bims-cytox1 2022-08-14 papers

Issue of 2022‒08‒14
three papers selected by
Gavin McStay
Liverpool John Moores University

Genome Biol. 2022 Aug 09. 23(1): 170

Balanced mitochondrial and cytosolic translatomes underlie the biogenesis of human respiratory complexes.

Iliana Soto, Mary Couvillion, Katja G Hansen, Erik McShane, J Conor Moran, Antoni Barrientos, L Stirling Churchman.

BACKGROUND: Oxidative phosphorylation (OXPHOS) complexes consist of nuclear and mitochondrial DNA-encoded subunits. Their biogenesis requires cross-compartment gene regulation to mitigate the accumulation of disproportionate subunits. To determine how human cells coordinate mitochondrial and nuclear gene expression processes, we tailored ribosome profiling for the unique features of the human mitoribosome.RESULTS: We resolve features of mitochondrial translation initiation and identify a small ORF in the 3' UTR of MT-ND5. Analysis of ribosome footprints in five cell types reveals that average mitochondrial synthesis levels correspond precisely to cytosolic levels across OXPHOS complexes, and these average rates reflect the relative abundances of the complexes. Balanced mitochondrial and cytosolic synthesis does not rely on rapid feedback between the two translation systems, and imbalance caused by mitochondrial translation deficiency is associated with the induction of proteotoxicity pathways.
CONCLUSIONS: Based on our findings, we propose that human OXPHOS complexes are synthesized proportionally to each other, with mitonuclear balance relying on the regulation of OXPHOS subunit translation across cellular compartments, which may represent a proteostasis vulnerability.

DOI: https://doi.org/10.1186/s13059-022-02732-9

Trends Biochem Sci. 2022 Aug 09. pii: S0968-0004(22)00187-6. [Epub ahead of print]

Cooperative assembly of the mitochondrial respiratory chain.

Erika Fernández-Vizarra, Cristina Ugalde.

Deep understanding of the pathophysiological role of the mitochondrial respiratory chain (MRC) relies on a well-grounded model explaining how its biogenesis is regulated. The lack of a consistent framework to clarify the modes and mechanisms governing the assembly of the MRC complexes and supercomplexes (SCs) works against progress in the field. The plasticity model was postulated as an attempt to explain the coexistence of mammalian MRC complexes as individual entities and associated in SC species. However, mounting data accumulated throughout the years question the universal validity of the plasticity model as originally proposed. Instead, as we argue here, a cooperative assembly model provides a much better explanation to the phenomena observed when studying MRC biogenesis in physiological and pathological settings.

Keywords: assembly factors; cooperative assembly model; mitochondria; plasticity model; respiratory chain organization; supercomplexes

DOI: https://doi.org/10.1016/j.tibs.2022.07.005

FEBS J. 2022 Aug 13.

Loss of Mitochondrial Fatty Acid β-Oxidation Protein Short Chain Enoyl-CoA Hydratase Disrupts Oxidative Phosphorylation Protein Complex Stability and Function.

Harrison Burgin, Alice J Sharpe, Shuai Nie, Mark Ziemann, Jordan J Crameri, Diana Stojanovski, James Pitt, Akira Ohtake, Kei Murayama, Matthew McKenzie.

Short chain enoyl-CoA hydratase 1 (ECHS1) is involved in the second step of mitochondrial fatty acid β-oxidation (FAO), catalysing the hydration of short chain enoyl-CoA esters to short chain 3-hyroxyl-CoA esters. Genetic deficiency in ECHS1 (ECHS1D) is associated with a specific subset of Leigh Syndrome, a disease typically caused by defects in oxidative phosphorylation (OXPHOS). Here, we examined the molecular pathogenesis of ECHS1D using a CRISPR/Cas9 edited human cell 'knockout' model and fibroblasts from ECHS1D patients. Transcriptome analysis of ECHS1 'knockout' cells showed reductions in key mitochondrial pathways, including the TCA cycle, receptor mediated mitophagy and nucleotide biosynthesis. Subsequent proteomic analyses confirmed these reductions and revealed additional defects in mitochondrial oxidoreductase activity and fatty acid β-oxidation. Functional analysis of ECHS1 'knockout' cells showed reduced mitochondrial oxygen consumption rates when metabolising glucose or OXPHOS complex I-linked substrates, as well as decreased complex I and complex IV enzyme activities. ECHS1 'knockout' cells also exhibited decreased OXPHOS protein complex steady-state levels (complex I, complex III2 , complex IV, complex V and supercomplexes CIII2 /CIV and CI/CIII2 /CIV), which were associated with a defect in complex I assembly. Patient fibroblasts exhibit varied reduction of mature OXPHOS complex steady-state levels, with defects detected in CIII2 , CIV, CV and the CI/CIII2 /CIV supercomplex. Overall, these findings highlight the contribution of defective OXPHOS function, in particular complex I deficiency, to the molecular pathogenesis of ECHS1D.

Keywords: ECHS1 deficiency; OXPHOS; fatty acid oxidation; mitochondria; mitochondrial disease; short chain enoyl-CoA hydratase

DOI: https://doi.org/10.1111/febs.16595