Cross-reactivity of amylin with calcitonin-gene-related peptide binding sites in rat liver and skeletal muscle membranes

A contemporary biological pathway of islet amyloid polypeptide for the management of diabetic dementia

Major challenges of dealing elder patients with diabetes mellitus (DM) are the individualization of consideration in persons with various comorbid types of conditions. In spite of the fact that microvascular and macrovascular problems associated with DM are well documented, there is only a few numbers of reports viewing different conditions, for example, cognitive dysfunction. Cognitive dysfunction is of specific significance due to its effect on self-care and quality of life. All in all, the etiology of cognitive dysfunction in the maturing populace is probably going to be the grouping of ischemic and degenerative pathology. It is likewise trusted that Hyperglycemia is engaged with the system of DM-related cognitive dysfunction. At present, it isn't certain in the case of enhancing glycemic control or utilizing therapeutic agents can enhance the risk of cognitive decay. Amylin was later characterized as an amyloidogenic peptide, confined from a beta cell tumor and called islet amyloid polypeptide (IAPP), and after that, amylin. Conversely, we investigate the beneficial role and hypothesizing the mechanism of amylin related expanding the level and activation of CGRP receptor to enhance the cognition declination amid diabetic dementia.

Pre-structured hydrophobic peptide β-strands: A universal amyloid trap?

Amyloid fibril formation has long been studied because of the variety of proteins that are capable of adopting this structure despite sharing little sequence homology. This makes amyloid fibrils a challenging focus for inhibition studies because the peptides and proteins that form amyloid fibrils cannot be targeted based on a sequence motif. Most peptide inhibitors that target specific amyloidogenic proteins rely heavily on sequence recognition to ensure that the inhibitory peptide is able to bind its target. This approach is limited to targeting one amyloidogenic protein at a time. However, there is increasing evidence of cross-reactivity between amyloid-forming polypeptides. It has therefore become more useful to study the similarities between these proteins that goes beyond their sequence homology. Indeed, the observation that amyloidogenic proteins adopt similar secondary structures along the pathway to fibril formation opens the way to an interesting investigation: the development of inhibitors that could be universal amyloid traps. The review below will analyze two specific amyloidogenic proteins, α-synuclein and human amylin, and introduce a small number of peptides that have been shown to be capable of inhibiting the amyloidogenesis of both of these very dissimilar polypeptides. Some of the inhibitory peptide motifs may indeed, be applicable to Aβ and other amyloidogenic systems.

Vascular Actions of Calcitonin Gene-Related Peptide and Adrenomedullin

This review summarizes the receptor-mediated vascular activities of calcitonin gene-related peptide (CGRP) and the structurally related peptide adrenomedullin (AM). CGRP is a 37-amino acid neuropeptide, primarily released from sensory nerves, whilst AM is produced by stimulated vascular cells, and amylin is secreted from the pancreas. They share vasodilator activity, albeit to varying extents depending on species and tissue. In particular, CGRP has potent activity in the cerebral circulation, which is possibly relevant to the pathology of migraine, whilst vascular sources of AM contribute to dysfunction in cardiovascular disease. Both peptides exhibit potent activity in microvascular beds. All three peptides can act on a family of CGRP receptors that consist of calcitonin receptor-like receptor (CL) linked to one of three receptor activity-modifying proteins (RAMPs) that are essential for functional activity. The association of CL with RAMP1 produces a CGRP receptor, with RAMP2 an AM receptor and with RAMP3 a CGRP/AM receptor. Evidence for the selective activity of the first nonpeptide CGRP antagonist BIBN4096BS for the CGRP receptor is presented. The cardiovascular activity of these peptides in a range of species and in human clinical conditions is detailed, and potential therapeutic applications based on use of antagonists and gene targeting of agonists are discussed.

Amylin-induced suppression of ANP secretion through receptors for CGRP1 and salmon calcitonin

Amylin cosecretes with insulin from pancreatic beta-cells and shows high sequence homology with CGRP, adrenomedullin, and salmon calcitonin. This study aimed to investigate the effect of amylin on the atrial hemodynamics and ANP release from rat atria and to identify its receptor subtypes. Isolated perfused left atria from either control or streptozotocin-treated rats were paced at 1.3 Hz. The concentration of ANP was measured by radioimmunoassay and the translocation of ECF was measured by [3H]-inulin clearance. Rat amylin increased atrial contractility and suppressed the release of ANP. Rat CGRP showed similar effects but was approximately 300-fold more potent than amylin. Pretreatment with receptor antagonist for CGRP1 [rat alpha-CGRP (8-37)] or salmon calcitonin [acetyl-(Asn30, Tyr32)-calcitonin(8-32), (AC 187)] blocked the suppressive effect of ANP release and the positive inotropic effect by rat amylin. However, receptor antagonists for amylin [amylin (8-37), acetyl-amylin] did not block those effects. Amylin (8-37), acetyl-amylin, or rat alpha-CGRP (8-37) alone accentuated the release of ANP with no changes in atrial contractility. The effect of rat amylin and rat amylin (8-37) on the ANP release was attenuated in streptozotocin-treated rats. We suggest that amylin suppressed ANP release with increased atrial contractility through receptors for CGRP1 and salmon calcitonin and the attenuation of amylin and its antagonist on ANP release from streptozotocin-treated rat atria may be due to the downregulation of amylin receptor.

