Analysis of ciliary neurotrophic factor and muscarinic receptor signaling in the adaptive plasticity of motor nerve terminals
Authors:   , Wright Megan Christine , Son Young-Jin 
Published: July 2024

Disruption of innervation or neural activity to adult muscle results in a naturally occurring compensatory process at the vertebrate neuromuscular junction (NMJ) known as "terminal sprouting." Terminal sprouting involves the formation of new neuritic processes, or sprouts, from intact motor nerve terminals that navigate the muscle fiber surface, form new synaptic connections on inactive muscle fibers, and reestablish muscle innervation. Examination of this robust nerve terminal sprouting and regeneration that occurs following injury to the NMJ provides the opportunity to advance our understanding of the compensatory capabilities of the adult nervous system and expands our knowledge of synaptic plasticity common throughout the nervous system. Additionally, findings from these studies may offer potential avenues to treat pathological conditions affecting nerve-muscle synapses and provide insight into the ways to promote synaptic regeneration elsewhere in our nervous system. Nevertheless, our understanding of the cellular and molecular processes underlying the induction and guidance of terminal sprouting remains insufficient. The goal of this thesis work was to examine how two putative sprouting inducers, ciliary neurotrophic factor (CNTF) and muscarinic acetylcholine receptors (mAChRs), contribute to the initiation of reactive plasticity of motor nerve terminals at the adult NMJ. Using anatomical, pharmacological and genetic approaches, I have performed a series of experiments that tested the hypothesis that initiation of nerve terminal sprouting is a process mediated by two possible mechanisms: injury-induced release of diffusible factors, specifically CNTF and/or reduced mAChR signaling at the NMJ. Results from Chapters 2 and 3 suggests that CNTF is dispensable for the initiation of terminal sprouting and that terminal sprouting is likely initiated by muscle associated, surface bound molecules rather than diffusible factors such as CNTF. First, terminal sprouting was similar in CNTF -/- and CNTF +/+ mice following a number of sprout-inducing conditions. Second, while CNTF was capable of inducing robust nerve terminal sprouting, the sprouting was spatially restricted to the original synaptic area. Third, comparison of sprouting elicited by Botulinum toxin-induced and CNTF-induced sprouting suggests that sprouting associated with muscle paralysis is initiated by surface bound molecules present on inactive muscle fibers. Finally, co-administration of CNTF and BoTX elicited exceptionally robust extrasynaptic sprouting, further supporting the role of inactive muscle in the initiation of terminal sprout. Results from chapter 4 demonstrate that inhibition of muscarinic acetylcholine receptor (mAChR) signaling, using both pharmacological and genetic means, can induce terminal sprouting. However this sprouting appears to differ from sprouting elicited by muscle paralysis and partial denervation. Quite unexpectedly, inhibition of mAChR signaling was also found to induce other neuromuscular effects including synapse disassembly and muscle atrophy. Taken together, results from these studies have significantly advanced our understanding of the cellular and molecular processes underlying the induction of compensatory sprouting and in addition have provided important insights into mechanisms regulating NMJ maintenance and plasticity.