Topological defects are familiar characters in the physics of liquid crystals and active matter. They are points or lines where the local orientation field of a material cannot be defined smoothly, forcing the system into characteristic patterns such as vortices, spirals and asters. Over the past decade, dense sheets of cells have been shown to behave like active nematic liquid crystals, with elongated cells playing the role of rod-like molecules that constantly burn chemical energy to move and exert forces. Earlier work had revealed that defects in such monolayers can act as mechanical organizing centres: they concentrate stresses, attract cell death and extrusion in epithelia, and even mark the sites where the freshwater polyp Hydra sprouts new tentacles. What remained unclear was how a defect could coordinate cell motion on large scales and drive the assembly of genuinely three-dimensional tissue.

The French team solved this problem by taking control of the defect itself. Instead of waiting for vortices to appear spontaneously in a disordered monolayer, they fabricated polydimethylsiloxane substrates engraved with concentric micro-rails. Myoblasts migrating on these tracks follow contact guidance, the tendency of elongated cells to align with underlying grooves, and so the pattern imposes a well-defined vortex of cell orientation with an integer topological charge of +1 at its centre. This experimental trick transformed a fleeting, hard-to-predict feature of active matter into a stationary, reproducible target that could be imaged for days with phase-contrast and confocal microscopy, and quantified with particle image velocimetry and custom orientation analysis.

The first surprise came before any three-dimensional structure appeared. For roughly the first twelve hours after the monolayer reached confluence, the cells did not simply circle the centre along the rails. Their velocity field underwent a dramatic reorganization: the azimuthal, ring-following motion gave way to a radial flow converging on the defect core, with cells migrating perpendicular to their long axis, along their short axis, toward the centre of the pattern. The researchers tracked this transition with an order parameter that distinguishes azimuthal from radial flow, and found that the convergent regime emerged across millimetre scales, a huge distance compared with the size of a single cell. A swirl in orientation, in other words, was broadcasting a coordinated traffic instruction to thousands of cells simultaneously.

That convergent traffic had immediate architectural consequences. Cells arriving at the core began to extrude out of the plane of the monolayer, climbing over their neighbours and forming a second layer roughly six micrometres thick. Crucially, this new layer was not a random heap. Confocal sections revealed that the second layer inherited the vortex orientation of the layer beneath it, and when a third layer appeared a day later, it too was vortex-aligned. The team found that the cells deposit their own extracellular matrix, including collagen IV, in patterns that mirror the orientation of the cells that laid it down. This secreted scaffold then guides the orientation of subsequent arrivals through contact guidance, creating a feedback loop in which each layer templates the one above it. The mound, in effect, was printing itself layer by layer, with the topological defect supplying the pattern.

As the structure matured over tens of hours, its internal architecture became even more sophisticated. Confocal reconstructions of fixed mounds showed that the nematic order varied systematically with altitude. Near the base, close to the core, cells adopted an aster-like arrangement pointing radially outward, while farther from the centre they retained the vortex configuration. At the apex of the mound, the orientation twisted into a spiral. Remarkably, despite these changes in appearance, the in-plane topological charge remained invariant and equal to +1 along the entire vertical axis of the mound, from the first layer to the summit. When the researchers compared mounds from different experiments and different ages, the profiles of their orientation order parameter collapsed onto a single master curve once rescaled by the maximum height, indicating that the mound possesses a self-similar, reproducible internal blueprint.

The growth mechanism itself proved equally elegant. By tracking fluorescently labelled cells shuttling between layers, the team documented a process they describe as interlayer permeation: cells do not simply stack passively at the core but percolate through the existing structure, moving between adjacent layers like fluid seeping through a porous medium. The flux between layers peaked at a specific radius, the boundary between the inner aster-like region and the outer vortex region, where the orientation order parameter crosses zero. Even more striking was a reversal of flow direction as the mound aged. Around thirty hours after confluence, cells near the axis of the mound, which had initially migrated inward, began to migrate outward, and this switch from convergent to divergent flow was mirrored by a local change in orientation from vortex to aster. The growing structure was not static; it continuously reorganized its own traffic patterns as it expanded.

