Neomorphic agouti mutations in obese yellow mice

Maternal methyl supplements in mice affect epigenetic variation and DNA methylation of offspring

This study was designed to determine if maternal dietary methyl supplements increase DNA methylation and methylation-dependent epigenetic phenotypes in mammalian offspring. Female mice of two strains were fed two levels of dietary methyl supplement or control diet prior to and during pregnancy. Offspring of these mice vary in phenotype, which is epigenetically determined and affects health and 2-y survival. Phenotype and DNA methylation of a long terminal repeat (LTR) controlling expression of the agouti gene were assayed in the resulting offspring. Methyl supplements increase the level of DNA methylation in the agouti LTR and change the phenotype of offspring in the healthy, longer-lived direction. This shows that methyl supplements have strong effects on DNA methylation and phenotype and are likely to affect long-term health. Optimum dietary supplements for the health and longevity of offspring should be intensively investigated. This should lead to public policy guidance that teaches optimal, rather than minimal, dose levels of maternal supplements.

Metastable epialleles in mammals

There are some mammalian alleles that display the unusual characteristic of variable expressivity in the absence of genetic heterogeneity. It has recently become evident that this is because the activity of these alleles is dependent on their epigenetic state. Interestingly, the epigenetic state is somewhat labile, resulting in phenotypic mosaicism between cells (variegation) and also between individuals (variable expressivity). The establishment of the epigenetic state occurs during early embryogenesis and is a probabilistic event that is influenced by whether the allele is carried on the paternal or maternal alleles. In addition, the epigenetic state determines whether these alleles are dominant. We propose that mammalian alleles with such characteristics should be termed metastable epialleles to distinguish them from traditional alleles. At this stage, it is unclear how common these alleles are, but an appreciation of their existence will aid in their identification.

Hyperleptinemia in A(y)/a mice upregulates arcuate cocaine- and amphetamine-regulated transcript expression

The effects of leptin on cocaine- and amphetamine-regulated transcript (CART) and agouti-related protein (AGRP) expression in the hypothalamic arcuate nucleus of obese A(y)/a mice were investigated. CART mRNA expression was upregulated by 41% and AGRP mRNA downregulated by 78% in hyperleptinemic A(y)/a mice relative to levels in lean a/a mice. The mRNA expression of these neuropeptides in either young nonobese A(y)/a mice or rats treated with SHU-9119, a synthetic melanocortin-4 receptor (MC4R) antagonist, did not differ significantly from that in the corresponding controls. After a 72-h fast, which decreased the concentration of serum leptin, CART and AGRP mRNA expression decreased and increased, respectively, in A(y)/a mice. The expression levels of these neuropeptides in leptin-deficient A(y)/a ob/ob double mutants were comparable to those in a/a ob/ob mice. Leptin thus modulates both CART and AGRP mRNA expression in obese A(y)/a mice, whereas leptin signals are blocked at the MCR4R level. Taken together, the present findings indicate that differential expression of these neuropeptides in A(y)/a and ob/ob mice results in dissimilar progression toward obesity.

Agouti: from mouse to man, from skin to fat

The agouti protein regulates pigmentation in the mouse hair follicle producing a black hair with a subapical yellow band. Its effect on pigmentation is achieved by antagonizing the binding of alpha-melanocyte stimulating hormone (alpha-MSH) to melanocortin 1 receptor (Mc1r), switching melanin synthesis from eumelanin (black/brown) to phaeomelanin (red/yellow). Dominant mutations in the non-coding region of mouse agouti cause yellow coat colour and ectopic expression also results in obesity, type 11 diabetes, increased somatic growth and tumourigenesis. At least some of these pleiotropic effects can be explained by antagonism of other members of the melanocortin receptor family by agouti protein. The yellow coat colour is the result of agouti chronically antagonizing the binding of alpha-MSH to Mc1r and the obese phenotype results from agouti protein antagonizing the binding of alpha-MSH to Mc3r and/or Mc4r. Despite the existence of a highly homologous agouti protein in humans, agouti signal protein (ASIP), its role has yet to be defined. However it is known that human ASIP is expressed at highest levels in adipose tissue where it may antagonize one of the melanocortin receptors. The conserved nature of the agouti protein combined with the diverse phenotypic effects of agouti mutations in mouse and the different expression patterns of human and mouse agouti, suggest ASIP may play a role in human energy homeostasis and possibly human pigmentation.

Molecular and phenotypic analysis of 25 recessive, homozygous-viable alleles at the mouse agouti locus

Agouti is a paracrine-acting, transient antagonist of melanocortin 1 receptors that specifies the subapical band of yellow on otherwise black hairs of the wild-type coat. To better understand both agouti structure/function and the germline damage caused by chemicals and radiation, an allelic series of 25 recessive, homozygous-viable agouti mutations generated in specific-locus tests were characterized. Visual inspection of fur, augmented by quantifiable chemical analysis of hair melanins, suggested four phenotypic categories (mild, moderate, umbrous-like, severe) for the 18 hypomorphs and a single category for the 7 amorphs (null). Molecular analysis indicated protein-coding alterations in 8 hypomorphs and 6 amorphs, with mild-moderate phenotypes correlating with signal peptide or basic domain mutations, and more devastating phenotypes resulting from C-terminal lesions. Ten hypomorphs and one null demonstrated wild-type coding potential, suggesting that they contain mutations elsewhere in the > or = 125-kb agouti locus that either reduce the level or alter the temporal/spatial distribution of agouti transcripts. Beyond the notable contributions to the field of mouse germ cell mutagenesis, analysis of this allelic series illustrates that complete abrogation of agouti function in vivo occurs most often through protein-coding lesions, whereas partial loss of function occurs slightly more frequently at the level of gene expression control.

Biology of mammalian L1 retrotransposons

L1 retrotransposons comprise 17% of the human genome. Although most L1s are inactive, some elements remain capable of retrotransposition. L1 elements have a long evolutionary history dating to the beginnings of eukaryotic existence. Although many aspects of their retrotransposition mechanism remain poorly understood, they likely integrate into genomic DNA by a process called target primed reverse transcription. L1s have shaped mammalian genomes through a number of mechanisms. First, they have greatly expanded the genome both by their own retrotransposition and by providing the machinery necessary for the retrotransposition of other mobile elements, such as Alus. Second, they have shuffled non-L1 sequence throughout the genome by a process termed transduction. Third, they have affected gene expression by a number of mechanisms. For instance, they occasionally insert into genes and cause disease both in humans and in mice. L1 elements have proven useful as phylogenetic markers and may find other practical applications in gene discovery following insertional mutagenesis in mice and in the delivery of therapeutic genes.

