The new X-ray/visible microscopy MAXWELL technique for fast three-dimensional nanoimaging with isotropic resolution

Microscopy by Achromatic X-rays With Emission of Laminar Light (MAXWELL) is a new X-ray/visible technique with attractive characteristics including isotropic resolution in all directions, large-volume imaging and high throughput. An ultrathin, laminar X-ray beam produced by a Wolter type I mirror irradiates the sample stimulating the emission of visible light by scintillating nanoparticles, captured by an optical system. Three-dimensional (3D) images are obtained by scanning the specimen with respect to the laminar beam. We implemented and tested the technique with a high-brightness undulator at SPring-8, demonstrating its validity for a variety of specimens. This work was performed under the Synchrotrons for Neuroscience-an Asia-Pacific Strategic Enterprise (SYNAPSE) collaboration.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Cerebrospinal fluid of chronic osteoarthritic patients induced interleukin-6 release in human glial cell-line T98G

BACKGROUND

Chronic osteoarthritic pain is not well understood in terms of its pathophysiological mechanism. Activated glial cells are thought to play a role in the maintenance of chronic pain. T98G glioblastoma cell line was previously observed to release higher amounts of interleukin-6 (IL-6) when treated with cerebrospinal fluid (CSF) from patients with another chronic pain condition, post-herpetic neuralgia. In this study, we investigated the ability of CSF from patients diagnosed with knee osteoarthritis suffering from chronic pain, to trigger the release of pro-inflammatory cytokines, IL-6, IL-1beta and tumour necrosis factor alpha (TNF-α) from T98G. Characterization of upstream signalling was also explored.

METHODS

Fifteen osteoarthritis patients undergoing total knee replacement due to chronic knee pain and 15 patients without pain undergoing other surgeries with spinal anaesthesia were prospectively recruited. CSF was collected during anaesthesia. CSF were added to cultured T98G cells in the presence of lipopolysaccharide. IL-6, IL-1β and TNF-α release from T98G cells were measured using enzyme immunoassay. Antibody array and western blotting were performed using CSF-triggered T98G cell lysates to identify possible signalling targets. Age, gender and pain scores were recorded. Mann-Whitney U test was used to compare IL-6 release and protein expression between groups. Association between IL-6 and pain score was analysed using linear regression.

RESULTS

Significant higher levels of IL-6 were released by T98G cells when induced by osteoarthritis patients' CSF in the presence of LPS. The IL-6 levels showed positive association with pain score (adjusted B estimate = 10.1 (95% Confidence Interval 4.3-15.9); p = 0.001). Antibody array conducted with 6 pooled T98G cell lysate induced with osteoarthritis pain patient CSF identified greater than 2-fold proteins including STE20-related kinase adaptor protein and spleen tyrosine kinase. Further validation done using western blotting of individual CSF-triggered T98G cell lysate showed non-significant increase.

CONCLUSION

Higher IL-6 release from T98G when triggered by OA-CSF, in the presence of LPS, suggest the presence of "unknown molecule" in CSF that may be crucial in the maintenance phase of chronic pain in our osteoarthritis population. Further studies on the signalling pathways involved in pain and relevance of IL-6 release from T98G cells in other pain models are needed.

Distinct roles of GRIN2A and GRIN2B variants in neurological conditions

Rapid advances in sequencing technology have led to an explosive increase in the number of genetic variants identified in patients with neurological disease and have also enabled the assembly of a robust database of variants in healthy individuals. A surprising number of variants in the GRIN genes that encode N-methyl-D-aspartate (NMDA) glutamatergic receptor subunits have been found in patients with various neuropsychiatric disorders, including autism spectrum disorders, epilepsy, intellectual disability, attention-deficit/hyperactivity disorder, and schizophrenia. This review compares and contrasts the available information describing the clinical and functional consequences of genetic variations in GRIN2A and GRIN2B. Comparison of clinical phenotypes shows that GRIN2A variants are commonly associated with an epileptic phenotype but that GRIN2B variants are commonly found in patients with neurodevelopmental disorders. These observations emphasize the distinct roles that the gene products serve in circuit function and suggest that functional analysis of GRIN2A and GRIN2B variation may provide insight into the molecular mechanisms, which will allow more accurate subclassification of clinical phenotypes. Furthermore, characterization of the pharmacological properties of variant receptors could provide the first opportunity for translational therapeutic strategies for these GRIN-related neurological and psychiatric disorders.

Curcumin Ameliorates Neuroinflammation, Neurodegeneration, and Memory Deficits in p25 Transgenic Mouse Model that Bears Hallmarks of Alzheimer's Disease

Several studies have indicated that neuroinflammation is indeed associated with neurodegenerative disease pathology. However, failures of recent clinical trials of anti-inflammatory agents in neurodegenerative disorders have emphasized the need to better understand the complexity of the neuroinflammatory process in order to unravel its link with neurodegeneration. Deregulation of Cyclin-dependent kinase 5 (Cdk5) activity by production of its hyperactivator p25 is involved in the formation of tau and amyloid pathology reminiscent of Alzheimer's disease (AD). Recent studies show an association between p25/Cdk5 hyperactivation and robust neuroinflammation. In addition, we recently reported the novel link between the p25/Cdk5 hyperactivation-induced inflammatory responses and neurodegenerative changes using a transgenic mouse that overexpresses p25 (p25Tg). In this study, we aimed to understand the effects of early intervention with a potent natural anti-inflammatory agent, curcumin, on p25-mediated neuroinflammation and the progression of neurodegeneration in p25Tg mice. The results from this study showed that curcumin effectively counteracted the p25-mediated glial activation and pro-inflammatory chemokines/cytokines production in p25Tg mice. Moreover, this curcumin-mediated suppression of neuroinflammation reduced the progression of p25-induced tau/amyloid pathology and in turn ameliorated the p25-induced cognitive impairments. It is widely acknowledged that to treat AD, one must target the early-stage of pathological changes to protect neurons from irreversible damage. In line with this, our results demonstrated that early intervention of inflammation could reduce the progression of AD-like pathological outcomes. Moreover, our data provide a rationale for the potential use of curcuminoids in the treatment of inflammation associated neurodegenerative diseases.

