Depolarization-dependent protein phosphorylation in synaptosomes: mechanisms and significance

A rapid Percoll gradient procedure for preparation of synaptosomes

Homogenization of fresh brain tissue in isotonic medium shears plasma membranes causing nerve terminals to become separated from their axons and postsynaptic connections. The nerve terminal membranes then reseal to form synaptosomes. The discontinuous Percoll gradient procedure described here is designed to isolate synaptosomes from brain homogenates in the minimum time to allow functional experiments to be performed. Synaptosomes are isolated using a medium-speed centrifuge, while maintaining isotonic conditions and minimizing mechanically damaging resuspension steps. This protocol has advantages over other procedures in terms of speed and by producing relatively homogeneous synaptosomes, minimizing the presence of synaptic and glial plasma membranes and extrasynaptosomal mitochondria. The purified synaptosomes are viable and take up and release neurotransmitters very efficiently. A typical yield of synaptosomes is between 2.5 and 4 mg of synaptosomal protein per gram rat brain. The procedure takes approximately 1 h from homogenization of the brain until collection of the synaptosomal suspension from the Percoll gradient.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Developmental regulation and effect of early undernutrition on phosphorylation of rat cortical synaptic membrane proteins

Undernutrition during early postnatal life was employed in rats by restricting the feeding time. The synaptic membrane fraction from cerebral cortex of normal and undernourished rats of various ages was prepared and endogenous protein phosphorylation studied. Many of the synaptic membrane proteins were found to be phosphorylated in an age-dependent manner. Early undernutrition affects the phosphorylation of various proteins in a complex way; most affected were 48-, 52-, 61- and 74-kDa proteins. These proteins were found to have phosphorylations mainly at tyrosine residues. This finding indicates that tyrosine phosphorylations may be affected most by early undernutrition. Adequate nutrition after early undernutrition removes most of the effects of undernutrition on synaptic protein phosphorylation. To address the question of how undernutrition may affect protein phosphorylation, we studied the lipid content of synaptic membrane fraction as it can affect membrane properties, including the fluidity. We found that undernutrition affects phosphorylation of most of the synaptic membrane proteins in the same manner in which it affects the cholesterol-phospholipid ratio of synaptic membrane and, hence, the fluidity of the membrane. This indicates that lipid biosynthesis is one of the ways by which undernutrition can affect synaptic membrane protein phosphorylation.

Protein tyrosine phosphorylation: implications for synaptic function

The phosphorylation of proteins on tyrosine residues, initially believed to be primarily involved in cell growth and differentiation, is now recognized as having a critical role in regulating the function of mature cells. The brain exhibits one of the highest levels of tyrosine kinase activity in the adult animal and the synaptic region is particularly rich in tyrosine kinases and tyrosine phosphorylated proteins. Recent studies have described the effects of tyrosine phosphorylation on the activities of a number of proteins which are potentially involved in the regulation of synaptic function. Furthermore, it is becoming apparent that tyrosine phosphorylation is involved in the modification of synaptic activity, such as occurs during depolarization, the induction of long-term potentiation or long-term depression, and ischemia. Changes in the activities of tyrosine kinases and/or protein tyrosine phosphatases which are associated with synaptic structures may result in altered tyrosine phosphorylation of proteins located at the synapse leading to both short-term and long-lasting changes in synaptic and neuronal function.

Biochemical evidence for the presence of NAP-22, a novel acidic calmodulin binding protein, in the synaptic vesicles of rat brain

NAP-22 is a neuronal tissue-enriched acidic protein which is expressed predominantly in rat brain. In the present study, we quantitated the amount of NAP-22 in a highly purified synaptic vesicle fraction. NAP-22 comprised 1.3 +/- 0.15% of the total protein in the fraction, and NAP-22 was located on the external surface of the synaptic vesicle membrane. The results suggest NAP-22 may be involved in the synaptic vesicle cycling.

Vesamicol, an inhibitor of acetylcholine vesicle packaging, increases synaptophysin phosphorylation in rat cortical synaptosomes

Vesamicol (AH5183) is an inhibitor (IC50, 50 nM) of acetylcholine (ACh) vesicle packaging. Vesamicol increases the phosphorylation pattern of synaptophysin (p38), identified as a vesicle-specific phosphoprotein involved in vesicle-mediated neurotransmitter release. Percoll fractionation of the rat cortex yielded a cholinergic-enriched synaptosomal Fraction 4. Fraction 4 contained the highest enrichment of cholineacetyl-transferase activity (86 +/- 4.6 mumole AcCh/g protein/hr.) in the Percoll gradient. Fraction 4 demonstrated oxygen consumption (108 +/- 23.4 nmole/mg protein), levels of adenosine triphosphate, ATP, (10.29 +/- 0.45 nmole/mg protein) and adenosine diphosphate, ADP, (10.54 +/- 2.72 nmole/mg protein), energy potential (ATP/[ADP] [Pi], (0.49) phosphate uptake (65-80 nmoles phosphate/mg tissue), 32Pi labelling (130 +/- 12 x 10(5) DPM/mg tissue; 74 +/- 9.8 x 10(2) nmoles phosphate/mg tissue). Synaptophysin was identified by Western blotting and confirmed by qualitative immunoprecipitation. Synaptophysin phosphorylation was confirmed by autoradiograph. Synaptophysin phosphorylation increased (225%) in the presence of vesamicol (ED50, 1 nM) in Fraction 4. Vesamicol (50 nM) and vanadate (54 microM) were compared for their effects on synaptophysin. This study suggests that during the inhibition of acetylcholine packaging by vesamicol that synaptophysin is phosphorylated. Therefore, the phosphorylation and dephosphorylation of synaptophysin may be involved in the transport of acetylcholine in or out of the synaptic vesicle.

