Energy Sparing Orexigenic Inflammation of Obesity

Cell Metabolism

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Energy Sparing Orexigenic Inflammation of Obesity Aileen Lee1,2 and Vishwa Deep Dixit1,2,3,* 1Department

of Comparative Medicine of Immunobiology 3Program on Integrative Cell Signaling and Neurobiology of Metabolism Yale School of Medicine, New Haven, CT 06520, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2017.06.013 2Department

The neuro-immune interactions that integrate host metabolism in health and disease are unclear. A new study by Valdearcos et al. (2017) describes how sensing of high-fat diet by microglia, brain’s resident innate immune cells, recruits additional bone-marrow-derived myeloid cells into the hypothalamus to produce inflammation and cause weight gain.

Smoldering metabolic-inflammation that emerges during obesity lacks classical signs of inflammation defined by redness, heat, swelling, pain, anorexia, and loss of function (Hotamisligil, 2017). It is well established that canonical inflammatory response leading to release of high levels of IL-1b and TNFa causes fever, anorexia, and sickness behavior through specific effects on hypothalamic and hindbrain circuits (Dantzer, 2001). On the contrary, obesity-induced metabolic inflammation in the periphery and hypothalamus is associated with reduced energy expenditure with an orexigenic response and not by reduced food intake. These paradoxical observations seen in acute inflammation versus chronic metabolic inflammation has remained a puzzle. How can anorexigenic pro-inflammatory thermogenic mediators controlled by NF-kB in acute inflammation flip their fundamental function in the opposite direction during obesity? Or do they? Clearly, our understanding of mechanisms that link neuroendocrine system to immunometabolism

is still in its infancy. A study from Valdearcos et al. (2017) published in this issue of Cell Metabolism moves this field forward by showing that, during obesity, microglia in an NF-kB-dependent manner recruits a unique population of myeloid cells from the bone marrow (BM) into medio-basal hypothalamus (MBH) to produce orexigenic inflammation, or metabolic inflammation, associated with increased food intake (Valdearcos et al., 2017). Homeostatic regulation via endocrine signaling in the hypothalamus controls the balance of food intake and energy expenditure to maintain a healthy body weight (Horvath et al., 2004). The orexigenic neurons in MBH are located outside of the blood-brain barrier (BBB) (Olofsson et al., 2013) and hence are uniquely positioned in the CNS to sense circulating metabolic hormones, fatty acids, and nutrients. Importantly, long-chain fatty acids (LCFAs), such as palmitate, cannot cross the intact BBB. However, the location of MBH outside of the BBB may allow microglia to sense the LCFAs that are increased

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in diet-induced obesity (DIO) to induce metabolic inflammation. Macrophages sense fatty acids, such as palmitate, and ceramides in an NLRP3 inflammasomedependent manner to induce metabolic inflammation in obesity (Kanneganti and Dixit, 2012). Similarly, inflammation of the hypothalamus is accompanied by activation of microglia, which precedes weight gain and peripheral inflammation (Thaler et al., 2012). However, the components of microglial innate sensing of obesity-derived inflammogens and subsequent metabolic consequences are unclear. Valdearcos et al. (2017) use multiple pharmacological and genetic approaches to demonstrate the role of microglial-specific inflammation in mediating hypothalamic neuroimmune response to nutrient excess and regulating susceptibility to obesity. These studies implicate NF-kBmediated signaling as a regulator of microglial activation during high-fat diet (HFD) intake to induce hypothalamic dysfunction (Figure 1). Microglia are

Cell Metabolism

Previews High-Fat Diet

Activated residential microglia Recruited myeloid-derived microglia

MBH Fatty acids? DAMPs? ?

NF-κB

STAT3

Food Intake +

A20

Leptin

Inflammation

+ ANS ? BAT ?

Obesity

Energy Expenditure 3V

microgliosis

Cytokines?

Figure 1. Microglia-Mediated Hypothalamic Inflammation in Obesity During dietary nutrient excess, NF-kB-mediated activation of long-lived residential microglia recruits bone-marrow-derived myeloid cells that adopt a microglial phenotype. Forced activation of NF-kB signaling in residential microglia by ablation of A20 recruits myeloid-derived microglia and suppresses neuronal responsiveness to metabolic hormones, such as leptin, to regulate food intake and energy expenditure and affect obesity susceptibility. ANS, autonomic nervous system; BAT, brown adipose tissue.

