Structure-based exploration and pharmacological evaluation of N-substituted piperidin-4-yl-methanamine CXCR4 chemokine receptor antagonists

Using the available structural information of the chemokine receptor CXCR4, we present hit finding and hit exploration studies that make use of virtual fragment screening, design, synthesis and structure-activity relationship (SAR) studies. Fragment 2 was identified as virtual screening hit and used as a starting point for the exploration of 31 N-substituted piperidin-4-yl-methanamine derivatives to investigate and improve the interactions with the CXCR4 binding site. Additionally, subtle structural ligand changes lead to distinct interactions with CXCR4 resulting in a full to partial displacement of CXCL12 binding and competitive and/or non-competitive antagonism. Three-dimensional quantitative structure-activity relationship (3D-QSAR) and binding model studies were used to identify important hydrophobic interactions that determine binding affinity and indicate key ligand-receptor interactions.

Detection of the secondary, low-affinity β1 -adrenoceptor site in living cells using the fluorescent CGP 12177 derivative BODIPY-TMR-CGP

Background and Purpose

CGP 12177 not only inhibits agonist effects mediated through the catecholamine site of the β₁-adrenoceptor with high affinity, but also exhibits agonist effects of its own at higher concentrations through a secondary, low-affinity β₁-adrenoceptor site or conformation. β-blocker affinities for this ‘CGP 12177’ site of the human β₁-adrenoceptor have thus far only been characterized in functional studies. Here, we used the fluorescent CGP 12177 analogue BODIPY-TMR-CGP to directly investigate receptor–ligand interactions at the secondary binding site of the β₁-adrenoceptor.

Experimental Approach

The human β₁-adrenoceptor was stably expressed in CHO cells containing a cAMP response element (CRE)-secreted placental alkaline phosphatase (SPAP) reporter gene construct. Functional responses of BODIPY-TMR-CGP were determined in the CRE-SPAP reporter gene assay, and manual and automated confocal microscopy platforms used to investigate the binding properties of BODIPY-TMR-CGP.

Key Results

BODIPY-TMR-CGP displayed a pharmacological profile similar to that of CGP 12177, retaining agonist activity at the secondary β₁-adrenoceptor site. In confocal microscopy studies, specific BODIPY-TMR-CGP binding allowed clear visualization of β₁-adrenoceptors in live cells. Using a wider concentration range of labelled ligand in a high-content fluorescence-based binding assay than is possible in radioligand binding assays, two-site inhibition binding curves of β-adrenoceptor antagonists were revealed in CHO cells expressing the human β₁-adrenoceptor, but not the β₂-adrenoceptor.

Conclusions and Implications

The fluorescent CGP 12177 analogue allowed the detection of the β₁-adrenoceptor secondary site in both functional and binding studies. This suggests that BODIPY-TMR-CGP presents an important and novel fluorescent tool to investigate the nature of the secondary β₁-adrenoceptor site.

Quantitative analysis of neuropeptide Y receptor association with beta-arrestin2 measured by bimolecular fluorescence complementation

BACKGROUND AND PURPOSE

beta-Arrestins are critical scaffold proteins that shape spatiotemporal signalling from seven transmembrane domain receptors (7TMRs). Here, we study the association between neuropeptide Y (NPY) receptors and beta-arrestin2, using bimolecular fluorescence complementation (BiFC) to directly report underlying protein-protein interactions.

EXPERIMENTAL APPROACH

Y1 receptors were tagged with a C-terminal fragment, Yc, of yellow fluorescent protein (YFP), and beta-arrestin2 fused with the complementary N-terminal fragment, Yn. After Y receptor-beta-arrestin association, YFP fragment refolding to regenerate fluorescence (BiFC) was examined by confocal microscopy in transfected HEK293 cells. Y receptor/beta-arrestin2 BiFC responses were also quantified by automated imaging and granularity analysis.

KEY RESULTS

NPY stimulation promoted association between Y1-Yc and beta-arrestin2-Yn, and the specific development of BiFC in intracellular compartments, eliminated when using non-interacting receptor and arrestin mutants. Responses developed irreversibly and were slower than for downstream Y1 receptor-YFP internalization, a consequence of delayed maturation and stability of complemented YFP. However, beta-arrestin2 BiFC measurements delivered appropriate ligand pharmacology for both Y1 and Y2 receptors, and demonstrated higher affinity of Y1 compared to Y2 receptors for beta-arrestin2. Receptor mutagenesis combined with beta-arrestin2 BiFC revealed that alternative arrangements of Ser/Thr residues in the Y1 receptor C tail could support beta-arrestin2 association, and that Y2 receptor-beta-arrestin2 interaction was enhanced by the intracellular loop mutation H155P.