Intrasynaptosomal free calcium levels in rat forebrain synaptosomes: modulation by sigma (sigma) receptor ligands

The sigma receptor ligands (+) and (-)pentazocine and BD1008 (1-100 microM) were added to rat forebrain synaptosomes. Their effects on intrasynaptosomal free calcium ([Ca2+(+)]i) levels under basal conditions and after depolarisation with high potassium buffer (45 mM KCl), veratridine (25 microM) and 4-aminopyridine (4-AP, 1 mM) were determined. The sigma ligands elicited significant, concentration-dependent decreases in basal [Ca2+]i levels with an order of potency (-)pentazocine > (+)pentazocine = BD1008. The sigma ligands (at the maximum effective concentrations) also significantly inhibited the rise in [Ca2+]i levels produced by depolarisation with KCl, veratridine and 4-AP. The effect of (+) and (-)pentazocine (100 microM) to inhibit the depolarisation-dependent increase in [Ca2+]i levels was greater when veratridine and 4-aminopyridine were used to depolarise the synaptosomes than with KCl, whereas the effect of BD1008 (100 microM) was approximately equipotent using all three depolarising agents. However, BD1008 was more potent to inhibit the KCl-induced rise in [Ca2+]i compared to (+) and (-)pentazocine. The data demonstrate for the first time that sigma ligands decrease [Ca2+]i levels in rat forebrain synaptosomes and this suggests a possible mechanism for the changes to neuronal protein phosphorylation and neurotransmitter release previously observed with sigma ligands.

Mn2+ can substitute for Ca2+ in causing catecholamine secretion but not for increasing tyrosine hydroxylase phosphorylation in bovine adrenal chromaffin cells

The ability of the divalent cation manganese (Mn2+) to substitute for calcium (Ca2+) both in triggering catecholamine release and in stimulating catecholamine synthesis, as indicated by an increase in tyrosine hydroxylase (TOH) phosphorylation, has been determined in bovine adrenal medullary chromaffin cells maintained in tissue culture. Mn2+ was found to enter chromaffin cells through pathways activated by nicotinic receptor stimulation and potassium depolarisation, and via the Na1:Ca0 exchange mechanism in Na(+)-loaded cells. Like Ca2+, entry of Mn2+ through these pathways triggered immediate catecholamine release and, like Ca2+, maintained quantitatively comparable release at least up to 40 min. Unlike Ca2+, Mn2+ did not stimulate an increase in TOH phosphorylation in intact chromaffin cells, even over a prolonged time course, but Mn2+ did stimulate increased TOH phosphorylation in lysed cell preparations showing that its lack of effect in the intact cells was not due to inhibition of the specific phosphorylation pathway. In lysed cell preparations, Mn2+ stimulated also phosphorylation of a different spectrum of proteins to Ca2+, and of the same proteins to different extents. In particular, P80 (MARCKS protein) was more intensely phosphorylated in the presence of Mn2+ than in the presence of Ca2+. Since TOH phosphorylation always occurs when intracellular Ca2+ is increased, the absence of an increase with Mn2+ indicates that none of its intracellular effects could have occurred as a consequence of Mn2+ mobilisation of intracellular Ca2+. In summary, the data show that Mn2+ is a surrogate for Ca2+ in triggering and maintaining catecholamine release, but does not substitute for Ca2+ in stimulating TOH phosphorylation.

Regulation of stimulus-dependent hippocampal acetylcholine release by okadaic acid-sensitive phosphoprotein phosphatases

Isolated nerve endings (synaptosomes) from rat hippocampus were used to characterize the influence by serine/threonine-specific phosphoprotein phosphatase (PP) inhibitors on acetylcholine release. Brief exposure to low concentrations of selective PP inhibitors (okadaic acid and calyculin A) caused a concentration-dependent attenuation of stimulus-dependent (calcium-evoked or potassium-evoked) [3H]acetylcholine ([3H]ACh) release, while having no effect on the rate of basal transmitter efflux. In view of the observed potencies for okadaic acid and calyculin A (pseudo-IC50 values near 3 nM), these data indicate that Type 1 (PP1) or Type 2A (PP2A) enzymes play a permissive role in exocytotic [3H]ACh release. In contrast, the absence of any measurable effect by sodium orthovanadate argues against a similar influence by tyrosine-specific phosphoprotein phosphatases. While the neuronal substrate(s) responsible for PP regulation of [3H]ACh release are unknown, the underlying mechanism clearly differs from that through which muscarinic autoreceptors act since inhibition by okadaic acid and oxotremorine (an autoreceptor agonist) are additive and the former is not blocked by the muscarinic receptor antagonist atropine. Based upon these results, we conclude that dephosphorylation steps catalyzed by okadaic acid-sensitive PP represent an important regulatory mechanism for stimulus-dependent transmitter release in septo-hippocampal cholinergic neurons.

Phosphorylation of synapsin I and dynamin in rat forebrain synaptosomes: modulation by sigma (sigma) ligands

The effects of sigma (sigma) ligands on protein phosphorylation were examined in crude, rat forebrain synaptosomes. Synaptosomes were prelabelled with 32P(i) and incubated with the sigma ligands 1,3-di-o-tolylguanidine (DTG), (+)pentazocine and (-)pentazocine (3, 10, 30, 100, 300 microM), or haloperidol, reduced haloperidol, and (+)SKF 10,047 (100 microM). Aliquots were then incubated for 10 s in control (5 mM K+) or depolarising buffer (41 mM K+). All the sigma ligands increased basal phosphorylation of synapsin Ib and other proteins including dynamin, and inhibited the depolarisation-dependent increase in phosphorylation of synapsin Ib in synaptosomes. The effects of these ligands are not directly on protein kinases or protein phosphatases. This indicates that the sigma ligands are mediating their effects via interaction with sigma binding sites, and suggest, for the first time, that protein phosphorylation may be one mechanism through which sigma ligands produce their biological effects.

