Identification of an adipogenic niche for adipose tissue remodeling and restoration

Abstract

The regulatory events guiding progenitor activation and differentiation in adult white adipose tissue are largely unknown. We report that induction of brown adipogenesis by β3-adrenergic receptor (ADRB3) activation involves the death of white adipocytes and their removal by M2-polarized macrophages. Recruited macrophages express high levels of osteopontin (OPN), which attracts a subpopulation of PDGFRα+ progenitors expressing CD44, a receptor for OPN. Preadipocyte proliferation is highly targeted to sites of adipocyte clearance and occurs almost exclusively in the PDGFRα+ CD44+ subpopulation. Knockout of OPN prevents formation of crown-like structures by ADRB3 activation and the recruitment, proliferation, and differentiation of preadipocytes. The recruitment and differentiation of PDGFRα+ progenitors are also observed following physical injury, during matrix-induced neogenesis, and in response to high-fat feeding. Each of these conditions recruits macrophages having a unique polarization signature, which may explain the timing of progenitor activation and the fate of these cells in vivo.

Figure 1. ADRB3 Stimulation Induces Proliferation and Differentiation of PDGFRα⁺ Cells in gWAT

(A) Flow cytometric analysis of depot-specific proliferation of PDGFRα⁺ cells from gWAT and iWAT of control mice and mice treated with CL for 3 days and injected with EdU 2 hr before analysis. Lineage-negative cells were gated and analyzed for EdU incorporation and PDGFRα expression. Representative density plots and quantification of three independent experiments are shown (values are mean ± SEM; ***p < 0.001). (B–E) Immunohistochemical analysis of proliferation and differentiation of PDGFRα⁺ cells in paraffin sections of gWAT and iWAT. Proliferating cells were flashed labeled on the third day of CL treatment, and cellular phenotypes were examined 2 or 96 hr later. (B) Representative low (left)- and high-magnification (right) images of gWAT paraffin sections stained for EdU and PDGFRα. Most actively dividing cells expressed PGDFRα (arrows). (C) The level of proliferation of PDGFRα⁺ cells was significantly greater in gWAT versus iWAT. Data are mean ± SEM, n = 4 (***p < 0.001). (D) Ninety-six hours after flash labeling, most cells that divided on day 3 have differentiated into multilocular adipocytes with a central nucleus surrounded by PLIN⁺ lipid droplets and expressed UCP1 (arrows). (E) Although the rate of proliferation was far greater in gWAT versus iWAT (left), the differentiation of EdU⁺ cells into PLIN1⁺ (middle) and UCP1⁺ (right) multilocular adipocytes did not differ between adipocyte depots. Data are mean ± SEM, n =4 (***p < 0.001). Sections were counterstained with DAPI (blue). Scale bars, 20 µm. See also Figure S1.

Figure 2. Depot-Specific ADRB3-Mediated Proliferation in WAT Correlates with CLS Formation and Macrophage Recruitment

(A) Representative images of paraffin sections of gWAT stained for EdU, F4/80, and PLIN1on D3 of CL treatment along with a magnified view of the boxed regions (the bottom row). CLSs are indicated by asterisks in high-magnification fields. (B) Representative low-magnification images of paraffin sections of gWAT and iWAT stained for F4/80 (left). Quantification of CLS in 200× microscopic fields of paraffin sections of gWAT and iWAT from mice treated with CL for 3 days (mean ± SEM; n = 6, ***p < 0.001). CLS were defined as circular regions with continuous rim of F4/80⁺ cells. (C) A positive correlation between CLS number and mitotic index. Symbols represent data from individual microscopic fields, with a minimum 5 of 200× fields of gWAT paraffin sections from individual mice treated with CL for 3 days (n = 6). (D) Quantitative PCR analysis of Emr1 expression of gWAT and iWAT on D3 and D7 of CL treatment (mean ± SEM; n = 4; ***p < 0.001). (E) Quantification of F4/80⁺ cells in paraffin sections of gWAT and iWAT (mean ± SEM; n = 3; *p < 0.05). Scale bars, 100 µm. See also Figure S2.

