Development of highly efficient NEG pumping system for EBIS

Ultrahigh vacuum inside the ion trap volume is crucial for stable and reliable operation of an Electron Beam Ion Source (EBIS). We have developed and tested a compact linear pumping system based on the ZAO Non-Evaporable Getter (NEG) module with high pumping speed and enhanced sorption capacity for all active gases. Due to its minimal transverse dimensions, the system can be mounted adjacent to the ion trap inside a superconducting solenoid bore and will provide a pumping speed of the order of 1000 l/s for all active gases in that area. An externally supplied current (100 A DC) is used to heat the ZAO NEG up to 650 °C for more than 1 h, which is required for pump activation and/or reactivation cycles. The pumping system is being developed for use in the Extended EBIS Upgrade which is presently in progress at BNL. The design of the system and results of multiple tests are presented and discussed.

Patterned neuronal networks using nanodiamonds and the effect of varying nanodiamond properties on neuronal adhesion and outgrowth

OBJECTIVE

Detonation nanodiamond monolayer coatings are exceptionally biocompatible substrates for in vitro cell culture. However, the ability of nanodiamond coatings of different origin, size, surface chemistry and morphology to promote neuronal adhesion, and the ability to pattern neurons with nanodiamonds have yet to be investigated.

APPROACH

Various nanodiamond coatings of different type are investigated for their ability to promote neuronal adhesion with respect to surface coating parameters and neurite extension. Nanodiamond tracks are patterned using photolithography and reactive ion etching.

MAIN RESULTS

Universal promotion of neuronal adhesion is observed on all coatings tested and analysis shows surface roughness to not be a sufficient metric to describe biocompatibility, but instead nanoparticle size and curvature shows a significant correlation with neurite extension. Furthermore, neuronal patterning is achieved with high contrast using patterned nanodiamond coatings down to at least 10 µm.

SIGNIFICANCE

The results of nanoparticle size and curvature being influential upon neuronal adhesion has great implications towards biomaterial design, and the ability to pattern neurons using nanodiamond tracks shows great promise for applications both in vitro and in vivo.

Pharmacological characterization of recombinant NR1/NR2A NMDA receptors with truncated and deleted carboxy termini expressed in Xenopus laevis oocytes

BACKGROUND AND PURPOSE

The carboxy terminal domain (CTD) of NR2 N-methyl-d-aspartate receptor (NMDAR) subunits interacts with numerous scaffolding and signal transduction proteins. Mutations of this region affect trafficking and downstream signalling of NMDARs. This study determines to what extent characteristic pharmacological properties of NR2A-containing NMDARs are influenced by this key functional domain.

EXPERIMENTAL APPROACH

Using recombinant receptor expression in Xenopus laevis oocytes and two electrode voltage clamp recordings we characterized pharmacological properties of rat NR1/NR2A NMDARs with altered CTDs. We assessed the effects of truncating [at residue Iso1098; NR2A(trunC)] and deleting [from residue Phe822; NR2A(delC)] the CTD of NR2A NMDAR subunits on agonist potencies, channel block by Mg(2+) and memantine and potentiation of NMDAR-mediated responses by chelating contaminating divalent cations.

KEY RESULTS

Truncation or deletion of the CTD of NR2A NMDAR subunits did not affect glutamate potency [EC(50) = 2.2 micromol.L(-1), NR2A(trunC); 2.7 micromol.L(-1), NR2A(delC) compared with 3.3 micromol.L(-1), NR2A(WT)] but did significantly increase glycine potency [EC(50) = 500 nmol.L(-1), NR2A(trunC); 900 nmol.L(-1), NR2A(delC) compared with 1.3 micromol.L(-1), NR2A(WT)]. Voltage-dependent Mg(2+) block of NR2A(WT)- and NR2A(trunC)-containing NMDARs was similar but low concentrations of Mg(2+) (1 micromol.L(-1)) potentiated NR1/NR2A(delC) NMDARs. Memantine block was not affected by changes to the structure of the NR2A CTD. EDTA-induced potentiation was similar at each of the three NMDAR constructs.

CONCLUSIONS AND IMPLICATIONS

Of the parameters studied only minor influences of the CTD were observed; these are unlikely to compromise interpretation of studies that make use of CTD-mutated recombinant receptors or transgenic mice in investigations of the role of the CTD in NMDAR signalling.

Structural and functional heterogeneity of nicotinic receptors

Three gene families of the ligand-gated ion channel gene superfamily encode proteins which bind cholinergic ligands: (1) nicotinic acetylcholine receptors (AChRs) from skeletal muscle, (2) AChRs from neurons, and (3) neuronal alpha-bungarotoxin-binding proteins (alpha BgtBPs). AChRs from muscles and nerves function as ACh-gated cation channels, but alpha BgtBPs do not appear to function in this way. A family of neuronal AChR subtypes has been characterized using monoclonal antibodies and cDNA probes. Neuronal AChRs exhibit sequence homologies with muscle AChRs, but differ in subunit composition, pharmacological and electrophysiological properties, and, in some cases, apparent functional roles. The genes that encode the subunits of the various purified AChR subtypes have been determined in several cases. Histological localization of AChR subunit mRNAs by in situ hybridization and of subunit proteins by immunohistochemistry is being conducted with increasing resolution. The subunit structure of alpha BgtBP is uncertain, but cDNAs have been identified for two subunits. Sequences of these cDNAs reveal that alpha BgtBPs are members of the ligand-gated ion channel gene family, and suggest that they could function as gated cation channels. Biochemical and molecular genetic approaches to studies of neuronal AChRs and related proteins are merging to provide a detailed description of a complex family of AChRs widely dispersed throughout the nervous system, which are probably important to many activities of the nervous system, but whose functional roles are not yet well characterized.

