Presynaptic localization of phosphoprotein B-50

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.

Calcium-Stimulated Protein Phosphorylation in Synaptic Membranes

Synaptic membranes from rat brain contain several calcium-requiring protein kinase (PK) activities with different substrate specificities: (a) an activity (CaH-PK) effective at high concentrations of Ca2+ ion in the absence of Mg2+ (active on class F substrates); (b) a (Ca + Mg)-PK activity that is mediated by Ca2+ ion in the presence of Mg2+ (active on class B substrates); (c) (Ca-CaM)-PK activities that exhibit simultaneous requirements for both Ca2+ ion and CaM (for class C and D substrates). Also described are three activities (d-f) that do not require Ca2+ ion: (d) a Mg-PK activity in which the presence of Ca2+ causes the inhibition of phosphorylation (active on class A substrates); (e) an activity affecting a diverse group of substrates (class E substrates), the phosphorylation of which occurs in the presence of Mg2+ ion alone (Mg-PK activity) and is unaffected by the addition of Ca2+ ion and CaM, the substrates of which show different responses to several types of inhibitors; and, finally, (f) the previously well characterized cAMP-dependent PK activities. Several of the substrates of these kinases have been identified in a fairly unambiguous manner: among them are P43 (class A), as the alpha subunit of pyruvate dehydrogenase; P54 (class B), as the presynaptic protein B50; and the doublet P75-P80, as proteins IA and IB of Ueda and Greengard. The most interesting activity is that requiring both Ca2+ and CaM. The half-maximal stimulation (K0.5) for Ca2+ in the presence of CaM was found to be 1.0 microM Ca2+F in untreated membranes. There is little change in this value on prior EGTA extraction of the membranes, which removes the bulk of its Ca2+ and reduces its residual CaM by greater than or equal to 50%. The apparent K0.5 for CaM in the presence of excess Ca2+ ion was found to equal 0.4 microgram per reaction mixture (8 micrograms/ml) or 1.35 micrograms per reaction mixture (27 micrograms/ml), for the untreated and EGTA-treated membranes, respectively.

Regulation of phosphatidylinositide transduction system in the rat spinal cord during aging

Age-related functional alterations in a variety of neurotransmitter systems result in modulation of interneuronal communications which has some relevance in neurological deficits observed in the aging process. The synergistic interactions between protein kinase and inositol 1,4,5-trisphosphate (insP3)/Ca2+ pathways underlie a variety of cellular responses to external stimuli. To determine whether age-dependent changes occur in the regulation of protein kinase C and inositol 1,4,5-trisphosphate/Ca2+ pathways, insP3 contents as a marker for the release of intracellular calcium, saturation binding analysis of Ins P3 receptor using [3H]inositol 1,4,5-trisphosphate, slot/northern blot analysis of Ins P3 receptor-encoding mRNA transcripts, and the activities of Ca2+/phospholipid-dependent protein kinase C isozymes were investigated in the rat spinal cord. Inositol 1,4,5-trisphosphate content and [3H]inositol 1,4,5-trisphosphate binding site density (Bmax) were quantified in the spinal cords of young (three months old), adult (12 months old) and senescent (25 months old) male Fischer 344 rats. Spinal cord content of inositol 1,4,5-trisphosphate was increased (P < 0.01) in the 25-month old compared to the three- and 12-month old animals. The density of Ins P3 receptor in particulate membranes derived from the 25-month old rats was reduced (P < or = 0.01), but the binding affinity (Kd) was increased (P < or = 0.04) by a factor of 2.2 and 3.2 at 25 months of age when compared with three- and 12-month old animals, respectively. Young and middle-aged animals showed no differences in both inositol 1,4,5-trisphosphate contents and [3H]inositol 1,4,5-trisphosphate binding site density. The quantity of Ins P3 receptor mRNA was significantly increased with age in the order 25 >> 12 > 3 months of age. Total functional cytosolic and membrane-associated PKC activities were decreased (P < or = 0.05) in the 25-month compared to the three- and 12-month old rats in which activity remained unchanged. Total membrane/cytosolic activity ratios were unchanged by the aging process. In all cases, the activities of membrane-associated conventional protein kinase C isozymes (alpha, beta and gamma), determined by immunoprecipitation followed by in situ quantification of protein kinase C activities in the immunoprecipitates, showed age-dependent decline. The activities of protein kinase C-alpha and beta were significantly decreased in age-related manner. However, the activity of the gamma-isozyme was not significantly changed at 12- and 25-months of age, although it was higher (P < or = 0.03) in young rats. Western blot analyses using affinity purified polyclonal antibodies specific for each isozyme indicated a single protein with an apparent molecular mass of approximately 80 x 10(3) molec. weight for all isozymes except for the beta isozyme that also had an appreciable immunoreactive band at approximately 36 x 10(3) molec. weight. Overall, the aging process did not affect the electropheretic mobility of each isozyme. With decreased protein kinase C activity, the present data suggest that the aging process would decrease protein kinase C-induced phosphorylation of membrane proteins including Ins P3 receptor. A significant change in Ins P3 receptor affinity combined with increased levels of Ins P3 receptor mRNA-encoding transcripts in senescent rats suggests not only a modification (possibly by phosphorylation) of Ins P3 receptor protein but also the existence of multiple (spliced) variants of Ins P3 receptor in spinal neurons with increasing age. The present data indicate that the spinal contents of inositol 1,4,5-trisphosphate increased with age, but with decreased efficacy and number of inositol 1,4,5-trisphosphate-activatable Ca2+ channels in the spinal cord of senescent rats. These age-related changes may contribute to the attenuated responsiveness of spinal cord neurons by phosphoinositide-coupled receptors during the aging process.

