Mutations in glycyl-tRNA synthetase impair mitochondrial metabolism in neurons

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.R.H. is a Wellcome Investigator (109915/Z/15/Z) supported by the Wellcome Centre for Mitochondrial Research (203105/Z/16/Z), who receives support from the Medical Research Council (UK) (MR/N025431/1), the European Research Council (309548) and the Mitochondrial European Educational Training (MEET), ITN MARIE CURIE PEOPLE, (317433). H.L. receives funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement No. 305444 (RD-Connect), 305121 (Neuromics), the Wellcome Trust Pathfinder Scheme (201064/Z/16/Z) and the Newton Fund (UK/Turkey, MR/N027302/1). H.J.G., A.R. and R.P.Z. acknowledge the financial support by the Ministeriumfür Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, the SenatsverwaltungfürWirtschaft, Technologie und Forschung des Landes Berlin, and the BundesministeriumfürBildung und Forschung. H.G.J. further thanks the CAPES Foundation for financial support