Knockout models resulting in the development of obesity

Our understanding of body weight regulation has been greatly advanced by the characterization of previously existing mutations in mice that cause obesity. Subsequent analysis of a number of mouse knockout models has greatly expanded the number of genes known to influence adiposity by affecting metabolic rate, physical activity, and/or appetite.

Gene-trap mutagenesis: past, present and beyond

Although at least 35,000 human genes have been sequenced and mapped, adequate expression or functional information is available for only approximately 15% of them. Gene-trap mutagenesis is a technique that randomly generates loss-of-function mutations and reports the expression of many mouse genes. At present, several large-scale, gene-trap screens are being carried out with various new vectors, which aim to generate a public resource of mutagenized embryonic stem (ES) cells. This resource now includes more than 8,000 mutagenized ES-cell lines, which are freely available, making it an appropriate time to evaluate the recent advances in this area of genomic technology and the technical hurdles it has yet to overcome.

Body and organ mass in agouti and non-agouti deer mice (Peromyscus maniculatus)

Body, adrenal, brain, heart, liver, kidney, spleen and testis masses were determined for agouti and non-agouti deer mice (Peromyscus maniculatus gracilis) of both sexes. Body mass was highest for non-agouti females and lowest for agouti females; and sex differences in body mass were significant for agouti, but not non-agouti, deer mice. Adrenal, brain and liver masses were similar between color morphs; heart mass was greater in agouti males; and kidney, spleen and testis masses were all significantly greater for non-agouti deer mice. Splenomegaly in non-agouti deer mice was prominent, as spleens of non-agouti deer mice were 50% larger than those of agouti animals. Sex differences varied across organs and color morphs. For both color morphs, males had heavier adrenals and brains, whereas females had heavier livers and spleens. Kidney and heart mass was greater for female non-agouti deer mice, but for agouti animals, heart mass was greater in males and kidney mass differed little between the sexes. For both color morphs, testes and spleen mass was altered by photoperiod in 72 deer mice housed under short- or long-day conditions and the effect was stronger in non-agouti animals. This is the first report of splenomegaly and sex-specific body mass differences associated with the non-agouti allele.

The marks, mechanisms and memory of epigenetic states in mammals

It is well recognized that there is a surprising degree of phenotypic variation among genetically identical individuals, even when the environmental influences, in the strict sense of the word, are identical. Genetic textbooks acknowledge this fact and use different terms, such as 'intangible variation' or 'developmental noise', to describe it. We believe that this intangible variation results from the stochastic establishment of epigenetic modifications to the DNA nucleotide sequence. These modifications, which may involve cytosine methylation and chromatin remodelling, result in alterations in gene expression which, in turn, affects the phenotype of the organism. Recent evidence, from our work and that of others in mice, suggests that these epigenetic modifications, which in the past were thought to be cleared and reset on passage through the germline, may sometimes be inherited to the next generation. This is termed epigenetic inheritance, and while this process has been well recognized in plants, the recent findings in mice force us to consider the implications of this type of inheritance in mammals. At this stage we do not know how extensive this phenomenon is in humans, but it may well turn out to be the explanation for some diseases which appear to be sporadic or show only weak genetic linkage.

A retrotransposon-derived gene, PEG10, is a novel imprinted gene located on human chromosome 7q21

A novel paternally expressed imprinted gene, PEG10 (Paternally Expressed 10), was identified on human chromosome 7q21. PEG10 is located near the SGCE (Sarcoglycan epsilon) gene, whose mouse homologue was recently shown to be imprinted. Therefore, it is highly possible that a new imprinted gene cluster exists on human chromosome 7q21. Analysis of two predicted open reading frames (ORF1 and ORF2) revealed that ORF1 and ORF2 have homology to the gag and pol proteins of some vertebrate retrotransposons, respectively. These data suggest that PEG10 is derived from a retrotransposon that was previously integrated into the mammalian genome. PEG10 is likely to be essential for understanding how exogenous genes become imprinted.

Genetic models of obesity and energy balance in the mouse

Obesity is a health problem of epidemic proportions in the industrialized world. The cloning and characterization of the genes for the five naturally occurring monogenic obesity syndromes in the mouse have led to major breakthroughs in understanding the physiology of energy balance and the contribution of genetics to obesity in the human population. However, the regulation of energy balance is an extremely complex process, and it is quickly becoming clear that hundreds of genes are involved. In this article, we review the naturally occurring monogenic and polygenic obese mouse strains, as well as the large number of transgenic and knockout mouse models currently available for the study of obesity and energy balance.

In vivo demethylation of a MoMuLV retroviral vector expressing the herpes simplex thymidine kinase suicide gene by 5' azacytidine

We constructed a functional MoMuLV-based bicistronic retroviral vector encoding the herpes simplex virus type I thymidine kinase gene, which induces sensitivity to the prodrug ganciclovir (gcv), and the reporter beta-galactosidase gene (MFG-tk-IRES-lacZ). The U937 histiocytic cell line was transduced with this vector, and a clone (VB71) with high-level transgene expression was selected. Severe combined immunodeficient (SCID) mice were injected with VB71 cells to evaluate the role of long terminal repeat methylation in transgene silencing in vivo and to see whether 5-azacytidine (5' aza-C) demethylating agent prevented it. We found 5' aza-C maintained gene expression at high level in vitro. In vivo, time to tumor onset was significantly longer in SCID mice receiving the VB71 cells, 5' aza-C, and gcv compared with animals treated with either 5' aza-C or gcv alone. The number of injected tumor cells influences tumor onset time and the efficacy of 5' aza-C and gcv treatment. The standard gcv treatment schedule (10 mg/kg from d + 1 until the onset of tumor) controlled tumor onset better than short-term treatment with high doses. In conclusion, the results extend our previous findings that transgene methylation in vivo may be prevented with an appropriate schedule of 5' aza-C and gcv.

