Calcitonin Receptor N-Glycosylation Enhances Peptide Hormone Affinity by Controlling Receptor Dynamics

Osteoporosis treatment: current drugs and future developments

Osteoporosis is a common systemic metabolic disease characterized by a decrease in bone density and bone mass, destruction of bone tissue microstructure, and increased bone fragility leading to fracture susceptibility. Pharmacological treatment of osteoporosis is the focus of current research, and anti-osteoporosis drugs usually play a role in inhibiting bone resorption, promoting bone formation, and having a dual role. However, most of the drugs have the disadvantages of single target and high toxic and side effects. There are many types of traditional Chinese medicines (TCM), from a wide range of sources and mostly plants. Herbal plants have unique advantages in regulating the relationship between osteoporosis and the immune system, acupuncture therapy has significant therapeutic effects in combination with medicine for osteoporosis. The target cells and specific molecular mechanisms of TCM in preventing and treating osteoporosis have not been fully elucidated. At present, there is a lack of comprehensive understanding of the pathological mechanism of the disease. Therefore, a better understanding of the pathological signaling pathways and key molecules involved in the pathogenesis of osteoporosis is crucial for the design of therapeutic targets and drug development. In this paper, we review the development and current status of anti-osteoporosis drugs currently in clinical application and under development to provide relevant basis and reference for drug prevention and treatment of osteoporosis, with the aim of promoting pharmacological research and new drug development.

Conformational dynamics underlying atypical chemokine receptor 3 activation

Atypical Chemokine Receptor 3 (ACKR3) belongs to the G protein-coupled receptor family but it does not signal through G proteins. The structural properties that govern the functional selectivity and the conformational dynamics of ACKR3 activation are poorly understood. Here, we combined hydrogen/deuterium exchange mass spectrometry, site-directed mutagenesis, and molecular dynamics simulations to examine the binding mode and mechanism of action of ACKR3 ligands of different efficacies. Our results show that activation or inhibition of ACKR3 is governed by intracellular conformational changes of helix 6, intracellular loop 2, and helix 7, while the DRY motif becomes protected during both processes. Moreover, we identified the binding sites and the allosteric modulation of ACKR3 upon β-arrestin 1 binding. In summary, this study highlights the structure-function relationship of small ligands, the binding mode of β-arrestin 1, the activation dynamics, and the atypical dynamic features in ACKR3 that may contribute to its inability to activate G proteins.

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

Classifying tetraspanins: A universal system for numbering residues and a proposal for naming structural motifs and subfamilies

All tetraspanins have four transmembrane domains (TMs). The large extracellular loop (LEL) that connects the third and fourth TMs contains multiple secondary structures together with the family's signature Cys-Cys-Gly motif. These intriguing membrane proteins are involved in diverse and incompletely understood cellular processes including cell adhesion, tissue differentiation, immune cell maturation and host-parasite interactions. Here we present a classification system that accurately describes the position of each amino acid within its primary sequence based on both sequence and topological conservation of the TMs and LEL. This builds on the numbering systems that have been used in the G protein-coupled receptor (GPCR) field for nearly three decades and which have aided the understanding of GPCR structure/activity relationships and ligand interactions. The high-resolution structures of the tetraspanins CD81, CD9, CD53 and Tspan15 were used to validate the structural relevance of our new tetraspanin classification system. Modelling of all tetraspanin LELs highlighted flexibility in LEL disulfide bonding across the family and suggests that the structural arrangement of tetraspanin LELs is more complex than previously thought. We therefore propose a new subfamily naming system that addresses this added complexity and facilitates the systematic classification of human tetraspanins, shedding light on all structural motifs within the family. We anticipate that our universal tetraspanin classification system will enable progress in defining how sequence and structure inform function.

Deficiency of N-linked glycosylation impairs immune function of B7-H6

B7-H6 is a novel immune checkpoint molecule that triggers NK cell cytotoxicity, but the role of N-glycosylation in B7-H6 is poorly understood. We here identified the existence of N-glycosylation of B7-H6 in different cell lines and exogenous expression cells by PNGase F digestion and tunicamycin blockage. Subsequently, we demonstrated that B7-H6 contains 6 functional N-linked glycosylation sites by single site mutation and electrophoresis. Phylogenetical and structural analysis revealed that N43 and N208 glycan are conserved in jawed vertebrates and may thus contribute more to the biological functions. We further demonstrated that N43 and N208 glycosylation are essential for B7-H6 to trigger NK cell activation. Mechanistically, we found that N43 and N208 glycan contributed to the stability and membrane expression of B7-H6 protein. Lack of N208 glycosylation led to membrane B7-H6 shedding, while N43 mutation resulted in impaired B7-H6/NKp30 binding affinity. Together, our findings highlight the significance of N-linked glycosylation in B7-H6 biological functions and suggest potential targets for modulating NK cell-mediated immunity.

