The Spindle Assembly Checkpoint Safeguards Genomic Integrity of Skeletal Muscle Satellite Cells

Abstract

To ensure accurate genomic segregation, cells evolved the spindle assembly checkpoint (SAC), whose role in adult stem cells remains unknown. Inducible perturbation of a SAC kinase, Mps1, and its downstream effector, Mad2, in skeletal muscle stem cells shows the SAC to be critical for normal muscle growth, repair, and self-renewal of the stem cell pool. SAC-deficient muscle stem cells arrest in G1 phase of the cell cycle with elevated aneuploidy, resisting differentiation even under inductive conditions. p21(CIP1) is responsible for these SAC-deficient phenotypes. Despite aneuploidy's correlation with aging, we find that aged proliferating muscle stem cells display robust SAC activity without elevated aneuploidy. Thus, muscle stem cells have a two-step mechanism to safeguard their genomic integrity. The SAC prevents chromosome missegregation and, if it fails, p21(CIP1)-dependent G1 arrest limits cellular propagation and tissue integration. These mechanisms ensure that muscle stem cells with compromised genomes do not contribute to tissue homeostasis.

Graphical abstract

Figure 1

Activated Satellite Cells Have a Functional SAC (A) Transcript levels of the satellite cell marker Pax7 and SAC genes Mps1 and Mad2 from FACS-purified SCs immediately after isolation (quiescent; QSC) or cultured in high-serum media (activated; ASC) for 3 days. Data were collected from three mice per time point. (B) Representative MAD2 and KI67 immunostaining pattern in PAX7⁺ SCs on single muscle fibers either fixed immediately or after 2 days in culture. 4′,6-diamidino-2-phenylindole (DAPI) stains nuclei. n = 3 mice. (C) MAD2 (red) and tubulin (green) immunostaining at key stages of the cell cycle on single muscle fibers after 2 days in culture. DAPI stains nuclei blue. (D) Number of cells that arrest in mitosis (pH3⁺ cells) after exposure to mitotic poisons. Triplicate data from n = 3 mice were pooled, with 800–2,000 cells per condition. (E) Representative images of MAD2 localization in prometaphase (characterized by the tubulin staining pattern) and in the presence of mitotic poisons. The scale bars represent 10 μm (B) and 5 μm (C and E). Data are presented as mean ± SEM. ^∗p < 0.05. n.d., not detectable. See also Figure S1.

Figure 2

Mps1 Is Required for Normal Muscle Regeneration and Replenishment of the Satellite Cell Pool (A) Experimental regime showing mouse strains, Tmx strategy, and injury paradigm. (B and C) Representative images (B) and histogram (C) showing EdU incorporation in control and Mps1Δ f/f myogenic cells collected 7 days after injury by FACS. n = 3 mice per genotype, with 200–300 cells per condition, performed in triplicate. (D) Transverse sections of regenerating muscle from control and Mps1Δ f/f mice collected 7 and 30 days after injury, stained for laminin, DAPI, and YFP to quantify muscle fiber size. Regenerated muscle fibers are characterized by centrally located nuclei. (E) Quantification of muscle fiber cross-sectional area (CSA) from regenerating control and Mps1Δ f/f muscle. n = 4 mice per genotype. (F and G) Number (F) and representative images (G) of PAX7⁺ SCs/single muscle fibers after 30 days of regeneration in control and Mps1Δ f/f mice. n = 3 mice, with 30–40 myofibers per condition. The scale bars represent 20 μm (B) and 50 μm (D and G). Data are presented as mean ± SEM. ^∗p < 0.05, ^∗∗p < 0.01, Student’s t test. See also Figures S2–S4.

Figure 3

Mps1 Function Is Required for Proper Postnatal Muscle Growth (A) Experimental regime showing mouse strains and experimental time course. (B) Representative images of transverse muscle sections from control and Mps1Δ f/f postnatal mice stained with anti-myogenin (red) (arrows), anti-GFP (green), and DAPI (blue). (C) Quantification of myogenic cells in transverse sections as assessed by staining for PAX7, MYOD, and myogenin. For Mps1Δ f/f animals, only eYFP⁺ cells were counted. n = 3 mice, with 8–10 muscle sections counted per tissue. (D) EdU incorporation of myogenic cells collected by FACS fromP7 control and Mps1Δ f/f mice. Only eYFP⁺ cells were counted in Mps1Δ f/f mice. n = 3 mice per group, with 200–300 cells counted per condition. (E) Transverse muscle in postnatal control and Mps1Δ f/f mice stained with anti-laminin and DAPI. (F and G) Quantification of mean extensor digitorum longus (EDL) muscle fiber CSA (F) and whole EDL muscle CSA (G). n = 3 mice. The scale bars represent 20 μm. Data are presented as mean ± SEM. ^∗p < 0.05, Student’s t test. See also Figures S2–S4.