Medial Septum Modulates Cellular Response Induced in Hippocampus on Microinjection of Cholinergic Agonists into Hypothalamic Lateral Supramammillary Nucleus

Cholinergic mechanisms in supramammillary nucleus (SuM), especially the lateral SuM (lSuM) modulates septo-hippocampal neural activity. The lSuM, as compared to the contiguous medial SuM (mSuM) has relatively dense projections to hippocampus and cingulate cortex (Cg). In the present study, we have investigated whether the effects of cholinergic activation of SuM on hippocampal and cortical neural activities involve a cooperative interaction with the medial septum (MS). Microinjection of the broad-spectrum cholinergic agonist, carbachol, or the cholinergic-nicotinic receptor agonist, nicotine, into the lSuM and the mSuM in urethane anesthetized rat evoked a similar pattern of hippocampal theta rhythm. Despite that, only the lSuM microinjections resulted in an increase in expression of c-Fos-like immunoreactivity (c-Fos-ir) in neurons, including interneurons, of the ipsilateral hippocampus with a very dense expression in dentate gyrus. Likewise, a robust induction of c-Fos-ir was also observed in the ipsilateral Cg. Inhibition of the MS with muscimol pre-treatment attenuated both carbachol-evoked c-Fos-ir and theta activation. The findings indicate that cholinergic–nicotinic mechanisms in lSuM evoke not only neural activation via the ascending synchronizing pathway but also an MS-modulated expression of the plasticity-related molecule c-Fos in cortical regions that are strongly innervated by the lSuM.

Fluorogenic probes to monitor cytosolic phospholipase A2 activity

Functionalization on the alkyl tail of arachidonyl trifluoromethylketone leads to the development of fluorogenic inhibitor and substrate probes of cPLA₂.

GluN2D-Containing N-methyl-d-Aspartate Receptors Mediate Synaptic Transmission in Hippocampal Interneurons and Regulate Interneuron Activity

N-methyl-d-aspartate receptors (NMDARs) are ionotropic glutamatergic receptors that have been implicated in learning, development, and neuropathological conditions. They are typically composed of GluN1 and GluN2A-D subunits. Whereas a great deal is known about the role of GluN2A- and GluN2B-containing NMDARs, much less is known about GluN2D-containing NMDARs. Here we explore the subunit composition of synaptic NMDARs on hippocampal interneurons. GluN2D mRNA was detected by single-cell PCR and in situ hybridization in diverse interneuron subtypes in the CA1 region of the hippocampus. The GluN2D subunit was detectable by immunoblotting and immunohistochemistry in all subfields of the hippocampus in young and adult mice. In whole-cell patch-clamp recordings from acute hippocampal slices, (+)-CIQ, the active enantiomer of the positive allosteric modulator CIQ, significantly enhanced the amplitude of the NMDAR component of miniature excitatory postsynaptic currents (mEPSCs) in CA1 interneurons but not in pyramidal cells. (+)-CIQ had no effect in slices from Grin2d-/- mice, suggesting that GluN2D-containing NMDARs participate in excitatory synaptic transmission onto hippocampal interneurons. The time course of the NMDAR component of the mEPSC was unaffected by (+)-CIQ potentiation and was not accelerated in slices from Grin2d-/- mice compared with wild-type, suggesting that GluN2D does not detectably slow the NMDAR EPSC time course at this age. (+)-CIQ increased the activity of CA1 interneurons as detected by the rate and net charge transfer of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from CA1 pyramidal cells. These data provide evidence that interneurons contain synaptic NMDARs possessing a GluN2D subunit, which can influence interneuron function and signal processing.

GABAergic neurons of the medial septum play a nodal role in facilitation of nociception-induced affect

The present study explored the functional details of the influence of medial septal region (MSDB) on spectrum of nociceptive behaviours by manipulating intraseptal GABAergic mechanisms. Results showed that formalin-induced acute nociception was not affected by intraseptal microinjection of bicuculline, a GABAA receptor antagonist, or on selective lesion of septal GABAergic neurons. Indeed, the acute nociceptive responses were dissociated from the regulation of sensorimotor behaviour and generation of theta-rhythm by the GABAergic mechanisms in MSDB. The GABAergic lesion attenuated formalin-induced unconditioned cellular response in the anterior cingulate cortex (ACC) and blocked formalin-induced conditioned place avoidance (F-CPA), and as well as the contextual fear induced on conditioning with brief footshock. The effects of lesion on nociceptive-conditioned cellular responses were, however, variable. Interestingly, the lesion attenuated the conditioned representation of experimental context in dorsal hippocampus field CA1 in the F-CPA task. Collectively, the preceding suggests that the MSDB is a nodal centre wherein the GABAergic neurons mediate nociceptive affect-motivation by regulating cellular mechanisms in ACC that confer an aversive value to the noxious stimulus. Further, in conjunction with a modulatory influence on hippocampal contextual processing, MSDB may integrate affect with context as part of associative learning in the F-CPA task.

Ionotropic GABA and Glutamate Receptor Mutations and Human Neurologic Diseases

The advent of whole exome/genome sequencing and the technology-driven reduction in the cost of next-generation sequencing as well as the introduction of diagnostic-targeted sequencing chips have resulted in an unprecedented volume of data directly linking patient genomic variability to disorders of the brain. This information has the potential to transform our understanding of neurologic disorders by improving diagnoses, illuminating the molecular heterogeneity underlying diseases, and identifying new targets for therapeutic treatment. There is a strong history of mutations in GABA receptor genes being involved in neurologic diseases, particularly the epilepsies. In addition, a substantial number of variants and mutations have been found in GABA receptor genes in patients with autism, schizophrenia, and addiction, suggesting potential links between the GABA receptors and these conditions. A new and unexpected outcome from sequencing efforts has been the surprising number of mutations found in glutamate receptor subunits, with the GRIN2A gene encoding the GluN2A N-methyl-d-aspartate receptor subunit being most often affected. These mutations are associated with multiple neurologic conditions, for which seizure disorders comprise the largest group. The GluN2A subunit appears to be a locus for epilepsy, which holds important therapeutic implications. Virtually all α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor mutations, most of which occur within GRIA3, are from patients with intellectual disabilities, suggesting a link to this condition. Similarly, the most common phenotype for kainate receptor variants is intellectual disability. Herein, we summarize the current understanding of disease-associated mutations in ionotropic GABA and glutamate receptor families, and discuss implications regarding the identification of human mutations and treatment of neurologic diseases.

