Effects of the neuronal phosphoprotein synapsin I on actin polymerization. II. Analytical interpretation of kinetic curves

Giving names to the actors of synaptic transmission: The long journey from synaptic vesicles to neural plasticity

More than a scientific paper or a review article, this is a remembrance of a unique time of science and life that the authors spent in Paul Greengard's laboratory at the Rockefeller University in New York in the 1980s and 1990s, forming the so-called synaptic vesicle group. It was a time in which the molecular mechanisms of synaptic transmission and the nature of the organelles in charge of storing and releasing neurotransmitter were just beginning to be understood. It was an exciting time in which the protein composition of synaptic vesicles started to be identified. It turned out that the interactions of synaptic vesicle proteins with the cytoskeleton and the presynaptic membrane and their modulation by protein phosphorylation represented an essential network regulating the efficiency of neurotransmitter release and thereby synaptic strength and plasticity. This is also a description of the distinct scientific journeys that the three authors took on going back to Europe and how they were strongly influenced by the generous and outstanding mentorship of Paul Greengard, his genuine interest in their lives and careers and the life-long friendship with him.

Early nucleation events in the polymerization of actin, probed by time-resolved small-angle x-ray scattering

Nucleators generating new F-actin filaments play important roles in cell activities. Detailed information concerning the events involved in nucleation of actin alone in vitro is fundamental to understanding these processes, but such information has been hard to come by. We addressed the early process of salt-induced polymerization of actin using the time-resolved synchrotron small-angle X-ray scattering (SAXS). Actin molecules in low salt solution maintain a monomeric state by an electrostatic repulsive force between molecules. On mixing with salts, the repulsive force was rapidly screened, causing an immediate formation of many of non-polymerizable dimers. SAXS kinetic analysis revealed that tetramerization gives the highest energetic barrier to further polymerization, and the major nucleation is the formation of helical tetramers. Filaments start to grow rapidly with the formation of pentamers. These findings suggest an acceleration mechanism of actin assembly by a variety of nucleators in cells.

Lysine methylation is an endogenous post-translational modification of tau protein in human brain and a modulator of aggregation propensity

In Alzheimer's disease, the microtubule-associated protein tau dissociates from the neuronal cytoskeleton and aggregates to form cytoplasmic inclusions. Although hyperphosphorylation of tau serine and threonine residues is an established trigger of tau misfunction and aggregation, tau modifications extend to lysine residues as well, raising the possibility that different modification signatures depress or promote aggregation propensity depending on site occupancy. To identify lysine residue modifications associated with normal tau function, soluble tau proteins isolated from four cognitively normal human brains were characterized by MS methods. The major detectable lysine modification was found to be methylation, which appeared in the form of mono- and di-methyl lysine residues distributed among at least 11 sites. Unlike tau phosphorylation sites, the frequency of lysine methylation was highest in the microtubule-binding repeat region that mediates both microtubule binding and homotypic interactions. When purified recombinant human tau was modified in vitro through reductive methylation, its ability to promote tubulin polymerization was retained, whereas its aggregation propensity was greatly attenuated at both nucleation and extension steps. These data establish lysine methylation as part of the normal tau post-translational modification signature in human brain, and suggest that it can function in part to protect against pathological tau aggregation.

Tau isoform composition influences rate and extent of filament formation

The risk of developing tauopathic neurodegenerative disease depends in part on the levels and composition of six naturally occurring Tau isoforms in human brain. These proteins, which form filamentous aggregates in disease, vary only by the presence or absence of three inserts encoded by alternatively spliced exons 2, 3, and 10 of the Tau gene (MAPT). To determine the contribution of alternatively spliced segments to Tau aggregation propensity, the aggregation kinetics of six unmodified, recombinant human Tau isoforms were examined in vitro using electron microscopy assay methods. Aggregation propensity was then compared at the level of elementary rate constants for nucleation and extension phases. We found that all three alternatively spliced segments modulated Tau aggregation but through differing kinetic mechanisms that could synergize or compete depending on sequence context. Overall, segments encoded by exons 2 and 10 promoted aggregation, whereas the segment encoded by exon 3 depressed it with its efficacy dependent on the presence or absence of a fourth microtubule binding repeat. In general, aggregation propensity correlated with genetic risk reported for multiple tauopathies, implicating aggregation as one candidate mechanism rationalizing the correlation between Tau expression patterns and disease.

