Antibodies against the bovine brain glutamate binding protein

Immunocytochemical and in situ hybridization studies of the expression and distribution of three subunits of a complex with N-methyl-D-aspartate receptor-like properties

A group of four proteins with recognition sites for L-glutamate, N-methyl-D-aspartate, glycine, and competitive and non-competitive inhibitors of N-methyl-D-aspartate receptors was previously purified from rat brain synaptic membranes. The biochemical and immunochemical characteristics of this complex, as well as the sequences of the complementary DNAs of three subunits, are distinct from those of other glutamate receptors, transporters, or enzymes. The function of this complex has not yet been defined, but it appears to be involved in glutamate-induced neuronal excitation and toxicity. It is not known whether all protein components of the complex are expressed in the same populations of brain cells. In the present study, immunohistochemical and in situ hybridization were used to map the distribution of the glutamate-binding, glycine/thienylcyclohexylpiperidine-binding, and carboxypiperazinyl-propylphosphonate-binding protein subunits of the complex. These proteins were abundantly expressed in pyramidal neurons of the hippocampus and cerebral cortex, and in granule cells of the dentate gyrus, cerebellum, and olfactory tubercle. Based on these results, it was concluded that the three subunits of the complex have similar patterns of expression in rat brain. The distribution of one subunit of the complex, glutamate-binding protein, was traced throughout the rat brain, thus providing a potential map of the expression of the complex in rodent brain. In addition, probes were developed in the present study that should be useful in future explorations of the role of these proteins in brain function and of the possible co-localization of the protein subunits in single cells or cell processes.

Calcium influx through NMDA receptors, chronic receptor inhibition by ethanol and 2-amino-5-phosponopentanoic acid, and receptor protein expression

Chronic treatment of neurons with either ethanol or competitive and noncompetitive antagonists of NMDA receptors leads to enhanced expression of NMDA receptor density and function in neurons. The signal transduction pathways for such receptor up-regulation are not known. The focus of the present study was on the role of Ca2+ entry into neurons, either through receptor or voltage-gated channels, in the expression of the NMDA receptor subunit NR1 and the 71-kDa glutamate-binding protein (GBP) of a glutamate/NMDA receptor-like complex. Chronic inhibition of NMDA receptors in cortical neurons in primary cultures by either 100 mM ethanol or 100 microM 2-amino-5-phosphonopentanoic acid (2-AP5) increased the expression of NR1 and GBP. The effect of 2-AP5 on the expression of the two proteins was not additive with that of ethanol when neuronal cultures were treated with both agents at the same time. However, the effects of ethanol on NR1 and GBP expression were blocked by the simultaneous treatment with NMDA (50 microM). Activation or inhibition of other glutamate ionotropic receptors had no effect on the expression of NR1 and GBP. The inhibition of L- or N-type voltage-sensitive Ca2+ channels and voltage-gated Na+ channels also had little effect on the expression of either protein; neither did exposure of neurons to elevated extracellular Ca2+ concentrations (3 or 5 mM). On the other hand, treatment of neurons for 48 h with the intracellular Ca2+ chelator BAPTA-AM as well as partial chelation of extracellular Ca2+ with EGTA caused an up-regulation in NR1 and GBP expression. The enhanced expression of NR1 in neurons treated for 48 h with either ethanol or EGTA was correlated with increases in the activity of NMDA receptors demonstrated as a doubling of the NMDA-stimulated rise in intracellular free Ca2+ concentration. The effects of chronic administration of EGTA on both NR1 expression as well as NMDA receptor function were probably related to an acute inhibition by EGTA of NMDA-induced Ca2+ influx into neurons. It appears that the expression of both the NR1 subunit of NMDA receptors and the GBP of a receptor-like complex is regulated by intracellular Ca2+, especially that entering through NMDA receptor ion channels.

Isolation of glutamate-binding proteins from rat and bovine brain synaptic membranes and immunochemical and immunocytochemical characterization

We previously isolated two glutamate-binding proteins from rat brain synaptic membranes of 71,000 and 63,000 mol. wt [Chen et al. (1988) J. biol. Chem. 263, 417-426]. In the present study, the 71,000 and 63,000 mol. wt glutamate-binding proteins were purified from rat and bovine brain synaptic membranes by affinity chromatographic separation on an L-glutamate-derived Trisacryl matrix. The major protein component in the purified fractions was a 71,000 mol. wt protein as judged by sodium dodecyl sulfate gel electrophoresis. This fraction represented enrichment of bovine brain glutamate-binding proteins by a factor of 7000-8000 when compared with brain homogenates. The ligand binding characteristics of the proteins purified by this new procedure are very similar to those of the proteins purified by the procedures we described in previous studies. Polyclonal antibodies were raised in mice against the purified, native rat and bovine brain glutamate-binding proteins. These two sets of antibodies interacted specifically with the glutamate-binding proteins but not with any glutamate-metabolizing enzymes. Both sets of antibodies labeled a 71,000 mol. wt protein in Western blots of synaptic membranes obtained from either rat or bovine brain, an indication of homology between these proteins. Both sets of antibodies produced immunoprecipitation of approximately 70-75% of all glutamate-binding proteins from solubilized synaptic membrane proteins. These observations are an indication that the glutamate-binding proteins described above are the most common glutamate-binding entities in both bovine and rat brain synaptic membranes and that they can be easily purified in a one-step chromatographic procedure.

