Development of an immobilized GPR17 receptor stationary phase for binding determination using frontal affinity chromatography coupled to mass spectrometry

Development of an Allostery Responsive Chromatographic Method for Screening Potential Allosteric Modulator of Beta2-adrenoceptor from a Natural Product-Derived DNA-Encoded Chemical Library

Allosteric ligands are promising drugs owing to their remote regulations of the orthosteric ligand signaling pathway. There are few allosteric ligands due to the lack of handy and efficacious method for the screening. Herein, we developed an affinity chromatographic method for allosteric ligand screening by immobilizing purified beta2 adrenoceptor (β₂-AR) onto macroporous silica gel by a two-point tethering method. The method relies on the occupation of the orthosteric site by an antagonist and the chelation of N-terminal His-tag of the receptor and Ni²⁺ coated on the gel. The immobilized β₂-AR demonstrated the greatest allosteric responsive feature when Cmpd-15 (0.25 μM) was included in the mobile phase. Under the same conditions, the association constants of three agonists (salbutamol, terbutaline, and tulobuterol) reduced to 47%, 19%, and 27% compared with the data without the inclusion of Cmpd-15 in the mobile phase. APF was screened as a potential allosteric modulator of β₂-AR by applying the immobilized receptor in a natural product-derived DNA-encoded chemical library (DEL). Relying on these results, we reasoned that the current method has potential in screening allosteric ligands of the receptor. We expect that it is applicable for the discovery of new allosteric binding sites of a target protein and screening allosteric modulators of the other receptors from complex samples.

Mechanistic insight on the role of leukotriene receptors in ischemic-reperfusion injury

Leukotrienes (LT) are a class of inflammatory mediators produced by the 5-lipoxygenase (5-LO) enzyme from arachidonic acid (AA). We discussed the various LT inhibitors and downstream pathway modulators, such as Mitogen-Activated Protein Kinases (MAPK), Phosphatidylinositol 3-Kinase/Protein Kinase B (PI3K/Akt), 5'-Adenosine Monophosphate-Activated Protein Kinase (AMPK), Protein Kinase C (PKC), Nitric Oxide (NO), Bradykinin, Early Growth Response-1 (Egr-1), Nuclear Factor-κB (NF-κB), and Tumor Necrosis Factor-Alpha (TNF-α), which in turn regulate various metabolic and physiological processes involving I/R injury. A systematic literature review of Bentham, Scopus, PubMed, Medline, and EMBASE (Elsevier) databases was carried out to understand the nature and mechanistic interventions of the leukotriene receptor modulations in ischemic injury. In the pathophysiology of I/R injuries, LT has been found to play an important role. I/R injury affects most of the vital organs and is characterized by inflammation, oxidative stress, cell death, and apoptosis leading to morbidity and mortality. sThis present review focuses on the various LT receptors, i.e., CysLT, LTC4, LTD4, and LTE4, involved in developing I/R injury in organs, such as the brain, spinal cord, heart, kidney, liver, and intestine.

Affinity chromatography: A review of trends and developments over the past 50 years

The field of affinity chromatography, which employs a biologically-related agent as the stationary phase, has seen significant growth since the modern era of this method began in 1968. This review examines the major developments and trends that have occurred in this technique over the past five decades. The basic principles and history of this area are first discussed. This is followed by an overview of the various supports, immobilization strategies, and types of binding agents that have been used in this field. The general types of applications and fields of use that have appeared for affinity chromatography are also considered. A survey of the literature is used to identify major trends in these topics and important areas of use for affinity chromatography in the separation, analysis, or characterization of chemicals and biochemicals.

Cell-membrane coated iron oxide nanoparticles for isolation and specific identification of drug leads from complex matrices

The lack of suitable tools for the identification of potential drug leads from complex matrices is a bottleneck in drug discovery. Here, we report a novel method to screen complex matrices for new drug leads targeting transmembrane receptors. Using α3β4 nicotinic receptors as a model system, we successfully demonstrated the ability of this new tool for the specific identification and effective extraction of binding compounds from complex mixtures. The formation of cell-membrane coated nanoparticles was confirmed by transmission electron microscopy. In particular, we have developed a direct tool to evaluate the presence of functional α3β4 nicotinic receptors on the cell membrane. The specific ligand binding to α3β4 nicotinic receptors was examined through ligand fishing experiments and confirmed by high-performance liquid chromatography coupled with diode-array detection and electrospray ionization mass spectrometry. This tool has a great potential to transform the drug discovery process focusing on identification of compounds targeting transmembrane proteins, as more than 50% of all modern pharmaceuticals use membrane proteins as prime targets.

GPR17 receptor modulators and their therapeutic implications: review of recent patents

INTRODUCTION

The GPR17 receptor, phylogenetically related to both purinergic P2Y and CysLT receptors, is mainly expressed in the CNS and, in general, in organs that can typically undergo ischemic damage. This receptor is involved in various pathologies including stroke, brain and spinal cord trauma, multiple sclerosis and in all diseases characterized by neuronal and myelin dysfunction. Therefore, there is a strong needed to identify molecules capable of binding specifically to GPR17 receptors.