Subunit Dependencies ofN-Methyl-d-aspartate (NMDA) Receptor-Induced α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptor Internalization

N-Methyl-D-aspartate (NMDA) receptor (NMDAR) activity regulates the net number of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPAR) at the cell surface by modulating the balance between AMPAR membrane insertion and endocytosis. In this study, we addressed the role of NMDAR subtypes and of NMDAR-mediated Ca2+ influx in the NMDAR-induced endocytosis of GluR2-containing AMPARs in primary murine hippocampal neurons. We found that NMDAR activation enhanced the endocytosis of AMPARs containing the GluR2 splice variants with short, but not long, cytoplasmic tails. NMDA-induced GluR2 endocytosis was completely inhibited by pharmacological block of NR2B-containing NMDARs. In turn, preferential block of NR2A-containing NMDARs did not affect NMDA-induced AMPAR endocytosis, indicating that AMPAR internalization is controlled by a restricted set of NMDARs. The NMDA-induced GluR2 internalization was also observed in the absence of extracellular Na+ ions, suggesting that membrane depolarization is not a prerequisite for this effect. Furthermore, the activation of Ca2+-impermeable NMDARs containing the mutant NR1(N598R) subunit failed to enhance AMPAR endocytosis, indicating a requirement of Ca2+ influx directly through the NMDAR channels. In summary, our findings suggest that the NMDAR-induced selective internalization of short C-terminal GluR2-containing AMPARs requires a Ca2+ signal that originates from NMDAR channels and is processed in an NMDAR subtype-restricted manner.

Chemically induced long-term potentiation increases the number of perforated and complex postsynaptic densities but does not alter dendritic spine volume in CA1 of adult mouse hippocampal slices

Examination of the morphological correlates of long-term potentiation (LTP) in the hippocampus requires the analysis of both the presynaptic and postsynaptic elements. However, ultrastructural measurements of synapses and dendritic spines following LTP induced via tetanic stimulation presents the difficulty that not all synapses examined are necessarily activated. To overcome this limitation, and to ensure that a very large proportion of the synapses and spines examined have been potentiated, we induced LTP in acute hippocampal slices of adult mice by addition of tetraethylammonium (TEA) to a modified CSF containing an elevated concentration of Ca(2+) and no Mg(+). Quantitative electron microscope morphometric analyses and three-dimensional (3-D) reconstructions of both dendritic spines and postsynaptic densities (PSDs) in CA1 stratum radiatum were made on serial ultrathin sections. One hour after chemical LTP induction the proportion of macular (unperforated) synapses decreased (50%) whilst the number of synapses with simple perforated and complex PSDs (nonmacular) increased significantly (17%), without significant changes in volume and surface area of the PSD. In addition, the surface area of mushroom spines increased significantly (13%) whilst there were no volume differences in either mushroom or thin spines, or in surface area of thin spines. CA1 stratum radiatum contained multiple-synapse en passant axons as well as multiple-synapse spines, which were unaffected by chemical LTP. Our results suggest that chemical LTP induces active dendritic spine remodelling and correlates with a change in the weight and strength of synaptic transmission as shown by the increase in the proportion of nonmacular synapses.

Phosphorylation state of postsynaptic density proteins

The postsynaptic density (PSD) is an electron-dense structure located at the synaptic contacts between neurons. Its considerable complexity includes cytoskeletal and scaffold proteins, receptors, ion channels and signaling molecules, in line with the role of PSDs in signal transduction and processing. The phosphorylation state of components of the PSD is central to synaptic transmission and is known to play a role in synaptic plasticity, learning and memory. The presence of a range of kinases and phosphatases in the PSD defines potential key players in this context. However, the substrates that these enzymes target have not been fully identified to date. We analyzed the protein composition of purified PSD samples from adult mouse brains by strong cation exchange chromatography fractionation of a tryptic digest followed by nano-reverse phase liquid chromatography coupled with electrospray ionization-quadrupole time of flight tandem mass spectrometry. This led to the identification of 244 proteins. To gain an insight into the phosphoproteome of the PSD we then purified phosphorylated tryptic peptides by immobilized metal ion affinity chromatography. This approach for the specific enrichment of phosphopeptides resulted in the identification of 42 phosphoproteins in the PSD preparation, 39 of which are known PSD components. Here we present a total of 83 in vivo phosphorylation sites.

Single-channel analysis of an NMDA receptor possessing a mutation in the region of the glutamate binding site

Recombinant NR1a/NR2A(T671A) N-methyl-D-aspartate (NMDA) receptor-channels, which carry a point mutation in the putative glutamate binding site that reduces glutamate potency by around 1000-fold, have been expressed in Xenopus laevis oocytes and their single-channel properties examined using patch-clamp recording techniques. Shut time distributions of channel activity were fitted with a mixture of five exponential components. The first three components in each distribution were considered to occur within a channel activation as they exhibited little or no dependence on agonist concentration. Bursts of single-channel openings were defined by a critical gap length with a mean of 5.65 +/- 0.37 ms. Shut intervals with a duration longer than this value were considered to occur between separate bursts of channel openings. Distributions of the lengths of bursts of openings were fitted with a mixture of four exponential components. The longest two components carried the majority of the charge transfer in the channel recordings and had means of 7.71 +/- 1.1 and 37.7 +/- 4.3 ms. The overall probability of a channel being open during a burst was high (mean 0.92 +/- 0.01). Brief concentration jumps (1 ms) of 10 mM glutamate were applied to outside-out patches so that a comparison between the macroscopic current relaxation and steady-state single-channel activity evoked by glutamate could be made. The decay of such macroscopic currents was fitted with a single exponential component with a mean of 32.0 +/- 3.53 ms. The good agreement between macroscopic current decay following brief agonist exposure and the value for the slowest component of the burst length distribution suggests that the bursts of openings that we identified in steady-state recordings represent individual activations of recombinant NR1a/NR2A(T671A) NMDA receptor-channels. A new way of displaying geometric distributions is suggested, and the utility of a modified definition of the 'probability of being open within a burst' is discussed. The single-channel data that we present in this paper support further the idea that the point mutation T671A in the NR2A NMDA receptor subunit affects mainly the ability of glutamate to remain bound to these channels.