Neocortex and hippocampus contain distinct distributions of calcium-calmodulin protein kinase II and GAP43 mRNA

Calcium-calmodulin protein kinase II and GAP43 are two molecules which have been linked to synaptic plasticity. Localization of mRNA for these molecules identifies the neuronal populations which have the potential to utilize these mechanisms. General descriptions for calcium-calmodulin protein kinase II or GAP43 mRNA have been previously reported. In light of recent evidence that suggests that at some sites these two molecules may interact, we sought to determine the cortical distribution in detail, and to examine the extent of overlap between neuronal populations containing each mRNA. To this end we have used in situ hybridization techniques to study the distribution of calcium-calmodulin protein kinase II and GAP43 mRNA in adjacent sections of adult rat forebrain. Overall, the distribution patterns were distinct but partially overlapping. For both calcium-calmodulin protein kinase II and GAP43, mRNA levels were highest in hippocampus, allo- and neocortex, compared to moderate to low levels in striatum and thalamic nuclei. Within the heavily labeled regions certain populations expressed both calcium-calmodulin protein kinase II and GAP43 mRNA at high levels, while other populations were selective for calcium-calmodulin protein kinase II. In the hippocampus, the stratum pyramidale of CA1-3 expressed high levels of both calcium-calmodulin protein kinase II and GAP43 mRNA. Granule cells of the fascia dentata and the stratum radiatum of CA3 both contained moderate to high levels of calcium-calmodulin protein kinase II mRNA, but near background levels of GAP43 mRNA label. Within the neocortex, deep layers were distinguished from superficial layers by their lack of calcium-calmodulin protein kinase II mRNA expression within the neuropil, and the presence of GAP43 mRNA in neurons located in layer V and the deepest part of layer VI. Thus, layer V and deep layer VI neurons showed high levels of label for both GAP43 and calcium-calmodulin protein kinase II mRNA, while neurons of superficial layers contained only calcium-calmodulin protein kinase II mRNA. These markers differentiate neuronal populations which can also be distinguished on the basis of their ability to undergo specific forms of synaptic plasticity. These different forms of plasticity may be due in part to the laminar-specific patterns of GAP43 and calcium-calmodulin protein kinase II mRNA that we have described.

Protein phosphatases 1 and 2A dephosphorylate B-50 in presynaptic plasma membranes from rat brain

The protein B-50 is dephosphorylated in rat cortical synaptic plasma membranes (SPM) by protein phosphatase type 1 and 2A (PP-1 and PP-2A)-like activities. The present studies further demonstrate that B-50 is dephosphorylated not only by a spontaneously active PP-1-like enzyme, but also by a latent form after pretreatment of SPM with 0.2 mM cobalt/20 micrograms of trypsin/ml. The activity revealed by cobalt/trypsin was inhibited by inhibitor-2 and by high concentrations (microM) of okadaic acid, identifying it as a latent form of PP-1. In the presence of inhibitor-2 to block PP-1, histone H1 (16-64 micrograms/ml) and spermine (2 mM) increased B-50 dephosphorylation. This sensitivity to polycations and the reversal of their effects on B-50 dephosphorylation by 2 nM okadaic acid are indicative of PP-2A-like activity. PP-1- and PP-2A-like activities from SPM were further displayed by using exogenous phosphorylase alpha and histone H1 as substrates. Both PP-1 and PP-2A in rat SPM were immunologically identified with monospecific antibodies against the C-termini of catalytic subunits of rabbit skeletal muscle PP-1 and PP-2A. Okadaic acid-induced alteration of B-50 phosphorylation, consistent with inhibition of protein phosphatase activity, was demonstrated in rat cortical synaptosomes after immunoprecipitation with affinity-purified anti-B-50 immunoglobulin G. These results provide further evidence that SPM-bound PP-1 and PP-2A-like enzymes that share considerable similarities with their cytosolic counterparts may act as physiologically important phosphatases for B-50.

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.

Noradrenaline release from streptolysin O-permeated rat cortical synaptosomes: effects of calcium, phorbol esters, protein kinase inhibitors, and antibodies to the neuron-specific protein kinase C substrate B-50 (GAP-43)

We studied the molecular mechanism of noradrenaline release from the presynaptic terminal and the involvement of the protein kinase C substrate B-50 (GAP-43) in this process. To gain access to the interior of the presynaptic terminal, we searched for conditions to permeate rat brain synaptosomes by the bacterial toxin streptolysin O. A crude synaptosomal/mitochondrial preparation was preloaded with [3H]noradrenaline. After permeation with 0.8 IU/ml streptolysin O, noradrenaline efflux could be induced in a concentration-dependent manner by elevating the free Ca2+ concentration from 10(-8) to 10(-5) M. Efflux of the cytosolic marker protein lactate dehydrogenase was not affected by this increase in Ca2+. Ca2(+)-induced efflux of noradrenaline was largely dependent on the presence of exogenous ATP. Changing the Na+/K+ ratio in the buffer did not affect Ca2(+)-induced noradrenaline release. Release of noradrenaline could also be evoked by phorbol esters, indicating the involvement of protein kinase C. Ca2(+)- and phorbol ester-induced release were not additive at higher phorbol ester concentrations (greater than 10(-7) M). We compared the sensitivities of Ca2(+)- and phorbol ester-induced release of noradrenaline to the protein kinase inhibitors H-7 and polymyxin B and to antibodies raised against synaptic protein kinase C substrate B-50. Ca2(+)-induced release was inhibited by B-50 antibodies and polymyxin B, but not by H-7; phorbol ester-induced release was inhibited by polymyxin B and by H-7, but only marginally by antibodies to B-50.(ABSTRACT TRUNCATED AT 250 WORDS)

Microencephaly reduces the phosphorylation of the PKC substrate B-50/GAP43 in rat cortex and hippocampus

The administration of the antimitotic agent methylazoxymethanol (MAM) to rats at day 15 of gestation results in a consistent loss of intrinsic neurons primarily in cortex and hippocampus. These animals when adult, show a cognitive impairment, if tested in specific behavioural tasks. B-50/GAP43 is a neuronal phosphoprotein, specific substrate for protein kinase C (PKC) and involved in the development and plasticity of synaptic connections. Since B-50/GAP43 has been implicated in functional modulation of synapses and in the molecular mechanism underlying cognitive processes, we studied the phosphorylation of B-50 in cortex and hippocampus of control and MAM-treated rats. Here we report that B-50 in MAM-treated rats shows a marked reduction in the phosphate incorporation in the areas affected by the prenatal treatment. In situ hybridization studies demonstrate that the mRNA levels for B-50 are not altered in MAM-treated rats and that the relative amount of the protein, as revealed by Western blot analysis, is also not affected in microencephalic rats. These results suggest that microencephalic animals might represent a useful experimental model to study biochemical correlates of cognitive impairment and synaptic plasticity.

