Unravelling the mysterious roles of melanocortin-3 receptors in metabolic homeostasis and obesity using mouse genetics.

The central nervous melanocortin system maintains body mass and adiposity within a 'healthy' range by regulating satiety and metabolic homeostasis. Neural melanocortin-4 receptors (MC4R) modulate satiety signals and regulate autonomic outputs governing glucose and lipid metabolism in the periphery. The functions of melanocortin-3 receptors (MC3R) have been less well defined. We have observed that food anticipatory activity (FAA) is attenuated in Mc3r-/- mice housed in light:dark or constant dark conditions. Mc3r-/- mice subjected to the restricted feeding protocol that was used to induce FAA also developed insulin resistance, dyslipidaemia, impaired glucose tolerance and evidence of a cellular stress response in the liver. MC3Rs may thus function as modulators of oscillator systems that govern circadian rhythms, integrating signals from nutrient sensors to facilitate synchronizing peak foraging behaviour and metabolic efficiency with nutrient availability. To dissect the functions of MC3Rs expressed in hypothalamic and extra-hypothalamic structures, we inserted a 'lox-stop-lox' (TB) sequence into the Mc3r gene. Mc3r (TB/TB) mice recapitulate the phenotype reported for Mc3r-/- mice: increased adiposity, accelerated diet-induced obesity and attenuated FAA. The ventromedial hypothalamus exhibits high levels of Mc3r expression; however, restoring the expression of the LoxTB Mc3r allele in this nucleus did not restore FAA. However, a surprising outcome came from studies using Nestin-Cre to restore the expression of the LoxTB Mc3r allele in the nervous system. These data suggest that 'non-neural' MC3Rs have a role in the defence of body weight. Future studies examining the homeostatic functions of MC3Rs should therefore consider actions outside the central nervous system.

Brown fat thermogenesis and body weight regulation in mice: relevance to humans.

Physiological, pharmacological and genetic studies in dogs, mice and rats have established that the uncoupling protein-1 (UCP1)-based brown adipose tissue system has an important role in the regulation of body temperature. Although it may be possible to create laboratory conditions in which mice with inactivated Ucp1 can survive in a modestly cooled environment, data overwhelmingly support the conclusion that the UCP1/BAT system has evolved to maintain body temperature at 37 °C. The corollary to this conclusion is that any influence UCP1/BAT might have on body weight regulation is a secondary function. The idea that BAT prevents obesity by burning off excess energy to maintain energy balance seems incompatible with evolutionary biology. Premodern humans spent an enormous amount of energy either running to catch their meal or avoiding becoming a meal themselves; consequently, there was no obesity. Nevertheless, although secondary to body temperature regulation, UCP1/BAT is extraordinarily effective at reducing adiposity and insulin resistance in mice and rats. Variation among mice in susceptibility to diet-induced obesity is correlated with the induction of brown adipocytes in traditional white fat depots (wBAT). Both genetic and cell biology-based experimentation have shown that the cellular origins of wBAT are different from those of interscapular-like brown adipocytes (iBAT). Do they have different functions? We have analyzed the effects of the early nutritional environment on the induction of brown adipocytes in inguinal fat to test the hypothesis that wBAT is primarily involved in body weight regulation. Although undernutrition during lactation severely suppresses wBAT at 21 days of age, undernourished mice fed a normal chow diet ad libitum at weaning recovered their normal wBAT and iBAT systems as young adults. The function of wBAT does not seem to be uniquely devoted to body weight regulation.

Synergistic gene interactions control the induction of the mitochondrial uncoupling protein (Ucp1) gene in white fat tissue.

Among a selected group of mouse strains susceptible to dietary obesity, those with an enhanced capacity for Ucp1 and brown adipocyte induction in white fat preferentially lost body weight following adrenergic stimulation. Based on the generality of this mechanism for reducing obesity, a genetic analysis was initiated to identify genes that control brown adipocyte induction in white fat depots in mice. Quantitative trait locus (QTL) analysis was performed using the variations of retroperitoneal fat Ucp1 mRNA expression in progeny of genetic crosses between the A/J and C57BL/6J parental strains and selected AXB recombinant inbred strains. Three A/J-derived loci on chromosomes 2, 3, and 8 and one C57BL/6J locus on chromosome 19 were linked to Ucp1 induction in retroperitoneal fat. Although A/J-derived alleles seemed to contribute to elevated Ucp1 expression, the C57BL/6J allele on chromosome 19 increased Ucp1 mRNA to levels higher than parental values. Thus, novel patterns of C57BL/6J and A/J recombinant genotypes among the four mapped loci resulted in a transgressive variation of Ucp1 phenotypes. Although the extent of the interchromosomal interactions have not been fully explored, strong synergistic interactions occur between a C57BL/6J allele on chromosome 19 and an A/J allele on chromosome 8. In addition to selective synergistic interactions between loci, variations in recessive and dominant effects also contribute to the final levels of Ucp1 expression.

