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

Proteomics of multiprotein complexes: answering fundamental questions in neuroscience

Proteomics tools offer new ways to analyse networks of proteins that control important neurobiological phenomena such as learning and memory. In this review, we discuss how a combined proteomic, pharmacological and genetic approach reveals that multiprotein complexes process neural information and encode memories. Simultaneous analysis of multiple proteins enables the development of new concepts and approaches for neuroscience research.

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.

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.

Calmodulin-dependent protein kinase II. Multifunctional roles in neuronal differentiation and synaptic plasticity

One of the most important mechanisms for regulating neuronal functions is through second messenger cascades that control protein kinases and the subsequent phosphorylation of substrate proteins. Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) is the most abundant protein kinase in mammalian brain tissues, and the alpha-subunit of this kinase is the major protein and enzymatic molecule of synaptic junctions in many brain regions. CaM-kinase II regulates itself through a complex autophosphorylation mechanism whereby it becomes calcium-independent following its initial activation. This property has implicated CaM-kinase II as a potential molecular switch at central nervous system (CNS) synapses. Recent studies have suggested that CaM-kinase II is involved in many diverse phenomena such as epilepsy, sensory deprivation, ischemia, synapse formation, synaptic transmission, long-term potentiation, learning, and memory. During brain development, the expression of CaM-kinase II at both protein and mRNA levels coincides with the active periods of synapse formation and, therefore, factors regulating the genes encoding kinase subunits may play a role in the cell-to-cell recognition events that underlie neuronal differentiation and the establishment of mature synaptic functions. Recent findings have demonstrated that the mRNA encoding the alpha-subunit of CaM-kinase II is localized in neuronal dendrites. Current speculation suggests that the localized translation of dendritic mRNAs encoding specific synaptic proteins may be responsible for producing synapse-specific changes associated with the processing, storage, and retrieval of information in neural networks.

Molecular characterization of GP50: a neuron-specific, synaptic-enriched glycoprotein

The molecular properties of the neuron-specific, synaptic-enriched glycoprotein GP50 have been investigated with the aid of the monoclonal antibody MabSM-GP50. GP50 immunoreactivity was detected in the brains of the frog, trout, pigeon, snake, rabbit, mouse, cow, and human, although variation in quantity and electrophoretic mobility of the immunoreactive protein between species was apparent. Deglycosylation of synaptic membranes (SMs) with endoglycosidase H, peptide:N-glycosidase F, trifluoromethane-sulfonic acid, and alkaline sodium borohydride indicated that GP50 is associated primarily, if not exclusively, with high-mannose and/or hybrid-type oligosaccharides and lacks complex N-linked and O-linked sugar chains. GP50 remained associated with the membrane fraction following extraction of SMs with alkaline sodium carbonate, was partially (55%) present in the detergent phase following the phase partitioning of SMs in the presence of Triton X-114, and was resistant to proteolytic digestion with trypsin when present as a component of intact membranes. Taken together, these results indicate that GP50 is an integral component of the SM. Sucrose gradient centrifugation of Triton X-100 extracts of SMs or of forebrain and cerebellar homogenates resolved GP50 into two fractions with sedimentation coefficients of 3.6S and 7.3S, which accounted for 45 and 55% of the total, respectively. The 7.3S form occurred exclusively in the aqueous phase following partitioning with Triton X-114, whereas the 3.6S species was found in both the aqueous and detergent phases.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Detection and characterization of some new basic proteins in chicken postsynaptic densities

Chicken brain postsynaptic density (PSD) polypeptides, obtained by treating synaptosomes with 0.5% Triton X-100 and then further purified on a sucrose gradient, are demonstrated to contain four basic proteins of 76K (pI greater than 9.2), 58K (pI 8.1-8.8, heterogeneous), 40K (pI 9.0), and 24K (pI 8.9). Nonequilibrium pH gradient-sodium dodecyl sulfate two-dimensional gels further reveal six more basic proteins with pI values higher than 9.2: 76K, 52K, 47K, 45K, 36K, and 34K. These basic proteins are a major part of the total chicken PSD polypeptides appearing on the gels. Some of these basic proteins (58K, 52K, 47K, 36K, 24K, and two at 76K) are distinguishable from those of brain mitochondria, the major contaminant. The 40K and 34K proteins may be common mitochondrial polypeptides. The 45K protein is probably a mitochondrial contaminant. A number of proteins including 76K (synapsin I-like protein) and 58K, along with some other minor ones, can be phosphorylated by endogenous protein kinase(s) in the presence of Ca2+, Mg2+, and [gamma-32P]ATP. No PSD basic proteins bind Ca2+.

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)

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.

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.

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.