Differential phosphorylation of multiple sites in purified protein I by cyclic AMP-dependent and calcium-dependent protein kinases

A role for synapsin tetramerization in synaptic vesicle clustering

Although synapsins have long been proposed to be key regulators of synaptic vesicle (SV) clustering, their mechanism of action has remained mysterious and somewhat controversial. Here, we review synapsins and their associations with each other and with SVs. We highlight the recent hypothesis that synapsin tetramerization is a mechanism for SV clustering. This hypothesis, which aligns with numerous experimental results, suggests that the larger size of synapsin tetramers, in comparison to dimers, allows tetramers to form optimal bridges between SVs that overcome the repulsive force associated with the negatively charged membrane of SVs and allow synapsins to form a reserve pool of SVs within presynaptic terminals.

Chemical Genetic Identification of PKC Epsilon Substrates in Mouse Brain

PKC epsilon (PKCε) plays important roles in behavioral responses to alcohol and in anxiety-like behavior in rodents, making it a potential drug target for reducing alcohol consumption and anxiety. Identifying signals downstream of PKCε could reveal additional targets and strategies for interfering with PKCε signaling. We used a chemical genetic screen combined with mass spectrometry to identify direct substrates of PKCε in mouse brain and validated findings for 39 of them using peptide arrays and in vitro kinase assays. Prioritizing substrates with several public databases such as LINCS-L1000, STRING, GeneFriends, and GeneMAINA predicted interactions between these putative substrates and PKCε and identified substrates associated with alcohol-related behaviors, actions of benzodiazepines, and chronic stress. The 39 substrates could be broadly classified in three functional categories: cytoskeletal regulation, morphogenesis, and synaptic function. These results provide a list of brain PKCε substrates, many of which are novel, for future investigation to determine the role of PKCε signaling in alcohol responses, anxiety, responses to stress, and other related behaviors.

Bassoon controls synaptic vesicle release via regulation of presynaptic phosphorylation and cAMP

Neuronal presynaptic terminals contain hundreds of neurotransmitter‐filled synaptic vesicles (SVs). The morphologically uniform SVs differ in their release competence segregating into functional pools that differentially contribute to neurotransmission. The presynaptic scaffold bassoon is required for neurotransmission, but the underlying molecular mechanisms are unknown. We report that glutamatergic synapses lacking bassoon feature decreased SV release competence and increased resting pool of SVs as assessed by imaging of SV release in cultured neurons. CDK5/calcineurin and cAMP/PKA presynaptic signalling are dysregulated, resulting in an aberrant phosphorylation of their downstream effectors synapsin1 and SNAP25, well‐known regulators of SV release competence. An acute pharmacological restoration of physiological CDK5 and cAMP/PKA activity fully normalises the SV pools in neurons lacking bassoon. Finally, we demonstrate that CDK5‐dependent regulation of PDE4 activity interacts with cAMP/PKA signalling and thereby controls SV release competence. These data reveal that bassoon organises SV pools in glutamatergic synapses via regulation of presynaptic phosphorylation and cAMP homeostasis and indicate a role of CDK5/PDE4/cAMP axis in the control of neurotransmitter release.

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

Protein Phosphorylation in Depolarized Synaptosomes: Dissecting Primary Effects of Calcium from Synaptic Vesicle Cycling

Synaptic transmission is mediated by the regulated exocytosis of synaptic vesicles. When the presynaptic membrane is depolarized by an incoming action potential, voltage-gated calcium channels open, resulting in the influx of calcium ions that triggers the fusion of synaptic vesicles (SVs) with the plasma membrane. SVs are recycled by endocytosis. Phosphorylation of synaptic proteins plays a major role in these processes, and several studies have shown that the synaptic phosphoproteome changes rapidly in response to depolarization. However, it is unclear which of these changes are directly linked to SV cycling and which might regulate other presynaptic functions that are also controlled by calcium-dependent kinases and phosphatases. To address this question, we analyzed changes in the phosphoproteome using rat synaptosomes in which exocytosis was blocked with botulinum neurotoxins (BoNTs) while depolarization-induced calcium influx remained unchanged. BoNT-treatment significantly alters the response of the synaptic phoshoproteome to depolarization and results in reduced phosphorylation levels when compared with stimulation of synaptosomes by depolarization with KCl alone. We dissect the primary Ca2+-dependent phosphorylation from SV-cycling-dependent phosphorylation and confirm an effect of such SV-cycling-dependent phosphorylation events on syntaxin-1a-T21/T23, synaptobrevin-S75, and cannabinoid receptor-1-S314/T322 on exo- and endocytosis in cultured hippocampal neurons.

Synaptic vesicle traffic is supported by transient actin filaments and regulated by PKA and NO

Synaptic vesicles (SVs) can be pooled across multiple synapses, prompting questions about their dynamic allocation for neurotransmission and plasticity. We find that the axonal traffic of recycling vesicles is not supported by ubiquitous microtubule-based motility but relies on actin instead. Vesicles freed from synaptic clusters undergo ~1 µm bouts of active transport, initiated by nearby elongation of actin filaments. Long distance translocation arises when successive bouts of active transport were linked by periods of free diffusion. The availability of SVs for active transport can be promptly increased by protein kinase A, a key player in neuromodulation. Vesicle motion is in turn impeded by shutting off axonal actin polymerization, mediated by nitric oxide-cyclic GMP signaling leading to inhibition of RhoA. These findings provide a potential framework for coordinating post-and pre-synaptic strength, using retrograde regulation of axonal actin dynamics to mobilize and recruit presynaptic SV resources.

Depolarization-dependent Induction of Site-specific Changes in Sialylation on N-linked Glycoproteins in Rat Nerve Terminals

Synaptic transmission leading to release of neurotransmitters in the nervous system is a fast and highly dynamic process. Previously, protein interaction and phosphorylation have been thought to be the main regulators of synaptic transmission. Here we show that sialylation of N-linked glycosylation is a novel potential modulator of neurotransmitter release mechanisms by investigating depolarization-dependent changes of formerly sialylated N-linked glycopeptides. We suggest that negatively charged sialic acids can be modulated, similarly to phosphorylation, by the action of sialyltransferases and sialidases thereby changing local structure and function of membrane glycoproteins. We characterized site-specific alteration in sialylation on N-linked glycoproteins in isolated rat nerve terminals after brief depolarization using quantitative sialiomics. We identified 1965 formerly sialylated N-linked glycosites in synaptic proteins and found that the abundances of 430 glycosites changed after 5 s depolarization. We observed changes on essential synaptic proteins such as synaptic vesicle proteins, ion channels and transporters, neurotransmitter receptors and cell adhesion molecules. This study is to our knowledge the first to describe ultra-fast site-specific modulation of the sialiome after brief stimulation of a biological system.

