Nerve impulses increase the phosphorylation state of protein I in rabbit superior cervical ganglion

Genetic mechanisms underlying brain functional homotopy: a combined transcriptome and resting-state functional MRI study

Functional homotopy, the high degree of spontaneous activity synchrony and functional coactivation between geometrically corresponding interhemispheric regions, is a fundamental characteristic of the intrinsic functional architecture of the brain. However, little is known about the genetic mechanisms underlying functional homotopy. Resting-state functional magnetic resonance imaging data from a discovery dataset (656 healthy subjects) and 2 independent cross-race, cross-scanner validation datasets (103 and 329 healthy subjects) were used to calculate voxel-mirrored homotopic connectivity (VMHC) indexing brain functional homotopy. In combination with the Allen Human Brain Atlas, transcriptome-neuroimaging spatial correlation analysis was conducted to identify genes linked to VMHC. We found 1,001 genes whose expression measures were spatially associated with VMHC. Functional enrichment analyses demonstrated that these VMHC-related genes were enriched for biological functions including protein kinase activity, ion channel regulation, and synaptic function as well as many neuropsychiatric disorders. Concurrently, specific expression analyses showed that these genes were specifically expressed in the brain tissue, in neurons and immune cells, and during nearly all developmental periods. In addition, the VMHC-associated genes were linked to multiple behavioral domains, including vision, execution, and attention. Our findings suggest that interhemispheric communication and coordination involve a complex interaction of polygenes with a rich range of functional features.

SAD-B Phosphorylation of CAST Controls Active Zone Vesicle Recycling for Synaptic Depression

Short-term synaptic depression (STD) is a common form of activity-dependent plasticity observed widely in the nervous system. Few molecular pathways that control STD have been described, but the active zone (AZ) release apparatus provides a possible link between neuronal activity and plasticity. Here, we show that an AZ cytomatrix protein CAST and an AZ-associated protein kinase SAD-B coordinately regulate STD by controlling reloading of the AZ with release-ready synaptic vesicles. SAD-B phosphorylates the N-terminal serine (S45) of CAST, and S45 phosphorylation increases with higher firing rate. A phosphomimetic CAST (S45D) mimics CAST deletion, which enhances STD by delaying reloading of the readily releasable pool (RRP), resulting in a pool size decrease. A phosphonegative CAST (S45A) inhibits STD and accelerates RRP reloading. Our results suggest that the CAST/SAD-B reaction serves as a brake on synaptic transmission by temporal calibration of activity and synaptic depression via RRP size regulation.

Phosphorylation of synapsin I by cAMP-dependent protein kinase controls synaptic vesicle dynamics in developing neurons

In developing neurons, synaptic vesicles (SVs) undergo cycles of exo-endocytosis along isolated axons. However, it is currently unknown whether SV exocytosis is regulated before synaptogenesis. Here, we show that cAMP-dependent pathways affect SV distribution and recycling in the axonal growth cone and that these effects are mediated by the SV-associated phosphoprotein synapsin I. The presence of synapsin I on SVs is necessary for the correct localization of the vesicles in the central portion of the growth cone. Phosphorylation of synapsin I by cAMP-dependent protein kinase (protein kinase A) causes the dissociation of the protein from the SV membrane, allowing diffusion of the vesicles to the periphery of the growth cone and enhancing their rate of recycling. These results provide new clues as to the bases of the well known activity of synapsin I in synapse maturation and indicate that molecular mechanisms similar to those operating at mature nerve terminals are active in developing neurons to regulate the SV life cycle before synaptogenesis.

Effects of neonatal cerebral hypoxia-ischemia on the in vitro phosphorylation of synapsin 1 in rat synaptosomes

Synapsins are phosphoproteins related to the anchorage of synaptic vesicles to the actin skeleton. Hypoxia-ischemia causes an increased calcium influx into neurons through ionic channels gated by activation of glutamate receptors. In this work seven-day-old Wistar rats were submitted to hypoxia-ischemia and sacrificed after 21 hours, 7, 30, or 90 days. Synaptosomal fractions were obtained by Percoll gradients and incubated with 32P (10 microCi/g). Proteins were analysed by SDS-PAGE and radioactivity incorporated into synapsin 1 was counted by liquid scintillation. Twenty-one hours after hypoxia-ischemia we observed a reduction on the in vitro phosphorylation of synapsin 1, mainly due to hypoxia, rather than to ischemia; this effect was reversed at day 7 after the insult. There was another decrease in phosphorylation 30 days after the event interpreted as a late effect of hypoxia-ischemia. No changes were observed at day 90. Our results suggest that decreased phosphorylation of synapsin 1 could be related to neuronal death that follows hypoxia-ischemia.

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.

Synaptophysin is phosphorylated in rat cortical synaptosomes treated with botulinum toxin A

Phosphorylation and dephosphorylation of neuronal proteins have been implicated in regulation of synaptic transmission. Studies were performed to determine if synaptophysin was phosphorylated or dephosphorylated during exposure of synaptosomes to botulinum toxin A (BoTX/A). Cholinergic-enriched synaptosomes were preincubated in the presence of 3H-choline to label newly synthesized acetylcholine (3H-ACh). This was followed by incubation with low or high potassium to stimulate release of newly synthesized 3H-ACh. BoTX/A inhibited total Ach release by 15-19% and inhibited release of newly synthesized 3H-ACh by 35%. A 165% increase in synaptophysin phosphorylation occurred in a dose-dependent manner over a range of doses (0.2 nM, 2 nM, 20 nM, 100 nM) of BoTX/A. When 4-Aminopyridine was added to synaptosomes that were BoTX/A treated, synaptophysin was dephosphorylated to control levels. Synaptosomes incubated with BoTX/A exhibited an inhibition of potassium stimulated ACh release and an increase in synaptophysin phosphorylation. Synaptophysin phosphorylation may be involved in inhibition of acetylcholine release.

