bims-cytox1 Biomed News
on Cytochrome oxidase subunit 1
Issue of 2020‒12‒20
six papers selected by
Gavin McStay
Staffordshire University

  1. FEBS Lett. 2020 Dec 12.
      Mitochondria control life and death in eukaryotic cells. Harboring a unique circular genome, a by-product of an ancient endosymbiotic event, mitochondria maintain a specialized and evolutionary divergent protein synthesis machinery, the mitoribosome. Mitoribosome biogenesis depends on elements encoded in both the mitochondrial genome (the RNA components) and the nuclear genome (all ribosomal proteins and assembly factors). Recent cryo-EM structures of mammalian mitoribosomes have illuminated their composition and provided hints regarding their assembly and elusive mitochondrial translation mechanisms. A growing body of literature involves the mitoribosome in inherited primary mitochondrial disorders. Mutations in genes encoding mitoribosomal RNAs, proteins, and assembly factors, impede mitoribosome biogenesis, causing protein synthesis defects that lead to respiratory chain failure and mitochondrial disorders such as encephalo- and cardiomyopathy, deafness, neuropathy, and developmental delays. In this article, we review the current fundamental understanding of mitoribosome assembly and function, and the clinical landscape of mitochondrial disorders driven by mutations in mitoribosome components and assembly factors, to portrait how basic and clinical studies combined help us better understand both mitochondrial biology and medicine.
    Keywords:  Mitochondrial disease; Mitochondrial ribosome; Mitochondrial translation; Mitoribosome assembly; OXPHOS deficiency
  2. Mol Genet Metab Rep. 2020 Dec;25 100687
      Mutations in LRPPRC cause Leigh Syndrome French Canadian (LSFC), an early onset neurodegenerative disease, with differential tissue involvement. The molecular basis for tissue specificity in this disease remains unknown. LRPPRC, an RNA binding protein, forms a stable complex with SLIRP, which binds to, and stabilizes mitochondrial mRNAs. In cell culture and animal models, loss of LRPPRC function results in transcript-specific alterations in the steady-state levels of mitochondrial mRNAs and poly (A) tail length, the mechanisms for which are not understood. The poly (A) tail length of mitochondrial mRNAs has not been investigated in human tissues from heathy subjects or LSFC patients. Here we have mapped the 3'-termini of mature mitochondrial mRNAs in three tissues (skeletal muscle, heart, and liver) from a healthy individual and an LSFC patient. We show that the poly (A) tail length of mitochondrial mRNAs varies amongst tissues, and that the missense mutation in LRPPRC that causes LSFC results in tissue- and transcript-specific deadenylation of a subset of mitochondrial mRNAs, likely contributing the nature and severity of the biochemical phenotype in different tissues. We also found a relatively large fraction of short transcripts lacking a stop codon, some with short poly (A) tails, in patient tissue, suggesting that mutations in LRPPRC may also impair proper 3' end processing of some mRNAs.
    Keywords:  LRPPRC; Leigh syndrome; Mitochondrial translation; Oxidative phosphorylation; Polyadenylation; RNA binding proteins
  3. Genes Dis. 2020 Dec;7(4): 578-584
      Allotopic expression of mitochondrial genes is a deliberate functional relocation of mitochondrial genes into the nucleus followed by import of the gene-encoded polypeptide from the cytoplasm into the mitochondria. For successful allotopic expression of a mitochondrial gene, several key aspects must be considered. These include the different codon dictionary used by the mitochondrial and nuclear genomes, different codon preferences between mitochondrial and nuclear-cytosolic translation systems, and the provision of an import signal to ensure that the newly translated protein in the cytosol is successfully imported into mitochondria. The allotopic expression strategy was first developed in yeast, a useful model organism for studying human and other eukaryotic cells. Currently, a number of mitochondrial genes have been successfully recoded and nuclearly expressed in yeast and human cells. In addition to its use in evolutionary and molecular biology studies, the allotopic expression strategy has been developed as a potential approach to treat mitochondrial genetic disorders. Substantial progress has been recently achieved, and the development of this technique for therapy of the mitochondrial disease Leber's hereditary optic neuropathy (LHON) has entered phase III clinical trials. However, a number of challenges remain to be overcome to accelerate the successful application of this technique. These include improvement of nuclear gene expression, import into mitochondria, processing, and functional integration of the allotopically expressed polypeptides into mitochondrial protein complexes. This review discusses the current basic strategy, progress, challenges, and prospects of the allotopic expression strategy for mitochondrial genes.
