Characterization of amylin and calcitonin receptor binding in the mouse alpha-thyroid-stimulating hormone thyrotroph cell line

Amylin, Another Important Neuroendocrine Hormone for the Treatment of Diabesity

Diabetes mellitus is a devastating chronic metabolic disease. Since the majority of type 2 diabetes mellitus patients are overweight or obese, a novel term-diabesity-has emerged. The gut-brain axis plays a critical function in maintaining glucose and energy homeostasis and involves a variety of peptides. Amylin is a neuroendocrine anorexigenic polypeptide hormone, which is co-secreted with insulin from β-cells of the pancreas in response to food consumption. Aside from its effect on glucose homeostasis, amylin inhibits homeostatic and hedonic feeding, induces satiety, and decreases body weight. In this narrative review, we summarized the current evidence and ongoing studies on the mechanism of action, clinical pharmacology, and applications of amylin and its analogs, pramlintide and cagrilintide, in the field of diabetology, endocrinology, and metabolism disorders, such as obesity.

The calcitonin receptor is the main mediator of LAAMA's body weight lowering effects in male mice

The anorectic action of the pancreatic hormone amylin is mainly mediated through the area postrema (AP). Amylin activates AP neurons using a heterodimeric receptor (AMY) composed of the calcitonin receptor (CTR) and the receptor activity modifying protein (RAMP 1, 2 or 3). The aim of the following experiments is to test the effects of the long acting amylin analogue (LAAMA) in RAMP1/3 knock-out (KO) male mice and in neuronal CTR KO Nestin-Cre^CTR male mice. In vitro, LAAMA exerted an equipotent effect on CTR and AMYs that was maintained across species. Following one week of 45% high fat diet, WT, RAMP1/3 KO and Nestin-Cre^CTR mice were injected daily for one week with vehicle or LAAMA. LAAMA decreased body weight gain in WT and in RAMP1/3 KO mice suggesting that RAMP1/3 are not necessary for LAAMA-induced effects. However, LAAMA was not able to produce any body lowering and anorectic effects in Nestin-Cre^CTR mice. This was accompanied by the absence of any c-Fos signal in the AP opposite to WT control mice. Together, these results suggest that LAAMA's effects are mainly mediated through CTR rather than specific AMY. The study of LAAMA or any amylin receptor agonist in different receptor KO mouse models helps disentangle the underlying mechanisms used by these molecules.

The expanding repertoire of receptor activity modifying protein (RAMP) function

Receptor activity modifying proteins (RAMPs) associate with G-protein-coupled receptors (GPCRs) at the plasma membrane and together bind a variety of peptide ligands, serving as a communication interface between the extracellular and intracellular environments. The collection of RAMP-interacting GPCRs continues to expand and now consists of GPCRs from families A, B and C, suggesting that RAMP activity is extremely prevalent. RAMP association with GPCRs can regulate GPCR function by altering ligand binding, receptor trafficking and desensitization, and downstream signaling pathways. Here, we elaborate on these RAMP-dependent mechanisms of GPCR regulation, which provide opportunities for pharmacological intervention.

Targeting a family B GPCR/RAMP receptor complex: CGRP receptor antagonists and migraine

The clinical effectiveness of antagonizing the calcitonin gene-related peptide (CGRP) receptor for relief of migraine pain has been clearly demonstrated, but the road to the development of these small molecule antagonists has been daunting. The key hurdle that needed to be overcome was the CGRP receptor itself. The vast majority of the current antagonists recognize similar epitopes on the calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1). RAMP1 is a relatively small, single, transmembrane-spanning protein and along with the G-protein-coupled receptor CLR comprise a functional CGRP receptor. The tri-helical extracellular domain of RAMP1 plays a key role in the high affinity binding of CGRP receptor antagonists and drives their species-selective pharmacology. Over the years, a significant amount of mutagenesis data has been generated to identify specific amino acids or regions within CLR and RAMP1 that are critical to antagonist binding and has directed attention to the CLR/RAMP1 extracellular domain (ECD) complex. Recently, the crystal structure of the CGRP receptor ECD has been elucidated and not only reinforces the early mutagenesis data, but provides critical insight into the molecular mechanism of CGRP receptor antagonism. This review will highlight the drug design hurdles that must be overcome to meet the desired potency, selectivity and pharmacokinetic profile while retaining drug-like properties. Although the development of these antagonists has proved challenging, blocking the CGRP receptor may one day represent a new way to manage migraine and offer hope to migraine sufferers.