Mitochondria uncoupling proteins and obesity: molecular and genetic aspects of UCP1.

Genetic variation in brwon fat specific mitochondrial uncoupling protein-1 (UCP1) expression and brown adipocyte morphology, have provided models to test the hypothesis that nonshivering thermogenesis is associated with the regulation of body weight. Genetic manipulation using transgenic animals and gene targeting, has resulted in mice with an over-expression of UCP1. These variant animals consistently show that over-expression of UCP1 reduced adiposity. On the other hand, less agreement is found in models that reduce nonshivering thermogenesis. Inactivation of the UCP1 gene, by gene targeting, does not increase adiposity when compared to control animals; however, a mouse expressing the UCP1-DTA transgene (UCPI-diphtheria toxin A chain), in which there is a modest reduction in the number of brown adipocytes, becomes obese. Other phenotypes of this mouse, the hyperphagia, extreme resistance to leptin administration, retinopathy and high residual content of brown adipocytes, suggest that the effects of the transgene may be more extensive than simply a 60% reduction in the number of brown adipocytes. Ectopic expression of UCP1-DTA in the brain could explain the phenotype of this mouse in a manner more consistent with the results of other models with altered UCP1 and brown adipocyte expression.

Abnormal nonshivering thermogenesis in mice with inherited defects of fatty acid oxidation.

When placed in the cold (4 degreesC), BALB/cByJ mice of both genders rapidly lose body temperature as compared with the control strain, C57BL/6J. This sensitivity to cold resembles that previously described for mice with a defect in nonshivering thermogenesis due to the targeted inactivation of the brown adipocyte-specific mitochondrial uncoupling protein gene, Ucp1. Genetic mapping of the trait placed the gene on chromosome 5 near Acads, a gene encoding the short chain acyl CoA dehydrogenase, which is mutated in BALB/cByJ mice. The analysis of candidate genes in the region indicated a defect only in the expression of Acads. Confirmation of the importance of fatty acid oxidation to thermogenesis came from our finding that mice carrying the targeted inactivation of the long chain acyl CoA dehydrogenase gene (Acadl) are also sensitive to the cold. Both of these mutations attenuate the induction of genes normally responsive to adrenergic signaling in brown adipocytes. These results suggest that the action of fatty acids as regulators of gene expression has been perturbed in the mutant mice. From a clinical perspective, it is important to determine whether defects in thermogenesis may be a phenotype in human neonates with inherited deficiencies in fatty acid beta-oxidation.

An edited linkage map for the AXB and BXA recombinant inbred mouse strains.

We have updated the history of the AXB and BXA recombinant inbred (RI) strains, typed additional loci, and edited the AXB, BXA RI database. Thirteen of the original 51 AXB and BXA RI strains are either extinct or genetically contaminated, leaving 33 living strains available from The Jackson Laboratory. However, we found a high degree of similarity among three sets of strains, indicating that these strains are not independent, which leaves 27 independent RI strains in the set. Accordingly, we modified the database by combining the AXB and BXA RI sets and eliminating strains that were genetically contaminated or extinct with no available DNA. We added 92 newly typed loci, retyped some questionable genotypings, and removed loci with excessive double crossovers or an insufficient number of typed strains. The edited strain distribution pattern (SDP) is available on the World Wide Web (WWW) (http://www. informatics.jax.org/riset.html) and now includes over 700 loci. Each locus is linked to adjacent loci with a LOD score of at least 3.0 with a few described exceptions. We also carried out a second editing designed for the analysis of quantitative trait loci by deleting extinct strains and loci with identical SDPs; this edited database is also available on the WWW.

Emergence of brown adipocytes in white fat in mice is under genetic control. Effects on body weight and adiposity.

The mRNA levels for the mitochondrial uncoupling protein (UCP1) in fat tissues of A/J and C57BL/6J inbred strains of mice varied in a regional-specific manner after stimulation of adrenergic signaling by cold exposure or treatment with a beta3-adrenergic agonist. While the differences between strains were minimal in interscapular brown fat, large differences occurred in white fat tissues, particularly in retroperitoneal fat. Among the AXB recombinant inbred strains, the Ucp1 mRNA levels varied up to 130-fold. This large induction at the mRNA level was accompanied by a corresponding increase in brown adipocytes as revealed by immunohistology with anti-UCP1 antibodies. A high capacity to induce brown fat in areas of traditional white fat had no impact on the ability to gain weight in response to high fat and sucrose diets, but did correlate with the loss of weight in response to treatment with a beta3-adrenergic agonist (CL 316,243). This genetic variation in mice provides an experimental approach to identify genes controlling the induction of brown adipocytes in white fat tissues.

