Human transfer RNA modopathies: diseases caused by aberrations in transfer RNA modifications

tRNA modifications: greasing the wheels of translation and beyond

Transfer RNA (tRNA) is one of the most abundant RNA types in cells, acting as an adaptor to bridge the genetic information in mRNAs with the amino acid sequence in proteins. Both tRNAs and small fragments processed from them play many nonconventional roles in addition to translation. tRNA molecules undergo various types of chemical modifications to ensure the accuracy and efficiency of translation and regulate their diverse functions beyond translation. In this review, we discuss the biogenesis and molecular mechanisms of tRNA modifications, including major tRNA modifications, writer enzymes, and their dynamic regulation. We also summarize the state-of-the-art technologies for measuring tRNA modification, with a particular focus on 2'-O-methylation (Nm), and discuss their limitations and remaining challenges. Finally, we highlight recent discoveries linking dysregulation of tRNA modifications with genetic diseases.

The Evolution and Implications of the Inosine tRNA Modification

Ever since the legendary publication by Francis Crick in JMB introducing the wobble hypothesis in 1966, inosine has been a permanent part of molecular biology. This review aims to integrate the rich array of novel insights emerging from subsequent research on the adenine-to-inosine modification of tRNA, with an emphasis on the results obtained during the last 5 years. Both the grand panorama of 4 billion years of evolution of life and the medical implications of defects in inosine modification will be reviewed. The most salient insights are that: (1) inosine at position 34 (the first position in the anticodon) is not universally present in the tree of life; (2) in many bacteria just a single homodimeric enzyme (TadA) is responsible for both tRNA inosine modification and mRNA inosine modification; (3) rapid progress is currently being made both in the molecular understanding of the heterodimeric ADAT2/ADAT3 enzyme responsible for inosine modifications in eukaryotes and in experimental capabilities for monitoring both the cytoplasmic tRNA pool and their modifications; (4) for selected tRNAs, inosine modification at position 37 has been demonstrated but this modification remains under-studied; (5) modification of tRNAs known to contain inosine can be incomplete; (6) the GC content of the T-stem is of great importance for wobble behavior, including wobbling behavior of inosine; and (7) the tRNA inosine modification is of direct relevance to human disease. In summary, research on inosine continues to yield important novel insights.

MapID-based quantitative mapping of chemical modifications and expression of human transfer RNA

Detection and quantification of tRNA chemical modifications are critical for understanding their regulatory functions in biology and diseases. However, tRNA-seq-based methods for modification mapping encountered challenges both experimentally (poor processivity of heavily modified tRNAs during reverse transcription or RT) and bioinformatically (frequent reads misalignment to highly similar tRNA genes). Here, we report "MapID-tRNA-seq" where we deployed an evolved reverse transcriptase (RT-1306) into tRNA-seq and developed "MapIDs" that reduce redundancy of the human tRNA genome and explicitly annotate genetic variances. RT-1306 generated robust mutations against m1A and m3C, and RT stops against multiple bulky roadblock modifications. MapID-assisted data processing enabled systematic exclusion of false-positive discoveries of modifications which arise from reads misalignment onto similar genes. We applied MapID-tRNA-seq into mapping m1A, m3C and expression levels of tRNAs in three mammary cell lines, which revealed cell-type dependent modification sites and potential translational regulation of the reduced mitochondrial activities in breast cancer.

Translation dysregulation in cancer as a source for targetable antigens

Aberrant peptides presented by major histocompatibility complex (MHC) molecules are targets for tumor eradication, as these peptides can be recognized as foreign by T cells. Protein synthesis in malignant cells is dysregulated, which may result in the generation and presentation of aberrant peptides that can be exploited for T cell-based therapies. To investigate the role of translational dysregulation in immunological tumor control, we disrupt translation fidelity by deleting tRNA wybutosine (yW)-synthesizing protein 2 (TYW2) in tumor cells and characterize the downstream impact on translation fidelity and immunogenicity using immunopeptidomics, genomics, and functional assays. These analyses reveal that TYW2 knockout (KO) cells generate immunogenic out-of-frame peptides. Furthermore, Tyw2 loss increases tumor immunogenicity and leads to anti-programmed cell death 1 (PD-1) checkpoint blockade sensitivity in vivo. Importantly, reduced TYW2 expression is associated with increased response to checkpoint blockade in patients. Together, we demonstrate that defects in translation fidelity drive tumor immunogenicity and may be leveraged for cancer immunotherapy.

A tRNA modification with aminovaleramide facilitates AUA decoding in protein synthesis

Modified tRNA anticodons are critical for proper mRNA translation during protein synthesis. It is generally thought that almost all bacterial tRNAsIle use a modified cytidine-lysidine (L)-at the first position (34) of the anticodon to decipher the AUA codon as isoleucine (Ile). Here we report that tRNAsIle from plant organelles and a subset of bacteria contain a new cytidine derivative, designated 2-aminovaleramididine (ava2C). Like L34, ava2C34 governs both Ile-charging ability and AUA decoding. Cryo-electron microscopy structural analyses revealed molecular details of codon recognition by ava2C34 with a specific interaction between its terminal amide group and an mRNA residue 3'-adjacent to the AUA codon. These findings reveal the evolutionary variation of an essential tRNA modification and demonstrate the molecular basis of AUA decoding mediated by a unique tRNA modification.

The Biology of tRNA t6A Modification and Hypermodifications-Biogenesis and Disease Relevance

The structure and function of transfer RNAs (tRNAs) are highly dependent on post-transcriptional chemical modifications that attach distinct chemical groups to various nucleobase atoms at selected tRNA positions via enzymatic reactions. In all three domains of life, the greatest diversity of chemical modifications is concentrated at positions 34 and 37 of the tRNA anticodon loops. N6-threonylcarbamoyladenosine (t6A) is an essential and universal modification occurring at position 37 of tRNAs that decode codons beginning with an adenine. In a subset of tRNAs from specific organisms, t6A is converted into a variety of hypermodified forms, including cyclic N6-threonylcarbamoyladenosine (ct6A), hydroxy-N6-threonylcarbamoyladenosine (ht6A), N6-methyl-N6-threonylcarbamoyladenosine (m6t6A), 2-methylthio-N6-threonylcarbamoyladenosine (ms2t6A) and 2-methylthio-cyclic N6-threonylcarbamoyladenosine (ms2ct6A). The tRNAs carrying t6A or one of its hypermodified derivatives are dubbed as the t6A family. The t6A family modifications pre-organize the anticodon loop in a conformation that enhances binding to the cognate mRNA codons, thereby promoting translational fidelity. The dysfunctional installation of modifications in the tRNA t6A family leads to translation errors, compromises proteostasis and cell viability, interferes with the growth and development of higher eukaryotes and is implicated in several human diseases, such as neurological disorders, mitochondrial encephalomyopathies, type 2 diabetes and cancers. In addition, loss-of-function mutations in KEOPS complex-the tRNA t6A-modifying enzyme-are associated with shortened telomeres, defects in DNA damage response and transcriptional dysregulation in eukaryotes. The chemical structures, the molecular functions, the known cellular roles and the biosynthetic pathways of the t6A tRNA family are described by integrating and linking biochemical and structural data on these modifications to their biological functions.

