Light-induced charge generation in polymeric nanoparticles restores vision in advanced-stage retinitis pigmentosa rats

Retinal dystrophies such as Retinitis pigmentosa are among the most prevalent causes of inherited legal blindness, for which treatments are in demand. Retinal prostheses have been developed to stimulate the inner retinal network that, initially spared by degeneration, deteriorates in the late stages of the disease. We recently reported that conjugated polymer nanoparticles persistently rescue visual activities after a single subretinal injection in the Royal College of Surgeons rat model of Retinitis pigmentosa. Here we demonstrate that conjugated polymer nanoparticles can reinstate physiological signals at the cortical level and visually driven activities when microinjected in 10-months-old Royal College of Surgeons rats bearing fully light-insensitive retinas. The extent of visual restoration positively correlates with the nanoparticle density and hybrid contacts with second-order retinal neurons. The results establish the functional role of organic photovoltaic nanoparticles in restoring visual activities in fully degenerate retinas with intense inner retina rewiring, a stage of the disease in which patients are subjected to prosthetic interventions.

Retinal dystrophies such as Retinitis pigmentosa are among the most prevalent causes of inherited incurable legal blindness. Here the authors demonstrate that conjugated polymer nanoparticles reinstate visual functions in aged rats with fully degenerated and rewired retinas.

Membrane Environment Enables Ultrafast Isomerization of Amphiphilic Azobenzene -INVITED

We report on the isomerization dynamics of an amphiphilic azobenzene that dwells within the plasma membrane. We found that in aggregates formed in water, the isomerization reaction is hindered, while radiative deactivation is favoured. However, once in the membrane, the molecule reacquires its isomerisation capacity. These data fully account for the recently reported experiments in neurons, showing that the amphiphilic azobenzenes are effective light actuators for the modification of the electrical state of the membrane.

Synapsin I Synchronizes GABA Release in Distinct Interneuron Subpopulations

Neurotransmitters can be released either synchronously or asynchronously with respect to action potential timing. Synapsins (Syns) are a family of synaptic vesicle (SV) phosphoproteins that assist gamma-aminobutyric acid (GABA) release and allow a physiological excitation/inhibition balance. Consistently, deletion of either or both Syn1 and Syn2 genes is epileptogenic. In this work, we have characterized the effect of SynI knockout (KO) in the regulation of GABA release dynamics. Using patch-clamp recordings in hippocampal slices, we demonstrate that the lack of SynI impairs synchronous GABA release via a reduction of the readily releasable SVs and, in parallel, increases asynchronous GABA release. The effects of SynI deletion on synchronous GABA release were occluded by ω-AgatoxinIVA, indicating the involvement of P/Q-type Ca2+channel-expressing neurons. Using in situ hybridization, we show that SynI is more expressed in parvalbumin (PV) interneurons, characterized by synchronous release, than in cholecystokinin or SOM interneurons, characterized by a more asynchronous release. Optogenetic activation of PV and SOM interneurons revealed a specific reduction of synchronous release in PV/SynIKO interneurons associated with an increased asynchronous release in SOM/SynIKO interneurons. The results demonstrate that SynI is differentially expressed in interneuron subpopulations, where it boosts synchronous and limits asynchronous GABA release.

REST-Dependent Presynaptic Homeostasis Induced by Chronic Neuronal Hyperactivity

Homeostatic plasticity is a regulatory feedback response in which either synaptic strength or intrinsic excitability can be adjusted up or down to offset sustained changes in neuronal activity. Although a growing number of evidences constantly provide new insights into these two apparently distinct homeostatic processes, a unified molecular model remains unknown. We recently demonstrated that REST is a transcriptional repressor critical for the downscaling of intrinsic excitability in cultured hippocampal neurons subjected to prolonged elevation of electrical activity. Here, we report that, in the same experimental system, REST also participates in synaptic homeostasis by reducing the strength of excitatory synapses by specifically acting at the presynaptic level. Indeed, chronic hyperactivity triggers a REST-dependent decrease of the size of synaptic vesicle pools through the transcriptional and translational repression of specific presynaptic REST target genes. Together with our previous report, the data identify REST as a fundamental molecular player for neuronal homeostasis able to downscale simultaneously both intrinsic excitability and presynaptic efficiency in response to elevated neuronal activity. This experimental evidence adds new insights to the complex activity-dependent transcriptional regulation of the homeostatic plasticity processes mediated by REST.

Imaging and structural studies of DNA-protein complexes and membrane ion channels

In bio-imaging by electron microscopy, damage of the sample and limited contrast are the two main hurdles for reaching high image quality. We extend a new preparation method based on nanofabrication and super-hydrophobicity to the imaging and structural studies of nucleic acids, nucleic acid-protein complexes (DNA/Rad51 repair protein complex) and neuronal ion channels (gap-junction, K⁺ and GABA_A channels) as paradigms of biological significance and increasing complexity. The preparation method is based on the liquid phase and is compatible with physiological conditions. Only in the very last stage, samples are dried for TEM analysis. Conventional TEM and high-resolution TEM (HRTEM) were used to achieve a resolution of 3.3 and 1.5 Å, respectively. The EM dataset quality allows the determination of relevant structural and metrological information on the DNA structure, DNA-protein interactions and ion channels, allowing the identification of specific macromolecules and their structure.

Chaoticity threshold in magnetized plasmas: numerical results in the weak coupling regime

The present paper is a numerical counterpart to the theoretical work [Carati et al., Chaos 22, 033124 (2012)]. We are concerned with the transition from order to chaos in a one-component plasma (a system of point electrons with mutual Coulomb interactions, in a uniform neutralizing background), the plasma being immersed in a uniform stationary magnetic field. In the paper [Carati et al., Chaos 22, 033124 (2012)], it was predicted that a transition should take place when the electron density is increased or the field decreased in such a way that the ratio ωp/ωc between plasma and cyclotron frequencies becomes of order 1, irrespective of the value of the so-called Coulomb coupling parameter Γ. Here, we perform numerical computations for a first principles model of N point electrons in a periodic box, with mutual Coulomb interactions, using as a probe for chaoticity the time-autocorrelation function of magnetization. We consider two values of Γ (0.04 and 0.016) in the weak coupling regime Γ ≪ 1, with N up to 512. A transition is found to occur for ωp/ωc in the range between 0.25 and 2, in fairly good agreement with the theoretical prediction. These results might be of interest for the problem of the breakdown of plasma confinement in fusion machines.

