Synapsin I is a highly surface-active molecule

Synapsins regulate brain-derived neurotrophic factor-mediated synaptic potentiation and axon elongation by acting on membrane rafts

In neurons, intracellular membrane rafts are essential for specific actions of brain-derived neurotrophic factor (BDNF), which include the regulation of axon outgrowth, growth cone turning and synaptic transmission. Virtually, all the actions of BDNF are mediated by binding to its receptor, TrkB. The association of TrkB with the tyrosine kinase, Fyn, is critical for its localization to intracellular membrane rafts. Here, we show that synapsins, a family of highly amphipathic neuronal phosphoproteins, regulate membrane raft lipid composition and consequently, the ability of BDNF to regulate axon/neurite development and potentiate synaptic transmission. In the brains of mice lacking all synapsins, the expression of both BDNF and TrkB were increased, suggesting that BDNF/TrkB-mediated signaling is impaired. Consistent with this finding, synapsin-depleted neurons exhibit altered raft lipid composition, deficient targeting of Fyn to rafts, attenuated TrkB activation, and abrogation of BDNF-stimulated axon outgrowth and synaptic potentiation. Conversely, overexpression of synapsins in neuroblastoma cells results in corresponding reciprocal changes in raft lipid composition, increased localization of Fyn to rafts and promotion of BDNF-stimulated neurite formation. In the presence of synapsins, the ratio of cholesterol to estimated total phospholipids converged to 1, suggesting that synapsins act by regulating the ratio of lipids in intracellular membranes, thereby promoting lipid raft formation. These studies reveal a mechanistic link between BDNF and synapsins, impacting early development and synaptic transmission.

The highly conserved synapsin domain E mediates synapsin dimerization and phospholipid vesicle clustering

Synapsins are abundant SV (synaptic vesicle)-associated phosphoproteins that regulate synapse formation and function. The highly conserved C-terminal domain E was shown to contribute to several synapsin functions, ranging from formation of the SV reserve pool to regulation of the kinetics of exocytosis and SV cycling, although the molecular mechanisms underlying these effects are unknown. In the present study, we used a synthetic 25-mer peptide encompassing the most conserved region of domain E (Pep-E) to analyse the role of domain E in regulating the interactions between synapsin I and liposomes mimicking the phospholipid composition of SVs (SV-liposomes) and other pre-synaptic protein partners. In affinity-chromatography and cross-linking assays, Pep-E bound to endogenous and purified exogenous synapsin I and strongly inhibited synapsin dimerization, indicating a role in synapsin oligomerization. Consistently, Pep-E (but not its scrambled version) counteracted the ability of holo-synapsin I to bind and coat phospholipid membranes, as analysed by AFM (atomic force microscopy) topographical scanning, and significantly decreased the clustering of SV-liposomes induced by holo-synapsin I in FRET (Förster resonance energy transfer) assays, suggesting a causal relationship between synapsin oligomerization and vesicle clustering. Either Pep-E or a peptide derived from domain C was necessary and sufficient to inhibit both dimerization and vesicle clustering, indicating the participation of both domains in these activities of synapsin I. The results provide a molecular explanation for the effects of domain E in nerve terminal physiology and suggest that its effects on the size and integrity of SV pools are contributed by the regulation of synapsin dimerization and SV clustering.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Influence of synapsin I on synaptic vesicles: an analysis by force-volume mode of the atomic force microscope and dynamic light scattering

Synaptic vesicles (SVs) are small neuronal organelles that store neurotransmitters and release them by exocytosis into the synaptic cleft for signal transmission between nerve cells. They consist of a highly curved membrane composed of different lipids containing several proteins with specific functions. A family of abundant extrinsic SV proteins, the synapsins, interact with SV proteins and phospholipids and play an important role in the regulation of SV trafficking and stability. We investigated the interactions of one these proteins with the SV membrane using atomic force microscope and dynamic light scattering. We examined SVs isolated from rat forebrain both under native conditions and after depletion of endogenous synapsin I. We used the atomic force microscope in two modes: imaging mode for characterizing the shape and size of SVs, and force-volume mode for characterizing their stiffness. Synapsin-depleted SVs were larger in size and showed a higher tendency to aggregate than native vesicles, although their stiffness was not significantly different. Because synapsins are believed to cross-link SV to each other and to the actin cytoskeleton, we also measured the SV aggregation kinetics induced by synapsin I by dynamic light scattering and atomic force microscopy and found that the addition of synapsin I promotes a rapid aggregation of SVs. The data indicate that synapsin directly affects SV stability and aggregation state and support the physiological role of synapsins in the assembly and regulation of SV pools within nerve terminals.

