Molecular Signature for Receptor Engagement in the Metabolic Peptide Hormone Amylin

Converting the Amyloidogenic Islet Amyloid Polypeptide into a Potent Nonaggregating Peptide Ligand by Side Chain-to-Side Chain Macrocyclization

The islet amyloid polypeptide (IAPP), also known as amylin, is a hormone playing key physiological roles. However, its aggregation and deposition in the pancreatic islets are associated with type 2 diabetes. While this peptide adopts mainly a random coil structure in solution, its secondary conformational conversion into α-helix represents a critical step for receptor activation and contributes to amyloid formation and associated cytotoxicity. Considering the large conformational landscape and high amyloidogenicity of the peptide, as well as the complexity of the self-assembly process, it is challenging to delineate the delicate interplay between helical folding, peptide aggregation, and receptor activation. In the present study, we probed the roles of helical folding on the function-toxicity duality of IAPP by restricting its conformational ensemble through side chain-to-side chain stapling via azide-alkyne cycloaddition. Intramolecular macrocyclization (i; i + 4) constrained IAPP into α-helix and inhibited its aggregation into amyloid fibrils. These helical derivatives slowed down the self-assembly of unmodified IAPP. Site-specific macrocyclization modulated the capacity of IAPP to perturb lipid bilayers and cell plasma membrane and reduced, or even fully inhibited, the cytotoxicity associated with aggregation. Furthermore, the α-helical IAPP analogs showed moderate to high potency toward cognate G protein-coupled receptors. Overall, these results indicate that macrocyclization represents a promising strategy to protect an amyloidogenic peptide hormone from aggregation and associated toxicity, while maintaining high receptor activity.

Understanding the structural dynamics of human islet amyloid polypeptide: Advancements in and applications of ion-mobility mass spectrometry

Human islet amyloid polypeptide (hIAPP) forms amyloid deposits that contribute to β-cell death in pancreatic islets and are considered a hallmark of Type II diabetes Mellitus (T2DM). Evidence suggests that the early oligomers of hIAPP formed during the aggregation process are the primary pathological agent in islet amyloid induced β-cell death. The self-assembly mechanism of hIAPP, however, remains elusive, largely due to limitations in conventional biophysical techniques for probing the distribution or capturing detailed structures of the early, structurally dynamic oligomers. The advent of Ion-mobility Mass Spectrometry (IM-MS) has enabled the characterisation of hIAPP early oligomers in the gas phase, paving the way towards a deeper understanding of the oligomerisation mechanism and the correlation of structural information with the cytotoxicity of the oligomers. The sensitivity and the rapid structural characterisation provided by IM-MS also show promise in screening hIAPP inhibitors, categorising their modes of inhibition through "spectral fingerprints". This review delves into the application of IM-MS to the dissection of the complex steps of hIAPP oligomerisation, examining the inhibitory influence of metal ions, and exploring the characterisation of hetero-oligomerisation with different hIAPP variants. We highlight the potential of IM-MS as a tool for the high-throughput screening of hIAPP inhibitors, and for providing insights into their modes of action. Finally, we discuss advances afforded by recent advancements in tandem IM-MS and the combination of gas phase spectroscopy with IM-MS, which promise to deliver a more sensitive and higher-resolution structural portrait of hIAPP oligomers. Such information may help facilitate a new era of targeted therapeutic strategies for islet amyloidosis in T2DM.

Investigation Into Novel Mukanadin B, Mukanadin D and Mukanadin F Derivatives as Antagonists of 5-HT1A Signalling

Marine bromopyrrole alkaloids are a diverse family of natural products with a large array of biological applications. The mukanadin family is a group of molecules consisting of seven members (mukanadin A-G) that possess a range of biological activities. Inhibition of serotonergic signaling has been demonstrated by mukanadin B derivatives, presenting this chemical scaffold as a candidate for further SAR exploration. A library of thirteen novel mukanadin B and D derivatives with structural variation targeted at the pyrrole ring, central linker and hydantoin ring, were synthesized. These analogues were subsequently assessed for serotonergic antagonism, in addition to natural products, mukanadin B, D, F and 9-hydroxy mukanadin B. A collection of compounds exhibited significant 5-HT1A signaling, including five of the novel derivatives and two of the naturally occurring bromopyrroles, mukanadin B and F. Particular SAR information could be determined from these results, such as modification of the pyrrole ring being a well-tolerated strategy for improving serotonergic inhibition. Other changes to the pharmacophore led to significant reduction in activity such as saturation of the linker region, or no conclusive improvement in inhibitory activity such as a 9-OH group or replacement of the hydantoin ring with a triazole moiety.

Cryo-EM Structure of the Human Amylin 1 Receptor in Complex with CGRP and Gs Protein

Inhibition of calcitonin gene-related peptide (CGRP) or its cognate CGRP receptor (CGRPR) has arisen as a major breakthrough in the treatment of migraine. However, a second CGRP-responsive receptor exists, the amylin (Amy) 1 receptor (AMY1R), yet its involvement in the pathology of migraine is poorly understood. AMY1R and CGRPR are heterodimers consisting of receptor activity-modifying protein 1 (RAMP1) with the calcitonin receptor (CTR) and the calcitonin receptor-like receptor (CLR), respectively. Here, we present the structure of AMY1R in complex with CGRP and Gs protein and compare it with the reported structures of the AMY1R complex with rat amylin (rAmy) and the CGRPR in complex with CGRP. Despite similar protein backbones observed within the receptors and the N- and C-termini of the two peptides bound to the AMY1R complexes, they have distinct organization in the peptide midregions (the bypass motif) that is correlated with differences in the dynamics of the respective receptor extracellular domains. Moreover, divergent conformations of extracellular loop (ECL) 3, intracellular loop (ICL) 2, and ICL3 within the CTR and CLR protomers are evident when comparing the CGRP bound to the CGRPR and AMY1R, which influences the binding mode of CGRP. However, the conserved interactions made by the C-terminus of CGRP to the CGRPR and AMY1R are likely to account for cross-reactivity of nonpeptide CGRPR antagonists observed at AMY1R, which also extends to other clinically used CGRPR blockers, including antibodies.

The processing intermediate of human amylin, pro-amylin(1-48), has in vivo and in vitro bioactivity

Amylin is released by pancreatic beta-cells in response to a meal and its major soluble mature form (37 amino acid-peptide) produces its biological effects by activating amylin receptors. Amylin is derived from larger propeptides that are processed within the synthesizing beta-cell. There are suggestions that a partially processed form, pro-amylin(1-48) is also secreted. We tested the hypothesis that pro-amylin(1-48) has biological activity and that human pro-amylin(1-48) may also form toxic pre-amyloid species. Amyloid formation, the ability to cross-seed and in vitro toxicity were similar between human pro-amylin(1-48) and amylin. Human pro-amylin(1-48) was active at amylin-responsive receptors, though its potency was reduced at rat, but not human amylin receptors. Pro-amylin(1-48) was able to promote anorexia by activating neurons of the area postrema, amylin's primary site of action, indicating that amylin can tolerate significant additions at the N-terminus without losing bioactivity. Our studies help to shed light on the possible roles of pro-amylin(1-48) which may be relevant for the development of future amylin-based drugs.

