Insulin signalling in tanycytes gates hypothalamic insulin uptake and regulation of AgRP neuron activity.

Insulin acts on neurons and glial cells to regulate systemic glucose metabolism and feeding. However, the mechanisms of insulin access in discrete brain regions are incompletely defined. Here we show that insulin receptors in tanycytes, but not in brain endothelial cells, are required to regulate insulin access to the hypothalamic arcuate nucleus. Mice lacking insulin receptors in tanycytes (IR∆Tan mice) exhibit systemic insulin resistance, while displaying normal food intake and energy expenditure. Tanycytic insulin receptors are also necessary for the orexigenic effects of ghrelin, but not for the anorexic effects of leptin. IR∆Tan mice exhibit increased agouti-related peptide (AgRP) neuronal activity, while displaying blunted AgRP neuronal adaptations to feeding-related stimuli. Lastly, a highly palatable food decreases tanycytic and arcuate nucleus insulin signalling to levels comparable to those seen in IR∆Tan mice. These changes are rooted in modifications of cellular stress responses and of mitochondrial protein quality control in tanycytes. Conclusively, we reveal a critical role of tanycyte insulin receptors in gating feeding-state-dependent regulation of AgRP neurons and systemic insulin sensitivity, and show that insulin resistance in tanycytes contributes to the pleiotropic manifestations of obesity-associated insulin resistance.

Structural Complexity and Plasticity of Signaling Regulation at the Melanocortin-4 Receptor.

The melanocortin-4 receptor (MC4R) is a class A G protein-coupled receptor (GPCR), essential for regulation of appetite and metabolism. Pathogenic inactivating MC4R mutations are the most frequent cause of monogenic obesity, a growing medical and socioeconomic problem worldwide. The MC4R mediates either ligand-independent or ligand-dependent signaling. Agonists such as α-melanocyte-stimulating hormone (α-MSH) induce anorexigenic effects, in contrast to the endogenous inverse agonist agouti-related peptide (AgRP), which causes orexigenic effects by suppressing high basal signaling activity. Agonist action triggers the binding of different subtypes of G proteins and arrestins, leading to concomitant induction of diverse intracellular signaling cascades. An increasing number of experimental studies have unraveled molecular properties and mechanisms of MC4R signal transduction related to physiological and pathophysiological aspects. In addition, the MC4R crystal structure was recently determined at 2.75 Å resolution in an inactive state bound with a peptide antagonist. Underpinned by structural homology models of MC4R complexes simulating a presumably active-state conformation compared to the structure of the inactive state, we here briefly summarize the current understanding and key players involved in the MC4R switching process between different activity states. Finally, these perspectives highlight the complexity and plasticity in MC4R signaling regulation and identify gaps in our current knowledge.

Macrocyclic Modalities Combining Peptide Epitopes and Natural Product Fragments.

"Hot loop" protein segments have variable structure and conformation and contribute crucially to protein-protein interactions. We describe a new hot loop mimicking modality, termed PepNats, in which natural product (NP)-inspired structures are incorporated as conformation-determining and -restricting structural elements into macrocyclic hot loop-derived peptides. Macrocyclic PepNats representing hot loops of inducible nitric oxide synthase (iNOS) and human agouti-related protein (AGRP) were synthesized on solid support employing macrocyclization by imine formation and subsequent stereoselective 1,3-dipolar cycloaddition as key steps. PepNats derived from the iNOS DINNN hot loop and the AGRP RFF hot spot sequence yielded novel and potent ligands of the SPRY domain-containing SOCS box protein 2 (SPSB2) that binds to iNOS, and selective ligands for AGRP-binding melanocortin (MC) receptors. NP-inspired fragment absolute configuration determines the conformation of the peptide part responsible for binding. These results demonstrate that combination of NP-inspired scaffolds with peptidic epitopes enables identification of novel hot loop mimics with conformationally constrained and biologically relevant structure.

Incorporation of Agouti-Related Protein (AgRP) Human Single Nucleotide Polymorphisms (SNPs) in the AgRP-Derived Macrocyclic Scaffold c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-dPro] Decreases Melanocortin-4 Receptor Antagonist Potency and Results in the Discovery of Melanocortin-5 Receptor Antagonists.

