Ira Gantz

Molecular Basis for the Interaction of [Nle4,d-Phe7]Melanocyte Stimulating Hormone with the Human Melanocortin-1 Receptor (Melanocyte α-MSH Receptor)

The melanocortin-1 receptor (MC1R) is a seven-transmembrane (TM) G-protein-coupled receptor whose natural ligands are the melanocortin peptides, adrenocorticotropic hormone, and α-, β-, and γ- melanocyte stimulating hormone (MSH). To test a previously constructed three-dimensional model of the molecular interaction between the long-acting, superpotent α-MSH analog [Nle4,d-Phe7]MSH (NDP-MSH) and the human MC1R we examined the effects of site-directed receptor mutagenesis on the binding affinity and potency of NDP-MSH. In addition, we also examined the effects of these same mutations on the binding affinity and potency of the structurally related agonists α-MSH, γ-MSH, and Ac-Nle4-cyclic-[Asp5,His6,d-Phe7,Arg8,Trp9,Lys10]NH2(MT-II). Mutagenesis of acidic receptor residues Glu94 in TM2 and Asp117 or Asp121in TM3 significantly altered the binding affinity and potency of all four agonists suggesting that these receptor residues are important to the ligand-receptor interactions of all. A disproportionate change in agonist potency versus affinity observed with simultaneous mutation of these acidic residues (mutant constructs D117A/D121A or E94A/D117A/D121A) or introduction of a single positive charge (mutant construct D121K) also implicates these residues in receptor activation. In addition, results from the individual mutation of aromatic receptor residues Phe175, Phe196, and Phe257, and simultaneous mutation of multiple TM4, -5, and -6 tyrosine and phenylalanine residues suggests that aromatic-aromatic ligand-receptor interactions also participate in binding these melanocortins to the MC1R. These experiments appear to have identified some of the critical receptor residues involved in the ligand-receptor interactions between these melanocortins and the hMC1R.

α-MSH and its receptors in regulation of tumor necrosis factor-α production by human monocyte/macrophages

The hypothesis that macrophages contain an autocrine circuit based on melanocortin [ACTH and α-melanocyte-stimulating hormone (α-MSH)] peptides has major implications for neuroimmunomodulation research and inflammation therapy. To test this hypothesis, cells of the THP-1 human monocyte/macrophage line were stimulated with lipopolysaccharide (LPS) in the presence and absence of α-MSH. The inflammatory cytokine tumor necrosis factor (TNF)-α was inhibited in relation to α-MSH concentration. Similar inhibitory effects on TNF-α were observed with ACTH peptides that contain the α-MSH amino acid sequence and act on melanocortin receptors. Nuclease protection assays indicated that expression of the human melanocortin-1 receptor subtype (hMC-1R) occurs in THP-1 cells; Southern blots of RT-PCR product revealed that additional subtypes, hMC-3R and hMC-5R, also occur. Incubation of resting macrophages with antibody to hMC-1R increased TNF-α concentration; the antibody also markedly reduced the inhibitory influence of α-MSH on TNF-α in macrophages treated with LPS. These results in cells known to produce α-MSH at rest and to increase secretion of the peptide when challenged are consistent with an endogenous regulatory circuit based on melanocortin peptides and their receptors. Targeting of this neuroimmunomodulatory circuit in inflammatory diseases in which myelomonocytic cells are prominent should be beneficial.

Novel insights into histamine H2 receptor biology.

Histamine exerts multiple biological actions through one of three receptor subtypes (H1, H2, and H3). This review focuses on new developments regarding the structure and function of the H2 receptor. In addition to the important role this receptor plays in stimulating gastric acid secretion, recent studies have demonstrated that it is also involved in regulating gastrointestinal motility and intestinal secretion. The potential role of the H2 receptor in regulating cell growth and differentiation has also been added to the list of actions this biogenic amine may exert in both normal and transformed tissues. Molecular cloning of the gene indicates that it has the structural characteristics of a heptahelical G protein-linked receptor. Site-directed mutagenesis studies of this receptor reveal the presence of key amino acids within the third and fifth transmembrane domains that are critical for ligand recognition. Molecular approaches have also shed light on the structural components of the H2 receptor important in regulating desensitization and internalization. Although the H2 receptor was classically thought to couple to the adenylate cyclase pathway, recent work with the cloned receptor indicates that it can also activate the phosphoinositide signaling cascade through an independent G protein-dependent mechanism. The novel observation that histamine may stimulate c- fos gene expression lends further support to the possible role of this receptor in regulating cell growth and differentiation.

