Calcium-Dependent 4-aminopyridine stimulation of protein phosphorylation in squid optic lobe synaptosomes

When intact synaptosomes were incubated with [gamma-32P]ATP, maximal protein phosphorylation was attained 2 min after the start of incubation. Protein phosphorylation under basal conditions was dependent on external Ca2+, and the dominant peak of phosphorylation was a 50-kd protein. Incubation of intact synaptosomes in the presence of 3-6 mM 4-aminopyridine (4-AP) caused a markedly enhanced phosphorylation of high molecular weight proteins of 90, 100, 130, and 180 kd, with no increase in the 50 or 38 kd proteins. This effect of 4-AP was dependent on external calcium ions in the incubation medium. The 4-AP effect on the high molecular weight proteins was also found in synaptosomal plasma membranes isolated from the synaptosomes. Tetraethylammonium (TEA) ions did not produce this enhancement of phosphorylation.     

Effects of Taurine on Calcium Ion Uptake and Protein Phosphorylation in Rat Retinal Membrane Preparations

The effects of taurine on ATP-dependent calcium ion uptake and protein phosphorylation of rat retinal membrane preparations were investigated. Taurine (20 mM) stimulates ATP-dependent calcium ion uptake by twofold in crude retinal homogenates. In contrast, it inhibits the phosphorylation of specific membrane proteins as shown by acrylamide gel electrophoresis and autoradiography. The close structural analogue of taurine, 2-aminoethylhydrogen sulfate, demonstrates similar effects in both systems, i.e., stimulation of ATP-dependent calcium ion uptake and inhibition of protein phosphorylation, whereas isethionic acid and guanidinoethanesulfonate have no effect on either system. A P1 subcellular fraction of the retinal membrane preparation that contains photoreceptor cell synaptosomes has a higher specific activity for the uptake of calcium ions. Phosphorylation of specific proteins in the P1 fraction is also inhibited by the addition of 20 mM taurine. Taurine has no effect on retinal ATPase activities or on phosphatase activity, thus suggesting that it directly affects a kinase system.     

Inhibition of Ca-transport ATPase from synaptosomal vesicles by flavonoids

The inhibitory action of the flavonoid quercetin has been examined on the calcium-transport ATPase of synaptosomal vesicles and compared to that of two other flavonoids, morin and rutin. We have found that while quercetin caused a 50% inhibition of calcium transport at a concentration of 15 μM, morin and rutin had similar effects at concentrations of about 200 μM. A similar order of potency was observed also for ATP hydrolysis, though at higher concentrations. Quercetin also strongly inhibited phosphorylation of membrane proteins by ATP in synaptosomal vesicles. Rutin and morin had an almost negligible effect on membrane protein phosphorylation. The order of inhibitory potency of the flavonoids on the Ca²⁺-transport ATPase from synaptosomal vesicles: quercetin > morin > rutin, could be linked to their possible solubility in the membrane lipid phase since: (1) it paralleled their partitioning between a mixture of oil and water; (2) it paralleled their uptake from the reaction mixture by synaptosomal vesicles and phosphatidylcholine liposomes; (3) they had almost equal potency as inhibitors of the water soluble system of histone phosphorylation by protein kinase.     

Inhibition of Ca2+-transport ATPase from synaptosomal vesicles by flavonoids

The inhibitory action of the flavonoid quercetin has been examined on the calcium-transport ATPase of synaptosomal vesicles and compared to that of two other flavonoids, morin and rutin. We have found that while quercetin caused a 50% inhibition of calcium transport at a concentration of 15 microM, morin and rutin had similar effects at concentrations of about 200 microM. A similar order of potency was observed also for ATP hydrolysis, though at higher concentrations. Quercetin also strongly inhibited phosphorylation of membrane proteins by ATP in synaptosomal vesicles. Rutin and morin had an almost negligible effect on membrane protein phosphorylation. The order of inhibitory potency of the flavonoids on the Ca2+-transport ATPase from synaptosomal vesicles: quercetin greater than morin greater than rutin, could be linked to their possible solubility in the membrane lipid phase since: (1) it paralleled their partitioning between a mixture of oil and water; (2) it paralleled their uptake from the reaction mixture by synaptosomal vesicles and phosphatidylcholine liposomes; (3) they had almost equal potency as inhibitors of the water soluble system of histone phosphorylation by protein kinase.     

