Evidence for a myogenic stem cell that is exhausted in dystrophic muscle

Injection of the myotoxin notexin, was found to induce regeneration In musc les that had been subjected to 18 Gy of radiation. This finding was unexpec ted as irradiation doses of this magnitude are known to block regeneration in dystrophic (mdx) mouse muscle. To investigate this phenomenon further we subjected mdx and normal (C57BI/10) muscle to irradiation and notexin trea tment and analysed them in two ways. First by counting the number of newly regenerated myofibres expressing developmental myosin in cryosections of da maged muscles. Second, by isolating single myofibres from treated muscles a nd counting the number of muscle precursor cells issuing from these over 2 day and 5 day periods. After irradiation neither normal nor dystrophic musc les regenerate to any significant extent. Moreover, single myofibres cultur ed from such muscles produce very few muscle precursor cells and these unde rgo little or no proliferation. However, when irradiated normal and mdx mus cles were subsequently treated with notexin, regeneration was observed. In addition, some of the single myofibres produced rapidly proliferative muscl e precursor cells when cultured. This occurred more frequently, and the myo genic cells proliferated more extensively, with fibres cultured from normal compared with dystrophic muscles. Even after 25 Gy, notexin induced some r egeneration but no proliferative myogenic cells remained associated with th e muscle fibres, Thus, skeletal muscles contain a number of functionally distinct population s of myogenic cells. Most are radiation sensitive. However, some survive 18 Gy as proliferative myogenic cells that can be evoked by extreme condition s of muscle damage; this population is markedly diminished in muscles of th e mdx mouse, A small third population survives 25 Gy and forms muscle but n ot proliferative myogenic cells.