Cardiomyopathy is a disease of the myocardium that could be either acquired or inherited (genetic). Our lab is focused on three types of inherited cardiomyopathy: hypertrophic (HCM), dilated (DCM) and restrictive (RCM) that occur 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.
The sarcomere is the basic contractile unit of muscle and is composed of the thick (myosin) and thin (actin) filaments. Myosin is the major molecular motor of contraction and its interaction with actin filaments coupled with the hydrolysis of ATP leads to force production and muscle contraction. Myosin head contains the ATP and actin binding sites and the lever arm domain with the attached regulatory (RLC) and essential (ELC) light chains. An α-helical coiled-coil tail domain of myosin forms the myosin backbone and the thick filaments. The RLC binds at the distal part of the lever arm and has a cation binding site for Ca2+ or Mg2+, and the myosin light chain kinase (MLCK)-phosphorylatable Serine 15, whose phosphorylation regulates cardiac muscle contraction. The ELC is positioned closer to the myosin motor domain within the head region, and its ventricular isoform contains an N-terminal extension shown to interact with actin as well as with the MHC. The myosin lever arm domain has been proposed to amplify small conformational changes that occur in the myosin head motor domain into large movements driving the sliding of the thick and thin filaments and muscle contraction. Hence, any mutation in either RLC or ELC may obstruct the function of the lever arm and influence the myosin motor function and thus affect cardiac muscle contraction leading to cardiomyopathy. Our lab is interested in studying the molecular determinants of HCM, DCM and RCM using a combination of molecular biology, X-ray, proteomic and physiological approaches to study the underlying mechanisms of cardiomyopathy. We measure contractile force in papillary muscle fibers from transgenic mice, myosin ATPase activity, myofilament calcium sensitivity, cross-bridge kinetics, and other contractile parameters. We perform fluorescence spectroscopy experiments monitoring the effects of RLC or ELC mutations on the mechanical and biophysical properties of myosin. We study the effect of cardiomyopathy mutations on the structure of myocardium and sarcomere lattice spacing using small angle X-ray diffraction methods at Advance Photon Source, Argonne National Laboratory, Chicago, IL. Transgenic mouse hearts are also tested in vivo by echocardiography and invasive hemodynamics to determine the effect of mutations on heart function and potential development of a hypertrophic phenotype.