Asc-1 Transporter Regulation of Synaptic Activity via the Tonic Release of d-Serine in the Forebrain

d-Serine is a co-agonist of NMDA receptors (NMDARs) whose activity is potentially regulated by Asc-1 (SLC7A10), a transporter that displays high affinity for d-serine and glycine. Asc-1 operates as a facilitative transporter and as an antiporter, though the preferred direction of d-serine transport is uncertain. We developed a selective Asc-1 blocker, Lu AE00527, that blocks d-serine release mediated by all the transport modes of Asc-1 in primary cultures and neocortical slices. Furthermore, d-serine release is reduced in slices from Asc-1 knockout (KO) mice, indicating that d-serine efflux is the preferred direction of Asc-1. The selectivity of Lu AE00527 is assured by the lack of effect on slices from Asc-1-KO mice, and the lack of interaction with the co-agonist site of NMDARs. Moreover, in vivo injection of Lu AE00527 in P-glycoprotein-deficient mice recapitulates a hyperekplexia-like phenotype similar to that in Asc-1-KO mice. In slices, Lu AE00527 decreases the long-term potentiation at the Schaffer collateral-CA1 synapses, but does not affect the long-term depression. Lu AE00527 blocks NMDAR synaptic potentials when typical Asc-1 extracellular substrates are present, but it does not affect AMPAR transmission. Our data demonstrate that Asc-1 mediates tonic co-agonist release, which is required for optimal NMDAR activation and synaptic plasticity.

PDE1A inhibition elicits cGMP-dependent relaxation of rat mesenteric arteries

BACKGROUND AND PURPOSE

PDE1, a subfamily of cyclic nucleotide PDEs consisting of three isoforms, PDE1A, PDE1B and PDE1C, has been implicated in the regulation of vascular tone. The PDE1 isoform(s) responsible for tone regulation is unknown. This study used isoform-preferring PDE1 inhibitors, Lu AF58027, Lu AF64196, Lu AF66896 and Lu AF67897, to investigate the relative contribution of PDE1 isoforms to regulation of vascular tone.

EXPERIMENTAL APPROACH

In rat mesenteric arteries, expression and localization of Pde1 isoforms were determined by quantitative PCR and in situ hybridization, and physiological impact of PDE1 inhibition was evaluated by isometric tension recordings.

KEY RESULTS

In rat mesenteric arteries, Pde1a mRNA expression was higher than Pde1b and Pde1c. In situ hybridization revealed localization of Pde1a to vascular smooth muscle cells (VSMCs) and only minor appearance of Pde1b and Pde1c. The potency of the PDE1 inhibitors at eliciting relaxation showed excellent correlation with their potency at inhibiting PDE1A. Thus, Lu AF58027 was the most potent at inhibiting PDE1A and was also the most potent at eliciting relaxation in mesenteric arteries. Inhibition of NOS with l-NAME, soluble GC with ODQ or PKG with Rp-8-Br-PET-cGMP all attenuated the inhibitory effect of PDE1 on relaxation, whereas PKA inhibition with H89 had no effect.

CONCLUSIONS AND IMPLICATIONS

Pde1a is the dominant PDE1 isoform present in VSMCs, and relaxation mediated by PDE1A inhibition is predominantly driven by enhanced cGMP signalling. These results imply that isoform-selective PDE1 inhibitors are powerful investigative tools allowing examination of physiological and pathological roles of PDE1 isoforms.

Novel selective PDE type 1 inhibitors cause vasodilatation and lower blood pressure in rats

BACKGROUND AND PURPOSE

The PDE enzymes (PDE1-11) hydrolyse and thus inactivate cyclic nucleotides and are important in the regulation of the cardiovascular system. Here,we have investigated the effects on the cardiovascular system, of two novel selective PDE1 inhibitors, Lu AF41228 and Lu AF58027.

EXPERIMENTAL APPROACH

We used rat mesenteric small arteries (internal diameters of 200-300 μm), RT-PCR and measured isometric wall tension. Effects of Lu AF41228 and Lu AF58027 on heart rate and BP were assessed in both anaesthetized and conscious male rats.

