Obesity is often discussed in terms of excess body fat, blood glucose, and cardiovascular risk, but its effects extend deep into the body’s machinery of movement. As obesity progresses, skeletal muscle can lose mass, strength, and metabolic flexibility, producing a condition known as obesity-related sarcopenia. The decline can restrict mobility, reduce independence, and make it harder for people to exercise or recover from illness. A new study from the University of Toyama in Japan suggests that a specific immune-cell signal may help drive this muscle deterioration—and that interrupting it could protect muscle even when body weight remains unchanged.
The research, led by Distinguished Research Professor Kazuyuki Tobe at the University of Toyama’s Research Center for Pre-Disease Science, focuses on CD206-positive M2 macrophages. Macrophages are immune cells that respond to injury, inflammation, and changes in tissue metabolism. M2 macrophages are generally associated with tissue repair and remodeling, including the recovery of injured muscle. However, the new findings indicate that these cells may become harmful in the metabolic environment created by obesity by releasing excessive amounts of transforming growth factor-beta 1, or TGF-β1.
TGF-β1 is a powerful signaling molecule involved in cell growth, tissue remodeling, immune regulation, and fibrosis. In healthy amounts, it contributes to normal repair processes. But persistent or excessive TGF-β1 activity can alter the behavior of muscle-resident cells, promote scar-like tissue formation, and interfere with regeneration. To determine whether macrophage-derived TGF-β1 was directly involved in obesity-associated muscle dysfunction, the researchers created genetically engineered mice in which the Tgf-β1 gene could be selectively deleted in CD206-positive M2 macrophages.
The animals were then fed a high-fat diet for 12 weeks, a standard experimental approach for inducing obesity and metabolic dysfunction in mice. The researchers compared the modified animals with control mice that retained TGF-β1 production in the targeted macrophage population. Their analysis included endurance running, grip-strength testing, hanging tests, glucose and insulin tolerance assessments, and molecular examination of muscle and adipose tissue. The design allowed the team to distinguish the effects of macrophage-specific TGF-β1 deletion from the effects of body weight alone.
The results were striking. Although the modified and control mice reached similar body weights, the animals lacking TGF-β1 in CD206-positive M2 macrophages performed considerably better in physical tests. They ran approximately twice as far before exhaustion, demonstrated stronger grip, and remained suspended for longer during hanging tests. They also showed improved glucose tolerance and insulin sensitivity, suggesting that the intervention protected not only muscle performance but also broader metabolic function.
Measurements of body composition and muscle tissue provided a biological explanation for the improved performance. The modified mice retained more skeletal muscle, possessed larger muscle fibers, and had a greater proportion of lean mass than the control animals. These findings point to protection from obesity-induced muscle wasting rather than simply an improvement in motivation or exercise capacity. The muscle tissue also showed lower expression of genes linked to fibrosis, indicating that the absence of macrophage-derived TGF-β1 may help preserve the structural environment required for effective regeneration.
The researchers identified two interconnected mechanisms behind the protective effect. First, removing TGF-β1 activated fibro-adipogenic progenitors, or FAPs, which are support cells located within skeletal muscle. FAPs can influence the formation and repair of muscle fibers, although abnormal activation can also contribute to fibrosis. In this study, the altered FAP response was associated with increased production of follistatin and follistatin-like protein 1, molecules that support myogenesis—the process through which muscle precursor cells develop into new muscle fibers. This suggests that TGF-β1 normally restrains a regenerative program in the obese muscle environment.
The second mechanism involved communication between fat tissue and muscle. Adipose tissue from the modified mice released more adiponectin, a hormone with important effects on energy metabolism and insulin sensitivity. Adiponectin activated the AdipoR1 receptor in skeletal muscle, triggering the AMPK/SIRT1/PGC-1α signaling pathway. This pathway is a central regulator of mitochondrial biogenesis, fatty-acid oxidation, and cellular energy production. By increasing activity through this system, the muscle cells were better able to use fatty acids as fuel and maintain mitochondrial performance. The researchers connect this metabolic improvement to the enhanced endurance observed in the engineered mice.
Together, the findings reveal that TGF-β1 from a narrowly defined population of M2 macrophages may contribute to obesity-related muscle decline through two distinct but complementary routes. It appears to suppress the cellular signals needed for muscle regeneration while also weakening the hormone-driven pathways that support mitochondrial energy production. Blocking the signal in these macrophages improved muscle size, fiber structure, exercise performance, and metabolic health without reducing obesity itself. The study therefore raises the possibility that future treatments could target the complications of obesity independently of weight loss.
The work remains an experimental finding in mice, and translating it into a human therapy will require substantial further research. TGF-β1 performs many essential functions throughout the body, so broadly blocking the molecule could produce unwanted effects. Any future intervention would likely need to act selectively within the relevant macrophage population or downstream pathways. Nevertheless, the study offers a new framework for treating obesity-related sarcopenia: rather than focusing exclusively on body weight or blood sugar, therapies might also restore the immune, regenerative, and mitochondrial signals that keep skeletal muscle functional. With obesity and aging increasingly occurring together worldwide, targeting this intersection could become an important strategy for preserving strength and independence.
Subject of Research: Animals
Article Title: Deletion of Tgf-β1 From CD206+ M2 Macrophages Ameliorates Obesity-Induced Suppression of Myogenesis and AMPK Phosphorylation in Skeletal Muscle
Web References: https://doi.org/10.1002/jcsm.70322
References: DOI: 10.1002/jcsm.70322
Image Credits: Distinguished Research Professor Kazuyuki Tobe, University of Toyama
Keywords: obesity-related sarcopenia, skeletal muscle, M2 macrophages, CD206, TGF-β1, muscle regeneration, mitochondria, adiponectin, AMPK, myogenesis