Reduced neuronal signaling in the ageing apolipoprotein-E4 targeted replacement female mice

The effect of ApoE on NMDAR-dependent ERK/CREB signaling is isoform-dependent, and ApoE4 accelerates memory decline in ageing. However, this isoform-dependent function on neuronal signaling during ageing is unclear. In this study, we have examined NMDAR-associated ERK/CREB signal transduction in young and aged huApoE3 and huApoE4 targeted replacement (TR) mice. At 12 weeks huApoE4 mouse brain, increased NR1-S896 phosphorylation was linked to higher protein kinase C (PKC) activation. This up-regulation was accompanied by higher phosphorylation of AMPA GluR1-S831, CaMKII, ERK1/2 and CREB. But at 32 weeks, there was no significant difference between huApoE3 and huApoE4 TR mice on NMDAR-associated ERK/CREB signaling. Interestingly, in 72-week-old huApoE4 TR mice, protein phosphorylation that were increased in younger mice were significantly reduced. Lower NR1-S896 phosphorylation was linked to reduced PKC, GluR1-S831, CaMKII, ERK1/2 and CREB phosphorylation in huApoE4 TR mice as compared to huApoE3 TR mice. Furthermore, we have consistently detected lower ApoE levels in young and aged huApoE4 TR mouse brain, and this was associated with reduced expression of the ApoE receptor, LRP1 and NR2A-Y1246 phosphorylation. These results suggest age-specific, isoform-dependent effects of ApoE on neuronal signaling.

Cleavage of the NR2B subunit amino terminus of N-methyl-D-aspartate (NMDA) receptor by tissue plasminogen activator: identification of the cleavage site and characterization of ifenprodil and glycine affinities on truncated NMDA receptor

Thrombolysis using tissue plasminogen activator (tPA) has been the key treatment for patients with acute ischemic stroke for the past decade. Recent studies, however, suggest that this clot-busting protease also plays various roles in brain physiological and pathophysiological glutamatergic-dependent processes, such as synaptic plasticity and neurodegeneration. In addition, increasing evidence implicates tPA as an important neuromodulator of the N-methyl-d-aspartate (NMDA) receptors. Here, we demonstrate that recombinant human tPA cleaves the NR2B subunit of NMDA receptor. Analysis of NR2B in rat brain lysates and cortical neurons treated with tPA revealed concentration- and time-dependent degradation of NR2B proteins. Peptide sequencing studies performed on the cleaved-off products obtained from the tPA treatment on a recombinant fusion protein of the amino-terminal domain of NR2B revealed that tPA-mediated cleavage occurred at arginine 67 (Arg(67)). This cleavage is tPA-specific, plasmin-independent, and removes a predicted ~4-kDa fragment (Arg(27)-Arg(67)) from the amino-terminal domain of the NR2B protein. Site-directed mutagenesis of putative cleavage site Arg(67) to Ala(67) impeded tPA-mediated degradation of recombinant protein. This analysis revealed that NR2B is a novel substrate of tPA and suggested that an Arg(27)-Arg(67)-truncated NR2B-containing NMDA receptor could be formed. Heterologous expression of NR2B with Gln(29)-Arg(67) deleted is functional but exhibits reduced ifenprodil inhibition and increased glycine EC(50) with no change in glutamate EC(50). Our results confirmed NR2B as a novel proteolytic substrate of tPA, where tPA may directly interact with NR2B subunits leading to a change in pharmacological properties of NR2B-containing NMDA receptors.

Nicotinic receptor mechanism in supramammillary nucleus mediates physiological regulation of neural activity in dorsal hippocampal field CA1 of anaesthetized rat

Evidence suggests that stimulation of the region of the rostral pontine oralis (RPO) nucleus and the peripheral application of a noxious stimulus activates an ascending system that also modulates hippocampal neural responses during behavioral arousal. Indeed, the two stimuli and behavioral arousal elicit theta activation and the suppression of population spikes (PS) in dorsal hippocampus field CA1. Interestingly, such neural responses in CA1 are also elicited by microinjection of the cholinergic agonist carbachol into the hypothalamic supramammillary nucleus (SuM). In the present in vivo electrophysiological study, we tested the hypothesis that cholinergic neural elements in the SuM modulate the neural drive to CA1 on RPO stimulation or the peripheral application of a noxious stimulus. Pharmacological investigation showed that intra-SuM microinjection of either a muscarinic or a nicotinic receptor antagonist attenuated the SuM carbachol-induced neural effects in CA1, namely, theta activation and PS suppression. However, neither antagonist attenuated the CA1 effects of intra-SuM microinjection of the excitatory neurotransmitter glutamate. Subsequent investigations revealed that microinjection of only the nicotinic antagonist, mecamylamine, into the lateral SuM selectively attenuated the responses elicited in CA1 by stimulation of the RPO or on nociceptive stimulation with hind paw injection of formalin (5%, 0.05 ml); whereas, microinjection of mecamylamine into the medial SuM did not affect the hippocampal responses elicited by either type of stimulation. Furthermore, application of mecamylamine into the lateral SuM attenuated the CA1 responses induced by injection of formalin into the contralateral, but not the ipsilateral hind paw. The lateralization of drug effect is consistent with the predominant unilateral anatomical connections between the SuM and the septohippocampal region. These findings provide novel evidence that nicotinic cholinoceptive neurons in the lateral SuM are common elements of the neural drive(s) to the hippocampus on RPO activation and noxious stimulation.

