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Pathophysiology and Genetics of Obesity and Diabetes in the New Zealand Obese Mouse: A Model of the Human Metabolic Syndrome
Abstract and Figures
The New Zealand Obese (NZO) mouse is one of the most thoroughly investigated polygenic models for the human metabolic syndrome and type 2 diabetes. It presents the main characteristics of the disease complex, including early-onset obesity, insulin resistance, dyslipidemia, and hypertension. As a consequence of this syndrome, a combination of lipotoxicity and glucotoxicity produces beta-cell failure and apoptosis resulting in hypoinsulinemia and diabetic hyperglycemia. With NZO as a breeding partner, several adipogenic and diabetogenic gene variants have been identified by hypothesis-free positional cloning (Tbc1d1, Zfp69) or by combining genetic screens and candidate gene approaches (Pctp, Abcg1, Nmur2, Lepr). This chapter summarizes the present knowledge of the NZO strain and describes its pathophysiology as well as the known underlying genetic defects.
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... The NZO mouse is one of the most thoroughly studied polygenic mouse models of both human metabolic syndrome as well as T2DM [66]. The inbred strain, generated in 1948 by Marianne and Franz Bielschowsky at the University of Otago in New Zealand, was obtained by mating mice with an agouti coat color. ...
... Lower spontaneous activity in the NZO mouse leads to reduced energy consumption and thus contributes to the development of obesity [69]. Studies using this strain and the crossing of NZO mice with lean mouse strains, such as the Swiss Jim Lambert (SJL) or the C57BL/6 mouse strain, have identified diabetogenic and adipogenic gene variants [66]. It should be noted that these gene variants may originate from the lean mouse strain, therefore becoming active once the mice are rendered obese by crossbreeding with a polygenic strain such as the NZO [70][71][72]. ...
... Since diabetes mellitus is a polygenic disease in humans, NZO mice are a suitable model for the study of the pathophysiology [73]. Development of overt T2DM is limited to males in all known NZO sublines [66,74]. In contrast, female mice of this strain are protected from hyperglycemia and β-cell death partly due to the sex hormone estrogen [66]. ...
Gestational diabetes mellitus (GDM) is currently the most common complication of pregnancy and is defined as a glucose intolerance disorder with recognition during pregnancy. GDM is considered a uniform group of patients in conventional guidelines. In recent years, evidence of the disease’s heterogeneity has led to a growing understanding of the value of dividing patients into different subpopulations. Furthermore, in view of the increasing incidence of hyperglycemia outside pregnancy, it is likely that many cases diagnosed as GDM are in fact patients with undiagnosed pre-pregnancy impaired glucose tolerance (IGT). Experimental models contribute significantly to the understanding of the pathogenesis of GDM and numerous animal models have been described in the literature. The aim of this review is to provide an overview of the existing mouse models of GDM, in particular those that have been obtained by genetic manipulation. However, these commonly used models have certain limitations in the study of the pathogenesis of GDM and cannot fully describe the heterogeneous spectrum of this polygenic disease. The polygenic New Zealand obese (NZO) mouse is introduced as a recently emerged model of a subpopulation of GDM. Although this strain lacks conventional GDM, it exhibits prediabetes and an IGT both preconceptionally and during gestation. In addition, it should be emphasized that the choice of an appropriate control strain is of great importance in metabolic studies. The commonly used control strain C57BL/6N, which exhibits IGT during gestation, is discussed in this review as a potential model of GDM.
... In addition to apparent RGC loss, aged NZO mice have been shown to exhibit most features of metabolic syndrome (69)(70)(71). It is well established that retinal microvascular dysfunction coincides with metabolic stressincluding diabetes and hypertension (72,73). ...
