Mutations in glycyl-tRNA synthetase impair mitochondrial metabolism in neurons

Abstract

The nuclear-encoded glycyl-tRNA synthetase gene (GARS) is essential for protein translation in both cytoplasm and mitochondria. In contrast, different genes encode the mitochondrial and cytosolic forms of most other tRNA synthetases. Dominant GARS mutations were described in inherited neuropathies, while recessive mutations cause severe childhood-onset disorders affecting skeletal muscle and heart. The downstream events explaining tissue-specific phenotype–genotype relations remained unclear. We investigated the mitochondrial function of GARS in human cell lines and in the GarsC210R mouse model. Human-induced neuronal progenitor cells (iNPCs) carrying dominant and recessive GARS mutations showed alterations of mitochondrial proteins, which were more prominent in iNPCs with dominant, neuropathy-causing mutations. Although comparative proteomic analysis of iNPCs showed significant changes in mitochondrial respiratory chain complex subunits, assembly genes, Krebs cycle enzymes and transport proteins in both recessive and dominant mutations, proteins involved in fatty acid oxidation were only altered by recessive mutations causing mitochondrial cardiomyopathy. In contrast, significant alterations of the vesicle-associated membrane protein-associated protein B (VAPB) and its downstream pathways such as mitochondrial calcium uptake and autophagy were detected in dominant GARS mutations. The role of VAPB has been supported by similar results in the GarsC210R mice. Our data suggest that altered mitochondria-associated endoplasmic reticulum (ER) membranes (MAM) may be important disease mechanisms leading to neuropathy in this condition.

Neurodegenerative Charcot–Marie–Tooth disease as a case study to decipher novel functions of aminoacyl-tRNA synthetases

“…It is worth noting that the two anticodon binding domain mutations S581L and G598A induce much stronger aberrant interactions with HDAC6 than other patient mutations in the catalytic domain of GlyRS (96). The S581L and G598A patients have more severe distal weakness and wasting in the lower limbs (33)(34)(35)(36)(37), in contrast to the upper limb predominance found in other CMT2D patients (22,28). Thus, the aberrant GlyRS-HDAC6 interaction appears to correlate with the divergent clinical presentation among CMT2D patients.…”

“…Two anticodon binding domain mutations (i.e. S581L and G598A) lack strong genetic evidence but are recurrently identified in patients with neuropathy phenotypes (33)(34)(35)(36)(37).…”

“…For example, Gars P234KY/P234KY mice are not viable; Gars C157R/C157R mice can be born, but they show reduced viability and early death (60,61). Compared with heterozygous Gars C157R/ϩ mice, Gars C157R/C157R mice have more severe neuropathies and also present abnormalities in the central nervous system, possibly related to the central nervous system phenotypes observed in patients with bi-allelic GlyRS mutations (35,37,69). Importantly, overexpression of WT GARS was able to rescue the perinatal death, but not the neuropathy phenotypes of the homozygous Gars C157R/C157R mice, again supporting that the neuropathy phenotype is caused by a toxic gain-of-function effect, whereas the phenotypes beyond neuropathy are caused by a loss of function, which may include enzymatic and/or nonenzymatic activities (Fig.…”

“…4 In the cytosol, TyrRS CMT and GlyRS CMT repress protein translation in motor and sensory neurons through an unknown mechanism independent of tRNA aminoacylation (41). The dual-localized GlyRS CMT also inhibits translation in mitochondria in mice and patient-induced neuronal progenitor cells (35). GlyRS CMT aberrantly interacts with HDAC6, leading to hypo-acetylation of ␣-tubulin and axonal transport deficits (96,97).…”

“…A neuron-specific mitochondrial defect was detected in the induced neuronal progenitor cells of a CMT2D patient carrying a dominant mono-allelic mutation H162R GlyRS (reported as H216R due to the inclusion of the mitochondrial targeting sequence) (35). The defects include reduced levels of both mtDNA and nucleus-encoded mitochondrial respiratory chain complexes and decreased mitochondrial respiration and a reduced level of vesicle-associated membrane protein-associated protein B (as part of the mitochondria-associated endoplasmic reticulum membrane complex) and its downstream signaling, including mitochondrial calcium uptake and autophagy.…”