On the importance of being amidated: Analysis of the role of the conserved C-terminal amide of amylin in amyloid formation and cytotoxicity

The polypeptide hormone Amylin (also known as islet amyloid polypeptide) plays a role in regulation of glucose metabolism, but forms pancreatic islet amyloid deposits in type 2 diabetes. The process of islet amyloid formation contributes to β-cell dysfunction and the development of the disease. Amylin is produced as a pro-from and undergoes processing prior to secretion. The mature hormone contains an amidated C-terminus. Analysis of an alignment of vertebrate amylin sequences reveals that the processing signal for amidation is strictly conserved. Furthermore, the enzyme responsible for C-terminal amidation is found in all of these organisms. Comparison of the physiologically relevant amidated form to a variant with a free C-terminus (Amylin-COO-) shows that replacement of the C-terminal amide with a carboxylate slows, but does not prevent amyloid formation. Pre-fibrillar species produced by both variants are toxic to cultured β-cells, although hAmylin-COO- is moderately less so. Amyloid fibrils produced by either peptide are not toxic. Prior work (ACS Pharmacol. Translational. Sci. 1, 132-49 (2018)) shows that Amylin- COO- exhibits a 58-fold reduction in activation of the Amylin1 receptor and 20-fold reduction in activation of the Amylin3 receptor. Thus, hAmylin-COO- exhibits significant toxicity, but significantly reduced activity and offers a reagent for studies which aim to decouple hAmylin's toxic effects from its activity. The different behaviours of free and C-terminal amidated Amylin should be considered when designing systems to produce the polypeptide recombinantly.

Pharmacological characterisation of erenumab, Aimovig, at two calcitonin gene-related peptide responsive receptors

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP) is involved in migraine pathophysiology. CGRP can signal through two receptors. The canonical CGRP receptor comprises the calcitonin receptor-like receptor and receptor activity-modifying protein 1 (RAMP1); the AMY1 receptor comprises the calcitonin receptor with RAMP1. Drugs that reduce CGRP activity, such as receptor antagonists, are approved for the treatment and prevention of migraine. Despite being designed to target the canonical CGRP receptor, emerging evidence suggests that these antagonists, including erenumab (a monoclonal antibody antagonist) can also antagonise the AMY1 receptor. However, it is difficult to estimate its selectivity because direct comparisons between receptors under matched conditions have not been made. We therefore characterised erenumab at both CGRP-responsive receptors with multiple ligands, including αCGRP and βCGRP.

EXPERIMENTAL APPROACH

Erenumab antagonism was quantified through IC50 and pKB experiments, measuring cAMP production. We used SK-N-MC cells which endogenously express the human CGRP receptor, and HEK293S and Cos7 cells transiently transfected to express either human CGRP or AMY1 receptors.

KEY RESULTS

Erenumab antagonised both the CGRP and AMY1 receptors with an ~20-120-fold preference for the CGRP receptor, depending on the cells, agonist, analytical approach and/or assay format. Erenumab antagonised both forms of CGRP equally, and appeared to act as a competitive reversible antagonist at both receptors.

CONCLUSION AND IMPLICATIONS

Despite being designed to target the CGRP receptor, erenumab can antagonise the AMY1 receptor. Its ability to antagonise CGRP activity at both receptors may be useful in better understanding the clinical profile of erenumab.

Shared and independent roles of CGRP and PACAP in migraine pathophysiology

The neuropeptides calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) have emerged as mediators of migraine pathogenesis. Both are vasodilatory peptides that can cause migraine-like attacks when infused into people and migraine-like symptoms when injected into rodents. In this narrative review, we compare the similarities and differences between the peptides in both their clinical and preclinical migraine actions. A notable clinical difference is that PACAP, but not CGRP, causes premonitory-like symptoms in patients. Both peptides are found in distinct, but overlapping areas relevant to migraine, most notably with the prevalence of CGRP in trigeminal ganglia and PACAP in sphenopalatine ganglia. In rodents, the two peptides share activities, including vasodilation, neurogenic inflammation, and nociception. Most strikingly, CGRP and PACAP cause similar migraine-like symptoms in rodents that are manifested as light aversion and tactile allodynia. Yet, the peptides appear to act by independent mechanisms possibly by distinct intracellular signaling pathways. The complexity of these signaling pathways is magnified by the existence of multiple CGRP and PACAP receptors that may contribute to migraine pathogenesis. Based on these differences, we suggest PACAP and its receptors provide a rich set of targets to complement and augment the current CGRP-based migraine therapeutics.

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

On the plasticity of amyloid formation: The impact of destabilizing small to large substitutions on islet amyloid polypeptide amyloid formation

Amyloids are partially ordered, proteinaceous, β-sheet rich deposits that have been implicated in a wide range of diseases. An even larger set of proteins that do not normally form amyloid in vivo can be induced to do so in vitro. A growing number of structures of amyloid fibrils have been reported and a common feature is the presence of a tightly packed core region in which adjacent monomers pack together in extremely tight interfaces, often referred to as steric zippers. A second common feature of many amyloid fibrils is their polymorphous nature. We examine the consequences of disrupting the tight packing in amyloid fibrils on the kinetics of their formation using the 37 residue polypeptide hormone islet amyloid polypeptide (IAPP, amylin) as a model system. IAPP forms islet amyloid in vivo and is aggressively amyloidogenic in vitro. Six Cryo-EM structures of IAPP amyloid fibrils are available and in all Gly24 is in the core of the structured region and makes tight contacts with other residues. Calculations using the ff14SBonlysc forcefield in Amber20 show that substitutions with larger amino acids significantly disrupt close packing and are predicted to destabilize the various fibril structures. However, Gly to 2-amino butyric acid (2-carbon side chain) and Gly to Leu substitutions actually enhance the rate of amyloid formation. A Pro substitution slows, but does not prevent amyloid formation.

Elucidating the Interactome of G Protein-Coupled Receptors and Receptor Activity-Modifying Proteins

G protein-coupled receptors (GPCRs) are known to interact with several other classes of integral membrane proteins that modulate their biology and pharmacology. However, the extent of these interactions and the mechanisms of their effects are not well understood. For example, one class of GPCR-interacting proteins, receptor activity-modifying proteins (RAMPs), comprise three related and ubiquitously expressed single-transmembrane span proteins. The RAMP family was discovered more than two decades ago, and since then GPCR-RAMP interactions and their functional consequences on receptor trafficking and ligand selectivity have been documented for several secretin (class B) GPCRs, most notably the calcitonin receptor-like receptor. Recent bioinformatics and multiplexed experimental studies suggest that GPCR-RAMP interactions might be much more widespread than previously anticipated. Recently, cryo-electron microscopy has provided high-resolution structures of GPCR-RAMP-ligand complexes, and drugs have been developed that target GPCR-RAMP complexes. In this review, we provide a summary of recent advances in techniques that allow the discovery of GPCR-RAMP interactions and their functional consequences and highlight prospects for future advances. We also provide an up-to-date list of reported GPCR-RAMP interactions based on a review of the current literature. SIGNIFICANCE STATEMENT: Receptor activity-modifying proteins (RAMPs) have emerged as modulators of many aspects of G protein-coupled receptor (GPCR)biology and pharmacology. The application of new methodologies to study membrane protein-protein interactions suggests that RAMPs interact with many more GPCRs than had been previously known. These findings, especially when combined with structural studies of membrane protein complexes, have significant implications for advancing GPCR-targeted drug discovery and the understanding of GPCR pharmacology, biology, and regulation.

