Bassoon, a novel zinc-finger CAG/glutamine-repeat protein selectively localized at the active zone of presynaptic nerve terminals

Molecular organization and assembly of the presynaptic active zone of neurotransmitter release

At chemical synapses, neurotransmitter is released at a restricted region of the presynaptic plasma membrane, called the active zone. At the active zone, a matrix of proteins is assembled, which is termed the presynaptic grid or cytomatrix at the active zone (CAZ). Components of the CAZ are thought to localize and organize the synaptic vesicle cycle, a series of membrane trafficking events underlying regulated neurotransmitter exocytosis. This review is focused on a set of specific proteins involved in the structural and functional organization of the CAZ. These include the multi-domain Rab3-effector proteins RIM1alpha and RIM2alpha; Bassoon and Piccolo, two multi-domain CAZ scaffolding proteins of enormous size; as well as members of the CAST/ERC family of CAZ-specific structural proteins. Studies on ribbon synapses of retinal photoreceptor cells have fostered understanding the molecular design of the CAZ. In addition, the analysis of the delivery pathways for Bassoon and Piccolo to presynaptic sites during development has produced new insights into assembly mechanisms of brain synapses during development. Based on these studies, the active zone transport vesicle hypothesis was formulated, which postulates that active zones, at least in part, are pre-assembled in neuronal cell bodies and transported as so-called Piccolo-Bassoon transport vesicles (PTVs) to sites of synaptogenesis. Several PTVs can fuse on demand with the presynaptic membrane to rapidly form an active zone.

Assembly of active zone precursor vesicles: obligatory trafficking of presynaptic cytomatrix proteins Bassoon and Piccolo via a trans-Golgi compartment

Neurotransmitter release from presynaptic nerve terminals is restricted to specialized areas of the plasma membrane, so-called active zones. Active zones are characterized by a network of cytoplasmic scaffolding proteins involved in active zone generation and synaptic transmission. To analyze the modes of biogenesis of this cytomatrix, we asked how Bassoon and Piccolo, two prototypic active zone cytomatrix molecules, are delivered to nascent synapses. Although these proteins may be transported via vesicles, little is known about the importance of a vesicular pathway and about molecular determinants of cytomatrix molecule trafficking. We found that Bassoon and Piccolo co-localize with markers of the trans-Golgi network in cultured neurons. Impairing vesicle exit from the Golgi complex, either using brefeldin A, recombinant proteins, or a low temperature block, prevented transport of Bassoon out of the soma. Deleting a newly identified Golgi-binding region of Bassoon impaired subcellular targeting of recombinant Bassoon. Overexpressing this region to specifically block Golgi binding of the endogenous protein reduced the concentration of Bassoon at synapses. These results suggest that, during the period of bulk synaptogenesis, a primordial cytomatrix assembles in a trans-Golgi compartment. They further indicate that transport via Golgi-derived vesicles is essential for delivery of cytomatrix proteins to the synapse. Paradigmatically this establishes Golgi transit as an obligatory step for subcellular trafficking of distinct cytoplasmic scaffolding proteins.

RIM function in short- and long-term synaptic plasticity

RIM1alpha (Rab3-interacting molecule 1alpha) is a large multidomain protein that is localized to presynaptic active zones [Wang, Okamoto, Schmitz, Hofmann and Südhof (1997) Nature (London) 388, 593-598] and is the founding member of the RIM protein family that also includes RIM2alpha, 2beta, 2gamma, 3gamma and 4gamma [Wang and Südhof (2003) Genomics 81, 126-137]. In presynaptic nerve termini, RIM1alpha interacts with a series of presynaptic proteins, including the synaptic vesicle GTPase Rab3 and the active zone proteins Munc13, liprins and ELKS (a protein rich in glutamate, leucine, lysine and serine). Mouse KOs (knockouts) revealed that, in different types of synapses, RIM1alpha is essential for different forms of synaptic plasticity. In CA1-region Schaffer-collateral excitatory synapses and in GABAergic synapses (where GABA is gamma-aminobutyric acid), RIM1alpha is required for maintaining normal neurotransmitter release and short-term synaptic plasticity. In contrast, in excitatory CA3-region mossy fibre synapses and cerebellar parallel fibre synapses, RIM1alpha is necessary for presynaptic long-term, but not short-term, synaptic plasticity. In these synapses, the function of RIM1alpha in presynaptic long-term plasticity depends, at least in part, on phosphorylation of RIM1alpha at a single site, suggesting that RIM1alpha constitutes a 'phosphoswitch' that determines synaptic strength. However, in spite of the progress in understanding RIM1alpha function, the mechanisms by which RIM1alpha acts remain unknown. For example, how does phosphorylation regulate RIM1alpha, what is the relationship of the function of RIM1alpha in basic release to synaptic plasticity and what is the physiological significance of different forms of RIM-dependent plasticity? Moreover, the roles of other RIM isoforms are unclear. Addressing these important questions will contribute to our view of how neurotransmitter release is regulated at the presynaptic active zone.

Proteomic comparison of two fractions derived from the transsynaptic scaffold

A fraction derived from presynaptic specializations (presynaptic particle fraction; PPF) can be separated from postsynaptic densities (PSD) by adjusting the pH of Triton X-100 (TX-100) extraction of isolated transsynaptic scaffolds. Solubilization of the PPF corresponds to disruption of the presynaptic specialization. We show that the PPF is insoluble to repeated TX-100 extraction at pH 6.0 but becomes soluble in detergent at pH 8.0. By immunolocalization, we find that the major proteins of the PPF, clathrin and dynamin, are concentrated in the presynaptic compartment. By using multidimensional protein identification technology, we compared the protein compositions of the PPF and the PSD fraction. We identified a total of 341 proteins, 50 of which were uniquely found in the PPF, 231 in the PSD fraction, and 60 in both fractions. Comparison of the two fractions revealed a relatively low proportion of actin and associated proteins and a high proportion of vesicle or intracellular compartment proteins in the PPF. We conclude that the PPF consists of presynaptic proteins not connected to the actin-based synaptic framework; its insolubility in pH 6 and solubility in pH 8 buffered detergent suggests that clathrin might be an anchorage scaffold for many proteins in the PPF.

Localization of myosin II and V isoforms in cultured rat sympathetic neurones and their potential involvement in presynaptic function

While vesicle transport is one of the principal functions of myosin motors in neurones, the role played by specific myosin subtypes in discrete vesicle trafficking is poorly understood. We conducted electrophysiological and morphological experiments to determine whether myosin isoforms II and V might be involved in the transport of small synaptic vesicles in presynaptic nerve terminals of a model cholinergic synapse. Electron microscopy revealed the presence of normal synaptic architecture and synaptic vesicle density in presynaptic terminals of cultured superior cervical ganglion neurones (SCGNs) from myosin Va null rats (dilute-opisthotonus, dop). Similarly, electrophysiological analyses of synaptic transmission and synaptic vesicle cycling at paired SCGN synapses failed to uncover any significant differences in synaptic development and function between normal and dop rats. Immunocytochemistry and in situ localization of green fluorescent protein (GFP)-fusion proteins in wild-type synapses revealed that myosins IIB and Va were distributed throughout the cell soma and processes of SCGNs, while myosins IIA and Vb were not detected in SCGNs. Myosin Va was conspicuously absent in presynaptic nerve terminals, but myosin IIB alone was found to be expressed. Furthermore, synaptic transmission was inhibited by introduction of myosin IIB heavy chain fragments into presynaptic terminals of SCGNs. Together these results suggest that only myosin IIB isoform participates in vesicle trafficking in presynaptic nerve terminals of cultured SCGNs.

