Mitochondrial aspartyl-tRNA Synthetase Research Articles

Targeted degradation of extracellular mitochondrial aspartyl-tRNA synthetase modulates immune responses

The severity of bacterial pneumonia can be worsened by impaired innate immunity resulting in ineffective pathogen clearance. We describe a mitochondrial protein, aspartyl-tRNA synthetase (DARS2), which is released in circulation during bacterial pneumonia in humans and displays intrinsic innate immune properties and cellular repair properties. DARS2 interacts with a bacterial-induced ubiquitin E3 ligase subunit, FBXO24, which targets the synthetase for ubiquitylation and degradation, a process that is inhibited by DARS2 acetylation. During experimental pneumonia, Fbxo24 knockout mice exhibit elevated DARS2 levels with an increase in pulmonary cellular and cytokine levels. In silico modeling identified an FBXO24 inhibitory compound with immunostimulatory properties which extended DARS2 lifespan in cells. Here, we show a unique biological role for an extracellular, mitochondrially derived enzyme and its molecular control by the ubiquitin apparatus, which may serve as a mechanistic platform to enhance protective host immunity through small molecule discovery.

Leukoencephalopathy with Brain stem and Spinal cord involvement and Lactate elevation (LBSL): Report of a new family and a novel DARS2 mutation

BackgroundLBSL is a mitochondrial disorder caused by mutations in the mitochondrial aspartyl-tRNA synthetase gene DARS2, resulting in a distinctive pattern on brain magnetic resonance imaging (MRI) and spectroscopy. Clinical presentation varies from severe infantile to chronic, slowly progressive neuronal deterioration in adolescents or adults. Most individuals with LBSL are compound heterozygous for one splicing defect in an intron 2 mutational hotspot and a second defect that could be a missense, non-sense, or splice site mutation or deletion resulting in decreased expression of the full-length protein. AimTo present a new family with two affected members with LBSL and report a novel DARS2 mutation. ResultsAn 8-year-old boy (Patient 1) was referred due to headaches and abnormal MRI, suggestive of LBSL. Genetic testing revealed a previously reported c.492 + 2 T > C mutation in the DARS2 gene. Sanger sequencing uncovered a novel variant c.228-17C > G in the intron 2 hotspot. Family studies found the same genetic changes in an asymptomatic 4-year-old younger brother (Patient 2), who was found on follow-up to have an abnormal MRI. mRNA extracted from patients' fibroblasts showed that the c.228-17C > G mutation caused skipping of exon 3 resulting in lower DARS2 mRNA level. Complete absence of DARS2 protein was also found in both patients. SummaryWe present a new family with two children affected with LBSL and describe a novel mutation in the DARS2 intron 2 hotspot. Despite findings of extensive white matter disease in the brain and spine, the proband in this family presented only with headaches, while the younger sibling, who also had extensive white matter changes, was asymptomatic. Our in-vitro results confirmed skipping of exon 3 in patients and family members carrying the intron 2 variant, which is consistent with previous reported mutations in intron 2 hotspots. DARS2 mRNA and protein levels were also reduced in both patients, further supporting the pathogenicity of the novel variant.

Mutations in DARS2 result in global dysregulation of mRNA metabolism and splicing

Leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) is a rare neurological disorder caused by the mutations in the DARS2 gene, which encodes the mitochondrial aspartyl-tRNA synthetase. The objective of this study was to understand the impact of DARS2 mutations on cell processes through evaluation of LBSL patient stem cell derived cerebral organoids and neurons. We generated human cerebral organoids (hCOs) from induced pluripotent stem cells (iPSCs) of seven LBSL patients and three healthy controls using an unguided protocol. Single cells from 70-day-old hCOs were subjected to SMART-seq2 sequencing and bioinformatic analysis to acquire high-resolution gene and transcript expression datasets. Global gene expression analysis demonstrated dysregulation of a number of genes involved in mRNA metabolism and splicing processes within LBSL hCOs. Importantly, there were distinct and divergent gene expression profiles based on the nature of the DARS2 mutation. At the transcript level, pervasive differential transcript usage and differential spliced exon events that are involved in protein translation and metabolism were identified in LBSL hCOs. Single-cell analysis of DARS2 (exon 3) showed that some LBSL cells exclusively express transcripts lacking exon 3, indicating that not all LBSL cells can benefit from the “leaky” nature common to splice site mutations. At the gene- and transcript-level, we uncovered that dysregulated RNA splicing, protein translation and metabolism may underlie at least some of the pathophysiological mechanisms in LBSL. To confirm hCO findings, iPSC-derived neurons (iNs) were generated by overexpressing Neurogenin 2 using lentiviral vector to study neuronal growth, splicing of DARS2 exon 3 and DARS2 protein expression. Live cell imaging revealed neuronal growth defects of LBSL iNs, which was consistent with the finding of downregulated expression of genes related to neuronal differentiation in LBSL hCOs. DARS2 protein was downregulated in iNs compared to iPSCs, caused by increased exclusion of exon 3. The scope and complexity of our data imply that DARS2 is potentially involved in transcription regulation beyond its canonical role of aminoacylation. Nevertheless, our work highlights transcript-level dysregulation as a critical, and relatively unexplored, mechanism linking genetic data with neurodegenerative disorders.