Figure 3. PDGFRα⁺ Progenitors Are Recruited to CLS, where They Form a Zone of Proliferation

(A) Confocal images of whole-mount gWAT from tamoxifen-induced Pdgfra-CreER^T2/tdTomato mice stained for lipid (LipidTox, yellow) and F4/80 (green), showing close association of PDGFRα⁺ cells with CLS. (B) F4/80, PDGFRα, and EdU staining in paraffin sections of gWAT from mice treated with CL for 3 days. Left panel merges images of PDGFRα (red) and EdU (green) fluorescence. Arrows indicate proliferating PDGFRα⁺ cells. Middle panel is the merged image of F4/80 (red) with EdU (green), showing close association between proliferating cells and F480⁺ macrophages that form CLS. (C) Frequency distribution of PDGFRα⁺ cell density near CLS in gWAT from mice treated with CL for 3 days. (D) Frequency distribution of proliferating and nonproliferating PDGFRα⁺ cells near CLS. Bins are centered at 10 µm intervals. Insert shows data plotted as distance versus cumulative frequency. (E and F) Confocal images of cryosectioned gWAT from tamoxifen-induced Pdgfra-CreER^T2/tdTomato mice stained for MGL1 and lipid (E) or PLIN1 (F). Magnified views of boxed regions show that tdTomato⁺ multilocular adipocytes near CLS were labeled with lipid (E) or PLIN1 (F). Double positive cells are indicated by arrows. Scale bars, 20 µm. Nuclei were counterstained with DAPI (blue). See also Figure S3.

Figure 4. CLSs Induced by ADRB3 Stimulation Contain M2-Polarized Macrophages

(A) Quantitative PCR analysis of anti- and proinflammatory gene expression in gWAT from control (CTL) and CL-treated mice (CLd3). CL treatment significantly upregulated anti-inflammatory genes (M2 markers) by 3 days, whereas expression of proinflammatory markers was not affected by CL treatment (mean ± SEM; n = 4; *p < 0.05). (B and C) 3D confocal projection images of whole-mount gWAT from control mice or mice treated with CL for 3 days, stained for lipid (nile red), MGL1 (B and C, left panel, green) or PLIN1 (C, right panel, green). (C) MGL1⁺ macrophages surround a lipid droplet devoid of PLIN1, forming CLS. (D) Dissociated gWAT SVC from CL treated-mice (CLd3), stained for lipid (LipidTox, red) and F4/80 (green). Arrows indicate double positive cells. Nuclei were counterstained with DAPI (blue). Scale bars, 20 µm. (E) Flow cytometric quantification of large (FSC-A ^hi) MGL⁺ cells with intense LipidTox staining (Lipid^hi) from SVC of control mice and mice treated with CL for 3 days (mean ± SEM; n = 4;*p = 0.014). See also Figure S4.

Figure 5. ADRB3 Treatment Upregulates OPN Expression in CLS-Associated Macrophages and CD44 Expression in a Subpopulation of PDGFRα-Expressing Cells

(A) Quantitative PCR analysis of Spp1 expression in gWAT from control mice and mice treated with CL at indicated days after treatment (mean ± SEM; n = 4–6, *p < 0.05). (B) Immunoblot analysis of gWAT from control mice and mice treated with CL at indicated points. (mean ± SEM; n = 3–5, *p < 0.05, **p < 0.01, ***p < 0.001). CL increased levels of full-length and cleaved OPN. (C) Immunohistochemical detection of OPN in F4/80⁺ cells in paraffin sections of gWAT from mice treated with CL for 3 days. Nuclei were counterstained with DAPI. Scale bar, 20 µm. (D) FACS analysis of PDGFRα⁺CD44⁺ cells in SVC obtained from gWAT and iWAT of control mice and CL-treated mice (mean ± SEM; n = 3, **p < 0.01). (E and F)FACS analysis of CD44 expression and EdU incorporation in PDGFRα+ cells in SVC of gWAT from control mice and mice treated with CL for 3 days. Mice were injected with EdU 2 hr before sacrifice. (E) Representative flow profiles of each condition are shown. (F) Quantification of CD44⁺PDGFRα⁺ cells and EdU⁺CD44⁺ cells by FACS (mean ± SEM; n = 3; ***p < 0.001). (G) Quantitative PCR analysis of proliferation maker expression in FACS-isolated cells from gWAT of control mice or mice treated with CL for 3 days (n = 3–4, mean ± SEM; *p < 0.05, **p < 0.01, ***p < 0.001). (H) Effect of OPN on in vitro migration of PDGFRα⁺ cells from Pdgfra-H2BeGFP mice (mean ± SEM; n = 3; *p < 0.05, **p < 0.01). See also Figure S5.