Glycosylation and Crowded Membrane Effects on Influenza Neuraminidase Stability and Dynamics

All protein simulations are conducted with varying degrees of simplification, oftentimes with unknown ramifications about how these simplifications affect the interpretability of the results. In this work, we investigated how protein glycosylation and lateral crowding effects modulate an array of properties characterizing the stability and dynamics of influenza neuraminidase. We constructed three systems: (1) glycosylated neuraminidase in a whole virion (i.e., crowded membrane) environment, (2) glycosylated neuraminidase in its own lipid bilayer, and (3) unglycosylated neuraminidase in its own lipid bilayer. We saw that glycans tend to stabilize the protein structure and reduce its conformational flexibility while restricting the solvent movement. Conversely, a crowded membrane environment encouraged exploration of the free energy landscape and a large-scale conformational change, while making the protein structure more compact. Understanding these effects informs what factors one must consider in attempting to recapture the desired level of physical accuracy.

Calcitonin/PAC1 receptor splice variants: a blind spot in migraine research

The neuropeptides calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) and their receptors are linked to migraine neurobiology. Recent antimigraine therapeutics targeting the signaling of these neuropeptides are effective; however, some patients respond suboptimally, indicating an incomplete understanding of migraine pathophysiology. The CGRP- and PACAP-responsive receptors can be differentially spliced. It is known that receptor splice variants can have different pathophysiological effects in other receptor-mediated pain pathways. Despite considerable knowledge on the structural and pharmacological differences of the CGRP- and PACAP-responsive receptor splice variants and their expression in migraine-relevant tissues, their role in migraine is rarely considered. Here we shine a spotlight on the calcitonin and PACAP (PAC1) receptor splice variants and examine what implications they may have for drug activity and design.

Glycosylation and Crowded Membrane Effects on Influenza Neuraminidase Stability and Dynamics

All protein simulations are conducted with varying degrees of simplifications, oftentimes with unknown ramifications on how these simplifications affect the interpretability of the results. In this work we investigated how protein glycosylation and lateral crowding effects modulate an array of properties characterizing the stability and dynamics of influenza neuraminidase. We constructed three systems: 1) Glycosylated neuraminidase in a whole virion (i.e. crowded membrane) environment 2) Glycosylated neuraminidase in its own lipid bilayer 3) Unglycosylated neuraminidase in its own lipid bilayer. We saw that glycans tend to stabilize the protein structure and reduce its conformational flexibility while restricting solvent movement. Conversely, a crowded membrane environment encouraged exploration of the free energy landscape and a large scale conformational change while making the protein structure more compact. Understanding these effects informs what factors one must consider while attempting to recapture the desired level of physical accuracy.

Enrichment and fragmentation approaches for enhanced detection and characterization of endogenous glycosylated neuropeptides

Glycosylated neuropeptides were recently discovered in crustaceans, a model organism with a well-characterized neuroendocrine system. Several workflows exist to characterize enzymatically digested peptides; however, the unique properties of endogenous neuropeptides require methods to be re-evaluated. We investigate the use of hydrophilic interaction liquid chromatography (HILIC) enrichment and different fragmentation methods to further probe the expression of glycosylated neuropeptides in Callinectes sapidus. During the evaluation of HILIC, we observed the necessity of a less aqueous solvent for endogenous peptide samples. This modification enabled the number of detected neuropeptide glycoforms to increase almost two-fold, from 18 to 36. Product ion-triggered electron-transfer/higher-energy collision dissociation enabled the site-specific detection of 55 intact N- and O-linked glycoforms, while the faster stepped collision energy higher-energy collisional dissociation resulted in detection of 25. Additionally, applying this workflow to five neuronal tissues enabled the characterization of 36 more glycoforms of known neuropeptides and 11 more glycoforms of nine putative novel neuropeptides. Overall, the database of glycosylated neuropeptides in crustaceans was largely expanded from 18 to 136 glycoforms of 40 neuropeptides from 10 neuropeptide families. Both macro- and micro-heterogeneity were observed, demonstrating the chemical diversity of this simple invertebrate, establishing a framework to use crustacean to probe modulatory effects of glycosylation on neuropeptides.