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.

The molecular pharmacology of CGRP and related peptide receptor subtypes

Calcitonin gene-related peptides (alpha and beta isoforms), better known as CGRPalpha and CGRPbeta, were isolated twenty years ago. In fact, these were the first peptides to be characterized using a molecular cloning strategy, which is not the traditional approach of biochemical extraction and purification. Paradoxically, progress in the characterization of CGRP receptor subtypes has been extremely slow as a result of difficulties in their cloning and the lack of selective receptor subtype agonists and antagonists. However, exciting progress has been made overthe pasttwo years and is briefly reviewed here.

Immunocytochemical study on the liver innervation in patients with cirrhosis

In the liver, the autonomic nervous system plays an important role in degenerative and inflammatory changes. The aim of the present study was to investigate the distribution of neuronal fibres containing neuropeptides in livers of 5 patients with cirrhosis by immunocytochemical localization at the light and electron microscopical level of substance P (SP), neuropeptide Y (NPY), somatostatin (SOM), and calcitonin gene-related peptide (CGRP). In patients with alcoholic cirrhosis, a decreased number of neuronal fibres was found in the portal tract and fibrous septa as well as in the sinusoids of regenerative nodules. NPY- and SP-immunoreactive neuronal fibres were more numerous than CGRP-containing fibres. They were located mainly in portal tracts. These findings led to the conclusion that peptidergic innervation plays a role in inflammatory and fibrotic changes in cirrhotic liver.

Synthesis of biologically active tritiated amylin and salmon calcitonin analogues

Amylin is a hormone belonging to the calcitonin protein family of peptides. To facilitate receptor screening studies, alternatively radiolabeled and biologically active amylin and salmon calcitonin analogues were synthesized by reductive methylation. Free amino groups of amylin and salmon calcitonin were methylated by reaction of peptides with formaldehyde and sodium [(3)H]borohydride. Radioactively labeled peptides were purified by size exclusion chromatography followed by HPLC. Analysis by MALDI-TOF mass spectrometry of purified amylin and salmon calcitonin peptides revealed incorporation of both two and four tritiated methyl groups per peptide molecule. Specific activities of 22.6 and 23.2 GBq/mmol were measured for amylin and salmon calcitonin, respectively. Methylation of rat amylin and salmon calcitonin did not affect their biological activities as both retained their potency to inhibit insulin-stimulated glycogen synthesis in isolated rat soleus muscle. The synthesis of these tritiated analogues provides an alternative chemically stable radiolabeled ligand which may be useful in exploring receptor interactions within the calcitonin peptide family.

Effects of altered tonicity by sodium chloride on L-tryptophan binding to hepatic nuclei

This study was concerned with the effects of NaCl administered in vivo or added in vitro to isolated nuclei on [(3)H]tryptophan binding to rat hepatic nuclei assayed in vitro. Hypertonic (10.7%) NaCl administered in vivo to rats caused at 10 min a marked decrease in in vitro binding (total and specific) of [(3)H]tryptophan to hepatic nuclei. In vitro incubation of isolated hepatic nuclei, but not of isolated nuclear envelopes, with added NaCl (particularly at 0.125 x 10(-4) M and 0.25 x 10(-4) M) revealed significant inhibition of [(3)H]tryptophan binding. However, isolated hepatic nuclear envelopes prepared after in vitro incubation of isolated nuclei with added NaCl did show inhibition of [(3)H]tryptophan binding (total and specific) compared with controls. Other salts (KCl, MgCl(2), NaHCO(3), NaC(2)H(3)O(2), NaF, or Na(2)SO(4)), at similar concentrations to that of NaCl except for MgCl(2), when added to isolated nuclei did not appreciably inhibit nuclear tryptophan binding. Kinetic studies of in vitro nuclear [(3)H]tryptophan binding in the presence of 0.125 x 10(-4) M NaCl revealed that binding decreased at 0.5 h and continued to 2 h compared with nuclear [(3)H]tryptophan binding with controls (without NaCl addition). The results obtained in vivo in rats and those obtained in vitro with isolated hepatic nuclei revealed NaCl-induced inhibitory effects on [(3)H]tryptophan binding to hepatic nuclei. Although the inhibitory effects were similar under the two different experimental conditions, the mechanism for each may be different in that the NaCl concentration in hepatic cells after administration of NaCl in vivo was appreciably higher than the low levels added in vitro to the isolated hepatic nuclei.

