Biochemical and morphological comparison of postsynaptic densities prepared from rat, hamster, and monkey brains by phase partitioning

The Neuroplastin adhesion molecules: key regulators of neuronal plasticity and synaptic function

The Neuroplastins Np65 and Np55 are neuronal and synapse-enriched immunoglobulin superfamily molecules that play important roles in a number of key neuronal and synaptic functions including, for Np65, cell adhesion. In this review we focus on the physiological roles of the Neuroplastins in promoting neurite outgrowth, regulating the structure and function of both inhibitory and excitatory synapses in brain, and in neuronal and synaptic plasticity. We discuss the underlying molecular and cellular mechanisms by which the Neuroplastins exert their physiological effects and how these are dependent upon the structural features of Np65 and Np55, which enable them to bind to a diverse range of protein partners. In turn this enables the Neuroplastins to interact with a number of key neuronal signalling cascades. These include: binding to and activation of the fibroblast growth factor receptor; Np65 trans-homophilic binding leading to activation of p38 MAPK and internalization of glutamate (GluR1) receptor subunits; acting as accessory proteins for monocarboxylate transporters, thus affecting neuronal energy supply, and binding to GABAA α1, 2 and 5 subunits, thus regulating the composition and localization of GABAA receptors. An emerging theme is the role of the Neuroplastins in regulating the trafficking and subcellular localization of specific binding partners. We also discuss the involvement of Neuroplastins in a number of pathophysiological conditions, including ischaemia, schizophrenia and breast cancer and the role of a single nucleotide polymorphism in the human Neuroplastin (NPTN) gene locus in impairment of cortical development and cognitive functions. Neuroplastins are neuronal cell adhesion molecules, which induce neurite outgrowth and play important roles in synaptic maturation and plasticity. This review summarizes the functional implications of Neuroplastins for correct synaptic membrane protein localization, neuronal energy supply, expression of LTP and LTD, animal and human behaviour, and pathophysiology and disease. It focuses particularly on Neuroplastin binding partners and signalling mechanisms, and proposes perspectives for future research on these important immunoglobulin superfamily members.

The neuroplastins: multifunctional neuronal adhesion molecules--involvement in behaviour and disease

The neuroplastins np65 and np55 are neuronal and synapse-enriched immunoglobulin (Ig) superfamily cell adhesion molecules that contain 3 and 2 Ig domains, respectively. Np65 is neuron specific whereas np55 is expressed in many tissues. They are multifunctional proteins whose physiological roles are defined by the partner proteins they bind to and the signalling pathways they activate. The neuroplastins are implicated in activity-dependent long-term synaptic plasticity. Thus neuroplastin-specific antibodies and a recombinant peptide inhibit long-term potentiation in hippocampal neurones. This is mediated by activation of the p38MAP kinase signalling pathway, resulting in the downregulation of the surface expression of GluR1 receptors. Np65, but not np55, exhibits trans-homophilic binding. Both np65 and np55 induce neurite outgrowth and both activate the FGF receptor and associated downstream signalling pathways. Np65 binds to and colocalises with GABA(A) receptor subtypes and may play a role in anchoring them to specific synaptic and extrasynaptic sites. Most recently the neuroplastins have been shown to chaperone and support the monocarboxylate transporter MCT2 in transporting lactate across the neuronal plasma membrane. Thus the neuroplastins are multifunctional adhesion molecules which support neurite outgrowth, modulate long-term activity-dependent synaptic plasticity, regulate surface expression of GluR1 receptors, modulate GABA(A) receptor localisation, and play a key role in delivery of monocarboxylate energy substrates both to the synapse and to extrasynaptic sites. The diverse functions and range of signalling pathways activated by the neuroplastins suggest that they are important in modulating behaviour and in relation to human disease.

Structural role of zinc ions bound to postsynaptic densities

Postsynaptic densities (PSDs) isolated from porcine cerebral cortices are large aggregates consisting of more than 30 different proteins. Inductively coupled plasma-mass spectrometric analyses revealed that isolated PSDs contained zinc at a concentration of 4.1 nmol per mg protein. Treatment with 8 m urea lead to dissociation of the PSDs into small components and, concomitantly, depletion of most of their bound zinc. After removal of the urea by dialysis, urea-dissociated PSD proteins did not reassemble into aggregates by themselves. Adding ZnCl2 to urea-treated PSD samples resulted in the assembly of urea-dissociated proteins into large aggregates with morphology and protein composition closely resembling those of the original PSDs. Mg2+, Ca2+, Co2+, Cd2+, Cu2+, Mn2+, Fe3+, K+ and Na+ ions at higher concentrations also induced the aggregation of urea-dissociated PSD protein. The structures of the K+-, Na+-, Mg2+- and Ca2+-induced aggregates were distinct from that of the original PSDs. Our results indicate that the structure of the PSD could be disassembled and reassembled under in vitro conditions. They further suggest that Zn2+ ions, by binding to certain zinc-binding proteins, play an important role in the formation and maintenance of the structure of the PSD.

A chromodomain-containing nuclear protein, MRG15 is expressed as a novel type of dendritic mRNA in neurons

On the basis of a hypothesis that proteins encoded by the mRNAs that are transported to and translated at the dendrites/synapses may play key roles in synaptic plasticity, this study reports on attempts to isolate mRNAs which are localizing at the dendrites/synapses from mouse cerebellar synaptosomal fractions. Among 100 pieces of dendritic mRNA candidates, 10 pieces of mRNAs were found to contain the cytoplasmic polyadenylation element (CPE)-like sequences which were contained in certain mRNAs translated in dendrites. We next examined the issue of whether the CPE-like sequence-containing mRNAs (CPERs) were localized in the synapses/dendrites by means of in situ hybridization. The findings indicate that CPER9 was actually localized at the apical dendrites of a portion of cerebral cortex layer V pyramidal cells, as well as at the proximal dendrites of some of the cerebellar Purkinje cells. CPER9 was found to encode a mouse homolog of MRG15, a nuclear protein which contains a chromodomain identified in several proteins that act as regulators of transcription. Immunohistochemistry with anti-MRG15 antibodies revealed that MRG15 was localized in dendrites as well as in the nuclei of Purkinje cells. These results suggest that MRG15 may serve as a link between synaptic activity and gene expression.

Protein 4.1 in forebrain postsynaptic density preparations: enrichment of 4.1 gene products and detection of 4.1R binding proteins

4.1 Proteins are a family of multifunctional cytoskeletal components (4.1R, 4.1G, 4.1N and 4.1B) derived from four related genes, each of which is expressed in the nervous system. Using subcellular fractionation, we have investigated the possibility that 4.1 proteins are components of forebrain postsynaptic densities, cellular compartments enriched in spectrin and actin, whose interaction is regulated by 4.1R. Antibodies to each of 4.1R, 4.1G, 4.1N and 4.1B recognize polypeptides in postsynaptic density preparations. Of these, an 80-kDa 4.1R polypeptide is enriched 11-fold in postsynaptic density preparations relative to brain homogenate. Polypeptides of 150 and 125 kDa represent 4.1B; of these, only the 125 kDa species is enriched (threefold). Antibodies to 4.1N recognize polypeptides of approximately 115, 100, 90 and 65 kDa, each enriched in postsynaptic density preparations relative to brain homogenate. Minor 225 and 200 kDa polypeptides are recognized selectively by specific anti-4.1G antibodies; the 200 kDa species is enriched 2.5-fold. These data indicate that specific isoforms of all four 4.1 proteins are components of postsynaptic densities. Blot overlay analyses indicate that, in addition to spectrin and actin, postsynaptic density polypeptides of 140, 115, 72 and 66 kDa are likely to be 4.1R-interactive. Of these, 72 kDa and 66 kDa polypeptides were identified as neurofilament L and alpha-internexin, respectively. A complex containing 80 kDa 4.1R, alpha-internexin and neurofilament L was immunoprecipitated with anti-4.1R antibodies from brain extract. We conclude that 4.1R interacts with the characteristic intermediate filament proteins of postsynaptic densities, and that the 4.1 proteins have the potential to mediate the interactions of diverse components of postsynaptic densities.