A non-muscle myosin II motor links NR1 to retrograde trafficking and proteasomal degradation in PC12 cells

Rat pheochromocytoma (PC12) cells have been shown to lack functional NMDA receptors; yet, these cells express NR1 subunits of the NMDA receptor. The reason for the lack of functional receptors has been attributed to the absence of significant levels of NR2 subunits to co-assemble with NR1. It is known that PC12 expresses very low levels of NR2C, with complete absence of other types of NR2 subunits. The purpose of the present study is to describe the molecular mechanism of trafficking and degradation of unassembled NR1 subunits in PC12 cells. The localization of NR1 subunits in PC12 cells were evaluated by immunofluorescence and co-immunoprecipitation, which showed that NR1 was present in the endoplasmic reticulum and cis-middle compartments of the Golgi apparatus. Upon treatment with a proteasome inhibitor, MG132, the ubiquitinylated species of NR1 subunit were detected, suggesting that NR1 is being targeted for endoplasmic reticulum-associated proteasomal degradation. Our previous studies suggest that NR1 subunits from the Golgi do not proceed to trans-Golgi, hence they will require re-routing to the endoplasmic reticulum for degradation. Further investigations on the factors involved in the trafficking of NR1 from Golgi to endoplasmic reticulum were performed using co-immunoprecipitation and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. These revealed the co-association of NR1 with non-muscle myosin heavy chain II isoforms A and B. We also demonstrate the functional significance of this interaction through the use of a myosin inhibitor, blebbistatin, to disrupt brefeldin A-induced Golgi-to-endoplasmic reticulum trafficking of NR1. In conclusion, our results suggest that non-muscle myosin II is involved in the retrograde trafficking of NR1 subunits from the cis/middle-Golgi to the endoplasmic reticulum for proteasomal degradation in PC12.

The serine protease plasmin cleaves the amino-terminal domain of the NR2A subunit to relieve zinc inhibition of the N-methyl-D-aspartate receptors

Zinc is hypothesized to be co-released with glutamate at synapses of the central nervous system. Zinc binds to NR1/NR2A N-methyl-d-aspartate (NMDA) receptors with high affinity and inhibits NMDAR function in a voltage-independent manner. The serine protease plasmin can cleave a number of substrates, including protease-activated receptors, and may play an important role in several disorders of the central nervous system, including ischemia and spinal cord injury. Here, we demonstrate that plasmin can cleave the native NR2A amino-terminal domain (NR2A(ATD)), removing the functional high affinity Zn(2+) binding site. Plasmin also cleaves recombinant NR2A(ATD) at lysine 317 (Lys(317)), thereby producing a approximately 40-kDa fragment, consistent with plasmin-induced NR2A cleavage fragments observed in rat brain membrane preparations. A homology model of the NR2A(ATD) predicts that Lys(317) is near the surface of the protein and is accessible to plasmin. Recombinant expression of NR2A with an amino-terminal deletion at Lys(317) is functional and Zn(2+) insensitive. Whole cell voltage-clamp recordings show that Zn(2+) inhibition of agonist-evoked NMDA receptor currents of NR1/NR2A-transfected HEK 293 cells and cultured cortical neurons is significantly reduced by plasmin treatment. Mutating the plasmin cleavage site Lys(317) on NR2A to alanine blocks the effect of plasmin on Zn(2+) inhibition. The relief of Zn(2+) inhibition by plasmin occurs in PAR1(-/-) cortical neurons and thus is independent of interaction with protease-activated receptors. These results suggest that plasmin can directly interact with NMDA receptors, and plasmin may increase NMDA receptor responses through disruption or removal of the amino-terminal domain and relief of Zn(2+) inhibition.

Structural insights into phenylethanolamines high-affinity binding site in NR2B from binding and molecular modeling studies

BACKGROUND

Phenylethanolamines selectively bind to NR2B subunit-containing N-methyl-D-aspartate-subtype of ionotropic glutamate receptors and negatively modulate receptor activity. To investigate the structural and functional properties of the ifenprodil binding domain on the NR2B protein, we have purified a soluble recombinant rat NR2B protein fragment comprising the first ~400 amino acid amino-terminal domain (ATD2B) expressed in E. coli. Spectral measurements on refolded ATD2B protein demonstrated specific binding to ifenprodil. We have used site-directed mutagenesis, circular dichroism spectroscopy and molecular modeling to obtain structural information on the interactions between critical amino acid residues and ifenprodil of our soluble refolded ATD2B proteins. Ligand-induced changes in protein structure were inferred from changes in the circular dichroism spectrum, and the concentration dependence of these changes was used to determine binding constants for ifenprodil and its analogues.

RESULTS

Ligand binding of ifenprodil, RO25,6981 and haloperidol on soluble recombinant ATD2B determined from circular dichroism spectroscopy yielded low-to-high micromolar equilibrium constants which concurred with functional IC₅₀ measurement determined in heterologously expressed NR1/NR2B receptors in Xenopus oocytes. Amino acid residue substitutions of Asp101, Ile150 and Phe176 with alanine residue within the ATD2B protein altered the recombinant protein dissociation constants for ifenprodil, mirroring the pattern of their functional phenotypes. Molecular modeling of ATD2B as a clam-shell-like structure places these critical residues near a putative ligand binding site.

CONCLUSION

We report for the first time biochemical measurements show that the functional measurements actually reflect binding to the ATD of NR2B subunit. Insights gained from this study help advance the theory that ifenprodil is a ligand for the ATD of NR2B subunit.

Immunolocalization of NMDA receptor subunit NR3B in selected structures in the rat forebrain, cerebellum, and lumbar spinal cord

N-methyl-D-aspartate (NMDA) receptors have been implicated in many neurological disorders. Although NMDA receptors are best known for their high calcium permeability, the recently discovered NR3 subunits, NR3A and NR3B, have been shown to reduce the calcium permeability of the NMDA receptor. Thus, NR3 subunits may be important players in modulating synaptic plasticity in neurons. Although NR3B expression in the rodent and human brain has been studied, little is known about its distribution in different cell types. Here we used immunolabeling with a specific NR3B antibody together with antibodies against established neurochemical markers to determine the cellular and subcellular localization of NR3B. The nucleus was concurrently stained with NR3B immunolabeling to show that NR3B is widely expressed by many cells in each brain region. Our findings indicate that NR3B is widely expressed in the structures examined in the rat forebrain (hippocampus, cerebral cortex, caudoputamen, and nucleus accumbens), cerebellum, and lumbar sections of the spinal cord. Within these regions NR3B was found to be expressed in all the substructures of the hippocampus (CA1, CA3, dentate gyrus), the various layers of the cerebral cortex, projection neurons and interneurons of the striatum, different cell types of the cerebellum, and motor neurons of the spinal cord. Furthermore, when stained with NR1-the obligatory subunit responsible for forming functional NMDA receptors-the distribution of NR3B appears to be as ubiquitous as NR1. Taken together, our data suggest that there may be a population of NR3B-containing NMDA receptors conferring new functional roles in the mammalian central nervous system.

