Muscle LIM protein/CSRP3: a mechanosensor with a role in autophagy

Muscle LIM protein (MLP) is a microtubule-associated protein expressed in cardiac and muscle tissues that belongs to the cysteine-rich protein (CSRP/CRP) family. MLP has a central role during muscle development and for architectural maintenance of muscle cells. However, muscle cells rely on autophagy during differentiation and for structural maintenance. To study the role of MLP in autophagy, we have used C2C12 mouse myoblasts silenced or overexpressing MLP. Our results show that MLP contributes to the correct autophagosome formation and flux by interacting with LC3 as demonstrated by co-immunoprecipitation and PLA assay. In fact, MLP silencing results in decreased LC3-II staining and absent degradation of long-lived proteins. Moreover, MLP silencing impaired myoblasts differentiation as measured by decreased expression of MyoD1, MyoG1 and myosin heavy chain. Ultrastructural analysis revealed the presence of large empty autophagosomes in myoblasts and multimembranous structures in myotubes from MLP-silenced clones. Impaired autophagy in MLP-silenced cells resulted in increased susceptibility to apoptotic cell death. In fact, treatment of MLP-silenced C2C12 myoblasts and myotubes with staurosporine resulted in increased caspase-3 and PARP cleavage as well as increased percentage of cell death. In conclusion, we propose that MLP regulates autophagy during muscle cell differentiation or maintenance through a mechanism involving MLP/LC3-II interaction and correct autophagosome formation.