... ; https://doi.org/10.1101/2024.04.16.589625 doi: bioRxiv preprint prior to overt DR (82). NZO mice are a polygenic model of metabolic syndrome exhibiting profound obesity, hypertension, hyperlipidemia, and insulin resistance, with approximately 50% of male mice becoming overtly hyperglycemic with age (69,71). We observed no clear sex-dependent differences in the progression of NZOassociated retinal phenotypes, suggesting these changes may be due to the milieu of metabolic impairments NZO animals develop beyond hyperglycemia alone. ...
Background
Age is the principal risk factor for neurodegeneration in both the retina and brain. The retina and brain share many biological properties; thus, insights into retinal aging and degeneration may shed light onto similar processes in the brain. Genetic makeup strongly influences susceptibility to age-related retinal disease. However, studies investigating retinal aging have not sufficiently accounted for genetic diversity. Therefore, examining molecular aging in the retina across different genetic backgrounds will enhance our understanding of human-relevant aging and degeneration in both the retina and brain—potentially improving therapeutic approaches to these debilitating conditions.
Methods
Transcriptomics and proteomics were employed to elucidate retinal aging signatures in nine genetically diverse mouse strains (C57BL/6J, 129S1/SvlmJ, NZO/HlLtJ, WSB/EiJ, CAST/EiJ, PWK/PhK, NOD/ShiLtJ, A/J, and BALB/cJ) across lifespan. These data predicted human disease-relevant changes in WSB and NZO strains. Accordingly, B6, WSB and NZO mice were subjected to human-relevant in vivo examinations at 4, 8, 12, and/or 18M, including: slit lamp, fundus imaging, optical coherence tomography, fluorescein angiography, and pattern/full-field electroretinography. Retinal morphology, vascular structure, and cell counts were assessed ex vivo .
Results
We identified common molecular aging signatures across the nine mouse strains, which included genes associated with photoreceptor function and immune activation. Genetic background strongly modulated these aging signatures. Analysis of cell type-specific marker genes predicted age-related loss of photoreceptors and retinal ganglion cells (RGCs) in WSB and NZO, respectively. Fundus exams revealed retinitis pigmentosa-relevant pigmentary abnormalities in WSB retinas and diabetic retinopathy (DR)-relevant cotton wool spots and exudates in NZO retinas. Profound photoreceptor dysfunction and loss were confirmed in WSB. Molecular analyses indicated changes in photoreceptor-specific proteins prior to loss, suggesting photoreceptor-intrinsic dysfunction in WSB. In addition, age-associated RGC dysfunction, loss, and concomitant microvascular dysfunction was observed in NZO mice. Proteomic analyses revealed an early reduction in protective antioxidant processes, which may underlie increased susceptibility to DR-relevant pathology in NZO.
Conclusions
Genetic context is a strong determinant of retinal aging, and our multi-omics resource can aid in understanding age-related diseases of the eye and brain. Our investigations identified and validated WSB and NZO mice as improved preclinical models relevant to common retinal neurodegenerative diseases.
... . CC-BY-NC-ND 4.0 International license made available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is 23 , C57BL/6J (Bl6), or global eNOS knockout (eNOS -/-) 24 mice. ...
Background
Endothelial function and cardioprotection through remote ischemic preconditioning (rIPC) are severely impaired in type 2 diabetes mellitus (T2DM). Proline-rich tyrosine kinase 2 (Pyk2), a downstream target of the insulin receptor, reduces endothelial nitric oxide synthase (eNOS) activity. Therapeutic options to rescue cardioprotection in T2DM and improve outcomes after acute myocardial infarction (AMI) are lacking. We hypothesized that vascular endothelium contributes to rIPC, and that inhibition of Pyk2 restores cardioprotection in T2DM through modulation of eNOS, thus limiting infarct size.
Methods
New Zealand Obese (NZO) mice were used as a polygenic model of T2DM. Effects of Pyk2-inhibition on endothelial function, remote ischemic preconditioning (rIPC), and infarct size (IS) after ischemia/reperfusion (I/R) were compared in NZO, eNOS KO, and C57Bl/6 (Bl6) mice. Plasma derived from mice and individuals with or without T2DM at baseline and after rIPC was transferred to isolated hearts and aortic rings to assess the effects of Pyk2-inhibition on remote tissue protection.