Antagonist and agonist activities of the mouse agouti protein fragment (91-131) at the melanocortin-1 receptor

Antagonist and agonist activities of chemically synthetized mouse agouti protein fragment (91-131) (AP91-131) at the melanocortin type-1 receptor (MC1-R) were assessed using B 16-F1 mouse melanoma cells in vitro and the following assay systems: (i) receptor binding, (ii) adenylate cyclase, (iii) tyrosinase, (iv) melanin production, and (v) cell proliferation. In competition binding studies AP91-131 was about 3-fold less potent than the natural agonist alpha-melanocyte-stimulating hormone (alpha-MSH) in displacing the radioligand [125I]-[Nle4, D-Phe7]-alpha-MSH (Ki 6.5 +/- 0.8 nmol/l). Alpha-MSH-induced tyrosinase activation and melanin production were completely inhibited by a 100-fold higher concentration of AP9 l -131; the IC50 values for AP91-131 in thetwo assay systems were 91 +/- 22 nM and 95 +/- 15 nM respectively. Basal melanin production and adenylate cyclase activity in the absence of agonist were decreased by AP91-131 with IC50 values of 9.6+/-1.8 nM and 5.0+/-2.4 nM, respectively. This indicates inverse agonist activity of AP91-131 similar to that of native AP. The presence of 10 nM melanin-concentrating hormone (MCH) slightly potentiated the inhibitory activity of AP91-131 in the adenylate cyclase and melanin assays. On the other hand, AP91-131 inhibited cell growth similar to alpha-MSH (IC50 11.0 +/- 2.1 nM; maximal inhibition 1.8-fold higher than that of alpha-MSH). Furthermore, MC1-R was down-regulated by AP91-131 with about the same potency and time-course as with alpha-MSH. These results demonstrate that AP91-131 displays both agonist and antagonist activities at the MC1-R and hence that it is the cysteine-rich region of agouti protein which inhibits and mimics the different alpha-MSH functions, most likely by simultaneous modulation of different intracellular signalling pathways.

Genomic characterization of the human and mouse protein tyrosine phosphatase-1B genes

PTP-1B is a ubiquitously expressed intracellular protein tyrosine phosphatase (PTP) that has been implicated in the negative regulation of insulin signaling. Mice deficient in PTP-1B were found to have an enhanced insulin sensitivity and a resistance to diet-induced obesity. Interestingly, the human PTP-1B gene maps to chromosome 20q13.1 in a region that has been associated with diabetes and obesity. Although there has been a partial characterization of the 3' end of the human PTP-1B gene, the complete gene organization has not been described. In order to further characterize the PTP-1B gene, we have cloned and determined the genomic organization for both the human and mouse PTP-1B genes including the promoter. The human gene spans >74 kb and features a large first intron of >54 kb; the mouse gene likewise contains a large first intron, although the exact size has not been determined. The organization of the human and mouse PTP-1B genes is identical except for an additional exon at the 3' end of the human that is absent in the mouse. The mouse PTP-1B gene maps to the distal arm of mouse chromosome 2 in the region H2-H3. This region is associated with a mouse obesity quantitative trait locus (QTL) and is syntenic with human chromosome 20. The promoter region of both the human and mouse genes contain no TATA box but multiple GC-rich sequences that contain a number of consensus SP-1 binding sites. The basal activity of the human PTP-1B promoter was characterized in Hep G2 cells using up to 8 kb of 5' flanking sequence. A 432 bp promoter construct immediately upstream of the ATG was able to confer maximal promoter activity. Within this sequence, there are at least three GC-rich sequences and one CCAAT box, and deletion of any of these elements results in decreased promoter activity. In addition, the promoter in a number of mouse strains contains, 3.5 kb upstream of the start codon, an insertion of an intracisternal a particle (IAP) element that possibly could alter the expression of PTP-1B mRNA in these strains.

Role of melanocortins in the central control of feeding

The injection of a melanocortin peptide or of melanocortin peptide analogues into the cerebrospinal fluid or into the ventromedial hypothalamus in nanomolar or subnanomolar doses induces a long-lasting inhibition of food intake. The effect keeps significant for up to 9 h and has been observed in all animal species so far tested, the most susceptible being the rabbit. The anorectic effect of these peptides is a primary one, not secondary to the shift towards other components of the complex melanocortin-induced behavioral syndrome, in particular grooming. The site of action is in the brain, and the effect is not adrenal-mediated because it is fully exhibited also by adrenalectomized animals. It is a very strong effect, because the degree of feeding inhibition is not reduced in conditions of hunger, either induced by 24 h starvation, or by insulin-induced hypoglycemia, or by stimulation of gamma-aminobutyric acid (GABA), noradrenergic or opioid systems. The microstructural analysis of feeding behavior suggests that melanocortins act as satiety-inducing agents, because they do not significantly modify the latencies to start eating, but shorten the latencies to stop eating. The mechanism of action involves the activation of melanocortin MC(4) receptors, because selective melanocortin MC(4) receptor antagonists inhibit the anorectic effect of melanocortins, while inducing per se a strong stimulation of food intake and a significant increase in body weight. Melanocortins seem to play an important role in stress-induced anorexia, because such condition, in rats, is significantly attenuated by the blockage of melanocortin MC(4) receptors; such a role is not secondary to an increased release of corticotropin-releasing factor (CRF), because, on the other hand, the CRF-induced anorexia is not affected at all by the blockage of melanocortin MC(4) receptors. The physiological meaning of the feeding inhibitory effect of melanocortins, and, by consequence, the physiological role of melanocortins in the complex machinery responsible for body weight homeostasis, is testified by the hyperphagia/obesity syndromes caused by mutations in the pro-opiomelanocortin (POMC) gene, or in the melanocortin MC(4) receptor gene, or in the agouti locus. Finally, recent evidences suggest that melanocortins could be involved in mediating the effects of leptin, and in controlling the expression of neuropeptide Y (NPY).

Complex patterns of inheritance of an imprinted murine transgene suggest incomplete germline erasure

Here we report a transgenic mouse line that exhibits significant deviations from a classic pattern of parental imprinting. When the transgene is passed through the female germline, it is completely silenced in some offspring while in others expression is reduced. This variable expressivity does not appear to be the result of differences in the presence of unlinked modifiers. Female transmission of the transgene is associated with hypermethylation. The transgene is generally reactivated on passage through the male germline. Extended pedigrees reveal complex patterns of inheritance of the phenotype. The most likely explanation for this result is that the imprint is not completely erased and reset when passed through the germline of either sex. FISH analysis reveals that the transgene has integrated into chromosome 3 band E3, a region not known to carry imprinted genes, and the integration site shows no sign of allele-specific differential methylation. These findings, in conjunction with other recent work, raise the possibility that the introduction of foreign DNA into the mammalian genome, either through retrotransposition or transgenesis, may be associated with parental imprinting that is not always erased and reset during meiosis.

