Overexpression of the mitochondrial methyltransferase TFB1M in the mouse does not impact mitoribosomal methylation status or hearing

Haplotype variability in mitochondrial rRNA predisposes to metabolic syndrome

Metabolic syndrome is a growing concern in developed societies and due to its polygenic nature, the genetic component is only slowly being elucidated. Common mitochondrial DNA sequence variants have been associated with symptoms of metabolic syndrome and may, therefore, be relevant players in the genetics of metabolic syndrome. We investigate the effect of mitochondrial sequence variation on the metabolic phenotype in conplastic rat strains with identical nuclear but unique mitochondrial genomes, challenged by high-fat diet. We find that the variation in mitochondrial rRNA sequence represents risk factor in the insulin resistance development, which is associated with diacylglycerols accumulation, induced by tissue-specific reduction of the oxidative capacity. These metabolic perturbations stem from the 12S rRNA sequence variation affecting mitochondrial ribosome assembly and translation. Our work demonstrates that physiological variation in mitochondrial rRNA might represent a relevant underlying factor in the progression of metabolic syndrome.

Illuminating mitochondrial translation through mouse models

Mitochondria are hubs of metabolic activity with a major role in ATP conversion by oxidative phosphorylation (OXPHOS). The mammalian mitochondrial genome encodes 11 mRNAs encoding 13 OXPHOS proteins along with 2 rRNAs and 22 tRNAs, that facilitate their translation on mitoribosomes. Maintaining the internal production of core OXPHOS subunits requires modulation of the mitochondrial capacity to match the cellular requirements and correct insertion of particularly hydrophobic proteins into the inner mitochondrial membrane. The mitochondrial translation system is essential for energy production and defects result in severe, phenotypically diverse diseases, including mitochondrial diseases that typically affect postmitotic tissues with high metabolic demands. Understanding the complex mechanisms that underlie the pathologies of diseases involving impaired mitochondrial translation is key to tailoring specific treatments and effectively targeting the affected organs. Disease mutations have provided a fundamental, yet limited, understanding of mitochondrial protein synthesis, since effective modification of the mitochondrial genome has proven challenging. However, advances in next generation sequencing, cryoelectron microscopy, and multi-omic technologies have revealed unexpected and unusual features of the mitochondrial protein synthesis machinery in the last decade. Genome editing tools have generated unique models that have accelerated our mechanistic understanding of mitochondrial translation and its physiological importance. Here we review the most recent mouse models of disease pathogenesis caused by defects in mitochondrial protein synthesis and discuss their value for preclinical research and therapeutic development.

Mitochondrial bioenergetics, metabolism, and beyond in pancreatic β-cells and diabetes

In Type 1 and Type 2 diabetes, pancreatic β-cell survival and function are impaired. Additional etiologies of diabetes include dysfunction in insulin-sensing hepatic, muscle, and adipose tissues as well as immune cells. An important determinant of metabolic health across these various tissues is mitochondria function and structure. This review focuses on the role of mitochondria in diabetes pathogenesis, with a specific emphasis on pancreatic β-cells. These dynamic organelles are obligate for β-cell survival, function, replication, insulin production, and control over insulin release. Therefore, it is not surprising that mitochondria are severely defective in diabetic contexts. Mitochondrial dysfunction poses challenges to assess in cause-effect studies, prompting us to assemble and deliberate the evidence for mitochondria dysfunction as a cause or consequence of diabetes. Understanding the precise molecular mechanisms underlying mitochondrial dysfunction in diabetes and identifying therapeutic strategies to restore mitochondrial homeostasis and enhance β-cell function are active and expanding areas of research. In summary, this review examines the multidimensional role of mitochondria in diabetes, focusing on pancreatic β-cells and highlighting the significance of mitochondrial metabolism, bioenergetics, calcium, dynamics, and mitophagy in the pathophysiology of diabetes. We describe the effects of diabetes-related gluco/lipotoxic, oxidative and inflammation stress on β-cell mitochondria, as well as the role played by mitochondria on the pathologic outcomes of these stress paradigms. By examining these aspects, we provide updated insights and highlight areas where further research is required for a deeper molecular understanding of the role of mitochondria in β-cells and diabetes.