Results
Transfer experiments with plasma drawn from non-diabetic humans and mice exposed to rIPC demonstrate that endothelium-dependent signals for remote tissue protection are conveyed by plasma. Key features reflecting the glucometabolic spectrum in T2DM were detected in NZO mice, including hyperinsulinemia, insulin resistance, obesity, and impaired glucose tolerance. Similar to T2DM patients, these mice also revealed endothelial dysfunction with decreased flow-mediated dilation (FMD), reduced circulating nitrite levels, elevated arterial blood pressure, and larger infarct size after I/R. Pyk2 increased the phosphorylation of eNOS on its inhibitory site (Tyr656). Cardioprotective effects by rIPC were lost in NZO mice. Inhibition of Pyk2 restored endothelial function and rescued endothelium-dependent cardioprotection after rIPC displayed by lower IS and improved LV function post I/R.
Conclusion
Endothelial function contributing to remote tissue protection is severely impaired in diabetes mellitus. Proline-rich tyrosine kinase 2 is a novel target to rescue cardioprotection through endothelium-dependent remote ischemic preconditioning, advocating its role in limiting infarct size in diabetes mellitus.
Clinical perspective What is new?
Vascular endothelium contributes to remote tissue protection in ischemic preconditioning, which is severely impaired in diabetes
Proline-rich tyrosine kinase 2 reduces eNOS-activity, causes endothelial dysfunction, and impairs cardioprotection through ischemic preconditioning
Inhibition of proline-rich tyrosine kinase 2 restores eNOS activity, endothelial function, and cardioprotective effects of remote ischemic preconditioning limiting infarct size in an experimental model of diabetes.
What are the clinical implications?
Proper endothelial function is cirtical to maintain cardiovascular health. Endothelial dysfunction contributes to impaired remote tissue protection in diabetes.
These data demonstrate for the first time that endothelium-dependent cardioprotection in myocardial ischemia/reperfusion through remote ischemic preconditioning can be restored in diabetes.
Proline-rich tyrosine kinase 2 is a novel target to restore endothelium-dependent remote cardioprotection to improve the outcome of diabetic patients with acute myocardial infarction.
... It was observed that NZO mice have already increased body and liver weight on a SD in comparison to C57BL/6J mice. Indeed, several adipogenic gene variants have been identified that are responsible for accumulation of body fat and hyperphagic behavior [181]. However, NZO mice fed with HFD, containing 30% fat, resulted in severe overweight, accompanied by pronounced high liver weight ( Figure 17). ...
Scope: Several studies show that excessive lipid intake can cause hepatic steatosis. To investigate lipotoxicity on cellular level, palmitate (PA) is often used to highly increase lipid droplets (LDs). One way to remove LDs is autophagy, while it is controversially discussed if autophagy is also affected by PA. It is aimed to investigate whether PA-induced LD accumulation can impair autophagy and punicalagin, a natural autophagy inducer from pomegranate, can improve it. Methods and results: To verify the role of autophagy in LD degradation, HepG2 cells are treated with PA and analyzed for LD and perilipin 2 content in presence of autophagy inducer Torin 1 and inhibitor 3-Methyladenine. PA alone seems to initially induce autophagy-related proteins but impairs autophagic-flux in a time-dependent manner, considering 6 and 24 h PA. To examine whether punicalagin can prevent autophagy impairment, cells are cotreated for 24 h with PA and punicalagin. Results show that punicalagin preserves expression of autophagy-related proteins and autophagic flux, while simultaneously decreasing LDs and perilipin 2. Conclusion: Data provide new insights into the role of PA-induced excessive LD content on autophagy and suggest autophagy-inducing properties of punicalagin, indicating that punicalagin can be a health-beneficial compound for future research on lipotoxicity in liver.