Fabrication of biocompatible free-standing nanopatterned films for primary neuronal cultures

A novel nanopatterned biocompatible PCL film promotes the growth of primary hippocampal neurons (rendered in blue).

Large-scale, high-resolution electrophysiological imaging of field potentials in brain slices with microelectronic multielectrode arrays

Multielectrode arrays (MEAs) are extensively used for electrophysiological studies on brain slices, but the spatial resolution and field of recording of conventional arrays are limited by the low number of electrodes available. Here, we present a large-scale array recording simultaneously from 4096 electrodes used to study propagating spontaneous and evoked network activity in acute murine cortico-hippocampal brain slices at unprecedented spatial and temporal resolution. We demonstrate that multiple chemically induced epileptiform episodes in the mouse cortex and hippocampus can be classified according to their spatio-temporal dynamics. Additionally, the large-scale and high-density features of our recording system enable the topological localization and quantification of the effects of antiepileptic drugs in local neuronal microcircuits, based on the distinct field potential propagation patterns. This novel high-resolution approach paves the way to detailed electrophysiological studies in brain circuits spanning spatial scales from single neurons up to the entire slice network.

Long-range and long-term interferometric tracking by static and dynamic force-clamp optical tweezers

Optical tweezers are recognized single-molecule technique to resolve forces and motion on the molecular scale. Complex biological phenomena, such as cell differentiation and locomotion, require long range tracking capabilities with nanometer resolution over an extended period, to resolve molecular processes on the cellular scale. Here we introduce a real-time control of the microscope stage position to perform long-term tracking, with sub-millisecond resolution, of a bead attached to a neuron, preserving sub-nanometer sensitivity on a spatial range of centimeters, seven orders of magnitude larger. Moreover, the suitability of the system is tested by time- modulating the force-clamp condition to study the role of statically and dynamically applied forces in neuronal differentiation.

Kidins220/ARMS is an essential modulator of cardiovascular and nervous system development

The growth factor family of neurotrophins has major roles both inside and outside the nervous system. Here, we report a detailed histological analysis of key phenotypes generated by the ablation of the Kinase D interacting substrate of 220 kDa/Ankyrin repeat-rich membrane spanning (Kidins220/ARMS) protein, a membrane-anchored scaffold for the neurotrophin receptors Trk and p75^NTR. Kidins220 is important for heart development, as shown by the severe defects in the outflow tract and ventricle wall formation displayed by the Kidins220 mutant mice. Kidins220 is also important for peripheral nervous system development, as the loss of Kidins220 in vivo caused extensive apoptosis of DRGs and other sensory ganglia. Moreover, the neuronal-specific deletion of this protein leads to early postnatal death, showing that Kidins220 also has a critical function in the postnatal brain.

Kidins220/ARMS mediates the integration of the neurotrophin and VEGF pathways in the vascular and nervous systems

Signaling downstream of receptor tyrosine kinases controls cell differentiation and survival. How signals from different receptors are integrated is, however, still poorly understood. In this work, we have identified Kidins220 (Kinase D interacting substrate of 220 kDa)/ARMS (Ankyrin repeat-rich membrane spanning) as a main player in the modulation of neurotrophin and vascular endothelial growth factor (VEGF) signaling in vivo, and a primary determinant for neuronal and cardiovascular development. Kidins220(-/-) embryos die at late stages of gestation, and show extensive cell death in the central and peripheral nervous systems. Primary neurons from Kidins220(-/-) mice exhibit reduced responsiveness to brain-derived neurotrophic factor, in terms of activation of mitogen-activated protein kinase signaling, neurite outgrowth and potentiation of excitatory postsynaptic currents. In addition, mice lacking Kidins220 display striking cardiovascular abnormalities, possibly due to impaired VEGF signaling. In support of this hypothesis, we demonstrate that Kidins220 constitutively interacts with VEGFR2. These findings, together with the data presented in the accompanying paper, indicate that Kidins220 mediates the integration of several growth factor receptor pathways during development, and mediates the activation of distinct downstream cascades according to the location and timing of stimulation.

Relaxation properties in classical diamagnetism

It is an old result of Bohr that, according to classical statistical mechanics, at equilibrium a system of electrons in a static magnetic field presents no magnetization. Thus a magnetization can occur only in an out of equilibrium state, such as that produced through the Foucault currents when a magnetic field is switched on. It was suggested by Bohr that, after the establishment of such a nonequilibrium state, the system of electrons would quickly relax back to equilibrium. In the present paper, we study numerically the relaxation to equilibrium in a modified Bohr model, which is mathematically equivalent to a billiard with obstacles, immersed in a magnetic field that is adiabatically switched on. We show that it is not guaranteed that equilibrium is attained within the typical time scales of microscopic dynamics. Depending on the values of the parameters, one has a relaxation either to equilibrium or to a diamagnetic (presumably metastable) state. The analogy with the relaxation properties in the Fermi Pasta Ulam problem is also pointed out.

Development of micro-electrode array based tests for neurotoxicity: assessment of interlaboratory reproducibility with neuroactive chemicals

Neuronal assemblies within the nervous system produce electrical activity that can be recorded in terms of action potential patterns. Such patterns provide a sensitive endpoint to detect effects of a variety of chemical and physical perturbations. They are a function of synaptic changes and do not necessarily involve structural alterations. In vitro neuronal networks (NNs) grown on micro-electrode arrays (MEAs) respond to neuroactive substances as well as the in vivo brain. As such, they constitute a valuable tool for investigating changes in the electrophysiological activity of the neurons in response to chemical exposures. However, the reproducibility of NN responses to chemical exposure has not been systematically documented. To this purpose six independent laboratories (in Europe and in USA) evaluated the response to the same pharmacological compounds (Fluoxetine, Muscimol, and Verapamil) in primary neuronal cultures. Common standardization principles and acceptance criteria for the quality of the cultures have been established to compare the obtained results. These studies involved more than 100 experiments before the final conclusions have been drawn that MEA technology has a potential for standard in vitro neurotoxicity/neuropharmacology evaluation. The obtained results show good intra- and inter-laboratory reproducibility of the responses. The consistent inhibitory effects of the compounds were observed in all the laboratories with the 50% Inhibiting Concentrations (IC(50)s) ranging from: (mean ± SEM, in μM) 1.53 ± 0.17 to 5.4 ± 0.7 (n = 35) for Fluoxetine, 0.16 ± 0.03 to 0.38 ± 0.16 μM (n = 35) for Muscimol, and 2.68 ± 0.32 to 5.23 ± 1.7 (n = 32) for Verapamil. The outcome of this study indicates that the MEA approach is a robust tool leading to reproducible results. The future direction will be to extend the set of testing compounds and to propose the MEA approach as a standard screen for identification and prioritization of chemicals with neurotoxicity potential.

