New leptin receptor mutations in mice: Lepr(db-rtnd), Lepr(db-dmpg) and Lepr(db-rlpy)

Diminished leptin signaling can alter circadian rhythm of metabolic activity and feeding

Leptin, a hormone mainly produced by fat cells, shows cell-specific effects to regulate feeding and metabolic activities. We propose that an important feature of metabolic dysregulation resulting in obesity is the loss of the circadian rhythm of biopotentials. This was tested in the pan-leptin receptor knockout (POKO) mice newly generated in our laboratory. In the POKO mice, leptin no longer induced pSTAT-3 signaling after intracerebroventricular injection. Three basic phenotypes were observed: the heterozygotes had similar weight and adiposity as the wild-type (WT) mice (>60% of the mice); the homozygotes were either fatter (∼30%), or rarely leaner (<5%) than the WT mice. By early adulthood, the POKO mice had higher average body weight and adiposity than their respective same-sex WT littermate controls, and this was consistent among different batches. The homozygote fat POKO showed significant reduction of midline estimating statistic of rhythm of circadian parameters, and shifts of ultradian rhythms. The blunted circadian rhythm of these extremely obese POKO mice was also seen in their physical inactivity, longer feeding bouts, and higher food intake. The extent of obesity correlated with the blunted circadian amplitude, accumulative metabolic and locomotor activities, and the severity of hyperphagia. This contrasts with the heterozygote POKO mice which showed little obesity and metabolic disturbance, and only subtle changes of the circadian rhythm of metabolic activity without alterations in feeding behavior. The results provide a novel aspect of leptin resistance, almost manifesting as an "all or none" phenomenon.

Rat thymectomy effects on leptin receptor and T-bet: erythroid hyperplasia with maturation arrest and suppressed T-cell-mediated hepatotoxicity

Thymectomy is an inevitable therapy for thymoma. Therefore, determining hemato-immune changes post-thymectomy is important. Twenty-six normal LEW/Sea rats thymectomized (Tx) at the ages of 38 +/- 5 days were followed without any treatment for 4 months (experiments [Exp] A and B). In addition, 16 LEW/Sea rats Tx at the age of 42 days (Exp C and D) and 10 non-Tx control LEW/Sea rats (Exp E) were immunized with syngeneic male liver cells 2 to 3 times and followed 2.7 months after the first immunization. Flow cytometric (FCM) analysis of mesenteric lymph nodes (MLN) and peripheral blood (PB) showed as follows: among the 26 Tx rats (Exp A and B), MLN lymphocyte population at 4 months postthymectomy was characterized by decreased numbers of CD4+ cells (22%-36%) or alphabeta T-cell receptor (TCR)-positive cells (16%-54%) and increased numbers of interleukin 2 receptors (IL-2R) (>90%). In the 16 Tx-immunized rats (Exp C and D), both alphabeta TCR (MLN) and CD45R (PB) expression on lymphocytes was suppressed with rather high numbers of CD4. Bone marrow (BM) and PB hematological studies of the partially and totally Tx rats indicated the following: 8 BM of the 10 totally Tx males (Exp A) and the 7/10 male spleen showed erythroid hyperplasia with maturation arrest at the stage of basophilic erythroblasts and reticulocytopenia in the PB. One of the 10 males, which had both BM myeloid to erythroid (M/E) ratio of 4.3 and spleen myeopoiesis, was in a more advanced stage, a prestage of pure red cell aplasia (PRCA) with 22% of CD4+ MLN cells. Syngeneic liver cell immunization resulted in the Tx rats as follows: hepatotoxicity based on the immunization was weaker in the 16 Tx rats (Exp C and D) than in the 10 non-Tx rats (Exp E). Polymerase chain reaction (PCR) of PB and MLN showed compensatory activated leptin receptor (LR) and T-bet DNA in 19 Tx rats (Exp A and B) with abnormal FCM findings. The ineffective erythropoiesis at 4 months after thymectomy was explained by the erythroblast LR/erythropoietin receptor (EPOR) dysfunction. The low grade hepatotoxicity in the Tx rats (Exp C and D) was explained by the disturbed Th1 reactions (or the disturbed T-bet gene transcription) at postthymectomy.

