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

Transfer RNA and small molecule therapeutics for aminoacyl-tRNA synthetase diseases

Aminoacyl-tRNA synthetases catalyze the ligation of a specific amino acid to its cognate tRNA. The resulting aminoacyl-tRNAs are indispensable intermediates in protein biosynthesis, facilitating the precise decoding of the genetic code. Pathogenic alleles in the aminoacyl-tRNA synthetases can lead to several dominant and recessive disorders. To date, disease-specific treatments for these conditions are largely unavailable. We review pathogenic human synthetase alleles, the molecular and cellular mechanisms of tRNA synthetase diseases, and emerging approaches to allele-specific treatments, including small molecules and nucleic acid-based therapeutics. Current treatment approaches to rescue defective or dysfunctional tRNA synthetase mutants include supplementation with cognate amino acids and delivery of cognate tRNAs to alleviate bottlenecks in translation. Complementary approaches use inhibitors to target the integrated stress response, which can be dysregulated in tRNA synthetase diseases.

AARS Online: A collaborative database on the structure, function, and evolution of the aminoacyl-tRNA synthetases

The aminoacyl-tRNA synthetases (aaRS) are a large group of enzymes that implement the genetic code in all known biological systems. They attach amino acids to their cognate tRNAs, moonlight in various translational and non-translational activities beyond aminoacylation, and are linked to many genetic disorders. The aaRS have a subtle ontology characterized by structural and functional idiosyncrasies that vary from organism to organism, and protein to protein. Across the tree of life, the 22 coded amino acids are handled by 16 evolutionary families of Class I aaRS and 21 families of Class II aaRS. We introduce AARS Online, an interactive Wikipedia-like tool curated by an international consortium of field experts. This platform systematizes existing knowledge about the aaRS by showcasing a taxonomically diverse selection of aaRS sequences and structures. Through its graphical user interface, AARS Online facilitates a seamless exploration between protein sequence and structure, providing a friendly introduction to the material for non-experts and a useful resource for experts. Curated multiple sequence alignments can be extracted for downstream analyses. Accessible at www.aars.online, AARS Online is a free resource to delve into the world of the aaRS.

Emc1 is essential for vision and zebrafish photoreceptor outer segment morphogenesis

Inherited retinal diseases (IRDs) are a rare group of eye disorders characterized by progressive dysfunction and degeneration of retinal cells. In this study, we characterized the raifteirí (raf) zebrafish, a novel model of inherited blindness, identified through an unbiased ENU mutagenesis screen. A mutation in the largest subunit of the endoplasmic reticulum membrane protein complex, emc1 was subsequently identified as the causative raf mutation. We sought to elucidate the cellular and molecular phenotypes in the emc1-/- knockout model and explore the association of emc1 with retinal degeneration. Visual behavior and retinal electrophysiology assays demonstrated that emc1-/- mutants had severe visual impairments. Retinal histology and morphometric analysis revealed extensive abnormalities, including thinning of the photoreceptor layer, in addition to large gaps surrounding the lens. Notably, photoreceptor outer segments were drastically smaller, outer segment protein expression was altered and hyaloid vasculature development was disrupted. Transcriptomic profiling identified cone and rod-specific phototransduction genes significantly downregulated by loss of emc1. These data shed light on why emc1 is a causative gene in inherited retinal disease and how outer segment morphogenesis is regulated.

TARS2 c.470 C > G is a chinese-specific founder mutation in three unrelated families with mitochondrial encephalomyopathy

Biallelic pathogenic variants in TARS2 lead to combined oxidative phosphorylation deficiency, subtype 21 (COXPD21, MIM #615918), which is a rare mitochondrial encephalomyopathy (ME) characterized by early-onset severe axial hypotonia, limb hypertonia, psychomotor developmental delay, epilepsy and brain anomalies. To date, approximately 28 individuals with COXPD21 and 28 TARS2 variants have been identified. In this study, we reported additional four individuals from three unrelated Chinese families with mitochondrial encephalomyopathy caused by pathogenic variants in TARS2, and described the novel clinical phenotypes and genotypic information. In addition to two novel variants (c.512G > A, p.Arg171Lys; c.988dup, p.Arg330Lysfs*4), one previously reported variant (c.470 C > G, p.Thr157Arg) recurred in six Chinese individuals with COXPD21 but was not present in populations of other races. Our findings expanded the mutation spectrum of TARS2 and confirmed that c.470 C > G is a Chinese-specific founder mutation. The novel phenotypes, including reduced fetal movement, eye anomalies and sleep irregularities, observed in our patients enriched the clinical characteristics of COXPD21.

The evolving spectrum of complex inherited neuropathies

PURPOSE OF REVIEW

Inherited peripheral neuropathies can be divided into those diseases in which peripheral neuropathy is the sole or main feature of the disease (Charcot-Marie-Tooth disease) and those in which peripheral neuropathy is just one feature of a more complex syndrome. In recent years there has been a substantial expansion in the number of genes associated with complex neuropathy syndromes.

RECENT FINDINGS

This review will focus on emerging themes in this group of diseases, namely the increasing number of diseases due to repeat expansions; the emergence of both recessive and dominant negative alleles in the same gene producing a common phenotype and diseases in which there is selective loss of the allele from haematopoietic stem cells making genetic diagnosis on blood derived DNA problematic.

SUMMARY

In this review we provide a practical approach to investigating and diagnosing patients with peripheral neuropathy as part of a complex syndrome and provide an updated table of the genes associated with this group of diseases.

Valine aminoacyl-tRNA synthetase promotes therapy resistance in melanoma

Transfer RNA dynamics contribute to cancer development through regulation of codon-specific messenger RNA translation. Specific aminoacyl-tRNA synthetases can either promote or suppress tumourigenesis. Here we show that valine aminoacyl-tRNA synthetase (VARS) is a key player in the codon-biased translation reprogramming induced by resistance to targeted (MAPK) therapy in melanoma. The proteome rewiring in patient-derived MAPK therapy-resistant melanoma is biased towards the usage of valine and coincides with the upregulation of valine cognate tRNAs and of VARS expression and activity. Strikingly, VARS knockdown re-sensitizes MAPK-therapy-resistant patient-derived melanoma in vitro and in vivo. Mechanistically, VARS regulates the messenger RNA translation of valine-enriched transcripts, among which hydroxyacyl-CoA dehydrogenase mRNA encodes for a key enzyme in fatty acid oxidation. Resistant melanoma cultures rely on fatty acid oxidation and hydroxyacyl-CoA dehydrogenase for their survival upon MAPK treatment. Together, our data demonstrate that VARS may represent an attractive therapeutic target for the treatment of therapy-resistant melanoma.

Mechanisms and Delivery of tRNA Therapeutics

Transfer ribonucleic acid (tRNA) therapeutics will provide personalized and mutation specific medicines to treat human genetic diseases for which no cures currently exist. The tRNAs are a family of adaptor molecules that interpret the nucleic acid sequences in our genes into the amino acid sequences of proteins that dictate cell function. Humans encode more than 600 tRNA genes. Interestingly, even healthy individuals contain some mutant tRNAs that make mistakes. Missense suppressor tRNAs insert the wrong amino acid in proteins, and nonsense suppressor tRNAs read through premature stop signals to generate full length proteins. Mutations that underlie many human diseases, including neurodegenerative diseases, cancers, and diverse rare genetic disorders, result from missense or nonsense mutations. Thus, specific tRNA variants can be strategically deployed as therapeutic agents to correct genetic defects. We review the mechanisms of tRNA therapeutic activity, the nature of the therapeutic window for nonsense and missense suppression as well as wild-type tRNA supplementation. We discuss the challenges and promises of delivering tRNAs as synthetic RNAs or as gene therapies. Together, tRNA medicines will provide novel treatments for common and rare genetic diseases in humans.