Depolarization-induced phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat cortical synaptosomes

We studied the molecular events underlying K(+)-induced phosphorylation of the neuron-specific protein kinase C substrate B-50. Rat cortical synaptosomes were prelabelled with 32P-labelled orthophosphate. B-50 phosphorylation was measured by an immunoprecipitation assay. In this system, various phorbol esters, as well as a synthetic diacylglycerol derivative, enhance B-50 phosphorylation. K+ depolarization induces a transient enhancement of B-50 phosphorylation, which is totally dependent on extracellular Ca2+. Also, the application of the Ca2+ ionophore A23187 induces B-50 phosphorylation, but the magnitude and kinetics of A23187-induced B-50 phosphorylation differ from those induced by depolarization. The protein kinase inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and staurosporine antagonize K(+)- as well as PDB-induced B-50 phosphorylation, whereas trifluoperazine and calmidazolium are ineffective under both conditions. We suggest that elevation of the intracellular Ca2+ level after depolarization is a trigger for activation of protein kinase C, which subsequently phosphorylates its substrate B-50. This sequence of events could be of importance for the mechanism of depolarization-induced transmitter release.

Evidence for a relationship between B-50 (GAP-43) and [3H]noradrenaline release in rat brain synaptosomes

Phosphorylation of the neuron-specific substrate of protein kinase C (PKC), B-50 (GAP-43), was studied parallel with noradrenaline release in rat brain synaptosomes. Both could be evoked by treating the synaptosomes with high K+ or veratridine. Phorbol 12,13-dibutyrate enhanced depolarization-induced B-50 phosphorylation and noradrenaline release. To investigate the involvement of PKC-mediated B-50 phosphorylation in noradrenaline release, we applied a variety of kinase inhibitors. Prior to measuring the effects of these inhibitors in intact synaptosomes, we determined their effectivity and specificity in a membrane phosphorylation assay. H-7 most specifically inhibited PKC-dependent phosphorylation, whereas calmidazolium inhibited calmodulin-dependent phosphorylation. Polymyxin B affected both protein kinase systems. Only polymyxin B effectively inhibited noradrenaline release in the intact synaptosomes. We conclude that PKC as well as calmodulin-dependent processes are important for the release event. Data are discussed in view of the presumed function of B-50 as a calmodulin-binding protein.

Dephosphorylation of B-50 in synaptic plasma membranes

Synaptic plasma membranes from rat brain cortex possess intrinsic ability to dephosphorylate the endogenous protein B-50. At low concentrations of [gamma-32P]ATP, B-50 phosphorylation in synaptic membranes is maximal at 30 seconds, followed by dephosphorylation for an additional 60 minutes. The dephosphorylation of 32P-labeled B-50 is not sensitive to the protease inhibitor leupeptin and not correlated with a loss of the B-50 content of synaptic membranes as measured with immunoblot analysis. Dephosphorylation of membrane-associated B-50 is stimulated to a small extent by Mg2+ but not by Ca2+. Heat-stable protein phosphatase inhibitors prevent dephosphorylation of 32P-labeled B-50. Dephosphorylation of B-50 in synaptic membranes is stimulated by ATP, ADP, or adenosine 5'-O-thiotriphosphate, but not by adenine, adenosine, other adenine or guanine nucleotides, nonhydrolyzable analogs of ATP or GTP, nor by adenosine 5'-O-(2-thiodiphosphate). B-50, phosphorylated by exogenous protein kinase C and purified to homogeneity, has been used as a substrate to follow the purification of B-50 phosphatase activity. B-50 phosphatase activity can be solubilized from synaptic membranes with 0.5% Triton X-100 and 75 mM KCl. Chromatography of the extract on DEAE-cellulose yields enhanced B-50 phosphatase activity.

4-Aminopyridine stimulates B-50 (GAP-43) phosphorylation in rat synaptosomes

Recently, we have shown that stimulation of [3H]-noradrenaline release from hippocampal slices by 4-aminopyridine (4-AP) is accompanied by an enhancement of the phosphorylation of B-50, a major presynaptic substrate of protein kinase C (PKC). PKC has been implicated in the regulation of transmitter release. In this study, we investigated the effects of 4-AP on B-50 phosphorylation in synaptosomes from rat brain and compared the effects of 4-AP with those of depolarization with K+, in order to gain more insight into the mechanism of action of 4-AP. B-50 phosphorylation was stimulated by incubation with 4-AP for 2 minutes at concentrations ranging from 10 microM to 5 mM. 4-AP (100 microM) stimulated B-50 phosphorylation already within 15 seconds; longer incubations revealed a sustained increase in the presence of 4-AP. B-50 phosphorylation was also stimulated by depolarization with 30 mM K+ for 15 seconds. The effects of both 4-AP or K+ depolarization on B-50 phosphorylation were abolished at low extracellular Ca2+ concentrations. The increase in B-50 phosphorylation induced by 4-AP seemed to be dependent on the state of depolarization, since the effect of 4-AP was largest under nondepolarizing conditions. Comparing the effects of 4-AP and K+ depolarization on B-50 phosphorylation suggests that a different mechanism of action is involved. These results indicate that the stimulation of B-50 phosphorylation by 4-AP in hippocampal slices can be attributed to a direct action of 4-AP on presynaptic terminals. In addition, our results support the hypothesis that B-50 phosphorylation by PKC is involved in Ca2(+)-dependent transmitter release evoked by 4-AP.

Phosphorylation of B-50 (GAP43) is correlated with neurotransmitter release in rat hippocampal slices

Recent studies have demonstrated that phorbol diesters enhance the release of various neurotransmitters. It is generally accepted that activation of protein kinase C (PKC) is the mechanism by which phorbol diesters act on neurotransmitter release. The action of PKC in neurotransmitter release is very likely mediated by phosphorylation of substrate proteins localized in the presynaptic nerve terminal. An important presynaptic substrate of PKC is B-50. To investigate whether B-50 mediates the actions of PKC in neurotransmitter release, we have studied B-50 phosphorylation in intact rat hippocampal slices under conditions that stimulate or inhibit PKC and neurotransmitter release. The slices were labelled with [32P]orthophosphate. After treatment, the slices were homogenized, B-50 was immunoprecipitated from the slice homogenate, and the incorporation of 32P into B-50 was determined. Chemical depolarization (30 mM K+) and the presence of phorbol diesters, conditions that stimulate neurotransmitter release, separately and in combination, also enhance B-50 phosphorylation. Polymyxin B, an inhibitor of PKC and neurotransmitter release, decreases concentration dependently the depolarization-induced stimulation of B-50 phosphorylation. The effects of depolarization are not detectable at low extracellular Ca2+ concentrations. It is concluded that in rat hippocampal slices B-50 may mediate the action of PKC in neurotransmitter release.