Neuromodulator Signaling Bidirectionally Controls Vesicle Numbers in Human Synapses

Neuromodulators bind to pre- and postsynaptic G protein-coupled receptors (GPCRs), are able to quickly change intracellular cyclic AMP (cAMP) and Ca²⁺ levels, and are thought to play important roles in neuropsychiatric and neurodegenerative diseases. Here, we discovered in human neurons an unanticipated presynaptic mechanism that acutely changes synaptic ultrastructure and regulates synaptic communication. Activation of neuromodulator receptors bidirectionally controlled synaptic vesicle numbers within nerve terminals. This control correlated with changes in the levels of cAMP-dependent protein kinase A-mediated phosphorylation of synapsin-1. Using a conditional deletion approach, we reveal that the neuromodulator-induced control of synaptic vesicle numbers was largely dependent on synapsin-1. We propose a mechanism whereby non-phosphorylated synapsin-1 "latches" synaptic vesicles to presynaptic clusters at the active zone. cAMP-dependent phosphorylation of synapsin-1 then removes the vesicles. cAMP-independent dephosphorylation of synapsin-1 in turn recruits vesicles. Synapsin-1 thereby bidirectionally regulates synaptic vesicle numbers and modifies presynaptic neurotransmitter release as an effector of neuromodulator signaling in human neurons.

A liquid phase of synapsin and lipid vesicles

Neurotransmitter-containing synaptic vesicles (SVs) form tight clusters at synapses. These clusters act as a reservoir from which SVs are drawn for exocytosis during sustained activity. Several components associated with SVs that are likely to help form such clusters have been reported, including synapsin. Here we found that synapsin can form a distinct liquid phase in an aqueous environment. Other scaffolding proteins could coassemble into this condensate but were not necessary for its formation. Importantly, the synapsin phase could capture small lipid vesicles. The synapsin phase rapidly disassembled upon phosphorylation by calcium/calmodulin-dependent protein kinase II, mimicking the dispersion of synapsin 1 that occurs at presynaptic sites upon stimulation. Thus, principles of liquid-liquid phase separation may apply to the clustering of SVs at synapses.

Functional distribution of synapsin I in human sperm

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Synapsin I was localized in the human sperm equatorial segment.

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Presence of synapsin I was confirmed by dot and Western blotting techniques.

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Treatment of sperm with anti-synapsin antibodies significantly decreased motility.

Proteins known to function during cell–cell communication and exocytosis in neurons and other secretory cells have recently been reported in human sperm. Synapsins are a group of proteins that have been very well characterized in neurons, but little is known about synapsin function in other cell types. Based upon previous findings and the known function of synapsin, we tested the hypothesis that synapsin I was present in human sperm. Washed, capacitated, and acrosome induced sperm preparations were used to evaluate the functional distribution of synapsin I using immunocytochemistry. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were used for protein blotting techniques. Immunolocalization revealed synapsin I was enriched in the sperm equatorial segment. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were positive for synapsin I using several different antibodies, and dot blot results were confirmed by Western blot analyses. Finally, treatment of capacitated and acrosome reaction induced samples with anti-synapsin antibodies significantly reduced sperm motility. Localization of synapsin I in human sperm is a novel finding. The association of synapsin I with the sperm equatorial segment and effects on motility are suggestive of a role associated with capacitation and/or acrosome reaction, processes that render sperm capable of fertilizing an oocyte.

Phospholipid dysregulation contributes to ApoE4-associated cognitive deficits in Alzheimer's disease pathogenesis

The apolipoprotein E4 (ApoE4) allele is the strongest genetic risk factor for developing sporadic Alzheimer's disease (AD). However, the mechanisms underlying the pathogenic nature of ApoE4 are not well understood. In this study, we have found that ApoE proteins are critical determinants of brain phospholipid homeostasis and that the ApoE4 isoform is dysfunctional in this process. We have found that the levels of phosphoinositol biphosphate (PIP2) are reduced in postmortem human brain tissues of ApoE4 carriers, in the brains of ApoE4 knock-in (KI) mice, and in primary neurons expressing ApoE4 alleles compared with those levels in ApoE3 counterparts. These changes are secondary to increased expression of a PIP2-degrading enzyme, the phosphoinositol phosphatase synaptojanin 1 (synj1), in ApoE4 carriers. Genetic reduction of synj1 in ApoE4 KI mouse models restores PIP2 levels and, more important, rescues AD-related cognitive deficits in these mice. Further studies indicate that ApoE4 behaves similar to ApoE null conditions, which fails to degrade synj1 mRNA efficiently, unlike ApoE3 does. These data suggest a loss of function of ApoE4 genotype. Together, our data uncover a previously unidentified mechanism that links ApoE4-induced phospholipid changes to the pathogenic nature of ApoE4 in AD.