Cortico-hippocampal hyperexcitability in synapsin I/II/III knockout mice: age-dependency and response to the antiepileptic drug levetiracetam

Synapsins (SynI, SynII, SynIII) are a multigene family of synaptic vesicle (SV) phosphoproteins implicated in the regulation of synaptic transmission and plasticity. Synapsin I, II, I/II and I/II/III knockout mice are epileptic and SYN1/2 genes have been identified as major epilepsy susceptibility genes in humans. We analyzed cortico-hippocampal epileptiform activity induced by 4-aminopyridine (4AP) in acute slices from presymptomatic (3-weeks-old) and symptomatic (1-year-old) Syn I/II/III triple knockout (TKO) mice and aged-matched triple wild type (TWT) controls and assessed the effect of the SV-targeted antiepileptic drug (AED) levetiracetam (LEV) in reverting the epileptic phenotype. Both fast and slow interictal (I-IC) and ictal (IC) events were observed in both genotypes. The incidence of fast I-IC events was higher in presymptomatic TKO slices, while frequency and latency of I-IC events were similar in both genotypes. The major age and genotype effects were observed in IC activity, that was much more pronounced in 3-weeks-old TKO and persisted with age, while it disappeared from 1-year-old TWT slices. LEV virtually suppressed fast I-IC and IC discharges from 3-weeks-old TWT slices, while it only increased the latency of fast I-IC and IC activity in TKO slices. Analysis of I-IC events in patch-clamped CA1 pyramidal neurons revealed that LEV increased the inhibitory/excitatory ratio of I-IC activity in both genotypes. The lower LEV potency in TKO slices of both ages was associated with a decreased expression of SV2A, a SV protein acting as LEV receptor, in cortex and hippocampus. The results demonstrate that deletion of Syn genes is associated with a higher propensity to 4AP-induced epileptic paroxysms that precedes the onset of epilepsy and consolidates with age. LEV ameliorates such hyper excitability by enhancing the inhibition/excitation ratio, although the effect is hindered in TKO slices which exhibit a concomitant decrease in the levels of the LEV receptor SV2A.

The synapsins: key actors of synapse function and plasticity

The synapsins are a family of neuronal phosphoproteins evolutionarily conserved in invertebrate and vertebrate organisms. Their best-characterised function is to modulate neurotransmitter release at the pre-synaptic terminal, by reversibly tethering synaptic vesicles (SVs) to the actin cytoskeleton. However, many recent data have suggested novel functions for synapsins in other aspects of the pre-synaptic physiology, such as SV docking, fusion and recycling. Synapsin activity is tightly regulated by several protein kinases and phosphatases, which modulate the association of synapsins to SVs as well as their interaction with actin filaments and other synaptic proteins. In this context, synapsins act as a link between extracellular stimuli and the intracellular signalling events activated upon neuronal stimulation. Genetic manipulation of synapsins in various in vivo models has revealed that, although not essential for the basic development and functioning of neuronal networks, these proteins are extremely important in the fine-tuning of neuronal plasticity, as shown by the epileptic phenotype and behavioural abnormalities characterising mouse lines lacking one or more synapsin isoforms. In this review, we summarise the current knowledge about how the various members of the synapsin family are involved in the modulation of the pre-synaptic physiology. We give a comprehensive description of the molecular basis of synapsin function, as well as an overview of the more recent evidence linking mutations in the synapsin proteins to the onset of severe central nervous system diseases such as epilepsy and schizophrenia.

Low-frequency stimulation enhances burst activity in cortical cultures during development

The intact brain is continuously targeted by a wealth of stimuli with distinct spatio-temporal patterns which modify, since the very beginning of development, the activity and the connectivity of neuronal networks. In this paper, we used dissociated neuronal cultures coupled to microelectrode arrays (MEAs) to study the response of cortical neuron assemblies to low-frequency stimuli constantly delivered over weeks in vitro. We monitored the spontaneous activity of the cultures before and after the stimulation sessions, as well as their evoked response to the stimulus. During in vitro development, the vast majority of the cultures responded to the stimulation by significantly increasing the bursting activity and a widespread stabilization of electrical activity was observed after the third week of age. A similar trend was present between the spontaneous activity of the networks observed over 30 min after the stimulus and the responses evoked by the stimulus itself, although no significant differences in spontaneous activity were detected between stimulated and non-stimulated cultures belonging to the same preparations. The data indicate that the stimulation had a delayed effect modulating responsiveness capability of the network without directly affecting its intrinsic in vitro development.

The synapsin cycle: a view from the synaptic endocytic zone

Although the synapsin phosphoproteins were discovered more than 30 years ago and are known to play important roles in neurotransmitter release and synaptogenesis, a complete picture of their functions within the nerve terminal is lacking. It has been shown that these proteins play an important role in the clustering of synaptic vesicles (SVs) at active zones and function as modulators of synaptic strength by acting at both pre- and postdocking levels. Recent studies have demonstrated that synapsins migrate to the endocytic zone of central synapses during neurotransmitter release, which suggests that there are additional functions for these proteins in SV recycling.

Heterogeneity of glutamatergic and GABAergic release machinery in cerebral cortex

We investigated whether cortical glutamatergic and GABAergic release machineries can be differentiated on the basis of the proteins they express, by studying the degree of co-localization of synapsin (SYN) I and II, synaptophysin (SYP) I and II, synaptosomal-associated protein (SNAP)-25 and SNAP-23 in vesicular glutamate transporter (VGLUT) 1-, VGLUT2- and vesicular GABA transporter (VGAT)-positive (+) puncta in the rat cerebral cortex. Co-localization studies showed that SYNI and II were expressed in approximately 90% of VGLUT1+, approximately 30% of VGLUT2+ and 30-50% of VGAT+ puncta; SYPI was expressed in approximately 95% of VGLUT1+, 30% of VGLUT2+, and 45% of VGAT+ puncta; SYPII in approximately 7% of VGLUT1+, 3% of VGLUT2+, and 20% of VGAT+ puncta; SNAP-25 in approximately 94% of VGLUT1+, 5% of VGLUT2+, and 1% of VGAT+ puncta, and SNAP-23 in approximately 3% of VGLUT1+, 86% of VGLUT2+, and 22% of VGAT+ puncta. Since SYPI, which is considered ubiquitous, was expressed in about half of GABAergic axon terminals, we studied its localization electron microscopically and in immunoisolated synaptic vesicles: these studies showed that approximately 30% of axon terminals forming symmetric synapses were SYPI-negative, and that immunoisolated VGAT-positive synaptic vesicles were relatively depleted of SYPI as compared with VGLUT1+ vesicles. Overall, the present investigation shows that in the cerebral cortex of rats distinct presynaptic proteins involved in neurotransmitter release are differentially expressed in GABAergic and in the two major types of glutamatergic axon terminals in the cerebral cortex of rats.

