MME+ fibro-adipogenic progenitors are the dominant adipogenic population during fatty infiltration in human skeletal muscle

Fatty infiltration, the ectopic deposition of adipose tissue within skeletal muscle, is mediated via the adipogenic differentiation of fibro-adipogenic progenitors (FAPs). We used single-nuclei and single-cell RNA sequencing to characterize FAP heterogeneity in patients with fatty infiltration. We identified an MME+ FAP subpopulation which, based on ex vivo characterization as well as transplantation experiments, exhibits high adipogenic potential. MME+ FAPs are characterized by low activity of WNT, known to control adipogenic commitment, and are refractory to the inhibitory role of WNT activators. Using preclinical models for muscle damage versus fatty infiltration, we show that many MME+ FAPs undergo apoptosis during muscle regeneration and differentiate into adipocytes under pathological conditions, leading to their depletion. Finally, we utilized the varying fat infiltration levels in human hip muscles to show the depletion of MME+ FAPs in fatty infiltrated human muscle. Altogether, we have identified the dominant adipogenic FAP subpopulation in skeletal muscle. Overall design: For bulk RNAseq, mRNA from sorted skeletal muscle FAPs (PDGFRA/eGFP+ MME+/-). 5 biological replicates per group. For human sn- and scRNA-seq, we harvested muscles from fatty (GM) and non-fatty infiltrated (RF) muscles. One sample per condition (scRNAseq), and pooled muscles (n=5) in 2 technical replicates (snRNAseq). For mouse scRNAseq, we harvested muscles from WT mouse. One biological sample.