Phosphorylation of protein B-50 (GAP-43) from adult rat brain cortex by casein kinase II

The phosphoprotein B-50 (GAP-43) was purified from adult rat brain cortex and phosphorylated by casein kinase II. Phosphorylation of B-50 by casein kinase II approached 1.2 mol phosphate/mol B-50. The apparent Km of casein kinase II for B-50 was 4 microM with an apparent Vmax of 13 nmol.min-1.mg-1. A tryptic phosphopeptide map on reversed phase HPLC and phosphoamino acid analysis of [32P]B-50 showed that casein kinase II phosphorylated in serine residue(s) which were located in a single tryptic peptide. Phosphorylation of B-50 by casein kinase II was inhibited more than 90% by 5 micrograms heparin/ml or 2.4 mM peptide substrate specific for casein kinase II (RRREEETEEE). The initial phosphorylation rate was increased about 2-fold by 1 mM spermine.

Biogenesis of presynaptic terminal proteins

The delivery of proteins to the presynaptic terminals of guinea pig retinal ganglion cells by two of the major components of axonal transport, and the subsequent persistence and turnover of those proteins were examined in this study. Ganglion cell proteins were radiolabeled by intravitreal injection of radiolabeled amino acids and radioactive axonally transported proteins were analyzed in synaptosomes prepared from the superior colliculi. This procedure allowed examination of presynaptic components of ganglion cell synapses without having to compensate for postsynaptic or other unidentified contaminants. Each of the two major axonal transport components supplies a large number of proteins to the presynaptic terminal, in relative quantities similar although not identical to those seen in the axon. Proteins conveyed by the fast component of axonal transport reached the terminals by 3 h after intraocular injection, peaked by 24 h, and were largely undetectable by 15 days. Slow component b proteins reached the terminals by 12 days, peaked around 21 days, and persisted up to 63 days in the terminals. Proteins in both components demonstrated differential turnover relative to cotransported proteins once they reached the terminals. Differential turnover may account for change in relative concentration of a particular protein required to meet new functional demands on that protein once it enters the terminal.

Effects of arginine vasopressin on protein phosphorylation in rat hippocampal synaptic membranes

Our laboratory has reported previously the characteristics of specific AVP binding to rat hippocampal synaptic membranes (SPM) in the presence of Ni2+ [Costantini MG, Pearlmutter AF: J Biol Chem 259: 11739-11745, 1984]. We extended our investigation to determine the effects of Ni2+, (AVP), and AVP analogs on SPM protein phosphorylation. Ni2+ (5 mM) caused a dramatic reduction in phosphorylation of most SPM phosphoproteins. The most prominent protein which is phosphorylated in SPM has a molecular weight of 48 kilodaltons (KDa) and has been named B50 or F1; this protein shows altered phosphorylation in vitro in response to long-term potentiation in vivo as well as changes induced by exposure of SPM to ACTH (1-24), dopamine, and somatostatin. AVP and related peptides reduced phosphorylation of this pre-synaptic phosphoprotein in the following order of potency: AVP = oxytocin greater than DG-AVP greater than dDAVP greater than d(CH2)5Tyr(Me)AVP = [pGlu4,Cyt6]AVP-(4-9). Except for the pressor antagonist d(CH2)5Tyr(Me)AVP, this corresponds to their relative efficacy in displacing 3H-AVP from high-affinity specific binding sites on rat hippocampal synaptic membranes. Ni2+ did not alter the degree of inhibition caused by the peptides. When SPM were treated with AVP after the attainment of maximum 32P incorporation, AVP inhibited dephosphorylation over a 30-min period. Our results show that AVP can alter both phosphorylation and dephosphorylation of hippocampal SPM phosphoproteins in vitro; the direction of these effects depends upon experimental conditions. Since B50/F1 is known to be a substrate for protein kinase C, AVP may act by inhibition of protein kinase C activity, either directly or indirectly.

The kinase C substrate protein B-50 and axonal regeneration

As reported previously the prominent protein kinase C substrate protein B-50 is present in growth cones isolated from fetal rat brain and in outgrowing hippocampal neurites. These findings suggest that B-50 plays a role in axonal growth during development of the nervous system. In the present paper the fate of B-50 is investigated in regenerating rat sciatic nerve. Using affinity-purified anti-B-50 antibodies B-50 levels have been compared in crushed and contralateral intact nerves by means of immunoblotting and radioimmunoassay. B-50 levels in the crushed nerve increased 5.3-fold as compared to non-crushed controls. Furthermore, the cellular localization of B-50 has been assessed by immunohistochemistry. Virtually no B-50 immunoreactivity was seen in control nerves, but bright immunofluorescence appeared in regenerating sprouts. Our data are in line with current evidence from several laboratories that B-50 is a member of a small family of growth-associated proteins and support the hypothesis that B-50 is involved in axonal growth.

Protein phosphorylation in rat hippocampal synaptic plasma membranes in response to neurohypophyseal peptides

The effect of arginine vasopressin (AVP) on protein phosphorylation in rat hippocampal synaptic plasma membranes (SPM) was examined. With a crude SPM preparation, AVP (10(-8)-10(-5) M) stimulated phosphorylation of a number of proteins which included a brain-specific protein of 48 kDa called B50 or protein F1, which is thought to be related to synaptic plasticity. Equimolar levels of oxytocin also stimulated B50/F1 phosphorylation. AVP and oxytocin at higher concentrations (10(-4)-10(-3) M) reduced SPM protein phosphorylation. When SPM was treated with both AVP and oxytocin, the effects were not additive; on the other hand, the effects of the phorbol ester (TPA) and AVP were additive. With SPM, partially purified by sucrose density centrifugation, only the inhibitory effect of AVP on B50/F1 phosphorylation was seen. These results suggest that AVP and oxytocin stimulation of B50/F1 phosphorylation requires cellular factors which are removed from SPM during membrane purification. In contrast, the inhibitory mechanism triggered by AVP and oxytocin appears to be associated with, or an integral part of, the synaptic membrane itself. Because the effects on membrane protein phosphorylation with maximal amounts of AVP and oxytocin were not additive, they must bind to the same sites on the membrane. This conclusion is supported by the additivity of the effects of AVP and phorbol ester, since the phorbol ester can act directly on the kinase and does not require a membrane recognition site.