A model for central synaptic junctional complex formation based on the differential adhesive specificities of the cadherins

Cadherins control critical developmental events through well-documented homophilic interactions. In epithelia, they are hallmark constituents of junctions that mediate intercellular adhesion. Brain tissue expresses several cadherins, and we now show that two of these, neural (N)- and epithelial (E)-cadherin, are localized to synaptic complexes in mutually exclusive distributions. In cerebellum, N-cadherin is frequently found associated with synapses, some of which are perforated, and in hippocampus, N- and E-cadherin-containing synapses are found aligned along dendritic shafts within the stratum lucidum of CA3. We propose that the cadherins function as primary adhesive moieties between pre- and postsynaptic membranes in the synaptic complex. According to this model, once neurites have been guided to the vicinity of their cognate targets, it is the differential distribution of cadherins along the axonal and dendritic plasma membranes, and ultimately cadherin self-association, that "locks in" nascent synaptic connections.

Existence of a putative specific postsynaptic density protein produced during Purkinje cell spine maturation

This study identified a 140 kDa polypeptide as a putative specific component of Purkinje cell spines' postsynaptic densities and which began to appear during the critical period of cerebellar cortex synaptogenesis. Mouse cerebellar cortices at postnatal days 5, 7, 9, 11, 15 and young adult, between days 30 and 40, were used to purify subcellular fractions of synaptosomes, synaptic membranes and postsynaptic densities. The purity of the subcellular fractions was assessed by electron microscopy and the protein composition of the different fractions was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polypeptides of apparent molecular weights of 25, 26, 27, 30, 33, 37, 43, 45, 52, 64, 74, 85, 94, 110, 125, 130, 165 and 174 kDa were found in the synaptosomal fractions of all the ages studied, even before the critical period of synaptogenesis, at postnatal day 7, when the postsynaptic densities were still nonexistent, indicating that the polypeptides are nonspecific constituents of these structures. On the other hand, a 140 kDa polypeptide was detected in the postsynaptic density fractions at postnatal day 11, immediately after postsynaptic structures began to appear, suggesting the possibility that this protein is a specific component of the cerebellar cortex postsynaptic densities. The 140 kDa polypeptide was electroeluted from the gel and analysed for its amino acid composition by reverse-phase high-pressure liquid chromatography. The analysis showed that this protein has a high content of nonpolar amino acid residues, such as leucine, isoleucine, glycine, phenylalanine and valine. A hypothetical model relative to the participation of the 140 kDa protein in the molecular organization of the postsynaptic density is suggested which may contribute to the understanding of the role played by this structure in synaptic function.

Subcellular localization of the F5 protein to the neuronal membrane-associated cytoskeleton

F5 was identified originally as an interleukin-2-regulated gene in the murine helper T-lymphocyte clone L2. Subsequent studies demonstrated high levels of F5 mRNA and protein in mature neurons in adult mouse central and peripheral nervous systems. The F5 protein was present in dendrites and perikarya but not in axons. In the present studies, the intracellular localization of the F5 protein in adult mouse brain was determined by subcellular fractionation and Western blotting. Although the deduced F5 sequence predicts a soluble protein, virtually no F5 immunoreactivity was found in the cytosol. The F5 protein was restricted to the P2 crude mitochondrial and P3 crude microsomal particulate fractions. Within the P2 fraction, F5 protein was enriched in the P2B synaptosomal subfraction. The results of temperature-dependent phase separation with Triton X-114 and alkaline extraction with sodium carbonate of the P2 and P3 fractions were consistent with the F5 protein being an extrinsic membrane-associated protein. Although essentially all of the F5 protein in the P3 fraction was membrane-associated, a substantial proportion of P2-associated F5 protein and nearly all of the synaptosomal F5 protein was detergent-insoluble. Direct isolation and subfractionation of brain cytoskeleton confirmed colocalization of F5 immunoreactivity with the membrane-associated cytoskeleton and postsynaptic densities. These studies suggest that the F5 protein, which has a large number of potential phosphorylation sites, plays a role in membrane-cytoskeletal interactions and in dynamic aspects of synaptic structure or function.

Tyrosine phosphorylation of glycoproteins in the adult and developing rat brain

The tyrosine phosphorylation of glycoproteins in the adult and developing rat brain was investigated. Immunoblotting with anti-tyr(P) antibodies identified a glycoprotein with an apparent Mr of 180,000 (GP180) as the major tyrosine-phosphorylated protein in the concanavalin A (con A)-binding fraction prepared from forebrain homogenates. This glycoprotein had the same electrophoretic mobility as the postsynaptic density (PSD)-associated glycoprotein PSD-GP180. Tyrosine-phosphorylated GP180 was enriched 24-fold in isolated PSDs relative to homogenates. Digestion with endoglycosidase F/N-glycosidase F demonstrated that GP180 present in total homogenates and PSD-GP180 present in isolated PSDs contained similar amounts of N-linked oligosaccharide suggesting that they are the same glycoprotein. The tyrosine phosphorylation of GP180 in homogenates varied between brain regions with the highest levels occurring in cortical areas and the amygdala and low or undetectable amounts being present in hindbrain regions. Incubation of homogenates with adenosine triphosphate (ATP) resulted in the tyrosine phosphorylation of GP180 in all regions examined except the cerebellum and identified a second con A-binding glycoprotein, GP110, which was phosphorylated on tyrosine. GP180 was not phosphorylated on tyrosine following the incubation of cerebellar homogenate, synaptic membranes, or PSDs and ATP. Tyr(P)-GP180 was not detected prior to the onset of synaptogenesis, increased in parallel with the formation of synapses during the first 4 weeks of postnatal development of the frontal cortex and hippocampus, and then decreased 50-60% to adult levels. The results suggest that GP180 corresponds to the PSD glycoprotein PSD-GP180 and are consistent with a role for this glycoprotein in synaptic development and signal transduction at the synapse.

Multiple ubiquitin conjugates are present in rat brain synaptic membranes and postsynaptic densities

The pattern of ubiquitin-protein conjugates present in a range of adult rat forebrain subcellular fractions has been investigated by immunoblotting with a monoclonal antibody specific for ubiquitin and its conjugates. Each fraction contains a complex and characteristic pattern of ubiquitin conjugates. Many integral synaptic membrane proteins are ubiquitinated, including a subset of high M(r) (> 120 kD) concanavalin A-binding glycoproteins. Postsynaptic densities are also enriched in ubiquitin conjugates, the profile being distinct from that of synaptic membranes. These results suggest that many plasma membrane and synaptic proteins are ubiquitinated.

