Prolonged intermittent hypoxia causes mitochondrial dysfunction due to O-GlcNacylation of DRP1 in models of COPD (chronic obstructive pulmonary disease)
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Background: Sarcopenia, or skeletal muscle loss, is common complication in COPD and contributes to adverse clinical outcomes including mortality. As the disease progresses, a significant proportion of COPD patients develop nocturnal hypoxemia which causes tissue-level hypoxia in the skeletal muscle which we have termed prolonged intermittent hypoxia (PIH). We have demonstrated sarcopenia, mitochondrial dysfunction, and dysregulated hypoxia-inducible factor 1- (HIF1-) responses both in vitro and in vivo in response to PIH. We also noted multiple transcriptional targets of HIF1- were upregulated in the glycolytic pathway. We hypothesized that PIH upregulates the Hexosamine Biosynthetic (HB) pathway, a branch of glycolysis responsible for post-translational glycosylation of proteins, to cause O-GlcNAcylation of DRP1 leading to increased mitochondrial fission, fragmented mitochondrial networks, mitochondrial dysfunction, and sarcopenia. Methods: Our in vitro model of PIH exposed differentiated, murine C2C12 myotubes to 16h normoxia/8h hypoxia [1% oxygen] for 3 days, while our in vivo model exposed C57BL/6 mice to 12h normoxia/12h hypoxia [10% oxygen] for 21 days. Protein extraction and immunoblots were performed to probe for enzymes in the HB, mTORC1 pathways, O-GlcNAcylation, and puromycin incorporation. Imaging of mitochondrial structure was determined by staining with MitoTracker orange and electron microscopy. Immunoprecipitation and detection of Drp1 O-GlcNAcylation was performed by incubating protein lysates with Wheat Germ Agglutinin coated Agarose beads. To determine responses to inhibition of O-GlcNAcylation, we performed CRISPR-Cas9 RNP deletion of GFAT, the first enzyme in the HB pathway. Results: We found significant increase in O-GlcNAcylated proteins in our PIH models . This was accompanied by increased expression of HIF1-, mitochondrial dysfunction, and upregulated enzymes in the HB pathway. O-GlcNAcylated modification of DRP1 was significantly upregulated due to PIH in vitro. GFAT KO myotubes demonstrated significant reductions in global O-GlcNAcylation of proteins and O-GlcNAcylated Drp1 with PIH. Interestingly, protein synthesis was rescued in GFAT KO myotubes as determined by puromycin incorporation, restoration of mTORC1 signaling, and reversal of a sarcopenic phenotype. Conclusion: Our findings demonstrate a potential mechanism for mitochondrial dysfunction and sarcopenia due to PIH which may be translationally relevant for patients with COPD. Future studies focused on O-GlcNAcylated modification of DRP1 and the functional consequences are needed to identify novel therapeutic targets to reverse sarcopenia in COPD. Supported in part by: NIH RO1 GM119174; RO1 DK113196; P50 AA024333; RO1 AA021890; 3U01AA026976 - 03S1; UO1 AA 026976; R56HL141744;UO1 DK061732; 5U01DK062470-17S2; R21 AR 071046; Howard and Helen Trevey Endowment; (SD); K12 HL141952 (AA) and the American College of Gastroenterology Clinical Research Award and NIH KO8 AAAA028794 (NW). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.
