Aging Muscle Weakens When a Key Exercise Enzyme Stays Switched On

Skeletal muscle has a remarkable capacity to adapt to the demands placed upon it, and at the center of that adaptability sits a signaling enzyme called calcium/calmodulin-dependent protein kinase II, or CaMKII. In young, healthy muscle, CaMKII acts as a molecular translator: it converts the calcium pulses and redox signals generated by each contraction into the metabolic and transcriptional programs that make muscle stronger and more resilient. Crucially, this activation is self-limiting. It rises with the intensity of exercise and then resolves once the workout ends. A new study published in Aging Cell now suggests that when this switch fails to turn off, the consequences for muscle are profound, offering one of the clearest mechanistic accounts yet of why aged muscles grow weak even before they shrink.

The research team, working with mouse models spanning young adults to very old animals, first documented that aging is accompanied by an increase in CaMKII abundance. In the tibialis anterior muscles of 33-month-old mice, total CaMKIIβ and CaMKIIδ/γ protein levels were significantly elevated compared with 3.7-month-old controls, and so were the T287 autophosphorylation signals that serve as a biochemical marker of kinase activation. These aged animals also showed the expected reductions in normalized muscle mass across the tibialis anterior, gastrocnemius, and quadriceps. The authors were careful to note, however, that steady-state autophosphorylation may not fully capture chronic CaMKII signaling, because the kinase is also regulated by calcium/calmodulin binding, oxidation, nitrosylation, and O-GlcNAcylation. That ambiguity set the stage for a more direct experimental question: is sustained CaMKII activation sufficient to drive muscle dysfunction on its own?

To answer it, the researchers used adeno-associated virus serotype 9 to deliver a constitutively active form of CaMKIIγ, carrying a phosphomimetic T287D mutation, into the tibialis anterior muscles of young mice under a muscle-specific promoter. Each animal received the active kinase in one leg and a green fluorescent protein control in the other, allowing paired comparisons within the same mouse. The construct expressed at levels comparable to endogenous CaMKII, and although the transgene triggered a feedback reduction in endogenous CaMKII abundance and autophosphorylation, kinase assays and a fluorescent CaMKII activity reporter called CaMKAR confirmed that net enzymatic activity was clearly elevated in the injected muscles.

The functional consequences were striking. Six weeks after gene transfer, muscles expressing the constitutively active kinase produced significantly less force than their paired controls across the entire 1–150 Hz stimulation range, and the deficit persisted even after normalizing force to muscle mass. Mediation analysis showed that the early loss of contractile strength was almost entirely a direct effect of CaMKII signaling rather than a byproduct of the modest 7 percent reduction in muscle mass that had developed by seven weeks. In other words, the muscle was genuinely weaker, not merely smaller. Only with much longer activation did atrophy become a dominant factor: nine months after injection, muscle mass had fallen by 24.2 percent, and mediation analysis then attributed the bulk of the force decline to the loss of tissue itself.

Under the microscope, the CaMKII-activated muscles revealed a distinctive pattern of structural remodeling. There was no inflammation or necrosis, but affected fibers showed abnormal basophilic staining, intensified subsarcolemmal and intrafiber signals on modified Gomori’s trichrome staining, and aggregated mitochondrial enzymatic activity on cytochrome c oxidase and succinate dehydrogenase stains. Electron microscopy made the picture explicit: mitochondria clumped beneath the sarcolemma and between myofibrils, and a quantitative Z-line mitochondrial occupancy score dropped significantly at both seven weeks and five months. Importantly, this was a problem of geography rather than quantity. Respiratory chain subunit abundance and the mitochondrial-to-nuclear DNA ratio were unchanged, indicating that sustained CaMKII signaling scatters mitochondria away from their normal posts near the calcium release units and Z-lines without depleting them.

That mislocalization matters because intermyofibrillar mitochondria in adult muscle are normally positioned to deliver ATP locally for cross-bridge cycling and calcium reuptake, and to take up calcium during activity in a process known as excitation–metabolism coupling. Aging is known to disrupt precisely this arrangement, reducing the apposition between calcium release units and mitochondria in both aged mouse and human muscle. The authors propose that CaMKII-driven disruption of cytoskeletal anchoring, possibly through phosphorylation of desmin and related intermediate filament proteins, or altered mitochondrial dynamics via the fission regulator Drp1, could explain how a calcium-activated kinase ends up scrambling the muscle’s power grid.