Molecular cloning and characterization of a novel ATP P2X receptor subtype from embryonic chick skeletal muscle

We have cloned a new P2X ligand-gated ion channel receptor from embryonic chick skeletal muscle, which is tentatively named as chick P2X(8) (cP2X(8)) receptor. The cloned cDNA encodes a protein with 402 amino acids. Electrophysiological study of the recombinant cP2X(8) receptor expressed in Xenopus oocytes showed that 10 microm ATP induced a fast inward current followed by rapid and long lasting desensitization in medium containing 1.8 mm Ca(2+). In medium with 0. 3 mm Ca(2+) ATP induced a bi-phasic response as follows: a slower inward current succeeded the initial fast one. 2-Methylthio-ATP, alpha,beta-methylene-ATP, and adenosine 5'-O-(thio)triphosphate were potent agonists, whereas ADP was a very weak agonist. ATP-induced currents were blocked by 100 microm suramin and pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid. Northern blot analysis and reverse transcription-polymerase chain reaction showed that cP2X(8) RNA transcripts were mainly expressed in skeletal muscle, brain, and heart of Day 10 chick embryos. A moderate level of expression was also detected in gizzard and retina. Whole mount in situ hybridization showed that cP2X(8) RNA transcripts were expressed mainly in neurotube, notochord, and stomach in Day 3 embryos. In Day 4 and Day 6 embryos, the cP2X(8) RNA transcripts were highly expressed in the myotome and premuscle mass. The physiological role of this receptor in the establishment of the skeletal muscle innervation will be studied.

Properties of human glycine receptors containing the hyperekplexia mutation alpha1(K276E), expressed in Xenopus oocytes

1. Inherited defects in human glycine receptors give rise to hyperekplexia (startle disease). We expressed human glycine receptors in Xenopus oocytes, in order to examine the pharmacological and single-channel properties of receptors that contain a mutation, alpha1(K276E), associated with an atypical form of hyperekplexia. 2. Equilibrium concentration-response curves showed that recombinant human alpha1(K276E)beta receptors had a 29-fold lower glycine sensitivity than wild-type alpha1beta receptors, and a greatly reduced Hill coefficient. The maximum response to glycine also appeared much reduced, whereas the equilibrium constant for the glycine receptor antagonist strychnine was unchanged. 3. Both wild-type and mutant channels opened to multiple conductance levels with similar main conductance levels (33 pS) and weighted mean conductances (41.5 versus 49.8 pS, respectively). 4. Channel openings were shorter for the alpha1(K276E)beta mutant than for the wild-type alpha1beta, with mean overall apparent open times of 0.82 and 6.85 ms, respectively. 5. The main effect of the alpha1(K276E) mutation is to impair the opening of the channel rather than the binding of glycine. This is shown by the results of fitting glycine dose-response curves with particular postulated mechanisms, the shorter open times of mutant channels, the properties of single-channel bursts, and the lack of an effect of the mutation on the strychnine-binding site.

Identification of amino acid residues of the NR2A subunit that control glutamate potency in recombinant NR1/NR2A NMDA receptors

The NMDA type of ligand-gated glutamate receptor requires the presence of both glutamate and glycine for gating. These receptors are hetero-oligomers of NR1 and NR2 subunits. Previously it was thought that the binding sites for glycine and glutamate were formed by residues on the NR1 subunit. Indeed, it has been shown that the effects of glycine are controlled by residues on the NR1 subunit, and a "Venus flytrap" model for the glycine binding site has been suggested by analogy with bacterial periplasmic amino acid binding proteins. By analysis of 10 mutant NMDA receptors, we now show that residues on the NR2A subunit control glutamate potency in recombinant NR1/NR2A receptors, without affecting glycine potency. Furthermore, we provide evidence that, at least for some mutated residues, the reduced potency of glutamate cannot be explained by alteration of gating but has to be caused primarily by impairing the binding of the agonist to the resting state of the receptor. One NR2A mutant, NR2A(T671A), had an EC50 for glutamate 1000-fold greater than wild type and a 255-fold reduced affinity for APV, yet it had single-channel openings very similar to those of wild type. Therefore we propose that the glutamate binding site is located on NR2 subunits and (taking our data together with previous work) is not on the NR1 subunit. Our data further imply that each NMDA receptor subunit possesses a binding site for an agonist (glutamate or glycine).

Regional, developmental and interspecies expression of the four NMDAR2 subunits, examined using monoclonal antibodies

Mouse monoclonal antibodies were raised against bacterially expressed protein sequences of the NR2A, NR2B, NR2C and NR2D subunits of the rat NMDA receptor. From immunoblots of rat brain proteins, the apparent molecular weights of these subunits were 165, 170, 135 and 145 kDa, respectively. Proteins of similar masses were observed on immunoblots of specifically transfected HEK293 cells. Deglycosylation with endoglycosidase F reduced the mass of each endogenous NR2 subunit by approximately 10 kDa. In distribution studies, NR2A-immunoreactive protein (IRP) was located throughout the adult rat brain, NR2B-IRP was primarily in the forebrain, NR2C-IRP was predominantly in the cerebellum and NR2D-IRP was mainly found in the thalamus, midbrain and brainstem. Whereas NR2A- and NR2C-IRPs increased during rat brain post-natal development, NR2B- and NR2D-IRPs were abundant at birth and declined with age, especially in cerebellum. NR2-IRPs of mouse, rabbit, frog and human brain were of sizes similar to those of the corresponding rat subunits and were similarly distributed. In summary, NR2 subunits are large glycoproteins whose specific expression profiles in the brain are developmentally and regionally regulated and which are similarly expressed in a variety of species.

Recombinant nicotinic receptors, expressed in Xenopus oocytes, do not resemble native rat sympathetic ganglion receptors in single-channel behaviour

1. In order to establish the subunit composition of neuronal nicotinic receptors in rat superior cervical ganglia (SCG), their single-channel properties were compared with those of recombinant receptors expressed in Xenopus oocytes, using outside-out excised patch recording. 2. The mean main conductance of SCG channels from adult and 1-day-old rats was 34.8 and 36.6 pS, respectively. Less frequent openings to lower conductances occurred both as isolated bursts and as events connected to the main level by direct transitions. There was considerable interpatch variability in the values of the lower conductances. 3. Nicotinic receptors from oocytes expressing alpha3beta4 and alpha4beta4 subunits had chord conductances lower than that of SCG neurones (22 pS for alpha3beta4 and 29 pS for alpha4beta4). 4. Prolonged recording from both native and recombinant channels was precluded by 'run-down', i.e. channel activity could be elicited for only a few minutes after excision. Nevertheless, SCG channel openings were clearly seen to occur as short bursts (slowest component, 38 ms), whereas recombinant channels opened in very prolonged bursts of activity, the major component being the slowest (480 ms). 5. Addition of the alpha5 subunit to the alpha3beta4 pair produced channels with a higher conductance than those observed after injection of the pair alone (24.9 vs. 22 pS), suggesting incorporation of alpha5 into the channel. Addition of the beta2 subunit did not change alpha3beta4 single-channel properties. In one out of fourteen alpha3alpha5beta4 patches, both ganglion-like, high conductance, short burst openings and recombinant-type, low conductance, slow burst openings were observed. 6. Channels produced by expression in Xenopus oocytes of neuronal nicotinic subunits present in rat SCG as a rule differ from native ganglion receptors in single-channel conductance and gross kinetics. While it is possible that an essential nicotinic subunit remains to be cloned, it is perhaps more likely that oocytes either cannot assemble neuronal nicotinic subunits efficiently into channels with the correct composition and stoichiometry, or that they produce post-translational channel modifications which differ from those of mammalian neurones.

