Subcellular relocalization and nuclear redistribution of the RNA methyltransferases TRMT1 and TRMT1L upon neuronal activation

RNA modifications are dynamic chemical entities that expand the RNA lexicon and regulate RNA fate. The most abundant modification present in mRNAs, N6-methyladenosine (m⁶A), has been implicated in neurogenesis and memory formation. However, whether additional RNA modifications may be playing a role in neuronal functions and in response to environmental queues is largely unknown. Here we characterize the biochemical function and cellular dynamics of two human RNA methyltransferases previously associated with neurological dysfunction, TRMT1 and its homolog, TRMT1-like (TRMT1L). Using a combination of next-generation sequencing, LC-MS/MS, patient-derived cell lines and knockout mouse models, we confirm the previously reported dimethylguanosine (m^2,2G) activity of TRMT1 in tRNAs, as well as reveal that TRMT1L, whose activity was unknown, is responsible for methylating a subset of cytosolic tRNA^Ala(AGC) isodecoders at position 26. Using a cellular in vitro model that mimics neuronal activation and long term potentiation, we find that both TRMT1 and TRMT1L change their subcellular localization upon neuronal activation. Specifically, we observe a major subcellular relocalization from mitochondria and other cytoplasmic domains (TRMT1) and nucleoli (TRMT1L) to different small punctate compartments in the nucleus, which are as yet uncharacterized. This phenomenon does not occur upon heat shock, suggesting that the relocalization of TRMT1 and TRMT1L is not a general reaction to stress, but rather a specific response to neuronal activation. Our results suggest that subcellular relocalization of RNA modification enzymes may play a role in neuronal plasticity and transmission of information, presumably by addressing new targets.

An intellectual disability-associated missense variant in TRMT1 impairs tRNA modification and reconstitution of enzymatic activity

The human TRMT1 gene encodes an RNA methyltransferase enzyme responsible for catalyzing dimethylguanosine (m2,2G) formation in transfer RNAs (tRNAs). Frameshift mutations in TRMT1 have been shown to cause autosomal-recessive intellectual disability (ID) in the human population but additional TRMT1 variants remain to be characterized. Here, we describe a homozygous TRMT1 missense variant in a patient displaying developmental delay, ID, and epilepsy. The missense variant changes an arginine residue to a cysteine (R323C) within the methyltransferase domain and is expected to perturb protein folding. Patient cells expressing TRMT1-R323C exhibit a deficiency in m2,2G modifications within tRNAs, indicating that the mutation causes loss of function. Notably, the TRMT1 R323C mutant retains tRNA binding but is unable to rescue m2,2G formation in TRMT1-deficient human cells. Our results identify a pathogenic point mutation in TRMT1 that perturbs tRNA modification activity and demonstrate that m2,2G modifications are disrupted in the cells of patients with TRMT1-associated ID disorders.

TRMT1-Catalyzed tRNA Modifications Are Required for Redox Homeostasis To Ensure Proper Cellular Proliferation and Oxidative Stress Survival

Mutations in the tRNA methyltransferase 1 (TRMT1) gene have been identified as the cause of certain forms of autosomal-recessive intellectual disability (ID). However, the molecular pathology underlying ID-associated TRMT1 mutations is unknown, since the biological role of the encoded TRMT1 protein remains to be determined. Here, we have elucidated the molecular targets and function of TRMT1 to uncover the cellular effects of ID-causing TRMT1 mutations. Using human cells that have been rendered deficient in TRMT1, we show that TRMT1 is responsible for catalyzing the dimethylguanosine (m2,2G) base modification in both nucleus- and mitochondrion-encoded tRNAs. TRMT1-deficient cells exhibit decreased proliferation rates, alterations in global protein synthesis, and perturbations in redox homeostasis, including increased endogenous ROS levels and hypersensitivity to oxidizing agents. Notably, ID-causing TRMT1 variants are unable to catalyze the formation of m2,2G due to defects in RNA binding and cannot rescue oxidative stress sensitivity. Our results uncover a biological role for TRMT1-catalyzed tRNA modification in redox metabolism and show that individuals with TRMT1-associated ID are likely to have major perturbations in cellular homeostasis due to the lack of m2,2G modifications.

tRNA Modifications: Impact on Structure and Thermal Adaptation

Transfer RNAs (tRNAs) are central players in translation, functioning as adapter molecules between the informational level of nucleic acids and the functional level of proteins. They show a highly conserved secondary and tertiary structure and the highest density of post-transcriptional modifications among all RNAs. These modifications concentrate in two hotspots-the anticodon loop and the tRNA core region, where the D- and T-loop interact with each other, stabilizing the overall structure of the molecule. These modifications can cause large rearrangements as well as local fine-tuning in the 3D structure of a tRNA. The highly conserved tRNA shape is crucial for the interaction with a variety of proteins and other RNA molecules, but also needs a certain flexibility for a correct interplay. In this context, it was shown that tRNA modifications are important for temperature adaptation in thermophilic as well as psychrophilic organisms, as they modulate rigidity and flexibility of the transcripts, respectively. Here, we give an overview on the impact of modifications on tRNA structure and their importance in thermal adaptation.

Cap analog substrates reveal three clades of cap guanine-N2 methyltransferases with distinct methyl acceptor specificities

The Tgs proteins are structurally homologous AdoMet-dependent eukaryal enzymes that methylate the N2 atom of 7-methyl guanosine nucleotides. They have an imputed role in the synthesis of the 2,2,7-trimethylguanosine (TMG) RNA cap. Here we exploit a collection of cap-like substrates to probe the repertoire of three exemplary Tgs enzymes, from mammalian, protozoan, and viral sources, respectively. We find that human Tgs (hTgs1) is a bona fide TMG synthase adept at two separable transmethylation steps: (1) conversion of m(7)G to m(2,7)G, and (2) conversion of m(2,7)G to m(2,2,7)G. hTgs1 is unable to methylate G or m(2)G, signifying that both steps require an m(7)G cap. hTgs1 utilizes a broad range of m(7)G nucleotides, including mono-, di-, tri-, and tetraphosphate derivatives as well as cap dinucleotides with triphosphate or tetraphosphate bridges. In contrast, Giardia lamblia Tgs (GlaTgs2) exemplifies a different clade of guanine-N2 methyltransferase that synthesizes only a dimethylguanosine (DMG) cap structure and cannot per se convert DMG to TMG under any conditions tested. Methylation of benzyl(7)G and ethyl(7)G nucleotides by hTgs1 and GlaTgs2 underscored the importance of guanine N7 alkylation in providing a key pi-cation interaction in the methyl acceptor site. Mimivirus Tgs (MimiTgs) shares with the Giardia homolog the ability to catalyze only a single round of methyl addition at guanine-N2, but is distinguished by its capacity for guanine-N2 methylation in the absence of prior N7 methylation. The relaxed cap specificity of MimiTgs is revealed at alkaline pH. Our findings highlight both stark and subtle differences in acceptor specificity and reaction outcomes among Tgs family members.