Passive avoidance learning induced change in GAP43 phosphorylation in day-old chicks

Day-old chicks trained on a single trial passive discriminated avoidance task demonstrated a significant increase in in vitro phosphorylation of a 50 kDa protein in P2M fractions of total forebrain. The increase occurred 30 min posttraining, at a time when previous reports suggest that mechanisms for triggering protein synthesis-dependent long-term memory consolidation are activated. These changes in phosphorylation rates were accompanied by a substantial enhancement of total kinase activity. Immunoblotting studies with monoclonal anti-GAP43 antibody indicate that this protein is GAP43. These results contradict previous reports of a decrease in in vitro GAP43 phosphorylation following the same learning paradigm. A number of procedural differences may account for this discrepancy. The results suggest that changes in the phosphorylation state may be associated with mechanisms triggering long-term memory consolidation.

Synaptosomal amino acid release: effect of inhibiting protein phosphatases with okadaic acid

The protein phosphatase inhibitor okadaic acid was used to investigate the role of protein phosphatases in regulating the release of amino acids from synaptosomes. Okadaic acid increased the basal release of the amino acids glutamate, aspartate and GABA. The effect was specific in that taurine was not released by either KCl or okadaic acid and there was no synaptosomal lysis or change in ATP/ADP ratios in the presence of okadaic acid. The okadaic acid-stimulated release of amino acids was, however, only a small proportion of that produced by KCl depolarisation. Since okadaic acid raised synaptosomal protein phosphorylation levels to those equivalent to that produced by KCl depolarisation, it is unlikely therefore that there is a direct causal relationship between protein phosphorylation and the release of amino acids. Nevertheless, that release of amino acids from synaptosomes can be elevated under basal conditions by okadaic acid treatment does suggest that okadaic acid-sensitive protein phosphatases have a modulatory role in this process.

Frequency dependence of vasoactive intestinal polypeptide release and vagally induced tachycardia in the canine heart

We evaluated the frequency dependence of vasoactive intestinal polypeptide (VIP) release from the parasympathetic nerves to the canine heart. In intact animals in the presence of beta-adrenergic receptor blockade (propranolol, 0.5 mg/kg), the cervical vagosympathetic trunks were stimulated at various frequencies before and after the administration of atropine (0.1 mg/kg). The stimulations before atropine produced a classical bradycardia that progressed to cardiac arrest when the stimulation frequency was raised above 10 to 15 Hz. After atropine, vagal stimulation at various frequencies increased heart rate. The heart rate reached a maximum increase of 21 +/- 3 beats per min at a stimulation frequency of 20 Hz. In an isolated atrial preparation in which the VIP outflow was measured, the tachycardia elicited after atropine had a frequency dependence similar to that obtained in vivo. The peak increase of 23 +/- 3% above the basal rate (95 +/- 8 beats per min) occurred at a stimulation frequency of 20 Hz. The VIP outflow paralleled the tachycardia response (r = 0.95); the maximum outflow of VIP was 172 +/- 54 pg/(min . 100 g wet wt) and was evoked at a stimulation frequency of 20 Hz. This suggests that the vagally induced tachycardia is mediated, at least partly, by VIP.

The effect of neurocatin on protein phosphorylation in striatal synaptosomes from rat brain

Neurocatin, a neuroregulatory factor isolated from mammalian brain, is a powerful affector of protein phosphorylation in rat striatal synaptosomes. Two major synaptosomal phosphoproteins of approximately 80 and approximately 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 approximately 7.5 ng/100 microliters 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 microliters caused progressive decrease in incorporation of 32P into many synaptosomal proteins; by a concentration of neurocatin of approximately 45 ng/100 microliters, the level of 32P incorporation into many proteins was < or = 70% of control. The effects of neurocatin on synaptosomal protein phosphorylation were also dependent on the time of incubation. At a constant concentration of approximately 7.5 ng/100 microliters 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.

Studies on the role of B-50 (GAP-43) in the mechanism of Ca(2+)-induced noradrenaline release: lack of involvement of protein kinase C after the Ca2+ trigger

The involvement of B-50, protein kinase C (PKC), and PKC-mediated B-50 phosphorylation in the mechanism of Ca(2+)-induced noradrenaline (NA) release was studied in highly purified rat cerebrocortical synaptosomes permeated with streptolysin-O. Under optimal permeation conditions, 12% of the total NA content (8.9 pmol of NA/mg of synaptosomal protein) was released in a largely (> 60%) ATP-dependent manner as a result of an elevation of the free Ca2+ concentration from 10(-8) to 10(-5) M Ca2+. The Ca2+ sensitivity in the micromolar range is identical for [3H]NA and endogenous NA release, indicating that Ca(2+)-induced [3H]NA release originates from vesicular pools in noradrenergic synaptosomes. Ca(2+)-induced NA release was inhibited by either N- or C-terminal-directed anti-B-50 antibodies, confirming a role of B-50 in the process of exocytosis. In addition, both anti-B-50 antibodies inhibited PKC-mediated B-50 phosphorylation with a similar difference in inhibitory potency as observed for NA release. However, in a number of experiments, evidence was obtained challenging a direct role of PKC and PKC-mediated B-50 phosphorylation in Ca(2+)-induced NA release. PKC pseudosubstrate PKC19-36, which inhibited B-50 phosphorylation (IC50 value, 10(-5) M), failed to inhibit Ca(2+)-induced NA release, even when added before the Ca2+ trigger. Similar results were obtained with PKC inhibitor H-7, whereas polymyxin B inhibited B-50 phosphorylation as well as Ca(2+)-induced NA release. Concerning the Ca2+ sensitivity, we demonstrate that PKC-mediated B-50 phosphorylation is initiated at a slightly higher Ca2+ concentration than NA release. Moreover, phorbol ester-induced PKC down-regulation was not paralleled by a decrease in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Finally, the Ca(2+)- and phorbol ester-induced NA release was found to be additive, suggesting that they stimulate release through different mechanisms. In summary, we show that B-50 is involved in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Evidence is presented challenging a role of PKC-mediated B-50 phosphorylation in the mechanism of NA exocytosis after Ca2+ influx. An involvement of PKC or PKC-mediated B-50 phosphorylation before the Ca2+ trigger is not ruled out. We suggest that the degree of B-50 phosphorylation, rather than its phosphorylation after PKC activation itself, is important in the molecular cascade after the Ca2+ influx resulting in exocytosis of NA.

