Lessons in obesity from transgenic animals

Fuel homeostasis and locomotor behavior: role of leptin and melanocortin pathways

BACKGROUND

While it is now accepted that genes and their products affect food intake, the concept that locomotor behavior or the propensity for physical activity is controlled by neuro hum oral regulators is frequently underappreciated. In mammals, complex interactions have developed to allow the cross-talk between fuel homeostasis and physical activity.

AIM

The aim of this review is to provide a synopsis of the influence of the leptin-melanocortin pathway, a well-studied pivotal player in body weight regulation, on locomotor behaviors.

CONCLUSIONS

In rodents, reductions in leptin levels that physiologically occur following acute food deprivation or a reduction of the fat mass consequent to prolonged caloric restrictions are associated with a decrease in total locomotor activity and simultaneous increase in food-anticipatory activity, a locomotor behavior which reflects a foraging attitude. These actions can be prevented by leptin administration and are at least partially mediated by the neurons of the melanocortin pathway. In humans, twin studies have attributed to genetic factors approximately 50% of the variance of physical activity. An elevated number of the genes or loci which may affect physical activity are involved in body weight homeostasis. Polymorphisms of the melanocortin-4 and leptin receptors have repeatedly been associated with the level of physical activity. Unraveling the complexity of the regulation of locomotor behavior and the interconnections with the pathways involved in energy homeostasis may help explain the substantial individual variability in physical activities in humans and disentangle the harmful effects of sedentary lifestyle, which may be distinct from the detrimental effects of obesity.

Early differentiated CD138(high) MHCII+ IgG+ plasma cells express CXCR3 and localize into inflamed kidneys of lupus mice

Humoral responses are central to the development of chronic autoimmune diseases such as systemic lupus erythematosus. Indeed, autoantibody deposition is responsible for tissue damage, the kidneys being one of the main target organs. As the source of pathogenic antibodies, plasma cells are therefore critical players in this harmful scenario, both at systemic and local levels. The aim of the present study was to analyze plasma cells in NZB/W lupus mice and to get a better understanding of the mechanisms underlying their involvement in the renal inflammation process. Using various techniques (i.e. flow cytometry, quantitative PCR, ELISpot), we identified and extensively characterized three plasma cell intermediates, according to their B220/CD138/MHCII expression levels. Each of these cell subsets displays specific proliferation and antibody secretion capacities. Moreover, we evidenced that the inflammation-related CXCR3 chemokine receptor is uniquely expressed by CD138^highMHCII⁺ plasma cells, which encompass both short- and long-lived cells and mostly produce IgG (auto)antibodies. Expression of CXCR3 allows efficient chemotactic responsiveness of these cells to cognate chemokines, which production is up-regulated in the kidneys of diseased NZB/W mice. Finally, using fluorescence and electron microscopy, we demonstrated the presence of CD138⁺CXCR3⁺IgG⁺ cells in inflammatory areas in the kidneys, where they are very likely involved in the injury process. Thus, early differentiated CD138^highMHCII⁺ rather than terminally differentiated CD138^highMHCII^low plasma cells may be involved in the renal inflammatory injury in lupus, due to CXCR3 expression and IgG secretion.

Diabetic retinopathy and signaling mechanism for activation of matrix metalloproteinase-9

In the pathogenesis of diabetic retinopathy, H-Ras (a small molecular weight G-protein) and matrix metalloproteinase-9 (MMP9) act as pro-apoptotic, accelerating the apoptosis of retinal capillary cells, a phenomenon that predicts its development and the activation of MMP9 is under the control of H-Ras. The goal of this study is to elucidate the cellular mechanism by which H-Ras activates MMP9 culminating in the development of diabetic retinopathy. Using isolated retinal endothelial cells, the effect of regulation of H-Ras downstream signaling cascade, Raf-1, MEK, and ERK, was investigated on glucose-induced activation of MMP9. In vitro results were confirmed in the retina obtained from diabetic mice manipulated for MMP9 gene, and also in the retinal microvasculature obtained from human donors with diabetic retinopathy. Regulation of Raf-1/MEK/ERK by their specific siRNAs and pharmacologic inhibitors prevented glucose-induced activation of MMP9 in retinal endothelial cells. In MMP9-KO mice, diabetes had no effect on retinal MMP9 activation, and H-Ras/Raf-1/MEK signaling cascade remained normal. Similarly, donors with diabetic retinopathy had increased MMP9 activity in their retinal microvessels, the site of histopathology associated with diabetic retinopathy, and this was accompanied by activated H-Ras signaling pathway (Raf-1/ERK). Collectively, these results suggest that Ras/Raf-1/MEK/ERK cascade has an important role in the activation of retinal MMP9 resulting in the apoptosis of its capillary cells. Understanding the upstream mechanism responsible for the activation of MMP9 should help identify novel molecular targets for future pharmacological interventions to inhibit the development/progression of diabetic retinopathy.