AREAS COVERED

The review provides a summary of patents, published between 2009 and 2018, on chemicals and biologics and their clinical use. In this work, information is reported about the representative structures and biological activity of recently developed GPR17 receptor ligands.

EXPERT OPINION

The GPR17 receptor is an enigmatic receptor and an interesting therapeutic target in a variety of brain disorders and demyelinating diseases such as multiple sclerosis, stroke, schizophrenia, and depression. The modulation of this receptor could also be potentially useful in obesity treatment. Unfortunately, so far, there are no compounds under investigation in clinical trials but many researchers and companies are investing in the discovery of future potential GPR17 receptor drugs.

GPR17 mediates ischemia-like neuronal injury via microglial activation

GPR17 is a G (i)-coupled dual receptor, linked to P2Y and CysLT receptors stimulated by uracil nucleotides and cysteinyl leukotrienes, respectively. Recent evidence has demonstrated that GPR17 inhibition ameliorates the progression of cerebral ischemic injury by regulating neuronal death and microglial activation. The present study aimed to assess the detailed regulatory roles of this receptor in oxygen-glucose deprivation/recovery (OGD/R)-induced ischemia-like injury in vitro and explore the underlying mechanism. The results demonstrated that OGD/R induced ischemic neuronal injury and microglial activation, including enhanced phagocytosis and increased inflammatory cytokine release in neuron‑glial mixed cultures of cortical cells. GPR17 upregulation during OGD/R was spatially and temporally correlated with neuronal injury and microglial activation. In addition, GPR17 knockdown inhibited OGD/R-induced responses in neuron-glial mixed cultures. GPR17 knockdown also attenuated cell injury induced by the agonist leukotriene D₄ (LTD₄) or uridine 5′-diphosphate (UDP) in neuron-glial mixed cultures. However, GPR17 knockdown did not affect OGD/R-induced ischemic neuronal injury in primary cultures of neurons. In primary astrocyte cultures, neither GPR17 nor OGD/R induced injury. By contrast, GPR17 knockdown ameliorated OGD/R-induced microglial activation, boosting phagocytosis and inflammatory cytokine release in primary microglia cultures. Finally, the results demonstrated that the conditioned medium of microglia pretreated with OGD/R induced neuronal death, and the neuronal injury was significantly inhibited by GPR17 knockdown. These findings suggested that GPR17 may mediate ischemia-like neuronal injury and microglial activation in vitro; however, the protective effects on ischemic neuronal injury might depend upon microglial activation. Whether GPR17 regulates neuronal injury mediated by oligodendrocyte linkage remains to be investigated.

Pharmacological Properties and Biological Functions of the GPR17 Receptor, a Potential Target for Neuro-Regenerative Medicine

In 2006, cells heterologously expressing the "orphan" receptor GPR17 were shown to acquire responses to both uracil nucleotides and cysteinyl-leukotrienes, two families of signaling molecules accumulating in brain or heart as a result of hypoxic/traumatic injuries. In subsequent years, evidence of GPR17 key role in oligodendrogenesis and myelination has highlighted it as a "model receptor" for new therapies in demyelinating and neurodegenerative diseases. The apparently contrasting evidence in the literature about the role of GPR17 in promoting or inhibiting myelination can be due to its transient expression in the intermediate stages of differentiation, exerting a pro-differentiating function in early oligodendrocyte precursor cells (OPCs), and an inhibitory role in late stage maturing cells. Meanwhile, several papers extended the initial data on GPR17 pharmacology, highlighting a "promiscuous" behavior of this receptor; indeed, GPR17 is able to respond to other emergency signals like oxysterols or the pro-inflammatory cytokine SDF-1, underlying GPR17 ability to adapt its responses to changes of the surrounding extracellular milieu, including damage conditions. Here, we analyze the available literature on GPR17, in an attempt to summarize its emerging biological roles and pharmacological properties.

Cysteinyl Leukotrienes as Potential Pharmacological Targets for Cerebral Diseases

Cysteinyl leukotrienes (CysLTs) are potent lipid mediators widely known for their actions in asthma and in allergic rhinitis. Accumulating data highlights their involvement in a broader range of inflammation-associated diseases such as cancer, atopic dermatitis, rheumatoid arthritis, and cardiovascular diseases. The reported elevated levels of CysLTs in acute and chronic brain lesions, the association between the genetic polymorphisms in the LTs biosynthesis pathways and the risk of cerebral pathological events, and the evidence from animal models link also CysLTs and brain diseases. This review will give an overview of how far research has gone into the evaluation of the role of CysLTs in the most prevalent neurodegenerative disorders (ischemia, Alzheimer's and Parkinson's diseases, multiple sclerosis/experimental autoimmune encephalomyelitis, and epilepsy) in order to understand the underlying mechanism by which they might be central in the disease progression.