Early postnatal undernutrition impairs protein kinase C-dependent phosphorylation in rat brain synaptosomes

Undernutrition in rat pups was established by restricting feeding time daily. Protein kinase C-dependent phosphorylation in vitro was studied by incubating the mitochondrial-synaptosomal membrane fractions from adult, 18-day-old control and undernourished rats with gamma-[32P]ATP in presence of Ca2+ and phosphatidylserine. In adult and 18-day-old control rats, an increased phosphorylation of three major proteins (49, 53, 84 kDa) were detected in presence of calcium and phosphatidylserine. However, in 18-day-old undernourished rats, calcium/phosphatidylserine activated phosphorylation was found to be significantly impaired with only a slightly increased labelling detected in the 49 kDa protein.

Tetanus toxin inhibits depolarization-stimulated protein phosphorylation in rat cortical synaptosomes: effect on synapsin I phosphorylation and translocation

Synapsin I, a prominent phosphoprotein in nerve terminals, is proposed to modulate exocytosis by interaction with the cytoplasmic surface of small synaptic vesicles and cytoskeletal elements in a phosphorylation-dependent manner. Tetanus toxin (TeTx), a potent inhibitor of neurotransmitter release, attenuated the depolarization-stimulated increase in synapsin I phosphorylation in rat cortical particles and in synaptosomes. TeTx also markedly decreased the translocation of synapsin I from the small synaptic vesicles and the cytoskeleton into the cytosol, on depolarization of synaptosomes. The effect of TeTx on synapsin I phosphorylation was both time and TeTx concentration dependent and required active toxin. One- and two-dimensional peptide maps of synapsin I with V8 proteinase and trypsin, respectively, showed no differences in the relative phosphorylation of peptides for the control and TeTx-treated synaptosomes, suggesting that both the calmodulin- and the cyclic AMP-dependent kinases that label this protein are equally affected. Phosphorylation of synapsin IIb and the B-50 protein (GAP43), a known substrate of protein kinase C, was also inhibited by TeTx. TeTx affected only a limited number of phosphoproteins and the calcium-dependent decrease in dephosphin phosphorylation remained unaffected. In vitro phosphorylation of proteins in lysed synaptosomes was not influenced by prior TeTx treatment of the intact synaptosomes or by the addition of TeTx to lysates, suggesting that the effect of TeTx on protein phosphorylation was indirect. Our data demonstrate that TeTx inhibits neurotransmitter release, the phosphorylation of a select group of phosphoproteins in nerve terminals, and the translocation of synapsin I. These findings contribute to our understanding of the basic mechanism of TeTx action.

Depolarization-dependent tyrosine phosphorylation in rat brain synaptosomes

Synaptosomes from rat forebrain were analyzed for the presence of phosphotyrosine-containing proteins by immunoblotting with antiphosphotyrosine antibodies. Using this technique, 10-11 phosphotyrosine-containing proteins were detected. Depolarization of synaptosomes by transfer to a high (41 mM) K+ medium resulted in increases in the phosphotyrosine content of several synaptosomal proteins, the most pronounced increase being associated with a membrane protein of M(r) 117,000 (ptp117). Additional proteins exhibiting depolarization-dependent increases in phosphotyrosine content had molecular weights of 39,000, 104,000, 135,000, and 160,000. The depolarization-dependent increase in the phosphotyrosine content of ptp117 was apparent within 30 s of the onset of depolarization, reached a maximum between 3 and 5 min, and then decreased to near control values by 30 min. The increase in tyrosine phosphorylation of ptp117 was dependent on the concentration of K+ in the depolarizing medium and was maximal with [K+] in excess of 50 mM. It was also calcium dependent and did not occur in the absence of extracellular calcium. The addition of veratridine to the incubation medium also resulted in an increase in the tyrosine phosphorylation of ptp117. The results suggest that the phosphorylation of synaptic proteins on tyrosine residues may be involved in the regulation or modulation of synaptic activity.

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.

