Translocation of synapsin I in response to depolarization of isolated nerve terminals

Depolarization of isolated nerve terminals (synaptosomes) has been shown to stimulate neurotransmitter release and to increase the phosphorylation state of a number of proteins, including synapsin I, in a Ca2+-dependent manner. Synapsin I, a prominent nerve terminal phosphoprotein, interacts with the cytoplasmic surface of small synaptic vesicles and with cytoskeletal elements in a phosphorylation-dependent manner. In the present study we have found that depolarization of synaptosomes resulted in a rapid (2-5 sec) translocation of synapsin I from the particulate to the cytosolic (soluble) fraction. This translocation of synapsin I correlated with its phosphorylation state and was dependent on the presence of Ca2+ in the incubation medium. The stoichiometry of phosphorylation of soluble synapsin I was considerably higher than that of synapsin I in the particulate fraction, under both basal and depolarizing conditions. These data are consistent with the hypothesis that, in situ, the phosphorylation of synapsin I promotes its translocation from synaptic vesicles/cytoskeleton to the cytosol. This phosphorylation/translocation may be instrumental in regulating the release of neurotransmitter.

ARPP-21, a cyclic AMP-regulated phosphoprotein enriched in dopamine-innervated brain regions. I. Amino acid sequence of ARPP-21B from bovine caudate nucleus

ARPP-21 (cAMP-regulated phosphoprotein, Mr = 21,000 as determined by SDS-PAGE) is a major cytosolic substrate for cAMP-stimulated protein phosphorylation in dopamine-innervated regions of the rat CNS. It has recently been purified to homogeneity from bovine caudate nucleus and characterized (Hemmings and Greengard, 1989). ARPP-21 is isolated as 2 isoforms, ARPP-21A and ARPP-21B. The amino acid sequence of purified bovine ARPP-21B has now been determined by gas-phase sequencing. The S-14C-carboxymethylated protein was subjected to enzymatic cleavage with trypsin, chymotrypsin, subtilisin, and endoproteinase Lys-C. The resulting peptides were purified by high-performance liquid chromatography, and selected peptides were subjected to amino acid analysis and/or amino acid sequencing by automated Edman degradation. ARPP-21B consists of a single NH2-terminal blocked polypeptide chain of 88 residues, with a calculated molecular mass of 9561 Da, including an NH2-terminal acetyl group inferred by deblocking with an acylaminopeptidase. This molecular mass is significantly lower than earlier estimates based on SDS-PAGE or hydrodynamic measurements. The seryl residue phosphorylated by cAMP-dependent protein kinase (Hemmings et al., 1989) is located at position 55. The molecule contains 1 cysteinyl residue, at position 71, and contains no methionyl, tyrosyl, phenylalanyl, tryptophanyl, or histidinyl residues. Determination of the primary structure of ARPP-21, one of several phosphoproteins localized to dopaminoceptive neurons in the basal ganglia, provides a framework for further investigations into the molecular mechanisms involved in dopaminergic neurotransmission.

Dopamine- and cAMP-regulated phosphoprotein (DARPP-32) and dopamine DA1 agonist-sensitive Na+,K+-ATPase in renal tubule cells

The cellular localization of DARPP-32, a dopamine- and cAMP-regulated phosphoprotein of Mr 32,000 that appears to mediate certain actions of dopamine in the mammalian brain by acting as an inhibitor of protein phosphatase 1, was studied in the kidney of several species. DARPP-32 mRNA and DARPP-32-like immunoreactivity were found in the cytoplasm of cells in the thick ascending limb of the loop of Henle. The specific dopamine DA1 agonist SKF 82526 caused a dose-dependent inhibition of Na+,K+-ATPase activity, which could be blocked by SCH 23390, a specific DA1 antagonist, and by PKI-(5-24) amide, a specific inhibitor of cAMP-dependent protein kinase. The results indicate that DA1 dopamine receptors and DARPP-32, an intracellular third messenger for dopamine, are part of the signal-transduction process for dopamine acting on renal tubule cells.