Affinity chromatographic methodologies based on immobilized voltage dependent anion channel isoform 1 and application in protein-ligand interaction analysis and bioactive compounds screening from traditional medicine

Voltage dependent anion channel isoform 1 (VDAC-1) serves as an attractive target of anti-cancer drugs by mediating the entry and exit of metabolites between cytoplasm and mitochondria. This work reports on the preparation of a VDAC-1-based bioaffinity chromatographic stationary phase by linking the protein on lecithin modified microspheres. An assay of chromatographic methods including frontal analysis, zonal elution, injection dependent analysis and nonlinear chromatography were utilized to investigate the bindings of ATP, NADH and NADPH to VDAC-1. Electrostatic interactions were found to be main forces during these bindings. The calculated association constants of the three ligands to VDAC-1 showed good agreements between diverse chromatographic methods. Validated application of the stationary phase was performed by screening anti-cancer compounds of Rheum officinale Baill. using high performance affinity chromatography coupled with electrospray ionization-quadrupole time of flight mass spectrometry. Chrysophanol, emodin, rhein, aloe-emodin and catechin were identified as the bioactive components of the herb. These compounds targeted VDAC-1 through Thr207 and the N-terminal region of the protein. Taken together, the current stationary phase was possible to become a promising tool for protein-ligand interaction analysis and anti-cancer drug screening from complex matrices.

Discovery and characterization of EIIB, a new α-conotoxin from Conus ermineus venom by nAChRs affinity capture monitored by MALDI-TOF/TOF mass spectrometry

Animal toxins are peptides that often bind with remarkable affinity and selectivity to membrane receptors such as nicotinic acetylcholine receptors (nAChRs). The latter are, for example, targeted by α-conotoxins, a family of peptide toxins produced by venomous cone snails. nAChRs are implicated in numerous physiological processes explaining why the design of new pharmacological tools and the discovery of potential innovative drugs targeting these receptor channels appear so important. This work describes a methodology developed to discover new ligands of nAChRs from complex mixtures of peptides. The methodology was set up by the incubation of Torpedo marmorata electrocyte membranes rich in nAChRs with BSA tryptic digests (>100 peptides) doped by small amounts of known nAChRs ligands (α-conotoxins). Peptides that bind to the receptors were purified and analyzed by MALDI-TOF/TOF mass spectrometry which revealed an enrichment of α-conotoxins in membrane-containing fractions. This result exhibits the binding of α-conotoxins to nAChRs. Negative controls were performed to demonstrate the specificity of the binding. The usefulness and the power of the methodology were also investigated for a discovery issue. The workflow was then applied to the screening of Conus ermineus crude venom, aiming at characterizing new nAChRs ligands from this venom, which has not been extensively investigated to date. The methodology validated our experiments by allowing us to bind two α-conotoxins (α-EI and α-EIIA) which have already been described as nAChRs ligands. Moreover, a new conotoxin, never described to date, was also captured, identified and sequenced from this venom. Classical pharmacology tests by radioligand binding using a synthetic homologue of the toxin confirm the activity of the new peptide, called α-EII_B. The K_i value of this peptide for Torpedo nicotinic receptors was measured at 2.2 ± 0.7 nM.

Synthesis and Ability of New Ligands for G Protein-Coupled Receptors 17 (GPR17)

Background

GPR17 is believed to be a novel target for the development of new therapeutic approaches to human stroke and multiple sclerosis. Hence, the selection of GPR17 ligands may be a potent way to reduce the progression of ischemic damage.

Material/Methods

New potential ligands for GPR17, mono-, di-, and triphosphate adenosine nucleotides substituted at N6-position with a methyl and a cyclopentyl group were synthesized. The ability of new ligands to bind GPR17 was evaluated using frontal affinity chromatography-mass spectrometry (FAC-MS) method. Cangrelor, MRS2179, and uridine diphosphate were selected as the reference compounds.

Results

The new triphosphate derivatives 9 and 10 were considered as the new GPR17 ligands. The compound 10 was eluted with breakthrough time (bt) between cangrelor and MRS 2179 (compound 10, bt=12.25; cangrelor, bt=24.55, and MRS 2179, bt=7.10), while the breakthrough volume of compound 9 was similar to that of MRS 2179 (compound 9, bt=7.53 and MRS 2179, bt=7.10).

Conclusions

N6-cyclopentyATP 10 is medium-high affinity ligand of GPR17, while the corresponding N6-methyl derivative 9 is a medium affinity ligand similar to MRS 2179. Hence, the new N6-cyclopentylATP 10 might be a good candidate for the pharmacological characterization of GPR17.

