Dopamine-induced changes in protein phosphorylation and polyphosphoinositide metabolism in rat hippocampus

Time course and involvement of protein kinase C-mediated phosphorylation of F1/GAP-43 in area CA3 after mossy fiber stimulation

1. Protein kinase C (PKC) activity and phosphorylation of F1/growth associated protein (GAP)-43, a PKC substrate, have been proposed to play key roles in the maintenance of long-term potentiation (LTP) at the synapses of Schaffer collateral/commissural on pyramidal neurons in CA1 (Akers et al., 1986). We have studied in the involvement of PKC and PKC-dependent protein phosphorylation of F1/GAP-3 in in vitro LTP observed at the synapses of mossy fiber (MF) on CA3 pyramidal neurons of rat hippocampus by post hoc in vitro phosphorylation. 2. After LTP was induced in CA3 in either the presence or absence of D-2-amino-5-phosphonovaleric acid (AP5), an NMDA receptor antagonist, the CA3 region was dissected for in vitro phosphorylation assay. In vivo phosphorylation of F1/GAP-43 was increased in membranes at 1 and 5 min after tetanic stimulation (TS) but not at 60 min after TS. 3. The degree of phosphorylation of F1/GAP-43 in the cytosol was inversely related to that in membranes at each time point after LTP. 4. The similar biochemical changes obtained from either control slices or AP5-treated slices indicate that LTP and the underlying biochemical changes are independent of the NMDA receptor. Immunoreactivity of the phosphorylated F1/GAP-43 in LTP slices was not significantly different from control, indicating that results from western blotting and post hoc in vitro phosphorylation are consistent. 5. Post hoc in vitro phosphorylation of F1/GAP-43 was PKC-mediated since phosphorylation of F1/GAP-43 was altered by the PKC activation cofactors, Ca2+, phosphatidylserine and phorbol ester. 6. Calmodulin (CaM) at > 5 microM inhibited phosphorylation, consistent with the presence of CaM-binding activity at the site on F1/GAP-43 acted upon by PKC. 7. We conclude that phosphorylation of F1/GAP-43 is associated with the induction but not the maintenance phase of MF-CA3 LTP.

Phorbol ester-induced changes in rat hippocampal glycoprotein fucosylation

Studies on glycoprotein fucosylation were carried out using hippocampal slices from rat brain. These slices were incubated in the presence of the protein kinase C (PKC) activating phorbol ester, 4 beta-phorbol-12,13-dibutyrate (PDBu), or an inactive isoform, 4 alpha-phorbol-12,13-didecanoate (PDD), respectively, for 7 min followed by a 60 min pulse of [3H]fucose. PDBu caused an increase in [3H]fucose incorporation into glycoproteins by 29% as well as an activation of the fucokinase enzyme reaction by 21%. The PDBu-induced stimulation of [3H]fucose insertion into hippocampal glycoproteins was abolished by the PKC inhibitors, staurosporine and H7. The importance of a PKC-regulated glycoprotein fucosylation in mechanisms underlying changes in neuronal plasticity is discussed.

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

Dopamine D2 receptors attenuate phosphatidylinositol 4,5-bisphosphate in synaptosomal membranes from rat neostriatum

The effects of dopamine on [32P]ATP-labelled phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate, and phosphatidic acid were analyzed by TLC in synaptosomal membranes of the rat neostriatum. The incorporation of 32P into these compounds was found to be stable within 1 min and was maintained during the 30 min of incubation. Dopamine (0.1-10 microM) was found to attenuate the levels of phosphatidylinositol 4,5-bisphosphate without affecting the levels of phosphatidylinositol 4-phosphate or phosphatidic acid. The maximal decrease (-35 +/- 4%) was reached at 10 microM of dopamine after 30 min of incubation. The dopamine (0.1 microM)-induced decrease was blocked by the D2 selective antagonist raclopride (1 microM), but not by the D1 selective antagonist SCH 23390 (1 microM). These findings indicate the existence of an intramembrane coupling of dopamine D2 receptors to phosphoinositide turnover and may underlie some of the physiological effects of D2 receptor stimulation.