While the melanocortin receptors (MCRs) are known to be involved in numerous biological pathways, the potential roles of the MC5R have not been clearly elucidated in humans. Agouti-related protein (AgRP), an MC3R/MC4R antagonist and MC4R inverse agonist, contains an exposed ß-hairpin loop composed of six residues (Arg-Phe-Phe-Asn-Ala-Phe) that is imperative for binding and function. Within this active loop of AgRP, four human missense polymorphisms were deposited into the NIH Variation Viewer database. These polymorphisms, Arg111Cys, Arg111His, Phe112Tyr, and Ala115Val (AgRP full-length numbering), were incorporated into the peptide macrocycles c[Pro1-Arg2-Phe3-Phe4-Xaa5-Ala6-Phe7-dPro8], where Xaa was Dap5 or Asn5, to explore the functional effects of these naturally occurring substitutions in a simplified AgRP scaffold. All peptides lowered potency at least 10-fold in a cAMP accumulation assay compared to the parent sequences at the MC4Rs. Compounds MDE 6-82-3c, ZMK 2-82, MDE 6-82-1c, ZMK 2-85, and ZMK 2-112 are also the first AgRP-based chemotypes that antagonize the MC5R.

Arg-Phe-Phe d-Amino Acid Stereochemistry Scan in the Macrocyclic Agouti-Related Protein Antagonist Scaffold c[Pro-Arg-Phe-Phe-Xxx-Ala-Phe-DPro] Results in Unanticipated Melanocortin-1 Receptor Agonist Profiles.

The melanocortin-3 and melanocortin-4 receptors (MC3R and MC4R), endogenous agonists derived from the proopiomelanocortin gene transcript, and naturally occurring antagonists agouti and agouti-related protein (AGRP) have been linked to biological pathways associated with energy homeostasis. The active tripeptide sequence of AGRP, Arg111-Phe112-Phe113, is located on a hypothesized ß-hairpin loop. Herein, stereochemical modifications of the Arg-Phe-Phe sequence were examined in the octapeptide AGRP-derived macrocyclic scaffold c[Pro-Arg-Phe-Phe-Xxx-Ala-Phe-DPro], where Xxx was Asn or diaminopropionic acid (Dap). Macrocyclic peptides were synthesized with one, two, or three residues of the Arg-Phe-Phe sequence substituted with the corresponding d-isomer(s), generating a 14 compound library. While l-to-d inversions of the Arg-Phe-Phe sequence in a 20-residue AGRP-derived ligand previously resulted in agonist activity at the MC1R, MC3R, MC4R, and MC5R, only the MC1R was consistently stimulated by the macrocyclic ligands in the present study, with varying ligand potencies and efficacies observed at the MC1R. A general trend of increased MC4R antagonist potency was observed for Dap-containing compounds, while MC5R inverse agonist activity was observed for select ligands. It was observed that stereochemical modification of the Arg-Phe-Phe active tripeptide sequence was insufficient to convert melanocortin antagonist into agonists. Overall, these observations are important in the design of melanocortin ligands possessing potent and selective agonist and antagonist activities.

Structure-Activity Relationship Studies on a Macrocyclic Agouti-Related Protein (AGRP) Scaffold Reveal Agouti Signaling Protein (ASP) Residue Substitutions Maintain Melanocortin-4 Receptor Antagonist Potency and Result in Inverse Agonist Pharmacology at the Melanocortin-5 Receptor.

The melanocortin system consists of five reported receptors, agonists from the proopiomelanocortin gene transcript, and two antagonists, agouti-signaling protein (ASP) and agouti-related protein (AGRP). For both ASP and AGRP, the hypothesized Arg-Phe-Phe pharmacophores are on exposed ß-hairpin loops. In this study, the Asn and Ala positions of a reported AGRP macrocyclic scaffold (c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-DPro]) were explored with 14-compound and 8-compound libraries, respectively, to generate more potent, selective melanocortin receptor antagonists. Substituting diaminopropionic acid (Dap), DDap, and His at the Asn position yielded potent MC4R ligands, while replacing Ala with Ser maintained MC4R potency. Since these substitutions correlate to ASP loop residues, an additional Phe to Ala substitution was synthesized and observed to maintain MC4R potency. Seventeen compounds also possessed inverse agonist activity at the MC5R, the first report of this pharmacology. These findings are useful in developing molecular probes to study negative energy balance conditions and unidentified functions of the MC5R.

