EFFECTS OF DIVALENT-CATIONS ON M-CADHERIN EXPRESSION AND DISTRIBUTIONDURING PRIMARY RAT MYOGENESIS IN-VITRO

In the process of myogenesis, cadherins are thought to be involved in the initial cell-cell recognition and possible initiation of myoblast fusion to form multinucleated myotubes. Of the cadherins, M-cadherin, but not N-cadherin, is down-regulated upon inhibition of myogenesis, s uggesting that M-cadherin may be a key receptor involved in myogenesis . M-cadherin binds in a calcium-dependent manner, and depletion of div alent cations inhibits myoblast fusion. We analyzed the regulation of M-cadherin protein and mRNA levels in primary rat myogenic cultures in the presence and absence of divalent cations. In untreated cultures M -cadherin was localized to various myogenic cell-cell contacts. M-cadh erin protein and mRNA levels showed a peak at day 2 after the initiati on of growth. When divalent cations were removed from the cell culture medium, myoblast fusion was inhibited and immunocytochemical analysis revealed a failure of M-cadherin to localize to cell-cell contacts. A nalysis of M-cadherin protein and mRNA in fusion-inhibited cultures st ill revealed a peak at day 2. However, by day 3, M-cadherin protein le vels in the fusion-inhibited cultures were reduced in both the deterge nt-soluble and -insoluble fractions in comparison with the untreated c ultures. Interestingly, beta-catenin, a protein associated with cadher ins, was frequently observed at intercellular contacts in the fusion-i nhibited cultures. We could also show that the intracellular levels of beta-catenin protein remained constant regardless of the presence or absence of divalent cations. In summary, the dynamic regulation of M-c adherin in muscle-fusion-related events is an indication of the import ance of M-cadherin for myoblast fusion and myogenic differentiation.