A thermodynamic study of GPI-anchored and soluble form of alkaline phosphatase films at the air-water interface

Glycosylphosphatidyl inositol (GPI) anchored proteins are localized and clustered on the outer layer of the plasma membranes forming microdomains. Among them, mammalian alkaline phosphatases (AP-GPI) are widely distributed enzymes. They can also exist as soluble proteins without anchor (APs). Using the Langmuir film technique, we study the thermodynamic properties of monolayers for both protein forms at the air-buffer interface. The enzymatic activity is maintained at the interface but the adsorption of the two forms of AP is very different. AP-GPI presents a higher surface activity and a larger molecular area than the soluble form. The molecular area deduced for high surface pressures suggests a different organization of the monolayers for these two forms. APs molecules seem to adsorb as a multilayer at the interface while AP-GPI appear to be orientated with the major axis parallel to the interface. This orientation allows the accessibility of AP-GPI enzymatic sites that are turned in direction of the subphase as in vivo where the active sites must be turned outside of the membrane.

Absence of synapsin I and II is accompanied by decreases in vesicular transport of specific neurotransmitters

Studies of synapsin-deficient mice have shown decreases in the number of synaptic vesicles but knowledge about the consequences of this decrease, and which classes of vesicles are being affected, has been lacking. In this study, glutamatergic, GABAergic and dopaminergic transport has been analysed in animals where the genes encoding synapsin I and II were inactivated. The levels of the vesicular glutamate transporter (VGLUT) 1, VGLUT2 and the vesicular GABA transporter (VGAT) were decreased by approximately 40% in adult forebrain from mice devoid of synapsin I and II, while vesicular monoamine transporter (VMAT) 2 and VGLUT3 were present in unchanged amounts compared with wild-type mice. Functional studies on synaptic vesicles showed that the vesicular uptake of glutamate and GABA was decreased by 41 and 23%, respectively, while uptake of dopamine was unaffected by the lack of synapsin I and II. Double-labelling studies showed that VGLUT1 and VGLUT2 colocalized fully with synapsin I and/or II in the hippocampus and neostriatum, respectively. VGAT showed partial colocalization, while VGLUT3 and VMAT2 did not colocalize with either synapsin I or II in the brain areas studied. In conclusion, distinct vesicular transporters show a variable degree of colocalization with synapsin proteins and, hence, distinct sensitivities to inactivation of the genes encoding synapsin I and II.

Using the atomic force microscope to study the interaction between two solid supported lipid bilayers and the influence of synapsin I

To measure the interaction between two lipid bilayers with an atomic force microscope one solid supported bilayer was formed on a planar surface by spontaneous vesicle fusion. To spontaneously adsorb lipid bilayers also on the atomic force microscope tip, the tips were first coated with gold and a monolayer of mercapto undecanol. Calculations indicate that long-chain hydroxyl terminated alkyl thiols tend to enhance spontaneous vesicle fusion because of an increased van der Waals attraction as compared to short-chain thiols. Interactions measured between dioleoylphosphatidylcholine, dioleoylphosphatidylserine, and dioleoyloxypropyl trimethylammonium chloride showed the electrostatic double-layer force plus a shorter-range repulsion which decayed exponentially with a decay length of 0.7 nm for dioleoylphosphatidylcholine, 1.2 nm for dioleoylphosphatidylserine, and 0.8 nm for dioleoyloxypropyl trimethylammonium chloride. The salt concentration drastically changed the interaction between dioleoyloxypropyl trimethylammonium chloride bilayers. As an example for the influence of proteins on bilayer-bilayer interaction, the influence of the synaptic vesicle-associated, phospholipid binding protein synapsin I was studied. Synapsin I increased membrane stability so that the bilayers could not be penetrated with the tip.