The agouti-related peptide binds heparan sulfate through segments critical for its orexigenic effects.

Syndecans potently modulate agouti-related peptide (AgRP) signaling in the central melanocortin system. Through heparan sulfate moieties, syndecans are thought to anchor AgRP near its receptor, enhancing its orexigenic effects. Original work proposed that the N-terminal domain of AgRP facilitates this interaction. However, this is not compatible with evidence that this domain is posttranslationally cleaved. Addressing this long-standing incongruity, we used calorimetry and magnetic resonance to probe interactions of AgRP peptides with glycosaminoglycans, including heparan sulfate. We show that mature, cleaved, C-terminal AgRP, not the N-terminal domain, binds heparan sulfate. NMR shows that the binding site consists of regions distinct from the melanocortin receptor-binding site. Using a library of designed AgRP variants, we find that the strength of the syndecan interaction perfectly tracks orexigenic action. Our data provide compelling evidence that AgRP is a heparan sulfate-binding protein and localizes critical regions in the AgRP structure required for this interaction.

A Macrocyclic Agouti-Related Protein/[Nle4,DPhe7]α-Melanocyte Stimulating Hormone Chimeric Scaffold Produces Subnanomolar Melanocortin Receptor Ligands.

The melanocortin system consists of five receptor subtypes, endogenous agonists, and naturally occurring antagonists. These receptors and ligands have been implicated in numerous biological pathways including processes linked to obesity and food intake. Herein, a truncation structure-activity relationship study of chimeric agouti-related protein (AGRP)/[Nle4,DPhe7]α-melanocyte stimulating hormone (NDP-MSH) ligands is reported. The tetrapeptide His-DPhe-Arg-Trp or tripeptide DPhe-Arg-Trp replaced the Arg-Phe-Phe sequence in the AGRP active loop derivative c[Pro-Arg-Phe-Phe-Xxx-Ala-Phe-DPro], where Xxx was the native Asn of AGRP or a diaminopropionic (Dap) acid residue previously shown to increase antagonist potency at the mMC4R. The Phe, Ala, and Dap/Asn residues were successively removed to generate a 14-member library that was assayed for agonist activity at the mouse MC1R, MC3R, MC4R, and MC5R. Two compounds possessed nanomolar agonist potency at the mMC4R, c[Pro-His-DPhe-Arg-Trp-Asn-Ala-Phe-DPro] and c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro], and may be further developed to generate novel melanocortin probes and ligands for understanding and treating obesity.

Discovery of a ß-Hairpin Octapeptide, c[Pro-Arg-Phe-Phe-Dap-Ala-Phe-DPro], Mimetic of Agouti-Related Protein(87-132) [AGRP(87-132)] with Equipotent Mouse Melanocortin-4 Receptor (mMC4R) Antagonist Pharmacology.

Agouti-related protein (AGRP) is a potent orexigenic peptide that antagonizes the melanocortin-3 and -4 receptors (MC3R and MC4R). While the C-terminal domain of AGRP, AGRP(87-132), is equipotent to the full-length peptide, further truncation decreases potency at the MC3R and MC4R. Herein, we report AGRP-derived peptides designed to mimic the active ß-hairpin secondary structure that contains the hypothesized Arg-Phe-Phe pharmacophore. The most potent scaffold, c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-DPro], comprised the hexa-peptide ß-hairpin loop from AGRP cyclized through a DPro-Pro motif. A 20 compound library was synthesized from this scaffold for further structure-activity relationship studies. The most potent peptide from this library was an asparagine to diaminopropionic acid substitution that possessed sub-nanomolar antagonist activity at the mMC4R and was greater than 160-fold selective for the mMC4R versus the mMC3R. The reported ligands may serve as probes to characterize the melanocortin receptors in vivo and leads in the development of novel therapeutics.