The development and endocrine functions of adipose tissue

White adipose tissue is a mesenchymal tissue that begins developing in the fetus. Classically known for storing the body's fuel reserves, adipose tissue is now recognized as an endocrine organ. As such, the secretions from adipose tissue are known to affect several systems such as the vascular and immune systems and play major roles in metabolism. Numerous studies have shown nutrient or hormonal manipulations can greatly influence adipose tissue development. In addition, the associations between various disease states, such as insulin resistance and cardiovascular disease, and disregulation of adipose tissue seen in epidemiological and intervention studies are great. Evaluation of known adipokines suggests these factors secreted from adipose tissue play roles in several pathologies. As the identification of more adipokines and determination of their role in biological systems, and the interactions between adipocytes and other cells types continues, there is little doubt that we will gain a greater appreciation for a tissue once thought to simply store excess energy.

Long-term Fenretinide treatment prevents high-fat diet-induced obesity, insulin resistance, and hepatic steatosis

The synthetic retinoid Fenretinide (FEN) increases insulin sensitivity in obese rodents and is in early clinical trials for treatment of insulin resistance in obese humans with hepatic steatosis (46). We aimed to determine the physiological mechanisms for the insulin-sensitizing effects of FEN. Wild-type mice were fed a high-fat diet (HFD) with or without FEN from 4-5 wk to 36-37 wk of age (preventive study) or following 22 wk of HF diet-induced obesity (12 wk intervention study). Retinol-binding protein-4 (RBP4) knockout mice were also fed the HFD with or without FEN in a preventive study. FEN had minimal effects on HFD-induced body weight gain but markedly reduced HFD-induced adiposity and hyperleptinemia in both studies. FEN-HFD mice gained epididymal fat but not subcutaneous or visceral fat mass in contrast to HFD mice without FEN. FEN did not have a measurable effect on energy expenditure, food intake, physical activity, or stool lipid content. Glucose infusion rate during hyperinsulinemic-euglycemic clamp was reduced 86% in HFD mice compared with controls and was improved 3.6-fold in FEN-HFD compared with HFD mice. FEN improved insulin action on glucose uptake and glycogen levels in muscle, insulin-stimulated suppression of hepatic glucose production, and suppression of serum FFA levels in HFD mice. Remarkably, FEN also reduced hepatic steatosis. In RBP4 knockout mice, FEN reduced the HFD-induced increase in adiposity and hyperleptinemia. In conclusion, long-term therapy with FEN partially prevents or reverses obesity, insulin resistance, and hepatic steatosis in mice on HFD. The anti-adiposity effects are independent of the RBP4 lowering effect.

Endogenous peroxisome proliferator-activated receptor-gamma augments fatty acid uptake in oxidative muscle

In the setting of insulin resistance, agonists of peroxisome proliferator-activated receptor (PPAR)-gamma restore insulin action in muscle and promote lipid redistribution. Mice with muscle-specific knockout of PPARgamma (MuPPARgammaKO) develop excess adiposity, despite reduced food intake and normal glucose disposal in muscle. To understand the relation between muscle PPARgamma and lipid accumulation, we studied the fuel energetics of MuPPARgammaKO mice. Compared with controls, MuPPARgammaKO mice exhibited significantly increased ambulatory activity, muscle mitochondrial uncoupling, and respiratory quotient. Fitting with this latter finding, MuPPARgammaKO animals compared with control siblings exhibited a 25% reduction in the uptake of the fatty acid tracer 2-bromo-palmitate (P < 0.05) and a 13% increase in serum nonesterified fatty acids (P = 0.05). These abnormalities were associated with no change in AMP kinase (AMPK) phosphorylation, AMPK activity, or phosphorylation of acetyl-CoA carboxylase in muscle and occurred despite increased expression of fatty acid transport protein 1. Palmitate oxidation was not significantly altered in MuPPARgammaKO mice despite the increased expression of several genes promoting lipid oxidation. These data demonstrate that PPARgamma, even in the absence of exogenous activators, is required for normal rates of fatty acid uptake in oxidative skeletal muscle via mechanisms independent of AMPK and fatty acid transport protein 1. Thus, when PPARgamma activity in muscle is absent or reduced, there will be decreased fatty acid disposal leading to diminished energy utilization and ultimately adiposity.

