Heterogeneous macrophage activation in injured skeletal muscle

Macrophages are the predominant inflammatory cells during skeletal muscle injury repair. They are required for the complete repair of acutely injured skeletal muscle, but detrimentally contribute to the pathological progression of dystrophic muscle in Duchenne muscular dystrophy (DMD) patients and DMD mouse model mdx5cv mice. The cellular and molecular mechanism underlying this difference has yet to be fully elucidated. To this end, the present study compared the single-cell transcriptomes of intramuscular monocytes/macrophages from uninjured and acutely injured quadriceps of 14 weeks-old wild-typed (WT) mice, and from quadriceps and diaphragm of age-matched mdx5cv mice, using single cell-based RNA sequencing (scRNA-seq) analysis. Our results identified multiple functionally diverse monocyte/macrophage clusters co-existing in each injured muscle sample. Both Ly6Chi and Ly6Clo intramuscular monocytes/macrophages were heterogenous, containing different clusters among different injury conditions. These clusters did not conform to strict M1/M2 activation, and were involved differentially in inflammation, extracellular matrix (ECM) remodeling, and myogenesis during skeletal muscle injury repair. These findings uncovered the heterogeneity of monocyte/macrophage activation in both acutely injured and dystrophic skeletal muscle, shedding light on the molecular and cellular mechanisms underlying the differential roles of monocytes/macrophages in acute skeletal muscle injury and muscular dystrophy in DMD. Single cell suspensions were prepared from 14 weeks-old quadriceps of WT mice day 1 and day 3 post BaCl2-induced acute muscle injury, and were then used for single cell-based RNA sequencing.