Characterization of Antibodies against Receptor Activity-Modifying Protein 1 (RAMP1): A Cautionary Tale

Calcitonin gene-related peptide (CGRP) is a key component of migraine pathophysiology, yielding effective migraine therapeutics. CGRP receptors contain a core accessory protein subunit: receptor activity-modifying protein 1 (RAMP1). Understanding of RAMP1 expression is incomplete, partly due to the challenges in identifying specific and validated antibody tools. We profiled antibodies for immunodetection of RAMP1 using Western blotting, immunocytochemistry and immunohistochemistry, including using RAMP1 knockout mouse tissue. Most antibodies could detect RAMP1 in Western blotting and immunocytochemistry using transfected cells. Two antibodies (844, ab256575) could detect a RAMP1-like band in Western blots of rodent brain but not RAMP1 knockout mice. However, cross-reactivity with other proteins was evident for all antibodies. This cross-reactivity prevented clear conclusions about RAMP1 anatomical localization, as each antibody detected a distinct pattern of immunoreactivity in rodent brain. We cannot confidently attribute immunoreactivity produced by RAMP1 antibodies (including 844) to the presence of RAMP1 protein in immunohistochemical applications in brain tissue. RAMP1 expression in brain and other tissues therefore needs to be revisited using RAMP1 antibodies that have been comprehensively validated using multiple strategies to establish multiple lines of convincing evidence. As RAMP1 is important for other GPCR/ligand pairings, our results have broader significance beyond the CGRP field.

Analysis of Sheep and Goat IAPP Provides Insight into IAPP Amyloidogenicity and Cytotoxicity

Human islet amyloid polypeptide (hIAPP) plays a role in glucose regulation but forms pancreatic amyloid deposits in type 2 diabetes, and that process contributes to β-cell dysfunction. Not all species develop diabetes, and not all secrete an IAPP that is amyloidogenic in vitro under normal conditions, a perfect correlation currently exists between both. Studies of IAPPs from such organisms can provide clues about the high amyloidogenicity of hIAPP and can inform the design of soluble analogues of hIAPP. Sheep and goat IAPP are among the most divergent from hIAPP, with 13 and 11 substitutions, respectively, including an unusual Tyr to His substitution at the C-terminus. The properties of sheep and goat IAPP were examined in solution and in the presence of anionic vesicles, resulting in no observed amyloid formation, even at increased concentrations. Furthermore, both peptides are considerably less toxic to cultured β-cells than hIAPP. The effect of the Y37H replacements was studied in the context of hIAPP, as was a Y37R substitution. Buffer- and salt-dependent effects were observed. There was little impact on the time to form amyloid in phosphate-buffered saline; however, a significant deceleration was observed in Tris buffer, and amyloid formation was slower in the absence of added salt. The Y37H substitution had little impact on toxicity, while the Y37R replacement led to a 30% decrease in toxicity compared with that of hIAPP. The implications for the amyloidogenicity of hIAPP and the design of soluble analogues of the human peptide are discussed.

Differential Effects of Aromatic Residues on Amyloid Formation and Cytotoxicity of Human IAPP

Islet amyloid polypeptide (IAPP) is a 37-residue polypeptide hormone secreted by the pancreatic β-cells. IAPP plays a role in glycemic regulation, but in the pre-type-2 diabetic state, it aggregates to form an islet amyloid. The process of islet amyloid formation contributes to β-cell dysfunction and disease progression. The features of the IAPP sequence that modulate amyloid formation are still not understood. Human IAPP contains three aromatic residues, F15, F23, and Y37. F15 and Y37 are highly conserved, while F23 is more commonly a Leu or Ile in other species. The role of the aromatic residues in modulating the time course of amyloid formation and the cytotoxicity was examined using aromatic to Leu mutations. All three single and double mutants and the triple mutant were studied. F23 plays a dominant role in both amyloid formation and toxicity. An F15L mutant accelerated amyloid formation, a Y37L mutant had little effect, while an F23L replacement slowed amyloid formation by a factor of 2.6. Double mutants, which contained an F23L replacement, had a larger effect than those that did not, and there are non-additive effects between pairs of aromatic residues. F23 also plays a key role in toxicity. Single or multiple mutants that contain the F23L replacement were noticeably less toxic than the wild-type or mutants which did not include the F23L substitution. In contrast, the F15L mutant was more toxic than the wild-type one. The implications for IAPP amyloid formation and for the design of non-aggregating analogues of IAPP are discussed.

CGRP and the Calcitonin Receptor are Co-Expressed in Mouse, Rat and Human Trigeminal Ganglia Neurons

The neuropeptide calcitonin gene-related peptide (CGRP) is expressed in the trigeminal ganglia, a key site in craniofacial pain and migraine. CGRP potently activates two receptors: the CGRP receptor and the AMY₁ receptor. These receptors are heterodimers consisting of receptor activity-modifying protein 1 (RAMP1) with either the calcitonin receptor-like receptor (CLR) to form the CGRP receptor or the calcitonin receptor (CTR) to form the AMY₁ receptor. The expression of the CGRP receptor in trigeminal ganglia has been described in several studies; however, there is comparatively limited data available describing AMY₁ receptor expression and in which cellular subtypes it is found. This research aimed to determine the relative distributions of the AMY₁ receptor subunit, CTR, and CGRP in neurons or glia in rat, mouse and human trigeminal ganglia. Antibodies against CTR, CGRP and neuronal/glial cell markers were applied to trigeminal ganglia sections to investigate their distribution. CTR-like and CGRP-like immunoreactivity were observed in both discrete and overlapping populations of neurons. In rats and mice, 30–40% of trigeminal ganglia neurons displayed CTR-like immunoreactivity in their cell bodies, with approximately 78–80% of these also containing CGRP-like immunoreactivity. Although human cases were more variable, a similar overall pattern of CTR-like immunoreactivity to rodents was observed in the human trigeminal ganglia. CTR and CGRP appeared to be primarily colocalized in small to medium sized neurons, suggesting that colocalization of CTR and CGRP may occur in C-fiber neurons. CGRP-like or CTR-like immunoreactivity were not typically observed in glial cells. Western blotting confirmed that CTR was expressed in the trigeminal ganglia of all three species. These results confirm that CTR is expressed in trigeminal ganglia neurons. The identification of populations of neurons that express both CGRP and CTR suggests that CGRP could act in an autocrine manner through a CTR-based receptor, such as the AMY₁ receptor. Overall, this suggests that a trigeminal ganglia CTR-based receptor may be activated during migraine and could therefore represent a potential target to develop treatments for craniofacial pain and migraine.

Calcitonin receptor antibody validation and expression in the rodent brain

BACKGROUND AND AIM

Therapeutics that reduce calcitonin gene-related peptide activity are effective migraine treatments. However, gaps remain in our understanding of the molecular mechanisms that link calcitonin gene-related peptide to migraine. The amylin 1 receptor responds potently to calcitonin gene-related peptide, and to the related peptide amylin, but its role in relation to either peptide or to migraine is unclear. We sought to better understand the expression of the amylin 1 receptor protein subunit, the calcitonin receptor, in the rodent brain.

