Pengpeng Bi

Myostatin knockout drives browning of white adipose tissue through activating the AMPK-PGC1α-Fndc5 pathway in muscle

Myostatin (Mstn) is predominantly expressed in skeletal muscles and plays important roles in regulating muscle growth and development, as well as fat deposition. Mstn‐knockout (Mstn–/–) mice exhibit increased muscle mass due to both hypertrophy and hyperplasia, and leaner body composition due to reduced fat mass. Here, we show that white adipose tissue (WAT) of Mstn–/– develops characteristics of brown adipose tissue (BAT) with dramatically increased expression of BAT signature genes, including Ucp1 and Pgc1α, and beige adipocyte markers Tmem26 and CD137. Strikingly, the observed browning phenotype is non‐cell autonomous and is instead driven by the newly defined myokine irisin (Fndc5) secreted from Mstn–/– skeletal muscle. Within the muscle, Mstn–/– leads to increased expression of AMPK and its phosphorylation, which subsequently activates PGC1α and Fndc5. Together, our study defines a paradigm of muscle‐fat crosstalk mediated by Fndc5, which is up‐regulated and secreted from muscle to induce beige cell markers and the browning of WAT in Mstn–/– mice. These results suggest that targeting muscle Mstn and its downstream signaling represents a therapeutic approach to treat obesity and type 2 diabetes.—Shan, T., Liang, X., Bi, P., Kuang, S. Myostatin knockout drives browning of white adipose tissue through activating the AMPK‐PGC1α‐Fndc5 pathway in muscle. FASEB J. 27, 1981–1989 (2013). www.fasebj.org

Constitutive Notch Activation Upregulates Pax7 and Promotes the Self-Renewal of Skeletal Muscle Satellite Cells

ABSTRACT Notch signaling is a conserved cell fate regulator during development and postnatal tissue regeneration. Using skeletal muscle satellite cells as a model and through myogenic cell lineage-specific NICDOE (overexpression of constitutively activated Notch 1 intracellular domain), here we investigate how Notch signaling regulates the cell fate choice of muscle stem cells. We show that in addition to inhibiting MyoD and myogenic differentiation, NICDOE upregulates Pax7 and promotes the self-renewal of satellite cell-derived primary myoblasts in culture. Using MyoD−/− myoblasts, we further show that NICDOE upregulates Pax7 independently of MyoD inhibition. In striking contrast to previous observations, NICDOE also inhibits S-phase entry and Ki67 expression and thus reduces the proliferation of primary myoblasts. Overexpression of canonical Notch target genes mimics the inhibitory effects of NICDOE on MyoD and Ki67 but not the stimulatory effect on Pax7. Instead, NICD regulates Pax7 through interaction with RBP-Jκ, which binds to two consensus sites upstream of the Pax7 gene. Importantly, satellite cell-specific NICDOE results in impaired regeneration of skeletal muscles along with increased Pax7+ mononuclear cells. Our results establish a role of Notch signaling in actively promoting the self-renewal of muscle stem cells through direct regulation of Pax7.

Inhibition of Notch signaling promotes browning of white adipose tissue and ameliorates obesity

Pengpeng Bi1, Tizhong Shan1, Weiyi Liu1, Feng Yue1, Xin Yang1, Xin-Rong Liang1, Jinghua Wang1, Jie Li2, Nadia Carlesso3, Xiaoqi Liu2,4, and Shihuan Kuang1,4,* 1Department of Animal Sciences, Purdue University, West Lafayette, IN 47907, USA. 2Department of Biochemistry, Purdue University, West Lafayette, IN 47907, USA. 3Department of Pediatrics, Indiana University School of Medicine, Indianapolis, IN 46202, USA. 4Center for Cancer Research, Purdue University, West Lafayette, IN 47907, USA.Beige adipocytes in white adipose tissue (WAT) are similar to classical brown adipocytes in that they can burn lipids to produce heat. Thus, an increase in beige adipocyte content in WAT browning would raise energy expenditure and reduce adiposity. Here we report that adipose-specific inactivation of Notch1 or its signaling mediator Rbpj in mice results in browning of WAT and elevated expression of uncoupling protein 1 (Ucp1), a key regulator of thermogenesis. Consequently, as compared to wild-type mice, Notch mutants exhibit elevated energy expenditure, better glucose tolerance and improved insulin sensitivity and are more resistant to high fat diet–induced obesity. By contrast, adipose-specific activation of Notch1 leads to the opposite phenotypes. At the molecular level, constitutive activation of Notch signaling inhibits, whereas Notch inhibition induces, Ppargc1a and Prdm16 transcription in white adipocytes. Notably, pharmacological inhibition of Notch signaling in obese mice ameliorates obesity, reduces blood glucose and increases Ucp1 expression in white fat. Therefore, Notch signaling may be therapeutically targeted to treat obesity and type 2 diabetes.

