MTO1 mediates tissue specificity of OXPHOS defects via tRNA modification and translation optimization, which can be bypassed by dietary intervention.

Mitochondrial diseases often exhibit tissue-specific pathologies, but this phenomenon is poorly understood. Here we present regulation of mitochondrial translation by the Mitochondrial Translation Optimization Factor 1, MTO1, as a novel player in this scenario. We demonstrate that MTO1 mediates tRNA modification and controls mitochondrial translation rate in a highly tissue-specific manner associated with tissue-specific OXPHOS defects. Activation of mitochondrial proteases, aberrant translation products, as well as defects in OXPHOS complex assembly observed in MTO1 deficient mice further imply that MTO1 impacts translation fidelity. In our mouse model, MTO1-related OXPHOS deficiency can be bypassed by feeding a ketogenic diet. This therapeutic intervention is independent of the MTO1-mediated tRNA modification and involves balancing of mitochondrial and cellular secondary stress responses. Our results thereby establish mammalian MTO1 as a novel factor in the tissue-specific regulation of OXPHOS and fine tuning of mitochondrial translation accuracy.

Defining the action spectrum of potential PGC-1α activators on a mitochondrial and cellular level in vivo.

Previous studies have demonstrated a therapeutic benefit of pharmaceutical PGC-1α activation in cellular and murine model of disorders linked to mitochondrial dysfunction. While in some cases, this effect seems to be clearly associated with boosting of mitochondrial function, additional alterations as well as tissue- and cell-type-specific effects might play an important role. We initiated a comprehensive analysis of the effects of potential PGC-1α-activating drugs and pharmaceutically targeted the PPAR (bezafibrate, rosiglitazone), AMPK (AICAR, metformin) and Sirt1 (resveratrol) pathways in HeLa cells, neuronal cells and PGC-1α-deficient MEFs to get insight into cell type specificity and PGC-1α dependence of their working action. We used bezafibrate as a model drug to assess the effect on a tissue-specific level in a murine model. Not all analyzed drugs activate the PGC pathway or alter mitochondrial protein levels. However, they all affect supramolecular assembly of OXPHOS complexes and OXPHOS protein stability. In addition, a clear drug- and cell-type-specific influence on several cellular stress pathways as well as on post-translational modifications could be demonstrated, which might be relevant to fully understand the action of the analyzed drugs in the disease state. Importantly, the effect on the activation of mitochondrial biogenesis and stress response program upon drug treatment is PGC-1α dependent in MEFs demonstrating not only the pleiotropic effects of this molecule but points also to the working mechanism of the analyzed drugs. The definition of the action spectrum of the different drugs forms the basis for a defect-specific compensation strategy and a future personalized therapeutic approach.

Mitochondrial protein acetylation mediates nutrient sensing of mitochondrial protein synthesis and mitonuclear protein balance.

Changes in nutrient supply require global metabolic reprogramming to optimize the utilization of the nutrients. Mitochondria as a central component of the cellular metabolism play a key role in this adaptive process. Since mitochondria harbor their own genome, which encodes essential enzymes, mitochondrial protein synthesis is a determinant of metabolic adaptation. While regulation of cytoplasmic protein synthesis in response to metabolic challenges has been studied in great detail, mechanisms which adapt mitochondrial translation in response to metabolic challenges remain elusive. Our results suggest that the mitochondrial acetylation status controlled by Sirt3 and its proposed opponent GCN5L1 is an important regulator of the metabolic adaptation of mitochondrial translation. Moreover, both proteins modulate regulators of cytoplasmic protein synthesis as well as the mitonuclear protein balance making Sirt3 and GCN5L1 key players in synchronizing mitochondrial and cytoplasmic translation. Our results thereby highlight regulation of mitochondrial translation as a novel component in the cellular nutrient sensing scheme and identify mitochondrial acetylation as a new regulatory principle for the metabolic competence of mitochondrial protein synthesis.

Prevalence of oral corticosteroid use in the German severe asthma population.

AIMS: We investigated the prevalence of severe asthma, its comorbidities, and especially the use of oral corticosteroid (OCS) therapy in patients with severe asthma. METHODS: Pooled data from 3 961 429 patients insured (with statutory health insurance) during the year 2015 were analysed. Prevalence rates of severe asthma and its OCS-associated comorbidities in patients on high-dosage (HD) inhaled corticosteroid (ICS) in combination with a long-acting ß agonist (LABA) therapy were compared with those of patients who were also treated with OCSs. RESULTS: The asthma prevalence was 7.3%, of which 8.7% (0.6% absolute) were treated with HD-ICS/LABAs. Of these, 33.6% received additional OCSs with calculated dosages between 0.9 and 9.1 mg·day-1. More than 80% of patients on HD-ICS/LABAs had at least one comorbidity. Disorders of the heart (67.5%), metabolism/ nutrition (51.4%), psychiatric disorders (36.0%), skeletal muscle/connective tissue and bone disorders (20.3%), and eye disorders (20.0%) were predominant. The prevalence of these disorders increased for patients also receiving OCS therapy, depending on the length of treatment. Mean therapy costs ranged from €4266 per patient without OCS therapy to €11 253 per patient on long-term OCS treatment. The largest share of costs was attributable to inpatient care. CONCLUSION: The analyses show that OCSs are frequently prescribed in patients receiving HD-ICS/LABAs because of severe asthma and are they are frequently associated with adverse effects commonly reported with steroid usage. These data support a necessary change in severe asthma treatment, which is reflected in current treatment guidelines.

Translation and Assembly of Radiolabeled Mitochondrial DNA-Encoded Protein Subunits from Cultured Cells and Isolated Mitochondria.

In higher eukaryotes, the mitochondrial electron transport chain consists of five multi-subunit membrane complexes responsible for the generation of cellular ATP. Of these, four complexes are under dual genetic control as they contain subunits encoded by both the mitochondrial and nuclear genomes, thereby adding another layer of complexity to the puzzle of respiratory complex biogenesis. These subunits must be synthesized and assembled in a coordinated manner in order to ensure correct biogenesis of different respiratory complexes. Here, we describe techniques to (1) specifically radiolabel proteins encoded by mtDNA to monitor the rate of synthesis using pulse labeling methods, and (2) analyze the stability, assembly, and turnover of subunits using pulse-chase methods in cultured cells and isolated mitochondria.

Post-translational modification of mitochondria as a novel mode of regulation.

Mitochondria not only form the metabolic hub, but also are crucial players in many cellular pathways, like apoptosis and innate immune response, putting the organelle in a central position in controlling cellular function and fate. As novel and powerful regulators of mitochondrial processes and hence mitochondrial-controlled pathways, post-translational modifications (PTMs) have emerged in the last years. In this review, we will summarize the current state of knowledge on PTMs occurring in mammalian mitochondria with a focus on phosphorylation, acetylation, succinylation and ubiquitination. We will highlight their regulatory role in metabolism, autophagy and apoptosis as well as communicating element to cellular stress response pathways such as the immune response. Finally, we will discuss open questions in this exciting research area and point out how mitochondrial PTMs might impact age-associated pathologies.