CGRP and adrenomedullin binding correlates with transcript levels for calcitonin receptor-like receptor (CRLR) and receptor activity modifying proteins (RAMPs) in rat tissues

1. Putative receptors for CGRP and adrenomedullin have been investigated in the rat. Calcitonin Receptor-Like Receptor (CRLR), in combination with Receptor Activity Modifying Proteins (RAMPs) is hypothesized to bind either CGRP or adrenomedullin. The receptors known as RDC1 and L1 have also been shown to bind CGRP and adrenomedullin respectively. 2. In this study it is shown that rat CRLR cDNA specifies a CGRP receptor when co-transfected with RAMP-1 cDNA and an adrenomedullin receptor when co-transfected with either RAMP-2 or RAMP-3 cDNA in human embryonic kidney 293 cells. 3. CRLR, RAMP, RCD1 and L1 mRNA levels and CGRP and adrenomedullin receptor densities have been measured and correlated with each other in eight rat tissues selected for their distinctive patterns of CGRP and adrenomedullin binding. 4. The data are consistent with the predictions of the CRLR/RAMP model. CGRP binding correlates well with RAMP-1 mRNA levels (R=1.0, P=0.007), adrenomedullin binding shows a tendency to vary with RAMP-2 mRNA levels (R=0.85, P=0.14) and total binding is correlated with CRLR mRNA levels (R=0.94, P=0.03). The data do not support the hypothesis that RDC1 and L1 account for the majority of CGRP and adrenomedullin binding respectively.

Adrenomedullin, a multifunctional regulatory peptide

Since the discovery of adrenomedullin in 1993 several hundred papers have been published regarding the regulation of its secretion and the multiplicity of its actions. It has been shown to be an almost ubiquitous peptide, with the number of tissues and cell types synthesizing adrenomedullin far exceeding those that do not. In Section II of this paper we give a comprehensive review both of tissues and cell lines secreting adrenomedullin and of the mechanisms regulating gene expression. The data on circulating adrenomedullin, obtained with the various assays available, are also reviewed, and the disease states in which plasma adrenomedullin is elevated are listed. In Section III the pharmacology and biochemistry of adrenomedullin binding sites, both specific sites and calcitonin gene-related peptide (CGRP) receptors, are discussed. In particular, the putative adrenomedullin receptor clones and signal transduction pathways are described. In Section IV the various actions of adrenomedullin are discussed: its actions on cellular growth, the cardiovascular system, the central nervous system, and the endocrine system are all considered. Finally, in Section V, we consider some unresolved issues and propose future areas for research.

Cultured astrocytes express functional receptors for galanin

The neuropeptides galanin and calcitonin gene-related peptide (CGRP) are strongly up-regulated in motoneurons following axotomy. Earlier reports have suggested that peptides might be released from injured neurons to recruit surrounding glia. In this study, the effects of galanin and CGRP on cultured rat astrocytes were investigated using the expression of immediate early genes as a model for receptor-mediated transcriptional activation. Galanin was found to induce c-fos, junB, and Tis11 mRNA in cultured astrocytes, providing evidence for the presence of functional galanin receptors on neuroglial cells. In contrast, CGRP only led to the induction of c-fos and junB mRNA. Cholecystokinin (CCK-8) and substance P, which are also up-regulated in select motoneuron populations following axotomy, fail to induce immediate early genes in astrocytes, indicating specificity of neuropeptides in their ability to stimulate glial cells. The differential induction of immediate early gene expression by galanin and CGRP in astrocytes points to differences in intracellular signal transduction mechanisms. Whereas CGRP was found to stimulate the accumulation of cyclic AMP by 10- to 20-fold, galanin had no effect on basal cyclic AMP content. The effect of CGRP on cyclic AMP accumulation was completely reversed by the CGRP receptor antagonist, CGRP(8-37). These results suggest roles for galanin and CGRP in the transcriptional activation of astrocytes.

Islet amyloid polypeptide decreases the effects of insulin-like growth factor-I on glucose transport and glycogen synthesis in skeletal muscle

Previous studies have shown that islet amyloid polypeptide (IAPP) is co-secreted with insulin from the beta-cell. IAPP reduces insulin-stimulated rates of glycogen synthesis in skeletal muscle but the mechanisms are unclear. Insulin-like growth factor I (IGF-I) is an important regulator of glucose metabolism in skeletal muscle and acts through its own receptor, which has many structural and functional similarities with the insulin receptor. Despite this, the effects of IGF-I on glucose utilization are not identical to those of insulin. The aim of the study was to determine the effects of IAPP on IGF-I-stimulated rates of glucose transport and metabolism (measured by 3-O-methyl[3H]glucose and [U-14C]glucose, respectively) in rat soleus muscle, and compare them with those simulated by insulin. IAPP (10 nM) decreased the sensitivity of 3-O-methylglucose transport, the flux of glucose to hexosemonophosphate and the sensitivity of glycogen synthesis to IGF-I. In contrast, IAPP had no effect on IGF-I-stimulated rates of lactate formation (i.e., glycolysis). IAPP decreased the sensitivity of 3-O-methylglucose transport and glycogen synthesis to insulin. It is concluded that IAPP blunts the stimulation of glucose uptake and deposition by IGF-I or insulin in skeletal muscle. These observations expand those made initially for IAPP and insulin and suggest that IAPP affects IGF-I- or insulin-stimulated glucose metabolism in muscle by a mechanism which is common for both hormones. These experiments may serve as a framework for future studies in order to clarify the mechanisms by which IAPP affects glucose metabolism in skeletal muscle.

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.