Spectrum of Magnetic Resonance Spectroscopy (MRS) in Differentiating Paediatric Leukodystrophies in Pakistani Population

Aim: To determine the frequency of findings of MRS in differentiating paediatric leukodystrophies in Pakistani population. Study design: Cross sectional descriptive study. Place and duration of study: Armed Forces Institute of Radiology and Imaging, Military Hospital Rawalpindi from 25thAugust 2018 to 24th February 2019. Methodology: A total of 110 individuals who were coming in for an MRI test and were either male or female, older than one month of age, and younger than 12 years of age, were chosen. A 3.0T MRI scanner equipped with MRS techniques was used for each scan. Axial T1 and T2 Weighted spin-echo, FLAIR, coronal, and sagittal T2 Weighted spin-echo images were all part of the MRI imaging protocol. During the same session, a single section multivoxel 2D chemical shift spectroscopy sequence was also carried out. In MRS, the metabolites that were detected in each of our patients were choline, creatine, lactate, N-acetyl aspartate, and myo-inositol. Data were examined utilizing SPSS 21. Results: The average age was 5.24±1.89 year. 56 (50.91%) of the 110 patients were male children, compared to 54 (49.09%) female children, for a male to female ratio of 1.1:1. The following paediatric leukodystrophies were more often found with MRS in this study: Krabbes disease in 13 (11.82%), Canavan's disease in 19 (17.27%), Adrenoleukodystrophy in 41 (37.27%), Leigh disease in 25 (22.73%), and Metachromatic leukodystrophy in 12 (10.91%). Practical Implication: The underlying issue has recently been linked to mutations in the DARS2 gene, which codes for mitochondrial aspartyl-tRNA synthetase. Conclusion: MRS, which is more affordable than other existing tests, should be utilized in every patient with clinical leukodystrophies. Keywords: Leukodystrophies, Magnetic resonance spectroscopy, MRS

Functional analysis of missense DARS2 variants in siblings with leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation.

Biallelic pathogenic variants in the nuclear gene DARS2 (MIM# 610956), encoding the mitochondrial enzyme aspartyl-tRNA synthetase (MT-ASPRS) cause leukoencephalopathy with Brain Stem and Spinal Cord Involvement and Lactate Elevation (LBSL) (MIM# 611105), a neurometabolic disorder characterized by progressive ataxia, spasticity, developmental arrest or regression and characteristic brain MRI findings. Most patients exhibit a slowly progressive disease course with motor deterirartion that begins in childhood or adolescence, but can also occasionaly occur in adulthood. More severe LBSL presentations with atypical brain MRI findings have been recently described. Baker's yeast orthologue of DARS2, MSD1, is required for growth on oxidative carbon sources. A yeast with MSD1 knockout (msd1Δ) demonstrated a complete lack of oxidative growth which could be rescued by wild-type MSD1 but not MSD1 with pathogenic variants. Here we reported two siblings who exhibited developmental regression and ataxia with different age of onset and phenotypic severity. Exome sequencing revealed 2 compound heterozygous missense variants in DARS2: c.473A>T (p.Glu158Val) and c.829G>A (p.Glu277Lys); this variant combination has not been previously reported. The msd1Δ yeast transformed with plasmids expressing p.Glu259Lys, equivalent to human p.Glu277Lys, showed complete loss of oxidative growth and oxygen consumption, while the strain carrying p.Gln137Val, equivalent to human p.Glu158Val, showed a significant reduction of oxidative growth, but a residual ability to grow was retained. Structural analysis indicated that p.Glu158Val may interfere with protein binding of tRNAAsp, while p.Glu277Lys may impact both homodimerization and catalysis of MT-ASPRS. Our data illustrate the utility of yeast model and in silico analysis to determine pathogenicity of DARS2 variants, expand the genotypic spectrum and suggest intrafamilial variability in LBSL.