Interaction of synapsin I with membranes

The synapsins (I, II, and III) comprise a family of peripheral membrane proteins that are involved in both regulation of neurotransmitter release and synaptogenesis. Synapsins are concentrated at presynaptic nerve terminals and are associated with the cytoplasmic surface of synaptic vesicles. Membrane-binding of synapsins involves interaction with both protein and lipid components of synaptic vesicles. Synapsin I binds rapidly and with high affinity to liposomes containing anionic lipids. The binding of bovine synapsin I to liposomes was studied using fluoresceinphosphatidyl-ethanolamine (FPE) to measure membrane electrostatic potential. Synapsin binding to liposomes caused a rapid increase in FPE fluorescence, indicating an increase in positive charge at the membrane surface. Synapsin I binding to monolayers resulted in a substantial increase in monolayer surface pressure. At higher initial surface pressures, the synapsin-induced increase in monolayer surface pressure is dependent on the presence of anionic lipids in the monolayer. Synapsin I also induced rapid aggregation of liposomes, but did not induce leakage of entrapped carboxyfluorescein, while other aggregation-inducing agents promoted extensive leakage. These results are in agreement with the presence of amphipathic stretches of amino acids in synapsin I that exhibit both electrostatic and hydrophobic interactions with membranes, and offer a molecular explanation for the high affinity binding of synapsin I to liposomes and for stabilization of membranes by synapsin I.

Thymosin beta 4 interactions

Thymosin beta 4 is a small, 5-kDa protein with a diverse range of activities, including its function as an actin monomer sequestering protein, an antiinflammatory agent, and an inhibitor of bone marrow stem cell proliferation. Only the effects of thymosin beta 4 on the actin cytoskeleton have an explanation based on identified molecular interactions. Thymosin beta 4 is largely unfolded or perhaps completely unfolded in solution. Based on the paradigm introduced by Wright and Dyson (1999) that unfolded proteins may have multiple functions based on their ability to recognize numerous ligands, the flexible structure of thymosin beta 4 may facilitate the recognition of a variety of molecular targets, thus explaining the plethora of functions attributed to thymosin beta 4. Furthermore, if multiple ligands bind to thymosin beta 4, then it is possible that thymosin beta 4 has a unique integrative function that links the actin cytoskeleton to important immune and cell growth-signaling cascades.

Lipid droplet binding and oligomerization properties of the Parkinson's disease protein alpha-synuclein

alpha-Synuclein is a major component of the fibrillary lesion known as Lewy bodies and Lewy neurites that are the pathologic hallmarks of Parkinson's disease (PD). In addition, point mutations in the alpha-synuclein gene imply alpha-synuclein dysfunction in the pathology of inherited forms of PD. alpha-Synuclein is a member of a family of proteins found primarily in the brain and is concentrated within presynaptic terminals. Here, we address the localization and membrane binding characteristics of wild type and PD mutants of alpha-synuclein in cultured cells. In cells treated with high concentrations of fatty acids, wild type alpha-synuclein accumulated on phospholipid monolayers surrounding triglyceride-rich lipid droplets and was able to protect stored triglycerides from hydrolysis. PD mutant synucleins showed variable distributions on lipid droplets and were less effective in regulating triglyceride turnover. Chemical cross-linking demonstrated that synuclein formed small oligomers within cells, primarily dimers and trimers, that preferentially associated with lipid droplets and cell membranes. Our results suggest that the initial phases of synuclein aggregation may occur on the surfaces of membranes and that pathological conditions that induce cross-linking of synuclein may enhance the propensity for subsequent synuclein aggregation.