Multiple receptors for calcitonin gene-related peptide and amylin on guinea-pig ileum and vas deferens

1. The responses of the electrically stimulated guinea-pig ileum and vas deferens to human and rat calcitonin gene-related peptide (CGRP) and amylin were investigated. 2. The inhibition of contraction of the ileum produced by human alpha CGRP was antagonized by human alpha CGRP8-37 (apparent pA2 estimated at 7.15 +/- 0.23) > human alpha CGRP19-37 (apparent pA2 estimated as 6.67 +/- 0.33) > [Tyr0]-human alpha CGRP28-37. The amylin antagonist, AC187, was three fold less potent than CGRP8-37 in antagonizing human alpha CGRP. 3. Both human beta- and rat alpha CGRP inhibited contractions of the ileum, but this was less sensitive to inhibition by CGRP8-37 than the effect of human alpha CGRP. However, CGRP19-37 was twenty times more effective in inhibiting the response to rat alpha CGRP (apparent pA2 estimated as 8.0 +/- 0.1) compared to human alpha CGRP. 4. Rat amylin inhibited contractions in about 10% of ileal preparations; this effect was not antagonized by any CGRP fragment. Human amylin had no action on this preparation. 5. Both human and rat alpha CGRP inhibited electrically stimulated contractions of the vas deferens, which were not antagonized by 3 microM CGRP8-37 or 10 microM AC187. 6. Rat amylin inhibited the stimulated contractions of the vas deferens (EC50 = 77 +/- 9 nM); human amylin was less potent (EC50 = 213 +/- 22 nM). The response to rat amylin was antagonized by 10 microM CGRP8-37 (EC50 = 242 +/- 25 nM) and 10 microM AC187 (EC50 = 610 +/- 22 nM). 7. It is concluded that human alpha CGRP relaxes the guinea-pig ileum via CGRP1-like receptors, but that human beta CGRP and rat alpha CGRP may use additional receptors. These are distinct CGRP2-like and amylin receptors on guinea-pig vas deferens.

Amylin mobilizes [Ca2+]i and stimulates the release of pancreatic digestive enzymes from rat acinar AR42J cells: evidence for an exclusive receptor system of amylin

Amylin dose-dependently stimulated the secretion of amylase and cholesterol esterase from rat pancreatic acinar AR42J cells. The biochemical basis of this action was investigated using fura-2-loaded AR42J cells. Amylin increased intracellular free calcium. [Ca2+]i, in a dose-dependent manner. The Ca2+ signal persisted even in Ca(2+)-free medium, suggesting mobilization from intracellular stores rather than influx. Consistently, thapsigargin abolished amylin-induced responses, suggesting that Ca2+ is released from an inositol 1,4,5-triphosphate (IP3)-sensitive pool. This was confirmed by the finding that amylin elevated IP3 levels. AR42J cells pretreated with amylin did not respond to amylin, suggesting that the receptors mediating this response undergo homologous desensitization. However, pretreatment with related peptides, calcitonin gene-related peptide (CGRP) and salmon calcitonin, did not diminish [Ca2+]i mobilization by amylin. CGRP and calcitonin also failed to mobilize [Ca2+]i even at 10 microM. These results suggests that the stimulatory effects of amylin on pancreatic digestive enzyme secretion from AR42J cells are mediated by a G-protein-linked membrane receptor coupled to IP3-dependent calcium pools.

Activity of amylin at CGRP1-preferring receptors coupled to positive contractile response in rat ventricular cardiomyocytes

Calcitonin gene-related peptide (CGRP) exerts a positive contractile response directly in rat ventricular cardiomyocytes. This response is mediated by receptors of the CGRP1-subtype. Amylin is 46% homologous with CGRP and binds to receptors selective for CGRP in a range of tissues. The ability of amylin to influence ventricular contractility has been assessed using cardiomyocytes isolated from the ventricles of adult rats. Cardiomyocytes were subjected to biphasic electrical stimulation at 0.5 Hz. CGRP produced a concentration-dependent positive contractile response which became maximal 4 min after initial stimulation. CGRP increased the contractile amplitude maximally at 1 nM and to a value which was 23.3% greater than in the absence of peptide (EC50 value = 21 pM). Amylin increased the contractile amplitude maximally at 20 nM and to a value which was 17.3% greater than in the absence of peptide (EC50 value = 216 pM). In the presence of amylin (20 nM), the concentration-dependence of the contractile response to CGRP was shifted to the left, so that the response became maximal when CGRP was present at 50 pM. In the presence of CGRP8-37 (100 nM), a selective antagonist at CGRP1-preferring receptors, the concentration-dependence of the contractile response to CGRP was shifted to the right (dose ratio = 54). Similarly, in the presence of CGRP8-37 (100 nM), the contractile response to amylin was inhibited significantly (P < or = 0.01). Amylin8-37 (100 nM) did not inhibit the concentration-dependence of the contractile responses to CGRP and amylin significantly (dose ratios = 4.2 and 2.4, respectively). In conclusion, these data indicate that amylin exerts a contractile response directly in rat ventricular cardiomyocytes via CGRP1-preferring receptors. This effect could assume greater significance in non-insulin-dependent diabetes mellitus and in hypertensive states, in which the concentration of amylin is elevated in plasma.