To make sense of these observations, Carles Blanch-Mercader and Jacques Prost developed a theoretical model treating the mound as a stack of active nematic fluid layers. The framework incorporates the active anisotropic stresses generated by the cells, the stresses in the external single layer, and an interfacial stress between layers, and it reproduces the measured radial velocity profiles and permeation fluxes with remarkable fidelity. The radial velocity in the monolayer follows a one-over-radius power law characteristic of a converging flow, while the flux from the mound to the external medium decays exponentially. In the late stages, the bell-shaped profile of the mound can be modelled entirely by dynamics driven by the converging flows, without invoking cell proliferation as the dominant driver. Drug perturbations confirmed this hierarchy: when proliferation was blocked with mitomycin C, the mounds retained their wild-type organization, but when contractility was inhibited with blebbistatin or the ROCK inhibitor Y-27632, the ordered alignment was almost completely destroyed. The architecture, it turns out, is a mechanical phenomenon built on cellular forces, not a by-product of cell division.

The implications reach well beyond the culture dish. Cellular whorls and swirls have long been noted by pathologists as diagnostic features in brain tumours, ovarian cancer surfaces and fine-needle aspirates of papillary thyroid carcinoma, and defects in the mesothelium have been shown to suppress the clearance of ovarian cancer cells. The new results suggest a physical mechanism by which such swirling patterns could actively organize cells into multilayered, tumour-like masses, with convergent defect-driven flows feeding cells into the core and permeation spreading them through the structure. The same physics may operate in normal morphogenesis, where embryonic tissues fold, bud and layer themselves with exquisite precision, and recent work on lumen nucleation in embryos has already implicated topological defect transitions controlled by boundary geometry. By demonstrating that an integer topological defect alone, imposed by nothing more than a patterned surface, can orchestrate millimetre-scale flows and the assembly of a highly ordered three-dimensional tissue from a featureless monolayer, the study provides both a conceptual framework and an experimental platform. It suggests that the grammar of tissue construction may be written partly in the language of topology, and that learning to read and perhaps rewrite that grammar could one day inform strategies in regenerative medicine, tissue engineering and cancer therapy.

Subject of Research: Topological-defect-driven morphogenesis of three-dimensional cell mounds in active nematic monolayers

Article Title: Morphogenesis of a stratified cell mound at a vortex defect

Article References: Chen, H.-Y., Blanch-Mercader, C., Giuglaris, C., Prost, J., & Silberzan, P. (2026). Morphogenesis of a stratified cell mound at a vortex defect. Nature Physics. https://doi.org/10.1038/s41567-026-03427-9

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03427-9

Keywords: topological defects, active nematics, cell monolayers, morphogenesis, collective cell migration, myoblasts, interlayer permeation, biological physics, tissue engineering, tumorigenesis, contact guidance, Institut Curie

Arginine kinase belongs to a family of enzymes known as phosphagen kinases, which act as cellular energy buffers. In vertebrates, the analogous enzyme is creatine kinase, familiar to anyone who has taken a fitness supplement. These enzymes work by catalyzing the transfer of a phosphate group from a storage molecule, phosphoarginine in flies or phosphocreatine in humans, onto adenosine diphosphate, instantly regenerating adenosine triphosphate, the universal currency of cellular energy. This buffering system is particularly critical in tissues with high and fluctuating energy demands, such as muscle, where bursts of contraction can deplete ATP faster than mitochondria can replenish it. In Drosophila, Argk1 has long been appreciated as a guardian of ATP homeostasis during intense activity, but its function outside of acute stress conditions has remained poorly understood.

The new study set out to change that by asking a deceptively simple question: what happens to a developing flight muscle if you remove its only phosphagen kinase? The researchers found that Argk1 is the sole phosphagen kinase expressed dynamically throughout flight muscle development, making it uniquely positioned to shape the energetic landscape of this tissue. Using genetic tools that allow targeted depletion of specific genes, the team knocked down Argk1 expression specifically in muscle tissue and observed dramatic consequences. The animals died, and their muscles showed depleted ATP levels alongside a reduced NAD-plus to NADH ratio, a biochemical signature of compromised energy homeostasis that points to deep metabolic distress within the tissue.

One of the most intriguing findings concerns the subcellular location of the enzyme. The researchers discovered that at least one isoform of Argk1 localizes to mitochondria within developing myofibers. This is significant because mitochondria are the power plants of the cell, generating the bulk of ATP through oxidative phosphorylation. Placing an energy-buffering enzyme at the site of ATP production suggests a sophisticated strategy: rather than waiting for energy demands to outpace supply, the cell positions its buffer directly where energy is made, presumably to sustain local ATP concentrations during the energetically expensive process of building muscle. This mitochondrial association hints at a level of metabolic coordination that goes well beyond the textbook picture of arginine kinase as a simple cytoplasmic ATP reservoir.

