Expression of calcitonin receptor-like receptor and receptor-activity-modifying proteins in human cranial arteries

The effects of CGRP in vascular tissue - Classical vasodilation, shadowed effects and systemic dilemmas

Vascular tissue consists of endothelial cells, vasoactive smooth muscle cells and perivascular nerves. The perivascular sensory neuropeptide CGRP has demonstrated potent vasodilatory effects in any arterial vasculature examined so far, and a local protective CGRP-circuit of sensory nerve terminal CGRP release and smooth muscle cell CGRP action is evident. The significant vasodilatory effect has shadowed multiple other effects of CGRP in the vascular tissue and we therefore thoroughly review vascular actions of CGRP on endothelial cells, vascular smooth muscle cells and perivascular nerve terminals. The actions beyond vasodilation includes neuronal re-uptake and neuromodulation, angiogenic, proliferative and antiproliferative, pro- and anti-inflammatory actions which vary depending on the target cell and anatomical location. In addition to the classical perivascular nerve-smooth muscle CGRP circuit, we review existing evidence for a shadowed endothelial autocrine pathway for CGRP. Finally, we discuss the impact of local and systemic actions of CGRP in vascular regulation and protection from hypertensive and ischemic heart conditions with special focus on therapeutic CGRP agonists and antagonists.

Molecular studies of CGRP and the CGRP family of peptides in the central nervous system

BACKGROUND

Calcitonin gene-related peptide is an important target for migraine and other painful neurovascular conditions. Understanding the normal biological functions of calcitonin gene-related peptide is critical to understand the mechanisms of calcitonin gene-related peptide-blocking therapies as well as engineering improvements to these medications. Calcitonin gene-related peptide is closely related to other peptides in the calcitonin gene-related peptide family of peptides, including amylin. Relatedness in peptide sequence and in receptor biology makes it difficult to tease apart the contributions that each peptide and receptor makes to physiological processes and to disorders.

SUMMARY

The focus of this review is the expression of calcitonin gene-related peptide, related peptides and their receptors in the central nervous system. Calcitonin gene-related peptide is expressed throughout the nervous system, whereas amylin and adrenomedullin have only limited expression at discrete sites in the brain. The components of two receptors that respond to calcitonin gene-related peptide, the calcitonin gene-related peptide receptor (calcitonin receptor-like receptor with receptor activity-modifying protein 1) and the AMY₁ receptor (calcitonin receptor with receptor activity-modifying protein 1), are expressed throughout the nervous system. Understanding expression of the peptides and their receptors lays the foundation for more deeply understanding their physiology, pathophysiology and therapeutic use.

Blockade of CGRP Receptors in the Intracranial Vasculature: A New Target in the Treatment of Headache

In primary headaches, there is a clear association between the headache and the release of calcitonin gene-related peptide (CGRP) but not with any of the other neuronal messengers. The purpose of this review is to describe the role of CGRP in the intracranial circulation and to elucidate a possible role for a specific CGRP receptor antagonist in the treatment of primary headaches. Acute treatment with a 5-HT1B/1D agonist (triptan) results in alleviation of the headache and normalization of the cranial venous CGRP levels, in part due to a presynaptic inhibitory effect on sensory nerves. The central role of CGRP in migraine and cluster headache pathophysiology has led to the search for small molecule CGRP antagonists with few cardiovascular side-effects. The initial pharmacological profile of such a group of compounds has recently been disclosed. One of these compounds has been found to be efficacious in the relief of acute attacks of migraine.

CGRP in the trigeminovascular system: a role for CGRP, adrenomedullin and amylin receptors?

The neuropeptide calcitonin gene-related peptide (CGRP) is reported to play an important role in migraine. It is expressed throughout the trigeminovascular system. Antagonists targeting the CGRP receptor have been developed and have shown efficacy in clinical trials for migraine. However, no CGRP antagonist is yet approved for treating this condition. The molecular composition of the CGRP receptor is unusual because it comprises two subunits; one is a GPCR, the calcitonin receptor-like receptor (CLR). This associates with receptor activity-modifying protein (RAMP) 1 to yield a functional receptor for CGRP. However, RAMP1 also associates with the calcitonin receptor, creating a receptor for the related peptide amylin but this also has high affinity for CGRP. Other combinations of CLR or the calcitonin receptor with RAMPs can also generate receptors that are responsive to CGRP. CGRP potentially modulates an array of signal transduction pathways downstream of activation of these receptors, in a cell type-dependent manner. The physiological significance of these signalling processes remains unclear but may be a potential avenue for refining drug design. This complexity has prompted us to review the signalling and expression of CGRP and related receptors in the trigeminovascular system. This reveals that more than one CGRP responsive receptor may be expressed in key parts of this system and that further work is required to determine their contribution to CGRP physiology and pathophysiology.

LINKED ARTICLES

This article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.

Calcitonin gene-related peptide inhibits Langerhans cell-mediated HIV-1 transmission

Upon its mucosal entry, human immunodeficiency virus type 1 (HIV-1) is internalized by Langerhans cells (LCs) in stratified epithelia and transferred locally to T cells. In such epithelia, LCs are in direct contact with peripheral neurons secreting calcitonin gene-related peptide (CGRP). Although CGRP has immunomodulatory effects on LC functions, its potential influence on the interactions between LCs and HIV-1 is unknown. We show that CGRP acts via its receptor expressed by LCs and interferes with multiple steps of LC-mediated HIV-1 transmission. CGRP increases langerin expression, decreases selected integrins, and activates NF-κB, resulting in decreased HIV-1 intracellular content, limited formation of LC-T cell conjugates, and elevated secretion of the CCR5-binding chemokine CCL3/MIP-1α. These mechanisms cooperate to efficiently inhibit HIV-1 transfer from LCs to T cells and T cell infection. In vivo, HIV-1 infection decreases CGRP plasma levels in both vaginally SHIV-challenged macaques and HIV-1-infected individuals. CGRP plasma levels return to baseline after highly active antiretroviral therapy. Our results reveal a novel path by which a peripheral neuropeptide acts at the molecular and cellular levels to limit mucosal HIV-1 transmission and suggest that CGRP receptor agonists might be used therapeutically against HIV-1.

