Activated Inositol Phosphate, Substrate for Synthesis of Prostaglandylinositol Cyclic Phosphate (Cyclic PIP)-The Key for the Effectiveness of Inositol-Feeding.

The natural cyclic AMP antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP), is biosynthesized from prostaglandin E (PGE) and activated inositol phosphate (n-Ins-P), which is synthesized by a particulate rat-liver-enzyme from GTP and a precursor named inositol phosphate (pr-Ins-P), whose 5-ring phosphodiester structure is essential for n-Ins-P synthesis. Aortic myocytes, preincubated with [3H] myo-inositol, synthesize after angiotensin II stimulation (30 s) [3H] pr-Ins-P (65% yield), which is converted to [3H] n-Ins-P and [3H] cyclic PIP. Acid-treated (1 min) [3H] pr-Ins-P co-elutes with inositol (1,4)-bisphosphate in high performance ion chromatography, indicating that pr-Ins-P is inositol (1:2-cyclic,4)-bisphosphate. Incubation of [3H]-GTP with unlabeled pr-Ins-P gave [3H]-guanosine-labeled n-Ins-P. Cyclic PIP synthase binds the inositol (1:2-cyclic)-phosphate part of n-Ins-P to PGE and releases the [3H]-labeled guanosine as [3H]-GDP. Thus, n-Ins-P is most likely guanosine diphospho-4-inositol (1:2-cyclic)-phosphate. Inositol feeding helps patients with metabolic conditions related to insulin resistance, but explanations for this finding are missing. Cyclic PIP appears to be the key for explaining the curative effect of inositol supplementation: (1) inositol is a molecular constituent of cyclic PIP; (2) cyclic PIP triggers many of insulin's actions intracellularly; and (3) the synthesis of cyclic PIP is decreased in diabetes as shown in rodents.

EP4 Receptor Conformation Sensor Suited for Ligand Screening and Imaging of Extracellular Prostaglandins.

Prostaglandins are important lipid mediators with a wide range of functions in the human body. They act mainly via plasma membrane localized prostaglandin receptors, which belong to the G-protein coupled receptor class. Due to their localized formation and short lifetime, it is important to be able to measure the distribution and abundance of prostaglandins in time and/or space. In this study, we present a Foerster resonance energy transfer (FRET)-based conformation sensor of the human prostaglandin E receptor subtype 4 (EP4 receptor), which was capable of detecting prostaglandin E2 (PGE2)-induced receptor activation in the low nanomolar range with a good signal-to-noise ratio. The sensor retained the typical selectivity for PGE2 among arachidonic acid products. Human embryonic kidney cells stably expressing the sensor did not produce detectable amounts of prostaglandins making them suitable for a coculture approach allowing us, over time, to detect prostaglandin formation in Madin-Darby canine kidney cells and primary mouse macrophages. Furthermore, the EP4 receptor sensor proved to be suited to detect experimentally generated PGE2 gradients by means of FRET-microscopy, indicating the potential to measure gradients of PGE2 within tissues. In addition to FRET-based imaging of prostanoid release, the sensor allowed not only for determination of PGE2 concentrations, but also proved to be capable of measuring ligand binding kinetics. The good signal-to-noise ratio at a commercial plate reader and the ability to directly determine ligand efficacy shows the obvious potential of this sensor interest for screening and characterization of novel ligands of the pharmacologically important human EP4 receptor. SIGNIFICANCE STATEMENT: The authors present a biosensor based on the prostaglandin E receptor subtype 4, which is well suited to measure extracellular prostaglandin E2 (PGE2) concentration with high temporal and spatial resolution. It can be used for the imaging of PGE2 levels and gradients by means of Foerster resonance energy transfer microscopy, and for determining PGE2 release of primary cells as well as for screening purposes in a plate reader setting.

Lifetime of muscarinic receptor-G-protein complexes determines coupling efficiency and G-protein subtype selectivity.

