Regulation of axonal transport by neurotrophic factors in health and disease

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Charcot-Marie-Tooth disease (CMT) is a debilitating, inherited neuromuscular condition that is characterised by muscle weakness and sensation deficiencies mainly in the hands and feet. The mechanisms linking gene alterations (mutations) to this neurodegeneration remain unresolved, and there are currently no effective treatments for the disease. To better understand CMT and develop therapies, I study the 2D subtype (CMT2D) caused by mutations in the gene GARS, which produces glycyl-tRNA synthetase (GlyRS) protein. GlyRS is found in almost all cells of the body, yet it is the nerves responsible for movement (motor neurons) and sensation (sensory neurons) that are affected by the disease. By understanding exactly why these peripheral nerves deteriorate, we will be better able to engineer targeted, molecular therapies.It has previously been shown that CMT2D-linked mutations in GARS affect the structure of the resulting GlyRS protein. I recently discovered that this conformational change causes mutant GlyRS to aberrantly interact with specific proteins called tropomyosin receptor kinase (Trk) receptors found on the surface of nerve cells. Trk receptors are crucial to life because they bind to survival molecules called neurotrophins, hence they are also known as neurotrophin receptors. I have shown that mutant GlyRS binding to Trk proteins disturbs the usual nerve response to neurotrophins, impairing sensory nervous system development in CMT2D mice. In addition, this non-physiological interaction leads to defects in a process called axonal transport, which is also essential for nerve cell function and survival. I have successfully treated these defects in CMT2D mice by injecting purified neurotrophic factors to overcome the detrimental impact of mutant GlyRS binding. This work has not only identified a possible way to treat CMT2D, but it has uncovered a previously unappreciated role for neurotrophic factors in regulating the dynamics of axonal transport.I now propose to unravel the molecular mechanism responsible for the modulation performed by neurotrophic factors on axonal transport in healthy nerves and, in the process, determine exactly how mutant GlyRS binding to Trk receptors reduces nerve survival in CMT2D.I will begin by testing the effects of a variety of neurotrophic factors on axonal transport in healthy and CMT2D mice in order to identify the signalling nodes most relevant to this essential cellular process. Complementing this, I will assess where the neurotrophic factor receptors are found in the muscle, and determine whether receptor localisation and levels are linked with the susceptibility of different muscles to weakness and degeneration in CMT2D mice. I will then grow nerve cells in tissue culture and use them to determine exactly how impairments in neurotrophic factor-regulated pathways disrupt axonal transport. Finally, I will design gene therapies, which will be delivered by harmless viruses, to boost levels of specific neurotrophic factors in CMT2D mice muscles and test this strategy as a potential therapy for the disease.This proposal will not only enhance our understanding of how neurotrophic factors regulate axonal transport in healthy nerves, but it has the very real potential of generating innovative gene therapies for CMT.