Monocular deprivation delays the dynamic changes of phosphorylated synapsin Ia/b at site-1 in contralateral visual cortex of juvenile mice

Synapsins as a family of presynaptic terminal phosphoprotein participates in neuronal development, but their role in the synaptic plasticity of visual cortex is unclear. In this study, the impact of monocular deprivation (MD) on dynamic changes of isoform-specific protein expression and site 1 phosphorylation of synapsins in visual cortex of the postnatal mice were observed by using the technique of Western blot analysis. The results showed that the total (T-) protein levels of synapsins including the isoform of Ia/b, IIa/b and IIIa were about 21-26% of adult level in visual cortex of mice at postnatal 7 days (P7), and then the T-synapsin Ia/b and IIb could quickly reach adult level at P35. However, the T-synapsin IIa and IIIa increased more slowly (71-74% at P35), and then kept increasing in the visual cortex of mice at P60. Unlike to the changes of T-synapsins, the level of phosphorylated (P-) synapsin Ia/b (not IIa/b and IIIa) at site 1 increased with development to the highest level at P21, and then decreased rapidly to a low level in visual cortex of mice at P35-60. In addition, we found that the levels of P-synapsin Ia/b increased significantly in left visual cortex of P28 and P35 (not P21 and P42) mice with 1-week MD of right eye; and no significant changes of T-synapsins were observed in both left and right sides of visual cortex in P21-42 mice with MD treatment. These results suggested that the isoform-specific protein expression and site-1 phosphorylation of synapsins might play a different role in the synaptic plasticity of visual cortex, and MD delays the dynamic changes of phosphorylated synapsin Ia/b at site-1 in contralateral visual cortex of juvenile mice.

Synaptic proteins as multi-sensor devices of neurotransmission

Neuronal communication is tightly regulated in time and space. Following neuronal activation, an electrical signal triggers neurotransmitter (NT) release at the active zone. The process starts by the signal reaching the synapse followed by a fusion of the synaptic vesicle (SV) and diffusion of the released NT in the synaptic cleft. The NT then binds to the appropriate receptor and induces a membrane potential change at the target cell membrane. The entire process is controlled by a fairly small set of synaptic proteins, collectively called SYCONs. The biochemical features of SYCONs underlie the properties of NT release. SYCONs are characterized by their ability to detect and respond to changes in environmental signals. For example, consider synaptotagmin I (Syt1), a prototype of a protein family with over 20 gene and variants in mammals. Syt1 is a specific example of a multi-sensor device with a large repertoire of discrete states. Several of these states are stimulated by a local concentration of signaling molecules such as Ca2+. The ability of this protein to sense signaling molecules and to adopt multiple biochemical states is shared by other SYCONs such as the synapsins (Syns). Specific biochemical states of Syns determine the accessibility of SV for NT release. Each of these states is defined by a specific alternative spliced variant with a unique profile of phosphorylation modified sites. The plasticity of the synapse is a direct reflection of SYCON's multiple biochemical states. State transitions occurs in a wide range of time scales, and therefore these molecules need to cope with events that last milliseconds (i.e., exocytosis in fast responding synapses) and with events that can carry on for many minutes (i.e., organization of SV pools). We suggest that SYCONs are optimized throughout evolution as multi-sensor devices. A full repertoire of the switches leading to alternation of protein states and a detailed characterization of protein-protein network within the synapse is critical for the development of a dynamic model of synaptic transmission.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Deletion of synapsins I and II genes alters the size of vesicular pools and rabphilin phosphorylation

Previous studies established that genetic deletion of synapsins, synaptic vesicle-associated phosphoproteins that regulate neurotransmitter release, decreases the number of synaptic vesicles in nerve terminals. To investigate whether these changes affect the release properties of the remaining synaptic vesicles, we used a radioactive labeling technique to measure release independently of the total number of synaptic vesicles. 3H-glutamate and 14C-gamma-amino-butyric-acid (GABA) release from isolated nerve terminals prepared from the neocortex of synapsins I and II double knock-out mice (DKO) was assayed and compared to wild-type preparations. Hyperosmotic shock-evoked 3H-glutamate was reduced by 20+/-3% from DKO nerve terminals and potassium depolarization-evoked glutamate release was also decreased by 28+/-2%. Surprisingly, sucrose or potassium depolarization-evoked release of 14C-GABA was increased by 32+/-4% and 29+/-5%, respectively. The basal efflux of both 3H-glutamate and 14C-GABA increased by 17+/-2% and 12+/-2% from DKO nerve terminals. As lack of synapsins I and II, major phosphoproteins of synaptic vesicles, may lead to deregulation of phosphorylation events, we compared phosphorylation state of another synaptic vesicle protein, rabphilin. In DKO nerve terminals, membrane-associated rabphilin level was reduced by approximately 0.28-fold, its phosphorylation at 234serine was increased by approximately 1.61-fold whereas cytosolic rabphilin levels showed both more dramatic reduction in abundance, approximately 16.5-fold, and increase in phosphorylation, approximately 4.8-fold. Collectively, these data suggest that deletion of major synapsin isoforms leads to (1) deregulation of basal neurotransmission causing "leaky" basal release, (2) changes in either the size or mobilization of releasable or reserve pools, and (3) a decrease in rabphilin abundance accompanied by an increase in basal phosphorylation of the remaining rabphilin.

Synapsin I is a major endogenous substrate for protein L-isoaspartyl methyltransferase in mammalian brain

The accumulation of potentially deleterious L-isoaspartyl linkages in proteins is prevented by the action of protein L-isoaspartyl O-methyltransferase, a widely distributed enzyme that is particularly active in mammalian brain. Methyltransferase-deficient (knock-out) mice exhibit greatly increased levels of isoaspartate and typically succumb to fatal epileptic seizures at 4-10 weeks of age. The link between isoaspartate accumulation and the neurological abnormalities of these mice is poorly understood. Here, we demonstrate that synapsin I from knock-out mice contains 0.9 +/- 0.3 mol of isoaspartate/mol of synapsin, whereas the levels in wild-type and heterozygous mice are undetectable. Transgenic mice that selectively express methyltransferase only in neurons show reduced levels of synapsin damage, and the degree of reduction correlates with the phenotype of these mice. Isoaspartate levels in synapsin from the knock-out mice are five to seven times greater than those in the average protein from brain cytosol or from a synaptic vesicle-enriched fraction. The isoaspartyl sites in synapsin from knock-out mice are efficiently repaired in vitro by incubation with purified methyltransferase and S-adenosyl-L-methionine. These findings demonstrate that synapsin I is a major substrate for the isoaspartyl methyltransferase in neurons and suggest that isoaspartate-related alterations in the function of presynaptic proteins may contribute to the neurological abnormalities of mice deficient in this enzyme.