Confirming therapeutic target of protopine using immobilized β2 -adrenoceptor coupled with site-directed molecular docking and the target-drug interaction by frontal analysis and injection amount-dependent method

Drug-protein interaction analysis is pregnant in designing new leads during drug discovery. We prepared the stationary phase containing immobilized β₂ -adrenoceptor (β₂ -AR) by linkage of the receptor on macroporous silica gel surface through N,N'-carbonyldiimidazole method. The stationary phase was applied in identifying antiasthmatic target of protopine guided by the prediction of site-directed molecular docking. Subsequent application of immobilized β₂ -AR in exploring the binding of protopine to the receptor was realized by frontal analysis and injection amount-dependent method. The association constants of protopine to β₂ -AR by the 2 methods were (1.00 ± 0.06) × 10⁵ M^-1 and (1.52 ± 0.14) × 10⁴ M^-1 . The numbers of binding sites were (1.23 ± 0.07) × 10^-7 M and (9.09 ± 0.06) × 10^-7 M, respectively. These results indicated that β₂ -AR is the specific target for therapeutic action of protopine in vivo. The target-drug binding occurred on Ser¹⁶⁹ in crystal structure of the receptor. Compared with frontal analysis, injection amount-dependent method is advantageous to drug saving, improvement of sampling efficiency, and performing speed. It has grave potential in high-throughput drug-receptor interaction analysis.

The G Protein-Coupled Receptor GPR17: Overview and Update

The GPR17 receptor is a G protein-coupled receptor (GPCR) that seems to respond to two unrelated families of endogenous ligands: nucleotide sugars (UDP, UDP-galactose, and UDP-glucose) and cysteinyl leukotrienes (LTD₄ , LTC₄ , and LTE₄ ), with significant affinity at micromolar and nanomolar concentrations, respectively. This receptor has a broad distribution at the level of the central nervous system (CNS) and is found in neurons and in a subset of oligodendrocyte precursor cells (OPCs). Unfortunately, disparate results emerging from different laboratories have resulted in a lack of clarity with regard to the role of GPR17-targeting ligands in OPC differentiation and in myelination. GPR17 is also highly expressed in organs typically undergoing ischemic damage and has various roles in specific phases of adaptations that follow a stroke. Under such conditions, GPR17 plays a crucial role; in fact, its inhibition decreases the progression of ischemic damage. This review summarizes some important features of this receptor that could be a novel therapeutic target for the treatment of demyelinating diseases and for repairing traumatic injury.

Lipophilicity and biomimetic properties measured by HPLC to support drug discovery

HPLC methods that use chromatographic retention times for gaining information about the properties of compounds for the purpose of designing drug molecules are reviewed. Properties, such as lipophilicity, protein binding, phospholipid binding, and acid/base character can be incorporated in the design of molecules with the right biological distribution and pharmacokinetic profile to become an effective drug. Standardization of various methodologies is suggested in order to obtain data suitable for inter-laboratory comparison. The published HPLC methods for lipophilicity, acid/base character, protein and phospholipid binding are critically reviewed and compared with each other using the solvation equation approach. One of the most important discussion points is how these data can be used in models and how they can influence the drug discovery process. Therefore, the published models for volume of distribution, unbound volume of distribution and drug efficiency are also discussed. The general relationships between the chemical structure and biomimetic HPLC properties are described in view of ranking and selecting putative drug molecules.

α,β-meATP mimics the effects of the purinergic neurotransmitter in the human and rat colon

The purine receptor involved in inhibitory responses in the gastrointestinal tract has been recently identified. P2Y1 receptor activation mediates the fast component of the inhibitory junction potential (IJPf) and the non-nitrergic relaxation. The aim of the present work has been to investigate which purinergic agonist better mimics endogenous responses. We used different agonist and antagonist of P2 receptors. Contractility and microelectrode experiments were used to compare the effects of exogenously added purines and electrical field stimulation (EFS)-induced nerve mediated effects in rat and human colonic strips. In rat colon, the IJPf and EFS-induced inhibition of contractions were concentration-dependently inhibited by the P2Y1 antagonist MRS2500 but not by iso-PPADS or NF023 (P2X antagonists) up to 1 μM. In samples from human colon, EFS-induced inhibition of contractions was inhibited by either MRS2500 or apamin (1 μM) but not by iso-PPADS. In both species, α,β-meATP, a stable analog of ATP, caused inhibition of spontaneous contractions. α,β-meATP effect was concentration-dependent (EC50: 2.7 μM rat, 4.4 μM human) and was antagonized by either MRS2500 or apamin but unaffected by P2X antagonists. ATP, ADP, β-NAD and ADP-ribose inhibited spontaneous contractions but did not show the same sensitivity profile to purine receptor antagonists as EFS-induced inhibition of contractions. The effect of α,β-meATP is due to P2Y1 receptor activation leading the opening of sKca channels. Accordingly, α,β-meATP mimics the endogenous purinergic mediator. In contrast, exogenously added putative neurotransmitters do not exactly mimic the endogenous mediator. Quick degradation by ecto-nuclease or different distribution of receptors (junctionally vs extrajunctionally) might explain these results.