Receptor activity-modifying proteins differentially modulate the G protein-coupling efficiency of amylin receptors

Receptor activity-modifying proteins (RAMPs) 1, 2, and 3 are prototypic G protein-coupled receptor accessory proteins that can alter not only receptor trafficking but also receptor phenotype. Specific RAMP interaction with the calcitonin receptor (CTR) generates novel and distinct receptors for the peptide amylin; however, the role of RAMPs in receptor signaling is not understood. The current study demonstrates that RAMP interaction with the CTRa in COS-7 or HEK-293 cells leads to selective modulation of signaling pathways activated by the receptor complex. There was a 20- to 30-fold induction in amylin potency at CTR/RAMP1 (AMY1) and CTR/RAMP3 (AMY3) receptors, compared with CTR alone, for formation of the second-messenger cAMP that parallels an increase in amylin binding affinity. In contrast, only 2- to 5-fold induction of amylin potency was seen for mobilization of intracellular Ca++ or activation of ERK1/2. In addition, in COS-7 cells, the increase in amylin potency for Ca++ mobilization was 2-fold greater for AMY3 receptors, compared with AMY1 receptors and this paralleled the relative capacity of overexpression of Galphaq proteins to augment induction of high affinity 125I-amylin binding. These data demonstrate that RAMP-complexed receptors have a different signaling profile to CTRs expressed in the absence of RAMPs, and this is likely due to direct effects of the RAMP on G protein-coupling efficiency.

Amylin receptors: molecular composition and pharmacology

Several receptors which bind the hormone AMY (amylin) with high affinity have now been identified. The minimum binding unit is composed of the CT (calcitonin) receptor at its core, plus a RAMP (receptor activity modifying protein). The receptors have been named AMY1(a), AMY2(a) and AMY3(a) in accordance with the association of the CT receptor (CT(a)) with RAMP1, RAMP2 and RAMP3 respectively. The challenge is now to determine the localization and pharmacological nature of each of these receptors. Recent attempts to achieve these aims will be briefly discussed.

Calcitonin receptor isoforms expressed in the developing rat kidney

BACKGROUND

Development in the metanephric-kidney transition period involves the precise expression of paracrine and autocrine events in an ordered spatio-temporal manner. Expression of these molecular events is tightly controlled and includes positive and negative growth factors and cognate receptors within close proximity in developing structures in the expanding renal cortex and medulla. The expression of calcitonin receptor (CTR) isoforms C1a and C1b in this context has not previously been described. Our current study also explored the relationship between the expression of CTR isoforms and amylin binding sites.

METHODS

Techniques included immunohistochemistry with novel antibodies that detect CTR isoforms, real time PCR for the quantification of CTR isoforms, Western blot and in vitro autoradiography, on tissues from embryo day 18 to postnatal day 30.

RESULTS

The CTR C1a isoform is expressed in the ureteric ducts of the metanephros and both isoforms are expressed in the developing distal convoluted tubules, ascending limbs of the loop of Henle and collecting ducts in the postnatal rat kidney. There was a 60-fold excess of C1a versus C1b isoforms. An apparent molecular weight of 63 kD was found. In vitro autoradiography demonstrated that while amylin binding sites were predominantly in the cortex, CTR expression was largely localized in the medulla in an earlier event, followed by cortical expression.

CONCLUSIONS

CTR C1a protein expression has been identified in the ureteric ducts in the metanephros and both isoforms expressed in the distal portions of the developing nephrons and collecting ducts. Since amylin binding sites have been localized on the proximal tubules of the cortex, it is unlikely that amylin receptors can be represented by modification of CTR affinity with receptor activity modifying proteins in the kidney.