Dynamics of secretion

In this short review, kinetic aspects of exocytosis are discussed. A special emphasis is put on recent data that highlight dynamic differences between neurotransmission and other forms of secretion.

The human hypothalamus: a morpho-functional perspective

Historical investigation suggests that the role of the hypothalamus as a site of integration for endocrine with autonomic and behavioral responses in man rises from ideas and observations first appearing between the 14th and 18th centuries. Research on human, post-mortem brains and by in vivo magnetic resonance techniques reveal that the functional morphology of the hypothalamus in man is very similar to that in Rodents and Primates. As such, the adult human hypothalamus can be subdivided in three longitudinal zones, representing the source and target of neural informations traveling back and forth the brain stem, thalamus, limbic system, basal ganglia and neocortex. In addition, the human hypothalamus can be further partitioned in three anterior-posterior regions, of which the rostral one exerts a prominent regulation in predictive homeostasis, as opposed to the two caudal ones, primarily involved in reactive homeostasis. Finally, nuclear distribution in the human hypothalamus largely coincides with that in higher Mammals. Nevertheless, it is still unclear how the hypothalamus may give rise to specific homeostatic behaviors like hunger, thirst, reproductive and parental attitude, thermoregulation, aggressive-defensive performance, affective-motivational tone, circadian rhythmicity, sleep-wake cycle and immune regulation. The recent advent of new theories for nervous communication, like volume transmission and neural Darwinism, is progressively enlightening our understanding of the role played by the hypothalamic architecture in homeostatic responses, both in Mammals and man.

Kinetics of neuronal and endocrine secretion

Calcium (Ca2+) regulated secretion/exocytosis is a key mechanism for cell-cell communication. Neurotransmission and hormone release are the most studied and the best characterized of all secretion systems so far. Here, some dynamic aspects of secretory vesicle trafficking will be briefly reviewed with special emphasis on the differences between synaptic vesicle and dense-core vesicle turnover.

Cellular mechanisms of the acute increase of glutamate release induced by nerve growth factor in rat cerebral cortex

Nerve growth factor (NGF) was found to increase glutamate release in the developing visual cortex. We investigated the cellular mechanisms of this effect and its dependence on extracellular and intracellular Ca2+. The NGF-induced enhancement of glutamate release from superfused rat visual cortex synaptosomes required mild depolarization. Removal of external Ca2+ during depolarization with 15 mM K+ only halved the effect of NGF on glutamate release. NGF increased [Ca2+]i in K+-depolarized synaptosomes preloaded with fura-2AM both in the presence and in the absence of external Ca2+. The effects of NGF on glutamate release and [Ca2+]i elevation were prevented by an anti-TrkA receptor monoclonal antibody. NGF increased synaptosomal inositol (1,4,5)-triphosphate (InsP3) during depolarization and the InsP3 receptor antagonist heparin abolished the effect of NGF on evoked glutamate release both in the presence and in the absence of external Ca2+. The effect of NGF on the evoked glutamate release in Ca2+-free medium was abolished by dantrolene, a ryanodine receptor blocker, by CGP 37157, a blocker of the mitochondrial Na+/Ca2+ exchanger and by pretreatment of synaptosomes with caffeine. NGF significantly increased the depolarization-induced activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and the subsequent phosphorylation of synapsin I in the absence of external Ca2+ and the NGF effect on evoked glutamate release was inhibited by the CaMKII inhibitors KN-93 and CaMKII 281-309 peptide but not by the MAP kinase inhibitor PD 98059. Thus, the effect of NGF on evoked glutamate release is linked to an increase in [Ca2+]i contributed by both Ca2+ entry and mobilization from InsP3-sensitive, ryanodine-sensitive and mitochondrial stores and to the subsequent activation of CaMKII.

Intracellular injection of synapsin I induces neurotransmitter release in C1 neurons of Helix pomatia contacting a wrong target

The contact with the postsynaptic target induces structural and functional modifications in the serotonergic cell C1 of Helix pomatia. In previous studies we have found that the presence of a non-physiological target down-regulates the number of presynaptic varicosities formed by cultured C1 neurons and has a strong inhibitory effect on the action potential-evoked Ca(2+) influx and neurotransmitter release at C1 terminals. Since a large body of experimental evidence implicates the synapsins in the development and functional maturation of synaptic connections, we have investigated whether the injection of exogenous synapsin I into the presynaptic neuron C1 could affect the inhibitory effect of the wrong target on neurotransmitter release. C1 neurons were cultured with the wrong target neuron C3 for three to five days and then injected with either dephosphorylated or Ca(2+)/calmodulin-dependent protein kinase II-phosphorylated Cy3-labeled synapsin I. The subcellular distribution of exogenous synapsin I, followed by fluorescence videomicroscopy, revealed that only synapsin I phosphorylated by Ca(2+)/calmodulin-dependent protein kinase II diffused in the cytoplasm and reached the terminal arborizations of the axon, while the dephosphorylated form did not diffuse beyond the cell body. Evoked neurotransmitter release was measured during C1 stimulation using a freshly dissociated neuron B2 (sniffer) micromanipulated in close contact with the terminals of C1. A three-fold increase in the amplitude of the sniffer depolarization with respect to the pre-injection amplitude (190+/-29% increase, n=10, P<0.006) was found 5 min after injection of Ca(2+)/calmodulin-dependent protein kinase II-phosphorylated synapsin I that lasted for about 30 min. No significant change was observed after injection of buffer or dephosphorylated synapsin I. These data indicate that the presence of synapsin I induces a fast increase in neurotransmitter release that overcomes the inhibitory effect of the non-physiological target and suggest that the expression of synapsins may play a role in the modulation of synaptic strength and neural connectivity.

