Aging muscles lose strength, water and the ability to repair themselves, and a new study points to a surprising culprit: a membrane protein better known for shuttling water in and out of cells. Researchers report that aquaporin-4, or AQP4, declines sharply in aged rat muscle and that silencing this protein in muscle cells is enough to derail proliferation, differentiation and protein turnover, producing a cellular phenotype that resembles sarcopenia. The findings, published in the Journal of Cachexia, Sarcopenia and Muscle, suggest that AQP4 may act as a mechanistic driver of age-related muscle decline rather than a passive bystander.
The study set out to answer a long-standing question in muscle biology: when aquaporin levels drop in diseased or wasting muscle, is that loss a cause of degeneration or simply a consequence of it? To find out, a team working with male Wistar rats compared young adults at 25 weeks of age with aged animals at 85 weeks. They measured grip strength, muscle water content, protein turnover signaling and the expression of aquaporins, and then isolated satellite cells, the stem cells responsible for regenerating damaged muscle fibers, from the gastrocnemius muscle of both age groups.
The contrast between young and old animals was striking. Grip strength and creatine kinase, a marker of late myoblast differentiation, were higher in the young adult rats. Muscles from older rats showed lower levels of the differentiation markers Pax7, Myf5 and Myf6, indicating a smaller pool of committed myoblasts and reduced regenerative potential. Protein turnover had also shifted in an unfavorable direction: phosphorylation of mTOR, a key driver of muscle protein synthesis, was reduced, while Atrogin and MurF1, markers of protein degradation, were elevated. Bioimpedance measurements revealed significantly lower intracellular and total body water in the aged animals, and the intracellular water to lean mass ratio, considered a biomarker of muscle quality and sarcopenia risk, was reduced by 35 percent.
Against this backdrop of declining function, AQP4 stood out. The aged muscles contained roughly a third less AQP4 than those of the young adults, while AQP1 levels remained similar between the two groups. The reduction appeared in both slow-twitch soleus and fast-twitch EDL muscles, with the fast-twitch fibers showing the greater loss. Because AQP4 is the predominant aquaporin in skeletal muscle, enriched in the sarcolemma of fast-twitch fibers, its selective decline alongside reduced muscle water content suggested a specific role in the aging process rather than a general breakdown of water handling.
Satellite cells isolated from the old rats told a similar story at the cellular level. These cells carried less AQP4 and, in contrast, more AQP1 than their counterparts from adult animals. They proliferated more slowly and expressed lower levels of differentiation markers. Immunocytochemistry revealed two functionally distinct subpopulations: cells co-expressing PAX7 with high levels of Myf5, which represent a committed myogenic population capable of differentiating and contributing to repair, and cells expressing PAX7 with low levels of Myf5, described as a quiescent reserve pool that can replenish the satellite cell compartment. Both subpopulations were reduced in the aged animals, pointing to a diminished capacity for regeneration and repair. When the researchers pushed the cells to differentiate in medium containing 2 percent horse serum, creatine kinase induction, a sign of successful myogenic differentiation, was significantly weaker in the cells from aged rats.
To test whether AQP4 loss could actually cause these defects, the team turned to the rat L6 myoblast cell line, a well-characterized model that differentiates into myotubes. During that differentiation, AQP4 expression rises, and the increase depends on the Akt, ERK and mTOR signaling pathways, as shown with specific inhibitors of each. Using CRISPR-Cas9, the researchers generated an isogenic line, called L6 ΔAQP4, in which AQP4 expression was obliterated, confirmed by Western blotting, immunocytochemistry and Sanger sequencing. Notably, AQP4 depletion triggered a compensatory rise in AQP1, mirroring the pattern seen in satellite cells from aged muscle.
The silenced cells reproduced nearly every aging-associated defect the researchers had observed in vivo. In non-differentiation media, L6 ΔAQP4 cells proliferated more slowly than wild-type cells, and similar results appeared when AQP4 was silenced in the murine C2C12 line. The knockout cells showed a higher rate of senescence in the beta-galactosidase assay and carried a morphology compatible with so-called zombie cells. Senescence marker analysis revealed a decrease in p16 expression alongside an increase in p21, suggesting that the cell cycle arrest was mediated predominantly through the p53-p21 pathway, a route recently implicated in animal and human models of muscle aging. When placed in differentiation media, the L6 ΔAQP4 cells failed to progress: while wild-type cells showed the expected profiles of PAX7, Myf5, Myf6, MEF2D, Troponin T and Myosin as they matured into myotubes, the silenced cells displayed no changes in these markers over time. In C2C12 cells, AQP4 silencing reduced Myf5 and Myf6 expression by 35 and 30 percent, respectively.
