Finding a Needle in a Haystack: Identification of a Beige Fat Progenitor

Oguri and colleagues use single-cell RNA sequencing to identify a beige adipocyte precursor cell that gives rise to thermogenic adipocytes in subcutaneous adipose tissue. These beige fat progenitors are marked by PDGFRα, Sca1, and CD81 and proliferate upon activation of FAK-signaling in response to the cold and irisin. Oguri and colleagues use single-cell RNA sequencing to identify a beige adipocyte precursor cell that gives rise to thermogenic adipocytes in subcutaneous adipose tissue. These beige fat progenitors are marked by PDGFRα, Sca1, and CD81 and proliferate upon activation of FAK-signaling in response to the cold and irisin. Adipose tissue can undergo dramatic remodeling in response to external cues such as low temperature. With prolonged cold exposure, thermogenic cells called beige adipocytes emerge within the white adipose tissue as clusters of cells (Wu et al., 2012Wu J. Boström P. Sparks L.M. Ye L. Choi J.H. Giang A.H. Khandekar M. Virtanen K.A. Nuutila P. Schaart G. et al.Beige adipocytes are a distinct type of thermogenic fat cell in mouse and human.Cell. 2012; 150: 366-376Abstract Full Text Full Text PDF PubMed Scopus (2018) Google Scholar). This adaptive response provides metabolic flexibility that allows organisms to shift metabolism from storing energy to utilizing energy to generate heat. This metabolic transformation is driven by the expansion of mitochondria, which are the major site of thermogenic potential in cells. The complex nature of the adipose tissue, which has many different cell types that include adipocytes, adipocyte precursors, endothelial cells, macrophages, T cells, B cells, and neutrophils, has thwarted efforts to understand which precursors undergo metabolic programming during cold exposure. In this issue of Cell, Oguri et al., 2020Oguri Y. Shinoda K. Kim H. Alba D.L. Bolus W.R. Wang Q. Brown Z. Pradhan R.N. Tajima K. Yoneshiro T. et al.CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.Cell. 2020; 182 (this issue): 563-577Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar pinpoint a beige adipocyte precursor cell (APC) that gives rise to thermogenic adipocytes in subcutaneous fat (Figure 1). Beige adipocytes were initially described as adipocytes rich in mitochondria, having multilocular lipid droplets, and expressing the mitochondrial uncoupling protein 1 (Bouillaud et al., 1985Bouillaud F. Ricquier D. Thibault J. Weissenbach J. Molecular approach to thermogenesis in brown adipose tissue: cDNA cloning of the mitochondrial uncoupling protein.Proc. Natl. Acad. Sci. USA. 1985; 82: 445-448Crossref PubMed Scopus (208) Google Scholar). They have been observed in people exposed to harsh winters and rodents challenged with cold exposure (Frontini and Cinti, 2010Frontini A. Cinti S. Distribution and development of brown adipocytes in the murine and human adipose organ.Cell Metab. 2010; 11: 253-256Abstract Full Text Full Text PDF PubMed Scopus (292) Google Scholar). Because of their inducible recruitment in humans, there has been an interest in understanding the origins of these cells and the impact on energy metabolism (Cypess et al., 2009Cypess A.M. Lehman S. Williams G. Tal I. Rodman D. Goldfine A.B. Kuo F.C. Palmer E.L. Tseng Y.H. Doria A. et al.Identification and importance of brown adipose tissue in adult humans.N. Engl. J. Med. 2009; 360: 1509-1517Crossref PubMed Scopus (2794) Google Scholar). Several studies have revealed that increasing the abundance of beige adipocytes improves glucose metabolism, prevents adipose tissue fibrosis, protects against hepatic steatosis and weight gain (Seale et al., 2011Seale P. Conroe H.M. Estall J. Kajimura S. Frontini A. Ishibashi J. Cohen P. Cinti S. Spiegelman B.M. Prdm16 determines the thermogenic program of subcutaneous white adipose tissue in mice.J. Clin. Invest. 2011; 121: 96-105Crossref PubMed Scopus (802) Google Scholar; Pearson et al., 2019Pearson S. Loft A. Rajbhandari P. Simcox J. Lee S. Tontonoz P. Mandrup S. Villanueva C.J. Loss of TLE3 promotes the mitochondrial program in beige adipocytes and improves glucose metabolism.Genes Dev. 2019; 33: 747-762Crossref PubMed Scopus (10) Google Scholar; Shao et al., 2016Shao M. Ishibashi J. Kusminski C.M. Wang Q.A. Hepler C. Vishvanath L. MacPherson K.A. Spurgin S.B. Sun K. Holland W.L. et al.Zfp423 Maintains White Adipocyte Identity through Suppression of the Beige Cell Thermogenic Gene Program.Cell Metab. 