METHODS

We profiled three antibodies for immunodetection of calcitonin receptor, using immunocytochemistry, western blotting, and calcitonin receptor conditional knockout mouse tissue. Selected migraine-relevant rat brain regions were then examined for calcitonin receptor-like immunoreactivity.

RESULTS

All three antibodies detected calcitonin receptor protein but only one (188/10) produced robust immunostaining in rodent brain, under the conditions used. Calcitonin receptor-like immunoreactivity was apparent in the rat brainstem and midbrain including the locus coeruleus, periaqueductal grey and spinal trigeminal nucleus.

CONCLUSIONS

Anti-calcitonin receptor antibodies require comprehensive profiling to ensure confidence in the detection of calcitonin receptor. Using a validated antibody, calcitonin receptor-like immunoreactivity was detected in several brain regions relevant to migraine. Further research is needed to understand the functional consequences of calcitonin receptor expression for calcitonin gene-related peptide or amylin physiology and pathophysiology.

A structural basis for amylin receptor phenotype

Amylin receptors (AMYRs) are heterodimers of the calcitonin (CT) receptor (CTR) and one of three receptor activity-modifying proteins (RAMPs), AMY₁R, AMY₂R, and AMY₃R. Selective AMYR agonists and dual AMYR/CTR agonists are being developed as obesity treatments; however, the molecular basis for peptide binding and selectivity is unknown. We determined the structure and dynamics of active AMYRs with amylin, AMY₁R with salmon CT (sCT), AMY₂R with sCT or human CT (hCT), and CTR with amylin, sCT, or hCT. The conformation of amylin-bound complexes was similar for all AMYRs, constrained by the RAMP, and an ordered midpeptide motif that we call the bypass motif. The CT-bound AMYR complexes were distinct, overlapping the CT-bound CTR complexes. Our findings indicate that activation of AMYRs by CT-based peptides is distinct from their activation by amylin-based peptides. This has important implications for the development of AMYR therapeutics.

Vasopressin-oxytocin-type signaling is ancient and has a conserved water homeostasis role in euryhaline marine planarians

Vasopressin/oxytocin (VP/OT)-related peptides are essential for mammalian antidiuresis, sociosexual behavior, and reproduction. However, the evolutionary origin of this peptide system is still uncertain. Here, we identify orthologous genes to those for VP/OT in Platyhelminthes, intertidal planarians that have a simple bilaterian body structure but lack a coelom and body-fluid circulatory system. We report a comprehensive characterization of the neuropeptide derived from this VP/OT-type gene, identifying its functional receptor, and name it the "platytocin" system. Our experiments with these euryhaline planarians, living where environmental salinities fluctuate due to evaporation and rainfall, suggest that platytocin functions as an "antidiuretic hormone" and also organizes diverse actions including reproduction and chemosensory-associated behavior. We propose that bilaterians acquired physiological adaptations to amphibious lives by such regulation of the body fluids. This neuropeptide-secreting system clearly became indispensable for life even without the development of a vascular circulatory system or relevant synapses.

Pharmacological characterisation of mouse calcitonin and calcitonin receptor-like receptors reveals differences compared with human receptors

BACKGROUND AND PURPOSE

The calcitonin (CT) receptor family is complex, comprising two receptors (the CT receptor [CTR] and the CTR-like receptor [CLR]), three accessory proteins (RAMPs) and multiple endogenous peptides. This family contains several important drug targets, including CGRP, which is targeted by migraine therapeutics. The pharmacology of this receptor family is poorly characterised in species other than rats and humans. To facilitate understanding of translational and preclinical data, we need to know the receptor pharmacology of this family in mice.

EXPERIMENTAL APPROACH

Plasmids encoding mouse CLR/CTR and RAMPs were transiently transfected into Cos-7 cells. cAMP production was measured in response to agonists in the absence or presence of antagonists.

KEY RESULTS

We report the first synthesis and characterisation of mouse adrenomedullin, adrenomedullin 2 and βCGRP and of mouse CTR without or with mouse RAMPs. Receptors containing m-CTR had subtly different pharmacology than human receptors; they were promiscuous in their pharmacology, both with and without RAMPs. Several peptides, including mouse αCGRP and mouse adrenomedullin 2, were potent agonists of the m-CTR:m-RAMP3 complex. Pharmacological profiles of receptors comprising m-CLR:m-RAMPs were generally similar to those of their human counterparts, albeit with reduced specificity.

CONCLUSION AND IMPLICATIONS

Mouse receptor pharmacology differed from that in humans, with mouse receptors displaying reduced discrimination between ligands. This creates challenges for interpreting which receptor may underlie an effect in preclinical models and thus translation of findings from mice to humans. It also highlights the need for new ligands to differentiate between these complexes.

LINKED ARTICLES

This article is part of a themed issue on Advances in Migraine and Headache Therapy (BJP 75th Anniversary).. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.3/issuetoc.

Hypervigilance, Allostatic Load, and Migraine Prevention: Antibodies to CGRP or Receptor

Migraine involves brain hypersensitivity with episodic dysfunction triggered by behavioral or physiological stressors. During an acute migraine attack the trigeminal nerve is activated (peripheral sensitization). This leads to central sensitization with activation of the central pathways including the trigeminal nucleus caudalis, the trigemino-thalamic tract, and the thalamus. In episodic migraine the sensitization process ends with the individual act, but with chronic migraine central sensitization may continue interictally. Increased allostatic load, the consequence of chronic, repeated exposure to stressors, leads to central sensitization, lowering the threshold for future neuronal activation (hypervigilance). Ostensibly innocuous stressors are then sufficient to trigger an attack. Medications that reduce sensitization may help patients who are hypervigilant and help to balance allostatic load. Acute treatments and drugs for migraine prevention have traditionally been used to reduce attack duration and frequency. However, since many patients do not fully respond, an unmet treatment need remains. Calcitonin gene-related peptide (CGRP) is a vasoactive neuropeptide involved in nociception and in the sensitization of peripheral and central neurons of the trigeminovascular system, which is implicated in migraine pathophysiology. Elevated CGRP levels are associated with dysregulated signaling in the trigeminovascular system, leading to maladaptive responses to behavioral or physiological stressors. CGRP may, therefore, play a key role in the underlying pathophysiology of migraine. Increased understanding of the role of CGRP in migraine led to the development of small-molecule antagonists (gepants) and monoclonal antibodies (mAbs) that target either CGRP or the receptor (CGRP-R) to restore homeostasis, reducing the frequency, duration, and severity of attacks. In clinical trials, US Food and Drug Administration-approved anti-CGRP-R/CGRP mAbs were well tolerated and effective as preventive migraine treatments. Here, we explore the role of CGRP in migraine pathophysiology and the use of gepants or mAbs to suppress CGRP-R signaling via inhibition of the CGRP ligand or receptor.

