Kate L. J. Ellacott

Mouse models of the metabolic syndrome

The metabolic syndrome (MetS) is characterized by obesity concomitant with other metabolic abnormalities such as hypertriglyceridemia, reduced high-density lipoprotein levels, elevated blood pressure and raised fasting glucose levels. The precise definition of MetS, the relationships of its metabolic features, and what initiates it, are debated. However, obesity is on the rise worldwide, and its association with these metabolic symptoms increases the risk for diabetes and cardiovascular disease (among many other diseases). Research needs to determine the mechanisms by which obesity and MetS increase the risk of disease. In light of this growing epidemic, it is imperative to develop animal models of MetS. These models will help determine the pathophysiological basis for MetS and how MetS increases the risk for other diseases. Among the various animal models available to study MetS, mice are the most commonly used for several reasons. First, there are several spontaneously occurring obese mouse strains that have been used for decades and that are very well characterized. Second, high-fat feeding studies require only months to induce MetS. Third, it is relatively easy to study the effects of single genes by developing transgenic or gene knockouts to determine the influence of a gene on MetS. For these reasons, this review will focus on the benefits and caveats of the most common mouse models of MetS. It is our hope that the reader will be able to use this review as a guide for the selection of mouse models for their own studies.

The role of the central melanocortin system in the regulation of food intake and energy homeostasis: lessons from mouse models

A little more than a decade ago, the molecular basis of the lipostat was largely unknown. At that time, many laboratories were at work attempting to clone the genes encoding the obesity, diabetes, fatty, tubby and agouti loci, with the hope that identification of these obesity genes would help shed light on the process of energy homeostasis, appetite and energy expenditure. Characterization of obesity and diabetes elucidated the nature of the adipostatic hormone leptin and its receptor, respectively, while cloning of the agouti gene eventually led to the identification and characterization of one of the key neural systems upon which leptin acts to regulate intake and expenditure. In this review, we describe the neural circuitry known as the central melanocortin system and discuss the current understanding of its role in feeding and other processes involved in energy homeostasis.

Melanocortin-4 Receptor Signaling Is Required for Weight Loss after Gastric Bypass Surgery

CONTEXT Roux-en-Y gastric bypass (RYGB) is one of the most effective long-term therapies for the treatment of severe obesity. Recent evidence indicates that RYGB effects weight loss through multiple physiological mechanisms, including changes in energy expenditure, food intake, food preference, and reward pathways. OBJECTIVE Because central melanocortin signaling plays an important role in the regulation of energy homeostasis, we investigated whether genetic disruption of the melanocortin-4 receptor (MC4R) in rodents and humans affects weight loss after RYGB. METHODS AND RESULTS Here we report that MC4R(-/-) mice lost substantially less weight after surgery than wild-type animals, indicating that MC4R signaling is necessary for the weight loss effects of RYGB in this model. Mice heterozygous for MC4R remain fully responsive to gastric bypass. To determine whether mutations affect surgically induced weight loss in humans, we sequenced the MC4R gene in 972 patients undergoing RYGB. Patients heterozygous for MC4R mutations exhibited the same magnitude and distribution of postoperative weight loss as patients without such mutations, suggesting that although two normal copies of the MC4R gene are necessary for normal weight regulation, a single normal copy of the MC4R gene is sufficient to mediate the weight loss effects of RYGB. CONCLUSIONS MC4R is the first gene identified that is required for the sustained effects of bariatric surgery. The need for MC4R signaling for the weight loss effects of RYGB in mice underscores the physiological mechanisms of action of this procedure and demonstrates that RYGB both influences and is dependent on the normal pathways that regulate energy balance.

Assessment of Feeding Behavior in Laboratory Mice

The global obesity epidemic has heightened the need for an improved understanding of how body weight is controlled, and research using mouse models is critical to this effort. In this perspective, we provide a conceptual framework for investigation of feeding behavior in this species, with an emphasis on factors that influence study design, data interpretation, and relevance to feeding behavior in humans. Although we focus on the mouse, the principles presented can be applied to most other animal models. This document represents the current consensus view of investigators from the National Institutes of Health (NIH)-funded Mouse Metabolic Phenotyping Centers (MMPCs).

Obesity induced by a high-fat diet is associated with increased immune cell entry into the central nervous system.

Obesity is associated with chronic low-grade inflammation in peripheral tissues caused, in part, by the recruitment of inflammatory monocytes into adipose tissue. Studies in rodent models have also shown increased inflammation in the central nervous system (CNS) during obesity. The goal of this study was to determine whether obesity is associated with recruitment of peripheral immune cells into the CNS. To do this we used a bone marrow chimerism model to track the entry of green-fluorescent protein (GFP) labeled peripheral immune cells into the CNS. Flow cytometry was used to quantify the number of GFP(+) immune cells recruited into the CNS of mice fed a high-fat diet compared to standard chow fed controls. High-fat feeding resulted in obesity associated with a 30% increase in the number of GFP(+) cells in the CNS compared to control mice. Greater than 80% of the GFP(+) cells recruited to the CNS were also CD45(+) CD11b(+) indicating that the GFP(+) cells displayed characteristics of microglia/macrophages. Immunohistochemistry further confirmed the increase in GFP(+) cells in the CNS of the high-fat fed group and also indicated that 93% of the recruited cells were found in the parenchyma and had a stellate morphology. These findings indicate that peripheral immune cells can be recruited to the CNS in obesity and may contribute to the inflammatory response.