Synapsin I is expressed in epithelial cells: localization to a unique trans-Golgi compartment

Synapsin I is abundant in neural tissues. Its phosphorylation is thought to regulate synaptic vesicle exocytosis in the pre-synaptic terminal by mediating vesicle tethering to the cytoskeleton. Using anti-synapsin antibodies, we detected an 85 kDa protein in liver cells and identified it as synapsin I. Like brain synapsin I, non-neuronal synapsin I is phosphorylated in vitro by protein kinase A and yields identical 32P-peptide maps after limited proteolysis. We also detected synapsin I mRNA in liver by northern blot analysis. These results indicate that the expression of synapsin I is more widespread than previously thought. Immunofluorescence analysis of several non-neuronal cell lines localizes synapsin I to a vesicular compartment adjacent to trans-elements of the Golgi complex, which is also labeled with antibodies against myosin II; no sub-plasma membrane synapsin I is evident. We conclude that synapsin I is present in epithelial cells and is associated with a trans-Golgi network-derived compartment; this localization suggests that it plays a role in modulating post-TGN trafficking pathways.

Is persistent activity of calcium/calmodulin-dependent kinase required for the maintenance of LTP?

Calcium/calmodulin-dependent protein kinase II (CaMKII) is concentrated in the postsynaptic density (PSD) and plays an important role in the induction of long-term potentiation (LTP). Because this kinase is persistently activated after the induction, its activity could also be important for LTP maintenance. Experimental tests of this hypothesis, however, have given conflicting results. In this paper we further explore the role of postsynaptic CaMKII in induction and maintenance of LTP. Postsynaptic application of a CaMKII inhibitor [autocamtide-3 derived peptide inhibitor (AC3-I), 2 mM] blocked LTP induction but had no detectable affect on N-methyl-D-aspartate (NMDA)-mediated synaptic transmission, indicating that the primary function of CaMKII in LTP is downstream from NMDA channel function. We next explored various methodological factors that could account for conflicting results on the effect of CaMKII inhibitors on LTP maintenance. In contrast to our previous work, we now carried out experiments at higher temperature (33 degrees C), used slices from adult animals, and induced LTP using a tetanic stimulation. However, we still found that LTP maintenance was not affected by postsynaptic application of AC3-I. Furthermore the inhibitor did not block LTP maintenance under conditions designed to enhance the Ca(2+)-dependent activity of protein phosphatases 1 and 2B (elevated Ca(2+), calmodulin, and an inhibitor of protein kinase A). We also tested the possibility that CaMKII inhibitor might not be able to affect CaMKII once it was inserted into the PSD. In whole-brain extracts, AC3-I blocked autophosphorylation of both soluble and particulate/PSD CaMKII with similar potencies although the potency of the inhibitor toward other CaMKII substrates varied. Thus we were unable to demonstrate a functional role of persistent Ca(2+)-independent CaMKII activity in LTP maintenance. Possible explanations of the data are discussed.

Use of phosphosynapsin I-specific antibodies for image analysis of signal transduction in single nerve terminals

We have developed a semi-quantitative method for indirectly revealing variations in the concentration of second messengers (Ca(2+), cyclic AMP) in single presynaptic boutons by detecting the phosphorylation of the synapsins, excellent nerve terminal substrates for cyclic AMP- and Ca(2+)/calmodulin-dependent protein kinases. For this purpose, we employed polyclonal, antipeptide antibodies recognising exclusively synapsin I phosphorylated by Ca(2+)/calmodulin-dependent protein kinase II (at site 3) or synapsins I/II phosphorylated by either cAMP-dependent protein kinase or Ca(2+)/calmodulin-dependent protein kinase I (at site 1). Cerebellar granular neurones in culture were double-labelled with a monoclonal antibody to synapsins I/II and either of the polyclonal antibodies. Digitised images were analysed to determine the relative phosphorylation stoichiometry at each individual nerve terminal. We have found that: (i) under basal conditions, phosphorylation of site 3 was undetectable, whereas site 1 exhibited some degree of constitutive phosphorylation; (ii) depolarisation in the presence of extracellular Ca(2+) was followed by a selective and widespread increase in site 3 phosphorylation, although the relative phosphorylation stoichiometry varied among individual terminals; and (iii) phosphorylation of site 1 was increased by stimulation of cyclic AMP-dependent protein kinase but not by depolarisation and often occurred in specific nerve terminal sub-populations aligned along axon branches. In addition to shedding light on the regulation of synapsin phosphorylation in living nerve terminals, this approach permits the spatially-resolved analysis of the activation of signal transduction pathways in the presynaptic compartment, which is usually too small to be studied with other currently available techniques.

A phospho-switch controls the dynamic association of synapsins with synaptic vesicles

Synapsins constitute a family of synaptic vesicle proteins essential for regulating neurotransmitter release. Only two domains are conserved in all synapsins: a short N-terminal A domain with a single phosphorylation site for cAMP-dependent protein kinase (PKA) and CaM Kinase I, and a large central C domain that binds ATP and may be enzymatic. We now demonstrate that synapsin phosphorylation in the A domain, at the only phosphorylation site shared by all synapsins, dissociates synapsins from synaptic vesicles. Furthermore, we show that the A domain binds phospholipids and is inhibited by phosphorylation. Our results suggest a novel mechanism by which proteins reversibly bind to membranes using a phosphorylation-dependent phospholipid-binding domain. The dynamic association of synapsins with synaptic vesicles correlates with their role in activity-dependent plasticity.

Homo- and heterodimerization of synapsins

In vertebrates, synapsins constitute a family of synaptic vesicle proteins encoded by three genes. Synapsins contain a central ATP-binding domain, the C-domain, that is highly homologous between synapsins and evolutionarily conserved in invertebrates. The crystal structure of the C-domain from synapsin I revealed that it constitutes a large (>300 amino acids), independently folded domain that forms a tight dimer with or without bound ATP. We now show that the C-domains of all synapsins form homodimers, and that in addition, C-domains from different synapsins associate into heterodimers. This conclusion is based on four findings: 1) in yeast two-hybrid screens with full-length synapsin IIa as a bait, the most frequently isolated prey cDNAs encoded the C-domain of synapsins; 2) quantitative yeast two-hybrid protein-protein binding assays demonstrated pairwise strong interactions between all synapsins; 3) immunoprecipitations from transfected COS cells confirmed that synapsin II heteromultimerizes with synapsins I and III in intact cells, and similar results were obtained with bacterial expression systems; and 4) quantification of the synapsin III level in synapsin I/II double knockout mice showed that the level of synapsin III is decreased by 50%, indicating that heteromultimerization of synapsin III with synapsins I or II occurs in vivo and is required for protein stabilization. These data suggest that synapsins coat the surface of synaptic vesicles as homo- and heterodimers in which the C-domains of the various subunits have distinct regulatory properties and are flanked by variable C-terminal sequences. The data also imply that synapsin III does not compensate for the loss of synapsins I and II in the double knockout mice.