Regulation of muscle glycogen metabolism by CGRP and amylin: CGRP receptors not involved

The aim of the present study was to determine whether amylin and calcitonin gene-related peptide (CGRP) act through shared or distinct receptors to inhibit insulin-stimulated incorporation of [14C]-glucose into glycogen. Rat amylin was 3 fold more potent than either rat alpha CGRP or rat beta CGRP at reducing glycogen synthesis from [14C]-glucose in insulin-treated rat soleus muscle. This action was blocked by peptide antagonists, with the rank order of potency being AC187 > salmon calcitonin8-32 (sCT8-32) > h-alpha CGRP8-37 for antagonism of either amylin or CGRP. The antagonist potency order correlated with affinity for amylin receptors measured in rat nucleus accumbens but not CGRP receptors measured in rat L6 muscle cells. Inhibition of glucose incorporation into glycogen by amylin and CGRP appears to be mediated by shared receptors that have the pharmacological characteristics of amylin receptors, and are distinct from previously described CGRP receptors.

Dose-dependent elevation of cyclic AMP, activation of glycogen phosphorylase, and release of lactate by amylin in rat skeletal muscle

We report here our investigation of the role of cyclic AMP (cAMP) in amylin signal transduction in isolated strips of soleus muscle. Rat amylin, at 100 nM, increased cAMP levels, from 0.431 +/- 0.047 to a peak of 1.24 +/- 0.01 pmol cAMP/mg wet wt. after 5 min, in the absence of added phosphodiesterase inhibitor. The EC50 of the response was 0.48 nM (+/- 0.12 log units) in the absence of insulin and 0.3 nM (+/- 0.18 log units) in the presence of 7.1 nM insulin. The response seen with a maximally effective concentration of amylin (10 nM) was similar to that seen with a maximally effective concentration of epinephrine (1 microM) under the same conditions. Consistent with the observed rise in cAMP there was an increase in glycogen phosphorylase a (EC50 2.2 nM +/- 0.25 log units), decreased glycogen content (EC50 0.9 nM +/- 0.22 log units) and enhanced production of lactate (EC50 1.5 nM +/- 0.33 log units). These data support the concept that amylin promotes glycogenolysis in skeletal muscle and enhances production of lactate through glycolysis as a result of activation of Gs coupled receptors, stimulation of adenylate cyclase, elevation of cAMP levels and activation of glycogen phosphorylase.

The role of the sensory peptide calcitonin-gene-related peptide(s) in skeletal muscle carbohydrate metabolism: effects of capsaicin and resiniferatoxin

1. The content of calcitonin-gene-related-peptide-like immunoreactivity (CGRP-LI) in various rat muscles was measured. Starvation for 24 h did not affect the content of CGRP-LI in these muscles, except for a decreased level in the starved-rat diaphragm. Higher contents of CGRP-LI were observed in well-vascularized muscles. 2. Capsaicin (at 1, 10 and 100 microM) inhibited insulin-stimulated rates of glycogen synthesis in isolated stripped incubated soleus muscle preparations by a mechanism independent of catecholamine release, since the effects of capsaicin were not altered by the beta-adrenoreceptor antagonist DL-propranolol. 3. Resiniferatoxin (10 nM), which is a potent capsaicin agonist, also significantly inhibited the insulin-stimulated rate of glycogen synthesis. Furthermore, the concentration of resiniferatoxin required to inhibit glycogen synthesis was 100 times less than the concentration of capsaicin needed for the same effect. 4. Capsaicin (10 microM) decreased the content of CGRP-LI in isolated stripped incubated soleus muscle preparations by about 40%. 5. Neonatal treatment of rats with capsaicin, which causes de-afferentation of some sensory nerves such, we hypothesize, that CGRP can no longer be released to counteract the effects of insulin in vivo, caused increased rates of glycogen synthesis and increased glycogen content in stripped soleus muscle preparations in vitro when muscles were isolated from the adult rats. 6. These findings support the hypothesis that capsaicin and resiniferatoxin elicit an excitatory response on sensory nerves in skeletal muscle in vitro to cause the efferent release of CGRP. Consequently, CGRP is delivered to skeletal muscle fibres to inhibit insulin-stimulated glycogen synthesis. The role of CGRP in recovery of blood glucose levels during hypoglycaemia is discussed.

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.

In vitro autoradiographic localization of amylin binding sites in rat brain

Amylin is a recently discovered 37 amino acid peptide which is co-secreted from the pancreas with insulin and acts to modulate carbohydrate metabolism. Recently, high-affinity binding sites for [125I]rat amylin have been identified in the rat central nervous system. These sites also have high affinity for the structurally related peptides calcitonin gene-related peptide and salmon calcitonin. In the present study we have used in vitro autoradiography to map the distribution of these [125I]rat amylin binding sites in rat brain. High to moderate levels of binding were present in mid-caudal accumbens nucleus, fundus striati and parts of the bed nucleus of the stria terminalis and substantia inominata. This binding extended caudally into parts of the amygdalostriatal transition zone and the central and medial amygdaloid nuclei. High to moderate levels of binding also occurred in much of the hypothalamus including the medial preoptic, dorsomedial hypothalamic and medial tuberal nuclei as well as the ventrolateral subnucleus of the ventromedial hypothalamic nucleus. Other regions of high level binding included the subfornical organ, the vascular organ of the lamina terminalis, area postrema, locus coeruleus, dorsal raphe and caudal parts of the nucleus of the solitary tract. The subfornical organ, vascular organ of the lamina terminalis and area postrema, which display some of the highest binding densities, lack a patent blood-brain barrier and thus could be responsive to blood-borne amylin. In conclusion we have mapped, in detail, the distribution of amylin binding sites in rat brain. The location of binding is consistent with potential roles for these sites in appetite, fluid and electrolyte homeostasis, autonomic function and regulation of mood.