Characterization, tissue distribution and regulation by fasting of the agouti family of peptides in the sea bass (Dicentrarchus labrax).

The melanocortin system is one of the most complex hormonal systems in vertebrates. Atypically, the signaling of melanocortin receptors is regulated by the binding of endogenous antagonists, named agouti-signaling protein (ASIP) and agouti-related protein (AGRP). Teleost specific genome duplication (TSGD) rendered new gene copies in teleost fish and up to four different genes of the agouti family of peptides have been characterized. In this paper, molecular cloning was used to characterize mRNA of the agouti family of peptides in sea bass. Four different genes were identified: AGRP1, ASIP1, AGRP2 and ASIP2. The AGRP1 gene is mainly expressed in the brain whereas ASIP1 is mainly expressed in the ventral skin. Both ASIP2 and AGRP2 are expressed in the brain and the pineal gland but also in some peripheral tissues. Immunocytochemical studies demonstrated that AGRP1 is exclusively expressed within the lateral tuberal nucleus, the homologue of the mammalian arcuate nucleus in fish. Long-term fasting (8-29 days) increased the hypothalamic expression of AGRP1 but depressed AGRP2 expression (15-29 days). In contrast, the hypothalamic expression of ASIP2 was upregulated during short-term fasting suggesting that this peptide could be involved in the short term regulation of food intake in the sea bass.

Repeated agouti related peptide (83-132) injections inhibit cocaine-induced locomotor sensitisation, but not via the nucleus accumbens.

Drug addiction is a chronic relapsing brain disease for which many of the underlying neuronal mechanisms are yet to be unravelled. There seems to be an interaction between the melanocortin system and drugs of abuse. For instance, infusion of the melanocortin MC4 receptor antagonist SHU9119 (Ac-Nle-cyclo(-Asp-His-D-2-Nal-Arg-Trp-Lys)-NH2) into the nucleus accumbens results in conditioned place avoidance, reduces the amount of lever presses for cocaine and blocks development of cocaine-induced locomotor sensitisation. The aim of this study is to determine whether the induction of locomotor sensitisation to repeated cocaine is inhibited by the melanocortin MC4 receptor inverse agonist Agouti Related Peptide (AgRP83-132). Rats were sensitised to daily cocaine injections for 5 consecutive days and 30 min prior to every daily cocaine injection, rats received an intracerebroventricular (i.c.v.) or intra nucleus accumbens injection with AgRP(83-132) or saline, to determine whether we could inhibit cocaine-induced locomotor sensitisation. We show that i.c.v. injections of AgRP(83-132) inhibit cocaine-induced locomotor sensitisation. This effect is not regulated via the nucleus accumbens, since injecting the melanocortin receptor inverse agonist AgRP(83-132) directly into the nucleus accumbens was unable to inhibit the cocaine-induced locomotor sensitisation. This implicates that the nucleus accumbens is an unlikely site to inhibit the induction of locomotor sensitisation via the melanocortin MC4 receptor. This is in contrast to other studies that show an effect of the melanocortin MC4 receptor antagonist SHU9119 on locomotor sensitisation when injected into the nucleus accumbens.

A novel radiofluorinated agouti-related protein for tumor angiogenesis imaging.