Inverse agonist activity of agouti and agouti-related protein

Agouti and agouti-related protein (AgRP) are endogenous antagonists of the melanocortin receptors (MCxR). Previous data showed that recombinant full-length agouti and a synthetic fragment of AgRP, AgRP (83-132), are inverse agonists at the MC1R and MC4R, respectively. This study demonstrates the smaller analogs AgRP (87-120) and ASIP [90-132 (L89Y)], and short peptides Yc[CRFFNAFC]Y and Qc[CRFFRSAC]S are also MC4R inverse agonists. Furthermore, the relative affinity of the series of MC4R ligands for displacement of radiolabeled antagonist 125I-AgRP (86-132) versus radiolabeled agonist 125I-NDP-MSH did not correlate with ligand efficacy, which is more consistent with an induced-fit model than a simple two-state model of MC4R activation. These data shed new light on the determinants and mechanism of inverse agonism at the MC4R.

NMR structure of a minimized human agouti related protein prepared by total chemical synthesis.

The structure of the chemically synthesized C‐terminal region of the human agouti related protein (AGRP) was determined by 2D 1H NMR. Referred to as inimized gouti elated rotein, MARP is a 46 residue polypeptide containing 10 Cys residues involved in five disulfide bonds that retains the biological activity of full length AGRP. AGRP is a mammalian signaling molecule, involved in weight homeostasis, that causes adult onset obesity when overexpressed in mice. AGRP was originally identified by homology to the agouti protein, another potent signaling molecule involved in obesity disorders in mice. While AGRPs exact mechanism of action is unknown, it has been identified as a competitive antagonist of melanocortin receptors 3 and 4 (MC3r, MC4r), and MC4r in particular is implicated in the hypothalamic control of feeding behavior. Full length agouti and AGRP are only 25% homologous, however, their active C‐terminal regions are ∼40% homologous, with nine out of the 10 Cys residues spatially conserved. Until now, 3D structures have not been available for either agouti, AGRP or their C‐terminal regions. The NMR structure of MARP reported here can be characterized as three major loops, with four of the five disulfide bridges at the base of the structure. Though its fold is well defined, no canonical secondary structure is identified. While previously reported structural models of the C‐terminal region of AGRP were attempted based on Cys homology between AGRP and certain toxin proteins, we find that Cys spacing is not sufficient to correctly determine the 3D fold of the molecule.

Contribution of Melanocortin Receptor Exoloops to Agouti-related Protein Binding

Agouti-related protein (AGRP) is an endogenous antagonist of melanocortin action that functions in the hypothalamic control of feeding behavior. Although previous studies have shown that AGRP binds three of the five known subtypes of melanocortin receptor, the receptor domains participating in binding and the molecular interactions involved are presently unknown. The present studies were designed to examine the contribution of extracytoplasmic domains of the melanocortin-4 receptor (MC4R) to AGRP binding by making chimerical receptor constructs of the human melanocortin-1 receptor (MC1R; a receptor that is not inhibited by AGRP) and the human MC4R (a receptor that is potently inhibited by AGRP). Substitutions of the extracytoplasmic NH2 terminus and the first extracytoplasmic loop (exoloop) of the MC4R with homologous domains of the MC1R had no effect on AGRP (87–132) binding affinity or inhibitory activity (the ability to inhibit melanocortin-stimulated cAMP generation). In contrast, cassette substitutions of exoloops 2 and 3 of the MC4R with the homologous exoloops of the MC1R resulted in a substantial loss of AGRP binding affinity and inhibitory activity. Conversely, the exchange of exoloops 2 and 3 of the MC1R with the homologous exoloops of the MC4R was found to confer AGRP binding and inhibitory activity to the basic structure of the MC1R. Importantly, these substitutions did not affect the ability of the α-melanocyte stimulating hormone analogue [Nle4,d-Phe7] melanocyte stimulating hormone to bind or activate the chimeric receptors. These data indicate that exoloops 2 and 3 of the melanocortin receptors are important for AGRP binding.

Agouti-related protein is a mediator of diabetic hyperphagia.

