The research, led by Dora Gjirlić, Anja Weber, Guido Posern, and Anurag Kumar Singh of the Institute for Physiological Chemistry, focuses on the actin-depolymerizing factor/cofilin family, a group of proteins that sever and depolymerize actin filaments, thereby controlling the constant turnover of the cytoskeleton. Vertebrates possess two major isoforms: Cofilin-1, which is broadly expressed in many cell types, and Cofilin-2, which is characteristic of muscle tissue. Although both proteins perform the same core biochemical task — cutting filamentous actin into globular subunits — their roles during myogenesis, the process by which muscle cells form, have remained incompletely understood. The new work provides the most detailed picture yet of how these isoforms are swapped during muscle differentiation and why that swap matters.

Using the immortalized mouse myoblast cell line C2C12, a workhorse of muscle biology, the team tracked the expression of both cofilin isoforms as cells transitioned from growth medium into differentiation medium. What emerged was a pronounced isoform switch. As differentiation proceeded, Cofilin-1 and the related Actin-Depolymerizing Factor (ADF) were progressively downregulated, while Cofilin-2 expression rose. Analyses of messenger RNA and protein stability indicated that this switching is controlled primarily at the level of gene expression rather than through altered degradation of the proteins themselves, suggesting that transcriptional reprogramming sits at the heart of the isoform transition.

To probe function rather than mere correlation, the researchers deployed an impressive arsenal of techniques. They used CRISPR/Cas9 genome editing to knock out the Cofilin-1 gene, shRNA-mediated knockdown to deplete it partially, immunofluorescence microscopy to visualize cellular morphology, quantitative gene expression analyses to monitor myogenic marker genes, MRTF reporter assays to measure the activity of the myocardin-related transcription factor A, and pharmacological inhibition of LIM kinase to block a key regulatory phosphorylation pathway. Each approach interrogated a different facet of the same question: what happens to differentiating muscle cells when the actin-severing machinery is perturbed?

The answer, for complete loss of Cofilin-1, was dramatic. Cells lacking the protein developed marked morphological abnormalities, failed to exit the cell cycle properly, showed elevated MRTF activity, and formed far fewer myotubes — the multinucleated structures that represent successful differentiation. This constellation of defects makes mechanistic sense. MRTF-A is a transcriptional coactivator whose nuclear activity is restrained by binding to globular actin; when actin polymerization dynamics are disturbed and the monomeric actin pool shifts, MRTF signaling can be unleashed inappropriately. Cofilin-1, by regulating the filament-monomer balance, normally helps keep this signaling pathway calibrated during the transition from proliferation to differentiation. Remove it entirely, and the cell’s transcriptional program falls out of register with its cytoskeletal state.

The most striking discovery, however, came from the knockdown experiments. When the researchers reduced Cofilin-1 only partially, something unexpected happened: myoblast fusion was enhanced. Cells with modestly lowered Cofilin-1 fused with their neighbors more readily than controls. But when depletion was pushed harder, the opposite occurred — differentiation collapsed and myotube formation was impaired. This bidirectional response demonstrates a dosage-sensitive requirement for Cofilin-1, a Goldilocks principle in which the protein must be present at precisely the right level. Too much Cofilin-1 appears to restrain fusion, while too little disrupts the cytoskeletal remodeling that fusion itself requires. The finding adds muscle formation to a growing list of biological processes in which gene dosage, rather than simple presence or absence, determines the outcome.

Cofilin-2 deficiency, by contrast, produced comparatively mild effects during early differentiation. This asymmetry is intriguing because Cofilin-2 is the isoform ultimately enriched in mature muscle. The results suggest that the two isoforms, despite their biochemical similarity, are not interchangeable in function or timing: Cofilin-1 is the critical regulator of the early, decision-making phase of myogenesis, whereas Cofilin-2 likely assumes importance later, in the context of mature contractile fibers. The progressive switch from one isoform to the other may therefore represent a carefully staged handover of cytoskeletal control, timed to the changing needs of the cell as it commits to the muscle lineage.

The study also illuminates how Cofilin-1’s activity, not just its abundance, is regulated during myogenesis. The researchers found that Cofilin-1 undergoes transient phosphorylation by LIM kinase during early differentiation. Phosphorylation by LIM kinase — which itself sits downstream of the Rho-associated kinase ROCK — inhibits cofilin’s actin-severing capacity, stabilizing actin filaments at specific moments. When the team blocked this pathway with the LIM kinase inhibitor LIMKi3 (BMS-5), myogenic progression was disrupted. The picture that emerges is one of dynamic, moment-to-moment control: the cell not only dials down Cofilin-1 expression over the course of differentiation but also rapidly toggles the remaining protein on and off through phosphorylation, fine-tuning actin turnover as the cell cycle exits and fusion machinery engages.