Sequence and tissue-dependent RNA expression of mouse FAD-linked glycerol-3-phosphate dehydrogenase.

A 2432-bp cDNA for mouse FAD-linked glycerol-3-phosphate dehydrogenase, a nuclear-encoded enzyme associated with the inner mitochondrial membrane, was isolated from a Lambda ZAP phage library generated from brown adipocyte mRNA. The amino acid sequence was 95 and 93% homologous to the rat and human enzymes. PvuII and SacI polymorphisms between Mus spretus and C57BL/6J were used to map the mouse FAD-linked glycerol-3-phosphate dehydrogenase gene (Gdm1) to chromosome 2, 33 cM from the centromere. Northern blot analysis showed that brown adipose tissue predominantly expressed a 6.5-kb mRNA with lower expression of 4.5- and 2.4-kb forms, whereas brain and pancreatic islets almost exclusively expressed a 6.5-kb transcript, muscle expressed a 4.5-kb transcript, and testis expressed a 2.4-kb RNA form. Analysis of poly(A)+ RNA from brown adipose tissue suggested that all RNA forms are polyadenylated. Among the tissues examined, FAD-linked glycerol-3-phosphate dehydrogenase protein levels and enzyme activity were highest in brown adipose tissue, and a consistent correlation between protein levels and enzyme activity in all tissues was observed. RNA levels corresponded to protein levels in all tissues except testis, where high levels of 2.4-kb mRNA and relatively low protein were expressed. Exposing mice to cold temperatures induced the 6.5-kb mRNA and enzyme activity only in brown adipose tissue, suggesting a role for thermogenesis in this tissue. Although the molecular basis for the formation of the 6.5- and 4.5-kb mRNA's is not known, data suggest that the regulation of FAD-linked glycerol-3-phosphate dehydrogenase is complex and tissue-specific.

Elevated ornithine decarboxylase activity, polyamines and cell proliferation in neoplastic and vacuolated liver cells of winter flounder (Pleuronectes americanus)

Liver neoplasms, including hepatocellular and cholangiocellular tumors, commonly occur in winter flounder (Pleuronectes americanus) caught from some chemically contaminated areas such as Boston Harbor. Hydropically vacuolated cells, very often associated with neoplasia in winter flounder liver, appear to represent the first cellular abnormality in animals that later develop frank neoplasms. The proliferative capacity of hydropically vacuolated cells was studied by analyzing both ornithine decarboxylase (ODC) activity and bromodeoxyuridine (BrdU) labeling indices. Liver of winter flounder with vacuolated cellular lesions had ODC activity more than 5- to 12-fold greater than that in liver that lacked such vacuolation, whether caught from Boston Harbor or Georges Bank. Large focal areas of hydropically vacuolated cells dissected from severely affected livers had ODC activity as high or higher than surrounding parenchymal tissue. Significant elevations in hepatic polyamine levels and ratios of putrescine/spermidine were also present in all Boston Harbor animals studied, especially those exhibiting vacuolated cellular lesions, as compared to Georges Bank fish. BrdU labeling techniques indicate that hydropically vacuolated cells, along with perivacuolar small basophilic cells and neoplastic cholangiocytes, appear to have the capacity to synthesize DNA and undergo mitosis. The frequent association of hydropically vacuolated cells with hepatic neoplasia, along with high ODC activity and DNA synthesis capability, suggest that the vacuolated cells and/or perivacuolar basophilic cells may be integral to the development of some neoplastic phenotypes in winter flounder liver.

Deficiencies in DNA replication and cell-cycle progression in polyamine-depleted HeLa cells.

