In striated muscle, the Muscle LIM protein (MLP, equivalent to cysteine and glycine-rich protein 3, CRP3) is located at Z-discs and M-lines of the sarcomere. MLP has multiple protein interaction partners and was also identified as a cytosolic protein. The MLP-W4R variant was shown to be associated with cardiomyopathy in humans; other mutations of MLP were associated with myopathy of myocardium and skeletal muscle. Mice with a knock-in of MLP-W4R were found to develop myopathic changes in cardiac and skeletal muscle. Since little is known about underlying functional changes at the sarcomeric level we investigated the influence of MLP-W4R on skeletal muscle function and ultrastructure.

We used skinned single muscle fibers of M. vastus lateralis of wildtype and MLP-W4R knock-in mice to study passive and active parameters like relaxed fiber stiffness, the passive force-sarcomere length relation, isometric force, the rate constant of force redevelopment, and maximal shortening velocity. Thin sections of the muscle fibers were analyzed by transmission electron microscopy.

MLP-W4R muscle fibers showed a significant decrease in relaxed fiber stiffness and passive force-sarcomere length relation compared to wildtype, whereas no significant changes occurred in isometric force generation, force redevelopment, and shortening velocity. Electron microscopy revealed broadened Z-discs and M-lines in MLP-W4R fibers, while the alignment of myofilaments within the sarcomere was normal.

The morphological changes of muscle fibers from MLP-W4R knock-in mice are consistent with a skeletal myopathy. We hypothesize that the disturbed passive properties of the muscle fibers contribute to the development of the disease.