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.

Regulation of hepatocyte adenylate cyclase by amylin and CGRP: a single receptor displaying apparent negative cooperatively towards CGRP and simple saturation kinetics for amylin, a requirement for phosphodiesterase inhibition to observe elevated hepatocyte cyclic AMP levels and the phosphorylation of Gi-2

Challenge of intact hepatocytes with amylin only succeeded in elevating intracellular cyclic AMP levels and activating phosphorylase in the presence of the cAMP phosphodiesterase inhibitor IBMX. Both amylin and CGRP similarly activated adenylate cyclase, around 5-fold, although approximately 400-fold higher levels of amylin were required to elicit half maximal activation. Amylin activated adenylate cyclase though apparently simple Michaelien kinetics whereas CGRP elicited activation by kinetics indicative of apparent negative co-operativity. Use of the antagonist CGPP(8-37) showed that both CGRP and amylin activated hepatocyte adenylate cyclase through a common receptor by a mnemonical mechanism where it was proposed that the receptor co-existed in interconvertible high and low affinity states for CGRP. It is suggested that this model may serve as a paradigm for G-protein linked receptors in general. Amylin failed to both stimulate inositol phospholipid metabolism in hepatocytes and to elicit the desensitization of glucagon-stimulated adenylate cyclase. Amylin did, however, elicit the phosphorylation of the inhibitory guanine nucleotide regulatory protein Gi-2 in hepatocytes and prevented the action of insulin in reducing the level of phosphorylation of this G-protein.

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.

Characterization of specific calcitonin gene-related peptide receptors present in hamster pancreatic beta cells

Calcitonin gene-related peptide (CGRP) shares about 46% and 20% amino acid sequence homology with islet amyloid polypeptide (IAPP) and salmon calcitonin (sCT). We investigated whether these related peptides could cross-react with the specific binding of 125I-[His]hCGRP I to the CGRP receptor in hamster insulinoma cell membranes. A rapid dissociation of membrane bound 125I-[His]hCGRP I could be induced in the presence of 1 microM chicken CGRP (cCGRP). The specific 125I-[His]hCGRP I binding was inhibited by the related peptides and their half-maximal inhibitory concentrations (IC50) were: cCGRP (0.1 nM), rat CGRP I and human CGRP I and II (1.0-2.0 nM), fragment of hCGRP I (8-37) (150 nM), human IAPP (440 nM). The non-amidated form of hIAPP; human diabetes-associated peptide (hDAP) did not inhibit the binding of 125I-[His]hCGRP I and sCT was only effective at a high concentration (1 microM). Binding of 125I-[His]hCGRP I was dose dependently inhibited by guanosine-5'-O-(3-thiotriphosphate) or (GTP gamma S) and a 70% reduction of binding was obtained with 0.1 mM GTP gamma S. The IC50 value of cCGRP (0.1 nM) was increased 100-fold in the presence of 0.1 mM GTP gamma S. Human CGRP I and cCGRP at 2.5 microM did not stimulate the activity of hamster insulinoma cell membranes adenylate cyclase, while glucagon (1 microM) induced a 2-fold increase. Thus, specific CGRP receptors present in hamster beta cells are associated with G protein (s) and IAPP can interact with these receptors. These results and the observation that cCGRP and hCGRP I did not influence adenylate cyclase activity provide further evidence for CGRP receptor subtypes.

A mnemonical or negative-co-operativity model for the activation of adenylate cyclase by a common G-protein-coupled calcitonin-gene-related neuropeptide (CGRP)/amylin receptor

Both amylin and calcitonin-gene-related neuropeptide (CGRP) activated adenylate cyclase activity in hepatocyte membranes around 5-fold in a dose-dependent fashion, with EC50 values of 120 +/- 14 and 0.3 +/- 0.14 nM respectively. Whereas amylin exhibited normal activation kinetics (Hill coefficient, h approximately 1), CGRP showed kinetics indicative of either multiple sites/receptor species having different affinities for this ligand or a single receptor species exhibiting apparent negative co-operativity (h approximately 0.21). The CGRP antagonist CGRP-(8-37)-peptide inhibited adenylate cyclase stimulated by EC50 concentrations of either amylin or CGRP. Inhibition by CGRP-(8-37) was selective in that markedly lower concentrations were required to block the action of amylin (IC50 = 3 +/- 1 nM) compared with that of CGRP itself (IC50 = 120 +/- 11 nM). Dose-effect data for inhibition of CGRP action by CGRP-(8-37) showed normal saturation kinetics (h approximately 1), whereas CGRP-(8-37) inhibited amylin-stimulated adenylate cyclase activity in a fashion which was indicative of either multiple sites or apparent negative co-operativity (h approximately 0.24). Observed changes in the kinetics of inhibition by CGRP-(8-37) of CGRP, but not amylin-stimulated adenylate cyclase, at concentrations of agonists below their EC50 values militated against a model of two distinct populations of non-interacting receptors each able to bind both amylin and CGRP. A kinetic model is proposed whereby a single receptor, capable of being activated by both CGRP and amylin, obeys either a mnemonical kinetic mechanism or one of negative co-operativity with respect to CGRP but not to amylin. The relative merits of these two models are discussed together with a proposal suggesting that the activation of adenylate cyclase by various G-protein-linked receptors may be described by a mnemonical model mechanism.