To understand precisely where in development Argk1 exerts its influence, the researchers turned to the wing disc, a larval structure that houses the myoblasts destined to become the adult indirect flight muscle. These muscle precursor cells must proliferate, differentiate, and fuse to form the multinucleated fibers that will eventually power flight. When the team depleted Argk1 in these myoblasts, they observed a reduction in cell size, but notably without any change in cell cycle progression. The cells continued to divide on schedule; they simply grew less. This dissociation between proliferation and growth is a telling clue, suggesting that Argk1’s role is not to drive the cell division machinery itself but to support the biosynthetic and energetic demands of cellular growth.

The researchers then deployed single-cell RNA sequencing, a powerful technique that captures the gene expression profile of individual cells, to probe whether Argk1 depletion derails the genetic program of muscle differentiation. The results were striking in their subtlety. The transcriptomes of both undifferentiated and differentiating Argk1-depleted myoblasts were not significantly affected compared to controls. Based on marker gene expression and the overall composition of the sequenced cell clusters, the early states of myoblast differentiation appeared largely intact. In other words, the blueprint for building muscle remained legible in the absence of Argk1. The cells knew what they were supposed to become; they simply lacked the energetic means to complete the construction.

Where the defects emerged was in the later stages of muscle development, and here the study delivers its most remarkable observation. Argk1-depleted muscles completely lacked spontaneous muscle contractions. This is not a trivial loss of movement. Spontaneous contractions are a critical developmental signal in the formation of the indirect flight muscle, providing the mechanical activity required for proper sarcomere maturation. Sarcomeres are the repeating contractile units of muscle, exquisitely ordered assemblies of actin and myosin filaments whose precise architecture determines muscle function. Without the rhythmic tension generated by spontaneous contraction, this architecture fails to mature correctly, a process known as myofibrillogenesis stalls in its final stages.

Consistent with this failure of mechanical maturation, the researchers documented defects in sarcomere structure and in mitochondrial morphogenesis within the Argk1-depleted muscles. Mitochondria in developing muscle must undergo their own remodeling program, changing shape and distribution to meet the demands of the mature fiber, and this process too was disrupted. The combined consequences of failed sarcomere maturation and defective mitochondrial development led to a severe reduction in overall muscle growth. The picture that emerges is one of cascading failure: an energy deficit at the top undermines contraction, contraction is needed to sculpt the sarcomere, and without sculpted sarcomeres and properly shaped mitochondria, the muscle never achieves its full size or function.

The broader significance of this work lies in reframing how we think about metabolic enzymes in development. Argk1 has traditionally been viewed through the lens of acute energy buffering, a molecule that springs into action when a flying insect’s flight muscles burn through ATP at extraordinary rates. The new findings reveal that the enzyme’s importance begins long before the first flight. By sustaining local ATP levels, Argk1 appears to meet the energetic demand of myofibrillogenesis, the intricate process of assembling contractile filaments, as well as the substantial biosynthetic costs of muscle growth. Development, in this view, is not just a matter of turning on the right genes at the right time; it is an energetically expensive construction project that requires a reliable power supply at every stage.

There are also tantalizing implications for human biology. Phosphagen kinases are conserved across animal evolution, and creatine kinase plays an analogous buffering role in vertebrate muscle and brain. Understanding how a phosphagen kinase supports tissue morphogenesis in flies may illuminate why these enzymes are so abundantly expressed in developing and regenerating tissues across species, and why their dysfunction is associated with muscle pathologies. The Drosophila model, with its powerful genetic toolkit and its rapid development, offers a tractable system for dissecting these questions. As the authors conclude, the data reveal an essential role for Argk1 in flight muscle development, presumably by sustaining the local ATP levels needed to support myofibrillogenesis, muscle growth, and proper flight muscle function. What began as a study of an energy buffer has ended as a story about how metabolism and morphogenesis are inseparably intertwined in the making of a muscle.

