Functional interactions between melanin-concentrating hormone, neuropeptide Y, and anorectic neuropeptides in the rat hypothalamus

A growing body of evidence indicates that a number of peptides expressed in the mammalian hypothalamus are involved in the regulation of food intake and energy balance. Among these, melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are potent appetite stimulants, whereas alpha-melanocyte-stimulating hormone (alpha-MSH), neurotensin, and glucagon-like peptide (GLP)-1(7-36) amide have appetite-suppressing properties. However, the functional interactions between pathways involving these neuropeptides remain incompletely understood. In the current study, we describe the functional interactions between orexigenic (appetite-stimulating: MCH and NPY) and anorectic (appetite-suppressing: alpha-MSH, neurotensin, and GLP-1) peptides after intracerebroventricular (i.c.v.) administration in the rat. The i.c.v. administration of GLP-1 completely prevents the orexigenic effects of both MCH and NPY. However, i.c.v. administration of alpha-MSH prevents only the orexigenic effect of MCH, as we have previously shown, but does not prevent the effect of NPY on food intake. Similarly, i.c.v. administration of neurotensin prevents only the orexigenic effect of MCH, but does not prevent the appetite-stimulating effect of NPY. Thus, our study suggests that the functional interactions between these neuropeptides are specific, although the underlying mechanisms are as yet unexplored.

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.

Absence of mutations in the human OB gene in obese/diabetic subjects

The product of the obese (ob) gene, leptin, is a secreted protein that is important in the regulation of body weight. Mice with mutations in the ob gene are obese and diabetic and manifest reduced physical as well as metabolic activity. In this study, we tested the possibility that mutations in the OB gene may contribute to human obesity. We report the isolation and partial sequence of the human OB gene and the screening of 105 obese patients for mutations in the protein coding sequence using the technique of single-strand conformational polymorphism. No coding sequence polymorphism was found, suggesting that mutations in the coding sequence of the OB gene do not constitute a common cause of increased body weight in humans. We also identified a highly polymorphic simple dinucleotide repeat DNA polymorphism in this gene that will be useful for genetic studies.

A role for melanin-concentrating hormone in the central regulation of feeding behaviour

The hypothalamus plays a central role in the integrated regulation of energy homeostasis and body weight, and a number of hypothalamic neuropeptides, such as neuropeptide Y (ref. 1), galanin, CRH (ref. 3) and GLP-1 (ref. 4), have been implicated in the mediation of these effects. To discover new hypothalmic peptides involved in the regulation of body weight, we used differential display polymerase chain reaction to identify messenger RNAs that are differentially expressed in the hypothalamus of ob/+ compared with ob/ob C57B1/6J mice. We show here that one mRNA that is overexpressed in the hypothalamus of ob/ob mice encodes the neuropeptide melanin-concentrating hormone (MCH). Fasting further increased expression of MCH mRNA in both normal and obese animals. Neurons containing MCH are located in the zona incerta and in the lateral hypothalamus. These areas are involved in regulation of ingestive behaviour, but the role of MCH in mammalian physiology is unknown. To determine whether MCH is involved in the regulation of feeding, we injected MCH into the lateral ventricles of rats and found that their food consumption increased. These findings suggest that MCH participates in the hypothalamic regulation of body weight.

Examination of the phosphoenolpyruvate carboxykinase gene promoter in patients with noninsulin-dependent diabetes mellitus

Expression of phosphoenolpyruvate carboxykinase (PEPCK), a rate-limiting enzyme in gluconeogenesis, is under dominant negative regulation by insulin. In this study, we sought to test the hypothesis that mutations in the PEPCK gene promoter may impair the ability of insulin to suppress hepatic glucose production, thereby contributing to both the insulin resistance and increased rate of gluconeogenesis characteristic of NIDDM. The proximal PEPCK promoter region in 117 patients with noninsulin-dependent diabetes mellitus and 20 obese Pima Indians was amplified by PCR and analyzed with single strand conformation polymorphism techniques. In addition, limited direct DNA sequencing was performed on the insulin response sequence and flanking regions. No DNA sequence polymorphisms were found in any patient. This result suggests that mutations in cis-acting PEPCK gene regulatory elements do not constitute a common cause of noninsulin-dependent diabetes mellitus. The significance of genetic variation in promoter regions to human disease is discussed.

Structure-function analysis of protein active sites with anti-idiotypic antibody

Antigen and internal image-bearing anti-idiotypic antibody, owing to potential differences in size and chemical nature, need not necessarily demonstrate identical binding specificities. Such differences, termed "dissociability," may be exploited in structure-function analysis of receptor-ligand interaction to identify functionally important amino acid residues, define receptor class, or distinguish receptor conformation. In this sense, ligand and the anti-idiotypes they elicit constitute alternative and complementary probes of protein active sites.