Nerve growth factor enhances the level of the protein kinase C substrate B-50 in pheochromocytoma PC12 cells

Exposure of PC12 cells to nerve growth factor results in arrest of cell growth and induction of differentiation to sympathetic neuron-like cells, bearing neurites. In this study we identify a 48 kDa PC12 phosphoprotein as the neuron-specific protein kinase C substrate B-50 (Mr 48 kDa; IEP 4.5) on basis of comigration with purified B-50, immunoreactivity and phosphopeptide mapping. B-50 is present in both undifferentiated and differentiated PC12 cells. Exposure of PC12 cells to nerve growth factor for two days results in a 2.5-fold increase in the amount of B-50 as measured by RIA. Indirect immunofluorescence microscopy reveals that B-50 is mainly localized at the cell membrane and in growth cones. Our data are in line with the hypothesis that B-50 plays a role in neurite outgrowth and indicate that PC12 cells provide a suitable model to study this hypothesis.

Phosphorylation of synaptic membrane glycoproteins: the effects of Ca2+ and calmodulin

Synaptic membranes were incubated with [gamma-32P]ATP, and glycoproteins were isolated by affinity chromatography on concanavalin A agarose. Glycoproteins accounted for 1.5-2.5% of the total 32P incorporated into synaptic membrane proteins. Ca2+ and calmodulin enhanced the phosphorylation of synaptic membrane glycoproteins approximately threefold. In the presence of Ca2+ and calmodulin, the rate of glycoprotein dephosphorylation was also increased three- to four-fold. Gel electrophoretic analysis identified several synaptic membrane glycoproteins that incorporated 32P, with the most highly labeled glycoprotein under basal phosphorylating conditions having an apparent Mr of 205,000 (gpiii). Ca2+ and calmodulin produced a marked increase in the phosphorylation of a glycoprotein with an apparent Mr of 180,000 (gpiv) and lesser increases in the labeling of three other glycoproteins. Membranes that had been labeled with [gamma-32P]ATP were extracted with Triton X-100 under conditions that yield a detergent-insoluble residue enriched in postsynaptic structures. The Triton X-100 insoluble residue accounted for 20-25% of the 32P associated with synaptic membrane glycoproteins. Gpiv and other glycoproteins, the phosphorylation of which was stimulated by calmodulin, were located exclusively in the Triton X-100 insoluble residue, whereas gpiii and other calmodulin-insensitive glycoproteins partitioned predominantly into the Triton X-100-soluble fraction. Phosphopeptide maps and phosphoamino acid analysis of gpiv isolated from synaptic membranes and a postsynaptic glycoprotein of apparent Mr of 180,000 (gp180) isolated from synaptic junctions indicated that the former protein was identical to the previously identified postsynaptic-specific gp180. In addition to phosphoserine and phosphothreonine, gpiv also contained phosphotyrosine, identifying it as a substrate for tyrosine-protein kinase as well as for Ca2+/calmodulin-dependent protein kinase.

Potentiation of synaptic transmission in the hippocampus by phorbol esters

Protein kinase C (PKC), a calcium-dependent phospholipid-sensitive kinase which is selectively activated by phorbol esters, is thought to play an important role in several cellular processes. In mammalian brain PKC is present in high concentrations and has been shown to phosphorylate several substrate phosphoproteins, one of which may be involved in the generation of long-term potentiation (LTP), a long-lasting increase in synaptic efficacy evoked by brief, high-frequency stimulation. Since the hippocampus contains one of the brain's highest levels of binding sites for phorbol esters and is the site where LTP has been most thoroughly characterized, we examined the effects of phorbol esters on hippocampal synaptic transmission and LTP. We found that phorbol esters profoundly potentiate excitatory synaptic transmission in the hippocampus in a manner that appears indistinguishable from LTP. Furthermore, after maximal synaptic enhancement by phorbol esters, LTP can no longer be elicited. Although the site of synaptic enhancement during LTP is not clearly established, phorbol esters appear to potentiate synaptic transmission by acting primarily at a presynaptic locus since changes in the postsynaptic responses to the putative transmitter, glutamate, cannot account for the increased synaptic responses induced by phorbol esters. These findings, in conjunction with previous biochemical studies, raise the possibility that, in mammalian brain, PKC plays a role in controlling the release of neurotransmitter and may be involved in the generation of LTP.

A radioimmunoassay for the phosphoprotein B-50: distribution in rat brain

A radioimmunoassay (RIA) for the B-50 protein was developed to determine B-50 in total homogenates of rat tissues. A tracer of purified B-50 was prepared at high activity (10-30 microCi/micrograms protein) by phosphorylating B-50 with carrier-free [gamma-32P]ATP, catalyzed by purified protein kinase C. The RIA was performed using affinity-purified anti-B-50 immunoglobulins G in a detergent containing medium and detected B-50 at levels of 0.1-10 ng. Specificity of the antibodies was ascertained by immunoprecipitation of B-50 from a crude mitochondrial membrane fraction from rat brain and by immunoblotting. For the B-50 content in rat brain the following distribution pattern was found: medulla spinalis less than cerebellum less than hippocampus; cerebral cortex less than periaqueductal gray less than septum. The septum contained 80 micrograms/g tissue weight. The level in liver homogenates was below detection. The regional distribution is in fair agreement with the pattern of the endogenous B-50 phosphorylation in rat brain synaptosomal plasma membranes previously reported.

Phosphoprotein B-50 in nerve growth cones from fetal rat brain

The presynaptic, nervous tissue-specific phosphoprotein B-50 is present in infant and adult rat brain. In the present study we demonstrate that B-50 is a major phosphoprotein in nerve growth cones obtained from fetal rat brain. As this protein is an endogenous substrate for protein kinase C, an enzyme linked to cell growth and proliferation, a role for B-50 in nerve growth cone function is suggested.

Characteristics of [D-Trp8]-somatostatin-sensitive B50 phosphorylation

Inhibition of the phosphorylation of the synaptic plasma membrane (SPM) protein B50 by [D-Trp8]-somatostatin in vitro is time-dependent. Increasing the time of incubation of hippocampal synaptic plasma membranes with the peptide from 15 sec to 30 min prior to addition of 7.5 microM [gamma-32P]ATP results in a complete reduction of B50 phosphorylation. Incubation of synaptic plasma membranes for 30 min in the absence of peptide does not alter basal B50 phosphorylation. Neither ACTH nor beta-endorphin produces similar effects--inhibition of B50 phosphorylation by ACTH is maximal at 15 sec and beta-endorphin produces only a small inhibition, even after 30 min. [D-Trp8]-somatostatin is not activating a membrane-bound protease, since maximal inhibition of B50 phosphorylation by the peptide is seen in the presence of leupeptin or bacitracin. Hippocampal synaptic plasma membranes contain protein phosphatase activity. Assays of B50 phosphorylation in synaptic plasma membranes done under conditions that favor either net phosphorylation or dephosphorylation are consistent with inhibition of protein phosphatase activity by [D-Trp8]-somatostatin. This evidence suggests that [D-Trp8]-somatostatin interacts with SPM binding sites in the hippocampus, which may alter the activity of an endogenous protein phosphatase to determine the degree of B50 phosphorylation.