Thermogenic and metabolic antiobesity drugs: rationale and opportunities

Antiobesity drugs that target peripheral metabolism may avoid some of the problems that have been encountered with centrally acting anorectic drugs. Moreover, if they cause weight loss by increasing fat oxidation, they not only address a cause of obesity but also should promote loss of fat rather than lean tissue and improve insulin sensitivity. Weight loss may be slow but more sustained than with anorectic drugs, and thermogenesis may be insufficient to cause any discomfort. Some thermogenic approaches are the activation of adrenergic, thyroid hormone or growth hormone receptors and the inhibition of glucocorticoid receptors; the modulation of transcription factors [e.g. peroxisome proliferator-activated receptor delta (PPARdelta) activators] or enzymes [e.g. glutamine fructose-6-phosphate amidotransferase (GFAT) inhibitors] that promote mitochondrial biogenesis, and the modulation of transcription factors (PPAR alpha activators) or enzymes (AMP-activated protein kinase) that promote fatty acid oxidation. More surprisingly, studies on genetically modified animals and with enzyme inhibitors suggest that inhibitors of fatty acid synthesis [e.g. ATP citrate lyase, fatty acid synthase, acetyl-CoA carboxylase (ACC)], fatty acid interconversion [stearoyl-CoA desaturase (SCD)] and triglyceride synthesis (e.g. acyl-CoA : diacylglycerol acyltransferase) may all be thermogenic. Some targets have been validated only by deleting genes in the whole animal. In these cases, it is possible that deletion of the protein in the brain is responsible for the effect on adiposity, and therefore a centrally penetrant drug would be required. Moreover, whilst a genetically modified mouse may display resistance to obesity in response to a high fat diet, it requires a tool compound to demonstrate that a drug might actually cause weight loss. Even then, it is possible that differences between rodents and humans, such as the greater thermogenic capacity of rodents, may give a misleading impression of the potential of a drug.

Some mathematical and technical issues in the measurement and interpretation of open-circuit indirect calorimetry in small animals

Indirect calorimetry is increasingly used to investigate why compounds or genetic manipulations affect body weight or composition in small animals. This review introduces the principles of indirect (primarily open-circuit) calorimetry and explains some common misunderstandings. It is not widely understood that in open-circuit systems in which carbon dioxide (CO2) is not removed from the air leaving the respiratory chamber, measurement of airflow out of the chamber and its oxygen (O2) content paradoxically allows a more reliable estimate of energy expenditure (EE) than of O2 consumption. If the CO2 content of the exiting air is also measured, both O2 consumption and CO2 production, and hence respiratory quotient (RQ), can be calculated. Respiratory quotient coupled with nitrogen excretion allows the calculation of the relative combustion of the macronutrients only if measurements are over a period where interconversions of macronutrients that alter their pool sizes can be ignored. Changes in rates of O2 consumption and CO2 production are not instantly reflected in changes in the concentrations of O2 and CO2 in the air leaving the respiratory chamber. Consequently, unless air-flow is high and chamber size is small, or rates of change of O2 and CO2 concentrations are included in the calculations, maxima and minima are underestimated and will appear later than their real times. It is widely appreciated that bigger animals with more body tissue will expend more energy than smaller animals. A major issue is how to compare animals correcting for such differences in body size. Comparison of the EE or O2 consumption per gram body weight of lean and obese animals is misleading because tissues vary in their energy requirements or in how they influence EE in other ways. Moreover, the contribution of fat to EE is lower than that of lean tissue. Use of metabolic mass for normalisation, based on interspecific scaling exponents (0.75 or 0.66), is similarly flawed. It is best to use analysis of covariance to determine the relationship of EE to body mass or fat-free mass within each group, and then test whether this relationship differs between groups.

Central regulation of energy balance: inputs, outputs and leptin resistance

The regulation of energy balance is complex and, in man, imprecise. Nevertheless, in many individuals intake and expenditure are balanced with <1% error with little or no conscious effect. Essential components of such a regulatory system are signals, leptin and insulin, that reflect the size of lipid stores. Leptin receptors signal via phosphatidylinositol 3-kinase (as do insulin receptors) and via the transcription factor signal transducer and activator of transcription-3 to activate various types of neurone. Obese rodents, and possibly man, are resistant to leptin; in some cases because of genetic or perinatal programming (primary resistance), but commonly in response to high leptin levels (secondary resistance). Secondary leptin resistance may be a result of reduced transport of leptin to the brain or down-regulation of leptin signalling. Signals that reflect lipid stores form the tonic homeostatic regulatory system. They interact with episodic homeostatic signals carried by neurones, hormones and metabolites to regulate meal size and frequency. They also interact with signals related to the palatability of food, biorhythms and learning. Many neurotransmitters and hormones mediate responses to more than one input (e.g. gastric and adipocyte leptin), but are nevertheless most involved with particular inputs (e.g. leptin with adipocyte fat stores). Feeding can be divided into appetitive (preparation for feeding) and consummatory phases, which can both be further subdivided. Different sets of neurotransmitters and hormones are involved at each stage. In the long term it may be possible to customise obesity therapies according to those inputs and outputs that are most disturbed and most amenable to intervention in individual subjects.

Using Obese Mouse Models in Research: Special Considerations for IACUC Members, Animal Care Technicians, and Researchers

The mouse is the animal most commonly used to study the underlying causes of and treatments for obesity. The author reviews many of the issues that should be considered by all involved in research with mice expressing this phenotype, and describes some procedures exclusive to obesity research.