Trk receptor signaling and sensory neuron fate are perturbed in human neuropathy caused by Gars mutations

Abstract

Charcot-Marie-Tooth disease type 2D (CMT2D) is a peripheral nerve disorder caused by dominant, toxic, gain-of-function mutations in the widely expressed, housekeeping gene, GARS. The mechanisms underlying selective nerve pathology in CMT2D remain unresolved, as does the cause of the mild-to-moderate sensory involvement that distinguishes CMT2D from the allelic disorder distal spinal muscular atrophy type V. To elucidate the mechanism responsible for the underlying afferent nerve pathology, we examined the sensory nervous system of CMT2D mice. We show that the equilibrium between functional subtypes of sensory neuron in dorsal root ganglia is distorted by Gars mutations, leading to sensory defects in peripheral tissues and correlating with overall disease severity. CMT2D mice display changes in sensory behavior concordant with the afferent imbalance, which is present at birth and nonprogressive, indicating that sensory neuron identity is prenatally perturbed and that a critical developmental insult is key to the afferent pathology. Through in vitro experiments, mutant, but not wild-type, GlyRS was shown to aberrantly interact with the Trk receptors and cause misactivation of Trk signaling, which is essential for sensory neuron differentiation and development. Together, this work suggests that both neurodevelopmental and neurodegenerative mechanisms contribute to CMT2D pathogenesis, and thus has profound implications for the timing of future therapeutic treatments.aminoacyl-tRNA synthetase | Charcot-Marie-Tooth disease | distal spinal muscular atrophy type V | neuromuscular disease | neurodevelopment C harcot-Marie-Tooth disease (CMT) is a group of genetically diverse peripheral neuropathies that share the main pathological feature of progressive motor and sensory degeneration (1). Although lifespan is usually unaffected, patients display characteristic muscle weakness and wasting predominantly in the extremities, leading to difficulty walking, foot deformities, and reduced dexterity (2). CMT is traditionally divided into type 1/ demyelinating CMTs that display loss of peripheral nerve myelin causing reduced nerve conduction velocity (NCV), type 2/axonal CMTs typified by axon loss with relatively normal NCVs, and intermediate CMTs that share clinical features of CMT1 and -2 (1). Over 80 different genetic loci have been linked to CMT, which is known to affect ∼1/2,500 people, making it the most common group of hereditary neuromuscular disorders (3).Dominant mutations in the glycyl-tRNA synthetase (GlyRS) gene, GARS, are causative of CMT type 2D (CMT2D) [Online Mendelian Inheritance in Man (OMIM) 601472], which normally manifests during adolescence and presents with muscle weakness in the extremities (4). The 2D subtype is one of a number of CMTs associated with mutation of an aminoacyl-tRNA synthetase (ARS) gene (5-8). Humans possess 37 ARS proteins, which covalently link amino acids to their partner transfer RNAs (tRNAs), thereby charging and priming the tRNAs for protein synthesis.This housekeeping function of gl...

Regulation of mRNA Translation in Neurons—A Matter of Life and Death

“…However, the underlying molecular mechanism of these disorders has remained elusive. Many CMT-linked mutations seem to have no significant effect on the aminoacylation activity of the synthetase, and recent studies support a toxic gain of function mechanism, possibly involving the exposure of a neomorphic binding surface and resulting in aberrant protein interactions (He et al, 2015; Motley et al, 2011; Seburn et al, 2006; Sleigh et al, 2017). In contrast, other CMT-linked mutations have been reported to cause a loss of charging activity, suggesting that haploinsufficiency of the enzyme may be a possible contributing factor (Griffin et al, 2014; McLaughlin et al, 2012).…”

SIRT2-knockdown Rescues GARS-induced Charcot-Marie-Tooth Neuropathy

“…Currently, two genetics experiments in flies and mice have demonstrated that dominant mutations in GARS cause CMT through toxic gain-of-function effects [10,18]. Several binding partner of CMT mutants have been discovered, including Nrp1 [25], Trk [26] and HDAC6 [27,28]. Here we presented that SIRT2, the acetylated tubulin deacetylase, is able to bind tightly with wildtype GARS comparing with CMT mutants, we didn't rule out the possibility that CMT mutants might gain the functions to increase deacetylation activity of SIRT2 which leading to decrease the acetylated tubulin.…”

Altered sensory neuron development in CMT2D mice is site-specific and linked to increased GlyRS levels

“…In addition to muscle weakness, this is followed by compromised reflexes and loss of sensation to vibration, touch, temperature and pin-prick (Sivakumar et al, 2005). These symptoms are well replicated in Gars-neuropathy mice, highlighting their potential for studying sensory pathomechanisms (Sleigh et al, 2017a).…”

“…Moreover, all assessed GlyRS mutants possess a similar conformational opening that excavates neomorphic surfaces usually buried within the structure of the wildtype enzyme (He et al, 2011(He et al, , 2015. Given that GlyRS is secreted from several different cell types (Park et al, 2012;Grice et al, 2015;He et al, 2015;Park et al, 2018), these uncovered protein regions can mediate aberrant deleterious interactions both inside and outside the cell (He et al, 2015;Sleigh et al, 2017a;Mo et al, 2018), likely accounting for non-cell autonomous aspects of pathology (Grice et al, 2015(Grice et al, , 2018. While some of these misinteractions are with neuronally-enriched proteins, the pathomechanisms underlying neuronal selectivity in CMT2D remain unresolved.…”

“…These mice display loss of lower motor neuron connectivity and disturbed neurotransmission at the neuromuscular junction (NMJ), causing muscle weakness and motor function deficits (Sleigh et al, 2014a;Spaulding et al, 2016). Furthermore, there appears to be a pre-natal perturbation of sensory neuron fate in dorsal root ganglia (DRG), such that CMT2D mice have more nociceptive (noxious stimulus-sensing) neurons and fewer mechanosensitive (touch-sensing) and proprioceptive (body position-sensing) neurons (Sleigh et al, 2017a). Perhaps causing this and providing rationale for neuronal selectivity, mutant GlyRS mis-interacts with the extracellular region of tropomyosin receptor kinase (Trk) receptors.…”