Long-Term In Vitro Expansion of Salivary Gland Stem Cells Driven by Wnt Signals

Abstract

Adult stem cells are the ultimate source for replenishment of salivary gland (SG) tissue. Self-renewal ability of stem cells is dependent on extrinsic niche signals that have not been unraveled for the SG. The ductal compartment in SG has been identified as the location harboring stem cells. Here, we report that rare SG ductal EpCAM(+) cells express nuclear β-catenin, indicating active Wnt signaling. In cell culture experiments, EpCAM(high) cells respond potently to Wnt signals stimulating self-renewal and long-term expansion of SG organoids, containing all differentiated SG cell types. Conversely, Wnt inhibition ablated long-term organoid cultures. Finally, transplantation of cells pre-treated with Wnt agonists into submandibular glands of irradiated mice successfully and robustly restored saliva secretion and increased the number of functional acini in vivo. Collectively, these results identify Wnt signaling as a key driver of adult SG stem cells, allowing extensive in vitro expansion and enabling restoration of SG function upon transplantation.

Graphical abstract

Figure 1

EpCAM-Expressing Ductal Cells Have the Potential to Be Wnt Activated (A) Immunofluorescence staining of EpCAM in striated (SD), intercalated (ID), and excretory ducts (ED). (B–D) β-Catenin (red) co-localizes with EpCAM (green) in salivary gland ducts. (E) Scattergram of co-localization of EpCAM and β-catenin. Different regions of interests (ROI) show which pixels (arrows) are included in the analysis. For each pixel in the fluorescent image, the two intensities (green, red) are used as coordinates in the scattergram. Images analyzed with the ImageJ “Colocalization_Finder” plugin (Christophe Laummonerie; 2006/08/29: Version 1.2). (F) Nuclear localization of β-catenin in rare basal cells (arrow) of excretory ducts. Top: β-catenin; center: DAPI; bottom: overlay. Scale bars represent 20 μm (A–E) and 5 μm (F).

Figure 2

Single EpCAM^high Cells Generate Spheres and Miniglands (A) Representative FACS gating strategy for the analysis of ductal cells in the salivary gland. Left panel shows the exclusion of lineage marker-expressing cells. Right panel depicts the distribution of EpCAM^high, EpCAM^med, and EpCAM^neg cells in dissociated adult mouse salivary gland. FSC, forward scatter. (B) Sphere-forming efficiency of EpCAM^high, EpCAM^med, and EpCAM^neg populations (^∗∗p < 0.005). Data are expressed as the mean ± SEM of three independent experiments. (C) Differential interference contrast image of a growing minigland until 9 days (d) of culture. (D) Representative example of a salisphere and a minigland originating from single EpCAM^high in 9-day-old culture. (E) Toluidine blue staining shows uniform lumen formation throughout minigland (arrows). Scale bars represent 100 μm (C, D) and 10 μm (E).

Figure 3

Single-Cell-Derived Miniglands Acquire Fate of Salivary Gland Cells (A–D) Confocal images (z-stack projection) for salivary gland specific markers. (A) CK18 (red, ductal cells); (B) Aqp5 (green, acinar cells); (C) Overlay of CK18 (red) and Aqp5 (green); (D) α-SMA (purple, myoepithelial cells). Counterstain, Hoechst 33342 (blue). (E) Electron microscopy demonstrates serous (arrows) and mucous (arrowheads) acinar cells and myoepithelial cells (asterisk). Data are accessible at http://www.nanotomy.org, Salivary gland organoid. Lu, lumen. (F) Confocal images (z-stack projection) for CK5 (red, embryonic SG progenitor cells). Counterstain, Hoechst 33342 (blue). Scale bars represent 30 μm (A–D [upper panel], F [left]); 10 μm (A–D [lower panel], F [right]); and 10 μm, 5 μm, and 2 μm in left, center, and right panels of (E), respectively.

Figure 4

In Vitro Expansion of Salivary Gland Stem Cells (A) Scheme showing isolation method of SGSCs and establishment of long-term salivary gland organoid culture. (B–F) Differential interference contrast (DIC) images of salivary gland organoid cultures grown in enriched medium (EM) (B) (Nanduri et al., 2014) in combination with DMSO (C) or Wnt antagonists IWR-1-endo (D), IWP-2 (E), and sFRP1 (F). Scale bars, 100 μm. (G and H) Population kinetics of salisphere-derived salivary gland organoid cultures during inhibition of Wnt pathway grown in EM representing population doubling (G) and sphere-forming capability (H). ^∗∗∗p < 0.001. Data are expressed as the mean ± SEM of three independent experiments. n.s., not significant. (I–M) DIC images of salivary gland organoid cultures grown in WRY medium (I) in combination with DMSO (J) or Wnt antagonists IWR-1-endo (K), IWP-2 (L), and sFRP1 (M). Scale bars, 100 μm. (N and O) Population kinetics of salisphere-derived salivary gland organoid cultures during inhibition of Wnt pathway grown in WRY medium representing population doubling (N) and sphere-forming capability (O). ^∗∗∗p < 0.001. Data are expressed as the mean ± SEM of three independent experiments. n.s., not significant. (P) Population dynamics plot of salisphere self-renewal culture (for formula see Experimental Procedures). WRY, minimal media supplemented with Wnt3a, R-spondin1, and y-27632. Data are expressed as the mean of ± SEM of three independent experiments (G, H, N–P).

Figure 5

Transplantation of Cultured Wnt-Induced Cells Improves Function of Irradiated Salivary Gland Tissue (A) Scheme representing the transplantation protocol. (B) Transplants of 10,000 (blue) and 100 (cyan) passage 1 Wnt-induced cells in time-course analysis of relative saliva production in comparison with irradiated control animals (black). Irradiation time point (left dashed line), transplantation time point (right dashed line). Statistical analysis is shown in comparison with irradiated control group (^∗∗∗p < 0.001, ^∗∗p < 0.01 at relevant time points). Data are expressed as the mean ± SEM, n = 8 animals per time point. (C) Transplants of 10,000 (blue), 1,000 (purple), and 100 (cyan) passage 7 Wnt-induced cells in time-course analysis of relative saliva production in comparison with irradiated control animals (black). Statistical analysis is shown in comparison with irradiated control group (^∗∗∗p < 0.001 at relevant time points). Data are expressed as the mean ± SEM, n = 8 animals per time point. (D) Relative saliva production at 120 days after irradiation in animals transplanted with 100 and 10,000 passage 1, or 100, 1,000, and 10,000 passage 7 Wnt-induced cells per animal. Each data point represents a recipient animal. Note the uniform response of animals transplanted with passage 7 Wnt-induced cells.

Figure 6

Donor-Derived Cells Regenerate Destroyed Salivary Gland Tissue (A–E) H&E staining of SG tissue irradiated and transplanted with 100 (A), 1,000 (B) or 10,000 (C) Wnt-induced cells, irradiated control (D), and untreated control (E), showing presence of acini (asterisk). (F) Immunohistochemical staining for DsRed reveals incorporation of transplanted Wnt-induced cells into donor tissue and formation of ducts (arrows) and acini (arrowheads). Scale bars represent 100 μm (zoom-out panels) and 20 μm (zoom-in panels).