Dephosphorylation of Synaptosomal Proteins P96 and P139 Is Regulated by Both Depolarization and Calcium, but Not by a Rise in Cytosolic Calcium Alone

Depolarization of intact synaptosomes activates calcium channels, leads to an influx of calcium, and increases the phosphorylation of several neuronal proteins. In contrast, there are two synaptosomal phosphoproteins labeled in intact synaptosomes with 32Pi, termed P96 and P139, which appear to be dephosphorylated following depolarization. Within intact synaptosomes P96 was found in the cytosol whereas P139 was present largely in membrane fractions. Depolarization-stimulated dephosphorylation was fully reversible and continued for up to five cycles of depolarization/repolarization, suggesting a physiological role for the phenomenon. The basal phosphorylation of these proteins was at least partly regulated by cyclic AMP, since dibutyryl cyclic AMP produced small but significant increases in P96 and P139 labeling, even in the presence of fluphenazine at concentrations that inhibited calcium-stimulated protein kinases. Depolarization-dependent dephosphorylation was independent of a rise in intracellular calcium, since agents such as guanidine and low concentrations of A23187, which increase intracellular calcium without activating the calcium channel, did not initiate P96 or P139 dephosphorylation. These agents did sustain increases in the phosphorylation of a number of other proteins including synapsin I and protein III. The results suggest that the phosphorylation of these two synaptosomal proteins is intimately linked to the membrane potential and that their dephosphorylation is dependent on both the mechanism of calcium entry and calcium itself, rather than simply on a rise in intracellular free calcium.     

ACTH and brain membrane phosphorylation: a model for modulation by neuropeptides

It has been hypothesized that changes in the phosphorylation of synaptic membrane constituents (proteins and lipids) may affect transmission in certain types of synapses. In this paper some of the recent evidence that neuropeptides like ACTH may bring about their behavioral activity by influencing brain protein and lipid phosphorylation is reviewed. An ACTH-sensitive, cAMP-independent protein kinase was isolated from rat brain synaptosomal plasma membranes. This enzyme was partially characterized and it was observed that its activity greatly depended on the presence of calcium ions. One of its substrate proteins B-50 (MW 48,000; IEP 4.5) may play a key role in the turnover of a special class of membrane phospholipids i.e. the (poly)phosphoinositides. Evidence was obtained to suggest that the degree of phosphorylation of the B-50 protein determines the conversion of diphosphoinositol to triphosphoinositol. A model which links the protein phosphorylation to lipid phosphorylation and which points to a functional role for peptides in the regulation of the permeability of brain membranes for calcium ions will be discussed. As the structure-activity relationship for the peptide effects on grooming behavior closely resembles that on phosphorylation, it is assumed that this neurochemical event may indeed be of relevance to the biological activity of the peptide. As the ion permeability may be altered by the peptide it can be suggested that this may lead to modulation of transynaptic information processing in the brain.     

Taurine Inhibits Protein Kinase C-Catalyzed Phosphorylation of Specific Proteins in a Rat Cortical P2 Fraction

Abstract: We previously reported that taurine inhibits the phosphorylation of specific proteins in a P₂ synaptosomal fraction prepared from the rat cortex. In the present study, the regulation of the phosphorylation of an ～20K M_r protein whose phosphorylation is inhibited by taurine was further investigated. The phosphorylation of the ～20K M_r protein in a hypo-osmotically shocked P₂ fraction from rat cortex was dependent on the free Ca²⁺ in the reaction medium. Depolarization induced by 30 mM K⁺ stimulated the phosphorylation of the ～20K M_r protein in an intact synaptosomal P₂ preparation by 30-fold. This stimulation was inhibited 35% by taurine, whereas guanidinoethanesulfonic acid, a taurine analogue, did not have any effect, thereby indicating the specificity of taurine. Addition of phorbol 12-myristate 13-acetate, a phorbol ester, together with phosphatidylserine, stimulated the phosphorylation of the ～20K M_r protein in the hypo-osmotically shocked P₂ synaptosomal fraction by fivefold, whereas cyclic AMP, cyclic GMP, and calmodulin did not have any effect on the phosphorylation of this particular protein. Phorbol 12-myristate 13-acetate–stimulated phosphorylation of the ～20K M_r protein is blocked 30% by taurine. Taurine also inhibited phorbol 12-myristate 13-acetate-activated phosphorylation of two other proteins that were similar in molecular weight and isoelectric point to the ～20K M_r protein on two-dimensional gels. These results suggest that taurine modulates the phosphorylation of specific proteins regulated by the signal transduction system in the brain. Thus, taurine may modulate neuroactivity by inhibiting the phosphorylation of specific proteins involved in regulatory function.     