A novel protein scaffold based on the cystine knot domain of the agouti-related protein (AgRP) has been used to engineer mutants that can bind to the α(v)ß(3) integrin receptor with high affinity and specificity. In the current study, an (18)F-labeled AgRP mutant (7C) was prepared and evaluated as a positron emission tomography (PET) probe for imaging tumor angiogenesis. AgRP-7C was synthesized by solid phase peptide synthesis and site-specifically conjugated with 4-nitrophenyl 2-(18/19)F-fluoropropionate ((18/19)F-NFP) to produce the fluorinated peptide, (18/19)F-FP-AgRP-7C. Competition binding assays were used to measure the relative affinities of AgRP-7C and (19)F-FP-AgRP-7C to human glioblastoma U87MG cells that overexpress α(v)ß(3) integrin. In addition, biodistribution, metabolic stability, and small animal PET imaging studies were conducted with (18)F-FP-AgRP-7C using U87MG tumor-bearing mice. Both AgRP-7C and (19)F-FP-AgRP-7C specifically competed with (125)I-echistatin for binding to U87MG cells with half maximal inhibitory concentration (IC(50)) values of 9.40 and 8.37 nM, respectively. Non-invasive small animal PET imaging revealed that (18)F-FP-AgRP-7C exhibited rapid and good tumor uptake (3.24 percentage injected dose per gram [% ID/g] at 0.5 h post injection [p.i.]). The probe was rapidly cleared from the blood and from most organs, resulting in excellent tumor-to-normal tissue contrasts. Tumor uptake and rapid clearance were further confirmed with biodistribution studies. Furthermore, co-injection of (18)F-FP-AgRP-7C with a large molar excess of blocking peptide c(RGDyK) significantly inhibited tumor uptake in U87MG xenograft models, demonstrating the integrin-targeting specificity of the probe. Metabolite assays showed that the probe had high stability, making it suitable for in vivo applications. (18)F-FP-AgRP-7C exhibits promising in vivo properties such as rapid tumor targeting, good tumor uptake, and excellent tumor-to-normal tissue ratios, and warrants further investigation as a novel PET probe for imaging tumor angiogenesis.

Identification of distant Agouti-like sequences and re-evaluation of the evolutionary history of the Agouti-related peptide (AgRP).

The Agouti-like peptides including AgRP, ASIP and the teleost-specific A2 (ASIP2 and AgRP2) peptides have potent and diverse functional roles in feeding, pigmentation and background adaptation mechanisms. There are contradictory theories about the evolution of the Agouti-like peptide family as well the nomenclature. Here we performed comprehensive mining and annotation of vertebrate Agouti-like sequences. We identified A2 sequences from salmon, trout, seabass, cod, cichlid, tilapia, gilt-headed sea bream, Antarctic toothfish, rainbow smelt, common carp, channel catfish and interestingly also in lobe-finned fish. Moreover, we surprisingly found eight novel homologues from the kingdom of arthropods and three from fungi, some sharing the characteristic C-x(6)-C-C motif which are present in the Agouti-like sequences, as well as approximate sequence length (130 amino acids), positioning of the motif sequence and sharing of exon-intron structures that are similar to the other Agouti-like peptides providing further support for the common origin of these sequences. Phylogenetic analysis shows that the AgRP sequences cluster basally in the tree, suggesting that these sequences split from a cluster containing both the ASIP and the A2 sequences. We also used a novel approach to determine the statistical evidence for synteny, a sinusoidal Hough transform pattern recognition technique. Our analysis shows that the teleost AgRP2 resides in a chromosomal region that has synteny with Hsa 8, but we found no convincing synteny between the regions that A2, AgRP and ASIP reside in, which would support that the Agouti-like peptides were formed by whole genome tetraplodization events. Here we suggest that the Agouti-like peptide genes were formed through classical subsequent gene duplications where the AgRP is the most distantly related to the three other members of that group, first splitting from a common ancestor to ASIP and A2, and then later the A2 split from ASIP followed by a split resulting in ASIP2 and AgRP2.

111In-labeled cystine-knot peptides based on the Agouti-related protein for targeting tumor angiogenesis.