Signaling at the vertebrate synapse: new roles for embryonic morphogens?

The formation of synapses is critical for functional neuronal connectivity. The coordinated assembly at both sides of the synapse is fundamental for the proper apposition of the neurotransmitter release machinery on the presynaptic neuron and the clustering of neurotransmitter receptors and ion channels on the receptive postsynaptic cell. This process requires bidirectional communication between the presynaptic neuron and its postsynaptic target, another neuron, or muscle fiber. Extracellular signals such as WNT, TGF-beta, and FGF factors are emerging as key target-derived signals required for the initial stages of synaptic assembly. Studies in invertebrates are also providing new insights into the function of these signals in synaptic growth and homeostasis. During early embryonic patterning, WNT, TGF-beta, and FGF factors function as typical morphogens in a concentration-dependent manner to regulate cell fate decisions. This mode of action raises the provocative idea that these same morphogens might also provide a coordinate system for axons to establish the distance to their targets during axon guidance and synapse formation.

Proteomic analysis of synaptosomes using isotope-coded affinity tags and mass spectrometry

Synaptosomes are isolated synapses produced by subcellular fractionation of brain tissue. They contain the complete presynaptic terminal, including mitochondria and synaptic vesicles, and portions of the postsynaptic side, including the postsynaptic membrane and the postsynaptic density (PSyD). A proteomic characterisation of synaptosomes isolated from mouse brain was performed employing the isotope-coded affinity tag (ICAT) method and tandem mass spectrometry (MS/MS). After isotopic labelling and tryptic digestion, peptides were fractionated by cation exchange chromatography and cysteine-containing peptides were isolated by affinity chromatography. The peptides were identified by microcapillary liquid chromatography-electrospray ionisation MS/MS (muLC-ESI MS/MS). In two experiments, peptides representing a total of 1131 database entries were identified. They are involved in different presynaptic and postsynaptic functions, including synaptic vesicle exocytosis for neurotransmitter release, vesicle endocytosis for synaptic vesicle recycling, as well as postsynaptic receptors and proteins constituting the PSyD. Moreover, a large number of soluble and membrane-bound molecules serving functions in synaptic signal transduction and metabolism were detected. The results provide an inventory of the synaptic proteome and confirm the suitability of the ICAT method for the assessment of synaptic structure, function and plasticity.

Differential distribution of release-related proteins in the hippocampal CA3 area as revealed by freeze-fracture replica labeling

Synaptic vesicle release occurs at a specialized membrane domain known as the presynaptic active zone (AZ). Several membrane proteins are involved in the vesicle release processes such as docking, priming, and exocytotic fusion. Cytomatrix at the active zone (CAZ) proteins are structural components of the AZ and are highly concentrated in it. Localization of other release-related proteins including target soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (t-SNARE) proteins, however, has not been well demonstrated in the AZ. Here, we used sodium dodecyl sulfate-digested freeze-fracture replica labeling (SDS-FRL) to analyze quantitatively the distribution of CAZ and t-SNARE proteins in the hippocampal CA3 area. The AZ in replicated membrane was identified by immunolabeling for CAZ proteins (CAZ-associated structural protein [CAST] and Bassoon). Clusters of immunogold particles for these proteins were found on the P-face of presynaptic terminals of the mossy fiber and associational/commissural (A/C) fiber. Co-labeling with CAST revealed distribution of the t-SNARE proteins syntaxin and synaptosomal-associated protein of 25 kDa (SNAP-25) in the AZ as well as in the extrasynaptic membrane surrounding the AZ (SZ). Quantitative analysis demonstrated that the density of immunoparticles for CAST in the AZ was more than 100 times higher than in the SZ, whereas that for syntaxin and SNAP-25 was not significantly different between the AZ and SZ in both the A/C and mossy fiber terminals. These results support the involvement of the t-SNARE proteins in exocytotic fusion in the AZ and the role of CAST in specialization of the membrane domain for the AZ.

Semaphorin 4B interacts with the post-synaptic density protein PSD-95/SAP90 and is recruited to synapses through a C-terminal PDZ-binding motif

The semaphorins are a large family of proteins that act as guidance signals for axons and dendrites. The class 4 semaphorins are integral membrane proteins that are widely expressed throughout the nervous system. Here, we show that a subclass of these semaphorins is characterized by a PDZ-binding motif at their carboxy-terminus. This sequence mediates the interaction with the post-synaptic density protein PSD-95/SAP90. Co-expression of Sema4B with PSD-95 in COS 7 cells results in the clustering of Sema4B. Sema4B co-localizes with PSD-95 at synaptic contacts between cultured hippocampal neurons. This synaptic localization depends on the presence of the PDZ-binding motif.

Analysis of the dorsal spinal cord synaptic architecture by combined proteome analysis and in situ hybridization

The proteomic analysis of tissue samples is an analytical challenge, because identified gene products not only have to be assigned to subcellular structures, but also to cell subpopulations. We here report a strategy of combined subcellular proteomic profiling and in situ hybridization to assign proteins to subcellular sites in subsets of cells within the dorsal region of rat spinal cord. With a focus on synaptic membranes, which represent a complex membrane protein structure composed of multiple integral membrane proteins and networks of accessory structural proteins, we also compared different two-dimensional gel electrophoresis systems for the separation of the proteins. Using MALDI mass spectrometric protein identification based on peptide mass fingerprints, we identified in total 122 different gene products within the different synaptic membrane subfractions. The tissue structure of the dorsal region of the spinal cord is complex, and different layers of neurons can be distinguished neuroanatomically. Proteomic data combined with an in situ hybridization analysis for the detection of mRNA was used to assign selected gene products, namely the optical atrophy protein OPA-1, the presynaptic cytomatrix protein KIAA0378/CAST1, and the uncharacterized coiled-coil-helix-coiled-coil-helix domain containing protein 3 (hypothetical protein FLJ20420), to cell subsets of the dorsal area of the spinal cord. Most striking, KIAA0378/CAST1 mRNA was found only sparsely within the dorsal horn of the spinal cord, but highly abundant within the dorsal root ganglion. This finding, combined with the identification of KIAA0378/CAST1 within the synaptic membrane fraction of the spinal cord at the protein level, are consistent with the reported presynaptic localization of CAST, predominantly within the tissue we investigated primarily attributable to primary afferent sensory neurons. Our approach may be of use in broader studies to characterize the proteomes of neural tissue.

A spinal cord pathway connecting primary afferents to the segmental sympathetic outflow system

The sympathetic innervation of lumbar dorsal root ganglia (DRGs) and the possible presence of spinal cord circuits connecting primary sensory afferents to the sympathetic outflow to DRGs were investigated. We used simultaneous tracing of the sympathetic input to and sensory output from DRGs. Adult male rats received unilateral microinjections of the Bartha strain of pseudorabies virus into four lumbar DRGs. At 24 h post-inoculation, productive infection was detected in both DRG neurons and sympathetic postganglionic neurons. Infection of spinal cord neurons was first observed in sympathetic preganglionic neurons of the intermediolateral column. Subsequently, the infection spread to the contralateral intermediolateral column, the area around the central canal and the superficial dorsal horn layers. To investigate the relationship between infected spinal cord neurons and primary afferents from the corresponding DRGs, we injected pseudorabies virus for retrograde tracing together with cholera toxin B for anterograde tracing. We found that infected LIV/LV and LX neurons were in close apposition to cholera toxin B labeled afferents. Importantly, immunohistochemical detection of bassoon, a pre-synaptic zone protein, identified such contacts as synapses. Together, this suggests synaptic contacts between primary sensory afferents and neurons regulating sympathetic outflow to corresponding DRGs.