CONCLUSIONS AND IMPLICATIONS

The BiFC approach quantifies Y receptor ligand pharmacology focused on the beta-arrestin2 pathway, and provides insight into mechanisms of beta-arrestin2 recruitment by activated and phosphorylated 7TMRs, at the level of protein-protein interaction.

Antagonist selective modulation of adenosine A1 and A3 receptor pharmacology by the food dye Brilliant Black BN: evidence for allosteric interactions

Allosteric binding sites on the adenosine receptor family represent potential therapeutic targets for a number of conditions involving metabolic stress. This study has identified Brilliant Black BN as a novel allosteric modulator of the adenosine A(1) and A(3) receptors. In addition to being a food dye and pharmaceutical excipient, Brilliant Black BN is commonly used within calcium mobilization assays to quench extracellular fluorescence. Brilliant Black BN (5-500 microM) had no significant effect on the calcium mobilization stimulated by the nonselective adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine in Chinese hamster ovary cells stably transfected with the human adenosine A(1) or A(3) receptor. Likewise, calcium mobilization and radioligand binding assays found that Brilliant Black BN (5-500 microM) did not significantly influence the antagonism mediated by 8-cyclopentyl-1,3-dipropylxanthine (100 nM) at the A(1) receptor. In contrast, the affinity of N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-yl]benzene acetamide (MRS1220) at the A(3) receptor and xanthine amine congener (XAC) and XAC-X-BY630 at the A(1) and A(3) receptors was significantly decreased in the presence of 500 muM Brilliant Black BN. A reduction in XAC potency at the A(1) and A(3) receptor was achieved within 1 min of Brilliant Black BN addition, despite receptors having been pre-equilibrated with antagonist. Dissociation kinetics of the fluorescent XAC derivative, XAC-X-BY630, revealed that the decrease in affinity is probably due to a significant increase in dissociation rate of the antagonist in the presence of Brilliant Black BN. Taken together, these results suggest that Brilliant Black BN can act allosterically to modify ligand affinity at A(1) and A(3) receptors.

Agonist-occupied A3 adenosine receptors exist within heterogeneous complexes in membrane microdomains of individual living cells

G protein-coupled receptors are known to be organized within different membrane compartments or microdomains of individual cells. Here, we have used a fluorescent A3 adenosine receptor (A3-AR) agonist, ABEA-X-BY630, and the technique of fluorescence correlation spectroscopy (FCS) to investigate the diffusional characteristics of functional agonist-occupied A3-AR complexes in single living cells. In Chinese hamster ovary cells expressing the human A3-AR, the fluorescent A3-AR agonist was able to inhibit forskolin-stimulated [3H]cAMP production (pEC50=8.57), and this was antagonized by the A3-selective antagonist MRS1220 (pK(B)=9.32). The fluorescent ligand also stimulated phosphoinositide hydrolysis (pEC50=7.34). Ligand binding to the A3-AR on the membranes of single cells and subsequent increases in single cell [Ca2+]i were monitored simultaneously in real time using confocal microscopy. FCS measurements in small-membrane microdomains (approximately 0.2 microm2) revealed two agonist-occupied A3-AR components with differing diffusion characteristics (diffusion coefficients=2.65x10(-8) and 1.19x10(-9) cm2/s, respectively). The binding of ligand to these two components was reduced from 5.1 and 14.9 to 2.6 and 3.3 receptors/microm2, respectively, by MRS1220 (100 nM). These data provide direct evidence for at least two populations of agonist-occupied A3-receptor complexes, showing different motilities within the membrane of single living cells.