Single-channel currents from recombinant NMDA NR1a/NR2D receptors expressed in Xenopus oocytes

We have investigated the single-channel and whole-cell behaviour of recombinant N-methyl-D-aspartate (NMDA) receptors formed from NR1a and NR2D receptor subunits expressed in Xenopus oocytes. The EC50 for apparent steady-state activation of NR1a/NR2D receptors by glutamate was 450 nM, while extracellular MG2+ produced a voltage-dependent block of glutamate responses with an IC50 of 440 microM at -70 mV. At negative holding potentials glutamate-activated NR1a/NR2D single-channel currents, in 0.85 mM external Ca2+, had slope conductances of 35 pS for the main level, and 17 pS for the sublevel; direct transitions occurred between these two conductance levels. On average 35 pS events had mean open times of 1.01 +/- 0.04 ms, whereas the mean open times of 17 pS events were consistently longer (1.28 +/- 0.06 ms). In 5 mM external Ca2+ the larger conductance level was reduced to 20 pS whereas in Ca(2+)-free solutions it was increased to 50 pS. The frequency of transitions between the main and subconductance levels showed temporal asymmetry: 35-17 pS transitions were more frequent (61%) than 17-35 pS transitions. This asymmetry was not affected by alterations in the external Ca2+ concentration (up to 5 mM). In conclusion, the NR1a/NR2D channel is, like NR1a/NR2C, a 'low conductance' NMDA channel, but it can be distinguished from NR1a/NR2C channels on the basis of transition asymmetry and differences in the open times of its main and sub-conductance levels.

Multiple structural elements determine subunit specificity of Mg2+ block in NMDA receptor channels

In NMDA receptor channels, subtype-specific differences of Mg2+ block are determined by the NR2 subunits. Channels assembled from the NR1-NR2A or NR1-NR2B subunits are blocked more strongly than channels formed by the NR1-NR2C or NR1-NR2D subunits, predominantly reflecting a difference in voltage dependence. A determinant of Mg2+ block common to the NR2 subunits is located in the M2 domain (N-site or Q/R/N-site). However, subunit-specific differences of block suggested that additional structural elements exist. Chimeric NR2 subunits were constructed by replacing segments of the least sensitive NR2C subunit with homologous segments of the most sensitive NR2B subunit. Mutant NR2 subunits were coexpressed with wild-type NR1 in Xenopus oocytes, and Mg2+ block was quantified. Replacement of the entire M1-M4 region resulted in a chimera with a sensitivity of Mg2+ block similar to that of the NR2B wild type. Replacing smaller segments or introducing point mutations did not generate channels with Mg2+ block characteristic of NR2B wild type. However, combining in a single chimera three small segments (M1, M2-M3 linker, M4), each independently mediating an increase in Mg2+ block, produced channels close to NR2B wild type. Thus, differences in Mg2+ block as controlled by the NR2 subunits cannot be explained by a single structural determinant in addition to the N-site. Moreover, three elements of the NR2 subunit are the major determinants of subtype-specific differences of Mg2+ block in heteromeric NMDA receptor channels.

Phosphorylation of the predicted major intracellular domains of the rat and chick neuronal nicotinic acetylcholine receptor alpha 7 subunit by cAMP-dependent protein kinase

The predicted major intracellular domains of the chick and rat neuronal nicotinic acetylcholine receptor alpha 7 subunits were expressed in E. coli as glutathione-S-transferase fusion proteins. These proteins were then purified to near homogeneity by chromatography on immobilized glutathione. The intracellular domains of the alpha 7 subunit from both species were phosphorylated to high stoichiometry by cAMP-dependent protein kinase, but not by protein kinase C, cGMP-dependent protein kinase, or calcium/calmodulin-dependent protein kinase. Phosphorylation occurred on serine residues only within an identical single tryptic peptide for both proteins. This conserved phosphorylation site was identified as Ser 342 utilizing site-directed mutagenesis. These results demonstrate that the intracellular domain of the alpha 7 subunit is a substrate of PKA, and suggest a role for protein phosphorylation in mediating cellular regulation upon neuronal AChRs containing this subunit.

Determination of NMDA NR1 subunit copy number in recombinant NMDA receptors

Co-expression of wild-type and mutated NMDA NR1 (N598R) subunits in Xenopus oocytes has been used to determine the stoichiometry of the NMDA receptor-channel. When expressed together, wild-type NR2A and mutant NR1 (N598R) subunits produced channels with a main conductance of 2.6 pS and a sublevel of 1.2 pS. These conductances were clearly different from those obtained from wild-type NR1 and wild-type NR2A channels which gave characteristic 50 pS events with a 40 pS sublevel. When wild-type and mutant NR1 subunits were co-expressed together with NR2A subunits a different channel type with a main conductance of 15.2 pS and a sublevel of 11.4 pS was obtained, as well as the 'all wild-type' and 'all mutant' channels described above. These results indicate that there are likely to be two copies of the NR1 subunit in each NMDA receptor complex.

