Inositol Phosphate Accumulation Research Articles

Using Native and Synthetic Genes to Disrupt Inositol Pyrophosphates and Phosphate Accumulation in Plants.

Inositol pyrophosphates are eukaryotic signaling molecules that have been recently identified as key regulators of plant phosphate sensing and homeostasis. Given the importance of phosphate to current and future agronomic practices, we sought to design plants which could be used to sequester phosphate, as a step in a phytoremediation strategy. To achieve this, we expressed Diadenosine and Diphosphoinositol Polyphosphate Phosphohydrolase (DDP1), a yeast (Saccharomyces cerevisiae) enzyme demonstrated to hydrolyze inositol pyrophosphates, in Arabidopsis thaliana and pennycress (Thlaspi arvense), a spring annual cover crop with emerging importance as a biofuel crop. DDP1 expression in Arabidopsis decreased inositol pyrophosphates, activated Phosphate Starvation Response marker genes, and increased phosphate accumulation. These changes corresponded with alterations in plant growth and sensitivity to exogenously applied phosphate. Pennycress plants expressing DDP1 displayed increases in phosphate accumulation, suggesting that these plants could potentially serve to reclaim phosphate from phosphate-polluted soils. We also identified a native Arabidopsis gene, Nucleoside diphosphate-linked moiety X 13 (NUDIX13), which we show encodes an enzyme homologous to DDP1 with similar substrate specificity. Arabidopsis transgenics overexpressing NUDIX13 had lower inositol pyrophosphate levels and displayed phenotypes similar to DDP1-overexpressing transgenics, while nudix13-1 mutants had increased levels of inositol pyrophosphates. Taken together, our data demonstrates that DDP1 and NUDIX13 can be used in strategies to regulate plant inositol pyrophosphates and could serve as potential targets for engineering plants to reclaim phosphate from polluted environments.

Novel orexin receptor agonists based on arene- or pyridine-fused 1,3-dihydro-2H-imidazole-2-imines

A structurally novel class of benzo- or pyrido-fused 1,3-dihydro-2H-imidazole-2-imines was designed and evaluated in an inositol phosphate accumulation assay for Gq signaling to measure agonistic activation of the orexin receptor type 2 (OX2R). These compounds were synthesized in 4–9 steps overall from readily available starting materials. Analogs that contain a stereogenic methyl or cyclopropyl substituent at the benzylic center, and a correctly configured alkyl ether, alkoxyalkyl ether, cyanoalkyl ether, or α-hydroxyacetamido substituted homobenzylic sidechain were identified as the most potent activators of OX2R coupled Gq signaling. Our results also indicate that agonistic activity was stereospecific at both the benzylic and homobenzylic stereogenic centra. We identified methoxyethoxy-substituted pyrido-fused dihydroimidazolimine analog 63c containing a stereogenic benzylic methyl group was the most potent agonist, registering a respectable EC50 of 339 nM and a maximal response (Emax) of 96 % in this assay. In vivo pharmacokinetic analysis indicated good brain exposure for several analogs. Our combined results provide important information towards a structurally novel class of orexin receptor agonists distinct from current chemotypes.

Enzyme‐assisted HPTLC method for the simultaneous analysis of inositol phosphates and phosphate

Background The analysis of myo-inositol phosphates (InsPx) released by phytases during phytic acid degradation is challenging and time-consuming, particularly in terms of sample preparation, isomer separation, and detection. However, a fast and robust analysis method is crucial when screening for phytases during protein engineering approaches, which result in a large number of samples, to ensure reliable identification of promising novel enzymes or target variants with improved characteristics, for example, pH range, thermal stability, and phosphate release kinetics. Results The simultaneous analysis of several InsPx (InsP1-InsP4 and InsP5 + 6) as well as free phosphate was established on cellulose HPTLC plates using a buffered mobile phase. Inositol phosphates were subsequently stained using a novel enzyme-assisted staining procedure. Immobilized InsPx were hydrolyzed by a phytase solution of Quantum® Blueliquid 5G followed by a molybdate reagent derivatization. Resulting blue zones were captured by DAD scan. The method shows good repeatability (intra-day and intra-lab) with maximum deviations of the Rf value of 0.01. The HPTLC method was applied to three commercially available phytases at two pH levels relevant to the gastrointestinal tract of poultry (pH 5.5 and pH 3.6) to observe their phytate degradation pattern and thus visualize their InsPx fingerprint. Conclusion This HPTLC method presents a semi-high-throughput analysis for the simultaneous analysis of phytic acid and the resulting lower inositol phosphates after its enzymatic hydrolysis and is also an effective tool to visualize the InsPx fingerprints and possible accumulations of inositol phosphates.