Cocaine- and amphetamine-regulated transcript, glucagon-like peptide-1 and corticotrophin releasing factor inhibit feeding via agouti-related protein independent pathways in the rat

The melanocortin-4 receptor (MC4-R) appears to be an important downstream mediator of the action of leptin. We examined to what extent the anorectic effects of cocaine- and amphetamine-regulated transcript (CART), glucagon-like peptide-1 (GLP-1) and corticotrophin releasing factor (CRF) might be mediated via MC4-R. alpha-Melanocyte stimulating hormone (alpha-MSH), the MC4-R agonist, administered intracerebroventricularly (ICV) at a dose of 1 nmol reduced food intake by approximately half. Agouti-related protein (Agrp) (83-132), a biologically active fragment of the endogenous MC4-R antagonist, administered ICV at a dose of 1 nmol completely blocked the anorectic effect of 1 nmol alpha-MSH. CART (55-102) (0.2 nmol), GLP-1 (3 nmol) and CRF (0.3 nmol) produced a reduction in feeding of approximately the same magnitude as 1 nmol alpha-MSH. Agrp (83-132) (1 nmol) administered ICV did not block the anorectic effects of CART (55-102) (1 h food intake, 0.2 nmol CART (55-102), 2.7+/-0.8 g vs. CART (55-102)+Agrp (83-132), 2.6+/-0.6 g, P=0.87; saline control 5.4+/-0.3 g, P<0.001 vs. both groups). Agrp (83-132) also did not block the anorectic effects of GLP-1 or CRF (1 h food intake, 0.3 nmol CRF, 0.7+/-0.3 g vs. CRF+Agrp (83-132), 0.7+/-0.3 g, P=0.91; 3 nmol GLP-1, 1.9+/-0.4 g vs. GLP-1+Agrp (83-132), 1.1+/-0. 5 g, P=0.23; saline control 5.0+/-0.6 g, P<0.001 vs. all four groups). Thus, as previous data suggests, GLP-1 and CRF do not appear to reduce food intake predominantly via MC4-R, we here demonstrate for the first time that CART, in addition to GLP-1 and CRF primarily acts via Agrp independent pathways.

Melanocortins and feeding behavior

The melanocortin (ACTH/MSH) peptides exert a number of central effects. In the eighties, we described for the first time a role for melanocortins in the central control of appetite. We showed that the injection of ACTH-(1-24) into a brain lateral ventricle reduced food intake up to 76.6% in starved rats. Injections into the ventromedial hypothalamus during the nocturnal feeding phase also markedly inhibited food intake. These effects were also confirmed in mice and rabbits. Targeted disruption of the MC4 receptor resulting in obesity in mice explained the role of this receptor in mediating effects of melanocortins on food intake. Administration of MC4 receptor agonists leads to acute reduction in food intake and body weight, while the reverse effects are observed after administration of selective MC4 receptor antagonists, confirming the role of the melanocortins in mediating a tonic inhibition on feeding behavior. Moreover, immobilization stress-induced anorexia may be partially reversed by single and repeated intracerebroventricular administration of selective MC4 receptor antagonists. It is thus evident that MC4 receptor blockage can reduce stress-induced anorexia and that repeated injections of selective MC4 receptor antagonists have a sustained effect on food intake without any sign of tachyphylaxis. However, we have also shown that the behavioral effects of CRF (anorexia and grooming) are not influenced by MC4 receptor blockage. These effects of CRF are thus not due to an indirect mechanism caused by an increased release of melanocortins acting on the central MC receptors.

Epigenetic inheritance at the agouti locus in the mouse

Epigenetic modifications have effects on phenotype, but they are generally considered to be cleared on passage through the germ line in mammals, so that only genetic traits are inherited. Here we describe the inheritance of an epigenetic modification at the agouti locus in mice. In viable yellow ( A(vy)/a) mice, transcription originating in an intra-cisternal A particle (IAP) retrotransposon inserted upstream of the agouti gene (A) causes ectopic expression of agouti protein, resulting in yellow fur, obesity, diabetes and increased susceptibility to tumours. The pleiotropic effects of ectopic agouti expression are presumably due to effects of the paracrine signal on other tissues. Avy mice display variable expressivity because they are epigenetic mosaics for activity of the retrotransposon: isogenic Avy mice have coats that vary in a continuous spectrum from full yellow, through variegated yellow/agouti, to full agouti (pseudoagouti). The distribution of phenotypes among offspring is related to the phenotype of the dam; when an A(vy) dam has the agouti phenotype, her offspring are more likely to be agouti. We demonstrate here that this maternal epigenetic effect is not the result of a maternally contributed environment. Rather, our data show that it results from incomplete erasure of an epigenetic modification when a silenced Avy allele is passed through the female germ line, with consequent inheritance of the epigenetic modification. Because retrotransposons are abundant in mammalian genomes, this type of inheritance may be common.

Molecular pharmacology of Agouti protein in vitro and in vivo

Agouti protein and Agouti-related protein (Agrp) are paracrine signaling molecules that act by antagonizing the effects of melanocortins, and several alternatives have been proposed to explain their mechanisms of action. Genetic crosses in a sensitized background uncover a phenotypic difference between overexpression of Agouti and loss of Mc1r function, demonstrate that a functional Mc1r is required for the pigmentary effects of Agouti, and suggest that Agouti protein can act as an agonist of the Mc1r in a way that differs from alpha-MSH stimulation. In vitro, Agouti protein inhibits melanocortin action by two mechanisms: competitive antagonism that depends on the carboxyterminus of the protein, and downregulation of melanocortin receptor signaling that depends on the aminoterminus. Our findings provide evidence of a novel signaling mechanism whereby alpha-MSH and Agouti protein function as independent ligands that inhibit each other's binding and transduce opposite signals through a single receptor.