C-terminal synaptic targeting elements for postsynaptic density proteins ProSAP1/Shank2 and ProSAP2/Shank3

Synapses are specialized contact sites mediating communication between neurons. Synaptogenesis requires the specific assembly of protein clusters at both sides of the synaptic contact by mechanisms that are barely understood. We studied the synaptic targeting of multi-domain proteins of the ProSAP/Shank family thought to serve as master scaffolding molecules of the postsynaptic density. In contrast to Shank1, expression of green-fluorescent protein (GFP)-tagged ProSAP1/Shank2 and ProSAP2/Shank3 deletion constructs in hippocampal neurons revealed that their postsynaptic localization relies on the integrity of the C-termini. The shortest construct that was perfectly targeted to synaptic sites included the last 417 amino acids of ProSAP1/Shank2 and included the C-terminal sterile alpha motif (SAM) domain. Removal of 54 residues from the N-terminus of this construct resulted in a diffuse distribution in the cytoplasm. Altogether, our data delineate a hitherto unknown targeting signal in both ProSAP1/Shank2 and ProSAP2/Shank3 and provide evidence for an implication of these proteins and their close homologue, Shank1, in distinct molecular pathways.

ELKS, a protein structurally related to the active zone-associated protein CAST, is expressed in pancreatic beta cells and functions in insulin exocytosis: interaction of ELKS with exocytotic machinery analyzed by total internal reflection fluorescence microscopy

The cytomatrix at the active zone (CAZ) has been implicated in defining the site of Ca2+-dependent exocytosis of neurotransmitters. Here, we demonstrate the expression and function of ELKS, a protein structurally related to the CAZ protein CAST, in insulin exocytosis. The results of confocal and immunoelectron microscopic analysis showed that ELKS is present in pancreatic beta cells and is localized close to insulin granules docked on the plasma membrane-facing blood vessels. Total internal reflection fluorescence microscopy imaging in insulin-producing clonal cells revealed that the ELKS clusters are less dense and unevenly distributed than syntaxin 1 clusters, which are enriched in the plasma membrane. Most of the ELKS clusters were on the docking sites of insulin granules that were colocalized with syntaxin 1 clusters. Total internal reflection fluorescence images of single-granule motion showed that the fusion events of insulin granules mostly occurred on the ELKS cluster, where repeated fusion was sometimes observed. When the Bassoon-binding region of ELKS was introduced into the cells, the docking and fusion of insulin granules were markedly reduced. Moreover, attenuation of ELKS expression by small interfering RNA reduced the glucose-evoked insulin release. These data suggest that the CAZ-related protein ELKS functions in insulin exocytosis from pancreatic beta cells.

Hair cell synaptic ribbons are essential for synchronous auditory signalling

Hearing relies on faithful synaptic transmission at the ribbon synapse of cochlear inner hair cells (IHCs). At present, the function of presynaptic ribbons at these synapses is still largely unknown. Here we show that anchoring of IHC ribbons is impaired in mouse mutants for the presynaptic scaffolding protein Bassoon. The lack of active-zone-anchored synaptic ribbons reduced the presynaptic readily releasable vesicle pool, and impaired synchronous auditory signalling as revealed by recordings of exocytic IHC capacitance changes and sound-evoked activation of spiral ganglion neurons. Both exocytosis of the hair cell releasable vesicle pool and the number of synchronously activated spiral ganglion neurons co-varied with the number of anchored ribbons during development. Interestingly, ribbon-deficient IHCs were still capable of sustained exocytosis with normal Ca2+-dependence. Endocytic membrane retrieval was intact, but an accumulation of tubular and cisternal membrane profiles was observed in ribbon-deficient IHCs. We conclude that ribbon-dependent synchronous release of multiple vesicles at the hair cell afferent synapse is essential for normal hearing.

Dopaminergic stimulation of local protein synthesis enhances surface expression of GluR1 and synaptic transmission in hippocampal neurons

The use-dependent modification of synapses is strongly influenced by dopamine, a transmitter that participates in both the physiology and pathophysiology of animal behavior. In the hippocampus, dopaminergic signaling is thought to play a key role in protein synthesis-dependent forms of synaptic plasticity. The molecular mechanisms by which dopamine influences synaptic function, however, are not well understood. Using a GFP-based reporter, as well as a small-molecule reporter of endogenous protein synthesis, we show that dopamine D1/D5 receptor activation stimulates local protein synthesis in the dendrites of hippocampal neurons. We also identify the GluR1 subunit of AMPA receptors as one protein upregulated by dopamine receptor activation, with increased incorporation of surface GluR1 at synaptic sites. The insertion of new GluRs is accompanied by an increase in the frequency of miniature synaptic events. Together, these data suggest a local protein synthesis-dependent activation of previously silent synapses as a result of dopamine receptor stimulation.

Generation of Silent Synapses by Acute In Vivo Expression of CaMKIV and CREB

The transcription factor CREB is critical for several forms of experience-dependent plasticity in a range of species and is commonly activated in neurons by calcium/calmodulin-dependent protein kinase IV (CaMKIV). Surprisingly, little is known about the neural circuit adaptations caused by activation of CaMKIV and CREB. Here, we use viral-mediated gene transfer in vivo to examine the consequences of acute expression of constitutively active forms of CaMKIV and CREB on synaptic function in the rodent hippocampus. Acute expression of active CaMKIV or CREB caused an enhancement of both NMDA receptor-mediated synaptic responses and long-term potentiation (LTP). This was accompanied by electrophysiological and morphological changes consistent with the generation of "silent synapses," which provide an ideal substrate for further experience-dependent modifications of neural circuitry and which may also be important for the consolidation of long-term synaptic plasticity and memories.

Molecular dissection of the photoreceptor ribbon synapse: physical interaction of Bassoon and RIBEYE is essential for the assembly of the ribbon complex

The ribbon complex of retinal photoreceptor synapses represents a specialization of the cytomatrix at the active zone (CAZ) present at conventional synapses. In mice deficient for the CAZ protein Bassoon, ribbons are not anchored to the presynaptic membrane but float freely in the cytoplasm. Exploiting this phenotype, we dissected the molecular structure of the photoreceptor ribbon complex. Identifiable CAZ proteins segregate into two compartments at the ribbon: a ribbon-associated compartment including Piccolo, RIBEYE, CtBP1/BARS, RIM1, and the motor protein KIF3A, and an active zone compartment including RIM2, Munc13-1, a Ca²⁺ channel α1 subunit, and ERC2/CAST1. A direct interaction between the ribbon-specific protein RIBEYE and Bassoon seems to link the two compartments and is responsible for the physical integrity of the photoreceptor ribbon complex. Finally, we found the RIBEYE homologue CtBP1 at ribbon and conventional synapses, suggesting a novel role for the CtBP/BARS family in the molecular assembly and function of central nervous system synapses.

