Reversal of the inhibitory effects of calcitonin gene-related peptide (CGRP) and amylin on insulin secretion by the 8-37 fragment of human CGRP

Inhibition of insulin secretion

Reports of the effects of amylin and amylin agonists on insulin secretion have varied widely. Some confusion can be attributed to the use of human amylin, which has been shown to readily fall out of solution resulting in low estimates of bioactivity. Some confusion can be resolved by assessing the probability that this had happened. The view taken here, supported by authors using reliable and well-characterized ligands (representing the preponderance of recent studies), is that exogenously administered amylin agonists inhibit insulin secretion, at least partly via activation of an amylin-like receptor linked to Gi-mediated inhibition of cAMP in islets. There may additionally be autonomic extrapancreatic effects of amylin on insulin secretion that derive from its action at the area postrema. Studies with amylin receptor antagonists, including human studies, indicate that endogenously secreted amylin may physiologically inhibit beta-cell secretion (insulin and amylin) via feedback inhibition that is characteristic of many other hormones. Part of this inhibition may be local (paracrine), as indicated by the amylin sensitivity of isolated preparations and the fact that the concentration of secreted products in the islet interstitium can be over 100-fold higher than in the circulation (Bendayan, 1993).

Receptor autoradiography as mean to explore the possible functional relevance of neuropeptides: focus on new agonists and antagonists to study natriuretic peptides, neuropeptide Y and calcitonin gene-related peptides

Over the past 20 years, receptor autoradiography has proven most useful to provide clues as to the role of various families of peptides expressed in the brain. Early on, we used this method to investigate the possible roles of various brain peptides. Natriuretic peptide (NP), neuropeptide Y (NPY) and calcitonin (CT) peptide families are widely distributed in the peripheral and central nervous system and induced multiple biological effects by activating plasma membrane receptor proteins. The NP family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The NPY family is composed of at least three peptides NPY, peptide YY (PYY) and the pancreatic polypeptides (PPs). The CT family includes CT, calcitonin gene-related peptide (CGRP), amylin (AMY), adrenomedullin (AM) and two newly isolated peptides, intermedin and calcitonin receptor-stimulating peptide (CRSP). Using quantitative receptor autoradiography as well as selective agonists and antagonists for each peptide family, in vivo and in vitro assays revealed complex pharmacological responses and radioligand binding profile. The existence of heterogeneous populations of NP, NPY and CT/CGRP receptors has been confirmed by cloning. Three NP receptors have been cloned. One is a single-transmembrane clearance receptor (NPR-C) while the other two known as CG-A (or NPR-A) and CG-B (or NPR-B) are coupled to guanylate cyclase. Five NPY receptors have been cloned designated as Y(1), Y(2), Y(4), Y(5) and y(6). All NPY receptors belong to the seven-transmembrane G-protein coupled receptors family (GPCRs; subfamily type I). CGRP, AMY and AM receptors are complexes which include a GPCR (the CT receptor or CTR and calcitonin receptor-like receptor or CRLR) and a single-transmembrane domain protein known as receptor-activity-modifying-proteins (RAMPs) as well as an intracellular protein named receptor-component-protein (RCP). We review here tools that are currently available in order to target each NP, NPY and CT/CGRP receptor subtype and establish their respective pathophysiological relevance.

Reduced nociceptive behavior in islet amyloid polypeptide (amylin) knockout mice

Islet amyloid polypeptide (IAPP or amylin) is predominantly expressed by insulin cells, but occurs also in primary sensory neurons in the rat. Here, using mice targeted for a null mutation in the IAPP gene, we establish murine expression of IAPP in sensory neurons; its distribution in a population of calcitonin gene-related peptide-containing neurons in the spinal cord and dorsal root ganglion is similar to that previously described in the rat. We also report the IAPP mutant mice display a reduced pain response in the paw formalin test. Adjuvant-induced joint inflammation was not altered in IAPP mutants, arguing against a peripheral inflammatory abnormality. These findings lead us to suggest that IAPP has a pro-nociceptive function in primary sensory neurons.