G-protein-coupled receptors (GPCRs) are essential for the detection of extracellular stimuli by cells and transfer the encoded information via the activation of functionally distinct subsets of heterotrimeric G proteins into intracellular signals. Despite enormous achievements toward understanding GPCR structures, major aspects of the GPCR-G-protein selectivity mechanism remain unresolved. As this can be attributed to the lack of suitable and broadly applicable assays, we set out to develop a quantitative FRET-based assay to study kinetics and affinities of G protein binding to activated GPCRs in membranes of permeabilized cells in the absence of nucleotides. We measured the association and dissociation kinetics of agonist-induced binding of Gi/o, Gq/11, Gs, and G12/13 proteins to muscarinic M1, M2, and M3 receptors in the absence of nucleotides between fluorescently labeled G proteins and receptors expressed in mammalian cells. Our results show a strong quantitative correlation between not the on-rates of G-protein-M3-R interactions but rather the affinities of Gq and Go proteins to M3-Rs, their GPCR-G-protein lifetime and their coupling efficiencies determined in intact cells, suggesting that the G-protein subtype-specific affinity to the activated receptor in the absence of nucleotides is, in fact, a major determinant of the coupling efficiency. Our broadly applicable FRET-based assay represents a fast and reliable method to quantify the intrinsic affinity and relative coupling selectivity of GPCRs toward all G-protein subtypes.

A benzothiadiazine derivative and methylprednisolone are novel and selective activators of transient receptor potential canonical 5 (TRPC5) channels.

The transient receptor potential canonical channel 5 (TRPC5) is a Ca2+-permeable ion channel, which is predominantly expressed in the brain. TRPC5-deficient mice exhibit a reduced innate fear response and impaired motor control. In addition, outgrowth of hippocampal and cerebellar neurons is retarded by TRPC5. However, pharmacological evidence of TRPC5 function on cellular or organismic levels is sparse. Thus, there is still a need for identifying novel and efficient TRPC5 channel modulators. We, therefore, screened compound libraries and identified the glucocorticoid methylprednisolone and N-[3-(adamantan-2-yloxy)propyl]-3-(6-methyl-1,1-dioxo-2H-1λ6,2,4-benzothiadiazin-3-yl)propanamide (BTD) as novel TRPC5 activators. Comparisons with closely related chemical structures from the same libraries indicate important substructures for compound efficacy. Methylprednisolone activates TRPC5 heterologously expressed in HEK293 cells with an EC50 of 12µM, while BTD-induced half-maximal activation is achieved with 5-fold lower concentrations, both in Ca2+ assays (EC50=1.4µM) and in electrophysiological whole cell patch clamp recordings (EC50=1.3 µM). The activation resulting from both compounds is long lasting, reversible and sensitive to clemizole, a recently established TRPC5 inhibitor. No influence of BTD on homotetrameric members of the remaining TRPC family was observed. On the main sensory TRP channels (TRPA1, TRPV1, TRPM3, TRPM8) BTD exerts only minor activity. Furthermore, BTD can activate heteromeric channel complexes consisting of TRPC5 and its closest relatives TRPC1 or TRPC4, suggesting a high selectivity of BTD for channel complexes bearing at least one TRPC5 subunit.

Impact of human autoantibodies on ß1-adrenergic receptor conformation, activity, and internalization.