β-Cell Death in Diabetes: Past Discoveries, Present Understanding, and Potential Future Advances

β-cell death is regarded as a major event driving loss of insulin secretion and hyperglycemia in both type 1 and type 2 diabetes mellitus. In this review, we explore past, present, and potential future advances in our understanding of the mechanisms that promote β-cell death in diabetes, with a focus on the primary literature. We first review discoveries of insulin insufficiency, β-cell loss, and β-cell death in human diabetes. We discuss findings in humans and mouse models of diabetes related to autoimmune-associated β-cell loss and the roles of autoreactive T cells, B cells, and the β cell itself in this process. We review discoveries of the molecular mechanisms that underlie β-cell death-inducing stimuli, including proinflammatory cytokines, islet amyloid formation, ER stress, oxidative stress, glucotoxicity, and lipotoxicity. Finally, we explore recent perspectives on β-cell death in diabetes, including: (1) the role of the β cell in its own demise, (2) methods and terminology for identifying diverse mechanisms of β-cell death, and (3) whether non-canonical forms of β-cell death, such as regulated necrosis, contribute to islet inflammation and β-cell loss in diabetes. We believe new perspectives on the mechanisms of β-cell death in diabetes will provide a better understanding of this pathological process and may lead to new therapeutic strategies to protect β cells in the setting of diabetes.

Characterisation of agonist signalling profiles and agonist-dependent antagonism at PACAP-responsive receptors: Implications for drug discovery

BACKGROUND AND PURPOSE

The pituitary adenylate cyclase-activating peptide (PACAP) family is of clinical interest for the treatment of migraine. These peptides activate three different PACAP-responsive class B G protein-coupled receptors: the PAC₁ , VPAC₁ and VPAC₂ receptors. The PAC₁ receptor may be alternatively spliced, generating variants that can differ in their pharmacological or signalling profiles. To inform drug discovery efforts targeting migraine, we need to better understand how the different PACAP-responsive receptors signal and how effectively these responses can be blocked by antagonists.

EXPERIMENTAL APPROACH

The signalling profiles of the human PAC_1n , PAC_1s , VPAC₁ and VPAC₂ receptors were examined in transfected Cos7 cells for cAMP, IP₁ , pAkt, pERK and pCREB. Biased signalling was then quantified. The ability of antagonists to block PACAP-38, PACAP-27 or VIP stimulated cAMP accumulation at PACAP-responsive receptors was also determined.

KEY RESULTS

PACAP-responsive receptors exhibited varied pharmacological profiles but activated signalling in a similar manner. The PAC_1n and PAC_1s receptors displayed distinct pharmacology. At the PAC_1s receptor, VIP and PHM were more potent than at the PAC_1n receptor. PACAP-responsive receptors displayed agonist-dependent antagonism where PACAP-38 was less effectively antagonised compared to PACAP-27 and VIP.

CONCLUSIONS AND IMPLICATIONS

The distinct pharmacological profile displayed by the PAC_1s receptor suggests that it can act as a dual receptor for VIP and PACAP. Furthermore, the effectiveness of blocking a signalling pathway can be influenced by which endogenous PACAP family agonist is present. These effects have potential implications for the development and effectiveness of drugs targeting the PACAP system.

Development of High Affinity Calcitonin Analog Fragments Targeting Extracellular Domains of Calcitonin Family Receptors

The calcitonin and amylin receptors (CTR and AMY receptors) are the drug targets for osteoporosis and diabetes treatment, respectively. Salmon calcitonin (sCT) and pramlintide were developed as peptide drugs that activate these receptors. However, next-generation drugs with improved receptor binding profiles are desirable for more effective pharmacotherapy. The extracellular domain (ECD) of CTR was reported as the critical binding site for the C-terminal half of sCT. For the screening of high-affinity sCT analog fragments, purified CTR ECD was used for fluorescence polarization/anisotropy peptide binding assay. When three mutations (N26D, S29P, and P32HYP) were introduced to the sCT(22–32) fragment, sCT(22–32) affinity for the CTR ECD was increased by 21-fold. CTR was reported to form a complex with receptor activity-modifying protein (RAMP), and the CTR:RAMP complexes function as amylin receptors with increased binding for the peptide hormone amylin. All three types of functional AMY receptor ECDs were prepared and tested for the binding of the mutated sCT(22–32). Interestingly, the mutated sCT(22–32) also retained its high affinity for all three types of the AMY receptor ECDs. In summary, the mutated sCT(22–32) showing high affinity for CTR and AMY receptor ECDs could be considered for developing the next-generation peptide agonists.

Exploring Ligand Binding to Calcitonin Gene-Related Peptide Receptors

Class B1 G protein-coupled receptors (GPCRs) are important targets for many diseases, including cancer, diabetes, and heart disease. All the approved drugs for this receptor family are peptides that mimic the endogenous activating hormones. An understanding of how agonists bind and activate class B1 GPCRs is fundamental for the development of therapeutic small molecules. We combined supervised molecular dynamics (SuMD) and classic molecular dynamics (cMD) simulations to study the binding of the calcitonin gene-related peptide (CGRP) to the CGRP receptor (CGRPR). We also evaluated the association and dissociation of the antagonist telcagepant from the extracellular domain (ECD) of CGRPR and the water network perturbation upon binding. This study, which represents the first example of dynamic docking of a class B1 GPCR peptide, delivers insights on several aspects of ligand binding to CGRPR, expanding understanding of the role of the ECD and the receptor-activity modifying protein 1 (RAMP1) on agonist selectivity.

Development of Cagrilintide, a Long-Acting Amylin Analogue

A hallmark of the pancreatic hormone amylin is its high propensity toward the formation of amyloid fibrils, which makes it a challenging drug design effort. The amylin analogue pramlintide is commercially available for diabetes treatment as an adjunct to insulin therapy but requires three daily injections due to its short half-life. We report here the development of the stable, lipidated long-acting amylin analogue cagrilintide (23) and some of the structure-activity efforts that led to the selection of this analogue for clinical development with obesity as an indication. Cagrilintide is currently in clinical trial and has induced significant weight loss when dosed alone or in combination with the GLP-1 analogue semaglutide.

CGRP receptor antagonists for migraine. Are they also AMY1 receptor antagonists?

The development of several drugs that target the calcitonin gene-related peptide (CGRP) system has been a major breakthrough in the pharmacological management of migraine. These are divided into two major classes, antibodies which bind to the CGRP peptide, preventing it from activating CGRP receptors and receptor antagonists. Within the receptor antagonist class, there are two mechanisms of action, small molecule receptor antagonists and an antibody antagonist. This mini-review considers the pharmacology of these receptor targeted antagonist drugs at the CGRP receptor and closely related AMY₁ receptor, at which CGRP may also act. The antagonists are most potent at the CGRP receptor but can also show antagonism of the AMY₁ receptor. However, important data are missing and selectivity parameters cannot be provided for all antagonists. The clinical implications of AMY₁ receptor antagonism are unknown, but we urge consideration of this receptor as a potential contributing factor to CGRP and antagonist drug actions.

Amylin antibodies frequently display cross-reactivity with CGRP: characterization of eight amylin antibodies

Amylin is a 37-amino acid endocrine hormone secreted from the pancreas in response to nutrient intake, acting centrally to promote meal-ending satiation. With many studies linking amylin action to the nervous system, determining the distribution or expression of amylin in the nervous system is critical. However, amylin shares sequence identity and structural homology to the related neuropeptide calcitonin gene-related peptide (CGRP). This creates challenges in identifying selective amylin antibodies that do not cross-react with CGRP, especially in neural tissues, where CGRP is densely packed into secretory vesicles. Here, we characterized eight amylin antibodies to determine their ability to detect amylin and cross-react with rat or human αCGRP, using immunoblots and preabsorption controls in rat pancreas. We observed that amylin antibodies frequently cross-reacted with αCGRP and are therefore not suitable for use in tissues that highly express CGRP. Earlier work using these antibodies should be revisited in light of our findings.