Hypoxia promotes satellite cell self-renewal and enhances the efficiency of myoblast transplantation

Microenvironmental oxygen (O2) regulates stem cell activity, and a hypoxic niche with low oxygen levels has been reported in multiple stem cell types. Satellite cells are muscle-resident stem cells that maintain the homeostasis and mediate the regeneration of skeletal muscles. We demonstrate here that hypoxic culture conditions favor the quiescence of satellite cell-derived primary myoblasts by upregulating Pax7, a key regulator of satellite cell self-renewal, and downregulating MyoD and myogenin. During myoblast division, hypoxia promotes asymmetric self-renewal divisions and inhibits asymmetric differentiation divisions without affecting the overall rate of proliferation. Mechanistic studies reveal that hypoxia activates the Notch signaling pathway, which subsequently represses the expression of miR-1 and miR-206 through canonical Hes/Hey proteins, leading to increased levels of Pax7. More importantly, hypoxia conditioning enhances the efficiency of myoblast transplantation and the self-renewal of implanted cells. Given the robust effects of hypoxia on maintaining the quiescence and promoting the self-renewal of cultured myoblasts, we predict that oxygen levels in the satellite cell niche play a central role in precisely balancing quiescence versus activation, and self-renewal versus differentiation, in muscle stem cells in vivo.

miR-133a Regulates Adipocyte Browning In Vivo

Prdm16 determines the bidirectional fate switch of skeletal muscle/brown adipose tissue (BAT) and regulates the thermogenic gene program of subcutaneous white adipose tissue (SAT) in mice. Here we show that miR-133a, a microRNA that is expressed in both BAT and SATs, directly targets the 3′ UTR of Prdm16. The expression of miR-133a dramatically decreases along the commitment and differentiation of brown preadipocytes, accompanied by the upregulation of Prdm16. Overexpression of miR-133a in BAT and SAT cells significantly inhibits, and conversely inhibition of miR-133a upregulates, Prdm16 and brown adipogenesis. More importantly, double knockout of miR-133a1 and miR-133a2 in mice leads to elevations of the brown and thermogenic gene programs in SAT. Even 75% deletion of miR-133a (a1−/−a2+/−) genes results in browning of SAT, manifested by the appearance of numerous multilocular UCP1-expressing adipocytes within SAT. Additionally, compared to wildtype mice, miR-133a1−/−a2+/− mice exhibit increased insulin sensitivity and glucose tolerance, and activate the thermogenic gene program more robustly upon cold exposure. These results together elucidate a crucial role of miR-133a in the regulation of adipocyte browning in vivo.

Notch signaling as a novel regulator of metabolism

Evolutionarily unprepared for modern high-calorie diets and sedentary lifestyles, humans are now unprecedentedly susceptible to metabolic disorders such as obesity, type 2 diabetes (T2D), nonalcoholic fatty liver, and cardiovascular disease. These metabolic conditions are intertwined, together known as metabolic syndrome, and compromise human life quality as well as lives. Notch signaling, a fundamental signal transduction pathway critical for cell-cell communication and development, has recently been recognized as a key player in metabolism. This review summarizes the emerging roles of Notch signaling in regulating the metabolism of various cell and tissue types, with emphasis on the underlying molecular mechanisms and the potential of targeting this signal axis to treat metabolic diseases.

Distinct populations of adipogenic and myogenic Myf5-lineage progenitors in white adipose tissues

Brown adipose tissues (BAT) are derived from a myogenic factor 5 (Myf5)-expressing cell lineage and white adipose tissues (WAT) predominantly arise from non-Myf5 lineages, although a subpopulation of adipocytes in some WAT depots can be derived from the Myf5 lineage. However, the functional implication of the Myf5- and non-Myf5-lineage cells in WAT is unclear. We found that the Myf5-lineage constitution in subcutaneous WAT depots is negatively correlated to the expression of classical BAT and newly defined beige/brite adipocyte-specific genes. Consistently, fluorescent-activated cell sorting (FACS)-purified Myf5-lineage adipo-progenitors give rise to adipocytes expressing lower levels of BAT-specific Ucp1, Prdm16, Cidea, and Ppargc1a genes and beige adipocyte-specific CD137, Tmem26, and Tbx1 genes compared with the non-Myf5-lineage adipocytes from the same depots. Ablation of the Myf5-lineage progenitors in WAT stromal vascular cell (SVC) cultures leads to increased expression of BAT and beige cell signature genes. Strikingly, the Myf5-lineage cells in WAT are heterogeneous and contain distinct adipogenic [stem cell antigen 1(Sca1)-positive] and myogenic (Sca1-negative) progenitors. The latter differentiate robustly into myofibers in vitro and in vivo, and they restore dystrophin expression after transplantation into mdx mouse, a model for Duchenne muscular dystrophy. These results demonstrate the heterogeneity and functional differences of the Myf5- and non-Myf5-lineage cells in the white adipose tissue.