Modifications of RNA in cancer: a comprehensive review

RNA modifications play essential roles in post-transcriptional gene regulation and have emerged as significant contributors to cancer biology. Major chemical modifications of RNA include N6-methyladenosine (m6A), 5-methylcytosine (m5C), N1-methyladenosine (m1A), pseudouridine (ψ), and N7-methylguanosine (m7G). Their dynamic regulation highlights their roles in gene expression modulation, RNA stability, and translation. Advanced high-throughput detection methods, ranging from liquid chromatography-mass spectrometry and high-performance liquid chromatography to next-generation sequencing (NGS) and nanopore direct RNA sequencing, have enabled detailed studies of RNA modifications in cancer cells. Aberrant RNA modifications are associated with the dysregulation of tumor suppressor genes and oncogenes, influencing cancer progression, therapy resistance, and immune evasion. Emerging research suggests the therapeutic potential of targeting RNA-modifying enzymes and their inhibitors in cancer treatment. This review compiles and analyzes the latest findings on RNA modifications, presenting an in-depth discussion of the diverse chemical alterations that occur in RNA and their profound implications in cancer biology. It integrates fundamental principles with cutting-edge research, offering a holistic perspective on how RNA modifications influence gene expression, tumor progression, and therapeutic resistance. It emphasizes the need for further studies to elucidate the complex roles of RNA modifications in cancer, as well as the potential for multimodality therapeutic strategies that exploit the dynamic and reversible nature of these epitranscriptomic marks. It also attempts to highlight the challenges, gaps, and limitations of RNA modifications in cancer that should be tackled before their functional implications. Understanding the interplay between RNA modifications, cancer pathways, and their inhibitors will be crucial for developing promising RNA-based therapeutic approaches to cancer and personalized medicine strategies.

tRNA synthetase activity is required for stress granule and P-body assembly

In response to stress, translation initiation is suppressed and ribosome runoff via translation elongation drives mRNA assembly into ribonucleoprotein (RNP) granules including stress granules and P-bodies. Defects in translation elongation activate the integrated stress response. If and how stalled ribosomes are removed from mRNAs during translation elongation stress to drive RNP granule assembly is not clear. We demonstrate the integrated stress response is induced upon tRNA synthetase inhibition in part via ribosome collision sensing. However, saturating levels of tRNA synthetase inhibitors do not induce stress granules or P-bodies and prevent RNP granule assembly upon exogenous stress. The loss of tRNA synthetase activity causes persistent ribosome stalls that can be released with puromycin but are not rescued by ribosome-associated quality control pathways. Therefore, tRNA synthetase activity is required for ribosomes to run off mRNAs during stress to scaffold cytoplasmic RNP granules. Our findings suggest ribosome stalls can persist in human cells and uniquely uncouple ribonucleoprotein condensate assembly from the integrated stress response.

Structural basis for aminoacylation of cellular modified tRNALys3 by human lysyl-tRNA synthetase

The average eukaryotic transfer ribonucleic acid (tRNA) contains 13 post-transcriptional modifications; however, their functional impact is largely unknown. Our understanding of the complex tRNA aminoacylation machinery in metazoans also remains limited. Herein, using a series of high-resolution cryo-electron microscopy (cryo-EM) structures, we provide the mechanistic basis for recognition and aminoacylation of fully modified cellular tRNALys3 by human lysyl-tRNA synthetase (h-LysRS). The tRNALys3 anticodon loop modifications S34 (mcm5s2U) and R37 (ms2t6A) play an integral role in recognition by h-LysRS. Modifications in the T-, variable-, and D-loops of tRNALys3 are critical for ordering the metazoan-specific N-terminal domain of LysRS. The two catalytic steps of tRNALys3 aminoacylation are structurally ordered; docking of the 3'-CCA end in the active site cannot proceed until the lysyl-adenylate intermediate is formed and the pyrophosphate byproduct is released. Association of the h-LysRS-tRNALys3 complex with a multi-tRNA synthetase complex-derived peptide shifts the equilibrium toward the 3'-CCA end "docked" conformation and allosterically increases h-LysRS catalytic efficiency. The insights presented here have broad implications for understanding the role of tRNA modifications in protein synthesis, the human aminoacylation machinery, and the growing catalog of metabolic and neurological diseases linked to it.

Mitochondrial translation regulates terminal erythroid differentiation by maintaining iron homeostasis

Mitochondrial tRNA taurine modifications mediated by mitochondrial tRNA translation optimization 1 (Mto1) is essential for the mitochondrial protein translation. Mto1 deficiency was shown to induce proteostress in embryonic stem cells. A recent finding that a patient with MTO1 gene mutation showed severe anemia led us to hypothesize that Mto1 dysfunctions may result in defective erythropoiesis. Hematopoietic-specific Mto1 conditional knockout (cKO) mice were embryonic lethal and showed niche-independent defect in erythroblast proliferation and terminal differentiation. Mechanistically, mitochondrial oxidative phosphorylation complexes were severely impaired in the Mto1 cKO fetal liver, and this was followed by cytosolic iron accumulation. Overloaded cytosolic iron promoted heme biosynthesis, which induced an unfolded protein response (UPR) in Mto1 cKO erythroblasts. An iron chelator or UPR inhibitor rescued erythroid terminal differentiation in the Mto1 cKO fetal liver in vitro. This mitochondrial regulation of iron homeostasis revealed the indispensable role of mitochondrial tRNA modification in fetal hematopoiesis.

HPLC Analysis of tRNA-Derived Nucleosides

Transfer RNAs (tRNAs), the essential adapter molecules in protein translation, undergo various post-transcriptional modifications. These modifications play critical roles in regulating tRNA folding, stability, and codon-anticodon interactions, depending on the modified position. Methods for detecting modified nucleosides in tRNAs include isotopic labeling combined with chromatography, antibody-based techniques, mass spectrometry, and high-throughput sequencing. Among these, high-performance liquid chromatography (HPLC) has been a cornerstone technique for analyzing modified nucleosides for decades. In this protocol, we provide a detailed, streamlined approach to purify and digest tRNAs from yeast cells and analyze the resulting nucleosides using HPLC. By assessing UV absorbance spectra and retention times, modified nucleosides can be reliably quantified with high accuracy. This method offers a simple, fast, and accessible alternative for studying tRNA modifications, especially when advanced technologies are unavailable. Key features • A streamlined protocol for purifying total tRNAs from yeast cells. • Adaptable for other RNA species and organisms, provided sufficient input material. • Enables the quantification of approximately 20 types of tRNA modifications. • Offers a cost-effective and rapid alternative for analyzing tRNA modifications by HPLC method.