Boosting BDNF in muscle rescues impaired axonal transport in a mouse model of DI-CMTC peripheral neuropathy

Charcot-Marie-Tooth disease (CMT) is a genetic peripheral neuropathy caused by mutations in many functionally diverse genes. The aminoacyl-tRNA synthetase (ARS) enzymes, which transfer amino acids to partner tRNAs for protein synthesis, represent the largest protein family genetically linked to CMT aetiology, suggesting pathomechanistic commonalities. Dominant intermediate CMT type C (DI-CMTC) is caused by YARS1 mutations driving a toxic gain-of-function in the encoded tyrosyl-tRNA synthetase (TyrRS), which is mediated by exposure of consensus neomorphic surfaces through conformational changes of the mutant protein. In this study, we first showed that human DI-CMTC-causing TyrRSE196K mis-interacts with the extracellular domain of the BDNF receptor TrkB, an aberrant association we have previously characterised for several mutant glycyl-tRNA synthetases linked to CMT type 2D (CMT2D). We then performed temporal neuromuscular assessments of YarsE196K mice modelling DI-CMT. We determined that YarsE196K homozygotes display a selective, age-dependent impairment in in vivo axonal transport of neurotrophin-containing signalling endosomes, phenocopying CMT2D mice. This impairment is replicated by injection of recombinant TyrRSE196K, but not TyrRSWT, into muscles of wild-type mice. Augmenting BDNF in DI-CMTC muscles, through injection of recombinant protein or muscle-specific gene therapy, resulted in complete axonal transport correction. Therefore, this work identifies a non-cell autonomous pathomechanism common to ARS-related neuropathies, and highlights the potential of boosting BDNF levels in muscles as a therapeutic strategy.

Beyond protein synthesis: non-translational functions of threonyl-tRNA synthetases

Aminoacyl-tRNA synthetases (AARSs) play an indispensable role in the translation of mRNAs into proteins. It has become amply clear that AARSs also have non-canonical or non-translational, yet essential, functions in a myriad of cellular and developmental processes. In this mini-review we discuss the current understanding of the roles of threonyl-tRNA synthetase (TARS) beyond protein synthesis and the underlying mechanisms. The two proteins in eukaryotes - cytoplasmic TARS1 and mitochondrial TARS2 - exert their non-canonical functions in the regulation of gene expression, cell signaling, angiogenesis, inflammatory responses, and tumorigenesis. The TARS proteins utilize a range of biochemical mechanisms, including assembly of a translation initiation complex, unexpected protein-protein interactions that lead to activation or inhibition of intracellular signaling pathways, and cytokine-like signaling through cell surface receptors in inflammation and angiogenesis. It is likely that new functions and novel mechanisms will continue to emerge for these multi-talented proteins.

Boosting BDNF in muscle rescues impaired axonal transport in a mouse model of DI-CMTC peripheral neuropathy

Charcot-Marie-Tooth disease (CMT) is a genetic peripheral neuropathy caused by mutations in many functionally diverse genes. The aminoacyl-tRNA synthetase (ARS) enzymes, which transfer amino acids to partner tRNAs for protein synthesis, represent the largest protein family genetically linked to CMT aetiology, suggesting pathomechanistic commonalities. Dominant intermediate CMT type C (DI-CMTC) is caused by YARS1 mutations driving a toxic gain-of-function in the encoded tyrosyl-tRNA synthetase (TyrRS), which is mediated by exposure of consensus neomorphic surfaces through conformational changes of the mutant protein. In this study, we first showed that human DI-CMTC-causing TyrRSE196K mis-interacts with the extracellular domain of the BDNF receptor TrkB, an aberrant association we have previously characterised for several mutant glycyl-tRNA synthetases linked to CMT type 2D (CMT2D). We then performed temporal neuromuscular assessments of YarsE196K mice modelling DI-CMT. We determined that YarsE196K homozygotes display a selective, age-dependent impairment in in vivo axonal transport of neurotrophin-containing signalling endosomes, phenocopying CMT2D mice. This impairment is replicated by injection of recombinant TyrRSE196K, but not TyrRSWT, into muscles of wild-type mice. Augmenting BDNF in DI-CMTC muscles, through injection of recombinant protein or muscle-specific gene therapy, resulted in complete axonal transport correction. Therefore, this work identifies a non-cell autonomous pathomechanism common to ARS-related neuropathies, and highlights the potential of boosting BDNF levels in muscles as a therapeutic strategy.

Protein-Protein Interactions of Seryl-tRNA Synthetases with Emphasis on Human Counterparts and Their Connection to Health and Disease

Seryl-tRNA synthetases (SerRSs), members of the aminoacyl-tRNA synthetase family, interact with diverse proteins, enabling SerRSs to enhance their role in the translation of the genetic message or to perform alternative functions in cellular processes beyond translation. Atypical archaeal SerRS interacts with arginyl-tRNA synthetase and proteins of the ribosomal P-stalk to optimize translation through tRNA channeling. The complex between yeast SerRS and peroxin Pex21p provides a connection between translation and peroxisome function. The partnership between Arabidopsis SerRS and BEN1 indicates a link between translation and brassinosteroid metabolism and may be relevant in plant stress response mechanisms. In Drosophila, the unusual heterodimeric mitochondrial SerRS coordinates mitochondrial translation and replication via interaction with LON protease. Evolutionarily conserved interactions of yeast and human SerRSs with m3C32 tRNA methyltransferases indicate coordination between tRNA modification and aminoacylation in the cytosol and mitochondria. Human cytosolic SerRS is a cellular hub protein connecting translation to vascular development, angiogenesis, lipogenesis, and telomere maintenance. When translocated to the nucleus, SerRS acts as a master negative regulator of VEGFA gene expression. SerRS alone or in complex with YY1 and SIRT2 competes with activating transcription factors NFκB1 and c-Myc, resulting in balanced VEGFA expression important for proper vascular development and angiogenesis. In hypoxia, SerRS phosphorylation diminishes its binding to the VEGFA promoter, while the lack of nutrients triggers SerRS glycosylation, reducing its nuclear localization. Additionally, SerRS binds telomeric DNA and cooperates with the shelterin protein POT1 to regulate telomere length and cellular senescence. As an antitumor and antiangiogenic factor, human cytosolic SerRS appears to be a promising drug target and therapeutic agent for treating cancer, cardiovascular diseases, and possibly obesity and aging.

