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

By employing photoactivatable RAC1 (PA-RAC1), the team demonstrated that ARP2/3-mediated actin assembly at neurite tips promotes growth cone activation and neurite extension. Local activation of PA-RAC1 triggered robust growth cone dynamics, driving neurite elongation. Conversely, activation of a dominant-negative RAC1 mutant led to collapse of the growth cone and neurite retraction, underscoring ARP2/3’s role in sustaining neurite advancement.

Intriguingly, PA-RAC1 activation at the soma induced lamellipodia formation reminiscent of nascent actin waves, which were associated with neurite retraction events. This phenomenon mirrored intrinsic cellular behaviors where somatic actin waves correspond with retracting neurites. The study elegantly showed that the emergence of these waves reduced somatic actin patch intensity, indicating a tightly regulated cytoskeletal oscillator mechanism.

The authors further explored the effects of ARP2/3 inhibition using CK-666 perfusion at distinct cellular locales. When applied over the soma and proximal neurites, CK-666 induced neurite retraction, whereas neurites outside this region continued growing unabated. Notably, actin waves were initiated in neurites outside the inhibited zone, propagating retrogradely toward the soma but dissipating upon entering CK-666-exposed areas. This highlights a spatial dependence of ARP2/3 activity for wave initiation and propagation.

Remarkably, simultaneous activation of PA-RAC1 at multiple growth cones revealed competitive dynamics in actin assembly, with enhanced growth cone activity limited to one or two sites. This suggests an intrinsic cellular mechanism balancing cytoskeletal resources and neurite growth across the complex neuronal arborization.

Together, these findings articulate a dual role for ARP2/3: driving actin wave initiation at the soma that triggers neurite retraction, and supporting growth cone expansion at neurite tips to promote extension. This oscillatory interplay mediated by ARP2/3 and RAC1 underscores the cytoskeleton’s fundamental role in establishing neuronal polarity.

This study not only provides a mechanistic framework for how neurons spatially regulate growth and retraction but also suggests that intrinsic cytoskeletal oscillators underpin the dynamic remodeling necessary for proper neuronal circuit formation. These insights pave the way for future investigations into cytoskeletal dysregulation in neurodevelopmental disorders and neuronal regeneration strategies.

The precise spatial control of actin dynamics mediated by ARP2/3 complex activity advances our understanding of neuronal development, highlighting how local cytoskeletal modulators can dictate overall neuronal architecture through oscillatory growth behaviors.

Subject of Research: Neuronal cytoskeletal dynamics and polarity establishment

Article Title: An intrinsic cytoskeletal oscillator establishes neuronal polarity

Article References:
Lin, Tc., Coles, C.H., Alfadil, E. et al. An intrinsic cytoskeletal oscillator establishes neuronal polarity. Nature (2026). https://doi.org/10.1038/s41586-026-10755-6

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41586-026-10755-6