Mutations in the Caenorhabditis elegans orthologs of human genes required for mitochondrial tRNA modification cause similar electron transport chain defects but different nuclear responses

Abstract

Several oxidative phosphorylation (OXPHOS) diseases are caused by defects in the post-transcriptional modification of mitochondrial tRNAs (mt-tRNAs). Mutations in MTO1 or GTPBP3 impair the modification of the wobble uridine at position 5 of the pyrimidine ring and cause heart failure. Mutations in TRMU affect modification at position 2 and cause liver disease. Presently, the molecular basis of the diseases and why mutations in the different genes lead to such different clinical symptoms is poorly understood. Here we use Caenorhabditis elegans as a model organism to investigate how defects in the TRMU, GTPBP3 and MTO1 orthologues (designated as mttu-1, mtcu-1, and mtcu-2, respectively) exert their effects. We found that whereas the inactivation of each C. elegans gene is associated with a mild OXPHOS dysfunction, mutations in mtcu-1 or mtcu-2 cause changes in the expression of metabolic and mitochondrial stress response genes that are quite different from those caused by mttu-1 mutations. Our data suggest that retrograde signaling promotes defect-specific metabolic reprogramming, which is able to rescue the OXPHOS dysfunction in the single mutants by stimulating the oxidative tricarboxylic acid cycle flux through complex II. This adaptive response, however, appears to be associated with a biological cost since the single mutant worms exhibit thermosensitivity and decreased fertility and, in the case of mttu-1, longer reproductive cycle. Notably, mttu-1 worms also exhibit increased lifespan. We further show that mtcu-1; mttu-1 and mtcu-2; mttu-1 double mutants display severe growth defects and sterility. The animal models presented here support the idea that the pathological states in humans may initially develop not as a direct consequence of a bioenergetic defect, but from the cell’s maladaptive response to the hypomodification status of mt-tRNAs. Our work highlights the important association of the defect-specific metabolic rewiring with the pathological phenotype, which must be taken into consideration in exploring specific therapeutic interventions.

Spaceflight Activates Autophagy Programs and the Proteasome in Mouse Liver

“…Defects in tRNA modifications and modification enzymes are associated with oxidative stress [ 40 ] and human diseases including cancer, diabetes and cardiomyopathy [ 38 ]. Indeed, results from a recent study in Caenorhabditis elegans associating defects in post-transcriptional modification of mitochondrial tRNAs with dysfunctional oxidative phosphorylation suggest that the cell's maladaptive response to hypomodified mitochondrial tRNAs may be a mechanism underlying disease development [ 41 ]. Although speculative, the idea that exposure to the space environment may lead to aberrant tRNA post-transcriptional modifications is provocative and warrants further investigation.…”

Human Elp3/Kat9 is a mitochondrial tRNA modifying enzyme

“…To evaluate a possible role of mt-ELP3 in modifying mt-tRNAs, we analyzed the sensitivity to digestion with the tRNA-specific RNase angiogenin of mt-tRNA Glu , mt-tRNA Lys and mt-tRNA Leu (with the τm5U modification) and mt-tRNA Val (without the τm5U modification) from control cells and from cells overexpressing mt-ELP3. tRNAs lacking modification at the wobble position are more sensitive toward angiogenin-mediated digestion than the corresponding modified tRNAs (Lefkimmiatis et al, 2019) (Navarro-González et al, 2017)(R. Boutoual et al, 2018). After in vitro angiogenin digestion of total RNA purified from control cells and cells overexpressing mt-ELP3, the digested products were analyzed using northern blot analysis with a specific digoxigenin (DIG)-labeled oligodeoxynucleotide probe for the mentioned mt-tRNAs.…”

Defects in the mitochondrial-tRNA modification enzymes MTO1 and GTPBP3 promote different metabolic reprogramming through a HIF-PPARγ-UCP2-AMPK axis

“…This approach was based on previous findings indicating that loss of the U34 modification at position 5 increases the angiogenin-mediated cleavage of the E. coli tRNA Lys , which is a substrate for the MTO1 and GTPBP3 bacterial orthologs 18 . This qualitative approach has proven to be useful in analysing the modification status of mt-tRNAs obtained from GTPBP3 knocked-down cells 18 and Caenorhabditis elegans strains carrying a deletion mutation in the GTPBP3 or MTO1 homolog 17 . As shown in Fig.…”

“…The reasons why TRMU and GTPBP3 or MTO1 mutations produce different clinical outcomes is currently unknown. We have proposed that these defects affect retrograde signalling from mitochondria to nucleus in a different manner, which results in different tissue-dependent nuclear responses and, consequently, in different phenotypes 17 . In this respect, we have reported that stable silencing of GTPBP3 triggers an AMPK-dependent retrograde signalling pathway, which down-regulates the mitochondrial pyruvate carrier (MPC), while up-regulating the expression of the uncoupling protein 2 (UCP2) and genes involved in glycolysis and fatty acid oxidation 18 .…”