Biogenesis and roles of tRNA queuosine modification and its glycosylated derivatives in human health and diseases

Various types of post-transcriptional modifications contribute to physiological functions by regulating the abundance and function of RNAs. In particular, tRNAs have the widest variety and largest number of modifications, with crucial roles in protein synthesis. Queuosine (Q) is a characteristic tRNA modification with a 7-deazaguanosine core structure bearing a bulky side chain with a cyclopentene group. Q and its derivatives are found in the anticodon of specific tRNAs in both bacteria and eukaryotes. In metazoan tRNAs, Q is further glycosylated with galactose or mannose. The functions of these glycosylated Qs remained unknown for nearly half a century since their discovery. Recently, our group identified the glycosyltransferases responsible for these tRNA modifications and elucidated their biological roles. We, here, review the biochemical and physiological functions of Q and its glycosylated derivatives as well as their associations with human diseases, including cancer and inflammatory and neurological diseases.

Mitochondrial tRNA modifications: functions, diseases caused by their loss, and treatment strategies

Mitochondrial tRNA (mt-tRNA) modifications play pivotal roles in decoding and sustaining tRNA stability, thereby enabling the synthesis of essential respiratory complex proteins in mitochondria. Consequently, loss of human mt-tRNA modifications caused by mutations in the mitochondrial or nuclear genome can cause life-threatening mitochondrial diseases such as encephalopathy and cardiomyopathy. In this article, we first provide a comprehensive overview of the functions of mt-tRNA modifications, the responsible modification enzymes, and the diseases caused by the loss of mt-tRNA modifications. We then discuss progress and potential strategies to treat these diseases, including taurine supplementation for MELAS patients, targeted deletion of mtDNA variants, and overexpression of modification-related proteins. Finally, we discuss factors that need to be overcome to cure "mitochondrial tRNA modopathies."

Human TRMT1 and TRMT1L paralogs ensure the proper modification state, stability, and function of tRNAs

The tRNA methyltransferase 1 (TRMT1) enzyme catalyzes the N2,N2-dimethylguanosine (m2,2G) modification in tRNAs. Intriguingly, vertebrates encode an additional tRNA methyltransferase 1-like (TRMT1L) paralog. Here, we use a comprehensive tRNA sequencing approach to decipher targets of human TRMT1 and TRMT1L. We find that TRMT1 methylates all known tRNAs containing guanosine at position 26, while TRMT1L represents the elusive enzyme catalyzing m2,2G at position 27 in tyrosine tRNAs. Surprisingly, TRMT1L is also necessary for maintaining 3-(3-amino-3-carboxypropyl)uridine (acp3U) modifications in a subset of tRNAs through a process that can be uncoupled from methyltransferase activity. We also demonstrate that tyrosine and serine tRNAs are dependent upon m2,2G modifications for their stability and function in translation. Notably, human patient cells with disease-associated TRMT1 variants exhibit reduced levels of tyrosine and serine tRNAs. These findings uncover unexpected roles for TRMT1 paralogs, decipher functions for m2,2G modifications, and pinpoint tRNAs dysregulated in human disorders caused by tRNA modification deficiency.

TRMT1L-catalyzed m22G27 on tyrosine tRNA is required for efficient mRNA translation and cell survival under oxidative stress

tRNA modifications are critical for several aspects of their functions, including decoding, folding, and stability. Using a multifaceted approach encompassing eCLIP-seq and nanopore tRNA-seq, we show that the human tRNA methyltransferase TRMT1L interacts with the component of the Rix1 ribosome biogenesis complex and binds to the 28S rRNA as well as to a subset of tRNAs. Mechanistically, we demonstrate that TRMT1L is responsible for catalyzing N2,N2-dimethylguanosine (m22G) solely at position 27 of tRNA-Tyr-GUA. Surprisingly, TRMT1L depletion also impaired the deposition of 3-(3-amino-3-carboxypropyl) uridine (acp3U) and dihydrouridine on tRNA-Tyr-GUA, Cys-GCA, and Ala-CGC. TRMT1L knockout cells have a marked decrease in tRNA-Tyr-GUA levels, coinciding with a reduction in global translation rates and hypersensitivity to oxidative stress. Our results establish TRMT1L as the elusive methyltransferase catalyzing the m22G27 modification on tRNA Tyr, resolving a long-standing gap of knowledge and highlighting its potential role in a tRNA modification circuit crucial for translation regulation and stress response.

Epileptic encephalopathy linked to a DALRD3 missense variant that impairs tRNA modification

Epileptic encephalopathies are severe epilepsy syndromes characterized by early onset and progressive cerebral dysfunction. A nonsense variant in the DALR anticodon binding domain containing 3 (DALRD3) gene has been implicated in epileptic encephalopathy, but no other disease-associated variants in DALRD3 have been described. In human cells, the DALRD3 protein forms a complex with the METTL2 methyltransferase to generate the 3-methylcytosine (m3C) modification in specific arginine tRNAs. Here, we identify an individual with a homozygous missense variant in DALRD3 who displays developmental delay, cognitive deficiencies, and multifocal epilepsy. The missense variant substitutes an arginine residue to cysteine (R517C) within the DALR domain of the DALRD3 protein that is required for binding tRNAs. Cells derived from the individual homozygous for the DALRD3-R517C variant exhibit reduced levels of m3C modification in arginine tRNAs, indicating that the R517C variant impairs DALRD3 function. Notably, the DALRD3-R517C protein displays reduced association with METTL2 and loss of interaction with substrate tRNAs. Our results uncover another loss-of-function variant in DALRD3 linked to epileptic encephalopathy disorders. Importantly, these findings underscore DALRD3-dependent tRNA modification as a key contributor to proper brain development and function.

Bacteria-specific modified nucleoside is released and elevated in urine of patients with bacterial infections

Over 170 types of chemical modifications have been identified in cellular RNAs across the three domains of life. Modified RNA is eventually degraded to constituent nucleosides, and in mammals, modified nucleosides are released into the extracellular space. By contrast, the fate of modified nucleosides in bacteria remains unknown. In this study, we performed liquid chromatography-mass spectroscopy (LC-MS) analysis of modified nucleosides from the RNA of 23 pathogenic bacteria, revealing 2-methyladenosine (m2A) as a common bacteria-specific modified nucleoside detected in all bacterial RNAs. Under normal culture conditions, bacteria did not actively release most modified nucleoside species, but robustly released nucleosides, including m2A, following addition of antibiotics or immune cells. These results indicate that m2A is released following bacterial lysis. Intraperitoneal injection of mice with m2A increased detectable levels of m2A in the urine, indicating that mammals can effectively excrete m2A. Additionally, mice infected with wild-type E. coli showed higher levels of m2A in their urine than mice infected by m2A-deficient rlmN KO E. coli. This suggests that m2A from the infected bacteria is excreted in the urine. Lastly, clinical studies using urine samples from febrile patients revealed significantly elevated levels of m2A during bacterial infections, and these values did not correlate with inflammation severity markers, such as white blood count (WBC) and C-reactive protein (CRP). This study reports the mammalian metabolism of modified nucleosides derived from bacterial RNA, and the elevation of urinary m2A in patients with bacterial infections.