Hydrogen sulphide reduces insulin secretion from HIT-T15 cells by a KATP channel-dependent pathway

Hydrogen sulphide (H(2)S), a naturally occurring gas exerts physiological effects by opening K(ATP) channels. Anti-diabetic drugs (e.g. glibenclamide) block K(ATP) channels and abrogate H(2)S-mediated physiological responses which suggest that H(2)S may also regulate insulin secretion by pancreatic beta-cells. To investigate this hypothesis, insulin-secreting (HIT-T15) cells were exposed to NaHS (100 microM) and the K(ATP) channel-driven pathway of insulin secretion was tracked with various fluorescent probes. The concentration of insulin released from HIT-T15 cells decreased significantly after NaHS exposure and this effect was reversed by the addition of glibenclamide (10 microM). Cell viability and intracellular ATP and glutathione (GSH) levels remained unchanged, suggesting that changes in insulin secretion were not ATP linked or redox dependent. Through fluorescence imaging studies, it was found that K(+) efflux occurs in cells exposed to NaHS. The hyperpolarised cell membrane, a result of K(+) leaving the cell, prevents the opening of voltage-gated Ca(2+) channels. This subsequently prevents Ca(2+) influx and the release of insulin from HIT-T15 cells. This data suggest that H(2)S reduces insulin secretion by a K(ATP) channel-dependent pathway in HIT-T15 cells. This study reports the molecular mechanism by which H(2)S reduces insulin secretion and provides further insight into a recent observation of increased pancreatic H(2)S production in streptozotocin-diabetic rats.

Improving solubility of NR2B amino-terminal domain of N-methyl-d-aspartate receptor expressed in Escherichia coli

The amino-terminal domains (ATDs) of N-methyl-d-aspartate (NMDA) receptors contain binding sites for modulators and may serve as potential drug targets in neurological diseases. Here, three fusion tags (6xHis-, GST-, and MBP-) were fused to the ATD of NMDA receptor NR2B subunit (ATD2B) and expressed in Escherichia coli. Each tag's ability to confer enhanced solubility to ATD2B was assessed. Soluble ATD2B was successfully obtained as a MBP fusion protein. Dynamic light scattering revealed the protein (1mg/ml) exists as monodispersed species at 25 degrees C. Functional studies using circular dichroism showed that the soluble MBP-ATD2B bound ifenprodil in a dose-dependent manner. The dissociation constants obtained for ifenprodil were similar in the absence (64nM) and presence (116nM) of saturating concentration of maltose. Moreover, the yield of soluble MBP-ATD2B is 18 times higher than the refolded 6xHis-ATD2B. We have reported a systematic comparison of three different affinity tagging strategies and identified a rapid and efficient method to obtain large amount of ATD2B recombinant protein for biochemical and structural studies.

Expression and characterization of soluble amino-terminal domain of NR2B subunit of N-methyl-D-aspartate receptor

N-methyl-D-aspartate (NMDA) receptors are involved in mediating excitatory synaptic transmissions in the brain and have been implicated in numerous neurologic disorders. The proximal amino-terminal domains (ATDs) of NMDA receptors constitute many modulatory binding sites that may serve as potential drug targets. There are few biochemical and structural data on the ATDs of NMDA receptors, as it is difficult to produce the functional proteins. Here an optimized method was established to reconstitute the insoluble recombinant ATD of NMDA receptor NR2B subunit (ATD2B) through productive refolding of 6xHis-ATD2B protein from inclusion bodies. Circular dichroism and dynamic light scattering characterizations revealed that the solubilized and refolded 6xHis-ATD2B adopted well-defined secondary structures and monodispersity. More significantly, the soluble 6xHis-ATD2B specifically bound ifenprodil to saturation. Ifenprodil bound to 6xHis-ATD2B with a dissociation constant (KD) of 127.5+/-45 nM, which was within the range of the IC50 determined electrophysiologically. This is the first report on a functional recombinant ATD2B with a characterized KD.

Differential binding properties of [3H]dextrorphan and [3H]MK-801 in heterologously expressed NMDA receptors

The N-methyl-D-aspartate receptor (NMDAR) antagonists: MK-801, phencyclidine and ketamine are open-channel blockers with limited clinical value due to psychotomimetic effects. Similarly, the psychotomimetic effects of the dextrorotatory opioids, dextromethorphan and its metabolite dextrorphan, derive from their NMDAR antagonist actions. Differences in the use dependency of blockade, however, suggest that the binding sites for MK-801 and dextrorphan are distinct. In the absence of exogenous glutamate and glycine, the rate of association of [3H]MK-801 with wild-type NR1-1a/NR2A receptors was considerably slower than that for [3H]dextrorphan. Glutamate individually, and in the presence of the co-agonist glycine, had substantial effects on the specific binding of [3H]MK-801, while the binding of [3H]dextrorphan was not affected. Mutation of residues N616 and A627 in the NR1 subunit had a profound effect on [3H]MK-801 binding affinity, while that of [3H]dextrorphan was unaltered. In contrast, NR1 residues, W611 and N812, were critical for specific binding of [3H]dextrorphan to NR1-1a/NR2A complexes with no corresponding influence on that of [3H]MK-801. Thus, [3H]dextrorphan and [3H]MK-801 have distinct molecular determinants for high-affinity binding. The ability of [3H]dextrorphan to bind to a closed channel, moreover, indicates that its recognition site is shallower in the ion channel domain than that of MK-801 and may be associated with the extracellular vestibule of the NMDAR.