Regulation of quantal transmitter secretion by ATP and protein kinases at developing neuromuscular synapses

The effects of endogenously released ATP on the maturation of developing neuromuscular synapses were investigated in Xenopus nerve-muscle co-cultures. The potentiating action of ATP (1 mM) on spontaneous acetylcholine release was inhibited by P2-purinoceptor antagonists suramin (0.3 mM) and reactive blue 2 (RB-2, 3 microM) in day 1 cultures. Bath application of suramin (10 microM) or RB-2 in day 1 cultures and prolonged treatment for 2 days dramatically decreased the amplitude of both spontaneous synaptic currents (SSCs) and evoked synaptic currents (ESCs) in the same cultures on day 3. Chronic treatment with 8-cyclopentyltheophylline (4 microM) or 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX, 10 microM), P1-purinoceptor and glutamate receptor antagonists respectively, did not exert such an inhibitory effect. Chronic treatment with suramin or RB-2 for 2 days had no significant effect on the amplitude of either iontophoretic acetylcholine-induced whole-cell currents or single acetylcholine channel measurements in 3-day-old cultured myocytes. In addition, prolonged treatment for 2 days with various kinase inhibitors such as H-8 (10 microM), KN-62 (5 microM) and H-7 (10 microM) also decreased the amplitudes of both spontaneous and evoked synaptic currents in natural synapses, but not those of iontophoretic acetylcholine-induced currents. Furthermore, suramin and these protein kinase inhibitors also decreased the amplitude of spontaneous synaptic currents in manipulated synapses of 'vacated' nerve terminals. The results suggest that endogenously released ATP, acting in concert with various protein kinases, is involved in the maintenance and/or development of the quantum size of synaptic vesicles at embryonic neuromuscular synapses.

Noradrenaline synthesis after sympathetic nerve activation in rat atria and its dependence on calcium but not CAM kinase II and protein kinases A or C

1. The biosynthesis of noradrenaline following sympathetic nerve activation was investigated in rat atria. In particular the time course of noradrenaline synthesis changes, the relationship of changes in synthesis to transmitter release and the possible roles of second messengers and protein kinases were examined. 2. Rat atria incubated with the precursor [3H]-tyrosine synthesized [3H]-noradrenaline. Synthesis was enhanced following pulsatile electrical field stimulation (3 Hz for 5 min) with the bulk of the increase occurring in the first 45 min after the commencement of electrical stimulation. In separate experiments rat atria were pre-incubated with [3H]-noradrenaline and the radioactive outflow in response to electrical field stimulation (3 Hz for 5 min) was taken as an index of noradrenaline release. 3. Stimulation-induced (S-I) noradrenaline synthesis was significantly correlated to S-I noradrenaline release for a variety of procedures which modulate noradrenaline release by mechanisms altering Ca2+ entry into the neurone (r2 = 0.99): those which decreased release: tetrodotoxin (0.3 microM), Ca(2+)-free medium, lowering the frequency of nerve activation to 1 Hz, and those which increased release, tetraethylammonium (0.3 mM), phentolamine (1 microM) and the combination of phentolamine (1 microM) and adenosine (10 microM). On the strength of this relationship we suggest that Ca2+ entry is a determining factor in S-I synthesis changes rather than the amount of noradrenaline released. Indeed the reduction in noradrenaline release with the calmodulin-dependent protein (CAM) kinase II inhibitor KN-62 (10 microM) which acts subsequent to Ca2+ entry, did not affect S-I synthesis. 4. The cell permeable cyclic AMP analogue, 8-bromoadenosine 3',5'-monophosphate (BrcAMP, 90 and 270 microM), dose-dependently increased basal [3H]-noradrenaline synthesis in unstimulated rat atria. This effect was antagonized by the selective protein kinase A (PKA) antagonist, Rp-8-chloroadenosine 3',5'-cyclic monophosphorothioate (RClcAMPS, 300 microM), suggesting that PKA activation enhances basal noradrenaline biosynthesis in sympathetic nerve terminals. 5. The protein kinase inhibitors, KN-62 (CAM kinase II, 10 microM), RClcAMPS (PKA, 300 microM), polymyxin B (protein kinase C (PKC), 21 microM) and staurosporine (PKC, PKA and CAM kinase II, (0.1 microM) did not affect S-I synthesis, although KN-62, polymyxin B and staurosporine decreased S-I release. We conclude that S-I synthesis is triggered by Ca2+ entering the neurone but that the signalling pathway does not involve classical protein kinases and appears distinct from the steps involved in transmitter release.

Site-specific phosphorylation of synapsin I by mitogen-activated protein kinase and Cdk5 and its effects on physiological functions

Posttranslational modifications of synapsin I, a major phosphoprotein in synaptic terminals, were studied by mass spectrometry. In addition to a well known phosphorylation site by calmodulin-dependent protein kinase II (CaM kinase II), a hitherto unrecognized site (Ser553) was found phosphorylated in vivo. The phosphorylation site is immediately followed by a proline, suggesting that the protein is an in vivo substrate of so-called proline-directed protein kinase(s). To identify the kinase involved, three proline-directed protein kinases expressed highly in the brain, i.e. mitogen-activated protein (MAP) kinase, Cdk5-p23, and glycogen synthase kinase 3beta, were tested for the in vitro phosphorylation of synapsin I. Only MAP kinase and Cdk5-p23 phosphorylated synapsin I stoichiometrically. The phosphorylation sites were determined to be Ser551 and Ser553 with Cdk5-p23, and Ser62, Ser67, and Ser551 with MAP kinase. Upon phosphorylation with MAP kinase, synapsin I showed reduced F-actin bundling activity, while no significant effect on the interaction was observed with the protein phosphorylated with Cdk5-p23. These results raise the possibility that the so-called proline-directed protein kinases together with CaM kinase II and cAMP-dependent protein kinase play an important role in the regulation of synapsin I function.