These findings connect several threads of muscle biology that had previously run in parallel. The actin cytoskeleton must be dismantled and rebuilt for a myoblast to elongate and fuse; the MRTF and serum response factor (SRF) transcriptional pathway reads actin dynamics and translates them into gene expression; and the myogenic regulatory factors MyoD and myogenin drive the differentiation program, with myomaker and myomixer/myomerger executing the fusion step itself. The new work positions Cofilin-1 as a molecular link between the physical and the transcriptional layers of this process — a dosage-sensitive node where cytoskeletal remodeling is converted into signals that govern proliferation, differentiation, and ultimately cell fusion. Caveolin-3, myosin heavy chain, and other differentiation markers tracked in the study provided the readouts confirming that these signaling changes translate into real changes in myogenic identity.

Beyond its immediate significance for understanding how skeletal muscle forms, the research carries potential implications for regenerative medicine and muscle disease. Muscle regeneration after injury recapitulates many steps of embryonic myogenesis, including the fusion of satellite-cell-derived myoblasts onto damaged fibers. If Cofilin-1 dosage and LIM kinase signaling set the efficiency of that fusion, they become candidate levers for improving muscle repair — or potential culprits in conditions where repair fails. The Halle team, whose work was supported by internal faculty core funding from Martin Luther University Halle-Wittenberg and enabled by open-access funding through Projekt DEAL, emphasizes that coordinated regulation of Cofilin-1 expression and activity, together with the timely transition to Cofilin-2, is required for efficient myoblast fusion and muscle formation. In revealing that a humble actin-severing protein must be tuned like an instrument rather than simply switched on or off, the study adds a subtle but essential rule to the growing rulebook of how cells build tissue.

Subject of Research: Dosage-sensitive regulation of Cofilin-1 and LIM kinase signaling in myoblast differentiation and fusion

Article Title: Dosage-sensitive regulation of Cofilin-1 and LIMK signaling controls myoblast differentiation and fusion

Article References: Gjirlić, D., Weber, A., Posern, G., & Singh, A. K. (2026). Dosage-sensitive regulation of Cofilin-1 and LIMK signaling controls myoblast differentiation and fusion. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06437-1

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06437-1

Keywords: myogenesis, Cofilin-1, Cofilin-2, actin cytoskeleton, LIM kinase, MRTF signaling, myoblast differentiation, cell fusion, cytoskeletal regulation, skeletal muscle, Dosage-sensitive, regulation

Eccentric exercise occurs when a muscle lengthens under load, as it does when lowering a weight or running downhill, generating high mechanical tension at a comparatively low metabolic cost. Concentric exercise, by contrast, shortens the muscle under load, as in uphill running or lifting. Physiotherapists and sports physicians have long observed that eccentric training builds strength efficiently and reduces the incidence of hamstring injuries by as much as 56.8 to 70 percent in some reported cohorts, yet the underlying mechanism linking this unique stress profile to enhanced tissue repair has remained elusive. The new research, conducted in mice using a barium chloride-induced injury model that reproducibly triggers myofibre necrosis while preserving the satellite cell niche, set out to close that gap.

The team subjected injured mice to five days of treadmill running, with one group performing downhill running at a minus 15-degree incline to simulate eccentric exercise and another performing uphill running at plus 15 degrees to simulate concentric exercise. Seven days after injury, the downhill-running animals displayed markedly better organised regenerating tissue in the tibialis anterior muscle, less collagen Iα deposition, and elevated expression of the myogenic regulators Myod, Myog and embryonic myosin heavy chain. Immunofluorescence revealed more newly formed eMyhc-positive fibres, and the extensor digitorum longus muscles of the eccentric group generated greater maximal tetanic force. In short, eccentric contraction was the more effective healer.

To explain the difference, the researchers turned to extracellular vesicles, nanoscale particles typically ranging from roughly 50 to 150 nanometres that cells release to ferry proteins, lipids, RNAs and metabolites between cells. Skeletal muscle is known to shed these vesicles during exercise, and the team hypothesised that the two contraction modes might load them differently. Western blotting showed that Rab27a, a key regulator of vesicle secretion, was more strongly expressed after eccentric exercise, and ex vivo experiments using the Aurora Scientific 1200A system confirmed that isolated soleus, extensor digitorum longus and tibialis anterior muscles secreted more vesicles during eccentric than concentric contractions. Transmission electron microscopy and nanoparticle tracking analysis characterised the vesicles, while canonical markers such as CD63, ALIX, TSG101 and CD9 were elevated in the eccentric group.