Actin-binding proteins in a postsynaptic preparation: Lasp-1 is a component of central nervous system synapses and dendritic spines

CNS synapses are complex sites of cell-cell communication. Identification and characterization of the protein components of synapses will lead to a better understanding of the mechanisms of neurotransmission and plasticity. We applied multidimensional protein identification technology (MudPIT) to purified, guanidine-solubilized postsynaptic fractions to identify novel synaptically localized molecules. We identified several actin-associated proteins known to regulate actin polymerization and control cell motility in nonneural cells that have not previously been associated with CNS synaptic function. One of these is lasp-1, an actin-associated LIM and SH3 domain-containing protein. We show that lasp-1 is strongly expressed by CNS neurons and is concentrated at synaptic sites. Overall, the preponderance of actin-associated proteins in postsynaptic density fractions, and specifically those involved in actin reorganization, suggests that there are many modes by which the state of synaptic F-actin polymerization and, hence, synaptic physiology are affected.

Integration of ATP, cAMP, and Ca2+ signals in insulin granule exocytosis

Intracellular ATP, cAMP, and Ca2+ are major signals involved in the regulation of insulin secretion in the pancreatic beta-cell. We recently found that the ATP-sensitive K+ channel (KATP channel) as an ATP sensor, cAMP-GEFII as a cAMP sensor, Piccolo as a Ca2+ sensor, and L-type voltage-dependent Ca2+ channel (VDCC) can interact with each other. In the present study, we examined the effects of cAMP and ATP on the interaction of cAMP-GEFII and sulfonylurea receptor-1 (SUR1). Interaction of cAMP-GEFII with SUR1 was inhibited by the cAMP analog 8-bromo-cAMP but not by ATP, and the inhibition by 8-bromo-cAMP persisted in the presence of ATP. In addition, SUR1, cAMP-GEFII, and Piccolo could form a complex. Piccolo also interacted with the alpha1 1.2 subunit of VDCC in a Ca2+-independent manner. These data suggest that the interactions of the KATP channel, cAMP-GEFII, Piccolo, and L-type VDCC are regulated by intracellular signals such as cAMP and Ca2+ and that the ATP, cAMP, and Ca2+ signals are integrated at a specialized region of pancreatic beta-cells.

Proteomic analysis of native metabotropic glutamate receptor 5 protein complexes reveals novel molecular constituents

We used a proteomic approach to identify novel proteins that may regulate metabotropic glutamate receptor 5 (mGluR5) responses by direct or indirect protein interactions. This approach does not rely on the heterologous expression of proteins and offers the advantage of identifying protein interactions in a native environment. The mGluR5 protein was immunoprecipitated from rat brain lysates; co-immunoprecipitating proteins were analyzed by mass spectrometry and identified peptides were matched to protein databases to determine the correlating parent proteins. This proteomic approach revealed the interaction of mGluR5 with known regulatory proteins, as well as novel proteins that reflect previously unidentified molecular constituents of the mGluR5-signaling complex. Immunoblot analysis confirmed the interaction of high confidence proteins, such as phosphofurin acidic cluster sorting protein 1, microtubule-associated protein 2a and dynamin 1, as mGluR5-interacting proteins. These studies show that a proteomic approach can be used to identify candidate interacting proteins. This approach may be particularly useful for neurobiology applications where distinct protein interactions within a signaling complex can dramatically alter the outcome of the response to neurotransmitter release, or the disruption of normal protein interactions can lead to severe neurological and psychiatric disorders.

The synaptic vesicle cycle

Neurotransmitter release is mediated by exocytosis of synaptic vesicles at the presynaptic active zone of nerve terminals. To support rapid and repeated rounds of release, synaptic vesicles undergo a trafficking cycle. The focal point of the vesicle cycle is Ca2+-triggered exocytosis that is followed by different routes of endocytosis and recycling. Recycling then leads to the docking and priming of the vesicles for another round of exo- and endocytosis. Recent studies have led to a better definition than previously available of how Ca2+ triggers exocytosis and how vesicles recycle. In particular, insight into how Munc18-1 collaborates with SNARE proteins in fusion, how the vesicular Ca2+ sensor synaptotagmin 1 triggers fast release, and how the vesicular Rab3 protein regulates release by binding to the active zone proteins RIM1 alpha and RIM2 alpha has advanced our understanding of neurotransmitter release. The present review attempts to relate these molecular data with physiological results in an emerging view of nerve terminals as macromolecular machines.

The Architecture of the Active Zone in the Presynaptic Nerve Terminal

Active zones are highly specialized sites for release of neurotransmitter from presynaptic nerve terminals. The architecture of the active zone is exquisitely designed to facilitate the regulated tethering, docking, and fusing of the synaptic vesicles with the plasma membrane. Here we present our view of the structural and molecular organization of active zones across species and propose that all active zones are organized according to a common principle in which the structural differences correlate with the kinetics of transmitter release.

Physical and functional interaction of the active zone proteins, CAST, RIM1, and Bassoon, in neurotransmitter release

We have recently isolated a novel cytomatrix at the active zone (CAZ)–associated protein, CAST, and found it directly binds another CAZ protein RIM1 and indirectly binds Munc13-1 through RIM1; RIM1 and Munc13-1 directly bind to each other and are implicated in priming of synaptic vesicles. Here, we show that all the CAZ proteins thus far known form a large molecular complex in the brain, including CAST, RIM1, Munc13-1, Bassoon, and Piccolo. RIM1 and Bassoon directly bind to the COOH terminus and central region of CAST, respectively, forming a ternary complex. Piccolo, which is structurally related to Bassoon, also binds to the Bassoon-binding region of CAST. Moreover, the microinjected RIM1- or Bassoon-binding region of CAST impairs synaptic transmission in cultured superior cervical ganglion neurons. Furthermore, the CAST-binding domain of RIM1 or Bassoon also impairs synaptic transmission in the cultured neurons. These results indicate that CAST serves as a key component of the CAZ structure and is involved in neurotransmitter release by binding these CAZ proteins.

Postsynaptic density assembly is fundamentally different from presynaptic active zone assembly

The cellular mechanisms involved in the formation of the glutamatergic postsynaptic density (PSD) are mainly unknown. Previous studies have indicated that PSD assembly may occur in situ by a gradual recruitment of postsynaptic molecules, whereas others have suggested that the PSD may be assembled from modular transport packets assembled elsewhere. Here we used cultured hippocampal neurons and live cell imaging to examine the process by which PSD molecules from different layers of the PSD are recruited to nascent postsynaptic sites. GFP-tagged NR1, the essential subunit of the NMDA receptor, and ProSAP1/Shank2 and ProSAP2/Shank3, scaffolding molecules thought to reside at deeper layers of the PSD, were recruited to new synaptic sites in gradual manner, with no obvious involvement of discernible discrete transport particles. The recruitment kinetics of these three PSD molecules were remarkably similar, which may indicate that PSD assembly rate is governed by a common upstream rate-limiting process. In contrast, the presynaptic active zone (AZ) molecule Bassoon was observed to be recruited to new presynaptic sites by means of a small number of mobile packets, in full agreement with previous studies. These findings indicate that the assembly processes of PSDs and AZs may be fundamentally different.