An Adapted GeneSwitch Toolkit for Comparable Cellular and Animal Models: A Proof of Concept in Modeling Charcot-Marie-Tooth Neuropathy

Investigating the impact of disease-causing mutations, their affected pathways, and/or potential therapeutic strategies using disease modeling often requires the generation of different in vivo and in cellulo models. To date, several approaches have been established to induce transgene expression in a controlled manner in different model systems. Several rounds of subcloning are, however, required, depending on the model organism used, thus bringing labor-intensive experiments into the technical approach and analysis comparison. The GeneSwitch™ technology is an adapted version of the classical UAS-GAL4 inducible system, allowing the spatial and temporal modulation of transgene expression. It consists of three components: a plasmid encoding for the chimeric regulatory pSwitch protein, Mifepristone as an inducer, and an inducible plasmid. While the pSwitch-containing first plasmid can be used both in vivo and in cellulo, the inducible second plasmid can only be used in cellulo. This requires a specific subcloning strategy of the inducible plasmid tailored to the model organism used. To avoid this step and unify gene expression in the transgenic models generated, we replaced the backbone vector with standard pUAS-attB plasmid for both plasmids containing either the chimeric GeneSwitch™ cDNA sequence or the transgene cDNA sequence. We optimized this adapted system to regulate transgene expression in several mammalian cell lines. Moreover, we took advantage of this new system to generate unified cellular and fruit fly models for YARS1-induced Charco-Marie-Tooth neuropathy (CMT). These new models displayed the expected CMT-like phenotypes. In the N2a neuroblastoma cells expressing YARS1 transgenes, we observed the typical "teardrop" distribution of the synthetase that was perturbed when expressing the YARS1CMT mutation. In flies, the ubiquitous expression of YARS1CMT induced dose-dependent developmental lethality and pan-neuronal expression caused locomotor deficit, while expression of the wild-type allele was harmless. Our proof-of-concept disease modeling studies support the efficacy of the adapted transgenesis system as a powerful tool allowing the design of studies with optimal data comparability.

Aminoacyl-tRNA synthetase - a molecular multitasker

Aminoacyl-tRNA synthetases (AaRSs) are valuable "housekeeping" enzymes that ensure the accurate transmission of genetic information in living cells, where they aminoacylated tRNA molecules with their cognate amino acid and provide substrates for protein biosynthesis. In addition to their translational or canonical function, they contribute to nontranslational/moonlighting functions, which are mediated by the presence of other domains on the proteins. This was supported by several reports which claim that AaRS has a significant role in gene transcription, apoptosis, translation, and RNA splicing regulation. Noncanonical/ nontranslational functions of AaRSs also include their roles in regulating angiogenesis, inflammation, cancer, and other major physio-pathological processes. Multiple AaRSs are also associated with a broad range of physiological and pathological processes; a few even serve as cytokines. Therefore, the multifunctional nature of AaRSs suggests their potential as viable therapeutic targets as well. Here, our discussion will encompass a range of noncanonical functions attributed to Aminoacyl-tRNA Synthetases (AaRSs), highlighting their links with a diverse array of human diseases.

Physiological and engineered tRNA aminoacylation

Aminoacyl-tRNA synthetases form the protein family that controls the interpretation of the genetic code, with tRNA aminoacylation being the key chemical step during which an amino acid is assigned to a corresponding sequence of nucleic acids. In consequence, aminoacyl-tRNA synthetases have been studied in their physiological context, in disease states, and as tools for synthetic biology to enable the expansion of the genetic code. Here, we review the fundamentals of aminoacyl-tRNA synthetase biology and classification, with a focus on mammalian cytoplasmic enzymes. We compile evidence that the localization of aminoacyl-tRNA synthetases can be critical in health and disease. In addition, we discuss evidence from synthetic biology which made use of the importance of subcellular localization for efficient manipulation of the protein synthesis machinery. This article is categorized under: RNA Processing Translation > Translation Regulation RNA Processing > tRNA Processing RNA Export and Localization > RNA Localization.

Translation dysregulation in neurodegenerative diseases: a focus on ALS

RNA translation is tightly controlled in eukaryotic cells to regulate gene expression and maintain proteome homeostasis. RNA binding proteins, translation factors, and cell signaling pathways all modulate the translation process. Defective translation is involved in multiple neurological diseases including amyotrophic lateral sclerosis (ALS). ALS is a progressive neurodegenerative disorder and poses a major public health challenge worldwide. Over the past few years, tremendous advances have been made in the understanding of the genetics and pathogenesis of ALS. Dysfunction of RNA metabolisms, including RNA translation, has been closely associated with ALS. Here, we first introduce the general mechanisms of translational regulation under physiological and stress conditions and review well-known examples of translation defects in neurodegenerative diseases. We then focus on ALS-linked genes and discuss the recent progress on how translation is affected by various mutant genes and the repeat expansion-mediated non-canonical translation in ALS.

Dominant aminoacyl-tRNA synthetase disorders: lessons learned from in vivo disease models

Aminoacyl-tRNA synthetases (ARSs) play an essential role in protein synthesis, being responsible for ligating tRNA molecules to their corresponding amino acids in a reaction known as 'tRNA aminoacylation'. Separate ARSs carry out the aminoacylation reaction in the cytosol and in mitochondria, and mutations in almost all ARS genes cause pathophysiology most evident in the nervous system. Dominant mutations in multiple cytosolic ARSs have been linked to forms of peripheral neuropathy including Charcot-Marie-Tooth disease, distal hereditary motor neuropathy, and spinal muscular atrophy. This review provides an overview of approaches that have been employed to model each of these diseases in vivo, followed by a discussion of the existing animal models of dominant ARS disorders and key mechanistic insights that they have provided. In summary, ARS disease models have demonstrated that loss of canonical ARS function alone cannot fully account for the observed disease phenotypes, and that pathogenic ARS variants cause developmental defects within the peripheral nervous system, despite a typically later onset of disease in humans. In addition, aberrant interactions between mutant ARSs and other proteins have been shown to contribute to the disease phenotypes. These findings provide a strong foundation for future research into this group of diseases, providing methodological guidance for studies on ARS disorders that currently lack in vivo models, as well as identifying candidate therapeutic targets.

Recessive aminoacyl-tRNA synthetase disorders: lessons learned from in vivo disease models

Protein synthesis is a fundamental process that underpins almost every aspect of cellular functioning. Intriguingly, despite their common function, recessive mutations in aminoacyl-tRNA synthetases (ARSs), the family of enzymes that pair tRNA molecules with amino acids prior to translation on the ribosome, cause a diverse range of multi-system disorders that affect specific groups of tissues. Neurological development is impaired in most ARS-associated disorders. In addition to central nervous system defects, diseases caused by recessive mutations in cytosolic ARSs commonly affect the liver and lungs. Patients with biallelic mutations in mitochondrial ARSs often present with encephalopathies, with variable involvement of peripheral systems. Many of these disorders cause severe disability, and as understanding of their pathogenesis is currently limited, there are no effective treatments available. To address this, accurate in vivo models for most of the recessive ARS diseases are urgently needed. Here, we discuss approaches that have been taken to model recessive ARS diseases in vivo, highlighting some of the challenges that have arisen in this process, as well as key results obtained from these models. Further development and refinement of animal models is essential to facilitate a better understanding of the pathophysiology underlying recessive ARS diseases, and ultimately to enable development and testing of effective therapies.

Boosting peripheral BDNF rescues impaired in vivo axonal transport in CMT2D mice

Gain-of-function mutations in the housekeeping gene GARS1, which lead to the expression of toxic versions of glycyl-tRNA synthetase (GlyRS), cause the selective motor and sensory pathology characterizing Charcot-Marie-Tooth disease (CMT). Aberrant interactions between GlyRS mutants and different proteins, including neurotrophin receptor tropomyosin receptor kinase receptor B (TrkB), underlie CMT type 2D (CMT2D); however, our pathomechanistic understanding of this untreatable peripheral neuropathy remains incomplete. Through intravital imaging of the sciatic nerve, we show that CMT2D mice displayed early and persistent disturbances in axonal transport of neurotrophin-containing signaling endosomes in vivo. We discovered that brain-derived neurotrophic factor (BDNF)/TrkB impairments correlated with transport disruption and overall CMT2D neuropathology and that inhibition of this pathway at the nerve-muscle interface perturbed endosome transport in wild-type axons. Accordingly, supplementation of muscles with BDNF, but not other neurotrophins, completely restored physiological axonal transport in neuropathic mice. Together, these findings suggest that selectively targeting muscles with BDNF-boosting therapies could represent a viable therapeutic strategy for CMT2D.