Acetylcholinesterase immobilized capillary reactors coupled to protein coated magnetic beads: a new tool for plant extract ligand screening

The use of immobilized capillary enzyme reactors (ICERs) and enzymes coated to magnetic beads ((NT or CT)-MB) for ligand screening has been adopted as a new technique of high throughput screening (HTS). In this work the selected target was the enzyme acetylcholinesterase (AChE), which acts on the central nervous system and is a validated target for the treatment of Alzheimer's disease, as well as for new insecticides. A new approach for the screening of plant extracts was developed based on the ligand fishing experiments and zonal chromatography. For that, the magnetic beads were used for the ligand fishing experiments and capillary bioreactors for the activity assays. The latter was employed also under non-linear conditions to determine the affinity constants of known ligands, for the first time, as well as for the active fished ligand.

UDP-glucose enhances outward K(+) currents necessary for cell differentiation and stimulates cell migration by activating the GPR17 receptor in oligodendrocyte precursors

In the developing and mature central nervous system, NG2 expressing cells comprise a population of cycling oligodendrocyte progenitor cells (OPCs) that differentiate into mature, myelinating oligodendrocytes (OLGs). OPCs are also characterized by high motility and respond to injury by migrating into the lesioned area to support remyelination. K(+) currents in OPCs are developmentally regulated during differentiation. However, the mechanisms regulating these currents at different stages of oligodendrocyte lineage are poorly understood. Here we show that, in cultured primary OPCs, the purinergic G-protein coupled receptor GPR17, that has recently emerged as a key player in oligodendrogliogenesis, crucially regulates K(+) currents. Specifically, receptor stimulation by its agonist UDP-glucose enhances delayed rectifier K(+) currents without affecting transient K(+) conductances. This effect was observed in a subpopulation of OPCs and immature pre-OLGs whereas it was absent in mature OLGs, in line with GPR17 expression, that peaks at intermediate phases of oligodendrocyte differentiation and is thereafter downregulated to allow terminal maturation. The effect of UDP-glucose on K(+) currents is concentration-dependent, blocked by the GPR17 antagonists MRS2179 and cangrelor, and sensitive to the K(+) channel blocker tetraethyl-ammonium, which also inhibits oligodendrocyte maturation. We propose that stimulation of K(+) currents is responsible for GPR17-induced oligodendrocyte differentiation. Moreover, we demonstrate, for the first time, that GPR17 activation stimulates OPC migration, suggesting an important role for this receptor after brain injury. Our data indicate that modulation of GPR17 may represent a strategy to potentiate the post-traumatic response of OPCs under demyelinating conditions, such as multiple sclerosis, stroke, and brain trauma.

Open tubular columns containing the immobilized ligand binding domain of peroxisome proliferator-activated receptors α and γ for dual agonists characterization by frontal affinity chromatography with mass spectrometry detection

The peroxisome proliferator-activated receptors (PPARs) belong to the nuclear receptor superfamily. In the last years novel PPARs ligands have been identified and these include PPARα/γ dual agonists. To rapidly identify novel PPARs dual ligands, a robust binding assay amenable to high-throughput screening toward PPAR isoforms would be desirable. In this work we describe a parallel assay based on the principles of frontal affinity chromatography coupled to mass spectrometry (FAC-MS) that can be used to characterize dual agonists. For this purpose the ligand binding domain of PPARα receptor was immobilized onto the surface of open tubular capillaries to create new PPAR-alpha-OT columns to be used in parallel with PPAR-gamma-OT columns. The two biochromatographic systems were used in both ranking and Kd experiments toward new ureidofibrate-like dual agonists for subtype selectivity ratio determination. In order to validate the system, the Kd values determined by frontal analysis chromatography were compared to the affinity constants obtained by ITC experiments. The results of this study strongly demonstrate the specific nature of the interaction of the ligands with the two immobilized receptor subtypes.

Development of new chromatographic tools based on A2A adenosine receptor subtype for ligand characterization and screening by FAC-MS

A liquid chromatographic stationary phase containing immobilized membranes from cells expressing A(2A) adenosine receptor (A(2A)AR) is firstly described. Cellular membranes from CHO cells stably transfected with human A(2A)AR vector (A(2A)(+)) and from the same cell line transfected with the corresponding empty vector (A(2A)(-)) were entrapped on immobilized artificial membrane (IAM) support and packed into 6.6 mm I.D. glass columns to create A(2A)(+)-IAM and A(2A)(-)-IAM stationary phases. Frontal chromatography experiments on both A(2A)(+)-IAM and A(2A)(-)-IAM columns demonstrated the presence of a low specific interaction with the receptor. However, immobilized A(2A) retained its ability to specifically bind known ligands as demonstrated by the agreement of the calculated K(d) values with two different chromatographic protocols in comparison to previously reported data. In order to maximize the specific interaction, the same cellular membranes were immobilized on the inner surface of a silica capillary (40 cm × 100 μm I.D.) by non-covalent interactions using the avidin-biotin coupling system to create two open tubular columns A(2A)(+)-OT and A(2A)(-)-OT. The open tubular system was characterized by ranking experiments for affinity studies in mixture useful for the selection of new potential candidates.