Agouti-related protein (AgRP) is a 4-kDa cystine-knot peptide of human origin with four disulfide bonds and four solvent-exposed loops. The cell adhesion receptor integrin α(v)ß(3) is an important tumor angiogenesis factor that determines the invasiveness and metastatic ability of many malignant tumors. AgRP mutants have been engineered to bind to integrin α(v)ß(3) with high affinity and specificity using directed evolution. Here, AgRP mutants 7C and 6E were radiolabeled with (111)In and evaluated for in vivo targeting of tumor integrin α(v)ß(3) receptors. AgRP peptides were conjugated to the metal chelator 1, 4, 7, 10-tetra-azacyclododecane- N, N', N″, N'''-tetraacetic acid (DOTA) and radiolabeled with (111)In. The stability of the radiopeptides (111)In-DOTA-AgRP-7C and (111)In-DOTA-AgRP-6E was tested in phosphate-buffered saline (PBS) and mouse serum, respectively. Cell uptake assays of the radiolabeled peptides were performed in U87MG cell lines. Biodistribution studies were performed to evaluate the in vivo performance of the two resulting probes using mice bearing integrin-expressing U87MG xenograft tumors. Both AgRP peptides were easily labeled with (111)In in high yield and radiochemical purity (>99%). The two probes exhibited high stability in phosphate-buffered saline and mouse serum. Compared with (111)In-DOTA-AgRP-6E, (111)In-DOTA-AgRP-7C showed increased U87MG tumor uptake and longer tumor retention (5.74 ± 1.60 and 1.29 ± 0.02%ID/g at 0.5 and 24 h, resp.), which was consistent with measurements of cell uptake. Moreover, the tumor uptake of (111)In-DOTA-AgRP-7C was specifically inhibited by coinjection with an excess of the integrin-binding peptidomimetic c(RGDyK). Thus, (111)In-DOTA-AgRP-7C is a promising probe for targeting integrin α(v)ß(3) positive tumors in living subjects.

Agouti-related protein segments outside of the receptor binding core are required for enhanced short- and long-term feeding stimulation.

The agouti-related protein (AgRP) plays a central role in energy balance by reducing signaling through the hypothalamic melanocortin receptors (McRs) 3 and 4, in turn stimulating feeding and decreasing energy expenditure. Mature AgRP(83-132), produced by endoproteolytic processing, contains a central region that folds as an inhibitor cystine knot (ICK) stabilized by a network of disulfide bonds; this domain alone carries the molecular features for high affinity McR binding and inverse agonism. Outside of the ICK domain are two polypeptide segments, an N-terminal extension and a C-terminal loop, both completely conserved but of unknown function. Here we examine the physiological roles of these non-ICK segments by developing a panel of modified AgRPs that were administered to rats through intracerebroventricular (ICV) injection. Analysis of food consumption demonstrates that basic (positively charged) residues are essential for potent short- and long-term AgRP stimulated feeding. Moreover, we demonstrate an approximate linear relationship between protein charge density and 24 h food intake. Next, we developed artificial AgRP(83-132) analogues with increased positive charge and found that these species were substantially more potent than wild type. A single dose of one protein, designated AgRP-4K, results in enhanced feeding for well over a week and weight gain that is nearly double that of AgRP(83-132). These studies suggest new strategies for the development of potent orexigenic species and may serve as leads for the development of therapeutics for treating wasting conditions such as cachexia.

Cyclic lactam hybrid α-MSH/Agouti-related protein (AGRP) analogues with nanomolar range binding affinities at the human melanocortin receptors.

A novel hybrid melanocortin pharmacophore was designed based on the topographical similarities between the pharmacophores of Agouti related protein (AGRP) an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. When employed in two different 23-membered macrocyclic lactam peptide templates, the designed hybrid AGRP/MSH pharmacophore yielded non-competitive ligands with nanomolar range binding affinities. The topography-based pharmacophore hybridization strategy will prove useful in development of unique non-competitive melanocortin receptor modulators.

Incorporation of a bioactive reverse-turn heterocycle into a peptide template using solid-phase synthesis to probe melanocortin receptor selectivity and ligand conformations by 2D 1H NMR.

By use of a solid-phase synthetic approach, a bioactive reverse turn heterocycle was incorporated into a cyclic peptide template to probe melanocortin receptor potency and ligand structural conformations. The five melanocortin receptor isoforms (MC1R-MC5R) are G-protein-coupled receptors (GPCRs) that are regulated by endogenous agonists and antagonists. This pathway is involved in pigmentation, weight, and energy homeostasis. Herein, we report novel analogues of the chimeric AGRP-melanocortin peptide template integrated with a small molecule moiety to probe the structural and functional consequences of the core His-Phe-Arg-Trp peptide domain using a reverse-turn heterocycle. A series of six compounds are reported that result in inactive to full agonists with nanomolar potency. Biophysical structural analysis [2D (1)H NMR and computer-assisted molecular modeling (CAMM)] were performed on selected analogues, resulting in the identification that these peptide-small molecule hybrids possessed increased flexibility and fewer discrete conformational families compared to the reference peptide and result in a novel template for further structure-function studies.