AIMS: Autoantibodies against second extracellular loops of ß(1)-adrenergic receptors frequent in dilated cardiomyopathy confer myocardial dysfunction presumably via cAMP stimulation. Here, we investigate the autoantibody impact on receptor conformation and function. METHODS AND RESULTS: IgG was prepared from patients with dilated cardiomyopathy, matched healthy donors (10 each) or commercial IgG preparations (2). IgG binding to ß(1)-adrenergic receptor peptides was detected in 5 of 10 patients and 2 of 10 controls. IgG colocalization with the native receptor was detected in 8 of 10 patients and 1 of 10 controls (10 of 10 patients and 7 of 10 controls at >30 mg IgG/L). All IgGs exhibiting receptor colocalization triggered changes in receptor conformation (determined with fluorescent sensors) not stringently correlated to cAMP stimulation, suggesting the induction of more or less active receptor conformations. Receptor-activating IgG was detected in 8 of 10 patients but only 1 of 10 controls. In addition, IgG from 8 of 10 patients and 3 of 10 controls attenuated receptor internalization (measured by total internal reflection fluorescence microscopy). IgG-inducing inactive receptor conformations had no effect on subsequent cAMP stimulation by isoproterenol. IgG-inducing active receptor conformations dampened or augmented subsequent cAMP stimulation by isoproterenol, depending on whether receptor internalization was attenuated or not. Corresponding IgG effects on the basal beating rate and chronotropic isoproterenol response of embryonic human cardiomyocytes were observed. CONCLUSIONS: (i) Autoantibodies trigger conformation changes in the ß(1)-adrenergic receptor molecule. (ii) Some also attenuate receptor internalization. (iii) Combinations thereof increase the basal beating rate of cardiomyocytes and optionally entail dampening of their chronotropic catecholamine responses. (iv) The latter effects seem specific for patient autoantibodies, which also have higher levels.

Synthesis of benzofuran, benzothiophene, and benzothiazole-based thioamides and their evaluation as K(ATP) channel openers.

Several series of benzofurans, benzothiophenes, and benzothiazoles, all featuring the thioamide group, were synthesized and tested as novel K(ATP) channel openers in artificial cell systems: CHO cells transfected with SUR1/Kir6.2, and HEK 293 cells transfected with SUR2B/Kir6.1; these served as model systems for insulin-secreting pancreatic ß cells and smooth muscle cells, respectively. All compounds were investigated with respect to their binding affinity for the SUR2B-type K(ATP) channels using [(3)H]P1075 as radioligand. Selected compounds were also tested as agonists in intact cells using DiBAC(4)(3) and DyeB (R7260) as membrane potential dyes. Remarkable affinity for SUR2B/Kir6.1 channels in the single-digit micromolar range was observed. In addition, benzothiazole-derived thioamides with sterically demanding, lipophilic substituents showed >100-fold selectivity in favor of SUR2B/Kir6.1. A one-carbon spacer between the heterocyclic skeleton and the thioamide moiety was observed to be crucial for affinity and selectivity. Two of the most potent and selective compounds were studied by crystal structure analyses.

Synthesis of modified 4H-1,2,4-benzothiadiazine-1,1-dioxides and determination of their affinity and selectivity for different types of K(ATP) channels.

4H-1,2,4-Benzothiadiazine-1,1-dioxides with various substituents in positions 3, 5, and 7 were synthesized and tested as K(ATP) channel agonists in artificial cell systems (CHO cells transfected with SUR1/Kir6.2, and HEK 293 transfected with SUR2B/Kir6.1) as model systems for insulin-secreting pancreatic beta-cells and for smooth muscle cells, respectively. The effects of agonists were tested in intact cells using DiBAC4(3) [bis-(1,3-dibarbituric acid)trimethine oxanol] as a membrane potential dye, and the results compared with their binding affinity for the SUR2B-type K(ATP) channels using the radioligand [(3)H]P1075. Compounds with cycloalkyl and (cycloalkyl)methyl side chains in position 3 had higher affinities towards the SUR2B/Kir6.1 receptor compared with the parent compound diazoxide (1 a). Compounds with bulky, nonpolar residues in position 3 exhibited remarkable selectivity for SUR2B-type K(ATP) channels. The compound substituted with a bulky (1-adamantyl)methyl residue exhibited micromolar affinity and activity on SUR2B-type K(ATP) channels without being able to activate the SUR1-type K(ATP) channels.

Binding studies and GRIND/ALMOND-based 3D QSAR analysis of benzothiazine type K(ATP)-channel openers.