Expression of PAC 1, an epitope associated with two synapse-enriched glycoproteins and a neuronal cytoskeleton-associated polypeptide in developing forebrain neurons

The monoclonal antibody PAC 1 (postsynaptic density and cytoskeleton enriched) recognizes an epitope present on two postsynaptic density-enriched glycoproteins of 130,000 (postsynaptic density-enriched glycoprotein 130) and 117,000 mol. wt (postsynaptic density-enriched glycoprotein 117), and a cytoskeleton-enriched polypeptide of 155,000 mol. wt (cp155). The PAC 1 antibody has been used to study the development of the PAC 1 antigens in the developing rat forebrain in vivo and in tissue culture. cp155 is detected by embryonic day 14 and its level continues to rise until the sixth postnatal week. Postsynaptic density-enriched glycoproteins 130 and 117 are also expressed in embryonic brain although the level of postsynaptic density-enriched glycoprotein 130 initially increases more rapidly than that of postsynaptic density-enriched glycoprotein 117. Peak values are observed at postnatal days 4 (postsynaptic density-enriched glycoprotein 117) and 9 (postsynaptic density-enriched glycoprotein 130). The level of post synaptic density-enriched glycoprotein 117 subsequently decreases to some 50% of the peak value by postnatal day 42. Immunocytochemical studies show that PAC 1 immunoreactivity in developing cerebral cortex, detectable by postnatal day 0, is primarily associated with the perikarya and dendrites of pyramidal cells. The immunoreactivity develops as patches of PAC 1-positive neurons, uniform staining of the cortex only being fully established after postnatal day 9. Double-immunofluorescence labelling studies of forebrain cultures prepared from embryonic day 18 animals shows that many, but not all, growth-associated protein 43-positive neurons exhibit PAC 1 immunoreactivity. Some non-neuronal cells also stain with the PAC 1 monoclonal antibody. The growth cones of cultured neurons exhibit PAC 1 immunoreactivity and the PAC 1 antigens are detected on immunodeveloped western blots of isolated growth cones. The PAC 1 epitope is intracellular, but immunoreactivity does not co-localize with F-actin as detected by rhod-amine-phalloidin or with tubulin immunoreactivity. Postsynaptic density-enriched glycoprotein 130 is readily detected on PAC 1 immunodeveloped western blots of forebrain cultures maintained for up to 14 days in vitro. Postsynaptic density-enriched glycoprotein 117 is only poorly expressed by these cultures. The PAC 1 glycoproteins are present in forebrain synaptic membranes and postsynaptic densities at an early stage of development. The synaptic membrane level of postsynaptic density-enriched glycoprotein 130 and postsynaptic density-enriched glycoprotein 117 increases markedly between postnatal days 3 and 8. The level of both glycoproteins detected in postsynaptic densities remain virtually constant from postnatal days 9-90. These results are consistent with functional roles for these molecules in neuronal and synapse development.

Impaired long-term potentiation, spatial learning, and hippocampal development in fyn mutant mice

Mice with mutations in four nonreceptor tyrosine kinase genes, fyn, src, yes, and abl, were used to study the role of these kinases in long-term potentiation (LTP) and in the relation of LTP to spatial learning and memory. All four kinases were expressed in the hippocampus. Mutations in src, yes, and abl did not interfere with either the induction or the maintenance of LTP. However, in fyn mutants, LTP was blunted even though synaptic transmission and two short-term forms of synaptic plasticity, paired-pulse facilitation and post-tetanic potentiation, were normal. In parallel with the blunting of LTP, fyn mutants showed impaired spatial learning, consistent with a functional link between LTP and learning. Although fyn is expressed at mature synapses, its lack of expression during development resulted in an increased number of granule cells in the dentate gyrus and of pyramidal cells in the CA3 region. Thus, a common tyrosine kinase pathway may regulate the growth of neurons in the developing hippocampus and the strength of synaptic plasticity in the mature hippocampus.

The postsynaptic density: constituent and associated proteins characterized by electrophoresis, immunoblotting, and peptide sequencing

The proteins of the postsynaptic density (PSD) fraction of cerebral cortex were resolved by two-dimensional electrophoresis (2DE) and more than 30 proteins identified by characteristic 2DE mobility, immunoblotting with specific antibodies, and N-terminal and peptide sequencing. The PSD fraction is enriched for spectrin, actin, tublin and microtubule associated protein II, myosin, enzymes of glycolysis, creatine kinase, elongation factor 1 alpha, and receptor protein. The three neurofilament proteins are detected but a 58-kDa protein is prominent and is, by peptide sequencing, the bovine homolog of the recently cloned 66-kDa neurofilament protein; in contrast to the latter, however, it is enriched in cerebrum compared with spinal cord. A 68-kDa protein is identified as a member of the hsp70/BiP family of proteins. A protein, designated dynamin, indicating its putative role as a microtubule motor, is identified as a major protein, is found, however, greatly enriched in the particulate fraction, and is significantly denaturant and detergent insoluble. A protein designated N-ethylmaleimide-sensitive factor is also detected. Thus, two proteins implicated in vesicular transport are present in the PSD fraction. Seven polyclonal antibodies were produced to 2DE separated and electroeluted proteins of the PSD and were identified by peptide sequence analysis and 2DE profile as the hsp70/BiP homologous protein, the novel neurofilament protein synapsin IIa, pyruvate kinase, dynamin, aconitase and an unknown contaminating protein, and a 115-kDa protein that by subcellular fractionation and immunoblotting is a diagnostic PSD molecule. In addition, peptide sequences are obtained for four additional higher molecular weight proteins of the PSD that are not related at the level of primary structure to any known proteins.

Molecular characterisation and structural relationship of the synapse-enriched glycoproteins gp65 and gp55

gp65 and gp55 are glycoprotein components of CNS synapses that are recognised by a single monoclonal antibody, SMgp65. This antibody has now been used to investigate the molecular properties of these two glycoproteins and the structural relationship between them. Both gp65 and gp55 occur in most brain regions as doublets of apparent molecular masses of 63 and 67 kDa, and 52 and 57 kDa, respectively. Striatal samples, however, are enriched in a novel gp65 isoform of 69 kDa. Removal of oligosaccharide residues from gp65 and gp55 with trifluoromethanesulphonic acid shows that gp65 and gp55 are composed of single polypeptide chains of 40 and 28 kDa, respectively. Removal of sialic acid residues with neuraminidase lowers the apparent molecular mass of both glycoproteins by 5-6 kDa. Triton X-114 phase partitioning and alkaline extraction of synaptic membranes indicate that both gp65 and gp55 are integral membrane glycoproteins. Treatment of synaptic membranes with phosphatidylinositol-specific phospholipase C does not solubilise either glycoprotein. One-dimensional peptide and epitope maps obtained by digestion of gp65 and gp55 with endoproteinase lys C or subtilisin are consistent with a close structural relationship between the two molecules. Tryptic digestion of samples enriched in gp65 and/or gp55 results in the formation of a novel immunoreactive 53-kDa species that is resistant to further trypsin degradation except in the presence of 0.1% (wt/vol) sodium dodecyl sulphate. Trypsin treatment of cultures of forebrain neurones in situ lowers the apparent molecular mass of gp65 to 53 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterisation and regional localisation of pre- and postsynaptic glycoproteins of the chick forebrain showing changed fucose incorporation following passive avoidance training