Optimized experimental pre-treatment strategy for temporary inhibition of islet amyloid polypeptide aggregation

Islet amyloid polypeptide (IAPP) is a neuroendocrine hormone from pancreatic β-cells. Misfolded, aggregated IAPP is believed to be toxic to islet cells and amyloid deposits in the pancreas are pathological hallmarks of type 2 diabetes. Rapid fibrillization of this peptide makes it difficult to study in its soluble form, impeding a better understanding of its role. In this study, a variety of popular pretreatment methods were tested for their ability to delay aggregation of IAPP, including solutions of hexafluoroisopropanol, sodium hydroxide, hydrochloric acid, phosphate buffered saline, ammonium hydroxide, as well as tris buffer at different pH and containing either calcium (II), zinc (II), or iron (II). Aggregation was assessed using the thioflavin T fluorescence assay as well as by transmission electron microscopy. Tris buffer at pH 8.1 containing Zn(II) was found to have the best balance of temporary inhibition of aggregation and biological relevance.

Multisite Model of Allosterism for the Adenosine A1 Receptor

Despite being a target for about one-third of approved drugs, G protein-coupled receptors (GPCRs) still represent a tremendous reservoir for therapeutic strategies against diseases. For example, several cardiovascular and central nervous system conditions could benefit from clinical agents that activate the adenosine 1 receptor (A₁R); however, the pursuit of A₁R agonists for clinical use is usually impeded by both on- and off-target side effects. One of the possible strategies to overcome this issue is the development of positive allosteric modulators (PAMs) capable of selectively enhancing the effect of a specific receptor subtype and triggering functional selectivity (a phenomenon also referred to as bias). Intriguingly, besides enforcing the effect of agonists upon binding to an allosteric site, most of the A₁R PAMs display intrinsic partial agonism and orthosteric competition with antagonists. To rationalize this behavior, we simulated the binding of the prototypical PAMs PD81723 and VCP171, the full-agonist NECA, the antagonist 13B, and the bitopic agonist VCP746. We propose that a single PAM can bind several A₁R sites rather than a unique allosteric pocket, reconciling the structure-activity relationship and the mutagenesis results.

Asn-linked N-acetylglucosamine of the amylin receptor 2 extracellular domain enhances peptide ligand affinity

The calcitonin receptor (CTR) has a large extracellular domain (ECD) with multiple N‐glycosylation sites. An asparagine (Asn)‐linked N‐acetylglucosamine (GlcNAc) of CTR ECD N130 was previously reported to enhance peptide hormone binding affinity for CTR ECD. CTR forms a complex with an accessory protein RAMP, and the RAMP:CTR complex gains affinity for peptide hormone amylin as the amylin receptor (AMY). Although N‐glycosylation of AMY ECD was reported to enhance peptide hormone affinity, it remains underexplored which N‐glycosites of AMY ECD are responsible for peptide affinity enhancement and it is unclear whether an Asn‐linked GlcNAc of the N‐glycosites plays a critical role. Here, I investigated the role of the Asn‐linked GlcNAc of CTR N130 in the affinity of an antagonistic amylin analog (AC413) for AMY₂ ECD (the RAMP2 ECD:CTR ECD complex). I used Endo H‐treated CTR ECD in which N‐glycans were trimmed to an Asn‐linked GlcNAc on each of the N‐glycosites. I incubated Endo H‐treated CTR ECD with excess of glycan‐free RAMP2 ECD to produce the RAMP2 ECD:CTR ECD complex. Using this coincubation system, I found that the RAMP2 ECD complex with Endo H‐treated CTR ECD with N130D mutation showed a fourfold decrease in AC413 affinity compared with the RAMP2 ECD complex with Endo H‐treated CTR ECD WT. In contrast, RAMP2 ECD N‐glycosylation did not affect peptide binding affinity. These results indicate that the Asn‐linked GlcNAc of CTR N130 is an important peptide affinity enhancer for AMY₂ ECD and reveals a significant role of the Asn‐linked GlcNAc in AMY₂ function.

Low concentration IL-1β promotes islet amyloid formation by increasing hIAPP release from humanised mouse islets in vitro

AIMS/HYPOTHESIS

Aggregation of the beta cell secretory product human islet amyloid polypeptide (hIAPP) results in islet amyloid deposition, a pathological feature of type 2 diabetes. Amyloid formation is associated with increased levels of islet IL-1β as well as beta cell dysfunction and death, but the mechanisms that promote amyloid deposition in situ remain unclear. We hypothesised that physiologically relevant concentrations of IL-1β stimulate beta cell islet amyloid polypeptide (IAPP) release and promote amyloid formation.

METHODS

We used a humanised mouse model of endogenous beta cell hIAPP expression to examine whether low (pg/ml) concentrations of IL-1β promote islet amyloid formation in vitro. Amyloid-forming islets were cultured for 48 h in the presence or absence of IL-1β with or without an IL-1β neutralising antibody. Islet morphology was assessed by immunohistochemistry and islet mRNA expression, hormone content and release were also quantified. Cell-free thioflavin T assays were used to monitor hIAPP aggregation kinetics in the presence and absence of IL-1β.

RESULTS

Treatment with a low concentration of IL-1β (4 pg/ml) for 48 h increased islet amyloid prevalence (93.52 ± 3.89% vs 43.83 ± 9.67% amyloid-containing islets) and amyloid severity (4.45 ± 0.82% vs 2.16 ± 0.50% amyloid area/islet area) in hIAPP-expressing mouse islets in vitro. This effect of IL-1β was reduced when hIAPP-expressing islets were co-treated with an IL-1β neutralising antibody. Cell-free hIAPP aggregation assays showed no effect of IL-1β on hIAPP aggregation in vitro. Low concentration IL-1β did not increase markers of the unfolded protein response (Atf4, Ddit3) or alter proIAPP processing enzyme gene expression (Pcsk1, Pcsk2, Cpe) in hIAPP-expressing islets. However, release of IAPP and insulin were increased over 48 h in IL-1β-treated vs control islets (IAPP 0.409 ± 0.082 vs 0.165 ± 0.051 pmol/5 islets; insulin 87.5 ± 8.81 vs 48.3 ± 17.3 pmol/5 islets), and this effect was blocked by co-treatment with IL-1β neutralising antibody.

CONCLUSIONS/INTERPRETATION

Under amyloidogenic conditions, physiologically relevant levels of IL-1β promote islet amyloid formation by increasing beta cell release of IAPP. Neutralisation of this effect of IL-1β may decrease the deleterious effects of islet amyloid formation on beta cell function and survival.

