Exploring the role of a neuro-repellent protein, Semaphorin3A, in functional muscle regeneration through premature satellite cell activation
Communications between satellite cells (SCs) and their niche are important during regeneration after skeletal muscle injury. Semaphorin (Sema) 3A, a neuro-chemorepellent, is thought to regulate axon guidance to neuromuscular junctions (NMJs) during myotube differentiation. In this study I explored the NMJ reformation and the effect of Wallerian degeneration and SC-niche remodeling on SC-TSC interaction which may advance our ability to find new types of treatment for skeletal muscle diseases that could promote regeneration including NMJ restoration. The first part of this research tested whether "premature" SC activation (SC activation before injury) by a NO donor (isosorbide dinitrate) would disrupt early myogenesis and/or NMJs. NMJ maturation was delayed by pretreatment, consistent with a day-6 rise in the denervation marker γ-AchR. With the NO pretreatment, S100B from terminal Schwann cells (TSCs) increased at the post-synaptic area. Premature SC activation before injury both accelerated myogenic repair and disrupted NMJ remodeling and maturation, possibly by reducing Sema3A neuro-repulsion and altering S100B. Since active TSCs generate local cues, facilitating nerve terminal sprouting during re-innervation and muscle regeneration, the next part of my PhD examined whether the loss of SC-specific Sema3A expression would disrupt TSC gene and protein expression after nerve or muscle injury. In situ hybridization experiments were used to evaluate the transcript expression of Sema3A, S100B, P75NGFR and Pax7 with or without immunostaining to identify SCs and TSCs. Western blotting was used to assess levels of Sema3A, γAchR, and S100B proteins. Colocalization studies analyzed by principal component analysis (PCA) of TSC and SC expression. Findings suggested that the number of TSCs increased only with the loss of SC-derived Sema3A. Results, therefore, demonstrate the dynamic nature of TSC populations at the nerve terminal after injury, and their novel signaling with SCs using SC-derived Sema3A. Findings improve our understanding of possible roles for Sema3A-mediated coordination of motor neurite connections to newly formed myofibers during the formation of NMJs. Moreover, these results open the door to future investigation of features in the SC niche that could be applied in designing novel therapeutic approaches to promote muscle regeneration and reinnervation.
Satellite cells, Semaphorin 3A, Myogenesis, reinnervation, Neuromuscular junction, Terminal Schwann cells