KEY RESULTS

Nanomolar concentrations of Lu AF41228 and Lu AF58027 inhibited PDE1A, PDE1B and PDE1C enzyme activity, while micromolar concentrations were required to observe inhibitory effects at other PDEs. RT-PCR revealed expression of PDE1A, PDE1B and PDE1C in rat brain, heart and aorta, but only PDE1A and PDE1B in mesenteric arteries. In rat isolated mesenteric arteries contracted with phenylephrine or U46619, Lu AF41228 and Lu AF58027 induced concentration-dependent relaxations which were markedly reduced by inhibitors of guanylate cyclase, ODQ, and adenylate cyclase, SQ22536, and in preparations without endothelium. In anaesthetized rats, Lu AF41228 and Lu AF58027 dose-dependently lowered mean BP and increased heart rate. In conscious rats with telemetric pressure transducers, repeated dosing with Lu AF41228 lowered mean arterial BP 10-15 mmHg and increased heart rate.

CONCLUSIONS AND IMPLICATIONS

These novel PDE1 inhibitors induce vasodilation and lower BP, suggesting a potential use of these vasodilators in the treatment of hypertension and vasospasm.

Discovery and development of 11C-Lu AE92686 as a radioligand for PET imaging of phosphodiesterase10A in the brain

Phosphodiesterase 10A (PDE10A) plays a key role in the regulation of brain striatal signaling, and several pharmaceutical companies currently investigate PDE10A inhibitors in clinical trials for various central nervous system diseases. A PDE10A PET ligand may provide evidence that a clinical drug candidate reaches and binds to the target. Here we describe the successful discovery and initial validation of the novel radiolabeled PDE10A ligand 5,8-dimethyl-2-[2-((1-(11)C-methyl)-4-phenyl-1H-imidazol-2-yl)-ethyl]-[1,2,4]triazolo[1,5-a]pyridine ((11)C-Lu AE92686) and its tritiated analog (3)H-Lu AE92686.

METHODS

Initial in vitro experiments suggested Lu AE92686 as a promising radioligand, and the corresponding tritiated and (11)C-labeled compounds were synthesized. (3)H-Lu AE92686 was evaluated as a ligand for in vivo occupancy studies in mice and rats, and (11)C-Lu AE92686 was evaluated as a PET tracer candidate in cynomolgus monkeys and in humans.

RESULTS

(11)C-Lu AE92686 displayed high specificity and selectivity for PDE10A-expressing regions in the brain of cynomolgus monkeys and humans. Similar results were found in rodents using (3)H-Lu AE92686. The binding of (11)C-Lu AE92686 and (3)H-Lu AE92686 to striatum was completely and dose-dependently blocked by the structurally different PDE10A inhibitor 2-[4-(1-methyl-4-pyridin-4-yl-1H-pyrazol-3-yl)-phenoxymethyl]-quinoline (MP-10) in rodents and in monkeys. In all species, specific binding of the radioligand was seen in the striatum but not in the cerebellum, supporting the use of the cerebellum as a reference region. The binding potentials (BPND) of (11)C-Lu AE92686 in the striatum of both cynomolgus monkeys and humans were evaluated by the simplified reference tissue model with the cerebellum as the reference tissue, and BPND was found to be high and reproducible-that is, BPNDs were 6.5 ± 0.3 (n = 3) and 7.5 ± 1.0 (n = 12) in monkeys and humans, respectively.

CONCLUSION

Rodent, monkey, and human tests of labeled Lu AE92686 suggest that (11)C-Lu AE92686 has great potential as a human PET tracer for the PDE10A enzyme.

Revision of the classical dopamine D2 agonist pharmacophore based on an integrated medicinal chemistry, homology modelling and computational docking approach

The scientific advances during the 1970ies and 1980ies within the field of dopaminergic neurotransmission enabled the development of a pharmacophore that became the template for design and synthesis of dopamine D2 agonists during the following four decades. A major drawback, however, is that this model fails to accommodate certain classes of restrained dopamine D2 agonists including ergoline structures. To accommodate these, a revision of the original model was required. The present study has addressed this by an extension of the original model without compromising its obvious qualities. The revised pharmacophore contains an additional hydrogen bond donor feature, which is required for it to accommodate ergoline structures in a low energy conformation and in accordance with the steric restrictions dictated by the original model. The additional pharmacophore feature suggests ambiguity in the binding mode for certain compounds, including a series of ergoline analogues, which was reported recently. The ambiguity was confirmed by docking to a homology model of the D2 receptor as well as by pharmacological characterization of individual enantiomers of one of the analogues. The present research also addresses the potential of designing ligands that interact with the receptor in a large, distal cavity of the dopamine D2 receptor that has not previously been studied systematically. The pharmacological data indicate that this area may be a major determinant for both the dopamine D2 affinity and efficacy, which remains to be explored in future studies.