Molecular interaction of an antagonistic amylin analog with the extracellular domain of receptor activity-modifying protein 2 assessed by fluorescence polarization

The peptide hormone amylin receptor is a complex of the calcitonin receptor (CTR) and an accessory protein called receptor activity-modifying proteins (RAMPs). The soluble extracellular domain (ECD) of CTR is an important binding site of peptide hormone calcitonin. RAMPs also have an ECD and the association of CTR ECD with RAMP ECD enhances the affinity of peptide hormone amylin. However, the mechanism of how RAMP ECD association enhances amylin affinity remains elusive. Here, we report evidence supporting direct molecular interaction between an antagonistic amylin analog AC413 and RAMP2 ECD. We measured FITC-labeled peptide affinity for purified receptor ECD using fluorescence polarization (FP). We first found that RAMP2 ECD addition to maltose-binding protein (MBP)-tagged CTR ECD and an engineered MBP-tagged RAMP2 ECD-CTR ECD fusion protein (MBP-RAMP2-CTR ECD fusion) enhanced AC413 affinity. This suggests that these recombinant ECD systems represent functional amylin receptors. Interestingly, AC413 C-terminal residue Tyr25 (Y25) to Pro mutation eliminated its selective affinity for the MBP-RAMP2-CTR ECD fusion suggesting the critical role of the AC413 C-terminal residue in amylin receptor selectivity. Our structural model of the RAMP2 ECD:CTR ECD complex predicted molecular interaction of AC413 C-terminal residue Y25 with RAMP2 Glu101 (E101). Our FP peptide-binding assay showed that the RAMP2 E101A mutation of MBP-RAMP2-CTR ECD fusion decreased AC413 affinity by 7-fold, while the affinity of AC413 with the Y25P mutation was minimally changed. Consistently, AC413 binding affinity for the MBP-free RAMP2-CTR ECD fusion protein was also markedly decreased by the RAMP2 E101A mutation, while the affinity of AC413 with the Y25P mutation was moderately decreased. Together, our results support the molecular interaction between the AC413 C-terminal residue Y25 and RAMP2 E101 expanding our understanding of how the accessory protein RAMP2 enhances affinity of peptide hormone amylin for its receptor.

Antagonism of CGRP Signaling by Rimegepant at Two Receptors

The “gepants” are a class of calcitonin gene-related peptide (CGRP) receptor antagonist molecules that have been developed for the prevention and treatment of migraine. Rimegepant is reported to act at the CGRP receptor, has good oral bioavailability, and has had positive clinical trial results. However, there is very little data available describing its receptor pharmacology. Importantly, rimegepant activity at the AMY₁ receptor, a second potent CGRP receptor that is known to be expressed in the trigeminovascular system, has not been reported. The ability of rimegepant to antagonize activation of human CGRP, AMY₁, and related adrenomedullin receptors was determined in transfected in Cos7 cells. Rimegepant was an effective antagonist at both the CGRP and AMY₁ receptor. The antagonism of both CGRP and AMY₁ receptors may have implications for our understanding of the mechanism of action of rimegepant in the treatment of migraine.

Analysis of Amylin Consensus Sequences Suggests That Human Amylin Is Not Optimized to Minimize Amyloid Formation and Provides Clues to Factors That Modulate Amyloidogenicity

The neuropancreatic polypeptide hormone amylin forms pancreatic islet amyloid in type-2 diabetes. Islet amyloid formation contributes to β-cell death in the disease and to the failure of islet transplants, but the features which influence amylin amyloidogenicity are not understood. We constructed an amino acid sequence alignment of 202 sequences of amylin and used the alignment to design consensus sequences of vertebrate amylins, mammalian amylins, and primate amylins. Amylin is highly conserved, but there are differences between human amylin and each consensus sequence, ranging from one to six substitutions. Biophysical analysis shows that all of the consensus sequences form amyloid but do so more slowly than human amylin in vitro. The rate of amyloid formation by the primate consensus sequence is 3- to 4-fold slower than human amylin; the mammalian consensus sequence is approximately 20- to 25-fold slower, and the vertebrate consensus sequence is approximately 6-fold slower. All of the consensus sequences are moderately less toxic than human amylin toward a cultured β-cell line, with the vertebrate consensus sequence displaying the largest reduction in toxicity of 3- to 4-fold. All of the consensus sequences activate a human amylin receptor and exhibit only modest reductions in activity, ranging from 3- to 4-fold as judged by a cAMP production assay. The analysis argues that there is no strong selective evolutionary pressure to avoid the formation of islet amyloid and provides information relevant to the design of less amyloidogenic amylin variants.

Molecular Mechanisms of Class B GPCR Activation: Insights from Adrenomedullin Receptors

Adrenomedullin (AM) is a 52 amino acid peptide that plays a regulatory role in the vasculature. Receptors for AM comprise the class B G protein-coupled receptor, the calcitonin-like receptor (CLR), in complex with one of three receptor activity-modifying proteins (RAMPs). The C-terminus of AM is involved in binding to the extracellular domain of the receptor, while the N-terminus is proposed to interact with the juxtamembranous portion of the receptor to activate signaling. There is currently limited information on the molecular determinants involved in AM signaling, thus we set out to define the importance of the AM N-terminus through five signaling pathways (cAMP production, ERK phosphorylation, CREB phosphorylation, Akt phosphorylation, and IP1 production). We characterized the three CLR:RAMP complexes through the five pathways, finding that each had a distinct repertoire of intracellular signaling pathways that it is able to regulate. We then performed an alanine scan of AM from residues 15-31 and found that most residues could be substituted with only small effects on signaling, and that most substitutions affected signaling through all receptors and pathways in a similar manner. We identify F18, T20, L26, and I30 as being critical for AM function, while also identifying an analogue (AM15-52 G19A) which has unique signaling properties relative to the unmodified AM. We interpret our findings in the context of new structural information, highlighting the complementary nature of structural biology and functional assays.

Identification of Small-Molecule Positive Modulators of Calcitonin-like Receptor-Based Receptors

Class B G protein-coupled receptors are highly therapeutically relevant but challenges remain in identifying suitable small-molecule drugs. The calcitonin-like receptor (CLR) in particular is linked to conditions such as migraine, cardiovascular disease, and inflammatory bowel disease. The CLR cannot act as a cell-surface receptor alone but rather must couple to one of three receptor activity-modifying proteins (RAMPs), forming heterodimeric receptors for the peptides adrenomedullin and calcitonin gene-related peptide. These peptides have extended binding sites across their receptors. This is one reason why there are few small-molecule ligands that can modulate these receptors. Here we describe small molecules that are able to positively modulate the signaling of the CLR with all three RAMPs but are not active at the related calcitonin receptor. These compounds were selected from a β-arrestin recruitment screen, coupled with rounds of medicinal chemistry to improve their activity. Translational potential is shown as the compounds can positively modulate cAMP signaling in a vascular cell line model. Binding experiments do not support an extracellular domain binding site; however, molecular modeling reveals potential allosteric binding sites in multiple receptor regions. These are the first small-molecule positive modulators described for the CLR:RAMP complexes.

Distinct Patterns of Internalization of Different Calcitonin Gene-Related Peptide Receptors

Calcitonin gene-related peptide (CGRP) is a neuropeptide that is involved in the transmission of pain. Drugs targeting CGRP or a CGRP receptor are efficacious in the treatment of migraine. The canonical CGRP receptor is a complex of a G protein-coupled receptor, the calcitonin-like receptor (CLR), with an accessory protein, receptor activity-modifying protein 1 (RAMP1). A second receptor, the AMY₁ receptor, a complex of the calcitonin receptor with RAMP1, is a dual high-affinity receptor for CGRP and amylin. Receptor regulatory processes, such as internalization, are crucial for controlling peptide and drug responsiveness. Given the importance of CGRP receptor activity in migraine we compared the internalization profiles of both receptors for CGRP using novel fluorescent probes and a combination of live cell imaging, fixed cell imaging, and ELISA. This revealed stark differences in the regulation of each receptor with the AMY₁ receptor unexpectedly showing little internalization.