resident brain macrophages that are derived from yolk sac and do not get replaced by BM progenitors (Ginhoux et al., 2010). Using pharmacological agents, Valdearcos et al. (2017) first deplete microglia in the context of HFD to reveal that microglia play a role in maintaining elevated levels of food intake during obesity. The role of neuronal IKKb/NFkB signaling has emerged as an important mediator of hypothalamic inflammation that leads to energy imbalance (Cai, 2013). Valdearcos et al. (2017) further demonstrate that IKKb/NF-kB activation in microglia of the hypothalamus also contributes to HFD-induced hyperphagia. By allowing the replenishment of peripheral CX3CR1+ monocytes after tamoxifen treatment in CX3CR1CreER/+ Ikbkbf/f mice, Valdearcos et al. (2017) conditionally delete IKKb in long-lived microglia and find that microgliosis in the MBH, hyperphagia, and weight gain in response to HFD are reduced. Furthermore, in mice where yolk-sac-derived microglia are indelibly marked through genetic means, additional markers, such as P2Y12, Tmem119, and CD169, allowed Valdear-

cos et al. (2017) to identify that microglia-like cells that accumulate in MBH of obese mice are from a distinct non-yolk sac origin and appear to be recruited from the BM (Figure 1). Valdearcos et al. (2017) show that recruitment of this BMderived myeloid subpopulation in MBH is dependent on the inflammatory status of residential microglia as conditional IKKb deletion prevents the accumulation of CD169+ myeloid cells in the MBH with HFD. Finally, Valdearcos et al. (2017) deleted A20, a primary negative regulator of NF-kB activity, in long-lived microglia and found that forced activation of residential microglia is sufficient to induce microgliosis and rapid weight gain by impairing leptin signaling in neurons (Figure 1). Interestingly, in this model of hypothalamic inflammation, peripheral inflammation and whole-body glucose tolerance were unaffected, suggesting that microglia-mediated inflammation in MBH alone is insufficient to drive insulin resistance. This work introduces a new subset of peripherally derived myeloid cells in microgliosis in response to DIO. These

interesting findings by Valdearcos et al. (2017) raise several important issues that remain unexplored. For example, does the MBH microenvironment educate these recruited microglial-like cells to assume an inflammatory phenotype? Given the lack of specific Cre drivers to target this unique cell type, the exact role of these cells in induction or resolution of inflammation is unclear. Nonetheless, given that Valdearcos et al. (2017) provide a tractable model to further isolate and characterize these infiltrating myeloid cells in MBH, future investigation could reveal the cell types that cause canonical anorexigenic inflammation versus non-canonical orexigenic inflammation in disease models. Interestingly, Valdearcos et al. (2017) also note changes in thermogenic genes in the brown adipose tissue (BAT) with lower energy expenditure (EE) induced by microglial activation. While this may indeed suggest that microglial signaling influences the sympathetic nervous system, the specificity of genetic manipulation in microglia versus other tissue residential macrophages complicates these interpretations. Notably, a recent study also using CX3CR1CreER/+ mice by Wolf and colleagues has identified that approximately 50%–60% of macrophages in BAT are long-lived, tissue-resident macrophages of yolk sac origin and regulate sympathetic innervation, which could impact EE (Wolf et al., 2017). Thus, NF-kB signaling in yolk-sacderived resident macrophages outside of MBH in other adipose tissue locations could also impact metabolic inflammation and energy homeostasis. Overall, the study by Valdearcos et al. (2017) reporting the identification of distinct microglia and BM-derived myeloid cell populations suggests that targeting NF-kB-mediated orexigenic inflammation may offer additional approaches for obesity treatment. Exploration of methods to regulate the scale of microglia-mediated inflammation may also prove relevant in the elderly, where adiposity is associated with additional increased risk of metabolic impairment and neurodegenerative disease. REFERENCES Cai, D. (2013). Trends Endocrinol. Metab. 24, 40–47.

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Cell Metabolism

Previews Dantzer, R. (2001). Brain Behav. Immun. 15, 7–24. Ginhoux, F., Greter, M., Leboeuf, M., Nandi, S., See, P., Gokhan, S., Mehler, M.F., Conway, S.J., Ng, L.G., Stanley, E.R., et al. (2010). Science 330, 841–845. Horvath, T.L., Diano, S., and Tscho¨p, M. (2004). Neuroscientist 10, 235–246. Hotamisligil, G.S. (2017). Nature 542, 177–185.

Kanneganti, T.D., and Dixit, V.D. (2012). Nat. Immunol. 13, 707–712. Olofsson, L.E., Unger, E.K., Cheung, C.C., and Xu, A.W. (2013). Proc. Natl. Acad. Sci. USA 110, E697–E706. Thaler, J.P., Yi, C.X., Schur, E.A., Guyenet, S.J., Hwang, B.H., Dietrich, M.O., Zhao, X., Sarruf, D.A., Izgur, V., Maravilla, K.R., et al. (2012). J. Clin. Invest. 122, 153–162.