Tyrosyl-tRNA synthetase has a noncanonical function in actin bundling

Dominant mutations in tyrosyl-tRNA synthetase (YARS1) and six other tRNA ligases cause Charcot-Marie-Tooth peripheral neuropathy (CMT). Loss of aminoacylation is not required for their pathogenicity, suggesting a gain-of-function disease mechanism. By an unbiased genetic screen in Drosophila, we link YARS1 dysfunction to actin cytoskeleton organization. Biochemical studies uncover yet unknown actin-bundling property of YARS1 to be enhanced by a CMT mutation, leading to actin disorganization in the Drosophila nervous system, human SH-SY5Y neuroblastoma cells, and patient-derived fibroblasts. Genetic modulation of F-actin organization improves hallmark electrophysiological and morphological features in neurons of flies expressing CMT-causing YARS1 mutations. Similar beneficial effects are observed in flies expressing a neuropathy-causing glycyl-tRNA synthetase. Hence, in this work, we show that YARS1 is an evolutionary-conserved F-actin organizer which links the actin cytoskeleton to tRNA-synthetase-induced neurodegeneration.

Fundamentals behind the specificity of Cysteinyl-tRNA synthetase: MD and QM/MM joint investigations

Cysteinyl-tRNA synthetase (CysRS) catalyzes the aminoacylation reaction of cysteine to its cognate tRNA^Cys in the first step of protein translation. It is found that CysRS is different from other aaRSs as it transfers cysteine without the need for an editing reaction, which is not applicable in the case of serine despite the similarity in their structures. Surprisingly, the reasons why CysRS has high amino acid specificity are not clear yet. In this research, the binding configurations of Cys-AMP and its near-cognate amino acid Ser-AMP with CysRS are compared by Molecular Dynamics (MD). The results reveal that CysRS screens the substrate Cys-AMP to a certain extent in the process of combination and recognition, thus providing a guarantee for the high selectivity of the next reaction. While Ser-AMP is in a folded state in CysRS. In the meanwhile, the interaction between Cys-AMP and Zn963 in CysRS is much stronger than Ser-AMP. The substrate-assisted aminoacylation mechanism in CysRS is also explored by Quantum Mechanics/Molecular Mechanics (QM/MM) modeling. According to the QM/MM potential energies, the energy barrier of TS_Cys-AMP is 91.75 kJ/mol, while that of TS_Ser-AMP is close to 150 kJ/mol. Based on thermochemistry calculations, it is found that the product of Cys-AMP is more stable than the reactant. In contrast, Ser-AMP has a reactant that is more stable than its product. As a result, it reflects that the specificity of CysRS originates from both the kinetic and thermodynamical perspectives of the reaction. Our investigations demonstrate comprehensively on how CysRS recognizes and catalyzes the substrate Cys-AMP, hoping to provide some guidance for researchers in this area.

Diverse CMT2 neuropathies are linked to aberrant G3BP interactions in stress granules

Complex diseases often involve the interplay between genetic and environmental factors. Charcot-Marie-Tooth type 2 neuropathies (CMT2) are a group of genetically heterogeneous disorders, in which similar peripheral neuropathology is inexplicably caused by various mutated genes. Their possible molecular links remain elusive. Here, we found that upon environmental stress, many CMT2-causing mutant proteins adopt similar properties by entering stress granules (SGs), where they aberrantly interact with G3BP and integrate into SG pathways. For example, glycyl-tRNA synthetase (GlyRS) is translocated from the cytoplasm into SGs upon stress, where the mutant GlyRS perturbs the G3BP-centric SG network by aberrantly binding to G3BP. This disrupts SG-mediated stress responses, leading to increased stress vulnerability in motoneurons. Disrupting this aberrant interaction rescues SG abnormalities and alleviates motor deficits in CMT2D mice. These findings reveal a stress-dependent molecular link across diverse CMT2 mutants and provide a conceptual framework for understanding genetic heterogeneity in light of environmental stress.

Towards a Cure for HARS Disease

Histidyl-tRNA synthetase (HARS) ligates histidine to its cognate transfer RNA (tRNA^His). Mutations in HARS cause the human genetic disorders Usher syndrome type 3B (USH3B) and Charcot-Marie-Tooth syndrome type 2W (CMT2W). Treatment for these diseases remains symptomatic, and no disease specific treatments are currently available. Mutations in HARS can lead to destabilization of the enzyme, reduced aminoacylation, and decreased histidine incorporation into the proteome. Other mutations lead to a toxic gain-of-function and mistranslation of non-cognate amino acids in response to histidine codons, which can be rescued by histidine supplementation in vitro. We discuss recent advances in characterizing HARS mutations and potential applications of amino acid and tRNA therapy for future gene and allele specific therapy.

Novel TARS2 variant identified in a Chinese patient with mitochondrial encephalomyopathy and a systematic review

Biallelic pathogenic variants in the TARS2 gene cause combined oxidative phosphorylation deficiency, subtype 21 (COXPD21, MIM #615918), which is a rare mitochondrial encephalomyopathy (ME) characterized by early-onset severe axial hypotonia, limb hypertonia, delayed psychomotor development, epilepsy, and brain anomalies. Currently, eight COXPD21 patients have been reported in the literature, and 11 pathogenic variants in TARS2 have been identified. Here, we report a 2-year-6-month-old Chinese female who presented with severe dystonia, developmental regression, absent speech, and intractable epilepsy. Laboratory examination showed persistently increased serum lactate. Brain MRI showed that the head of the caudate and partial lenticular nucleus were bilateral symmetrical T2-weighted imaging (T2WI) hyperintense and the corpus callosum was very thin. The clinical characteristics pointed to a ME. Trio-based whole-exome sequencing (WES) was employed to detect the causative variants. WES revealed novel compound heterozygous variants, c.470G>C (p.Thr157Arg) and c.2051C>T (p.Arg684Gln), in TARS2 in our patient that were inherited from the mother and father, respectively. Next, we systematically reviewed the available clinical features of COXPD21 patients and noticed that the reduced fetal movement observed in our patient may be a novel phenotype of COXPD21. These findings expand the mutation spectrum of TARS2 and provide insights into the genotype-phenotype relationship in COXPD21 as well as a foundation for its genetic counseling, diagnosis and treatment.

Clinical and genetic analyses of premature mitochondrial encephalopathy with epilepsia partialis continua caused by novel biallelic NARS2 mutations

Biallelic NARS2 mutations can cause various neurodegenerative diseases, leading to growth retardation, intractable epilepsy, and hearing loss in early infancy and further progressing to spastic paraplegia, neurodegeneration, and even death. NARS2 mutations are associated with mitochondrial dysfunction and cause combined oxidative phosphorylation deficiency 24 (COXPD24). Relatively few cases have been reported worldwide; therefore, the pathogenesis of COXPD24 is poorly understood. We studied two unrelated patients with COXPD24 with similar phenotypes who presented with intractable refractory epilepsia partialis continua, hearing loss, and growth retardation. One patient died from epilepsy. Three novel NARS2 variants (case 1: c.185T > C and c.251 + 2T > G; case 2: c.185T > C and c.509T > G) were detected with whole-exome sequencing. c.251 + 2T > G is located at the donor splicing site in the non-coding sequence of the gene. The minigene experiment further verified that c.251 + 2T > G caused variable splicing abnormalities and produced truncated proteins. Molecular dynamics studies showed that c.185T > C and c.509T > G reduced the binding free energy of the NARS2 protein dimer. The literature review revealed fewer than 30 NARS2 variants. These findings improved our understanding of the disease phenotype and the variation spectrum and revealed the potential pathogenic mechanism of non-coding sequence mutations in COXPD24.