Melanocortins and body weight regulation: glucocorticoids, Agouti-related protein and beyond.

In the intervening three decades since Panksepp observed for the first time that centrally administered α-melanocyte stimulating hormone decreased food intake (Panksepp and Meeker, 1976), a wealth of data have accrued to firmly establish melanocortin signaling as a central regulator of food intake and fat mass. Advances in molecular biology have not only allowed detailed studies of spontaneously occurring obese mice with altered melanocortin signaling to be undertaken but also permitted the generation of a plethora of mouse models with precise perturbations at critical steps in the melanocortin system to finesse further the cellular and molecular architecture of relevant pathways. In this article we focus in upon a number of these mouse models which continue to help us tease apart the complexities of this critical system. Further, we review data on the important interaction between pro-opiomelanocortin derived peptides and the adrenal system and the relationship between agonist and antagonist peptides acting at central melanocortin receptors.

The early origin of melanocortin receptors, agouti-related peptide, agouti signalling peptide, and melanocortin receptor-accessory proteins, with emphasis on pufferfishes, elephant shark, lampreys, and amphioxus.

There are conflicting theories about the evolution of melanocortin MC receptors while only few studies have addressed the evolution of agouti-related peptide (AgRP) and agouti signalling peptide (ASIP), which are antagonists at the melanocortin receptors (MCRs), or the melanocortin MC(2) receptor accessory proteins (MRAP1 and MRAP2). Previously we have cloned melanocortin MC receptors (MC(a) and MC(b)) genes in river lamprey and here we identify orthologues to these melanocortin MC receptor sequences in the sea lamprey. We investigate the putative presence of the melanocortin MC receptor genes in lancelet (amphioxus; Branchiostoma floridae) but we find it unlikely that such gene exists, due to a sharp drop in sequence similarity beyond sequence clusters of known receptors. We show the presence of AgRP and ASIP in elephant shark, a cartilaginous fish belonging to the subclass of Elasmobranchii. However, we do not find any of these genes in lamprey or lancelet after detailed analysis of both targeted and whole proteome regular expression scans. We found MRAP2, but not MRAP1, to be present in elephant shark and sea lamprey while Fugu (T. rubripes) has both genes. This study shows that the most ancient presence of these melanocortin-related sequences is found in elephant shark and lampreys considering the current available sequence data.

Cystine-knot peptides engineered with specificities for α(IIb)ß(3) or α(IIb)ß(3) and α(v)ß(3) integrins are potent inhibitors of platelet aggregation.

A truncated form of the Agouti-related protein (AgRP), a member of the cystine-knot family, has shown promise as a scaffold for engineering novel peptides with new molecular recognition properties. In this study, we replaced a constrained six amino acid loop in AgRP with a nine amino acid loop containing an Arg-Gly-Asp integrin recognition motif, and randomized the neighboring residues to create a library of approximately 20 million AgRP variants. We displayed the AgRP mutants as fusions on the surface of yeast and used high-throughput fluorescence-activated cell sorting (FACS) to isolate peptides that bound specifically to the platelet integrin α(IIb)ß(3), a clinically important target for the prevention and treatment of thrombosis. These AgRP peptides had equilibrium dissociation (K(D)) constants for α(IIb)ß(3) integrin ranging from 60 to 90 nM, and did not bind to α(v)ß(3), α(v)ß(5), or α(5)ß(1) integrins. Using an alternate library screening strategy, we identified AgRP peptides that bound to both α(IIb)ß(3) and α(v)ß(3) integrins with K(D) values ranging from 40 to 70 nM and 20 to 30 nM, respectively, and did not bind to α(v)ß(5) or α(5)ß(1) integrins. Unique consensus sequences were identified within both series of AgRP peptides suggesting alternative molecular recognition events that dictate different integrin binding specificities. In addition, the engineered AgRP peptides prevented platelet aggregation as well as or slightly better than the FDA-approved cyclic peptide eptifibatide. Collectively, these data demonstrate that cystine-knot peptides can be generated with high affinity and specificity to closely-related integrins, and provide insights into molecular interactions between small, structured peptide ligands and their receptors.