A P2X purinoceptor cDNA conferring a novel pharmacological profile

We have cloned P2X4, a member of the P2-purinoceptor family, which has a new pharmacological profile. Rat P2X4 is distantly related to P2X1, P2X2 and P2X3 and is expressed in brain, spinal cord, lung, thymus, bladder, adrenal, testis and vas deferens. This ligand gated ion channel is activated by ATP and analogs with the potency order of ATP > ATP gamma S > 2-methylthio ATP > ADP approximately alpha beta-methylene ATP. However, none of the currently used P2X purinoceptor antagonists suramin, reactive blue 2 and PPADS blocked ATP evoked currents; in contrast their application resulted in potentiation of the agonist response. Due to lack of any known antagonist for P2X4 it is unlikely that native P2X4 has previously been recognized as a P2X purinoceptor.

A molecular determinant for submillisecond desensitization in glutamate receptors

The decay of excitatory postsynaptic currents in central neurons mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors is likely to be shaped either by receptor desensitization or by offset after removal of glutamate from the synaptic cleft. Native AMPA receptors show desensitization time constants of 1 to about 10 milliseconds, but the underlying molecular determinants of these large differences are unknown. Cloned AMPA receptors carrying the "flop" splice variants of glutamate receptor subtype C (GluR-C) and GluR-D are shown to have desensitization time constants of around 1 millisecond, whereas those with the "flip" variants are about four times slower. Cerebellar granule cells switch their expression of GluR-D splice variants from mostly flip forms in early stages to predominantly flop forms in the adult rat brain. These findings suggest that rapid desensitization of AMPA receptors can be regulated by the expression and alternative splicing of GluR-D gene transcripts.

Molecular biology of glutamate receptors

The ligand-gated receptors for L-glutamate play a central role in acute neuronal degeneration. Recently cDNAs have been isolated for subunits of several glutamate receptor subtypes. By sequence homology all these subunits clearly belong to one large gene family. Several subfamilies exist and match roughly previously pharmacologically and electrophysiologically defined subtypes of glutamate receptors. Currently four genes (GluR A, B, C and D) are known that code for the AMPA subtypes of glutamate receptors. Recombinant expression of wild type and mutated sequences identified a critical residue in the putative TM2 channel-lining segment that controls Ca2+ ion permeability. The arginine (R) found in GluR B subunits at that position renders AMPA channels impermeable for Ca2+ ions, whereas glutamine (Q) containing GluR A, C and D subunits give rise to Ca2+ permeable channels. RNA editing converts the genomically encoded glutamine codon into the arginine codon found in GluR B cDNAs for the Q/R site. NMDA subtypes of glutamate receptors are formed after coexpression of the NR1 cDNA with a cDNA of the NR2 family. Depending on the member of the NR2 family used, NMDA receptors with different kinetical and pharmacological properties are generated. Common to all channels of these NMDA receptors is a high permeability for Ca2+ ions and a voltage dependent block by Mg2+ ions. All currently known NMDA receptor subunits have an asparagine at the Q/R homologous position. We found that this residue is critical for Mg2+ block and Ca2+ permeability of NMDA receptor channels.

Ethanol inhibits glutamate-induced currents in heteromeric NMDA receptor subtypes

Maximal L-glutamate/glycine-evoked currents were inhibited by ethanol in Xenopus laevis oocytes expressing recombinant heteromeric NMDA receptors consisting of NR1-NR2A, NR1-NR2B, and NR1-NR2C subunit combinations. Concentration-dependent inhibition was observed at ethanol concentrations of > or = 50 mM both in Ca(2+)-containing and Ca(2+)-deficient, Ba(2+)-containing Mg(2+)-free media. The NR1-NR2C channels were slightly less sensitive to ethanol inhibition than the other heteromeric channels in Ca(2+)-deficient, Ba(2+)-containing medium. The inhibition was unaffected by the clamping-voltage and by a mutation [NR1-NR2A(N595Q)] that prevents the Mg(2+)-blockade of the channels, indicating that the mechanism of action of ethanol differs from that of Mg2+. The results are consistent with the hypothesis that the NMDA receptor subtypes can mediate many behavioural actions of ethanol.

Studying block in cloned N-methyl-D-aspartate (NMDA) receptors

The biophysics of block of NMDA receptor channels has been investigated extensively during the past 8 years. In the last few years, cloned NMDA receptor channels have become available. Here we have discussed advantages and disadvantages of studying block phenomena in cloned NMDA receptors. Some recent work on the pore block of the cloned NMDA receptor channels was critically reviewed and extended by data about the calcium block. Novel effects of kainate on cloned NMDA receptors and of NMDA on cloned AMPA receptors were reported and discussed with respect to recent work concerning possible occurrence of NMDA-AMPA hybrid channels.

Subunit-specific block of cloned NMDA receptors by argiotoxin636

Cloned NMDA receptor channels of the NR1-NR2A, NR1-NR2B and NR1-NR2C type show differences in argiotoxin636 block. Mutations of an asparagine residue located at a homologous position in the TM2 region of all NMDA receptor subunits, which corresponds to the Q/R site of the AMPA receptors, alters the argiotoxin636-induced block. The results suggest that the toxin interacts at this amino acid position with the putative pore forming TM2 region of the NMDA receptor subunits. Sequence differences in the TM2 segment of NR2A and NR2C subunits are not responsible for the subtype-specific sensitivity to argiotoxin636 as revealed by site-directed mutagenesis.

Argiotoxin detects molecular differences in AMPA receptor channels

Argiotoxin, a component of the spider venom from Argiope lobata, blocks AMPA receptor channels expressed in homomeric and heteromeric configuration in Xenopus oocytes. Argiotoxin acts as an open channel blocker in a voltage-dependent manner and discriminates between the functionally diverse AMPA receptors. Importantly, a transmembrane region 2 determinant for divalent cation permeability also determines argiotoxin sensitivity. Subunit-specific differences in the time courses of block and recovery demonstrate that heteromeric AMPA receptors can assemble in variable ratios. Thus, argiotoxin can be used as a tool in analyzing the subunit composition of AMPA receptors in native membranes.

The pRSET family of T7 promoter expression vectors for Escherichia coli

A family of eight T7 promoter-based expression plasmids is presented. These are high-copy-number vectors featuring translational start and stop elements and a multiple cloning site (polylinker) with eleven unique restriction sites in all six reading frames. Depending on the cloning strategy used, recombinant proteins may contain either short vector-encoded fusion fragments or no fusion fragments at all. Following promoter induction, proteins are usually produced at a high level.

