G.P.13.13 Age-related abnormalities and reduced expression of the Notch ligand Delta in IBM primary muscle cultures. A clue for diminished regenerative potential of satellite cells in IBM muscle?

Satellite cell activation is controlled by Notch signalling pathway with rapid increase of the ligand Delta following injury. In old muscle, upregulation of Delta is blunted and satellite cells activation is markedly reduced. Inclusion-body myositis is considered an age-related myopathy and various muscle changes of normal aging are thought to participate in its pathogenesis. It is known that satellite-cell derived cultured IBM myoblasts proliferate and differentiate normally and can be properly innervated. Although regenerative potential of IBM muscle is reduced, an increased age-related decline in muscle repair ability of IBM resident stem cells has not been demonstrated. We studied primary muscle cultures from 5 IBM patients and 3 age-matched controls. IBM myoblasts did not show any morphological abnormality and maintained their proliferation ability up to 6 passages. However, they started to undergo senescence earlier than controls, as indicated by telomerase assays showing shorter telomeres. Moreover, proliferation rate of IBM myoblasts was significantly lower than normal controls, as well as their doubling time and their clonogenicity. All IBM myoblasts fully differentiated into multinucleated myosin-positive myotubes, as confirmed also by normal activation of skeletal muscle differentiation pathways. By bromodeoxyuridine (BrdU) incorporation we demonstrated that virtually all myotubes were terminally differentiated with no BrdU-positive nuclei. Increased levels of the Notch-ligand Delta were detected in normal myoblasts, while significantly lower levels were observed in IBM myoblasts at the same time points. Our data show that when compared with cultured muscle of similar age, IBM cultured cells show premature senescence and lower clonogenicity and indicate that disregulation of the Notch patway may account for in vivo defective satellite cell activation and diminished regenerative potential. Modulation of extrinsic cues acting on Notch pathway may have important therapeutic implications in IBM both to restore endogenous regenerative potential of the aged muscle and to avoid a diminished response of transplanted stem cells in the aged muscle environment.