Possible sites of action of the new calcitonin gene-related peptide receptor antagonists

Migraine is considered a neurovascular disease affecting more than 10% of the general population. Currently available drugs for the acute treatment of migraine are vasoconstrictors, which have limitations in their therapeutic use. The calcitonin gene-related peptide (CGRP) has a key role in migraine, where levels of CGRP are increased during acute migraine attacks. CGRP is expressed throughout the central and peripheral nervous system, consistent with control of vasodilatation and transmission of nociceptive information. In migraine, CGRP is released from the trigeminal system. At peripheral synapses CGRP results in vasodilatation via receptors on the smooth muscle cells. At central synapses, CGRP acts postjunctionally on second-order neurons to transmit pain centrally via the brainstem and midbrain to higher cortical pain regions. The recently developed CGRP-receptor antagonists have demonstrated clinical efficacy in the treatment of acute migraine attacks. A remaining question is their site of action. The CGRP-receptor components (calcitonin receptor-like receptor, receptor activity modifying protein 1 and receptor component protein) are found to colocalize in the smooth muscle cells of intracranial arteries and in large-sized neurons in the trigeminal ganglion. The CGRP receptor has also been localized within parts of the brain and the brainstem. The aim of this paper is to review recent localization studies of CGRP and its receptor components within the nervous system and to discuss whether these sites could be possible targets for the CGRP-receptor antagonists.

Calcitonin gene-related peptide receptor antagonists for migraine

IMPORTANCE OF THE FIELD

Migraine is a highly prevalent disabling condition, and the current treatment options are not satisfactory. The role of calcitonin gene-related peptide (CGRP) in migraine pathophysiology is well established. CGRP receptor antagonists address this new target and have the potential to improve therapy for both responders and non-responders to previous options.

AREAS COVERED IN THIS REVIEW

This review describes CGRP, its receptors and their role in the pathophysiology of migraine. CGRP receptor antagonists are a recent development; all reported antagonists are reported in chronological order. The experimental evidence, as well as all clinical trials since the first proof-of-concept study in 2004, is discussed.

WHAT THE READER WILL GAIN

An overview of the CGRP system and why it provides an attractive drug target for headache. The main focus is on the currently presented CGRP receptor antagonists and clinical evidence for this new therapeutic option.

TAKE HOME MESSAGE

CGRP receptor antagonists will provide an additional and valuable therapeutic option for the treatment of headaches.

Animal models of headache: from bedside to bench and back to bedside

In recent years bench-based studies have greatly enhanced our understanding of headache pathophysiology, while facilitating the development of new headache medicines. At present, established animal models of headache utilize activation of pain-producing cranial structures, which for a complex syndrome, such as migraine, leaves many dimensions of the syndrome unstudied. The focus on modeling the central nociceptive mechanisms and the complexity of sensory phenomena that accompany migraine may offer new approaches for the development of new therapeutics. Given the complexity of the primary headaches, multiple approaches and techniques need to be employed. As an example, recently a model for trigeminal autonomic cephalalgias has been tested successfully, while by contrast, a satisfactory model of tension-type headache has been elusive. Moreover, although useful in many regards, migraine models are yet to provide a more complete picture of the disorder.

CGRP receptor antagonism and migraine

Calcitonin gene-related peptide (CGRP) is expressed throughout the central and peripheral nervous systems, consistent with control of vasodilatation, nociception, motor function, secretion, and olfaction. alphaCGRP is prominently localized in primary spinal afferent C and ADelta fibers of sensory ganglia, and betaCGRP is the main isoform in the enteric nervous system. In the CNS there is a wide distribution of CGRP-containing neurons, with the highest levels occurring in striatum, amygdala, colliculi, and cerebellum. The peripheral projections are involved in neurogenic vasodilatation and inflammation, and central release induces hyperalgesia. CGRP is released from trigeminal nerves in migraine. Trigeminal nerve activation results in antidromic release of CGRP to cause non-endothelium-mediated vasodilatation. At the central synapses in the trigeminal nucleus caudalis, CGRP acts postjunctionally on second-order neurons to transmit pain signals centrally via the brainstem and midbrain to the thalamus and higher cortical pain regions. Recently developed CGRP receptor antagonists are effective at aborting acute migraine attacks. They may act both centrally and peripherally to attenuate signaling within the trigeminovascular pathway.

Expression and effect of adrenomedullin in pheochromocytoma

This study investigates the expression of human adrenomedullin (ADM) and its receptor-receptor activity modifying protein 2/calcitonin receptor-like receptor (RAMP2/CRLR) mRNA in pheochromocytoma by reverse transcriptase polymerase chain reaction (RT-PCR) and its effect on the proliferation of pheochromocytoma cells by MTT. The mRNA expression of ADM and its receptor RAMP2/CRLR was present in normal adrenal medulla and pheochromocytoma tissues. The mRNA expression of ADM, RAMP2, and CRLR is markedly higher in pheochromocytomas than in normal medulla. ADM inhibits the proliferation of human pheochromocytoma cells and exerts a possible autocrine or paracrine effect in the adrenal.

GPCR modulation by RAMPs

Our conceptual understanding of the molecular architecture of G-protein coupled receptors (GPCRs) has transformed over the last decade. Once considered as largely independent functional units (aside from their interaction with the G-protein itself), it is now clear that a single GPCR is but part of a multifaceted signaling complex, each component providing an additional layer of sophistication. Receptor activity-modifying proteins (RAMPs) provide a notable example of proteins that interact with GPCRs to modify their function. They act as pharmacological switches, modifying GPCR pharmacology for a particular subset of receptors. However, there is accumulating evidence that these ubiquitous proteins have a broader role, regulating signaling and receptor trafficking. This article aims to provide the reader with a comprehensive appraisal of RAMP literature and perhaps some insight into the impact that their discovery has had on those who study GPCRs.