Synapsin controls both reserve and releasable synaptic vesicle pools during neuronal activity and short-term plasticity in Aplysia

Neurotransmitter release is a highly efficient secretory process exhibiting resistance to fatigue and plasticity attributable to the existence of distinct pools of synaptic vesicles (SVs), namely a readily releasable pool and a reserve pool from which vesicles can be recruited after activity. Synaptic vesicles in the reserve pool are thought to be reversibly tethered to the actin-based cytoskeleton by the synapsins, a family of synaptic vesicle-associated phosphoproteins that have been shown to play a role in the formation, maintenance, and regulation of the reserve pool of synaptic vesicles and to operate during the post-docking step of the release process. In this paper, we have investigated the physiological effects of manipulating synapsin levels in identified cholinergic synapses of Aplysia californica. When endogenous synapsin was neutralized by the injection of specific anti-synapsin antibodies, the amount of neurotransmitter released per impulse was unaffected, but marked changes in the secretory response to high-frequency stimulation were observed, including the disappearance of post-tetanic potentiation (PTP) that was substituted by post-tetanic depression (PTD), and increased rate and extent of synaptic depression. Opposite changes on post-tetanic potentiation were observed when synapsin levels were increased by injecting exogenous synapsin I. Our data demonstrate that the presence of synapsin-dependent reserve vesicles allows the nerve terminal to release neurotransmitter at rates exceeding the synaptic vesicle recycling capacity and to dynamically change the efficiency of release in response to conditioning stimuli (e.g., post-tetanic potentiation). Moreover, synapsin-dependent regulation of the fusion competence of synaptic vesicles appears to be crucial for sustaining neurotransmitter release during short periods at rates faster than the replenishment kinetics and maintaining synchronization of quanta in evoked release.

Identification of synapsin I peptides that insert into lipid membranes

The synapsins constitute a family of synaptic vesicle-associated phosphoproteins essential for regulating neurotransmitter release and synaptogenesis. The molecular mechanisms underlying the selective targeting of synapsin I to synaptic vesicles are thought to involve specific protein-protein interactions, while the high-affinity binding to the synaptic vesicle membrane may involve both protein-protein and protein-lipid interactions. The highly hydrophobic N-terminal region of the protein has been shown to bind with high affinity to the acidic phospholipids phosphatidylserine and phosphatidylinositol and to penetrate the hydrophobic core of the lipid bilayer. To precisely identify the domains of synapsin I which mediate the interaction with lipids, synapsin I was bound to liposomes containing the membrane-directed carbene-generating reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine and subjected to photolysis. Isolation and N-terminal amino acid sequencing of 125I-labelled synapsin I peptides derived from CNBr cleavage indicated that three distinct regions in the highly conserved domain C of synapsin I insert into the hydrophobic core of the phospholipid bilayer. The boundaries of the regions encompass residues 166-192, 233-258 and 278-327 of bovine synapsin I. These regions are surface-exposed in the crystal structure of domain C of bovine synapsin I and are evolutionarily conserved among isoforms across species. The present data offer a molecular explanation for the high-affinity binding of synapsin I to phospholipid bilayers and synaptic vesicles.

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.

Specificity of the binding of synapsin I to Src homology 3 domains

Synapsins are synaptic vesicle-associated phosphoproteins involved in synapse formation and regulation of neurotransmitter release. Recently, synapsin I has been found to bind the Src homology 3 (SH3) domains of Grb2 and c-Src. In this work we have analyzed the interactions between synapsins and an array of SH3 domains belonging to proteins involved in signal transduction, cytoskeleton assembly, or endocytosis. The binding of synapsin I was specific for a subset of SH3 domains. The highest binding was observed with SH3 domains of c-Src, phospholipase C-gamma, p85 subunit of phosphatidylinositol 3-kinase, full-length and NH(2)-terminal Grb2, whereas binding was moderate with the SH3 domains of amphiphysins I/II, Crk, alpha-spectrin, and NADPH oxidase factor p47(phox) and negligible with the SH3 domains of p21(ras) GTPase-activating protein and COOH-terminal Grb2. Distinct sites in the proline-rich COOH-terminal region of synapsin I were found to be involved in binding to the various SH3 domains. Synapsin II also interacted with SH3 domains with a partly distinct binding pattern. Phosphorylation of synapsin I in the COOH-terminal region by Ca(2+)/calmodulin-dependent protein kinase II or mitogen-activated protein kinase modulated the binding to the SH3 domains of amphiphysins I/II, Crk, and alpha-spectrin without affecting the high affinity interactions. The SH3-mediated interaction of synapsin I with amphiphysins affected the ability of synapsin I to interact with actin and synaptic vesicles, and pools of synapsin I and amphiphysin I were shown to associate in isolated nerve terminals. The ability to bind multiple SH3 domains further implicates the synapsins in signal transduction and protein-protein interactions at the nerve terminal level.

The inhibitory effects of interleukin-6 on synaptic plasticity in the rat hippocampus are associated with an inhibition of mitogen-activated protein kinase ERK

Several cytokines have short-term effects on synaptic transmission and plasticity that are thought to be mediated by the activation of intracellular protein kinases. We have studied the effects of interleukin-6 (IL-6) on the expression of paired pulse facilitation (PPF), posttetanic potentiation (PTP), and long-term potentiation (LTP) in the CA1 region of the hippocampus as well as on the activation of the signal transducer and activator of transcription-3 (STAT3), the mitogen-activated protein kinase ERK (MAPK/ERK), and the stress-activated protein kinase/c-Jun NH(2)-terminal kinase (SAPK/JNK). IL-6 induced a marked and dose-dependent decrease in the expression of PTP and LTP that could be counteracted by the simultaneous treatment with the tyrosine kinase inhibitor lavendustin A (LavA) but did not significantly affect PPF. The IL-6-induced inhibition of PTP and LTP was accompanied by a simulation of STAT3 tyrosine phosphorylation and an inhibition of MAPK/ERK dual phosphorylation, in the absence of changes in the state of activation of SAPK/JNK. Both effects of IL-6 on STAT3 and MAPK/ERK activation were effectively counteracted by LavA treatment. The results indicate the tyrosine kinases and MAPK/ERK are involved in hippocampal synaptic plasticity and may represent preferential intracellular targets for the actions of IL-6 in the adult nervous system.