Chloride conductance regulated by cyclic AMP-dependent protein kinase in cardiac myocytes

In heart cells, cyclic AMP-dependent protein kinase (PKA) regulates calcium- and potassium-ion current by phosphorylating the ion channels or closely associated regulatory proteins. We report here that isoprenaline induced large chloride-ion currents in voltage-clamped, internally-dialysed myocytes from guinea-pig ventricles. The Cl- current could be activated by intracellular dialysis with cAMP or the catalytic subunit of PKA, indicating regulation by phosphorylation. In approximately symmetrical solutions of high Cl- concentration, the macroscopic cardiac Cl- current showed little rectification, unlike the single-channel current in PKA-regulated Cl- channels of airway epithelial cells. But, like epithelial Cl- -channel currents, the cardiac Cl- current was sensitive to the distilbene,4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS). In the absence of kinase activation, cardiac sarcolemmal Cl- conductance was negligible. During beta-adrenergic stimulation of the heart, this novel Cl- conductance should accelerate action-potential repolarization and so protect impulse propagation in the face of the possibly arrhythmogenic increases in heart rate and in calcium entry into the cells.

Spatial relationship of the striatonigral and mesostriatal pathways: double-label immunocytochemistry for DARPP-32 and tyrosine hydroxylase

Antibodies to tyrosine hydroxylase and DARPP-32 were used to examine the spatial arrangement between mesostriatal dopamine projections and the reciprocal pathway from DARPP-32-containing neurons in the basal forebrain. Use of a double-labeling immunocytochemical procedure demonstrated that the mesostriatal and striatonigral pathways run in close proximity throughout the rostral mesencephalon and basal forebrain. The majority of descending axons immunoreactive for DARPP-32 appear to originate in the striatum, including the nucleus accumbens, and run through the internal capsule to innervate the globus pallidus, entopeduncular nucleus, and all subdivisions of the substantia nigra. The ventral tegmental area is sparsely invested with DARPP-32-immunoreactive axons. At all levels, there are also fascicles of DARPP-32-containing fibers which run ventromedial to the internal capsule in the medial forebrain bundle, and which are coextensive with ascending axons of the mesencephalic dopamine the internal capsule in the medial forebrain bundle, and which are coextensive with ascending axons of the mesencephalic dopamine cell groups. Tyrosine hydroxylase-immunoreactive axons are coextensive with DARPP-32-immunoreactive axons in the internal capsule entopeduncular nucleus, and globus pallidus, as well as much of the remainder of the basal forebrain. Although the main source of descending DARPP-32 immunoreactive axons would appear to be the striatum, other possible sources are also discussed.

Regulation of chloride channels by protein kinase C in normal and cystic fibrosis airway epithelia

Apical membrane chloride channels control chloride secretion by airway epithelial cells. Defective regulation of these channels is a prominent characteristic of cystic fibrosis. In normal intact cells, activation of protein kinase C (PKC) by phorbol ester either stimulated or inhibited chloride secretion, depending on the physiological status of the cell. In cell-free membrane patches, PKC also had a dual effect: at a high calcium concentration, PKC inactivated chloride channels; at a low calcium concentration, PKC activated chloride channels. In cystic fibrosis cells, PKC-dependent channel inactivation was normal, but activation was defective. Thus it appears that PKC phosphorylates and regulates two different sites on the channel or on an associated membrane protein, one of which is defective in cystic fibrosis.

Bacterial lipopolysaccharide regulates the phosphorylation of the 68K protein kinase C substrate in macrophages