Calcium signaling in neurons: molecular mechanisms and cellular consequences

Neuronal activity can lead to marked increases in the concentration of cytosolic calcium, which then functions as a second messenger that mediates a wide range of cellular responses. Calcium binds to calmodulin and stimulates the activity of a variety of enzymes, including calcium-calmodulin kinases and calcium-sensitive adenylate cyclases. These enzymes transduce the calcium signal and effect short-term biological responses, such as the modification of synaptic proteins and long-lasting neuronal responses that require changes in gene expression. Recent studies of calcium signal-transduction mechanisms have revealed that, depending on the route of entry into a neuron, calcium differentially affects processes that are central to the development and plasticity of the nervous system, including activity-dependent cell survival, modulation of synaptic strength, and calcium-mediated cell death.

Cytochemical polarity in lateral geniculate interneurons

To determine the cytochemical composition of presynaptic dendrites, we have examined the distribution of synapsin 1, calcium and calmodulin-dependent protein kinase II (CaM-II), microtubule-associated protein 2 (MAP-2) and spectrin in cat lateral geniculate (LGN) class III cells by immune-EM. Special attention was paid to the dendrites of these interneurons because they are both pre- and postsynaptic. The dendritic proteins MAP-2 and RBC spectrin were not observed in interneuron dendrites but these proteins were localized in relay cell dendrites. The synaptic vesicle-associated protein synapsin 1 was present in all synaptic vesicle containing profiles, including dendritic terminals. CaM-II, the major postsynaptic density protein, was found in all dendrites. Thus, the LGN interneuron dendritic compartment displays both axonal and dendritic cytochemical properties. The results suggest the possibility of unique molecular interactions in interneuron dendritic terminals.

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

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

Multifunctional Ca2+/calmodulin-dependent protein kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is a prominent mediator of neurotransmitters which elevate Ca2+. It coordinates cellular responses to external stimuli by phosphorylating proteins involved in neurotransmitter synthesis, neurotransmitter release, carbohydrate metabolism, ion flux and neuronal plasticity. Structure/function studies of CaM kinase have provided insights into how it decodes Ca2+ signals. The kinase is kept relatively inactive in its basal state by the presence of an autoinhibitory domain. Binding of Ca2+/calmodulin eliminates this inhibitory constraint and allows the kinase to phosphorylate its substrates, as well as itself. This autophosphorylation significantly slows dissociation of calmodulin, thereby trapping calmodulin even when Ca2+ levels are subthreshold. The kinase may respond particularly well to multiple Ca2+ spikes since trapping may enable a spike frequency-dependent recruitment of calmodulin with each successive Ca2+ spike leading to increased activation of the kinase. Once calmodulin dissociates, CaM kinase remains partially active until it is dephosphorylated, providing for an additional period in which its response to brief Ca2+ transients is potentiated.

Regulation and role of brain calcium/calmodulin-dependent protein kinase II

Ca2+/calmodulin-dependent protein kinase II (CaMKII) exhibits a broad substrate specificity and regulates diverse responses to physiological changes of intracellular Ca2+ concentrations. Five isozymic subunits of the highly abundant brain kinase are encoded by four distinct genes. Expression of each gene is tightly regulated in a cell-specific and developmental manner. CaMKII immunoreactivity is broadly distributed within neurons but is discretely associated with a number of subcellular structures. The unique regulatory properties of CaMKII have attracted a lot of attention. Ca2+/calmodulin-dependent autophosphorylation of a specific threonine residue (alpha-Thr286) within the autoinhibitory domain generates partially Ca(2+)-independent CaMKII activity. Phosphorylation of this threonine in CaMKII is modulated by changes in intracellular Ca2+ concentrations in a variety of cells, and may prolong physiological responses to transient increases in Ca2+. Additional residues within the calmodulin-binding domain are autophosphorylated in the presence of Ca2+ chelators and block activation by Ca2+/calmodulin. This Ca(2+)-independent autophosphorylation is very rapid following prior Ca2+/calmodulin-dependent autophosphorylation at alpha-Thr286 and generates constitutively active, Ca2+/calmodulin-insensitive CaMKII activity. Ca(2+)-independent autophosphorylation of CaMKII also occurs at a slower rate when alpha-Thr286 is not autophosphorylated and results in inactivation of CaMKII. Thus, Ca(2+)-independent autophosphorylation of CaMKII generates a form of the kinase that is refractory to activation by Ca2+/calmodulin. CaMKII phosphorylates a wide range of neuronal proteins in vitro, presumably reflecting its involvement in the regulation of diverse functions such as postsynaptic responses (e.g. long-term potentiation), neurotransmitter synthesis and exocytosis, cytoskeletal interactions and gene transcription. Recent evidence indicates that the levels of CaMKII are altered in pathological states such as Alzheimer's disease and also following ischemia.