The new P2Y-like receptor G protein-coupled receptor 17 mediates acute neuronal injury and late microgliosis after focal cerebral ischemia in rats

G protein-coupled receptor 17 (GPR17), the new P2Y-like receptor, is phylogenetically related to the P2Y and cysteinyl leukotriene receptors, and responds to both uracil nucleotides and cysteinyl leukotrienes. GPR17 has been proposed to be a damage sensor in ischemic stroke; however, its role in brain inflammation needs further detailed investigation. Here, we extended previous studies on the spatiotemporal profiles of GPR17 expression and localization, and their implications for brain injury after focal cerebral ischemia. We found that in the ischemic core, GPR17 mRNA and protein levels were upregulated at both 12-24 h and 7-14 days, but in the boundary zone the levels increased 7-14 days after reperfusion. The spatiotemporal pattern of GPR17 expression well matched the acute and late (subacute/chronic) responses in the ischemic brain. According to previous findings, in the acute phase, after ischemia (24 h), upregulated GPR17 was localized in injured neurons in the ischemic core and in a few microglia in the ischemic core and boundary zone. In the late phase (14 days), it was localized in microglia, especially in activated (ED1-positive) microglia in the ischemic core, but weakly in most microglia in the boundary zone. No GPR17 was detectable in astrocytes. GPR17 knockdown by a small interfering RNA attenuated the neurological dysfunction, infarction, and neuron loss at 24 h, and brain atrophy, neuron loss, and microglial activation at 14 days after reperfusion. Thus, GPR17 might mediate acute neuronal injury and late microgliosis after focal cerebral ischemia.

Frontal affinity chromatography with MS detection of the ligand binding domain of PPARγ receptor: ligand affinity screening and stereoselective ligand-macromolecule interaction

In this study we report the development of new chromatographic tools for binding studies based on the gamma isoform ligand binding domain (LBD) of peroxisome proliferator-activated receptor (PPARγ) belonging to the nuclear receptor superfamily of ligand-activated transcription factors. PPARγ subtype plays important roles in the functions of adipocytes, muscles, and macrophages with a direct impact on type 2 diabetes, dyslipidemia, atherosclerosis, and cardiovascular disease. In order to set up a suitable immobilization chemistry, the LBD of PPARγ receptor was first covalently immobilized onto the surface of aminopropyl silica particles to create a PPARγ-Silica column for zonal elution experiments and then onto the surface of open tubular (OT) capillaries to create PPARγ-OT capillaries following different immobilization conditions. The capillaries were used in frontal affinity chromatography coupled to mass spectrometry (FAC-MS) experiments to determine the relative binding affinities of a series of chiral fibrates. The relative affinity orders obtained for these derivatives were consistent with the EC(50) values reported in literature. The optimized PPARγ-OT capillary was validated by determining the K(d) values of two selected compounds. Known the role of stereoselectivity in the binding of chiral fibrates, for the first time a detailed study was carried out by analysing two enantioselective couples on the LBD-PPARγ capillary by FAC and a characteristic two-stairs frontal profile was derived as the result of the two saturation events. All the obtained data indicate that the immobilized form of PPARγ-LBD retained the ability to specifically bind ligands.

Agonist-induced desensitization/resensitization of human G protein-coupled receptor 17: a functional cross-talk between purinergic and cysteinyl-leukotriene ligands

G protein-coupled receptor (GPR) 17 is a P2Y-like receptor that responds to both uracil nucleotides (as UDP-glucose) and cysteinyl-leukotrienes (cysLTs, as LTD(4)). By bioinformatic analysis, two distinct binding sites have been hypothesized to be present on GPR17, but little is known on their putative cross-regulation and on GPR17 desensitization/resensitization upon agonist exposure. In this study, we investigated in GPR17-expressing 1321N1 cells the cross-regulation between purinergic- and cysLT-mediated responses and analyzed GPR17 regulation after prolonged agonist exposure. Because GPR17 receptors couple to G(i) proteins and adenylyl cyclase inhibition, both guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding and the cAMP assay have been used to investigate receptor functional activity. UDP-glucose was found to enhance LTD(4) potency in mediating activation of G proteins and vice versa, possibly through an allosteric mechanism. Both UDP-glucose and LTD(4) induced a time- and concentration-dependent GPR17 loss of response (homologous desensitization) with similar kinetics. GPR17 homologous desensitization was accompanied by internalization of receptors inside cells, which occurred in a time-dependent manner with similar kinetics for both agonists. Upon agonist removal, receptor resensitization occurred with the typical kinetics of G protein-coupled receptors. Finally, activation of GPR17 by UDP-glucose (but not vice versa) induced a partial heterologous desensitization of LTD(4)-mediated responses, suggesting that nucleotides have a hierarchy in producing desensitizing signals. These findings suggest a functional cross-talk between purinergic and cysLT ligands at GPR17. Because of the recently suggested key role of GPR17 in brain oligodendrogliogenesis and myelination, this cross-talk may have profound implications in fine-tuning cell responses to demyelinating and inflammatory conditions when these ligands accumulate at lesion sites.