RAMPs as allosteric modulators of the calcitonin and calcitonin-like class B G protein-coupled receptors

Receptor activity-modifying proteins (RAMPs) are a family of three single span transmembrane proteins in humans that interact with many GPCRs and can modulate their function. RAMPs were discovered as key components of the calcitonin gene-related peptide and adrenomedullin receptors. They are required for transport of this class B GPCR, calcitonin receptor-like receptor (CLR), to the cell surface and determine its peptide ligand binding preferences. Soon thereafter RAMPs were shown to modulate the binding of calcitonin and amylin peptides to the related calcitonin receptor (CTR) and in the years since an ever-growing number of RAMP-interacting receptors have been identified including most if not all of the 15 class B GPCRs and several GPCRs from other families. Studies of CLR, CTR, and a handful of other GPCRs revealed that RAMPs are able to modulate various aspects of receptor function including trafficking, ligand binding, and signaling. Here, we review RAMP interactions and functions with an emphasis on class B receptors for which our understanding is most advanced. A key focus is to discuss recent evidence that RAMPs serve as endogenous allosteric modulators of CLR and CTR. We discuss structural studies of RAMP-CLR complexes and CTR and biochemical and pharmacological studies that collectively have significantly expanded our understanding of the mechanistic basis for RAMP modulation of these class B GPCRs. Last, we consider the implications of these findings for drug development targeting RAMP-CLR/CTR complexes.

Pharmacological Characterization and Investigation of N-Terminal Loop Amino Acids of Adrenomedullin 2 That Are Important for Receptor Activation

Adrenomedullin 2 (AM2) is a peptide hormone with potent effects in the cardiovascular system. The N-terminal disulfide loop of AM2 is thought to be important for interacting with its receptors to initiate a signaling response. However, the relative contribution of each amino acid within this region is currently unknown. Thus, the region was investigated using an alanine scanning approach. Two AM2 peptides (AM2-47 and AM2-40) were directly compared at the CGRP, AM₁, and AM₂ receptors in transfected Cos7 cells and found to have equivalent activity. Analogues of AM2-40 were then synthesized, substituting each individual amino acid within the disulfide loop with alanine. The ability of these analogues to stimulate a cAMP response was evaluated at the CGRP, AM₁, and AM₂ receptors. AM2-40 L12A and T14A were less able to elicit cAMP responses through all tested receptors. In contrast, AM2-40 G13A was slightly more potent than the unmodified peptide at all tested receptors. Thus, it appears that residues within the disulfide loop region play differential roles in the ability of AM2 to stimulate cAMP production. The data provide the first structure-function investigation of AM2 agonism.

Peeking at G-protein-coupled receptors through the molecular dynamics keyhole

Molecular dynamics is a state of the art computational tool for the investigation of biophysics phenomenon at a molecular scale, as it enables the modeling of dynamic processes, such as conformational motions, molecular solvation and ligand binding. The recent advances in structural biology have led to a bloom in published G-protein-coupled receptor structures, representing a solid and valuable resource for molecular dynamics studies. During the last decade, indeed, a plethora of physiological and pharmacological facets of this membrane protein superfamily have been addressed by means of molecular dynamics simulations, including the activation mechanism, allosterism and, very recently, biased signaling. Here, we try to recapitulate some of the main contributions that molecular dynamics has recently produced in the field.

Molecular Determinants for Ligand Selectivity of the Cell-Free Synthesized Human Endothelin B Receptor

Extracellular domains of G-protein-coupled receptors act as initial molecular selectivity filters for subtype specific ligands and drugs. Chimeras of the human endothelin-B receptor containing structural units from the extracellular domains of the endothelin-A receptor were analyzed after their co-translational insertion into preformed nanodiscs. A short β-strand and a linker region in the second extracellular loop as well as parts of the extracellular N-terminal domain were identified as molecular discrimination sites for the endothelin-B receptor-selective agonists IRL1620, sarafotoxin 6c, 4Ala-ET-1 and ET-3, but not for the non-selective agonist ET-1 recognized by both endothelin receptors. A proposed second disulfide bridge in the endothelin-B receptor tethering the N-terminal domain with the third extracellular loop was not essential for ET-1 recognition and binding, but increased the receptor thermostability. We further demonstrate an experimental approach with cell-free synthesized engineered agonists to analyze the differential discrimination of peptide ligand topologies by the two endothelin receptors. The study is based on the engineering and cell-free insertion of G-protein-coupled receptors into defined membranes and may become interesting also for other targets as an alternative platform to reveal molecular details of ligand selectivity and ligand binding mechanisms.

Amyloidogenicity, Cytotoxicity, and Receptor Activity of Bovine Amylin: Implications for Xenobiotic Transplantation and the Design of Nontoxic Amylin Variants

Islet amyloid formation contributes to β-cell death and dysfunction in type-2 diabetes and to the failure of islet transplants. Amylin (islet amyloid polypeptide, IAPP), a normally soluble 37 residue polypeptide hormone produced in the pancreatic β-cells, is responsible for amyloid formation in type-2 diabetes and is deficient in type-1 diabetes. Amylin normally plays an adaptive role in metabolism, and the development of nontoxic, non-amyloidogenic, bioactive variants of human amylin are of interest for use as adjuncts to insulin therapy. Naturally occurring non-amyloidogenic variants are of interest for xenobiotic transplantation and because they can provide clues toward understanding the amyloidogenicity of human amylin. The sequence of amylin is well-conserved among species, but sequence differences strongly correlate with in vitro amyloidogenicity and with islet amyloid formation in vivo. Bovine amylin differs from the human peptide at 10 positions and is one of the most divergent among known amylin sequences. We show that bovine amylin oligomerizes but is not toxic to cultured β-cells and that it is considerably less amyloidogenic than the human polypeptide and is only a low-potency agonist at human amylin-responsive receptors. The bovine sequence contains several nonconservative substitutions relative to human amylin, including His to Pro, Ser to Pro, and Asn to Lys replacements. The effect of these substitutions is analyzed in the context of wild-type human amylin; the results provide insight into their role in receptor activation, the mode of assembly of human amylin, and the design of soluble amylin analogues.

Cryo-EM structure of the active, Gs-protein complexed, human CGRP receptor

Calcitonin gene-related peptide (CGRP) is a widely expressed neuropeptide that has a major role in sensory neurotransmission. The CGRP receptor is a heterodimer of the calcitonin receptor-like receptor (CLR) class B G-protein-coupled receptor and a type 1 transmembrane domain protein, receptor activity-modifying protein 1 (RAMP1). Here we report the structure of the human CGRP receptor in complex with CGRP and the Gs-protein heterotrimer at 3.3 Å global resolution, determined by Volta phase-plate cryo-electron microscopy. The receptor activity-modifying protein transmembrane domain sits at the interface between transmembrane domains 3, 4 and 5 of CLR, and stabilizes CLR extracellular loop 2. RAMP1 makes only limited direct contact with CGRP, consistent with its function in allosteric modulation of CLR. Molecular dynamics simulations indicate that RAMP1 provides stability to the receptor complex, particularly in the positioning of the extracellular domain of CLR. This work provides insights into the control of G-protein-coupled receptor function.