Three subtypes of alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors are expressed in chick retina

A recent report described the isolation of cDNA clones encoding alpha 7 and alpha 8 subunits of alpha-bungarotoxin-sensitive nicotinic ACh receptors (alpha BgtAChRs) from chick brain and demonstrated that they were related to, but distinct from, the alpha subunits of nicotinic ACh receptors (nAChRs) from muscles and neurons. Monoclonal antibodies against the two alpha BgtAChR subunits were used to demonstrate that at least two subtypes are present in embryonic day 18 chicken brain. The predominant brain subtype contains alpha 7 subunits, while a minor subtype contains both alpha 7 and alpha 8 subunits. Both subtypes may also contain other subunits. Here we report the results of immune precipitation studies and immunohistochemical studies of alpha BgtAChRs in the chick retina. In addition to the two subtypes found in brain, a new alpha BgtAChR subtype that contains alpha 8 subunits, but not alpha 7 subunits, was identified and was found to be the major subtype in chick retina. This subtype has a lower affinity for alpha-bungarotoxin (alpha Bgt) than does the subtype containing only alpha 7 subunits. Small amounts of this alpha 8 subtype were also detected in brain by labeling with higher concentrations of 125I-alpha Bgt than had been used previously. The subtype containing only alpha 7 subunits comprised 14% of the alpha BgtAChRs in hatchling chick retina. The subtype containing alpha 8 subunits (but no alpha 7 subunits) accounted for 69%, and the alpha 7 alpha 8 subtype accounted for 17%. Amacrine, bipolar, and ganglion cells displayed alpha 8 subunit immunoreactivity, and a complex pattern of labeling was evident in both the inner and outer plexiform layers. In contrast, only amacrine and ganglion cells exhibited alpha 7 subunit immunoreactivity, and the pattern of alpha 7 subunit labeling in the inner plexiform layer differed from that of alpha 8 subunit labeling. These disparities suggest that the alpha BgtAChR subunits are differentially expressed by different populations of retinal neurons. In addition, the distribution of alpha BgtAChR subunit immunoreactivity was found to differ from that of alpha-Bgt-insensitive nAChR subunits.

Single-channel conductances of NMDA receptors expressed from cloned cDNAs: comparison with native receptors

To cast light on the subunit composition of native NMDA-type glutamate receptors, four cloned subunits of the NMDA receptor have been expressed, in pairs, in Xenopus oocytes, and their single-channel properties have been measured. The conductances of the channels, and their characteristic patterns of sublevel transitions, turn out to be useful diagnostic criteria for subunit composition. The NR1-NR2A and NR1-NR2B combinations (which have identical TM2 sequences) are very similar to each other. Both have 50 pS openings and brief 40 pS sublevels (in 1 mM external Ca2+), with similar mean lifetimes and frequencies. They also show close quantitative resemblance to the channels of hippocampal CA1 and dentate gyrus cells and of cerebellar granule cells, except that the NR1-NR2A combination has a lower glycine sensitivity than the native channels. In contrast, the NR1-NR2C combination produces a channel with 36 pS and 19 pS conductances of similar (brief) duration; these closely resemble the 38-18 pS channels that have previously been observed in large cerebellar neurons in culture (together with 50 pS channels).

Control by asparagine residues of calcium permeability and magnesium blockade in the NMDA receptor

The N-methyl-D-aspartate (NMDA) receptor forms a cation-selective channel with a high calcium permeability and sensitivity to channel block by extracellular magnesium. These properties, which are believed to be important for the induction of long-term changes in synaptic strength, are imparted by asparagine residues in a putative channel-forming segment of the protein, transmembrane 2 (TM2). In the NR1 subunit, replacement of this asparagine by a glutamine residue decreases calcium permeability of the channel and slightly reduces magnesium block. The same substitution in NR2 subunits strongly reduces magnesium block and increases the magnesium permeability but barely affects calcium permeability. These asparagines are in a position homologous to the site in the TM2 region (Q/R site) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that is occupied by either glutamine (Q) or arginine (R) and that controls divalent cation permeability of the AMPA receptor channel. Hence AMPA and NMDA receptor channels contain common structural motifs in their TM2 segments that are responsible for some of their ion selectivity and conductance properties.

Heteromeric NMDA Receptors: Molecular and Functional Distinction of Subtypes

The N-methyl D-aspartate (NMDA) receptor subtype of glutamate-gated ion channels possesses high calcium permeability and unique voltage-dependent sensitivity to magnesium and is modulated by glycine. Molecular cloning identified three complementary DNA species of rat brain, encoding NMDA receptor subunits NMDAR2A (NR2A), NR2B, and NR2C, which are 55 to 70% identical in sequence. These are structurally related, with less than 20% sequence identity, to other excitatory amino acid receptor subunits, including the NMDA receptor subunit NMDAR1 (NR1). Upon expression in cultured cells, the new subunits yielded prominent, typical glutamate- and NMDA-activated currents only when they were in heteromeric configurations with NR1. NR1-NR2A and NR1-NR2C channels differed in gating behavior and magnesium sensitivity. Such heteromeric NMDA receptor subtypes may exist in neurons, since NR1 messenger RNA is synthesized throughout the mature rat brain, while NR2 messenger RNA show a differential distribution.

T cells from normal and myasthenic individuals recognize the human acetylcholine receptor: heterogeneity of antigenic sites on the alpha-subunit

The alpha-subunit of the nicotinic acetylcholine receptor is the major target of the autoimmune response in myasthenia gravis. We investigated the proliferative response of T cells from patients with myasthenia gravis and healthy volunteers to recombinant polypeptides of the human acetylcholine receptor including the full-length alpha-subunit (alpha 1-437). T cells from 20 (71%) of 28 patients and 7 (37%) of 19 healthy volunteers responded in primary cultures. Subsequently, specific T-cell lines were established: CD4+, CD8-, UCHL1+, and major histocompatibility complex (MHC) class II-restricted. Using a set of fragments of the alpha-subunit, major antigenic sites could be localized on the extracellular, N-terminal part of the molecule as well as close to the C-terminus. The T-cell response was heterogeneous, both among different individuals and among T-cell lines from a single donor. These T cells did not cross-react with Torpedo acetylcholine receptor, which was previously used as a substitute for human muscle acetylcholine receptor, suggesting that the T cells had a bias for unique human sequences. A single antigenic fragment could be presented in the context of different MHC class II molecules, and different fragments could be presented in the context of the same MHC molecule. This supports earlier observations of considerable heterogeneity in dealing with acetylcholine receptor as an autoantigen on the level of both T cells and antigen-presenting cells. The data also demonstrate that acetylcholine receptor-specific T cells are present in the normal immune repertoire, and emphasize the role of immune regulation for maintaining a state of tolerance.

