially in the heart and the somites of embryos and subsequently in the arteries of the vascular system, but i n none of the visceral muscles, nor in the veins, Expression in the he art was spatially restricted to the presumptive right ventricle and ou tflow tract and disappeared in the adult, Likewise, expression in the somites was only transitory and was not observed after about 14.5 days post coitum in the embryo, In the adult mouse, SM 22 promoter activit y persisted in the smooth muscle cells of the arteries and was still n otably absent from other smooth muscles, despite the ubiquitous presen ce of the endogenous SM 22 protein. These findings on the transcriptio nal activity of a smooth muscle promoter in vivo reveal the existence of different differentiation programmes for smooth muscle cells in the veins and the arteries and raise the expectation of a further subdivi sion of programmes among the visceral muscles.