Molecular determinants of proton-sensitive N-methyl-D-aspartate receptor gating

Extracellular protons inhibit N-methyl-D-aspartate (NMDA) receptors with an IC50 value in the physiological pH range. To identify the molecular determinants of proton sensitivity, we used scanning mutagenesis of the NR1 subunit to search for residues that control proton inhibition of NMDA receptors. Homology modeling of the extracellular domains suggested that residues at which mutations perturbed pH sensitivity were localized in discrete regions. The majority of mutations that strongly affected proton sensitivity were clustered in the extracellular end of the second transmembrane domain (M3) and adjacent linker leading to the S2 portion of the glycine-binding domain of NR1. Mutations in NR2A confirmed that the analogous region controls the pH sensitivity of this subunit and also identified the linker region between the third transmembrane domain (M4) and the S2 portion of the NR2 glutamate binding domain as an additional determinant of proton sensitivity. One mutant receptor, NR1(A649C)/NR2A(A651T), showed a 145-fold reduction in the IC50 for protons (IC50, 17.3 microM corresponding to pH 4.9). The M3-S2 linker region has been suggested to control NMDA receptor gating, leading to the hypothesis that the proton sensor and receptor gate may be structurally and functionally integrated.

Allosteric interaction between the amino terminal domain and the ligand binding domain of NR2A

Fast desensitization is an important regulatory mechanism of neuronal NMDA receptor function. Only recombinant NMDA receptors composed of NR1/NR2A exhibit a fast component of desensitization similar to neuronal NMDA receptors. Here we report that the fast desensitization of NR1/NR2A receptors is caused by ambient zinc, and that a positive allosteric interaction occurs between the extracellular zinc-binding site located in the amino terminal domain and the glutamate-binding domain of NR2A. The relaxation of macroscopic currents reflects a shift to a new equilibrium due to increased zinc affinity after binding of glutamate. We also show a similar interaction between the ifenprodil binding site and the glutamate binding site of NR1/NR2B receptors. These data raise the possibility that there is an allosteric interaction between the amino terminal domain and the ligand-binding domain of other glutamate receptors. Our findings may provide insight into how zinc and other extracellular modulators regulate NMDA receptor function.

Quantitative determination of intermolecular interactions with fluorinated aromatic rings

The chemical double mutant cycle approach has been used to investigate substituent effects on intermolecular interactions between aromatic rings and pentafluorophenyl pi-systems. The complexes have been characterised using 1H and 19F NMR titrations, X-ray crystal structures of model compounds and molecular mechanics calculations. In the molecular zipper system used for these experiments, H-bonds and the geometries of the interacting surfaces favour the approach of the edge of the aromatic ring with the face of the pentafluorophenyl pi-system. The interactions are generally repulsive and this repulsion increases with more electron-withdrawing substituents up to a limit of +2.2 kJ mol(-1), when the complex distorts to minimise the unfavourable interaction. Strongly electron-donating groups cause a change in the geometry of the aromatic interaction and attractive stacking interactions are found (-1.6 kJ mol(-1) for NMe2). These results are generally consistent with an electrostatic model: the polarisation of the pentafluorophenyl ring leads to a partial positive charge located at the centre and this leads to repulsive interactions with the positive charges on the protons on the edge of the aromatic ring; when the aromatic ring has a high pi-electron density there is a large electrostatic driving force in favour of the stacked geometry which places this pi-electron density over the centre of the positive charge on the pentafluorophenyl group.

Nonpeptide cholecystokinin-2 receptor agonists

In the course of structural explorations around a series of potent CCK2 receptor antagonists, it was noted that simple N-methylation of the indolic N-H in the parent molecule gave rise to behavior in vivo that was consistent with the compound acting as an agonist. Exploration in vitro confirmed this property, and it was shown that the agonist action could be blocked by the reference CCK2 receptor antagonist, L-365,260. Further examples of this type of modification were explored, and a common theme with regard to agonist behavior was uncovered. Some molecular modeling is also presented in an attempt to throw light on the nature of the ligand receptor interactions that may be giving rise to the differing properties of these, apparently, structurally similar molecules.

Molecular determinants of coordinated proton and zinc inhibition of N-methyl-D-aspartate NR1/NR2A receptors

Modulation of the N-methyl-d-aspartate (NMDA)-selective glutamate receptors by extracellular protons and Zn(2+) may play important roles during ischemia in the brain and during seizures. Recombinant NR1/NR2A receptors exhibit a much higher apparent affinity for voltage-independent Zn(2+) inhibition than receptors with other subunit combinations. Here, we show that the mechanism of this apparent high-affinity, voltage-independent Zn(2+) inhibition for NR2A-containing receptors results from the enhancement of proton inhibition. We also show that the N-terminal leucine/isoleucine/valine binding protein (LIVBP)-like domain of the NR2A subunit contains critical determinants of the apparent high-affinity, voltage-independent Zn(2+) inhibition. Mutations H42A, H44G, or H128A greatly increase the Zn(2+) IC(50) (by up to approximately 700-fold) with no effect on the potencies of glutamate and glycine or on voltage-dependent block by Mg(2+). Furthermore, the amino acid residue substitution H128A, which mediates the largest effect on the apparent high-affinity Zn(2+) inhibition among all histidine substitutions we tested, is also critical to the pH-dependency of Zn(2+) inhibition. Our data revealed a unique interaction between two important extracellular modulators of NMDA receptors.

