Localization and Function of Xinα in Mouse Skeletal Muscle

Abstract

Xin repeat-containing proteins were originally found in the intercalated discs of cardiac muscle with proposed roles in cardiac development and function. A pair of paralogous genes, Xinα (Xirp1) and Xinβ (Xirp2), is present in mammals. Ablation of the mouse Xinα (mXinα) did not affect heart development but caused late-onset adulthood cardiac hypertrophy and cardiomyopathy with conductive defects. Both mXinα and mXinβ are also found in the myotendinous junctions (MTJs) of skeletal muscle. In the present study, we investigated the structural and functional significance of mXinα in skeletal muscles. In addition to MTJs and the contact sites between muscle and perimysium, mXinα but not mXinβ was found in the blood vessel walls, whereas both proteins were absent in neuromuscular junctions or the nerve fascicles. Co-localization and co-immunoprecipitation suggested association of mXinα with talin, vinculin and filamin but not β-catenin in MTJs of adult skeletal muscle. Complete loss of mXinα in mXinα-null mice had subtle effects on the MTJ structure and the expression of other known MTJ components. Diaphragm muscle fibers of mXinα-null mice showed significant hypertrophy. In comparison with wild type controls, mouse extensor digitorum longus (EDL) muscle lacking mXinα exhibited no overt change in contraction and relaxation velocities or in maximum force development. Its fatigability and recovery from fatigue were similar to that of wild type control. Loaded fatigue contractions generated stretch injury in wild type EDL muscle as indicated by an adaptive restrictive truncation of troponin T. However, this effect was blunted in mXinα-null EDL muscle. The results suggest that mXinα may play a role in MTJ conductance of contractile and stretching forces, essential to skeletal muscle function.