RELAXANT EFFECT OF ADRENOMEDULLIN ON BOVINE ISOLATED IRIS SPHINCTER MUSCLE UNDER RESTING CONDITIONS

1. The mechanisms involved in the fine adjustment of iris sphincter muscle tone are largely unknown. The aim of the present study was to clarify the effects of adrenomedullin on the resting tension of the bovine isolated iris sphincter muscle. 2. The motor activity of the bovine isolated iris sphincter muscle was measured isometrically. The effects of adrenomedullin on resting tension were analysed in the presence of indomethacin. The presence of adrenomedullin mRNA in the preparation was determined by reverse transcription-polymerase chain reaction. Immunolabelling for adrenomedullin was also performed. 3. Adrenomedullin significantly decreased the resting tension of the muscle. The relaxant effect of adrenomedullin was significantly inhibited by adrenomedullin (22-52), a putative antagonist for the adrenomedullin receptor, or calcitonin gene-related peptide (CGRP) (8-37), a putative antagonist for the CGRP1 receptor. The relaxant effect was almost completely blocked by a combination of adrenomedullin (22-52) and CGRP (8-37). 4. The relaxant effect of adrenomedullin was also significantly diminished by 2',5'-dideoxyadenosine, an inhibitor of adenylate cyclase, N(G)-nitro-L-arginine, an inhibitor of nitric oxide synthesis, or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylate cyclase. 5. Reverse transcription-polymerase chain reaction analysis showed that adrenomedullin mRNA was expressed in the muscle strip. Immunopositive staining for adrenomedullin was detected in blood vessel cells and in the iris sphincter muscle cells. 6. These results suggest that adrenomedullin may be an autocrine and paracrine regulator of the resting tension of the iris sphincter muscle. Its biological effects may be due to the direct involvement of adrenomedullin receptors and also to the stimulation of CGRP1 receptors. The stimulation of these receptors by the peptide leads to the activation of adenylate cyclase and soluble guanylate cyclase and subsequent relaxation of the muscle strip.

BIBN4096BS and CGRP(8-37) antagonize the relaxant effects of alpha-CGRP more than those of beta-CGRP in human extracranial arteries

We hypothesize that dilatation of extracranial arteries during migraine could be caused by CGRP. We compared the relaxant effects of alpha-calcitonin gene-related peptide (alpha-CGRP) and beta-calcitonin gene-related peptide (beta-CGRP) and the antagonism by BIBN4096BS and CGRP(8-37) on rings of human temporal and occipital arteries precontracted with KCl. beta-CGRP relaxed temporal (-logEC50M = 8.1) and occipital arteries (-logEC50M = 7.6) with 19-fold and 29-fold lower potencies respectively than alpha-CGRP. Nearly maximal effective concentrations of alpha-CGRP (4 nM) and beta-CGRP (50 nM) caused stable relaxations of the temporal artery for 4 h without fading. BIBN4094BS antagonized the effects of alpha-CGRP (pK(B) = 10.1 and 9.9, respectively) more than beta-CGRP (pK(B) = 9.3 and 9.2 respectively) on both temporal and occipital arteries. CGRP(8-37) antagonized the effects of alpha-CGRP (pK(B) = 6.6 and 6.4 respectively) more than beta-CGRP (pK(B) = 5.7 and 5.5 respectively) on both temporal and occipital arteries. Antagonism of the relaxant effects of alpha-CGRP (4 nM) and beta-CGRP (50 nM) by BIBN4096BS (10 and 100 nM) was reversible for beta-CGRP, but irreversible for alpha-CGRP, 1 h after BIBN4096BS washout. We conclude that alpha-CGRP and beta-CGRP interact either at different binding sites of the same CGRP receptor system or all together with different receptor systems in human extracranial arteries. BIBN4096BS binds more firmly to the receptor activated by alpha-CGRP than to the receptor activated by beta-CGRP.

Noncompetitive antagonism of BIBN4096BS on CGRP-induced responses in human subcutaneous arteries

We investigated the antagonistic effect of 1-piperidinecarboxamide, N-[2-[[5amino-l-[[4-(4-pyridinyl)-l-piperazinyl]carbonyl]pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl) (BIBN4096BS) on the calcitonin gene-related peptide (CGRP)-induced responses by using isometric myograph and FURA-2 technique in human subcutaneous arteries removed in association with abdominal surgery. BIBN4096BS, at the concentration of 1 pm, had no significant effect on the CGRP-induced relaxation in these vessels. At the concentration of 10 pM, BIBN4096BS had a competitive antagonistic-like behaviour characterized by parallel rightward shift in the log CGRP concentration-tension curve with no depression of the E(max). At the higher concentrations (0.1 and 1 nM), BIBN4096BS had a concentration-dependent noncompetitive antagonistic effect on the CGRP-induced responses. The efficacy and potency of CGRP was significantly greater in the smaller (lumen diameter approximately 200 microM) human subcutaneous arteries compared to the larger ones. The apparent agonist equilibrium dissociation constant, K(A), for CGRP(1) receptors in the human subcutaneous arteries was approximately 1 nM. Analysis of the relationship between receptor occupancy and response to CGRP indicates that the receptor reserve is relatively small. Using reverse transcriptase-polymerase chain reaction (RT-PCR), the presence of mRNA sequences encoding the calcitonin receptor-like receptor, receptor activity modifying protein (RAMP1, RAMP2, RAMP3) and receptor component protein were demonstrated in human subcutaneous arteries, indicating the presence of CGRP(1)-like receptor and the necessary component for the receptor activation. In conclusion, the inhibitory action of BIBN4096BS at the low concentration (10 pM) on the CGRP-tension curve (but not intracellular calcium concentration ([Ca(2+)](i)) resembles what is seen with a reversible competitive antagonist. However, at the higher concentrations (0.1 and 1 nM), BIBN4096BS acts as a selective noncompetitive inhibitor at CGRP(1) receptors in human subcutaneous arteries.