Molecular constituents of the postsynaptic density fraction revealed by proteomic analysis using multidimensional liquid chromatography-tandem mass spectrometry

Protein constituents of the postsynaptic density (PSD) fraction were analysed using an integrated liquid chromatography (LC)-based protein identification system, which was constructed by coupling microscale two-dimensional liquid chromatography (2DLC) with electrospray ionization (ESI) tandem mass spectrometry (MS/MS) and an automated data analysis system. The PSD fraction prepared from rat forebrain was solubilized in 6 m guanidium hydrochloride, and the proteins were digested with trypsin after S-carbamoylmethylation under reducing conditions. The tryptic peptide mixture was then analysed with the 2DLC-MS/MS system in a data-dependent mode, and the resultant spectral data were automatically processed to search a genome sequence database for protein identification. In triplicate analyses, the system allowed assignments of 5264 peptides, which could finally be attributed to 492 proteins. The PSD contained various proteins involved in signalling transduction, including receptors, ion channel proteins, protein kinases and phosphatases, G-protein and related proteins, scaffold proteins, and adaptor proteins. Structural proteins, including membrane proteins involved in cell adhesion and cell-cell interaction, proteins involved in endocytosis, motor proteins, and cytoskeletal proteins were also abundant. These results provide basic data on a major protein set associated with the PSD and a basis for future functional studies of this important neural machinery.

CAST2: identification and characterization of a protein structurally related to the presynaptic cytomatrix protein CAST

The cytomatrix at the active zone (CAZ) is thought to define the site of Ca2+-dependent exocytosis of neurotransmitters. We have recently identified a novel CAZ protein from rat brain which we have named CAST (CAZ-associated structural protein). CAST forms a large molecular complex with other CAZ proteins such as Bassoon, RIM1 and Munc13-1, at least through direct binding to RIM1. Here, we have identified a rat protein that is structurally related to CAST and named it CAST2. Subcellular fractionation analysis of rat brain shows that CAST2 is also tightly associated with the postsynaptic density fraction. Like CAST, CAST2 directly binds RIM1 and forms a hetero-oligomer with CAST. In primary cultured rat hippocampal neurones, CAST2 co-localizes with Bassoon at synapses. Furthermore, immunoelectron microscopy reveals that CAST2 localizes to the vicinity of the presynaptic membrane of synapses in mouse brain. Sequence analysis reveals that CAST2 is a rat orthologue of the human protein ELKS. ELKS has also recently been identified as Rab6IP2 and ERC1. Accordingly, the original CAST is tentatively re-named CAST1. These results indicate that CAST2 is a new component of the CAZ and, together with CAST1, may be involved in the formation of the CAZ structure.

Regressive and reactive changes in the connectivity patterns of rod and cone pathways of P23H transgenic rat retina

We have used the P23H line 1 homozygous albino rat to study how progressive photoreceptor degeneration affects rod and cone relay pathways. We examined P23H retinas at different stages of degeneration by confocal microscopy of immunostained sections and electroretinogram (ERG) recordings. By 21 days of age in the P23H rat retina, there is already substantial loss of rods and reduction in rod bipolar dendrites along with reduction of metabotropic glutamate receptor 6 (mGluR6) and rod-associated bassoon staining. The cone pathway is relatively unaffected. By 150 days, when rods are absent from much of the retina, some rod bipolars remain and dendrites of rod and cone bipolar cells form synaptic complexes associated with cones and horizontal cell processes. These complexes include foci of mGluR6 and bassoon staining; they develop further by 270 days of age. Over the course of degeneration, beginning at 21 days, bipolar axon terminals atrophy and the inner retina undergoes further changes including a reduced and disorganized AII amacrine cell population and thinning of the inner plexiform layer. Electroretinogram (ERG) results at 23 days show reductions in a-wave amplitude, in rod and cone-associated b-waves (using a double flash paradigm) and in the amplitude of oscillatory potentials (OPs). By 38 days, rod scotopic a-wave responses and OPs are lost. B-wave amplitudes decline until 150 days, at which point they are purely cone-driven and remain stable up to 250 days. The results show that during the course of photoreceptor loss in the P23H rat, there are progressive degenerative changes, particularly in the rod relay pathway, and these are reflected in the changing ERG response patterns. Later reactive changes involving condensation of cone terminals and neurotransmitter receptors associated with rod and cone bipolar dendrites and with horizontal cell processes suggest that at this stage, there are likely to be complex changes in the relay of sensory information through the retina.

Kalirin, a multifunctional Rho guanine nucleotide exchange factor, is necessary for maintenance of hippocampal pyramidal neuron dendrites and dendritic spines

The structures of dendritic spines and the dendritic tree, key determinants of neuronal function, are regulated by diverse inputs that affect many scaffolding and signaling molecules. Nevertheless, here we show that reduced expression of a single gene results in loss of dendritic spines and a decrease in dendritic complexity. Kalirin, a dual Rho GDP-GTP exchange factor, causes spine formation when overexpressed. Reduced expression of Kalirin in CA1 hippocampal neurons resulted in a reduction in linear spine density, with dispersion of postsynaptic density markers and elimination of presynaptic endings. Simplification of the apical dendritic tree preceded simplification of basal dendrites. Pyramidal cell axons were not dramatically altered. Although many factors determine dendrite shape and spine formation, expression of Kalirin is necessary for the normal function of these many regulatory elements.

Interaction of ATP sensor, cAMP sensor, Ca2+ sensor, and voltage-dependent Ca2+ channel in insulin granule exocytosis

ATP, cAMP, and Ca(2+) are the major signals in the regulation of insulin granule exocytosis in pancreatic beta cells. The sensors and regulators of these signals have been characterized individually. The ATP-sensitive K(+) channel, acting as the ATP sensor, couples cell metabolism to membrane potential. cAMP-GEFII, acting as a cAMP sensor, mediates cAMP-dependent, protein kinase A-independent exocytosis, which requires interaction with both Piccolo as a Ca(2+) sensor and Rim2 as a Rab3 effector. l-type voltage-dependent Ca(2+) channels (VDCCs) regulate Ca(2+) influx. In the present study, we demonstrate interactions of these molecules. Sulfonylurea receptor 1, a subunit of ATP-sensitive K(+) channels, interacts specifically with cAMP-GEFII through nucleotide-binding fold 1, and the interaction is decreased by a high concentration of cAMP. Localization of cAMP-GEFII overlaps with that of Rim2 in plasma membrane of insulin-secreting MIN6 cells. Localization of Rab3 co-incides with that of Rim2. Rim2 mutant lacking the Rab3 binding region, when overexpressed in MIN6 cells, is localized exclusively in cytoplasm, and impairs cAMP-dependent exocytosis in MIN6 cells. In addition, Rim2 and Piccolo bind directly to the alpha(1)1.2-subunit of VDCC. These results indicate that ATP sensor, cAMP sensor, Ca(2+) sensor, and VDCC interact with each other, which further suggests that ATP, cAMP, and Ca(2+) signals in insulin granule exocytosis are integrated in a specialized domain of pancreatic beta cells to facilitate stimulus-secretion coupling.

Kalirin, a multifunctional Rho guanine nucleotide exchange factor, is necessary for maintenance of hippocampal pyramidal neuron dendrites and dendritic spines

The structures of dendritic spines and the dendritic tree, key determinants of neuronal function, are regulated by diverse inputs that affect many scaffolding and signaling molecules. Nevertheless, here we show that reduced expression of a single gene results in loss of dendritic spines and a decrease in dendritic complexity. Kalirin, a dual Rho GDP-GTP exchange factor, causes spine formation when overexpressed. Reduced expression of Kalirin in CA1 hippocampal neurons resulted in a reduction in linear spine density, with dispersion of postsynaptic density markers and elimination of presynaptic endings. Simplification of the apical dendritic tree preceded simplification of basal dendrites. Pyramidal cell axons were not dramatically altered. Although many factors determine dendrite shape and spine formation, expression of Kalirin is necessary for the normal function of these many regulatory elements.