To identify those glycoproteins whose synthesis or modification is necessary for memory formation, we have studied the uptake of radiolabelled fucose into synaptic plasma membranes (SPMs) and postsynaptic densities (PSDs) derived from two specific left and right forebrain loci, at two different times after training of 1-day-old chicks on a one-trial passive avoidance learning task. To increase the reliability of the comparison, a double-labelling method was used. Tissue samples from intermediate medial hyperstriatum ventrale (IMHV) and lobus parolfactorius (LPO) were isolated at 6 and 24 h after training. At both times, training resulted in region-specific changes, both increases and decreases, in incorporated radioactivity into pre- and postsynaptic glycoproteins. After 6 h, there was a relative decline in incorporation into both SPMs and PSDs of the right IMHV of trained chicks, a decline that persisted in the PSDs until 24 h. A small decline in incorporation in SPMs from the right LPO of trained chicks at 6 h was reversed by 24 h, by which time there was a 64% increase in incorporation into SPMs and a 24% increase into PSDs of the left LPO. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis of left and right hemisphere samples containing LPO revealed that 6 h after training the main effect was presynaptic, including a reduction of incorporation into high molecular mass glycoproteins, of 150-180 kDa, and an increase in a lower molecular mass (41 kDa) fraction. By 24 h after training, a left hemisphere presynaptic glycoprotein of molecular mass approximately 50 kDa showed the biggest increase in fucosylation. In addition, a wide group of postsynaptic glycoproteins of both hemispheres, in the ranges 150-180, 100-120, and 33 kDa now showed increases in incorporation. Some other fractions showed decreases. These results are in accord with previous data on incorporation obtained using the amnesic agent 2-deoxygalactose. They also support the hypothesis that memory formation involves the strengthening of connections between pre- and postsynaptic neurons of the LPO by growth or modulation of pre- and postsynaptic structures.

Stimulation of protein-tyrosine phosphorylation in rat striatum after lesion of dopamine neurons or chronic neuroleptic treatment

Even though the short-term actions of dopamine on postsynaptic receptors are well-characterized, the molecular bases for long-term trophic interactions between dopamine neurons and their targets remain unclear. Since protein-tyrosine phosphorylation plays a key role in the action of trophic factors, we have investigated its possible involvement in the interactions between dopamine neurons and their striatal targets. Lesioning rat nigrostriatal dopamine neurons by using 6-hydroxydopamine increased the phosphorylation on tyrosine of several proteins, including a major 180-kDa protein (pp180) in the ipsilateral striatum. Protein-tyrosine kinase activity was also increased in the striatum ipsilateral to the lesion, whereas no change in phosphotyrosine phosphatase activity was detected. The stimulation of pp180 phosphorylation was observed 1, 2, and 8 weeks after 6-hydroxydopamine lesion, was selective for the destruction of dopamine neurons, and was mimicked by chronic blockade of dopamine receptors with neuroleptics. Additional lesion experiments and subcellular fractionation showed that pp180 is located in neuronal postsynaptic densities, suggesting that pp180 is a postsynaptic component of corticostriatal synapses. Our results indicate that lesion of specific afferent fibers can activate tyrosine phosphorylation in central neurons and suggest that tyrosine phosphorylation is involved in the long-term consequences of dopamine deficiency and may play a role in synaptic plasticity.

Subfractions of membranes from calf brain synaptosomes obtained and studied by liquid-liquid partitioning

Synaptosomes isolated from calf brain cortex were lysed and fragmented by Yeda press treatment. The obtained membranes have previously been fractionated in a counter-current distribution process using a liquid-liquid two-phase system consisting of water, dextran, Ficoll and poly(ethylene glycol) [J. Chromatogr., 358 (1986) 147]. Using the fact that there are discrete membrane populations, a rapid preparative method for isolation of the two main fractions is presented in the present work, as well as a subfractionation of one of them using liquid-liquid extraction with dextran-bound Procion yellow HE-3G. The content of several membrane constituents, i.e. protein, acetylcholinesterase, succinate dehydrogenase and ATPase, as well as opiate binding, were determined for the three fractions. Counter-current distribution of the fractions elucidates their heterogeneity and the effectiveness of the purification.

Heterogeneity of synaptosomal membrane preparations from different regions of calf brain studied by partitioning and counter-current distribution

1. Membranes obtained by lysis and Yeda-press treatment of synaptosomes (nerve endings) from cortex, caudateus nucleus, and hippocampal region of calf brain have been studied by partitioning within a liquid-liquid aqueous two-phase system consisting of water, dextran, Ficoll, and poly(ethylene glycol). 2. The partitioning of membranes was sensitive to the presence of a dextran-bound dye, Procion yellow HE-3G, in the lower phase. 3. The two-phase system was used for counter-current distribution to study the heterogeneity of the synaptic membranes from the three regions of the brain and to separate the membranes into fractions. 4. The obtained counter-current distribution profiles strongly depended on the region of the brain from which the membranes were isolated. 5. The membrane fractions obtained showed marked differences in their SDS electrophoresis pattern.

PAC 1: an epitope associated with two novel glycoprotein components of isolated postsynaptic densities and a novel cytoskeleton-associated polypeptide

A monoclonal antibody has been raised which recognizes an epitope, PAC 1 (postsynaptic density and cytoskeleton enriched), which is specifically associated with two novel glycoprotein components of forebrain postsynaptic density preparations and a novel neuronal cytoskeletal-associated polypeptide. The monoclonal antibody has been used to study the cellular and subcellular localization of these molecules and for the partial characterization of all three PAC 1 antigens in the rat. The PAC 1 epitope is present on two concanavalin A binding glycoproteins of apparent molecular weights 130,000 (pgp130) and 117,000 (pgp117). Both species are enriched in preparations of rat forebrain postsynaptic densities and to a lesser extent in synaptic membranes. The epitope is also expressed by a polypeptide of 155,000 mol. wt, cp155. This molecule is highly enriched in cytoskeleton rather than membrane preparations. Enzymic removal of N-linked carbohydrate lowers the molecular weights of the PAC 1 glycoproteins pgp130 and pgp117 by 11,000 and 14,000 respectively, and suggests that cp155 is not glycosylated. Detergent, alkaline and salt extractions of postsynaptic densities and synaptic membranes indicate that pgp130 and pgp117 are integral membrane glycoproteins and are tightly bound components of postsynaptic density preparations. Immunocytochemical studies of adult rat forebrain show prominent staining of pyramidal cell dendrites and perikarya. There is no evidence of glial staining. Electron microscope studies show staining of microtubules together with punctate deposits of plasma membrane-associated reaction product. Several criteria have been used to show that pgp130 and pgp117 do not correspond to other known neuronal glycoproteins of similar molecular weight. We conclude that the PAC 1 epitope is expressed by two novel synaptic glycoproteins which are very probably integral components of the postsynaptic density and by a novel neuronal cytoskeleton-associated protein.

Phosphorylation of proteins of the postsynaptic density: effect of development on protein tyrosine kinase and phosphorylation of the postsynaptic density glycoprotein, PSD-GP180

The effect of development on the tyrosine kinase activity of postsynaptic densities (PSDs) has been determined. PSDs were prepared from the forebrains of rats ranging in postnatal age from 13 to 90 days and the phosphorylation of both exogenous and endogenous substrates by tyrosine kinase measured. PSDs exhibited tyrosine kinase activity at all ages examined. Phosphorylation of the exogenous substrates polyglutamyltyrosine (4:1) and [val5] angiotensin II increased twofold between days 17 and 22 and then decreased between days 30 and 90 to levels slightly lower than those present at 13 days. The phosphorylation of endogenous PSD proteins on tyrosine residues, assessed by alkali digestion of polyacrylamide gels of 32P-labelled PSD proteins and by measuring the formation of [32P] phosphotyrosine by PSDs incubated in the presence of [gamma-32P] ATP, closely paralleled the changes in total tyrosine kinase activity. Tyrosine phosphorylation of the PSD-specific glycoprotein, PSD-GP180, also showed a transient increase between days 22 and 30, although its concentration within the PSD continued to increase slowly up to 90 days. The results indicate that the tyrosine kinase activity of PSDs is developmentally regulated and that tyrosine phosphorylation of PSD proteins is limited by enzyme rather than substrate availability.