A microassay for measuring synaptosomal 3H-dopamine and 3H-metabolite release

A modified synaptosomal superfusion apparatus is described which uses less than 10 micrograms of tissue per replicate sample and facilitates the routine separation of 3H-DA, 3H-DOPAC, and 3H-HVA. A flow rate of 1.5 ml/min allows superfusion without the use of reuptake or monoamine oxidase inhibitors. Superfusate samples are collected directly onto alumina columns for the separation of 3H-DA and its acid metabolites. Total recovery of authentic 3H-DA applied via superfusion was 87.63(1.10) percent [Mean(SEM)]. Contamination of the acetic acid eluate fraction, containing 80.98(1.15)% of the total DA, by DOPAC and HVA was less than 0.1%. To illustrate the utility of this technique, the relative proportions of 3H-DA and 3H-metabolites released from synaptosomes by 6 mM potassium and 1 microM reserpine were compared.

Acute and subchronic corticosterone treatment differentially modulates subcortical limbic and neostriatal nicotinic cholinergic receptors

The effects of acute and subchronic corticosterone treatment were analyzed on the competition by (-)nicotine ((-)nicotine hydrogen(+)-tartrate) on N-[3H]methylcarbamyl choline iodide ([ 3H]MCC) binding sites in membranes from the subcortical limbic forebrain and the neostriatum. Acute treatment with corticosterone (5 mg/kg, i.p., 2 h) increased the IC50 values of (-)nicotine by 230% in the subcortical limbic areas but not in the neostriatum. Subchronic corticosterone treatment (5 mg/kg, twice a day, 7 days) increased the IC50 values of (-)nicotine by 50% and slightly decreased the specific binding of [3H]MCC (5 nM) in the subcortical limbic area. In the neostriatum, subchronic treatment with corticosterone instead decreased by 50% the IC50 values of (-)nicotine and slightly increased the specific binding of [3H]MCC. The results indicate that corticosterone treatment selectively reduces the affinity of nicotinic cholinergic receptors within the subcortical limbic forebrain.

Determination of changes in the phosphorylation state of the neuron-specific protein kinase C substrate B-50 (GAP43) by quantitative immunoprecipitation

To determine changes in the degree of phosphorylation of the protein kinase C substrate B-50 in vivo, a quantitative immunoprecipitation assay for B-50 (GAP43, F1, pp46) was developed. B-50 was phosphorylated in intact hippocampal slices with 32Pi or in synaptosomal plasma membranes with [gamma-32P]ATP. Phosphorylated B-50 was immunoprecipitated from slice homogenates or synaptosomal plasma membranes using polyclonal anti-B-50 antiserum. Proteins in the immunoprecipitate were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the incorporation of 32P into B-50 was quantified by densitometric scanning of the autoradiogram. Only a single 48-kilodalton phosphoband was detectable in the immunoprecipitate, but this band was absent when preimmune serum was used. The B-50 immunoprecipitation assay was quantitative under the following condition chosen, as (1) recovery of purified 32P-labelled B-50 added to slice homogenates or synaptosomal plasma membranes was greater than 95%; and (2) modulation of B-50 phosphorylation in synaptosomal plasma membranes with adrenocorticotrophic hormone, polymyxin B, or purified protein kinase C in the presence of phorbol diester resulted in EC50 values identical to those obtained without immunoprecipitation. With this immunoprecipitation assay we found that treatment of hippocampal slices with 4 beta-phorbol 12,13-dibutyrate stimulated B-50 phosphorylation, whereas 4 alpha-phorbol 12,13-didecanoate was inactive. Thus, we conclude that the B-50 immunoprecipitation assay is suitable to monitor changes in B-50 phosphorylation in intact neuronal tissue.

