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Hypoxia-reoxygenation stress response in the marine facultative anaerobe Mytilus edulis

ABSTRACT: Hypoxia is a characteristic feature of marine environments and a major stressor for marine organisms inhabiting benthic and intertidal zones. Several studies have explored the responses of these organisms to hypoxic stress at the whole organism level with a focus on energy metabolism and mitochondrial response, but the instrinsic mitochondrial responses that support the organelle’s function under hypoxia and reoxygenation (H/R) stress are not well understood. We studied the effects of acute H/R stress (10 min anoxia followed by 15 min reoxygenation) on mitochondrial respiration, production of reactive oxygen species (ROS) and posttranslational modifications (PTM) of the proteome in a marine facultative anaerobe, the blue mussel Mytilus edulis. The mussels’ mitochondria showed increased OXPHOS respiration and suppressed proton leak resulting in a higher coupling efficiency after H/R stress. ROS production decreased in both the resting (LEAK) and phosphorylating (OXPHOS) state indicating that M. edulis is able to prevent oxidative stress and mitochondrial damage during reoxygenation. Hypoxia did not stimulate the rearrangement of the mitochondrial supercomplexes but impacted the mitochondrial phosphoproteome including the proteins involved in OXPHOS, amino acid and fatty acid catabolism, and protein quality control. This study indicates that mussels’ mitochondria possess intrinsic mechanisms (including regulation via PTM mechanisms such as reversible protein phosphorylation) that ensure high respiratory flux and mitigate oxidative damage during H/R stress and contribute to the hypoxia-tolerant mitochondrial phenotype of this metabolically plastic species.

INSTRUMENT(S): Q Exactive

ORGANISM(S): Mytilus Edulis (blue Mussel)

TISSUE(S): Digestive Gland

SUBMITTER: Stephanie Markert

LAB HEAD: Stephanie Markert

PROVIDER: PXD027955 | Pride | 2021-11-25

REPOSITORIES: Pride

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Project description:Intermittent hypoxia is a common stressor in estuarine and coastal habitats due to the diurnal and tidal cycles of oxygen availability. Oxygen deficiency results in energy stress by limiting aerobic ATP production, while reoxygenation may lead to oxidative injury due to the excessive production of reactive oxygen species (ROS) in mitochondria. Mitochondria are a key intracellular target of hypoxia-reoxygenation (H/R) stress due to their central role in ATP production, ROS generation and stress signaling. Marine intertidal bivalves such as the Pacific oyster Crassostrea gigas are adapted to frequent H/R cycles in the intertidal zone and maintain mitochondrial integrity and aerobic function despite frequent oxygen fluctuations. To gain insight into the molecular responses of mitochondria of the hypoxia-tolerant Pacific oyster to H/R stress, we studied changes in oxidative phosphorylation (OXPHOS) capacity, proton leak and activity of the electron transport system enzymes (ETS) in gill mitochondria of C. gigas. We furthermore investigated shifts in mitochondrial proteome and phosphoproteome after 24 h of hypoxia and 1 h of post-hypoxic recovery. Oyster mitochondria maintained normal ETS and OXPHOS capacity during H/R stress, despite a slight but significant decline in cytochrome c oxidase (Complex IV) activity after reoxygenation. Fifty total proteins and 36 phosphoproteins were differentially abundant in mitochondria of H/R exposed oysters compared with the controls. Rearrangements of the mitochondrial (phospho)proteome during H/R stress involved upregulation of mitochondrial ETS (most notably Complexes I and IV) and iron-binding proteins (frataxin and ferrochelatase) as well as suppression of the metabolic pathways that channel electrons to ubiquinone, possibly as a mechanism to limit ROS production due to the iron overload and reverse flow of electrons through ETS. H/R stress also led to upregulation of a mitophagic activator serine/threonine protein phosphatase PGAM5 and dephosphorylation of metalloendopeptidase OMA1 indicating stimulation of mitochondrial quality control mechanisms during reoxygenation. Changes in the overall abundance and phosphorylation levels of several key proteins involved in the mitochondrial protein homeostasis (including subunits of the mitochondrial ribosome, mitochondrial inner membrane translocase and elongation factor G) are consistent with the suppression of the protein synthesis during hypoxia, likely as an energy-saving mechanism. Overall, our study indicates that shifts in the mitochondrial (phospho-)proteome play an important role in responses of oysters to H/R stress and might complement adaptations of anaerobic metabolism and metabolic rate depression in ensuring hypoxic survival and rapid recovery in this hypoxia-tolerant intertidal species.

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Hypoxia-reoxygenation stress response in the marine facultative anaerobe Mytilus edulis

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