Subject of Research: The role of Arginine Kinase 1 in energy homeostasis during Drosophila flight muscle development

Article Title: Arginine Kinase 1 supports energy homeostasis in Drosophila flight muscle development

Article References: Zappia, M. P., Westacott, A., Cooke, H., Geary, R., Travers, L., de Castro, L., Carty, O., & Frolov, M. V. (2026). Arginine Kinase 1 supports energy homeostasis in Drosophila flight muscle development. PLOS Genetics, 22(9), e1012304. https://doi.org/10.1371/journal.pgen.1012304

Image Credits: AI Generated

DOI: 10.1371/journal.pgen.1012304

Keywords: Arginine Kinase 1, Drosophila, flight muscle, ATP homeostasis, mitochondria, sarcomere maturation, myofibrillogenesis, myoblasts, single-cell RNA sequencing, phosphagen kinase, muscle development, PLOS Genetics

The research team, led by investigators at Fujian Medical University, began with a straightforward clinical observation. Analyzing serum samples from three groups of ten participants each—healthy controls, COPD patients without sarcopenia, and COPD patients with sarcopenia—they found that TNF-α stood out as one of the most significantly upregulated cytokines in the sarcopenic group. To model the disease in animals, the researchers exposed mice to cigarette smoke for 24 weeks and added two weeks of hindlimb unloading at the end, a two-hit design that reproduces both the chronic inflammatory burden of COPD and the physical deconditioning that advanced patients experience when breathlessness forces them into inactivity. These Se-Copd mice showed clear pulmonary inflammation, shrunken muscle fibers with a leftward shift in fiber size distribution, and dramatically elevated TNF-α both in serum and in gastrocnemius muscle tissue, confirming that the model faithfully captured the human phenotype.

With the clinical and animal foundations in place, the team turned to high-throughput RNA sequencing to discover what TNF-α actually does inside muscle cells. They treated C2C12 mouse myoblasts with the cytokine and profiled the resulting changes in gene expression. Principal component analysis cleanly separated treated from untreated cells, and pathway enrichment analysis delivered a striking result: the p53 signaling pathway, famous as the genome’s guardian and a central hub of cellular stress responses, was strongly activated among the differentially expressed genes. Both KEGG enrichment and gene set enrichment analysis confirmed the pattern. Among the genes responding to TNF-α, one stood out for its peculiar biochemistry: Prodh2, a mitochondrial enzyme that degrades hydroxyproline and, in doing so, directly generates mitochondrial reactive oxygen species.

Prodh2 had never before been implicated in skeletal muscle disease, making it a genuinely unexplored territory. The enzyme occupies a unique intersection of metabolism, oxidative stress, and signal transduction. Its catalytic reaction is an established source of mitochondrial superoxide, and the Prodh2 gene is a known transcriptional target of p53, the very pathway that TNF-α had just been shown to activate. The researchers confirmed that TNF-α treatment raised Prodh2 mRNA and protein levels in myoblasts in a dose-dependent fashion, with cellular damage rising in parallel as concentrations climbed from 5 to 20 nanograms per milliliter. The question then became whether Prodh2 was merely a bystander or the actual engine of destruction.

The answer came from a series of knockdown experiments that proved decisively that Prodh2 is necessary for nearly everything TNF-α does to muscle cells. When the researchers silenced Prodh2 with siRNA and then exposed the cells to the cytokine, the catastrophic cascade simply failed to launch. Mitochondrial ROS production, measured with MitoSox Red fluorescence, was blunted. The collapse of mitochondrial membrane potential, tracked by the JC-1 dye that shifts from red aggregates to green monomers as mitochondria fail, was prevented. The phosphorylation of the signaling molecules STING, IRF3, and p53 was abrogated. Cytoplasmic release of mitochondrial DNA, a hallmark of mitochondrial damage, was reversed. Cytotoxicity measured by lactate dehydrogenase release, apoptosis detected by TUNEL staining and cleaved caspase-3, and the suppression of proliferation measured by EdU incorporation were all rescued. Even the upregulation of the atrophy genes Atrogin-1 and MuRF1 vanished, and myotubes that would otherwise have shortened dramatically under TNF-α treatment retained their normal length.

To map the complete signaling chain, the team deployed a pharmacological toolkit. The p53 inhibitor Pifithrin-α completely blocked TNF-α’s ability to induce Prodh2 transcription, establishing that the cytokine upregulates the enzyme through p53-dependent transcription. Scavenging mitochondrial ROS with MitoTEMPO rescued cells from mitochondrial stress, proliferation arrest, apoptosis, and myotube atrophy, confirming that reactive oxygen species are the proximate mediators of the damage. Most remarkably, when the researchers overexpressed Prodh2 in cells whose p53 had been pharmacologically inhibited, the phosphorylation of STING and IRF3 returned, causally linking this single metabolic enzyme to the innate immune machinery. The pathway, in essence, runs in a straight line: TNF-α activates p53, p53 drives Prodh2 expression, Prodh2 floods mitochondria with ROS, damaged mitochondria release their DNA into the cytoplasm, and that mislocalized DNA triggers the cGAS/STING innate immune pathway.