Quantitative analysis of the formation and diffusion of A1-adenosine receptor-antagonist complexes in single living cells

The A1-adenosine receptor (A1-AR) is a G protein-coupled receptor that mediates many of the physiological effects of adenosine in the brain, heart, kidney, and adipocytes. Currently, ligand interactions with the A1-AR can be quantified on large cell populations only by using radioligand binding. To increase the resolution of these measurements, we have designed and characterized a previously undescribed fluorescent antagonist for the A1-AR, XAC-BY630, based on xanthine amine congener (XAC). This compound has been used to quantify ligand-receptor binding at a single cell level using fluorescence correlation spectroscopy (FCS). XAC-BY630 was a competitive antagonist of A1-AR-mediated inhibition of cAMP accumulation [log10 of the affinity constant (pKb) = 6.7)] and stimulation of inositol phosphate accumulation (pKb = 6.5). Specific binding of XAC-BY630 to cell surface A1-AR could also be visualized in living Chinese hamster ovary (CHO)-A1 cells by using confocal microscopy. FCS analysis of XAC-BY630 binding to the membrane of CHO-A1 cells revealed three components with diffusion times (tauD) of 62 micros (tauD1, free ligand), 17 ms (tauD2, A1-AR-ligand), and 320 ms (tauD3). Confirmation that tauD2 resulted from diffusion of ligand-receptor complexes came from the similar diffusion time observed for the fluorescent A1-AR-Topaz fusion protein (15 ms). Quantification of tauD2 showed that the number of receptor-ligand complexes increased with increasing free ligand concentration and was decreased by the selective A1-AR antagonist, 8-cyclopentyl-1,3-dipropylxanthine. The combination of FCS with XAC-BY630 will be a powerful tool for the characterization of ligand-A1-AR interactions in single living cells in health and disease.

Genetic and pharmacological analyses of involvement of Src-family, Syk and Btk tyrosine kinases in platelet shape change. Src-kinases mediate integrin alphaIIb beta3 inside-out signalling during shape change

Platelet shape change was found to be associated with an increase in protein tyrosine phosphorylation upon stimulation of thrombin-, ADP- and thromboxane A2-G-protein coupled receptors in human platelets and thromboxane A2 receptors in mouse platelets. By using PP1 and PD173956, two structurally unrelated specific inhibitors of Src-family tyrosine kinases, and mouse platelets deficient in the Src-kinase Fyn or Lyn, we show that Src-family kinases cause the increase in protein tyrosine phosphorylation. We further detected that the non-Src tyrosine kinase Syk was activated during shape change in a manner dependent on Src-family kinaseactivation. The pharmacological experiments and the studies on Fyn-, Lyn- and Syk-deficient mouse platelets showed that neither Src-family kinases nor Syk are functionally involved in shape change. Also human platelets deficient of the tyrosine kinase Btk showed a normal shape change. Binding of PAC-1 that recognizes activated integrin alphaIIb beta3 complexes on the platelet surface was enhanced during shape change and blocked by inhibition of Src-kinases. We conclude that the activation of Src-kinases and the subsequent Syk stimulation upon activation of G-protein coupled receptors are not involved in the cytoskeletal changes underlying shape change of human and mouse platelets, but that the stimulation of this evolutionary conserved pathway leads to integrin alphaIIb beta3 exposure during shape change.

Influence of receptor number on functional responses elicited by agonists acting at the human adenosine A(1) receptor: evidence for signaling pathway-dependent changes in agonist potency and relative intrinsic activity

Activation of A(1) adenosine receptors leads to the inhibition of cAMP accumulation and the stimulation of inositol phosphate accumulation via pertussis toxin-sensitive G-proteins. In this study we have investigated the signaling of the A(1) adenosine receptor in Chinese hamster ovary (CHO) cells, when expressed at approximately 203 fmol/mg (CHOA1L) and at approximately 3350 fmol/mg (CHOA1H). In CHOA1L cells, the agonists N(6)-cyclopentyladenosine (CPA), (R)-N(6)-(2-phenylisopropyl)adenosine, and 5'-(N-ethylcarboxamido)adenosine (NECA) inhibited cAMP production in a concentration-dependent manner. After pertussis toxin treatment, the agonist NECA produced a stimulation of cAMP production, whereas CPA and (R)-N(6)-(2-phenylisopropyl)adenosine were ineffective. In CHOAIH cells, however, all three agonists produced both an inhibition of adenylyl cyclase and a pertussis toxin-insensitive stimulation of adenylyl cyclase. All three agonists were more potent at inhibiting adenylyl cyclase in CHOA1H cells than in CHOA1L cells. In contrast, A(1) agonists (and particularly NECA) were less potent at stimulating inositol phosphate accumulation in CHOA1H cells than in CHOA1L cells. After pertussis toxin treatment, agonist-stimulated inositol phosphate accumulation was reduced in CHOA1H cells and abolished in CHOA1L cells. The relative intrinsic activity of NECA in stimulating inositol phosphate accumulation, compared to CPA (100%), was much greater in the presence of pertussis toxin (289.6%) than in the absence of pertussis toxin (155.2%). These data suggest that A(1) adenosine receptors can couple to both pertussis toxin-sensitive and -insensitive G-proteins in an expression level-dependent manner. These data also suggest that the ability of this receptor to activate different G-proteins is dependent on the agonist present.