Rim, a component of the presynaptic active zone and modulator of exocytosis, binds 14-3-3 through its N terminus

Rim1, a brain-specific Rab3a-binding protein, localizes to the presynaptic cytomatrix and plays an important role in synaptic transmission and synaptic plasticity. Rim2, a homologous protein, is more ubiquitously expressed and is found in neuroendocrine cells as well as in brain. Both Rim1 and Rim2 contain multiple domains, including an N-terminal zinc finger, which in Rim1 strongly enhances secretion in chromaffin and PC12 cells. The yeast two-hybrid technique identified 14-3-3 proteins as ligands of the N-terminal domain. In vitro protein binding experiments confirmed a high-affinity interaction between the N terminus of Rim1 and 14-3-3. The N-terminal domain of Rim2 also bound 14-3-3. The binding domains were localized to a short segment just C-terminal to the zinc finger. 14-3-3 proteins bind to specific phosphoserine residues. Alkaline phosphatase treatment of N-terminal domains of Rim1 and Rim2 almost completely inhibited the binding of 14-3-3. Two serine residues in Rim1 (Ser-241 and Ser-287) and one serine residue in Rim2 (Ser-335) were required for 14-3-3 binding. Incubation with Ca2+/calmodulin-dependent protein kinase II greatly stimulated the interaction of recombinant N-terminal Rim but not the S241/287A mutant with 14-3-3, again indicating the importance of the phosphorylation of these residues for the binding. Rabphilin3, another Rab3a effector, also bound 14-3-3. Serine-to-alanine mutations identified Ser-274 as the likely phosphorylated residue to which 14-3-3 binds. Because the phosphorylation of this residue had been shown to be stimulated upon depolarization in brain slices, the interaction of 14-3-3 with Rabphilin3 may be important in the dynamic function of central nervous system neurons.

Synapse-associated protein-97 isoform-specific regulation of surface AMPA receptors and synaptic function in cultured neurons

Members of the synapse-associated protein-97 (SAP97) family of scaffold proteins have been implicated as central organizers of synaptic junctions to build macromolecular signaling complexes around specific postsynaptic neurotransmitter receptors. In this regard, SAP97 has been suggested to regulate the synaptic localization of glutamate receptor type 1 subunits of the AMPA-type glutamate receptors. To test this hypothesis directly, we assessed the effects of SAP97 overexpression on surface expression of synaptic AMPA receptors. We find that recombinant SAP97 not only becomes concentrated at synaptic junctions but also leads to an increase in synaptic AMPA receptors, spine enlargement, and an increase in miniature EPSC (mEPSC) frequency, indicating that SAP97 has both postsynaptic and presynaptic effects on synaptic transmission. Synaptic targeting of SAP97, increased surface AMPA receptors, and increased mEPSC frequency are dependent on the presence of specific alternatively spliced sequences in SAP97 that encode a protein 4.1 binding site. These results suggest that SAP97 can affect the synaptic recruitment of AMPA receptors and spine morphology and that these effects may be regulated by alternative splicing.

Nectin-dependent localization of ZO-1 at puncta adhaerentia junctions between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of adult mouse hippocampus

Nectin and afadin constitute a novel intercellular adhesion system that organizes adherens junctions in cooperation with the cadherin-catenin system in epithelial cells. Nectin is a Ca(2+)-independent immunoglobulin-like adhesion molecule and afadin is an actin filament (F-actin)-binding protein that connects nectin to the actin cytoskeleton. At the puncta adhaerentia junctions (PAs) between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of the adult mouse hippocampus, the nectin-afadin system also colocalizes with the cadherin-catenin system and has a role in the formation of synapses. ZO-1 is another F-actin-binding protein that localizes at tight junctions (TJs) and connects claudin to the actin cytoskeleton in epithelial cells. The nectin-afadin system is able to recruit ZO-1 to the nectin-based cell-cell adhesion sites in nonepithelial cells that have no TJs. In the present study, we investigated the localization of ZO-1 in the mouse hippocampus. Immunofluorescence and immunoelectron microscopy revealed that ZO-1 also localized at the PAs between the mossy fiber terminals and the dendrites of the pyramidal cells in the CA3 area of the adult mouse hippocampus, as described for afadin. ZO-1 colocalized with afadin during the development of synaptic junctions and PAs. Microbeads coated with the extracellular fragment of nectin, which interacts with cellular nectin, recruited both afadin and ZO-1 to the bead-cell contact sites in cultured rat hippocampal neurons. These results indicate that ZO-1 colocalizes with nectin and afadin at the PAs and that the nectin-afadin system is involved in the localization of ZO-1.

Functional regions of the presynaptic cytomatrix protein bassoon: significance for synaptic targeting and cytomatrix anchoring

Exocytosis of neurotransmitter from synaptic vesicles is restricted to specialized sites of the presynaptic plasma membrane called active zones. A complex cytomatrix of proteins exclusively assembled at active zones, the CAZ, is thought to form a molecular scaffold that organizes neurotransmitter release sites. Here, we have analyzed synaptic targeting and cytomatrix association of Bassoon, a major scaffolding protein of the CAZ. By combining immunocytochemistry and transfection of cultured hippocampal neurons, we show that the central portion of Bassoon is crucially involved in synaptic targeting and CAZ association. An N-terminal region harbors a distinct capacity for N-myristoylation-dependent targeting to synaptic vesicle clusters, but is not incorporated into the CAZ. Our data provide the first experimental evidence for the existence of distinct functional regions in Bassoon and suggest that a centrally located CAZ targeting function may be complemented by an N-terminal capacity for targeting to membrane-bounded synaptic organelles.

Synapse-associated protein-97 isoform-specific regulation of surface AMPA receptors and synaptic function in cultured neurons

Members of the synapse-associated protein-97 (SAP97) family of scaffold proteins have been implicated as central organizers of synaptic junctions to build macromolecular signaling complexes around specific postsynaptic neurotransmitter receptors. In this regard, SAP97 has been suggested to regulate the synaptic localization of glutamate receptor type 1 subunits of the AMPA-type glutamate receptors. To test this hypothesis directly, we assessed the effects of SAP97 overexpression on surface expression of synaptic AMPA receptors. We find that recombinant SAP97 not only becomes concentrated at synaptic junctions but also leads to an increase in synaptic AMPA receptors, spine enlargement, and an increase in miniature EPSC (mEPSC) frequency, indicating that SAP97 has both postsynaptic and presynaptic effects on synaptic transmission. Synaptic targeting of SAP97, increased surface AMPA receptors, and increased mEPSC frequency are dependent on the presence of specific alternatively spliced sequences in SAP97 that encode a protein 4.1 binding site. These results suggest that SAP97 can affect the synaptic recruitment of AMPA receptors and spine morphology and that these effects may be regulated by alternative splicing.