Pre- versus postsynaptic localization of benzodiazepine and beta-carboline binding sites

[3H]Flunitrazepam (FNP) and [3H]methyl beta-carboline-3-carboxylate (MCC) binding was examined in soluble and particulate fractions from membranes solubilized with Triton X-100 or in subfractions of synaptosomal membranes obtained by a physical separation technique. Results using both methods demonstrate that benzodiazepine and beta-carboline sites reside on both pre- and postsynaptic membranes. Further, subfractionation experiments indicate that the binding sites for both ligands are unequally distributed within the synapse and among brain regions. For example, in cerebral cortical presynaptic membranes there are twice as many FNP as MCC sites whereas in postsynaptic membranes this ratio is reversed. The number of FNP and MCC sites are equal in the presynaptic fraction from cerebellum. The postsynaptic membranes derived from cerebellum have three times the number of FNP compared to MCC sites. In hippocampus this ratio varies between 1.5 and 2.8 in each subfraction. These results support the idea that benzodiazepine and beta-carboline binding sites represent different recognition sites.

Resolution of rat brain synaptic phosphoprotein B-50 into multiple forms by two-dimensional electrophoresis: evidence for multisite phosphorylation

Phosphoprotein B-50 was extracted from rat brain membranes by alkaline extraction and purified by ammonium sulphate precipitation and flat-bed isoelectric focusing. The purified protein shows microheterogeneity upon isoelectric focusing in a narrow pH gradient (pH 3.5-5.0). As visualized by two-dimensional gel electrophoresis, B-50 resolved into four clearly separated forms which differ slightly in isoelectric point. The forms are in part mutually convertible by exhaustive phosphorylation (using protein kinase C) and dephosphorylation (using Escherichia coli alkaline phosphatase). Proteolysis with Staphylococcus aureus protease yielded two radioactive peptides. Analysis of their molecular weights and the time course of their formation suggests that B-50 was cleaved at only one specific site. Our data indicate the presence of more than one phosphorylatable site. The possibility that the heterogeneity of B-50 was in part due to a glycoprotein nature of B-50 was studied extensively. However, none of the six different methods used revealed the presence of glyco-moieties in B-50.

A simple and economical method for studying protein phosphorylation in vivo in the rat brain

A simple and economical procedure, capable of routine application, is described for the labelling of cerebral phosphoproteins in vivo. [32P]Orthophosphate, in high concentration, was infused into selected brain areas of anaesthetised rats under stereotaxic control. The animals were frozen with liquid N2 and the labelled tissue punched out of frozen thick sections. [32P]Polypeptides were analysed by high-resolution two-dimensional gel electrophoresis. Several phosphoproteins on the gels were provisionally identified, including synapsin I, MAP-2 and an 82-87 kdalton substrate of protein kinase 'C'.

Presynaptic localization of B-50 phosphoprotein: the (ACTH)-sensitive protein kinase substrate involved in rat brain polyphosphoinositide metabolism

This study describes the ultrastructural localization in rat hippocampal tissue in situ and in isolated synaptosomes of the brain-specific phosphoprotein B-50, using affinity purified anti-B-50 immunoglobulins (IgGs). Evidence is presented for the presynaptic localization of B-50 in rat brain. Given this specific localization a model is presented outlining the presumed function of the B-50 protein in the membrane and describing possible neuromodulation by adrenocorticotropin hormone (ACTH)-like peptides.

Regulation of phosphate incorporation into four brain phosphoproteins that are affected by experience

Various regulators of protein kinase activities were tested for their effects on the in vitro transfer of phosphate from [gamma-32P]ATP to four proteins of rat brain synaptic particulate preparations. One protein, of apparent molecular weight 44,000, accepted 32P in the presence of 8 mM EDTA and no added Mg2+. It was the major phosphoprotein of brain mitochondria. Its phosphorylation was inhibited by pyruvate and stimulated by K+, and it comigrated in electrophoretic gels with authentic alpha-subunit of pyruvate: lipoamide oxidoreductase (decarboxylating) (EC 1.2.4.1) from bovine heart. The major kinase acting on three proteins of apparent molecular weights 24,000, 21,000, and 19,000 was stimulated by Ca2+, by preincubation with phospholipase C, and by 12-tetradecanoyl 4-beta-phorbol 13-acetate. Phosphorylation of these lower-molecular-weight proteins was inhibited by ACTH1-24, by cyclic 3',5'-adenosine monophosphate, and by 50 microM trifluoperazine. The stimulatory effect of Ca2+ was antagonized by calmodulin. The kinase in question appears to be B-50 protein kinase or protein kinase C.

Cross-reaction of anti-rat B-50: characterization and isolation of a "B-50 phosphoprotein" from bovine brain

Antibodies to the phosphoprotein B-50 of rat brain were used to trace cross-reacting brain proteins of vertebrates. With the SDS-gel-immunoperoxidase method, a cross-reacting protein (CP) of apparent Mr 53,000 was demonstrated in the homogenate and the synaptic plasma membrane fraction of bovine brain. Sequence 1-24 of adrenocorticotropin (ACTH1-24) (10(-5) M and 10(-4) M) inhibited endogenous phosphorylation of CP in synaptic plasma membranes. The protein was partially characterized and purified to homogeneity from bovine brain by procedures previously described for rat B-50. CP was enriched in ammonium sulfate precipitated protein (ASP) fractions and phosphorylated by an endogenous protein kinase. Two-dimensional gel analysis of bovine and rat ASP showed that the cross-reacting protein had an isoelectric point less acidic than B-50. Limited proteolysis by Staphylococcus aureus protease yielded a "peptide map" analogous to B-50. Two major fragments of Mr 30,000 and 17,000 were produced. In addition, CP exhibited other similarities to rat B-50: phosphorylation by rat brain protein kinase C, microheterogeneity observed after isoelectric focusing, and possibly degradation by endogenous proteolysis. Cross-reaction of proteins in brain homogenates of other mammalian species and of chicken was demonstrated: the Mr of the proteins ranged from 47,000 to 53,000. We conclude that (1) the cross-reacting bovine protein is a "B-50 protein," and (2) the Mr of the "B-50 protein" varies from species to species.

