The events leading to cell-cell fusion are key to the development and homeostasis of bone and muscle. The broad objective of this study is to understand the mechanisms that lead to the differentiation and fusion of osteoclasts and myotubes. Results from the study could lead to new treatments for osteoporosis and muscular dystrophy. The hypothesis of the proposal is that a recently discovered family of transmembrane proteins, called ADAMs, are important in the differentiation of bone and muscle. The ADAMS contain A Disintegrin And Metalloproteinase domain. The study will initially focus on meltrin-alpha, an ADAM expressed by myoblasts and osteoclasts. One objective of the study is to perform the first characterization of the biosynthesis and cellular localization of meltrin-alpha. These experiments will determine if the metalloproteinase domain of meltrin is released from the cell surface, and whether meltrin-alpha co-localizes with integrin in focal adhesion sites. A second objective will be to determine how each domain of meltrin-alpha is involved in the fusion of myoblasts. In this analysis, site-directed mutagenesis and polyclonal antibodies will be applied to ablate the biochemical activity of domains of meltrin, and the effects of these manipulations on myogenesis will be assessed. A third objective is to examine the structure-function relationships of the metalloproteinase and disintegrin domains of meltrin-alpha. Phage-display will be used to build inhibitors of the metalloproteinase. Studies will be conducted to identify the cell surface receptor for the disintegrin domain of meltrin-alpha. A final objective is to identify the ADAM proteins present in osteoclasts and their precursors. Homology-based PCR will be used to clone osteoclast ADAMs. Antibodies against these ADAMs will be used in attempts to block osteoclast differentiation.