SMOOTH MUSCLE-SPECIFIC EXPRESSION OF THE SMOOTH-MUSCLE MYOSIN HEAVY-CHAIN GENE IN TRANSGENIC MICE REQUIRES 5'-FLANKING AND FIRST INTRONIC DNA-SEQUENCE

The smooth muscle myosin heavy chain (SM-MHC) gene encodes a major con tractile protein whose expression exclusively marks the smooth muscle cell (SMC) lineage. To batter understand smooth muscle differentiation at the transcriptional level, we have initiated studies to identify t hose DNA sequences critical for expression of the SM-MHC gene. Here we report the identification of an SM-MHC promoter-intronic DNA fragment that directs smooth muscle-specific expression in transgenic mice. Tr ansgenic mice harboring an SM-MHC-lacZ reporter construct containing a pproximate to 16 kb of the SM-MHC genomic region from -4.2 to +11.6 kb (within the first intron) expressed the lacZ transgene in all smooth muscle tissue types. The inclusion of the intronic sequence was requir ed for transgene expression, since 4.2 kb of the 5'-flanking region al one was not sufficient for expression. In the adult mouse, transgene e xpression was observed in both arterial and venous smooth muscle, in a irway smooth muscle of the trachea and bronchi, and in the smooth musc le layers of all abdominal organs, including the stomach, intestine, u reters, and bladder. During development, transgene expression was firs t detected in airway SMCs at embryonic day 12.5 and in vascular and vi sceral SMC tissues by embryonic day 14.5. Of interest, expression of t he SM-MHC transgene was markedly reduced or absent in some SMC tissues , including the pulmonary circulation. Moreover, the transgene exhibit ed a heterogeneous pattern between individual SMCs within a given tiss ue, suggesting the possibility of the existence of different SM-MHC ge ne regulatory programs between SMC subpopulations and/or of episodic r ather than continuous expression of the SM-MHC gene. To our knowledge, results of these studies are the first to identify a promoter region that confers complete SMC specificity in vivo, thus providing a system with which to define SMC-specific transcriptional regulatory mechanis ms and to design vectors for SMC-specific gene targeting.