Valdearcos, M., Douglass, J.D., Robblee, M.M., Dorfman, M.D., Stifler, D.R., Bennett, M.L., Gerritse, I., Fasnacht, R., Barres, B.A., Thaler, J.P., and Koliwad, S.K. (2017). Cell Metab. 26, this issue, 185–197. Wolf, Y., Boura-Halfon, S., Cortese, N., Haimon, Z., Sar Shalom, H., Kuperman, Y., Kalchenko, V., Brandis, A., David, E., Segal-Hayoun, Y., et al. (2017). Nat. Immunol. 18, 665–674.

Is Glycogenin Essential for Glycogen Synthesis? Anders Oldfors1,* 1Department of Pathology and Genetics, University of Gothenburg, Sahlgrenska Hospital, 413 45 Gothenburg, Sweden *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2017.06.017

Glycogen synthesis requires a priming oligosaccharide, formed by autoglucosylation of glycogenin, a core protein in glycogen particles. In this edition of Cell Metabolism, Testoni et al. (2017) challenge this generally accepted concept by demonstrating that glycogenin inactivation in mice results in an increased amount of glycogen and not glycogen depletion. Glycogen provides storage of readily accessible glucose for maintenance of blood glucose levels between meals, for strenuous muscle exercise, and for other cellular functions. The discovery of glycogenin as a core protein in glycogen particles and its function as a glycosyl transferase with the ability to autoglucosylate has led to the generally accepted concept that de novo glycogen synthesis is initiated by autoglucosylation of glycogenin, thereby forming an oligosaccharide primer that functions as a substrate for glycogen synthase. Elongation and branching is catalyzed by glycogen synthase and branching enzyme, respectively (Figure 1A) (Berg et al., 2015; Lomako et al., 1988; Pitcher et al., 1988). The requirement of glycogenin for glycogen synthesis is challenged by the results from a study on mice where the glycogenin gene was inactivated by a knockout (KO) approach (Testoni et al., 2017). Contrary to what was expected, the glycogenin KO mice showed accumulation of glycogen instead of glycogen depletion, and no protein that functioned as a substitute for glycogenin could be identified. The lack of glycogenin was associated with reduced endurance and a metabolic shift toward glycolytic meta-

bolism in the otherwise fatigue-resistant oxidative muscle fibers. Considering the generally accepted function and importance of glycogenin, the findings are surprising but in line with recent observations that glycogenin probably is not necessary for glycogen synthesis in humans, either. Unlike mice, humans and other primates have a second variant of glycogenin called glycogenin-2, which is mainly expressed in the liver. Glycogenin-1 is ubiquitously expressed and the only isoform in human skeletal muscle. Several recessive pathogenic mutations have been identified in the glycogenin-1 gene, GYG1. Complete absence of glycogenin-1 protein secondary to bi-allelic truncating mutations in GYG1 causes a rare muscle disease that is characterized by accumulation of glycogen (Figure 1B). This glycogen is, in addition to lack of a glycogenin-1 core, abnormal with regard to its ultrastructure. Many glycogen particles show uneven size and irregular shape, and some of the storage material has a fibrillar structure (Figure 1C). Most of the material is digested by alpha-amylase, similar to normal glycogen, but some of it remains undigested as polyglucosan (Malfatti et al., 2014; Hedberg-Oldfors and Oldfors, 2015). The glycogenin-defi-

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cient mice described by Testoni et al. may serve as a model for this human muscle disease associated with lack of glycogenin-1. However, in the mice the accumulated glycogen did not reveal fibrillar structure and no alpha-amylase resistant material as seen in humans, but the glycogen particles were abnormal in shape with marked variability in size. In humans, complete lack of glycogenin-1 is usually associated with lateonset muscle weakness, indicating that lack of glycogenin-1 has no major impact on muscle energy metabolism. The muscle weakness that appears later in life is associated with muscle fiber degeneration; replacement of muscle tissue by fat and fibrous connective tissue explains the weakness. However, the studies in mice by Testoni et al. demonstrate that overproduction of glycogen secondary to glycogenin deficiency is associated with altered metabolism, affecting mainly oxidative muscle fibers and causing impaired endurance. Such effects of glycogenin-1 deficiency may occur also in humans and would be of interest to study further. A more severe heart disease associated with missense GYG1 mutations has been described in several individuals