Discovery of novel PDE10 inhibitors by a robust homogeneous screening assay

AIM

To develop a homogeneous assay for high-throughput screening (HTS) of inhibitors of phosphodiesterase 10 (PDE10).

METHODS

Purified human PDE10 enzyme derived from E coli, [(3)H]-cAMP and yttrium silicate microbeads were used to develop an HTS assay based on the scintillation proximity assay (SPA) technology. This method was applied to a large-scale screening campaign against a diverse compound library and subsequent confirmation studies. Preliminary structure-activity relationship (SAR) studies were initiated through limited structural modifications of the hits.

RESULTS

The IC50 value of the control compound (papaverine) assessed with the SPA approach was comparable and consistent with that reported in the literature. Signal to background (S/B) ratio and Z' factor of the assay system were evaluated to be 5.24 and 0.71, respectively. In an HTS campaign of 71 360 synthetic and natural compounds, 67 hits displayed reproducible PDE10 inhibition, of which, 8 were chosen as the scaffold for structural modifications and subsequent SAR analysis.

CONCLUSION

The homogeneous PDE10 SPA assay is an efficient and robust tool to screen potential PDE10 inhibitors. Preliminary SAR studies suggest that potent PDE10 inhibitors could be identified and developed through this strategy.

Hit-to-lead investigation of a series of novel combined dopamine D2 and muscarinic M1 receptor ligands with putative antipsychotic and pro-cognitive potential

We describe the discovery of a series of compounds based on 1-{3-[4-(2-oxo-2,3-dihydro-benzoimidazol-1-yl)-piperidin-1-yl]-propyl}-3,4-dihydro-1H-quinolin-2-one (3), showing combined D(2) receptor affinity and M(1) receptor agonism. Based on a strategy of controlling logP, we herein describe a hit-to-lead investigation with the aim of retaining the combined D(2)/M(1) profile, while removing the propensity of the compounds to inhibit the hERG channel, as well as at obtaining acceptable pharmacokinetic properties. Although a SAR was evident for all four parameters in question, it was not possible to separate hERG channel inhibition and D(2) receptor affinity by this effort; whilst it was feasible to obtain compounds with M(1) receptor agonism, acceptable clearance, and weak hERG inhibition.

Identification of novel KCNQ4 openers by a high-throughput fluorescence-based thallium flux assay

To develop a real-time thallium flux assay for high-throughput screening (HTS) of human KCNQ4 (Kv7.4) potassium channel openers, we used CHO-K1 cells stably expressing human KCNQ4 channel protein and a thallium-sensitive dye based on the permeability of thallium through potassium channels. The electrophysiological and pharmacological properties of the cell line expressing the KCNQ4 protein were found to be in agreement with that reported elsewhere. The EC(50) values of the positive control compound (retigabine) determined by the thallium and (86)rubidium flux assays were comparable to and consistent with those documented in the literature. Signal-to-background (S/B) ratio and Z factor of the thallium influx assay system were assessed to be 8.82 and 0.63, respectively. In a large-scale screening of 98,960 synthetic and natural compounds using the thallium influx assay, 76 compounds displayed consistent KCNQ4 activation, and of these 6 compounds demonstrated EC(50) values of less than 20 μmol/L and 2 demonstrated EC(50) values of less than 1 μmol/L. Taken together, the fluorescence-based thallium flux assay is a highly efficient, automatable, and robust tool to screen potential KCNQ4 openers. This approach may also be expanded to identify and evaluate potential modulators of other potassium channels.

The synthesis and SAR of 2-arylsulfanyl-phenyl piperazinyl acetic acids as glyT-1 inhibitors

Elevation of glycine levels and activation of the NMDA receptor by inhibition of the glycine transporter 1 (GlyT-1) is a potential strategy for the treatment of schizophrenia. A novel series of GlyT-1 inhibitors have been identified containing the 2-arylsulfanyl-phenylpiperazine motif. The most prominent member of this series, (R)-4-[5-chloro-2-(4-methoxy-phenylsulfanyl)-phenyl]-2-methyl-piperazin-1-yl-acetic acid (31) is a potent glycine transporter-1 inhibitor (IC(50)=150 nM), which elevated glycine levels in rat ventral hippocampus as measured by microdialysis in vivo at doses of 1.2-4.6 mg/kg s.c.

