Premature satellite cell activation before injury accelerates myogenesis and disrupts neuromuscular junction maturation in regenerating muscle.

: Satellite cell (SC) activation, mediated by nitric oxide (NO) is essential to myogenic repair, while myotube function requires innervation. Semaphorin (Sema)3A, a neuro-chemorepellent, is thought to regulate axon guidance to neuromuscular junctions (NMJs) during myotube differentiation. We tested whether "premature" SC activation (SC activation before injury) by a NO donor (isosorbide dinitrate) would disrupt early myogenesis and/or NMJs. Adult muscle was examined during regeneration in two models of injury: myotoxic cardiotoxin (CTX) and traumatic crush (CR) (n=4-5/group). Premature SC activation was confirmed by increased DNA synthesis by SCs immediately in pretreated mice after CTX injury. Myotubes grew faster after CTX than CR; growth was accelerated by pretreatment. NMJ maturation, classified by silver histochemistry (neurites) and acetylcholinesterase (AchE), and alpha-bungarotoxin staining (Ach receptors, AchRs) was delayed by pretreatment, consistent with a day-6 rise in the denervation marker, γAchR. With pretreatment, S100B from terminal Schwann cells (TSCs) increased 10-20-fold at days 0 and 10 after CTX and doubled 6 days after CR. Premature SC activation disrupted motoneuritogenesis 8-10 days post-CTX, as pretreatment reduced colocalization of pre- and post-synaptic NMJ features and increased Sema3A-65. Premature SC activation before injury both accelerated myogenic repair and disrupted NMJ remodeling and maturation, possibly by reducing Sema3A neuro-repulsion and altering S100B. This interpretation extends the model of Sema3A-mediated motoneuritogenesis during muscle regeneration. Manipulating the timing and type of Sema3A by brief NO effects on SCs suggests an important role for TSCs and Sema3A-65 processing in axon guidance and NMJ restoration during muscle repair.