Effects of phosphorylation and neuronal activity on the control of synapse formation by synapsin I

Synapsins are synaptic vesicle (SV)-associated proteins that regulate synaptic transmission and neuronal differentiation. At early stages, Syn I and II phosphorylation at Ser9 by cAMP-dependent protein kinase (PKA) and Ca2+/calmodulin-dependent protein kinase I/IV modulates axon elongation and SV-precursor dynamics. We evaluated the requirement of Syn I for synapse formation by siRNA-mediated knockdown as well as by overexpression of either its wild-type (WT) form or its phosphorylation mutants. Syn1 knockdown at 14 days in vitro caused a decrease in the number of synapses, accompanied by a reduction of SV recycling. Although overexpression of WT Syn I was ineffective, overexpression of its phosphorylation mutants resulted in a complex temporal regulation of synapse density. At early stages of synaptogenesis, phosphomimetic Syn I S9E significantly increased the number of synapses. Conversely, dephosphomimetic Syn I S9A decreased synapse number at more advanced stages. Overexpression of either WT Syn I or its phosphomimetic S9E mutant rescued the decrease in synapse number caused by chronic treatment with tetrodotoxin at early stages, suggesting that Syn I participates in an alternative PKA-dependent mechanism that can compensate for the impairment of the activity-dependent synaptogenic pathway. Altogether these results indicate that Syn I is an important regulator of synapse formation, which adjusts synapse number in response to extracellular signals.

The synapsins: multitask modulators of neuronal development

Neurons are examples of specialized cells that evolved the extraordinary ability to transmit electrochemical information in complex networks of interconnected cells. During their development, neurons undergo precisely regulated processes that define their lineage, positioning, morphogenesis and pattern of activity. The events leading to the establishment of functional neuronal networks follow a number of key steps, including asymmetric cell division from neuronal precursors, migration, establishment of polarity, neurite outgrowth and synaptogenesis. Synapsins are a family of abundant neuronal phosphoproteins that have been extensively studied for their role in the regulation of neurotransmission in presynaptic terminals. Beside their implication in the homeostasis of adult cells, synapsins influence the development of young neurons, interacting with cytoskeletal and vesicular components and regulating their dynamics. Although the exact molecular mechanisms determining synapsin function in neuronal development are still largely unknown, in this review we summarize the most important literature on the subject, providing a conceptual framework for the progress of present and future research.

Pseudophosphorylation of tau protein directly modulates its aggregation kinetics

Hyperphosphorylation of tau protein is associated with neurofibrillary lesion formation in Alzheimer's disease and other tauopathic neurodegenerative diseases. It fosters lesion formation by increasing the concentration of free tau available for aggregation and by directly modulating the tau aggregation reaction. To clarify how negative charge incorporation into tau directly affects aggregation behavior, the fibrillization of pseudophosphorylation mutant T212E prepared in a full-length four-repeat tau background was examined in vitro as a function of time and submicromolar tau concentrations using electron microscopy assay methods. Kinetic constants for nucleation and extension phases of aggregation were then estimated by direct measurement and mathematical simulation. Kinetic analysis revealed that pseudophosphorylation increased tau aggregation rate by increasing the rate of filament nucleation. In addition, it increased aggregation propensity by stabilizing mature filaments against disaggregation. The data suggest that incorporation of negative charge into the T212 site can directly promote tau filament formation at multiple steps in the aggregation pathway.