Effect of zinc on calmodulin-stimulated protein kinase II and protein phosphorylation in rat cerebral cortex

The effect of increasing concentrations of Zn2+ (1 microM-5 mM) on protein phosphorylation was investigated in cytosol (S3) and crude synaptic plasma membrane (P2-M) fractions from rat cerebral cortex and purified calmodulin-stimulated protein kinase II (CMK II). Zn2+ was found to be a potent inhibitor of both protein kinase and protein phosphatase activities, with highly specific effects on CMK II. Only one phosphoprotein band (40 kDa in P2-M phosphorylated under basal conditions) was unaffected by addition of Zn2+. The vast majority of phosphoprotein bands in both basal and calcium/calmodulin-stimulated conditions showed a dose-dependent inhibition of phosphorylation, which varied with individual phosphoproteins. Two basal phosphoprotein bands (58 and 66 kDa in S3) showed a significant stimulation of phosphorylation at 100 microM Zn2+ with decreased stimulation at higher concentrations, which was absent by 5 mM Zn2+. A few Ca2+/calmodulin-stimulated phosphoproteins in P2-M and S3 showed biphasic behavior; inhibition at less than 100 microM Zn2+ and stimulation by millimolar concentrations of Zn2+ in the presence or absence of added Ca2+/calmodulin. The two major phosphoproteins in this group were identified as the alpha and beta subunits of CMK II. Using purified enzyme, Zn2+ was shown to have two direct effects on CMK II: an inhibition of Ca2+/calmodulin-stimulated autophosphorylation and substrate phosphorylation activity at low concentrations and the creation of a new Zn(2+)-stimulated, Ca2+/calmodulin-independent activity at concentrations of greater than 100 microM that produces a redistribution of activity biased toward autophosphorylation and an alpha subunit with an altered mobility on sodium dodecyl sulfate-containing gels.

The regulation and function of protein phosphatases in the brain

Data emerging from a number of different systems indicate that protein phosphatases are highly regulated and potentially responsive to changes in the levels of intracellular second messengers produced by extracellular stimulation. They may therefore be involved in the regulation of many cell functions. The protein phosphatases in the nervous system have not been well studied. However, a number of neuronal-specific regulators (such as DARPP-32 and G-substrate) exist, and brain protein phosphatases appear to have particularly low specific activity, suggesting that neuronal protein phosphatases possess considerable and unique potential for regulation. Several early events following depolarization or receptor activation appear to involve specific dephosphorylations, indicating that regulation of protein phosphatase activity is important for the control of many neuronal functions. This article reviews the current literature concerning the identification, regulation, and function of serine/threonine protein phosphatases in the brain, with particular emphasis on the regulation of the major protein phosphatases, PP1 and PP2A, and their potential roles in modulating neurotransmitter release and postsynaptic responses.

Autophosphorylation of neuronal calcium/calmodulin-stimulated protein kinase II

A unique feature of neuronal calcium/calmodulin-stimulated protein kinase II (CaM-PK II) is its autophosphorylation. A number of sites are involved and, depending on the in vitro conditions used, three serine and six threonine residues have been tentatively identified as autophosphorylation sites in the alpha subunit. These sites fall into three categories. Primary sites are phosphorylated in the presence of calcium and calmodulin, but under limiting conditions of temperature, ATP, Mg2+, or time. Secondary sites are phosphorylated in the presence of calcium and calmodulin under nonlimiting conditions. Autonomous sites are phosphorylated in the absence of calcium and calmodulin after initial phosphorylation of Thr-286. Mechanisms that lead to a decrease in CaM-PK II autophosphorylation include the thermolability of the enzyme and the activity of protein phosphatases. A range of in vitro inhibitors of CaM-PK II autophosphorylation have recently been identified. Autophosphorylation of CaM-PK II leads to a number of consequences in vitro, including generation of autonomous activity and subcellular redistribution, as well as alterations in conformation, activity, calmodulin binding, substrate specificity, and susceptibility to proteolysis. It is established that CaM-PK II is autophos-phorylated in neuronal cells under basal conditions. Depolarization and/or activation of receptors that lead to an increase in intracellular calcium induces a marked rise in the autophosphorylation of CaM-PK II in situ. The incorporation of phosphate is mainly found on Thr-286, but other sites are also phosphorylated at a slower rate. One consequence of the increase in CaM-PK II autophosphorylation in situ is an increase in the level of autonomous kinase activity. It is proposed that the formation of an autonomous enzyme is only one of the consequences of CaM-PK II autophosphorylation in situ and that some of the other consequences observed in vitro will also be seen. CaM-PK II is involved in the control of neuronal plasticity, including neurotransmitter release and long-term modulation of postreceptor events. In order to understand the function of CaM-PK II, it will be essential to ascertain more fully the mechanisms of its autophosphorylation in situ, including especially the sites involved, the consequences of this autophosphorylation for the kinase activity, and the relationships between the state of CaM-PK II autophosphorylation and the physiological events within neurons.

Phosphoprotein B-50: localization of proteolytic sites for S. aureus V8 protease using truncated cRNAs for cell-free translation

B-50 (= GAP-43, F1, and P-57 or neuromodulin) is a nervous tissue-specific, growth-associated protein, localized in the presynaptic membrane. Phosphorylation by protein kinase C at Ser41 appears to play a role in B-50/calmodulin interaction and neurotransmitter release. Previous studies have shown that digestion of the phosphorylated protein with S. aureus V8 protease (SAP) resulted consecutively in 28- and 15-kDa phospho fragments, the latter containing all incorporated phosphate. These proteolytic products of digestion with SAP have frequently been used to identify B-50 in various systems. Therefore we were interested to find out the location of these fragments in the rat B-50 molecule. For this purpose, the rat cDNA for B-50 was used to generate full-length and truncated cRNAs for cell-free translation. B-50 and B-50 peptides were either N-terminally labeled with [35S]methionine (residues 1 and 5) as a tracer, or they were phosphorylated in vitro by protein kinase C. SAP digestion of the immunoprecipitated, 35S-labeled translation products produced similar 28- and 15-kDa fragments as were obtained from 32P-labeled B-50, indicating that these fragments are N-terminal. Relative mobilities of the N-terminal B-50 fragments of known length were used as internal standards for the calculation of the length of SAP and phospho fragments. Comparing the 35S- and 32P-labeled products, four SAP sites at Glu12, Glu28, Glu65, and Glu132 could be deduced. The latter two sites are in accordance with sequence data of C-terminal fragments from the literature. All available data could be fitted into one scheme.

