Abstract: Cardiac fibrosis impairs heart function and increases risk for cardiac arrhythmias. The transforming growth factor beta (TGFβ) family is major driver of fibrosis, including cardiac fibrosis. Latent TGFβ binding proteins (LTBPs) are extracellular matrix proteins that restrict latent TGFβ release and activity. We previously identified LTBP4 as a genetic modifier of muscular dystrophy, where we showed that LTBP4’s ability to bind TGFβ was strongly linked to sarcolemmal stability and fibrosis. LTBP4 is found along the exterior surface of the sarcolemma in myofibers, and LTBP4 is similarly found on the exterior surface of cardiomyocytes in a striated pattern. Because LTBP4 is highly expressed in the heart, LTBP4 is well positioned to regulate latent TGFβ release in cardiac fibrosis. The LTBP4 genes in mice and humans have naturally occurring protective and deleterious forms which produce proteins associated with differential TGFβ activity and downstream TGFβ signaling. Mouse strains bearing the protective Ltbp4 allele have 12 amino acids inserted into LTBP4’s hinge region, rendering the protein more resistant to proteolysis and latent TGFβ release. Correspondingly, mouse strains harboring the deleterious allele of Ltbp4, lacking 12 amino acids, produce an LTBP4 protein that is more susceptible to proteolysis leading to excess latent TGFβ release, signaling and fibrosis. In chronic progressive cardiomyopathies, there is dysregulation of matrix remodeling, which can further enhance maladaptive matrix shifts and adversely alter heart function and promote arrhythmia risk. We will now study LTBP4 in the heart by probing TGFβ’s interaction with LTBP4 using three approaches. In Aim 1, we will use decellularized matrices, called dECMs, from mouse hearts to define components and activity necessary for cellular communication between cardiomyocytes and cardiac fibroblasts. In Aim 2, we will evaluate human induced pluripotent stem cell-derived cardiomyocytes, and we will also conduct in vivo assessment of blocking TGFβ release in mice using an anti-LTBP4 antibody to promote cardiac sarcolemmal stability and reduce cardiac fibrosis. In Aim 3, we will evaluate cellular crosstalk mediated by LTBP4 in human engineered heart tissues (EHTs). Through this work, we will expand the mechanistic understanding of LTBP’s regulation of TGFβ with the goal of therapeutically modifying the matrix.