Wearable sensors detect impaired gait and coordination in LBSL during remote assessments

BackgroundLeukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) is a rare leukodystrophy with motor impairment due to biallelic mutations in DARS2, which encodes mitochondrial aspartyl tRNA synthetase. Progressive ataxia is the primary feature.ObjectiveThe study objective is to determine the feasibility of remotely collecting quantitative gait and balance measures in LBSL.MethodsThe study design uses wearable accelerometers and the scale for the assessment and rating of ataxia (SARA) scale to assess gait and postural sway in LBSL and control participants' homes through video conferencing.ResultsLateral step variability (LSV), which indicates stride variability, and elevation of the step at mid‐swing are increased for LBSL patients during brief walking tests. During stance with the eyes closed, LBSL participants show rapid accelerations and decelerations of body movement covering a large sway area and path. Both the LSV and sway area during stance with the feet together and eyes closed correlate strongly with the SARA.ConclusionsWearable accelerometers are valid and sensitive for detecting ataxia in LBSL patients during remote assessments. The finding of large increases in the sway area during stance with the eyes closed is intriguing since dorsal column dysfunction is universally seen in LBSL. This approach can be applied to related rare diseases that feature ataxia.

Mitochondrial metabolism coordinates stage-specific repair processes in macrophages during wound healing

Wound healing is a coordinated process that initially relies on pro-inflammatory macrophages, followed by a pro-resolution function of these cells. Changes in cellular metabolism likely dictate these distinct activities, but the nature of these changes has been unclear. Here, we profiled early- versus late-stage skin wound macrophages in mice at both the transcriptional and functional levels. We found that glycolytic metabolism in the early phase is not sufficient to ensure productive repair. Instead, by combining conditional disruption of the electron transport chain with deletion of mitochondrial aspartyl-tRNA synthetase, followed by single-cell sequencing analysis, we found that a subpopulation of early-stage wound macrophages are marked by mitochondrial ROS (mtROS) production and HIF1α stabilization, which ultimately drives a pro-angiogenic program essential for timely healing. In contrast, late-phase, pro-resolving wound macrophages are marked by IL-4Rα-mediated mitochondrial respiration and mitohormesis. Collectively, we identify changes in mitochondrial metabolism as a critical control mechanism for macrophage effector functions during wound healing.

LBSL: Case Series and DARS2 Variant Analysis in Early Severe Forms With Unexpected Presentations.

ObjectiveLeukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) is regarded a relatively mild leukodystrophy, diagnosed by characteristic long tract abnormalities on MRI and biallelic variants in DARS2, encoding mitochondrial aspartyl-tRNA synthetase (mtAspRS). DARS2 variants in LBSL are almost invariably compound heterozygous; in 95% of cases, 1 is a leaky splice site variant in intron 2. A few severely affected patients, still fulfilling the MRI criteria, have been described. We noticed highly unusual MRI presentations in 15 cases diagnosed by WES. We examined these cases to determine whether they represent consistent novel LBSL phenotypes.MethodsWe reviewed clinical features, MRI abnormalities, and gene variants and investigated the variants' impact on mtAspRS structure and mitochondrial function.ResultsWe found 2 MRI phenotypes: early severe cerebral hypoplasia/atrophy (9 patients, group 1) and white matter abnormalities without long tract involvement (6 patients, group 2). With antenatal onset, microcephaly, and arrested development, group 1 patients were most severely affected. DARS2 variants were severer than for classic LBSL and severer for group 1 than group 2. All missense variants hit mtAspRS regions involved in tRNAAsp binding, aspartyl-adenosine-5′-monophosphate binding, and/or homodimerization. Missense variants expressed in the yeast DARS2 ortholog showed severely affected mitochondrial function.ConclusionsDARS2 variants are associated with highly heterogeneous phenotypes. New MRI presentations are profound cerebral hypoplasia/atrophy and white matter abnormalities without long tract involvement. Our findings have implications for diagnosis and understanding disease mechanisms, pointing at dominant neuronal/axonal involvement in severe cases. In line with this conclusion, activation of biallelic DARS2 null alleles in conditional transgenic mice leads to massive neuronal apoptosis.