Interactions between Piccolo and the actin/dynamin-binding protein Abp1 link vesicle endocytosis to presynaptic active zones

Piccolo is a high molecular weight multi-domain protein shown to be a structural component of the presynaptic CAZ (cytoskeletal matrix assembled at active zones). These features indicate that Piccolo may act to scaffold proteins involved in synaptic vesicle endo- and exocytosis near their site of action. To test this hypothesis, we have utilized a functional cell-based endocytosis assay and identified the N-terminal proline-rich Q domain in Piccolo as a region that interferes with clathrin-mediated endocytosis. Utilizing the Piccolo Q domain as bait in a yeast two-hybrid screen, we have identified the F-actin-binding protein Abp1 (also called SH3P7 or HIP-55) as a potential binding partner for this domain. The physiological relevance of this interaction is supported by in vitro binding studies, colocalization in nerve terminals, in vivo recruitment studies, and immunoprecipitation experiments. Intriguingly, Abp1 binds to both F-actin and the GTPase dynamin and has been implicated in linking the actin cytoskeleton to clathrin-mediated endocytosis. Our results suggest that Piccolo, as a structural protein of the CAZ, may serve to localize Abp1 at active zones where it can actively participate in creating a functional connection between the dynamic actin cytoskeleton and synaptic vesicle recycling.

Unitary assembly of presynaptic active zones from Piccolo-Bassoon transport vesicles

Recent studies indicate that active zones (AZs)-sites of neurotransmitter release-may be assembled from preassembled AZ precursor vesicles inserted into the presynaptic plasma membrane. Here we report that one putative AZ precursor vesicle of CNS synapses-the Piccolo-Bassoon transport vesicle (PTV)-carries a comprehensive set of AZ proteins genetically and functionally coupled to synaptic vesicle exocytosis. Time-lapse imaging reveals that PTVs are highly mobile, consistent with a role in intracellular transport. Quantitative analysis reveals that the Bassoon, Piccolo, and RIM content of individual PTVs is, on average, half of that of individual presynaptic boutons and shows that the synaptic content of these molecules can be quantitatively accounted for by incorporation of integer numbers (typically two to three) of PTVs into presynaptic membranes. These findings suggest that AZs are assembled from unitary amounts of AZ material carried on PTVs.

Functional inactivation of a fraction of excitatory synapses in mice deficient for the active zone protein bassoon

Mutant mice lacking the central region of the presynaptic active zone protein Bassoon were generated to establish the role of this protein in the assembly and function of active zones as sites of synaptic vesicle docking and fusion. Our data show that the loss of Bassoon causes a reduction in normal synaptic transmission, which can be attributed to the inactivation of a significant fraction of glutamatergic synapses. At these synapses, vesicles are clustered and docked in normal numbers but are unable to fuse. Phenotypically, the loss of Bassoon causes spontaneous epileptic seizures. These data show that Bassoon is not essential for synapse formation but plays an essential role in the regulated neurotransmitter release from a subset of glutamatergic synapses.

The presynaptic active zone protein bassoon is essential for photoreceptor ribbon synapse formation in the retina

The photoreceptor ribbon synapse is a highly specialized glutamatergic synapse designed for the continuous flow of synaptic vesicles to the neurotransmitter release site. The molecular mechanisms underlying ribbon synapse formation are poorly understood. We have investigated the role of the presynaptic cytomatrix protein Bassoon, a major component of the photoreceptor ribbon, in a mouse retina deficient of functional Bassoon protein. Photoreceptor ribbons lacking Bassoon are not anchored to the presynaptic active zones. This results in an impaired photoreceptor synaptic transmission, an abnormal dendritic branching of neurons postsynaptic to photoreceptors, and the formation of ectopic synapses. These findings suggest a critical role of Bassoon in the formation and the function of photoreceptor ribbon synapses of the mammalian retina.

Abl family nonreceptor tyrosine kinases modulate short-term synaptic plasticity

Abl family nonreceptor tyrosine kinases regulate cell morphogenesis through functional interactions with the actin cytoskeleton. The vertebrate Abl family kinases, Abl and Arg, are expressed in the adult mouse brain, where they may regulate actin cytoskeletal dynamics in mature neurons. Using immunoelectron microscopy, we have localized Abl and Arg to the pre- and postsynaptic compartments of synapses in the mouse hippocampal area CA1. Paired-pulse facilitation (PPF) was significantly reduced at the Schaffer collateral-CA1 (SC-CA1) excitatory synapses in hippocampal slices from abl-/- and arg-/- mice as compared with wild-type mice. Furthermore, treatment of wild-type slices with the specific Abl family kinase inhibitor STI571 also reduced PPF. Basal synaptic transmission, posttetanic potentiation (PTP), long-term potentiation (LTP), and long-term depression (LTD) were similar to wild-type controls in abl-/- and arg-/- slices and in STI571-treated wild-type slices. These results indicate that an important function of Abl and Arg is to modulate synaptic efficacy via a presynaptic mechanism during repetitive activation.

CELLBIOLOGY OF THEPRESYNAPTICTERMINAL

The chemical synapse is a specialized intercellular junction that operates nearly autonomously to allow rapid, specific, and local communication between neurons. Focusing our attention on the presynaptic terminal, we review the current understanding of how synaptic morphology is maintained and then the mechanisms in synaptic vesicle exocytosis and recycling.

The GIT family of proteins forms multimers and associates with the presynaptic cytomatrix protein Piccolo

The cytoskeletal matrix assembled at active zones (CAZ) is implicated in defining neurotransmitter release sites. However, little is known about the molecular mechanisms by which the CAZ is organized. Here we report a novel interaction between Piccolo, a core component of the CAZ, and GIT proteins, multidomain signaling integrators with GTPase-activating protein activity for ADP-ribosylation factor small GTPases. A small region (approximately 150 amino acid residues) in Piccolo, which is not conserved in the closely related CAZ protein Bassoon, mediates a direct interaction with the Spa2 homology domain (SHD) domain of GIT1. Piccolo and GIT1 colocalize at synaptic sites in cultured neurons. In brain, Piccolo forms a complex with GIT1 and various GIT-associated proteins, including betaPIX, focal adhesion kinase, liprin-alpha, and paxillin. Point mutations in the SHD of GIT1 differentially interfere with the association of GIT1 with Piccolo, betaPIX, and focal adhesion kinase, suggesting that these proteins bind to the SHD by different mechanisms. Intriguingly, GIT proteins form homo- and heteromultimers through their C-terminal G-protein-coupled receptor kinase-binding domain in a tail-to-tail fashion. This multimerization enables GIT1 to simultaneously interact with multiple SHD-binding proteins including Piccolo and betaPIX. These results suggest that, through their multimerization and interaction with Piccolo, the GIT family proteins are involved in the organization of the CAZ.

Temporal and spatial coordination of exocytosis and endocytosis

In secretory cells, exocytosis and compensatory endocytosis are tightly coupled membrane trafficking processes that control the surface area and composition of the plasma membrane. While exocytic and endocytic processes have been studied independently in great detail, at present there is much interest in understanding the mode of their coupling. This review discusses emerging insights into the coupling of these processes, both in the chemical synapses of neurons and in non-neuronal cells.

Subcellular proteomics

The step from the analysis of the genome to the analysis of the proteome is not just a matter of numerical complexity in terms of variants of gene products that can arise from a single gene. A significant further level of complexity is introduced by the supramolecular organization of gene products because of protein-protein interactions or targeting of proteins to specific subcellular structures. There is currently no single proteome analysis strategy that can sufficiently address all levels of the organization of the proteome. To approach an appropriate analytical complement for the interrogation of the proteome at all of the levels at which it is organized, there emerges the need for a whole arsenal of proteomics strategies. The proteome analysis at the level of subcellular structures (that can be enriched by subcellular fractionation) represents an analytical strategy that combines classic biochemical fractionation methods and tools for the comprehensive identification of proteins. Among the key potentials of this strategy is the capability to screen not only for previously unknown gene products but also to assign them, along with other known, but poorly characterized gene products, to particular subcellular structures. Furthermore, the analysis at the subcellular level is a prerequisite for the detection of important regulatory events such as protein translocation in comparative studies. This review is meant to give an overview on recent key studies in the field of proteome analysis at the level of subcellular structures, and to highlight potentials and requirements.

