Mutations of pigment type switching have provided basic insight into melanocortin physiology and evolutionary adaptation. In all vertebrates that have been studied to date, two key genes, Agouti and Melanocortin 1 receptor (Mc1r), encode a ligand-receptor system that controls the switch between synthesis of red-yellow pheomelanin vs. black-brown eumelanin. However, in domestic dogs, historical studies based on pedigree and segregation analysis have suggested that the pigment type-switching system is more complicated and fundamentally different from other mammals. Using a genomewide linkage scan on a Labrador x greyhound cross segregating for black, yellow, and brindle coat colors, we demonstrate that pigment type switching is controlled by an additional gene, the K locus. Our results reveal three alleles with a dominance order of black (K(B)) > brindle (k(br)) > yellow (k(y)), whose genetic map position on dog chromosome 16 is distinct from the predicted location of other pigmentation genes. Interaction studies reveal that Mc1r is epistatic to variation at Agouti or K and that the epistatic relationship between Agouti and K depends on the alleles being tested. These findings suggest a molecular model for a new component of the melanocortin signaling pathway and reveal how coat-color patterns and pigmentary diversity have been shaped by recent selection. (+info)
(50/158) Diet-induced hypermethylation at agouti viable yellow is not inherited transgenerationally through the female.
The effects of nonmutagenic environmental exposures can sometimes be transmitted for several generations, suggesting transgenerational inheritance of induced epigenetic variation. Methyl donor supplementation of female mice during pregnancy induces CpG hypermethylation at the agouti viable yellow (A(vy)) allele in A(vy)/a offspring. Epigenetic inheritance occurs at A(vy); when passed through the female germ line, A(vy) epigenotype is not completely "reset." We therefore tested whether diet-induced epigenetic alterations at A(vy) are inherited transgenerationally. Female A(vy)/a mice were weaned onto either control (n=6) or a methyl-supplemented diet (n=5). These F0 dams were mated with a/a males. All F1 and F2 A(vy)/a females were weaned onto the same diet as their mothers, then mated with a/a males. F1, F2, and F3 A(vy)/a offspring were classified for coat color, an indicator of A(vy) methylation. In total, 62 F1, 98 F2, and 209 F3 A(vy)/a mice were studied. As expected, average A(vy)/a coat color was darker in the supplemented group (P<0.01). However, there was no cumulative effect of supplementation across successive generations. These results suggest that, in the female germ line, diet-induced A(vy) hypermethylation occurs in the absence of additional epigenetic modifications that normally confer transgenerational epigenetic inheritance at the locus. (+info)
(51/158) Effects of weaning and weaning weight on neuroendocrine regulators of feed intake in pigs.
A depression in feed intake and growth often occurs in the weaned pig. Spray-dried plasma is often added to nursery diets in an attempt to stimulate feed intake during this lag. The current study evaluated gene expression of appetite regulators in hypothalamus and adipose tissue 4 d after weaning. Barrows (2 wk of age) were cross-fostered to a sow (SOW, n = 8) or weaned and fed a nursery diet containing either 0 or 7% spray-dried plasma (NP, n = 8, and SDP, n = 8, respectively). Piglets were allocated such that 2 size groups existed within each experimental group: small (3.5 to 4.3 kg of BW piglets) and large (4.6 to 5.7 kg of BW piglets) subsets, based on weaning weight (WW), existed within each experimental group: small (3.5 to 4.3 kg piglets) and large (4.6 to 5.7 kg piglets). Animals were killed 4 d after weaning for tissue collection. There was a weaning group x WW interactive effect (P < 0.05) on hypothalamic neuropeptide Y messenger RNA expression, such that expression was least in the small SDP piglets. No WW or weaning group effects were seen on adipose leptin, hypothalamic leptin receptor, or hypothalamic proopiomelanocortin gene expression. An effect of WW was seen on hypothalamic neuropeptide Y, agouti-related protein, orexin, and type 2 orexin receptor gene expression, such that large pigs expressed greater amounts of these transcripts (P < 0.002). Strong positive correlations in gene expression were found among all of these genes, whose products are known to stimulate appetite. Partial correlation controlling for initial WW revealed that preweaning size explained most if not all of these associations. These data suggest that the postweaning expression of appetite-regulating genes is more dependent on preweaning conditions than on weaning diet. (+info)
(52/158) Obesity potentiates development of fatty liver and insulin resistance, but not atherosclerosis, in high-fat diet-fed agouti LDLR-deficient mice.