Bacterial lipopolysaccharide (LPS) potentiates protein kinase C (PKC)-dependent responses such as the activation of arachidonic acid metabolism in macrophages (Aderem, A. A., Cohen, D. S., Wright, S. D., and Cohn, Z. A. (1986) J. Exp. Med. 164, 165-179). Concomitantly, LPS promotes the myristoylation of a 68K PKC substrate, shown to be equivalent to the 80/87K PKC substrate found in brain and fibroblasts (Aderem, A. A., Albert, K. A., Keum, M. M., Wang, J. K., Greengard, P., and Cohn, Z. A. (1988) Nature 332, 362-364). We have now examined the effect of LPS on the phosphorylation of this 68K PKC substrate. We report here that LPS modifies the kinetics and extent of phosphorylation of the 68K protein. While treatment with LPS alone induces low level phosphorylation of the 68K protein, it markedly increases the rate of subsequent phorbol 12-myristate 13-acetate (PMA)-dependent phosphorylation of this protein. Phosphorylation in LPS-treated macrophages was maximal 1-2 min after administration of PMA, while maximal phosphorylation in macrophages not exposed to LPS was only achieved 6 min after addition of PMA. In addition to increasing the rate of PMA-dependent phosphorylation of the 68K protein in macrophages, LPS also promoted the phosphorylation of a novel peptide on the 68K protein. Thus while PMA stimulated the phosphorylation of two thermolytic phosphopeptides (phosphopeptides 1 and 2), the low level of phosphorylation observed with LPS alone was found to occur on phosphopeptides 1 and 2 as well as on a novel phosphopeptide (phosphopeptide 3). Furthermore, LPS treatment of macrophages potentiated phosphorylation of all three phosphopeptides when the cells were subsequently stimulated with PMA. While phosphorylation stimulated by LPS and PMA was slightly more than additive for phosphopeptides 1 and 2, it was markedly synergistic (increased 14.5-fold) for phosphopeptide 3. Phosphorylation of all three phosphopeptides occurred exclusively on serine. It is possible that LPS-induced myristoylation of the 68K protein directs it to the membrane where its phosphorylation is enhanced by its close association with PKC.

Protein phosphorylation systems in postmortem human brain

Protein phosphorylation systems regulated by cyclic adenosine 3',5'-monophosphate (cyclic AMP), or calcium in conjunction with calmodulin or phospholipid/diacylglycerol, have been studied by phosphorylation in vitro of particulate and soluble fractions from human postmortem brain samples. One-dimensional or two-dimensional gel electrophoretic protein separations were used for analysis. Protein phosphorylation catalyzed by cyclic AMP-dependent protein kinase was found to be highly active in both particulate and soluble preparations throughout the human CNS, with groups of both widely distributed and region-specific substrates being observed in different brain nuclei. Dopamine-innervated parts of the basal ganglia and cerebral cortex contained the phosphoproteins previously observed in rodent basal ganglia. In contrast, calcium/phospholipid-dependent and calcium/calmodulin-dependent protein phosphorylation systems were less prominent in human postmortem brain than in rodent brain, and only a few widely distributed substrates for these protein kinases were found. Protein staining indicated that postmortem proteolysis, particularly of high-molecular-mass proteins, was prominent in deeply located, subcortical regions in the human brain. Our results indicate that it is feasible to use human postmortem brain samples, when obtained under carefully controlled conditions, for qualitative studies on brain protein phosphorylation. Such studies should be of value in studies on human neurological and/or psychiatric disorders.

Identification of two protein kinases that phosphorylate the neural cell-adhesion molecule, N-CAM

The neural cell-adhesion molecule (N-CAM) is detected as at least 3 related polypeptides generated by alternative splicing of a single gene. In vivo the 2 larger polypeptides are phosphorylated, but the smallest polypeptide, which lacks a cytoplasmic domain, is not. We have found that the 2 larger polypeptides are phosphorylated in vivo on several common phosphorylation sites. Furthermore, the largest polypeptide has additional sites, suggesting that some phosphorylation occurs in that portion of the intracellular region unique to it. In vitro N-CAM is not a substrate for cyclic AMP-dependent protein kinase, cyclic GMP-dependent protein kinase, calcium/calmodulin-dependent protein kinase I, II, or III, protein kinase C, or casein kinase II. However, we have isolated 2 protein kinases from mammalian and avian brain that phosphorylate rodent and chicken N-CAM. On the basis of their chromatographic behavior and substrate specificity, the 2 kinases are glycogen synthase kinase 3 (GSK-3) and casein kinase I (CK I). The 2 kinases phosphorylate N-CAM rapidly, to a high stoichiometry and with a low Km for N-CAM, suggesting that the phosphorylation of N-CAM by these kinases is physiologically relevant. Both enzymes phosphorylate the 2 larger N-CAM polypeptides in vitro in the cytoplasmic domain on threonyl residues that are phosphorylated to a low level in vivo. In addition, the threonyl residues are close to seryl residues phosphorylated to a high level in vivo. Prior phosphorylation at the in vivo sites appears to be a prerequisite for phosphorylation by GSK-3 and CK I. Taken together, the results suggest that N-CAM may be physiologically phosphorylated on 2 sets of interrelated sites, one demonstrable in vivo and one in vitro. Phosphorylation on the "in vivo" sites is resistant to dephosphorylation and may be constitutive, while phosphorylation on the "in vitro" sites is much more labile.