Structural basis for the autoinhibition of calcium/calmodulin-dependent protein kinase I

The crystal structure of calcium/calmodulin-dependent protein kinase I has been determined in the autoinhibited form. The C-terminal regulatory region of the enzyme forms a helix-loop-helix segment that extends across the two domains of the catalytic core, making multiple inhibitory interactions. Elements of the first regulatory alpha helix and the loop interfere with the binding site for peptide substrates, while the loop and the second helix interact with the ATP-binding domain to induce conformational changes that obstruct the nucleotide binding pocket. One part of the calmodulin recognition element protrudes away from the catalytic domain and is potentially available for an initial interaction with calmodulin. The structure provides a view of an intact calmodulin target and suggests that substantial structural changes will accompany kinase activation by calmodulin binding to the regulatory region.

Localization of mRNA for Ca2+/calmodulin-dependent protein kinase I in the brain of developing and mature rats

The gene expression of Ca2+/calmodulin-dependent protein kinase I (CaM kinase I) in the brain of developing and adult rats was examined by in situ hybridization histochemistry. During the development, CaM kinase I showed two chronological expression patterns; the persistent and relative high expression as observed in the olfactory bulb and cerebellar cortex, and the gradual decrease in the expression during the postnatal development as observed in most other brain regions. The gene expression was not detected in the germinal ventricular zone and cerebellar external granular layer. In the mature brain, CaM kinase I mRNA was expressed widely, though weakly in general, in almost all neurons, except for the olfactory bulb, cerebellum and hippocampus expressing at high intensity. These findings suggest that CaM kinase I may play a variety of neuronal Ca2+/calmodulin-mediated signaling processes in the developing and mature brains.

Effects of KS-501, KS-502 and their enantiomers on calmodulin-sensitive enzyme activity and cellular proliferation

Calmodulin plays an important role in cellular proliferation as part of a signal transduction pathway activated by phospholipase C. Drugs that block the ability of calmodulin to bind to and activate its target enzymes inhibit the growth of a wide variety of malignant cells. To identify more potent and selective inhibitors of this potential target for new drug development, we studied two recently synthesized compounds, KS-501 and KS-502, for their activity against calmodulin-sensitive enzymes and for their ability to block the growth of parental and multidrug-resistant leukemic cells. KS-501 and KS-502 inhibited the activation of a calmodulin-sensitive cyclic nucleotide phosphodiesterase. The mechanism of enzyme inhibition was through interfering with calmodulin activation rather than through a direct effect on the enzyme. KS-501 was more potent than KS-502 and was studied in greater detail. This compound inhibited the activation of calmodulin kinase I and II, but had less effect against cyclic adenosine 3',5'-monophosphate (cyclic AMP)-sensitive kinase. KS-501 was also more effective than KS-502 in inhibiting the growth of sensitive L1210 leukemic lymphocytes. Both compounds were less effective inhibitors of multidrug-resistant L1210 leukemia than of the parental line. These studies identify a new class of calmodulin inhibitor, with selectivity for calmodulin-dependent kinases over cyclic AMP-dependent protein kinase. Since the total synthesis of the KS-compounds has been accomplished, it should now be possible to develop derivatives with greater activity and selectivity.

Increase in synapsin I phosphorylation implicates a presynaptic component in septal kindling

Synaptic plasticity in the CNS is thought to be an important component of learning and memory. Kindling is an animal model of synaptic plasticity in which repetitive local electrical stimulation eventually leads to a generalized motor seizure. Once established, the sensitivity of kindled animals to this epileptic condition is long-lasting. An increase in synaptic efficacy appears to underlie the plastic changes observed in kindling but the molecular mechanisms involved remain unknown. Here we demonstrate that the phosphorylation state of synapsin I, a synaptic vesicle-associated protein which has been implicated in the regulation of neurotransmitter release, is significantly increased in hippocampus and parietal cortex of rats two weeks after the establishment of septal kindling. Furthermore, K(+)-evoked release of L-glutamate is significantly increased in synaptosomes prepared from cerebral cortex of kindled animals. Thus, changes within the presynaptic nerve terminal may contribute, at least in part, to the long-lasting modification in neuronal function induced by kindling.

Haloperidol-induced morphological alterations are associated with changes in calcium/calmodulin kinase II activity and glutamate immunoreactivity

Administration of haloperidol for 2 weeks causes an increase within the caudate nucleus of asymmetrical synapses associated with a discontinuous or perforated, postsynaptic density (PSD) [Meshul et al. (1992), Psychopharmacology, 106:45-52; Meshul et al. (1992), Neuropsychopharmacology, 7:285-293]. Coadministration of the N-methyl-D-aspartate noncompetitive antagonist, MK-801, with haloperidol blocked the increase in striatal synapses containing a perforated PSD [Meshul et al. (1994), Brain Res., 648:181-195]. Examination of the caudate using immuno-gold electron microscopy revealed the vast majority (90%) of asymmetrical synapses were labelled with a glutamate antibody [Meshul et al. (1994), Brain Res., 648:181-195]. The purpose of this study was to determine if there were any changes in the density of glutamate immunoreactivity within presynaptic terminals of asymmetric synapses within the striatum following treatment with haloperidol for 1 month that would correlate with the previously observed increase in synapses with perforated PSDs. We also determined the activity of striatal calcium/calmodulin kinase II (CaMK II), an enzyme known to be localized within the synaptic region, after administration of haloperidol. We report here that haloperidol causes an increase in the activity of CaMK II and a decrease in the density of immuno-gold labelling for glutamate within the nerve terminals of asymmetrical synapses containing a perforated or nonperforated PSD. These results are consistent with the hypothesis that the haloperidol-induced increase in activity of CaMK II and the increase in glutamate release, as suggested by the decrease in presynaptic glutamate immunoreactivity, may ultimately lead to an increase in the number of synapses displaying a perforated PSD. These results support the speculation that the haloperidol-induced increase in synapses containing a perforated PSD may be associated with enhanced activity at excitatory synapses.