Tachykinin Receptor-Selectivity of the Potential Glioblastoma-Targeted Therapy, DOTA-[Thi8,Met(O2)11]-Substance P.

Radiopharmaceutical development hinges on the affinity and selectivity of the biological component for the intended target. An analogue of the neuropeptide Substance P (SP), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-[Thi8,Met(O2)11]-SP (DOTA-[Thi8,Met(O2)11]SP), in the theranostic pair [68Ga]Ga-/ [213Bi]Bi-DOTA-[Thi8,Met(O2)11]SP has shown promising clinical results in the treatment of inoperable glioblastoma. As the theranostic targeting component, modifications to SP that affect the selectivity of the resulting analogue for the intended target (neurokinin-1 receptor [NK1R]) could be detrimental to its therapeutic potential. In addition to other closely related tachykinin receptors (neurokinin-2 receptor [NK2R] and neurokinin-3 receptor [NK3R]), SP can activate a mast cell expressed receptor Mas-related G protein-coupled receptor subtype 2 (MRGPRX2), which has been implicated in allergic-type reactions. Therefore, activation of these receptors by SP analogues has severe implications for their therapeutic potential. Here, the receptor selectivity of DOTA-[Thi8,Met(O2)11]SP was examined using inositol phosphate accumulation assay in HEK293-T cells expressing NK1R, NK2R, NK3R or MRGPRX2. DOTA-[Thi8,Met(O2)11]SP had similar efficacy and potency as native SP at NK1R, but displayed greater NK1R selectivity. DOTA-[Thi8,Met(O2)11]SP was unable to elicit significant activation of the other tachykinin receptors nor MRGPRX2 at high concentrations nor did it display antagonistic behaviour at these receptors. DOTA-[Thi8,Met(O2)11]SP, therefore has high potency and selectivity for NK1R, supporting its potential for targeted theranostic use in glioblastoma multiforme and other conditions characterised by NK1R overexpression.

Initial Pharmacological Characterization of a Major Hydroxy Metabolite of PF-5190457: Inverse Agonist Activity of PF-6870961 at the Ghrelin Receptor.

Preclinical and clinical studies have identified the ghrelin receptor [growth hormone secretagogue receptor (GHSR)1a] as a potential target for treating alcohol use disorder. A recent phase 1a clinical trial of a GHSR1a antagonist/inverse agonist, PF-5190457, in individuals with heavy alcohol drinking identified a previously undetected major hydroxy metabolite of PF-5190457, namely PF-6870961. Here, we further characterized PF-6870961 by screening for off-target interactions in a high-throughput screen and determined its in vitro pharmacodynamic profile at GHSR1a through binding and concentration-response assays. Moreover, we determined whether the metabolite demonstrated an in vivo effect by assessing effects on food intake in male and female rats. We found that PF-6870961 had no off-target interactions and demonstrated both binding affinity and inverse agonist activity at GHSR1a. In comparison with its parent compound, PF-5190457, the metabolite PF-6870961 had lower binding affinity and potency at inhibiting GHSR1a-induced inositol phosphate accumulation. However, PF-6870961 had increased inhibitory potency at GHSR1a-induced β-arrestin recruitment relative to its parent compound. Intraperitoneal injection of PF-6870961 suppressed food intake under conditions of both food restriction and with ad libitum access to food in male and female rats, demonstrating in vivo activity. The effects of PF-6870961 on food intake were abolished in male and female rats knockout for GHSR, thus demonstrating that its effects on food intake are in fact mediated by the GHSR receptor. Our findings indicate that the newly discovered major hydroxy metabolite of PF-5190457 may contribute to the overall activity of PF-5190457 by demonstrating inhibitory activity at GHSR1a. SIGNIFICANCE STATEMENT: Antagonists or inverse agonists of the growth hormone secretagogue receptor (GHSR)1a have demonstrated substantial potential as therapeutics for alcohol use disorder. We here expand understanding of the pharmacology of one such GHSR1a inverse agonist, PF-5190457, by studying the safety and pharmacodynamics of its major hydroxy metabolite, PF-6870961. Our data demonstrate biased inverse agonism of PF-6870961 at GHSR1a and provide new structure-activity relationship insight into GHSR1a inverse agonism.