Turnover of inositol phospholipids and signal transduction

Various extracellular informational signals such as those from a group of hormones and some neurotransmitters appear to be passed from the cell surface into the cell interior by two routes, protein kinase C activation and Ca2+ mobilization. Both routes usually become available as the result of an interaction of a single ligand and a receptor and act synergistically to evoke subsequent cellular responses such as release reactions. The signal-dependent breakdown of inositol phospholipids, particularly phosphatidylinositol bisphosphate, now appears to be a key event for initiating these processes.

Adrenocorticotropic hormone (ACTH)-lipid interactions. Implications for involvement of amphipathic helix formation

ACTH-lipid interactions were investigated by: (1) lipid-monolayer studies using several zwitterionic and anionic phospholipids and gangliosides, (2) permeability experiments by following the swelling rate of liposomes in isotonic glycerol solutions by light scattering, using liposomes of synthetic lipids and liposomes made of lipids extracted from light synaptic plasma membranes, and (3) by steady-state fluorescence anisotropy measurements on liposomes derived from light synaptic plasma membranes employing 1,6-diphenyl-1,3,5-hexatriene as fluorescent probe. (1) The monolayer experiments demonstrated an interaction with gangliosides GT1, GM1, dioleoylphosphatidic acid and phosphatidylserine, but little or no interaction with phosphatidylcholine or sphingomyelin. The interaction with monolayers of GT1 or phosphatidic acid decreased for ACTH1-13-NH2 and ACTH1-10. (2) The liposome experiments showed that 2 X 10(-5) M ACTH1-24 increased the glycerol permeability by 20% and decreased the activation energy only when liposomes derived from light synaptic plasma membranes were used. Treatment of the liposomes with neuraminidase abolished the ACTH-induced permeability increase. (3) Steady-state fluorescence depolarization measurements revealed that ACTH1-24, ACTH1-16-NH2 and ACTH1-10 did not change the fluidity of liposomes derived from light synaptic plasma membranes as sensed by diphenylhexatriene. It is concluded that ACTH1-24 can bind to negatively charged lipids and can form an amphipathic helix aligned parallel to the membrane surface involving the N-terminal residues 1 to 12, possibly to 16. Polysialogangliosides will favorably meet the condition of a high local surface charge density under physiological circumstances. It is suggested that ACTH-ganglioside interactions will participate in ACTH-receptor interactions.

Alteration of hippocampal B50 phosphorylation after adrenalectomy

Adrenalectomy alters endogenous phosphorylation of the presynaptic protein, B50, in the hippocampus. Three and four days following adrenalectomy decreases are seen, relative to control values, in the in vitro phosphorylation of B50 when a synaptic plasma membrane fraction from the hippocampus is incubated with [gamma-32P]ATP. At four days post-adrenalectomy, the percent decrease in B50 phosphorylation is -49.8 +/- 6.8%. No alteration is seen in the level of B50 phosphorylation when comparing hippocampal membrane preparations from sham-operated and intact animals. Fourteen days following adrenalectomy, hippocampal B50 phosphorylation was restored to normal levels. Hypophysectomy did not alter the degree of in vitro B50 phosphorylation, but the effect of adrenalectomy occurred in hypophysectomized rats. Following adrenalectomy, no differences are seen in the phosphorylation of any hippocampal cytosolic proteins. Changes in B50 phosphorylation seen in hypothalamic synaptic plasma membranes are likely due to the effects of sham-operation. The results indicate that a transient neurochemical or neuroendocrine event following adrenalectomy modulates in vivo the degree of B50 phosphorylation in hippocampal synaptic membranes.

Kindling alters the calcium/calmodulin-dependent phosphorylation of synaptic plasma membrane proteins in rat hippocampus

Septal kindling was associated with an inhibition of the post hoc phosphorylation of several synaptic plasma membrane proteins of rat hippocampus. In control rats, the 32P incorporation into proteins of molecular weights 50,000, 58,000, and 60,000 was markedly stimulated by combined calcium/calmodulin, whereas in kindled animals, the response to combined calcium/calmodulin was reduced. Calcium alone, cAMP, or cGMP modulated 32P incorporation into several synaptic plasma membrane proteins but did not differentiate control from kindled tissues. Both control and kindled rats showed nonspecific inhibition of calcium/calmodulin-stimulated phosphorylation in the post hoc assay by corticotropin and by [Leu]enkephalin. The differences between control and kindled animals were most striking in hippocampus and in the amygdaloid-entorhinal area; less pronounced in cortex, basal ganglia, and brain stem; and not significant in cerebellum, a region where kindling cannot be elicited. An 8-wk period of rest after kindling did not reduce these changes, suggesting that they may be a persistent as the kindling behavior itself.

Effects of calcium, strontium, and barium ions on phosphorylation of hippocampal proteins in vitro

Calcium ion alone or in the presence of added calmodulin stimulated in vitro transfer of 32P from [gamma 32P]ATP into several proteins of mitochondrial and synaptosomal particulate fractions from rat brain. Strontium ion was capable of substituting for calcium ion in this stimulation, but barium ion lacked this capacity. These results bring into question the hypothesis that calcium-dependent protein phosphorylation of synaptic proteins is intrinsic to neurotransmitter release during neurotransmission, but they do not rule out that possibility.