Phenotypic changes, signaling pathway, and functional correlates of GPR17-expressing neural precursor cells during oligodendrocyte differentiation

The developing and mature central nervous system contains neural precursor cells expressing the proteoglycan NG2. Some of these cells continuously differentiate to myelin-forming oligodendrocytes; knowledge of the destiny of NG2(+) precursors would benefit from the characterization of new key functional players. In this respect, the G protein-coupled membrane receptor GPR17 has recently emerged as a new timer of oligodendrogliogenesis. Here, we used purified oligodendrocyte precursor cells (OPCs) to fully define the immunophenotype of the GPR17-expressing cells during OPC differentiation, unveil its native signaling pathway, and assess the functional consequences of GPR17 activation by its putative endogenous ligands, uracil nucleotides and cysteinyl leukotrienes (cysLTs). GPR17 presence was restricted to very early differentiation stages and completely segregated from that of mature myelin. Specifically, GPR17 decorated two subsets of slowly proliferating NG2(+) OPCs: (i) morphologically immature cells expressing other early proteins like Olig2 and PDGF receptor-α, and (ii) ramified preoligodendrocytes already expressing more mature factors, like O4 and O1. Thus, GPR17 is a new marker of these transition stages. In OPCs, GPR17 activation by either uracil nucleotides or cysLTs resulted in potent inhibition of intracellular cAMP formation. This effect was counteracted by GPR17 antagonists and receptor silencing with siRNAs. Finally, uracil nucleotides promoted and GPR17 inhibition, by either antagonists or siRNAs, impaired the normal program of OPC differentiation. These data have implications for the in vivo behavior of NG2(+) OPCs and point to uracil nucleotides and cysLTs as main extrinsic local regulators of these cells under physiological conditions and during myelin repair.

Expression of the new P2Y-like receptor GPR17 during oligodendrocyte precursor cell maturation regulates sensitivity to ATP-induced death

The P2Y-like receptor GPR17 is expressed by adult neural progenitor cells, suggesting a role in lineage determination. Here, we characterized GPR17 expression and function in mouse cortical primary astrocytes/precursor cell cultures. GPR17 is expressed by a subpopulation of oligodendrocyte precursor cells (OPCs), but not by astrocytes. This expression pattern was also confirmed in vivo. In vitro, GPR17 expression was markedly influenced by culturing conditions. In the presence of growth factors (GFs), no significant GPR17 expression was found. When cultures were shifted to a differentiating medium, a dramatic, time-dependent increase in the number of highly branched GPR17-positive cells was observed. Under these conditions, GPR17 was induced in the totality of O4-positive immature oligodendrocytes. Instead, in cultures originally grown in the absence of GFs, GPR17 was already expressed in morphologically more mature OPCs. Shifting of these cultures to differentiating conditions induced GPR17 only in a subpopulation of O4-positive cells. Under both culture protocols, appearance of more mature CNPase- and MBP-positive cells was associated to a progressive loss of GPR17. GPR17 expression also sensitized cells to adenine nucleotide-induced cytotoxicity, whereas activation with uracil nucleotides promoted differentiation towards a more mature phenotype. We suggest that GFs may keep OPCs in a less differentiated stage by restraining GPR17 expression, and that, under permissive conditions, GPR17 contributes to OPCs differentiation. However, upon high extracellular adenine nucleotide concentrations, as during trauma and ischemia, GPR17 sensitizes cells to cytotoxicity. This double-edged sword role may be exploited to unveil new therapeutic approaches to acute and chronic brain disorders.

Recent developments in protein-ligand affinity mass spectrometry

This review provides an overview of direct and indirect technologies to screen protein–ligand interactions with mass spectrometry. These technologies have as a key feature the selection or affinity purification of ligands in mixtures prior to detection. Specific fields of interest for these technologies are metabolic profiling of bioactive metabolites, natural extract screening, and the screening of libraries for bioactives, such as parallel synthesis libraries and small combichem libraries. The review addresses the principles of each of the methods discussed, with a focus on developments in recent years, and the applicability of the methods to lead generation and development in drug discovery.

Figure

Frontal affinity chromatography-mass spectrometry useful for characterization of new ligands for GPR17 receptor

The application of frontal affinity chromatography-mass spectrometry (FAC-MS), along with molecular modeling studies, to the screening of potential drug candidates toward the recently deorphanized G-protein-coupled receptor (GPCR) GPR17 is shown. GPR17 is dually activated by uracil nucleotides and cysteinyl-leukotrienes, and is expressed in organs typically undergoing ischemic damage (i.e., brain, heart and kidney), thus representing a new pharmacological target for acute and chronic neurodegeneration. GPR17 was entrapped on an immobilized artificial membrane (IAM), and this stationary phase was used to screen a library of nucleotide derivatives by FAC-MS to select high affinity ligands. The chromatographic results have been validated with a reference functional assay ([(35)S]GTPgammaS binding assay). The receptor nucleotide-binding site was studied by setting up a column where a mutated GPR17 receptor (Arg255Ile) has been immobilized. The chromatographic behavior of the tested nucleotide derivatives together with in silico studies have been used to gain insights into the structure requirement of GPR17 ligands.