Molecular cloning, characterization, and expression of a cDNA encoding the "80- to 87-kDa" myristoylated alanine-rich C kinase substrate: a major cellular substrate for protein kinase C

We isolated and sequenced a cDNA clone encoding the bovine "80- to 87-kDa" protein, a major cellular substrate for protein kinase C. An open reading frame of 1005 base pairs predicted a protein of 335 amino acids (Mr, 31,949). Despite this predicted size, the protein migrated on SDS/polyacrylamide gels with an apparent molecular weight of 80-87,000 after expression of the cDNA in cells lacking the protein. It was highly enriched in alanine (28.4 mol %), contained an amino-terminal myristoylation consensus sequence, and included a 25-residue basic domain containing the known protein kinase C phosphorylation sites. Two mRNA species (2.6 and 4.4 kilobases) were most highly expressed in brain, spinal cord, spleen, and lung, in parallel with the distribution of immuno-reactive protein. Genomic blot analysis indicated the likelihood of a single gene coding for this mRNA. We propose the name myristoylated alanine-rich C kinase substrate (MARCKS) for this protein.

ARPP-21, a cyclic AMP-regulated phosphoprotein (Mr = 21,000) enriched in dopamine-innervated brain regions. Amino acid sequence of the site phosphorylated by cyclic AMP in intact cells and kinetic studies of its phosphorylation in vitro

ARPP-21 (cyclic AMP-regulated phosphoprotein, Mr = 21,000) is a cytosolic neuronal phosphoprotein that is highly enriched in regions of mammalian brain that receive dopaminergic innervation, in particular the striatum. The state of phosphorylation of ARPP-21 in brain slices prepared from rat striatum was shown to be regulated by 8-bromo-cyclic AMP. Phosphorylation occurred exclusively on seryl residues contained within a single tryptic phosphopeptide as analyzed by two-dimensional thin layer electrophoresis/chromatography. The tryptic phosphopeptide derived from ARPP-21 phosphorylated in intact cells comigrated with the tryptic phosphopeptide derived from purified ARPP-21 phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase in vitro. Purified cyclic AMP-dependent protein kinase catalyzed the incorporation of 1.1 mol of [32P]phosphate/mol of ARPP-21 exclusively on seryl residues. The amino acid sequence surrounding the site in purified ARPP-21 phosphorylated by cyclic AMP-dependent protein kinase in vitro was determined by analyzing two overlapping chymotryptic peptides isolated from [32P]phospho-ARPP-21 by reverse phase high performance liquid chromatography. A combination of gas phase and solid phase amino acid sequencing yielded a phosphorylation site sequence of -Glu-Arg-Arg-Lys-Ser(P)-Lys-Ser-Gly-Ala-Gly-. Initial rate studies of the phosphorylation of purified ARPP-21 by the catalytic subunit of cyclic AMP-dependent protein kinase yielded an apparent Km of 0.78 microM and a kcat of 2.2 s-1. A synthetic peptide based on the phosphorylation site of ARPP-21 was phosphorylated on the corresponding seryl residue with an apparent Km of 40 microM and a kcat of 4.0 s-1. These results are compatible with a physiological role for the phosphorylation of ARPP-21 by cyclic AMP-dependent protein kinase in vivo, regulated by first messengers acting via cyclic AMP, e.g. dopamine and vasoactive intestinal peptide.

Characterization of synapsin I fragments produced by cysteine-specific cleavage: a study of their interactions with F-actin

Synapsin I is a neuron-specific phosphoprotein that is concentrated in the presynaptic nerve terminal in association with the cytoplasmic surface of synaptic vesicles. It has been demonstrated to bundle F-actin in a phosphorylation-dependent manner in vitro, a property consistent with its proposed role in linking synaptic vesicles to the cytoskeleton and its involvement in the regulation of neurotransmitter release. Synapsin I is composed of two distinct domains, a COOH terminal, collagenase-sensitive, hydrophilic, and strongly basic tail region, and an NH2 terminal, collagenase-resistant head region relatively rich in hydrophobic amino acids. To elucidate the structural basis for the interactions between synapsin I and F-actin and how it relates to other characteristics of synapsin I, we have performed a structure-function analysis of fragments of synapsin I produced by cysteine-specific cleavage with 2-nitro-5-thiocyanobenzoic acid. The fragments were identified and aligned with the parent molecule using the deduced primary structure of synapsin I and the known phosphorylation sites as markers. We have purified these fragments and examined their interactions with F-actin. Two distinct fragments, a 29-kD NH2-terminal fragment and a 15-kD middle fragment, were shown to contain F-actin binding sites. A 51/54-kD middle/tail fragment retained the F-actin binding and bundling activity of synapsin I, but the isolated tail fragment did not retain either activity. In contrast to phosphorylation of sites two and three in intact synapsin I, which abolishes F-actin bundling activity, phosphorylation of these sites in the middle/tail fragment failed to abolish this activity. In conclusion, three domains of synapsin I appear to be involved in F-actin binding and bundling.