Functional interaction of G protein-coupled receptors of the adrenomedullin peptide family with accessory receptor-activity-modifying proteins (RAMP)

Adrenomedullin (AM), alpha- and beta-calcitonin gene-related peptide (CGRP), calcitonin (CT), and amylin are homologous polypeptides with overlapping biological actions such as vasodilatation and inhibition of bone resorption. They are brought about through receptors that include the CT receptor (CTR) and an initially orphan CT receptor-like receptor (CRLR) in association with receptor-activity-modifying proteins (RAMP)1, -2, and -3. Co-expression of CRLR with RAMP1 or -2 revealed CGRP or AM receptors, respectively. The CTR interacts with CT and does not require a known RAMP for functional expression. The same CTR is a CGRP/amylin or an amylin receptor upon co-expression with RAMP1 or -3, respectively. Interactions between CRLR and RAMP are thought to be required for their delivery to the cell surface. There, heterodimeric complexes between CRLR or CTR and the corresponding RAMP reveal high-affinity receptors for AM, CGRP, and amylin. Here we review the current knowledge on interactions of G protein-coupled receptors with defined associated proteins.

Lipopolysaccharide supports survival and fusion of preosteoclasts independent of TNF-alpha, IL-1, and RANKL

Lipopolysaccharide (LPS), a cell component of Gram-negative bacteria, is a pathogen of inflammatory bone loss. To examine the effects of LPS on the survival and fusion of osteoclasts, mononuclear osteoclasts (preosteoclasts, pOCs) were collected from a mouse co-culture system and cultured in the presence or absence of LPS. Most pOCs died within 24 h in the absence of any stimulus. LPS as well as receptor activator of NF-kappaB ligand (RANKL) supported the survival of pOCs, and induced their fusion to form multinucleated cells (MNCs). Like authentic osteoclasts, MNCs induced by LPS expressed calcitonin receptors, and formed actin rings on culture plates. LPS-induced MNC formation in pOC cultures was observed even in the presence of osteoprotegerin and interleukin (IL)-1-receptor antagonists. MNC formation was also stimulated by LPS in pOC cultures prepared from tumor necrosis factor (TNF)-receptor-I or TNF-receptor-II deficient mice. LPS induced the degradation of IkappaB in pOCs within 20 min. Lactacystin, an inhibitor of NF-kappaB activation, and wortmannin, an inhibitor of phosphatidylinositol-3 kinase, strongly inhibited LPS-induced MNC formation in pOC cultures. LPS induced pit-forming activity of pOCs in the presence of macrophage-colony stimulating factor (M-CSF). These findings suggest that LPS stimulates the survival and fusion of pOCs, independent of RANKL, IL-1 or TNF-alpha action. Activation of NF-kappaB and phosphatidylinositol-3 kinase appeared to be involved in LPS-induced effects on pOCs. These observations suggest that LPS is involved directly in inflammatory bone loss, and also indirectly through the production of LPS-induced host factors such as IL-1 and TNF-alpha.

Calcitonin and calcitonin receptors: bone and beyond

Calcitonin (CT), a 32 amino acid peptide hormone produced primarily by the thyroid, and its receptor (CTR) are well known for their ability to regulate osteoclast mediated bone resorption and enhance Ca2+ excretion by the kidney. However, recent studies now suggest that CT and CTRs may play an important role in a variety of processes as wide ranging as embryonic/foetal development and sperm function/physiology. In this review article, CT and CTR gene transcription, signal transduction and function are addressed. The effects of CT on the physiology of a variety of organ systems are discussed and the relationship between polymorphisms in the CTR gene and bone mineral density (BMD)/osteoporosis is examined. Recent studies demonstrating the ability of receptor activity modifying proteins (RAMPs) to post-translationally modify the calcitonin receptor-like receptor (CRLR) are detailed and studies employing transgenic mouse technology to determine the temporal and tissue specific transcriptional activity of the CTR gene in vivo are discussed.

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.

