DESCRIPTION (appended verbatim from investigator's abstract): The broad goals of this research are to extend our knowledge of how skeletal myoblasts form muscle fibers. Although much evidence demonstrates a critical role for protein tyrosine kinases (PTKs) in myogenesis, the role played by protein tyrosine phosphatases (PTPs) in this process is unknown. This grant focuses on how PTPs participate in three fundamental areas of myogenesis. First, how do PTPs participate in growth factor signaling pathways that negatively regulate myogenesis? Second, what is the contribution of PTPs to myogenic progression? Third, are PTPs involved in the mechanisms that mediate conversion of a mononucleated myocyte to a multinucleated myotube? Our preliminary data on the SH2 domain containing PTP (SHP 2) and the mitogen activated protein kinase (MAPK) phosphatase 1 (MKP 1), which function as positive and negative regulators of the MAPK pathway respectively, has led us to the following hypotheses: (1) MKP 1 functions as a critical mediator of multinucleated myotube formation and (2) that SHP 2 plays both a positive and negative signaling role in regulating myogenesis. Mutants of MKP 1 that form stable enzyme substrate complexes (substrate trapping mutants) will be utilized to identify the MAPK(s) involved in myotube formation. How SHP 2 participates in FGF 2 regulation of myogenesis will be determined. A combination of SHP 2 overexpression studies and SHP 2 "loss of function" myoblasts will be used to elucidate its role in myogenic progression. Finally, we report that in myoblasts, SHP 2 interacts with the signal regulatory protein 1cc(SIRP 1a), a recently discovered member of a family of transmembrane glycoproteins. The protein expression and tyrosyl phosphorylation of SIRP 1 a are induced during myogenesis, prior to myotube formation. We will define the function of SIRP 1a by identifying its cellular localization both in cultured myoblasts during myogenic differentiation and in vivo in striated skeletal muscle. We will test whether the extracellular domain of SlRP 1a is involved in myotube formation and determine using anti sense methods its requirement for myogenic progression. Many musculoskeletal diseases, such as the myasthenic syndromes and muscular dystrophies, lead to muscle degeneration and atrophy. The information gained from this research can be harnessed to provide insight into new therapies designed to either prevent or retard these pathological conditions.