Calcitonin gene-related peptide regulates phosphorylation of the nicotinic acetylcholine receptor in rat myotubes

The nicotinic acetylcholine receptor (AChR) is a substrate for at least three different protein kinases, and phosphorylation of the receptor has been shown to increase its rate of desensitization. However, the first messengers that regulate AChR phosphorylation have not yet been identified. This study demonstrates that calcitonin gene-related peptide (CGRP), a neuropeptide present in the axon terminals of the neuromuscular junction, regulates phosphorylation of the AChR in primary rat myotube cultures. CGRP, in the presence of the phosphodiesterase inhibitor Ro 20-1724, increased phosphorylation of the alpha and delta subunits of the AChR. CGRP-induced phosphorylation of the AChR had the same subunit specificity and temporal sequence as previously observed using forskolin or cAMP analogs. Phosphorylation of the AChR in the presence of CGRP appears to be mediated by CGRP-stimulated increases in cAMP levels leading to activation of cAMP-dependent protein kinase. The present results, taken together with the recent demonstration that CGRP increases the rate of AChR desensitization in mouse myotubes, suggest that CGRP may play a physiological role as a regulator of AChR desensitization by modulating AChR phosphorylation at the neuromuscular junction.

Interactions of synapsin I with small synaptic vesicles: distinct sites in synapsin I bind to vesicle phospholipids and vesicle proteins

Synapsin I is a major neuron-specific phosphoprotein that is specifically localized to the cytoplasmic surface of small synaptic vesicles. In the present study, the binding of synapsin I to small synaptic vesicles was characterized in detail. The binding of synapsin I was preserved when synaptic vesicles were solubilized and reconstituted in phosphatidylcholine. After separation of the protein and lipid components of synaptic vesicles under nondenaturing conditions, synapsin I bound to both components. The use of hydrophobic labeling procedures allowed the assessment of interactions between phospholipids and synapsin I in intact synaptic vesicles. Hydrophobic photolabeling followed by cysteine-specific cleavage of synapsin I demonstrated that the head domain of synapsin I penetrates into the hydrophobic core of the bilayer. The purified NH2-terminal fragment, derived from the head domain by cysteine-specific cleavage, bound to synaptic vesicles with high affinity confirming the results obtained from hydrophobic photolabeling. Synapsin I binding to synaptic vesicles could be inhibited by the entire molecule or by the combined presence of the NH2-terminal and tail fragments, but not by an excess of either NH2-terminal or tail fragment alone. The purified tail fragment bound with relatively high affinity to synaptic vesicles, though it did not significantly interact with phospholipids. Binding of the tail fragment was competed by holosynapsin I; was greatly decreased by phosphorylation; and was abolished by high ionic strength conditions or protease treatment of synaptic vesicles. The data suggest the existence of two sites of interaction between synapsin I and small synaptic vesicles: binding of the head domain to vesicle phospholipids and of the tail domain to a protein component of the vesicle membrane. The latter interaction is apparently responsible for the salt and phosphorylation dependency of synapsin I binding to small synaptic vesicles.