Modern views of brainstem and spinal cord leukoencephalopathy with increased lactate content

The review considers modern evidence on a rare genetic disease, leukoencephalopathy of brainstem and spinal cord associated with elevated lactate content. The research history of the disease is described. The epidemiology of the disease prevalence in a population is described. Emphasis is placed on aetiology of leukoencephalopathy and peculiar genetic and pathomorphological changes associated with a DARS2 gene mutation leading to mitochondrial aspartyl-tRNA synthetase deficiency. The review details the clinical picture, manifestations specific in various age groups and identifies diseases with similar clinical symptoms and morbid changes that require differential diagnosis. Modern relevant diagnostic approaches are outlined based primarily on neuroimaging techniques. The major neuroimaging approach to leukoencephalopathy is magnetic resonance imaging that allows detection of disease-specific lesion patterns in the brainstem and spinal cord. Magnetic resonance spectrometry is used in turn to detect a usually higher lactate content in the affected white matter of the brain. Definitive diagnosis is based on detection of the marker DARS2 mutation in patients with characteristic clinical picture and MRI signatures in the brain and spinal cord. The review describes contemporary treatment strategies utilising pathogenetic effects to potentially contain the disease. However, its genetic determination renders symptomatic therapy yet a common treatment.

DARS2 is indispensable for Purkinje cell survival and protects against cerebellar ataxia.

Leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation disorder (LBSL) arises from mutations in mitochondrial aspartyl-tRNA synthetase (DARS2) gene. The disease has a childhood or juvenile-onset and is clinically characterized by cerebellar ataxia, cognitive decline and distinct morphological abnormalities upon magnetic resonance imaging. We previously demonstrated that neurons and not adult myelin-producing cells are specifically sensitive to DARS2 loss, hence likely the primary culprit in LBSL disorder. We used conditional Purkinje cell (PCs)-specific Dars2 deletion to elucidate further the cell-type-specific contribution of this class of neurons to the cerebellar impairment observed in LBSL. We show that DARS2 depletion causes a severe mitochondrial dysfunction concomitant with a massive loss of PCs by the age of 15weeks, thereby rapidly deteriorating motor skills. Our findings conclusively show that DARS2 is indispensable for PC survival and highlights the central role of neuroinflammation in DARS2-related PC degeneration.

Neuronal ablation of mt-AspRS in mice induces immune pathway activation prior to severe and progressive cortical and behavioral disruption

Leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) is a rare, slowly progressive white matter disease caused by mutations in the mitochondrial aspartyl-tRNA synthetase (mt-AspRS, or DARS2). While patients show characteristic MRI T2 signal abnormalities throughout the cerebral white matter, brainstem, and spinal cord, the phenotypic spectrum is broad and a multitude of gene variants have been associated with the disease. Here, Dars2 disruption in CamKIIα-expressing cortical and hippocampal neurons results in slowly progressive increases in behavioral activity at five months, and culminating by nine months as severe brain atrophy, behavioral dysfunction, reduced corpus callosum thickness, and microglial morphology indicative of neuroinflammation. Interestingly, RNAseq based gene expression studies performed prior to the presentation of this severe phenotype reveal the upregulation of several pathways involved in immune activation, cytokine production and signaling, and defense response regulation. RNA transcript analysis demonstrates that activation of immune and cell stress pathways are initiated in advance of a behavioral phenotype and cerebral deficits. An understanding of these pathways and their contribution to significant neuronal loss in CamKII-Dars2 deficient mice may aid in deciphering mechanisms of LBSL pathology.

Mitochondrial dysfunctions in leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL).

