Weiwei Fan, Tae Gyu Oh, Hui J Wang, Lillian Crossley, Mingxiao He, Hunter Robbins, Chandra Koopari, Yang Dai, Morgan L Truitt, Christopher Liddle, Ruth T Yu, Annette R Atkins, Michael Downes, Ronald M Evans
8 min
Mitochondrial energy metabolism is vital for muscle function and is tightly controlled at the transcriptional level, both in the basal state and during adaptive muscle remodeling. The importance of the transcription factors estrogen-related receptors (ERRs) in controlling innate mitochondrial energetics has been recently demonstrated. However, whether different ERR isoforms display distinct functions in glycolytic versus oxidative myofibers is largely unknown. Moreover, their roles in regulating exercise-induced adaptive mitochondrial biogenesis remain unclear. Using muscle-specific single and combinatorial knockout mouse models, we have identified both cooperative and distinct roles of the ERR isoforms ERRα and ERRγ in regulating mitochondrial energy metabolism in different muscles. We demonstrate the essential roles of both these ERRs in mediating adaptive mitochondrial biogenesis in response to exercise training. We further show that PGC1α-induced mitochondrial biogenesis is completely abolished in primary myotubes with ERRα deletion but not ERRγ, highlighting distinct roles of these two isoforms in adaptive mitochondrial remodeling. Mechanistically, we find that both ERRs directly bind to the majority of mitochondrial energetic genes and control their expression, largely through collaborative binding to the same genomic loci. Collectively, our findings reveal critical and direct regulatory roles of ERRα and ERRγ in governing both innate and adaptive mitochondrial energetics in skeletal muscle.
Skeletal muscle mitochondrial function is critical for exercise capacity and overall metabolic health. The transcriptional coactivator PGC1α is well-known for driving adaptive mitochondrial biogenesis, but its direct DNA-binding partners in muscle have remained incompletely understood. This study investigates the roles of Estrogen-related receptors (ERRα and ERRγ) in both innate mitochondrial maintenance and adaptive, exercise-induced remodeling.
Using muscle-specific knockout mouse models, the researchers depleted ERRα, ERRβ, and ERRγ individually and in combination. They assessed mitochondrial structure (using electron microscopy) and function (via oxygen consumption and enzymatic activity) across different muscle types (oxidative soleus vs. glycolytic quadriceps). The authors performed RNA-seq and CUT&TAG-seq to identify the genomic binding sites (cistrome) of ERRα and ERRγ, and utilized exercise training and PGC1α overexpression assays to determine the transcriptional dependencies of adaptive muscle remodeling.
The study demonstrates that ERRα and ERRγ serve as critical, cooperative regulators of mitochondrial energetics. In oxidative muscle, these isoforms act redundantly to maintain mitochondrial health. In glycolytic muscle, ERRα is the dominant isoform; however, even minimal levels of ERRγ are sufficient to sustain basal mitochondrial function if ERRα is absent.
Crucially, the study identifies a functional split in adaptive responses: ERRα is indispensable for PGC1α-mediated mitochondrial biogenesis, whereas ERRγ contributes to adaptive exercise responses through potentially distinct, PGC1-independent pathways. Genomic profiling revealed that these receptors largely colocalize at promoter regions of mitochondrial genes, often binding as heterodimers to control aerobic respiration and fatty acid oxidation programs.
These results provide a definitive transcriptional mechanism for how exercise reprograms skeletal muscle. By characterizing the specific, essential role of ERRα in PGC1α-mediated adaptation, this study identifies these nuclear receptors as prime targets for pharmacological interventions designed to treat muscle-related mitochondrial disorders or to mimic the benefits of exercise in patients with limited physical mobility.
Alex: What did the alpha-versus-gamma comparison show before any exercise training?
Sam: In oxidative and mixed-type muscles, removing either alone caused relatively mild mitochondrial defects. Removing both caused much larger losses in mitochondrial content, respiratory activity, and energy-related proteins. The structural evidence agreed: mitochondria were reduced, some had damaged internal folds, and their normally organized network was disrupted.
Alex: That supports compensation where both are abundant. But if gamma is scarce in glycolytic muscle, does alpha become indispensable there?
Sam: Not for basic maintenance. Gamma makes up only about four percent of the receptors in the glycolytic muscle they studied. Yet in alpha’s absence, that small gamma pool could maintain approximately seventy percent of total energetic capacity. Deleting gamma as well caused a much steeper decline.