Distinct expression and ligand-binding profiles of two constitutively active GPR17 splice variants

Background and purpose:

In humans and non-human primates, the 7TM receptor GPR17 exists in two isoforms differing only by the length of the N-terminus. Of these, only the short isoform has previously been characterized. Hence, we investigated gene expression and ligand-binding profiles of both splice variants and furthermore uncovered and characterized constitutive activity of both isoforms.

Experimental approach:

Expression levels of the hGPR17 isoforms were determined in several brain regions as well as heart and kidney using quantitative RT-PCR. A CREB reporter assay and [³⁵S]-GTPγS binding were employed to assess the constitutive activity and the activation by UDP, UDP-glucose and -galactose and the cysteinyl leukotrienes LTC₄ and LTD₄. Leukotriene binding and induction of internalization were furthermore tested using homologous competition binding and antibody-feeding experiments respectively.

Key results:

The short isoform (hGPR17-S) was expressed more abundantly (eight- to 23-fold) in the brain than the long isoform (hGPR17-L), whereas the opposite was observed in heart and kidney. As previously reported, the uracil nucleotides activated hGPR17-S with micromolar potencies. However, much lower potencies were observed for hGPR17-L with a 50- to 170-fold increase in EC₅₀. Furthermore, contrary to previous reports, neither of the isoforms was activated or bound by the cysteinyl leukotrienes. Finally, both receptors were demonstrated to be constitutively active through Gα_i.

Conclusions and implications:

We present the first isoform-specific characterization of GPR17 and show that differences exist between the isoforms, in both expression pattern and pharmacological profile. In turn, our results indicate that the two human isoforms might serve tissue-specific functions.

Regulation of PC12 cell survival and differentiation by the new P2Y-like receptor GPR17

The P2Y-like receptor GPR17 has been reported to respond to both uracil nucleotides and cysteinyl-leukotrienes (cysLTs), such as UDP-glucose and LTD(4). Our previous data suggest a potential role for GPR17 in regulation of both cell viability and differentiation state of central nervous system cells. On this basis, in the present paper we investigated the effect of GPR17 receptor ligands on PC12 cell viability, following induction of morphological differentiation by nerve growth factor (NGF). In addition, the role of GPR17 ligands, either alone or in combination with growth factors, on the degree of PC12 cell differentiation was investigated. GPR17, which was not basally expressed in undifferentiated PC12 cells, was specifically induced by a 10day-treatment with NGF, suggesting a role in the control of neuronal specification. Both UDP-glucose and LTD(4), agonists at the nucleotide and cysLT GPR17 binding sites, respectively, induced a significant pro-survival effect on PC12 cells after priming with NGF. By in vitro silencing experiments with specific small interfering RNAs and by using receptor antagonists, we confirmed that the agonist effects are indeed mediated by the selective activation of GPR17. We also demonstrated that GPR17 agonists act, both alone and synergistically with NGF, to promote neurite outgrowth in PC12 cells. In addition, GPR17 ligands were able to confer an NGF-like activity to the epidermal growth factor (EGF), that, under these experimental conditions, also promoted cell differentiation and neurite elongation. Finally, we show that GPR17 ligands activate the intracellular phosphorylation of both ERK 1/2 and p38 kinases, that have been identified as important signalling pathways for neurotrophins in PC12 cells. Our results establish GPR17 as a neurotrophic regulator for neuronal-like cells and suggest a possible interplay between endogenous uracil derivatives, cysLTs and NGF in the signalling pathways involved in neuronal survival and differentiation. They also represent the first direct demonstration, in a native system, that GPR17 can indeed be activated by uracil nucleotides and cysLTs, in line with what previously demonstrated in recombinant expression systems.

Synthesis and characterization of a cellular membrane affinity chromatography column containing histamine 1 and P2Y(1) receptors: a multiple G-protein coupled receptor column

A cellular membrane affinity chromatography (CMAC) column has been created using cellular membrane fragments from a 1321N1 cell line stably transfected with the P2Y(1) receptor. The CMAC(1321N1(P2Y1)) column contained functional P2Y(1) and histamine 1 receptors, which independently bound receptor-specific ligands. The data obtained with the CMAC(1321N1(P2Y1)) column demonstrate that multiple-G-protein coupled receptor (GPCR) columns can be developed and used to probe interactions with the immobilized receptors and that endogenously expressed GPCRs can be used to create CMAC columns. The results also establish that the histamine 1 receptor can be immobilized with retention of ligand-specific binding.