The causal evidence was compelling. When the researchers blocked vesicle production with GW4869, an inhibitor of vesicle biogenesis, the benefits of eccentric exercise largely vanished: regenerating muscles showed disorganised morphology, increased fibrosis, reduced myogenic marker expression and fewer newborn fibres. Conversely, when vesicles harvested from the muscles of exercised mice were injected directly into injured muscles, the vesicles entered satellite cells, as demonstrated by PKH26 labelling, and accelerated repair. Critically, vesicles from eccentrically exercised donors outperformed those from concentric donors and sedentary controls, producing better tissue architecture, less collagen deposition, higher Myog protein and more eMyhc-positive regenerating fibres.

What made the eccentric vesicles so potent? The answer appears to lie in their cargo. Non-targeted metabolomic profiling using liquid chromatography-mass spectrometry revealed that lipid metabolites, particularly phosphatidylcholines and sphingomyelins, were the most enriched class of upregulated molecules in vesicles from eccentrically exercised muscle. Quantitative PCR showed corresponding upregulation of lipid metabolism genes, and KEGG pathway analysis confirmed that lipid metabolism was significantly more active in the eccentric vesicles. Because phosphatidylcholines supply substrates for membrane remodelling, a process essential for myoblast fusion and fibre growth, while sphingomyelins populate lipid rafts that organise pro-differentiation signalling, the authors propose that these lipids act as both building blocks and signals for regeneration.

Functional tests strengthened the case. When the researchers blocked lipid metabolism in exercising muscle using BMS-309403, an inhibitor of the fatty acid binding protein AP2, vesicles extracted from those muscles lost nearly all of their regenerative advantage, both in injured mice and in cultures of C2C12 myoblasts. Supplementation experiments with PI (18:1/18:1), a phosphatidylinositol species highly enriched in eccentric vesicles, activated the energy-sensing AMPK pathway, boosted MyoD expression and rescued the pro-myogenic capacity of otherwise weaker vesicle preparations, while the AMPK inhibitor Compound C attenuated these effects. Vesicles from eccentric exercise also raised ATP levels and mitochondrial membrane potential in recipient cells.

Transcriptomics added a further layer of mechanism. RNA sequencing of myoblasts treated with the different vesicle populations showed activation of fatty acid metabolism pathways and a stepwise, exercise-dependent increase in nine genes, including Prkag3, a known supporter of muscle repair, and the mitochondrial energy-sensing genes Cox7a1, Cox6a2 and Cox8b. Together with the metabolomic and pharmacological data, the picture that emerges is of vesicles acting as lipid-rich metabolic packages that reprogramme satellite cells and myoblasts toward an energetically primed, differentiation-ready state, thereby accelerating the reconstruction of damaged tissue.

The therapeutic implications are considerable, particularly for patients who cannot exercise after injury. Because improper eccentric training can itself cause secondary muscle damage, the prospect of isolating or mimicking the vesicles released during downhill running, and administering them exogenously, offers a way to capture the regenerative benefits of exercise without the movement that injured limbs may not tolerate. The authors caution that significant hurdles remain before clinical translation, including scalable production with consistent quality, batch-to-batch variability, maintaining vesicle stability during storage, achieving targeted delivery to injured muscle, and excluding immunogenicity or long-term adverse effects. The study also used only male mice, leaving open whether the findings extend to females, and other exercise-induced factors beyond vesicles may contribute to regeneration. Nevertheless, by demonstrating that the healing power of eccentric contraction travels, at least in part, inside lipid-laden extracellular vesicles, the work transforms these nanoscale particles from a curiosity of intercellular communication into a promising natural therapeutic candidate, one that could eventually shorten recovery times for athletes and patients alike and reshape rehabilitation medicine around the molecular conversation that exercising muscles hold with their own stem cells.

Subject of Research: Muscle-derived extracellular vesicles released during eccentric exercise that promote skeletal muscle regeneration after injury

Article Title: Skeletal Muscle–Derived Extracellular Vesicles During Eccentric and Concentric Exercise Promote Muscle Regeneration After Injury

Article References: Zhou, Y., Shao, X., Zhang, P., Lin, J., Chen, X., An, X., Jiang, Z., Wang, H., Fang, D., Xian, Y., Liu, B., Shen, T., Chen, Y., Li, K., Liu, H., Li, Y., Jiang, Q., & Guo, B. (2026). Skeletal Muscle–Derived Extracellular Vesicles During Eccentric and Concentric Exercise Promote Muscle Regeneration After Injury. Journal of Cachexia, Sarcopenia and Muscle, 17(5), Article e70374. https://doi.org/10.1002/jcsm.70374

Image Credits: AI Generated

DOI: 10.1002/jcsm.70374

Keywords: extracellular vesicles, eccentric exercise, concentric exercise, skeletal muscle regeneration, satellite cells, lipid metabolites, myokines, muscle injury, metabolomics, AMPK signalling, myoblast differentiation, rehabilitation