A naturally occurring membrane-anchored Gαs variant, XLαs, activates phospholipase Cβ4

Extra-large stimulatory Gα (XLαs) is a large variant of G protein αs subunit (Gαs) that uses an alternative promoter and thus differs from Gαs at the first exon. XLαs activation by G protein–coupled receptors mediates cAMP generation, similarly to Gαs; however, Gαs and XLαs have been shown to have distinct cellular and physiological functions. For example, previous work suggests that XLαs can stimulate inositol phosphate production in renal proximal tubules and thereby regulate serum phosphate levels. In this study, we show that XLαs directly and specifically stimulates a specific isoform of phospholipase Cβ (PLCβ), PLCβ4, both in transfected cells and with purified protein components. We demonstrate that neither the ability of XLαs to activate cAMP generation nor the canonical G protein switch II regions are required for PLCβ stimulation. Furthermore, this activation is nucleotide independent but is inhibited by Gβγ, suggesting a mechanism of activation that relies on Gβγ subunit dissociation. Surprisingly, our results indicate that enhanced membrane targeting of XLαs relative to Gαs confers the ability to activate PLCβ4. We also show that PLCβ4 is required for isoproterenol-induced inositol phosphate accumulation in osteocyte-like Ocy454 cells. Taken together, we demonstrate a novel mechanism for activation of phosphoinositide turnover downstream of Gs-coupled receptors that may have a critical role in endocrine physiology.

In vitro assays for the functional characterization of (psychedelic) substances at the serotonin receptor 5-HT2A R.

Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5-HT) 2A receptors (5-HT2A Rs) as a main pharmacological mechanism. Besides their appearance on the (illicit) drug market, e.g. as new psychoactive substances, their potential therapeutic application is increasingly explored. This group of substances demonstrates a broad structural variety, leading to insufficiently described structure-activity relationships, hence illustrating the need for better functional characterization. This review therefore elaborates on the in vitro molecular techniques that have been used the most abundantly for the characterization of (psychedelic) 5-HT2A R agonists. More specifically, this review covers assays to monitor the canonical G protein signaling pathway (e.g. measuring G protein recruitment/activation, inositol phosphate accumulation, or Ca2+ mobilization), assays to monitor non-canonical G protein signaling (such as arachidonic acid release), assays to monitor β-arrestin recruitment or signaling, and assays to monitor receptor conformational changes. In particular, focus lies on the mechanism behind the techniques, and the specific advantages and challenges that are associated with these. Additionally, several variables are discussed that one should consider when attempting to compare functional outcomes from different studies, both linked to the specific assay mechanism and linked to its specific execution, as these may heavily impact the assay outcome.

Protease-Triggered Release of Stabilized CXCL12 from Coated Scaffolds in an Ex Vivo Wound Model.

Biomaterials are designed to improve impaired healing of injured tissue. To accomplish better cell integration, we suggest to coat biomaterial surfaces with bio-functional proteins. Here, a mussel-derived surface-binding peptide is used and coupled to CXCL12 (stromal cell-derived factor 1α), a chemokine that activates CXCR4 and consequently recruits tissue-specific stem and progenitor cells. CXCL12 variants with either non-releasable or protease-mediated-release properties were designed and compared. Whereas CXCL12 was stabilized at the N-terminus for protease resistance, a C-terminal linker was designed that allowed for specific cleavage-mediated release by matrix metalloproteinase 9 and 2, since both enzymes are frequently found in wound fluid. These surface adhesive CXCL12 derivatives were produced by expressed protein ligation. Functionality of the modified chemokines was assessed by inositol phosphate accumulation and cell migration assays. Increased migration of keratinocytes and primary mesenchymal stem cells was demonstrated. Immobilization and release were studied for bioresorbable PCL-co-LC scaffolds, and accelerated wound closure was demonstrated in an ex vivo wound healing assay on porcine skin grafts. After 24 h, a significantly improved CXCL12-specific growth stimulation of the epithelial tips was already observed. The presented data display a successful application of protein-coated biomaterials for skin regeneration.