Piccolo, a Ca2+ sensor in pancreatic beta-cells. Involvement of cAMP-GEFII.Rim2. Piccolo complex in cAMP-dependent exocytosis

We have previously shown that cAMP-binding protein cAMP-guanidine nucleotide exchange factor II (GEFII) (or Epac2) interacting with Rim2 is involved in cAMP-dependent, protein kinase A-independent exocytosis in pancreatic beta-cells. The action of the cAMP-GEFII.Rim2 complex requires both intracellular cAMP and Ca(2+). Although Rim2 has C(2) domains, its role as a Ca(2+) sensor has remained unclear. In the present investigation, we have discovered that Piccolo, a CAZ (cytoskeletal matrix associated with the active zone) protein in neurons that is structurally related to Rim2, also binds to cAMP-GEFII and that it forms both homodimer and heterodimer with Rim2 in a Ca(2+)-dependent manner, whereas Rim2 alone does not form the homodimer. The association of Piccolo.Rim2 heterodimerization is stronger than Piccolo. Piccolo homodimerization. Treatment of pancreatic islets with antisense oligodeoxynucleotides against Piccolo inhibits insulin secretion induced by cAMP analog 8-bromo-cyclic AMP plus high glucose stimulation. These results suggest that Piccolo serves as a Ca(2+) sensor in exocytosis in pancreatic beta-cells and that the formation of a cAMP-GEFII.Rim2. Piccolo complex is important in cAMP-induced insulin secretion. In addition, this study suggests that CAZ proteins similar to those in neurons are also function in pancreatic beta-cells.

Regulation by glycogen synthase kinase-3beta of the arborization field and maturation of retinotectal projection in zebrafish

The retinotectal projection is one of the best systems to study the molecular basis of synapse formation in the CNS because of the well characterized topographic connections and activity-dependent refinement. Here, we developed a presynaptic neuron-specific gene manipulation system in the zebrafish retinotectal projection in vivo using the nicotinic acetylcholine receptor beta3 (nAChRbeta3) gene promoter. Enhanced green fluorescent protein (EGFP) expression signals in living transgenic zebrafish lines carrying the nAChRbeta3 gene promoter-directed EGFP expression vector visualized the development of entire retinal ganglion cell (RGC) axon projection to the tectum. Microinjection of the nAChRbeta3 gene promoter-driven double-cassette vectors directing the expression of both dominant-negative glycogen synthase kinase-3beta (dnGSK-3beta) and EGFP enabled us to follow the development of individual RGCs and to examine the effect of the molecule on the axonal arborization and maturation of the same neurons in living zebrafish. We found that the expression of the dominant-negative form of zebrafish GSK-3beta suppressed the arborization field of RGC axon terminals in the tectum as estimated by the reduction of arbor branch length and arbor areas. Furthermore, the suppression of GSK-3beta activity increased the size of vesicle-associated membrane protein 2-EGFP puncta in RGC axon terminals at the early stage of innervation to the tectum. These results suggest that GSK-3beta regulates the arborization field and maturation of RGC axon terminals in vivo.

Regulation by glycogen synthase kinase-3beta of the arborization field and maturation of retinotectal projection in zebrafish

The retinotectal projection is one of the best systems to study the molecular basis of synapse formation in the CNS because of the well characterized topographic connections and activity-dependent refinement. Here, we developed a presynaptic neuron-specific gene manipulation system in the zebrafish retinotectal projection in vivo using the nicotinic acetylcholine receptor beta3 (nAChRbeta3) gene promoter. Enhanced green fluorescent protein (EGFP) expression signals in living transgenic zebrafish lines carrying the nAChRbeta3 gene promoter-directed EGFP expression vector visualized the development of entire retinal ganglion cell (RGC) axon projection to the tectum. Microinjection of the nAChRbeta3 gene promoter-driven double-cassette vectors directing the expression of both dominant-negative glycogen synthase kinase-3beta (dnGSK-3beta) and EGFP enabled us to follow the development of individual RGCs and to examine the effect of the molecule on the axonal arborization and maturation of the same neurons in living zebrafish. We found that the expression of the dominant-negative form of zebrafish GSK-3beta suppressed the arborization field of RGC axon terminals in the tectum as estimated by the reduction of arbor branch length and arbor areas. Furthermore, the suppression of GSK-3beta activity increased the size of vesicle-associated membrane protein 2-EGFP puncta in RGC axon terminals at the early stage of innervation to the tectum. These results suggest that GSK-3beta regulates the arborization field and maturation of RGC axon terminals in vivo.

Evidence that synaptically released beta-amyloid accumulates as extracellular deposits in the hippocampus of transgenic mice

A neuropathological hallmark of Alzheimer's disease is the deposition of amyloid-beta (Abeta) peptides in senile plaques in the hippocampus and cerebral cortex. Abeta is derived from larger integral membrane proteins termed amyloid precursor proteins (APP). We demonstrated previously that APP, synthesized by neurons in the entorhinal cortex, is transported via the perforant pathway to presynaptic terminals in the dentate gyrus. We reported that, although full-length APP and membrane-tethered, C-terminal APP derivatives (APP-CTFs) accumulate at terminal fields, the production of Abeta peptides at these sites was indeterminate. To test the hypothesis that APP-CTFs, generated from axonally transported APP, are further metabolized to Abeta peptides that are subsequently released and deposited proximal to nerve terminals, we created unilateral knife lesions of the perforant pathway of transgenic mice that exhibit hippocampal amyloid deposits. We observed pronounced reductions in amyloid burden in the ipsilateral dentate gyrus, findings that lead us to conclude that axonally transported APP gives rise to Abeta peptides that are released from presynaptic sites in the dentate gyrus and deposited in extracellular plaques. Moreover, our findings are consistent with the view that Abeta deposits are dynamic structures and that the perforant path lesion alters the equilibrium between Abeta production-deposition toward clearance as a consequence of blocked axonal transport of APP from the entorhinal cortex to terminal fields in the hippocampus.

Syndapins integrate N-WASP in receptor-mediated endocytosis

Syndapins are potential links between the cortical actin cytoskeleton and endocytosis because this family of dynamin-associated proteins can also interact with the Arp2/3 complex activator N-WASP. Here we provide evidence for involvement of N-WASP interactions in receptor-mediated endocytosis. We reveal that the observed dominant-negative effects of N-WASP are dependent exclusively on the proline-rich domain, the binding interface of syndapins. Our results therefore suggest that syndapins integrate N-WASP functions in endocytosis. Both proteins co-localize in neuronal cells. Consistent with a crucial role for syndapins in endocytic uptake, co-overexpression of syndapins rescued the endocytosis block caused by N-WASP. An in vivo reconstitution of the syndapin-N-WASP interaction at cellular membranes triggered local actin polymerization. Depletion of endogenous N-WASP by sequestering it to mitochondria or by introducing anti-N-WASP antibodies impaired endocytosis. Our data suggest that syndapins may act as important coordinators of N-WASP and dynamin functions during the different steps of receptor-mediated endocytosis and that local actin polymerization induced by syndapin-N-WASP interactions may be a mechanism supporting clathrin-coated vesicle detachment and movement away from the plasma membrane.