Project Summary Cells in the human body are exposed to a broad range of mechanical forces. Cells respond to such mechanical stimuli with the expression of specific mechanoresponsive genes, enabling the cells to adapt to their physical environment. This ‘mechanotransduction’ process is particularly important in tissues subjected to large and highly variable mechanical stresses, such as skeletal muscle, cardiac muscle, and skin, where impaired mechanotransduction can lead to muscular dystrophy, heart disease, and other pathologies. Research on mechanotransduction mechanisms has typically focused on proteins at the cell surface and in the cytoskeleton, along with the signaling pathways activated by these proteins. Recent studies and our preliminary data using advanced techniques to detect rapid changes in gene expression, however, found that mechanical stimulation induces expression of mechanoresponsive genes faster than the time needed for cytoplasmic signaling cascades to reach the nuclear interior, suggesting the existence of novel, yet to be determined mechanotransduction mechanisms. The overall objective of this proposal is to identify the mechanism responsible for this ‘ultra-rapid’ induction of mechanoresponsive genes and to determine the functional consequences of impaired nuclear mechanotransduction. Given the importance of mechanotransduction in muscle development, maintenance, and disease, the proposed research will focus on skeletal muscle cells. Nonetheless, insights gained from this research are expected to be also broadly applicable to many other cell types. The central hypothesis of this proposal is that the nucleus is not just a receiver of cytoplasmic mechanotransduction signals, but actively participates in transducing mechanical forces in changes in gene expression. Supporting this idea, deletion or mutation of nuclear envelope proteins that physically connect the nucleus to the cytoskeleton, such as lamins and components of the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, lead to impaired activation of mechanoresponsive genes and cause various muscle diseases. Nonetheless, how these proteins, and the nucleus in general, participate in cellular mechanotransduction and interface with established mechanotransduction pathways remains unresolved. The specific aims of the proposed work are to (1) determine the molecular mechanisms for the ultra-rapid mechanically induced gene expression and (2) define the role of nucleo-cytoskeletal force transmission, the LINC complex, and lamins in the mechanotransduction process in muscle cells. The long-term goal is to understand the fundamental mechanisms by which cells sense and respond to their physical environment, and to determine the effect of disease-causing mutations on this process. Gaining better mechanistic insights into how mechanical stimulation activates mechanoresponsive genes in skeletal muscle is critical to the development of new targeted therapeutic approaches for d...