Evidence that two non-overlapping high-affinity calmodulin-binding sites are present in the head region of synapsin I

Calmodulin is an important element in the regulation of nerve terminal exocytosis by Ca2+. Calmodulin has been shown to interact with the synaptic vesicle phosphoproteins synapsins Ia and Ib [Okabe, T. & Sobue, K. (1987) FEBS Lett. 213, 184-188; Hayes, N. V. L., Bennett, A. F. & Baines, A. J. (1991) Biochem. J. 275, 93-97]. These proteins are thought to provide regulated linkages between synaptic vesicles and cytoskeletal elements. It is well established that calmodulin modulates synapsin I activities via calmodulin-dependent protein-kinase-II-catalysed phosphorylation. The direct binding of calmodulin to synapsin I suggests a second mode of regulation in addition to phosphorylation. In this study, we present evidence indicating that two sites for calmodulin binding exist in the N-terminal head region of synapsins Ia and Ib. In unphosphorylated synapsin I, these sites had a Kd value of = 36 +/- 14 nM for binding to calmodulin labelled with acetyl-N'-(5-sulpho-1-naphthyl)ethylene diamine. The Kd values for synapsin I phosphorylated at various sites were as follows: site I 18 +/- 11 nM; sites II and III 35 +/- 14 nM; sites I-III 16 +/- 9 nM. The fluorescence data indicated a stoichiometry of not less than 2 mol calmodulin bound to 1 mol synapsin I at saturation in each case. Consistent with this stoichiometry, two chemically cross-linked species (96 kDa and 116 kDa) containing calmodulin and synapsin I were generated in vitro, corresponding to one and two calmodulin molecules bound/synapsin I. Defined fragments of synapsin I were generated with the reagent 2-nitro-5-thiocyanobenzoic acid, which cleaves at cysteine residues. Cysteine-specific cleavage of whole synapsin I after cross-linking to biotinylated calmodulin generated a pair of polypeptide complexes (approximately 46 kDa and 38 kDa), the masses of which indicated cross-linking of calmodulin to the N-terminal and middle regions of synapsin I. Purified N-terminal and middle fragments each showed a Ca(2+)-dependent interaction with calmodulin affinity columns. Two calmodulin-binding fragments (7.4 kDa and 6.5 kDa) were generated using Staphylococcus aureus V8 protease digestion of synapsin I. These fragments were isolated by calmodulin affinity chromatography and reverse-phase HPLC. N-terminal sequence analysis indicated that each was contained within one of the 2-nitro-5-thiocyanobenzoic-acid-derived calmodulin-binding fragments.(ABSTRACT TRUNCATED AT 400 WORDS)

Rectal epithelial expression of protein kinase A phosphorylation of cystic fibrosis transmembrane conductance regulator

BACKGROUND/AIMS

Human rectal epithelium in cystic fibrosis (CF) shows impaired ion transport in response to theophylline or bethanechol, although it possesses regulatory subunits of adenosine cyclic 3',5'-monophosphate (cAMP)-dependent protein kinase (protein kinase A). Protein kinase A-specific phosphorylation of CF transmembrane conductance regulator (CFTR) in rectal tissues of control and CF volunteers was examined in this study.

METHODS

CFTR was evaluated using a polyclonal antiserum (pre-NBF) raised against a peptide corresponding to residues 415-427 of CFTR. Microsomal membranes from normal and CF rectal mucosa and from T-84 cells were incubated with [gamma 32P]-adenosine triphosphate +/- protein kinase A and subjected to immunoblotting with pre-NBF and autoradiography.

RESULTS

Pre-NBF recognized a single band of 180 kilodaltons. Protein kinase A altered phosphorylation of this 180-kilodalton band 1.4-, 2.2- and 0.9-fold in T-84, normal, and CF rectal membranes, respectively. Catalytic activities of protein kinase A, Ca2+ calmodulin protein kinase, or protein kinase C in control and CF tissues were similar.

CONCLUSIONS

cAMP and Ca(2+)-signaling pathways are normal up to the kinases in CF rectal mucosa. Our results suggest differences in CFTR phosphorylation in normal and CF rectal mucosal membranes.

Phosphorylation of neuronal kinesin heavy and light chains in vivo

The microtubule-based motor protein kinesin is thought to drive anterograde organelle transport in axons, but nothing is known about how its force-generating activity or organelle-binding properties are regulated. Studies in other motility systems suggest that protein phosphorylation is a reasonable candidate for this function. I report here that the kinesin heavy chain (HC) and light chain (LC), as well as the 160-kDa kinesin-associated protein kinectin, are phosphorylated in vivo in cultures of chick sympathetic neurons and PC12 cells labeled metabolically with 32P. In neurons, both kinesin chains are phosphorylated exclusively on serine residues, and limiting tryptic digestion demonstrated that the phosphorylation sites are clustered in a region of < or = 5 kDa for the HC and < or = 14 kDa for the LC. Partial tryptic digestion of 32P-labeled HC followed by immunoblotting with SUK4 monoclonal anti-HC and fluorography showed that the sites of HC phosphorylation are outside the globular N-terminal head region where kinesin's microtubule-binding and mechanochemical activities reside. Treatment of metabolically labeled neurons with forskolin, phorbol esters, or calcium ionophore did not alter the extent of phosphorylation, the phosphoamino acid composition, or the V8 protease phosphopeptide maps of the HC, LC, and 160-kDa protein, with one exception: treatment with calcium ionophore reduced the specific activity of the LC. In addition, when kinesin from PC12 cells was compared with that from PC12-derived cell lines lacking protein kinase A activity, neither the extent of phosphorylation nor the phosphopeptide maps were altered for either chain. Phosphopeptide mapping experiments also showed that postlysis kinase activity can phosphorylate both the neuronal HC and LC at sites not phosphorylated in vivo.