2016; 23: 1167-1184Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar). Lineage tracing studies have suggested that beige adipocytes arise from APCs expressing cell surface markers like platelet-derived growth factor receptor (PDGFRa) and Ly6a (Stem cell antigen 1 [Sca1]) (Lee et al., 2012Lee Y.H. Petkova A.P. Mottillo E.P. Granneman J.G. In vivo identification of bipotential adipocyte progenitors recruited by β3-adrenoceptor activation and high-fat feeding.Cell Metab. 2012; 15: 480-491Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar). However, these progenitor cells can be found in multiple adipose tissue depots, and yet the inguinal white adipose tissue is one of the few depots where beige adipocytes emerge (Wu et al., 2012Wu J. Boström P. Sparks L.M. Ye L. Choi J.H. Giang A.H. Khandekar M. Virtanen K.A. Nuutila P. Schaart G. et al.Beige adipocytes are a distinct type of thermogenic fat cell in mouse and human.Cell. 2012; 150: 366-376Abstract Full Text Full Text PDF PubMed Scopus (2018) Google Scholar). This observation has prompted the search for beige adipocyte precursors that acquire thermogenic characteristics. Here, Oguri et al., 2020Oguri Y. Shinoda K. Kim H. Alba D.L. Bolus W.R. Wang Q. Brown Z. Pradhan R.N. Tajima K. Yoneshiro T. et al.CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.Cell. 2020; 182 (this issue): 563-577Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar use single cell transcriptomic analysis to identify precursor cells within the white adipose tissue that give rise to beige adipocytes. Using lineage negative (Lin−) stromal cells in the subvascular fraction of mouse adipose tissue depots from interscapular brown adipose tissue (iBAT), inguinal white adipose tissue (iWAT), and epididymal white adipose tissue (eWAT), they analyzed a subpopulation of cells with progenitor markers, including Acta2 (encoding alpha smooth muscle actin [a-SMA]), PDGFRa, and Sca1. To isolate this population, they searched for cell surface markers and found enrichment of 370 transcripts that encode for cell surface proteins, including PDGFRa, Ly6a (Sca1), and CD81. Isolation and subsequent differentiation of CD81+ APCs from iWAT generated thermogenic beige adipocytes in vitro, and in vivo when transplanted into immunodeficient nude mice. CD81+ APC transplants contained multilocular adipocytes that expressed thermogenic genes, including UCP1, Cidea, Pgc1α, and Cox8b, and responded to norepinephrine, a β3-adrenergic receptor ligand, whereas CD81 negative transplants failed to express the thermogenic gene expression program. Lineage tracing studies using CD81-CreERT2; Rosa26-mTmG showed that cold-induced UCP1+ beige adipocytes in the iWAT were derived from CD81+ cells, whereas iBAT showed limited labeling of UCP1+ brown adipocytes. These findings were consistent with observations that beige and brown adipocytes have distinct precursors (Kajimura et al., 2009Kajimura S. Seale P. Kubota K. Lunsford E. Frangioni J.V. Gygi S.P. Spiegelman B.M. Initiation of myoblast to brown fat switch by a PRDM16-C/EBP-beta transcriptional complex.Nature. 2009; 460: 1154-1158Crossref PubMed Scopus (492) Google Scholar). Gene ontology analysis revealed notable characteristics in CD81+ APCs from iWAT, including enrichment of pathways related to cell growth and the cell cycle. Consistent with these observations, Oguri et al., 2020Oguri Y. Shinoda K. Kim H. Alba D.L. Bolus W.R. Wang Q. Brown Z. Pradhan R.N. Tajima K. Yoneshiro T. et al.CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.Cell. 2020; 182 (this issue): 563-577Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar found that CD81+ APCs were highly proliferative and had a higher number of Ki67+ precursors when compared to CD81- APCs, indicating that cells were engaged in the cell cycle. CD81+ precursors had a greater proliferative advantage when isolated from iWAT but not eWAT. There were notable strain differences in the number of CD81+ APCs that could explain why some strains are protected against diet-induced obesity. The observation that the 129SVE strain has a greater number of beige adipocytes within the subcutaneous white adipose tissue when compared to the obesity- and diabetes-prone C57BL6J