To explore the role of agouti-related protein (AGRP) in diabetic hyperphagia changes in hypothalamic AGRP mRNA levels were examined in diabetic rats. Rats rendered diabetic by streptozotocin displayed marked hyperglycemia (blood glucose 456.0+/-8.4 mg/dl versus 71.8+/-1.9 mg/dl) and hyperphagia (36.9+/-1.0 g/day versus 22.0+/-0.4 g/day), that was associated with a 286.6+/-4.4% increase in hypothalamic AGRP mRNA and a 178.9+/-13.5% increase in hypothalamic NPY mRNA. Insulin treatment of diabetic rats partially corrected blood glucose (147.4+/-13.1 mg/dl) and ameliorated hyperphagia (26.6+/-2.0 g/day). Insulin replacement was also associated with a return of hypothalamic AGRP mRNA (111.7+/-8.3% of controls) and NPY mRNA (125.0+/-8.9% of controls) from the elevated levels that were observed in untreated diabetic rats. In contrast to insulin treated rats, sodium orthovanadate treated diabetic rats remained significantly hyperglycemic (361.5+/-12.5 mg/dl). However, despite their persistent hyperglycemia, orthovanadate treated diabetic rats were still observed to have a significant reduction of hypothalamic AGRP mRNA (138.7+/-11.4%) and NPY mRNA (129.9+/-9.8%). Simultaneous measurement of serum leptin revealed suppressed levels in both untreated diabetic (0.5+/-0.1 ng/ml) and sodium orthovanadate treated rats (0.5+/-0.1 ng/ml) compared to non-diabetic controls (2.1+/-0.1 ng/ml). These data indicate that AGRP is a mediator of diabetic hyperhpagia and suggest that insulin can directly influence hypothalamic AGRP and NPY mRNA expression.

Peptoid mimics of agouti related protein

The Agouti Related Protein (AGRP) is an endogenous antagonist of melanocortin-3 and -4 receptors, each of which plays a key role in body weight homeostasis. We designed a peptoid trimer based on AGRP 111-113 in which a single chiral atom is used to partially restrain the backbone structure. Peptoid 5 displaced both radiolabeled Nle4-alpha-MSH (IC(50)=3.1 microM) and AGRP (86-132) (IC(50)=1.9 microM) from the human melanocortin-4 receptor and functioned as an antagonist of alpha-MSH stimulated cAMP generation, thus providing an important lead in the development of AGRP mimetics.

Loops and Links: Structural Insights into the Remarkable Function of the Agouti-Related Protein

Abstract: The agouti‐related protein (AGRP) is an endogenous antagonist of the melanocortin receptors MC3R and MC4R found in the hypothalamus and exhibits potent orexigenic activity. The cysteine‐rich C‐terminal domain of this protein, corresponding to AGRP(87–132), exhibits receptor binding affinity and antagonism equivalent to that of the full‐length protein. We recently determined the NMR structure of AGRP(87–132) and demonstrated that a portion of the domain adopts the inhibitor cystine‐knot fold. Remarkably, this is the first identification of a mammalian protein with this specific architecture. Further analysis of the structure suggests that melanocortin receptor contacts are made primarily by two loops presented within the cystine knot. 10 To test this hypothesis we designed a 34‐residue AGRP analogue corresponding to only the cystine knot. We found that this designed miniprotein folds to a homogeneous product, retains the desired cystine‐knot architecture, functions as a potent antagonist, and maintains the melanocortin receptor pharmacological profile of AGRP(87–132). 26 The AGRP‐like activity of this molecule supports the hypothesis that indeed the cystine‐knot region possesses the melanocortin receptor contacts. Based on these design and structure studies, we propose that the N‐terminal loop of AGRP(87–132) makes contact with a receptor exoloop and helps confer AGRPs selectivity for the central MCRs.

Cloning of Canine Gastrin cDNA’s Encoding Variant Amino Acid Sequences

We have isolated 2 clones encoding gastrin from a canine antral mucosa cDNA library. The clones differed in nucleotide sequence at 2 sites in the coding region. One of the clones confirmed the amino acid sequence of dog gastrin obtained by peptide purification and analysis. An A-G substitution in the other clone resulted in a Thr-Ala substitution in the region encoding the biologically active carboxyl-terminal portion of gastrin. The source of the observed heterogeneity in cDNA sequences is not known but its location may have implications with regard to the physiology of gastrin in dogs.