Electrostatic and hydrophobic interactions of synapsin I and synapsin I fragments with phospholipid bilayers

Synapsin I, a major neuron-specific phosphoprotein, is localized on the cytoplasmic surface of small synaptic vesicles to which it binds with high affinity. It contains a collagenase-resistant head domain and a collagenase-sensitive elongated tail domain. In the present study, the interaction between synapsin I and phospholipid vesicles has been characterized, and the protein domains involved in these interactions have been identified. When lipid vesicles were prepared from cholesterol and phospholipids using a lipid composition similar to that found in native synaptic vesicle membranes (40% phosphatidylcholine, 32% phosphatidylethanolamine, 12% phosphatidylserine, 5% phosphatidylinositol, 10% cholesterol, wt/wt), synapsin I bound with a dissociation constant of 14 nM and a maximal binding capacity of about 160 fmol of synapsin I/microgram of phospholipid. Increasing the ionic strength decreased the affinity without greatly affecting the maximal amount of synapsin I bound. When vesicles containing cholesterol and either phosphatidylcholine or phosphatidylcholine/phosphatidylethanolamine were tested, no significant binding was detected under any conditions examined. On the other hand, phosphatidylcholine vesicles containing either phosphatidylserine or phosphatidylinositol strongly interacted with synapsin I. The amount of synapsin I maximally bound was directly proportional to the percentage of acidic phospholipids present in the lipid bilayer, whereas the Kd value was not affected by varying the phospholipid composition. A study of synapsin I fragments obtained by cysteine-specific cleavage showed that the collagenase-resistant head domain actively bound to phospholipid vesicles; in contrast, the collagenase-sensitive tail domain, though strongly basic, did not significantly interact. Photolabeling of synapsin I was performed with the phosphatidylcholine analogue 1-palmitoyl-2-[11-[4-[3-(trifluoromethyl)diazirinyl]phenyl] [2-3H]undecanoyl]-sn-glycero-3-phosphocholine; this compound generates a highly reactive carbene that selectively interacts with membrane-embedded domains of membrane proteins. Synapsin I was significantly labeled upon photolysis when incubated with lipid vesicles containing acidic phospholipids and trace amounts of the photoactivatable phospholipid. Proteolytic cleavage of photolabeled synapsin I localized the label to the head domain of the molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Striatal phosphoproteins in Parkinson disease and progressive supranuclear palsy

This study was undertaken to evaluate the levels of cAMP-regulated phosphoproteins in the striatum of patients with neurodegenerative diseases of the dopaminergic system. Postmortem samples of caudate nucleus and putamen from 24 control subjects, 23 patients with Parkinson disease, and 13 patients with progressive supranuclear palsy were studied with immunoblotting techniques. The levels of tyrosine hydroxylase were reduced in patients with Parkinson disease (levels were 24% and 10% of controls in caudate nucleus and putamen, respectively) and with progressive supranuclear palsy (levels were 11% and 6% of controls in caudate nucleus and putamen, respectively). Five phosphoproteins, which are present in striatal neurons and are likely to play a role in the postsynaptic actions of dopamine, were measured. These included ARPP-16, ARPP-19, ARPP-21 (cAMP-regulated phosphoproteins of Mr 16,000, 19,000, and 21,000, respectively), DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of Mr 32,000), and phosphatase inhibitor I. The levels of these phosphoproteins were inversely correlated with postmortem delay. In brains of patients with Parkinson disease or progressive supranuclear palsy with postmortem delays comparable to those of controls, the levels of these proteins as well as those of synaptic (synapsin I and synaptophysin) and glial (glial fibrillary acidic protein and myelin basic protein) markers were not significantly modified. We conclude that the levels of several phosphoproteins involved in signal transduction in striatal neurons are not altered in Parkinson disease and progressive supranuclear palsy. This observation supports the view that the striatal output neurons are intact in both diseases.

Phosphorylation and associated translocation of the 87-kDa protein, a major protein kinase C substrate, in isolated nerve terminals

A protein of 87 kilodaltons (87 kDa) was previously identified as a major specific substrate for protein kinase C in neuronal and other tissues. We have now studied the effect of protein kinase C-catalyzed phosphorylation of this protein on its association with membranes in isolated nerve terminals (synaptosomes) from rat cerebral cortex. Incubation of synaptosomal membranes under conditions associated with activation of protein kinase C led to the release of the phosphorylated 87-kDa protein into the incubation medium. In intact synaptosomes, activation of protein kinase C by phorbol esters or by depolarization-induced Ca2+ influx caused an increased phosphorylation of the 87-kDa protein and its translocation from membrane to cytosol. This translocation showed time courses, calcium dependency, and reversibility similar to those observed for the protein kinase C-induced phosphorylation of the protein. These results suggest that protein kinase C-catalyzed phosphorylation of the 87-kDa protein is responsible for its subcellular translocation into the cytosol of nerve terminals.