In-depth characterization of CGRP receptors in human intracranial arteries

The purpose of the present study was to characterize the effects of human (h) alpha- and beta-calcitonin gene-related peptide (CGRP) on intracranial arteries from man and to investigate the presence of mRNA for the calcitonin receptor like receptor (CRLR) and the receptor activity modifying proteins (RAMPs) 1, 2 and 3, in cerebral and middle meningeal arteries with and without endothelium, in microvessels and in the endothelial cells isolated from the human basilar artery. Reverse transcriptase-polymerase chain reaction (RT-PCR) revealed the presence of CRLR, RAMP 1, RAMP 2 and RAMP 3 in cerebral and middle meningeal arteries with and without endothelium as well as in microvessels and in the endothelial cells. Human and rat alpha- and beta-CGRP, amylin, adrenomedullin and [acetamidomethyl-Cys(2,7)]human CGRP induced strong concentration-dependent relaxation of human cerebral and middle meningeal arteries. Removal of the endothelium neither changed the maximum relaxant response nor the pIC(50) values for alpha- and beta-CGRP as compared to the responses in arteries with an intact endothelium. Human alpha-CGRP-(8-37) caused a shift of h alpha- and h beta-CGRP-induced relaxations in cerebral and middle meningeal arteries. Calculation of pK(B) values revealed that h alpha-CGRP-(8-37) could not significantly discriminate between relaxations induced by h alpha-CGRP (pK(B) around 6.8) and h beta-CGRP (pK(B) around 5.4). There was no significant difference in pK(B) value of h alpha-CGRP-(8-37) on h beta-CGRP-induced relaxation of human cerebral and middle meningeal arteries with and without endothelium. In conclusion, our molecular and pharmacological data support the existence of a single type of CGRP(1) receptors in the human intracranial circulation.

New therapeutic target in primary headaches - blocking theCGRP receptor

The primary headaches are among the most prevalent neurological disorders, afflicting up to 16% of the adult population. The associated pain originates from intracranial blood vessels that are innervated by sensory nerves storing several neurotransmitters. In primary headaches, there is a clear association between the headache and the release of calcitonin gene-related peptide (CGRP), but not other neuronal messengers. The specific purpose of this review is to describe CGRP in the human cranial circulation and to elucidate a possible role for a specific antagonist in the treatment of primary headaches. Acute treatment with administration of a 5-HT(1B/1D) agonist (triptan) results in alleviation of the headache and normalisation of the CGRP level. The mechanism of action of triptans involves vasoconstriction of intracranial vessels and a presynaptic inhibitory effect of sensory nerves. The central role of CGRP in migraine and cluster headache pathophysiology has led to the search for small-molecule CGRP antagonists, which are predicted to have fewer cardiovascular side effects in comparison to the triptans. The initial pharmacological profile of such a group of compounds has recently been disclosed. These compounds have high selectivity for human CGRP receptors and are reportedly efficacious in the relief of acute attacks of migraine.

Cell and molecular biology of the multifunctional peptide, adrenomedullin

Adrenomedullin (AM) is a recently discovered regulatory peptide involved in many functions including vasodilatation, electrolyte balance, neurotransmission, growth, and hormone secretion regulation, among others. This 52-amino acid peptide is expressed by specific cell types in many organs throughout the body. A complex receptor system has been described for AM; it requires at least the presence of a seven-transmembrane-domain G-protein-coupled receptor, a single-transmembrane-domain receptor activity modifying protein, and a receptor component protein needed to establish the connection with the downstream signal transduction pathway, which usually involves cyclicAMP. In addition, a serum-binding protein regulates the biological actions of AM, frequently by increasing AM functional attributes. Changes in levels of circulating AM correlate with several critical diseases, including cardiovascular and renal disorders, sepsis, cancer, and diabetes. Whether AM is a causal agent, a protective reaction, or just a marker for these diseases is currently under investigation. New technologies seeking to elevate and/or reduce AM levels are being investigated as potential therapeutic avenues.

Expression of adrenomedullin and its receptor by chondrocyte phenotype cells

For clarifying a process of de-differentiation in culturing chondrocytes, the present study was undertaken to investigate the secretion of adrenomedullin (AM) by chondrocyte phenotype cells and whether or not AM effects this proliferation in a cAMP-dependent fashion. Chondrocyte phenotype cells expressed AM and the AM receptor, and secreted high concentration of AM into the culture medium. When added to cultures, AM increased the intracellular cAMP level and decreased the number of these cells in a similar concentration-dependent fashion. Addition of forskolin and dibutyryl-cAMP caused a significant decrease in the number of these cells. Furthermore, the effect of AM was inhibited by a cAMP-dependent protein kinase A inhibitor (H89). The present findings indicate that AM has an autocrine/paracrine type of anti-proliferative effect on these cells mediated via a cAMP-dependent pathway and raise the possibility that AM plays a role in the local modulation of a process of de-differentiation by culturing chondrocyte phenotype cells.