IMPORTANCE

This study reveals the differences in the fate and release of modified nucleosides in bacteria and mammals. Additionally, our study highlights that external bacteria-damaging factors, such as antibiotics and phagocytosis by host immune cells, promote the release of bacteria-specific modified nucleosides. Furthermore, we found that m2A was elevated in the urine from animal models of bacterial infection and the urine of patients with bacterial infections. Collectively, this work spans basic biology and clinical science, offering valuable insights into the fate of modified nucleosides in bacterial systems and their relevance to infectious diseases.

Aminoglycoside tolerance in Vibrio cholerae engages translational reprogramming associated with queuosine tRNA modification

Tgt is the enzyme modifying the guanine (G) in tRNAs with GUN anticodon to queuosine (Q). tgt is required for optimal growth of Vibrio cholerae in the presence of sub-lethal aminoglycoside concentrations. We further explored here the role of the Q34 in the efficiency of codon decoding upon tobramycin exposure. We characterized its impact on the overall bacterial proteome, and elucidated the molecular mechanisms underlying the effects of Q34 modification in antibiotic translational stress response. Using molecular reporters, we showed that Q34 impacts the efficiency of decoding at tyrosine TAT and TAC codons. Proteomics analyses revealed that the anti-SoxR factor RsxA is better translated in the absence of tgt. RsxA displays a codon bias toward tyrosine TAT and overabundance of RsxA leads to decreased expression of genes belonging to SoxR oxidative stress regulon. We also identified conditions that regulate tgt expression. We propose that regulation of Q34 modification in response to environmental cues leads to translational reprogramming of transcripts bearing a biased tyrosine codon usage. In silico analysis further identified candidate genes which could be subject to such translational regulation, among which DNA repair factors. Such transcripts, fitting the definition of modification tunable transcripts, are central in the bacterial response to antibiotics.

Structural basis for aminoacylation of cellular modified tRNALys3 by human lysyl-tRNA synthetase

The average eukaryotic tRNA contains 13 posttranscriptional modifications; however, their functional impact is largely unknown. Our understanding of the complex tRNA aminoacylation machinery in metazoans also remains limited. Herein, using a series of high-resolution cryo-electron microscopy (cryo-EM) structures, we provide the mechanistic basis for recognition and aminoacylation of fully-modified cellular tRNALys3 by human lysyl-tRNA synthetase (h-LysRS). The tRNALys3 anticodon loop modifications S34 (mcm5s2U) and R37 (ms2t6A) play an integral role in recognition by h-LysRS. Modifications in the T-, variable-, and D-loops of tRNALys3 are critical for ordering the metazoan-specific N-terminal domain of LysRS. The two catalytic steps of tRNALys3 aminoacylation are structurally ordered; docking of the 3'-CCA end in the active site cannot proceed until the lysyl-adenylate intermediate is formed and the pyrophosphate byproduct is released. Association of the h-LysRS-tRNALys3 complex with a multi-tRNA synthetase complex-derived peptide shifts the equilibrium towards the 3'-CCA end 'docked' conformation and allosterically enhances h-LysRS catalytic efficiency. The insights presented here have broad implications for understanding the role of tRNA modifications in protein synthesis, the human aminoacylation machinery, and the growing catalog of metabolic and neurological diseases linked to it.

Differential redox sensitivity of tRNA dihydrouridylation

Various transfer RNA (tRNA) modifications have recently been shown to regulate stress-dependent gene expression by modulating messenger RNA translation. Among these modifications, dihydrouridine stands out for its increase of tRNA structural flexibility. However, whether and how dihydrouridine synthesis reacts to environmental stimuli is largely unknown. In this study, we manipulated the intracellular redox state of Escherichia coli using paraquat, revealing differential sensitivities of the three tRNA-dihydrouridine synthases towards oxidative stress. Using liquid chromatography-mass spectrometry quantification of dihydrouridine in various knockout strains, we validated the use of a specific RNA sequencing method, namely AlkAnilineSeq, for the precise mapping of dihydrouridines throughout E. coli tRNAs. We found DusA showing high activity, followed by DusB and DusC, whose activity was decreased under paraquat treatment. The relative sensitivity is most plausibly explained by a paraquat-dependent drop of NADPH availability. These findings are substantiated by in vitro kinetics, revealing DusA as the most active enzyme, followed by DusB, while DusC showed little activity, likely related to the efficacy of the redox reaction of the flavin coenzyme with NADPH. Overall, our study underscores the intricate interplay between redox dynamics and tRNA modification processes, revealing a new facet of the regulatory mechanisms influencing cellular responses to oxidative stress.

Structural insights into the agonist selectivity of the adenosine A3 receptor

Adenosine receptors play pivotal roles in physiological processes. Adenosine A3 receptor (A3R), the most recently identified adenosine receptor, is expressed in various tissues, exhibiting important roles in neuron, heart, and immune cells, and is often overexpressed in tumors, highlighting the therapeutic potential of A3R-selective agents. Recently, we identified RNA-derived N6-methyladenosine (m6A) as an endogenous agonist for A3R, suggesting the relationship between RNA-derived modified adenosine and A3R. Despite extensive studies on the other adenosine receptors, the selectivity mechanism of A3R, especially for A3R-selective agonists such as m6A and namodenoson, remained elusive. Here, we identify tRNA-derived N6-isopentenyl adenosine (i6A) as an A3R-selective ligand via screening of modified nucleosides against the adenosine receptors. Like m6A, i6A is found in the human body and may be an endogenous A3R ligand. Our cryo-EM analyses elucidate the A3R-Gi complexes bound to adenosine, 5'-N-ethylcarboxamidoadenosine (NECA), m6A, i6A, and namodenoson at overall resolutions of 3.27 Å (adenosine), 2.86 Å (NECA), 3.19 Å (m6A), 3.28 Å (i6A), and 3.20 Å (namodenoson), suggesting the selectivity and activation mechanism of A3R. We further conduct structure-guided engineering of m6A-insensitive A3R, which may aid future research targeting m6A and A3R, providing a molecular basis for future drug discovery.

Post-transcriptional methylation of mitochondrial-tRNA differentially contributes to mitochondrial pathology

Human mitochondrial tRNAs (mt-tRNAs), critical for mitochondrial biogenesis, are frequently associated with pathogenic mutations. These mt-tRNAs have unusual sequence motifs and require post-transcriptional modifications to stabilize their fragile structures. However, whether a modification that stabilizes a wild-type (WT) mt-tRNA would also stabilize its pathogenic variants is unknown. Here we show that the N1-methylation of guanosine at position 9 (m1G9) of mt-Leu(UAA), while stabilizing the WT tRNA, has a destabilizing effect on variants associated with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes). This differential effect is further demonstrated, as removal of the m1G9 methylation, while damaging to the WT tRNA, is beneficial to the major pathogenic variant, improving the structure and activity of the variant. These results have therapeutic implications, suggesting that the N1-methylation of mt-tRNAs at position 9 is a determinant of pathogenicity and that controlling the methylation level is an important modulator of mt-tRNA-associated diseases.