Vasopressin decreases immunogold labeling of apical actin in the toad bladder granular cell

Studies with the confocal microscope have shown that arginine vasopressin (AVP) depolymerizes F-actin in the apical region of the toad bladder granular cell. However, the resolution of the fluorescence microscope is not great enough to reveal the exact pattern of depolymerization or the relative extent to which microvillar and subapical membrane actin pools contribute to overall depolymerization. We have developed an electron microscopic immunogold method that shows a significant decrease in immunogold labeling of actin in the region just below the apical membrane, with the decrease most pronounced in regions adjacent to the microvilli. There was no significant change of immunogold labeling within the microvilli themselves. Our studies show a reorganization of the actin cytoskeleton in the region of the granular cell, where water channel-carrying vesicles are positioned and fuse in response to AVP.

Conditions restricting depolarization-dependent calcium influx in synaptosomes reveal a graded response of P96 dephosphorylation and a transient dephosphorylation of P65

Temporal changes in the phosphorylation level of synaptosomal phosphoproteins following depolarization of synaptosomes were investigated under conditions restricting calcium influx. High-K+ depolarization in media of low [Na+]o (32 mM during preincubation and depolarization) at pH 6.5 resulted in a pronounced fall in the cytosolic free calcium concentration transient, and in a reduction in the initial K(+)-stimulated 45Ca2+ uptake and endogenous acetylcholine release relative to the values obtained with control synaptosomes (preincubated and depolarized in Na(+)-based media). This reduction was paralleled by a decrease in the rate of dephosphorylation of the synaptosomal protein P96. A slower dephosphorylation of P96 also was observed on exposure to 20 microM veratridine at 0.5 mM external calcium. Our results indicate that, similar to synapsin I phosphorylation, P96 dephosphorylation shows a graded response to the amount of calcium entering the presynaptic terminal. Depolarization of synaptosomes under conditions restricting the influx of calcium revealed a transient dephosphorylation (reversed within 10 s) of the phosphoprotein P65. The possible significance of this finding to the process of neurotransmitter release is discussed.

Norepinephrine and isoproterenol increase the phosphorylation of synapsin I and synapsin II in dentate slices of young but not aged Fisher 344 rats

A number of recent reports have suggested that norepinephrine (NE) produces a form of synaptic enhancement that resembles long-term potentiation (LTP). LTP, thought to be an electrophysiological correlate of memory, in part involves an augmentation of transmitter release. Although the effects of NE have not been unequivocally linked to LTP, it is clear that NE can produce increased transmitter release in the dentate gyrus of the hippocampus. The purpose of this study was to determine whether NE was capable of enhancing the phosphorylation of synapsin I and synapsin II, two homologous phosphoproteins thought to be involved in modulation of neurotransmitter release. NE (10 microM) and isoproterenol (250 nM) produced an increase in the phosphorylation of synapsin I and synapsin II in dentate slices from young rats. Phosphorylation site analysis of synapsin I, performed by limited proteolysis, indicated that NE and isoproterenol increased the phosphorylation of synapsin I at sites modified by Ca2+/calmodulin-dependent protein kinase II as well as cAMP-dependent protein kinase. These data demonstrate that NE stimulates the phosphorylation of synapsin I at its Ca2+/calmodulin-dependent protein kinase II site, which is a site that has been shown to regulate the effect of synapsin I on neurotransmitter release. We have also examined the effects of NE and isoproterenol on synapsin phosphorylation in dentate slices prepared from aged animals. Such animals have previously been shown to exhibit deficits in NE sensitivity as well as significant impairment in their ability to exhibit LTP. Neither NE nor isoproterenol stimulated synapsin phosphorylation in slices prepared from aged animals. Interestingly, the basal level of phosphorylation of the synapsin proteins was higher in slices prepared from aged animals. This higher basal level of phosphorylation may underlie the failure of aged animals to exhibit NE-stimulated increases in phosphorylation of the synapsin proteins. We hypothesize that the beta-adrenergic agonist-stimulated phosphorylation of synapsin I and synapsin II in young rats plays a role in the increase in transmitter release produced by NE in the dentate. Thus, the failure of the aged rats to show such phosphorylation may underlie, in part, their failure to exhibit normal responsiveness to NE. Moreover, these deficits in synapsin phosphorylation may also play some role in the deficits in plasticity seen in aged rats.

Computer modeling of synapsin I binding to synaptic vesicles and F-actin: implications for regulation of neurotransmitter release

Synapsin I is a neuron-specific phosphoprotein that binds to small synaptic vesicles and actin filaments in a phosphorylation-dependent fashion. It has been hypothesized that dephosphorylated synapsin I inhibits neurotransmitter release either by forming a cage around synaptic vesicles (cage model) or by anchoring them to the F-actin cytoskeleton of the nerve terminal (crosslinking model). Computer modeling was performed with the aim of testing the impact of phosphorylation on the molecular interactions of synapsin I within the nerve terminal. The results of the simulation experiments demonstrate that in the crosslinking model the phosphorylation of synapsin I causes a severalfold increase in the number of vesicles released from the cytoskeleton and that in the cage model the phosphorylation induces a 2-fold increase in the number of vesicles bearing one or more unsaturated synapsin I binding sites. These data are compatible with the view that the function of synapsin I in the short-term regulation of neurotransmitter release is to induce a phosphorylation-dependent transition of synaptic vesicles from a "reserve pool" to a readily "releasable pool" of vesicles.