Structural and pharmacological characterization of the major brain nicotinic acetylcholine receptor subtype stably expressed in mouse fibroblasts

Previously, we purified the predominant subtype of brain nicotinic acetylcholine receptor (AChR), analyzed its structure, and found that it was composed of two kinds of subunit, with sequences encoded by cDNAs termed alpha 4 and beta 2. Here we express these cDNAs from chicken brain in stably transfected fibroblasts. We demonstrate by synthesis that these cDNAs encode subunit polypeptides of the expected sizes, which coassemble to form receptor macromolecules having the same size as native AChRs. Additionally, we demonstrate that the expressed AChRs exhibit the ligand-binding pharmacology of native brain AChRs and function as acetylcholine-gated ion channels.

Identification of sequence segments forming the alpha-bungarotoxin binding sites on two nicotinic acetylcholine receptor alpha subunits from the avian brain

The relationship between neuronal alpha-bungarotoxin binding proteins (alpha BGTBPs) and nicotinic acetylcholine receptor function in the brain of higher vertebrates has remained controversial for over a decade. Recently, the cDNAs for two homologous putative ligand binding subunits, designated alpha BGTBP alpha 1 and alpha BGTBP alpha 2, have been isolated on the basis of their homology to the N terminus of an alpha BGTBP purified from chick brain. In the present study, a panel of overlapping synthetic peptides corresponding to the complete chick brain alpha BGTBP alpha 1 subunit and residues 166-215 of the alpha BGTBP alpha 2 subunits were tested for their ability to bind 125I-alpha BGT. The sequence segments corresponding to alpha BGTBP alpha 1-(181-200) and alpha BGTBP alpha 2-(181-200) were found to consistently and specifically bind 125I-alpha BGT. The ability of these peptides to bind alpha BGT was significantly decreased by reduction and alkylation of the Cys residues at positions 190/191, whereas oxidation had little effect on alpha BGT binding activity. The relative affinities for alpha BGT of the peptide sequences alpha BGTBP alpha 1-(181-200) and alpha BGTBP alpha 2-(181-200) were compared with those of peptides corresponding to the sequence segments Torpedo alpha 1-(181-200) and chick muscle alpha 1-(179-198). In competition assays, the IC50 for alpha BGTBP alpha 1-(181-200) was 20-fold higher than that obtained for the other peptides (approximately 2 versus 40 microM). These results indicate that alpha BGTBP alpha 1 and alpha BGTBP alpha 2 are ligand binding subunits able to bind alpha BGT at sites homologous with nAChR alpha subunits and that these subunits may confer differential ligand binding properties on the two alpha BGTBP subtypes of which they are components.

Neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes have a pentameric quaternary structure

We have determined the subunit stoichiometry of chicken neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes by quantitation of the amount of radioactivity in individual subunits of [35S] methionine-labeled receptors. The chicken neuronal nicotinic acetylcholine receptor appears to be a pentamer of two alpha 4 acetylcholine-binding subunits and three beta 2 structural subunits. We also show that these expressed receptors bind L-[3H]nicotine with high affinity, are transported to the surface of the oocyte outer membrane, and cosediment on sucrose gradients with acetylcholine receptors isolated from chicken brain. Using this unique and generally applicable method of determining subunit stoichiometry of receptors expressed in oocytes, we obtained the expected (alpha 1) 2 beta 1 gamma delta stoichiometry for muscle-type acetylcholine receptors assembled from coexpression of either Torpedo alpha 1 or human alpha 1 subunits, with Torpedo beta 1, gamma, and delta subunits.

Identification of a Single Melanin-Concentrating Hormone Messenger Ribonucleic Acid in Coho Salmon: Structural Relatedness with 7SL Ribonucleic Acid

Abstract Melanin-concentrating hormone (MCH) is a cyclic neuropeptide possessing antagonistic function to alpha-melanocyte-stimulating hormone and corticotropin-releasing factor in the control of melanosome dispersion within melanophores and adrenocorticotropin release in fish. We have isolated and characterized MCH cDNAs from coho salmon (Oncorhyncus kisutch). The precursor protein predicted by the longest cDNA consists of 132 amino-acids with a characteristic signal peptide at the N-terminus and the biologically active salmon MCH (sMCH) peptide at the C-terminus. The coho sMCH mRNA and protein sequences are very similar but not identical to the previously reported chum or chinook salmon counterparts, suggesting the existence of species polymorphism. Sequence similarities were revealed between alpha-melanocyte-stimulating hormone and part of the C-terminal domain of sMCH precursor. Two sMCH genes were found in coho salmon. By contrast to other salmon species, only one major sMCH mRNA was detected in coho species suggesting that differential MCH gene expression might occur in salmon. In addition, under low stringency oligoprobes complementary to the sMCH RNA recognize a 0.3 kb RNA which was identified as the 7SL RNA. The regions conserved between those RNAs fold in a similar secondary structure. These similarities might reflect common ancestry which may have functional significance.

Expression of nicotinic acetylcholine receptor subtypes in brain and retina

Neuronal nicotinic acetylcholine receptors (AChRs) are composed of two types of subunits: ACh-binding (termed alpha 2, alpha 3, alpha 4 ...) and structural (termed beta 2, beta 3, beta 4 ...). AChR subtypes composed of combinations of subunits of these two types encoded by several related genes are expressed in different parts of the nervous system, where they presumably serve different functional roles. Here we identify the ACh-binding subunit of the most prominent chicken brain AChR subtype by N-terminal amino acid sequence and show that it corresponds to the alpha 4 gene. Previously we identified the structural subunit for this AChR subtype from chicken brain as beta 2 by N-terminal amino acid sequence. Thus, this identifies both genes which encode subunits of the major nicotinic AChR subtype in avian brains. By immunoprecipitation, immunohistochemistry, and northern blot analysis we show that alpha 3 (or a very closely related sequence) is expressed at low levels in the brain and relatively high levels in the retina, while alpha 4 is expressed at high levels in the brain and lower levels in the retina. This differential expression indicates that alpha 3-containing 'ganglionic-type' AChRs may be an important AChR subtype in avian retina.