... The polygenetic New Zealand Obese (NZO) mouse strain has been frequently used as a model for spontaneous polygenetic obesity and T2D. Both genders develop impaired glucose tolerance and obesity in response to a high fat diet (15); however, subsequent T2D progressing into pancreatic islet failure is limited to the males, whereas the females benefit from the protective influence of the hormone oestrogen (16,17). ...
To nominate novel disease genes for obesity and type 2 diabetes (T2D), we recently generated two mouse backcross populations of the T2D-susceptible New Zealand Obese (NZO/HI) mouse strain and two genetically different, lean and T2D-resistant strains, 129P2/OlaHsd and C3HeB/FeJ. Comparative linkage analysis of our two female backcross populations identified seven novel body fat-associated quantitative trait loci (QTL). Only the locus Nbw14 (NZO body weight on chromosome 14) showed linkage to obesity-related traits in both backcross populations, indicating that the causal gene variant is likely specific for the NZO strain as NZO allele carriers in both crosses displayed elevated body weight and fat mass. To identify candidate genes for Nbw14, we used a combined approach of gene expression and haplotype analysis to filter for NZO-specific gene variants in gonadal white adipose tissue (gWAT), defined as the main QTL-target tissue. Only two genes, Arl11 and Sgcg, fulfilled our candidate criteria. In addition, expression QTL analysis revealed cis-signals for both genes within the Nbw14 locus. Moreover, retroviral overexpression of Sgcg in 3 T3-L1 adipocytes resulted in increased insulin-stimulated glucose uptake. In humans, mRNA levels of SGCG correlated with BMI and body fat mass exclusively in diabetic subjects, suggesting that SGCG may present a novel marker for metabolically unhealthy obesity. In conclusion, our comparative-cross analysis could substantially improve the mapping resolution of the obesity locus Nbw14. Future studies will shine light on the mechanism by which Sgcg may protect from the development of obesity.
Diabetes mellitus is one of the world's most prevalent and complex metabolic disorders, and it is a rapidly growing global public health issue. It is characterized by hyperglycemia, a condition involving a high blood glucose level brought on by deficiencies in insulin secretion, decreased activity of insulin, or both. Prolonged effects of diabetes include cardiovascular problems, retinopathy, neuropathy, nephropathy, and vascular alterations in both macro‐ and micro‐blood vessels. In vivo and in vitro models have always been important for investigating and characterizing disease pathogenesis, identifying targets, and reviewing novel treatment options and medications. Fully understanding these models is crucial for the researchers so this review summarizes the different experimental in vivo and in vitro model options used to study diabetes and its consequences. The most popular in vivo studies involves the small animal models, such as rodent models, chemically induced diabetogens like streptozotocin and alloxan, and the possibility of deleting or overexpressing a specific gene by knockout and transgenic technologies on these animals. Other models include virally induced models, diet/nutrition induced diabetic animals, surgically induced models or pancreatectomy models, and non‐obese models. Large animals or non‐rodent models like porcine (pig), canine (dog), nonhuman primate, and Zebrafish models are also outlined. The in vitro models discussed are murine and human beta‐cell lines and pancreatic islets, human stem cells, and organoid cultures. The other enzymatic in vitro tests to assess diabetes include assay of amylase inhibition and inhibition of α‐glucosidase activity.
Obesity, defined by excessive body fat, is a highly complex condition affecting numerous physiological processes, such as metabolism, proliferation, and cellular homeostasis. These multifaceted effects impact cells and tissues throughout the host, including immune cells as well as cancer biology. Because of the multifaceted nature of obesity, common parameters used to define it (such as body mass index in humans) can be problematic, and more nuanced methods are needed to characterize the pleiotropic metabolic effects of obesity. Obesity is well-accepted as an overall negative prognostic factor for cancer incidence, progression, and outcome. This is in part due to the meta-inflammatory and immunosuppressive effects of obesity. Immunotherapy is increasingly used in cancer therapy, and there are many different types of immunotherapy approaches. The effects of obesity on immunotherapy have only recently been studied with the demonstration of an "obesity paradox", in which some immune therapies have been demonstrated to result in greater efficacy in obese subjects despite the direct adverse effects of obesity and excess body fat acting on the cancer itself. The multifactorial characteristics that influence the effects of obesity (age, sex, lean muscle mass, underlying metabolic conditions and drugs) further confound interpretation of clinical data and necessitate the use of more relevant preclinical models mirroring these variables in the human scenario. Such models will allow for more nuanced mechanistic assessment of how obesity can impact, both positively and negatively, cancer biology, host metabolism, immune regulation, and how these intersecting processes impact the delivery and outcome of cancer immunotherapy.
