Peripheral neuropathy via mutant tRNA synthetases: Inhibition of protein translation provides a possible explanation

Abstract

Recent evidence indicates that inhibition of protein translation may be a common pathogenic mechanism for peripheral neuropathy associated with mutant tRNA synthetases (aaRSs). aaRSs are enzymes that ligate amino acids to their cognate tRNA, thus catalyzing the first step of translation. Dominant mutations in five distinct aaRSs cause Charcot-Marie-Tooth (CMT) peripheral neuropathy, characterized by length-dependent degeneration of peripheral motor and sensory axons. Surprisingly, loss of aminoacylation activity is not required for mutant aaRSs to cause CMT. Rather, at least for some mutations, a toxic-gain-of-function mechanism underlies CMT-aaRS. Interestingly, several mutations in two distinct aaRSs were recently shown to inhibit global protein translation in Drosophila models of CMT-aaRS, by a mechanism independent of aminoacylation, suggesting inhibition of translation as a common pathogenic mechanism. Future research aimed at elucidating the molecular mechanisms underlying the translation defect induced by CMT-mutant aaRSs should provide novel insight into the molecular pathogenesis of these incurable diseases.

Keywords: Charcot-Marie-Tooth peripheral neuropathy; aminoacylation; animal model; axonal degeneration; gain-of-toxic-function; tRNA synthetase; translation.

Figure 1

Mutations in tRNA synthetases cause CMT peripheral neuropathy. A: aaRSs catalyze tRNA aminoacylation in a two‐step reaction. In the first step (1), the amino acid (AA) is activated with ATP, resulting in the formation of an enzyme‐bound aminoacyl‐adenylate (AA‐AMP), with the concomitant release of pyrophosphate (PPi). At this stage, aaRSs with pre‐transfer editing activity can remove misacetylated aminoacyl adenylate. In the second step (2), the aminoacyl‐adenylate is transferred to the tRNA, with release of AMP. aaRSs which posses posttransfer editing activity can hydrolyze misacetylated tRNAs to prevent amino acid misincorporation in nascent proteins. B: Distribution of CMT‐associated mutations in aaRSs. Schematic representation of GlyRS, TyrRS, AlaRS, HisRS, and MetRS proteins and their functional domains. CMT‐associated mutations found in single patients are indicated in black, while mutations that co‐segregate with disease in CMT families are labeled red. Mutations labeled green are equivalent to mutations in mouse CMT2D models. For GlyRS, the positions of the mutations refer to the cytoplasmic form of the human protein.

Figure 2

Impaired protein translation in Drosophila CMT‐aaRS models. A: Non‐canonical amino acid tagging (NCAT) for cell‐type‐specific labeling of proteomes in Drosophila. In contrast to endogenous MetRS, a modified MetRS (MetRS*) is able to aminoacylate tRNA^Met with the non‐canonical amino acid azidonorleucine (ANL). When transgenic Drosophila that cell‐type specifically express MetRS* are fed with ANL, ANL will be incorporated in newly synthesized proteins (NSPs) in cells that express MetRS*. After a defined labeling time, relevant tissues are dissected and ANL‐containing proteins are labeled by “click chemistry” with either a fluorescent (FUNCAT) or a biotin tag (BONCAT). Quantification of tagged proteins by fluorescence microscopy or western blot allows to determine the relative amounts of NSPs, which are proportional to the protein synthesis rate. B: CMT‐mutant tRNA synthetases inhibit translation independent of aminoacylation, leading to degeneration of peripheral motor and sensory axons.