GRP is the mammalian homolog of the amphibian peptide bombesin. Bombesin is a 14-amino acid peptide that was initially isolated from the skin of the frog Bombina bombina in 1971 by Anastasi and collaborators. Three different families of bombesin-like peptides exist in amphibians (bombesins, ranaten-sins, and phyllolitorins). These peptides are localized to myoepithelial poison glands that are under adrenergic control and therefore likely function in defence against predators. During the 1970s it became evident that bombesin-like peptides also exist in mammals, and in 1979 a mammalian homolog to bombesin was isolated from the porcine gastrointestinal tract by Dr. McDonald in the laboratory of Professor Mutt in Stockholm. Because this peptide was found to stimulate gastrin release from a porcine stomach model, it was called GRP. A few years later, a second bombesin-like peptide was isolated in mammals and called neuromedin B (NMB). These two peptides show a resemblance to different bombesin families. Thus, whereas GRP is structurally related to bombesin, NMB is structurally related to the ranatensins. GRP consists of a 27-amino acid residue that is a-amidated at its C-terminal methionine. The peptide was highly conserved during evolution, and the human and porcine forms of the peptide differ in only two residues (Fig. 2). The gene for GRP is located on chromosome 18q21 and consists of three exons (Fig. 2). The first exon encodes for the signal peptide and for the first 23 N-terminally located amino acids of GRP, whereas the second exon encodes for the remaining amino acids in GRP and for the first 74 amino acids in a C-terminal extension peptide of GRP. Finally, the third exon encodes for the remaining 21 amino acid residues in this long extension peptide. Due to alternative donor and acceptor sites in the second intron, three different GRP mRNAs are formed by differential splicing. These three GRP mRNAs therefore arise from the same gene and they code for three different proGRPs; however, all consist of 125 residues and differ only in their C-terminal parts. The proform of GRP contains, besides GRP (=proGRP1 27), the C-terminal extension peptide of GRP (=proGRP31 125). This extension peptide contains a 95-amino acid residue, but the
15 10 15 20 25 hGRP VPLPAGGGTVLTK.MYPRGNHWAVGHLM* pGRP APVSVGGGTVLAKMYPRGNHWAVGHLM* bombesin EQRLGNQWAVGHLM*
Figure 2 Schematic representation of the GRP gene and proGRP. The first exon encodes for the signal peptide and the N-terminal portion of the sequence of GRP, exon 2 encodes for the remaining portion of the GRP sequence and the major, N-terminal portion of the C-terminal extension peptide of GRP, and exon 3 encodes for the remaining sequence of this extension peptide. At the bottom of figure are the amino acid sequences of human and porcine GRP and bombesin. * indicates a C-terminal NH2 group. The amino acids in this and the other figures are abbreviated according to the one-letter system: A, Ala; C, Cys; D, Asp; E, Glu; F, Phe; G, Gly; H, His; I Ile; K, Lys; L, Leu; M, Met; N, Asn; P, Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; W, Trp; Y, Tyr.
amino acids are different depending on which mRNA is translated. This is in contrast to the GRP sequence, which is identical in all three GRP gene-derived mRNAs. proGRP31 125 is posttranslationally modified to several different smaller peptides. GRP itself is also posttranslationally modified, and the major molecular forms in various tissues are GRP14, i.e., GRP271 14, and GRP27. The NMB gene is located on chromosome 15 and encodes for a 76-residue proNMB, which is processed to NMB (10 and 22 residues) and a 17-residue C-terminal extension pep-tide of NMB. It is thought that the GRP and NMB genes developed during evolution by duplication ofthe bombesin gene.
GRP is widely distributed in mammalian tissues, with the highest concentrations in the lung, central nervous system, and gut. In the peripheral nervous system, GRP is involved in the regulation of a variety of physiological processes, such as exocrine and endocrine secretions and smooth muscle contractions, and GRP is a powerful trophic agent as well. GRP is also localized to the brain with particular density in the hypothalamus, and centrally the peptide has been shown to be involved in the control of food intake and behavior. In the pancreas, GRP is localized to nerves with particular density in the ganglia, and GRP has also been shown to be released from the isolated pig pancreas when the attached vagal nerve is activated.