For seventeen 1,4-benzothiazine potassium channel openers, we performed binding studies in rat aortic smooth muscle cells and cardiomyocytes, compared their binding affinities with published relaxation data, and derived 3D-QSAR models using GRIND/ALMOND descriptors. Binding affinities in smooth muscle cells range from a pK(D) of 4.76 for compound 3e to 9.10 for compound 4c. Comparison of data for smooth muscle relaxation and binding shows preferentially higher pEC(50)s for the former. In cardiomyocytes, pK(D) values range from 4.21 for 3e to 8.16 for 4c. 3D-QSAR analysis resulted in PLS models of two latent variables for all three activities with determination coefficients of 0.97 (smooth muscle relaxation) and 0.94 (smooth muscle cells- and cardiomyocytes-binding). Internal validation yielded q(2) values of 0.69, 0.66, and 0.64. The carbonyl on the N-4 substituent, the hydrogen bond acceptor at C-6, the five-membered ring at N-4, and the gem-dimethyls mainly guide strong binding and strong smooth muscle relaxation.

Molecular modeling and QSAR studies on K(ATP) channel openers of the benzopyran type.

The present paper describes our molecular modeling and quantitative structure-activity relationships (QSARs) studies on K(ATP) channel openers (KCOs) of the benzopyran type. In the first part we performed molecular modeling investigations with seven benzopyrans, varied at the C3- and C4-positions, in order to understand which molecular features at these positions are essentially effecting the biological activity. The impact of C6-substitution on biological activity was studied in the second part via HANSCH analysis. For this purpose physicochemical properties (charge distributions, lowest unoccupied molecular orbital (LUMO) energies, desolvation energies, volumes and dipole moments) were calculated for a set of 50 C6-varied benzopyrans. A QSAR equation was developed showing a relationship between the vasodilator activity and the direction of the dipole vector of the ligands. The conclusion can be drawn that a direct interaction between the C6-substituents and the receptor structure is not of primary importance. However, the substitutents influence the orientation of the whole ligand approaching the binding site. An unfavorably oriented ligand cannot bind to the binding site, thus exhibiting weak activity.

6-Sulfonylchromenes as highly potent K(ATP)-channel openers.

We synthesized K(ATP)-channel openers (KCOs) composed of the 4-pyridonechromene moiety of bimakalim (1) and a variety of sulfonyl-containing 6-substituents 4-29. Dilator potencies were measured in rat aorta and trachea. In both test systems the KCOs exhibit potency ranges of roughly 3 log units. The 6-N-phenyl-N-methylsulfonamido derivative 24 shows the highest potency. In rat aorta the potency spectrum ranges from a pEC(50) value of 8.76 to 5.68; in rat trachea it ranges from 8.01 to 4.99. On average, the dilator activity is about 0.8 log units stronger in the aorta. Aortic relaxation by chromene 13 is markedly retarded, the clinical relevance of which (e.g., preventing tachycardia) remains to be clarified. Binding affinities were determined in myocardial membranes and aortic smooth muscle cells of the rat. The affinity spectrum in myocardial membranes ranges from a pK(D) of 7.83 to 5.18; the highest affinity in aortic smooth muscle cells is measured for compound 28 (pK(D) = 8.55), whereas the lowest affinity is measured for 4 (pK(D) = 4.51). Significant selectivities discriminating between K(ATP)-channels of different organs could not be detected. PLS analysis yielded no significant correlation between vasodilator activity in aorta and chemical descriptors (GRIND). Compounds 13, 24, and 28 represent the most potent KCOs of the 4-pyridonechromene type published so far. Their 6-substituents exhibit a phenyl ring with a congruent conformational orientation in relation to the sulfonylchromene. From SAR data and conformational analysis we postulate that these new 6-substituents extend the binding site for chromene KCOs. Correspondingly, we assume that the receptor area exhibits two separate interaction sites with the capacity to bind 6-substituents: (a) one site interacting with negatively polarized partial structures (e.g., CN, NO(2), SO(2)) and (b) one spatially restricted site enabling favorable pi-interactions.