Alternative splicing of exon 17 and a missense mutation in exon 20 of the insulin receptor gene in two brothers with a novel syndrome of insulin resistance (congenital fiber-type disproportion myopathy)

The insulin receptor (IR) in two brothers with a rare syndrome of congenital muscle fiber type disproportion myopathy (CFTDM) associated with diabetes and severe insulin resistance was studied. By direct sequencing of Epstein-Barr virus-transformed lymphocytes both patients were found to be compound heterozygotes for mutations in the IR gene. The maternal allele was alternatively spliced in exon 17 due to a point mutation in the -1 donor splice site of the exon. The abnormal skipping of exon 17 shifts the amino acid reading frame and leads to a truncated IR, missing the entire tyrosine kinase domain. In the correct spliced variant, the point mutation is silent and results in a normally translated IR. The paternal allele carries a missense mutation in the tyrosine kinase domain. All three cDNA variants were present in the lymphocytes of the patients. Purified IR from 293 cells overexpressing either of the two mutated receptors lacked basal or stimulated IR beta-subunit autophosphorylation. A third brother who inherited both normal alleles has an normal muscle phenotype and insulin sensitivity, suggesting a direct linkage of these IR mutations with the CFTDM phenotype.

Insulin and insulin-like growth factor-I receptor mediated differentiation of 3T3-F442A cells into adipocytes: effect of PI 3-kinase inhibition

The ability of insulin and insulin-like growth factors (IGF-I and IGF-II) to induce differentiation of 3T3-F442A cells into adipocytes was examined at various hormone concentrations. Both insulin and the IGFs promoted differentiation at concentrations compatible with binding to their cognate receptors, suggesting that both insulin and IGF-I receptors are capable of promoting this differentiation. Adipocyte conversion of 3T3-F442A cells was completely blocked in the presence of LY294002, a specific inhibitor of PI 3-kinase, indicating that PI 3-kinase activity plays a crucial role in the initial signalling events that trigger this differentiation process.

Insulin receptor-deficient cells as a new tool for dissecting complex interplay in insulin and insulin-like growth factors

Cell systems derived from knockout mice for the insulin receptor (IR) or the IGF-1 receptor (IGF-1R) represent unique tools for dissecting complex interplay in the actions of insulin and insulin-like growth factors through their cognate versus non-cognate receptor. In this study, we used a fibroblast cell line derived from IR-deficient mice to investigate metabolic and mitogenic effects of IGF-1 and insulin. IGF-1 was able to stimulate glucose uptake, glucose incorporation into glycogen and thymidine incorporation in such cells. Phosphatidylinositol 3-kinase and mitogen-activated protein kinase, two enzymes of major metabolic-mitogenic signaling pathways, were activated upon stimulating these cells with IGF-1. All these effects were also achieved when IR-deficient cells were stimulated with insulin. Thus, IGF-1R can represent an alternative receptor through which insulin might exert some of its effects.

Mitogenic properties of insulin and insulin analogues mediated by the insulin receptor

Insulin has traditionally been considered as a hormone essential for metabolic regulation, while the insulin-like growth factors (IGF-I and IGF-II) are postulated to be more specifically involved in growth regulation. The conventional wisdom is that they share each other's effects only at high concentrations, due to their weak affinity for the heterologous receptor. We discuss here the evidence that in the proper cellular context, insulin can be mitogenic at physiologic concentrations through its own receptor. We studied the insulin and IGF-I binding characteristics of a new model suitable for analysing insulin receptor mediated mitogenesis; that is, a T-cell lymphoma line that depends on insulin for growth, but is unresponsive to IGFs. The cells showed no specific binding of 125I-IGF-I and furthermore, no IGF-I receptor mRNA was detected by RNAse protection assay in the LB cells, in contrast with mouse brain and thymus. The cells bound at saturation about 3000 insulin molecules to receptors that had normal characteristics in terms of affinity, kinetics, pH dependence and negative co-operativity. A series of insulin analogues competed for 125I-insulin binding with relative potencies comparable to those observed in other insulin target cells. The full sequence of the insulin receptor cDNA was determined and found to be identical to the published sequence of the murine insulin receptor cDNA. The LB cell line is therefore an ideal model with which to investigate insulin mitogenic signalling without interference from the IGF-I receptor. Using this model, we have started approaching the molecular basis of insulin-induced mitogenesis, in particular the role of signalling kinetics in choosing between mitogenic and metabolic pathways.

