MFN2 deficiency promotes cardiac response to hypobaric hypoxia by reprogramming cardiomyocyte metabolism

Metabolomic analysis of rat arterial serum under hypobaric hypoxia: Adaptive regulation of physiological systems by metabolic reprogramming

Objective

To investigate the associations between metabolic changes and functions, including energy metabolism, immune response, and redox balance, under short-term hypobaric hypoxia exposure. Non-targeted metabolomics and bioinformatics analysis were applied to explore the adaptive mechanisms of organisms in hypobaric hypoxia.

Methods

Healthy adult male Sprague-Dawley rats were placed in environments simulating altitudes of 6500 m (HC group) and 1588 m (Control group). After 14 days, arterial serum samples were analyzed using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS). Significant metabolites (P < 0.05, VIP >1) were identified, and KEGG enrichment analysis was conducted. Differential metabolites were globally analyzed with MetaboAnalyst 5.0.

Results

A total of 117 significantly altered metabolites were identified. In the HC group, 84 metabolites significantly increased, while 33 metabolites significantly decreased compared to the Control group. KEGG enrichment analysis revealed significant metabolic pathways, including the PPAR signaling pathway, bile secretion, arginine biosynthesis, alcoholism, and cholesterol metabolism (P < 0.05). Global analysis indicated that these differential metabolites were involved in various pathways, such as energy metabolism, amino acid metabolism, nucleotide metabolism, lipid metabolism, vitamin and cofactor metabolism, steroid metabolism, neurotransmitter metabolism, and heme metabolism, all of which play crucial roles in multiple biological processes.

Conclusion

Short-term hypobaric hypoxia exposure significantly altered the metabolite profiles in the arterial serum samples of rats, revealing adaptive metabolic reprogramming in energy metabolism, redox balance, immune function, endocrine regulation, and neurological systems.

HIF-1α limits myocardial infarction by promoting mitophagy in mouse hearts adapted to chronic hypoxia

AIM

The transcriptional factor HIF-1α is recognized for its contribution to cardioprotection against acute ischemia/reperfusion injury. Adaptation to chronic hypoxia (CH) is known to stabilize HIF-1α and increase myocardial ischemic tolerance. However, the precise role of HIF-1α in mediating the protective effect remains incompletely understood.

METHODS

Male wild-type (WT) mice and mice with partial Hif1a deficiency (hif1a +/-) were exposed to CH for 4 weeks, while their respective controls were kept under normoxic conditions. Subsequently, their isolated perfused hearts were subjected to ischemia/reperfusion to determine infarct size, while RNA-sequencing of isolated cardiomyocytes was performed. Mitochondrial respiration was measured to evaluate mitochondrial function, and western blots were performed to assess mitophagy.

RESULTS

We demonstrated enhanced ischemic tolerance in WT mice induced by adaptation to CH compared with their normoxic controls and chronically hypoxic hif1a +/- mice. Through cardiomyocyte bulk mRNA sequencing analysis, we unveiled significant reprogramming of cardiomyocytes induced by CH emphasizing mitochondrial processes. CH reduced mitochondrial content and respiration and altered mitochondrial ultrastructure. Notably, the reduced mitochondrial content correlated with enhanced autophagosome formation exclusively in chronically hypoxic WT mice, supported by an increase in the LC3-II/LC3-I ratio, expression of PINK1, and degradation of SQSTM1/p62. Furthermore, pretreatment with the mitochondrial division inhibitor (mdivi-1) abolished the infarct size-limiting effect of CH in WT mice, highlighting the key role of mitophagy in CH-induced cardioprotection.

CONCLUSION

These findings provide new insights into the contribution of HIF-1α to cardiomyocyte survival during acute ischemia/reperfusion injury by activating the selective autophagy pathway.

Mfn2/Hsc70 Complex Mediates the Formation of Mitochondria-Lipid Droplets Membrane Contact and Regulates Myocardial Lipid Metabolism

The heart primarily derives its energy through lipid oxidation. In cardiomyocytes, lipids are stored in lipid droplets (LDs) and are utilized in mitochondria, although the structural and functional connections between these two organelles remain largely unknown. In this study, visible evidence have presented indicating that a complex is formed at the mitochondria-LD membrane contact (MLC) site, involving mitochondrion-localized Mfn2 and LD-localized Hsc70. This complex serves to tether mitochondria to LDs, facilitating the transfer of fatty acids (FAs) from LDs to mitochondria for β-oxidation. Reduction of Mfn2 induced by lipid overload inhibits MLC, hinders FA transfer, and results in lipid accumulation. Restoring Mfn2 reinstates MLC, alleviating myocardial lipotoxicity under lipid overload conditions both in-vivo and in-vitro. Additionally, prolonged lipid overload induces Mfn2 degradation through the ubiquitin-proteasome pathway, following Mfn2 acetylation at the K243 site. This leads to the transition from adaptive lipid utilization to maladaptive lipotoxicity. The experimental findings are supported by clinical data from patients with obesity and age-matched non-obese individuals. These translational results make a significant contribution to the molecular understanding of MLC in the heart, and offer new insights into its role in myocardial lipotoxicity.

Novel Relationship between Mitofusin 2-Mediated Mitochondrial Hyperfusion, Metabolic Remodeling, and Glycolysis in Pulmonary Arterial Endothelial Cells

The disruption of mitochondrial dynamics has been identified in cardiovascular diseases, including pulmonary hypertension (PH), ischemia-reperfusion injury, heart failure, and cardiomyopathy. Mitofusin 2 (Mfn2) is abundantly expressed in heart and pulmonary vasculature cells at the outer mitochondrial membrane to modulate fusion. Previously, we have reported reduced levels of Mfn2 and fragmented mitochondria in pulmonary arterial endothelial cells (PAECs) isolated from a sheep model of PH induced by pulmonary over-circulation and restoring Mfn2 normalized mitochondrial function. In this study, we assessed the effect of increased expression of Mfn2 on mitochondrial metabolism, bioenergetics, reactive oxygen species production, and mitochondrial membrane potential in control PAECs. Using an adenoviral expression system to overexpress Mfn2 in PAECs and utilizing 13C labeled substrates, we assessed the levels of TCA cycle metabolites. We identified increased pyruvate and lactate production in cells, revealing a glycolytic phenotype (Warburg phenotype). Mfn2 overexpression decreased the mitochondrial ATP production rate, increased the rate of glycolytic ATP production, and disrupted mitochondrial bioenergetics. The increase in glycolysis was linked to increased hypoxia-inducible factor 1α (HIF-1α) protein levels, elevated mitochondrial reactive oxygen species (mt-ROS), and decreased mitochondrial membrane potential. Our data suggest that disrupting the mitochondrial fusion/fission balance to favor hyperfusion leads to a metabolic shift that promotes aerobic glycolysis. Thus, therapies designed to increase mitochondrial fusion should be approached with caution.