Modulation of B-50 phosphorylation and polyphosphoinositide metabolism in synaptic plasma membranes by protein kinase C, phorbol diesters and ACTH

One of the major phosphoproteins in synaptic plasma membranes (SPM) is the neuron-specific protein B-50 (Mr 48 kDa, IEP 4.5). Addition of purified protein kinase C (PKC) to native SPM increases B-50 phosphorylation. Exogenous PKC also phosphorylates B-50 in heat-inactivated SPM. Endogenous phosphorylation of B-50 in SPM is enhanced in a concentration-dependent manner by the tumor-promoting phorbol diesters 4 beta-phorbol 12-myristate, 13-acetate, 4 beta-phorbol 12,13-dibutyrate (PDB) and 4 beta-phorbol 12,13-diacetate, with an EC50 of 7 x 10(-8) M, 3 x 10(-7) M and 10(-6) M, respectively. This increase in the B-50 phosphorylation can be inhibited by ACTH1-24. PDB (10(-6) M) also stimulates B-50 phosphorylation by exogenous PKC in native and heat-inactivated SPM (204 and 712%, respectively). The increase in B-50 phosphorylation induced by the addition of PKC to SPM is accompanied by a decrease in the [32P]-incorporation into phosphatidylinositol 4,5-bisphosphate (PIP2). These data support the hypothesis that in neuronal membranes the degree of B-50 phosphorylation exerts a negative control on receptor-mediated hydrolysis of PIP2 in receptor systems coupled to phospholipase C.

Activation of dopamine receptors does not affect phosphoinositide turnover in NCB-20 cells

Dopamine inhibits and serotonin stimulates adenylate cyclase activity in a neuroblastoma X Chinese hamster brain explant cell line (NCB-20). The inhibition of cyclic AMP accumulation by dopamine was blocked by pretreatment of the cells with pertussis toxin. Carbachol and bradykinin stimulated the accumulation of water-soluble inositol phosphates whereas thyrotropin-releasing hormone, vasopressin, neurotensin, and phenylephrine were without effect. Dopamine and serotonin had no significant effect on carbachol-induced phosphoinositide hydrolysis or the levels of the parent lipids within the membrane. Forskolin induced a much larger stimulation of cyclic AMP than did serotonin, and caused an increase in the levels of phosphatidylinositol-4-phosphate and phosphatidyl inositol-4,5-bisphosphate in the cell membrane.

Incorporation of [3H]fucose in rat hippocampal structures after conditioning by perforant path stimulation and after LTP-producing tetanization

The contribution of glycoprotein synthesis to functional synaptic changes and to the formation of memory traces was investigated by autoradiographic determination of the incorporation of [3H]fucose into the hippocampal structures of rats. In the first experiment, the fucose incorporation was measured after induction of post-tetanic long-term potentiation (LTP) in granular cell synapses by repeated tetanization (200 cps) of the perforant path, and after stimulation of this hippocampal input by the same number of impulses with very low frequency (0.2 cps) not producing LTP. In the second experiment, the incorporation of fucose was determined after an active avoidance training using the stimulation of the perforant path by impulse trains of 15 cps as conditioning stimuli, and after a session of corresponding unpaired stimulations of the perforant path. Unstimulated animals were used in both experiments to measure the basal glycosylation. LTP-producing tetanization resulted only in a slight increase of incorporation into the ipsilateral hippocampal structures without significant differences to similar changes after the corresponding control stimulation with single impulses. After a session of unpaired stimulation of the perforant path with impulse trains of 15 cps only slight and inconsistent changes of incorporation occurred in the hippocampus too. However, after conditioning by the corresponding perforant path stimulation as conditioned stimulus, considerable increases of incorporation were observed in all structures of the ipsilateral hippocampus, when compared to the unpaired control stimulation. An enhanced labeling occurred also in some structures of the contralateral hippocampus mainly receiving commissural inputs. The results suggest again, that the activation of one single hippocampal afferent, even if producing LTP, would not be sufficient to induce an increased glycosylation of neuronal proteins. The increase of glycoprotein formation seems to require the convergence of several inputs, which can be assumed to occur during learning. Therefore, LTP of a single synaptic population seems not to represent the complete long-lasting memory trace, but only one of its components, or a preceding transient storage mechanism.