Obesity is increasing at an alarming rate, and its related disorders are placing a considerable strain on our healthcare system. Although they are not always coincident, obesity is often accompanied by hyperlipidemia. Both obesity and hyperlipidemia are independently associated with atherosclerosis, nonalcoholic fatty liver disease (NAFLD), and insulin resistance (IR). Thus, we sought to determine the relative contributions of obesity and hyperlipidemia to these associated pathologies. Obese agouti (A(y)/a) mice and their littermate controls (a/a) were placed on an LDL receptor (LDLR)(-/-) background. At 4 mo of age, mice were either maintained on chow diet (CD) or placed on Western diet (WD) for 12 wk. These genetic and dietary manipulations yielded four experimental groups: 1) lean, a/a;LDLR(-/-)CD; 2) genetic-induced obesity (GIO), A(y)/a;LDLR(-/-)CD; 3) diet-induced obesity (DIO), a/a;LDLR(-/-)WD; and 4) genetic- plus diet-induced obesity (GIO/DIO), A(y)/a;LDLR(-/-)WD. Lipoprotein profiles revealed increased VLDL and LDL particles in WD-fed mice compared with CD-fed controls. The hyperlipidemia present in this mouse model was the result of both increased hepatic triglyceride production and delayed lipoprotein clearance from the plasma. Both WD-fed groups exhibited similar levels of atherosclerotic lesion area, with increased obesity in the GIO/DIO group having no impact on atherogenesis. However, the severe obesity in the GIO/DIO group did aggravate NAFLD and IR. These findings suggest that, although obesity and hyperlipidemia exert individual pathological effects, the combination of the two has the potential to exert an additive effect on NAFLD and IR but not atherosclerosis in this mouse model. (+info)
(53/158) Agouti sequence polymorphisms in coyotes, wolves and dogs suggest hybridization.
Domestic dogs have been shown to have multiple alleles of the Agouti Signal Peptide (ASIP) in exon 4 and we wished to determine the level of polymorphism in the common wild canids of Canada, wolves and coyotes, in comparison. All Canadian coyotes and most wolves have banded hairs. The ASIP coding sequence of the wolf did not vary from the domestic dog but one variant was detected in exon 4 of coyotes that did not alter the arginine at this position. Two other differences were found in the sequence flanking exon 4 of coyotes compared with the 45 dogs and 1 wolf. The coyotes also demonstrated a relatively common polymorphism in the 3' UTR sequence that could be used for population studies. One of the ASIP alleles (R96C) in domestic dogs causes a solid black coat color in homozygotes. Although some wolves are melanistic, this phenotype does not appear to be caused by this same mutation. However, one wolf, potentially a dog-wolf hybrid or descendant thereof, was heterozygous for this allele. Likewise 2 coyotes, potentially dog-coyote or wolf-coyote hybrid descendants, were heterozygous for the several polymorphisms in and flanking exon 4. We could conclude that these were coyote-dog hybrids because both were heterozygous for 2 mutations causing fawn coat color in dogs. (+info)
(54/158) Humanized MC1R transgenic mice reveal human specific receptor function.