The synapsins and the regulation of synaptic function

Synapsin I and II are a family of synaptic vesicle-associated phosphoproteins involved in the short-term regulation of neurotransmitter release. In this review, we discuss a working model for the molecular mechanisms by which the synapsins act. We propose that synapsin I links synaptic vesicles to actin filaments in the presynaptic nerve terminal and that these interactions are modulated by the reversible phosphorylation of synapsin I through various signal transduction pathways. The high degree of homology between the synapsins suggests that some of the functional properties of synapsin I are also shared by synapsin II.

Calcium/calmodulin-dependent protein kinase II increases glutamate and noradrenaline release from synaptosomes

A variety of evidence indicates that calcium-dependent protein phosphorylation modulates the release of neurotransmitter from nerve terminals. For instance, the injection of rat calcium/calmodulin-dependent protein kinase II (Ca2+/CaM-dependent PK II) into the preterminal digit of the squid giant synapse leads to an increase in the release of a so-far unidentified neurotransmitter induced by presynaptic depolarization. But until now, it has not been demonstrated that Ca2+/CaM-dependent PK II can also regulate neurotransmitter release in the vertebrate nervous system. Here we report that the introduction of Ca2+/CaM-dependent PK II, autoactivated by thiophosphorylation, into rat brain synaptosomes (isolated nerve terminals) increases the initial rate of induced release of two neurotransmitters, glutamate and noradrenaline. We also show that introduction of a selective peptidergic inhibitor of Ca2+/CaM-dependent PK II inhibits the initial rate of induced glutamate release. These results support the hypothesis that activation of Ca2+/CaM-dependent PK II in the nerve terminal removes a constraint on neurotransmitter release.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.

Calcium/calmodulin-dependent protein kinase II

There is a great deal known about the in vitro properties of CaM kinase II, both in terms of its substrate specificity and its regulation by calmodulin and autophosphorylation. Much of this characterization is based on experiments performed with the rat brain isozyme of CaM kinase II, although in the aspects examined to date isozymes of the kinase from other tissues appear to behave in a broadly similar manner in vitro. However, relatively little is known about the functions of the kinase in vivo. The proteins phosphorylated by the kinase (with the probable exception of synapsin I and tyrosine hydroxylase) and the role of kinase autophosphorylation in vivo remain largely unknown. Investigation of the physiological role of the kinase in brain and other tissues will be a particularly exciting area for future work. The current knowledge of the in vitro properties and the availability of cDNA clones will hopefully expedite this research.

An analysis of postmortem brain samples from 32 alcoholic and nonalcoholic individuals for protein III, a neuronal phosphoprotein

Protein phosphorylation is a primary mechanism of intracellular signal transduction, and abnormalities in protein phosphorylation have been implicated in the pathogenesis of several specific diseases. Protein III is a neuronal phosphoprotein that is associated with synaptic vesicles and is probably involved in the regulation of neurotransmitter release. Analysis of 32 postmortem brains has confirmed our previous report that variant forms of protein III with higher apparent molecular weights are found frequently in the brains of alcoholic individuals but rarely in the brains of nonalcoholic individuals who did not suffer from any other medical or neuropsychiatric disorders. Eight of 14 (57%) brain samples from alcoholic individuals and four of eight (50%) brain samples from suspected alcoholic individuals had variant forms, while none of 10 samples from nonalcoholic individuals had variant forms. Previous data indicate that variant forms of protein III are also associated with other neurodegenerative conditions, including various dementias, and, possibly, normal aging.

Synapsins: mosaics of shared and individual domains in a family of synaptic vesicle phosphoproteins

Synapsins are neuronal phosphoproteins that coat synaptic vesicles, bind to the cytoskeleton, and are believed to function in the regulation of neurotransmitter release. Molecular cloning reveals that the synapsins comprise a family of four homologous proteins whose messenger RNA's are generated by differential splicing of transcripts from two genes. Each synapsin is a mosaic composed of homologous amino-terminal domains common to all synapsins and different combinations of distinct carboxyl-terminal domains. Immunocytochemical studies demonstrate that all four synapsins are widely distributed in nerve terminals, but that their relative amounts vary among different kinds of synapses. The structural diversity and differential distribution of the four synapsins suggest common and different roles of each in the integration of distinct signal transduction pathways that modulate neurotransmitter release in various types of neurons.

Divergent effects of acute depolarization on somatostatin release and protein synthesis in cultured fetal and neonatal rat brain cells

The influence of membrane depolarization on somatostatin secretion and protein synthesis by fetal and neonatal cerebrocortical neurons was studied. Cortical cells obtained by mechanical dispersion were maintained as monolayer cultures for 8 days. The ability of fetal cerebrocortical and hypothalamic cells to release immunoreactive somatostatin (IR-SRIF) was confirmed. Total protein synthesis was determined by the incorporation of [3H]phenylalanine into trichloroacetic acid-precipitable proteins. To study the effect of acute depolarization on protein synthesis, cells were incubated for 30 min with [3H]phenylalanine or [3H]leucine and the depolarizing agent. In fetal cerebrocortical cells, potassium (30 and 56 mM) decreased protein synthesis and RNA levels and increased IR-SRIF release. Depolarization by veratridine, a sodium channel activator, induced a similar effect. The effect of veratridine on IR-SRIF and protein synthesis was reversed by tetrodotoxin, a sodium channel blocker, or verapamil, a calcium channel blocker. These findings suggest that protein synthesis by cerebrocortical cells is decreased in fetal brain cells by membrane depolarization and is dependent on Na+ and Ca2+ entry into cells. In postnatal (day 7) cerebrocortical cells, depolarization induced by high potassium concentrations led to a concomitant increase in protein synthesis, RNA content, and somatostatin release. These findings indicate that depolarization of the cellular membrane is coupled to an increase in protein synthesis in neonatal, but not in fetal, dispersed brain cells.