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.

Modulation of synaptosomal protein phosphorylation/dephosphorylation by calcium is antagonised by inhibition of protein phosphatases with okadaic acid

The protein phosphatase inhibitor okadaic acid was used to investigate the protein phosphatases involved in the endogenous dephosphorylation of proteins in intact synaptosomes. Despite the fact that the calcium-dependent protein phosphatase (calcineurin) is most concentrated in synaptosomes and accounts for approximately 0.3% of synaptoplasmic protein, the majority of the dephosphorylation activity under both basal and depolarisation conditions is due to protein phosphatase type 1 (PP1) and/or protein phosphatase type 2A (PP2A). Nevertheless our results do suggest that calcineurin is active in synaptosomes and has 2 effects: a rapid, direct dephosphorylation of a limited range of substrates and an indirect activation of PP1 presumably by dephosphorylation of protein phosphatase 1 inhibitor-1.

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)

Protein phosphorylation in guinea-pig myenteric ganglia and brain: presence of calmodulin kinase II. protein kinase C and cyclic AMP kinase and characterization of major phosphoproteins

The aim of this study was to demonstrate the presence of calmodulin-stimulated protein kinase II, protein kinase C, and cyclic AMP-stimulated protein kinase in isolated myenteric ganglia and to characterize the major ganglia phosphoproteins using biochemical and immunochemical techniques. Ganglia from the small intestine of guinea-pigs were isolated, disrupted by sonication in Triton X-100, and phosphorylated. The phosphoprotein patterns obtained were compared with those of synaptosomes from guinea-pig and rat cerebral cortex. Myenteric ganglia were as rich in protein kinase C and cyclic AMP-stimulated protein kinase as brain tissue, but the level of calmodulin-stimulated protein kinase II was relatively lower. The alpha subunit of calmodulin-stimulated protein kinase II was detected by immunoblotting and the beta subunit by autophosphorylation. The ratio of beta to alpha subunit was considerably higher in ganglia than in brain and ganglia beta subunit had a lower apparent molecular weight than the brain enzyme. A number of neuronal phosphoproteins were found in ganglia including the 87,000 mol. wt phosphoprotein, synapsins 1a and 1b, and proteins IIIa and IIIb. A phosphoprotein of 48,000 mol. wt had many of the characteristics of the B-50 protein but was not the same. In addition, a number of other phosphoproteins not previously identified in neurons were found in ganglia including those with apparent molecular weights of 60,000 and 58,000 that were the major calmodulin kinase substrates. The guinea-pig enteric nervous system has been extensively studied but, unlike other parts of the mammalian nervous system, little is known about the intracellular mechanisms underlying its functions. A technique for isolating myenteric ganglia is now available and we have used this preparation to characterize the major protein kinase and phosphoproteins present in this tissue. The results obtained will allow the phosphorylation of the various proteins to be investigated after physiological or pharmacological manipulation of myenteric ganglia in situ and in vivo.

Cytoplasmic matrix proteins in central nervous system presynaptic terminals: turnover and effects of osmotic lysis

Cytomatrix proteins, of primary functional importance in central nervous system neuron terminals, are provided to their site of action in the terminal by axonal transport. Slow component b (SCb) of axonal transport has been proposed to be the biochemical counterpart of the moving cytoplasmic matrix, or cytomatrix, in axons. In the current study, axonally transported SCb proteins destined for neuron terminals were pulse-radiolabeled with [35S]methionine in guinea pig retinal ganglion cells. After SCb proteins reached the terminals in the superior colliculi, synaptosomes were prepared to distinguish between SCb proteins in the preterminal axons and those of the presynaptic terminals. Study of the initial entry and turnover of individual SCb proteins in presynaptic terminals revealed different residence times of certain SCb proteins in comparison with their cohorts. Preliminary information about the structural relationships of the proteins comprising the presynaptic cytomatrix was obtained by examining the solubility of individual SCb proteins relative to other SCb proteins, or membranes from osmotically lysed terminals. Last, treatment of those radiolabeled synaptosomes with varying concentrations of salts was performed to determine possible effects on observed structural relationships.

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.

Selective alterations in presynaptic cytomatrix protein organization induced by calcium and other divalent cations that modulate exocytosis

Rises in intracellular calcium cause several events of physiological significance, including the regulated release of neuronal transmitters. In this study, the effects of divalent cations on the structural organization of cytomatrix in presynaptic terminals was examined. [35S]Methionine-radiolabeled guinea pig retinal ganglion cell cytomatrix proteins were axonally transported [in slow component b (SCb) of axonal transport] to the neuron terminals in the superior colliculus. When the peak of radiolabeled cytomatrix proteins reached the terminals, synaptosomes containing the radiolabeled cytomatrix proteins were prepared. Approximately 40% of each SCb protein was soluble after hypoosmotic lysis of the radiolabeled synaptosomes in the presence of divalent cation chelators. Lysis of synaptosomes in the presence of calcium ions over a range of concentrations, however, caused a dramatic decrease in solubility of the presynaptic SCb proteins. The cytoplasmic effects may result from a calcium-dependent condensation of cytoplasm around presynaptic terminal membrane systems. There are two major presynaptic SCb proteins (at 60 and 35 kDa), that exhibited exceptional behavior: they remained as soluble in the presence of calcium as under control conditions, suggesting that they were relatively unaffected by the mechanism causing the decrease in SCb protein solubility. Also examined were the effects of other alkaline earth and transition metal divalent cations on the presynaptic SCb proteins.