Pathogenic missense MAPT mutations differentially modulate tau aggregation propensity at nucleation and extension steps

Mutations in the MAPT gene encoding tau protein lead to neurofibrillary lesion formation, neurodegeneration, and cognitive decline associated with frontotemporal lobar degeneration. While some pathogenic mutations affect MAPT introns, resulting in abnormal splicing patterns, the majority occur in the tau coding sequence leading to single amino acid changes in tau primary structure. Depending on their location within the polypeptide chain, tau missense mutations have been reported to augment aggregation propensity. To determine the mechanisms underlying mutation-associated changes in aggregation behavior, the fibrillization of recombinant pathogenic mutants R5L, G272V, P301L, V337M, and R406W prepared in a full-length four-repeat human tau background was examined in vitro as a function of time and submicromolar tau concentrations using electron microscopy assay methods. Kinetic constants for nucleation and extension phases of aggregation were then estimated by direct measurement and mathematical simulation. Results indicated that the mutants differ from each other and from wild-type tau in their aggregation propensity. G272V and P301L mutations increased the rates of both filament nucleation and extension reactions, whereas R5L and V337M increased only the nucleation phase. R406W did not differ from wild-type in any kinetic parameter. The results show that missense mutations can directly promote tau filament formation at different stages of the aggregation pathway.

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.

Mass Spectrometric Studies on Mouse Hippocampal Synapsins Ia, IIa, and IIb and Identification of a Novel Phosphorylation Site at Serine-546

Synapsins are key phosphoproteins in the mammalian brain, and structural research on synapsins is still holding center stage. Proteins were extracted from hippocampal tissue and separated on two-dimensional gel electrophoresis (2-DE), and the spots were analyzed by MALDI-TOF-TOF and nano-LC-ESI-MS/MS. Synapsins Ia, IIa, and IIb were unambiguously identified and represented by 15 individual spots on 2-DE. Several serine phosphorylation sites were confirmed, and a novel phosphorylation site was observed at Ser-546 in synapsin IIa in all gels analyzed.

C-terminal truncation modulates both nucleation and extension phases of τ fibrillization

Proteolytic post-translational modification has been proposed as an early stage event in the aggregation of tau protein and formation of neurofibrillary lesions in Alzheimer's disease. Caspases and other proteases cleave tau in vivo at discrete locations including Asp421 and Glu391. Both cleavage products are prone to aggregation relative to wild-type, full-length tau protein. To determine the mechanism underlying this effect, the fibrillization of tau truncated after Asp421 and Glu391 residues was characterized in a full-length four-repeat tau background using quantitative electron microscopy methods under homogeneous nucleation conditions. Both C-terminal truncations decreased critical concentration relative to full-length tau, resulting in more filament mass at reaction plateau. Moreover, truncation directly augmented the efficiency of the nucleation reaction. The results suggest the mechanism through which C-terminal proteolysis can modulate tau filament accumulation depending on whether it precedes or follows nucleation.

Role of phospholipase D1 in neurite outgrowth of neural stem cells

Employing neural stem cells from the brain cortex of E12 rat embryos, we investigated the possible role of phospholipase D (PLD) in the synaptogenesis and neurite formation of neural cells during differentiation. Expression level of PLD1 increased during neuronal differentiation of the neural stem cells, resulting in increased PLD activity. Expression level of synapsin I, a marker of synaptogenesis, also increased as the differentiation of neural stem cells progressed. To figure out the effect of PLD on synapsin I expression, we treated the neural stem cells with phorbol myristate acetate (PMA) to stimulate PLD activity. Increased PLD activity induced by PMA treatment resulted in elevated synapsin I expression and neurite outgrowth during neuronal differentiation. To further confirm the role of PLD in neurite outgrowth, we transfected the dominant-negative form of rat PLD1 cDNA (DN-rPLD1) into neural stem cells to downregulate PLD activity. Overexpression of DN-rPLD1 showed the complete inhibition of neurite outgrowth of neural stem cells under differentiation condition. While transfection of DN-rPLD1 did not affect the synapsin I expression, overexpression of rPLD1 resulted in increased synapsin I expression of the neural cells. These results suggest that PLD1 plays a critical role in neurite outgrowth during differentiation of the neural stem cells. In conclusion, this is the first evidence to show that PLD1 acts as an important regulator of neurite outgrowth in neural stem cell by promoting neuronal differentiation via increase of synapsin I expression.

