Regulation of cell proliferation by fast Myosin light chain 1 in myoblasts derived from extraocular muscle, diaphragm and gastrocnemius.

The extraocular muscle (EOM) suffers much less injury from Duchenne muscular dystrophy (DMD) than other skeletal muscles such as diaphragm and gastrocnemius. The present study was undertaken to test the hypothesis that differential expression of regulatory proteins between the EOM and other skeletal muscles is responsible for the observed difference in the sensitivity to DMD-associated damage. Protein expression in the tissue samples obtained from EOM, diaphragm or gastrocnemius of C57BL/6 mice was analyzed by two-dimensional gel electrophoresis and mass spectrometry. There were 35 proteins that were identified to be differentially expressed among different skeletal muscle tissues. Among the 35 proteins, a fast skeletal muscle isoform myosin light chain 1 (MLC1f) protein was further studied in relation to muscle cell proliferation. The EOM-derived myoblasts had much lower levels of MLC1f and higher rate of cell proliferation in contrast to the myoblasts derived from diaphragm or gastrocnemius, which displayed a higher expression of MLC1f along with a slow proliferation. Deletion of MLC1f using siRNA targeting MLC1f resulted in an increased rate of cell proliferation in the myoblasts. Cell cycle analysis revealed that MLC1f inhibited the transition of the cell cycle from the G1 to the S phase. Therefore, the present study demonstrates that MLC1f may negatively regulate proliferation of myoblasts through inhibition of the transition from the G1 to the S phase of the cell cycle. Low levels of MLC1f in myoblasts of EOM may ensure cell proliferation and enhance the repair process for EOM under the DMD disease condition, thus making EOM suffer less injury from DMD.