Effects of leptin and melanocortin signaling interactions on pubertal development and reproduction.

Leptin and melanocortin signaling control ingestive behavior, energy balance, and substrate utilization, but only leptin signaling defects cause hypothalamic hypogonadism and infertility. Although GnRH neurons do not express leptin receptors, leptin influences GnRH neuron activity via regulation of immediate downstream mediators including the neuropeptides neuropeptide Y and the melanocortin agonist and antagonist, α-MSH, agouti-related peptide, respectively. Here we show that modulation of melanocortin signaling in female db/db mice through ablation of agouti-related peptide, or heterozygosity of melanocortin 4 receptor, restores the timing of pubertal onset, fertility, and lactation. Additionally, melanocortin 4 receptor activation increases action potential firing and induces c-Fos expression in GnRH neurons, providing further evidence that melanocortin signaling influences GnRH neuron activity. These studies thus establish melanocortin signaling as an important component in the leptin-mediated regulation of GnRH neuron activity, initiation of puberty and fertility.

Hematopoietic cell-specific deletion of toll-like receptor 4 ameliorates hepatic and adipose tissue insulin resistance in high-fat-fed mice.

Chronic low-grade inflammation, particularly in adipose tissue, is an important modulator of obesity-induced insulin resistance. The Toll-like receptor 4 (Tlr4) is a key initiator of inflammatory responses in macrophages. We performed bone marrow transplantation (BMT) of Tlr4lps-del or control C57Bl/10J donor cells into irradiated wild-type C57Bl6 recipient mice to generate hematopoietic cell-specific Tlr4 deletion mutant (BMT-Tlr4(-/-)) and control (BMT-WT) mice. After 16 weeks of a high-fat diet (HFD), BMT-WT mice developed obesity, hyperinsulinemia, glucose intolerance, and insulin resistance. In contrast, BMT-Tlr4(-/-) mice became obese but did not develop fasting hyperinsulinemia and had improved hepatic and adipose insulin sensitivity during euglycemic clamp studies, compared to HFD BMT-WT controls. HFD BMT-Tlr4(-/-) mice also showed markedly reduced adipose tissue inflammatory markers and macrophage content. In summary, our results indicate that Tlr4 signaling in hematopoietic-derived cells is important for the development of hepatic and adipose tissue insulin resistance in obese mice.

Collective and individual functions of leptin receptor modulated neurons controlling metabolism and ingestion.

Two known types of leptin-responsive neurons reside within the arcuate nucleus: the agouti gene-related peptide (AgRP)/neuropeptide Y (NPY) neuron and the proopiomelanocortin (POMC) neuron. By deleting the leptin receptor gene (Lepr) specifically in AgRP/NPY and/or POMC neurons of mice, we examined the several and combined contributions of these neurons to leptin action. Body weight and adiposity were increased by Lepr deletion from AgRP and POMC neurons individually, and simultaneous deletion in both neurons (A+P LEPR-KO mice) further increased these measures. Young (periweaning) A+P LEPR-KO mice exhibit hyperphagia and decreased energy expenditure, with increased weight gain, oxidative sparing of triglycerides, and increased fat accumulation. Interestingly, however, many of these abnormalities were attenuated in adult animals, and high doses of leptin partially suppress food intake in the A+P LEPR-KO mice. Although mildly hyperinsulinemic, the A+P LEPR-KO mice displayed normal glucose tolerance and fertility. Thus, AgRP/NPY and POMC neurons each play mandatory roles in aspects of leptin-regulated energy homeostasis, high leptin levels in adult mice mitigate the importance of leptin-responsiveness in these neurons for components of energy balance, suggesting the presence of other leptin-regulated pathways that partially compensate for the lack of leptin action on the POMC and AgRP/NPY neurons.

Inflammation and insulin resistance.

Obesity-induced chronic inflammation is a key component in the pathogenesis of insulin resistance and the Metabolic syndrome. In this review, we focus on the interconnection between obesity, inflammation and insulin resistance. Pro-inflammatory cytokines can cause insulin resistance in adipose tissue, skeletal muscle and liver by inhibiting insulin signal transduction. The sources of cytokines in insulin resistant states are the insulin target tissue themselves, primarily fat and liver, but to a larger extent the activated tissue resident macrophages. While the initiating factors of this inflammatory response remain to be fully determined, chronic inflammation in these tissues could cause localized insulin resistance via autocrine/paracrine cytokine signaling and systemic insulin resistance via endocrine cytokine signaling all of which contribute to the abnormal metabolic state.

