Fibro-Adipogenic Progenitor Ablation Triggers Muscle Atrophy Through Cell Death-Induced Inflammation

Skeletal muscle mass maintenance involves coordination of myofibers with mononuclear cell populations, including satellite cells, resident macrophages and fibro-adipogenic progenitors (FAPs). FAPs, identified by surface expression of PDGFRα, are important contributors to muscle homeostasis, as genetic ablation of FAPs induces rapid muscle atrophy. However, mechanisms underlying this response remain poorly understood.

We utilized tamoxifen-inducible FAP specific diptheria toxin (DTA) mice (Pdgfra^Cre-ERT2/+; Rosa26^{^DTA/+}) to evaluate consequences of FAP deletion, measuring body and muscle mass, and isolated function on the extensor digitorum longus (EDL) and soleus (SOL). Neuromuscular junction (NMJ) patency was assessed via whole-mount staining, nerve vs. direct muscle stimulated function, and denervation-responsive expression targeting AchRα/β/γ. Immunofluorescence of muscle sections was used to investigate cellular and morphological changes. RT-qPCR measured expression of atrogenes, inflammatory chemokines/cytokines, and growth factors in tibialis anterior (TA) muscles. To modulate the immune response, mice were subjected to immune cell depletion, pharmacological Cxcr2 inhibition, or treatment with the anti-inflammatory steroid Vamorolone (VBP15).

Following tamoxifen administration, FAP density decreased 90%, coupled with a 30% loss of lean mass (p < 0.001). Isolated contractile measurements showed that FAP-deleted muscles exhibited 25% reductions in maximal tetanic force in the EDL and SOL, while specific force and vulnerability to contractile damage remained unchanged. Whole-mount staining revealed preserved NMJ structural integrity, and functional testing showed no differences between nerve and direct muscle stimulation. RT-qPCR showed no change in genes encoding AchR subunits. Longitudinal body composition tracking revealed that FAP-deletion-induced muscle and fat loss occurred during tamoxifen administration and coincided with a 10-fold increase in the infiltration of macrophages and neutrophils. Robust 10- to 150-fold increases (p < 0.0001) in chemokine transcript levels was observed soon after FAP deletion but preceded atrogene upregulation (Trim63, Fbxo32, Sqstm1, Ulk1). Ccl2 increased ~80-fold, Ccl12 ~ 80-fold, Cxcl1 ~ 50-fold, and Cxcl2 ~ 50-fold. While immune cell depletion exacerbated muscle atrophy by worsening mass loss and further increasing atrogene expression 5-fold, Cxcr2 inhibition or VBP15 treatment restored muscle mass by 15% (p < 0.01). VBP15 treatment also resulted in a 50% reduction in atrogene expression and a 70% reduction in Cxcl1/2 expression levels.

Our findings demonstrate that FAP deletion results in simple muscle atrophy without affecting muscular contractile properties and NMJ function. The atrophy induced by the loss of FAPs occurs through an inflammation-mediated, Cxcl1/2-dependent mechanism, caused by the response to FAP cell death potentially coupled with the absence of FAP actions on the inflammatory environment.