Modeling neuromuscular diseases in zebrafish

Neuromuscular diseases are a diverse group of conditions that affect the motor system and present some overlapping as well as distinct clinical manifestations. Although individually rare, the combined prevalence of NMDs is similar to Parkinson's. Over the past decade, new genetic mutations have been discovered through whole exome/genome sequencing, but the pathogenesis of most NMDs remains largely unexplored. Little information on the molecular mechanism governing the progression and development of NMDs accounts for the continual failure of therapies in clinical trials. Different aspects of the diseases are typically investigated using different models from cells to animals. Zebrafish emerges as an excellent model for studying genetics and pathogenesis and for developing therapeutic interventions for most NMDs. In this review, we describe the generation of different zebrafish genetic models mimicking NMDs and how they are used for drug discovery and therapy development.

Clinicopathological features in two families with MARS-related Charcot-Marie-Tooth disease

Mutations in MARS gene cause dominant Charcot-Marie-Tooth disease (CMT) 2U. The aim of this study is to investigate phenotypic heterogeneities and peripheral neuropathology of MARS-related CMT patients. We identified a heterozygous p. R199Q mutation and an already reported heterozygous p. P800T mutation of MARS gene in two unrelated families using targeted next-generation sequencing. The first pedigree comprised three patients over three generations and the second pedigree comprised two patients over two generations. In addition of an asymptomatic carrier in the second pedigree, all patients presented with childhood-onset length dependent sensorimotor neuropathy with pes cavus. Nerve conduction studies revealed slowing of motor nerve conduction velocities (MNCV) of the median nerve indicating intermediate neuropathy in the patient with the p. R199Q mutation, and normal MNCV with reduced compound muscle action potential indicating axonal neuropathy in the patient with the p. P800T mutation. Magnetic resonance imaging detected a pattern of nerve changes similar to those in demyelinating polyneuropathies in intermediate type (p. R199Q mutation) patients compared with normal in the axonal type (p. P800T mutation) patients. Additionally, sural nerve biopsy revealed loss of myelinated axons with onion bulb formation in both mutations. By electron microscopy, a marked decrease of myelinated and unmyelinated fiber, neurofilaments aggregate with degenerating mitochondria and microtubule loss in axons were frequently found. Denervated Schwann cell complexes and few collagen pockets indicated involvement of unmyelinated Schwann cells. Therefore, the investigated MARS mutations cause not only the known axonal type but also intermediate type neuropathy with involvement of both axons and Schwann cells. Those findings are useful for the differential diagnosis of CMT patients with unknown MARS variants.

Aminoacyl-tRNA synthetases in human health and disease

The Aminoacyl-tRNA Synthetases (aaRSs) are an evolutionarily ancient family of enzymes that catalyze the esterification reaction linking a transfer RNA (tRNA) with its cognate amino acid matching the anticodon triplet of the tRNA. Proper functioning of the aaRSs to create aminoacylated (or “charged”) tRNAs is required for efficient and accurate protein synthesis. Beyond their basic canonical function in protein biosynthesis, aaRSs have a surprisingly diverse array of non-canonical functions that are actively being defined. The human genome contains 37 genes that encode unique aaRS proteins. To date, 56 human genetic diseases caused by damaging variants in aaRS genes have been described: 46 are autosomal recessive biallelic disorders and 10 are autosomal dominant monoallelic disorders. Our appreciation of human diseases caused by damaging genetic variants in the aaRSs has been greatly accelerated by the advent of next-generation sequencing, with 89% of these gene discoveries made since 2010. In addition to these genetic disorders of the aaRSs, anti-synthetase syndrome (ASSD) is a rare autoimmune inflammatory myopathy that involves the production of autoantibodies that disrupt aaRS proteins. This review provides an overview of the basic biology of aaRS proteins and describes the rapidly growing list of human diseases known to be caused by genetic variants or autoimmune targeting that affect both the canonical and non-canonical functions of these essential proteins.

Alanyl-tRNA Synthetase 1 Gene Variants in Hereditary Neuropathy

Background and Objectives

Our objective was to report 2 novel variants and to reclassify previously reported alanyl-tRNA synthetase 1 (AARS1) variants associated with hereditary neuropathy and to summarize the clinical features of a previously published cohort of patients.

Methods

We performed detailed neurologic and electrophysiologic assessments and segregation analysis of 2 unrelated families with Charcot-Marie-Tooth (CMT) disease with novel variants in the AARS1 gene. Via literature search, we found studies that included neuropathy cases with AARS1 variants; we then reviewed and reclassified these variants.

Results

We identified 2 CMT families harboring previously unreported likely pathogenic AARS1 variants: c.1823C>A p.(Thr608Lys) and c.1815C>G p.(His605Gln). In addition, we reinterpreted a total of 35 different AARS1 variants reported in cases with neuropathy from the literature: 9 variants fulfilled the current criteria for being (likely) pathogenic. We compiled and summarized standardized clinical and genotypic information for 90 affected individuals from 32 families with (likely) pathogenic AARS1 variants. Most experienced motor weakness and sensory loss in the lower limbs.

Discussion

In total, 11 AARS1 variants can currently be classified as pathogenic or likely pathogenic and are associated with sensorimotor axonal or intermediate, slowly progressive polyneuropathy with common asymmetry and variable age of symptom onset with no apparent involvement of other organ systems.

Clinical characteristics and proteome modifications in two Charcot-Marie-Tooth families with the AARS1 Arg326Trp mutation

BACKGROUND

Aminoacyl tRNA-synthetases are ubiquitously-expressed enzymes that attach amino acids to their cognate tRNA molecules. Mutations in several genes encoding aminoacyl tRNA-synthetases, have been associated with peripheral neuropathy, i.e. AARS1, GARS1, HARS1, YARS1 and WARS1. The pathogenic mechanism underlying AARS1-related neuropathy is not known.

METHODS

From 2012 onward, all probands presenting at Telemark Hospital (Skien, Norway) with peripheral neuropathy were screened for variants in AARS1 using an "in-house" next-generation sequencing panel. DNA from patient's family members was examined by Sanger sequencing. Blood from affected family members and healthy controls were used for quantification of AARS1 mRNA and alanine. Proteomic analyses were conducted in peripheral blood mononuclear cells (PBMC) from four affected family members and five healthy controls.

RESULTS

Seventeen individuals in two Norwegian families affected by Charcot-Marie-Tooth disease (CMT) were characterized in this study. The heterozygous NM_001605.2:c.976C > T p.(Arg326Trp) AARS1 mutation was identified in ten affected family members. All living carriers had a mild to severe length-dependent sensorimotor neuropathy. Three deceased obligate carriers aged 74-98 were reported to be unaffected, but were not examined in the clinic. Proteomic studies in PBMC from four affected individuals suggest an effect on the immune system mediated by components of a systemic response to chronic injury and inflammation. Furthermore, altered expression of proteins linked to mitochondrial function/dysfunction was observed. Proteomic data are available via ProteomeXchange using identifier PXD023842.