Synaptophysin expression during synaptogenesis in the rat cerebellar cortex

In order to study the mechanisms of synaptogenesis in the rat cerebellar cortex, a library of monoclonal antibodies has been generated against proteins of the isolated synapse. One recognizes a glycosylated 38 kDa protein that is concentrated in the synaptic vesicle fraction and resembles synaptophysin biochemically in its molecular weight, charge, and pattern of glycosylation. In the adult cerebellar cortex, the antisynaptophysin(mabQ155) immunoreactivity is codistributed with synapses. Immunoreactivity is strongest in the molecular layer where punctate deposits of reaction product outline the Purkinje cell dendrites. Discrete small profiles, consistent with the distribution of basket cell axon terminals, surround the Purkinje cells, and in the granular layer the synaptic glomeruli are intensely stained. There is no immunoreactivity in the white matter axon tracts. Electron microscope immunocytochemistry confirms the synaptic location of the antigen and suggests that the reaction product is associated with synaptic vesicles. Both round and flat vesicle populations are immunoreactive. Antisynaptophysin(mabQ155) has been used to follow synaptogenesis in the developing rat cerebellum. In the newborn rat (P0), despite the paucity of synapses, there is some specific immunoreactivity, especially in the subcortical white matter. Electron microscopy shows that the antigenicity is associated with vesicles within growth cones, filopodia, and immature axon profiles. During development, antisynaptophysin immunoreactivity increases progressively, along with the maturing cell populations, for both the granule cell-Purkinje cell and the mossy fiber-granule cell synapses. Quantitative biochemical analysis confirms the cytochemical results. These data suggest that neuronal growth cones express a synapse-specific antigen before complete morphological synapses are present.

Characterization of novel glycoprotein components of synaptic membranes and postsynaptic densities, gp65 and gp55, with a monoclonal antibody

A monoclonal antibody, mab SMgp65, which recognises two major glycoprotein components of isolated forebrain synaptic subfractions has been raised. The mab has been used to study the cellular and subcellular localisation of these novel glycoproteins and for the partial characterisation of both molecular species. Western blots show that the mab reacts with two diffuse glycoprotein bands (gp) of apparent Mr 65,000, gp65, and 55,000, gp55. Both glycoproteins are membrane-bound, only detectable in CNS tissue and exist solely in a concanavalin A (con A) binding form. Digestion with endoglycosidase H lowers the Mr of both glycoproteins by some 5-7 kDa. Gp65 and gp55 are enriched in synaptic membrane (SM), light membrane (LM) and microsomal fractions. However, whilst gp65 is enriched in isolated postsynaptic densities (psds) gp55 is conspicuously absent from this fraction. Regional distribution studies show a marked variation in the level of gp65. Gp65 is concentrated in several forebrain regions notably cerebral cortex, hippocampus and striatum, is present only in low levels in cerebellum and is barely detectable in pons and medulla. In contrast gp55 is present in all regions studied, but is most concentrated in cerebellum. Immunocytochemical studies show intense staining of regions rich in gp65, but no staining of regions deficient in this glycoprotein. This suggests that the mab recognises gp65, but not gp55 in fixed tissue sections. Exposure of tissue sections to Triton X-100 increases the intensity of gp65-like immunoreactivity, but does not alter its pattern of subcellular distribution. Higher resolution studies show the immunoreactivity to be localised to subsets of neurites, many being axonal. The reaction deposits also extend into the synaptic region of the immunoreactive neurones. Cultured cerebellar granule cells, but not astrocytes express gp55. The results are discussed in terms of the molecular properties and localisation of these two novel glycoproteins.

411B: a monoclonal postsynaptic marker for modulations of synaptic connectivity in the rat brain

Using 411B, a monoclonal marker raised to chick forebrain postsynaptic densities (PSDs), we have been able to demonstrate by enzyme-linked immunosorbent assay that the antigen recognised by this monoclonal exists in brain tissue from adult Wistar rats but not in liver, heart, or lung. Moreover, 411B immunoreactivity estimated in various cortical and subcortical brain structures exhibited remarkable differences. The pattern of subcellular distribution of 411B antiserum titre in rats was found to be qualitatively similar to that in day-old chicks, indicating an enrichment of the antigen concentration in the PSD fraction by about 60 times over that observed in the lysed homogenates. One aim of this study was to investigate whether 411B is a useful biochemical marker for plastic changes of postsynaptic structures in the rat brain. Antigen was assayed in lysed homogenates from various brain regions dissected from dopaminergically supersensitive rats. Dopaminergic supersensitivity induced by treating animals with haloperidol (1 mg/kg i.p.) for 21 consecutive days resulted in a significant increase in the titre of 411B in corpus striatum (+21%) and hippocampus (+45%) whereas the titre of Q155, a monoclonal marker for an integral synaptic vesicle protein, was unchanged. Our results support the hypothesis that drug-induced dopaminergic supersensitivity is based on plastic changes at the postsynaptic site. In addition, monoclonal antibody 411B does appear to be a useful tool for further investigation of plastic changes occurring in postsynaptic brain components.

Synaptic protein tyrosine kinase: partial characterization and identification of endogenous substrates

The subcellular distribution of protein tyrosine kinase in rat forebrain was determined using [Val5]-angiotensin II as exogenous substrate. Enzyme activity was present in each of the fractions analyzed and was enriched in synaptic membranes (SMs) and the synaptosomal soluble fraction (2.2- and 2.5-fold over the homogenate, respectively). SMs also phosphorylated polyglutamyltyrosine (pGT; molar ratio of 4:1), the Vmax for angiotensin and pGT phosphorylation being 26.3 +/- 1.6 and 142 +/- 4 pmol/min/mg, respectively. Extraction of SMs with several different detergents resulted in enhanced enzyme activity and the solubilization of 33-37% of the angiotensin and 43-70% of the pGT-phosphorylating activity. Isolated postsynaptic densities (PSDs) contained tyrosine kinase and phosphorylated angiotensin and pGT. The Vmax values for angiotensin and pGT phosphorylation by PSDs were 17 +/- 5 and 23 +/- 1 pmol/min/mg, respectively. Six putative endogenous substrates for SM tyrosine kinase, with molecular weights of 205K, 180K, 76K, 60K, 50K, and 45K, were identified. Each of these proteins, except p76, was phosphorylated in the detergent-insoluble residue obtained following the extraction of SMs with Triton X-100 as well as in PSDs, indicating that the postsynaptic apparatus is an active site of tyrosine phosphorylation. The phosphorylation of p76 was localized to the Triton X-100 extract and also occurred in the synaptosomal soluble fraction. The results indicate that tyrosine kinase and its substrates are located in both pre- and postsynaptic compartments and suggest a role for this enzyme in synaptic function.