New insights into the structure and function of class B1 GPCRs

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors. Class B1 GPCRs constitute a subfamily of 15 receptors that characteristically contain large extracellular domains (ECDs) and respond to long polypeptide hormones. Class B1 GPCRs are critical regulators of homeostasis, and as such, many are important drug targets. While most transmembrane proteins, including GPCRs, are recalcitrant to crystallization, recent advances in electron cryo-microscopy (cryo-EM) have facilitated a rapid expansion of the structural understanding of membrane proteins. As a testament to this success, structures for all the class B1 receptors bound to G proteins have been determined by cryo-EM in the past five years. Further advances in cryo-EM have uncovered dynamics of these receptors, ligands, and signalling partners. Here, we examine the recent structural underpinnings of the class B1 GPCRs with an emphasis on structure-function relationships.

Towards Precision Oncology: The Role of Smoothened and Its Variants in Cancer

The G protein-coupled receptor Smoothened (Smo) is a central signal transducer of the Hedgehog (Hh) pathway which has been linked to diverse forms of tumours. Stimulated by advancements in structural and functional characterisation, the Smo receptor has been recognised as an important therapeutic target in Hh-driven cancers, and several Smo inhibitors have now been approved for cancer therapy. This receptor is also known to be an oncoprotein itself and its gain-of-function variants have been associated with skin, brain, and liver cancers. According to the COSMIC database, oncogenic mutations of Smo have been identified in various other tumours, although their oncogenic effect remains unknown in these tissues. Drug resistance is a common challenge in cancer therapies targeting Smo, and data analysis shows that healthy individuals also harbour resistance mutations. Based on the importance of Smo in cancer progression and the high incidence of resistance towards Smo inhibitors, this review suggests that detection of Smo variants through tumour profiling could lead to increased precision and improved outcomes of anti-cancer treatments.

Hyaluronic acid-GPRC5C signalling promotes dormancy in haematopoietic stem cells

Bone marrow haematopoietic stem cells (HSCs) are vital for lifelong maintenance of healthy haematopoiesis. In inbred mice housed in gnotobiotic facilities, the top of the haematopoietic hierarchy is occupied by dormant HSCs, which reversibly exit quiescence during stress. Whether HSC dormancy exists in humans remains debatable. Here, using single-cell RNA sequencing, we show a continuous landscape of highly purified human bone marrow HSCs displaying varying degrees of dormancy. We identify the orphan receptor GPRC5C, which enriches for dormant human HSCs. GPRC5C is also essential for HSC function, as demonstrated by genetic loss- and gain-of-function analyses. Through structural modelling and biochemical assays, we show that hyaluronic acid, a bone marrow extracellular matrix component, preserves dormancy through GPRC5C. We identify the hyaluronic acid-GPRC5C signalling axis controlling the state of dormancy in mouse and human HSCs.

Mediators of Amylin Action in Metabolic Control

Amylin (also called islet amyloid polypeptide (IAPP)) is a pancreatic beta-cell hormone that is co-secreted with insulin in response to nutrient stimuli. The last 35 years of intensive research have shown that amylin exerts important physiological effects on metabolic control. Most importantly, amylin is a physiological control of meal-ending satiation, and it limits the rate of gastric emptying and reduces the secretion of pancreatic glucagon, in particular in postprandial states. The physiological effects of amylin and its analogs are mediated by direct brain activation, with the caudal hindbrain playing the most prominent role. The clarification of the structure of amylin receptors, consisting of the calcitonin core receptor plus receptor-activity modifying proteins, aided in the development of amylin analogs with a broad pharmacological profile. The general interest in amylin physiology and pharmacology was boosted by the finding that amylin is a sensitizer to the catabolic actions of leptin. Today, amylin derived analogs are considered to be among the most promising approaches for the pharmacotherapy against obesity. At least in conjunction with insulin, amylin analogs are also considered important treatment options in diabetic patients, so that new drugs may soon be added to the only currently approved compound pramlintide (Symlin^®). This review provides a brief summary of the physiology of amylin’s mode of actions and its role in the control of the metabolism, in particular energy intake and glucose metabolism.