Inhibition of Ca2+-transport ATPase from synaptosomal vesicles by flavonoids

The inhibitory action of the flavonoid quercetin has been examined on the calcium-transport ATPase of synaptosomal vesicles and compared to that of two other flavonoids, morin and rutin. We have found that while quercetin caused a 50% inhibition of calcium transport at a concentration of 15 μM, morin and rutin had similar effects at concentrations of about 200 μM. A similar order of potency was observed also for ATP hydrolysis, though at higher concentrations. Quercetin also strongly inhibited phosphorylation of membrane proteins by ATP in synaptosomal vesicles. Rutin and morin had an almost negligible effect on membrane protein phosphorylation. The order of inhibitory potency of the flavonoids on the Ca²⁺-transport ATPase from synaptosomal vesicles: quercetin > morin > rutin, could be linked to their possible solubility in the membrane lipid phase since: (1) it paralleled their partitioning between a mixture of oil and water; (2) it paralleled their uptake from the reaction mixture by synaptosomal vesicles and phosphatidylcholine liposomes; (3) they had almost equal potency as inhibitors of the water soluble system of histone phosphorylation by protein kinase.     

Effect of Oral Administration of Tri-o-cresyl Phosphate on In Vitro Phosphorylation of Membrane and Cytosolic Proteins from Chicken Brain

Abstract: The effects of a single oral dose of 750 mg/kg tri- o -cresyl phosphate (TOCP) on the endogenous phosphorylation of specific brain proteins were assessed in male adult chickens following the development of delayed neurotoxicity. Phosphorylation of crude synaptosomal (P₂) membrane and synaptosomal cytosolic proteins was assayed in vitro by using [γ-³²P]ATP as phosphate donor. Following resolution of brain proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis, specific protein phosphorylation was detected by autoradiography and quantified by microdensitometry. TOCP administration enhanced the phosphorylation of both cytosolic (M_r 65,000 and 55,000) and membrane (20,000) proteins by as much as 146% and 200%, respectively.     

Depolarisation-Dependent Protein Phosphorylation and Dephosphorylation in Rat Cortical Synaptosomes Is Modulated by Calcium

The effect of calcium on protein phosphorylation was investigated using intact synaptosomes isolated from rat cerebral cortex and prelabelled with 32Pi. For nondepolarised synaptosomes a group of calcium-sensitive phosphoproteins were maximally labelled in the presence of 0.1 mM calcium. The phosphorylation of these proteins was slightly decreased in the presence of strontium and absent in the presence of barium, consistent with the decreased ability of these cations to activate calcium-stimulated protein kinases. Addition of calcium alone to synaptosomes prelabelled in its absence increased phosphorylation of a number of proteins. On depolarisation in the presence of calcium certain of the calcium-sensitive phosphoproteins were further increased in labelling above nondepolarised levels. These increases were maximal and most sustained after prelabelling at 0.1 mM calcium. On prolonged depolarisation at this calcium concentration a slow decrease in labelling was observed for most phosphoproteins, whereas a greater rate and extent of decrease occurred at higher calcium concentrations. At 2.5 mM calcium a rapid and then a subsequent slow dephosphorylation was observed, indicating two distinct phases of dephosphorylation. Of all the phosphoproteins normally stimulated by depolarisation, only phosphoprotein 59 did not exhibit the rapid phase of dephosphorylation at high calcium concentrations. Replacing calcium with strontium markedly decreased the extent of change observed on depolarisation whereas barium decreased phosphorylation changes even further. Taken together these data suggest that an influx of calcium into synaptosomes initially activates protein phosphorylation, but as the levels of intrasynaptosomal calcium rise protein dephosphorylation predominates. Other phosphoproteins were dephosphorylated immediately on depolarisation in the presence of calcium. The fine control of protein phosphorylation levels exerted by calcium supports the idea that the synaptosomal phosphoproteins could play a role in modulating events such as neurotransmitter release in the nerve terminal.     

