Involvement of transportin 2-mediated HuR import in muscle cell differentiation.

Muscle fiber formation requires the sequential expression of myogenic regulatory factors (MRFs) such as MyoD and myogenin. The messenger RNAs encoding these two proteins are regulated posttranscriptionally through their ability to associate with the RNA-binding protein HuR. HuR localizes first to the nucleus and then to the cytoplasm during muscle differentiation. Therefore, we examined the link between this localization and the promyogenic function of HuR. We show that early in muscle differentiation, HuR is localized to the nucleus of myoblasts by active Transportin 2 (TRN2)-mediated import. In differentiated muscle fibers, however, the TRN2-HuR complex is disrupted, leading to the cytoplasmic localization of HuR, as well as to the stabilization of MyoD and myogenin mRNAs. Interrupting the TRN2-HuR complex using RNA interference against TRN2, or the cell-permeable peptides (AP) fused to the HuR nucleocytoplasmic shuttling domain (HNS), enhanced the efficiency of myofiber formation. Together, our data suggest that HuR import is disrupted in differentiated muscle fibers and this event constitutes an important regulatory step during myogenesis.