Nuclear receptors as potential therapeutic targets in peripheral arterial disease and related myopathy

Concerted regulation of skeletal muscle metabolism and contractile properties by the orphan nuclear receptor Nr2f6

BACKGROUND

The maintenance of skeletal muscle plasticity upon changes in the environment, nutrient supply, and exercise depends on regulatory mechanisms that couple structural and metabolic adaptations. The mechanisms that interconnect both processes at the transcriptional level remain underexplored. Nr2f6, a nuclear receptor, regulates metabolism and cell differentiation in peripheral tissues. However, its role in the skeletal muscle is still elusive. Here, we aimed to investigate the effects of Nr2f6 modulation on muscle biology in vivo and in vitro.

METHODS

Global RNA-seq was performed in Nr2f6 knockdown C2C12 myocytes (N = 4-5). Molecular and metabolic assays and proliferation experiments were performed using stable Nr2f6 knockdown and Nr2f6 overexpression C2C12 cell lines (N = 3-6). Nr2f6 content was evaluated in lipid overload models in vitro and in vivo (N = 3-6). In vivo experiments included Nr2f6 overexpression in mouse tibialis anterior muscle, followed by gene array transcriptomics and molecular assays (N = 4), ex vivo contractility experiments (N = 5), and histological analysis (N = 7). The conservation of Nr2f6 depletion effects was confirmed in primary skeletal muscle cells of humans and mice.

RESULTS

Nr2f6 knockdown upregulated genes associated with muscle differentiation, metabolism, and contraction, while cell cycle-related genes were downregulated. In human skeletal muscle cells, Nr2f6 knockdown significantly increased the expression of myosin heavy chain genes (two-fold to three-fold) and siRNA-mediated depletion of Nr2f6 increased maximal C2C12 myocyte's lipid oxidative capacity by 75% and protected against lipid-induced cell death. Nr2f6 content decreased by 40% in lipid-overloaded myotubes and by 50% in the skeletal muscle of mice fed a high-fat diet. Nr2f6 overexpression in mice resulted in an atrophic and hypoplastic state, characterized by a significant reduction in muscle mass (15%) and myofibre content (18%), followed by an impairment (50%) in force production. These functional phenotypes were accompanied by the establishment of an inflammation-like molecular signature and a decrease in the expression of genes involved in muscle contractility and oxidative metabolism, which was associated with the repression of the uncoupling protein 3 (20%) and PGC-1α (30%) promoters activity following Nr2f6 overexpression in vitro. Additionally, Nr2f6 regulated core components of the cell division machinery, effectively decoupling muscle cell proliferation from differentiation.

CONCLUSIONS

Our findings reveal a novel role for Nr2f6 as a molecular transducer that plays a crucial role in maintaining the balance between skeletal muscle contractile function and oxidative capacity. These results have significant implications for the development of potential therapeutic strategies for metabolic diseases and myopathies.

A cell competition system with one gene expression from a single-copy gene in one cell

Even with advanced plasmid and viral vectors, attaining copy numbers of multiple genes among different transfected cells is challenging. We achieved one gene expression from a single-copy gene in one cell using a transgene competition system, a combination of the Kazusa cDNA clones and our dual recombinase-mediated cassette exchange system. All 48 nuclear receptors were simultaneously expressed in one dish at the same expression level in HEK293 using this system, and the cell proliferation rate was compared. Significant differences were observed between cells transfected with CMV- or EF1 promoter-driven expression of the 48 nuclear receptors after 8 weeks. The EF1-NR1I2 cell line, which exhibited the highest increase from 2 to 8 weeks, showed 1.13-fold higher proliferation than the EF1-DsRed line. On the other hand, the EF1-NR4A1 cell line, which showed the maximum decrease at 8 weeks, showed 0.88-fold lower proliferation than the EF1-DsRed line. The results were confirmed in both our transgene competition system and long-term growth experiments. Our transgene competition system offers a wide-range, simple, and accurate cell competition method.

PPARs as Key Transcription Regulators at the Crossroads of Metabolism and Inflammation

The metabolic and immune systems are complex networks of organs, cells, and proteins that are involved in the extraction of energy from food; this is to run complex cellular processes and defend the body against infections while protecting its own tissues, respectively [...].

Mitochondrial modulation of amplified preconditioning influences of remote ischemia plus erythropoietin against skeletal muscle ischemia/reperfusion injury in rats

AIMS

Skeletal muscle ischemia and reperfusion (S-I/R) injury is relieved by interventions like remote ischemic preconditioning (RIPC). Here, we tested the hypothesis that simultaneous exposure to a minimal dose of erythropoietin (EPO) boosts the protection conferred by RIPC against S-I/R injury and concomitant mitochondrial oxidative and apoptotic defects.

MAIN METHODS

S-I/R injury was induced in rats by 3-h right hindlimb ischemia followed by 3-h of reperfusion, whereas RIPC involved 3 brief consecutive I/R cycles of the contralateral hindlimb.