Ventral tegmental (A10) system: neurobiology. 1. Anatomy and connectivity

The VTA contains the A10 group of DA containing neurons. These neurons have been grouped into nuclei to be found on the floor of the midbrain tegmentum--Npn, Nif, Npbp and Nln rostralis and caudalis. The VTA is traversed by many blood vessels and nerve fibers. Close to its poorly defined borders are found DA (A8, A9, A11) and 5-HT containing neurons (B8). Efferent projections of the VTA can be divided into 5 subsystems. The mesorhombencephalic projects to other monoaminergic nuclei, the cerebellum and a fine projection descends to other tegmental nuclei as far as the inferior olive. Fibers to the spinal cord have not been demonstrated. The mesodiencephalic path projects to several thalamic and hypothalamic nuclei and possibly the median eminence. Functionally important examples are the anterior hypothalamic-preoptic area, N. medialis dorsalis and reuniens thalami. These two subsystems are largely non-dopaminergic. A minor mesostriatal projection is overshadowed by the large mesolimbic projection to the accumbens, tuberculum olfactorium, septum lateralis and n. interstitialis stria terminalis. There are also mesolimbic connections with several amygdaloid nuclei (especially centralis and basolateralis), the olfactory nuclei and entorhinal cortex. A minor projection to the hippocampus has been detected. The mesocortical pathway projects to sensory (e.g. visual), motor, limbic (e.g. retrosplenial) and polysensory association cortices (e.g. prefrontal). Prefrontal, orbitofrontal (insular) and cingulate cortices receive the most marked innervation from the VTA. A more widespread presence of DA in other cortices of rodents becomes progressively more evident in carnivores and primates. Most but not all projections are unilateral. Some neurons project to more than one area in mesodiencephalic, limbic and cortical systems. The majority of these fibers ascend in the MFB. Most areas receiving a projection from the VTA (DA or non-DA) project back to the VTA. The septohippocampal complex in particular and the limbic system in general provide quantitatively much less feedback than other areas. The role of the VTA as a mediator of dialogue with the frontostriatal and limbic/extrapyramidal system is discussed under the theme of circuit systems. The large convergence of afferents to certain VTA projection areas (prefrontal, entorhinal cortices, lateral septum, central amygdala, habenula and accumbens) is discussed under the theme of convergence systems.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Impairment of glycoprotein fucosylation in rat hippocampus and the consequences on memory formation

The intraventricular injection of 2-deoxy-D-galactose led to a dose- and time-dependent decrease in the fucosylation of hippocampal glycoproteins in rats whereas the incorporation of 3H-N-acetyl-glucosamine was not influenced. This effect is not related to an interference with fucose activating or transferring enzymes but can be abolished by an application of D-galactose. Thus, it seems likely that also in brain tissue a deoxy-galactose induced decrease in the fucosylation is due to a hindering of a glycosidic linkage of fucose to the deoxy-sugar incorporated into glycoprotein chains. As a consequence of an intrahippocampal injection of the deoxy-sugar the retention performance of the animals in a foot-shock motivated brightness discrimination task was considerably impaired. But deoxy-galactose is effective only when administered before and immediately after training whereas either a pre- or a post-training injection did not influence the retention performance of the rats. Thus, an effective metabolic inhibition of the glycoprotein completion by the deoxy-sugar starting at the time of training seems to be crucial to interfere with such morphofunctional alterations in the neuronal network underlying the formation of a memory trace.

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.