Isolation, purification, and characterization of calcitonin gene-related peptide receptor

Intact calcitonin gene-related peptide (CGRP) receptors were solubilized from porcine neural membranes using sodium cholate: potassium buffer. The solubilized receptors were purified sequentially by hydrophobic interaction and ion-exchange chromatography followed by specific affinity chromatography. Using these procedures, we have isolated 2 nmol of highly purified active CGRP receptor to a homogeneity (5 x 10(8)-fold purification). The isolated receptors retained their specificity and the capacity to bind to 125I-CGRP, and showed no cross-reactivity with a number of other peptides, except with amylin having 46% amino acid sequence homology to h-CGRP. The solubilized receptors were adsorbed by WGA-agarose and concanavalin-A, suggesting a glycoprotein nature. SDS-PAGE, size-exclusion HPLC, and autoradiography confirmed that CGRP receptor is a monomeric membrane protein with M(r) 66 kDa.

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

The 8-37 fragment of human calcitonin gene-related peptide [(8-37)hCGRP] antagonizes the effects of calcitonin gene-related peptide (CGRP) and amylin in a number of tissues. We have studied the influence of (8-37)hCGRP on the effects of both CGRP and amylin on insulin secretion. In the perfused rat pancreas, homologous CGRP and amylin, at 75 pM, exerted comparable inhibitory effects on the insulin response to 9 mM glucose (ca. 70%; P < 0.025). These effects were antagonized by (8-37)hCGRP (1 microM). Our results suggest that CGRP and amylin act on the B-cell, at least in part, through a common receptor.

Structure and biology of amylin

Amylin is a recently discovered 37 amino acid peptide secreted into the bloodstream, along with insulin, from pancreatic beta-cells. It is about 50% identical to calcitonin gene-related peptides (CGRP alpha and CGRP beta) and structurally related to the calcitonins. Amylin can elicit the vasodilator effects of CGRP and the hypocalcaemic actions of calcitonin, while these peptides can mimic newly discovered actions of amylin on carbohydrate metabolism. The different relative potencies of these peptides suggest that they act with different selectivities at a family of receptors. Amylin is deficient in insulin-dependent diabetes mellitus, while plasma levels are elevated in insulin-resistant conditions such as obesity and impaired glucose tolerance. In this Viewpoint article, Tim Rink and colleagues propose that amylin is an endocrine partner to insulin and glucagon; deficiency or excess of amylin may therefore contribute to important metabolic diseases.

Dose response characteristics for the hyperglycemic, hyperlactemic, hypotensive and hypocalcemic actions of amylin and calcitonin gene-related peptide-I (CGRP alpha) in the fasted, anaesthetized rat

Amylin, a 37 amino-acid peptide secreted from the pancreatic beta-cells, exerts marked effects on carbohydrate metabolism in intact rats. It has approximately 50% amino-acid identity with the calcitonin gene-related peptides (CGRP) as well as certain shared biological actions. In vivo potencies were determined for four responses (increases in plasma glucose, increases in plasma lactate, decreases in plasma calcium, and depression of arterial pressure). These responses were measured in fasted, lightly anaesthetized rats given single intravenous bolus injections of rat amylin or rat CGRP alpha at doses of 0.01, 0.1, 1, 10, 100 and 1000 micrograms (about 7 pmol/kg-700 nmol/kg). Control animals received an equal volume of saline. The order of potency for the different responses was as follows: (i) increase in plasma glucose concentration, amylin approximately 2 times more potent than CGRP (by ED50) with detectable responses occurring at doses 100-fold less; (ii) decrease in plasma total calcium concentration, CGRP of equal or greater potency than amylin; and (iii) decrease in arterial pressure, CGRP 44-fold more potent than amylin. An increase in plasma lactate occurred with amylin doses 1000-fold lower than the CGRP doses producing such effects. Saturation of the dose-dependent increase in lactate was not observed, so median effective doses (ED50) were not obtained. These results are consistent with the existence of separate receptor systems for amylin and CGRP. The effects of amylin on plasma glucose and lactate concentrations were demonstrable at doses of 0.1-1.0 micrograms (70-700 pmol/kg). These doses produced plasma levels that were within the concentration range previously reported for insulin-resistant rats, supporting the proposal that amylin is a physiologic endocrine regulator of carbohydrate metabolism in vivo.