Interleukin-6 inhibits neurotransmitter release and the spread of excitation in the rat cerebral cortex

Cytokines are extracellular mediators that have been reported to affect neurotransmitter release and synaptic plasticity phenomena when applied in vitro. Most of these effects occur rapidly after the application of the cytokines and are presumably mediated through the activation of protein phosphorylation processes. While many cytokines have an inflammatory action, interleukin-6 (IL-6) has been found to have a neuroprotective effect against ischaemia lesions and glutamate excitotoxicity, and to increase neuronal survival in a variety of experimental conditions. In this paper, the functional effects of IL-6 on the spread of excitation visualized by dark-field/infrared videomicroscopy in rat cortical slices and on glutamate release from cortical synaptosomes were analysed and correlated with the activation of the STAT3, mitogen-activated protein kinase ERK (MAPK/ERK) and stress-activated protein kinase/cJun NH2-terminal kinase (SAPK/JNK) pathways. We have found that IL-6 depresses the spread of excitation and evoked glutamate release in the cerebral cortex, and that these effects are accompanied by a stimulation of STAT3 tyrosine phosphorylation, an inhibition of MAPK/ERK activity, a decreased phosphorylation of the presynaptic MAPK/ERK substrate synapsin I and no detectable effects on SAPK/JNK. The effects of IL-6 were effectively counteracted by treatment of the cortical slices with the tyrosine kinase inhibitor lavendustin A. The inhibitory effects of IL-6 on glutamate release and on the spread of excitation in the rat cerebral cortex indicate that the protective effect of IL-6 on neuronal survival could be mediated by a downregulation of neuronal activity, release of excitatory neurotransmitters and MAPK/ERK activity.

Neurite extension occurs in the absence of regulated exocytosis in PC12 subclones

We have investigated the process leading to differentiation of PC12 cells. This process is known to include extension of neurites and changes in the expression of subsets of proteins involved in cytoskeletal rearrangements or in neurosecretion. To this aim, we have studied a PC12 clone (trk-PC12) stably transfected with the nerve growth factor receptor TrkA. These cells are able to undergo both spontaneous and neurotrophin-induced morphological differentiation. However, both undifferentiated and nerve growth factor-differentiated trk-PC12 cells appear to be completely defective in the expression of proteins of the secretory apparatus, including proteins of synaptic vesicles and large dense-core granules, neurotransmitter transporters, and neurotransmitter-synthesizing enzymes. These results indicate that neurite extension can occur independently of the presence of the neurosecretory machinery, including the proteins that constitute the fusion machine, suggesting the existence of differential activation pathways for the two processes during neuronal differentiation. These findings have been confirmed in independent clones obtained from PC12-27, a previously characterized PC12 variant clone globally incompetent for regulated secretion. In contrast, the integrity of the Rab cycle appears to be necessary for neurite extension, because antisense oligonucleotides against the neurospecific isoform of Rab-guanosine diphosphate-dissociation inhibitor significantly interfere with process formation.

Protein-protein interactions and protein modules in the control of neurotransmitter release

Information transfer among neurons is operated by neurotransmitters stored in synaptic vesicles and released to the extracellular space by an efficient process of regulated exocytosis. Synaptic vesicles are organized into two distinct functional pools, a large reserve pool in which vesicles are restrained by the actin-based cytoskeleton, and a quantitatively smaller releasable pool in which vesicles approach the presynaptic membrane and eventually fuse with it on stimulation. Both synaptic vesicle trafficking and neurotransmitter release depend on a precise sequence of events that include release from the reserve pool, targeting to the active zone, docking, priming, fusion and endocytotic retrieval of synaptic vesicles. These steps are mediated by a series of specific interactions among cytoskeletal, synaptic vesicle, presynaptic membrane and cytosolic proteins that, by acting in concert, promote the spatial and temporal regulation of the exocytotic machinery. The majority of these interactions are mediated by specific protein modules and domains that are found in many proteins and are involved in numerous intracellular processes. In this paper, the possible physiological role of these multiple protein-protein interactions is analysed, with ensuing updating and clarification of the present molecular model of the process of neurotransmitter release.

A third member of the synapsin gene family

Synapsins are a family of neuron-specific synaptic vesicle-associated phosphoproteins that have been implicated in synaptogenesis and in the modulation of neurotransmitter release. In mammals, distinct genes for synapsins I and II have been identified, each of which gives rise to two alternatively spliced isoforms. We have now cloned and characterized a third member of the synapsin gene family, synapsin III, from human DNA. Synapsin III gives rise to at least one protein isoform, designated synapsin IIIa, in several mammalian species. Synapsin IIIa is associated with synaptic vesicles, and its expression appears to be neuron-specific. The primary structure of synapsin IIIa conforms to the domain model previously described for the synapsin family, with domains A, C, and E exhibiting the highest degree of conservation. Synapsin IIIa contains a novel domain, termed domain J, located between domains C and E. The similarities among synapsins I, II, and III in domain organization, neuron-specific expression, and subcellular localization suggest a possible role for synapsin III in the regulation of neurotransmitter release and synaptogenesis. The human synapsin III gene is located on chromosome 22q12-13, which has been identified as a possible schizophrenia susceptibility locus. On the basis of this localization and the well established neurobiological roles of the synapsins, synapsin III represents a candidate gene for schizophrenia.

Anti-synapsin monoclonal antibodies: epitope mapping and inhibitory effects on phosphorylation and Grb2 binding

The synapsins are a family of major neuron-specific synaptic vesicle-associated phosphoproteins which play important roles in synaptic function. In an effort to identify molecular tools which can be used to perturb the activity of the synapsins in in vitro as well as in vivo experiments, we have localized the epitopes of a panel of monoclonal antibodies (mAbs) raised against synapsins I and II and have characterized their ability to interfere with the interactions of the synapsins with protein kinases, actin and Src homology-3 (SH3) domains. The epitopes of the six mAbs were found to be concentrated in the N-terminal region within domains A and B for the synapsin II-reactive mAbs 19.4, 19.11, 19.51 and 19.21, and in two C-terminal clusters in the proline-rich domains D for synapsin I (mAbs 10.22, 19.51, 19.11 and 19.8) and G for synapsin II (mAb 19.8). The synapsin II-specific mAbs 19.4 and 19.21, whose overlapping epitopes are adjacent to phosphorylation site 1, specifically inhibited synapsin II phosphorylation by endogenous or exogenous cAMP-dependent protein kinase. While all the anti-synapsin I mAbs were unable to affect the interactions of synapsin I both with Ca2+/calmodulin-dependent protein kinase II and with actin monomers and filaments, mAbs 19.8 and 19.51 were found to inhibit the binding of Grb2 SH3 domains to the proline-rich C-terminal region of synapsin I.