Insulin and cortisol promote leptin production in cultured human fat cells

The aim of this study was to investigate the regulation of leptin expression and production in cultured human adipocytes using the model of in vitro differentiated human adipocytes. Freshly isolated human preadipocytes did not exhibit significant leptin mRNA and protein levels as assessed by reverse transcriptase (RT)-polymerase chain reaction (PCR) and radioimmunoassay (RIA). However, during differentiation induced by a defined adipogenic serum-free medium, cellular leptin mRNA and leptin protein released into the medium increased considerably in accordance with the cellular lipid accumulation. In fully differentiated human fat cells, insulin provoked a dose-dependent rise in leptin protein. Cortisol at a near physiological concentration of 10(-8) mol/l was found to potentiate this insulin effect by almost threefold. Removal of insulin and cortisol, respectively, was followed by a rapid decrease in leptin expression, which was reversible after readdition of the hormones. These results clearly indicate that both insulin and cortisol are potent and possibly physiological regulators of leptin expression in human adipose tissue.

Role of the time factor in signaling specificity: application to mitogenic and metabolic signaling by the insulin and insulin-like growth factor-I receptor tyrosine kinases

The signal transduction pathways activated by hormones, growth factors, and cytokines show an extraordinary degree of cross-talk and redundancy. This review addresses the question of how the specificity conferred at the binding step is maintained through the signaling network despite the convergence of multiple signals on common efferent pathways such as mitogen-activated protein (MAP) kinase. The mechanism of receptor activation by ligand-induced dimerization provides a signaling device with both a switch and a timer. The role of the time factor, ie, of signaling kinetics, as a determinant of selectivity is discussed with emphasis on the receptor tyrosine kinases and cytokine receptors, and especially mitogenic versus metabolic signaling by insulin and insulin-like growth factor-I (IGF-I).

Mitogenic potential of insulin on lymphoma cells lacking IGF-1 receptor

We have characterized an insulin-dependent T-cell lymphoma, LB, devoid of IGF-I receptor, which undergoes insulin stimulation and cell proliferation both in vitro and in vivo. In these cells, the mitogenic response can be evoked only through binding of insulin to its own receptor. This lymphoma is thus a good model for studying the molecular mechanisms involved in insulin mitogenicity. The high level of activated Ras in LB cells, even under nonproliferative conditions, shows that activation of Ras is insufficient for mitogenicity. It has been suggested earlier that separate pathways of signal transduction may emerge from Ras. The decision to activate a certain signaling pathway may depend on the activation state of other signaling routes in the cell. This may be the case in LB cells, where a signaling component activated by insulin works in concert with the Ras signaling pathway to induce mitogenesis. Yet it is still unclear whether activated Ras is a prerequisite for the insulin-induced response in LB cells.

Negative cooperativity in the insulin-like growth factor-I receptor and a chimeric IGF-I/insulin receptor

Insulin and insulin-like growth factor-I (IGF-I) share a spectrum of metabolic and growth-promoting effects, mediated through homologous receptors that belong to the tyrosine kinase family. The dissociation rate of insulin from its receptor is affected by negative cooperativity, i.e. accelerates with increased receptor occupancy. The dose-response curve for the acceleration of tracer dissociation by unlabeled insulin has a distinct bell-shaped curve, with a progressive slowing down at insulin concentrations greater than 100 nM. The kinetics of the IGF-I interaction with its receptor has not been studied in such detail. In the present work, we report that while the IGF-I receptor exhibits negative cooperativity like the insulin receptor, the concentration dependence of the dissociation kinetics is distinct from that of native human insulin by not being bell-shaped, but monophasic like that of insulin analogues mutated at the hexamer-forming surface; it is changed to an insulin-type curve by substitution of IGF-I receptor's sequence including residues 382-565 with the homologous insulin receptor domain. The data suggest that like insulin, IGF-I has a bivalent binding mode and crosslinks two distinct areas of the two alpha subunits that are close, but distinct from the equivalent insulin receptor binding sites.

The insulin-like growth factor-I receptor. Structure, ligand-binding mechanism and signal transduction

The nonclassical binding kinetics of IGF-I and insulin to their respective receptors, suggestive of negative cooperativity, can be readily explained by our recently proposed novel binding mechanism whereby the bivalent ligand bridges the two receptor alpha-subunits alternatively at opposite sites in a symmetrical receptor structure. The bivalent binding mechanism also explains bell-shaped bioactivity curves. The possible role of different binding modes versus differences in downstream signaling by insulin and IGF-I in producing specific mitogenic or metabolic responses is discussed.