Investigation of CGRP receptors and peptide pharmacology in human coronary arteries. Characterization with a nonpeptide antagonist

Calcitonin gene-related peptide (CGRP), adrenomedullin (AM), and amylin are structurally related peptides mediating vasorelaxation in the coronary circulation possibly via CGRP receptors (subtypes 1 or 2). Functional CGRP1 receptors appear to consist of at least three different kinds of proteins: the calcitonin receptor-like receptor (CRLR), receptor-activity-modifying proteins (RAMPs) and the receptor component protein (RCP). No CGRP2 receptor has yet been cloned. Using reverse transcriptase - polymerase chain reaction, the presence of mRNA sequences encoding CRLR, RCP and RAMPs was demonstrated in human coronary arteries. Relaxant responses were studied on isolated segments of coronary arteries after precontraction with U46619 (9,11-dideoxy-11alpha,9alpha-epoxymethano-prostaglandin F(2alpha)). The human peptides alphaCGRP, AM, and amylin induced relaxation with mean pEC50 values of 8.6, 6.8, and 6.3 M, respectively. Preincubation with alphaCGRP(8-37) (10(-7) -10(-5) M) and a novel nonpeptide CGRP antagonist "Compound 1" (WO98/11128) (10(-7)-10(-5) M) caused a dose-dependent rightward shift of the concentration-response curves for alphaCGRP with pA(2) values of 7.0 and 7.1, respectively. Preincubation with alphaCGRP(8-37) (10(-6) M) and Compound 1 (10(-6) M) caused significant rightward shift of the concentration-response curves for AM and amylin as well with pK B values between 6.6 and 7.5. Preincubation with AM(22-52) had no antagonistic effect on the AM and amylin response, neither did diacetoamidomethyl cysteine CGRP cause any concentration dependent (10(-11)-10(-6) M) dilatation. In conclusion, mRNA for the components forming CGRP1 and AM receptors was detected in the human left anterior descending coronary arteries. alphaCGRP, AM, and amylin mediated vasorelaxation via the CGRP1 receptor. Compound 1 acted as a nonpeptide antagonist at the CGRP1 receptor and could thus become a tool for the study of CGRP-mediated functional responses in human tissue.

Immunohistochemical localization of calcitonin receptor-like receptor and receptor activity-modifying proteins in the human cerebral vasculature

Calcitonin gene-related peptide and adrenomedullin belong to a structurally related neuropeptide family and are potent vasodilators expressed in the trigeminovascular system. The molecular identity of receptors for these proteins has only recently been elucidated. Central to functional binding of these neuropeptides is the G-protein-coupled receptor, the calcitonin receptor-like receptor (CRLR), whose cell surface expression and pharmacology is determined by coexpression of a receptor activity-modifying protein (RAMP). CRLR combined with RAMP binds calcitonin gene-related peptide with high affinity, whereas CRLR coexpression with RAMP2 or -3 confers high-affinity binding of adrenomedullin. The authors investigated the expression of these receptor components in human cerebral vasculature to further characterize neuropeptide receptor content and the potential functions of these receptors. Localization has been carried out using specific antisera raised against immunogenic peptide sequences that were subsequently applied using modern immunohistochemical techniques and confocal microscopy. The results are the first to show the presence of these receptor component proteins in human middle meningeal, middle cerebral, pial, and superficial temporal vessels, and confirm that both calcitonin gene-related peptide and adrenomedullin receptors may arise from the coassembly of RAMPs with CRLR in these vessel types. These novel data advance the understanding of the molecular function of the trigeminovascular system, its potential role in vascular headache disorders such as migraine, and may lead to possible ways in which future synthetic ligands may be applied to manage these disorders.

Efficacy of the non-peptide CGRP receptor antagonist BIBN4096BS in blocking CGRP-induced dilations in human and bovine cerebral arteries: potential implications in acute migraine treatment

Calcitonin gene-related peptide (CGRP) is a potent vasodilator in brain vessels and it has been implicated in the pathogenesis of migraine headache. Blocking post-junctional CGRP receptors, mediators of trigeminal-induced vasodilation, has been suggested as a potential antimigraine strategy. In this study, we tested the ability of a new non-peptide CGRP receptor antagonist, BIBN4096BS, to inhibit the CGRP-induced dilation in human and/or bovine brain vessels and compared it to that of the antagonist alpha-CGRP(8-37). BIBN4096BS and alpha-CGRP(8-37) both blocked the alpha-CGRP-induced dilation in bovine middle artery segments with respective potency (pK(B) values) of 6.3 and 7.8. In human pial vessels, BIBN4096BS was particularly potent. When tested at 10(-14)-10(-9) M concentrations, it induced a rightward shift in the alpha-CGRP concentration-response curve and yielded a biphasic Schild plot suggesting interaction with more than one receptor population, as was also indicated by the significant best fit of the alpha-CGRP-induced dilation in human brain vessels with a two receptor site interaction. Schild plot analysis in the linear portion of the BIBN4096BS inhibition curve revealed interaction with one high affinity site (pA(2) value approximately 14). In bovine vessels, both alpha-CGRP(8-37) and BIBN4096BS concentration-dependently reversed a pre-established CGRP-induced dilation ( approximately 59 and 85%, respectively), BIBN4096BS being approximately tenfold more potent than alpha-CGRP(8-37) (respective pIC(50) values of 7.5 and 6.75). In human middle cerebral and middle meningeal arteries, BIBN4096BS reversed the alpha-CGRP-induced dilation (> or =70%) by interaction with two different receptor populations: it exhibited a high affinity for one population (pIC(50) value approximately 13) and a lower affinity for the other (pIC(50) value approximately 8). The present data demonstrate that BIBN4096BS is a very potent antagonist that could, depending on its bioavailability and in vivo affinity, be of potential benefit in the acute treatment of migraine headache by blocking and/or reversing the CGRP-mediated dilation of intracranial vessels induced by activation of trigeminovascular afferents.

