Asymmetric self-renewal and commitment of satellite stem cells in muscle

Abstract

Satellite cells play a central role in mediating the growth and regeneration of skeletal muscle. However, whether satellite cells are stem cells, committed progenitors, or dedifferentiated myoblasts has remained unclear. Using Myf5-Cre and ROSA26-YFP Cre-reporter alleles, we observed that in vivo 10% of sublaminar Pax7-expressing satellite cells have never expressed Myf5. Moreover, we found that Pax7(+)/Myf5(-) satellite cells gave rise to Pax7(+)/Myf5(+) satellite cells through apical-basal oriented divisions that asymmetrically generated a basal Pax7(+)/Myf5(-) and an apical Pax7(+)/Myf5(+) cells. Prospective isolation and transplantation into muscle revealed that whereas Pax7(+)/Myf5(+) cells exhibited precocious differentiation, Pax7(+)/Myf5(-) cells extensively contributed to the satellite cell reservoir throughout the injected muscle. Therefore, we conclude that satellite cells are a heterogeneous population composed of stem cells and committed progenitors. These results provide critical insights into satellite cell biology and open new avenues for therapeutic treatment of neuromuscular diseases.

Figure 1. Satellite Cells Are a Heterogeneous Population Based on Myf5 Expression

(A) 87% of Pax7⁺ satellite cells expressed β-Gal (left column), and 13% of Pax7⁺ cells were β-Gal⁻ (right column, n = 3 mice) on single fibers isolated from Myf5-nLacZ mice. (B) 90% of Pax7⁺ cells expressed YFP (left column), and 10% of Pax7⁺ cells were YFP⁻ (right column, n = 18 mice) on single fibers isolated from Myf5-Cre/ROSA-YFP mice. (C and D) Total number of Pax7⁺ satellite cells per EDL myofiber and percentage of Pax7⁺/YFP⁻ cells at different ages (n = 3, 4, 6, and 4 mice for 0.5-, 1-, 2-, and 6-month-old mice, respectively). Error bars indicate standard error of mean (SEM). (E) Sublaminar localization of Pax7⁺/YFP⁻ (left) and Pax7⁺/YFP⁺ (right) cells. Scale bars are 10 μm in (A) and (B) and 25 μm in (E).

Figure 2. Isolation of Pax7⁺/YFP⁻ and Pax7⁺/YFP⁺ Cells from Myf5-Cre/ROSA-YFP Muscle

(A) Top panels: FACS profile of cells from control mice with no staining. Bottom panels: FACS profile of Myf5-Cre/ROSA-YFP cells stained with PE conjugated to Sca-1, CD31, CD45 (PE-Lin), α7-integrin, and β1-integrin. The gates used to prospectively isolate α7-int⁺/β1-int⁺/Lin⁻/YFP⁻ and α7-int⁺/β1-int⁺/Lin⁻/YFP⁺ cells are shown by boxed or circled areas. The bottom right panel shows the α7-int and YFP profile after gating. (B) Pax7 expression in FACS-isolated YFP⁺ (upper panels) and YFP⁻ (lower panels) cells. (C) RT-PCR analysis of sorted cells showing the absence of Myf5, Cre, and YFP expression in the YFP⁻ cells. Myoblasts indicates cultured primary myoblasts isolated from Myf5-Cre/ROSA-YFP mice. NTC indicates no template control. (D) Absence of Myf5 protein in FACS-isolated YFP⁻ cells. Scale bars are 25 μm.

Figure 3. Clonal Analysis Implies a Developmental Relationship between Pax7⁺/Myf5⁻ and Pax7⁺/Myf5⁺ Satellite Cells

(A) Satellite-cell-derived clonal clusters from Myf5-Cre/ROSA-YFP reporter mice after 3 days in suspended culture. Shown is a clonal cluster of cells that uniformly express YFP. (B) Shown is a clonal cluster containing both Pax7⁺/YFP⁺ (arrow) and Pax7⁺/YFP⁻ (arrowhead) cells. (C) A couplet of sister cells with one Pax7⁺/YFP⁻ cell (arrow head) and one Pax7⁺/YFP⁺ cell (arrow) after 2 days in culture. (D) Extensive in vivo proliferation of satellite cells on a regenerating EDL myofiber 4 days post CTX injection. The majority of satellite cells have undergone mitosis as indicated by the presence of two adjacent Pax7⁺ nuclei (double arrows) within a single satellite cell niche. (E) Couplet of identical sister cells that were both Pax7⁺/YFP⁺. (F) Couplet of sister cells with one Pax7⁺/YFP⁻ cell (arrowhead) and one Pax7⁺/YFP⁺ cell (arrow). (G) Satellite cell couplets expressed the meta-phase mitosis marker phosphorylated his-tone-H3 (H3-P). (H) Satellite cell couplets expressed the proliferation marker Ki67. β-Gal labeling denotes Myf5-nLacZ expression. Scale bars are 20 μm in (A), (B), and (D) and 10 μm in (C), (E), (F), (G), and (H).