There is growing evidence that sex and gender differences play an important role in risk and pathophysiology of type 2 diabetes (T2D). Men develop T2D earlier than women, even though there is more obesity in young women than men. This difference in T2D prevalence is attenuated after the menopause. However, not all women are equally protected against T2D before the menopause, and gestational diabetes represents an important risk factor for future T2D. Biological mechanisms underlying sex and gender differences on T2D physiopathology are not yet fully understood. Sex hormones affect behavior and biological changes, and can have implications on lifestyle; thus, both sex-specific environmental and biological risk factors interact within a complex network to explain the differences in T2D risk and physiopathology in men and women. In addition, lifetime hormone fluctuations and body changes due to reproductive factors are generally more dramatic in women than men (ovarian cycle, pregnancy, and menopause). Progress in genetic studies and rodent models have significantly advanced our understanding of the biological pathways involved in the physiopathology of T2D. However, evidence of the sex-specific effects on genetic factors involved in T2D is still limited, and this gap of knowledge is even more important when investigating sex-specific differences during the life course. In this narrative review, we will focus on the current state of knowledge on the sex-specific effects of genetic factors associated with T2D over a lifetime, as well as the biological effects of these different hormonal stages on T2D risk. We will also discuss how biological insights from rodent models complement the genetic insights into the sex-dimorphism effects on T2D. Finally, we will suggest future directions to cover the knowledge gaps.
The review presents modern ideas about peripheral microhemodynamics, approaches to the ana-lysis of skin blood flow oscillations and their diagnostic significance. Disorders of skin microhemodynamics in type 2 diabetes mellitus (DM) and the possibility of their interpretation from the standpoint of external and internal interactions between systems of skin blood flow regulation, based on a comparison of couplings in normal and pathological conditions, including models of pathologies on animals, are considered. The factors and mechanisms of vasomotor regulation, among them receptors and signaling events in endothelial and smooth muscle cells considered as models of microvessels are discussed. Attention was drawn to the disturbance of Ca2+-dependent regulation of coupling between vascular cells and NO-dependent regulation of vasodilation in diabetes mellitus. The main mechanisms of insulin resistance in type 2 DM are considered to be a defect in the number of insulin receptors and impaired signal transduction from the receptor to phosphatidylinositol-3-kinase and downstream targets. Reactive oxygen species plays an important role in vascular dysfunction in hyperglycemia. It is assumed that the considered molecular and cellular mechanisms of microhemodynamics regulation are involved in the formation of skin blood flow oscillations. Parameters of skin blood microcirculation can be used as diagnostic and prognostic markers for assessing the state of the body.
Background
The prevalence of gestational diabetes mellitus (GDM) is increasing worldwide. There is increasing evidence that GDM is a heterogeneous disease with different subtypes. An important question in this context is whether impaired glucose tolerance (IGT), which is a typical feature of the disease, may already be present before pregnancy and manifestation of the disease. The latter type resembles in its clinical manifestation prediabetes that has not yet manifested as type 2 diabetes (T2DM). Altered lipid metabolism plays a crucial role in the disorder's pathophysiology. The aim was to investigate the role of lipids which are relevant in diabetes-like phenotypes in these both models with different time of initial onset of IGT.