Structural insights into the specific recognition of mitochondrial ribosome-binding factor hsRBFA and 12 S rRNA by methyltransferase METTL15

Mitochondrial rRNA modifications are essential for mitoribosome assembly and its proper function. The m4C methyltransferase METTL15 maintains mitochondrial homeostasis by catalyzing m4C839 located in 12 S rRNA helix 44 (h44). This modification is essential to fine-tuning the ribosomal decoding center and increasing decoding fidelity according to studies of a conserved site in Escherichia coli. Here, we reported a series of crystal structures of human METTL15-hsRBFA-h44-SAM analog, METTL15-hsRBFA-SAM, METTL15-SAM and apo METTL15. The structures presented specific interactions of METTL15 with different substrates and revealed that hsRBFA recruits METTL15 to mitochondrial small subunit for further modification instead of 12 S rRNA. Finally, we found that METTL15 deficiency caused increased reactive oxygen species, decreased membrane potential and altered cellular metabolic state. Knocking down METTL15 caused an elevated lactate secretion and increased levels of histone H4K12-lactylation and H3K9-lactylation. METTL15 might be a suitable model to study the regulation between mitochondrial metabolism and histone lactylation.

Our current understanding of the toxicity of altered mito-ribosomal fidelity during mitochondrial protein synthesis: What can it tell us about human disease?

Altered mito-ribosomal fidelity is an important and insufficiently understood causative agent of mitochondrial dysfunction. Its pathogenic effects are particularly well-known in the case of mitochondrially induced deafness, due to the existence of the, so called, ototoxic variants at positions 847C (m.1494C) and 908A (m.1555A) of 12S mitochondrial (mt-) rRNA. It was shown long ago that the deleterious effects of these variants could remain dormant until an external stimulus triggered their pathogenicity. Yet, the link from the fidelity defect at the mito-ribosomal level to its phenotypic manifestation remained obscure. Recent work with fidelity-impaired mito-ribosomes, carrying error-prone and hyper-accurate mutations in mito-ribosomal proteins, have started to reveal the complexities of the phenotypic manifestation of mito-ribosomal fidelity defects, leading to a new understanding of mtDNA disease. While much needs to be done to arrive to a clear picture of how defects at the level of mito-ribosomal translation eventually result in the complex patterns of disease observed in patients, the current evidence indicates that altered mito-ribosome function, even at very low levels, may become highly pathogenic. The aims of this review are three-fold. First, we compare the molecular details associated with mito-ribosomal fidelity to those of general ribosomal fidelity. Second, we gather information on the cellular and organismal phenotypes associated with defective translational fidelity in order to provide the necessary grounds for an understanding of the phenotypic manifestation of defective mito-ribosomal fidelity. Finally, the results of recent experiments directly tackling mito-ribosomal fidelity are reviewed and future paths of investigation are discussed.

[Progress in genetic susceptibility to aminoglycoside-induced deafness]

Aminoglycoside antibiotics can cause irreversible hearing loss, but they are still widely used because of their low production cost and broad-spectrum effect on most infections. Although it has been studied for decades, the mechanism of aminoglycoside-induced deafness has not been fully elucidated. Since patients'individual susceptibility to aminoglycoside-ototoxicity varies considerably, it is necessary to identify high-risk patients. This review summarizes the genetic mutations linked to aminoglycoside-induced deafness, in order to provide reference for further prevention and treatment of aminoglycoside-induced deafness.