Several inherited human diseases have been linked to mitochondrial aminoacyl-tRNA synthetases (mtARSs). Leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) is a leukodystrophy caused by mutations in the DARS2 gene which encodes mitochondrial aspartyl-tRNA synthetase. As mitochondrial ARSs are key components of the mitochondrial translation apparatus, we investigated the effects of DARS2 mutations on mitochondrial functions and mitochondrial morphology in an LBSL patient. In fibroblasts from the patient with LBSL, biosynthesis of respiratory chain complex proteins encoded by mitochondrial DNA was decreased, while those encoded by nuclear DNA were not. Cellular oxygen consumption rates and respiratory control ratio were decreased in the LBSL patient; in addition, fragmentation of mitochondria was increased, while their tubular elongation and interconnectivity were decreased. Taken together, these findings suggest that DARS2 mutations impair translations of mitochondrial DNA-encoded respiratory chain complex proteins, consequently causing dysfunction of cellular respiration and impediment of mitochondrial dynamics, which highlights the role of mtARSs in the maintenance of normal mitochondrial bioenergetics and dynamics.

An Aminoacyl tRNA Synthetase, OKI1, Is Required for Proper Shoot Meristem Size in Arabidopsis.

In plants, the stem cells that form the shoot system reside within the shoot apical meristem (SAM), which is regulated by feedback signaling between the WUSCHEL (WUS) homeobox protein and CLAVATA (CLV) peptides and receptors. WUS-CLV feedback signaling can be modulated by various endogenous or exogenous factors, such as chromatin state, hormone signaling, reactive oxygen species (ROS) signaling and nutrition, leading to a dynamic control of SAM size corresponding to meristem activity. Despite these insights, however, the knowledge of genes that control SAM size is still limited, and in particular, the regulation by ROS signaling is only beginning to be comprehended. In this study, we report a new function in maintenance of SAM size, encoded by the OKINA KUKI1 (OKI1) gene. OKI1 is expressed in the SAM and encodes a mitochondrial aspartyl tRNA synthetase (AspRS). oki1 mutants display enlarged SAMs with abnormal expression of WUS and CLV3 and overaccumulation of ROS in the meristem. Our findings support the importance of normal AspRS function in the maintenance of the WUS-CLV3 feedback loop and SAM size.

Лейкоэнцефалопатия с преимущественным поражением ствола мозга, спинного мозга и повышенным содержанием лактата (клинический случай)

Firstly described by Van Berge et al. in 2003, leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation (LBSL) is an autosomal recessive inherited leukoencephalopathy caused by compound heterozygous mutations in the DARS2gene, which codes the mitochondrial aspartyl-tRNA synthetase. The disease is included in a list of rare diseases that have been established by TheMinistry of Health. LBSL mainly affects children and juvenile patients with a slow progressive cerebellar ataxia with spastic paraparesis and proprioceptive loss. The diagnosis of LBSL can be made with confidence in persons with characteristic abnormalities observed on brain and spinal cord MRI and genetic test. The treatment of LBSL doesn’t exist.

A novel DARS2 mutation in a Japanese patient with leukoencephalopathy with brainstem and spinal cord involvement but no lactate elevation

The mitochondrial aspartyl-tRNA synthetase 2 gene (DARS2) is responsible for leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL). A Japanese patient with LBSL showed compound heterozygous DARS2 mutations c.358_359delinsTC (p.Gly120Ser) and c.228-15C>G (splicing error). This provides further evidence that most patients with LBSL show compound heterozygous mutations in DARS2 in association with a common splicing mutation in the splicing acceptor site of intron 2.

Two proteomic methodologies for defining N-termini of mature human mitochondrial aminoacyl-tRNA synthetases.

Human mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs) are encoded in the nucleus, synthesized in the cytosol and targeted for importation into mitochondria by a N-terminal mitochondrial targeting sequence. This targeting sequence is presumably cleaved upon entry into the mitochondria, following a process still not fully deciphered in human, despite essential roles for the mitochondrial biogenesis. Maturation processes are indeed essential both for the release of a functional enzyme and to route correctly the protein within mitochondria. The absence of consensus sequences for cleavage sites and the discovery of possible multiple proteolytic steps render predictions of N-termini difficult. Further, the knowledge of the cleavages is key for the design of protein constructions compatible with efficient production in bacterial strains. Finally, full comprehension becomes essential because a growing number of mutations are found in genes coding for mt-aaRS. In the present study, we take advantage of proteomic methodological developments and identified, in mitochondria, three N-termini for the human mitochondrial aspartyl-tRNA synthetase. This first description of the co-existence of different forms opens new perspectives in the biological understanding of this enzyme. Those methods are extended to the whole set of human mt-aaRSs and methodological advice are provided for further investigations.