An agouti mutation lacking the basic domain induces yellow pigmentation but not obesity in transgenic mice

Chronic antagonism of melanocortin receptors by the paracrine-acting agouti gene product induces both yellow fur and a maturity-onset obesity syndrome in mice that ubiquitously express wild-type agouti. Functional analysis of agouti mutations in transgenic mice indicate that the cysteine-rich C terminus, signal peptide, and glycosylation site are required for agouti activity in vivo. In contrast, no biological activity has been ascribed to the conserved basic domain. To examine the functional significance of the agouti basic domain, the entire 29-aa region was deleted from the agouti cDNA, and the resulting mutation (agoutiDeltabasic) was expressed in transgenic mice under the control of the beta-actin promoter (BAPaDeltabasic). Three independent lines of BAPaDeltabasic transgenic mice all developed some degree of yellow pigment in the fur, indicating that the agoutiDeltabasic protein was functional in vivo. However, none of the BAPaDeltabasic transgenic mice developed completely yellow fur, obesity, hyperinsulinemia, or hyperglycemia. High levels of agoutiDeltabasic expression in relevant tissues exceeded the level of agouti expression in obese viable yellow mice, suggesting that suboptimal activity or synthesis of the agoutiDeltabasic protein, rather than insufficient RNA synthesis, accounts for the phenotype of the BAPaDeltabasic transgenic mice. These findings implicate a functional role for the agouti basic domain in vivo, possibly influencing the biogenesis of secreted agouti protein or modulating protein-protein interactions that contribute to effective antagonism of melanocortin receptors.

Mechanisms of mutation induction in germ cells of the mouse as assessed by the specific locus test

Mouse germ cell specific locus mutagenesis data and a molecular characterization of mutant alleles have been reviewed to arrive at an understanding of the mechanism of mutation induction in mammals. (a) The spermatogenic stage specificity for the sensitivity to mutation induction by 20 chemical mutagens is considered. (b) The effects of a saturable repair process and its recovery over time are examined for the mutagenic efficiency of ethylnitrosourea. (c) The mutagenic events following methylnitrosourea and chlorambucil are shown to be mainly deletions. In contrast the mutations recovered after ethylnitrosourea treatment are almost exclusively base pair substitutions. (d) It is emphasized that to date very few specific locus experiments have been designed to test for mutagenic events outside the interval stem cell spermatogonia-mature spermatozoa. A specific locus mutation has recently been shown to be due to loss of heterozygosity via mitotic recombination in an early zygote stage and suggests a broader range of possible mechanisms of mutation when these stages are considered. (e) With the cloning of all 7 marker loci mutation analysis at the molecular level will allow a more direct assessment of the mutation process in future studies.

The role of agouti-related protein in regulating body weight

Defects in signaling by leptin, a hormone produced primarily by adipose tissue that informs the brain of the body's energy reserves, result in obesity in mice and humans. However, the majority of obese humans do not have abnormalities in leptin or its receptor but instead exhibit leptin resistance that could result from defects in downstream mediators of leptin action. Recently, two potential downstream mediators, agouti-related protein (Agrp) and its receptor, the melanocortin-4 receptor (Mc4r), have been identified. Agrp and Mc4r are excellent candidates for human disorders of body weight regulation and represent promising targets for pharmacological intervention in the treatment of these disorders.

Down-regulation of melanocortin receptor signaling mediated by the amino terminus of Agouti protein in Xenopus melanophores

Agouti protein and Agouti-related protein (Agrp) regulate pigmentation and body weight, respectively, by antagonizing melanocortin receptor signaling. A carboxyl-terminal fragment of Agouti protein, Ser73-Cys131, is sufficient for melanocortin receptor antagonism, but Western blot analysis of skin extracts reveals that the electrophoretic mobility of native Agouti protein corresponds to the mature full-length form, His23-Cys131. To investigate the potential role of the amino-terminal residues, we compared the function of full-length and carboxyl-terminal fragments of Agrp and Agouti protein in a sensitive bioassay based on pigment dispersion in Xenopus melanophores. We find that carboxyl-terminal Agouti protein, and all forms of Agrp tested, act solely by competitive antagonism of melanocortin action. However, full-length Agouti protein acts by an additional mechanism that is time- and temperature-dependent, depresses maximal levels of pigment dispersion, and is therefore likely to be mediated by receptor down-regulation. Apparent down-regulation is not observed for a mixture of amino-terminal and carboxyl-terminal fragments. We propose that the phenotypic effects of Agouti in vivo represent a bipartite mechanism: competitive antagonism of agonist binding by the carboxyl-terminal portion of Agouti protein and down-regulation of melanocortin receptor signaling by an unknown mechanism that requires residues in the amino terminus of the Agouti protein.

Inner ear and kidney anomalies caused by IAP insertion in an intron of the Eya1 gene in a mouse model of BOR syndrome

A spontaneous mutation causing deafness and circling behavior was discovered in a C3H/HeJ colony of mice at the Jackson Laboratory. Pathological analysis of mutant mice revealed gross morphological abnormalities of the inner ear, and also dysmorphic or missing kidneys. The deafness and abnormal behavior were shown to be inherited as an autosomal recessive trait and mapped to mouse chromosome 1 near the position of the Eya1 gene. The human homolog of this gene, EYA1, has been shown to underly branchio-oto-renal (BOR) syndrome, an autosomal dominant disorder characterized by hearing loss with associated branchial and renal anomalies. Molecular analysis of the Eya1 gene in mutant mice revealed the insertion of an intracisternal A particle (IAP) element in intron 7. The presence of the IAP insertion was associated with reduced expression of the normal Eya1 message and formation of additional aberrant transcripts. The hypomorphic nature of the mutation may explain its recessive inheritance, if protein levels in homozygotes, but not heterozygotes, are below a critical threshold needed for normal developmental function. The new mouse mutation is designated Eya1(bor) to denote its similarity to human BOR syndrome, and will provide a valuable model for studying mutant gene expression and etiology.

The mouse mahogany locus encodes a transmembrane form of human attractin

Agouti protein and agouti-related protein are homologous paracrine signalling molecules that normally regulate hair colour and body weight, respectively, by antagonizing signalling through melanocortin receptors. Expression of Agouti is normally limited to the skin, but rare alleles from which Agouti is expressed ubiquitously, such as lethal yellow, have pleiotropic effects that include a yellow coat, obesity, increased linear growth, and immune defects. The mahogany (mg) mutation suppresses the effects of lethal yellow on pigmentation and body weight, and results of our previous genetic studies place mg downstream of transcription of Agouti but upstream of melanocortin receptors. Here we use positional cloning to identify a candidate gene for mahogany, Mgca. The predicted protein encoded by Mgca is a 1,428-amino-acid, single-transmembrane-domain protein that is expressed in many tissues, including pigment cells and the hypothalamus. The extracellular domain of the Mgca protein is the orthologue of human attractin, a circulating molecule produced by activated T cells that has been implicated in immune-cell interactions. These observations provide new insight into the regulation of energy metabolism and indicate a molecular basis for crosstalk between melanocortin-receptor signalling and immune function.