CONCLUSION

This study describes clinical and neurophysiological features linked to the p.(Arg326Trp) variant of AARS1 in CMT-affected members of two Norwegian families. Proteomic analyses based on of PBMC from four CMT-affected individuals suggest that involvement of inflammation and mitochondrial dysfunction might contribute to AARS1 variant-associated peripheral neuropathy.

Neuropilin 1 and its inhibitory ligand mini-tryptophanyl-tRNA synthetase inversely regulate VE-cadherin turnover and vascular permeability

The formation of a functional blood vessel network relies on the ability of endothelial cells (ECs) to dynamically rearrange their adhesive contacts in response to blood flow and guidance cues, such as vascular endothelial growth factor-A (VEGF-A) and class 3 semaphorins (SEMA3s). Neuropilin 1 (NRP1) is essential for blood vessel development, independently of its ligands VEGF-A and SEMA3, through poorly understood mechanisms. Grounding on unbiased proteomic analysis, we report here that NRP1 acts as an endocytic chaperone primarily for adhesion receptors on the surface of unstimulated ECs. NRP1 localizes at adherens junctions (AJs) where, interacting with VE-cadherin, promotes its basal internalization-dependent turnover and favors vascular permeability initiated by histamine in both cultured ECs and mice. We identify a splice variant of tryptophanyl-tRNA synthetase (mini-WARS) as an unconventionally secreted extracellular inhibitory ligand of NRP1 that, by stabilizing it at the AJs, slows down both VE-cadherin turnover and histamine-elicited endothelial leakage. Thus, our work shows a role for NRP1 as a major regulator of AJs plasticity and reveals how mini-WARS acts as a physiological NRP1 inhibitory ligand in the control of VE-cadherin endocytic turnover and vascular permeability.

Aminoacyl-tRNA Synthetases: On Anti-Synthetase Syndrome and Beyond

Anti-synthetase syndrome (ASSD) is an autoimmune disease characterized by the presence of autoantibodies targeting one of several aminoacyl t-RNA synthetases (aaRSs) along with clinical features including interstitial lung disease, myositis, Raynaud’s phenomenon, arthritis, mechanic’s hands, and fever. The family of aaRSs consists of highly conserved cytoplasmic and mitochondrial enzymes, one for each amino acid, which are essential for the RNA translation machinery and protein synthesis. Along with their main functions, aaRSs are involved in the development of immune responses, regulation of transcription, and gene-specific silencing of translation. During the last decade, these proteins have been associated with cancer, neurological disorders, infectious responses, and autoimmune diseases including ASSD. To date, several aaRSs have been described to be possible autoantigens in different diseases. The most commonly described are histidyl (HisRS), threonyl (ThrRS), alanyl (AlaRS), glycyl (GlyRS), isoleucyl (IleRS), asparaginyl (AsnRS), phenylalanyl (PheRS), tyrosyl (TyrRS), lysyl (LysRS), glutaminyl (GlnRS), tryptophanyl (TrpRS), and seryl (SerRS) tRNA synthetases. Autoantibodies against the first eight autoantigens listed above have been associated with ASSD while the rest have been associated with other diseases. This review will address what is known about the function of the aaRSs with a focus on their autoantigenic properties. We will also describe the anti-aaRSs autoantibodies and their association to specific clinical manifestations, and discuss their potential contribution to the pathogenesis of ASSD.

The Antiepileptic Valproic Acid Ameliorates Charcot-Marie-Tooth 2W (CMT2W) Disease-Associated HARS1 Mutation-Induced Inhibition of Neuronal Cell Morphological Differentiation Through c-Jun N-terminal Kinase

Hereditary peripheral neuropathies called Charcot-Marie-Tooth (CMT) disease affect the sensory nerves as well as motor neurons. CMT diseases are composed of a heterogeneous group of diseases. They are characterized by symptoms such as muscle weakness and wasting. Type 2 CMT (CMT2) disease is a neuropathy with blunted or disrupted neuronal morphological differentiation phenotypes including process formation of peripheral neuronal axons. In the early stages of CMT2, demyelination that occurs in Schwann cells (glial cells) is rarely observed. CMT2W is an autosomal-dominant disease and is responsible for the gene encoding histidyl-tRNA synthetase 1 (HARS1), which is a family molecule of cytoplasmic aminoacyl-tRNA synthetases and functions by ligating histidine to its cognate tRNA. Despite increasing knowledge of the relationship of mutations on responsible genes with diseases, it still remains unclear how each mutation affects neuronal differentiation. Here we show that in neuronal N1E-115 cells, a severe Asp364-to-Tyr (D364Y) mutation of HARS1 leads to formation of small aggregates of HARS1 proteins; in contrast, wild type proteins are distributed throughout cell bodies. Expression of D364Y mutant proteins inhibited process formation whereas expression of wild type proteins possessed the normal differentiation ability to grow processes. Pretreatment with the antiepileptic valproic acid recovered inhibition of process formation by D364Y mutant proteins through the c-Jun N-terminal kinase signaling pathway. Taken together, these results indicate that the D364Y mutation of HARS1 causes HARS1 proteins to form small aggregates, inhibiting process growth, and that these effects are recovered by valproic acid. This could be a potential therapeutic drug for CMT2W at the cellular levels.

HDAC6 Inhibition Corrects Electrophysiological and Axonal Transport Deficits in a Human Stem Cell-Based Model of Charcot-Marie-Tooth Disease (Type 2D)

Charcot-Marie-Tooth disease type 2D (CMT2D), is a hereditary peripheral neuropathy caused by mutations in the gene encoding glycyl-tRNA synthetase (GARS1). Here, human induced pluripotent stem cell (hiPSC)-based models of CMT2D bearing mutations in GARS1 and their use for the identification of predictive biomarkers amenable to therapeutic efficacy screening is described. Cultures containing spinal cord motor neurons generated from this line exhibit network activity marked by significant deficiencies in spontaneous action potential firing and burst fire behavior. This result matches clinical data collected from a patient bearing a GARS1^P724H mutation and is coupled with significant decreases in acetylated α-tubulin levels and mitochondrial movement within axons. Treatment with histone deacetylase 6 inhibitors, tubastatin A and CKD504, improves mitochondrial movement and α-tubulin acetylation in these cells. Furthermore, CKD504 treatment enhances population-level electrophysiological activity, highlighting its potential as an effective treatment for CMT2D.

Systematic expression profiling of neuropathy-related aminoacyl-tRNA synthetases in zebrafish during development

Aminoacyl tRNA synthetases (ARSs) are a group of proteins, acting as transporters to transfer and attach the appropriate amino acids onto their cognate tRNAs for translation. So far, 18 out of 20 cytoplasmic ARSs are reported to be connected to different neuropathy disorders with multi-organ defects that are often accompanied with developmental delays. Thus, it is important to understand functions and impacts of ARSs at the whole organism level. Here, we systematically analyzed the spatiotemporal expression of 14 ars and 2 aimp genes during development in zebrafish that have not be previously reported. Not only in the brain, their dynamic expression patterns in several tissues such as in the muscles, liver and intestine suggest diverse roles in a wide range of development processes in addition to neuronal function, which is consistent with potential involvement in multiple syndrome diseases associated with ARS mutations. In particular, hinted by its robust expression pattern in the brain, we confirmed that aimp1 is required for the formation of cerebrovasculature by a loss-of-function approach. Overall, our systematic profiling data provides a useful basis for studying roles of ARSs during development and understanding their potential functions in the etiology of related diseases.

