Removing TGF-β1 from M2 macrophages restores muscle growth impaired by obesity

Obesity has long been known to erode muscle mass and strength, quietly setting the stage for a condition clinicians call sarcopenic obesity, in which the metabolic burden of excess fat converges with the loss of regenerative capacity in skeletal muscle. Now, a study published in the Journal of Cachexia, Sarcopenia and Muscle has identified a surprisingly specific culprit within the immune system: transforming growth factor-beta 1 (TGF-β1) produced by a subset of anti-inflammatory macrophages. By deleting the gene encoding this single signalling molecule from CD206-positive M2 macrophages in mice, researchers were able to dramatically reverse obesity-induced muscle dysfunction, doubling the distance animals could run on a treadmill and restoring molecular pathways that govern both muscle growth and mitochondrial energy production.

The research, conducted at the University of Toyama, focused on macrophages, the versatile immune cells that populate nearly every tissue in the body. Within injured or inflamed skeletal muscle, macrophages transition from a pro-inflammatory M1 state, which initiates the response to damage, to an anti-inflammatory M2 state, which supports repair and remodelling. M2 macrophages are supposed to be the good guys, secreting factors that help satellite cells, the resident muscle stem cells, differentiate and rebuild tissue. Yet previous work had shown that CD206-positive macrophages accumulate in ageing muscle in both mice and humans, and that their presence correlates with declining muscle mass. The paradox — repair-promoting cells associated with deterioration — suggested that one or more of the molecules these cells produce might, under conditions of chronic metabolic stress such as obesity, turn from healer to saboteur. TGF-β1, a potent cytokine with well-known fibrotic and anti-myogenic effects, emerged as the prime suspect.

To test this hypothesis directly, the team engineered a conditional knockout mouse model by crossing CD206-CreER T2 mice, in which Cre recombinase activity is restricted to CD206-expressing cells and can be pharmacologically controlled, with mice carrying floxed alleles of the Tgf-β1 gene. Administering tamoxifen at six weeks of age selectively excised the Tgf-β1 gene from CD206-positive macrophages, leaving the cytokine production of all other cell types untouched. One week later, the mice — both knockout animals and floxed controls — were placed on a high-fat diet deriving sixty percent of calories from fat for twelve weeks, a regimen that reliably produces obesity, insulin resistance and measurable skeletal muscle impairment. Crucially, body weight and food intake remained comparable between the two groups throughout the experiment, meaning that any differences in muscle function could not be attributed to differences in how much the animals ate or weighed.

The functional results were striking. In an exhaustive treadmill protocol in which running distance and time were recorded alongside the number of electric shocks received, the knockout mice ran roughly twice as far before exhaustion, a difference the authors report with a p-value of 0.0008. The knockout animals also hung from a wire mesh for significantly longer before dropping and generated measurably greater forelimb grip strength. Gene expression analysis confirmed the molecular validity of the model: Tgfb1 transcript levels were markedly reduced in the tibialis anterior muscle of knockout mice, and immunohistochemistry revealed a loss of the CD206 and TGF-β1 double-positive signal that characterised macrophages in control tissue. In other words, removing one cytokine from one immune cell population was sufficient to produce a whole-animal improvement in physical performance under obesogenic conditions.

The mechanisms behind this improvement proved to operate through two distinct and complementary routes. The first involves fibro-adipogenic progenitors, or FAPs, a population of mesenchymal-like cells resident in skeletal muscle that normally support repair by secreting paracrine factors. Using magnetic-activated cell sorting to isolate PDGFRα-positive FAPs from muscle tissue, the researchers found that deleting macrophage-derived TGF-β1 unleashed these progenitors: expression of follistatin (Fst) rose 1.70-fold and follistatin-like protein 1 (Fstl1) rose 2.60-fold in the tibialis anterior. Follistatin is a well-characterised antagonist of myostatin and activin, two powerful brakes on muscle growth, and its induction is a textbook signature of enhanced myogenesis. Consistent with this, myogenic regulatory genes were upregulated across both soleus and tibialis anterior muscles, and fibrosis-related gene expression declined, suggesting that removing TGF-β1 also relieved the profibrotic pressure that stiffens and scars obese muscle.

