SMOOTH-MUSCLE MYOSIN HEAVY-CHAIN EXCLUSIVELY MARKS THE SMOOTH-MUSCLE LINEAGE DURING MOUSE EMBRYOGENESIS

We cloned a portion of the mouse smooth muscle myosin heavy chain (SM- MHC) cDNA and analyzed its mRNA expression in adult tissues, several c ell lines, and developing mouse embryos to determine the suitability o f the SM-MHC promoter as a tool for identifying smooth muscle-specific transcription factors and to define the spatial and temporal pattern of smooth muscle differentiation during mouse development. RNase prote ction assays showed SM-MHC mRNA in adult aorta, intestine, lung, stoma ch, and uterus, with little or no signal in brain, heart, kidney, live r, skeletal muscle, spleen, and testes. From an analysis of 14 differe nt cell lines, including endothelial cells, fibroblasts, and rhabdomyo sarcomas, we failed to detect any SM-MHC mRNA; all of the cell lines i nduced to differentiate also showed no detectable SM-MHC. In situ hybr idization of staged mouse embryos first revealed SM-MHC transcripts in the early developing aorta at 10.5 days post coitum (dpc). No hybridi zation signal was demonstrated beyond the aorta and its arches until 1 2.5 to 13.5 dpc, when SM-MHC mRNA appeared in smooth muscle cells (SMC s) of the developing gut and lungs as well as peripheral blood vessels . By 17.5 dpc, SM-MHC transcripts had accumulated in esophagus, bladde r, and ureters. Except for blood vessels, no SM-MHC transcripts were e ver observed in developing brain, heart, or skeletal muscle. These res ults indicate that smooth muscle myogenesis begins by 10.5 days of emb ryonic development in the mouse and establish SM-MHC as a highly speci fic marker for the SMC lineage. The SM-MHC promoter should therefore s erve as a useful model for defining the mechanisms that govern SMC tra nscription during development and disease.