4-Aminopyridine inhibits synaptosomal plasma membrane protein phosphorylation in vitro: effect of the selective NMDA-antagonist 2-amino-5-phosphonovalerate

Phosphorylation of synaptosomal plasma membranes from rat hippocampus in the presence of the convulsant drug 4-aminopyridine resulted in the inhibition of the phosphorylation of the nervous tissue specific protein kinase C substrate protein B-50 (48 kDa) and the alpha-subunit of calcium/calmodulin-dependent protein kinase II (50 kDa). Preincubation of SPM with 2-amino-5-phosphonovalerate prevents the inhibition of B-50 phosphorylation by 4-aminopyridine, but had no effect on the inhibition of 50 kDa phosphorylation. 2-Amino-5-phosphonovalerate is known to be a specific N-methyl-D-aspartate antagonist and has anti-epileptic activity in vitro and in vivo. Several other anti-epileptic drugs tested did not influence the 4-aminopyridine-induced inhibition of protein phosphorylation.     

Non-genomic effect of L-triiodothyronine on calmodulin-dependent synaptosomal protein phosphorylation in adult rat cerebral cortex

The present study was undertaken to examine calmodulin-dependent effect of thyroid hormones (THs) on synaptosomal protein phosphorylation in mature rat brain. Effect of L-triiodothyronine (L-T3) on in vitro protein phosphorylation was measured in a hypotonic lysate of synaptosomes prepared from adult male rat cerebral cortex, incubated in presence and absence of calcium ion (Ca2+) and calmodulin. L-T3 significantly enhanced incorporation of 32P into synaptosomal proteins as compared to basal level of phosphorylation in the presence of Ca2+ and calmodulin. Under these conditions, increase in protein phosphorylation was 47, 74 and 52% for 10 nM, 100 nM and 1 microM L-T3, respectively. Chelation of Ca2+ using ethylene glycol-bis (2-aminoethylether)-N, N, N', N'-tetraacetic acid (EGTA) inhibited the effects of Ca2+/calmodulin on TH-stimulated protein phosphorylation levels. This study suggests that a high proportion of L-T3-stimulated protein phosphorylation involves Ca2+/calmodulin-dependent pathways in adult rat cerebrocortical synaptosomes.     

Phosphorylation in skeletal myosin light chain modulates the actin--myosin interaction in the presence of regulatory proteins in vitro

The effect of phosphorylation in skeletal myosin light chain (LC2) on the actomyosin and acto-heavymeromyosin (HMM) ATPase activities was investigated in the presence or absence of regulatory proteins (tropomyosin-troponin complex). Phosphorylation in LC2 did not modulate the actin-myosin and actin-HMM interactions over a wide range of KCl concentrations from 30 to 150 mM without regulatory proteins. In the presence of regulatory proteins, phosphorylation in myosin LC2 enhanced the ATPase activity of actomyosin with calcium ions, but the removal of calcium ions made little difference in the ATPase activity between phosphorylated and dephosphorylated myosins. Ca2+-sensitivity of the regulated actomyosin was slightly changed by phosphorylation in myosin LC2. However, both the ATPase activity and Ca2+-sensitivity of the regulated acto-HMM were unaffected by phosphorylation in HMM LC2.     