Phosphorylation-dependent effects of synapsin IIa on actin polymerization and network formation

The synapsins are a family of synaptic vesicle phosphoproteins which play a key role in the regulation of neurotransmitter release and synapse formation. In the case of synapsin I, these biological properties have been attributed to its ability to interact with both synaptic vesicles and the actin-based cytoskeleton. Although synapsin II shares some of the biological properties of synapsin I, much less is known of its molecular properties. We have investigated the interactions of recombinant rat synapsin Ila with monomeric and filamentous actin and the sensitivity of those interactions to phosphorylation, and found that: i) dephosphorylated synapsin II stimulates actin polymerization by binding to actin monomers and forming actively elongating nuclei and by facilitating the spontaneous nucleation/elongation processes; ii) dephosphorylated synapsin II induces the formation of thick and ordered bundles of actin filaments with greater potency than synapsin I; iii) phosphorylation by protein kinase A markedly inhibits the ability of synapsin II to interact with both actin monomers and filaments. The results indicate that the interactions of synapsin II with actin are similar but not identical to those of synapsin I and suggest that synapsin II may play a major structural role in mature and developing nerve terminals, which is only partially overlapping with the role played by synapsin I.

Downregulation of MAP kinase activity signalled by HIV-1-gp120 coat protein in granular neurons and glial cells from rat cerebellum

We have studied the effect of gp120 coat protein from HIV-1 on tyrosine phosphorylation processes in primary cultures of granular neurons or glial cells from the cerebellum of neonatal rats. The extracellular application of recombinant gp120 (200 pM) was able to reduce the phosphotyrosine content and the immunoreactivity for active form-specific antibodies of MAP kinase. Whereas in neurons MAP kinase appeared to be the only protein whose phosphotyrosine content was decreased, in glial cultures the inhibitory effect of gp120 on tyrosine phosphorylation processes appeared to be more widespread. In neuronal cultures, the effect of the viral protein was prevented by the concomitant treatment with depolarizing agents.

Kinetic analysis of the phosphorylation-dependent interactions of synapsin I with rat brain synaptic vesicles

1. Synapsin I, a major synaptic vesicle (SV)-associated phosphoprotein, is involved in the regulation of neurotransmitter release and synapse formation. By binding to both phospholipid and protein components of SV with high affinity and in a phosphorylation-dependent fashion, synapsin I is believed to cluster SV and to attach them to the actin-based cytoskeleton of the nerve terminal. 2. In the present study we have investigated the kinetic aspects of synapsin I-SV interactions and the mechanisms of their modulation by ionic strength and site-specific phosphorylation, using fluorescence resonance energy transfer between suitable fluorophores linked to synapsin I and to the membrane bilayer. 3. The binding of synapsin I to the phospholipid and protein components of SV has fast kinetics: mean time constants ranged between 1 and 4 s for association and 9 and 11's for ionic strength-induced dissociation at 20 degrees C. The interaction with the phospholipid component consists predominantly of a hydrophobic binding with the core of the membrane which may account for the membrane stabilizing effect of synapsin I. 4. Phosphorylation of synapsin I by either SV-associated or purified exogenous Ca2+/calmodulin-dependent protein kinase II (CaMPKII) inhibited the association rate and the binding to SV at steady state by acting on the ionic strength-sensitive component of the binding. When dephosphorylated synapsin I was previously bound to SV, exposure of SV to Ca2+/calmodulin in the presence of ATP triggered a prompt dissociation of synapsin I with a time constant similar to that of ionic strength-induced dissociation. 5. In conclusion, the reversible interactions between synapsin I and SV are highly regulated by site-specific phosphorylation and have kinetics of the same order of magnitude as the kinetics of SV recycling determined in mammalian neurons under comparable temperature conditions. These findings are consistent with the hypothesis that synapsin I associates with, and dissociates from, SV during the exo-endocytotic cycle. The on-vesicle phosphorylation of synapsin I by the SV-associated CaMPKII, and the subsequent dissociation of the protein from the vesicle membrane, though not involved in mediating exocytosis of primed vesicles evoked by a single stimulus, may represent a prompt and efficient mechanism for the modulation of neurotransmitter release and presynaptic plasticity.

Synapsin I interacts with c-Src and stimulates its tyrosine kinase activity

Synapsin I is a synaptic vesicle-associated phosphoprotein that has been implicated in the formation of presynaptic specializations and in the regulation of neurotransmitter release. The nonreceptor tyrosine kinase c-Src is enriched on synaptic vesicles, where it accounts for most of the vesicle-associated tyrosine kinase activity. Using overlay, affinity chromatography, and coprecipitation assays, we have now shown that synapsin I is the major binding protein for the Src homology 3 (SH3) domain of c-Src in highly purified synaptic vesicle preparations. The interaction was mediated by the proline-rich domain D of synapsin I and was not significantly affected by stoichiometric phosphorylation of synapsin I at any of the known regulatory sites. The interaction of purified c-Src and synapsin I resulted in a severalfold stimulation of tyrosine kinase activity and was antagonized by the purified c-Src-SH3 domain. Depletion of synapsin I from purified synaptic vesicles resulted in a decrease of endogenous tyrosine kinase activity. Portions of the total cellular pools of synapsin I and Src were coprecipitated from detergent extracts of rat brain synaptosomal fractions using antibodies to either protein species. The interaction between synapsin I and c-Src, as well as the synapsin I-induced stimulation of tyrosine kinase activity, may be physiologically important in signal transduction and in the modulation of the function of axon terminals, both during synaptogenesis and at mature synapses.

Tat protein from HIV-1 activates MAP kinase in granular neurons and glial cells from rat cerebellum

We have investigated the effect of extracellularly applied Tat protein of the human immunodeficiency virus type 1 (HIV-1) on tyrosine phosphorylation processes, which represent a major signal transduction pathway of cells of the central nervous system. Primary cultures of rat cerebellar astrocytes or granule cells were incubated with synthetic Tat (10 ng/ml) for various periods of time and analyzed for their phosphotyrosine content by Western blotting. In both types of cultures Tat was able to induce the phosphorylation of mitogen-activated protein kinase (MAP kinase) on tyrosine residues, although with different kinetics and isoform specificity. In addition, in neuronal cells, but not in astrocytes, Tat increased the phosphotyrosine content of Shc, a protein involved in signal transduction downstream of receptor tyrosine kinase activation. This study shows that Tat applied extracellularly is able to induce the generation of intracellular signals in neuronal as well as glial cells.