Elucidation of Inverse Agonist Activity of Bilastine.

H1-antihistamines antagonize histamine and prevent it from binding to the histamine H1 receptor (H1R). Some of them also act as inverse agonists, which are more potent than pure antagonists because they suppress the constitutive H1R activity. Bilastine is a non-sedative antihistamine which is one of the most satisfy the requirements for oral antihistamines. However, there is no information to show the inverse agonist activity of bilastine including inositol phosphates accumulation, and its inverse agonist activity is yet to be elucidated. Here we evaluated whether bilastine has inverse agonist activity or not. Intracellular calcium concentration was measured using Fluo-8. Inositol phosphates accumulation was assayed using [3H]myo-inositol. The H1R mRNA level was measured using real-time RT-PCR. At rest, Ca2+ oscillation was observed, indicating that H1R has intrinsic activity. Bilastine attenuated this fluorescence oscillation. Bilastine suppressed the increase in IPs formation in a dose-dependent manner and it was about 80% of the control level at the dose of 3 μM. Bilastine also suppressed histamine-induced increase in IPs formation to the control level. Furthermore, bilastine suppressed basal H1R gene expression in a dose-dependent manner. Data suggest that bilastine is an inverse agonist. Preseasonal prophylactic administration with bilastine could down-regulate basal H1R gene expression in the nasal mucosa and ameliorate the nasal symptoms during the peak pollen period.

Rescue of Function of Mutant Luteinising Hormone Receptors with Deficiencies in Cell Surface Expression, Hormone Binding, and Hormone Signalling

Introduction: G protein-coupled receptor (GPCR) mutations are implicated in many diseases. Most inactivating mutations cause receptor misfolding and prevent trafficking to the plasma membrane. Pharmacological chaperones can “rescue” cell surface expression of such mutants, presumably by stabilising correct folding of the nascent protein. Objective: Here we examine the scope of intracellularly retained luteinising hormone receptor (LHR) mutants that can be “rescued” by the pharmacological chaperone LHR-Chap, and whether this allosteric agonist can also restore the function of mutant LHRs with deficiencies in hormone binding or hormone-induced signalling. Methods: Mutant LHRs were expressed in HEK 293-T cells. Cell surface expression/localisation, hormone binding, and hCG/LHR-Chap signalling were determined by ELISA, radioligand binding, and inositol phosphate accumulation assays, respectively. Molecular modelling predicted LHR-Chap interactions. Results: LHR-Chap increased cell surface expression of a subset of retained mutants located in transmembrane helices predicted to be stabilised by LHR-Chap binding. For 3 (T461^3.47I, L502^4.61P, and S616^7.46Y) hCG-responsiveness was increased following treatment. LHRs with mutations in the hormone-binding site (C131^ECDR and I152^ECDT) or in the hinge region (E354^HingeK) had good cell surface expression but poor response to hormone stimulation, yet were responsive to allosteric activation by LHR-Chap. Conclusions: LHR-Chap, in addition to rescuing cell surface expression of intracellularly retained LHR mutants, can rescue function in mutant receptors with binding and signalling deficiencies that have normal cell surface expression. This demonstration of rescue of multiple elements of LHR dysfunction arising from inactivating mutations offers exceptional potential for treating patients with diseases arising from GPCR mutations in general.

Suppression of histamine H1 receptor gene expression by antihistamines with inverse agonist activities

Allergic rhinitis is an incurable disease whose prevalence rate is over 30% in Japan. Antihistamines are first‐choice drugs for the therapy of the disease. However, sole treatment of antihistamine is not enough to obtain satisfactory control. We elucidated histamine‐induced histamine H1 receptor gene expression as a pathological mechanism of allergic rhinitis. Treatment of epinastine, an antihistamine, to allergic rhinitis patients showed alleviation of nasal symptoms and correlated suppression of histamine H1 receptor mRNA elevation in nasal mucosa. We observed inverse agonist activities in some antihistamines. Suppression of inositol phosphates accumulation and suppression of histamine H1 receptor gene expression showed good correlation. These results suggested that antihistamines with inverse agonist activities have advantage for the therapy of allergic rhinitis.Support or Funding InformationResearch Grant from The Osaka Medical Research Foundation for Intractable Diseases

Effects of Mlx-8, a phospholipase A2 from Brazilian coralsnake Micrurus lemniscatus venom, on muscarinic acetylcholine receptors in rat hippocampus.