TRMT1L-catalyzed m2 2G27 on tyrosine tRNA is required for efficient mRNA translation and cell survival under oxidative stress

tRNA modifications are critical for several aspects of their functions, including decoding, folding, and stability. Using a multifaceted approach encompassing eCLIP-seq and Nanopore tRNA-seq, we show that the human tRNA methyltransferase TRMT1L interacts with component of the Rix1 ribosome biogenesis complex and binds to the 28S rRNA, as well as to a subset of tRNAs. Mechanistically, we demonstrate that TRMT1L is responsible for catalyzing m2 2G solely at position 27 of tRNA-Tyr-GUA. Surprisingly, TRMT1L depletion also impaired the deposition of acp3U and dihydrouridine on tRNA-Tyr-GUA, Cys-GCA, and Ala-CGC. TRMT1L knockout cells have a marked decrease in tRNA-Tyr-GUA levels, coinciding with a reduction in global translation rates and hypersensitivity to oxidative stress. Our results establish TRMT1L as the elusive methyltransferase catalyzing the m2 2G27 modification on tRNA Tyr, resolving a long-standing gap of knowledge and highlighting its potential role in a tRNA modification circuit crucial for translation regulation and stress response.

Implications of RNA pseudouridylation for cancer biology and therapeutics: a narrative review

BACKGROUND

Pseudouridine (Ψ), a C5-glycoside isomer of uridine, stands as one of the most prevalent RNA modifications in all RNA types. Distinguishing from the C-N bond linking uridine to ribose, the link between Ψ and ribose is a C-C bond, endowing Ψ modified RNA distinct properties and functions in various biological processes. The conversion of uridine to Ψ is governed by pseudouridine synthases (PUSs). RNA pseudouridylation is implicated in cancer biology and therapeutics.

OBJECTIVES

In this review, we will summarize the methods for detecting Ψ, the process of Ψ generation, the impact of Ψ modification on RNA metabolism and gene expression, the roles of dysregulated Ψ and pseudouridine synthases in cancers, and the underlying mechanism.

METHODS

We conducted a comprehensive search of PubMed from its inception through February 2024. The search terms included "pseudouridine"; "pseudouridine synthase"; "PUS"; "dyskerin"; "cancer"; "tumor"; "carcinoma"; "malignancy"; "tumorigenesis"; "biomarker"; "prognosis" and "therapy". We included studies published in peer-reviewed journals that focused on Ψ detection, specific mechanisms involving Ψ and PUSs, and prognosis in cancer patients with high Ψ expression. We excluded studies lacking sufficient methodological details or appropriate controls.

RESULTS

Ψ has been recognized as a significant biomarker in cancer diagnosis and prognosis. Abnormal Ψ modifications mediated by various PUSs result in dysregulated RNA metabolism and impaired RNA function, promoting the development of various cancers. Overexpression of PUSs is common in cancer cells and predicts poor prognosis. PUSs inhibition arrests cell proliferation and enhances apoptosis in cancer cells, suggesting PUS-targeting cancer therapy may be a potential strategy in cancer treatment.

DISCUSSION

High Ψ levels in serum, urine, and saliva may suggest cancer, but do not specify the type, requiring additional lab markers and imaging for accurate diagnosis. Standardized detection methods are also crucial for reliable results. PUSs are linked to cancer, but more researches are needed to understand their mechanisms in different cancers. Anticancer treatments targeting PUSs are still under developed.

Human DUS1L catalyzes dihydrouridine modification at tRNA positions 16/17, and DUS1L overexpression perturbs translation

Human cytoplasmic tRNAs contain dihydrouridine modifications at positions 16 and 17 (D16/D17). The enzyme responsible for D16/D17 formation and its cellular roles remain elusive. Here, we identify DUS1L as the human tRNA D16/D17 writer. DUS1L knockout in the glioblastoma cell lines LNZ308 and U87 causes loss of D16/D17. D formation is reconstituted in vitro using recombinant DUS1L in the presence of NADPH or NADH. DUS1L knockout/overexpression in LNZ308 cells shows that DUS1L supports cell growth. Moreover, higher DUS1L expression in glioma patients is associated with poorer prognosis. Upon vector-mediated DUS1L overexpression in LNZ308 cells, 5' and 3' processing of precursor tRNATyr(GUA) is inhibited, resulting in a reduced mature tRNATyr(GUA) level, reduced translation of the tyrosine codons UAC and UAU, and reduced translational readthrough of the near-cognate stop codons UAA and UAG. Moreover, DUS1L overexpression increases the amounts of several D16/D17-containing tRNAs and total cellular translation. Our study identifies a human dihydrouridine writer, providing the foundation to study its roles in health and disease.

Mettl1-dependent m7G tRNA modification is essential for maintaining spermatogenesis and fertility in Drosophila melanogaster

Modification of guanosine to N7-methylguanosine (m7G) in the variable loop region of tRNA is catalyzed by the METTL1/WDR4 heterodimer and stabilizes target tRNA. Here, we reveal essential functions of Mettl1 in Drosophila fertility. Knockout of Mettl1 (Mettl1-KO) causes no major effect on the development of non-gonadal tissues, but abolishes the production of elongated spermatids and mature sperm, which is fully rescued by expression of a Mettl1-transgene, but not a catalytic-dead Mettl1 transgene. This demonstrates that Mettl1-dependent m7G is required for spermatogenesis. Mettl1-KO results in a loss of m7G modification on a subset of tRNAs and decreased tRNA abundance. Ribosome profiling shows that Mettl1-KO led to ribosomes stalling at codons decoded by tRNAs that were reduced in abundance. Mettl1-KO also significantly reduces the translation efficiency of genes involved in elongated spermatid formation and sperm stability. Germ cell-specific expression of Mettl1 rescues disrupted m7G tRNA modification and tRNA abundance in Mettl1-KO testes but not in non-gonadal tissues. Ribosome stalling is much less detectable in non-gonadal tissues than in Mettl1-KO testes. These findings reveal a developmental role for m7G tRNA modification and indicate that m7G modification-dependent tRNA abundance differs among tissues.

tRNA modifications and tRNA-derived small RNAs: new insights of tRNA in human disease

tRNAs are codon decoders that convert the transcriptome into the proteome. The field of tRNA research is excited by the increasing discovery of specific tRNA modifications that are installed at specific, evolutionarily conserved positions by a set of specialized tRNA-modifying enzymes and the biogenesis of tRNA-derived regulatory fragments (tsRNAs) which exhibit copious activities through multiple mechanisms. Dysregulation of tRNA modification usually has pathological consequences, a phenomenon referred to as "tRNA modopathy". Current evidence suggests that certain tRNA-modifying enzymes and tsRNAs may serve as promising diagnostic biomarkers and therapeutic targets, particularly for chemoresistant cancers. In this review, we discuss the latest discoveries that elucidate the molecular mechanisms underlying the functions of clinically relevant tRNA modifications and tsRNAs, with a focus on malignancies. We also discuss the therapeutic potential of tRNA/tsRNA-based therapies, aiming to provide insights for the development of innovative therapeutic strategies. Further efforts to unravel the complexities inherent in tRNA biology hold the promise of yielding better biomarkers for the diagnosis and prognosis of diseases, thereby advancing the development of precision medicine for health improvement.