Peptide ligand interaction with maltose-binding protein tagged to the calcitonin gene-related peptide receptor: The inhibitory role of receptor N-glycosylation

Calcitonin gene-related peptide (CGRP) and adrenomedullin (AM) are peptide hormones and their receptors play a critical role in migraine progression and blood pressure control, respectively. CGRP and AM receptors are structurally related since they are the complex of the calcitonin receptor-like receptor (CLR) with the different types of receptor activity-modifying protein (RAMP). Several crystal structures of the CGRP and AM receptor extracellular domain (ECD) used maltose-binding protein (MBP) as a tag protein to facilitate crystallization. Unexpectedly, the recent crystal structures of CGRP receptor ECD showed that the N-terminal tag MBP located in proximity of bound/mutated peptide ligands. This study provided evidence that MBP N-terminally tagged to the CGRP receptor ECD formed chemical interaction with the mutated peptide ligands. Interestingly, N-glycosylation of the CGRP receptor ECD was predicted to prevent MBP docking to the mutated peptide ligands. I found that the N-glycosylation of CLR ECD N123 was the most critical for inhibiting MBP interaction with the mutated peptide ligands. The MBP tag protein interaction was also dependent on the sequence of the peptide ligands. In contrast to the CGRP receptor, the MBP tag was not involved in peptide ligand binding at AM receptor ECD. Here, I provided evidence that N-glycosylation of the CGRP receptor ECD inhibited the tag protein interaction suggesting an additional function of N-glycosylation in the MBP-fused CGRP receptor ECD. This study reveals the importance of using tag protein-free versions of the CGRP receptor for the accurate assessment of peptide binding affinity.

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.

Computational Modeling of Molecular Structures Guided by Hydrogen-Exchange Data

Data produced by hydrogen-exchange monitoring experiments have been used in structural studies of molecules for several decades. Despite uncertainties about the structural determinants of hydrogen exchange itself, such data have successfully helped guide the structural modeling of challenging molecular systems, such as membrane proteins or large macromolecular complexes. As hydrogen-exchange monitoring provides information on the dynamics of molecules in solution, it can complement other experimental techniques in so-called integrative modeling approaches. However, hydrogen-exchange data have often only been used to qualitatively assess molecular structures produced by computational modeling tools. In this paper, we look beyond qualitative approaches and survey the various paradigms under which hydrogen-exchange data have been used to quantitatively guide the computational modeling of molecular structures. Although numerous prediction models have been proposed to link molecular structure and hydrogen exchange, none of them has been widely accepted by the structural biology community. Here, we present as many hydrogen-exchange prediction models as we could find in the literature, with the aim of providing the first exhaustive list of its kind. From purely structure-based models to so-called fractional-population models or knowledge-based models, the field is quite vast. We aspire for this paper to become a resource for practitioners to gain a broader perspective on the field and guide research toward the definition of better prediction models. This will eventually improve synergies between hydrogen-exchange monitoring and molecular modeling.

G Protein-Coupled Receptors in Osteoarthritis: A Novel Perspective on Pathogenesis and Treatment

G protein-coupled receptors (GPCRs) are transmembrane receptor proteins that trigger numerous intracellular signaling pathways in response to the extracellular stimuli. The GPCRs superfamily contains enormous structural and functional diversity and mediates extensive biological processes. Until now, critical roles have been established in many diseases, including osteoarthritis (OA). Existing studies have shown that GPCRs play an important role in some OA-related pathogenesis, such as cartilage matrix degradation, synovitis, subchondral bone remodeling, and osteophyte formation. However, current pharmacological treatments are mostly symptomatic and there is a paucity of disease-modifying OA drugs so far. Targeting GPCRs is capable of inhibiting cartilage matrix degradation and synovitis and up-regulating cartilage matrix synthesis, providing a new therapeutic strategy for OA. In this review, we have comprehensively summarized the structures, biofunctions, and the novel roles of GPCRs in the pathogenesis and treatment of OA, which is expected to lay the foundation for the development of novel therapeutics against OA. Even though targeting GPCRs may ameliorate OA progression, many GPCRs-related therapeutic strategies are still in the pre-clinical stage and require further investigation.

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.