Characterization of calcitonin gene-related peptide (CGRP) receptors and their receptor-activity-modifying proteins (RAMPs) in human brain microvascular and astroglial cells in culture

1. In the present study, we examined the expression of the CGRP receptor-activity-modifying proteins (RAMP1, RAMP2 and RAMP3) and receptor component protein (RCP) in human brain astrocytes (AST), cerebromicrovascular endothelial (EC) and smooth muscle (SMC) cells in culture. Further, we pharmacologically characterized CGRP receptors in these cells by assessing the potency of the CGRP receptor antagonists h-alpha CGRP(8-37) and the new non-peptide compound BIBN4096BS to block the production of cAMP elicited by CGRP(1) and CGRP(2) receptor agonists. 2. AST, EC and SMC all expressed mRNAs for RAMP1, RAMP2 and RCP. In contrast, message for RAMP3 was detected in AST, but not in SMC and in only one out of four preparations of EC. 3. h-alpha CGRP, h-beta CGRP and [Cys (Et)(2,7)]-h-alpha CGRP exerted concentration-dependent production of cAMP in all cultures, with a maximal effect at 25-50 nM (20-60-fold increase from basal levels). In contrast, 50 nM [Cys (Acm)(2,7)]-h-alpha CGRP only induced a weak stimulatory effect on cAMP formation, especially in SMC and AST (1.5- and 5-fold increase above baseline, respectively). 4. h-alpha CGRP(8-37) and BIBN4096BS concentration-dependently inhibited cAMP formation evoked by CGRP receptor agonists. Depending on the agonists used, h-alpha CGRP(8-37) distinguished two different CGRP receptors for which it exhibited low (pIC(50)< or =6.4) and high (pIC(50) approximately 7.3) affinity, respectively. BIBN4096BS was much more potent (>2.5 orders of magnitude) than h-alpha CGRP(8-37). Further, BIBN4096BS was able to discriminate three different CGRP receptor sites for which it exhibited low (pIC(50) approximately 9.3-9.9), intermediate (pIC(50) approximately 10.9), and a very high (pIC(50) approximately 13.7) affinity, respectively. Together, these results suggest the presence of CGRP(1) and/or CGRP(2) receptors in human brain AST, EC and SMC, and of an additional population of CGRP receptors in AST, possibly associated to the combined expression of RAMP3 and RCP in these cells, for which BIBN4096BS exhibits an exquisitely high affinity.

Calcitonin gene-related peptide (CGRP) and the pathophysiology of headache: therapeutic implications

Cerebral blood vessels are innervated by sensory nerves that store several neurotransmitters. In primary headaches, there is a clear association between head pain and the release of the neuropeptide calcitonin gene-related peptide (CGRP). Furthermore, when triptan antimigraine agents are administered, headache subsides and the neuropeptide release normalises, in part via a presynaptic effect. The central role of CGRP in primary headaches has led to the search for suitable antagonists of the receptors for this neuropeptide, which it is hoped will have less cardiovascular adverse effects than the triptans. Recently, the initial pharmacological profile of such a group of compounds has been disclosed. These compounds are small molecules with high selectivity for human CGRP receptors. Hypothetically, these agents will be efficacious in the relief of migraine headaches via blockade of the effects of CGRP.

Effect of the CGRP receptor antagonist BIBN4096BS in human cerebral, coronary and omental arteries and in SK-N-MC cells

Several lines of evidence suggest that a calcitonin-gene related peptide (CGRP) receptor antagonist may serve as a novel abortive migraine treatment. Here we present data on a human cell line and isolated human vessels for such an antagonist, BIBN4096BS. On SK-N-MC membranes, radiolabelled CGRP was displaced by both CGRP-(8-37) and BIBN4096BS, yielding pK(i) values of 8.5 and 11.4, respectively. Functional studies with SK-N-MC cells demonstrated that CGRP-induced cAMP production was antagonised by both CGRP-(8-37) and BIBN4096BS with pA(2) values of 7.8 and 11.2, respectively. Isolated human cerebral, coronary, and omental arteries were studied with a sensitive myograph technique. CGRP induced a concentration-dependent relaxation that was antagonized by both CGRP-(8-37) and BIBN4096BS in a competitive manner. CGRP was a weaker agonist on coronary arteries as compared to intracranial arteries; however, BIBN4096BS was an equally effective antagonist. In human omental arteries, CGRP did not induce relaxation. BIBN4096 had a pA(2) value of 10.1 in cerebral and 10.4 in coronary arteries. The results of clinical trials with BIBN4096BS for acute migraine attacks are awaited with great interest.

Adrenomedullin receptors: molecular identity and function

Since its discovery in 1993 adrenomedullin (AM) has been the subject over 600 published articles. This multifunctional peptide has powerful vasodilator actions and recent evidence from AM gene-deleted mice suggest that AM plays an essential role in vascular development. However the lack of valid AM receptor clones and non-peptide receptor ligands has considerably slowed research progress on this important peptide. In this review we have focused on the proposition that the calcitonin receptor-like receptor (CRLR) is a receptor both for AM and the related vasoactive peptide calcitonin gene-related peptide (CGRP). The receptor activity modifying proteins (RAMPs) that are essential for defining CRLR pharmacology will also be discussed. We will describe how AM receptors have been reported to signal and be regulated and to consider whether further receptors for AM beyond CRLR/RAMP combinations might exist.

CGRP receptors mediating CGRP-, adrenomedullin- and amylin-induced relaxation in porcine coronary arteries. Characterization with 'Compound 1' (WO98/11128), a non-peptide antagonist