The second route ran through metabolism rather than directly through muscle. Analysis of muscle fibre type genes revealed that knockout mice had significantly elevated expression of Myh7 (type I fibres, 2.40-fold), Myh2 (type IIa, 1.50-fold) and Myh1 (type IIx, 2.35-fold) in the soleus, along with increased Myh4 (type II, 1.76-fold) in the tibialis anterior. Because type I fibres are rich in mitochondria and resistant to fatigue, this shift pointed toward enhanced oxidative metabolism, and downstream analyses confirmed it: genes governing mitochondrial biogenesis, all five oxidative phosphorylation complexes, fatty acid oxidation and fatty acid uptake were broadly upregulated in the muscles of knockout animals. At the centre of this metabolic reprogramming sat the AMPK/SIRT1/PGC-1α axis, the canonical energy-sensing cascade that orchestrates mitochondrial biogenesis. Phosphorylation of the AMPKα subunit at threonine 172 increased 1.5-fold, PGC-1α protein rose 2.2-fold, and Sirt1 expression climbed in parallel — a coordinated activation pattern that, in the knockout mice, restored the mitochondrial programme that obesity normally suppresses.

What connected an immune-cell gene deletion in muscle to mitochondrial activation in muscle? The answer turned out to lie in adipose tissue. Adiponectin, a hormone secreted by fat cells that sensitises tissues to insulin and activates AMPK in muscle through its receptor AdipoR1, was significantly elevated in the knockout mice. In epididymal white adipose tissue, Tgfb1 expression dropped while Adipoq expression rose, and serum adiponectin measured by ELISA increased 1.23-fold. Correspondingly, AdipoR1 mRNA increased 1.8-fold in both soleus and tibialis anterior muscle, closing the loop: macrophage-derived TGF-β1 was suppressing adiponectin production in fat, and its removal liberated the adiponectin–AdipoR1–AMPK signalling axis that drives mitochondrial function in muscle. Insulin signalling improved as well, with insulin-stimulated Akt phosphorylation rising 2.14-fold in adipose tissue and 1.62-fold in liver, and both glucose tolerance and insulin tolerance tests showed significantly better metabolic profiles in the knockout animals. Histology of adipose tissue told a matching story, with fewer crown-like structures — the histological scars of dying, inflamed adipocytes — and a trend toward smaller, metabolically healthier fat cells.

The significance of these findings extends beyond the mouse cage. Sarcopenic obesity is a growing public health concern as populations age and obesity rates climb, and current therapeutic options are limited largely to exercise and nutritional intervention, neither of which fully restores regenerative capacity in metabolically compromised muscle. By identifying macrophage-derived TGF-β1 as a node that simultaneously suppresses FAP-mediated myogenesis, dampens adiponectin secretion, blunts insulin sensitivity and throttles mitochondrial biogenesis, the study reframes sarcopenic obesity not simply as a passive consequence of carrying excess fat, but as an actively maintained state orchestrated in part by misbehaving immune cells. It also resolves the earlier paradox of CD206-positive macrophage accumulation in ageing muscle: these cells may indeed be present to repair, but the TGF-β1 they secrete in an obese or aged environment may prevent them from doing so effectively, or even actively contribute to fibrosis and metabolic dysfunction.

The authors are careful to note the limitations of their work. The study does not disentangle the relative contributions of the myogenic and adiponectin-mediated mechanisms to the overall improvement in muscle strength, and the experiments were performed exclusively in mice on a defined high-fat diet protocol. Whether human CD206-positive macrophages behave identically, whether pharmacological TGF-β1 blockade — an approach already in clinical use for other fibrotic conditions — could reproduce the benefits without unacceptable side effects, and how the two mechanisms interact over longer time courses all remain open questions. TGF-β1 is a pleiotropic molecule with essential roles in immune regulation and wound healing, so systemic inhibition carries real risks; the appeal of the macrophage-specific strategy demonstrated here is precisely its selectivity.

Even with those caveats, the study delivers a conceptually important message: the immune system is not a bystander in metabolic muscle disease but an active participant whose output can be reprogrammed. The demonstration that deleting a single cytokine from a single macrophage subset can simultaneously enhance regeneration, improve whole-body glucose metabolism, boost circulating adiponectin and reactivate mitochondrial biogenesis suggests that carefully targeted immunomodulation could one day complement or even substitute for lifestyle interventions in patients whose muscle function is collapsing under the combined weight of obesity and age. For now, the treadmill mice — running twice as far on the same obese body mass — offer the most vivid evidence yet that the key to rescuing failing muscle may lie not in the muscle fibres themselves, but in the immune cells that surround them.