Cellular Mechanisms Regulating Neuromuscular Junction Stability and Plasticity in Normal and Diseased Mice
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The stability of the neuromuscular junction (NMJ) is critical for maintaining independence and quality of life. Therefore, understanding the mechanisms that underlie this stability is of paramount importance to be able to recover function lost due to disease or injury. This thesis seeks to investigate these NMJ stability mechanisms in three states: disease, homeostasis, and repair. First, amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by distal neuropathy at the NMJ followed by paralysis and motor neuron (MN) death. I found that terminal Schwann cells (TSCs) at a subset of muscle fibres (type IIb, the most susceptible to disease progression in ALS) vanished from denervated endplates after a partial denervation injury in a mouse model of ALS. These endplates were therefore unable to become reinnervated via a compensatory process called collateral reinnervation. I also found that infiltrating macrophages were interacting with these TSCs, and, when inhibited with masitinib, was able to recover TSC loss and reinnervation in ALS mice. Second, I investigated whether muscular BMP4 in a mouse model had similar roles in NMJ homeostasis as had been previously reported in Drosophila. NMJs lacking BMP4 had impaired synaptic transmission and fragmented acetylcholine receptor clusters. This resulted in mice with locomotor, strength, and proprioceptive impairments. These results correlate with the previous Drosophila literature and provide evidence that the role of this protein at the NMJ is highly conserved. Third, I examined whether muscular BMP4 has a role in neuromuscular reinnervation following Tibial n. crush injury. I found that reinnervated NMJs at muscles lacking BMP4 had impaired reinnervation and delayed withdrawal of polyneuronal innervation. These results were consistent with a delayed maturation phenotype, indicating that BMP4 has similar roles during repair as it does during development. Together, this thesis highlights the complexity of mechanisms that exist at the NMJ to maintain proper function and reveals potential targets to exploit to improve outcomes in ALS as well as following peripheral nerve injury.