Figure 4

Disruption of Mps1 Function Induces a Rapid G1 Arrest in Myogenic Progenitors Top: experimental regime showing myoblast treatment and phenotypic assays. Satellite cell-derived myoblasts from Mps1Δ f/f adult mice treated with either Adeno-GFP (control) or Adeno-GFP-CRE (Mps1Δ f/f) virus and recovered in high-serum media. (A) Cell growth relative to initial plating density (n = 100–200 cells/well on day 0). n = 3 mice per condition, with 100–200 cells plated in triplicate wells. (B) Quantification of BrdU incorporation in control and Mps1Δ f/f myoblasts after a 24-hr pulse; n = 3 mice per condition, with 200–500 cells counted in triplicate wells. (C) Representative images of control and Mps1Δ f/f myoblasts treated with nocodazole showing GFP (to detect viral infection) and cells in mitosis (pH3⁺). DAPI stains nuclei. (D) Quantification of progenitors stalled in mitosis (pH3⁺) from control and Mps1Δ f/f myoblasts after nocodazole treatment. Between 1,000 and 2,000 cells were counted per condition. The experiment was done in biological triplicate. (E) FACS analysis of the cell-cycle distribution of control and Mps1Δ f/f myoblasts. (F) Relative transcript levels of CyclinD1 between control and Mps1Δ f/f myoblasts Triplicate data are from n = 3 mice. (G and H) Aneuploidy index scored from interphase FISH analysis of chromosomes 15 (green) and chromosome 19 (red). Arrows indicate aneuploid cells; n = 100–150 cells per condition. The experiment was done in biological triplicate. The scale bars represent 50 μm (C) and 10 μm (G). Data are presented as mean ± SEM. ^∗p < 0.05, ^∗∗p < 0.01, Student’s t test. See also Figures S5 and S6.

Figure 5

Differentiation Is Aborted in Myoblasts Lacking Kinetochore Binding of Mps1 Top: experimental regime showing myoblast treatment and phenotypic assays. Satellite cell-derived myoblasts from Mps1Δ f/f adult mice treated with either Adeno-GFP (control) or Adeno-GFP-CRE (Mps1Δ f/f) virus and subjected to cell-fate assays in either high- or low-serum culture conditions after recovery. (A) FACS plots showing Annexin-V and 7AAD staining to quantify necrotic and apoptotic cells in control and Mps1Δ f/f myoblasts 24 hr after virus treatment. Experiments were performed in triplicate; n = 3 mice. (B and C) Representative images (B) and quantification (C) of senescence-associated β-galactosidase (SA-B gal) assays. Arrows highlight senescent cells. Experiments were counted in triplicate; n = 3 mice. (D and E) Representative images (D) and quantification (E) of low-density cultures from control and Mps1Δ f/f myoblasts switched to low-serum conditions. Cultures were stained with anti-GFP (green), anti-myogenin (white), anti-PAX7 (violet), and DAPI (blue). Experiments were counted in triplicate; n = 3 mice. (F) Relative transcript levels of myogenic-fate genes in high-serum (proliferation) and low-serum (differentiation) conditions. Triplicate data from n = 3 mice. (G and H) Images (G) and quantification (H) of control and Mps1Δ f/f myoblasts after switching to low-serum high-density conditions to test myogenic differentiation (anti-myosin heavy chain; green) and fusion potential. n = 3 trials for each experiment from three mice. (I and J) Immunofluorescence of pRB under differentiation conditions. n = 3 trials for each experiment from three mice. The scale bars represent 10 μm (B and I), 50 μm (D), and 20 μm (G). Data are presented as mean ± SEM. ^∗p < 0.05, ^∗∗p < 0.01, Student’s t test. See also Figure S6.

Figure 6

Mps1Δ f/f Myoblasts Undergo a p21-Mediated G1 Arrest (A–C) Satellite cell-derived myoblasts from Mps1Δ f/f adult mice treated with either Adeno-GFP (control) or Adeno-GFP-CRE (Mps1Δ f/f) virus and recovered in high-serum media. (A) Relative transcript levels of p21^CIP1 (p21). Triplicate data are from n = 3 mice. (B and C) Representative images (B) and quantification (C) of p21⁺ cells from control and Mps1Δ f/f cultures. The experiment was performed in biological triplicate, with 200–300 cells counted per condition. (D) Relative transcript levels of p21 from FACS-sorted myogenic cells from Mps1Δ f/f and control P7 pups as per Figure 3A. Triplicate data are from n = 3 mice. Only eYFP⁺ myogenic cells were isolated from Mps1Δ f/f pups. (E) Experimental regime to knock down p21 and p53 in Mps1Δ f/f myoblasts. (F and H) Cell proliferation (% EdU⁺ cells). (G and I) Terminal myogenic differentiation (% myogenin⁺ cells); 200–300 cells were counted per condition. Experiments were performed in biological triplicate. (J) Representative images of control and Mps1Δ f/f cultures treated with Scr-shRNA or p53KD and pulsed with EdU to mark cycling cells. Arrows indicate red (p53⁺, EdU⁺), white (p53⁺, EdU⁻), and yellow and blue (p53⁻, EdU⁺). (K) Mitotic arrest (pH3⁺) after nocodazole treatment; 1,000–2,000 cells were counted per condition. Experiments were performed in biological triplicate. (L) Aneuploidy index scored from interphase FISH analysis of chromosomes 15 (green) and chromosome 19 (red). n = 100–150 cells per condition. The experiment was performed in biological triplicate. The scale bars represent 5 μm (B) and 20 μm (J). Data are presented as mean ± SEM. ^∗p < 0.05, ^∗∗p < 0.01, Student’s t test. See also Figures S6 and S7.

Figure 7

Aging Does Not Impact SAC Function and Aneuploidy in Satellite Cells (A and B) Representative images of MAD2 protein on activated SCs during interphase (A) and mitosis (B). (C) Mitotic poison assay using nocodazole and monastrol treatment on adult and aged activated SCs; n = 3 mice, with 800–2,000 cells counted per condition. (D) Representative images of MAD2 localization in mitotic cells in the presence of nocodazole. (E) Aneuploidy index on activated satellite cells (2 days postisolation) and progenitor cells (5 days postisolation) measured by FISH on chromosomes 15 and 19. n = 3 mice, with 200 cells counted per condition. The scale bars represent 20 μm (A) and 5 μm (B and D). Data are presented as mean ± SEM. ^∗p < 0.05.