Vascular smooth muscle-derived Trpv1+ progenitors are a source of cold-induced thermogenic adipocytes.

Brown adipose tissue (BAT) and beige fat function in energy expenditure in part due to their role in thermoregulation, making these tissues attractive targets for treating obesity and metabolic disorders. While prolonged cold exposure promotes de novo recruitment of brown adipocytes, the exact sources of cold-induced thermogenic adipocytes are not completely understood. Here, we identify transient receptor potential cation channel subfamily V member 1 (Trpv1)+ vascular smooth muscle (VSM) cells as previously unidentified thermogenic adipocyte progenitors. Single-cell RNA sequencing analysis of interscapular brown adipose depots reveals, in addition to the previously known platelet-derived growth factor receptor (Pdgfr)α-expressing mesenchymal progenitors, a population of VSM-derived adipocyte progenitor cells (VSM-APC) expressing the temperature-sensitive cation channel Trpv1. We demonstrate that cold exposure induces the proliferation of Trpv1+ VSM-APCs and enahnces their differentiation to highly thermogenic adipocytes. Together, these findings illustrate the landscape of the thermogenic adipose niche at single-cell resolution and identify a new cellular origin for the development of brown and beige adipocytes.

FGF6 and FGF9 regulate UCP1 expression independent of brown adipogenesis.

Uncoupling protein-1 (UCP1) plays a central role in energy dissipation in brown adipose tissue (BAT). Using high-throughput library screening of secreted peptides, we identify two fibroblast growth factors (FGF), FGF6 and FGF9, as potent inducers of UCP1 expression in adipocytes and preadipocytes. Surprisingly, this occurs through a mechanism independent of adipogenesis and involves FGF receptor-3 (FGFR3), prostaglandin-E2 and interaction between estrogen receptor-related alpha, flightless-1 (FLII) and leucine-rich-repeat-(in FLII)-interacting-protein-1 as a regulatory complex for UCP1 transcription. Physiologically, FGF6/9 expression in adipose is upregulated by exercise and cold in mice, and FGF9/FGFR3 expression in human neck fat is significantly associated with UCP1 expression. Loss of FGF9 impairs BAT thermogenesis. In vivo administration of FGF9 increases UCP1 expression and thermogenic capacity. Thus, FGF6 and FGF9 are adipokines that can regulate UCP1 through a transcriptional network that is dissociated from brown adipogenesis, and act to modulate systemic energy metabolism.

Loss of BMP receptor type 1A in murine adipose tissue attenuates age-related onset of insulin resistance.

AIMS/HYPOTHESIS: Adipose tissue dysfunction is a prime risk factor for the development of metabolic disease. Bone morphogenetic proteins (BMPs) have previously been implicated in adipocyte formation. Here, we investigate the role of BMP signalling in adipose tissue health and systemic glucose homeostasis. METHODS: We employed the Cre/loxP system to generate mouse models with conditional ablation of BMP receptor 1A in differentiating and mature adipocytes, as well as tissue-resident myeloid cells. Metabolic variables were assessed by glucose and insulin tolerance testing, insulin-stimulated glucose uptake and gene expression analysis. RESULTS: Conditional deletion of Bmpr1a using the aP2 (also known as Fabp4)-Cre strain resulted in a complex phenotype. Knockout mice were clearly resistant to age-related impairment of insulin sensitivity during normal and high-fat-diet feeding and showed significantly improved insulin-stimulated glucose uptake in brown adipose tissue and skeletal muscle. Moreover, knockouts displayed significant reduction of variables of adipose tissue inflammation. Deletion of Bmpr1a in myeloid cells had no impact on insulin sensitivity, while ablation of Bmpr1a in mature adipocytes partially recapitulated the initial phenotype from aP2-Cre driven deletion. Co-cultivation of macrophages with pre-adipocytes lacking Bmpr1a markedly reduced expression of proinflammatory genes. CONCLUSIONS/INTERPRETATION: Our findings show that altered BMP signalling in adipose tissue affects the tissue's metabolic properties and systemic insulin resistance by altering the pattern of immune cell infiltration. The phenotype is due to ablation of Bmpr1a specifically in pre-adipocytes and maturing adipocytes rather than an immune cell-autonomous effect. Mechanistically, we provide evidence for a BMP-mediated direct crosstalk between pre-adipocytes and macrophages.

Increased mitochondrial activity in BMP7-treated brown adipocytes, due to increased CPT1- and CD36-mediated fatty acid uptake.

