Fig 2: Proposed hypothesis schematic. Tortuous collagen fibrils in the young myomatrix are compliant and experience deformation (ey) with loading during muscle contraction. The compliance of the young ECM triggers mechanotransductive signaling in fibroblasts via integrins to promote the secretion of biochemical matrix factors that are more favorable for muscle stem cell myogenesis. Conversely, in aging, collagen fibrils become more taut and display a decreased deformation in response to contractile activity (eo; ey > eo), thereby rendering the muscle more stiff. The resulting mechanotransductive cascade in fibroblasts increases nuclear translocation of YAP/TAZ and triggers the expression of matrix-associated biochemical factors that promote muscle stem cell fibrogenesis.

Fig 3: The effect of age and substrate stiffness on YAP/TAZ signaling in fibroblasts and its influence on MuSC fate. MuSCs cultured on matrices derived from old fibroblasts demonstrated a significant decrease in the percentage of desmin-positive cells (young = 26.8 ± 1.5%; old = 11.9 ± 1.9%; *P < 0.0001) and a significant increase in the percentage of Tcf4-positive cells (young = 27.8 ± 1.4%; old = 74.5 ± 1.2%; *P < 0.0001), relative to MuSCs cultured on matrices derived from young fibroblasts (A-C). Representative immunofluorescence images of YAP/TAZ expression from old and young fibroblasts, and from young fibroblasts cultured on soft (8 kPA) and stiff (32 kPA) silicone gel substrates. Compared to young fibroblasts, old fibroblasts demonstrate a significant increase in the nuclear translocation of both YAP (young = 214.7 ± 9.5; old = 271.1 ± 16.5; *P = 0.0003 and TAZ (young = 152.0 ± 5.3; old = 204.5 ± 10.3; *P = 0.0008). Similarly, young fibroblasts cultured on stiff substrates exhibited a significant increase in YAP/TAZ nuclear translocation, relative to those cultured on soft substrates. When these preconditioned fibroblasts were allowed to elaborate a matrix for 2 days, cells previously cultured on the stiff substrate demonstrated a significant decrease in the percentage of desmin-positive cells (soft surface = 14.0 ± 1.4; stiff surface 7.3 ± 0.6; P = 0.0004), and a significant increase in the percentage of Tcf4-positive cells (soft surface = 10.78 ± 1.0; stiff surface 32.3 ± 1.4; P = 0.0008), consistent with that observed in fibroblasts cultured on young as compared to old ECM (E, F). Bar = 100 µm.