M-cadherin, a candidate gene for type 2 diabetes and related phenotypes in a KK/Ta mouse model

Suppression of GSK-3β activation by M-cadherin protects myoblasts against mitochondria-associated apoptosis during myogenic differentiation

Apoptosis occurs concurrently with differentiation of muscle progenitor cells (MPCs) before they fuse to form myotubes. Dysregulated apoptosis in MPCs contributes to the low regeneration capability in aged muscle and decreases the survival rate of donor cells in stem cell-based therapies for muscular dystrophies. This study investigated the role of the M-cadherin/PI3K/Akt/GSK-3β signaling pathway in regulating apoptosis during differentiation of MPCs. Disruption of M-cadherin-dependent cell-cell adhesion by M-cadherin RNA interference in confluent C2C12 myoblasts sensitized the cells to mitochondria-associated intrinsic apoptosis induced by cell confluence or serum starvation. Further investigation of this pathway revealed that M-cadherin-mediated signaling suppressed GSK-3β activation by enhancing the PI3K/AKT-dependent inhibitory phosphorylation of Ser9 in GSK-3β. Overexpression of wild-type GSK-3β in confluent C2C12 myoblasts exacerbated the apoptosis, whereas chemical inhibition of GSK-3β using TDZD-8, or forced expression of constitutively active Akt (myrAkt), or a kinase-deficient GSK-3β mutant [GSK-3β(K85R)], attenuated apoptosis and rescued the impaired myogenic differentiation that is caused by M-cadherin RNA interference. These data suggest that M-cadherin-mediated signaling prevents acceleration of mitochondria-associated intrinsic apoptosis in MPCs by suppressing GSK-3β activation during myogenic differentiation.

Myogenic regulatory factors regulate M-cadherin expression by targeting its proximal promoter elements

M- and N-cadherin are members of the Ca(2+)-dependent cell-cell adhesion molecule family. M-cadherin is expressed predominantly in developing skeletal muscles and has been implicated in terminal myogenic differentiation, particularly in myoblast fusion. N-cadherin-mediated cell-cell adhesion also plays an important role in skeletal myogenesis. In the present study, we found that both genes were differentially expressed in C2C12 and Sol8 myoblasts during myogenic differentiation and that the expression of M-cadherin was preferentially enhanced in slow-twitch muscle. Interestingly, most MRFs (myogenic regulatory factors) significantly activated the promoter of M-cadherin, but not that of N-cadherin. In line with this, overexpression of MyoD in C3H10T1/2 fibroblasts strongly induced endogenous M-cadherin expression. Promoter analysis in silico and in vitro identified an E-box (from -2 to +4) abutting the transcription initiation site within the M-cadherin promoter that is bound and differentially activated by different MRFs. The activation of the M-cadherin promoter by MRFs was also modulated by Bhlhe40 (basic helix-loop-helix family member e40). Finally, chromatin immunoprecipitation proved that MyoD as well as myogenin binds to the M-cadherin promoter in vivo. Taken together, these observations identify a molecular mechanism by which MRFs regulate M-cadherin expression directly to ensure the terminal differentiation of myoblasts.