Brain alpha-bungarotoxin binding protein cDNAs and MAbs reveal subtypes of this branch of the ligand-gated ion channel gene superfamily

alpha-Bungarotoxin (alpha Bgt) is a potent, high-affinity antagonist for nicotinic acetylcholine receptors (AChRs) from muscle, but not for AChRs from neurons. Both muscle and neuronal AChRs are thought to be formed from multiple homologous subunits aligned around a central cation channel whose opening is regulated by ACh binding. In contrast, the exact structure and function of high-affinity alpha Bgt binding proteins (alpha BgtBPs) found in avian and mammalian neurons remain unknown. Here we show that cDNA clones encoding alpha BgtBP alpha 1 and alpha 2 subunits define alpha BgtBPs as members of a gene family within the ligand-gated ion channel gene superfamily, but distinct from the gene families of AChRs from muscles and nerves. Subunit-specific monoclonal antibodies raised against bacterially expressed alpha BgtBP alpha 1 and alpha 2 subunit fragments reveal the existence of at least two different alpha BgtBP subtypes in embryonic day 18 chicken brains. More than 75% of all alpha BgtBPs have the alpha 1 subunit, but no alpha 2 subunit, and a minor alpha BgtBP subtype (approximately 15%) has both the alpha 1 and alpha 2 subunits.

Cloning and expression of bovine brain inositol monophosphatase

Inositol monophosphatase is a key enzyme of the inositol phosphate second messenger signaling pathway. It is responsible for the provision of inositol required for synthesis of phosphatidylinositol and polyphosphoinositides and has been implicated as the pharmacological target for lithium action in brain. Using oligonucleotide probes based on partial amino acid sequence data for the bovine brain enzyme, several overlapping cDNA clones of 2-3 kilobases in length have been isolated. All contain an open reading frame encoding a 277-amino acid protein. No significant sequence homology was found with any known protein. The open reading frame was inserted into a bacterial expression vector in order to confirm the presumed identity of the protein. The expressed protein reacted with an anti-inositol monophosphatase monoclonal antibody. In addition, the protein was enzymically active and indistinguishable from the bovine brain enzyme with respect to Km values for substrate and Li+ sensitivity of inositol 1-phosphate hydrolysis.

Antisera against an acetylcholine receptor alpha 3 fusion protein bind to ganglionic but not to brain nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptor (AChR) subtypes have been defined pharmacologically, immunologically, and by DNA cloning, but the correlations between these approaches are incomplete. Vertebrate neuronal AChRs that have been isolated are composed of structural subunits and ACh-binding subunits. A single kind of subunit can be used in more than one AChR subtype. Monoclonal antibody (mAb) 35 binds to structural subunits of subtypes of AChRs from both chicken brain and ganglia. By using antisera to a unique sequence of alpha 3 ACh-binding subunits expressed in bacteria, we show that ganglionic AChRs contain alpha 3 ACh-binding subunits, whereas the brain AChR subtype that binds mAb 35 does not. Subunit-specific antisera raised against recombinant proteins should be a valuable approach for identifying the subunit composition of receptors in multigene, multisubunit families.

A muscle acetylcholine receptor is expressed in the human cerebellar medulloblastoma cell line TE671

The human neuromedulloblastoma cell line TE671 is shown by single-channel recordings to express nicotinic acetylcholine receptors (AChRs) that are blocked by alpha-bungarotoxin (alpha Bgt). These AChRs do not react with antisera to the alpha Bgt-binding protein of brain or with monoclonal antibodies (mAbs) to brain nicotinic AChRs that do not bind alpha Bgt. TE671 AChRs do react with autoantibodies to muscle AChRs from myasthenia gravis patients and with mAbs to muscle AChRs, including mAbs specific for extrajunctional AChRs. AChRs. AChRs purified from TE671 cells are composed of 4 kinds of subunits corresponding to those of muscle AChR. Sequences of cDNAs for the ACh-binding alpha subunit and the delta subunit of this AChR further identify it as muscle AChR. Expression of TE671 AChR can be up-regulated by nicotine and dexamethasone, and down-regulated by forskolin.

cDNA clones coding for the structural subunit of a chicken brain nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors (AChRs) immunoaffinity-purified from brains are composed of only two kinds of subunits rather than the four kinds present in muscle-type AChRs. Here we report the N-terminal protein sequences of the structural subunits of AChRs from rat and chicken brains and the cloning of full-length cDNAs for the chicken brain AChR structural subunit. Previously, the N-terminal amino acid sequence of the ACh-binding subunit of AChR immunoaffinity-purified from rat brain was shown to correspond to the cDNA alpha 4. Thus, cDNA sequences are now known for both of the subunits that form one AChR subtype in vivo.

The human medulloblastoma cell line TE671 expresses a muscle-like acetylcholine receptor. Cloning of the alpha-subunit cDNA

Nicotinic acetylcholine receptors (AChRs) from muscle bind alpha-bungarotoxin (alpha Bgt) and are composed of four kinds of subunits, whereas AChRs from mammalian brains do not bind alpha Bgt and are composed of two kinds of subunits. alpha Bgt-binding proteins whose function is unknown are also found in brain. All these proteins belong to the same gene family. The human medulloblastoma cell line TE671 expresses a functional AChR which binds alpha Bgt. Surprisingly, the AChR of this neuron-derived cell line has electrophysiological, immunological and biochemical properties different from neuronal AChRs and very similar to muscle AChRs. The TE671 AChR binds alpha Bgt, but is different from alpha Bgt-binding proteins in brain. Here we show that TE671 expresses the alpha-subunit mRNA coding for the muscle AChR, thereby proving that TE671 expresses a muscle-type AChR that is not expressed in adult brain. The isolated cDNA clones should prove useful for expression of large amounts of human muscle-type AChR alpha-subunit protein for studies of the autoimmune response to muscle AChRs in human myasthenia gravis.