strain could be explained by their differences in abundance of CD81+ APCs. Remarkably, Oguri et al., 2020Oguri Y. Shinoda K. Kim H. Alba D.L. Bolus W.R. Wang Q. Brown Z. Pradhan R.N. Tajima K. Yoneshiro T. et al.CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.Cell. 2020; 182 (this issue): 563-577Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar found that CD81+ APCs from the 129SVE strain had greater proliferative advantage when compared to those from C57BL6J. In addition, older mice, who have few beige adipocytes, had fewer proliferating CD81+ APCs when compared to young mice, suggesting that CD81+ APCs are more likely to expand and contribute to newly differentiating beige adipocytes. There were some clues that suggested that CD81+ precursors from iWAT were unique. Using transcriptional profiling to compare CD81+ and CD81− cells. They found that CD81+ cells from iWAT were enriched for signaling pathways involved in focal adhesion and elevated levels of focal adhesion kinase (FAK). Pharmacological inhibition of FAK or short hairpin RNAs targeting FAK reduced cell proliferation. CD81 forms a complex with αV/β1 and αV/β5 integrins and mediates the activation of integrin-FAK signaling to promote cell proliferation. This pathway directly responds to the hormone Irisin, an exercise-induced myokine secreted by muscle that was previously shown to induce beige fat biogenesis (Wu et al., 2012Wu J. Boström P. Sparks L.M. Ye L. Choi J.H. Giang A.H. Khandekar M. Virtanen K.A. Nuutila P. Schaart G. et al.Beige adipocytes are a distinct type of thermogenic fat cell in mouse and human.Cell. 2012; 150: 366-376Abstract Full Text Full Text PDF PubMed Scopus (2018) Google Scholar). Irisin treatment of iWAT APCs rapidly activates FAK signaling and cell proliferation. To test the impact of CD81 on in vivo beige fat biogenesis, Oguri et al., 2020Oguri Y. Shinoda K. Kim H. Alba D.L. Bolus W.R. Wang Q. Brown Z. Pradhan R.N. Tajima K. Yoneshiro T. et al.CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.Cell. 2020; 182 (this issue): 563-577Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar created CRISPRi-Cd81 mice. Compared to controls, iWAT of CRISPRi-Cd81 had few beige adipocytes in iWAT when mice were cold challenged. iWAT from CRISPRi-Cd81 mice had reduced expression of thermogenic genes and oxygen consumption, whereas adipogenic genes like PPARγ were largely unaffected. As a result, loss of CD81 made mice susceptible to high-fat diet-induced weight gain, glucose intolerance, insulin resistance, and adipose tissue inflammation. Upon examination of human subcutaneous adipose tissue, Oguri et al., 2020Oguri Y. Shinoda K. Kim H. Alba D.L. Bolus W.R. Wang Q. Brown Z. Pradhan R.N. Tajima K. Yoneshiro T. et al.CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.Cell. 2020; 182 (this issue): 563-577Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar found that individuals with a greater number of CD81+ APCs were more likely to be lean and have improved fasting blood glucose levels and measures of insulin sensitivity. Together these findings would suggest that an increase in CD81+ APCs has predictive value in determining which individuals will be protected against acquiring metabolic risk factors. Together, Oguri et al., 2020Oguri Y. Shinoda K. Kim H. Alba D.L. Bolus W.R. Wang Q. Brown Z. Pradhan R.N. Tajima K. Yoneshiro T. et al.CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.Cell. 2020; 182 (this issue): 563-577Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar identify a unique CD81 cell surface marker on beige APCs from subcutaneous white adipose tissue. CD81 is required to drive proliferation of beige APCs in response to external cues. The proliferation of these cells is regulated by cold exposure and contributes to beige fat biogenesis in vivo. Therapeutic interventions that increase the number of thermogenic adipocytes could be used to enhance energy expenditure and promote weight loss. C.J.V. acknowledges support from the National Institutes of Health through funding of R01DK103930 . CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK SignalingOguri et al.CellJuly 1, 2020In BriefA subset of adipocyte progenitor cells give rise to beige fat through signaling responses to irisin through the action of specific integrins and the co-receptor CD81. Full-Text PDF Open Archive