Connective tissue cells expressing fibro/adipogenic progenitor markers increase under chronic damage: relevance in fibroblast-myofibroblast differentiation and skeletal muscle fibrosis

Abstract

Fibrosis occurs in skeletal muscle under various pathophysiological conditions such as Duchenne muscular dystrophy (DMD), a devastating disease characterized by fiber degeneration that results in progressive loss of muscle mass, weakness and increased extracellular matrix (ECM) accumulation. Fibrosis is also observed after skeletal muscle denervation and repeated cycles of damage followed by regeneration. The ECM is synthesized largely by fibroblasts in the muscle connective tissue under normal conditions. Myofibroblasts, cells that express α-smooth muscle actin (α-SMA), play a role in many tissues affected by fibrosis. In skeletal muscle, fibro/adipogenic progenitors (FAPs) that express cell-surface platelet-derived growth factor receptor-α (PDGFR-α) and the transcription factor Tcf4 seem to be responsible for connective tissue synthesis and are good candidates for the origin of myofibroblasts. We show that cells positive for Tcf4 and PDGFR-α are expressed in skeletal muscle under normal conditions and are increased in various skeletal muscles of mdx mice, a murine model for DMD, wild type muscle after sciatic denervation and muscle subjected to chronic damage. These cells co-label with the myofibroblast marker α-SMA in dystrophic muscle but not in normal tissue. The Tcf4-positive cells lie near macrophages mainly concentrated in dystrophic necrotic-regenerating foci. The close proximity of Tcf4-positive cells to inflammatory cells and their previously described role in muscle regeneration might reflect an active interaction between these cell types and growth factors, possibly resulting in a muscular regenerative or fibrotic condition.

ALS skeletal muscle shows enhanced TGF-β signaling, fibrosis and induction of fibro/adipogenic progenitor markers

“…We previously showed that the number of FAPs is augmented in models of increased fibrosis [42]. Therefore, we evaluated their presence by western blot analyses using markers of activated FAPs.…”

“…We have previously demonstrated that the number of connective tissue cells expressing FAPs markers are increased in three different fibrotic models: mdx mice, BaCl 2 -induced chronic damage and sciatic nerve denervation [42], suggesting a role of these cells in fibrosis associated to skeletal muscle. It seems that FAPs cells are required for correct regeneration in skeletal muscle.…”

“…However, under conditions where persistent damage occurs, these cells appear to play a rather deleterious role [62]. Current evidence suggests that mesenchymal progenitors that reside in connective tissue are probably the major contributors of ECM deposition in skeletal muscle, and that expression of fibrotic genes including different types of collagens, CTGF and α-SMA, take place in PDGFRα + cells [42, 44]. We showed that PDGFRα + cells co-localize with p-Smad3 + nuclei, suggesting that these cells are sensitive to TGF-β and might be responsible of ECM deposition in muscles of hSOD1 G93A mice.…”

“…The corresponding Alexa Fluor 568 or 488-conjugated anti IgGs (Invitrogen, Carlsbad, CA, USA) were used as secondary antibodies. For nuclear staining, sections were incubated with 1 μg/mL Hoechst 33258 in PBS for 10 minutes [42]. Slices were then washed in water and mounted in fluorescent mounting medium (DAKO, Santa Clara, CA, USA).…”

“…These mesenchymal progenitors specifically express PDGFRα and Tcf4 [40, 41], and are increased in numbers in different models of chronic muscle damage [42–45]. Moreover, FAPs play a supporting role in healthy muscle regeneration [41, 46, 47].…”