Actin filament cross-linking by MARCKS: characterization of two actin-binding sites within the phosphorylation site domain

We recently identified conformational changes that occur upon phosphorylation of myristoylated alanine-rich protein kinase C substrate (MARCKS) that preclude efficient cross-linking of actin filaments (Bubb, M. R., Lenox, R. H., and Edison, A. S. (1999) J. Biol. Chem. 274, 36472-36478). These results implied that the phosphorylation site domain of MARCKS has two actin-binding sites. We now present evidence for the existence of two actin-binding sites that not only mutually compete but also specifically compete with the actin-binding proteins thymosin beta(4) and actobindin to bind to actin. The effects of substitution of alanine for phenylalanine within a repeated hexapeptide segment suggest that the noncharged region of the domain contributes to binding affinity, but the binding affinity of peptides corresponding to each binding site has a steep dependence on salt concentration, consistent with presumed electrostatic interactions between these polycationic peptides and the polyanionic N terminus of actin. Phosphorylation decreases the site-specific affinity by no more than 0.7 kcal/mol, which is less than the effect of alanine substitution. However, phosphorylation has a much greater effect than alanine substitution on the loss of actin filament cross-linking activity. These results are consistent with the hypothesis that the compact structure resulting from conformational changes due to phosphorylation, in addition to modest decreases in site-specific affinity, explains the loss of cross-linking activity in phosphorylated MARCKS.

Surface active properties of amphiphilic sequential isopeptides: Comparison between alpha-helical and beta-sheet conformations

Poly(Leu-Lys-Lys-Leu) and poly(Leu-Lys) are sequential amphiphilic peptide isomers that adopt respectively an alpha-helical conformation and a beta-sheet structure in saline solutions and at the air/water interface. The surface active properties of LKKL and LK sequential isopeptides containing 16, 20, and n residues have been compared in order to evaluate the contributions of the alpha-helical and beta-sheet conformations. Both have a natural tendency to spread at the surface of a saline solution and the values of the equilibrium spreading pressure pi(e) lie in the same range. When dissolved in a saline solution, alpha-helical peptides diffuse faster and adsorb faster at the interface than the beta-sheet isomers. From the compression isotherms of LKKL and LK peptide monolayers it is possible to extract parameters that characterize the behavior of alpha-helical and beta-sheet conformations: beta-sheet peptide monolayers are more stable and less compressible than the monolayers formed with the alpha-helical isomers. The LK peptides differ also by their high degree of self-association at the air/water interface. Copyright 1999 John Wiley & Sons, Inc.

Synapsins I and II are ATP-binding proteins with differential Ca2+ regulation

Synapsins I and II are abundant phosphoproteins that are localized to synaptic vesicles and have essential functions in regulating synaptic vesicle exocytosis. Synapsins contain a single evolutionarily conserved, large central domain, the C-domain, that accounts for the majority of their sequences. Unexpectedly, the crystal structure of the C-domain from synapsin I revealed that it is structurally closely related to several ATPases despite the absence of sequence similarities (Esser, L., Wang, C.-R., Hosaka, M., Smagula, C. S., Südhof, T. C., and Deisenhofer, J. (1998) EMBO J., in press). We now show that the C-domains of both synapsin I and synapsin II constitute high affinity ATP-binding modules. The two C-domains exhibit similar ATP affinities but are differentially regulated; ATP binding to synapsin I is Ca(2+)-dependent whereas ATP binding to synapsin II is Ca(2+)-independent. In synapsin I, the Ca2+ requirement for ATP binding is mediated by a single, evolutionarily conserved glutamate residue (Glu373) at a position where synapsin II contains a lysine residue. Exchange of Glu373 for lysine converts synapsin I from a Ca(2+)-dependent protein into a Ca(2+)-independent ATP-binding protein. Our studies suggest that synapsins I and II function on synaptic vesicles as ATP-binding proteins that are differentially regulated by Ca2+.

Formation of synaptic vesicles

Synaptic vesicles (SVs) are specialized secretory organelles used for the fast and focal signaling between nerve cells. They are small and homogeneous in size (50 nm), and contain non-peptide neurotransmitters such as glutamate, gamma-aminobutyric acid (GABA) and acetylcholine. The exocytosis of SVs occurs at low rates in resting nerve terminals and is greatly stimulated by depolarization-induced Ca2+ influx. Following exocytosis, SV membranes are rapidly retrieved, refilled locally with neurotransmitters and reused for the assembly of new SVs. Over the past few years, significant progress has been made in characterizing the molecular composition of SVs. From these studies, we know that SVs share a conserved set of membrane proteins with transport vesicles involved in other pathways. Furthermore, these findings have provided us with a new understanding about the evolutionary origin of SVs from recycling vesicles present in all cells.

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

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