DEVELOPMENTAL PATTERN OF EXPRESSION AND GENOMIC ORGANIZATION OF THE CALPONIN-H1 GENE - A CONTRACTILE SMOOTH-MUSCLE CELL MARKER

Calponin-h1 is a 34-kDa myofibrillar thin filament, actin-binding prot ein that is expressed exclusively in smooth muscle cells (SMCs) in adu lt animals, To examine the molecular mechanisms that regulate SMC spec ific gene expression, we have examined the temporal, spatial, and cell cycle-regulated patterns of expression of calponin-h1 gene expression and isolated and structurally characterized the murine calponin-h1 ge ne. Calponin-h1 mRNA is expressed exclusively in SMC-containing tissue s in adult animals, During murine embryonic development, calponin-h1 g ene expression is (i) detectable in E9.5 embryos in the dorsal aorta, cardiac outflow tract, and tubular heart, (ii) sequentially up-regulat ed in SMC containing tissues, and (iii) downregulated to non-detectabl e levels in the heart during late fetal development, In addition, the gene is expressed in resting rat aortic SMCs, but its expression is ra pidly down-regulated when growth-arrested cells reenter phase G(1) of the cell cycle and proliferate, Calponin-h1 is encoded by a 10.7-kilob ase single copy gene composed of seven exons, which is part of a multi gene family, Transient transfection analyses demonstrated that 1.5 kil obases of calponin-h1 5'-flanking sequence is sufficient to program hi gh level transcription of a luciferase reporter gene in cultured prima ry rat aortic SMCs and the smooth muscle cell line, A7r5. Taken togeth er, these data suggest that the calponin-h1 gene will serve as an exce llent model system with which to examine the molecular mechanisms that regulate SMC lineage specification, differentiation, and phenotypic m odulation.