Charcot Marie Tooth 2D (CMT2D) is caused by dominant mutations in the glycine RNA synthetase gene (GlyRS). Despite the ubiquitous requirement for this cytoplasmic enzyme in generating glycine-tRNA for translation, mutations cause selective peripheral neuron axon degeneration leading to motor and sensory deficits. Although CMT2D was initially thought to be caused by inadequate GlyRS enzyme, previous genetic studies indicate that CMT2D is caused by a neomorphic function of GlyRS (i.e. gain-of-function). The mechanism of action by which mutant GlyRS causes CMT2D is not known and therefore is the focus of this grant. Preliminary in vitro and biochemical data from our collaborator Xiang-Lei Yang's lab has unmasked a heretofore unknown biological pathway in which GlyRS is secreted and binds competitively with VEGF to the Nrp1 receptor. Since motor neurons express Nrp1, and VEGF isoform 164 has neurotrophin-activity, they hypothesize that the novel extracellular interaction between mutant GlyRS and Nrp1 underpins CMT2D. The goal in this exploratory R21 grant is to establish whether key aspects of their hypothesis are correct in order to determine if VEGF can be used as a therapeutic agent for CMT2D disease. The four aims are (1) to establish whether GlyRS (normal and mutant protein) is secreted in mice, (2) to characterize the motor deficits in CMT2D mice and determine if GlyRS genetically interacts with VEGF and Nrp1 in vivo, (3) to assay the influence of GlyRS on Nrp1 signaling, and (4) to express VEGF in CMT2D mouse muscles using AAV-VEGF vectors and assay whether the progression of peripheral neuropathy (i.e. axon loss) is slowed or prevented. These studies may lead to a paradigm shift in our understanding of normal GlyRS function as an extracellular ligand in a broad array of biological contexts, but the main goal here is to determine whether mutant GlyRS has acquired a neomorphic activity as a VEGF antagonist of Nrp1 signaling in motor neurons. This grant aims to establish sufficient preliminary data to justify further investigation of VEGF as a feasible and novel treatment for CMT2D.
Hereditary motor and sensory neuropathies affect 1 in 2,500 people, but the treatments for this class of diseases are limited because the underlying causes are not well understood. We will investigate how mutations in the ubiquitous glycine aminoacyl-tRNA synthetase (GlyRS) causing Charcot-Marie Tooth (CMT) 2D neuropathy lead to deficits in distal motor function (i.e. hand weakness). Our studies will provide evidence for an entirely new mechanism of GlyRS activity based on the unexpected finding that this cytoplasmic protein is also secreted and competes for binding with VEGF to the neuropilin receptor ? a paradigm-shifting discovery that suggests CMT2D can be treated using VEGF.