Functional dissection of human protease μ-calpain in cell migration using RNAi

Calpains are a family of calcium-dependent cysteine proteases involved in a variety of cellular functions. Two isoforms, m-calpain and μ-calpain, have been implicated in cell migration. However, since conventional inhibitors used for the studies of the functions of these enzymes lack specificity, the individual physiological function and biochemical mechanism of these two isoforms, especially μ-calpain, are not clear. In contrast, RNA interference has the potential to allow a sequence-specific destruction of target RNA for functional assay of gene of interest. In the present study, we found that small interfering RNAs-mediated knockdown of μ-calpain expression in MCF-7 cells that do not express m-Calpain led to a reduction of cell migration. This isoform-specific function of μ-calpain was further confirmed by the rescue experiment as overexpression of μ-calpain but not m-calpain could restore the cell migration rate. Knockdown of μ-calpain also altered cell morphology with increased filopodial projections and a highly elongated tail that seemed to prevent cell spreading and migration with reduced rear detachment ability. Furthermore, knockdown of μ-calpain decreased the proteolytic products of filamin and talin, which were specifically rescued by overexpression of μ-calpain but not m-calpain, suggesting that their proteolysis could be one of the key mechanisms by which μ-calpain regulates cell migration.