Figure 4. Pax7⁺/Myf5⁻ Satellite Cells Give Rise to Pax7⁺/Myf5⁺ Satellite Cells

Time-lapse image analysis showing a YFP⁻ satellite cell (arrowhead at T = 0) give rise to YFP⁺ (arrows) and YFP⁻ (arrowheads) daughter cells. EDL myofibers isolated from Myf5-Cre/ROSA-YFP reporter mouse were plated on Matrigel, and satellite cell growth was monitored with a live imaging system. Cell division occurred at approximately 0 min (around 36 hr in real time), and YFP expression became apparent in the lower daughter cell at 240 min. Scale bar is 25 μm.

Figure 5. Orientation of Cell Division within the Satellite Cell Niche Determines Cell Fate

Regenerating EDL myofibers were isolated from Myf5-Cre/ROSA-YFP (C–F) or Myf5-nLacZ (G–H) reporter mice and fixed immediately for labeling of Pax7 and YFP or β-Gal. Myofibers were horizontally oriented in all panels. (A and B) Schematics of planar (A) and apical-basal (B) oriented sister cells and the corresponding frequency of symmetric (Sym) versus asymmetric (Asym) expression of Myf5 (based on YFP or β-Gal). Satellite cells (red and green cells) are located beneath the basal lamina (green) and are adjacent to the myofiber (brown). (C and D) Newly formed sister cells (double arrows) were both located under the basal lamina (Lam) but were separated from myofiber by M-cadherin (M-Cad) regardless of their relative orientation (C, planar; D, apical-basal). (E and F) Examples of planar (double arrows) and apical-basal (arrow and arrowhead)-oriented Pax7⁺ sister cells that displayed symmetric and asymmetric YFP expression, respectively. (G and H) Examples of planar (double arrows in G) and apical-basal (arrow and arrowhead in H)-oriented Pax7⁺ sister cells that displayed symmetric and asymmetric β-Gal expression, respectively. Centrally located myonuclei indicated the regenerating status of the myofibers. Scale bar is 12.5 μm.

Figure 6. A Role for Notch Signaling in Self-Renewal of Pax7⁺/Myf5⁻ Satellite Cells

(A) RT-PCR analysis of FACS-purified YFP⁻ and YFP⁺ satellite cells from Myf5-Cre/ROSA-YFP muscles. (B) Fresh isolated Myf5-nLacZ myofiber showing Pax7⁺/β-Gal⁻ (arrowhead) and Pax7⁺/β-Gal⁺ (arrow) satellite cells (B1) and their corresponding expressions of Delta-1 (B2). (C) Satellite-cell-derived cultures showing differential expression of Delta-1 in Pax7⁺/Myf5⁻ (arrowhead) and Pax7⁺/Myf5⁺ (arrows) cells. (D) High levels of Delta-1 were observed in Pax7⁺/Myf5⁺ cells and low levels of Delta-1 in Pax7⁺/Myf5⁻ cells in newly formed sister doublets. (E) Abrogation of satellite cell self-renewal by the γ-secretase inhibitor DAPT. (E1–E2) Expression of the self-renewing marker Pax7 and myogenic differentiation marker MyoD in colonies of satellite cell progenies in control (E1) and DAPT (E2)-treated (3 day) myofiber cultures. Notice the lack of Pax7 expression in DAPT-treated cells. (E3–E5) Quantification of cell number (E3) and the relative proportion of Pax7⁺ (E4) and MyoD⁺ (E5) cells. Numbers in the graphs indicate n. Error bars indicate SEM. Scale bar is 15 μm.

Figure 7. Transplanted Pax7⁺/Myf5⁻ Cells Extensively Contribute to the Satellite Cell Compartment

TA muscles of Pax7^−/− mice were engrafted with freshly sorted YFP⁺ and YFP⁻ cells from Myf5-Cre/ROSA-YFP mice. Three weeks later, transplanted muscles were examined for the expression of YFP and Pax7. (A and B) Transplanted YFP⁺ (A) and YFP⁻ (B) satellite cells both differentiated into YFP⁺ myofibers; however, YFP⁻ satellite cells also gave rise to several Pax7⁺/YFP⁻ cells associated with host (YFP⁻) myofibers. (C) Transplanted YFP⁺ cells gave rise to a satellite cell (arrowhead) that was associated with a YFP⁺ fiber. (D) Transplanted YFP⁻ satellite cells gave rise to several Pax7⁺/YFP⁻ (arrows) satellite cells that migrated onto host fibers (YFP⁻) and located beneath the basal lamina (white staining). Notice that one Pax7⁺/YFP⁺ (arrowhead) cell closely associated with a YFP⁺ myofiber. (E) TA muscle transplanted with YFP⁺ satellite cells showing numerous nuclei surrounding a YFP⁺ myofiber and two interstitial Pax7⁺/YFP⁺ cells (arrowheads) indicative of host immune rejection. (F) TA muscle transplanted with YFP⁻ satellite cells showing several Pax7⁺/YFP⁻ cells (arrows) and two YFP⁺ cells (arrowheads) associated with host (YFP⁻) myofibers and minimal immune rejection. (G and H) Number of donor-derived satellite cells/TA cross-section after transplantation of YFP⁺ (G) and YFP⁻ (H) satellite cells (n = 6 mice). Error bars indicate SEM. Scale bars are 25 μm in (A) and (B) and 12.5 μm in (C)–(F).