Analysis of isoactin gene expression during normal and abnormal gastrointestinal smooth muscle development

Normal gastrointestinal morphogenesis and cellular differentiation requires the precise interaction of elements from each of the three embryonic germ layers. Little is currently known regarding the molecular basis of the complex cellular interactions responsible for normal gastrointestinal development. In an effort to understand some of these interactions, the present studies focused on an analysis of the molecular basis of gastrointestinal smooth muscle cell differentiation and maturation. Prior studies have indicated that an increase in actin was one of the major factors contributing to the development of the classic morphology of an adult smooth muscle cell. Additional biochemical and molecular studies have demonstrated that there are six distinct actin isoforms in higher vertebrates including: the $\beta$-cytoplasmic and $\gamma$-cytoplasmic isoactins; the $\alpha$-skeletal and $\alpha$-cardiac isoactins; and the $\alpha$-smooth muscle and $\gamma$-smooth muscle isoactins. The initial studies described in this thesis demonstrated that the actin multigene family was differentially coexpressed in a segment specific manner throughout the adult rat gastrointestinal tract. This analysis, in conjunction with prior studies, clearly indicated that the $\gamma$-smooth muscle isoactin was an excellent molecular marker for the differentiation and maturation of gastrointestinal smooth muscle cells. Additional molecular analyses indicated that functionally distinct smooth muscle tissues differentially coexpressed the smooth muscle and cytoplasmic isoactins and that abnormal smooth muscle tissues display distinct alterations in isaoactin gene expression patterns. The results of these studies established that the $\gamma$-smooth muscle actin gene is responsive to a variety of elements involved in the differentiation and maturation processes of smooth muscle cells. Therefore, investigations were initiated to isolate the 5$\sp\prime$ promoter region of the rat $\gamma$-smooth muscle actin gene to begin to identify specific cellular elements involved in gastrointestinal smooth muscle cell differentiation. These investigations have led to the isolation of a promising positive clone which is currently being analyzed. In summary, these investigations have provided the basis for developing key models of smooth muscle cell development and have provided markers to continue investigation into smooth muscle cell differentiation and maturation.