Mitochondrial DNA homeostasis is essential for nigrostriatal integrity

Mitochondrial involvement in the pathogenesis of Parkinson's disease has been suggested by multiple studies, but the mechanisms involved remain unresolved. Here, we sought to identify which mitochondrial defects are associated with degeneration of the nigrostriatal system.Nigrostriatal integrity was assessed in vivo by dopamine transporter (DAT) imaging in twenty-one patients with mitochondrial disorders of different molecular aetiology including: maternally inherited mitochondrial DNA (mtDNA) point mutations, primary single mtDNA deletions, nuclear-encoded disorders of mtDNA replication and maintenance due to mutations in POLG or C10orf2 (Twinkle), and mutations in other nuclear mitochondrial genes including the mitochondrial aspartyl-tRNA synthetase (DARS2) and ADCK3 genes. Patients with mitochondrial disease were compared with twenty patients with Parkinson's disease and eighteen controls. Nigrostriatal degeneration occurred exclusively in patients with defective mtDNA replication and maintenance. In these patients, nigrostriatal degeneration was progressive and at least as severe as in patients with advanced Parkinson's disease. None of the patients with other mitochondrial defects showed evidence of nigral involvement.Our findings demonstrate that dopaminergic neurons of the substantia nigra are specifically vulnerable to defective mtDNA replication/repair or quality control and not to primary point mutations of mtDNA. These results support the hypothesis that accumulating somatic mtDNA damage plays an important role in neurodegeneration.

Clinical Cases of Leukoencephalopathy with Predominant Lesion of the Brain Stem, Spinal Cord and High Blood Lactate in the MR Spectroscopy (Mitochondrial Aspartyl-tRNASynthetase Deficiency)

Leukoencephalopathy with predominant lesion of the brain stem, spinal cord and high blood lactate in the MR spectroscopy is a hereditary disease characterized by slowly progressing cerebellar, pyramidal disorders, disorders involving the lesion of posterior columns of the spinal cord and highly specific changes according to the data of MRI and magnetic resonanse spectroscopy of the brain and spinal cord. This disease is one of nuclear inherited mitochondrial encephalomyopathies, caused by mutations in the DARS2 gene and characterized by mitochondrial aspartyl-tRNA-synthetase deficiency. This article presents the record of the disease description, its genetic basis, clinical features and diagnostic criteria. The article describes two clinical cases of the disease in 9-year and 17-year old girls.

Leukoencephalopathy

Just 11 years ago, a new genetic white matter disease, leukoencephalopathy with brainstem and spinal cord involvement and elevated lactate (LBSL), was described in 7 children and 1 teenager.1 Clinical features were dominated by progressive motor deterioration without sphincter dysfunction; half of the patients had loss of position and vibratory sensation and learning problems, but no vision or hearing loss. CSF was acellular without oligoclonal bands. Somatosensory evoked potentials, nerve conduction velocities, and a single sural nerve biopsy were normal. MRI showed a symmetrical leukoencephalopathy (figure, top) with a predilection for the brainstem and spinal cord. Magnetic resonance spectroscopy revealed decreased N -acetylaspartate and elevated lactate levels in brain white matter. Disease was due to a deficiency in mitochondrial aspartyl-tRNA synthetase.2

Tissue-Specific Loss of DARS2 Activates Stress Responses Independently of Respiratory Chain Deficiency in the Heart

Adaptive stress responses activated upon mitochondrial dysfunction are assumed to arise in order to counteract respiratory chain deficiency. Here, we demonstrate that loss of DARS2 (mitochondrial aspartyl-tRNA synthetase) leads to the activation of various stress responses in a tissue-specific manner independently of respiratory chain deficiency. DARS2 depletion in heart and skeletal muscle leads to the severe deregulation of mitochondrial protein synthesis followed by a strong respiratory chain deficit in both tissues, yet the activation of adaptive responses is observed predominantly in cardiomyocytes. We show that the impairment of mitochondrial proteostasis in the heart activates the expression of mitokine FGF21, which acts as a signal for cell-autonomous and systemic metabolic changes. Conversely, skeletal muscle has an intrinsic mechanism relying on the slow turnover of mitochondrial transcripts and higher proteostatic buffering capacity. Our results show that mitochondrial dysfunction is sensed independently of respiratory chain deficiency, questioning the current view on the role of stress responses in mitochondrial diseases.