To appreciate the significance of the discovery, it helps to understand how skeletal muscle is built. Muscle tissue is composed of bundles of fibers that come in distinct functional flavors. Fast-twitch fibers are tuned for quick, forceful bursts of effort but tire rapidly. Slow-twitch fibers are built for endurance and are packed with mitochondria, the organelles commonly described as the powerhouses of the cell. With age, muscle mitochondria work less efficiently, and yet the fiber population drifts toward the slow-twitch, mitochondria-dependent type. A similar shift occurs in many diseases that cause muscle wasting, which suggests that the phenomenon reflects a shared biological program rather than random wear and tear.

The Copenhagen-led team, working across more than 20 institutions and led by the NNF Center for Basic Metabolic Research at the University of Copenhagen together with the Center for Adipocyte Signaling at the University of Southern Denmark, focused their investigation on a fat molecule called cardiolipin. Cardiolipin occupies a singular position in cellular architecture: it is found essentially nowhere else in the cell but the inner membrane of mitochondria, where it is essential for maintaining that membrane’s characteristic folded structure, the cristae that give mitochondria their enormous effective surface area. Without adequate cardiolipin, mitochondria cannot produce sufficient energy, nor can they generate the metabolic signals and molecular building blocks that cells depend on to function.

When the researchers measured cardiolipin in muscle mitochondria from mice and humans, they found a consistent pattern: levels decline with both age and disease, and the mitochondria in cardiolipin-poor muscle appear distorted and unable to operate properly. That correlation alone, however, could not settle the question of causation. Declining cardiolipin might simply be a consequence of aging rather than a driver of the changes that accompany it. To distinguish between these possibilities, the team performed a decisive experiment in young mice, artificially lowering cardiolipin levels to mimic the drop seen naturally in aging. The result was striking: the young animals developed the same fast-to-slow-twitch fiber shift that occurs in aged mice and in aging humans, indicating that cardiolipin loss is sufficient to trigger the remodeling program on its own.

Even more consequential was what happened when the researchers partially restored cardiolipin. Bringing levels back to roughly two-thirds of normal was enough to begin reversing muscle wasting in the animals and to prevent their early deaths entirely. This partial-rescue result carries substantial therapeutic weight, because it suggests that a treatment does not need to fully normalize cardiolipin to deliver meaningful benefit. Senior author Zachary Gerhart-Hines, Associate Professor at the NNF Center for Basic Metabolic Research at the University of Copenhagen, emphasized this point in the announcement of the work, noting that even a partial recovery of cardiolipin was enough to bring the muscle back, and that the central question now is whether cardiolipin can be increased in aging muscle or whether the downstream pathway can be targeted to promote healthy adaptations.

But why would a muscle respond to mitochondrial damage by remodeling itself into a slower, more mitochondria-dependent fiber type? The answer the researchers uncovered reframes the entire phenomenon as a defense mechanism. As cardiolipin levels fall, the strained mitochondria generate far more reactive oxygen species, or ROS, the chemically reactive molecules that damage cellular components in excess. Yet ROS is not merely a destructive byproduct; it also functions as a signal. When the team used an antioxidant to mop up ROS in cardiolipin-depleted muscle cells, the shift toward slow-twitch fibers was blunted, demonstrating that the ROS signal is what initiates the remodeling program rather than an incidental side effect of it.

Tracing that signal to its destination, the researchers identified a protein called ERRγ, a nuclear receptor that acts as the switch driving the fiber-type transformation. ERRγ triggers muscle cells to remodel their mitochondria and convert from fast-twitch to slow-twitch identity. The causal role of the receptor was confirmed in the opposite direction: when the scientists blocked ERRγ in cultured muscle cells, the fiber switch was completely shut down. In other words, the entire adaptive program, from ROS sensing to mitochondrial remodeling to fiber-type conversion, runs through this single regulatory node, making it an unusually clean target for intervention.

The logic of the adaptation becomes clearer when one follows what the remodeled fibers do with sugar. Cardiolipin-deficient mice pulled much more glucose out of the bloodstream than expected, but not to burn it for energy. By tracing labeled sugar through the muscle, the researchers showed that the glucose was instead diverted into the manufacture of the cell’s own antioxidants, molecules that neutralize ROS and shield the stressed cell from further oxidative damage. The slow-twitch state, in this reading, is a metabolic bargain: the muscle trades raw power for biochemical protection. Fabian Finger, Assistant Professor at the University of Copenhagen and first and co-corresponding author of the study, described the switch as the muscle trading power for protection rather than failing, and cautioned that interfering with it can backfire. In a telling result, mice given antioxidants to clear ROS fared worse, not better, because the treatment suppressed a protective response the animals needed.