Increased insulin secretion and glucose tolerance in mice lacking islet amyloid polypeptide (amylin)

Islet amyloid polypeptide (IAPP or amylin) is costored and cosecreted with insulin and may regulate insulin secretion and blood glucose handling. However, the role and importance of endogenous IAPP in the regulation of insulin release and glucose homeostasis have been controversial. Here we report on the generation and phenotypic analysis of IAPP-deficient mice. These mice have normal, or near to normal, basal levels of circulating insulin and glucose. However, following glucose administration, IAPP-deficient males presented increased insulin responses paralleled with a more rapid blood glucose elimination compared to wild-type controls. Blood glucose elimination was also found to be enhanced in IAPP-deficient females, but the insulin response in this gender did not differ from controls. In a transgenic rescue experiment, using an insulin-promoter human-IAPP fusion gene, insulin responses and blood glucose elimination were reversed in IAPP-deficient males, whereas the female phenotype appeared unaffected. Our results provide the first firm evidence of a physiological role for endogenous IAPP and indicate that IAPP, apparently in a gender-dependent manner, limits the degree of glucose-induced insulin secretion and the rate of blood glucose elimination.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Influence of glucose concentration on the inhibitory effect of amylin on insulin secretion. Study in the perfused rat pancreas

The effect of amylin on insulin secretion is a matter of controversy. Short-term experiments have shown that amylin, at 75 pmol/l, inhibits the insulin release elicited by a modest increase in the perfusate glucose concentration (from 5.5 mmol/l to 9 mmol/l). The present work was undertaken to further investigate the effect of amylin on glucose-induced insulin release at different glucose concentrations. The study was performed in the isolated perfused rat pancreas. Amylin, at 75 pmol/l, markedly blocked the insulin response when the perfusate glucose concentration was increased from 3.2 mmol/l to 7 mmol/l (by 90%; P < 0.01) or from 5.5 mmol/l to 9 mmol/l (by 80%; P < 0.01). At the same amylin concentration, no significant inhibition of insulin output was observed when the perfusate glucose level was augmented from 5.5 mmol/l to 16.6 mmol/l, from 7 mmol/l to 11 mmol/l or from 9 mmol/l to 13 mmol/l. At a higher concentration (750 pmol/l), amylin also failed to inhibit the insulin response induced by increasing glucose levels from 5.5 mmol/l to 16.6 mmol/l or from 9 to 13 mmol/l. These findings indicate that, in the rat pancreas, amylin only inhibits insulin release when evoked by elevations of glucose levels comparable to those occurring in normal subjects under physiological conditions.

Effect of (8-32) salmon calcitonin, an amylin antagonist, on insulin, glucagon and somatostatin release: study in the perfused pancreas of the rat

1. The 8-32 fragment of salmon calcitonin ((8-32) sCT) has been proposed as a highly selective amylin receptor antagonist. 2. In the present study, we have studied the influence of (8-32) sCT on the inhibitory effect of both amylin and its structural congener, calcitonin gene-related peptide (CGRP), on insulin secretion in the rat perfused pancreas. 3. Both amylin and CGRP, at 75 pM, clearly inhibited glucose-induced insulin release (by 80% and by 70%, respectively). Simultaneous infusion of 10 microM (8-32) sCT reversed the inhibitory effect of amylin (by 80%; P < 0.05 vs. amylin experiments) but did not significantly affect the inhibition of glucose-induced insulin output elicited by CGRP. Furthermore, at the same concentration (10 microM), (8-32) sCT alone potentiated the insulin response to 7 mM glucose (2.5 fold; P < 0.05) whilst it did not affect glucagon or somatostatin secretion. 4. The observation that infusion of an amylin antagonist into the rat pancreas potentiates the insulin response to glucose, favours the concept of endogenous amylin as an inhibitor of insulin release. 5. Finally, as an amylin antagonist at the level of the beta-cell, (8-32) sCT might be considered of potential interest in experimental and clinical pharmacology.

