Hernandez-Ochoa, E. | bioRxiv Science

bioRxiv2025

Chronic activation of a key exercise signal transducer, CaMKII, drives skeletal muscle aging and sarcopenia

Sarcopenia, the age-related loss of muscle strength and mass, contributes to adverse health outcomes in older adults. While exercise mitigates sarcopenia by transiently activating calcium (Ca2+)- and reactive oxygen species (ROS)-dependent signaling pathways that enhance muscle performance and adaptation, these same signals become chronically elevated in aged skeletal muscle and promote functional decline. Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a key transducer of both Ca2+ and ROS signals during exercise. Here we show that CaMKII is chronically activated in aged muscles, promoting muscle dysfunction. Muscle-specific expression of a constitutively active CaMKII construct in young mice recapitulates features of aging muscles, including impaired contractility, progressive atrophy, mitochondrial disorganization, formation of tubular aggregates, and an older transcriptional profile characterized by the activation of inflammatory and stress response pathways. Mediation analysis identified altered heme metabolism as a potential mechanism of CaMKII-induced weakness, independent of muscle atrophy. Conversely, partial inhibition of CaMKII in aged muscle improved contractile function and shifted the transcriptome toward a more youthful state without inducing hypertrophy. These findings identify chronic CaMKII activation as a driver of functional and molecular muscle aging and support the concept that CaMKII exemplifies antagonistic pleiotropy, whereby its beneficial roles in promoting muscle performance and adaptation during youth may incur deleterious consequences in aging. We propose that persistent CaMKII activation in aged skeletal muscle reflects unresolved cellular stress and promotes maladaptive remodeling. Enhancing physiological reserve capacity through exercise, in combination with temporally targeted CaMKII inhibition, may help restore adaptive CaMKII signaling dynamics and preserve muscle function in aging.

physiology↗

bioRxiv2018

Optical recording of action potential initiation and propagation in mouse skeletal muscle fibers

Individual skeletal muscle fibers have been used to examine a wide variety of cellular functions and pathologies. Among other parameters, skeletal muscle action potential propagation has been measured to assess the integrity and function of skeletal muscle. In this paper, we utilize Di-8-ANEPPS, a potentiometric dye and mag-fluo-4, a low-affinity intracellular calcium indicator to non-invasively and reliably measure action potential conduction velocity in skeletal muscle. We used an extracellular bipolar electrode to generate an electric field that will initiate an action potential at one end of the fiber or the other. Using enzymatically dissociated flexor digitorum brevis (FDB) fibers, we demonstrate the strength and applicability of this technique. Using high-speed line scans, we estimate the conduction velocity to be approximately 0.4 m/s. In addition to measuring the conduction velocity, we can also measure the passive electrotonic potentials elicited by pulses by either applying tetrodotoxin (TTX) or reducing the bath sodium levels. We applied these methodologies to FDB fibers under elevated extracellular potassium conditions, and found that the conduction velocity is significantly reduced compared to our control concentration. Lastly, we have constructed a circuit model of a skeletal muscle in order to predict passive polarization of the fiber by the field stimuli. Our predictions from the model fiber closely resemble the recordings acquired from in vitro assays. With these techniques, we can examine how many different pathologies and mutations affect skeletal muscle action potential propagation. Our work demonstrates the utility of using Di-8-ANEPPS or mag-fluo-4 to non-invasively measure action potential conduction velocity.

biophysics↗