Methods
Two rodent models reflecting different characteristics of human GDM were used to characterize changes in lipid metabolism occurring during gestation. Since the New Zealand obese (NZO)-mice already exhibit IGT before and during gestation, they served as a subtype model for GDM with preexisting IGT (preIGT) and were compared with C57BL/6 N mice with transient IGT acquired during gestation (aqIGT). While the latter model does not develop manifest diabetes even under metabolic stress conditions, the NZO mouse is prone to severe disease progression later in life. Metabolically healthy Naval Medical Research Institute (NMRI) mice served as controls.
Results
In contrast to the aqIGT model, preIGT mice showed hyperlipidemia during gestation with elevated free fatty acids (FFA), triglycerides (TG), and increased atherogenic index. Interestingly, sphingomyelin (SM) concentrations in the liver decreased during gestation concomitantly with an increase in the sphingosine-1-phosphate (S1P) concentration in plasma. Further, preIGT mice showed impaired hepatic weight adjustment and alterations in hepatic FFA metabolism during gestation. This was accompanied by decreased expression of peroxisome proliferator-activated receptor alpha (PPARα) and lack of translocation of fatty acid translocase (FAT/CD36) to the hepatocellular plasma membrane.
Conclusion
The preIGT model showed impaired lipid metabolism both in plasma and liver, as well as features of insulin resistance consistent with increased S1P concentrations, and in these characteristics, the preIGT model differs from the common GDM subtype with aqIGT. Thus, concomitantly elevated plasma FFA and S1P concentrations, in addition to general shifts in sphingolipid fractions, could be an interesting signal that the metabolic disorder existed before gestation and that future pregnancies require more intensive monitoring to avoid complications.
Graphical Abstract
This graphical abstract was created with BioRender.com .
Background and aims: Centrally administered neuromedin U (NmU) suppresses food intake through a hypothalamic receptor (NmuR2). The goal of the project was to evaluate the effect of NmU in a polygenic mouse model, the New Zealand obese (NZO) mouse. Methods: NmU was injected intracerebroventricular (icv) in C57BL/6 and NZO mice and feeding behavior was monitored. NmuR2 wild-type and its variants were expressed in HEK293 cells and calcium flux was assayed by FLIPR experiments after stimulation with NmU. Results: The anorexigenic effect of an icv administration of NmU in lean C57BL/6J mice was mainly due to a reduction of the number of meals. This effect was markedly reduced in NZO mice (15% vs 60% reduction in C57BL/6J). NZO mice carry a variant NmU receptor gene 2 (nmur2) with 35 single nucleotide polymorphisms including two missense mutations (NmuR2(V190M/1202M)). In an [NZOxC57BL/6] out-cross population, carriers of NmuR2(V190M/1202M) exhibited significantly higher body weight than carriers of the wild type allele. Transfection of HEK293 cells with wild-type or NmuR2(1202M) cDNA resulted in a dose-dependent calcium increase in response to a stimulation with NmU with an effective concentration (EC(50)) of 3.0+/-1.3 and 3.2+/-0.9 nM, respectively. In contrast, cells expressing the NmuR2(V190M) or NmuR2(V190M/1202M) variant exhibited significantly (p<0.001) higher EC(50) of 6.9+/-2.9 and 8.7+/-3.9 nM, respectively. Conclusions: These data suggest that resistance to the anorexigenic effect of NmU contributes to the polygenic obesity of NZO mice, and that it may reflect an impaired signal transduction of the NmuR2(V190M/1202M) variant Thus, we propose that NmuR2 is a potential candidate gene for human obesity.