Preparation and characterisation of a monoclonal antibody to an antigen enriched in chick brain postsynaptic densities

The preparation and characterisation of a monoclonal antibody to an antigen enriched in day-old chick brain postsynaptic densities (PSDs), with respect to other subcellular loci, are described. Immunolabelling with this antibody produced a dendritic immunoprecipitate that was markedly stronger in PSDs than in other subcellular loci. Thus, the antiserum could be used as a marker for PSDs during their purification by subcellular fractionation, as well as in the study of PSD assembly. Monoclonal antibody 411B has already been shown to be a useful tool in the chemical determination of changes in synapse density after various experimental manipulations in both the chick and rat. In the present study, we have used the antiserum to monitor the appearance and maintenance or redundancy of synaptic components in the developing chick forebrain.

Phosphorylation of synaptic proteins in chick forebrain: changes with development and passive avoidance training

We have used synaptic plasma membranes (SPMs) and postsynaptic densities (PSDs) to study protein phosphorylation at the synapse in the developing chick forebrain and in 1-day-old chick forebrain following training on a passive avoidance task. Endogenous phosphorylation patterns in SPMs and PSDs prepared by extraction with n-octylglucoside isolated from chick forebrain were investigated by labelling with [32P]ATP. The phosphoprotein components of the SPM and PSD fractions were separated using sodium dodecyl sulphate gradient polyacrylamide gel electrophoresis. Autoradiography and densitometry of the Coomassie Blue protein staining pattern revealed phosphate incorporation into several SPM components including those of molecular mass 52, 37, and 29 kilodaltons (kDa). Bands of similar molecular mass were not phosphorylated in PSD fractions. This difference in phosphorylation between SPMs and PSDs was not due to the detergent n-octylglucoside. In a developmental study in which SPM and PSD fractions were prepared from 1-day-old, 14-day-old, and 21-day-old chickens, the phosphorylation patterns of SPMs were similar throughout, but striking differences occurred in PSDs, both in the level of phosphorylation and in the components phosphorylated. A time-course study was carried out in which phosphorylation of SPMs and PSDs from 1-day-old chicks trained on a passive avoidance task was compared with patterns from control chicks trained on a water-coated bead and untrained chicks. In SPMs prepared from forebrains removed 10 mins following training, a consistent but nonsignificant decrease (-21%) in phosphorylation of a 52 kDa band occurred in chicks with passive avoidance training compared with water-trained and untrained chicks.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic vesicle proteins and acetylcholine levels in chick forebrain nuclei are altered by passive avoidance training

In a search for biochemical markers of modified synaptic function following training of day-old chicks on a passive avoidance task, we have assayed two monoclonal antibodies to synaptic vesicle proteins (anti-p65 and anti-SV2) and one raised to postsynaptic densities (411B). We have also measured total acetylcholine (ACh) content. Measurements were made on three forebrain regions known to show metabolic and morphological change consequent on training--the lobus parolfactorius (LPO), paleostriatum augmentatum (PA), and medial hyperstriatum ventrale (MHV)--in the right and left hemispheres 2 and 24 h after training chicks on a passive avoidance task, in which they learn to avoid pecking a bead coated with methylanthranilate [methylanthranilate-trained (M-trained)]. Control chicks were trained on a water-coated bead [water-trained (W-trained)]. Twenty-four hours after training, 411B levels showed no differences between W-trained and M-trained chicks in any region. M-training reduced the titre of anti-p65 by 16% in the left PA and 15% in the left MHV and that of anti-SV2 by 19% in the left PA. M-trained chicks showed reduced total ACh content in the LPO by up to 40% and in the PA by up to 48% but had no change in ACh level in the MHV. The decreases in antibody titre were not seen in forebrains analysed 2 h after training, but tendencies toward increases in levels in the right PA and MHV were observed with all three antibodies. Significant differences between right and left hemispheric regions, independent of training, were observed for all the antibodies and for ACh content.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of gp 50, a major glycoprotein present in rat brain synaptic membranes, with a monoclonal antibody

Several cell lines secreting monoclonal antibodies (Mabs) against a major forebrain synaptic membrane (SM) glycoprotein, gp 50, have been raised. Western blots show that the Mabs react with a polypeptide doublet of Mrs 49 and 45 kDa. These polypeptides exist solely in a concanavalin A (Con A) binding form. Removal of the Con A receptors by digestion with endo-beta-N-acetylglucosaminidase H (endo H) lowers the Mrs of the glycoprotein doublet to 36.5 and 34 kDa. Western blots of 2D polyacrylamide gels indicate that gp 50 exists in several isoforms. Solid phase radioimmunoassay (RIA) and Western blots of brain subcellular fractions show the antigenic material to be concentrated in the SM fraction, but to be present in much lower amounts in synaptic junctions and postsynaptic densities. Gp 50 appears to be brain specific. Regional distribution studies show that it is present in all brain regions but is two-fold concentrated in cerebellum, brainstem and midbrain compared to forebrain. Immunocytochemical studies of several brain regions show that gp 50-like immunoreactivity is neuron specific and is concentrated in selected neuronal species, particularly granule cells. In both cerebellar and hippocampal granule cells gp 50-like immunoreactivity is localized in the perikarya and primary dendrites. Though immunocytochemistry did not show staining of synaptic regions this may be due to masking of the reactive epitope. The results are discussed in terms of the molecular properties of gp 50 and its subcellular localization in brain tissue.

Isolation of postsynaptic densities from day-old chicken brain

Synaptic plasma membranes from chicken brain were used to isolate a postsynaptic density (PSD) fraction using an aqueous two-phase polymer system and the detergent n-octyl glucoside. The protein and glycoprotein composition and the morphology of the day-old chicken brain PSD fraction were compared with a PSD fraction isolated from 12-week-old chicken brain. The PSD fraction from day-old chicken brain contained predominantly PSDs although, like the fraction from 12-week-old chicken, there was some membrane contamination. The major polypeptides in the day-old chicken fraction resolved by polyacrylamide gel electrophoresis comigrated with alpha- and beta-tubulin (Mr 57,000 and 55,000) and actin (Mr 45,000). The major PSD polypeptide (mPSDp) of 12-week-old chicken forebrain, which has a molecular weight of 52,000 was not a major component in day-old chicken. A polypeptide of molecular weight 63,000 was also far more prominent in the 12-week-old chicken PSD fraction whereas the reverse was true for a polypeptide of 31,000. Day-old chicken brain PSDs contained at least 14 concanavalin A-binding glycoproteins of high (greater than 85,000) molecular weight, the two most prominent having molecular weights of 170,000 and 180,000. In contrast to the polypeptide composition, the glycoprotein pattern of day-old chicken PSDs was very similar to that of the 12-week-old bird. Intraperitoneally injected [3H]fucose was incorporated into the glycoproteins of synaptic plasma membranes and PSDs from day-old chickens.(ABSTRACT TRUNCATED AT 250 WORDS)

Existence of a Ca2+-dependent K+ channel in synaptic membrane and postsynaptic density fractions isolated from canine cerebral cortex and cerebellum, as determined by apamin binding