The Effect of Neurocatin on Protein Phosphorylation in Striatal Synaptosomes from Rat Brain

Abstract: Neurocatin, a neuroregulatory factor isolated from mammalian brain, is a powerful affector of protein phosphorylation in rat striatal synaptosomes. Two major synaptosomal phosphoproteins of ～80 and ～60 kDa, possibly synapsin I and tyrosine hydroxylase, were especially sensitive to neurocatin. Immunoprecipitation experiments confirmed that the 60-kDa protein is the enzyme tyrosine hydroxylase. At low concentrations of neurocatin (to ～7.5 ng/100 μl of suspension), incorporation of ³²P orthophosphate into these proteins increased with increasing neurocatin concentration. At 7.5 ng of neurocatin, incorporation of the label into the two proteins increased by 22 and 26%, respectively. Concentrations of neurocatin >7.5 ng/100 μl caused progressive decrease in incorporation of ³²P into many synaptosomal proteins; by a concentration of neurocatin of ～45 ng/100 μ/l, the level of ³²P incorporation into many proteins was ≤70% of control. The effects of neurocatin on synaptosomal protein phosphorylation were also dependent on the time of incubation. At a constant concentration of ～7.5 ng/100 μl of neurocatin, increased incorporation of 32P into many proteins was measurable within 0.5 min and was maximal by 1 min. Incubation times >2.0 min, showed progressive decrease in ³²P incorporation. Removing extrasynaptosomal Ca²⁺ with EGTA attenuated the increased ³²P incorporation induced by low neurocatin concentrations, suggesting that calcium plays a role in neurocatin-induced phosphorylation of rat striatal synaptosomal proteins. The reduced incorporation of label induced by high neurocatin concentrations, however, was not calcium dependent. The effects of neurocatin on the level of ³²P incorporation into proteins were observed only in intact synaptosomes, consistent with this compound acting through receptors on the plasma membrane.     

Calcium dependent neurotransmitter release and protein phosphorylation in synaptic vesicles.

A highly purified preparation of synaptic vesicles was prepared to study the relationship between calcium-dependent neurotransmitter release and protein phosphorylation. Calcium ions simultaneously produced significant increases in both the endogenous release of norepinephrine from the synaptic vesicles and the endogenous incorporation of [³²p] phosphate into specific synaptic vesicle proteins. The results are compatible with the hypothesis that the action of calcium on the phosphorylation of specific synaptic vesicle proteins is the molecular mechanism mediating some of the effects of calcium on neurotransmitter release and synaptic vesicle function.     

Endorphin-regulated protein phosphorylation in brain membranes

This study was initiated to determine whether opioid peptides exert direct effects on the phosphorylation of specific proteins in membranes from rat neostriatum. It was found that low concentrations of β-endorphin (0.1–10nM) inhibit the phosphorylation of specific proteins designated F and H (M.W. 47,000 and 10–20,000 respectively). In addition, β-endorphin produced an overall stimulation of phosphate incorporation into other membrane proteins, the phosphorylation of which is dependent on calcium ions. The stimulatory effects were blocked by naloxone, but the inhibitory effects were not. The regulation of membrane protein-phosphorylation by endorphins may constitute a biochemical mechanism mediating for some of the physiological affects of these peptides on neuronal function.     

Dephosphin, a 96,000 Da substrate of protein kinase C in synaptosomal cytosol, is phosphorylated in intact synaptosomes

A 96,000 dalton phosphoprotein, called dephosphin, is phosphorylated in intact synaptosomes from rat brain and is rapidly dephosphorylated upon depolarisation-dependent calcium entry. A 96,000 dalton phosphoprotein is also a substrate of protein kinase C in synaptosomal cytosol, and the aim of the study was to determine whether the two proteins may be the same. Dephosphin in intact synaptosomes and the 96,000 dalton protein kinase C substrate comigrated on polyacrylamide gels. Both phosphoproteins had identical phosphopeptide maps after digestion with V8 protease. Both phosphoproteins ran on isoelectric focussing gels with a pI of 6.3-6.7 and focussed as a series of 5-6 spots. Both proteins were phosphorylated exclusively on serine. Both proteins could be resolved into a doublet on longer polyacrylamide gels. The two subunits were of 96 and 93 kDa in both phosphorylation conditions and had dissimilar phosphopeptide maps. However, phosphopeptide maps of either the 96 or 93 kDa subunits were identical in intact synaptosomes compared with synaptosomal cytosol. These results show that a phosphoprotein phosphorylated in intact synaptosomes and a 96,000 dalton protein kinase C substrate from rat brain synaptosomal cytosol are the same, and raise the possibility that protein kinase C is the protein kinase responsible for dephosphin phosphorylation in intact synaptosomes.     