KEY FINDINGS

S-I/R injury caused (i) rises in serum lactate dehydrogenase and creatine kinase and falls in serum pyruvate, (ii) structural deformities like sarcoplasm vacuolations, segmental necrosis, and inflammatory cells infiltration, and (iii) decreased amplitude and increased duration of electromyography action potentials. These defects were partially ameliorated by RIPC and dose-dependently by EPO (500 or 5000 IU/kg). Further, greater repairs of S-I/R-evoked damages were seen after prior exposure to the combined RIPC/EPO-500 intervention. The latter also caused more effective (i) preservation of mitochondrial number (confocal microscopy assessed Mitotracker red staining) and function (citrate synthase activity), (ii) suppression of mitochondrial DNA damage and indices of oxidative stress and apoptosis (succinate dehydrogenase, myeloperoxidase, cardiolipin, and cytochrome c), (iii) preventing calcium and nitric oxide metabolites (NOx) accumulation and glycogen consumption, and (iv) upregulating EPO receptors (EPO-R) gene expression.

SIGNIFICANCE

dual RIPC/EPO conditioning exceptionally mends structural, functional, and neuronal deficits caused by I/R injury and interrelated mitochondrial oxidative and apoptotic damage. Clinically, the utilization of relatively low EPO doses could minimize the hormone-related adverse effects.

Estrogen-Related Receptor Gamma Gene Therapy Promotes Therapeutic Angiogenesis and Muscle Recovery in Preclinical Model of PAD

Background Peripheral arterial disease and critical limb ischemia are cardiovascular complications associated with vascular insufficiency, oxidative metabolic dysfunction, and myopathy in the limbs. Estrogen-related receptor gamma (ERRγ) has emerged as a dual regulator of paracrine angiogenesis and oxidative metabolism through transgenic mouse studies. Here our objective was to investigate whether postischemic intramuscular targeting of ERRγ via gene therapy promotes ischemic recovery in a preclinical model of peripheral arterial disease/critical limb ischemia. Methods and Results Adeno-associated virus 9 (AAV9) Esrrg gene delivery vector was developed and first tested via intramuscular injection in murine skeletal muscle. AAV9-Esrrg robustly increased ERRγ protein expression, induced angiogenic and oxidative genes, and boosted capillary density and succinate dehydrogenase oxidative metabolic activity in skeletal muscles of C57Bl/6J mice. Next, hindlimb ischemia was induced via unilateral femoral vessel ligation in mice, followed by intramuscular AAV9-Esrrg (or AAV9-green fluorescent protein) gene delivery 24 hours after injury. ERRγ overexpression increased ischemic neoangiogenesis and markers of endothelial activation, and significantly improved ischemic revascularization measured using laser Doppler flowmetry. Moreover, ERRγ overexpression restored succinate dehydrogenase oxidative metabolic capacity in ischemic muscle, which correlated with increased mitochondrial respiratory complex protein expression. Most importantly, myofiber size to number quantification revealed that AAV9-Esrrg restores myofibrillar size and mitigates ischemia-induced myopathy. Conclusions These results demonstrate that intramuscular AAV9-Esrrg delivery rescues ischemic pathology after hindlimb ischemia, underscoring that Esrrg gene therapy or pharmacological activation could be a promising strategy for the management of peripheral arterial disease/critical limb ischemia.

Targeted ablation of Fn14 receptor improves exercise capacity and inhibits neurogenic muscle atrophy

Skeletal muscle atrophy is a prevalent complication in multiple chronic diseases and disuse conditions. Fibroblast growth factor-inducible 14 (Fn14) is a member of the TNF receptor superfamily and a bona fide receptor of the TWEAK cytokine. Accumulating evidence suggests that Fn14 levels are increased in catabolic conditions as well as during exercise. However, the role of Fn14 in the regulation of skeletal muscle mass and function remains poorly understood. In this study, through the generation of novel skeletal muscle-specific Fn14-knockout mice, we have investigated the muscle role of Fn14 in the regulation of exercise capacity and denervation-induced muscle atrophy. Our results demonstrate that there was no difference in skeletal muscle mass between control and muscle-specific Fn14-knockout mice. Nevertheless, the deletion of Fn14 in skeletal muscle significantly improved exercise capacity and resistance to fatigue. This effect of Fn14 deletion is associated with an increased proportion of oxidative myofibers and higher capillaries number per myofiber in skeletal muscle. Furthermore, our results demonstrate that targeted deletion of Fn14 inhibits denervation-induced muscle atrophy in adult mice. Deletion of Fn14 reduced the expression of components of the ubiquitin-proteasome system and non-canonical NF-kappa B signaling in denervated skeletal muscle, as well as increased the phosphorylation of Akt kinase and FoxO3a transcription factor. Collectively, our results demonstrate that targeted inhibition of Fn14 improves exercise tolerance and inhibits denervation-induced muscle atrophy in adult mice.