A monoclonal antibody that inhibits opioid binding to rat brain membranes

To understand the structure-function relationship and to probe the molecular characteristics of the purified opioid receptor, monoclonal antibodies (mab) were raised against a purified opioid receptor protein. After intensive screening of almost 1500 hybridoma cell lines, only 7 clones were shown to have very high immunoreactivity against the purified receptor. Moreover, out of these 7 clones, only 2, 3B4F11 and 3A27G, were found to inhibit the ligand binding property of the mu-opioid receptor. The mab 3B4F11 was found to inhibit 3H-diprenorphine binding to the purified receptor in a dose dependent manner with a maximal inhibition of 100% achieved with 20 micrograms of the antibody. Likewise, Fab fragments prepared from the mabs 3B4F11 inhibited 3H-diprenorphine binding to P2 membranes in a dose-dependent manner. In addition, it was found that the binding of 3H-DAGO, 3H-DPDPE and 3H-EKC was inhibited with approximately equal potency, suggesting that the Fabs prepared from the mab 3B4F11 interact with all 3 receptor types.

Different molecular weight forms of opioid receptors revealed by polyclonal antibodies

Polyclonal antibodies were raised against a purified opioid receptor from bovine brain (Cho, et. al., 1986), and shown to inhibit 3H-diprenorphine binding to this receptor in a dose-dependent fashion. These antibodies were then used to characterize opioid-binding material present in rat brain and in NG108-15 neuroblastoma-glioma hybrid cells. Western blot analysis revealed that the antibodies reacted with a single species of 58,000 molecular weight in rat brain membranes; this closely corresponds in size to the bovine opioid receptor used to raise the antibodies. In contrast, the polyclonal antibodies reacted with a 45,000 molecular weight species in NG108-15 neuroblastoma-glioma hybrid cells; moreover, this band was specifically reduced in NG108-15 cells in which opioid receptors had been down-regulated by incubation with D-ala2-D-leu5-enkephalin for 24 hours. Thus at least two distinct opioid receptor molecules have been identified, which have antigenic similarities.

Effects of anti-glutamate-binding protein antibodies on synaptic membrane ion flux, glutamate transport and release, and L-glutamate binding activities

Antibodies (Abs) raised against the L-glutamate-binding protein (GBP) purified from bovine brain were used to define the possible physiologic activity of GBP in synaptic membranes. Three processes were examined for their sensitivity to the Abs: the excitatory amino acid stimulation of thiocyanate (SCN-) flux, the transport of L-glutamic acid across the synaptic membrane, and the depolarization-induced release of L-glutamate. Only the amino acid-induced changes in ion flux were inhibited by the anti-GBP Abs. The change in membrane potential produced by exposure of synaptic membranes to excitatory amino acids was measured as the increase in the uptake of the lipophilic anion SCN-. The L-glutamate-induced SCN- influx was 40 times more sensitive to inhibition by the anti-GBP Abs than the stimulation of ion flux by kainate, and 60 times more sensitive than that produced by quisqualate. The anti-GBP Abs did not inhibit the activation of ion flux produced by N-methyl-D-aspartate. The inhibition of glutamate-stimulated ion fluxes by the Abs was complete, whereas the inhibition of L-glutamate binding to either the rat or bovine brain GBP was not. The results obtained indicated that although the majority of the anti-GBP Abs were not directed against the glutamate recognition site of the GBP and of presumed synaptic membrane receptors, they were effective in blocking the activation of receptor-associated ion channels. Thus, the GBP may be considered a component of some excitatory amino acid receptor complexes.

Functional characteristics of L-glutamate, N-methyl-D-aspartate and kainate receptors in isolated brain synaptic membranes

L-Glutamic acid (L-Glu) and other excitatory amino acids and amino acid analogs enhanced [35S]thiocyanate (SCN-) uptake in isolated-resealed synaptic membrane vesicles. The SCN- uptake was used as a measure of membrane depolarization to evaluate the characteristics of functional excitatory amino acid receptors in the synaptic membranes. N-Methyl-D-aspartate (NMDA) and L-Glu produced additive effects on SCN- accumulation indicating the presence of distinct L-Glu and NMDA receptors. On the other hand, kainic acid (KA) and L-Glu shared either common receptor sites or ion channels. The effects of antagonists on NMDA, L-Glu, and KA stimulation of SCN- influx were consistent with previously reported electrophysiologic observations in intact neurons.