Mvwf, a dominant modifier of murine von Willebrand factor, results from altered lineage-specific expression of a glycosyltransferase

We have identified altered lineage-specific expression of an N-acetylgalactosaminyltransferase gene, Galgt2, as the gain-of-function mechanism responsible for the action of the Mvwf locus, a major modifier of plasma von Willebrand factor (VWF) level in RIIIS/J mice. A switch of Galgt2 gene expression from intestinal epithelial cell-specific to a pattern restricted to the vascular endothelial cell bed leads to aberrant posttranslational modification and rapid clearance of VWF from plasma. Transgenic expression of Galgt2 directed to vascular endothelial cells reproduces the low VWF phenotype, confirming this switch in lineage-specific gene expression as the likely molecular mechanism for Mvwf. These findings identify alterations in glycosyltransferase function as a potential general mechanism for the genetic modification of plasma protein levels.

Melanocortin mediated inhibition of feeding behavior in rats

Melanocortinergic neurons are believed to play a role in the control of food intake. Melanocortin receptor agonists and antagonists modulate feeding in several mouse models of chemically and genetically induced hyperphagia. To date, little information is available describing the role of this neurological system in the control of the natural feeding cycle in genetically intact rats. To evaluate the involvement of melanocortins in spontaneous nocturnal feeding, the synthetic melanocortin receptor agonist, MTII and the antagonist, SHU9119 were administered ICV (third ventricle) alone and in combination. Dose-dependent inhibition or stimulation of food intake was observed with MTII or SHU9119, respectively. Co-injections containing equal concentrations of MTII and SHU9119 resulted in food intake that was indistinguishable from controls. Food intake patterns observed in studies in which various dose combinations of MTII and SHU9119 were co-injected are consistent with the concept that both affect feeding by acting on similar melanocortin receptors. The hypothesis that effects of melanocortins on feeding may be mediated via an NPY related pathway was tested by co-injecting MTII and NPY in a 2-h satiated food intake paradigm. MTII inhibited food intake induced by 5.0 microg hNPY in a dose dependent manner with the highest dose tested abolishing the NPY feeding response. The studies suggest that melanocortins act via specific receptors to control food intake in rats, possibly via an NPY related pathway. If similar neurochemical processes operate in humans, selectively modulating specific melanocortin receptor signaling may be an approach to the treatment of human obesity.

Intracisternal A-particle element transposition into the murine beta-glucuronidase gene correlates with loss of enzyme activity: a new model for beta-glucuronidase deficiency in the C3H mouse

The severity of human mucopolysaccharidosis type VII (MPS VII), or Sly syndrome, depends on the relative activity of the enzyme beta-glucuronidase. Loss of beta-glucuronidase activity can cause hydrops fetalis, with in utero or postnatal death of the patient. In this report, we show that beta-glucuronidase activity is not detectable by a standard fluorometric assay in C3H/HeOuJ (C3H) mice homozygous for a new mutation, gusmps2J. These gusmps2J/gusmps2J mice are born and survive much longer than the previously characterized beta-glucuronidase-null B6.C-H-2(bm1)/ByBir-gusmps (gusmps/gusmps) mice. Northern blot analysis of liver from gusmps2J/gusmps2J mice demonstrates a 750-bp reduction in size of beta-glucuronidase mRNA. A 5.4-kb insertion in the Gus-sh nucleotide sequence from these mice was localized by Southern blot analysis to intron 8. The ends of the inserted sequences were cloned by inverse PCR and revealed an intracisternal A-particle (IAP) element inserted near the 3' end of the intron. The sequence of the long terminal repeat (LTR) regions of the IAP most closely matches that of a composite LTR found in transposed IAPs previously identified in the C3H strain. The inserted IAP may contribute to diminished beta-glucuronidase activity either by interfering with transcription or by destabilizing the message. The resulting phenotype is much less severe than that previously described in the gusmps/gusmps mouse and provides an opportunity to study MPS VII on a genetic background that clearly modulates disease severity.

Mobile elements and disease

A substantial fraction of mammalian genomes is composed of mobile elements and their remnants. Recent insertions of LTR-retrotransposons, non-LTR retrotransposons, and non-autonomous retrotransposons have caused disease frequently in mice, but infrequently in humans. Although many of these elements are defective, a number of mammalian non-LTR retrotransposons of the L1 type are capable of autonomous retrotransposition. The mechanism by which they retrotranspose and in turn aide the retrotransposition of non-autonomous elements is being elucidated.

The impact of L1 retrotransposons on the human genome

The 'master' human mobile element, the L1 retrotransposon, has come of age as a biological entity. Knowledge of how it retrotransposes in vivo, how its proteins act to retrotranspose other poly A elements and the extent of its role in shaping the human genome should emerge rapidly over the next few years. We review the impact of retrotransposons and how new insight is likely to lead to important practical applications for these intriguing mobile elements.

Melanin-concentrating hormone: a functional melanocortin antagonist in the hypothalamus

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) demonstrate opposite actions on skin coloration in teleost fish. Both peptides are present in the mammalian brain, although their specific physiological roles remain largely unknown. In this study, we examined the interactions between MCH and alpha-MSH after intracerebroventricular administration in rats. MCH increased food intake in a dose-dependent manner and lowered plasma glucocorticoid levels through a mechanism involving ACTH. In contrast, alpha-MSH decreased food intake and increased glucocorticoid levels. MCH, at a twofold molar excess, antagonized both actions of alpha-MSH. alpha-MSH, at a threefold molar excess, blocked the orexigenic properties of MCH. MCH did not block alpha-MSH binding or the ability of alpha-MSH to induce cAMP in cells expressing either the MC3 or MC4 receptor, the principal brain alpha-MSH receptor subtypes. These data suggest that MCH and alpha-MSH exert opposing and antagonistic influences on feeding behavior and the stress response and may function in a coordinate manner to regulate metabolism through a novel mechanism mediated in part by an MCH receptor.