Lost in translation: How neurons cope with tRNA decoding

Post-transcriptional tRNA modifications contribute to the decoding efficiency of tRNAs by supporting codon recognition and tRNA stability. Recent work shows that the molecular and cellular functions of tRNA modifications and tRNA-modifying-enzymes are linked to brain development and neurological disorders. Lack of these modifications affects codon recognition and decoding rate, promoting protein aggregation and translational stress response pathways with toxic consequences to the cell. In this review, we discuss the peculiarity of local translation in neurons, suggesting a role for fine-tuning of translation performed by tRNA modifications. We provide several examples of tRNA modifications involved in physiology and pathology of the nervous system, highlighting their effects on protein translation and discussing underlying mechanisms, like the unfolded protein response (UPR), ribosome quality control (RQC), and no-go mRNA decay (NGD), which could affect neuronal functions. We aim to deepen the understanding of the roles of tRNA modifications and the coordination of these modifications with the protein translation machinery in the nervous system.

TRMT10A dysfunction perturbs codon translation of initiator methionine and glutamine and impairs brain functions in mice

In higher eukaryotes, tRNA methyltransferase 10A (TRMT10A) is responsible for N1-methylguanosine modification at position nine of various cytoplasmic tRNAs. Pathogenic mutations in TRMT10A cause intellectual disability, microcephaly, diabetes, and short stature in humans, and generate cytotoxic tRNA fragments in cultured cells; however, it is not clear how TRMT10A supports codon translation or brain functions. Here, we generated Trmt10a null mice and showed that tRNAGln(CUG) and initiator methionine tRNA levels were universally decreased in various tissues; the same was true in a human cell line lacking TRMT10A. Ribosome profiling of mouse brain revealed that dysfunction of TRMT10A causes ribosome slowdown at the Gln(CAG) codon and increases translation of Atf4 due to higher frequency of leaky scanning of its upstream open reading frames. Broadly speaking, translation of a subset of mRNAs, especially those for neuronal structures, is perturbed in the mutant brain. Despite not showing discernable defects in the pancreas, liver, or kidney, Trmt10a null mice showed lower body weight and smaller hippocampal postsynaptic densities, which is associated with defective synaptic plasticity and memory. Taken together, our study provides mechanistic insight into the roles of TRMT10A in the brain, and exemplifies the importance of universal tRNA modification during translation of specific codons.

Molecular pathways in mitochondrial disorders due to a defective mitochondrial protein synthesis

Mitochondria play a central role in cellular metabolism producing the necessary ATP through oxidative phosphorylation. As a remnant of their prokaryotic past, mitochondria contain their own genome, which encodes 13 subunits of the oxidative phosphorylation system, as well as the tRNAs and rRNAs necessary for their translation in the organelle. Mitochondrial protein synthesis depends on the import of a vast array of nuclear-encoded proteins including the mitochondrial ribosome protein components, translation factors, aminoacyl-tRNA synthetases or assembly factors among others. Cryo-EM studies have improved our understanding of the composition of the mitochondrial ribosome and the factors required for mitochondrial protein synthesis and the advances in next-generation sequencing techniques have allowed for the identification of a growing number of genes involved in mitochondrial pathologies with a defective translation. These disorders are often multisystemic, affecting those tissues with a higher energy demand, and often present with neurodegenerative phenotypes. In this article, we review the known proteins required for mitochondrial translation, the disorders that derive from a defective mitochondrial protein synthesis and the animal models that have been established for their study.

Activity reconstitution of Kre33 and Tan1 reveals a molecular ruler mechanism in eukaryotic tRNA acetylation

RNA acetylation is a universal post-transcriptional modification that occurs in various RNAs. Transfer RNA (tRNA) acetylation is found at position 34 (ac4C34) in bacterial tRNAMet and position 12 (ac4C12) in eukaryotic tRNASer and tRNALeu. The biochemical mechanism, structural basis and functional significance of ac4C34 are well understood; however, despite being discovered in the 1960s and identification of Kre33/NAT10 and Tan1/THUMPD1 as modifying apparatuses, ac4C12 modification activity has never been reconstituted for nearly six decades. Here, we successfully reconstituted the ac4C12 modification activity of yeast Kre33 and Tan1. Biogenesis of ac4C12 is primarily dependent on a minimal set of elements, including a canonical acceptor stem, the presence of the 11CCG13 motif and correct D-arm orientation, indicating a molecular ruler mechanism. A single A13G mutation conferred ac4C12 modification to multiple non-substrate tRNAs. Moreover, we were able to introduce ac4C modifications into small RNAs. ac4C12 modification contributed little to tRNA melting temperature and aminoacylation in vitro and in vivo. Collectively, our results realize in vitro activity reconstitution, delineate tRNA substrate selection mechanism for ac4C12 biogenesis and develop a valuable system for preparing acetylated tRNAs as well as non-tRNA RNA species, which will advance the functional interpretation of the acetylation in RNA structures and functions.

Cryo-EM structures of the human Elongator complex at work

tRNA modifications affect ribosomal elongation speed and co-translational folding dynamics. The Elongator complex is responsible for introducing 5-carboxymethyl at wobble uridine bases (cm5U34) in eukaryotic tRNAs. However, the structure and function of human Elongator remain poorly understood. In this study, we present a series of cryo-EM structures of human ELP123 in complex with tRNA and cofactors at four different stages of the reaction. The structures at resolutions of up to 2.9 Å together with complementary functional analyses reveal the molecular mechanism of the modification reaction. Our results show that tRNA binding exposes a universally conserved uridine at position 33 (U33), which triggers acetyl-CoA hydrolysis. We identify a series of conserved residues that are crucial for the radical-based acetylation of U34 and profile the molecular effects of patient-derived mutations. Together, we provide the high-resolution view of human Elongator and reveal its detailed mechanism of action.

Epigenetic mechanisms in depression: Implications for pathogenesis and treatment

The risk of depression is influenced by both genetic and environmental factors. It has been suggested that epigenetic mechanisms may mediate the risk of depression following exposure to adverse life events. Epigenetics encompasses stable alterations in gene expression that are controlled through transcriptional, post-transcriptional, translational, or post-translational processes, including DNA modifications, chromatin remodeling, histone modifications, RNA modifications, and non-coding RNA (ncRNA) regulation, without any changes in the DNA sequence. In this review, we explore recent research advancements in the realm of epigenetics concerning depression. Furthermore, we evaluate the potential of epigenetic changes as diagnostic and therapeutic biomarkers for depression.