2,7-Dioxo-2,3,4,5,6,7-hexahydro-1H-benzo[h][1,4]diazonine as a new template for the design of CCK(2) receptor antagonists

A novel series of nonpeptide CCK(2) receptor antagonists has been prepared, in which 2,7-dioxo-2,3,4,5,6,7-hexahydro-1H-benzo[h][1, 4]diazonine (5) was used as a chemical template. This uncommon ring system was obtained in a highly substituted form and in high yield by ozonolysis of the enamine bond of 1,2,3,4-tetrahydro-9H-pyrido[3, 4-b]indole derivatives (4), in which the configuration of the substituents was established stereoselectively via the Pictet-Spengler reaction. Further structural manipulation was guided by molecular modeling through comparison of fieldpoint-based structures of candidate compounds with a selected low-energy conformation of the representative CCK(2) receptor antagonist 5-[[[(1S)-[[(3, 5-dicarboxyphenyl)amino]carbonyl]-2-phenylethyl]amino]carbonyl]-6- [[( 1-adamantylmethyl)amino]carbonyl]indole (JB93182 (3)). By this approach compounds such as (3R, 5S)-4-acetyl-3-(1-adamantyl)methyl-1-(2-chlorobenzyl)-5-carboxymet hyl aminocarbonyl-2,7-dioxo-2,3,4,5,6,7-hexahydro-1H-benzo[h][1, 4]diazonine (32) were prepared. Compound 32 behaved as a competitive CCK(2) receptor antagonist in vitro as judged by its inhibition of pentagastrin-stimulated acid secretion in an isolated, lumen-perfused, immature rat stomach assay (pK(B) = 6.74 +/- 0.27) and by its displacement of [(125)I]CCK-8S from CCK(2) sites in mouse cortical homogenates (pK(i) = 6.99 +/- 0.05). Compound 32 was 100-fold selective for CCK(2) over CCK(1) receptors based on the affinity estimate obtained in a guinea pig pancreas radioligand binding assay (pK(i) = 5.0).

Development of peptide 3D structure mimetics: rational design of novel peptoid cholecystokinin receptor antagonists

The two hormones cholecystokinin and gastrin share the same C-terminal sequence of amino acids, namely Gly(29)-Trp(30)-Met(31)-Asp(32)-Phe(33)-NH(2). Nevertheless, this congruence has not precluded using this structure to develop selective ligands for either CCK(1) or CCK(2) receptors. Manipulation of the hydrophobic residues at positions 31 and 33 gave a series of CCK(1) tripeptide antagonists, typified by N-t-BOC-Trp-2-Nal-Asp-2-(phenyl)ethylamide (pK(B) 6.8 +/- 0.3). Molecular modeling was used to identify the bioactive conformation of these CCK(1)-selective compounds and prompted the design of new peptoid structures. We aimed to maintain the conformation of the parent series by exploiting patterns of hydrogen-bonding and pi-stacking interactions present in the original molecule, rather than introducing additional covalent bonds. The prototype, N-(succinyl-D-Asp-2-phenylethylamido)-L-Trp-2-(2-naphthyl)ethylami de, was a potent and selective CCK(1) antagonist (pK(B) 7.2 +/- 0.3). Furthermore, the new series showed patterns of biological activity that mirrored those of the parent tripeptides. These compounds contain elements of both peptide primary and secondary structure and represent a novel approach to designing peptidomimetics. Interesting results were obtained from comparing models of a representative tripeptide CCK(1) antagonist with a conformation of CCK(30)(-)(33) that others have proposed to be responsible for its activity at the CCK(2) receptor. The results suggest that CCK(1) and CCK(2) receptors recognize enatiomeric dispositions of the Trp(30) indole, Asp(32) carboxylic acid, and C-terminal phenyl groups arrayed about a common backbone configuration. This "functional chirality" may underpin the mechanism by which these closely related receptor systems bind CCK(30)(-)(33) and explain patterns of selectivity observed with optical isomers of a number of peptoid and nonpeptide ligands.

Changing the nature of the initial chaperonin capture complex influences the substrate folding efficiency

For the chaperonin substrates, rhodanese, malate dehydrogenase (MDH), and glutamine synthetase (GS), the folding efficiencies, and the lifetimes of folding intermediates were measured with either the nucleotide-free GroEL or the activated ATP.GroEL.GroES chaperonin complex. With both nucleotide-free and activated complex, the folding efficiency of rhodanese and MDH remained high over a large range of GroEL to substrate concentration ratios (up to 1:1). In contrast, the folding efficiency of GS began to decline at ratios lower than 8:1. At ratios where the refolding yields were initially the same, only a relatively small increase (1.6-fold) in misfolding kinetics of MDH was observed with either the nucleotide-free or activated chaperonin complex. For rhodanese, no change was detected with either chaperonin complex. In contrast, GS lost its ability to interact with the chaperonin system at an accelerated rate (8-fold increase) when the activated complex instead of the nucleotide-free complex was used to rescue the protein from misfolding. Our data demonstrate that the differences in the refolding yields are related to the intrinsic folding kinetics of the protein substrates. We suggest that the early kinetic events at the substrate level ultimately govern successful chaperonin-substrate interactions and play a crucial role in dictating polypeptide flux through the chaperonin system. Our results also indicate that an accurate assessment of the transient properties of folding intermediates that dictate the initial chaperonin-substrate interactions requires the use of the activated complex as the interacting chaperonin species.

Non-peptide cholecystokinin-B/gastrin receptor antagonists based on bicyclic, heteroaromatic skeletons

A series of potent and selective cholecystokinin-B/gastrin receptor antagonists based on the dibenzobicyclo[2.2.2]octane (BCO) skeleton which have recently been described were found to show species-dependent behavior when examined in rat and dog models. We now report the discovery of compounds in which the BCO skeleton has been replaced with bicyclic, heteroaromatic frameworks, such as a 5,6-disubstituted indole or benzimidazole. These new ligands maintain the affinity and selectivity profile of the previous compounds in vitro but show a much more consistent behavior pattern in vivo. Representative examples of this class of compound have been shown to inhibit pentagastrin-stimulated acid secretion when administered intravenously at doses of 0.1 mumol kg-1 or less.

Improving the affinity and selectivity of a nonpeptide series of cholecystokinin-B/gastrin receptor antagonists based on the dibenzobicyclo[2.2.2]octane skeleton

We have recently described a novel series of nonpeptidic cholecystokinin-B (CCKB)/gastrin receptor antagonists based on a dibenzobicyclo[2.2.2]octane skeleton. We wish now to report on compounds arising out of our earlier work which have substantially greater affinity as antagonists for the CCKB/gastrin receptor system and which maintain, or improve on, the already high selectivity with respect to CCKA receptors. Thus, cis-7-[[[(1S)-[[3,5-dicarboxy-phenyl)amino]carbonyl]-2- phenylethyl]amino]carbonyl]-8-[[(1-adamantylmethyl)amino]- carbonyl]-2,3:5,6-dibenzobicyclo[2.2.2]octane expressed a pKi of 8.80 in mouse cortical membranes at CCKB/gastrin receptors. The selectivity for these receptors over CCKA receptors was in the order of 1000-fold.

