Single-cell quantitative analysis of skeletal muscle cell population dynamics during regeneration and ageing

Abstract

The skeletal muscle is populated by a variety of different mononuclear cell types that actively contribute to tissue homeostasis. When the tissue is stressed by exercise or by an acute or chronic insult, the different cell types are activated, exchange signals and initiate a finely-orchestrated regeneration process to prevent the loss of muscle mass. This cell variety is exacerbated by an additional intra population heterogeneity, where the cell population boundaries often lose their significance. Here, we applied a high-dimensional single-cell mass cytometry analysis to solve the cellular and molecular complexity of the muscle tissue in different physiological and pathological conditions. Taken together, our results provide a comprehensive picture both of muscle cell population homeostasis during ageing and of the changes induced by a perturbation of the system, be it chronic (dystrophy) or acute (cardiotoxin).

Fibro-adipogenic progenitors of dystrophic mice are insensitive to NOTCH regulation of adipogenesis

“…In our model, in young mdx mice, FAPs are insensitive to NOTCH anti-adipogenic signals, but do not cause adipocyte infiltrations in vivo, thanks to the sustained inflammatory environment (Porter et al, 2002). We have observed a substantial decrease in macrophage infiltrations in muscle of old mdx mice (Petrilli et al, 2017, Preprint ), supporting the notion that, with aging, the decrease in inflammation makes the concentration of inflammatory cytokines inadequate to cooperate with NOTCH signaling, thus making adipogenesis control insufficient to prevent fat deposition.…”

TGF-β-driven downregulation of the Wnt/β-Catenin transcription factor TCF7L2/TCF4 in PDGFRα+fibroblasts

“…Dynamic regulation of stromal and/or fibroblast markers following damage has been also identified in resident stromal cardiac fibroblasts (Farbehi et al, 2019; Kanisicak et al, 2016; Tallquist and Molkentin, 2017) and skeletal muscle (Contreras et al, 2019c; Malecova et al, 2018; Petrilli et al, 2017 preprint; Soliman et al, 2019 preprint). We have recently reported that the in vivo and in vitro expression of PDGFRα is strongly downregulated by damage-associated TGF-β signaling in skeletal muscle and heart mesenchymal stromal cells (Contreras et al, 2019c).…”

“…In summary, the work of others and our results suggest that stromal stem cell and/or progenitor markers are often downregulated by TGF-β and probably after a complex array of niche signals during regeneration or disease, as cells change phenotypically towards an activated ECM-producing cell or myofibroblast. These downregulated stromal and/or fibroblast markers include Sca-1, PDGFRα, and Tcf21 (Asli et al, 2018 preprint; Contreras et al, 2019c; Fu et al, 2018; Kanisicak et al, 2016; Petrilli et al, 2017 preprint; Soliman et al, 2019 preprint). With our work, we added Tcf7l2 to the growing list.…”

Fibro-adipogenic progenitors in skeletal muscle homeostasis, regeneration and diseases

“…Upon acute muscle injury, FAPs rapidly enter cell cycle. The rise in BrDU + cells happens faster in FAPs than in the muscle stem cell population, which leads to an increase in the FAPs/ muscle stem cell ratio during the first few days after an injury [10,15]. The total number of FAPs peaks around 3-4 days post-injury, depending on the type and severity of injury [36,54].…”

“…Skeletal muscle injury induces the coordinated accumulation of different cell types. Single-cell RNAseq and single-cell mass cytometry have identified between nine and 15 distinct cell populations in the resting skeletal muscle and during the different phases of regeneration [8][9][10][11][12]. After an injury, there is a rapid accumulation of immune cells (neutrophils, pro-and anti-inflammatory macrophages, natural killer cells, B-and T-cells) and changes in the proportion of nonimmune cells (endothelial cells, smooth muscle cells, glial cells, tenocytes and fibro-adipogenic progenitors).…”