Background: Here, we described the presence of a neurotoxin with phospholipase A2 activity isolated from Micrurus lemniscatus venom (Mlx-8) with affinity for muscarinic acetylcholine receptors (mAChRs). Methods: The purification, molecular mass determination, partial amino acid sequencing, phospholipase A2 activity determination, inhibition of the binding of the selective muscarinic ligand [3H]QNB and inhibition of the total [3H]inositol phosphate accumulation in rat hippocampus of the Mlx-8 were determined. Results: Thirty-one fractions were collected from HPLC chromatography, and the Mlx-8 toxin was used in this work. The molecular mass of Mlx-8 is 13.628 Da. Edman degradation yielded the following sequence: NLYQFKNMIQCTNTRSWL-DFADYG-CYCGRGGSGT. The Mlx-8 had phospholipase A2 enzymatic activity. The pKi values were determined for Mlx-8 toxin and the M1 selective muscarinic antagonist pirenzepine in hippocampus membranes via [3H]QNB competition binding assays. The pKi values obtained from the analysis of Mlx-8 and pirenzepine displacement curves were 7.32 ± 0.15, n = 4 and 5.84 ± 0.18, n = 4, respectively. These results indicate that Mlx-8 has affinity for mAChRs. There was no effect on the inhibition ability of the [3H]QNB binding in hippocampus membranes when 1 µM Mlx-8 was incubated with 200 µM DEDA, an inhibitor of phospholipase A2. This suggests that the inhibition of the phospholipase A2 activity of the venom did not alter its ability to bind to displace [3H]QNB binding. In addition, the Mlx-8 toxin caused a blockade of 43.31 ± 8.86%, n = 3 and 97.42 ± 2.02%, n = 3 for 0.1 and 1 µM Mlx-8, respectively, on the total [3H]inositol phosphate content induced by 10 µM carbachol. This suggests that Mlx-8 inhibits the intracellular signaling pathway linked to activation of mAChRs in hippocampus. Conclusion: The results of the present work show, for the first time, that muscarinic receptors are also affected by the Mlx-8 toxin, a muscarinic ligand with phospholipase A2 characteristics, obtained from the venom of the Elapidae snake Micrurus lemniscatus, since this toxin was able to compete with muscarinic ligand [3H]QNB in hippocampus of rats. In addition, Mlx-8 also blocked the accumulation of total [3H]inositol phosphate induced by muscarinic agonist carbachol. Thus, Mlx-8 may be a new pharmacological tool for examining muscarinic cholinergic function.

Discovery of Gut Bacteria Specific to Alzheimer's Associated Diseases is a Clue to Understanding Disease Etiology: Meta-Analysis of Population-Based Data on Human Gut Metagenomics and Metabolomics.

Alzheimer's disease (AD)-associated sequence (ADAS) of cultured fecal bacteria was discovered in human gut targeted screening. This study provides important information to expand our current understanding of the structure/activity relationship of ADAS and putative inhibitors/activators that are potentially involved in ADAS appearance/disappearance. The NCBI database analysis revealed that ADAS presents at a large proportion in American Indian Oklahoman (C&A) with a high prevalence of obesity/diabetes and in colorectal cancer (CRC) patients from the US and China. An Oklahoman non-native group (NNI) showed no ADAS. Comparison of two large US populations reveals that ADAS is more frequent in individuals aged ≥66 and in females. Prevalence and levels of fecal metabolites are altered in the C&A and CRC groups versus controls. Biogenic amines (histamine, tryptamine, tyramine, phenylethylamine, cadaverine, putrescine, agmatine, spermidine) that present in food and are produced by gut microbiota are significantly higher in C&A (e.g., histamine/histidine 95-fold) versus NNI (histamine/histidine 16-fold). The majority of these bio-amines are cytotoxic at concentrations found in food. Inositol phosphate signaling implicated in AD is altered in C&A and CRC. Tryptamine stimulated accumulation of inositol phosphate. The seizure-eliciting tryptamine induced cytoplasmic vacuolization and vesiculation with cell fragmentation. Present additions of ADAS-carriers at different ages including infants led to an ADAS-comprising human sample size of 2,830 from 27 studies from four continents (North America, Australia, Asia, Europe). Levels of food-derived monoamine oxidase inhibitors and anti-bacterial compounds, the potential modulators of ADAS-bacteria growth and biogenic amine production, were altered in C&A versus NNI. ADAS is attributable to potentially modifiable risk factors of AD associated diseases.