Amylin compared with calcitonin: competitive binding studies in rat brain and antinociceptive activity

Binding studies for rat amylin (AMY) and salmon calcitonin (sCT) were performed on rat membranes prepared from pons and medulla oblongata of rats. The aim was to see whether specific binding sites for AMY and/or for sCT present in these areas could be relevant to some of the biological activities of the two peptides. Binding sites specific for [125I]AMY are present in the pons-medulla of rat brain as AMY, but not sCT, was able to displace radiolabeled AMY binding with an IC50 = 3.7+/-0.5x10(-10) M. In contrast, binding of [125I]sCT was displaced by both sCT and AMY, although with different potencies, the IC50 for sCT being 1+/-0.1x10(-11) M, and for AMY, 1.8+/-0.08x10(-7) M. The functional significance of the presence of these binding sites was evaluated in two different nociceptive tests, hot-plate and tail-flick. In the tail-flick test neither AMY (5-10 microg/rat, i.c.v.) nor sCT (10 microg/rat i.c.v.) showed antinociceptive activity, whereas in the hot-plate test AMY (10 microg/rat, i.c.v.) significantly increased the response latencies as did sCT (250 ng/rat, i.c.v.). These results demonstrated that a 40-fold greater dose of AMY is necessary to produce a comparable antinociceptive effect to that exerted by sCT. These findings are in accordance with the low affinity of AMY for sCT binding sites in rat pons-medulla. It is therefore suggested that the central inhibitory activity of AMY on pain perception involves interaction with sCT receptors whereas the selective AMY binding sites subserve other (as yet unknown) functions.

TGF-beta enhances osteoclast differentiation in hematopoietic cell cultures stimulated with RANKL and M-CSF

TGF-beta has been shown to inhibit and stimulate osteoclastogenesis. The purpose of this study was to evaluate the effects of TGF-beta in hematopoietic cell cultures stimulated with RANKL and M-CSF. In cocultures of hematopoietic cells and BALC cells (a calvarial-derived cell line), TGF-beta inhibited tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cell formation. In contrast, TGF-beta enhanced TRAP-positive multinucleated cell formation up to 10-fold in hematopoietic cell cultures containing few osteoblastic/stromal cells. Likewise, TGF-beta increased the number of calcitonin receptor (CTR)-positive multinucleated and mononucleated cells in a concentration-dependent manner. An increase in cell size and multinuclearity was also observed in the presence of TGF-beta. The stimulatory effects of TGF-beta were dependent on the presence of M-CSF and RANKL. When differentiated on bovine cortical bone slices, these cells formed resorption lacunae. These results suggest that TGF-beta has a direct stimulatory effect on osteoclastogenesis in hematopoietic cells treated with RANKL and M-CSF.

Amylin evokes phosphorylation of P20 in rat skeletal muscle

To investigate the signal transduction events underlying amylin's actions, the amylin-evoked protein phosphorylation cascade was analysed using two-dimensional gel electrophoresis. We found that phosphorylation of three isoelectric variants of P20 (termed ARPP1, ARPP2 and ARPP3) was associated with amylin's actions in rat skeletal muscle. Amylin decreased phosphorylation of ARPP1 and increased phosphorylation of ARPP2 and ARPP3 in a dose-dependent manner. Insulin inhibited amylin-evoked phosphorylation of ARPP2 and ARPP3. The amylin-selective antagonist rat amylin-(8-37) completely reversed amylin's action on ARPP3 and partially decreased phosphorylation of ARPP2. By contrast, the CGRP-selective antagonist, human CGRP-(8-37) blocked phosphorylation of ARPP2 but had little effect on ARPP3. These results suggest that amylin modifies phosphorylation of P20 via two independent mechanisms, and that P20 might be a molecule mediating amylin's biological functions.

The RAMP2/CRLR complex is a functional adrenomedullin receptor in human endothelial and vascular smooth muscle cells

Adrenomedullin, a potently hypotensive peptide isolated from human pheochromocytoma, is known to elicit a rise in cAMP levels within mammalian endothelial and smooth muscle cells. Until now, however, little has been known about the adrenomedullin receptor. Recently, a group called receptor activity-modifying proteins that complex with the calcitonin receptor-like receptor, and thereby regulate its transport and ligand specificity, were identified. Here we show that mRNA for both the calcitonin receptor-like receptor and the receptor activity-modifying protein 2, but not the receptor activity-modifying protein 1 or receptor activity-modifying protein 3, are expressed in human endothelial and vascular smooth muscle cells. We also found that adrenomedullin increased cAMP levels in HeLa EBNA and 293 EBNA cells, expressing both the receptor activity-modifying protein 2 and calcitonin receptor-like receptor proteins. Thus, the receptor activity-modifying protein 2/calcitonin receptor-like receptor complex apparently serves as a functional adrenomedullin receptor in human endothelial and vascular smooth muscle cells.