Three-dimensional structure of cholera toxin penetrating a lipid membrane

Two-dimensional crystals of cholera toxin bound to receptors in a lipid membrane give diffraction extending to 15 A resolution. Three-dimensional structure determination reveals a ring of five B subunits on the membrane surface, with one-third of the A subunit occupying the center of the ring. The remaining mass of the A subunit appears to penetrate the hydrophobic interior of the membrane. Cleavage of a disulfide bond in the A subunit, which activates the toxin, causes a major conformational change, with the A subunit mostly exiting from the B ring.

HLA-A2 peptides can regulate cytolysis by human allogeneic T lymphocytes

The class-I and class-II molecules encoded by the major histocompatibility complex (MHC) are homologous proteins which allow cytotoxic and helper T cells to recognize foreign antigens. Recent studies have shown that the form of the antigen recognized by T cells is generally not a native protein but rather a short peptide fragment and that class-II molecules specifically bind antigenic peptides. Furthermore, the three-dimensional structure of the human MHC class-I molecule, HLA-A2, is consistent with a peptide-binding function for MHC class-I molecules. An outstanding question concerns the molecular nature and involvement of MHC-bound peptides in antigens recognized by alloreactive T cells. In this study the effects of peptides derived from HLA-A2 on cytolysis of alloreactive cytotoxic T cells (TC) cells are presented. Peptides can inhibit lysis by binding to the T cell or sensitize to lysis by binding an HLA-A2-related class-I molecule (HLA-Aw69) on the target cell. Thus, allospecific TC cells can recognize HLA-derived peptides in the context of the MHC.

Antigenic determinants of the cholera/coli family of enterotoxins

Hybridoma-derived monoclonal antibodies were raised to enterotoxins of the cholera family and to chimeric B-subunit proteins in which individual amino acid residues of a heat-labile, cholera-related enterotoxin from an Escherichia coli strain of porcine origin (P-LT) were substituted with corresponding residues from such an enterotoxin from an E. coli strain of human origin (H-LT). Single amino acid substitutions were found to have profound effects on the physicochemical behavior of the proteins and on their immunologic reactivity. With the use of enzyme-linked immunosorption assays (ELISAs) with and without the GM1 ganglioside receptor for these toxins, several distinct epitopes in GM1-binding domains were identified by different monoclonal antibodies. Polyclonal rabbit antisera to synthetic peptides of the cholera enterotoxin B subunit were cross-reactive to various degrees with the proteins in our library, which include two different cholera enterotoxins, two H-LTs, P-LT, and four chimeric proteins. Some of these reactions were blocked by GM1 ganglioside but not the oligosaccharide of GM1, a finding suggesting that the peptides generated antibodies to epitopes near, but not in, a GM1-binding domain. A hypothetical evolutionary tree based on the reported amino acid sequences of the various enterotoxins is constructed. As additional enterotoxins are described, it will be interesting to determine if and where they fit in this scheme.

Anti-receptor antibodies designed to elicit "internal image"-bearing anti-idiotypes: a possible AIDS vaccine

Two obstacles hinder the development of an AIDS vaccine: (1) the AIDS virus exhibits extensive amino acid heterogeneity between isolates and (2) antibodies elicited by virus during the course of natural infection are often non-neutralizing. A vaccine designed to induce anti-idiotypic antibodies against the virus' receptor on T-cells, T4, should, in principle, overcome these obstacles. Such antibody could contain an "internal image" of T4 and bind the receptor binding domain of the virus. Since this domain is both critical to function and, therefore, poorly susceptible to antigenic variation, anti-receptor anti-idiotypic antibodies may demonstrate broad, strain-independent crossreactivity and block viral adherence.

Anti-idiotypic antibodies as probes of protein active sites: application to cholera toxin subunit B

Since Jerne proposed a "network" theory of immune regulation, the properties of anti-idiotypic antibodies (anti-IdAb) have been investigated widely. Anti-IdAb raised against antibodies to a variety of ligands have been shown to bind the ligands' receptors. Thus, the combining site of an anti-IdAb may contain information regarding the three-dimensional structure of an antigen. However, this remarkable property of "internal imagery" has not been exploited for structural investigation at the molecular level. In the present report, a monoclonal "auto"-anti-IdAb was raised against ganglioside GM1 (a cell-surface glycolipid that binds cholera toxin) and was shown to crossreact with the B subunit of cholera toxin. This antibody was presumed to recognize amino acid residues located within the GM1 binding domain. To identify these residues, the antibody was screened against homologous toxins purified from enterotoxigenic strains of Escherichia coli and chimeric peptides produced by recombinant methods. Amino acid variation at position 4 from the N terminus of these proteins was found to disrupt antibody binding. Since the toxins and chimera are all closely related in structure and function, the residue at position 4 (an asparagine in cholera toxin B subunit) appears to be in the epitope of the antibody and, by implication, in the GM1 binding site. Of particular significance, this structural detail could not be deduced with GM1 alone. It would seem that ligand and anti-ligand anti-IdAb encode similar stereochemical information but do so with different "chemical alphabets," giving rise to distinct binding specificities.

