9506346 Venuti Studies on the MyoD family of muscle specific gene regulatory factors have provided enormous insight into the mechanisms that regulate myoblast determination and differentiation. But a fundamental question that remains in muscle developmental biology is: what activates the myogenic transcription factors? The prime objective of this proposed research is to identify the earliest molecular events that commit a pluripotent mesodermal stem cell to a myogenic fate. These experiments are designed to identify factors that influence early embryonic muscle cell determination and explore the capacity of embryonic cells which are not part of the myogenic program to differentiate as muscle cells. The sea urchin embryo, whose development is extremely well understood and whose embryos are particularly amenable to micromanipulation during early development, will be the experimental organism. Dr. Venuti has developed an in vitro assay that will enable her to ask if and when extracellular signaling is required for the determination and differentiation of muscle precursors or if they develop cell autonomouqly. These experiments should identify the signals or cues that initiate the decision to follow a muscle fate and determine when muscle progenitors first become committed. Dr. Venuti has cloned and characterized the muscle specific basic helix-loop-helix (b-HLH) transcription factor, SUM-1, from the sea urchin embryo and shown that it is activated very early in the muscle differentiation program. She has shown that SUM-1 can activate muscle specific reporter elements in vertebrate tissue culture cells and in microinjected sea urchin embryos. She will examine the consequences of expressing SUM-1 at premature times and at ectopic locations in the embryo either alone or in combination with interacting factors. To identify factors that act directly to regulate the transcription of SUM-1, the cis regulatory elements and trans acting factors of the SUM-1 promoter wil l be characterized. Once important temporal and spatial regulatory elements of the SUM-1 promoter are identified, she will test the effects of the extracellular signals identified in in vitro assays on these elements. The combination of these experiments will help direct long term investigations into muscle cell fate determination and the role of extracellular signals in the decision of embryonic cells to commit to a myogenic fate. They will also provide a better understanding of the role of the myogenic transcription factor, SUM-1, and other regulatory molecules in these determination events. ***
