PROJECT SUMMARY Mutations in genes encoding nuclear envelope (NE) components cause an array of diseases referred to as nuclear envelopathies that often manifest as cardiomyopathies. The NE separates the nucleoplasm from cytoplasm and is composed of outer and inner nuclear membranes. The nuclear lamina (NL) is an extensive network of lamin polymers and associated proteins that are embedded in the inner nuclear membrane (INM). Functionally, NL and INM proteins not only provide mechanical stability to the nucleus but also serve as the anchoring point for chromatin at the nuclear periphery, playing a critical role in chromatin organization and regulation of gene expression via interaction with and modulation of epigenetic machinery components. The LAP2-Emerin-MAN1-domain (LEM-D) family of proteins play an important role in the association between the NL and chromatin. LEMD2 (LEM domain-containing protein 2), is a transmembrane protein located in the INM, involved in nuclear integrity and perinuclear tethering and transcriptional silencing of heterochromatin. Recently, it has been reported that a single amino acid substitution of leucine 13 to arginine (L13R) in LEMD2 leads to autosomal recessive human cardiomyopathy. However, despite its clinical relevance, little is known as to the specific role of LEMD2 in cardiomyocytes (CMs), and mechanisms by which the L13R mutation leads to cardiomyopathy. To address this gap in knowledge, we have generated three mouse models: constitutive CM- specific Lemd2 knockout (cKO), inducible CM-specific Lemd2 knockout (icKO), and LEMD2 L13R knock-in mice. We will also utilize a human induced pluripotent stem cell (iPSC)-derived CM model of the LEMD2 L13R mutation to address the impact of L13R mutation in LEMD2 on human CMs. Preliminary studies revealed that loss of LEMD2 in embryonic or adult CMs is detrimental, and LEMD2 L13R mutation affects cardiac function in a murine model, indicating that LEMD2 plays a critical role in maintaining normal CM structure and function in developing and adult hearts. Thus, we hypothesize that LEMD2-mediated maintenance of NE integrity and/or regulation of heterochromatin tethering and silencing is essential for CM structure and function, and LEMD2 L13R mutation impairs specific aspects of LEMD2 function leading to cardiomyopathy. Accordingly, Our Specific Aims are: 1. To determine the role of LEMD2 in the developing and adult myocardium by analyzing constitutive (cKO) and inducible (icKO) Lemd2 CM-specific knockout mice for heart morphogenesis, structure and function, and the progression of cardiomyopathy; and 2. To elucidate molecular mechanisms underlying cardiomyopathy consequent to the L13R mutation in LEMD2 by detailed analyses of LEMD2 L13R knock-in mice and human induced pluripotent stem cell (iPSC)-derived LEMD2 L13R mutant CMs.