A genome-wide scan to identify signatures of selection in two Iranian indigenous chicken ecotypes

Background

Various regions of the chicken genome have been under natural and artificial selection for thousands of years. The substantial diversity that exits among chickens from different geographic regions provides an excellent opportunity to investigate the genomic regions under selection which, in turn, will increase our knowledge about the mechanisms that underlie chicken diversity and adaptation. Several statistics have been developed to detect genomic regions that are under selection. In this study, we applied approaches based on differences in allele or haplotype frequencies (F_ST and hapFLK, respectively) between populations, differences in long stretches of consecutive homozygous sequences (ROH), and differences in allele frequencies within populations (composite likelihood ratio (CLR)) to identify inter- and intra-populations traces of selection in two Iranian indigenous chicken ecotypes, the Lari fighting chicken and the Khazak or creeper (short-leg) chicken.

Results

Using whole-genome resequencing data of 32 individuals from the two chicken ecotypes, approximately 11.9 million single nucleotide polymorphisms (SNPs) were detected and used in genomic analyses after quality processing. Examination of the distribution of ROH in the two populations indicated short to long ROH, ranging from 0.3 to 5.4 Mb. We found 90 genes that were detected by at least two of the four applied methods. Gene annotation of the detected putative regions under selection revealed candidate genes associated with growth (DCN, MEOX2 and CACNB1), reproduction (ESR1 and CALCR), disease resistance (S1PR1, ALPK1 and MHC-B), behavior pattern (AGMO, GNAO1 and PSEN1), and morphological traits (IHH and NHEJ1).

Conclusions

Our findings show that these two phenotypically different indigenous chicken populations have been under selection for reproduction, immune, behavioral, and morphology traits. The results illustrate that selection can play an important role in shaping signatures of differentiation across the genomic landscape of two chicken populations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12711-021-00664-9.

Advances in Hydrogen/Deuterium Exchange Mass Spectrometry and the Pursuit of Challenging Biological Systems

Solution-phase hydrogen/deuterium exchange (HDX) coupled to mass spectrometry (MS) is a widespread tool for structural analysis across academia and the biopharmaceutical industry. By monitoring the exchangeability of backbone amide protons, HDX-MS can reveal information about higher-order structure and dynamics throughout a protein, can track protein folding pathways, map interaction sites, and assess conformational states of protein samples. The combination of the versatility of the hydrogen/deuterium exchange reaction with the sensitivity of mass spectrometry has enabled the study of extremely challenging protein systems, some of which cannot be suitably studied using other techniques. Improvements over the past three decades have continually increased throughput, robustness, and expanded the limits of what is feasible for HDX-MS investigations. To provide an overview for researchers seeking to utilize and derive the most from HDX-MS for protein structural analysis, we summarize the fundamental principles, basic methodology, strengths and weaknesses, and the established applications of HDX-MS while highlighting new developments and applications.

Association of hyperglycaemia with hospital mortality in nondiabetic COVID-19 patients: A cohort study

Objective

Diabetes is a known risk factor for mortality in Coronavirus disease 2019 (COVID-19) patients. Our objective was to identify prevalence of hyperglycaemia in COVID-19 patients with and without prior diabetes and quantify its association with COVID-19 disease course.

Research design and methods

This observational cohort study included all consecutive COVID-19 patients admitted to John H Stroger Jr. Hospital, Chicago, IL from March 15, 2020 to May 3, 2020 and followed till May 15, 2020. The primary outcome was hospital mortality, and the studied predictor was hyperglycaemia [any blood glucose ≥7.78 mmol/L (140 mg/dL) during hospitalization].

Results

Of the 403 COVID-19 patients studied, 51 (12.7%) died; 335 (83.1%) were discharged while 17 (4%) were still in hospital. Hyperglycaemia occurred in 228 (56.6%) patients; 83 of these hyperglycaemic patients (36.4%) had no prior history of diabetes. Compared to the reference group no-diabetes/no-hyperglycaemia patients the no-diabetes/hyperglycaemia patients showed higher mortality [1.8% versus 20.5%, adjusted odds ratio 21.94 (95% confidence interval 4.04–119.0), P < 0.001]; improved prediction of death (P = 0.01) and faster progression to death (P < 0.01). Hyperglycaemia within the first 24 and 48 h was also significantly associated with mortality (odds ratio 2.15 and 3.31, respectively).

Conclusions

Hyperglycaemia without prior diabetes was common (20.6% of hospitalized COVID-19 patients) and was associated with an increased risk of and faster progression to death. Development of hyperglycaemia in COVID-19 patients who do not have diabetes is an early indicator of progressive disease.

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.

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.