ARPP-21, a cyclic AMP-regulated phosphoprotein enriched in dopamine-innervated brain regions. II. Immunocytochemical localization in rat brain

ARPP-21, a cAMP-regulated phosphoprotein, has been studied by immunocytochemistry to determine its cellular and regional distribution in rat brain. This study demonstrates that ARPP-21 immunoreactivity is present throughout the cytoplasm of immunoreactive neurons and that most of the immunoreactivity is associated with the basal ganglia. Within the caudatoputamen (CP), nucleus accumbens, olfactory tubercle, bed nucleus of the stria terminalis, and portions of the amygdaloid complex, ARPP-21 is present in neuronal somata and dendrites. In brain regions known to receive projections from these nuclei, immunoreactivity is present in puncta (presumed axons and axon terminals). These regions include the globus pallidus, ventral pallidum, entopeduncular nucleus, lateral preoptic area, and substantia nigra. Within the basal ganglia, ARPP-21 immunoreactivity is most intense in the olfactory tubercle, nucleus accumbens, medial portion of the CP, and the ventral retrochiasmatic pocket of the CP. These same areas comprise the limbic striatum, and ARPP-21 is the first substance found to be specifically enriched therein. The possibility is discussed that ARPP-21 mediates effects of multiple first messengers, including dopamine and vasoactive intestinal polypeptide, that act through cAMP.

Inhibitors of protein phosphatase-1. Inhibitor-1 of bovine adipose tissue and a dopamine- and cAMP-regulated phosphoprotein of bovine brain are identical

Protein phosphatase inhibitor-1 was purified from bovine adipose tissue. The protein had an apparent molecular mass of 32 kDa by SDS/PAGE and a Stokes' radius of 3.4 nm. It was phosphorylated by cAMP-dependent protein kinase on a threonyl residue; this phosphorylation was necessary for inhibition of protein phosphatase-1. Bovine adipose tissue inhibitor-1 was compared directly with rabbit skeletal muscle inhibitor-1 and with a 32000-Mr, dopamine- and cAMP-regulated phosphoprotein from bovine brain (DARPP-32), also an inhibitor of protein phosphatase-1. By the following biochemical and immunochemical criteria, bovine adipose tissue inhibitor-1 was found to be very similar and possibly identical to DARPP-32 and was clearly distinct from skeletal muscle inhibitor-1: molecular mass by SDS/PAGE; Stokes' radii; phosphorylation on threonine residues; Staphylococcus-aureus-V8-protease-generated peptide patterns analyzed by SDS/PAGE; tryptic phosphopeptide maps analysed by two-dimensional thin-layer electrophoresis/chromatography; elution on reverse-phase HPLC; chymotryptic peptide maps as analysed by reverse-phase HPLC; amino acid composition; antibody recognition by immunoprecipitation and immunoblotting; effect of cyanogen bromide cleavage on protein phosphatase inhibitor activity. Based on these results we conclude that bovine brain and adipose tissue contain an identical phosphoprotein inhibitor of protein phosphatase-1 (DARPP-32), which is distinct from that of skeletal muscle (inhibitor-1).

ARPP-21, a cyclic AMP-regulated phosphoprotein enriched in dopamine-innervated brain regions. I. Purification and characterization of the protein from bovine caudate nucleus