1. Calcitonin gene-related peptide (CGRP), amylin and adrenomedullin (AM) belong to the same family of peptides. Accumulating evidence indicate that the calcitonin (CT) receptor, the CT receptor-like receptor (CRLR) and receptor-activity-modifying proteins (RAMPs) form the basis of all the receptors in this family of peptides. 2. Using reverse transcriptase - polymerase chain reaction the presence of mRNA sequences encoding the CRLR, RAMP1 and RAMP2 were demonstrated in porcine left anterior descending (LAD) coronary arteries, whereas porcine calcitonin (CT) receptor mRNA was not present. The partial porcine mRNA sequences shared 82 - 92% nucleotide identity with human sequences. 3. The human peptides alphaCGRP, betaCGRP, AM and amylin induced relaxation with pEC(50) values of 8.1, 8.1, 6.7 and 6.1 M respectively. 4. The antagonistic properties of a novel non-peptide CGRP antagonist 'Compound 1' (WO98/11128), betaCGRP(8 - 37) and the proposed AM receptor antagonist AM(22 - 52) were compared to the well-known CGRP(1) receptor antagonist alphaCGRP(8 - 37). 5. The alphaCGRP(8 - 37) and betaCGRP(8 - 37) induced concentration-dependent (10(-7) - 10(-5) M) rightward shift of both the alphaCGRP and betaCGRP concentration-response curves. betaCGRP(8 - 37) (10(-6) M) had the same effect as alphaCGRP(8 - 37) (10(-6) M), but with less potent rightward shift of the concentration-response curves for alphaCGRP, AM and amylin. 6. Preincubation with 'Compound 1' (10(-7) - 10(-5) M) and AM(22 - 52) (10(-6) M) had no significant antagonistic effect. 7. In conclusion, the building blocks forming CGRP and AM receptors were present in the porcine LAD, whereas those of the amylin receptor were not. alphaCGRP, betaCGRP, AM and amylin mediated vasorelaxation via the CGRP receptors. No functional response was detected to adrenomedullin via the adrenomedullin receptor.

Characterisation of the effects of a non-peptide CGRP receptor antagonist in SK-N-MC cells and isolated human cerebral arteries

The cerebral circulation is innervated by calcitonin gene-related peptide (CGRP) containing fibers originating in the trigeminal ganglion. During a migraine attack, there is a release of CGRP in conjunction with the head pain, and triptan administration abolishes both the CGRP release and the pain at the same time. In the search for a novel treatment of migraine, a non-peptide CGRP antagonist has long been sought. Here, we present data on a human cell line and human and guinea-pig isolated cranial arteries for such an antagonist, Compound 1 (4-(2-Oxo-2,3-dihydro-benzoimidazol-1-yl)-piperidine-1-carboxylic acid [1-(3,5-dibromo-4-hydroxy-benzyl)-2-oxo-2-(4-phenyl-piperazin-1-yl)-ethyl]-amide). On SK-N-MC cell membranes, radiolabelled CGRP binding was displaced by both CGRP-(8-37) and Compound 1, yielding pK(i) values of 8.9 and 7.8, respectively. Functional studies with SK-N-MC cells showed that CGRP-induced cAMP production was antagonised by both CGRP-(8-37) and Compound 1 with pA(2) values of 7.8 and 7.7, respectively. Isolated human and guinea pig cerebral arteries were studied with a sensitive myograph technique. CGRP induced a concentration-dependent relaxation in human cerebral arteries which was antagonized by both CGRP-(8-37) and Compound 1 in a competitive manner. In guinea pig basilar arteries, CGRP-(8-37) antagonised the CGRP-induced relaxation while Compound 1 had a weak blocking effect. The clinical studies of non-peptide CGRP antagonists are awaited with great interest.

mRNA expression of novel CGRP1 receptors and their activity-modifying proteins in hypoxic rat lung

Calcitonin gene-related peptide (CGRP) is a potent vasodilator. Our group has reported that exogenous CGRP may prevent or reverse hypoxic pulmonary hypertension in rats. The vasodilatory action of CGRP is mediated primarily by CGRP1 receptors. The calcitonin receptor-like receptor (CRLR) and the orphan receptor RDC-1 have been proposed as CGRP1 receptors, and recent evidence suggests that CRLR can function as either a CGRP1 receptor or an adrenomedullin (ADM) receptor. Receptor activity-modifying proteins (RAMPs) determine the ligand specificity of CRLR: coexpression of CRLR and RAMP1 results in a CGRP1 receptor, whereas coexpression of CRLR and RAMP2 or -3 results in an ADM receptor. We used qualitative, semiquantitative, and real-time quantitative RT-PCR to detect and quantitate the relative expression of these agents in the lungs of rats exposed to normoxia (n = 3) and 1 and 2 wk of chronic hypobaric hypoxia (barometric pressure 380 mmHg, equivalent to an inspired O(2) level of 10%; n = 3/time period). Our results show upregulation of RDC-1, RAMP1, and RAMP3 mRNAs in hypoxic rat lung and no change in CRLR and RAMP2 mRNAs. These findings support a functional role for CGRP and ADM receptors in regulating the adult pulmonary circulation.

Human hepatic stellate cells secrete adrenomedullin: potential autocrine factor in the regulation of cell contractility

BACKGROUND/AIMS

Hepatic stellate cells (HSCs) are perisinusoidal pericytes which have receptors for vasoactive factors, such as endothelin-1, which can regulate cell contractility in an autocrine manner. It is unknown whether human HSCs have receptors for and are able to synthesize the vasodilator peptide adrenomedullin (ADM), a peptide produced by most contractile cells.

METHODS AND RESULTS

Stimulation of HSCs with ADM resulted in a dose-dependent raise in cAMP concentration (radioimmunoassay) and markedly blunted the endothelin-induced increase in [Ca2+]i and cell contraction, as assessed in cells loaded with fura-2 using a morphometric method. The existence of the receptor CRLR for ADM and their associated proteins RAMP-1 and RAMP-2 was demonstrated by reverse transcriptase-polymerase chain reaction (RT-PCR). Moreover, activated human HSCs spontaneously secreted ADM in the culture medium in a time-dependent manner. ADM secretion was markedly enhanced by tumour necrosis factor-alpha and interleukin-1beta. Specific mRNA for ADM (RT-PCR and Northern blot) was detected in HSCs and increased after incubation of cells with cytokines.

CONCLUSIONS

Human HSCs have functional receptors for ADM, the stimulation of which blunts the contractile effect of endothelin-1. Cultured human HSCs secrete ADM in baseline conditions. This secretion is markedly increased by cytokines. These results suggest that ADM can regulate HSCs' contractility in an autocrine manner.