Furthermore, when GRP is exogenously added to different experimental models, both in vivo and in vitro, and to both the intact pancreas and isolated islets, the peptide efficiently stimulates both insulin and glucagon secretion. Because the pancreatic density of GRP localization is highest in the ganglia, it is possible that the peptide exerts actions both locally in the islets and also through ganglionic activation. Experimental support for such a notion was presented because ganglionic blockade inhibited the insulin response to exogenously added GRP in mice. Therefore, morphological and functional evidence suggests that GRP is a pancreatic parasympathetic neurotransmitter, the effect of which may contribute to islet actions of parasympathetic nerve stimulation, and this may be achieved both by a direct islet action and by an indirect ganglionic action.
The bombesin-like peptides, like GRP, are known to bind to G-protein-coupled receptors that are of the seven transmembrane domain type. Three receptors for the bombesin-like peptides have been cloned in mammals: the GRP receptor, the NMB receptor, and the bombesin receptor subtype 3. It is also possible, although not yet demonstrated, that bombesin receptor subtypes 2 and 4 might exist in mammals. However, in contrast to the GRP and NMB receptors, no endogenous ligand for these receptors has been found in mammals. The genes for both the GRP receptor and the bombesin receptor subtype 3 are localized to the X chromosome, whereas the gene for the NMB receptor is localized to chromosome 6. The chromosomal site for the GRP receptor is chromosome Xp21.2-p21.3, and this gene contains three exons. The receptor consists of 384 amino acids and shows a high degree of conservation during evolution, with homology between species of approximately 90%. Interestingly, the GRP receptor shows 60% homology with the NMB receptor. It has not yet been established whether the GRP receptors are expressed in the islet endocrine cells, although it is known that the islets are equipped with specific GRP binding sites. In islets, activation by GRP has been shown to be coupled mainly to PLC and PLD, and the islet action of GRP therefore is related to changes in cytoplasmic Ca2 + , formation of DAG, and activation of PKC. Studies have also demonstrated that GRP induces typical patterns of oscillation in cytosolic Ca2+, which is due to both dynamic release of Ca2+ from intracellular stores and passage of Ca2+ through plasma membrane Ca2+ channels.
The relative importance of GRP for islet function has not yet been established. One study has used a specific GRP receptor antagonist, N-acetyl-GRP20 26-amide, which is a fragment of GRP in which methionine has been deleted from position 27, to examine the potential involvement of GRP in isiet function in mice. This antagonist has been used to inhibit the actions of GRP in a number of experimental systems and has aiso been shown to inhibit insulin secretion stimulated by exogenously added GRP in mice. However, the antagonist did not reduce islet hormone secretion induced by autonomic nerve activation, which suggests either that GRP does not contribute to islet function during autonomic nerve activation or that this antagonist is not a suitable tool for examining this contribution in vivo.
Another approach to the study of the physiological impact of GRP and GRP-related peptides is the use of receptor-deficient mice. Dr. Wada and collaborators in Japan have used genetic targeting to develop mice with deletion of either the bombesin receptor subtype 3 or the GRP receptor. Studies presented so far indicate that the mice with bombesin receptor subtype 3 deleted develop obesity. However, until the endogenous ligand for this receptor in mammals is found, the physiological relevance of this finding is not known. Furthermore, the GRP-receptor-deficient mice show impaired insulin response to gastric administration of glucose and to autonomic nerve activation, which suggests that GRP is involved in the parasympathetic regulation of islet function through activation of the GRP receptors. This would imply that GRP physiologically contributes to parasympathetically mediated insulin secretion during food intake, which supports a role for this neuropeptide in the regulation of islet function through the activation of GRP receptors.
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Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...