ARPP-21 (cAMP-regulated phosphoprotein, Mr = 21,000 as determined by SDS/PAGE) is a major cytosolic substrate for cAMP-stimulated protein phosphorylation in dopamine-innervated regions of rat CNS (Walaas et al., 1983c). This acidic phosphoprotein has now been identified in bovine caudate nucleus cytosol and purified to homogeneity from this source. The purification procedure involved diethylaminoethyl-cellulose chromatography, ammonium sulfate fractionation, phenyl-Sepharose CL-4B chromatography, and fast protein liquid chromatography using Mono Q anion-exchange resin. Two isoforms of ARPP-21 (ARPP-21A and ARPP-21B) were obtained, which were present in approximately equal amounts in the starting material. ARPP-21A was purified 2610-fold with a final yield of 20% and ARPP-21B was purified 2940-fold with a final yield of 21%. The purified preparations of both isoforms were judged to be homogenous by SDS/PAGE. ARPP-21A and ARPP-21B yielded identical 2-dimensional thin-layer tryptic phosphopeptide maps, identical amino acid compositions and closely related, but distinct, reverse-phase high-pressure liquid chromatograms of tryptic digests. The amino acid composition of ARPP-21 showed a high content of glutamic acid/glutamine, and no methionine, tryptophan, tyrosine, phenylalanine, or histidine. ARPP-21 was stable to heat denaturation and to 50% (vol/vol) ethanol treatment and was partially soluble at pH 2. The Mr determined for ARPP-21 by SDS/PAGE was 21,000. The Stokes radius of ARPP-21 was 26.3 A, and the sedimentation coefficient of ARPP-21 was 1.3 S; these values yield a calculated molecular mass of 13,700 Da and a frictional ratio of 1.7, indicative of an elongated tertiary structure. ARPP-21 was an excellent substrate for cAMP-dependent protein kinase and was either not phosphorylated or only poorly phosphorylated by cGMP-dependent protein kinase, calcium/calmodulin-dependent protein kinase I, calcium/calmodulin-dependent protein kinase II, casein kinase II, or protein kinase C. The purified catalytic subunit of cAMP-dependent protein kinase catalyzed the incorporation of 1.2 mol phosphate/mol purified ARPP-21. Phosphorylation occurred exclusively on seryl residues. Phospho-ARPP-21 was dephosphorylated effectively by protein phosphatase-1 or -2A, but not by protein phosphatase-2B or -2C. Rabbit polyclonal and mouse monoclonal antibodies were prepared to purified ARPP-21. These antibodies specifically immunoprecipitated ARPP-21, which was found to be highly enriched in the caudate nucleus and putamen of monkey brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of ARPP-90, a major 90 kiloDalton basal ganglion-enriched substrate for cyclic AMP-dependent protein kinase, in striatonigral neurons in the rat brain

Cyclic AMP-regulated phosphoproteins with specific cellular localizations in brain represent important targets through which this second messenger system can mediate or modulate distinct neurotransmitter signals. This study reports that two cyclic AMP-regulated phosphoproteins (Mr 90,000 and 93,000) found in brain share several properties, including similar isoelectric points and similar phosphopeptide maps. This protein doublet is particularly enriched in the forebrain basal ganglia, but it can also be found in the substantia nigra, a brainstem region which is a major target for fibers from the forebrain basal ganglia. Quinolinic acid lesions of neurons in the neostriatum decrease the levels of the 90/93 kDa phosphoprotein doublet to about the same extent as they reduce the levels of DARPP-32, a phosphoprotein specifically enriched in striatonigral medium-sized spiny neurons. These reductions are seen in both the neostriatum and the substantia nigra. Therefore, within the basal ganglia, the 90/93 kDa phosphoprotein doublet, termed adenosine 3':5'-monophosphate-regulated phosphoprotein, Mr = 90,000 (ARPP-90), is largely, if not solely, present in striatonigral cells and fibers. The specific localization in these neurons suggests that ARPP-90 could be important in receptor-regulated, cyclic AMP-mediated functions in the striatonigral neurons.

Role of protein phosphorylation in neuronal signal transduction

Protein phosphorylation is involved in the regulation of a wide variety of physiological processes in the nervous system. Studies in which purified protein kinases or kinase inhibitors have been microinjected into defined cells while a specific response is monitored have demonstrated that protein phosphorylation is both necessary and sufficient to mediate responses of excitable cells to extracellular signals. The precise molecular mechanisms involved in neuronal signal transduction processes can be further elucidated by identification and characterization of the substrate proteins for the various protein kinases. The roles of three such substrate proteins in signal transduction are described in this article: 1) synapsin I, whose phosphorylation increases neurotransmitter release and thereby modulates synaptic transmission presynaptically; 2) the nicotinic acetylcholine receptor, whose phosphorylation increases its rate of desensitization and thereby modulates synaptic transmission postsynaptically; and 3) DARPP-32, whose phosphorylation converts it to a protein phosphatase inhibitor and which thereby may mediate interactions between dopamine and other neurotransmitter systems. The characterization of the large number of additional phosphoproteins that have been found in the nervous system should elucidate many additional molecular mechanisms involved in signal transduction in neurons.