Regenerative
A muscle-support cell's death triggers inflammation and rapid atrophy in mice
By Life and Health Today Staff, . Life and Health Today.
When researchers deleted a specific population of muscle-resident support cells in mice, the animals lost nearly a third of their lean mass within days, and a new study has traced the mechanism behind that loss to an inflammatory chain reaction rather than to any failure of the nerves or the muscle fibres themselves.
The study, published in the Journal of Cachexia, Sarcopenia and Muscle, focused on cells called fibro-adipogenic progenitors, or FAPs. FAPs are a type of connective-tissue stem cell that lives inside skeletal muscle and helps maintain its normal structure. They are identified by a surface protein called PDGFRα. Scientists have known for some time that FAPs matter to muscle health, but the mechanism by which their loss causes atrophy, the medical term for muscle wasting, had not been worked out.
To find out, the researchers used a mouse model in which FAPs could be selectively destroyed by administering tamoxifen, a drug that triggers a genetic kill switch only in FAP cells. Following tamoxifen administration, FAP density dropped by 90%. Lean mass fell by 30%, a result the authors reported as statistically significant at p less than 0.001. Maximal force output in two isolated muscles, the extensor digitorum longus and the soleus, dropped by 25%. Crucially, specific force, meaning force per unit of muscle cross-section, did not change, and the muscles were no more vulnerable to contractile damage than normal. That distinction matters: the muscles were smaller, but the tissue that remained was functioning normally.
The researchers also checked whether the neuromuscular junction, the connection point where a nerve tells a muscle fibre to contract, had been damaged. Structural staining, functional nerve-versus-direct-stimulation tests, and gene expression analysis of acetylcholine receptor subunits all came back normal. The atrophy was not a nerve problem.
What the researchers did find was a massive inflammatory response. Coinciding with FAP loss, macrophages and neutrophils, two types of immune cell, infiltrated the muscle at ten times their normal density. Transcript levels of several inflammatory signalling molecules, called chemokines, surged before the muscle-wasting genes even switched on. The journal reports that Ccl2 increased roughly 80-fold, Ccl12 roughly 80-fold, Cxcl1 roughly 50-fold, and Cxcl2 roughly 50-fold, with the authors describing these as "robust 10- to 150-fold increases" that were statistically significant at p less than 0.0001. The muscle-atrophy genes, including Trim63 and Fbxo32, followed rather than led.
To test whether the inflammation was causing the atrophy rather than simply accompanying it, the team tried three interventions. Depleting immune cells entirely made things worse, worsening mass loss and increasing atrophy-gene expression fivefold, suggesting that some immune activity is protective. Blocking the receptor for Cxcl1 and Cxcl2, a protein called Cxcr2, restored muscle mass by 15%. Treatment with an anti-inflammatory steroid called Vamorolone, also known as VBP15, also restored muscle mass by 15%, while cutting atrophy-gene expression by 50% and Cxcl1 and Cxcl2 expression by 70%.
The authors conclude that FAP deletion causes muscle atrophy through an inflammation-driven, Cxcl1/2-dependent pathway, triggered by the immune response to dying FAP cells and possibly by the loss of whatever anti-inflammatory role FAPs normally play.
What this does not show is whether the same mechanism operates in human muscle-wasting conditions such as sarcopenia, the age-related loss of muscle mass, or cachexia, the severe wasting seen in cancer and chronic disease. The entire study was conducted in mice using a genetic deletion model that has no direct equivalent in human disease. Whether FAP numbers or function decline in ageing or illness in people, and whether the Cxcl1/2 pathway is similarly involved, remains an open question.
The finding that blocking a specific inflammatory signal, rather than suppressing immunity broadly, partially rescued muscle mass is the result most worth watching. If a similar pathway operates in people, it would suggest a more targeted approach to treating muscle wasting than broad immunosuppression. That would require human trials, which have not been reported.