Control of muscle formation by the fusogenic micropeptide myomixer

Micromanaging muscle cell fusion Adult skeletal muscles are characterized by long, multinucleated cells called myofibers. Myofibers form when muscle precursor cells, or myoblasts, differentiate and fuse together during embryogenesis. The fusion process is not fully understood. Studying cell culture and mouse models, Bi et al. identified an 84–amino acid peptide that promotes myoblast fusion. This small peptide, called Myomixer, physically interacts with and stimulates the activity of a fusogenic membrane protein called Myomaker. Notably, the Myomaker-Myomixer pair can also promote the fusion of nonmuscle cells, such as fibroblasts. Science, this issue p. 323 A small peptide expressed in developing skeletal muscle controls muscle cell fusion and myofiber formation. Skeletal muscle formation occurs through fusion of myoblasts to form multinucleated myofibers. From a genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) loss-of-function screen for genes required for myoblast fusion and myogenesis, we discovered an 84–amino acid muscle-specific peptide that we call Myomixer. Myomixer expression coincides with myoblast differentiation and is essential for fusion and skeletal muscle formation during embryogenesis. Myomixer localizes to the plasma membrane, where it promotes myoblast fusion and associates with Myomaker, a fusogenic membrane protein. Myomixer together with Myomaker can also induce fibroblast-fibroblast fusion and fibroblast-myoblast fusion. We conclude that the Myomixer-Myomaker pair controls the critical step in myofiber formation during muscle development.

Lkb1 controls brown adipose tissue growth and thermogenesis by regulating the intracellular localization of CRTC3

Brown adipose tissue (BAT) dissipates energy through Ucp1-mediated uncoupled respiration and its activation may represent a therapeutic strategy to combat obesity. Here we show that Lkb1 controls BAT expansion and UCP1 expression in mice. We generate adipocyte-specific Lkb1 knockout mice and show that, compared with wild-type littermates, these mice exhibit elevated UCP1 expression in BAT and subcutaneous white adipose tissue, have increased BAT mass and higher energy expenditure. Consequently, KO mice have improved glucose tolerance and insulin sensitivity, and are more resistant to high-fat diet (HFD)-induced obesity. Deletion of Lkb1 results in a cytoplasm to nuclear translocation of CRTC3 in brown adipocytes, where it recruits C/EBPβ to enhance Ucp1 transcription. In parallel, the absence of Lkb1 also suppresses AMPK activity, leading to activation of the mTOR signalling pathway and subsequent BAT expansion. These data suggest that inhibition of Lkb1 or its downstream signalling in adipocytes could be a novel strategy to increase energy expenditure in the context of obesity, diabetes and other metabolic diseases.

Plk1 Phosphorylation of PTEN Causes a Tumor-Promoting Metabolic State

ABSTRACT One outcome of activation of the phosphatidylinositol 3-kinase (PI3K) pathway is increased aerobic glycolysis, but the upstream signaling events that regulate the PI3K pathway, and thus the Warburg effect, are elusive. Increasing evidence suggests that Plk1, a cell cycle regulator, is also involved in cellular events in addition to mitosis. To test whether Plk1 contributes to activation of the PI3K pathway, and thus aerobic glycolysis, we examined potential targets of Plk1 and identified PTEN as a Plk1 substrate. We hypothesize that Plk1 phosphorylation of PTEN leads to its inactivation, activation of the PI3K pathway, and the Warburg effect. Our data show that overexpression of Plk1 leads to activation of the PI3K pathway and enhanced aerobic glycolysis. In contrast, inhibition of Plk1 causes markedly reduced glucose metabolism in mice. Mechanistically, we show that Plk1 phosphorylation of PTEN and Nedd4-1, an E3 ubiquitin ligase of PTEN, results in PTEN inactivation. Finally, we show that Plk1 phosphorylation of PTEN promotes tumorigenesis in both its phosphatase-dependent and -independent pathways, revealing potentially new drug targets to arrest tumor cell growth.