Date of Award

Fall 12-20-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical Sciences

Advisor

Robert Burgess

Second Committee Member

Gregory Cox

Third Committee Member

Susan Ackerman

Additional Committee Members

Sandra Rieger

Roger Sher

Abstract

Charcot-Marie-Tooth disease (CMT) is a debilitating inherited peripheral neuropathy resulting in progressive distal muscle atrophy and loss of sensation. CMT is genetically heterogeneous, with mutations in over 80 different genes leading to demyelinating or axonal forms. There are genetically similar subgroups, including the largest protein family implicated in the disease, the tRNA synthetases (ARSs). ARSs are responsible for aminoacylation of tRNAs during translation and are therefore ubiquitously expressed and essential proteins. Dominant mutations in at least five ARSs cause axonal forms of CMT. How mutations in ARSs cause CMT is unclear, however, the similar clinical presentation of patients suggests shared disease mechanisms. To investigate peripheral axon sensitivity to dominant mutations in ARSs, we first performed an extensive examination of motor axon terminals in two mouse models of CMT type 2D, caused by mutations in glycyl-ARS (GARS). Our findings reveal a progressive, presynaptic dysfunction at the mutant neuromuscular junction that correlates with fewer acetylcholine vesicles, release sites, and mitochondria. One of the proposed disease mechanisms of mutant ARSs is through gain-of-function impaiments in translation. Because all ARSs participate in translation, impairment in this process is an attractive disease mechanism to test in mammalian models of ARS-associated CMT. To this end we have profiled translation and transcription in motor neurons of three CMT2D mouse models. This profiling has revealed global impairments in translation in mutant Gars motor neurons. Identification of the integrated stress response (ISR) only in the largest motor and sensory peripheral neurons has further refined our understanding of the cell type-specificity of CMT2D. Activation of the ISR occurs in these cells through the translational homeostasis-sensing kinase, GCN2, indicating that GCN2 is responding to impairments in translation. Genetic removal of GCN2 alleviates mutant Gars neuropathy, suggesting that chronic activation of the ISR contributes to CMT2D. The ISR is also activated in motor neurons of mice with dominant mutations in Yars, a model of CMT type C. These data support impairments in translation as a disease mechanism in mice with dominant mutations in Gars and Yars and have increased our understanding of the cellular and molecular pathways leading to motor axon degeneration.

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