Picomolar Affinity Antagonist and Sustained Signaling Agonist Peptide Ligands for the Adrenomedullin and Calcitonin Gene-Related Peptide Receptors

The calcitonin receptor-like class B G protein-coupled receptor (CLR) mediates adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) functions including vasodilation, cardioprotection, and nociception. Receptor activity-modifying proteins (RAMP1-3) form heterodimers with CLR and determine its peptide ligand selectivity through an unresolved mechanism. The CGRP (RAMP1:CLR) and AM (RAMP2/3:CLR) receptors are proven or promising drug targets, but short AM and CGRP plasma half-lives limit their therapeutic utility. Here, we used synthetic peptide combinatorial library and rational design approaches to probe the ligand selectivity determinants and develop truncated AM and CGRP antagonist variants with receptor extracellular domain binding affinities that were enhanced ∼1000-fold into the low nanomolar range. Receptor binding studies and a high-resolution crystal structure of a novel library-identified AM variant bound to the RAMP2-CLR extracellular domain complex explained the increased affinities and defined roles for AM Lys46 and RAMP modulation of CLR conformation in the ligand selectivity mechanism. In longer AM and CGRP scaffolds that also bind the CLR transmembrane domain, the variants generated picomolar affinity antagonists, one with an estimated 12.5 h CGRP receptor residence time, and sustained signaling agonists "ss-AM" and "ss-CGRP" that exhibited persistent cAMP signaling after ligand washout. Sustained signaling was demonstrated in primary human umbilical vein endothelial cells and the SK-N-MC cell line, which endogenously express AM and CGRP receptors, respectively. This work clarifies the RAMP-modulated CLR ligand selectivity mechanism and provides AM and CGRP variants that are valuable pharmacological tools and may have potential as long-acting therapeutics.

Biochemical characterization of G protein coupling to calcitonin gene-related peptide and adrenomedullin receptors using a native PAGE assay

Calcitonin gene-related peptide (CGRP), adrenomedullin (AM), and adrenomedullin 2/intermedin (AM2/IMD) have overlapping and unique functions in the nervous and circulatory systems including vasodilation, cardioprotection, and pain transmission. Their actions are mediated by the class B calcitonin-like G protein-coupled receptor (CLR), which heterodimerizes with three receptor activity-modifying proteins (RAMP1-3) that determine its peptide ligand selectivity. How the three agonists and RAMPs modulate CLR binding to transducer proteins remains poorly understood. Here, we biochemically characterized agonist-promoted G protein coupling to each CLR·RAMP complex. We adapted a native PAGE method to assess the formation and thermostabilities of detergent-solubilized fluorescent protein-tagged CLR·RAMP complexes expressed in mammalian cells. Addition of agonist and the purified Gs protein surrogate mini-Gs (mGs) yielded a mobility-shifted agonist·CLR·RAMP·mGs quaternary complex gel band that was sensitive to antagonists. Measuring the apparent affinities of the agonists for the mGs-coupled receptors and of mGs for the agonist-occupied receptors revealed that both ligand and RAMP control mGs coupling and defined how agonist engagement of the CLR extracellular and transmembrane domains affects transducer recruitment. Using mini-Gsq and -Gsi chimeras, we observed a coupling rank order of mGs > mGsq > mGsi for each receptor. Last, we demonstrated the physiological relevance of the native gel assays by showing that they can predict the cAMP-signaling potencies of AM and AM2/IMD chimeras. These results highlight the power of the native PAGE assay for membrane protein biochemistry and provide a biochemical foundation for understanding the molecular basis of shared and distinct signaling properties of CGRP, AM, and AM2/IMD.

G Protein-Coupled Receptors in the Sweet Spot: Glycosylation and other Post-translational Modifications

Post-translational modifications (PTMs) are a fundamental phenomenon across all classes of life and several hundred different types have been identified. PTMs contribute widely to the biological functions of proteins and greatly increase their diversity. One important class of proteins regulated by PTMs, is the cell surface expressed G protein-coupled receptors (GPCRs). While most PTMs have been shown to exert distinct biological functions, we are only beginning to approach the complexity that the potential interplay between different PTMs may have on biological functions and their regulation. Importantly, PTMs and their potential interplay represent an appealing mechanism for cell and tissue specific regulation of GPCR function and may partially contribute to functional selectivity of some GPCRs. In this review we highlight examples of PTMs located in GPCR extracellular domains, with special focus on glycosylation and the potential interplay with other close-by PTMs such as tyrosine sulfation, proteolytic cleavage, and phosphorylation.