Wnt7a activates the planar cell polarity pathway to drive the symmetric expansion of satellite stem cells

Abstract

Satellite cells in skeletal muscle are a heterogeneous population of stem cells and committed progenitors. We found that quiescent satellite stem cells expressed the Wnt receptor Fzd7 and that its candidate ligand Wnt7a was upregulated during regeneration. Wnt7a markedly stimulated the symmetric expansion of satellite stem cells but did not affect the growth or differentiation of myoblasts. Silencing of Fzd7 abrogated Wnt7a binding and stimulation of stem cell expansion. Wnt7a signaling induced the polarized distribution of the planar cell polarity effector Vangl2. Silencing of Vangl2 inhibited Wnt7a action on satellite stem cell expansion. Wnt7a overexpression enhanced muscle regeneration and increased both satellite cell numbers and the proportion of satellite stem cells. Muscle lacking Wnt7a exhibited a marked decrease in satellite cell number following regeneration. Therefore, Wnt7a signaling through the planar cell polarity pathway controls the homeostatic level of satellite stem cells and hence regulates the regenerative potential of muscle.

Figure 1. Satellite Stem Cells Express the Wnt receptor Frizzled7

(A) Single myofibers isolated from Myf5-Cre/ROSA26-YFP mice. 90% of Pax7⁺ cells expressed YFP, and 10% of Pax7⁺ cells were YFP^-. Satellite cells uniformly expressed the stem cell marker CXCR4. (B) Gated satellite cells (α7-Integrin⁺, CD34⁺, CD45^-, CD31^-, CD11b^-, Sca1^-) extracted from resting limb skeletal muscle were separated on the basis of Myf5-Cre activated YFP fluorescence. (C) Real-time PCR analysis of sorted cells showing the absence of Myf5 and YFP transcripts as well as the expression of Fzd7 transcripts in YFP^- sorted cells (n=3). (D) Fzd7 was expressed specifically in quiescent Pax7⁺/YFP^- satellite stem cells (left) but not in Pax7⁺/YFP⁺ satellite myogenic cells (right) in freshly isolated Myf5-Cre/ROSA26-YFP myofibers (E) Proliferating satellite cells and myogenic precursor cells express Fzd7. Regenerating EDL myofibers were isolated 4 days after TA muscle injury. Both Pax7⁺/Myf5^- (left) and Pax7⁺/Myf5⁺ (right) dividing satellite cells expressed Fzd7. Bars are 10 μm. Errors bars represent SEM.

Figure 2. Wnt7a is Highly Upregulated During Muscle Regeneration

(A) Cryosections of resting (top) and freeze-injured TA muscles analyzed at 3 (middle) and 6 (bottom) days following injury. The basal lamina of myofibers is revealed by Laminin α2 chain staining and satellite cell nuclei were visualized by Pax7 staining. (B) Real-time PCR-array analysis of regenerating TA muscle 6-days following freeze-injury, revealed upregulation of Wnt7a mRNA at the time that satellite cells return to quiescence (n=3). (C) Recombinant Wnt7a protein binds Frizzled7 at the surface of myogenic cells, and this binding is abolished after knock-down of Frizzled7. (D) Wnt3a but not Wnt7a activates β-catenin/TCF target genes. Real-time PCR analysis of cultured myogenic cells after stimulation with BSA (control), and recombinant Wnt proteins. Only Wnt3a induced the transcription of the β-catenin/TCF target genes Tcf7 and Axin2 (n=5). (E) Wnt7a protein is expressed by regenerating myofibers, and not by vascular endothelial cells. Cryosections of 4-days cardiotoxin-induced regenerating (left) and resting contralateral (right) TA muscles. Sections were examined for the expression of Myogenin (differentiating myogenic cells), CD144 (endothelial cells) and Wnt7a proteins. Bars are 25 μm. Errors bars represent SEM.

Figure 3. Wnt7a-Frizzled7 Signaling Drives Satellite Stem Cell Expansion

(A) First division of Pax7⁺/YFP^- satellite stem cells, 42 hours after isolation of EDL single myofibers from Myf5-Cre/ROSA26-YFP mice, cultured in floating conditions. Satellite stem cells either give rise to one YFP^- stem cell and one YFP⁺ committed cell, via asymmetric cell division (left), or alternatively give rise to two YFP^- daughter cells by symmetric cell division (right). (B) Wnt7a but not Wnt3a stimulation markedly increased the proportion of symmetric cell divisions resulting in satellite stem cell expansion (n=3, *p=0.009). (C) Activated satellite cells on cultured myofibers at 42 h after isolation, do not express Fzd7 (bottom) after knock-down of Fzd7 with siRNA, as compared to cells in non-silencing conditions (top). (D) The Wnt7a-induced increase in the rate of symmetric satellite stem cell divisions was abrogated following silencing of Fzd7 on myofibers after 42 h of culture (n=3, *p<0.02). (E) The increase in symmetric satellite stem cell numbers induced by Wnt7a was blocked by silencing of Fzd7 on myofibers after 52 h of culture (n=3, *p<0.03). Bars are 10 μm. Errors bars represent SEM.