Endogenous protein phosphorylation in chick and rat brain synaptic membranes

The protein kinase activities endogenous to synaptic membranes prepared by an identical procedure from avian (chick) and mammalian (rat) brains were compared. Both species showed similar responses towards both protein kinase effector molecules cyclic adenosine monophosphate and Ca2+. Kapp for cyclic adenosine monophosphate-dependent protein kinase activity occurred at 0.4-0.8 microM cAMP and Kapp for Ca2+-dependent, calmodulin-requiring protein kinase activity occurred at 1-2 microM Ca2+ (free ion concentration) both in the absence or presence of calmodulin added to the reaction mixture. This suggests that endogenous calmodulin in these membranes was able to modulate the Ca2+-dependent, calmodulin requiring protein kinase activity. After EGTA-treatment of the membranes to remove endogenous Ca2+ and calmodulin, no significant response towards Ca2+ on the phosphorylation of the membrane polypeptides was measured unless exogenous calmodulin was added after which the Kapp for Ca2+ was increased to 15 microM Ca2+ (free ion concentration). There was a difference in the maximal levels of kinase activity in these membranes with chick membranes containing 57% less cyclic adenosine monophosphate-dependent protein kinase activity, but 65% more Ca2+-dependent, calmodulin-requiring protein kinase activity than the rat membranes. Similar results were determined when either low (5 microM) or high (5.8 microM) concentrations of adenosine 5'-triphosphate were added to the reaction mixtures. Besides certain species differences in the molecular weights of the resulting phosphoproteins, we observed several major differences with respect to the absence or presence of some of the phosphoproteins. Chick synaptic membranes may lack the cyclic adenosine monophosphate-requiring, microtubule-associated phosphoprotein, MAP2, one of the 2 neurospecific, cyclic adenosine monophosphate-requiring and Ca2+, calmodulin-requiring phosphoproteins (Protein Ib, although Protein Ia apparently is present), and the Ca2+-requiring, calmodulin-independent, ACTH-sensitive phosphoprotein, B50. The phenothiazines, trifluoperazine, fluphenazine and chlorpromazine were found to inhibit the Ca2+-dependent, calmodulin-requiring protein kinase activities of both the chick and rat synaptic membranes. This inhibition appeared to be specific for calmodulin because at the same concentrations the phenothiazine analogue, chlorpromazine-sulfoxide, had no effect on this activity. Also found to inhibit Ca2+-dependent calmodulin-requiring protein kinase activity were dibucaine and adrenocorticotropin. These data suggest that rat forebrain synaptic plasma membranes are activated by cyclic adenosine monophosphate

Phosphorylation of B-50 protein by calcium-activated, phospholipid-dependent protein kinase and B-50 protein kinase

B-50 is a brain-specific phosphoprotein, the phosphorylation state of which may play a role in the regulation of (poly)phosphoinositide metabolism. Several kinases were tested for their ability to phosphorylate purified B-50 protein. Only calcium-activated, phospholipid-dependent protein kinase (kinase C) and B-50 protein kinase were able to use B-50 protein as a substrate. Furthermore, kinase C specifically phosphorylates B-50 when added to synaptic plasma membranes. We further characterized the sensitivity of kinase C and B-50 kinase to ACTH (and various fragments), phospholipids, chlorpromazine, and proteolytic activation. Since the sensitivities of both kinases were similar, we conclude that B-50 protein kinase is a calcium-dependent, phospholipid-stimulated protein kinase of the same type as kinase C.

Affinity-purified anti-B-50 protein antibody: interference with the function of the phosphoprotein B-50 in synaptic plasma membranes

Affinity-purified anti-B-50 protein antibodies were used to study the previously proposed relationship of the phosphorylation state of B-50 protein and polyphosphoinositide metabolism in synaptic plasma membranes. Antibodies were raised against a membrane extract enriched in the B-50 protein and its adrenocorticotropin-sensitive protein kinase, obtained from rat brain. Anti-B-50 protein immunoglobulins were purified by affinity chromatography on a solid immunosorbent prepared from B-50 protein isolated by an improved procedure. The purified antibodies reacted only with the B-50 and B-60 protein, a proteolysis derivative (of B-50), as assessed by the sodium dodecyl sulfate-gel immunoperoxidase method. These antibodies inhibited specifically the endogenous phosphorylation of B-50 protein in synaptic plasma membranes, without affecting notably the phosphorylation of other membrane proteins. This inhibition was accompanied by changes of the formation of phosphatidylinositol 4,5-diphosphate and phosphatidic acid in synaptic plasma membranes, whereas formation of phosphatidylinositol 4-phosphate was not altered. Inhibition by ACTH 1-24 of the endogenous phosphorylation of B-50 protein in membranes was associated only with an enhancement of the phosphorylation of phosphatidyl-inositol 4-phosphate to phosphatidylinositol 4,5-diphosphate. These data support our hypothesis on the functional interaction of B-50 protein and phosphatidylinositol 4-phosphate kinase in rat brain membranes. The evidence shows that purified anti-B-50 protein antibodies can be used to probe specifically the function of B-50 protein in membranes.

Isolation of postsynaptic densities retaining their membrane attachment

A method is described for the preparation of a subcellular fraction, 30-50% pure, of intact postsynaptic units from rat cerebral cortex. The isolation procedure is based on chemical dissociation of the synaptic cleft as described by Crawford, Osborne & Potter followed by sonication of the extracted membranes and separation of the postsynaptic units on a discontinuous sucrose gradient. This preparation provides the first practical procedure for the isolation of postsynaptic densities, prominent organelles of unknown function, without the use of detergents, enabling retention of the postsynaptic membrane in association with the postsynaptic density. The preparation shows enhanced binding of spiroperidol, a dopamine agonist, which, in conjunction with morphological evidence, indicates that the preparation is sufficiently intact to enable study of the interaction of the postsynaptic membrane with the postsynaptic density. Actin, alpha- and beta-tubulin and postsynaptic density protein constitute the major proteins in the preparation; they are present in amounts of 41, 54, 57 and 74 micrograms per mg protein, respectively; as compared to 54, 59, 55 and 9 micrograms per mg protein of the synaptic junctional membrane used as starting material. The utility of the preparation for a number of localization studies, including ion translocating adenosine 5'-triphosphatases, protein kinases and their substrates is discussed.

Somatostatin and analogs inhibit endogenous synaptic plasma membrane protein phosphorylation in vitro

The addition of somatostatin to hippocampal synaptic plasma membrane (SPM) preparations in vitro decreased subsequent phosphorylation of specific protein bands. 10(-4)M somatostatin inhibited the phosphorylation of protein bands with apparent molecular weights between 10 000 and 20 000 daltons and, to a lesser extent, 48 000 daltons (B-50) and 52 000. Increasingly greater degrees of inhibition were seen in response to somatostatin-28 and [D-Trp8]somatostatin. Inhibition of B-50 protein phosphorylation in the presence of [D-Trp8]somatostatin was most prominent in SPM preparations from the hippocampus and amygdala, with lesser degrees of inhibition seen in the cortex and hypothalamus. Addition of [D-Trp8]somatostatin to an ammonium sulfate-precipitated fraction (ASP 55-80) from cortex only slightly inhibited endogenous B-50 phosphorylation. The injection of [D-Trp8] somatostatin intracerebroventricularly into rats did not induce excessive grooming behavior but resulted in barrel rotation. These results suggest that somatostatin and congeners affect SPM protein phosphorylation in a manner different from that of ACTH, presumably involving membrane sites that bind somatostatin.