Characterization of amylin binding sites in a human hepatoblastoma cell line

Amylin binding sites in a human hepatoblastoma cell line (HepG2) have been characterized in detail. 125I-Amylin (rat) bound to HepG2 cells with high affinity. Binding was reversible and selective, and dependent on time and temperature. Scatchard analysis revealed the presence of high (Kd = 0.11 +/- 0.04 nM) and low (Kd = 1.3 +/- 0.4 microM) affinity binding sites for 125I-amylin in HepG2 cells. The dissociation experiments also showed that 125I-amylin dissociated from high- and low-affinity sites. The association data, however, indicated the presence of only one binding site. Rat amylin was more potent than human amylin and rat calcitonin gene-related peptide (CGRP) in displacing 125I-amylin bound to HepG2 cells. Nonhomologous peptides did not displace 125I-amylin. Rat amylin was, however, less potent than rat CGRP in displacing 125I[Tyr0]CGRP from HepG2 cells. Pretreatment of HepG2 cells with rat amylin (10 nM) reduced the specific binding of 125I-amylin by 75%, whereas rat CGRP (10 nM) pretreatment had no effect on amylin binding. Calcitonin gene-related peptide, as well as rat and human amylin, stimulated the adenylate cyclase activity of HepG2 cell membrane preparation in a dose-dependent manner, with an order of potency of CGRP > rat amylin > human amylin. A CGRP antagonist, CGRP(8-37), significantly attenuated the stimulatory effect of both amylin and CGRP on adenylate cyclase activity. These investigations show that distinct receptors of amylin and CGRP are present in HepG2 cells, and that amylin stimulates adenylate cyclase activity through CGRP receptors. This system could now be exploited for studying amylin receptors and amylin-mediated signal transduction.

Involvement of multiple receptors in the biological effects of calcitonin gene-related peptide and amylin in rat and guinea-pig preparations

1. The activity of rat alpha and beta calcitonin gene-related peptide (CGRP) as compared to the structurally related peptide, rat amylin, has been investigated in the guinea-pig isolated left atrium (electrically driven), in mucosa-free strips from the base of the guinea-pig urinary bladder and in the rat isolated vas deferens (pars prostatica). The antagonist activity of the C-terminal fragment of human alpha CGRP, alpha CGRP(8-37), was also investigated. 2. In the guinea-pig isolated left atrium the three peptides produced a concentration-related positive inotropic effect, amylin being about 16 and 31 times less potent than alpha or beta CGRP, respectively. Human alpha CGRP(8-37) produced a rightward displacement of the log concentration-response curve to the three agonists tested, without depression of maximal response attainable. Apparent pKB values calculated on the basis of the displacement produced by 1 microM human alpha CGRP(8-37) indicated an agonist-independent affinity of the antagonist (6.66 +/- 0.11 for alpha CGRP, 6.42 +/- 0.17 for beta CGRP and 6.95 +/- 0.11 for amylin). 3. In the guinea-pig isolated urinary bladder, alpha or beta CGRP or amylin produce a concentration-related inhibition of twitch contractions evoked by train electrical field stimulation (10 Hz frequency, 0.25 ms duration at 100 V for 0.5 s every 60 s). Amylin was about 100 times less potent than alpha or beta CGRP. Human alpha CGRP(8-37) (3 microM) did not significantly affect the inhibitory action of the three agonists tested.4. In the rat isolated vas deferens, a or PCGRP or amylin produced a concentration-related inhibition of twitch contractions evoked by electrical field stimulation (0.2 Hz frequency, 0.5 ms duration at 60 volts). Amylin was about 100 times less potent than a or PCGRP. Human aCGRP(8-37) at 3 microM did not significantly affect the inhibitory action of amylin and at 3 microM antagonized the responses to rat a and PCGRP with apparent pKB values of 5.86 +/- 0.15 and 6.11 +/- 0.13, respectively.5. These findings indicate that multiple receptors mediate the actions of peptides of the CGRP/amylin family in the preparations investigated. In the guinea-pig atrium both a and P forms of rat CGRP as well as amylin act by stimulating a single class of receptors which are sensitive to the inhibitory action of human aCGRP(8-37). In rat isolated vas deferens, at least two receptors could be present, one activated by a and PCGRP and partially sensitive to human aCGRP(8-37) and another which is sensitive to amylin but not recognised by human aCGRP(8-37). This latter type of receptor could be entirely responsible for the action of the agonists in the guinea-pig urinary bladder.

Biotinyl analogues of amylin as biologically active probes for amylin/CGRP receptor recognition

Biotinyl analogues of rat amylin were synthesised with sulfosuccinimidyl 2-(biotinamido) ethyl-1,3-dithiopropionate (NHS-SS-Biotin). Biotinylated amylin peptides were purified by HPLC, quantitated, and the presence of the biotin group at Lys-1 confirmed by peroxidase-labelled avidin and FAB mass spectroscopy. Amylin-biotin retained a similar affinity for binding to rat liver plasma membranes compared with rat amylin and also completely inhibited insulin-stimulated glycogen synthesis in rat soleus muscle incubated in vitro. These biologically active amylin probes will enable a complete analysis of amylin/CGRP receptor expression in various cell types and facilitate the isolation and characterisation of the hormone-receptor complex.

Calcitonin gene-related peptide: multiple actions, multiple receptors

Calcitonin gene-related peptide (CGRP) shows diversity both in its effects and its receptors. It is likely to have roles as a neurotransmitter, neuromodulator, local hormone and trophic factor. Its effects include rapid changes in neuronal activity, relaxation of many types of smooth muscle, actions on metabolism and changes in gene expression. Receptor heterogeneity has been revealed from experiments comparing agonist potency ratios and antagonist affinities. The evidence from these approaches is reviewed in this article and a speculative receptor classification scheme is proposed. Some of the likely future directions for CGRP research are discussed.