Attenuation of the anorectic effects of glucagon, cholecystokinin, and bombesin by the amylin receptor antagonist CGRP(8-37)

The anorectic effect of IP injection of amylin (1 microgram/kg) was abolished by simultaneous IP injection of the amylin receptor antagonist calcitonin gene-related peptide-(8-37) [CGRP(8-37), 10 micrograms/kg]. The IP injection of pancreatic glucagon (400 micrograms/kg) at dark onset also reduced food intake in 24-h food-deprived rats, and this effect was also totally blocked by coadministration of CGRP(8-37) (10 micrograms/kg). In another feeding paradigm with glucagon (540 micrograms/kg IP 3 h into the light phase in 3 h-prefed rats), however, the anorectic effect of glucagon was not significantly antagonized by CGRP(8-37). The anorectic effect of cholecystokinin (CCK) (0.25 microgram/kg) and bombesin (BBS) (2 micrograms/kg) was partly neutralized by CGRP(8-37). In contrast, the anorectic effect of vasopressin (VP) (2.5 micrograms/kg) was not influenced by CGRP(8-37). As glucagon has been shown previously to increase the secretion of amylin, we conclude that the anorectic effect of peripherally administered glucagon is mediated by the release of amylin, at least under certain conditions. This may also be true for CCK and BBS, as these peptides are insulinotropic and may therefore be presumed to increase amylin release.

Binding sites for islet amyloid polypeptide in mammalian lung: species variation and effects on adenylyl cyclase

Islet amyloid polypeptide (IAPP) and calcitonin gene related peptide (CGRP) share a 47% sequence homology. IAPP can interact with adenylyl cyclase coupled CGRP receptors. We have examined [125I]IAPP binding in mouse, pig, and guinea pig lung membranes in competition with IAPP, CGRP, and CGRP(8-37). Three types of site were shown by order of potency: (i) mouse, IAPP > CGRP(8-37) >> CGRP; (ii) pig, CGRP > IAPP > CGRP(8-37); and (iii) guinea pig, CGRP = IAPP = CGRP(8-37). Chemical cross-linking of [125I]IAPP and [125I]CGRP binding sites in lung demonstrated that both sites had similar molecular weights in any one species but differed across species, i.e., mouse M(r) = 70,000 and 98,000; pig M(r) = 68,000, 56,000, and 47,000; and guinea pig M(r) = 106,000 and 56,000. Adenylyl cyclase activity was stimulated by forskolin and AlCl3-NaF in rat, mouse, pig, and guinea pig membranes. Only in mouse and pig were CGRP and IAPP able to stimulate adenylyl cyclase activity. In mouse lung CGRP and IAPP stimulated adenylyl cyclase activity with EC50 values of 642 +/- 222 nM (n = 4) and 325 +/- 115 nM (n = 4), respectively. In pig lung membranes EC50 values were 5.7 +/- nM (n = 4) for CGRP and 1230 +/- 1130 nM (n = 4) for IAPP. Thus IAPP either did not stimulate adenylyl cyclase activity in these lung membranes or did so with a low potency.

Pathogenesis of feline diabetes mellitus

Cats are one of the few species that develop a form of diabetes mellitus that is clinically and histologically analogous to human type 2 diabetes mellitus. Figure 9 summarizes the etiologic factors thought to be involved in the development of feline and human type 2 diabetes. The main metabolic characteristics of type 2 diabetes mellitus are impaired insulin secretion and resistance to the action of insulin in its target tissues. Impaired beta cell function occurs before histologic changes become evident. The characteristic histologic finding in cats with type 2 diabetes is deposition of amyloid in pancreatic islets. Amyloid deposition occurs before the onset of clinical signs, but does not seem to be the primary defect. Pancreatic amyloid is derived form the recently discovered pancreatic hormone amylin. Amylin is synthesized in pancreatic beta cells, and is co-stored and co-secreted with insulin. Amylin has been postulated to be involved in the pathogenesis of feline diabetes mellitus both through its metabolic effects, which include inhibition of insulin secretion and induction of insulin resistance, and via progressive amyloid deposition and beta cell degeneration. Increased amylin concentration has been documented intracellularly in cats with impaired glucose tolerance and in the plasma of diabetic cats, and supports the hypothesis that amylin is involved in the pathogenesis of type 2 diabetes. Obesity is a common finding in diabetic felines and is a contributing factor to the insulin resistance present in type 2 diabetes. Clinical signs of diabetes develop once total insulin secretion decreases to 20% to 25% of normal levels. Many diabetic cats have been treated successfully with oral hypoglycemics, but 50% to 70% of diabetic cats are insulin dependent. Based on histologic evidence, this is the result of extensive amyloid deposition and subsequent beta cell degeneration, rather than autoimmune destruction of pancreatic beta cells associated with type 1 diabetes. Alternative ways of treating type 2 diabetes currently are being investigated. Amylin antagonists recently have been proposed as a novel treatment to reverse the deleterious effects of excessive amylin concentrations. The gastrointestinal hormone glucagon-like peptide-1 may also prove useful in treating diabetic cats, because of its stimulatory effect on insulin secretion and synthesis, and the absence of significant hypoglycemic effect.