The translational implications are unusually concrete for basic mitochondrial biology. Cardiolipin is already a pharmacological target: the FDA recently granted accelerated approval to elamipretide, a drug proposed to stabilize cardiolipin, for the rare genetic disorder Barth syndrome. Whether that class of compounds can be repurposed or adapted for age-related muscle decline remains an open question, but the existence of an approved pathway demonstrates that the target is druggable. ERRγ offers a second point of entry. It belongs to the nuclear receptor family, a class of molecules targeted by roughly 10 to 15 percent of all FDA-approved small-molecule medications, and activators of ERRγ are already in preclinical development for other indications. The study was funded by the Novo Nordisk Foundation, the European Research Council, the Lundbeck Foundation and the US National Institutes of Health, among others, and the published work carries a disclosure that senior author Zachary Gerhart-Hines works in some capacity for Embark Laboratories ApS, a company developing therapeutics for diabetes and obesity.

Important caveats remain. The causal experiments were carried out in mice, and the human samples served only to confirm that cardiolipin also declines with age in people; whether the same ROS-driven, ERRγ-mediated switch operates identically in human muscle has yet to be demonstrated directly. Nor is it yet known whether raising cardiolipin in aging human muscle, or modulating ERRγ activity, would improve function without unintended metabolic consequences, particularly given the finding that blocking the pathway entirely shuts down what appears to be a protective program. Still, the study resolves a genuine paradox with a coherent mechanism: the fast-to-slow-twitch shift of aging is not the muscle surrendering to decay but a regulated trade of performance for resilience, governed by a lipid whose loss destabilizes mitochondria and a receptor that reorganizes the cell in response. If future work can safely tilt that trade back toward function, the hidden switch behind aging muscle may become one of the first points of genuine therapeutic control over a process long accepted as inevitable.

Subject of Research: The role of the mitochondrial lipid cardiolipin and the nuclear receptor ERRγ in driving age-related muscle fiber-type switching

Article Title: The hidden switch behind one of the biggest paradoxes in aging muscle

Article References: The hidden switch behind one of the biggest paradoxes in aging muscle. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: aging, muscle, mitochondria, cardiolipin, ERRγ, reactive oxygen species, muscle fibers, nuclear receptors, elamipretide, Nature Aging, sarcopenia, University of Copenhagen

The biological problem the researchers set out to tackle is well known to reproductive medicine. As women age, the quality of their oocytes deteriorates, driving up rates of miscarriage, chromosomal abnormalities, preterm birth and fetal growth restriction, and making women aged 35 to 39 more prone to infertility than any other age group. Oocyte quality hinges on two tightly coordinated processes: nuclear maturation, in which chromosomes separate during the resumption of meiosis, and cytoplasmic maturation, in which the egg stockpiles the messenger RNAs, proteins and substrates that fertilization and early embryonic development will demand. Aging disrupts the cytoplasmic arm of this program through mitochondrial dysfunction, disturbed calcium signaling, malformed smooth endoplasmic reticulum and cytoskeletal disarray.

Mitochondria sit at the center of that failure. The oocyte is one of the most energy-hungry cells in the body, relying on oxidative phosphorylation through the five complexes of the electron transport chain to power chromosome segregation, spindle migration and organelle repositioning. In aging oocytes, the respiratory capacity of each mitochondrion falls and the mitochondrial membrane potential drops, a shift typically accompanied by a surge in reactive oxygen species. That oxidative burst damages nuclear and mitochondrial DNA, destabilizes the meiotic spindle and can ultimately produce aneuploid eggs. Previous metabolic surveys of aging ovaries found depletion of NAD+, purines and pyrimidines alongside an accumulation of glycolysis substrates and glutamine, painting a consistent picture of failing mitochondrial metabolism.

Researchers have tried a succession of interventions to prop up the aging egg: the CD38 inhibitor 78c to raise NAD+ levels, rapamycin to restore protein homeostasis, melatonin to quench oxidative stress, spermidine to enhance mitophagy, and mitochondria-targeted antioxidants such as MitoQ and SkQ1, along with NAD+ boosters like nicotinamide mononucleotide and the electron carrier CoQ10. Each has shown promise, but low targeting efficiency and poor bioavailability have limited clinical translation. Elamipretide, a membrane-penetrating aromatic cationic tetrapeptide with the sequence Arg-Dmt-Lys-Phe-NH2, was attractive to the team precisely because it is small, water-soluble, resistant to peptidase degradation and stable in solution—properties that make it a plausible candidate for real-world clinical use.

The peptide’s mechanism is unusually specific. At physiological pH it carries three positive charges and accumulates in the inner mitochondrial membrane through electrostatic and hydrophobic interactions, where it binds with high affinity to cardiolipin, a signature phospholipid of that membrane. Cardiolipin is central to respiratory chain supercomplex assembly, membrane stability and energy transduction, but it is also vulnerable to oxidation under oxidative stress, which disrupts the membrane, saps electron transport efficiency and diminishes ATP output. By binding cardiolipin, elamipretide inhibits the peroxidase activity of cytochrome c, prevents cardiolipin oxidation and preserves the cristae architecture of the inner membrane, stabilizing energy production and cutting back the generation of reactive oxygen species.