    Keywords:  Allotopic expression; Gene therapy; Mitochondria; Mitochondrial gene; Protein targeting
  4. Front Cell Dev Biol. 2020 ;8 600079
      The concept of a mitochondrial disorder was initially described in 1962, in a patient with altered energy metabolism. Over time, mitochondrial energy metabolism has been discovered to be influenced by a vast number of proteins with a multitude of functional roles. Amongst these, defective oxidative phosphorylation arose as the hallmark of mitochondrial disorders. In the premolecular era, the diagnosis of mitochondrial disease was dependent on biochemical criteria, with inherent limitations such as tissue availability and specificity, preanalytical and analytical artifacts, and secondary effects. With the identification of the first mitochondrial disease-causing mutations, the genetic complexity of mitochondrial disorders began to unravel. Mitochondrial dysfunctions can be caused by pathogenic variants in genes encoded by the mitochondrial DNA or the nuclear DNA, and can display heterogenous phenotypic manifestations. The application of next generation sequencing methodologies in diagnostics is proving to be pivotal in finding the molecular diagnosis and has been instrumental in the discovery of a growing list of novel mitochondrial disease genes. In the molecular era, the diagnosis of a mitochondrial disorder, suspected on clinical grounds, is increasingly based on variant detection and associated statistical support, while invasive biopsies and biochemical assays are conducted to an ever-decreasing extent. At present, there is no uniform biochemical or molecular definition for the designation of a disease as a "mitochondrial disorder". Such designation is currently dependent on the criteria applied, which may encompass clinical, genetic, biochemical, functional, and/or mitochondrial protein localization criteria. Given this variation, numerous gene lists emerge, ranging from 270 to over 400 proposed mitochondrial disease genes. Herein we provide an overview of the mitochondrial disease associated genes and their accompanying challenges.
    Keywords:  challenges; diagnostic; genetics; mitochondrial disease; mutation
  5. FEBS Lett. 2020 Dec 14.
      The majority of proteins localised to mitochondria are encoded by the nuclear genome, with approximately 1500 proteins imported into mammalian mitochondria. Dysfunction in this fundamental cellular process is linked to a variety of pathologies including neuropathies, cardiovascular disorders, myopathies, neurodegenerative diseases and cancer, demonstrating the importance of mitochondrial protein import machinery for cellular function. Correct import of proteins into mitochondria requires the co-ordinated activity of multimeric protein translocation and sorting machineries located in both the outer and inner mitochondrial membranes, directing the imported proteins to the destined mitochondrial compartment. This dynamic process maintains cellular homeostasis, and its dysregulation significantly affects cellular signalling pathways and metabolism. This review summarises current knowledge of the mammalian mitochondrial import machinery and the pathological consequences of mutation of its components. In addition, we will discuss the role of mitochondrial import in cancer, and our current understanding of the role of mitochondrial import in neurodegenerative diseases including Alzheimer's disease, Huntington's disease, Parkinson's disease.
    Keywords:  Alzheimer’s disease; Cancer; Huntington’s disease; Mitochondria; Neurodegeneration; Parkinson’s disease; Protein Import; TIM; TOM
  6. Anal Chem. 2020 Dec 15.
      Assessment of activities of mitochondrial electron transport enzymes is important for understanding mechanisms of metabolic diseases, but structural organization of mitochondria and low sample availability pose distinctive challenges for in situ functional studies. We report the development of a tandem microfluidic respirometer that simultaneously tracks both the reduction of mediators on the electrode and the ensuing reduction of O2 by complex IV in the inner mitochondrial membrane. The response time of O2 consumption to multiple alternating potential steps is of approximately 10 s for a 150 μm-thick sample. Steady O2 depletion shows good quantitative correlation with the supplied electric charge, Pearson's r = 0.994. Reduction of mediators on biocompatible gold electrodes modified with carbon ink or fumed silica can compete with the oxidation of mediators by mitochondria, yielding an overall respiratory activity comparable to that upon chemical reduction by ascorbate. The dependence of O2 consumption on mediator and mitochondrial suspension concentrations shows that mass transport between the electrode and mitochondria does not limit biological activity of the latter. The mediated electrochemical approach is validated by the radiometric measurements of simulated changes in the intrinsic mitochondrial activity upon partial inhibition of complex IV by NaN3. This approach enables the development of O2-independent, biomimetic electrochemical assays narrowly targeting components of the electron transport chains in their native environments.