Bone marrow-specific Cap gene deletion protects against high-fat diet-induced insulin resistance.

Cbl-associated protein (Cap) is a member of a phosphatidylinositol 3-kinase-independent pathway for insulin-stimulated translocation of the glucose transporter GLUT4. Despite this positive role of Cap in glucose uptake, here we show that deletion of the gene encoding Cap (official gene name: Sorbs1) protects against high-fat diet (HFD)-induced insulin resistance in mice while also having an opposite, insulin-sensitizing effect, accompanied by reduced tissue markers of inflammation. Given the emerging role of chronic inflammation in insulin resistance and the macrophage in initiating this inflammatory process, we considered that Sorbs1 deletion from macrophages may have resulted in the observed protection from HFD-induced insulin resistance. Using bone marrow transplantation to generate functional Sorbs1-null macrophages, we show that the insulin-sensitive phenotype can be transferred to wild-type mice by transplantation of Sorbs1-null bone marrow. These studies show that macrophages are an important cell type in the induction of insulin resistance and that Cap has a modulatory role in this function.

Complete rescue of obesity, diabetes, and infertility in db/db mice by neuron-specific LEPR-B transgenes.

We have generated mice that carry a neuron-specific leptin receptor (LEPR) transgene whose expression is driven by the rat synapsin I promoter synapsin-LEPR B (SYN-LEPR-B). We have also generated mice that are compound hemizygotes for the transgenes SYN-LEPR-B and neuron-specific enolase-LEPR B (NSE-LEPR-B). We observed a degree of correction in db/db mice that are hemizygous (Syn db/db) and homozygous (Syn/Syn db/db) for the SYN-LEPR-B transgene similar to that previously reported for the NSE-LEPR-B transgene. We also show complete correction of the obesity and related phenotypes of db/db mice that are hemizygous for both NSE-LEPR-B and SYN-LEPR-B transgenes (Nse+Syn db/db). Body composition, insulin sensitivity, and cold tolerance were completely normalized in Nse+Syn db/db mice at 12 weeks of age compared with lean controls. In situ hybridization for LEPR B isoform expression in Nse+Syn db/db mice showed robust expression in the energy homeostasis-relevant regions of the hypothalamus. Expression of 3 neuropeptide genes, agouti-related peptide (Agrp), neuropeptide Y (Npy), and proopiomelanocortin (Pomc), was fully normalized in dual transgenic db/db mice. The 2 transgenes in concert conferred normal fertility to male and female db/db mice. Male mice with partial peripheral deletion of Lepr, induced in the periweaning phase, did not show alterations in body composition or mass. In summary, we show that brain-specific leptin signaling is sufficient to reverse the obesity, diabetes, and infertility of db/db mice.

Quantitative trait locus mapping of genetic modifiers of metabolic syndrome and atherosclerosis in low-density lipoprotein receptor-deficient mice: identification of a locus for metabolic syndrome and increased atherosclerosis on chromosome 4.

OBJECTIVE: The purpose of this study was to examine genetic factors responsible for metabolic syndrome and atherosclerosis in a setting of low-density lipoprotein (LDL) receptor deficiency in a cross between C57BL/6J (B6) and PERA/Ei (PERA) inbred mouse strains. METHODS AND RESULTS: Comparison of metabolic phenotypes in B6 and PERA strains revealed the PERA genetic background to be dramatically more susceptible to hyperleptinemia, hyperglycemia, hypertriglyceridemia, elevated insulin levels, and body fat increase than the B6 background. To facilitate genetic analysis, metabolic syndrome-related traits and atherosclerotic lesion area were measured in 167 [(PERAxB6.129S7-Ldlr(tm1Her))xB6.129S7-Ldlr(tm1Her)]N2 male and female backcross mice that were homozygous for the Ldlr null allele. Quantitative trait locus analysis was performed using 153 polymorphic microsatellite markers spanning the genome. On chromosome 4, we identified a locus influencing plasma triglyceride, insulin, and leptin concentrations, body weight, and atherosclerosis. Several other genetic loci were identified with separate effects on plasma insulin, body weight, high-density lipoprotein cholesterol, and atherosclerosis. CONCLUSIONS: The PERA strain is highly susceptible to the development of metabolic syndrome after feeding a Western-type diet. This susceptibility is due, in part, to a locus on murine chromosome 4 in which PERA alleles predispose to adiposity, increased insulin, and accelerated atherogenesis in the absence of marked hyperlipidemia.