Posthoc phosphorylation of proteins derived from ischemic rat hippocampus, striatum and neocortex

Disruption of the brain's protein phosphorylation system by ischemia may cause irreversible metabolic and structural alterations leading eventually to cell death. To examine the effect of ischemia on the phosphorylation state of brain proteins, tissue homogenates derived from the hippocampus, striatum and neocortex of normal rats and rats subjected to severe forebrain ischemia were phosphorylated with [gamma-32P]ATP. The phosphorylated proteins were separated by two-dimensional polyacrylamide gel electrophoresis and changes were assessed by autoradiography. Cerebral ischemia caused marked alterations of the phosphorylation state of many brain proteins; phosphorylation of some proteins was increased while phosphorylation of others was decreased. Despite differences in the sensitivity of the hippocampus, striatum and neocortex to ischemic injury the direction and approximate magnitude of protein phosphorylation changes caused by ischemia were similar in all three regions. Since the pattern of protein phosphorylation in the ischemia-vulnerable hippocampus was identical to that in the ischemia-resistant paramedian neocortex we conclude that abnormalities of protein phosphorylation may be necessary for ischemic injury to neurons but none are sufficient to explain the selective vulnerability of certain brain regions to ischemic damage.

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.

Purification and characterization of calmodulin-stimulated protein kinase II from two-day and adult chicken forebrain

Soluble calmodulin-stimulated protein kinase II has been purified from 2-day and adult chicken forebrain. At both ages the holoenzyme eluted from a Superose-6B column with an apparent molecular weight of approximately 700,000 daltons and contained three subunits. The subunits were found to be the counterparts of the alpha, beta, and beta' subunits of the enzyme purified from adult rat brain in that they had one-dimensional phosphopeptide maps that were indistinguishable from those of the corresponding subunit in the rat enzyme and they migrated in SDS-polyacrylamide gels with the same apparent molecular weights. However, the doublet formed by the beta subunit was much more clearly resolved in the chicken enzyme and the beta' subunit, which was much more abundant in the adult chicken than in the adult rat, was also found to be a doublet. The ratio of the concentrations of the alpha and beta subunits changed during development. By autoradiography following autophosphorylation, the alpha:beta ratios of the 2-day and adult enzymes were 0.89 +/- 0.07 and 1.92 +/- 0.26, respectively; by silver staining the alpha:beta ratios were 0.95 +/- 0.11 and 1.85 +/- 0.17, respectively. The concentration of the beta' subunit was equal to that of the beta subunit at both ages. Autophosphorylation produced a decrease in the electrophoretic mobility of the alpha and beta subunits in SDS-polyacrylamide gels and a marked decrease in the calcium dependence of the substrate phosphorylation activity of the enzyme at both ages. The purified enzyme from chicken brain appeared to be more stable under standard in vitro assay conditions than the rat enzyme, and this was particularly so for the enzyme from 2-day forebrain.

Calcium uptake and protein phosphorylation in myenteric neurons, like the release of vasoactive intestinal polypeptide and acetylcholine, are frequency dependent

The mechanism of the electrical-to-chemical decoding involved in the preferential release of the transmitters acetylcholine and vasoactive intestinal polypeptide (VIP) by electrical field stimulation at low (5 Hz) and high (50 Hz) frequencies was studied in superfused myenteric neurons. The stimulation-induced uptake of 45Ca2+ accompanying high frequency stimulation was markedly reduced by 10 microM nifedipine, a specific blocker of L-type voltage-sensitive Ca2+ channels (VSCCs), as was also the preferential high-frequency release of VIP. By contrast, the 45Ca2+ uptake during low-frequency stimulation was somewhat lower per pulse, and neither this uptake nor the preferential release of acetylcholine occurring at this frequency was significantly reduced by nifedipine. These findings suggest that the release of acetylcholine and VIP involve different VSCCs. The pattern of in vitro protein thiophosphorylation in tissue extracts of differentially stimulated myenteric neurons involved polypeptides of 205, 173, 86, 73, 57, 54, 46, 32, 28, and 24 kDa and was also markedly stimulus and nifedipine dependent. This suggests that different phosphoproteins are involved during the frequency-dependent activation of the different Ca2+ channels and exocytotic mechanisms.

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.

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.

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.

Distribution of calmodulin- and cyclic AMP-stimulated protein kinases in synaptosomes

The subcellular location of calmodulin- and cyclic AMP stimulated protein kinases was assessed in synaptosomes which were prepared on Percoll density gradients. The distribution of the protein kinases between the outside and the inside and between the soluble and membrane fractions was determined by incubating intact and lysed synaptosomes, as well as supernatant and pellet fractions obtained from lysed synaptosomes, in the presence of [gamma-32P]ATP. Protein kinase activity was assessed by the labelling of endogenous proteins, or exogenous peptide substrates, under conditions optimized for either calmodulin- or cyclic AMP-stimulated protein phosphorylation. When assessed by calmodulin-stimulated autophosphorylation of the alpha subunit of calmodulin kinase II, 44% of this enzyme was on the outside of synaptosomes, and 41% was in the 100,000 g supernatant. Using an exogenous peptide substrate, the distribution of total calmodulin-stimulated kinase activity was 27% on the outside and 34% in the supernatant. The high proportion of calmodulin kinase II on the outside of synaptosomes is consistent with its known localization at postsynaptic densities. The proportion of calmodulin kinase II which was soluble depended on the ionic strength conditions used to prepare the supernatant, but the results suggest that a major proportion of this enzyme which is inside synaptosomes is soluble. When assessed by cyclic AMP-stimulated phosphorylation of endogenous substrates, no cyclic AMP-stimulated kinase activity was observed on the outside of synaptosomes, whereas 21% was found with an exogenous peptide substrate. This suggests that if endogenous substrates are present on the outside of synaptosomes, then the enzyme does not have access to them. The cyclic AMP-stimulated protein kinase present inside synaptosomes was largely bound to membranes and/or the cytoskeleton, with only 10% found in the supernatant when assessed by endogenous protein phosphorylation and 25% with an exogenous substrate. The markedly different distribution of the calmodulin- and cyclic AMP-stimulated protein kinases presumably reflects differences in the functions of these enzymes at synapses.