Synapsin Ia, synapsin Ib, protein IIIa, and protein IIIb, four related synaptic vesicle-associated phosphoproteins, share regional and cellular localization in rat brain

The regional and cellular distribution of four synaptic vesicle-associated proteins, synapsins Ia and Ib (Mr 86,000 and 80,000, collectively referred to as synapsin I) and proteins IIIa and IIIb (Mr 74,000 and 55,000, collectively referred to as protein III), has been compared in selected rat brain regions, using both radioimmunoassays and back-phosphorylation assays. Lesions of several neuronal populations in the basal ganglia (corticostriatal fibers, intrinsic striatal neurons, striatonigral fibers, nigrostriatal fibers) induced decreases in the levels of these various proteins that were highly correlated (r = 0.96-0.97). Moreover, the synaptic vesicle-associated phosphoproteins displayed a similar and widespread distribution throughout the CNS. This evidence for colocalization indicates that the majority of, and possibly all, CNS neurons and nerve terminals may contain both forms of synapsin I and both forms of protein III.

Depolarization-stimulated protein phosphorylation in pure cholinergic nerve endings

Cholinergic synaptosomes obtained from the electric organ of Torpedo marmorata have been used to study chemical stimulation-stimulated protein phosphorylation. Cholinergic synaptosomes were exposed to elevated K+0 concentrations or other chemical depolarizing agents such as gramicidin or secretagogues as the calcium ionophore A23187. During depolarization several synaptosomal proteins increase their state of phosphorylation. This phenomenon depends on the presence of Ca2+ in the external medium. These results suggest that stimulation of protein phosphorylation may be implicated in the acetylcholine release process and could represent a modulation mechanism in the neurotransmitter release machinery at this cholinergic synapse.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

The synaptic vesicle and the cytoskeleton

Images

Calcium/calmodulin-dependent protein kinase II in squid synaptosomes

The Ca2+/calmodulin (CaM)-dependent protein kinase II system in squid nervous tissue was investigated. The Ca2+/CaM-dependent protein kinase II was found to be very active in the synaptosome preparation from optic lobe, where it was associated with the high-speed particulate fraction. Incubation of the synaptosomal homogenate with calcium, calmodulin, magnesium, and ATP resulted in partial and reversible conversion of the Ca2+/CaM-dependent protein kinase II from its calcium-dependent form to a calcium-independent species. The magnitude of this conversion reaction could be increased by inclusion of the protein phosphatase inhibitor NaF or by substitution of adenosine 5'-O-(3-thiotriphosphate) for ATP. When [gamma-32P]ATP was used, proteins of 54 and 58 kilodaltons (kDa) as well as proteins greater than 100 kDa were rapidly 32P-labeled in a calcium-dependent manner. Major 125I-CaM binding proteins in the synaptosome membrane fraction were 38 and 54 kDa. The Ca2+/CaM-dependent protein kinase II was purified from the squid synaptosome and was shown to consist of 54- and 58-60-kDa subunits. The purified kinase, like Ca2+/CaM-dependent protein kinase II from rat brain, catalyzed autophosphorylation associated with formation of the calcium-independent form. These studies, characterizing the Ca2+/CaM-dependent protein kinase II in squid neural tissue, are supportive of the putative role of this kinase in regulating calcium-dependent synaptic functions.

Botulinum neurotoxin inhibits depolarization-stimulated protein phosphorylation in pure cholinergic synaptosomes

Botulinum neurotoxin, a strong blocker of acetylcholine release at peripheral cholinergic synapses, inhibits depolarization-stimulated protein phosphorylation in pure cholinergic synaptosomes isolated from the electric organ of Torpedo marmorata. Moreover, tetrodotoxin has the same effect on protein phosphorylation when cholinergic synaptosomes are depolarized by veratridine. Correlation between presynaptic protein phosphorylation and acetylcholine release is suggested by the fact that botulinum neurotoxin blocks specifically neurotransmitter release without affecting membrane depolarization and calcium fluxes in our synaptosomal preparation.

Translocations of fodrin and its binding proteins

Fodrin, a protein related to erythrocyte spectrin, redistributes within the cell in certain situations. We compare such movements of fodrin and several fodrin binding proteins during the processes of axonal transport in neurons, and capping of surface proteins in lymphocytes. In neurons, three different populations of newly synthesized fodrin appear to be transported down the axons at different velocities corresponding to those of groups of transported proteins designated II, IV, and V. Actin, which can interact with fodrin, is transported at the velocity of group IV. Synapsin, a component of synaptic vesicles, is also reported to bind to fodrin. One population of synapsin is transported more rapidly than fodrin, at the velocity of group I: two additional populations of transported synapsin may overlap fodrin in groups II and IV. We consider possible functional associations of these different populations of fodrin and fodrin binding proteins. We note that the transport of group IV proteins resembles in certain respects the process of capping in lymphocytes, suggesting the possibility of a common mechanism. We outline one of several possible mechanisms.

