Obesity is the predominant risk factor for an expanding array of diseases including: type 2 diabetes, heart disease, stroke and cancer. Our lab investigates the transcriptional pathways that control the development, differentiation and function of adipose cells in normal development and in obesity. We are particularly interested in early determination and specification events; this involves the commitment of mesenchymal stem cells to a preadipose cell fate. We are also exploring pathways that determine the fate (and thus the function) of different types of fat cells.
Mammals have two main subtypes of adipose tissue, white and brown. White adipose tissue is specialized for energy storage, whereas brown adipose expends chemical energy in the form of heat. White adipose tissue is found in the subcutaneous layer and in distinct intra-abdominal depots. Excess abdominal adiposity is associated with metabolic dysfunction, insulin resistance and heart disease. By contrast, expansion of subcutaneous fat is not correlated with insulin resistance or metabolic disease.
Brown fat can counteract obesity by safely burning off excess energy. Increased brown adipose function promotes a lean and healthy phenotype. Conversely, animals lacking brown adipose develop obesity and type 2 diabetes. Recent PET-based imaging studies suggests that the amount of activated brown adipose in humans is inversely correlated with body mass index and age. These results suggest that brown adipose plays an important and unappreciated role in human energy balance. Moreover, drug or cell-based approaches that increase the amount or function of brown adipose could provide novel therapies for obesity and its metabolic complications.
Stem Cells, Embryonic development, Adipocyte progenitors, Brown adipose tissue, White adipose tissue, PRDM16, PPARgamma
1. Brown Adipocyte Development and PRDM16
Brown fat cells are packed with mitochondria that express Uncoupling Protein-1 (UCP1) in their inner membrane. Brown fat tissue is also highly vascular, enabling it to efficiently distribute heat via the circulation. The molecular pathways that control the process of adipogenic differentiation from preadipocytes have been extensively studied in animal models and cultured cells. Preadipocytes are specialized fibroblast-like cells contained in white and brown fat tissues that differentiate into mature fat storing adipocytes in response to hormonal cues. PPARgamma and members of the c/EBP family of transcription factors orchestrate adipocyte differentiation in both white and brown cell types. Recently, several factors have been shown to influence the white versus brown adipose cell phenotype including: PGC-1alpha; FoxC2; pRb; p107 and RIP140.
In a global expression screen of all known mouse transcriptional components, we identified PRDM16 as a gene expressed selectively in brown adipose cells. PRDM16 is necessary and sufficient for the brown adipose differentiation and function. Ectopic expression of PRDM16 in white fat- or skeletal muscle- progenitors induces a complete program of brown adipogenesis; this includes: mitochondrial biogenesis; activation of thermogenic genes and uncoupled respiration in a cAMP-dependent manner; and suppression of white adipose or muscle development. Loss of PRDM16 from brown adipogenic precursors results in skeletal muscle development (Figure 1).
Primary mouse brown adipocyte precursors were infected with adenovirus expressing a shRNA directed against PRDM16 to knock-down its expression. PRDM16-depleted cells (marked by GFP (green) also expressed by the adenoviral vector) differentiated into skeletal myocytes marked in red by staining for Myosin Heavy Chain protein expression. Therefore, PRDM16 is required in brown adipocytes to suppress skeletal muscle development.
These data suggest that PRDM16 controls brown adipocyte versus skeletal muscle cell fate. Lineage tracing reveal that skeletal myogenic cells and brown adipose cells arise from similar (or common) progenitors during embryonic development (Figure 2). Elucidating the extrinsic factors that regulate brown adipose versus muscle cell development may uncover novel therapeutic avenues to expand brown adipose mass.
We are investigating the mechanistic basis by which PRDM16 influences brown adipogenic cell fate using cell-based and mouse genetic approaches.