Structural and functional evidence supporting a role for leptin in central neural pathways influencing blood pressure in rats

Leptin, a peptide hormone normally associated with body weight homeostasis, is implicated in the generation of obesity-induced hypertension. Administration of leptin increases sympathetic nerve activity and blood pressure; however, the neural circuity involved in this pressor effect is not clearly defined. In this review we describe experiments in which pseudorabies virus was injected into the heart, kidney and the vasculature within skeletal muscle to reveal the distribution of neurones in the hypothalamus that project to these cardiovascular tissues. This distribution is compared to the well-documented distribution of leptin receptors. Finally we discuss microinjection studies designed to examine the effect of leptin, in these regions, on sympathetic nerve discharge and arterial blood pressure. Leptin injected directly into the ventromedial hypothalamus, arcuate nucleus and lateral hypothalamic area (particularly the perifornical area) increased lumbar sympathetic nerve activity. In addition, microinjection into the ventromedial hypothalamus and parvocellular paraventricular nucleus increased blood pressure. Our results demonstrate a discrete set of hypothalamic pathways that may underlie the involvement of leptin in obesity-induced hypertension.

Missense mutations in exon 4 of the porcine LEPR gene encoding extracellular domain and their association with fatness traits

Three non-synonymous single nucleotide polymorphisms (T221C, T232A and C233T) were detected in exon 4 of the porcine leptin receptor (LEPR) gene. The T232A substitution could be identified as a (Tsp509I) restriction fragment length polymorphism. The frequency of genotype TT varied in six genetic groups from 0.62 (Duroc) to 0.99 (Polish Large White). Sequencing of exon 4, performed for 30 animals, revealed that only two intragenic haplotypes (TC and AT at nucleotide position 232-233) were present. The phenotypic effect of the Tsp509I polymorphism was tested for the Polish Landrace (n = 241) and a synthetic line 990 (n = 243). There was no statistical evidence for the direct effect of the LEPR polymorphisms on fatness traits. However, in Polish Landrace allele A at position 232 was associated with thicker backfat over shoulder.

Diminished gallbladder motility in Rotund leptin-resistant obese mice

BACKGROUND

Obesity is a risk factor for cholesterol gallstone formation, but the pathogenesis of this phenomenon remains unclear. Most human obesity is associated with diabetes and leptin-resistance. Previous studies from this laboratory have demonstrated that diabetic leptin-resistant (Lep(db)) obese mice have low biliary cholesterol saturation indices, enlarged gallbladders and diminished gallbladder response to neurotransmitters. Recently, a novel leptin-resistant mouse strain Lepr(db-rtnd) (Rotund) has been discovered. Rotund mice are also obese, diabetic, and have an abnormal leptin receptor. Therefore, we tested the hypothesis that leptin-resistant obese Rotund mice would have large gallbladders and reduced biliary motility.

METHODS

Eight-week-old control (C57BL/6J, N=12) and Rotund leptin-resistant (Lepr(db-rnd), N=9) mice were fed a non- lithogenic diet for four weeks. Animals were fasted and underwent cholecystectomy. Gallbladder volumes were recorded, and contractile responses (N/cm(2)) to acetylcholine (10(-5) M), Neuropeptide Y (10(-8,-7,-6) M), and cholecystokinin (10(-10,-9,-8,-7) M) were measured. Results were analyzed using the Mann-Whitney Rank Sum Test.

RESULTS

Compared to control mice, Rotund mice had larger body weights, higher serum glucose levels, and greater gallbladder volumes (p<0.05). Rotund gallbladders had less contractility (p<0.05)) to acetylcholine and cholecystokinin than control mice. Responses to Neuropeptide Y were also less, but not statistically significant, in the Rotund mice.

CONCLUSIONS

These data suggest that leptin-resistant Rotund mice have (1) enlarged gallbladders with (2) diminished contractility compared to lean control mice. Therefore, this study confirms that leptin-resistance is associated with abnormal biliary motility and may lead to gallstone formation in leptin-resistant obesity.

Mouse models of diabetic nephropathy

Mice provide an experimental model of unparalleled flexibility for studying mammalian diseases. Inbred strains of mice exhibit substantial differences in their susceptibility to the renal complications of diabetes. Much remains to be established regarding the course of diabetic nephropathy (DN) in mice as well as defining those strains and/or mutants that are most susceptible to renal injury from diabetes. Through the use of the unique genetic reagents available in mice (including knockouts and transgenics), the validation of a mouse model reproducing human DN should significantly facilitate the understanding of the underlying genetic mechanisms that contribute to the development of DN. Establishment of an authentic mouse model of DN will undoubtedly facilitate testing of translational diagnostic and therapeutic interventions in mice before testing in humans.

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.