Toll-like Receptor 4 Deficiency Promotes the Alternative Activation of Adipose Tissue Macrophages

Obesity is characterized by adipose tissue (AT) macrophage (ATM) accumulation, which promotes AT inflammation and dysfunction. Toll-like receptor 4 (TLR4) deficiency attenuates AT inflammation in obesity but does not impede the accumulation of ATMs. The purpose of the current study was to determine whether TLR4 deficiency alters ATM polarization. TLR4−/− and wild-type mice were fed a low-fat, high-monounsaturated fat (HFMUFA), or a high-saturated fat (HFSFA) diet for 16 weeks. Further, we used a bone marrow transplant model to determine the influence of hematopoietic cell TLR4 signaling. The metabolic and inflammatory responses to high-fat feeding and ATM phenotype were assessed. Global and hematopoietic cell TLR4 deficiency, irrespective of recipient genotype, produced a shift in ATM phenotype toward an alternatively activated state, which was accompanied by reduced AT inflammation. Despite the observed shift in ATM phenotype, neither global nor hematopoietic cell TLR4 deficiency influenced systemic insulin sensitivity after high-fat feeding. Results of the current study suggest that TLR4 directly influences ATM polarization but question the relevance of TLR4 signaling to systemic glucose homeostasis in obesity.

Regional Astrogliosis in the Mouse Hypothalamus in Response to Obesity

Obesity is associated with chronic low‐grade inflammation in peripheral tissues, which contributes to the development of comorbidities such as insulin resistance and cardiovascular disease. While less extensively characterized, obesity also promotes inflammation in the central nervous system (CNS) and the consequences of this inflammation for CNS function are only beginning to be examined. In response to CNS insults such as inflammation, astrocytes undergo a process of hypertrophy and hyperplasia known as reactive astrogliosis. We used immunohistochemistry to examine the differential distribution of the astrocyte marker glial‐fibrillary acidic protein (GFAP) in the brains of diet‐induced or genetically obese mice compared with their respective lean controls to determine whether different nuclei of the hypothalamus showed comparable astrogliosis in response to obesity. The areas that showed the highest differential GFAP immunoreactivity between lean and obese animals include the medial preoptic, paraventricular, and dorsomedial nuclei. Comparatively, little astrogliosis was seen in the ventromedial nucleus, lateral hypothalamus, or anterior hypothalamic area. In obese animals high levels of GFAP immunoreactivity were often associated with the microvasculature. There were no differences in the differential distribution of GFAP staining between obese animals and their lean controls in the diet‐induced compared with the genetic model of obesity. The exact cause(s) of the astrogliosis in obesity is not known. The finding that obesity causes a distinct pattern of elevated GFAP immunoreactivity associated with microvessels suggests that the astrogliosis may be occurring as a response to changes at the blood–brain barrier and/or in the peripheral circulation. J. Comp. Neurol., 521:1322–1333, 2013.

High‐fat diet acutely affects circadian organisation and eating behavior

The organisation of timing in mammalian circadian clocks optimally coordinates behavior and physiology with daily environmental cycles. Chronic consumption of a high‐fat diet alters circadian rhythms, but the acute effects on circadian organisation are unknown. To investigate the proximate effects of a high‐fat diet on circadian physiology, we examined the phase relationship between central and peripheral clocks in mice fed a high‐fat diet for 1 week. By 7 days, the phase of the liver rhythm was markedly advanced (by 5 h), whereas rhythms in other tissues were not affected. In addition, immediately upon consumption of a high‐fat diet, the daily rhythm of eating behavior was altered. As the tissue rhythm of the suprachiasmatic nucleus was not affected by 1 week of high‐fat diet consumption, the brain nuclei mediating the effect of a high‐fat diet on eating behavior are likely to be downstream of the suprachiasmatic nucleus.

Neurovascular unit on a chip: implications for translational applications

The blood-brain barrier (BBB) dynamically controls exchange between the brain and the body, but this interaction cannot be studied directly in the intact human brain or sufficiently represented by animal models. Most existing in vitro BBB models do not include neurons and glia with other BBB elements and do not adequately predict drug efficacy and toxicity. Under the National Institutes of Health Microtissue Initiative, we are developing a three-dimensional, multicompartment, organotypic microphysiological system representative of a neurovascular unit of the brain. The neurovascular unit system will serve as a model to study interactions between the central nervous system neurons and the cerebral spinal fluid (CSF) compartment, all coupled to a realistic blood-surrogate supply and venous return system that also incorporates circulating immune cells and the choroid plexus. Hence all three critical brain barriers will be recapitulated: blood-brain, brain-CSF, and blood-CSF. Primary and stem cell-derived human cells will interact with a variety of agents to produce critical chemical communications across the BBB and between brain regions. Cytomegalovirus, a common herpesvirus, will be used as an initial model of infections regulated by the BBB. This novel technological platform, which combines innovative microfluidics, cell culture, analytical instruments, bioinformatics, control theory, neuroscience, and drug discovery, will replicate chemical communication, molecular trafficking, and inflammation in the brain. The platform will enable targeted and clinically relevant nutritional and pharmacologic interventions for or prevention of such chronic diseases as obesity and acute injury such as stroke, and will uncover potential adverse effects of drugs. If successful, this project will produce clinically useful technologies and reveal new insights into how the brain receives, modifies, and is affected by drugs, other neurotropic agents, and diseases.

Interactions between gut peptides and the central melanocortin system in the regulation of energy homeostasis

Genetic and pharmacological studies have shown that the central melanocortin system plays a critical role in the regulation of energy homeostasis. Animals and humans with defects in the central melanocortin system display a characteristic melanocortin obesity phenotype typified by increased adiposity, hyperphagia, metabolic defects and increased linear growth. In addition to interacting with long-term regulators of energy homeostasis such as leptin, more recent data suggest that the central melanocortin system also responds to gut-released peptides involved in mediating satiety. In this review, we discuss the interactions between these systems, with particular emphasis on cholecystokinin (CCK), ghrelin and PYY(3-36).