Synchronized HeLa cells depleted of polyamines by alpha-difluoromethylornithine exhibited substantially decreased DNA synthesis, and proliferation ceased after the release of the cells into S phase. Nuclei from these cells synthesized 70-80% less DNA than did nuclei from control cells. Extraction of isolated nuclei with 0.3 M-KCl decreased DNA synthesis by about 60%, which was recovered almost completely in control cell nuclei by reconstitution with the salt extracts of these nuclei. On the other hand, salt extracts of polyamine-depleted nuclei restored only 50% of DNA synthesis in extracted control nuclei. Salt extracts of control cell nuclei contained twice the DNA polymerase alpha activity of polyamine-depleted nuclear extracts. Extracts of cell lysates of both control and polyamine-depleted HeLa cells exhibited similar DNA polymerase alpha activity, suggesting that uptake of the enzyme or its retention by the nuclei of polyamine-depleted cells was decreased. Polyamine-depleted nuclei also showed altered phosphorylation of a 31 kDa protein as compared with control nuclei. Almost normal DNA synthesis, cell proliferation, DNA polymerase alpha activity and nuclear protein phosphorylation were restored in polyamine-depleted cells grown in medium supplemented with 20 microM-spermidine at least 10-12 h before S phase. Cultures in which proliferation was blocked by alpha-difluoromethylornithine did not exhibit synchronous growth after the block was removed. Thus it may be concluded that HeLa cells depleted of polyamines are not inhibited at a single control point in the cell cycle, but are arrested at diverse sites throughout G1 phase.

Constitutively elevated levels of ornithine and polyamines in mouse epidermal papillomas.

Epidermal papillomas were induced in CD-1 mice by a single topical application of 7,12-dimethylbenzanthracene (DMBA) followed by twice weekly applications of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in acetone. Control groups consisted of mice treated singly or chronically with acetone or TPA. TPA induced a rapid, yet transient 500- to 1000-fold increase in ornithine decarboxylase (ODC) activity which resulted in a 2- to 8.4-fold elevation of putrescine in both singly or chronically TPA-treated mouse epidermis 4-6 h after its application. After 24 h, levels of spermidine, but not spermine, were also elevated. The ODC and arginase activities in the 11 individual papillomas studied averaged 400- and 26-fold higher respectively than basal levels in epidermis. The activity of ODC in most papillomas, unlike ODC in epidermis, could be stimulated by guanosine 5'-triphosphate (GTP). Putrescine and spermidine levels in papillomas, especially those exhibiting highly GTP-stimulated ODC, were substantially higher compared to either normal or TPA-treated epidermis. Although epidermis contains a relatively high ornithine content, its level is even further elevated in papillomas, in some cases as much as 70-fold. The consequences of the constitutively elevated polyamine levels in papillomas caused by the loss of control over the normally tightly regulated polyamine biosynthetic pathway are not known, but could be important in regulating the balance between proliferation and differentiation in this self-renewing epithelial tissue.

Polyamines and HeLa-cell DNA replication.

HeLa cells were synchronized for S-phase DNA synthesis by the double thymidine-block procedure. A comparison was made of the polyamine content and S-phase DNA synthesis in cells from control cultures and cultures to which an inhibitor of polyamine biosynthesis, alpha-difluoromethylornithine, was added to the synchronization medium. Control cells showed a peak of synchronous DNA synthesis at 3 h and a maximum concentration of polyamines at 6-9 h after release of the second thymidine block. Cells from cultures containing the inhibitor were severely inhibited in the synthesis of DNA and contained no putrescine and only traces of spermidine while the spermine content was lowered by as much as 80%. Supplementation of cultures containing alpha-difluoromethylornithine with a polyamine, at the time of release of the second thymidine block, replenished the intracellular pool of the administered polyamine and partially restored S-phase DNA synthesis, with a lag of 3-6 h. Almost complete restoration of DNA synthesis in cells depleted of polyamines was achieved by the addition of a polyamine to cultures at least 10 h before release of the second thymidine block. The lag in initiation of synchronous S-phase DNA synthesis was eliminated in these cells. It is concluded that reversal by polyamines of the deficiency in S-phase DNA synthesis, in polyamine-depleted HeLa cells, is a time-dependent process indicative of the necessity for the replenishment of replication factors or their organization into an active replication complex.

Phase transitions in phosphatidylcholine dispersion observed with an interference refractometer.

An interferometer is used to measure the refractive index change accompanying the crystal-to-liquid-crystal phase transition in the dispersion of phosphatidylcholines. Two separate methods of obtaining the refractive index change are employed: the first method analyzes the intensity transmitted through a spatial filter and the second method utilizes a piezoeletric crystal-based electronic compensator. The results of the two methods agree well. The accuracy of the apparatus (6 X 10(-6)) permitted us to use a very dilute sample to detect the phase change. Only a fraction of a milligram of dry lecithin is needed to observe the change. The result confirms conclusively that the major reason for the turbidity change at the transition temperature is the alteration in the refractive index of the lipid membranes. The fractional change in the refractive index does not agree well with the fractional change in the density of lipid molecules in vesicles.