Figure 6. OPN Is Required for CLS Formation and ADRB3-Mediated Adipogenesis from PDGFRα⁺ Cells

(A) Reduction in weight of gWAT from WT and Spp1-KO mice after 3 days of CL treatment (mean ± SEM; n = 4–6; **p < 0.01, ***p < 0.001). (B) Reduction in size of gWAT adipocytes from WT and Spp1-KO mice after 3 days of CL treatment (mean ± SEM; n = 4–6; **p < 0.01). (C) Immunohistochemical analysis of CLS in paraffin sections of gWAT from wild-type (WT) and Spp1-KO mice under control condition (CTL) or after 3 days of CL treatment (CLd3), stained for F4/80 (green). (D) Quantitative PCR analysis of macrophage marker expression in gWAT from control and Spp1KO mice (mean ± SEM; n=10–11 from two independent cohorts; *p < 0.05, **p < 0.01, ***p < 0.001). (E) FACS analysis of EdU incorporation in PDGFRα+ cells from gWAT of WT and Spp1-KO mice treated with CL for 3 days and injected with EdU 2 hr before analysis (mean ± SEM; n = 3–4; **p < 0.01). (F) Representative images of paraffin sections of gWAT stained for PLIN1 and EdU and quantification of PLIN1⁺EdU⁺ cells (mean ± SEM; n = 4; **p < 0.01). Scale bars, 100 µm except high-magnification field in (F), where scale bar, 20 µm. See also Figure S6.

Figure 7. PDGFRα⁺ Cells Interact with Macrophages in Adult WAT and Contribute to Adipogenesis during Tissue Repair, Tissue Neogenesis, and Nutritional Hyperplasia

(A) 3D projection images of CLS in iWAT whole mounts from Pdgfra-CreER^T2/tdTomato mice fed HFD for 8 weeks. (B) tdTomato⁺ adipocyte clusters that were found in iWAT of HFD-fed mice were not associated with CLS. (C and D) Needle injury induced the appearance of tdTomato⁺ adipocyte clusters in iWAT. Shown are cryosections of Pdgfra-CreER^T2/tdTomato mice 10 days after needle injury. (D) A magnified view of the boxed region from (C). Injured sites were marked with DiO (green, left). tdTomato⁺ multilocular adipocytes (arrows) contained PLIN1⁺ lipid droplets. (E and F) Representative confocal images of Matrigel plugs 7 days or 4 weeks after injection. (E) tdTomato⁺ progenitors and F4/80⁺ macrophages were the major cells infiltrating Matrigel plugs 7 days after injection. (F) Numerous tdTomato⁺ adipocytes formed 4 weeks after Matrigel injection. (G) Heatmap showing the unique gene expression profiles of macrophage-associated genes under different adipogenic conditions. Gene expression values are relative to each control condition. (n = 4–6; bold, p < 0.05). (H) PCA score plot illustrates distinct gene expression profiles across adipogenic conditions (CLd3, gWAT of mice treated with CL for 3 days; injury, iWAT of mice 3 days after needle injury; HFD, gWAT of mice fed HFD for 8 weeks; neogenesis, Matrigel plug 7 days after injection). Scale bars, 20 µm in (A), (B), and (D). Scale bars, 100 µm in (C), (E), and (F). See also Figure S7.