Neuronal phosphoproteins. Mediators of signal transduction

This article summarizes some of our knowledge concerning intracellular protein phosphorylation pathways in nerve cells. It also summarizes, very briefly, recent direct experimental evidence involving intracellular injection of protein kinases, protein kinase inhibitors, and substrates, indicating that protein phosphorylation mediates the actions of a variety of neurotransmitters on their target cells. Finally, it summarizes in somewhat greater detail the results of studies of three different types of substrate proteins that appear to regulate different types of biological responses in nerve cells: synapsin I, a substrate protein present in virtually all nerve terminals, which appears to regulate neurotransmitter release from those nerve terminals; the acetylcholine receptor, the phosphorylation of which regulates its rate of desensitization in the presence of acetylcholine; and DARPP-32, the phosphorylation of which converts it into a very potent phosphoprotein phosphatase inhibitor that may be involved in the regulation by the neuromodulator dopamine of the effects of the neurotransmitter glutamate. The identification and characterization of additional neuronal phosphoproteins can be expected to lead to the clarification of numerous additional molecular mechanisms by which signal transduction is carried out in nerve cells.

Four synaptic vesicle-specific proteins: identification by monoclonal antibodies and distribution in the nervous tissue and the adrenal medulla

Synaptic vesicles from the guinea-pig cerebrum were isolated and administered to mice for the production of monoclonal antibodies (MAb). Four vesicle-associated proteins in the guinea-pig nervous tissue were specifically and differentially recognized by MAbs thus obtained. These proteins had molecular weights of 30,000, 36,000, 38,000 and 65,000 Da and were named SVPs (synaptic vesicle proteins) 30, 36, 38 and 65, respectively. Immunohistochemistry demonstrated that all SVPs were localized in the synaptic regions throughout the central nervous system and in the adrenal medulla. Nerve terminals in skeletal muscle, smooth muscle and sympathetic ganglion contained SVPs 36 and 38. Immunoelectron microscopy of the cerebellar cortex confirmed the localization of SVPs in the synaptic vesicles and the adjacent membranes of the presynaptic nerve terminals. Fractionation of the cerebral tissue and treatment with various agents showed that SVPs were localized in the synaptic vesicles and the synaptic plasma membrane and that SVPs were integrated within the membrane and liberated only after solubilization of the membrane.

Synapsin I, a phosphoprotein associated with synaptic vesicles: possible role in regulation of neurotransmitter release

The data presented here provide evidence that the study of neuronal phosphoproteins can lead to the identification of previously unknown proteins and that these proteins may play important roles in neuronal communication. Specifically, in the case of synapsin I, direct evidence has been obtained that this phosphoprotein is involved in regulating neurotransmitter release. A tentative explanation of the results obtained in the micro-injection studies is as follows: synapsin I, in the dephosphostate, is bound to the cytoplasmic surface of synaptic vesicles and inhibits the ability of the vesicle to interact with the plasma membrane; increases in intracellular calcium activate calmodulin kinase II which in turn phosphorylates synapsin I and the phosphorylated synapsin I dissociates from the synaptic vesicle thus removing a constraint on the release of neurotransmitter. Clearly, more studies need to be done to rigorously test this hypothesis. Nevertheless these studies of synapsin I suggest that the study of previously unknown phosphoproteins will lead to the elucidation of previously unknown regulatory processes in neurons.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Endogenous phosphorylation of proteins and phosphatidylinositol in the plasma membranes of a human astrocytoma

Incubation of plasma membrane preparations from several tissues with [gamma-32P]ATP resulted in the phosphorylation of phosphatidylinositol as well as of proteins. The presence of an active phosphatidylinositol kinase in these membranes was indicated by equal or greater incorporation of 32P into phosphatidylinositol phosphate than into proteins. Phosphorylation of endogenous protein and lipid substrates by protein and phosphatidylinositol kinases in the plasma membranes of a human astrocytoma was investigated in detail. Maximal protein phosphorylation required the presence of Nonidet-P40 and phosphatase inhibitors (vanadate or fluoride). The rate of protein phosphorylation was greater with Mg2+ than with Mn2+, and phosphoserine accounted for 60% of the radioactivity incorporated into proteins. In the presence of Mn2+, phosphorylation of tyrosine was increased and was equal to that of serine phosphorylation (40%). With one exception, the overall pattern of phosphorylated proteins was similar with either Mg2+ or Mn2+. Maximal phosphatidylinositol phosphorylation of the astrocytoma plasma membranes also required detergent and phosphatase inhibitors. However, the enzymatic characteristics of lipid phosphorylation differed from those of protein phosphorylation with respect to divalent cation activation, ATP dependence, and sensitivity to inhibition by p-chloromercuriphenyl sulfonate, quercetin, and nucleoside derivatives. These results suggest that phosphorylation of plasma membrane proteins and phosphatidylinositol is catalyzed by different enzymes. The fact that membrane preparations exhibited phosphatidylinositol kinase activity almost 100,000 times greater than that exhibited by the purified tyrosine kinase of ros gene would exclude this and similar oncogene proteins from making a significant contribution to the overall phosphatidylinositol phosphorylation of cell membranes.