A rapid Percoll gradient procedure for isolation of synaptosomes directly from an S1 fraction: homogeneity and morphology of subcellular fractions

A method for preparation of synaptosomes from rat cerebral cortex, on a discontinuous Percoll gradient, was previously developed for use with a P2 pellet (Brain Research, 372 (1986) 115-129). Here the Percoll method has been adapted for use with an S1-supernatant which eliminates a potentially damaging resuspension step and saves over 30 min, representing a third of the total preparation time. The homogeneity of the synaptosomes in each of the 5 subcellular fractions obtained with the S1-Percoll method was determined biochemically by analysis of the distribution of total protein, myelin basic protein, synapsin I and pyruvate dehydrogenase across the gradient. Electron microscopy was also used to determine the homogeneity of the synaptosomes, as well as to determine their morphological characteristics. Fraction 4 was the most enriched in synaptosomes and contained the lowest level of contamination by myelin, extrasynaptosomal mitochondria and plasma membranes. The yield of synaptosomes in fraction 4 with the S1-Percoll method was 1.4-fold greater than with the P2-Percoll method. While all other fractions contained some synaptosomes the major additional content in fractions 1-3 and 5 was, respectively, unidentified small membranes, myelin, synaptic plasma membranes and extrasynaptosomal mitochondria. Fraction 1 was enriched for very small synaptosomes (0.34 micron mean diameter) only 8% of which contained mitochondria, while fractions 2-4 progressively included larger synaptosomes containing more mitochondria. Fraction 5 synaptosomes were approximately the same size as those in fraction 4 (0.63 micron mean diameter), but 83% contained mitochondria, significantly more than in fraction 4. The synaptosomes in fraction 5 were found to be relatively resistant to hypotonic lysis, explaining a previously observed lack of phosphorylation of synapsin I in this fraction. The differences in homogeneity and morphological characteristics of the synaptosomes in fractions 1-5 suggest that the basis for their fractionation on Percoll gradients is different from that achieved with the more traditional procedures for isolating synaptosomes and that unique synaptosomal fractions are obtained with the S1-Percoll procedure.

A rapid Percoll gradient procedure for isolation of synaptosomes directly from an S1 fraction: viability of subcellular fractions

The metabolic and functional viability of synaptosomes was examined in 5 subcellular fractions obtained after centrifugation of an S1 fraction from rat cerebral cortex on a discontinuous Percoll gradient (Brain Research, this volume, 1987). Fraction 4 was the most enriched for viable synaptosomes since, although it accounted for only 11.8% of the total protein recovered from the gradient, this fraction contained 23.7% of the basal synapsin I phosphorylation activity, the greatest degree of depolarisation-stimulated increase in synapsin I phosphorylation, 36.1% of the total [3H]noradrenaline uptake capacity and 46.9% of the total [3H]noradrenaline release capacity. Noradrenaline release from fraction 4 was consistent with a neuronal mechanism as it was increased with increasing K+ concentrations and was dependent on calcium. Fractions 1 and 2 contained few viable synaptosomes as judged by their capacity for noradrenaline uptake and release, yet these fractions accounted for some 62.6% of the endogenous content of noradrenaline. In part their lack of viability was due to a low content of intrasynaptosomal mitochondria, while their high content of endogenous noradrenaline was due to the presence of synaptic vesicles released from damaged nerve terminals. The synaptosomes in fraction 3 were metabolically and functionally viable, but their capacity for uptake and release of noradrenaline was lower than for fraction 4. The synaptosomes in fraction 5 showed only a small depolarisation-stimulated release of noradrenaline, suggesting a lack of viability. Part of the capacity for uptake of [3H]noradrenaline into fraction 5 was attributed to the presence of extrasynaptosomal mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered protein phosphorylation in intact rat cortical synaptosomes after in vivo administration of fluphenazine

Phenothiazines such as fluphenazine are able to inhibit calcium-stimulated protein kinases in vitro in both lysed and intact synaptosomes. In this study protein phosphorylation was assayed in intact synaptosomes isolated from the cerebral cortex of rats treated chronically (21 days, 10 mg/kg, i.p.) or acutely (1 hr, 10 mg/kg, i.p.) with fluphenazine. When intact synaptosomes from chronically treated animals were prelabeled with 32Pi, there were two effects on protein phosphorylation: an increase in the basal labeling of many phosphoproteins and a decrease in depolarization-evoked protein phosphorylation. Acute injections had even more pronounced effects, but the direction and nature of the effects were the same. No effects on K+-stimulated calcium entry or on protein phosphatase activity were detected. When lysed synaptosomes from chronically treated animals were labeled in the presence of [gamma-32P]ATP, a small decrease in calmodulin-dependent and cAMP-dependent protein phosphorylation was observed. The results suggest that two different in vivo mechanisms may underlie these effects, and these are discussed. We proposed that intact synaptosomes may be a good model in which to study the in vivo mechanisms of the action of fluphenazine since they appear to retain at least some effects of the drug after subcellular fractionation.