Figure 4. PCP Components are Expressed by Myogenic Cells

(A) Quantitative real-time PCR analysis indicated expression of PCP core component transcripts by YFP⁺ and YFP^- satellite cell-derived myoblasts (n=3). (B) Immunostaining indicated that Vangl2 is upregulated during activation of Pax7⁺ satellite cells by 24 h on cultured myofibers. (C) Wnt7a induces polarized Vangl2 cellular localization on opposite poles of dividing Pax7⁺ satellite cells on cultured myofibers. EDL myofibers were cultured in control medium or medium supplemented with Wnt7a and fixed 42 hours after isolation. (D) Effects of Wnt treatment on Vangl2 polarization during initial division. Wnt7a signaling, but not Wnt3a, induces polarized localization of Vangl2 and Fzd7 during satellite cell division (n=3, *p=0.006). (E) Wnt7a-treated myofibers were immunolocalized for Vangl2 and the membrane marker α7-Integrin. Vangl2 is polarized and co-localize to the membrane in planar-dividing satellite cells (arrows). Note the polarized and upregulated expression of α7-integrin, which facilitates adhesion to the basal lamina of both daughter cells. Bars are 10 μm. Errors bars represent SEM.

Figure 5. Vangl2 is Required for Symmetric Expansion of Satellite Stem Cells

EDL single myofibers from Myf5-Cre/ROSA26-YFP mice were cultured in floating conditions and subjected to either non-silencing or Vangl2 siRNA transfection. (A) Orientation of Pax7⁺/Syndecan4⁺ satellite cell first cell division at 42 hours. Divisions were scored either as planar (top) or apical-basal (bottom). Note, in myofiber culture, satellite cells translocate to the outside surface of the basal lamina and apical-basal cell divisions are directed into the media. (B) Wnt7a induces a significant decrease in the proportion of apical-basal cell divisions after 42h of culture supporting its function in stimulating stem cell expansion. Knock down of Vangl2 inhibits the ability of Wnt7a to stimulate planar cell divisions (n=3, *p<0.02). (C) The Wnt7a-induced increase in symmetric satellite stem cell divisions was abrogated following silencing of Vangl2 on myofibers after 42 h of culture (n=3, *p<0.02). (D) Activated satellite cells on myofibers knocked-down for Vangl2 after 52 h of culture do not express Vangl2 (bottom) as compared to cells in non-silencing conditions (top). (E) Knock-down of Vangl2 increased the rate of apical-basal divisions (n=5, *p=0.001). (F) Knock-down of Vangl2 decreased the proportion of Pax7⁺/YFP^- stem cells (n=3, *p=0.03). (G) Knock-down of Vangl2 decreased the number of cells per fibers (n=5, *p=0.001). (H and I) Silencing of Vangl2 increased the proportion of differentiating Myogenin⁺/Pax7^- cells myofibers after 3 days of culture (n=4, * p=10^-5). (J) Silencing of Vangl2 depleted the satellite cells pool (n=4, *p=0.001). (K) Vangl2 silencing promotes myogenic differentiation as revealed by Real-time PCR analysis of gene expression in satellite cell-derived myoblasts (n=4). Bars are 10 μm. Errors bars represent SEM.

Figure 6. Ectopic Wnt7a Enhances Muscle Regeneration

(A) Representative histology of regenerated TA muscles of 3-month old mice, 8 days following electrotransfer-induced injury. Regenerated myofibers show centrally-located nuclei. Bar is 25 μm. (B) Representative cryosections of TA muscles 3 weeks following electroporation with CMV-Wnt7a plasmid exhibit accelerated regeneration as evidenced by increased mass, and number and caliber of fibers. Electroporation with CMV-Wnt3a resulted in malformed muscle with abnormal accumulation of matrix. The basal lamina of myofibers was detected by Laminin α2 chain immunostaining. Bars are 200 μm. (C) Quantification of muscle fiber caliber in TA muscles electroporated with either saline or a Wnt7a / Wnt3a expression plasmids, as compared to contralateral leg, 3 weeks after electroporation (n=4, *p≤0.008). Wnt7a and Wnt3a have divergent effects on myofiber caliber. (D) Quantification of muscle fiber number in TA muscles electroporated with either saline or a Wnt7a / Wnt3a expression plasmids, as compared to contralateral leg, 3 weeks after electroporation (n=4, *p≤0.03). Errors bars represent SEM.

Figure 7. Wnt7a Drives Satellite Stem Cell Expansion

(A) TA muscles of 3-month old mice were electroporated with either saline or a Wnt7a / Wnt3a expression plasmid, and dissected after 3 weeks. Sublaminar Pax7⁺ satellite cells were scored on cryosections of electroporated muscles. Note the increased numbers of Pax7⁺ satellite cells following electroporation with CMV-Wnt7a plasmid. Bar is 25 μm (B) The satellite cell population was increased by two-fold following electroporation of CMV-Wnt7a plasmid (n=4, *p≤0.03), as compared to Saline- or Wnt3a- electroporated samples. (C) Satellite cells were FACS-sorted from electroporated Myf5-Cre/ROSA26-YFP TA muscles, 3 weeks after electroporation, and plated in culture for 24 hours, fixed and stained for Pax7 and YFP. Bar is 10 μm. (D) The proportion of Pax7⁺/YFP^- satellite stem cells was significantly increased following overexpression of Wnt7a in electroporated TA muscles (n=5, *p≤0.0001). (E) Wnt7a^-/- myofibers showed a reduced population of Pax7+ satellite cells on myofibers were isolated from EDL muscle. (n=4, *p=0.03). (F) Cryosections of freeze-injured TA muscles of 3-month old Wnt7a^-/- null mice and their littermate controls analyzed at 3 weeks following injury. No significant difference in terms of structure or cross-sectional area was observed in the regenerated muscle. Bar is 20 μm. (n=3). (G) Decreased numbers of satellite cells were observed in regenerated Wnt7a ^-/- TA muscles normalized to the number of myofibers in cross-sectional area and to the contralateral leg. (n=3, *p=0.03). Errors bars represent SEM.