In vivo central actions of rat amylin

The purpose of the present study was to examine and compare the profile of neurobehavioral effects of rat amylin (r-amylin) and rat calcitonin gene-related peptide (rCGRP), two peptides having a 50% structural homology. The effects of synthetic r-amylin and rCGRP administered in several doses (0.312-80.0 micrograms) into the lateral cerebro-ventricle of rats on spontaneous activity, muscular tone, body temperature, nociception, food intake as well as their potential for inducing catalepsy, were investigated. Intraventricular administration of r-amylin or rCGRP significantly reduced spontaneous motor activity and markedly increased body temperature of animals in a dose-dependent related fashion. rCGRP produced a significant increase in muscular tone and induced cataleptic effect in animals, but r-amylin had no effect on these variables. Furthermore, neither r-amylin nor rCGRP were able to induce any significant effect on nociceptive response time of animals in the tail immersion test even with doses as large as 80.0 micrograms. Finally, the two peptides did not affect ad libitum food intake, but significantly reduced food consumption in 22 h food-deprived animals. Together, the results of the present study suggest that amylin may be involved in a diversity of neurophysiological processes but displays a different profile of neurobehavioral effects to that of CGRP which may involve different receptors.

Amylin (islet amyloid polypeptide) inhibition of insulin release in the perfused rat pancreas: implication of the adenylate cyclase/cAMP system

Amylin inhibits glucose-induced insulin secretion in the rat pancreas. To study the mechanism by which amylin acts on the B-cell, we have investigated, in the perfused rat pancreas, the effect of synthetic rat amylin (75 pM) on insulin release elicited by secretagogues acting on the B-cell via the adenylate cyclase/cAMP system, i.e., glucagon (10 nM), gastric inhibitory polypeptide (GIP, 1 nM), forskolin (1 microM) and isobutylmethylxanthine (IBMX, 75 microM). In addition, we examined the effect of amylin on GIP-induced insulin release in pancreata from rats pretreated with pertussis toxin, an agent which inactivates certain Gi proteins coupled to adenylate cyclase. Amylin inhibited the insulin response to glucagon (approx. 70%), GIP (approx. 90%), IBMX (approx. 75%) as well as the early phase of forskolin-induced insulin output (approx. 74%). However, amylin failed to modify GIP-induced insulin release in pancreata obtained from pertussis toxin pretreated rats. These results would indicate that the inhibitory effect of amylin on insulin secretion could be, at least in part, attributed to its interfering with the adenylate cyclase/cAMP system. Furthermore, prevention of the inhibitory effect of amylin on GIP-induced insulin output by pertussis toxin pretreatment, supports the concept that amylin can inhibit insulin release via a pertussis toxin-sensitive Gi protein coupled to the adenylate cyclase system.

Molecular physiology of amylin

Amylin is a 37-amino acid peptide first isolated, purified, and characterized from the amyloid deposits in the pancrease of type 2 diabetics. It is synthesized and secreted primarily from pancreatic beta cells along with insulin. The ability of amylin to potently reduce insulin-stimulated incorporation of glucose into glycogen in skeletal muscle requires both an intact 2Cys-7Cys disulfide bond and a COOH-terminal amide. Amylin has structural and functional relationships to two other messenger proteins, calcitonin and CGRP. Amylin has relatively potent calcitonin-like activity on bone metabolism and weaker CGRP-like activity on the vasculature. CGRP is a slightly weaker agonist than amylin for metabolic responses. Although rat calcitonins are weak, teleost fish calcitonins are very potent agonists for amylin's metabolic effects. This group of peptides appears to act on a family of related G protein-coupled receptors; several variant calcitonin receptors have recently been cloned and expressed. These receptors appear to be coupled to adenylyl cyclase in many instances; recent evidence supports the view that amylin's effects on skeletal muscle occur, at least in large part, through activation of the cAMP pathway.