Alex: So receptor abundance was a poor guide to how much function would disappear. Could losing alpha have made the muscle produce more gamma?
Sam: The authors checked that. Deleting a receptor did not increase expression of the remaining forms. They suggest alpha may normally be present in excess, leaving capacity available for adaptive responses. That’s an interpretation of the maintenance results, not something abundance measurements alone prove.
Alex: Let’s test that interpretation against training. Could muscle preserve much of its everyday function yet fail to improve?
Sam: That’s the important turn. The mice had voluntary running-wheel access for four weeks. Normal mice and gamma-deficient mice increased their wheel-running activity, but alpha-deficient mice did not. The researchers then assessed treadmill endurance and mitochondrial content, rather than relying on wheel use alone.
Alex: Wheel running and endurance aren’t identical outcomes. Did gamma-deficient mice really adapt normally?
Sam: Only on the wheel-running measure. Their improvements in treadmill performance and mitochondrial biogenesis—the buildup of mitochondrial machinery—were lower than in normal mice. Alpha deletion produced a more severe failure of adaptation. So both receptors contribute, but their roles aren’t interchangeable.
Alex: If alpha-deficient mice trained less, reduced adaptation could follow from reduced exercise exposure. How far do the experiments get beyond that objection?
Sam: The authors compared another knockout line with a similarly impaired starting level of running, which still improved somewhat. That argues against poor starting performance being the whole explanation. More decisively, they tested the proposed pathway directly in cultured muscle cells, where running behavior cannot explain the result.
Alex: What did they activate in those cells, and why was that informative?
Sam: They increased P G C one alpha, an exercise-responsive protein that helps other proteins activate genes. It cannot bind DNA itself, so it needs a DNA-binding partner. In normal muscle cells, increasing it boosted mitochondrial biogenesis and oxygen consumption. The boost also occurred without gamma, but disappeared without alpha.
Alex: That separates the exercise signal from the ability to execute it. Was the signal itself still present in the knockout mice?
Sam: Acute exercise still increased P G C one alpha in the receptor-knockout mice. And muscle-targeted gene delivery supported the cell result: increasing that protein activated mitochondrial genes without gamma, but not without alpha. Increasing gamma itself could boost mitochondrial content even in muscle initially lacking both receptors.
Alex: So gamma can promote mitochondrial growth, but it doesn’t replace alpha as the partner for this particular exercise-responsive protein. How directly do the receptors control the genes involved?
Sam: The researchers used CUT and TAG, a method that maps where proteins bind DNA. Across a reference set of mouse mitochondrial genes, roughly seven in ten had binding sites for at least one receptor. Many sites were shared by alpha and gamma. Shared binding was especially prominent near the starts of genes involved in energy production.
Alex: Binding alone can be hard to interpret. Did they connect those locations to changes in gene activity?
Sam: They combined the binding maps with gene-expression measurements after deletion. Most genes whose activity fell in the double-knockout oxidative muscle had receptor binding sites. They also used sequential protein-binding experiments, supporting alpha and gamma binding together at selected energy-related genes. That gives their functional cooperation a physical basis.
Alex: How much weight should we put on the whole package, given the scale and the treatment ambitions?
Sam: My judgment is that the mechanism is supported by converging methods, rather than one readout. But the main mitochondrial comparisons used five animals per group. The exercise and overexpression results were reported with four. These are modest experiments, and the functional interventions were in mice or mouse-derived cells—not patients receiving a drug.
Alex: And deleting both receptors didn’t eliminate mitochondria altogether. Does that limit the idea that they’re master regulators?
Sam: It limits any claim that they’re the only regulators. Substantial mitochondrial function remained, and oxidative muscle still differed from glycolytic muscle. The authors explicitly infer that other transcriptional mechanisms must contribute. Their treatment proposal is a rationale for targeting these receptors, not demonstrated therapeutic efficacy.
Alex: Who should make time for the full paper, and where should they start?
Sam: Researchers studying muscle adaptation, mitochondrial dysfunction, or gene regulation should start with the section on exercise and P G C one alpha-induced mitochondrial biogenesis. Compare the single deletions carefully, then read the direct DNA-binding analysis. For everyone else, remember this: regulators that substitute for each other during maintenance may have different jobs during adaptation.
Alex: Keeping muscle running and helping it improve are different tests.