Potentiation of sympathetic neurosecretion by forskolin and cyclic AMP in the rabbit iris-ciliary body

Forskolin has been reported to stimulate cAMP formation and reduce intraocular pressure in rabbit and primate eyes. In view of recent evidence for the involvement of cAMP in modulation of transmitter release at adrenergic synapses, we have investigated the presynaptic effects of forskolin and other cAMP activators on field-stimulated secretion of 3H-norepinephrine (3H-NE) in the isolated, perfused rabbit iris-ciliary body. Forskolin (10(-7)-10(-5) M) was found to markedly enhance stimulation-evoked 3H-NE release without affecting basal (spontaneous) release. The response to forskolin was potentiated by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX; 0.5 mM) and was mimicked by the cell-permeant cyclic nucleotide analog 8-bromo-cAMP. 8-bromo-cGMP also produce a small enhancement of stimulus-evoked 3H-NE secretion, whereas IBMX alone had little effect on either stimulated or basal secretion. These results suggest that cAMP may play an important neuromodulatory role in regulation of norepinephrine release at intraocular synapses, and raise the possibility that the ocular hypotensive response to forskolin in rabbit eyes may be mediated, in part, by enhanced adrenergic neurosecretion.

Identification of a synaptic vesicle-specific 38,000-dalton protein by monoclonal antibodies

Synaptic vesicles were purified from the guinea pig cerebrum by sucrose density gradient centrifugation, and monoclonal antibodies (MAbs) were produced against this vesicle fraction. Seven MAbs (171B5, 171E8, 174D12, 174H11, 177A2, 177H11 and 178D4) recognized a novel acidic protein of about 38,000 daltons which was specific to synaptic vesicles. In immunofluorescence microscopy, the staining pattern of these MAbs corresponded to the distribution of the synapses in the guinea pig central nervous system. These MAbs appeared to stain all synaptic regions, irrespective of their synaptic function or type of neurotransmitters. MAb 171B5 and 174H11 stained the rat, rabbit and bovine synapses similarly to the guinea pig. Two other MAbs (171E8 and 177H11) stained other mammals weakly but the remaining 3 MAbs reacted only with the guinea pig. In immunoelectron microscopy of both the cerebellar tissue and isolated vesicle fraction, these MAbs selectively labeled the synaptic vesicles but not other structures. Immunoblot analysis was performed on electrophoretically separated proteins in vesicle fraction and brain homogenate. All of 7 MAbs reacted with a band at a molecular weight of about 38,000 from the guinea pig. Isoelectric focussing disclosed that this protein was acidic (pI 4.5-5).

Long-term potentiation of transmitter release induced by adrenaline in bull-frog sympathetic ganglia

Long-term potentiation (l.t.p.) of transmitter release induced by adrenaline in bull-frog sympathetic ganglia was studied using intracellular recording techniques. The quantal content of the fast excitatory post-synaptic potentials (fast e.p.s.p.s: evoked by the nicotinic action of acetylcholine) was potentiated for more than several hours after treatment with adrenaline (1-100 microM). A similar l.t.p. of quantal content was produced consistently by isoprenaline (10 microM) and only in a certain fraction of cells by dopamine (10 microM). The l.t.p. induced by adrenaline (10 microM) was blocked by a beta-antagonist, propranolol (1 microM), but not by an alpha-antagonist, phenoxybenzamine (1 microM). Dibutyryl adenosine 3',5'-phosphate (dibutyryl cyclic AMP) (0.8-1.0 mM), adenosine 3',5'-phosphate (cyclic AMP) (4 mM), 3-isobutyl-1-methylxanthine (10 microM), caffeine (1-2 mM), and cholera toxin (2 micrograms ml-1) applied for 20-30 min, all caused the l.t.p. of quantal content. By contrast, adenosine 5'-phosphate (AMP) (4 mM) and adenosine (4 mM) had no potentiating action. Treatment of the ganglion with adrenaline (2.5-160 microM) or dibutyryl cyclic AMP (4 mM) for 15-30 min resulted in the l.t.p. of the frequency of miniature e.p.s.p.s. The l.t.p. of quantal content induced by adrenaline was markedly suppressed by lowering temperature from 20-25 degrees C to 11-13 degrees C, and blocked by dibutyryl guanosine 3',5'-phosphate (dibutyryl cyclic GMP) (100 microM) consistently when applied together, but inconsistently when given after adrenaline. The post-synaptic sensitivity to acetylcholine was unchanged for at least 1 h after exposure to adrenaline (2.5-160 microM) or dibutyryl cyclic AMP (0.8-4 mM). It can be concluded that adrenaline produces l.t.p. of transmitter release by activating a cyclic-AMP-dependent metabolic process through the activation of beta-adrenoceptors, and that this mechanism is presumably regulated by a process involving endogenous guanosine 3',5'-phosphate (cyclic GMP).