New Zealand Obese (NZO) male mice develop a polygenic juvenile-onset obesity and maturity-onset hyperinsulinemia and hyperglycemia (diabesity). Here we report on metabolic and molecular changes associated with the antidiabesity action of CL316,243 (CL), a β3-adrenergic receptor agonist. Dietary CL treatment initiated at weaning reduced the peripubertal rise in body weight and adiposity while promoting growth without suppressing hyperphagia. The changes in adiposity, in turn, suppressed development of hyperinsulinemia, hyperleptinemia, hyperlipidemia, and hyperglycemia. These CL-induced alterations were reflected by decreased adipose tissue mass, increased expression of transcripts for uncoupling protein-1 (UCP-1), peroxisome proliferator—activated receptor alpha (PPARα), peroxisome proliferater-activated receptor coactivator-1 (PGC-1), and robust development of brown adipocyte function in white fat. Increased drug-mediated energy dissipation elicited a 1.5°C increase in whole body temperature under conditions of increased food intake but with no change in physical activity. Indirect calorimetry of mice treated with CL showed both increased energy expenditure and a restoration of a prominent diurnal pattern in the respiratory exchange ratio suggesting improved nutrient sensing. Our data suggest that CL promotes increased energy dissipation in white and brown fat depots by augmenting thermogenesis and by metabolic re-partitioning of energy in a diabesity-protective fashion. This is the first report demonstrating the effects of dietary β3-agonist in preventing the onset of diabesity in a polygenic rodent model of type 2 diabetes.
Immunology and Cell Biology focuses on the general functioning of the immune system in its broadest sense, with a particular emphasis on its cell biology. Areas that are covered include but are not limited to: Cellular immunology, Innate and adaptive immunity, Immune responses to pathogens,Tumour immunology,Immunopathology, Immunotherapy, Immunogenetics, Immunological studies in humans and model organisms (including mouse, rat, Drosophila etc)
Detailed reference data are given for survival and for all types of neo-plastic lesions found in inbred NZO/B1 mice from the original colonies. NZO/B1 mice are unusual in producing malignant lymphomas of Peyer's patches, and a high incidence of duodenal and lung tumours. They are very responsive to most chemical carcinogens. The significance generally of the data is briefly discussed, and reference statistics for neoplasms with more than 1% incidence in untreated animals are tabulated to assist other laboratories studying NZO/B1 mice.
Recent data have identified leptin as an afferent signal in a negative-feedback loop regulating the mass of the adipose tissue.
High leptin levels are observed in obese humans and rodents, suggesting that, in some cases, obesity is the result of leptin
insensitivity. This hypothesis was tested by comparing the response to peripherally and centrally administered leptin among
lean and three obese strains of mice: diet-induced obese AKR/J, New Zealand Obese (NZO), and Ay. Subcutaneous leptin infusion to lean mice resulted in a dose-dependent loss of body weight at physiologic plasma levels.
Chronic infusions of leptin intracerebroventricularly (i.c.v.) at doses of 3 ng/hr or greater resulted in complete depletion
of visible adipose tissue, which was maintained throughout 30 days of continuous i.c.v. infusion. Direct measurement of energy
balance indicated that leptin treatment did not increase total energy expenditure but prevented the decrease that follows
reduced food intake. Diet-induced obese mice lost weight in response to peripheral leptin but were less sensitive than lean
mice. NZO mice were unresponsive to peripheral leptin but were responsive to i.c.v. leptin. Ay mice did not respond to subcutaneous leptin and were 1/100 as sensitive to i.c.v. leptin. The decreased response to leptin
in diet-induced obese, NZO, and Ay mice suggests that obesity in these strains is the result of leptin resistance. In NZO mice, leptin resistance may be the
result of decreased transport of leptin into the cerebrospinal fluid, whereas in Ay mice, leptin resistance probably results from defects downstream of the leptin receptor in the hypothalamus.
The insulin content of the pancreas and the insulin-like activity of the blood serum are considerably higher in mice of the New Zealand obese strain than in normal controls.
The rate of glucose uptake by adipose tissue and hemi-diaphragms incubated in the absence of added insulin did not differ significantly in tissues from obese and normal mice and the stimulation of glucose uptake caused by addition of insulin was similar in tissues from the two strains. Insulin extracted from the pancreas of New Zealand obese mice stimulated glucose uptake by adipose tissue and muscle from obese mice to the same extent as an equivalent amount of porcine insulin.