Pseudophosphorylation and Glycation of Tau Protein Enhance but Do Not Trigger Fibrillization in Vitro

Alzheimer's disease is defined in part by the intraneuronal aggregation of tau protein into filamentous lesions. The pathway is accompanied by posttranslational modifications including phosphorylation and glycation, each of which has been shown to promote tau fibrillization in vitro when present at high stoichiometry. To clarify the site-specific impact of posttranslational modification on tau fibrillization, the ability of recombinant full-length four repeat tau protein (htau40) and 11 pseudophosphorylation mutants to fibrillize in the presence of anionic inducer was assayed in vitro using transmission electron microscopy and laser light scattering assays. Tau glycated with d-glucose was examined as well. Both glycated tau and pseudophosphorylation mutants S199E, T212E, S214E, double mutant T212E/S214E, and triple mutant S199E/S202E/T205E yielded increased filament mass at equilibrium relative to wild-type tau. Increases in filament mass correlated strongly with decreases in critical concentration, indicating that both pseudophosphorylation and glycation promoted fibrillization by shifting equilibrium toward the fibrillized state. Analysis of reaction time courses further revealed that increases in filament mass were not associated with reduced lag times, indicating that these posttranslational modifications did not promote filament nucleation. The results suggest that site-specific posttranslational modifications can stabilize filaments once they nucleate, and thereby support their accumulation at low intracellular tau concentrations.

Nucleation-elongation: a mechanism for cooperative supramolecular polymerization

The kinetic and thermodynamic characteristics of polymerizations following a cooperative, nucleation-elongation mechanism are discussed in comparison to those of non-cooperative, isodesmic polymerizations. Nucleation-elongation polymerization is a relatively unexplored avenue of synthetic polymer chemistry and offers some unique and interesting thermodynamic and kinetic attributes not found in the more classical mechanisms of polymer chemistry.

Colocalization of synapsin and actin during synaptic vesicle recycling

It has been hypothesized that in the mature nerve terminal, interactions between synapsin and actin regulate the clustering of synaptic vesicles and the availability of vesicles for release during synaptic activity. Here, we have used immunogold electron microscopy to examine the subcellular localization of actin and synapsin in the giant synapse in lamprey at different states of synaptic activity. In agreement with earlier observations, in synapses at rest, synapsin immunoreactivity was preferentially localized to a portion of the vesicle cluster distal to the active zone. During synaptic activity, however, synapsin was detected in the pool of vesicles proximal to the active zone. In addition, actin and synapsin were found colocalized in a dynamic filamentous cytomatrix at the sites of synaptic vesicle recycling, endocytic zones. Synapsin immunolabeling was not associated with clathrin-coated intermediates but was found on vesicles that appeared to be recycling back to the cluster. Disruption of synapsin function by microinjection of antisynapsin antibodies resulted in a prominent reduction of the cytomatrix at endocytic zones of active synapses. Our data suggest that in addition to its known function in clustering of vesicles in the reserve pool, synapsin migrates from the synaptic vesicle cluster and participates in the organization of the actin-rich cytomatrix in the endocytic zone during synaptic activity.

