Cardiomyopathy is a disease of the myocardium that could be either acquired or inherited (genetic). This study is focused on three types of inherited cardiomyopathy: hypertrophic (HCM), dilated (DCM) and restrictive (RCM) due to mutations in the MYL2 and MYL3 genes encoding for the myosin regulatory (RLC) and essential (ELC) light chains, respectively. The normal myofilament structure of the heart muscle is important for its contractile function and any alterations in myofilament architecture may trigger abnormal cardiac remodeling leading to HCM, DCM and RCM. In this study, I will focus on the D166V (Aspartic Acid to Valine) and D94A (Aspartic Acid to Alanine) mutations in the RLC associated with HCM and DCM, respectively. In addition, I will focus on the E143K mutation in the ELC that was reported to cause RCM. We hypothesize that these mutations may differently alter sarcomeric structure including the interfilament lattice spacing (IFS) and affect the cross-bridge mass distribution between the thick and thin filaments. Structural changes are expected to be closely correlated with myofilament force transmission in vitro and the systolic and diastolic function in vivo. Left ventricular papillary muscles from mice will undergo the sarcomere structural studies by synchrotron X-ray diffraction while the mechanical properties of muscle will be measured by force transducer using the Guth Muscle Research System. Myofilament Ca2+ sensitivity and muscle activation and relaxation kinetics will also be measured (Aim1). I anticipate an increase in IFS for D166V (HCM) and D94A (DCM) mutations and a decrease in IFS for the E143K (RCM) mutation. These changes in IFS are expected to correlate with force production. HCM and DCM mutations are expected to reduce the maximal tension per cross-section of muscle, while RCM will elicit an increase in tension when compared to WT, non-mutated myocardium. The equatorial reflections’ ratio (I1,1/I1,0), an indicator of cross-bridge mass distribution, is expected to increase in HCM and RCM while decrease in DCM. This is because myosin cross-bridges are expected to move toward the thin filaments enhancing Ca+2 sensitivity of force in HCM and RCM. On the other hand, they are expected to move away from thin filaments decreasing the Ca+2 sensitivity in DCM. The cardiac function will be assessed by echocardiography and invasive pressure-volume loops to confirm the structure-function relationship in vivo in transgenic mice expressing these HCM, DCM and RCM mutations (Aim2). The pathologic phenotypes and disease-related gene expression will be investigated by qRT-PCR. Posttranslational modifications, mainly sarcomeric protein phosphorylation will be assessed by Western blotting and ProQ/Sypro stained gels (Aim3). Collectively, this study will explore, for the first time, the link between mutation-elicited structural modifications of the sarcomere that in turn lead to functional abnormalities and HCM, DCM and RCM cardiomyopathy.