Figure 2.(A) Lineage tracing experiments were performed using Myf5-CRE knock-in mice. In this system, Cre recombinase is expressed from a skeletal muscle selective gene, Myf5. These mice are intercrossed with indicator mice that have a YFP reporter gene integrated into the Rosa26 housekeeping gene locus downstream of a Floxed transcriptional stop sequence. Cre mediated excision of the Stop signal in Myf5-expressing cells allows permanent and heritable expression of YFP. Therefore, all cells that are descendant from Myf5-expressing cells are labeled by YFP (B) Direct immunofluorescence shows that the brown adipose tissue from Myf5-Cre+ reporter mice expresses YFP. (C) Immunohistochemistry for YFP expression (stained in red) shows that skeletal muscle and brown adipose (BAT) but not white adipose tissue (WAT) are descendant from a Myf5+ cell lineage.
2. Identification of regulatory genes and pathways in white adipogenic precursors
A major unresolved and central issue in the field of obesity and metabolism research is the identity of the adipogenic precursor cell(s) and pathways that regulate the proliferation and self-renewal of these cells in vivo. We are interested in developing approaches to characterize these precursors in normal development and in obesity. We seek to identify key regulatory components that control the activation, expansion and differentiation of these cells in response to developmental or metabolic cues.
|White preadipocytes were induced to differentiate in culture to form mature, lipid storing adipocytes. The cells are fixed and lipid droplets are stained red with oil-red-o.|
Please contact me if you are interested in discussing a rotation project in the lab.
Gupta RK, Arany Z, Seale P, Mepani RJ, Ye L, Conroe HM, Roby YA, Kulaga H, Reed RR, Spiegelman BM: Transcriptional Control of Preadipocyte Determination by
Zfp423. Nature March 2010.
Kajimura S, Seale P, Kubota K, Lunsford E, Frangioni JV, Gygi SP, Spiegelman BM.: Initiation of myoblast to brown fat switch by a PRDM16-C/EBP-beta transcriptional complex. Nature 460(7259): 1154-8, Aug 27 2009.
Seale P, Kajimura S, Spiegelman BM.: Transcriptional control of brown adipocyte development and physiological function--of mice and men. Genes Dev 23(7): 788-97, Apr 1 2009.
Seale P, Lazar MA.: Brown fat in humans: turning up the heat on obesity. Diabetes 58(7): 1482-4, Jul 2009.
Seale P, Bjork B, Yang W, Kajimura S, Chin S, Kuang S, Scimè A, Devarakonda S, Conroe HM, Erdjument-Bromage H, Tempst P, Rudnicki MA, Beier DR, Spiegelman BM.: PRDM16 controls a brown fat/skeletal muscle switch. Nature 454(7207): 961-7, Aug 21 2008.
Kajimura S, Seale P, Tomaru T, Erdjument-Bromage H, Cooper MP, Ruas JL, Chin S, Tempst P, Lazar MA, Spiegelman BM.: Regulation of the brown and white fat gene programs through a PRDM16/CtBP transcriptional complex. Genes Dev 22(10): 1397-409, May 15 2008.
Seale P, Kajimura S, Yang W, Chin S, Rohas LM, Uldry M, Tavernier G, Langin D, Spiegelman BM: Transcriptional control of brown fat determination by PRDM16. Cell Metab 6(1): 38-54, Jul 2007.
Handschin C, Kobayashi YM, Chin S, Seale P, Campbell KP, Spiegelman BM: PGC-1alpha regulates the neuromuscular junction program and ameliorates Duchenne muscular dystrophy. Genes Dev 21(7): 770-83, Apr 1 2007.
Uldry M, Yang W, St-Pierre J, Lin J, Seale P, Spiegelman BM: Complementary action of the PGC-1 coactivators in mitochondrial biogenesis and brown fat differentiation. Cell Metab 3(5): 333-41, May 2006.
Seale P, Ishibashi J, Holterman C, Rudnicki MA: Muscle satellite cell-specific genes identified by genetic profiling of MyoD-deficient myogenic cell. Dev Biol 275(2): 287-300, Nov 15 2004.
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Last updated: 09/22/2011
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