This final step carries particular significance for the biology of aging. Mitochondrial DNA, being of prokaryotic origin, is recognized by the cytosolic DNA sensor cGAS as a danger signal when it escapes its organelle, provoking a type I interferon response and what researchers call sterile inflammation. Aberrant activation of this mtDNA-cGAS/STING axis has been implicated in renal fibrosis, atherosclerosis, cerebral ischemia-reperfusion injury, and, critically, in the chronic low-grade inflammation of aging known as inflammaging. Recent work has shown STING activation in muscle wasting associated with cirrhosis and COPD-related muscle senescence, but the upstream trigger inside muscle cells had remained mysterious. By identifying Prodh2-mediated ROS production as the initiator of mtDNA release, the study provides the first complete molecular answer to how metabolic dysregulation ignites innate immunity to drive sarcopenia.

The findings also cast p53 in an unexpected and darker role. Best known as a tumor suppressor, p53 has increasingly been recognized as a context-dependent driver of pathology in noncancerous diseases, promoting senescence and tissue dysfunction in neurodegeneration, skin aging, and ischemia-reperfusion injury when activated by chronic rather than acute stress. The study’s authors note that while earlier work in cancer cell lines found Prodh2 to be at best a weak p53 target under acute genotoxic stress, the robust induction they observed in myoblasts reflects a fundamentally different biological context: chronic inflammatory signaling through TNF-α, likely engaging distinct transcriptional co-factors or epigenetic states that render the Prodh2 promoter highly responsive in muscle progenitor cells.

Crucially, the team then asked whether blocking Prodh2 could treat the disease in living animals. Using adeno-associated viruses carrying shRNA injected directly into the gastrocnemius muscles of Se-Copd mice, they achieved targeted knockdown of the enzyme over a 12-week intervention period. The results were striking: treated mice recovered significant grip strength, their muscle fiber cross-sectional areas increased, atrophy markers Atrogin-1 and MuRF1 declined, and phosphorylation of p53, IRF3, and STING in muscle tissue was suppressed. Immunohistochemistry revealed that Prodh2 was diffusely overexpressed throughout the cytoplasm of mature contractile myofibers in the diseased animals, indicating that the pathway operates universally in muscle tissue under chronic inflammatory stress. Importantly, knocking down Prodh2 did not lower the animals’ serum TNF-α levels, demonstrating that the intervention works downstream of inflammation, at the point where inflammatory signals are translated into mitochondrial catastrophe.

The authors acknowledge limitations, including the lack of cell-type specificity in the AAV vector and the absence of histological data from other muscles such as the diaphragm, and they plan cell-specific Cre-LoxP studies to dissect the contributions of progenitors versus mature fibers. Nevertheless, the study delivers a complete mechanistic arc from clinic to molecule to therapy. It establishes a metabolic-immune crosstalk framework for sarcopenia in which a chronic inflammatory cytokine, acting through the ancient stress hub p53, repurposes a ordinary mitochondrial enzyme into an engine of self-destruction. In doing so, it positions Prodh2 as a druggable upstream node, offering hope that future interventions could protect the muscles of millions of COPD patients even while the inflammation itself continues to smolder in their lungs.

Subject of Research: The role of Prodh2-mediated mitochondrial stress in TNF-α-induced myoblast dysfunction and sarcopenia in COPD

Article Title: Prodh2‐Mediated Mitochondrial Stress Drives TNF‐α‐Induced Myoblast Dysfunction and Sarcopenia in COPD

Article References: Chen, G., Shangguan, Z., Gong, J., Cheng, D., Chen, P., Li, J., & Lin, Q. (2026). Prodh2‐Mediated Mitochondrial Stress Drives TNF ‐α‐Induced Myoblast Dysfunction and Sarcopenia in COPD. Aging Cell, 25(10), Article e70739. https://doi.org/10.1111/acel.70739

Image Credits: AI Generated

DOI: 10.1111/acel.70739

Keywords: COPD, sarcopenia, TNF-alpha, Prodh2, mitochondrial ROS, p53, STING, mitochondrial DNA, muscle atrophy, inflammation, myoblasts, Aging Cell