Collagen mediates changes in intracellular calcium in primary mouse megakaryocytes through syk-dependent and -independent pathways

We have characterized changes in [Ca2+]i in primary mouse megakaryocytes in response to fibrillar collagen and in response to cross-linking of the collagen receptor, the integrin alpha2beta1. The response to collagen was markedly different from that seen to a triple helical collagen-related peptide (CRP), which signals via the tyrosine kinases p59(fyn) and p72(syk). This peptide binds to the collagen receptor glycoprotein VI (GPVI), but not to the integrin alpha2beta1. Collagen elicited a sustained increase in [Ca2+]i composed primarily of influx of extracellular Ca2+ with some Ca2+ release from internal stores. In contrast to CRP, this response was only partially (approximately 30%) inhibited by the src-family kinase inhibitor PP1 (10 micromol/L) or by microinjection of the tandem SH2 domains of p72(syk). Collagen also caused an increase in [Ca2+]i in megakaryocytes deficient in either p59(fyn) or p72(syk), although the response was reduced by approximately 40% in both cases: Cross-linking of the alpha2 integrin increased [Ca2+]i in these cells exclusively via Ca2+ influx. This response was reduced by approximately 50% after PP1 pretreatment, but was significantly increased in fyn-deficient megakaryocytes. Collagen therefore increases [Ca2+]i in mouse megakaryocytes via multiple receptors, including GPVI, which causes Ca2+ mobilization, and alpha2beta1, which stimulates a substantial influx of extracellular Ca2+.

Evidence for the involvement of p59fyn and p53/56lyn in collagen receptor signalling in human platelets

The binding of collagen to platelet glycoprotein VI (GPVI) leads to the subsequent activation of phospholipase Cgamma2 through a pathway that is dependent on the Fc receptor gamma (FcR gamma) chain and the tyrosine kinase p72syk. We have investigated the role of platelet Src-family kinases in this signalling pathway. The selective Src-family kinase inhibitor PP1 prevented collagen-stimulated increases in whole-cell tyrosine phosphorylation and tyrosine phosphorylation of the FcR gamma chain and p72syk. A similar set of observations was made for a collagen-related peptide (CRP), which binds to GPVI but not to the integrin alpha2beta1 (GPIa/IIa). These effects were seen at a concentration of PP1 that inhibited platelet aggregation, dense granule release and Ca2+ mobilization induced by CRP, but not aggregation and Ca2+ mobilization mediated by the G-protein-coupled receptor agonist thrombin. After stimulation by CRP or collagen, the Src-family kinases p59fyn and p53/56lyn became associated with several tyrosine-phosphorylated proteins including the FcR gamma chain. This was not true of the other platelet Src-family kinases. The association between the FcR gamma chain and p59fyn was also seen under basal conditions, and was stable only in the weak detergent Brij96 but not in Nonidet P40, suggesting a non-SH2-dependent interaction. These results provide strong evidence for the involvement of p59fyn and p53/56lyn in signalling via GPVI, with p59fyn possibly acting upstream of FcR gamma chain phosphorylation.

Comparative desensitization of the human 5-HT2A and 5-HT2C receptors expressed in the human neuroblastoma cell line SH-SY5Y

1. We have used previously characterized clones of the human neuroblastoma cell line, SH-SY5Y. constitutively expressing either the human 5-HT2A or 5-HT2C receptor to compare their desensitization profiles after exposure to 5-HT. 2. 5-HT stimulated [3H]-inositol phosphate ([3H]-IPx) production at both the 5-HT2C (pEC50=8.03+/-0.15) and 5-HT2A receptors (pEC50=7.15+/-0.08), with maximal responses occurring after exposure to 1 microM and 10 microM 5-HT, respectively. 3. Exposure of cells to maximally effective concentrations of 5-HT caused time- and concentration-dependent desensitization of [3H]-IPx formation. The 5-HT2A response desensitized slower (t1/2 = 110 min) and with lower sensitivity than that of the 5-HT2C receptor (t1/2 = 12.5 min). In each case, desensitization was blocked by co-administration of a specific receptor antagonist. Following exposure to 10 microM 5-HT for 2 h, both receptors exhibited extensive desensitization, with subsequent responses to 5-HT reduced by more than 80%. 4. 5-HT stimulated Ins(1,4,5)P3 production with a potency similar to that for [3H]-IPx production at each receptor. In both cases Ins(1,4,5)P3 levels peaked rapidly then returned to basal level within a short time. This peak consistently occurred earlier for the 5-HT2C receptor (5 s) than for the 5-HT2A receptor (20 s). 5. Prior exposure of SH-SY5Y/5-HT2C cells to 5-HT (1 microM/15 min) caused a significant decrease in the 5-HT-stimulated peak in Ins(1,4,5)P3 levels whereas no such change occurred in SH-SY5Y/5-HT2A cells following exposure to 10 microM 5-HT for 15 min. 6 These results indicate that the human 5-HT2A and 5-HT2C receptors both exhibit desensitization at the level of inositol phosphate formation when expressed in the same cellular environment, with the 5-HT2C receptor being more sensitive to 5-HT-mediated desensitization than the 5-HT2A receptor.