Epitopes of the cholera family of enterotoxins

Hybridoma-derived monoclonal antibodies were raised to enterotoxins of the cholera family and to chimeric B-subunit proteins in which individual amino acid residues of a heat-labile, cholera-related enterotoxin from an Escherichia coli strain of porcine origin (P-LT) were substituted with corresponding residues from such an enterotoxin from an E. coli strain of human origin (H-LT). Single amino acid substitutions were found to have profound effects on the physicochemical behavior of the proteins and on their immunologic reactivity. With the use of enzyme-linked immunosorption assays (ELISAs) with and without the GM1 ganglioside receptor for these toxins, several distinct epitopes in GM1-binding domains were identified by different monoclonal antibodies. Polyclonal rabbit antisera to synthetic peptides of the cholera enterotoxin B subunit were cross-reactive to various degrees with the proteins in our library, which include two different cholera enterotoxins, two H-LTs, P-LT, and four chimeric proteins. Some of these reactions were blocked by GM1 ganglioside but not by the oligosaccharide of GM1, a finding suggesting that the peptides generated antibodies to epitopes near, but not in, a GM1-binding domain. A hypothetical evolutionary tree based on the reported amino acid sequences of the various enterotoxins is constructed. As additional enterotoxins are described, it will be interesting to determine if and where they fit in this scheme.

Inhibition of alloreactive cytotoxic T lymphocytes by peptides from the alpha 2 domain of HLA-A2

Class I major histocompatibility complex (MHC) molecules function in the recognition of antigens by cytotoxic T lymphocytes (CTL). Although this biological role is firmly established and much has been learnt about their structure and polymorphic variation, little is known of the regions of class I molecules that are involved in functional interactions with components of the T-cell surface. Here we show that peptides derived from residues 98-113 of the alpha 2 domain of HLA-A2 specifically inhibit the recognition of target cells by many HLA-A2-specific CTL. In addition to identifying a region that is probably involved in binding the T-cell receptor these results raise the possibility that alloreactive CTL may recognize degraded fragments of class I histocompatibility antigens.

Two-dimensional crystals of cholera toxin B-subunit-receptor complexes: projected structure at 17-A resolution

The B subunit of cholera toxin forms two-dimensional crystals when bound to its membrane receptor, ganglioside GM1, in phospholipid layers. A rectangular crystal lattice gives diffraction extending to 15-A resolution in negative stain, and image-processing of electron micrographs reveals a ring of five protein densities. The diameter of the central hole and the outer diameter of the ring are about 20 and 60 A, respectively. These data are consistent with a pentameric, doughnut-shaped structure of the B subunit that lies flat on a membrane surface. A hexagonal crystal lattice is obtained as well, and results of image processing and chemical crosslinking allow two interpretations: the B subunit may exist in both pentameric and hexameric forms or, more likely, the hexagonal lattice may represent a disordered or liquid crystalline form, in which a pentamer undergoes rotational averaging about its 5-fold axis.

Chemical and immunochemical studies on the receptor binding domain of cholera toxin B subunit

The contributions of various amino acids to the structure and function of cholera toxin B subunit were assessed with quantifiable, chemically conservative, reversible derivatizations, and sensitive assays of activity. A panel of monoclonal antibodies was employed to monitor the conformational integrity of modified protein and help distinguish the direct from indirect effects of chemical derivatization. We describe a novel monoclonal antibody, which competes with the receptor GM1 for binding to cholera toxin B subunit, and use this reagent to help identify critically located residues. Our data support the hypothesis that tryptophan participates directly in binding GM1. In addition, we propose a dual role for lysine: first, these basic residues maintain an electrostatic attraction vital to receptor recognition; second, at least 1 lysine resides near the receptor binding domain and may interact with GM1. The influence of arginyl and tyrosyl residues upon activity is re-examined. Finally, we present data which suggest, in variance with previous studies, that the intramolecular disulfide bond is vital to the structure and function of cholera toxin B subunit.