Sowing Date and Seeding Rate Affect Bioactive Compound Contents of Chickpea Grains.

Simple SummaryChickpea seeds are commonly used in human diets in many areas of the world, and it can be a valid alternative to soybean meals and energetic feedstuff in animal nutrition. However, its excellent nutritional characteristics are accompanied by the occurrence of some bioactive substances with anti-nutritional effects. Usually, those secondary compounds are inactivated using appropriate processing techniques, but this increases costs and causes a loss of some nutritional traits. It is well-known that grain composition changes according to the genetic type and environmental conditions, and this study suggests that it is possible to modulate the presence of bioactive compounds in the seed also by varying some agronomic parameters, such as the sowing dates and the seeding rate. This study demonstrates that, under Mediterranean climate conditions, spring sowing reduces the content of trypsin inhibitors, and a high seeding rate can depress the content of α-galactosides. A controversial effect of the sowing period was observed on the content of inositol phosphates, probably because of the different climatic conditions during the two-year experimental period. Consequently, agronomic management can modulate the content of anti-nutritional factors, reducing the likelihood that they will avoid feeding treatments.Chickpea grains may represent an alternative to soybean meals and energetic concentrates in animal feeding, as their nutritional value can help to increase the sustainability of livestock systems. Unfortunately, the presence of bioactive compounds with anti-nutritional effects can prevent its direct use, especially in mono-gastrics. It is known that the synthesis of these compounds depends on genetic expression, which is also influenced by growth conditions. The objective of this two-year study was to assess the effect of sowing date (winter versus spring) and seeding rate (70 versus 110 seeds m−2) on the accumulation of soluble carbohydrates, α-galactosides, trypsin inhibitors, and inositol phosphates in the grains of two Kabuli cultivars, in the Mediterranean climate. The results showed that seeds collected from winter sowing contained more trypsin inhibitors than those seeded in spring (+ 4%, on average), reaching values between 16.1 and 18.6 TIU mg protein−1. The seeding rate affects only the α-galactosides content, which increases (+9%) at lower densities (70 seeds m−2). These findings suggest that agronomic management can be used to modulate the content of some anti-nutritional factors in the seeds, even though the genetic characteristics and phenotypic expression, in relation to the climatic conditions, seem to deeply affect the content of all the bioactive compounds investigated.

Rapid Aldosterone-Mediated Signaling in the DCT Increases Activity of the Thiazide-Sensitive NaCl Cotransporter.

The NaCl cotransporter NCC in the kidney distal convoluted tubule (DCT) regulates urinary NaCl excretion and BP. Aldosterone increases NaCl reabsorption via NCC over the long-term by altering gene expression. But the acute effects of aldosterone in the DCT are less well understood. Proteomics, bioinformatics, and cell biology approaches were combined with animal models and gene-targeted mice. Aldosterone significantly increases NCC activity within minutes in vivo or ex vivo. These effects were independent of transcription and translation, but were absent in the presence of high potassium. In vitro, aldosterone rapidly increased intracellular cAMP and inositol phosphate accumulation, and altered phosphorylation of various kinases/kinase substrates within the MAPK/ERK, PI3K/AKT, and cAMP/PKA pathways. Inhibiting GPR30, a membrane-associated receptor, limited aldosterone's effects on NCC activity ex vivo, and NCC phosphorylation was reduced in GPR30 knockout mice. Phosphoproteomics, network analysis, and in vitro studies determined that aldosterone activates EGFR-dependent signaling. The EGFR immunolocalized to the DCT and EGFR tyrosine kinase inhibition decreased NCC activity ex vivo and in vivo. Aldosterone acutely activates NCC to modulate renal NaCl excretion.

Identification of monellin as the first naturally derived proteinaceous allosteric agonist of metabotropic glutamate receptor 5.