Syk and Fyn are required by mouse megakaryocytes for the rise in intracellular calcium induced by a collagen-related peptide

Stimulation of platelets by collagen leads to activation of a tyrosine kinase cascade resulting in secretion and aggregation. We have recently shown that this pathway involves rapid tyrosine phosphorylation of an Fc receptor gamma chain, which contains an immunoreceptor tyrosine-based activation motif (ITAM), enabling interaction with the tandem SH2 domains of the tyrosine kinase Syk. Activation of Syk lies upstream of tyrosine phosphorylation of phospholipase Cgamma2. In the present study we sought to test directly the role of the ITAM/Syk interaction and the role of the Src-related kinases in collagen receptor signaling using mouse megakaryocytes. We demonstrate that the calcium-mobilizing action of a collagen-related peptide (CRP) is kinase-dependent, inhibited by the microinjection of the tandem SH2 domains of Syk and abolished in Syk-deficient mice. Furthermore, the CRP response is abolished by the Src family kinase inhibitor PP1 and inhibited in Fyn-deficient mice. In contrast, the calcium response to the G-protein-linked receptor agonist thrombin is not significantly altered under these conditions. These results provide direct evidence of the functional importance of Fyn and Syk in collagen receptor signaling and support the megakaryocyte as a model for the study of proteins involved in this pathway.

Characterization of the 5-hydroxytryptamine2A receptor-activated cascade in rat C6 glioma cells

We have investigated the identity and intracellular cascade of responses resulting from activation of the endogenous 5-hydroxytryptamine receptor in the C6 rat glioma cell line. Sequence analysis of reverse transcription-polymerase chain reaction products derived from C6 glioma cell messenger RNA revealed complete homology with a portion of the rat 5-hydroxytryptamine2A receptor. The binding of [3H]ketanserin to cell membranes demonstrated a significant correlation with the 5-hydroxytryptamine2A receptor in rat frontal cortex. On intact cells, 5-hydroxytryptamine stimulated a concentration-dependent increase in phosphatidyl inositide turnover and intracellular [Ca2+] mediated by 5-hydroxytryptamine2A receptors. In whole-cell patch-clamp recordings, 5-hydroxytryptamine induced an outward current mediated predominantly by K+ ions (reversal potential = -80 mV). Using caged molecules containing Ca2+ or inositol 1,4,5-trisphosphate in the patch electrode solution, we found that rapid photolytic release of Ca2+ and particularly inositol 1,4,5-trisphosphate within the cytosol induced an outward current with characteristics similar to those seen after application of 5-hydroxytryptamine. Comparison between differentiated and undifferentiated cells revealed significantly higher receptor density and maximal phosphoinositide response to 5-hydroxytryptamine in undifferentiated cells but the associated rise in [Ca2+]i and activation of an outward current was observed more frequently in differentiated cells. Prolonged exposure of the cells to 5-hydroxytryptamine led to a decrease in all responses and to the down-regulation of receptor number. We conclude that the rat C6 glioma cell expresses a 5-hydroxytryptamine2A receptor identical to that found in rat brain and that stimulation of the receptor in C6 cells leads to the activation of Ca2+ activated K+ channels via phosphoinositide hydrolysis and subsequent rise in cytosolic Ca2+ ion concentration. However, the contrasting effects of differentiation on receptor number and phosphoinositide response to 5-hydroxytryptamine compared to Ca2+ release and conductance change indicate that a complex relationship exists between the component parts of the receptor-activated cascade.