Apamin, a 18-amino acid neurotoxin isolated from bee venom, is a specific blocker of one class of the Ca2+-dependent K+ channels. The monoiodo derivative of the toxin with high specific radioactivity (1600 Ci/mmol) has been used to study its binding to synaptic membrane (SM) and postsynaptic density (PSD) fractions isolated from cerebral cortex (CTX) and cerebellum (CL) of canine brains. The Bmax (30.2 fmol/mg protein) for CTX-PSD is about twice that for CTX-SM (17.3 fmol/mg protein), suggesting a concentration of the apamin receptor protein in CTX-PSD over CTX-SM fractions. The lower value of Bmax for CL-PSD (12.3 fmol/mg protein), and the higher Kd value (51 pM) than for CTX-SM (33 pM), CTX-PSD (24 pM), and CL-SM (39 pM), may reflect the disruptive effect of Triton X-100 on these thin structures. The values of Bmax and Kd for CTX-SM are similar to those (22.0 fmol/mg protein and 33 pM) for rat CTX-SM. Both Ca2+ and Na+ inhibit apamin binding to CTX-PSD with K0.5 values of 14 and 31 mM, respectively, while the optimum concentration of KCl for activation is 5 mM. All these values are similar to those found for rat synaptosomes. Covalent labeling of the apamin binding protein, using the non-cleavable cross-linker, disuccinimidyl suberate, reveals an apamin binding polypeptide of 27 kdaltons under reducing and denaturing conditions in both the CTX-SM and CTX-PSD preparations, similar to that (28 kdaltons) reported for rat CTX-SM fractions. Prior phosphorylation of isolated CTX-PSD had no effect on apamin binding, nor did apamin binding influence subsequent phosphorylation of CTX-PSD. Calmodulin, an intrinsic PSD protein, may not play a role in apamin binding to PSD, since addition of calmodulin, or removal of the calmodulin by EGTA treatment, resulted in no change in the binding capacity of the PSD. The apamin binding protein seems to be bound quite firmly in the CTX-PSD fraction since treatments with 0.5% deoxycholate, 1% N-lauroyl sarcosinate, 4 M guanidine-HCl, pH 7.0, 0.5 M KCl and 1.0 M KCl, could only remove the apamin-receptor complexes from CTX-PSD by 40, 55, 52, 12 and 15%, respectively. These results contrast with the findings that the two detergents mentioned solubilize 80-93% of the receptor from synaptosomal or synaptic membrane fractions, indicating that a good deal of the receptor in these fractions is membrane-bound and not connected to the PSD.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphorylation of the postsynaptic density glycoprotein gp180 by Ca2+/calmodulin-dependent protein kinase

Postsynaptic densities (PSDs) were prepared by the aqueous two-phase extraction of synaptic membranes in the presence of n-octyl glucoside. Incubation of postsynaptic densities with [gamma-32P]ATP resulted in the incorporation of 32P into a range of proteins. Isolation of glycoproteins from 32P-labelled PSDs by affinity chromatography on concanavalin A-agarose identified the postsynaptic glycoprotein of apparent Mr 180,000 (gp180) as a substrate for endogenous protein kinase(s). When the phosphorylation reaction was performed in the presence of Ca2+ and calmodulin, there was an overall 13-fold increase in the phosphorylation of PSD proteins. The largest effects of calmodulin were associated with two proteins of molecular weights 51,000 and 60,000, which showed average calmodulin-dependent increases in phosphorylation of 68-fold. The phosphorylation of gp180 was increased 7.5-fold in the presence of calmodulin. Fifty percent of maximum phosphorylation of proteins and glycoproteins occurred with a free Ca2+ concentration of 0.3 X 10(-6) M. The amounts 12.6 micrograms/ml and 9.1 micrograms/ml of calmodulin were required for 50% of maximum phosphorylation of proteins and glycoproteins, respectively. Peptide mapping experiments identified three major phosphorylation sites in gp180. The phosphorylation of all three sites was increased in the presence of calmodulin. Phosphoamino acid analysis of gp180 revealed that [32P]phosphoserine and [32P]phosphothreonine were both produced during the phosphorylation reaction, with phosphoserine being the predominant product. The phosphorylation of both amino acids was increased in the presence of calmodulin. [32P]phosphotyrosine was also identified as a product of the phosphorylation of gp180.

In vivo phosphorylation of the postsynaptic density glycoprotein gp180

Rats received intraventricular injections of [32P]PO4 and were killed after 30 min for the preparation of postsynaptic densities (PSDs). Gel electrophoretic analysis identified a number of PSD proteins that incorporated 32P under these conditions. Major proteins that were labelled with 32P had Mr of 185,000, 165,000, 140,000, 92,000, and 51,000. Of these p185, p165, and p140 were also labelled when PSDs were incubated with [gamma-32P]ATP in vitro. In contrast p92 and p51 were relatively poorly labelled under in vitro conditions. Analysis of glycoproteins isolated by chromatography on concanavalin A (Con A)-agarose demonstrated that greater than 70-80% of the 32P present in the glycoproteins eluted from Con A-agarose with alpha-methyl-D-mannopyranoside (Con A+ glycoproteins) was associated with the PSD specific glycoprotein gp180 following both in vivo and in vitro labelling. Phosphopeptide maps and phosphoamino acid analysis of gp180 indicated that similar sites were labelled in vitro and in vivo. Analysis of the subcellular distribution of glycoproteins that incorporated 32P during in vivo labelling demonstrated that gp180 was highly concentrated in PSDs, in accord with the previously suggested exclusive association of this glycoprotein with postsynaptic structures.

Affinity partitioning and centrifugal counter-current distribution of membrane-bound opiate receptors using naloxone-poly(ethylene glycol)

Crude synaptic membranes isolated from calf brain cortex were subjected to an aqueous two-phase system and the partition of the various membrane constituents and activities between the phases were studied. These constituents were phosphate, cholesterol and protein. The activities measured were acetyl-cholinesterase, succinate dehydrogenase, 2',3'-cyclicnucleotide-3'-phosphohydrolase and stereospecific opiate-binding. The successful fractionation of the membranes was achieved by the use of an aqueous two-phase system in a counter-current distribution process. A ligand bound to poly(ethylene glycol) with an affinity for opiate receptors was synthesized by reacting 6-aminonaloxone with tresylpoly(ethylene glycol). The ligand-polymer was used to extract membrane-bound opiate receptors into the upper, poly(ethylene glycol)-rich phase. This use of affinity partitioning resulted in membrane fractions with a 3-4 fold higher ability to bind stereospecifically etorphine than the original preparations of synaptic membranes.

Ethanol tolerance and enhanced calcium/calmodulin-dependent phosphorylation of synaptic membrane proteins

The chronic effects of ethanol on synaptic membrane proteins was studied in ethanol-tolerant rats. Synaptic plasma membranes and postsynaptic densities were prepared from homogenates of forebrain and incubated in vitro with [gamma-32P]adenosine triphosphate in the presence and absence of calcium and calmodulin. In ethanol-tolerant animals, enhanced phosphorylation of synaptic plasma membrane but not postsynaptic density proteins was demonstrated in the presence of calcium and calmodulin. These results suggest that the development of tolerance to ethanol may involve alteration in neuronal sensitivity to calcium.

Phosphorylation of the postsynaptic density glycoprotein gp180 by endogenous tyrosine kinase

Incubation of postsynaptic densities (PSDs) with [gamma-32P]adenosine triphosphate (ATP) results in the phosphorylation of a number of proteins. Of these, phosphoproteins with apparent molecular weights (Mr) of 51,000, 180,000, 300,000, 320,000 and 370,000 contain 32P which is resistant to digestion with hot KOH suggesting the presence of [32P]phosphotyrosine residues. Phosphoamino acid analysis of total 32P-labelled PSDs identified [32P]phosphotyrosine as well as phosphoserine and phosphothreonine as products of the phosphorylation reaction. The PSD-specific glycoprotein gp180 was isolated from 32P-labelled PSDs and shown to contain [32P]phosphotyrosine. The results identify tyrosine kinase as a component of purified PSDs and gp180 as an endogenous substrate for this enzyme.