Nicotinic and muscarinic agonists, phorbol esters, and agents which raise cyclic AMP levels phosphorylate distinct groups of proteins in the superior cervical ganglion

The phosphorylation of proteins in the superior cervical ganglion of the rat was investigated. Ganglia were incubated with 32Pi, and the 32P-labeled proteins in the ganglion were separated by two-dimensional electrophoresis and visualized by autoradiography. Approximately 40 distinct phosphoproteins could be visualized by these methods. The most heavily labeled ganglionic protein was an acidic protein with an Mr of approximately 83,000. Tyrosine hydroxylase was identified as a doublet of two closely-migrating radioactive spots. Treatment of intact ganglia with depolarizing agents, nicotinic and muscarinic agonists, phorbol esters, and agents that increase the content of cyclic adenosine 3':5'-monophosphate in the ganglion stimulated the incorporation of 32Pi into distinct but overlapping groups of phosphoproteins. All of these agents increased the phosphorylation of tyrosine hydroxylase. In contrast, only phorbol esters and muscarinic agonists increased the phosphorylation of the 83,000 ganglionic phosphoprotein. Our data are consistent with the idea that the various classes of agonists may activate distinct protein kinases in the ganglion.

Ca2+ sensitivity of Ca2+-dependent protein kinase activities toward intrinsic proteins in synaptosomal membrane fragments from rat cerebral tissue

The Ca2+ and calmodulin sensitivity of endogenous protein kinase activity in synaptosomal membrane fragments from rat brain was studied in medium containing Ca2+ plus EGTA using a modified computer programme to calculate free Ca2+ concentrations that took into account the effect of all competing cations and chelators. The Ca2+-dependent phosphorylation of 10 major polypeptide acceptors with Mr values ranging from 50 to 360 kilodaltons required calmodulin in reactions that were all equally sensitive to Ca2+; half-maximal phosphorylation required a free Ca2+ concentration of 45 nM and maximal phosphorylation approximately 110 nM. The significance of these values in relation to published data on the intracellular concentration of free Ca2+ in the nervous system is discussed. One acceptor of 45 kilodaltons was phosphorylated in a Ca2+-dependent reaction that did not require calmodulin. This polypeptide appeared to correspond to the B-50 protein, an established substrate of the lipid-dependent protein kinase C. Further study of this phosphorylating system showed that the reaction was only independent of calmodulin at saturating concentrations of Ca2+; at subsaturating concentrations (in the range 50-130 nM), a small but significant stimulation of the enzyme by calmodulin was demonstrated. The possible significance of this finding is discussed.

Protein phosphorylation and neuronal function

Studies in the past several years have provided direct evidence that protein phosphorylation is involved in the regulation of neuronal function. Electrophysiological experiments have demonstrated that three distinct classes of protein kinases, i.e., cyclic AMP-dependent protein kinase, protein kinase C, and CaM kinase II, modulate physiological processes in neurons. Cyclic AMP-dependent protein kinase and kinase C have been shown to modify potassium and calcium channels, and CaM kinase II has been shown to enhance neurotransmitter release. A large number of substrates for these protein kinases have been found in neurons. In some cases (e.g., tyrosine hydroxylase, acetylcholine receptor, sodium channel) these proteins have a known function, whereas most of these proteins (e.g., synapsin I) had no known function when they were first identified as phosphoproteins. In the case of synapsin I, evidence now suggests that it regulates neurotransmitter release. These studies of synapsin I suggest that the characterization of previously unknown neuronal phosphoproteins will lead to the elucidation of previously unknown regulatory processes in neurons.

Phylogenetic survey of proteins related to synapsin I and biochemical analysis of four such proteins from fish brain

A phylogenetic survey of proteins immunologically related to Synapsin I, a major synaptic vesicle-associated phosphoprotein in mammals was carried out. Proteins antigenically related to Synapsin I were found by use of radioimmunoassay and other radioimmunochemical techniques in the nervous systems of several vertebrate and invertebrate species, which included birds, reptiles, amphibians, fish, echinoderms, arthropods, and mollusks. Four proteins present in fish brain, antigenically related to Synapsin I, were further studied and found to resemble mammalian Synapsin I in several respects. Like Synapsin I, the fish proteins were present in high amounts in nervous tissue, were enriched in synaptosomal fractions of brain where they were substrates for endogenous protein kinases, were acid extractable, and were sensitive to digestion by collagenase. In addition, two-dimensional peptide-mapping analysis revealed some homology between major phosphopeptide fragments of Synapsin I and the fish proteins. The results indicate that proteins related to Synapsin I are wide-spread in the animal kingdom.

Synapsin I is a spectrin-binding protein immunologically related to erythrocyte protein 4.1

The membrane-associated cytoskeleton is considered to be the apparatus by which cells regulate the properties of their plasma membranes, although recent evidence has indicated additional roles for the proteins of this structure, including an involvement in intracellular transport and exocytosis (see refs 1-3 for review). Of the membrane skeletal proteins, to date only spectrin (fodrin) and ankyrin have been purified and characterized from non-erythroid sources. Protein 4.1 in the red cell is a spectrin-binding protein that enhances the binding of spectrin to actin and can apparently bind to at least one transmembrane protein Immunoreactive forms of 4.1 have been detected in several cell types, including brain. Here we report the purification of brain 4.1 on the basis of its cross-reactivity with erythrocyte 4.1 and spectrin-binding activity. We further show that brain 4.1 is identical to the synaptic vesicle protein, synapsin I, one of the brain's major substrates for cyclic AMP and Ca2+-calmodulin-dependent kinases. Spectrin and synapsin are present in brain homogenates in an approximately 1:1 molar ratio. Although synapsin I has been implicated in synaptic transmission, no activity has been previously ascribed to it.