METTL15 introduces N4-methylcytidine into human mitochondrial 12S rRNA and is required for mitoribosome biogenesis

Post-transcriptional RNA modifications, the epitranscriptome, play important roles in modulating the functions of RNA species. Modifications of rRNA are key for ribosome production and function. Identification and characterization of enzymes involved in epitranscriptome shaping is instrumental for the elucidation of the functional roles of specific RNA modifications. Ten modified sites have been thus far identified in the mammalian mitochondrial rRNA. Enzymes responsible for two of these modifications have not been characterized. Here, we identify METTL15, show that it is the main N4-methylcytidine (m⁴C) methyltransferase in human cells and demonstrate that it is responsible for the methylation of position C839 in mitochondrial 12S rRNA. We show that the lack of METTL15 results in a reduction of the mitochondrial de novo protein synthesis and decreased steady-state levels of protein components of the oxidative phosphorylation system. Without functional METTL15, the assembly of the mitochondrial ribosome is decreased, with the late assembly components being unable to be incorporated efficiently into the small subunit. We speculate that m⁴C839 is involved in the stabilization of 12S rRNA folding, therefore facilitating the assembly of the mitochondrial small ribosomal subunits. Taken together our data show that METTL15 is a novel protein necessary for efficient translation in human mitochondria.

Structural insights into dimethylation of 12S rRNA by TFB1M: indispensable role in translation of mitochondrial genes and mitochondrial function

Mitochondria are essential molecular machinery for the maintenance of cellular energy supply by the oxidative phosphorylation system (OXPHOS). Mitochondrial transcription factor B1 (TFB1M) is a dimethyltransferase that maintains mitochondrial homeostasis by catalyzing dimethylation of two adjacent adenines located in helix45 (h45) of 12S rRNA. This m⁶₂A modification is indispensable for the assembly and maturation of human mitochondrial ribosomes. However, both the mechanism of TFB1M catalysis and the precise function of TFB1M in mitochondrial homeostasis are unknown. Here we report the crystal structures of a ternary complex of human (hs) TFB1M–h45–S-adenosyl-methionine and a binary complex hsTFB1M–h45. The structures revealed a distinct mode of hsTFB1M interaction with its rRNA substrate and with the initial enzymatic state involved in m⁶₂A modification. The suppression of hsTFB1M protein level or the overexpression of inactive hsTFB1M mutants resulted in decreased ATP production and reduced expression of components of the mitochondrial OXPHOS without affecting transcription of the corresponding genes and their localization to the mitochondria. Therefore, hsTFB1M regulated the translation of mitochondrial genes rather than their transcription via m⁶₂A modification in h45.

Mutant MRPS5 affects mitoribosomal accuracy and confers stress-related behavioral alterations

The 1555 A to G substitution in mitochondrial 12S A‐site rRNA is associated with maternally transmitted deafness of variable penetrance in the absence of otherwise overt disease. Here, we recapitulate the suggested A1555G‐mediated pathomechanism in an experimental model of mitoribosomal mistranslation by directed mutagenesis of mitoribosomal protein MRPS5. We first establish that the ratio of cysteine/methionine incorporation and read‐through of mtDNA‐encoded MT‐CO1 protein constitute reliable measures of mitoribosomal misreading. Next, we demonstrate that human HEK293 cells expressing mutant V336Y MRPS5 show increased mitoribosomal mistranslation. As for immortalized lymphocytes of individuals with the pathogenic A1555G mutation, we find little changes in the transcriptome of mutant V336Y MRPS5 HEK cells, except for a coordinated upregulation of transcripts for cytoplasmic ribosomal proteins. Homozygous knock‐in mutant Mrps5 V338Y mice show impaired mitochondrial function and a phenotype composed of enhanced susceptibility to noise‐induced hearing damage and anxiety‐related behavioral alterations. The experimental data in V338Y mutant mice point to a key role of mitochondrial translation and function in stress‐related behavioral and physiological adaptations.