Concanavalin A binding glycoproteins in subcellular fractions from the developing rat cerebral cortex

Synaptic plasma membrane (SPM) and mitochondrial fractions were prepared from 3-50-day rat cerebral cortex and their purity assessed. The fractions were subjected to electrophoresis on slab gels, stained for protein, and overlaid with 125I-concanavalin A (ConA). ConA binding glycoproteins (CABGs) were revealed by autoradiography. In the SPM fraction CABGs of MW 25,000, 63,000, 80,000, 115,000, 174,000, and 239,000 increased while those of MW 47,000, 75,000, and 190,000 decreased developmentally. In the mitochondrial fraction, CABGs of MW 25,000, 44,000, 115,000 and 174,000 increased while those of 34,000, 43,000, 47,000, 51,000, 80,000, 107,000, and 195,000 decreased developmentally. CABGs of MW 32,000, 63,000, 88,000, 153,000, 190,000, and 239,000 appear to be unique to the SPM fraction and those of MW 34,000, 107,000, and 195,000 are unique to the mitochondrial fraction.

The effects of detergents on the composition of postsynaptic densities

A method of purifying postsynaptic densities (PSD) of Cohen et al. (1977) has been modified, primarily by the substitution of octyl glucoside as the detergent used to solubilize synaptosomal fractions. Subsequent extraction with other detergents resulted in the selective removal of specific polypeptides. In particular sulphobetaine 3-14 removed most of the beta-tubulin but not alpha-tubulin. Sodium N-lauroyl sarcosinate completely destroyed the structural integrity of the PSD when the in vitro formation of intermolecular disulphide bonds was minimized. These results suggest that the structure of PSDs is more labile than previously thought and demonstrate a technique for further examining their composition.

Developmental changes in the oligosaccharide composition of synaptic junctional glycoproteins

Synaptic junctions (SJs) were isolated from the forebrains of rats ranging in postnatal age from 10 days to greater than 1 year. SJ glycoproteins that react with Concanavalin A (Con A) were isolated by chromatography on Con A-agarose and separated by gel electrophoresis. The concentrations of the major SJ Con A binding (Con A+) glycoproteins (apparent Mr 180,000, 130,000, and 110,000) increased between 10 and 28 days, with GP180 and GP110 showing greater relative increases than GP130. Con A binding oligosaccharides associated with 10-day SJs were sensitive to digestion with endoglycosidase C11 and alpha-mannosidase, indicating that they were of the high-mannose type, as previously shown for 28-day SJs. Con A+ oligosaccharides from rats of increasing postnatal age were analyzed by chromatography on Biogel P-4. Two major oligosaccharides, containing five and eight mannose residues, were present in SJs of all ages examined. During development the ratio of man5 to man8 oligosaccharides increased, so that man5 constituted the predominant species in 28-day and adult SJs. Peptide mapping experiments showed that GP180, GP130, and GP110 were each associated with a unique polypeptide composition. Little or no change in peptide composition of the major SJ glycoproteins occurred during development.

Major differences in the concanavalin A binding glycoproteins of postsynaptic densities from rat forebrain and cerebellum

A highly purified fraction of postsynaptic densities (PSDs) was isolated from rat forebrain and cerebellum. The glycoproteins in the PSDs from the two brain regions were compared by probing nitrocellulose blots with concanavalin A. This approach revealed the presence of 8, previously undetected PSD glycoproteins and a 230K M, glycoprotein unique to forebrain PSDs and 260K Mr glycoprotein unique to cerebellar PSDs. There were also variations in the relative abundances of glycoproteins common to both brain regions. These observations may indicate that individual PSDs differ in their complement of major glycoproteins.

Identification and localization of concanavalin A binding sites on isolated postsynaptic densities

Synaptic fractions of decreasing morphological complexity were prepared by phase partitioning of synaptic membranes in an aqueous two-phase polymer system containing increasing concentrations of the neutral detergent n-octylglucoside (OG). The morphology, distribution of concanavalin A binding sites (CABS) and protein and glycoprotein composition of the resultant fractions were examined. The lowest concentration of OG employed (0.5% w/w) gave fractions enriched in relatively intact junctions retaining both pre- and postsynaptic structures. Increasing the detergent concentration resulted in the stepwise solubilization of pre- and postsynaptic structures until purified postsynaptic densities (PSDs) were obtained with 1% (w/w) OG. CABS were generally distributed on all membrane structures present in the 0.5% OG fraction, were restricted to synaptic structures in the fraction obtained with 0.75% OG, and were localized to the convex (outer) surface of purified PSDs. Gel electrophoretic analysis showed that the restriction of CABS to the region of the synapse was associated with a marked increase in the concentration of glycoproteins with apparent molecular weights of 180,000 and 130,000. These glycoproteins were retained, and further concentrated in the purified PSD fraction.

Synaptic junctional glycoproteins are phosphorylated by cyclic-AMP-dependent protein kinase

Synaptic junctions (SJs) isolated from rat brain are associated with protein kinase activity and a unique complement of high molecular weight gglycoproteins. Incubation of SJs with [gamma-32P]A+ glycoproteins which were retained by concanavalin A agarose (con A+ glycoproteins). Three major (apparent mol. wt. 180 K, 130 K and 110 K) and 2 minor (apparent mol. wt. 230 K and 145 K) glycoproteins were identified in the con A+ fraction. Of these, GP180 incorporated the most 32P and GP145 was not labeled. Peptide mapping experiments showed that each molecular weight class of glycoprotein was associated with a unique set of phosphorylated peptides. Cyclic AMP stimulated the incorporation of 32P into total SJ proteins and con A+ lycoproteins by 38% and 58%, respectively. GP130 showed the greatest increase in labelling in the presence of cyclic AMP (198% of control levels) although incorporation into all 4 glycoproteins was increased. Cyclic AMP selectively stimulated the incorporation of 32P into only 2 of the 6 phosphorylated peptides derived from GP130. These studies demonstrate that endogenous glycoproteins serve as substrates for intrinsic SJ protein kinases and identify this reaction as a potential means of modifying postsynaptic membrane function.

Identification of intrinsic sialidase and sialoglycoprotein substrates in rat brain synaptic junctions

Sialidase activity associated with rat brain synaptic junctions (SJ) and synaptic membranes (SM) was determined. Both fractions released sialic acid from exogenous glycopeptides and gangliosides. SJ accounted for 5-10% of the total sialidase activity recovered from SM following extraction with Triton X-100, and the specific activity of SJ sialidase was 60% of that of the parent SM fraction. Intrinsic SJ sialidase hydrolysed 12-15% of the sialic acid associated with endogenous SJ glycoproteins. Sialic acid residues associated with SJ glycoproteins were labelled with sodium borotritide and SJ proteins fractionated by affinity chromatography on concanavalin A-agarose. SJ glycoproteins that reacted with concanavalin A (con A+ glycoproteins) accounted for 25% of the total SJ [3H]sialic acid. Intrinsic SJ sialidase hydrolysed 20% of the [3H]sialic acid associated with these glycoproteins. Each molecular weight class of con A+ glycoprotein previously shown to be a specific component of the postsynaptic apparatus contained sialic acid and was acted on by intrinsic SJ sialidase.