An Integrated Approach to Studying Rare Neuromuscular Diseases Using Animal and Human Cell-Based Models

As sequencing technology improves, the identification of new disease-associated genes and new alleles of known genes is rapidly increasing our understanding of the genetic underpinnings of rare diseases, including neuromuscular diseases. However, precisely because these disorders are rare and often heterogeneous, they are difficult to study in patient populations. In parallel, our ability to engineer the genomes of model organisms, such as mice or rats, has gotten increasingly efficient through techniques such as CRISPR/Cas9 genome editing, allowing the creation of precision human disease models. Such in vivo model systems provide an efficient means for exploring disease mechanisms and identifying therapeutic strategies. Furthermore, animal models provide a platform for preclinical studies to test the efficacy of those strategies. Determining whether the same mechanisms are involved in the human disease and confirming relevant parameters for treatment ideally involves a human experimental system. One system currently being used is induced pluripotent stem cells (iPSCs), which can then be differentiated into the relevant cell type(s) for in vitro confirmation of disease mechanisms and variables such as target engagement. Here we provide a demonstration of these approaches using the example of tRNA-synthetase-associated inherited peripheral neuropathies, rare forms of Charcot-Marie-Tooth disease (CMT). Mouse models have led to a better understanding of both the genetic and cellular mechanisms underlying the disease. To determine if the mechanisms are similar in human cells, we will use genetically engineered iPSC-based models. This will allow comparisons of different CMT-associated GARS alleles in the same genetic background, reducing the variability found between patient samples and simplifying the availability of cell-based models for a rare disease. The necessity of integrating mouse and human models, strategies for accomplishing this integration, and the challenges of doing it at scale are discussed using recently published work detailing the cellular mechanisms underlying GARS-associated CMT as a framework.

Comprehensive characterization of mRNAs associated with yeast cytosolic aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRSs) are a conserved family of enzymes with an essential role in protein synthesis: ligating amino acids to cognate tRNA molecules for translation. In addition to their role in tRNA charging, aaRSs have acquired non-canonical functions, including post-transcriptional regulation of mRNA expression. Yet, the extent and mechanisms of these post-transcriptional functions are largely unknown. Herein, we performed a comprehensive transcriptome analysis to define the mRNAs that are associated with almost all aaRSs present in S. cerevisiae cytosol. Nineteen (out of twenty) isogenic strains of GFP-tagged cytosolic aaRSs were subjected to immunoprecipitation with anti-GFP beads along with an untagged control. mRNAs associated with each aaRS were then identified by RNA-seq. The extent of mRNA association varied significantly between aaRSs, from MetRS in which none appeared to be statistically significant, to PheRS that binds hundreds of different mRNAs. Interestingly, many target mRNAs are bound by multiple aaRSs, suggesting co-regulation by this family of enzymes. Gene Ontology analyses for aaRSs with a considerable number of target mRNAs discovered an enrichment for pathways of amino acid metabolism and of ribosome biosynthesis. Furthermore, sequence and structure motif analysis revealed for some aaRSs an enrichment for motifs that resemble the anticodon stem loop of cognate tRNAs. These data suggest that aaRSs coordinate mRNA expression in response to amino acid availability and may utilize RNA elements that mimic their canonical tRNA binding partners.

Variants of aminoacyl-tRNA synthetase genes in Charcot-Marie-Tooth disease: A Korean cohort study

Charcot-Marie-Tooth disease (CMT) and related diseases are a genetically and clinically heterogeneous group of peripheral neuropathies. Particularly, mutations in several aminoacyl-tRNA synthetase (ARS) genes have been reported to cause axonal CMT (CMT2) or distal hereditary motor neuropathy (dHMN). However, the common pathogenesis among CMT subtypes by different ARS gene defects is not well understood. This study was performed to investigate ARS gene mutations in a CMT cohort of 710 Korean families. Whole-exome sequencing was applied to 710 CMT patients who were negative for PMP22 duplication. We identified 12 disease-causing variants (from 13 families) in GARS1, AARS1, HARS1, WARS1, and YARS1 genes. Seven variants were determined to be novel. The frequency of overall ARS gene mutations was 1.22% among all independent patients diagnosed with CMT and 1.83% in patients negative for PMP22 duplication. WARS1 mutations have been reported to cause dHMN; however, in our patients with WARS1 variants, CMT was associated with sensory involvement. We analyzed genotype-phenotype correlations and expanded the phenotypic spectrum of patients with CMT possessing ARS gene variants. We also characterized clinical phenotypes according to ARS genes. This study will be useful for performing exact molecular and clinical diagnoses and providing reference data for other population studies.

AAV1.NT-3 gene therapy in a CMT2D model: phenotypic improvements in GarsP278KY/+ mice

Glycyl-tRNA synthetase mutations are associated to the Charcot-Marie-Tooth disease type-2D. The Gars^P278KY/+ model for Charcot-Marie-Tooth disease type-2D is known best for its early onset severe neuropathic phenotype with findings including reduced axon size, slow conduction velocities and abnormal neuromuscular junction. Muscle involvement remains largely unexamined. We tested the efficacy of neurotrophin 3 gene transfer therapy in two Gars mutants with severe (Gars^P278KY/+ ) and milder (Gars^ΔETAQ/+ ) phenotypes via intramuscular injection of adeno-associated virus setoype-1, triple tandem muscle creatine kinase promoter, neurotrophin 3 (AAV1.tMCK.NT-3) at 1 × 10¹¹ vg dose. In the Gars^P278KY/+ mice, the treatment efficacy was assessed at 12 weeks post-injection using rotarod test, electrophysiology and detailed quantitative histopathological studies of the peripheral nervous system including neuromuscular junction and muscle. Neurotrophin 3 gene transfer therapy in Gars^P278KY/+ mice resulted in significant functional and electrophysiological improvements, supported with increases in myelin thickness and improvements in the denervated status of neuromuscular junctions as well as increases in muscle fibre size along with attenuation of myopathic changes. Improvements in the milder phenotype Gars^ΔETAQ/+ was less pronounced. Furthermore, oxidative enzyme histochemistry in muscles from Gars mutants revealed alterations in the content and distribution of oxidative enzymes with increased expression levels of Pgc1a. Cox1, Cox3 and Atp5d transcripts were significantly decreased suggesting that the muscle phenotype might be related to mitochondrial dysfunction. Neurotrophin 3 gene therapy attenuated these abnormalities in the muscle. This study shows that neurotrophin 3 gene transfer therapy has disease modifying effect in a mouse model for Charcot-Marie-Tooth disease type-2D, leading to meaningful improvements in peripheral nerve myelination and neuromuscular junction integrity as well as in a unique myopathic process, associated with mitochondria dysfunction, all in combination contributing to functional outcome. Based on the multiple biological effects of this versatile molecule, we predict neurotrophin 3 has the potential to be beneficial in other aminoacyl-tRNA synthetase-linked Charcot-Marie-Tooth disease subtypes.

The integrated stress response contributes to tRNA synthetase-associated peripheral neuropathy

Dominant mutations in ubiquitously expressed transfer RNA (tRNA) synthetase genes cause axonal peripheral neuropathy, accounting for at least six forms of Charcot-Marie-Tooth (CMT) disease. Genetic evidence in mouse and Drosophila models suggests a gain-of-function mechanism. In this study, we used in vivo, cell type–specific transcriptional and translational profiling to show that mutant tRNA synthetases activate the integrated stress response (ISR) through the sensor kinase GCN2 (general control nonderepressible 2). The chronic activation of the ISR contributed to the pathophysiology, and genetic deletion or pharmacological inhibition of Gcn2 alleviated the peripheral neuropathy. The activation of GCN2 suggests that the aberrant activity of the mutant tRNA synthetases is still related to translation and that inhibiting GCN2 or the ISR may represent a therapeutic strategy in CMT.

