SM22-ALPHA, A MARKER OF ADULT SMOOTH-MUSCLE, IS EXPRESSED IN MULTIPLEMYOGENIC LINEAGES DURING EMBRYOGENESIS

SM22 alpha is a calponin-related protein that is expressed specificall y in adult smooth muscle. To begin to define the mechanisms that regul ate the establishment of the smooth muscle lineage, we analyzed the ex pression pattern of the SM22 alpha gene during mouse embryogenesis. In situ hybridization demonstrated that SM22 alpha transcripts were firs t expressed in vascular smooth muscle cells at about embryonic day (E) 9.5 and thereafter continued to be expressed in all smooth muscle cel ls into adulthood. In contrast to its smooth muscle specificity in adu lt tissues, SM22 alpha was expressed transiently in the heart between E8.0 and E12.5 and in skeletal muscle cells in the myotomal compartmen t of the somites between E9.5 and E12.5. The expression of SM22 alpha in smooth muscle cells, as well as early cardiac and skeletal muscle c ells, suggests that there may be commonalities between the regulatory programs that direct muscle-specific gene expression in these three my ogenic cell types.