Signaling analysis explained part of the failure. L6 cells promote differentiation by enhancing Akt and ERK signaling, but in the AQP4-deficient cells, Akt and ERK activation was suppressed, as indicated by reduced phosphorylation and lower GLUT4 expression. Blocking the Akt pathway in these cells also decreased mTOR activation and protein synthesis, while protein degradation and the degradation markers Atrogin and MurF1 increased, producing a negative protein balance. The researchers describe this as a bidirectional regulatory circuit: AQP4 expression is controlled by Akt and mTOR signaling, yet AQP4 itself influences the activation of those same pathways, potentially amplifying even small changes in its levels into large effects on proliferation, differentiation and cellular function.
Water handling and membrane integrity completed the picture. Under normal isotonic conditions, intracellular water levels were comparable between wild-type and knockout cells, but when the researchers imposed osmotic stress with mannitol, the L6 ΔAQP4 cells lost more intracellular water and recovered incompletely once the stressor was removed. They also showed consistently lower survival across all stressor concentrations, and the same vulnerabilities appeared in differentiated cells. The stressed knockout cells displayed altered morphology and damaged plasma membrane integrity, detected with a membrane integrity marker, and the damage persisted even after the osmotic challenge ended. Confocal microscopy added a structural clue: in cells expressing AQP4, dystrophin, a protein that anchors the muscle cytoskeleton to the membrane, showed a diffuse distribution including the plasma membrane, cytosol and nucleus, whereas in AQP4-deficient cells dystrophin was confined to the nucleus. This shift matters because the AQP4-dystrophin partnership at the plasma membrane is thought to maintain water homeostasis, membrane integrity and the signaling that supports differentiation.
The most convincing evidence came from rescue experiments. The researchers built a vector encoding a human AQP4-GFP fusion protein and transfected it into the silenced cells. The transfected cells regained PAX7 expression, restored a well-defined plasma membrane, returned dystrophin to its normal subcellular distribution and, when grown in differentiation media, underwent cell fusion and formed myotubes, changes seen only in the green fluorescent cells carrying the fusion protein. Taken together with the in vivo findings, the authors conclude that AQP4 is a mechanistic driver of muscle aging rather than a passive marker of degeneration, and that its consistent decline in aged muscle may precede and potentially initiate key pathological changes associated with sarcopenia. Because AQP4 loss, unlike dystrophin deficiency, is a common feature of aging muscle rather than a rare genetic disorder, the researchers suggest that targeting AQP4 could represent a promising strategy to counteract age-related muscle atrophy and preserve function, though the work so far rests on rat models and cell lines and has not yet been tested in humans.
Subject of Research: The role of the water channel protein aquaporin-4 in muscle cell differentiation, satellite cell function and age-related muscle decline
Article Title: Aquaporin‐4 Orchestrates Muscle Cell Differentiation and Satellite Cell Preservation: A Novel Target in Age‐Related Muscle Decline
Article References: Girón, M. D., Manzano, M., Pereira, S. L., Reinoso, A., Burgio, C., Montero‐Martin, N., Jiménez‐Jiménez, G., Argilés, J. M., Rueda, R., López‐Pedrosa, J. M., & Salto, R. (2026). Aquaporin‐4 Orchestrates Muscle Cell Differentiation and Satellite Cell Preservation: A Novel Target in Age‐Related Muscle Decline. Journal of Cachexia, Sarcopenia and Muscle, 17(6), Article e70412. https://doi.org/10.1002/jcsm.70412
Image Credits: AI Generated
DOI: 10.1002/jcsm.70412
Keywords: aquaporin-4, sarcopenia, satellite cells, muscle differentiation, muscle aging, AQP4, L6 myoblasts, CRISPR-Cas9, protein turnover, membrane integrity, dystrophin, osmotic stress
News Source: Beatrice Stafford. (November 5, 2026). Water Channel Protein AQP4 Emerges as a Driver of Age-Related Muscle Loss. Scienmag.