Mapping the microtubule binding regions of calponin

The smooth muscle basic calponin interacts with F-actin and inhibits the actomyosin ATPase in a calmodulin or phosphorylation modulated manner. It also binds in vitro to microtubules and its acidic isoform, present in nonmuscle cells, and co-localizes with microfilaments and microtubules in cultured neurons. To assess the physiological significance and the molecular basis of the calponin-microtubule interaction, we have first studied the solution binding of recombinant acidic calponin to microtubules using quantitative cosedimentation analyses. We have also characterized, for the first time, the ability of both calponin isoforms to induce the inhibition of the microtubule-stimulated ATPase activity of the cytoskeletal, kinesin-related nonclaret dysjunctional motor protein (ncd) and the abolition of this effect by calcium calmodulin. This property makes calponin a potent inhibitor of all filament-activated motor ATPases and, therefore, a potential regulatory factor of many motor-based biological events. By combining the enzymatic measurements of the ncd-microtubules system with various in vitro binding assays employing proteolytic, recombinant and synthetic fragments of basic calponin, we further unambiguously identified the interaction of microtubules at two distinct calponin sites. One is inhibitory and resides in the segment 145-182, which also binds F-actin and calmodulin. The other one is noninhibitory, specific for microtubules, and is located on the COOH-terminal repeat-containing region 183-292. Finally, quantitative fluorescence studies of the binding of basic calponin to the skeletal pyrenyl F-actin in the presence of microtubules did not reveal a noticeable competition between the two sets of filaments for calponin. This result implies that calponin undergoes a concomitant binding to both F-actin and microtubules by interaction at the former site with actin and at the second site with microtubules. Thus, in the living cells, calponin could potentially behave as a cross-linking protein between the two major cytoskeletal filaments.

A protein kinase A-dependent molecular switch in synapsins regulates neurite outgrowth

Cyclic AMP (cAMP) promotes neurite outgrowth in a variety of neuronal cell lines through the activation of protein kinase A (PKA). We show here, using both Xenopus laevis embryonic neuronal culture and intact X. laevis embryos, that the nerve growth-promoting action of cAMP/PKA is mediated in part by the phosphorylation of synapsins at a single amino acid residue. Expression of a mutated form of synapsin that prevents phosphorylation at this site, or introduction of phospho-specific antibodies directed against this site, decreased basal and dibutyryl cAMP-stimulated neurite outgrowth. Expression of a mutation mimicking constitutive phosphorylation at this site increased neurite outgrowth, both under basal conditions and in the presence of a PKA inhibitor. These results provide a potential molecular approach for stimulating neuron regeneration, after injury and in neurodegenerative diseases.

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.

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.

Sequences of actin implicated in the polymerization process: a simplified mathematical approach to probe the role of these segments

Regulation of actin polymerization and depolymerization is essential for the functions of actin in non-muscle cells and is mediated by a large number of heterologous actin-binding proteins which questions their true impact on the polymerization process. As a model, we report here the modulating effect of monospecific antibody fragments (Fab) as in vitro effectors on actin polymerization kinetics. Polymerization curves were obtained through fluorescence measurements. They were fitted using analytical equations derived from classical models describing the actin polymerization process with the aim of identifying kinetic steps potentially altered by the effectors. The study was limited to three short segments bore by the 300-328 sequence which is located in actin subdomain 3 and implicated in one of the monomer-monomer interfaces. We observed that antibodies which inhibited actin polymerization reacted with both G- and F-actins, modulated both nucleation and elongation steps, enhanced actin monomer dissociation from the filament and apparently did not act as capping or sequestering proteins. Among the antibody populations specific for a restricted and selected sequence in subdomain 3 of actin (sequence 300-326), only those directed to epitopes located near Met 305 and 325 were effective. In contrast, antibodies directed towards the alpha-helix located between the two preceding epitopes had no effect. All the results analyzed here emphasize the important role of some discrete regions and their conformational state in regulation of the interconversion between monomeric and polymeric actins which could be controlled in different ways by the various actin-binding proteins.

