bims-cytox1 2020-12-06 papers

Issue of 2020–12–06
three papers selected by
Gavin McStay, Staffordshire University

Proc Natl Acad Sci U S A. 2020 Nov 30. pii: 202005877. [Epub ahead of print]

Mitochondrial Nuclear Retrograde Regulator 1 (MNRR1) rescues the cellular phenotype of MELAS by inducing homeostatic mechanisms.

Siddhesh Aras, Neeraja Purandare, Stephanie Gladyck, Mallika Somayajulu-Nitu, Kezhong Zhang, Douglas C Wallace, Lawrence I Grossman.

MNRR1 (CHCHD2) is a bi-organellar regulator of mitochondrial function that directly activates cytochrome c oxidase in the mitochondria and functions in the nucleus as a transcriptional activator for hundreds of genes. Since MNRR1 depletion contains features of a mitochondrial disease phenotype, we evaluated the effects of forced expression of MNRR1 on the mitochondrial disease MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes) syndrome. MELAS is a multisystem encephalomyopathy disorder that can result from a heteroplasmic mutation in the mitochondrial DNA (mtDNA; m.3243A > G) at heteroplasmy levels of ∼50 to 90%. Since cybrid cell lines with 73% m.3243A > G heteroplasmy (DW7) display a significant reduction in MNRR1 levels compared to the wild type (0% heteroplasmy) (CL9), we evaluated the effects of MNRR1 levels on mitochondrial functioning. Overexpression of MNRR1 in DW7 cells induces the mitochondrial unfolded protein response (UPRmt), autophagy, and mitochondrial biogenesis, thereby rescuing the mitochondrial phenotype. It does so primarily as a transcription activator, revealing this function to be a potential therapeutic target. The role of MNRR1 in stimulating UPRmt, which is blunted in MELAS cells, was surprising and further investigation uncovered that under conditions of stress the import of MNRR1 into the mitochondria was blocked, allowing the protein to accumulate in the nucleus to enhance its transcription function. In the mammalian system, ATF5, has been identified as a mediator of UPRmt MNRR1 knockout cells display an ∼40% reduction in the protein levels of ATF5, suggesting that MNRR1 plays an important role upstream of this known mediator of UPRmt.

Keywords: CHCHD2; cytochrome c oxidase; mitochondria; transcription; unfolded protein response

DOI: https://doi.org/10.1073/pnas.2005877117

EMBO Rep. 2020 Dec 03. e51830

Supercomplex formation boosts respiration.

Fabian den Brave, Thomas Becker.

Mitochondrial respiratory chain complexes associate in supercomplexes, but the physiological role of these assemblies remains controversial. Recent studies in EMBO Reports reveal that supercomplexes promote metabolic fitness. Berndtsson et al (2020) demonstrate that supercomplex formation enhances electron transport by reducing the distance for diffusion of cytochrome c between cytochrome bc1 complex and cytochrome c oxidase and thereby increases competitive fitness in yeast. Similarly, Garcia-Poyatos et al (2020) report that zebrafish lacking the supercomplex assembly factor SCAF1 display a reduced growth and decreased female fertility.

DOI: https://doi.org/10.15252/embr.202051830

FEBS Lett. 2020 Nov 28.

The Proteasome: Friend and Foe of Mitochondrial Biogenesis.

Lena Krämer, Carina Groh, Johannes M Herrmann.

Most mitochondrial proteins are synthesized in the cytosol and subsequently translocated as unfolded polypeptides into mitochondria. Cytosolic chaperones maintain precursor proteins in an import-competent state. This post-translational import reaction is under surveillance of the cytosolic ubiquitin-proteasome system, which carries out several distinguishable activities. On the one hand, the proteasome degrades non-productive protein precursors from the cytosol and nucleus, import intermediates that are stuck in mitochondrial translocases, and misfolded or damaged proteins from the outer membrane and the intermembrane space. These surveillance activities of the proteasome are essential for mitochondrial functionality, as well as cellular fitness and survival. On the other hand, the proteasome competes with mitochondria for non-imported cytosolic precursor proteins, which can compromise mitochondrial biogenesis. In order to balance the positive and negative effects of the cytosolic protein quality control system on mitochondria, mitochondrial import efficiency directly regulates the capacity of the proteasome via transcription factor Rpn4 in yeast and nuclear respiratory factor (Nrf) 1 and 2 in animal cells. In this review, we provide a thorough overview of how the proteasome regulates mitochondrial biogenesis.

Keywords: Aging; Mitochondria; Mitochondria-Associated Degradation; Mitoprotein-Induced Stress Response; Proteasome; Protein Quality Control; Protein degradation; Rpn4; Ubiquitin

DOI: https://doi.org/10.1002/1873-3468.14010