Depolarisation-Dependent Protein Phosphorylation in Rat Cortical Synaptosomes Is Inhibited by Fluphenazine at a Step After Calcium Entry

Abstract: The sequence of molecular events linking depolarisation-dependent calcium influx to calcium-stimulated protein phosphorylation is unknown. In this study the effect of the neuroleptic drug fluphenazine on depolarisation-dependent protein phosphorylation was investigated using an intact postmitochondrial pellet isolated from rat cerebral cortex. Fluphenazine, in a dose-dependent manner, completely inhibited the increases in protein phosphorylation observed previously. The concentration of fluphenazine required for 50% inhibition varied for different phosphoproteins but for synapsin I was 123 μ M. Other neuroleptics produced effects similar to fluphenazine with their order of potency being thioridazine > haloperidol > trifluoperazine > fluphenazine > chlorpromazine. Fluphenazine also increased the phosphorylation of proteins in nondepolarised controls at concentrations of 20 and 60 μ M. The inhibition of depolarisation-dependent phosphorylation was apparently not due to a loss of synaptosomal integrity or viability, a decrease in calcium uptake, a change in substrate availability, or to a change in protein phosphatase activity. The data are most consistent with an inhibition of protein kinase activity by blockade of calmodulin or phospholipid activation.     

Introduction of Impermeant Molecules into Synaptosomes Using Freeze/Thaw Permeabilization

Brief freezing as a means of transiently permeabilizing synaptosomes was explored. Rat brain synaptosomes frozen and thawed in the presence of 5% dimethyl sulfoxide, a cryoprotectant, were shown to release, in a calcium-dependent manner, previously accumulated [3H]norepinephrine and [14C]acetylcholine in response to elevated [K+]. In addition, synaptosomes subjected to freeze/thaw were shown to retain their ability to exhibit resting protein phosphorylation, as well as stimulated protein phosphorylation occurring in response to calcium influx. Brief freezing of synaptosomes in the presence of [gamma-32P]ATP and either the catalytic subunit of cyclic AMP-dependent protein kinase or calcium/calmodulin-dependent protein kinase II rendered the synaptosomal interior accessible to these agents, as reflected by the phosphorylation of substrate proteins, such as synapsin I, which reside within the nerve terminal. Inclusion of inhibitors of these protein kinases during freeze/thaw blocked synaptosomal protein phosphorylation, indicating that the inhibitors were also introduced. After freezing, the synaptosomes resealed rapidly and spontaneously, as shown by the inability of any of the agents to elicit an effect on phosphorylation when added at the end of the freezing period. The permeabilization procedure should contribute to an understanding of the functional roles of phosphoproteins, and of their associated protein kinases and protein phosphatases, in nerve terminals.     

Clearer demonstration of calcium/calmodulin-dependent events in synaptosomes by use of the differential effects of two calmodulin antagonists, N-(aminohexyl)-5-chloro-1-naphthalenesulfonamide and N-(6-aminohexyl)-1-naphthalenesulfonamide

1. In order to demonstrate more clearly calcium/calmodulin-dependent events, the differential effects of two calmodulin antagonists, W-7 and W-5, on synapsin I phosphorylation and norepinephrine release associated with calcium influx, were investigated using 32Pi in synaptosomes derived from rat cerebral cortex. 2. The calcium ionophore (A23187)-stimulatory effect on synapsin I phosphorylation and norepinephrine release was markedly reduced by W-7 and slightly reduced by W-5; whereas neither the strong nor the weak calmodulin antagonist had an effect on A23187-stimulated synaptosomal uptake of calcium. 3. Preincubation with H-8 reduced both W-5- and W-7-inhibited A23187-stimulated synapsin I phosphorylation by the same amount but did not affect their inhibitory effect nor the ionophore-stimulated norepinephrine release, thereby suggesting that W-5 may serve as an appropriate control for non-calmodulin-mediated effect of both calmodulin antagonists.