Muscle PGC-1α modulates satellite cell number and proliferation by remodeling the stem cell niche

Abstract

Background: The myogenic capacity of satellite cells (SCs), adult muscle stem cells, is influenced by aging, exercise, and other factors. In skeletal muscle, the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) is a key regulator of oxidative metabolism and endurance training adaptation. However, a link between PGC-1α and SC behavior remains unexplored.

Methods: We have now studied SC function in a PGC-1α fiber-specific gain-of-function animal model.

Results: In surprising contrast to bona fide exercise, muscle-specific PGC-1α transgenic mice have lower SC numbers. Nevertheless, SCs from these mice have a higher propensity for activation and proliferation. Intriguingly, muscle PGC-1α triggers a remodeling of the SC niche by altering the extracellular matrix composition, including the levels of fibronectin, which affects the proliferative output of SCs.

Conclusions: Taken together, PGC-1α indirectly affects SC plasticity in skeletal muscle and thereby might contribute to improved SC activation in exercise.

Keywords: Basal lamina; Fibronectin; PGC-1α; Satellite cell niche; Satellite cells; Skeletal muscle.

Fig 1

SC numbers, MRF expression, and SC proliferation after cardiotoxin injection. a SC numbers per area in TA (n = 4 per group), b SOL and EDL muscles (n = 4 per group) of mTGs and WTs. c Relative mRNA levels of Pax7 and MRFs in TA (n = 6 per group) of mTGs compared to controls (WT); mRNA levels in mTG were normalized to littermate control levels. d SC numbers per area 4 days post-CTX (n = 7–8 per group) and e 19 days post-CTX (n = 8–10 per group) in mTGs together with representative IHC images (laminin: green, DAPI/nuclei: blue; Pax7: red). Red rectangles denote enlarged areas of sections, and SCs are marked with white asterisks. Brightness/contrast was adjusted using ImageJ. Values are plotted as AV ± SEM; *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001

Fig 2

SC numbers and proliferation on fibers in culture. a Representative IHC images (DAPI/nuclei: blue; MyoD: green; Pax7: red) of SCs on freshly isolated fibers (T0) and b SC quantification per fiber in mTGs and WT mice. Approximately 60 fibers per mouse and 5 mice per group were used. c Representative IHC images (DAPI/nuclei: blue; MyoD: green; Pax7: red) of SC progeny on fibers kept for 3 days in culture media (T3). Quantification results: d Total proliferative output of SCs. e Normalized proliferative output (T3/T0). f Quantification of Pax7⁺/MyoD⁻, Pax7⁺/MyoD⁺, and Pax7⁻/MyoD⁺ cells per fiber expressed as percentages and g numbers of total proliferative output. Approximately 20 fibers per mouse and 5 mice per group were used. Brightness/contrast was adjusted using ImageJ. Values are plotted as AV ± SEM; *p ≤ 0.05, **p ≤ 0.01

Fig 3

Myoblast proliferation outside of the SC niche. a Assessment of live mTG and WT myoblasts (MBs) 1 and 2 days after plating equal numbers and fold change in MB numbers between these two days. Cells were quantified on 10 fields per dish, 2 dishes per mouse, and 3 mice per genotype. Representative bright-field image with the white rectangle indicating enlarged area. b Desmin-positive cell quantification 2 and 4 days after plating equal number of MBs and fold change between these two days. Cells were quantified on an area of 38.7 mm² per dish, 2 dishes per mouse, and 3 mice per genotype. Representative IHC image (DAPI/nuclei: red; desmin: green) with the white rectangle indicating enlarged area. Values are plotted as AV ± SEM

Fig 4

Extracellular matrix gene expression. a Representative images of freshly isolated fibers from mTGs and WT mice. b Images of WT and mTG fibers after 3 days in culture media, prior to fixation. WT fibers look translucent and silky while mTG fibers appear matte and with cracks on the surface. c Relative mRNA levels of ECM components in mTGs and WT mice (n = 6 per group). d Relative mRNA levels of basal lamina (BL) components in mTGs and WT mice (n = 6 per group). e Western blot analysis and quantification of fibronectin normalized to α-tubulin levels. Values are plotted as AV ± SEM; *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001