Pathophysiology of human mitochondrial tRNA metabolism

Mitochondria play multiple critical roles in cellular activity. In particular, mitochondrial translation is pivotal in the regulation of mitochondrial and cellular homeostasis. In this forum article, we discuss human mitochondrial tRNA metabolism and highlight its tight connection with various mitochondrial diseases caused by mutations in aminoacyl-tRNA synthetases, tRNAs, and tRNA-modifying enzymes.

Beyond the Anticodon: tRNA Core Modifications and Their Impact on Structure, Translation and Stress Adaptation

Transfer RNAs (tRNAs) are heavily decorated with post-transcriptional chemical modifications. Approximately 100 different modifications have been identified in tRNAs, and each tRNA typically contains 5-15 modifications that are incorporated at specific sites along the tRNA sequence. These modifications may be classified into two groups according to their position in the three-dimensional tRNA structure, i.e., modifications in the tRNA core and modifications in the anticodon-loop (ACL) region. Since many modified nucleotides in the tRNA core are involved in the formation of tertiary interactions implicated in tRNA folding, these modifications are key to tRNA stability and resistance to RNA decay pathways. In comparison to the extensively studied ACL modifications, tRNA core modifications have generally received less attention, although they have been shown to play important roles beyond tRNA stability. Here, we review and place in perspective selected data on tRNA core modifications. We present their impact on tRNA structure and stability and report how these changes manifest themselves at the functional level in translation, fitness and stress adaptation.

Elucidating the influence of RNA modifications and Magnesium ions on tRNA Phe conformational dynamics in S. cerevisiae : Insights from Replica Exchange Molecular Dynamics simulations

Post-transcriptional modifications in RNA can significantly impact their structure and function. In particular, transfer RNAs (tRNAs) are heavily modified, with around 100 different naturally occurring nucleotide modifications contributing to codon bias and decoding efficiency. Here, we describe our efforts to investigate the impact of RNA modifications on the structure and stability of tRNA Phenylalanine (tRNA Phe ) from S. cerevisiae using molecular dynamics (MD) simulations. Through temperature replica exchange MD (T-REMD) studies, we explored the unfolding pathway to understand how RNA modifications influence the conformational dynamics of tRNA Phe , both in the presence and absence of magnesium ions (Mg 2+ ). We observe that modified nucleotides in key regions of the tRNA establish a complex network of hydrogen bonds and stacking interactions which is essential for tertiary structure stability of the tRNA. Furthermore, our simulations show that modifications facilitate the formation of ion binding sites on the tRNA. However, high concentrations of Mg 2+ ions can stabilize the tRNA tertiary structure in the absence of modifications. Our findings illuminate the intricate interactions between modifications, magnesium ions, and RNA structural stability.

WITHDRAWN: The effect of ischemia on expression quantitative trait loci (eQTL) in human myocardium and insights into myocardial injury etiology

26 February, 2024. Research Square has withdrawn this preprint as it was submitted and made public without the full consent of all the authors and without the full consent of the principle investigator of the registered clinical trial. Therefore, this work should not be cited as a reference.

Tailoring the stress response of human skin cells by substantially limiting the nuclear localization of angiogenin

Human angiogenin (hANG) is the most studied stress-induced ribonuclease (RNase). In physiological conditions it performs its main functions in nucleoli, promoting cell proliferation by rDNA transcription, whereas it is strongly limited by its inhibitor (RNH1) throughout the rest of the cell. In stressed cells hANG dissociates from RNH1 and thickens in the cytoplasm where it manages the translational arrest and the recruitment of stress granules, thanks to its propensity to cleave tRNAs and to induce the release of active halves. Since it exists a clear connection between hANG roles and its intracellular routing, starting from our recent findings on heterologous ANG (ANG) properties in human keratinocytes (HaCaT cells), here we designed a variant unable to translocate into the nucleus with the aim of thoroughly verifying its potentialities under stress. This variant, widely characterized for its structural features and biological attitudes, shows more pronounced aid properties than unmodified protein. The collected evidence thus fully prove that ANG stress-induced skills in assisting cellular homeostasis are strictly due to its cytosolic localization. This study opens an interesting scenario for future studies regarding both the strengthening of skin defences and in understanding the mechanism of action of these special enzymes potentially suitable for any cell type.

[Abnormal transfer RNA epigenetic modifications and related impact on neurodegenerative diseases]

Neurodegenerative diseases are a heterogeneous group of neurological disorders characterized by progressive loss of neurons in the central or peripheral nervous system. Research on the pathogenesis and drug targets of these diseases still faces many challenges due to the complex etiology. In recent years, the role of epigenetic modifications in transfer RNA (tRNA) in neurodegenerative diseases has attracted widespread attention. The tRNA modifications are crucial for regulating codon recognition, maintaining molecular structural stability, and the generation of tRNA-derived fragments (tRFs). Recent studies have highlighted a close association between abnormal tRNA modifications and the pathogenesis of various neurodegenerative diseases, especially for abnormalities of elongator complex-dependent tRNA modification and methylation modification, which impact the translation process and tRFs levels. These changes regulate protein homeostasis and cellular stress responses, ultimately influencing the survival of neuronal cells. Moreover, significant changes in tRFs levels have been observed in neurodegenerative diseases, and specific tRFs show distinct effects on neurodegenerative diseases. This review aims to provide an overview of the physiological functions of tRNA epigenetic modifications and their regulatory mechanisms in neurodegenerative diseases, covering both classical functions such as codon recognition and non-classical functions such as tRFs biogenesis. Additionally, the potential of targeting tRNA modifications for therapeutic applications is discussed.

Mitochondrial RNA maturation

The vast majority of oxygen-utilizing eukaryotes need to express their own mitochondrial genome, mtDNA, to survive. In comparison to size of their nuclear genome, mtDNA is minimal, even in the most exceptional examples. Having evolved from bacteria in an endosymbiotic event, it might be expected that the process of mtDNA expression would be relatively simple. The aim of this short review is to illustrate just how wrong this assumption is. The production of functional mitochondrial RNA across species evolved in many directions. Organelles use a dizzying array of RNA processing, modifying, editing, splicing and maturation events that largely require the import of nuclear-encoded proteins from the cytosol. These processes are sometimes driven by the unusual behaviour of the mitochondrial genome from which the RNA is originally transcribed, but in many examples the complex processes that are essential for the production of functional RNA in the organelle, are fascinating and bewildering.

Amyloid pathology reduces ELP3 expression and tRNA modifications leading to impaired proteostasis

Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by accumulation of β-amyloid aggregates and loss of proteostasis. Transfer RNA (tRNA) modifications play a crucial role in maintaining proteostasis, but their impact in AD remains unclear. Here, we report that expression of the tRNA modifying enzyme ELP3 is reduced in the brain of AD patients and amyloid mouse models and negatively correlates with amyloid plaque mean density. We further show that SH-SY5Y neuronal cells carrying the amyloidogenic Swedish familial AD mutation (SH-SWE) display reduced ELP3 levels, tRNA hypomodifications and proteostasis impairments when compared to cells not carrying the mutation (SH-WT). Additionally, exposing SH-WT cells to the secretome of SH-SWE cells led to reduced ELP3 expression, wobble uridine tRNA hypomodification, and increased protein aggregation. Importantly, correcting tRNA deficits due to ELP3 reduction reverted proteostasis impairments. These findings suggest that amyloid pathology dysregulates proteostasis by reducing ELP3 expression and tRNA modification levels, and that targeting tRNA modifications may be a potential therapeutic avenue to restore neuronal proteostasis in AD and preserve neuronal function.

