Persistance of regenerative myogenesis in spite of down-regulation of activity-dependent genes in long-term denervated rat muscle.

Contrary to general expectation, in humans we have recently shown that after complete Conus Cauda lesion, the lower motoneuron denervated myofibers may survive several years. In adult rats, the sciatectomized muscle progresses in 4-6 months from severe atrophy to a dystrophic stage and undergoes a dramatic weight loss; during this process, myofiber death/regeneration processes maintain a decreasing population of very small, but vital myofibers. At the same time, in vitro electrophysiological recordings show that denervated fibers can maintain membrane excitability longer than they can retain contractile properties. A certain level of myofiber regeneration seems to have a role in the process, with the early re-expression of embryonic subunits of integrins and acetylcholine receptor subunits. In the present work, using the reliable Real-Time quantitative PCR, we confirm the long-lasting occurrence of myoblast proliferation-dependent events and their focal nature. In fact, we show here that in sciatectomized muscle the expression of 12 selected genes was differentially regulated at 3- and 9-month denervation. At both time points, indexes of muscle activity/inactivity and tissue re-modeling (proteolysis, energy usage, angiogenic factors) were down-regulated, while indexes of regenerative myogenesis (Myogenin, MyoD, Mrf4, MHCemb) were up-regulated. Immunohistochemistry with anti-MHCemb and anti-N-CAM monoclonal antibodies show that such regeneration events were focally distributed. We conclude that myofiber regeneration is a non-compensatory mechanism, which prolongs the chance of re-innervation during long lasting denervation. It may also contribute to muscle recovery in paraplegic patients, even when rehabilitation strategies based on Functional Electrical Stimulation (FES) start late after Spinal Cord Injury (SCI)