Molecular and functional diversity at synapses of individual neurons in vitro

We have quantified activity-dependent uptake of the fluorescent dye FM1-43 in combination with immunocytochemistry for synaptic vesicle-associated proteins (SVPs) at individual synapses in primary cultures of rat cortical neurons. We show that expression of synaptic proteins is highly variable and that the levels of synaptophysin (p38), synapsin I and sv2, but not synapsin II, correlate with the extent of FM1-43 labelling at synapses. The data indicate that SVP levels affect the uptake of FM1-43 with different efficacy (p38 > synapsin I > sv2 or synapsin II). We also found that the relative levels of SVPs vary at individual boutons of single neurons grown in isolation, which indicates that differential regulation of specific SVPs may contribute to the selective modulation of activity at synapses of the same neuron.

Effects of synaptic vesicles on actin polymerization

We have analyzed the effects of synaptic vesicles on actin polymerization by using a time-resolved spectrofluorometric assay. We have found that synaptic vesicles have complex effects on the kinetics of actin polymerization, which vary depending on whether the synaptic vesicle-specific phosphoprotein synapsin I is absent or present on their membrane. Synapsin I bound either to synaptic vesicles or to pure phospholipid vesicles exhibits phosphorylation-dependent actin-nucleating activity. Synaptic vesicles depleted of endogenous synapsin I decrease the rate and the final extent of actin polymerization, an effect which is not observed with pure phospholipid vesicles. Thus, the state of association of synapsin I with synaptic vesicles, which is modulated by its state of phosphorylation, may affect actin assembly and the physico-chemical characteristics of the synaptic vesicle microenvironment.

Synapsin-like molecules in Aplysia punctata and Helix pomatia: identification and distribution in the nervous system and during the formation of synaptic contacts in vitro

The distribution and biochemical features of the synapsin-like peptides recognized in Aplysia and Helix by various antibodies directed against mammalian synapsins were studied. The peptides can be extracted at low pH and are digested by collagenase; further, they can be phosphorylated by both protein kinase A and Ca2+/calmodulin-dependent protein kinase II. In the ganglia of both snails, they are associated with the soma of most neurons and with the neuropil; punctate immunostaining is present along the neurites. Using cocultures of a Helix serotoninergic neuron and of its target cell, we analysed the redistribution of the synapsin-like peptides during the formation of active synaptic contacts. When the presynaptic neuron is plated in isolation, both synapsin and serotonin immunoreactivities are restricted to the distal axonal segments and to the growth cones; in the presence of the target, the formation of a chemical connection is accompanied by redistribution of the synapsin and serotonin immunoreactivities that concentrate in highly fluorescent round spots scattered along the newly grown neurites located close to the target cell. Almost every spot that is stained for serotonin is also positive for synapsin. In the presynaptic cell plated alone, the number of these varicosity-like structures is substantially stable throughout the whole period; by contrast, when the presynaptic cell synapses the target, their number increases progressively parallel to the increase in the mean amplitude of cumulative excitatory postsynaptic potentials recorded at the same times. The data indicate that mollusc synapsin-like peptides to some extent resemble their mammalian homologues, although they are not exclusively localized in nerve terminals and their expression strongly correlates with the formation of active synaptic contacts.

Biochemical and functional characterization of the synaptic vesicle-associated form of CA2+/calmodulin-dependent protein kinase II

Ca+/calmodulin-dependent protein kinase II (CaMPKII) is a brain-enriched protein kinase that plays important roles in synaptic transmission and plasticity. In nerve terminals, a form of CaMPKII is associated with synaptic vesicles and binds the COOH-terminal region of synapsin I (SYNI). The biochemical properties of the vesicle-associated form of CAMPKII have been investigated and compared with those of the soluble forebrain enzyme. Both the alpha- and beta-subunits of CaMPKII copurifying with synaptic vesicles were tightly associated with the vesicle membrane. The vesicle-associated form of CaMPKII was indistinguishable from the soluble form with respect to sites of autophosphorylation, kinetics of both autophosphorylation and SYNI phosphorylation, and induction of autonomous activity upon autophosphorylation. Although both subunits of the soluble CaMPKII interacted with a photoactivatable SYNI derivative, only the alpha-subunit of the synaptic vesicle-associated CaMPKII bound to the COOH-terminal region of SYNI. The latter interaction was strongly dependent on the phosphorylation state of SYNI and on divalent cations, but appeared to be independent of autophosphorylation. These results demonstrate that, although the vesicle-associated form of CaMPKII is catalytically indistinguishable from the soluble form, it exhibits distinct characteristics concerning its association with the vesicle membrane and with SYNI.

Neurotrophins stimulate phosphorylation of synapsin I by MAP kinase and regulate synapsin I-actin interactions

The ability of neurotrophins to modulate the survival and differentiation of neuronal populations involves the Trk/MAP (mitogen-activated protein kinase) kinase signaling pathway. More recently, neurotrophins have also been shown to regulate synaptic transmission. The synapsins are a family of neuron-specific phosphoproteins that play a role in regulation of neurotransmitter release, in axonal elongation, and in formation and maintenance of synaptic contacts. We report here that synapsin I is a downstream effector for the neurotrophin/Trk/MAP kinase cascade. Using purified components, we show that MAP kinase stoichiometrically phosphorylated synapsin I at three sites (Ser-62, Ser-67, and Ser-549). Phosphorylation of these sites was detected in rat brain homogenates, in cultured cerebrocortical neurons, and in isolated presynaptic terminals. Brain-derived neurotrophic factor and nerve growth factor upregulated phosphorylation of synapsin I at MAP kinase-dependent sites in intact cerebrocortical neurons and PC12 cells, respectively, while KCl- induced depolarization of cultured neurons decreased the phosphorylation state at these sites. MAP kinase-dependent phosphorylation of synapsin I significantly reduced its ability to promote G-actin polymerization and to bundle actin filaments. The results suggest that MAP kinase-dependent phosphorylation of synapsin I may contribute to the modulation of synaptic plasticity by neurotrophins and by other signaling pathways that converge at the level of MAP kinase activation.