AIMS: Brown adipose tissue dissipates chemical energy in the form of heat and regulates triglyceride and glucose metabolism in the body. Factors that regulate fatty acid uptake and oxidation in brown adipocytes have not yet been fully elucidated. Bone morphogenetic protein 7 (BMP7) is a growth factor capable of inducing brown fat mitochondrial biogenesis during differentiation from adipocyte progenitors. Administration of BMP7 to mice also results in increased energy expenditure. To determine if BMP7 is able to affect the mitochondrial activity of mature brown adipocytes, independent of the differentiation process, we delivered BMP7 to mature brown adipocytes and measured mitochondrial activity. RESULTS: We found that BMP7 increased mitochondrial activity, including fatty acid oxidation and citrate synthase activity, without increasing the mitochondrial number. This was accompanied by an increase in fatty acid uptake and increased protein expression of CPT1 and CD36, which import fatty acids into the mitochondria and the cell, respectively. Importantly, inhibition of either CPT1 or CD36 resulted in a blunting of the mitochondrial activity of BMP7-treated cells. INNOVATION: These findings uncover a novel pathway regulating mitochondrial activities in mature brown adipocytes by BMP7-mediated fatty acid uptake and oxidation. CONCLUSION: In conclusion, BMP7 increases mitochondrial activity in mature brown adipocytes via increased fatty acid uptake and oxidation, a process that requires the fatty acid transporters CPT1 and CD36.

Insulin/IGF-I regulation of necdin and brown adipocyte differentiation via CREB- and FoxO1-associated pathways.

Brown adipose tissue plays an important role in obesity, insulin resistance, and diabetes. We have previously shown that the transition from brown preadipocytes to mature adipocytes is mediated in part by insulin receptor substrate (IRS)-1 and the cell cycle regulator protein necdin. In this study, we used pharmacological inhibitors and adenoviral dominant negative constructs to demonstrate that this transition involves IRS-1 activation of Ras and ERK1/2, resulting in phosphorylation of cAMP response element-binding protein (CREB) and suppression of necdin expression. This signaling did not include an elevation of intracellular calcium. A constitutively active form of CREB expressed in IRS-1 knockout cells decreased necdin promoter activity, necdin mRNA, and necdin protein levels, leading to a partial restoration of differentiation. By contrast, forkhead box protein (Fox)O1, which is regulated by the phosphoinositide 3 kinase-Akt pathway, increased necdin promoter activity. Based on reporter gene assays using truncations of the necdin promoter and chromatin immunoprecipitation studies, we demonstrated that CREB and FoxO1 are recruited to the necdin promoter, likely interacting with specific consensus sequences in the proximal region. Based on these results, we propose that insulin/IGF-I act through IRS-1 phosphorylation to stimulate differentiation of brown preadipocytes via two complementary pathways: 1) the Ras-ERK1/2 pathway to activate CREB and 2) the phosphoinositide 3 kinase-Akt pathway to deactivate FoxO1. These two pathways combine to decrease necdin levels and permit the clonal expansion and coordinated gene expression necessary to complete brown adipocyte differentiation.

New role of bone morphogenetic protein 7 in brown adipogenesis and energy expenditure.

Adipose tissue is central to the regulation of energy balance. Two functionally different types of fat are present in mammals: white adipose tissue, the primary site of triglyceride storage, and brown adipose tissue, which is specialized in energy expenditure and can counteract obesity. Factors that specify the developmental fate and function of white and brown adipose tissue remain poorly understood. Here we demonstrate that whereas some members of the family of bone morphogenetic proteins (BMPs) support white adipocyte differentiation, BMP7 singularly promotes differentiation of brown preadipocytes even in the absence of the normally required hormonal induction cocktail. BMP7 activates a full program of brown adipogenesis including induction of early regulators of brown fat fate PRDM16 (PR-domain-containing 16; ref. 4) and PGC-1alpha (peroxisome proliferator-activated receptor-gamma (PPARgamma) coactivator-1alpha; ref. 5), increased expression of the brown-fat-defining marker uncoupling protein 1 (UCP1) and adipogenic transcription factors PPARgamma and CCAAT/enhancer-binding proteins (C/EBPs), and induction of mitochondrial biogenesis via p38 mitogen-activated protein (MAP) kinase-(also known as Mapk14) and PGC-1-dependent pathways. Moreover, BMP7 triggers commitment of mesenchymal progenitor cells to a brown adipocyte lineage, and implantation of these cells into nude mice results in development of adipose tissue containing mostly brown adipocytes. Bmp7 knockout embryos show a marked paucity of brown fat and an almost complete absence of UCP1. Adenoviral-mediated expression of BMP7 in mice results in a significant increase in brown, but not white, fat mass and leads to an increase in energy expenditure and a reduction in weight gain. These data reveal an important role of BMP7 in promoting brown adipocyte differentiation and thermogenesis in vivo and in vitro, and provide a potential new therapeutic approach for the treatment of obesity.