Lang, Qiuhan; (2025) Investigating the role of neurotrophins in regulating axonal transport and morphology of mitochondria in Charcot-Marie-Tooth disease. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Charcot-Marie-Tooth disease (CMT) is a form of genetic peripheral neuropathy that affects motor and sensory nerves and usually manifests during adolescence. CMT type 2D (CMT2D) is caused by dominant mutations in GARS1, which encodes glycyl-tRNA synthetase (GlyRS). The Sleigh Laboratory recently showed that CMT2D-causing mutant GlyRS aberrantly interacts with the extracellular domain of the BDNF neurotrophin receptor TrkB on motor neuron terminals. This interrupts pro-survival BDNF-TrkB signalling, and causes early and persistent impairments in signalling endosome axonal transport in CMT2D mice.This thesis aims to investigate whether the transport disruption is organelle-specific by assessing the role of BDNF-TrkB signalling in regulating mitochondrial trafficking and morphology in CMT2D mice. GarsC201R/+ and Gars∆ETAQ/+ were crossed with Thy1-CFP-MitoS mice, which selectively express cyan fluorescent protein in neuronal mitochondria. In vivo imaging was performed in anaesthetised mice to visualise axonal mitochondria in sciatic nerves at one and three months of age – representing early and later symptomatic timepoints. For these studies, new methods were developed to enable transport assessment in forelimb-innervating nerves and to perform semi-automated evaluation of mitochondrial morphology. Anterograde mitochondrial transport speed was increased at one month in GarsC201R/+ mice, followed by a decrease at three months. In the more severe Gars∆ETAQ/+ mice, anterograde mitochondrial speed was also decreased at three months, but retrograde transport was slower at both timepoints. Only Gars∆ETAQ/+ mice showed differences in mitochondrial morphology within axons, while GarsC201R/+ mice displayed mitochondrial morphology differences in motor neuron terminals. Key neurotrophin proteins in sciatic nerves were studied by western blotting. Boosting BDNF levels at motor nerve terminals using AAV gene therapy was tested to see whether it rescues the identified mitochondrial phenotypes in CMT2D mice. Overall, this thesis aims to provide in vivo understanding of the role of neurotrophins in regulating axonal transport and morphology of mitochondria in CMT2D pathogenesis.

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