Accelerated structural maturation induced by synapsin I at developing neuromuscular synapses of Xenopus laevis

The role of synapsin I, a synaptic vesicle-associated phosphoprotein, in the maturation of nerve-muscle synapses was investigated in nerve-muscle co-cultures prepared from Xenopus embryos loaded with the protein by the early blastomere injection method. The stage of maturation of the synapses was analysed by electron microscopy as well as by whole-cell patch-clamp recording. The acceleration in the functional maturation of neuromuscular synapses induced by synapsin I was accompanied by a profound rearrangement in the ultrastructure of the nerve terminal. Nerve terminals formed by synapsin I-loaded neurons were characterized by a higher number of small synaptic vesicles organized in clusters and predominantly localized close to the nerve terminal plasma membrane, a smaller number of large dense-core vesicles and no significant change in the number of coated vesicles. Precocious development of active zone-like structures as well as deposition of basal lamina into the synaptic cleft were also observed at these synapses. These results support a role for synapsin I in the architectural changes which occur during synaptogenesis and lead to the maturation of quantal neurotransmitter release mechanisms.

The synaptic vesicle and its targets

Synaptic vesicles play the central role in synaptic transmission. They are regarded as key organelles involved in synaptic functions such as uptake, storage and stimulus-dependent release of neurotransmitter. In the last few years our knowledge concerning the molecular components involved in the functioning of synaptic vesicles has grown impressively. Combined biochemical and molecular genetic approaches characterize many constituents of synaptic vesicles in molecular detail and contribute to an elaborate understanding of the organelle responsible for fast neuronal signalling. By studying synaptic vesicles from the electric organ of electric rays and from the mammalian cerebral cortex several proteins have been characterized as functional carriers of vesicle function, including proteins involved in the molecular cascade of exocytosis. The synaptic vesicle specific proteins, their presumptive function and targets of synaptic vesicle proteins will be discussed. This paper focuses on the small synaptic vesicles responsible for fast neuronal transmission. Comparing synaptic vesicles from the peripheral and central nervous systems strengthens the view of a high conservation in the overall composition of synaptic vesicles with a unique set of proteins attributed to this cellular compartment. Synaptic vesicle proteins belong to gene families encoding multiple isoforms present in subpopulations of neurons. The overall architecture of synaptic vesicle proteins is highly conserved during evolution and homologues of these proteins govern the constitutive secretion in yeast. Neurotoxins from different sources helped to identify target proteins of synaptic vesicles and to elucidate the molecular machinery of docking and fusion. Synaptic vesicle proteins and their markers are useful tools for the understanding of the complex life cycle of synaptic vesicles.

Rapid binding of synapsin I to F- and G-actin. A study using fluorescence resonance energy transfer

Synapsin I is a nerve terminal phosphoprotein which interacts with synaptic vesicles and actin in a phosphorylation-dependent manner. By using fluorescence resonance energy transfer between purified components labeled with fluorescent probes, we now show that the binding of synapsin I to actin is a rapid phenomenon. Binding of synapsin I to actin can also be demonstrated when synaptic vesicles are present in the medium and appears to be modulated by ionic strength and synapsin I phosphorylation.

Synaptic vesicle phosphoproteins and regulation of synaptic function

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

Fluorescence approaches to the study of the actin-nucleating and bundling activities of synapsin I

Synapsin I is a neuron-specific phosphoprotein which binds to small synaptic vesicles and actin in a phosphorylation-dependent fashion. We have analyzed the ability of synapsin I to interact with actin monomers and filaments using purified proteins derivatized with fluorescent probes. Synapsin I accelerates the initial rate of actin polymerization and increases the final steady-state levels of polymerized actin. The fraction of total actin polymerized by synapsin I strongly depends on the synapsin I-actin ratio. We have visualized the actin-bundling activity of synapsin I using a non-perturbing method, video-enhanced microscopy of fluoresceinated synapsin I and actin filaments. Our findings suggest that synapsin I exerts a control on the physical characteristics of the cytoskeletal network of the nerve terminal and are consistent with the proposed role of synapsin I in mediating the interaction of synaptic vesicles with actin.

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

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