Allosteric modulators bind sites distinct from orthosteric ligands, allowing for improved spatiotemporal control of receptors and greater subtype selectivity. However, we recently showed that allosteric ligands previously classified as selective for select Class C G protein-coupled receptors (GPCRs) had unappreciated activity at other off-target receptors, in some cases higher affinity, within the class. Here, we extended our investigation of off-target activity of "selective" allosteric ligands for the sweet taste receptor. Using metabotropic glutamate receptor 5 (mGlu5 ) as a representative of Class C GPCR, we assessed the sweet protein, monellin and the small-molecule artificial sweetener, NHDC. We found that monellin, but not NHDC, is an agonist for mGlu5 . Radioligand binding and functional assays performed in cells expressing N-terminally truncated mGlu5 demonstrated that monellin agonism was not mediated via the "common" allosteric binding site in the transmembrane domain but required the presence of the large extracellular N-terminal domain of mGlu5 . Monellin displayed neutral functional cooperativity with orthosteric ligands. However, monellin positively modulated the mGlu5 PAM-agonist, VU0424465, activity in intracellular calcium assays, but the interaction was neutral in inositol phosphate accumulation assays. Furthermore, monellin mGlu5 agonism was positively modulated by the mGlu5 pure PAM, VU0360172. Taken together, these data indicate that monellin is an allosteric agonist for mGlu5 , binding to an allosteric binding site on the N-terminus that is functionally linked to the common Class C GPCR allosteric site in a biased manner. This is the first evidence of a naturally derived proteinaceous allosteric ligand for the mGlu receptor family.

An intact model for quantifying functional selectivity

A ligand that acts on a target receptor to activate particular multiple signalling pathways with activity that is distinct from other ligands is termed ligand bias. Quantification of ligand bias is based on applying the operational model to each pathway separately and subsequent calculation of the ligand bias metric (ΔΔlogR). This approach implies independence among different pathways and causes propagation of error in the calculation. Here, we propose a semi-mechanism-based model which allows for receptor selectivity across all the pathways simultaneously (termed the ‘intact operational model’). The power of the intact model for detecting ligand bias was evaluated via stochastic simulation estimation studies. It was also applied to two examples: (1) opposing effects of Gi/Gs signalling of α2-adrenergic receptors and (2) simultaneous measurement of arachidonic acid release and inositol phosphate accumulation following 5-HT2C receptor activation. The intact operational model demonstrated greater power to detect ligand bias in the simulation. In the applications, it provided better precision of estimation and identified biased ligands that were missed by analysis of traditional methods. Issues identified in both examples might lead to different interpretations of the data. The intact operational model may elucidate greater understanding of the underlying mechanisms of functional selectivity.

Kinetic and system bias as drivers of metabotropic glutamate receptor 5 allosteric modulator pharmacology

Allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGlu5) have been proposed as potential therapies for various CNS disorders. These ligands bind to sites distinct from the orthosteric (or endogenous) ligand, often with improved subtype selectivity and spatio-temporal control over receptor responses. We recently revealed that mGlu5 allosteric agonists and positive allosteric modulators exhibit biased agonism and/or modulation. To establish whether negative allosteric modulators (NAMs) engender similar bias, we rigorously characterized the pharmacology of eight diverse mGlu5 NAMs. Radioligand inhibition binding studies revealed novel modes of interaction with mGlu5 for select NAMs, with biphasic or incomplete inhibition of the radiolabeled NAM, [3H]methoxy-PEPy. We assessed mGlu5-mediated intracellular Ca2+ (iCa2+) mobilization and inositol phosphate (IP1) accumulation in HEK293A cells stably expressing low levels of mGlu5 (HEK293A-rat mGlu5-low) and mouse embryonic cortical neurons. The apparent affinity of acetylenic NAMs, MPEP, MTEP and dipraglurant, was dependent on the signaling pathway measured, agonist used, and cell type (HEK293A-rat mGlu5-low versus mouse cortical neurons). In contrast, the acetylenic partial NAM, M-5MPEP, and structurally distinct NAMs (VU0366248, VU0366058, fenobam), had similar affinity estimates irrespective of the assay or cellular background. Biased modulation was evident for VU0366248 in mouse cortical neurons where it was a NAM for DHPG-mediated iCa2+ mobilization, but neutral with DHPG in IP1 accumulation assays. Overall, this study highlights the inherent complexity in mGlu5 NAM pharmacology that we hypothesize may influence interpretation when translating into preclinical models and beyond in the design and development of novel therapeutics for neuropsychiatric and neurological disorders.