To test whether these properties could rescue the aging ovary, the team treated 10-month-old mice—well past the mouse reproductive prime—with daily intraperitoneal injections at 3, 5 or 10 milligrams per kilogram. The 5 mg/kg dose proved optimal, and a seven-day course was the most effective duration, improving reproductive performance in 8-, 10- and 12-month-old animals with the strongest gains in the 10-month group. Ovarian sections showed a marked increase in high-quality dominant follicles, fertilization rates of aged oocytes rose substantially, and early embryonic development recovered: aged oocytes normally stall between the two-cell and four-cell stages after fertilization, but elamipretide effectively reversed that block. The benefit, however, was transient, lasting only about a month, which the authors note defines both the promise and the limits of the regimen.

Molecular profiling revealed an unexpected metabolic handle on the effect. Metabolomics of treated ovaries showed that elamipretide shifted the aged metabolome back toward a young-like profile, and pathway analysis singled out vitamin B6 metabolism as the most dramatically altered route. When the team supplemented aged oocytes directly, vitamin B6—but not the other strongly altered metabolites, L-acetylcarnitine and aminovaleric acid—fully reversed the age-associated decline in first polar body extrusion, the hallmark of successful nuclear maturation. Transcriptomics then pointed to the vascular endothelial growth factor pathway, whose age-elevated expression was normalized by the peptide. Blocking VEGF-A transcription with the inhibitor PTC299 reproduced many of elamipretide’s benefits in aged oocytes, restoring mitochondrial positioning, damping the ROS surge, repairing spindle defects and correcting chromosome misalignment, while PI3K/AKT inhibitors failed to do so—placing VEGF-A, acting downstream of vitamin B6 metabolism, at the heart of the mechanism.

Under the microscope, the cellular rescue was striking. Aged oocytes typically display aberrant spindles and scattered chromosomes, loss of γ-tubulin foci at the spindle poles, and reduced acetylation of tubulin, a marker of microtubule instability. Elamipretide restored all of these, along with the cytoplasmic actin network that aging disrupts, and it recovered the expression of ROCK1/2, proteins involved in actin assembly. Mitochondria, which drift away from the spindle and clump in the cytoplasm of aged eggs, returned to their proper peri-spindle domain; ATP levels normalized, mitochondrial DNA copy number partially rebounded, membrane potential rose and ROS fell. The peptide also repaired organelle systems beyond the mitochondria: Golgi markers Rab10 and GM130, endoplasmic reticulum positioning and the ER stress marker GRP78, lysosomal distribution and LC3-mediated autophagic flux all returned toward youthful patterns, indicating a coordinated restoration of cytoplasmic maturation.

Crucially, the findings were not confined to mice. In a randomized in vitro experiment approved under the Declaration of Helsinki, germinal vesicle oocytes from 39 patients undergoing intracytoplasmic sperm injection were cultured with or without 500 micromolar elamipretide. Among women aged 35 and older—three patients contributing 54 oocytes—the peptide improved maturation at both 24 and 48 hours and raised fertilization and cleavage rates after ICSI, with similar gains in younger women’s oocytes. In pigs, whose oocytes are a standard translational model, elamipretide at 1 millimolar rescued maturation in eggs subjected to hydrogen-peroxide-induced oxidative stress, restoring mitochondrial distribution, LC3 expression and apoptosis levels. The authors conclude that short-term elamipretide treatment reverses age-related oocyte decline through a vitamin B6–VEGF axis that jointly rescues nuclear and cytoplasmic maturation. Because the peptide has already advanced into human trials for cardiac and neuromuscular indications, the path from mouse ovary to fertility clinic may be shorter than for most laboratory discoveries—though the transient one-month window of benefit and the small human sample mean considerable work remains before any clinical application.

Subject of Research: Mitochondria-targeted peptide therapy for age-related decline in oocyte quality and female fertility

Article Title: Elamipretide Rejuvenates Oocyte Quality and Restores Female Fertility During Reproductive Aging

Article References: Zhang, H.-L., Wang, Y., Wang, C., Guo, X., Chen, H., Hou, Y.-X., Wu, X., Wu, Z.-J., Pan, W.-L., Ma, R.-J., Lu, P.-S., Shu, J., & Sun, S.-C. (2026). Elamipretide Rejuvenates Oocyte Quality and Restores Female Fertility During Reproductive Aging. Aging Cell, 25(10), Article e70749. https://doi.org/10.1111/acel.70749

Image Credits: AI Generated

DOI: 10.1111/acel.70749

Keywords: elamipretide, oocyte aging, female fertility, mitochondria, cardiolipin, vitamin B6, VEGF, oxidative stress, meiotic spindle, reproductive aging, in vitro maturation, Aging Cell