Synaptic vesicle phosphoproteins and regulation of synaptic function

Complex brain functions, such as learning and memory, are believed to involve changes in the efficiency of communication between nerve cells. Therefore, the elucidation of the molecular mechanisms that regulate synaptic transmission, the process of intercellular communication, is an essential step toward understanding nervous system function. Several proteins associated with synaptic vesicles, the organelles that store neurotransmitters, are targets for protein phosphorylation and dephosphorylation. One of these phosphoproteins, synapsin I, by means of changes in its state of phosphorylation, appears to control the fraction of synaptic vesicles available for release and thereby to regulate the efficiency of neurotransmitter release. This article describes current understanding of the mechanism by which synapsin I modulates communication between nerve cells and reviews the properties and putative functions of other phosphoproteins associated with synaptic vesicles.

Characterization of cAMP-dependent protein kinase activation by CCK in rat pancreas

This study reports on the use of a new sensitive assay of cAMP-dependent protein kinase activity to examine the effect of cholecystokinin (CCK) on the cAMP second messenger cascade in rat pancreatic acini. Treatment of acini with both low (pM) and high (nM) concentrations of CCK was associated with an increase in cAMP-dependent protein kinase activity. The increases in kinase activity were detected in the absence of phosphodiesterase inhibition, a condition required to detect a measurable increase in cellular cAMP in these cells. Furthermore, the cAMP cascade was dissociated from the secretory effects of CCK, since the CCK analogue, OPE, mediates enzyme secretion but does not increase cellular cAMP levels or kinase activity.

Synapsin I, synapsin II, and synaptophysin: marker proteins of synaptic vesicles

The nerve terminal of neurons is filled with small synaptic vesicles, specialized secretory organelles involved in the storage and release of neurotransmitters. The synapsins are a family of four proteins that are the major peripheral proteins on the cytoplasmic face of synaptic vesicles. Synaptophysin is the major integral membrane protein of synaptic vesicles. The characterization of the synapsins and of synaptophysin during the last years has revealed exciting information about their structure, regulation and possible function. To understand the role of the synapsins and synaptophysin in the biology of a nerve cell means to elucidate the fundamental mechanism of brain function, the release of neurotransmitter.

Synapsin I partially dissociates from synaptic vesicles during exocytosis induced by electrical stimulation

The distribution of the synaptic vesicle-associated phosphoprotein synapsin I after electrical stimulation of the frog neuromuscular junction was investigated by immunogold labeling and compared with the distribution of the integral synaptic vesicle protein synaptophysin. In resting terminals both proteins were localized exclusively on synaptic vesicles. In stimulated terminals they appeared also in the axolemma and its infoldings, which however exhibited a lower synapsin I/synaptophysin ratio with respect to synaptic vesicles at rest. The value of this ratio was intermediate in synaptic vesicles of stimulated terminals, and an increased synapsin I labeling of the cytomatrix was observed. These results indicate that synapsin I undergoes partial dissociation from and reassociation with synaptic vesicles, following physiological stimulation, and are consistent with the proposed modulatory role of the protein in neurotransmitter release.

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.

Phosphorylation and activation of tyrosine hydroxylase in PC18 cells: a cell line derived from rat pheochromocytoma PC12 cells

Treatment of rat pheochromocytoma PC18 cells (a variant subclone of PC12 cells) with forskolin produced increased activity and phosphorylation of tyrosine hydroxylase. In contrast, treatment of the PC18 cells with 56 mM K+, A23187, phorbol-12-myristate-13-acetate (PMA) or phorbol-12,13-dibutyrate (PDB) did not affect the activity and only slightly increased the phosphorylation of tyrosine hydroxylase. None of the treatments except forskolin increased cyclic AMP levels in PC18 cells. Furthermore, 45Ca2+ uptake into PC18 cells was not affected by 56 mM K+, PDB or forskolin; however, A23187 increased 45Ca2+ uptake 4-fold over basal uptake. Nevertheless, no activation and little increase in phosphorylation of tyrosine hydroxylase was observed in PC18 cells treated with A23187. When tyrosine hydroxylase levels in PC18 cells were elevated by treatment with dexamethasone, activation of tyrosine hydroxylase by 56 mM K+, PDB or A23187 was still not observed. Both purified Ca2+/calmodulin-dependent protein kinase and cyclic AMP-dependent protein kinase catalyzed the phosphorylation of tyrosine hydroxylase purified from PC18 cells in vitro. Furthermore, crude cell extracts from PC12 cells and PC18 cells possessed Ca2+/calmodulin-dependent protein kinase activity that catalyzed the phosphorylation of purified tyrosine hydroxylase. These results suggest that tyrosine hydroxylase activity in PC18 cells is regulated by a cyclic AMP-dependent mechanism. However, due to a number of abnormalities the Ca(2+)-dependent mechanisms do not result in the activation of tyrosine hydroxylase and only slightly increase the phosphorylation of the enzyme in PC18 cells.

The morphological localization and biochemical characterization of a synapsin I-like antigen in the nervous system of Aplysia californica

Synapsins are a well-characterized class of phosphoproteins found at synapses in the mammalian nervous system. One member of this family, synapsin I, has been extensively studied and shown to associate in a phosphorylation-dependent manner with both small synaptic vesicles and cytoskeletal elements. Though the characteristics of synapsin I suggest an important function in synaptic transmission, its definitive role is still in question. In an effort to find a model system in which to test directly the function of synapsin I, we have looked in the nervous system of the marine mollusc Aplysia californica for synapsin I-like antigens (SILA). Light microscope immunocytochemical studies using polyclonal and monoclonal antibodies to bovine brain synapsin I demonstrate Aplysia SILA in neuronal somata, in the neuropil, and at some identified synapses. Though SILA were exclusively associated with neuronal structures in Aplysia, the pattern of staining suggested that they are not present at all synaptic terminals. This interpretation was corroborated by ultrastructural studies in which SILA were present at some synaptic terminals but absent, or in low abundance, in adjacent terminals. In axons, SILA were associated with vesicles of 120-150 nm diameter, as well as with filamentous structures. Biochemical studies identified small amounts of SILA of 40 and 50 kD molecular weight that are recognized by several antibodies to mammalian synapsin I, and are acid extractable, collagenase-sensitive phosphoproteins; these are criteria used to define synapsin I homologues in other species. Our studies indicate that SILA are present in neurons in Aplysia californica but suggested that they represent only a small percentage of the total protein within the nervous system.