Calcium-Sensing Receptor Autoantibodies in Patients with Autoimmune Polyendocrine Syndrome Type 1: Epitopes, Specificity, Functional Affinity, IgG Subclass, and Effects on Receptor Activity.

A major manifestation of autoimmune polyendocrine syndrome type 1 (APS1) is hypoparathyroidism, which is suggested to result from aberrant immune responses against the parathyroid glands. The calcium-sensing receptor (CaSR), which plays a pivotal role in maintaining calcium homeostasis by sensing blood calcium levels and regulating release of parathyroid hormone (PTH), is an autoantibody target in APS1. In this study, the aim was to characterize the binding sites, specificity, functional affinity, IgG subclass, and functional effects of CaSR autoantibodies using phage-display technology, ELISA, and bioassays. The results indicated that CaSR autoantibody binding sites were at aa 41-69, 114-126, 171-195, and 260-340 in the extracellular domain of the receptor. Autoantibodies against CaSR epitopes 41-69, 171-195, and 260-340 were exclusively of the IgG1 subclass. Autoantibody responses against CaSR epitope 114-126 were predominantly of the IgG1 with a minority of the IgG3 subclass. Only autoantibodies recognizing CaSR epitopes 114-126 and 171-195 affected receptor activity; inositol-phosphate accumulation was increased significantly in HEK293-CaSR cells, and PTH secretion from PTH-C1 cells was reduced significantly when either were incubated with purified Ab and Ca2+ compared with Ca2+ alone. In conclusion, although the majority of APS1 patients do not have CaSR-stimulating autoantibodies, the hypoparathyroid state in a small minority of patients is the result of functional suppression of the parathyroid glands.

Neurochemical pharmacology of psychoactive substituted N-benzylphenethylamines: High potency agonists at 5-HT2A receptors

The use of new psychoactive substituted 2,5-dimethoxy-N-benzylphenethylamines is associated with abuse and toxicity in the United States and elsewhere and their pharmacology is not well known. This study compares the mechanisms of action of 2-(2,5-dimethoxy-4-methylphenyl)-N-(2-methoxybenzyl)ethanamine (25D-NBOMe), 2-(4-ethyl-2,5-dimethoxyphenyl)-N-(2-methoxybenzyl)ethanamine (25E-NBOMe), 2-(2,5-dimethoxyphenyl)-N-(2-methoxybenzyl)ethanamine (25H-NBOMe), 2-(((4-iodo-2,5-dimethoxyphenethyl)amino)methyl)phenol (25I-NBOH); and 2-(2,5-dimethoxy-4-nitrophenyl)-N-(2-methoxybenzyl)ethanamine) (25N-NBOMe) with hallucinogens and stimulants. Mammalian cells heterologously expressing 5-HT1A, 5-HT2A, 5-HT2B or 5-HT2C receptors, or dopamine, serotonin or norepinephrine transporters (DAT, SERT and NET, respectively) were used to assess drug affinities at radioligand binding sites. Potencies and efficacies were determined using [35S]GTPγS binding assays (5-HT1A), inositol-phosphate accumulation assays (5-HT2A, 5-HT2B and 5-HT2C), and uptake and release assays (transporters). The substituted phenethylamines were very low potency and low efficacy agonists at the 5-HT1A receptor. 25D-NBOMe, 25E-NBOMe, 25H-NBOMe, 25I-NBOH and 25N-NBOMe had very high affinity for, and full efficacy at, 5-HT2A and 5-HT2C receptors. In the 5-HT2A receptor functional assay, 25D-NBOMe, 25E-NBOMe, 25I-NBOH and 25N-NBOMe had subnanomolar to low nanomolar potencies similar to (+)lysergic acid diethylamide (LSD) while 25H-NBOMe had lower potency, similar to serotonin. At the 5-HT2C receptor, four had very high potencies, similar to LSD and serotonin, while 25H-NBOMe had lower potency. At the 5-HT2B receptor, the compounds had lower affinity, potency and efficacy compared to 5-HT2A or 5-HT2C. The phenethylamines had low to mid micromolar affinities and potencies at the transporters. These results demonstrate that these –NBOMe and –NBOH substituted phenethylamines have a biochemical pharmacology consistent with hallucinogenic activity, with little psychostimulant activity.