The melanocortin receptor, MC1R, is a key regulator of pigmentation in mammals, and is necessary for production of dark eumelanin pigment. Human MC1R variants with reduced or absent function are associated with red hair; mouse mutants result in yellow fur. Previous reports indicate differences between mouse and human receptors in their sensitivity to, and requirement for, alphaMSH agonist. We have generated a transgenic mouse model in which coat pigmentation is mediated solely by human MC1R. Although the hair pigment pattern is superficially normal, we show the human receptor is more sensitive to exogenous ligand than mouse Mc1r. Furthermore, although the endogenous receptor antagonist, agouti signalling protein, blocks activation of human MC1R, its action is unlike that on the mouse receptor in that it does not generate an inverse signal. In transfected cells, both receptors show ligand independent signalling. However, in transgenic mice, the human receptor does not elicit significant eumelanin synthesis in absence of ligand, in contrast to the mouse receptor which gives normal eumelanogenesis without ligand. Thus, the mouse model recapitulates the observation that humans mutated in POMC, the melanocortin precursor gene, lack eumelanin and have red hair. We suggest this apparent paradox can be explained by the much lower receptor number expressed in human versus mouse melanocytes, resulting in a much lower endogenous signalling in vivo. (+info)
(55/158) Involvement of dynein and spectrin with early melanosome transport and melanosomal protein trafficking.
Melanosomes are unique membrane-bound organelles specialized for the synthesis and distribution of melanin. Mechanisms involved in the trafficking of proteins to melanosomes and in the transport of mature pigmented melanosomes to the dendrites of melanocytic cells are being characterized, but details about those processes during early stages of melanosome maturation are not well understood. Early melanosomes must remain in the perinuclear area until critical components are assembled. In this study, we characterized the processing of two distinct melanosomal proteins, tyrosinase (TYR) and Pmel17, to elucidate protein processing in early or late steps of the secretory pathway, respectively, and to determine mechanisms underlying the subcellular localization and transport of early melanosomes. We used immunological, biochemical, and molecular approaches to demonstrate that the movement of early melanosomes in the perinuclear area depends primarily on microtubules but not on actin filaments. In contrast, the trafficking of TYR and Pmel17 depends on cytoplasmic dynein and its interaction with the spectrin/ankyrin system, which is involved with the sorting of cargo from the plasma membrane. These results provide important clues toward understanding the processes involved with early events in melanosome formation and transport. (+info)
(56/158) Carbenoxolone treatment attenuates symptoms of metabolic syndrome and atherogenesis in obese, hyperlipidemic mice.
Glucocorticoids, which are well established to regulate body fat mass distribution, adipocyte lipolysis, hepatic gluconeogenesis, and hepatocyte VLDL secretion, are speculated to play a role in the pathology of metabolic syndrome. Recent focus has been on the activity of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1), which is capable of regenerating, and thus amplifying, glucocorticoids in key metabolic tissues such as liver and adipose tissue. To determine the effects of global 11beta-HSD1 inhibition on metabolic syndrome risk factors, we subcutaneously injected "Western"-type diet-fed hyperlipidemic mice displaying moderate or severe obesity [LDL receptor (LDLR)-deficient (LDLR(-/-)) mice and mice derived from heterozygous agouti (A(y)/a) and homozygous LDLR(-/-) breeding pairs (A(y)/a;LDLR(-/-) mice)] with the nonselective 11beta-HSD inhibitor carbenoxolone for 4 wk. Body composition throughout the study, end-point fasting plasma, and extent of hepatic steatosis and atherosclerosis were assessed. This route of treatment led to detection of high levels of carbenoxolone in liver and fat and resulted in decreased weight gain due to reduced body fat mass in both mouse models. However, only A(y)/a;LDLR(-/-) mice showed an effect of 11beta-HSD1 inhibition on fasting insulin and plasma lipids, coincident with a reduction in VLDL due to mildly increased VLDL clearance and dramatically decreased hepatic triglyceride production. A(y)/a;LDLR(-/-) mice also showed a greater effect of the drug on reducing atherosclerotic lesion formation. These findings indicate that subcutaneous injection of an 11beta-HSD1 inhibitor allows for the targeting of the enzyme in not only liver, but also adipose tissue, and attenuates many metabolic syndrome risk factors, with more pronounced effects in cases of severe obesity and hyperlipidemia. (+info)