The transcriptional evidence was equally compelling. RNA sequencing of young muscles expressing active CaMKII for seven weeks revealed 2,771 differentially expressed genes, and when these changes were correlated with the gene-expression signature of normal aging, the concordance was moderate but highly significant. Of the genes altered with age, 73.5 percent were shifted in the same direction by sustained CaMKII activation. Gene set enrichment analysis against the Reactome database showed that CaMKII activation in young muscle reproduced the aging-associated activation of immune, interferon-gamma, antigen-presentation, DNA-repair, and extracellular-matrix pathways, while suppressing metabolic and structural-remodeling programs. Sustained CaMKII signaling, in effect, pushed the young transcriptome toward an aged profile, though not a complete one, implying that additional pathways cooperate with CaMKII in the full aging program.

Two intervention experiments then tested causality from the opposite direction. When aged mice received AAV9-delivered CN19o, a well-characterized CaMKII inhibitory peptide, their tibialis anterior muscles generated significantly greater absolute force and significantly higher force normalized to muscle mass after five weeks, without any increase in muscle size or fiber diameter. A second cohort using phosphate-buffered saline controls confirmed the effect. Meanwhile, co-expression of a degradation-resistant IκBα super-repressor, which blocks canonical NF-κB signaling, did not prevent the early CaMKII-induced force deficit but partially preserved long-term force at ten months, independently of muscle mass. This places inflammatory NF-κB signaling downstream of CaMKII as one contributor to progressive decline, while pointing to other mechanisms for the acute weakness.

Among those other mechanisms, mediation analysis of matched transcriptomic and force data singled out heme metabolism as the principal transcriptomic correlate of CaMKII-induced weakness. Active CaMKII upregulated genes involved in heme and iron export, including Flvcr1, Abcg2, and the iron exporter Slc40a1, while downregulating genes supporting heme synthesis and iron uptake, such as Fech, Abcb10, Tfrc, and Fth1. Regulatory network inference identified the heme-responsive transcription factor Spic as the top activated factor in CaMKII-expressing young muscle and the top suppressed factor in CN19o-treated aged muscle, a reciprocal pattern consistent with dysregulated heme and iron handling. The authors suggest a model of functional iron deficiency coexisting with labile-iron accumulation, a state that could compromise mitochondrial metabolism and elevate oxidative stress, and one that echoes iron dyshomeostasis previously reported in aging human muscle.

The broader conceptual payoff is an example of what evolutionary biologists call antagonistic pleiotropy: a pathway that is adaptive in youth becomes harmful in old age. Transient CaMKII activation scales with physiological demand and resolves during recovery, supporting performance and exercise adaptation. With aging, declining functional capacity may make everyday activity a relatively greater stress, promoting more persistent CaMKII activation and shifting its effects toward dysfunction. Exercise, on this view, protects muscle not only by inducing beneficial adaptation but also by preserving the functional reserve that keeps chronic homeostatic stress in check. For people who cannot exercise enough, the findings raise the possibility of pharmacological CaMKII inhibition as a complementary strategy, a prospect sharpened by the recent identification of the FDA-approved JAK inhibitor ruxolitinib as a potent CaMKII inhibitor at therapeutic concentrations. The authors caution that their constitutively active kinase model does not perfectly reproduce the magnitude, duration, or isoform composition of endogenous CaMKII dysregulation, and that the relevance to human muscle aging remains to be established. Still, by connecting a self-amplifying calcium signal to mitochondrial mislocalization, an aged transcriptome, heme-iron disruption, and measurable weakness, the study gives sarcopenia research a concrete, druggable molecular thread to pull.

Subject of Research: The role of sustained CaMKII signaling in age-related skeletal muscle contractile dysfunction and sarcopenia

Article Title: Sustained Activation of CaMKII Promotes Skeletal Muscle Contractile Dysfunction in Aging

Article References: Bene, M. R., Chung, T., Luczak, E. D., Lopez‐Cecetaite, G., Fountain, W. A., Rosales‐Soto, G., Hernández‐Ochoa, E., Antonescu, C., Florea, L., Jeong, S. J., Elassal, E., Le, A., Xue, Q.-L., Hoke, A., Abadir, P. M., & Wang, Q. (2026). Sustained Activation of CaMKII Promotes Skeletal Muscle Contractile Dysfunction in Aging. Aging Cell, 25(9), Article e70704. https://doi.org/10.1111/acel.70704

Image Credits: AI Generated

DOI: 10.1111/acel.70704

Keywords: CaMKII, sarcopenia, skeletal muscle, aging, mitochondria, calcium signaling, NF-kB, heme metabolism, gene expression, AAV9, CN19o, contractile dysfunction