RAMPs regulate the transport and ligand specificity of the calcitonin-receptor-like receptor

Calcitonin-gene-related peptide (CGRP) and adrenomedullin are related peptides with distinct pharmacological profiles. Here we show that a receptor with seven transmembrane domains, the calcitonin-receptor-like receptor (CRLR), can function as either a CGRP receptor or an adrenomedullin receptor, depending on which members of a new family of single-transmembrane-domain proteins, which we have called receptor-activity-modifying proteins or RAMPs, are expressed. RAMPs are required to transport CRLR to the plasma membrane. RAMP1 presents the receptor at the cell surface as a mature glycoprotein and a CGRP receptor. RAMP2-transported receptors are core-glycosylated and are adrenomedullin receptors.

Purification of calcitonin-like peptides from rat brain and pituitary

Accumulating evidence supports the existence of nonthyroidal calcitonin (CT)-like peptides, more similar to fish CTs, which may act as endogenous regulators of CT receptors in brain and other tissues. In this study, we have carried out large-scale extractions from Sprague-Dawley rat brain diencephalon and pituitary, and purified a novel, biologically active, CT-like peptide from pituitary. Monitoring of the calcitonin-like activity of the peptides from rat brain and pituitary required different detection systems. While the brain CT cross-reacted with C-terminally directed salmon CT-specific antisera, the pituitary CT did not. However, the pituitary CT was biologically active, exhibiting specific interaction with CT receptors to activate adenylate cyclase. Conventional chromatographic techniques were employed to purify the CT-like peptides. Although the brain CT was not purified to homogeneity, size exclusion chromatography revealed the presence of multiple molecular weight forms of immunoreactive CT. Of these, only the lowest molecular weight form was biologically active. Purification from the pituitary resulted in the isolation of a biologically active peptide with a mass of 3267 Da. This mass differs from the mass of both salmon and thyroid-derived rat CT. Initial amino acid sequencing of the pituitary CT indicated that it was N-terminally blocked. Following aminopeptidase digestion, a unique six amino acid sequence, EKSQSP, was identified. Elucidation of the amino acid composition provided supporting evidence that the peptide was novel and was consistent with a full length peptide of approximately 30 amino acids. These data support the existence of novel, nonthyroidal, CTs which are potential regulators of CT receptor-mediated functions.

Characterization of binding sites for amylin, calcitonin, and CGRP in primate kidney

Analysis of receptor distributions for 125I-labeled amylin, 125I-labeled calcitonin, and 125I-labeled calcitonin gene-related peptide (CGRP) in Macaca fascicularis kidney by in vitro autoradiography revealed distinct patterns of binding for each peptide. 125I-rat amylin bound primarily to the cortex, being associated with the distal tubule, including apparent binding to the juxtaglomerular apparatus. 125I-salmon calcitonin displayed high-density binding in the cortex with low-density binding to the medulla. Emulsion autoradiography indicated that binding was associated with both distal tubule and thick ascending limb of the loop of Henle. Intense binding was also found often over juxtaglomerular apparatus. 125I-rat CGRP-alpha exhibited low- to moderate-density binding to the inner medulla/papilla with high-density binding over small-, medium-, and large-caliber arteries. Weak binding to the glomerulus was also seen, but no binding was associated with cortical tubules. Competition binding studies, performed with each of the radioligands, revealed peptide specificity profiles for CGRP and calcitonin receptors that were similar to those described in rat. However, the monkey amylin receptors differed from those in rat, exhibiting relatively higher affinity for calcitonin peptides but reduced affinity for CGRP peptides. These studies suggest potential roles for amylin, calcitonin, and CGRP in primate renal function.