Meis1: effects on motor phenotypes and the sensorimotor system in mice

MEIS1 encodes a developmental transcription factor and has been linked to restless legs syndrome (RLS) in genome-wide association studies. RLS is a movement disorder leading to severe sleep reduction and has a substantial impact on the quality of life of patients. In genome-wide association studies, MEIS1 has consistently been the gene with the highest effect size and functional studies suggest a disease-relevant downregulation. Therefore, haploinsufficiency of Meis1 could be the system with the most potential for modeling RLS in animals. We used heterozygous Meis1-knockout mice to study the effects of Meis1 haploinsufficiency on mouse behavioral and neurological phenotypes, and to relate the findings to human RLS. We exposed the Meis1-deficient mice to assays of motor, sensorimotor and cognitive ability, and assessed the effect of a dopaminergic receptor 2/3 agonist commonly used in the treatment of RLS. The mutant mice showed a pattern of circadian hyperactivity, which is compatible with human RLS. Moreover, we discovered a replicable prepulse inhibition (PPI) deficit in the Meis1-deficient animals. In addition, these mice were hyposensitive to the PPI-reducing effect of the dopaminergic receptor agonist, highlighting a role of Meis1 in the dopaminergic system. Other reported phenotypes include enhanced social recognition at an older age that was not related to alterations in adult olfactory bulb neurogenesis previously shown to be implicated in this behavior. In conclusion, the Meis1-deficient mice fulfill some of the hallmarks of an RLS animal model, and revealed the role of Meis1 in sensorimotor gating and in the dopaminergic systems modulating it.

Summary: Loss of Meis1 results in motor restlessness in mice, a phenotype resembling human restless legs syndrome, as well as altered sensorimotor gating and improved social discrimination memory.

Genetic Epidemiology of Mitochondrial Pathogenic Variants Causing Nonsyndromic Hearing Loss in a Large Cohort of South Indian Hearing Impaired Individuals

Mitochondria play a critical role in the generation of metabolic energy in the form of ATP. Tissues and organs that are highly dependent on aerobic metabolism are involved in mitochondrial disorders including nonsyndromic hearing loss (NSHL). Seven pathogenic variants leading to NSHL have so far been reported on two mitochondrial genes: MT-RNR1 encoding 12SrRNA and MT-TS1 encoding tRNA for Ser((UCN)) . We screened 729 prelingual NSHL subjects to determine the prevalence of MT-RNR1 variants at position m.961, m.1555A>G and m.1494C>T, and MT-TS1 m.7445A>G, m.7472insC m.7510T>C and m.7511T>C variants. Mitochondrial pathogenic variants were found in eight probands (1.1%). Five of them were found to have the m.1555A>G variant, two others had m.7472insC and one proband had m.7444G>A. The extended relatives of these probands showed variable degrees of hearing loss and age at onset. This study shows that mitochondrial pathogenic alleles contribute to about 1% prelingual hearing loss. This study will henceforth provide the reference for the prevalence of mitochondrial pathogenic alleles in the South Indian population, which to date has not been estimated. The m.1555A>G variant is a primary predisposing genetic factor for the development of hearing loss. Our study strongly suggests that mitochondrial genotyping should be considered for all hearing impaired individuals and particularly in families where transmission is compatible with maternal inheritance, after ruling out the most common variants.

Towards the Prevention of Aminoglycoside-Related Hearing Loss

Aminoglycosides are potent antibiotics deployed worldwide despite their known side-effect of sensorineural hearing loss. The main etiology of this sensory deficit is death of inner ear sensory hair cells selectively triggered by aminoglycosides. For decades, research has sought to unravel the molecular events mediating sensory cell demise, emphasizing the roles of reactive oxygen species and their potentials as therapeutic targets. Studies in recent years have revealed candidate transport pathways including the mechanotransducer channel for drug entry into sensory cells. Once inside sensory cells, intracellular targets of aminoglycosides, such as the mitochondrial ribosomes, are beginning to be elucidated. Based on these results, less ototoxic aminoglycoside analogs are being generated and may serve as alternate antimicrobial agents. In this article, we review the latest findings on mechanisms of aminoglycoside entry into hair cells, their intracellular actions and potential therapeutic targets for preventing aminoglycoside ototoxicity.