Translational Responses of Motor Neurons to Neurodegeneration in ALS Identifies FGF21 as a Critical Myogenic Regulatory Factor That Slows Disease Progression

Abstract

The neuromuscular junction (NMJ) is a chemical synapse that is the site of skeletal muscle innervation by spinal motor neurons and the maintenance of the NMJ is critical for preserving musculoskeletal homeostasis. Under normal physiological conditions, spinal motor neurons have significant regenerative potential and can regrow axons in response to peripheral nerve injury. In diseases such as amyotrophic lateral sclerosis (ALS), the NMJ is dismantled and motor neurons selectively degenerate resulting in progressive muscle wasting and eventual fatal paralysis. Interestingly, some motor neurons are more resistant to degeneration, with slow motor neurons persisting for longer and, in some cases, reinnervating fast, vacant NMJ endplates, underscoring the vital role of motor neurons in supporting skeletal muscle in disease states. In this dissertation we explore the role of motor neurons in skeletal muscle maintenance in ALS. We adapted the RiboTag methodology developed by Sanz et al. to perform ribosomal profiling of motor neurons in two mouse models of ALS. In chapter two we evaluated the translatome of spinal motor neurons in the Ubqln2P497S proteostasis model. The most significant finding from this study was the dramatic downregulation of muscle-related transcripts in motor neuron cell bodies in ALS, raising the possibility of motor neurons translating mRNAs previously thought to be muscle cell-type specific in direct support of the skeletal muscles they innervate. In chapter three, another RiboTag screen comparing a sciatic nerve crush model of acute injury and the Sod1G93A ALS model identified Fgf21, a metabolic and stress-inducible hormone, as one of the most upregulated ALS-specific transcripts. Transgenic mouse models where Fgf21 is conditionally knocked out in Sod1G93A motor neurons showed reduced motor neuron survival and NMJ innervation. Behavioral and survival trials with Sod1G93A mice showed a dramatic reduction in locomotion and lifespan when Fgf21 was conditionally knocked out of motor neurons. Taken together, these data suggest Fgf21 functions in a neuroprotective capacity in ALS pathology. Here we evaluate the functions of Fgf21 and the mechanisms by which it promotes motor neuron survival and skeletal muscle innervation and metabolism.