Detection and cloning of unique integration sites of retrotransposon, intracisternal A-particle element in the genome of acute myeloid leukemia cells in mice

We previously found retrotransposition of the intracisternal A-particle (IAP) element in the genome of acute myeloid leukemia (AML) cells induced by X-irradiation of C3H/He mice (FEBS 16333). To analyze the occurrence of the IAP-mediated retrotransposition in AML cells, we compared integration sites of the IAP element by polymerase chain reaction (PCR) in the genomes of five AML strains derived from different C3H mice. Unique PCR products were found in all of the above independent leukemia cells, whereas no such products were detected in normal cells. Results of cloning, sequencing and Southern analyses showed that the PCR products were derived from novel integration sites of the IAP element in the genome. The data suggest that IAP-mediated retrotransposition occurs frequently in radiation-induced AML cells from C3H/He mice.

Antagonism of central melanocortin receptors in vitro and in vivo by agouti-related protein

Expression of Agouti protein is normally limited to the skin where it affects pigmentation, but ubiquitous expression causes obesity. An expressed sequence tag was identified that encodes Agouti-related protein, whose RNA is normally expressed in the hypothalamus and whose levels were increased eightfold in ob/ob mice. Recombinant Agouti-related protein was a potent, selective antagonist of Mc3r and Mc4r, melanocortin receptor subtypes implicated in weight regulation. Ubiquitous expression of human AGRP complementary DNA in transgenic mice caused obesity without altering pigmentation. Thus, Agouti-related protein is a neuropeptide implicated in the normal control of body weight downstream of leptin signaling.

Obesity as a pleiotropic effect of gene action

Obesity, an easily detected and quantifiable phenotypic endpoint, is often considered, colloquially, as a disease. However, the study of obesity in rodents suggests that it is merely a convenient indicator of diverse underlying metabolic and physiologic dysregulations, rather than a disease entity in itself. To illustrate this concept, the differences between the murine Lepob/Lepob and Avy/- "obesity" syndromes are delineated. In both syndromes, pleiotropic effects of single mutations play a major role in altering the homeostatic regulation of energy metabolism and a myriad of extra- and intracellular processes in a diversity of tissues and cell types. The Lepob/Lepob syndrome mimics juvenile-onset obesity, whereas the Avy/- syndrome resembles maturity-onset obesity. The Avy/- syndrome has its basis in overabundance of agouti protein, whereas the Lepob/Lepob syndrome results from a lack of active leptin hormone. Lepob/Lepob mice have a smaller lean body mass, whereas Avy/- mice have a larger lean body mass than their respective lean siblings. Lepob/Lepob mice have fewer lung and mammary tumors than their lean Lep/- littermates, and Avy/- develop more mammary and lung tumors than their lean A/- or a/a siblings. Lepob/Lepob mice are infertile or sterile, whereas Avy/- mice are fertile. Thus, although adult Lepob/Lepob and Avy/- mice are both obese, many of the other morphologic and physiologic attributes of one mutant are diametrically opposite to those of the other.

The role of the agouti gene in the yellow obese syndrome

The yellow obese syndrome in mice encompasses many pleiotropic effects including yellow fur, maturity-onset obesity, hyperinsulinemia, insulin resistance, hyperglycemia, increased skeletal length and lean body mass, and increased susceptibility to neoplasia. The molecular basis of this syndrome is beginning to be unraveled and may have implications for human obesity and diabetes. Normally, the agouti gene is expressed during the hair-growth cycle in the neonatal skin where it functions as a paracrine regulator of pigmentation. The secreted agouti protein antagonizes the binding of the alpha-melanocyte-stimulating hormone to its receptor (melanocortin 1 receptor) on the surface of hair bulb melanocytes, causing alterations in intracellular cAMP levels. Widespread, ectopic expression of the mouse agouti gene is central to the yellow obese phenotype, as demonstrated by the molecular cloning of several dominant agouti mutations and the ubiquitous expression of the wild-type agouti gene in transgenic mice. Recent experiments have revealed that the hypothalamus and adipose tissue are biologically active target sites for agouti in the yellow obese mutant lines.

Aberrant splicing of a mouse disabled homolog, mdab1, in the scrambler mouse

Although accurate long-distance neuronal migration is a cardinal feature of cerebral cortical development, little is known about control of this migration. The scrambler (scm) mouse shows abnormal cortical lamination that is indistinguishable from reeler. Genetic and physical mapping of scm identified yeast artificial chromosomes containing an exon of mdab1, a homolog of Drosophila disabled, which encodes a phosphoprotein that binds nonreceptor tyrosine kinases. mdab1 transcripts showed abnormal splicing in scm homozygotes, with 1.5 kb of intracisternal A particle retrotransposon sequence inserted into the mdab1 coding region in antisense orientation, producing a mutated and truncated predicted protein. Therefore, mdab1 is most likely the scm gene, thus implicating nonreceptor tyrosine kinases in neuronal migration and lamination in developing cerebral cortex.

Reln(rl-Alb2), an allele of reeler isolated from a chlorambucil screen, is due to an IAP insertion with exon skipping

The reeler Albany2 mutation (Reln(rl-Alb2) in the mouse is an allele of reeler isolated during a chlorambucil mutagenesis screen. Homozygous animals had drastically reduced concentrations of reelin mRNA, in which an 85-nt exon was absent. At the genomic level, the mutation was shown to be due to an intracisternal A-particle insertion leading to exon skipping. This appears to be the first observation of retrotransposon insertion during chlorambucil mutagenesis.

Mouse pale ear (ep) is homologous to human Hermansky-Pudlak syndrome and contains a rare 'AT-AC' intron

Hermansky-Pudlak syndrome (HPS) is a rare, often fatal, autosomal recessive disorder in which albinism, bleeding and lysosomal storage are associated with defects of diverse cytoplasmic organelles, including melanosomes, platelet dense granules and lysosomes. Similar multi-organellar defects occur in the Chediak-Higashi syndrome (CHS), as well as in a large number of different mouse mutants. The HPS gene is located in 10q23, and two genetically distinct mouse loci, pale ear (ep) and ruby-eye (ru), both with mutant phenotypes similar to human HPS, map close together in the homologous region of murine chromosome 19, suggesting that one of these loci might be homologous to human HPS. We recently identified the human HPS gene, which encodes a novel ubiquitously-expressed transmembrane protein of unknown function. Here, we describe characterization of the mouse Hps cDNA and genomic locus, and identification of pathologic Hps gene mutations in ep but not in ru mice, establishing mouse pale ear as an animal model for human HPS. The phenotype of homozygous ep mutant mice encompasses those of both HPS and CHS, suggesting that these disorders may be closely related. In addition, the mouse and human HPS genes both contain a rare 'AT-AC' intron, and comparison of the sequences of this intron in the mouse and human genes identified conserved sequences that suggest a possible role for pre-mRNA secondary structure in excision of this rare class of introns.