Comparison of the immunocytochemical distribution of the phosphoprotein B-50 in the cerebellum and hippocampus of immature and adult rat brain

In this study we compare the distribution of the phosphoprotein B-50 in two regions of immature and adult rat brain using affinity-purified antibodies to B-50. In the cerebellum of the 8-day-old rat we observed distinct patterns of distribution of B-50 immunoreactivity (BIR) in the premigratory zone and the developing molecular layer, likely associated with outgrowing parallel and climbing fibers contacting Purkinje cells in the internal granular layer and in axons coursing through the cerebellar medulla. In contrast, in adult cerebellum, a sparcer distribution of BIR as punctuate deposits is observed in the molecular layer, outlining dendritic trees and the perikarya of neurons. At relatively lower density BIR is found dispersed between the cells of the granular layer and along fibers in the white matter. In the immature hippocampal formation, fibers penetrating between unstained cells of the stratum pyramidale and the subiculum, and neuropil areas are immunostained. In the adult rat a graded immunostaining pattern corresponding to the laminar structure of the hippocampal formation is found with high density of BIR in the strata oriens, radiatum, parts of stratum lacunosum molecular and in the stratum molecular adjoining the field of the proximal apical dendrites of the granule cells. BIR appears to be absent from the proximal part of the mossy fiber pathway. In neuropil areas of adult hippocampus and cerebellum BIR is fairly restricted to dot-like deposits indicating a synaptic localization. This is in correspondence with our previous ultrastructural findings. The present observations in developing brain of B-50-like components in fibers, as well, suggest that B-50 (and/or B-50-like precursors) are involved in neurite outgrowth.

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.

Modulation of the activity of purified phosphatidylinositol 4-phosphate kinase by phosphorylated and dephosphorylated B-50 protein

To investigate the modulation of phosphatidylinositol 4-phosphate kinase activity by the degree of phosphorylation of the B-50 protein, the enzyme was purified from rat brain cytosol by ammonium sulphate precipitation and DEAE-cellulose column chromatography. Purified rat brain B-50 was phosphorylated with protein kinase C and dephosphorylated with alkaline phosphatase. Incubation of the semi-purified phosphatidylinositol 4-phosphate kinase with 1 microgram of the B-50 preparation enriched in the dephospho-form, resulted in a small reduction of phosphatidylinositol 4-phosphate kinase activity (-16%), whereas incubation with the phospho B-50 preparation inhibited the enzyme activity by 40%. The effect of exogenous B-50 was studied in the presence of 10 micrograms albumin to minimize aspecific protein-protein interactions. The present data on the effect of exogenous B-50 protein on phosphatidylinositol 4-phosphate kinase activity, further support our hypothesis that the phosphorylation state of B-50 may be a regulatory factor in phosphoinositide metabolism in rat brain.

Purification and partial characterization of the phosphatidylinositol 4-phosphate kinase from rat brain

Phosphatidylinositol 4-phosphate (PtdIns4P) kinase was purified from cytosolic and particulate material of rat brain. The purification procedure of the enzyme from cytosol consisted of (NH4)2SO4 precipitation. DEAE-cellulose column chromatography and preparative isoelectric focusing. Other methods after DEAE-cellulose column chromatography failed to achieve further purification of the PtdIns4P kinase, probably caused by the tendency of the enzyme to aggregate with contaminating proteins. The final purification was 67-fold, and the recovery was 0.6%. After isoelectric focusing the fraction containing the highest PtdIns4P kinase activity showed only one protein as visualized by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and silver staining. The apparent Mr of this protein was 45 kDa and the isoelectric point about 5.8. The activity of PtdIns4P kinase was dependent on the concentration of divalent cations in the incubation medium. PtdIns4P kinase activity was found to be optimal at 10-30 mM-Mg2+. In an attempt to compare the cytosolic with the membrane-derived kinase activity, a Triton/KCl extract from synaptic membranes was subjected to the same purification procedure as the cytosolic enzyme. A difference in isoelectric focusing was observed, possibly due to a higher tendency to form aggregates. However, we tend to conclude that also in the membranes the PtdIns4P kinase activity is present as a 45 kDa protein, identical with that found in the cytosol.