Stressing out translation

[Figure: see text].

tRNA overexpression rescues peripheral neuropathy caused by mutations in tRNA synthetase

[Figure: see text].

X-shaped structure of bacterial heterotetrameric tRNA synthetase suggests cryptic prokaryote functions and a rationale for synthetase classifications

AaRSs (aminoacyl-tRNA synthetases) group into two ten-member classes throughout evolution, with unique active site architectures defining each class. Most are monomers or homodimers but, for no apparent reason, many bacterial GlyRSs are heterotetramers consisting of two catalytic α-subunits and two tRNA-binding β-subunits. The heterotetrameric GlyRS from Escherichia coli (EcGlyRS) was historically tested whether its α- and β-polypeptides, which are encoded by a single mRNA with a gap of three in-frame codons, are replaceable by a single chain. Here, an unprecedented X-shaped structure of EcGlyRS shows wide separation of the abutting chain termini seen in the coding sequences, suggesting strong pressure to avoid a single polypeptide format. The structure of the five-domain β-subunit is unique across all aaRSs in current databases, and structural analyses suggest these domains play different functions on α-subunit binding, ATP coordination and tRNA recognition. Moreover, the X-shaped architecture of EcGlyRS largely fits with a model for how two classes of tRNA synthetases arose, according to whether enzymes from opposite classes can simultaneously co-dock onto separate faces of the same tRNA acceptor stem. While heterotetrameric GlyRS remains the last structurally uncharacterized member of aaRSs, our study contributes to a better understanding of this ancient and essential enzyme family.

Associations between Neurological Diseases and Mutations in the Human Glycyl-tRNA Synthetase

Aminoacyl-RNA synthetases (aaRSs) are among the key enzymes of protein biosynthesis. They are responsible for conducting the first step in the protein biosynthesis, namely attaching amino acids to the corresponding tRNA molecules both in cytoplasm and mitochondria. More and more research demonstrates that mutations in the genes encoding aaRSs lead to the development of various neurodegenerative diseases, such as incurable Charcot–Marie–Tooth disease (CMT) and distal spinal muscular atrophy. Some mutations result in the loss of tRNA aminoacylation activity, while other mutants retain their classical enzyme activity. In the latter case, disease manifestations are associated with additional neuron-specific functions of aaRSs. At present, seven aaRSs (GlyRS, TyrRS, AlaRS, HisRS, TrpRS, MetRS, and LysRS) are known to be involved in the CMT etiology with glycyl-tRNA synthetase (GlyRS) being the most studied of them.

Regulation of ex-translational activities is the primary function of the multi-tRNA synthetase complex

During mRNA translation, tRNAs are charged by aminoacyl-tRNA synthetases and subsequently used by ribosomes. A multi-enzyme aminoacyl-tRNA synthetase complex (MSC) has been proposed to increase protein synthesis efficiency by passing charged tRNAs to ribosomes. An alternative function is that the MSC repurposes specific synthetases that are released from the MSC upon cues for functions independent of translation. To explore this, we generated mammalian cells in which arginyl-tRNA synthetase and/or glutaminyl-tRNA synthetase were absent from the MSC. Protein synthesis, under a variety of stress conditions, was unchanged. Most strikingly, levels of charged tRNA^Arg and tRNA^Gln remained unchanged and no ribosome pausing was observed at codons for arginine and glutamine. Thus, increasing or regulating protein synthesis efficiency is not dependent on arginyl-tRNA synthetase and glutaminyl-tRNA synthetase in the MSC. Alternatively, and consistent with previously reported ex-translational roles requiring changes in synthetase cellular localizations, our manipulations of the MSC visibly changed localization.

CMT2N-causing aminoacylation domain mutants enable Nrp1 interaction with AlaRS

Charcot-Marie-Tooth disease (CMT) is a devastating motor and sensory neuropathy with an estimated 100,000 afflicted individuals in the US. Unexpectedly, aminoacyl-tRNA synthetases are the largest disease-associated protein family. A natural explanation is that the disease is associated with weak translation or mistranslation (caused by editing defects). However, our results with six different disease-causing mutants in AlaRS ruled out defects in aminoacylation or editing as causal factors. Instead, specific mutant proteins gained a neuropilin 1 (Nrp1)-AlaRS interaction. Previously a gain of Nrp1 interaction with a different disease-causing tRNA synthetase was mechanistically linked to the pathology of CMT. Thus, our results raise the possibility that pathological engagement of Nrp1 is common to at least a subset of tRNA synthetase-associated cases of CMT.

Through dominant mutations, aminoacyl-tRNA synthetases constitute the largest protein family linked to Charcot-Marie-Tooth disease (CMT). An example is CMT subtype 2N (CMT2N), caused by individual mutations spread out in AlaRS, including three in the aminoacylation domain, thereby suggesting a role for a tRNA-charging defect. However, here we found that two are aminoacylation defective but that the most widely distributed R329H is normal as a purified protein in vitro and in unfractionated patient cell samples. Remarkably, in contrast to wild-type (WT) AlaRS, all three mutant proteins gained the ability to interact with neuropilin 1 (Nrp1), the receptor previously linked to CMT pathogenesis in GlyRS. The aberrant AlaRS-Nrp1 interaction is further confirmed in patient samples carrying the R329H mutation. However, CMT2N mutations outside the aminoacylation domain do not induce the Nrp1 interaction. Detailed biochemical and biophysical investigations, including X-ray crystallography, small-angle X-ray scattering, hydrogen-deuterium exchange (HDX), switchSENSE hydrodynamic diameter determinations, and protease digestions reveal a mutation-induced structural loosening of the aminoacylation domain that correlates with the Nrp1 interaction. The b1b2 domains of Nrp1 are responsible for the interaction with R329H AlaRS. The results suggest Nrp1 is more broadly associated with CMT-associated members of the tRNA synthetase family. Moreover, we revealed a distinct structural loosening effect induced by a mutation in the editing domain and a lack of conformational impact with C-Ala domain mutations, indicating mutations in the same protein may cause neuropathy through different mechanisms. Our results show that, as with other CMT-associated tRNA synthetases, aminoacylation per se is not relevant to the pathology.

Distinct pathogenic mechanisms of various RARS1 mutations in Pelizaeus-Merzbacher-like disease

Mutations of the genes encoding aminoacyl-tRNA synthetases are highly associated with various central nervous system disorders. Recurrent mutations, including c.5A>G, p.D2G; c.1367C>T, p.S456L; c.1535G>A, p.R512Q and c.1846_1847del, p. Y616Lfs*6 of RARS1 gene, which encodes two forms of human cytoplasmic arginyl-tRNA synthetase (hArgRS), are linked to Pelizaeus-Merzbacher-like disease (PMLD) with unclear pathogenesis. Among these mutations, c.5A>G is the most extensively reported mutation, leading to a p.D2G mutation in the N-terminal extension of the long-form hArgRS. Here, we showed the detrimental effects of R512Q substitution and ΔC mutations on the structure and function of hArgRS, while the most frequent mutation c.5A>G, p.D2G acted in a different manner without impairing hArgRS activity. The nucleotide substitution c.5A>G reduced translation of hArgRS mRNA, and an upstream open reading frame contributed to the suppressed translation of the downstream main ORF. Taken together, our results elucidated distinct pathogenic mechanisms of various RARS1 mutations in PMLD.