DNA methylation in genomic imprinting

As a reversible epigenetic modification which can affect gene expression, DNA methylation has been an attractive candidate for the biochemical mechanism of genomic imprinting. Many correlations in mice and humans link allele-specific DNA methylation to the allele-restricted RNA expression which is the hallmark of imprinted genes. Moreover, abnormal DNA methylation accompanies the pathological functional imprinting of certain human genes on chromosome 11p15.5 in Wilms' tumors and in the Beckwith-Weidemann syndrome and on chromosome 15q11-13 in the Prader-Willi and Angelman syndromes. A role for DNA methylation in maintaining the transcriptional silence of imprinted alleles at some loci has been supported by pharmacological manipulation with 5-aza-2'-deoxycytidine and by experiments with methyltransferase deletion mice. Gametic differences in DNA methylation could also account for the initiation of imprints, but this remains unproven. Comprehensive physical models for the role of DNA methylation in imprinting must account not only for local allele-restricted gene expression but also for the existence of large chromosomal domains containing multiple coordinately imprinted genes.

Formation of methylation patterns in the mammalian genome

Cytosine methylation in mammals is an epigenetic modification required for viability of the developing embryo. It has been suggested that DNA methylation plays important roles in X-chromosome inactivation, imprinting, protection of the genome from invasive DNA sequences, and compartmentalization of the genome into active and condensed regions. Despite the significance of DNA methylation in mammalian cells, the mechanisms used to establish methylation patterns during development are not understood. This review will summarize the current state of knowledge about potential roles for cis- and trans-acting factors in the formation of methylation patterns in the mammalian genome.

Effects of recombinant agouti-signaling protein on melanocortin action

Mouse agouti protein is a paracrine signaling molecule that has previously been demonstrated to be an antagonist of melanocortin action at several cloned rodent and human melanocortin receptors. In this study we report the effects of agouti-signaling protein (ASIP), the human homolog of mouse agouti, on the action of alpha-MSH or ACTH at the five known human melanocortin receptor subtypes (hMCR 1-5). When stably expressed in L cells (hMC1R, hMC3R, hMC4R, hMC5R) or in the adrenocortical cell line OS3 (hMC1R, hMC2R, hMC4R), purified recombinant ASIP inhibits the generation of cAMP stimulated by alpha-MSH (hMC1R, hMC3R, hMC4R, hMC5R) or by ACTH (hMC2R). However, dose-response and Schild analysis indicated that the degree of ASIP inhibition varied significantly among the receptor subtypes; ASIP is a potent inhibitor of the hMC1R, hMC2R, and hMC4R, but has relatively weak effects at the hMC3R and hMC5R. These analyses also indicated that the apparent mechanism of ASIP antagonism varied among receptor subtypes, with characteristics consistent with competitive antagonism observed only at the hMC1R, and more complex behavior observed at the other receptors. ASIP inhibition at these latter receptors, nonetheless, can be classified as surmountable (hMC3R, hMC4R and hMC5R) or nonsurmountable (hMC2R). Recombinant ASIP also inhibited binding of radiolabeled melanocortins, [125I-Nle4, D-Phe7] alpha-MSH and [125I-Phe2, Nle4]ACTH 1-24, to the hMCR 1-5 receptors, with a relative efficacy that paralleled the ability of ASIP to inhibit cAMP accumulation at the hMC1R, hMC2R, hMC3R, and hMC4R. These results provide new insight into the biochemical mechanism of ASIP action and suggest that ASIP may play an important role in modulating melanocortin signaling in humans.

Mouse U2af1-rs1 is a neomorphic imprinted gene

The mouse U2af1-rs1 gene is an endogenous imprinted gene on the proximal region of chromosome 11. This gene is transcribed exclusively from the unmethylated paternal allele, while the methylated maternal allele is silent. An analysis of genome structure of this gene revealed that the whole gene is located in an intron of the Murr1 gene. Although none of the three human U2af1-related genes have been mapped to chromosome 2, the human homolog of Murr1 is assigned to chromosome 2. The mouse Murr1 gene is transcribed biallelically, and therefore it is not imprinted in neonatal mice. Allele-specific methylation is limited to a region around U2af1-rs1 in an intron of Murr1. These results suggest that in chromosomal homology and genomic imprinting, the U2af1-rs1 gene is distinct from the genome region surrounding it. We have proposed the neomorphic origin of the U2af1-rs1 gene by retrotransposition and the particular mechanism of genomic imprinting of ectopic genes.

Targeted disruption of the melanocortin-4 receptor results in obesity in mice

The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R.

Obesity genes and the regulation of body fat content

Physiological investigation has demonstrated that the central nervous system monitors body composition and adjusts energy intake and expenditure to stabilize total adipose tissue mass. Genetic variations in the signalling molecules involved in this regulatory system account for the heritable component of body fat content. The application of molecular techniques to rodent models of Mendelian obesity has resulted in the characterization of five loci at which mutations produce an abnormal accumulation of body fat. The genes at these loci include agouti, which encodes a molecule that antagonizes the binding of alpha melanocyte-stimulating hormone to its receptor; fat, which encodes carboxypeptidase E; tubby, which encodes a putative phosphodiesterase; obese, which encodes a circulating satiety protein; and diabetes, which encodes the receptor for the obese gene product. A more detailed understanding of the functional interrelationships of these genes should lead to important new insights into the causes and potential therapies for human obesity.

The interaction of agouti signal protein and melanocyte stimulating hormone to regulate melanin formation in mammals

Important regulatory controls of melanogenesis that operate at the subcellular level to modulate the structural and/or the functional nature of the melanins and melanin granules produced in melanocytes are reviewed. Melanocyte stimulating hormone and agouti signal protein have antagonistic roles and possibly opposing mechanisms of action in the melanocyte. In the mouse, melanocyte stimulating hormone promotes melanogenic enzyme function and elicits increases in the amount of eumelanins produced, while agouti signal protein reduces total melanin production and elicits the synthesis of pheomelanin rather than eumelanin. We are now beginning to understand the complex controls involved in regulating this switch at the molecular and biochemical levels. The quality and quantity of melanins produced by melanocytes have important physiological consequences for melanocyte function and undoubtedly play important roles in the various functions of the melanins per se, including hair and skin coloration and photoprotection.