CGRP and adrenomedullin receptor populations in human cerebral arteries: in vitro pharmacological and molecular investigations in different artery sizes

The aim of the present study was to determine functional and molecular characteristics of receptors for calcitonin gene-related peptide (CGRP) and adrenomedullin in three different diameter groups of lenticulostriate arteries. Furthermore, the presence of perivascular neuronal sources of CGRP was evaluated in these arteries. In the functional studies, in vitro pharmacological experiments demonstrated that both CGRP and adrenomedullin induce alpha-CGRP-(8-37) sensitive vasodilation in artery segments of various diameters. The maximal amounts of vasodilation induced by CGRP and adrenomedullin were not different, whereas the potency of CGRP exceeded that of adrenomedullin by 2 orders of magnitude. Significant negative correlations between artery diameters and maximal responses were demonstrated for CGRP and adrenomedullin. In addition, the potency of both peptides tended to increase in decreasing artery diameter. In the molecular experiments, levels of mRNAs encoding CGRP receptors and receptor subunits were compared using reverse transcriptase polymerase chain reactions (RT-PCR). The larger the artery, the more mRNA encoding receptor activity-modifying proteins 1 and 2 (RAMP1 and RAMP2) was detected relative to the amount of mRNA encoding the calcitonin receptor-like receptor. By immunohistochemistry, perivascular CGRP containing nerve fibres were demonstrated in all the investigated artery sizes. In conclusion, both CGRP and adrenomedullin induced vasodilation via CGRP receptors in human lenticulostriate artery of various diameter. The artery responsiveness to the CGRP receptor agonists increased with smaller artery diameter, whereas the receptor-phenotype determining mRNA ratios tended to decrease. No evidence for CGRP and adrenomedullin receptor heterogeneity was present in lenticulostriate arteries of different diameters.

Positive inotropy mediated via CGRP receptors in isolated human myocardial trabeculae

Isometric contractile force were studied on isolated human myocardial trabeculae that were paced at 1.0 Hz in tissue baths. Alpha calcitonin gene-related peptide (alpha-CGRP) had a potent positive inotropic effect in most trabeculae from both the right atrium and left ventricle, and this effect was partially antagonized by the CGRP(1) receptor antagonist alpha-CGRP-(8-37) (10(-6) M). Amylin and the CGRP(2) receptor agonist [Cys(acetylmethoxy)(2, 7)]CGRP had a positive inotropic effect in some trabeculae, whereas adrenomedullin had no inotropic effect. Using reverse transcriptase-polymerase chain reaction (PCR) mRNAs encoding the human calcitonin receptor-like receptor and the receptor associated modifying proteins (RAMPs) RAMP1, RAMP2, and RAMP3 were detected in human myocardial trabeculae from both the right atrium and left ventricle. In conclusion, functional CGRP(1) and CGRP(2) receptors may mediate a positive inotropic effect at both the atrial and ventricular level of the human heart. mRNAs for calcitonin receptor-like receptor and specific RAMPs further support the presence of CGRP receptors.

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.

Equipotent in vitro actions of alpha- and beta-CGRP on guinea pig basilar artery are likely to be mediated via CRLR derived CGRP receptors

The aim of the present study was to investigate and compare specific in vitro pharmacological actions of human alpha- and beta-CGRP applied as single concentrations to prostaglandin F2alpha precontracted segments of guinea pig basilar artery. To support the suggestion of a possible link between the pharmacological actions of alpha- and beta-CGRP and a specific receptor, we wished to determine whether mRNAs required for the expression of calcitonin receptor-like receptor (CRLR) derived CGRP receptors were present in the guinea pig basilar artery. In the pharmacological experiments we demonstrated an increase in the cAMP content by 2.5-fold and a concomitant significant vasorelaxation of the precontracted basilar artery segments following 1 min of stimulation by 10(-7) M alpha- or beta-CGRP. In another set of experiments, the time course of alpha- and beta-CGRP induced vasodilatation was investigated and concentration dependent responses of the two peptides were demonstrated. No significant differences between the actions of alpha- and beta-CGRP regarding induction of cAMP formation, amount of vasodilatation, time course of vasodilatation and mode of inhibition by the CGRP receptor antagonist, human alpha-CGRP(8-37), could be detected. The presence of mRNA encoding the guinea pig CRLR and the guinea pig CGRP receptor component protein (RCP) in the guinea pig basilar artery was demonstrated by RT-PCR methods. Furthermore, a partial sequence of mRNA encoding the guinea pig CRLR was determined. The expression in this tissue of a CRLR derived CGRP receptor and a functional RCP is therefore likely, and the equipotent pharmacological actions of alpha- and beta-CGRP might be mediated via CRLR derived CGRP receptors.

Characterization of a putative calcitonin receptor in IMR 32 human neuroblastoma cells

In this study we characterized calcitonin (CT) receptors in human neuroblastoma IMR 32 cells. Saturation binding assays indicated that [125I]-human CT bound with high affinity to IMR 32 cell membranes (K(d) = 253.6 pM; Bmax = 3.84 fmol/ mg protein). In competition binding studies, human adrenomedullin displayed high affinity for these sites (IC50 = 30 nM) whereas human alpha calcitonin-gene related peptide (alphaCGRP; IC50 = 145 nM) and human amylin (IC50 = 415 nM) showed lower affinity. These peptides increased cAMP levels in viable cells; the relative potencies were: human CT > human adrenomedullin > human cCGRP > or = human amylin. The expression of mRNA coding for the published sequences of the human calcitonin receptor and of the human calcitonin receptor-like receptorwas evaluated by reverse transcriptase-polymerase chain reaction. Electrophoretic analysis did not confirm the occurrence of mRNA coding for the above mentioned receptors in these cells. This study suggests the presence of a novel, putative CT receptor in IMR 32 cells.