Post-Transcriptional Methylation of Mitochondrial-tRNA Differentially Contributes to Mitochondrial Pathology

Human mitochondrial tRNAs (mt-tRNAs), critical for mitochondrial biogenesis, are frequently associated with pathogenic mutations. These mt-tRNAs have unusual sequence motifs and require post-transcriptional modifications to stabilize their fragile structures. However, whether a modification that stabilizes a wild-type (WT) mt-tRNA structure would also stabilize its pathogenic variants is unknown. Here we show that the N 1 -methylation of guanosine at position 9 (m 1 G9) of mt-Leu(UAA), while stabilizing the WT tRNA, has an opposite and destabilizing effect on variants associated with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes). This differential effect is further demonstrated by the observation that demethylation of m 1 G9, while damaging to the WT tRNA, is beneficial to the major pathogenic variant, improving its structure and activity. These results have new therapeutic implications, suggesting that the N 1 -methylation of mt-tRNAs at position 9 is a determinant of pathogenicity and that controlling the methylation level is an important modulator of mt-tRNA-associated diseases.

Genome-Wide Profiling of tRNA Using an Unexplored Reverse Transcriptase with High Processivity

Monitoring the dynamic changes of cellular tRNA pools is challenging, due to the extensive post-transcriptional modifications of individual species. The most critical component in tRNAseq is a processive reverse transcriptase (RT) that can read through each modification with high efficiency. Here we show that the recently developed group-II intron RT Induro has the processivity and efficiency necessary to profile tRNA dynamics. Using our Induro-tRNAseq, simpler and more comprehensive than the best methods to date, we show that Induro progressively increases readthrough of tRNA over time and that the mechanism of increase is selective removal of RT stops, without altering the misincorporation frequency. We provide a parallel dataset of the misincorporation profile of Induro relative to the related TGIRT RT to facilitate the prediction of non-annotated modifications. We report an unexpected modification profile among human proline isoacceptors, absent from mouse and lower eukaryotes, that indicates new biology of decoding proline codons.

Cell type-specific translational regulation by human DUS enzymes

Dihydrouridine is an abundant and conserved modified nucleoside present on tRNA, but characterization and functional studies of modification sites and associated DUS writer enzymes in mammals is lacking. Here we use a chemical probing strategy, RNABPP-PS, to identify 5-chlorouridine as an activity-based probe for human DUS enzymes. We map D modifications using RNA-protein crosslinking and chemical transformation and mutational profiling to reveal D modification sites on human tRNAs. Further, we knock out individual DUS genes in two human cell lines to investigate regulation of tRNA expression levels and codon-specific translation. We show that whereas D modifications are present across most tRNA species, loss of D only perturbs the translational function of a subset of tRNAs in a cell type-specific manner. Our work provides powerful chemical strategies for investigating D and DUS enzymes in diverse biological systems and provides insight into the role of a ubiquitous tRNA modification in translational regulation.

Transfer RNAs as dynamic and critical regulators of cancer progression

Transfer RNAs (tRNAs) have been historically viewed as non-dynamic adaptors that decode the genetic code into proteins. Recent work has uncovered dynamic regulatory roles for these fascinating molecules. Advances in tRNA detection methods have revealed that specific tRNAs can become modulated upon DNA copy number and chromatin alterations and can also be perturbed by oncogenic signalling and transcriptional regulators in cancer cells or the tumour microenvironment. Such alterations in the levels of specific tRNAs have been shown to causally impact cancer progression, including metastasis. Moreover, sequencing methods have identified tRNA-derived small RNAs that influence various aspects of cancer progression, such as cell proliferation and invasion, and could serve as diagnostic and prognostic biomarkers or putative therapeutic targets in various cancers. Finally, there is accumulating evidence, including from genetic models, that specific tRNA synthetases - the enzymes responsible for charging tRNAs with amino acids - can either promote or suppress tumour formation. In this Review, we provide an overview of how deregulation of tRNAs influences cancer formation and progression.

Impacts of maternal microbiota and microbial metabolites on fetal intestine, brain, and placenta

BACKGROUND

The maternal microbiota modulates fetal development, but the mechanisms of these earliest host-microbe interactions are unclear. To investigate the developmental impacts of maternal microbial metabolites, we compared full-term fetuses from germ-free and specific pathogen-free mouse dams by gene expression profiling and non-targeted metabolomics.

RESULTS

In the fetal intestine, critical genes mediating host-microbe interactions, innate immunity, and epithelial barrier were differentially expressed. Interferon and inflammatory signaling genes were downregulated in the intestines and brains of the fetuses from germ-free dams. The expression of genes related to neural system development and function, translation and RNA metabolism, and regulation of energy metabolism were significantly affected. The gene coding for the insulin-degrading enzyme (Ide) was most significantly downregulated in all tissues. In the placenta, genes coding for prolactin and other essential regulators of pregnancy were downregulated in germ-free dams. These impacts on gene expression were strongly associated with microbially modulated metabolite concentrations in the fetal tissues. Aryl sulfates and other aryl hydrocarbon receptor ligands, the trimethylated compounds TMAO and 5-AVAB, Glu-Trp and other dipeptides, fatty acid derivatives, and the tRNA nucleobase queuine were among the compounds strongly associated with gene expression differences. A sex difference was observed in the fetal responses to maternal microbial status: more genes were differentially regulated in male fetuses than in females.

CONCLUSIONS

The maternal microbiota has a major impact on the developing fetus, with male fetuses potentially more susceptible to microbial modulation. The expression of genes important for the immune system, neurophysiology, translation, and energy metabolism are strongly affected by the maternal microbial status already before birth. These impacts are associated with microbially modulated metabolites. We identified several microbial metabolites which have not been previously observed in this context. Many of the potentially important metabolites remain to be identified.

Chemical Approaches To Investigate Post-transcriptional RNA Regulation

RNA plays a central role in biological processes, and its activity is regulated by a host of diverse chemical and biochemical mechanisms including post-transcriptional modification and interactions with RNA-binding proteins. Here, we describe our efforts to illuminate RNA biology through the application of chemical tools, focusing on post-transcriptional regulatory mechanisms. We describe the development of an activity-based protein profiling approach for discovery and characterization of RNA-modifying enzymes. Next, we highlight novel approaches for RNA imaging based upon metabolic labeling with modified nucleosides and engineering of the nucleotide salvage pathway. Finally, we discuss profiling RNA-protein interactions using small molecule-dependent RNA editing and synthetic photo-cross-linkable oligonucleotide probes. Our work provides enabling technologies for deciphering the complexity of RNA and its diverse functions in biology.