The recognition of a diarylimine as a metabonate produced during incubation of N-benzyl-4-chloroaniline with hepatic microsomal preparations

Evidence is presented for the formation of N-benzylidene-4-chloroaniline as a metabonate during the metabolism of N-benzyl-4-chloroaniline. Control studies suggest that the diarylimine is formed as a chemical artifact from the debenzylation products (benzaldehyde and 4-chloroaniline). This novel observation indicates a possible pathway to amide formation from N-benzylanilines via diarylimines as intermediates.

Echinomycin and distamycin induce rotation of nucleosome core DNA

When nucleosome cores reconstituted from chicken erythrocyte histones and a 160 bp DNA molecule are exposed to echinomycin, a bis-intercalating antitumour antibiotic, the DNA appears to rotate with respect to the histone octamer by about half a turn. New bands appear in patterns of DNAase I digestion at positions approximately mid-way between those characteristic of control core samples, while the control pattern is largely suppressed. Similar (but not identical) changes are produced when nucleosome cores are exposed to distamycin, a non-intercalating DNA-binding antibiotic. The effects of both ligands can be explained in terms of a change in rotational orientation of the core DNA, so as to place antibiotic binding sites on the inward-facing (concave) surface of the DNA supercoil. Presumably this serves to optimise non-bonded contacts with the polynucleotide backbone. These results establish that the positioning of DNA about the histone octamer is not absolutely determined by its nucleotide sequence, but may be modified by the binding of such relatively small molecules as antibiotics.

DNA sequence selectivity of three biosynthetic analogues of the quinoxaline antibiotics

Mono- and bis-quinoline analogues of echinomycin, and a bis-3-amino-quinoxaline analogue of triostin A, have been prepared by directed biosynthesis and investigated for sequence selectivity in binding to DNA. Binding isotherms for bis-3-amino triostin A interacting with four natural and two synthetic DNA species have been determined by using solvent-partition analysis with radiolabelled antibiotic. They reveal a similar pattern of preferences to that seen with the parent compound: tighter binding occurs with the more guanine and cytosine (GC)-rich DNAs and in every case the association constant is increased two- to five-fold over the value recorded for triostin A. Deoxyribonuclease I footprinting patterns measured for the quinoline analogues of echinomycin differ from those observed with the parent antibiotic in that an additional strong site of protection occurs around the CpG sequence at position 35 in the tyrT fragment. Footprints for bis-3-amino triostin A reveal a substantially more selective pattern of cleavage inhibition than seen with the natural antibiotics: only two or three distinct binding sites are identified in tyrT DNA and four in pTyr2 DNA. Each is centred around one or more CpG steps, but many more CpG-containing sequences are unprotected. The analogue seems to prefer CpG steps flanked by at least one adenine and thymine (AT) pair, optimally ACGN. Enhancement of cutting at AT-rich sequences surrounding their binding sites is seen with all three of the new antibiotics. The results lend weight to the idea that novel sequence selectivity can be attained by making appropriate substitution on the chromophores of quinoxaline antibiotics.

DNA sequence recognition by under-methylated analogues of triostin A

Two new analogues of TANDEM (des-N-tetramethyl triostin A) have been synthesised in an effort to elucidate the molecular basis of DNA nucleotide sequence recognition in this series of compounds. Their binding preferences have been investigated by DNAase I footprinting and differential inhibition of restriction nuclease attack. The presence of a single N-methyl group on only one valine residue (in [N-MeVal4] TANDEM) abolishes the ability to recognise DNA, presumably because this antibiotic analogue has suffered an unfavourable conformational change in the depsipeptide ring. A bis-methylated analogue, [N-MeCys3, N-MeCys7]TANDEM, was found to interact quite strongly with DNA and afforded binding sites, rich in AT residues, identical to those of TANDEM. Footprinting with various DNA fragments of known sequence showed that this analogue recognises sequences containing the dinucleotide TpA, although we cannot exclude the possibility that it binds to ApT as well. [N-MeCys3, N-MeCys7]TANDEM inhibits cutting by RsaI, a restriction enzyme that recognises GTAC but not by Sau3AI which recognises GATC. This provides further supportive evidence that the ligand (and, by extension, TANDEM itself) prefers binding to sequences containing the dinucleotide step TpA.

Sequence preferences in the binding to DNA of triostin A and TANDEM as reported by DNase I footprinting

Six or seven triostin-binding sites have been identified in a 160-base-pair DNA restriction fragment containing the tyr T promoter sequence. Each is centred round a CpG step, and the minimum binding site-size appears to be six base pairs. The sites are practically the same as those reported for echinomycin by DNase I digestion. Only two sites are protected by binding of TANDEM, the des-N-tetramethyl analogue of triostin A; they are centred around the sequences ATA or TAT.

Sequence-specific binding of echinomycin to DNA: evidence for conformational changes affecting flanking sequences

The technique of DNAase I footprinting has been used to investigate preferred binding sites for echinomycin on a 160-base-pair DNA fragment from E. coli containing the tyr T promoter sequence. Six binding sites have been precisely located in the sequence; a seventh has been partially identified. The minimum site-size is six base pairs. All the binding sites contain the dinucleotide sequence CpG but no other regularities can be discerned. When the protected regions on each complementary strand are compared it is evident that they are staggered by 2-3 base-pairs towards the 3' end at each site. Footprinting with DNAase II reports a similar, though less precise, pattern of protection. Cutting by both enzymes is markedly enhanced at AT-rich regions flanking the antibiotic-binding sites. This increased susceptibility to nuclease attack can be attributed to an altered helix conformation in the vicinity of the bis-intercalated echinomycin molecule. It seems that certain sequences, mainly runs of A or runs of T, switch from a nuclease-resistant to a nuclease-sensitive form when echinomycin binds nearby.