Preassembly and transport of nerve terminals: a new concept of axonal transport

CNS myelin sheath is stochastically built by homotypic fusion of myelin membranes within the bounds of an oligodendrocyte process

Myelin - the multilayer membrane that envelops axons - is a facilitator of rapid nerve conduction. Oligodendrocytes form CNS myelin; the prevailing hypothesis being that they do it by extending a process that circumnavigates the axon. It is pertinent to ask how myelin is built because oligodendrocyte plasma membrane and myelin are compositionally different. To this end, we examined oligodendrocyte cultures and embryonic avian optic nerves by electron microscopy, immuno-electron microscopy and three-dimensional electron tomography. The results support three novel concepts. Myelin membranes are synthesized as tubules and packaged into "myelinophore organelles" in the oligodendrocyte perikaryon. Myelin membranes are matured in and transported by myelinophore organelles within an oligodendrocyte process. The myelin sheath is generated by myelin membrane fusion inside an oligodendrocyte process. These findings abrogate the dogma of myelin resulting from a wrapping motion of an oligodendrocyte process and open up new avenues in the quest for understanding myelination in health and disease.

Regulation of neuronal GABA(B) receptor functions by subunit composition

GABA(B) receptors (GABA(B)Rs) are G protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the CNS. In the past 5 years, notable advances have been made in our understanding of the molecular composition of these receptors. GABA(B)Rs are now known to comprise principal and auxiliary subunits that influence receptor properties in distinct ways. The principal subunits regulate the surface expression and the axonal versus dendritic distribution of these receptors, whereas the auxiliary subunits determine agonist potency and the kinetics of the receptor response. This Review summarizes current knowledge on how the subunit composition of GABA(B)Rs affects the distribution of these receptors, neuronal processes and higher brain functions.

Stable mossy fiber long-term potentiation requires calcium influx at the granule cell soma, protein synthesis, and microtubule-dependent axonal transport

The synapses formed by the mossy fiber (MF) axons of hippocampal dentate gyrus granule neurons onto CA3 pyramidal neurons exhibit an intriguing form of experience-dependent synaptic plasticity that is induced and expressed presynaptically. In contrast to most other CNS synapses, long-term potentiation (LTP) at the MF-CA3 synapse is readily induced even during blockade of postsynaptic glutamate receptors. Furthermore, blocking voltage-gated Ca(2+) channels prevents MF-LTP, supporting an involvement of presynaptic Ca(2+) signaling via voltage-gated Ca(2+) channels in MF-LTP induction. We examined the contribution of activity in both dentate granule cell somata and MF terminals to MF-LTP. We found that the induction of stable MF-LTP requires tetanization-induced action potentials not only at MF boutons, but also at dentate granule cell somata. Similarly, blocking Ca(2+) influx via voltage-gated Ca(2+) channels only at the granule cell soma was sufficient to disrupt MF-LTP. Finally, blocking protein synthesis or blocking fast axonal transport mechanisms via disruption of axonal tubulin filaments resulted in decremental MF-LTP. Collectively, these data suggest that-in addition to Ca(2+) influx at the MF terminals-induction of MF synaptic plasticity requires action potential-dependent Ca(2+) signaling at granule cell somata, protein synthesis, and fast axonal transport along MFs. A parsimonious interpretation of these results is that somatic activity triggers protein synthesis at the soma; newly synthesized proteins are then transported to MF terminals, where they contribute to the stabilization of MF-LTP. Finally, the present data imply that synaptic plasticity at the MF-CA3 synapse can be affected by local modulation of somatic and presynaptic Ca(2+) channel activity.

The Sushi domains of GABAB receptors function as axonal targeting signals

GABA(B) receptors are the G-protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the brain. Two receptor subtypes, GABA(B(1a,2)) and GABA(B(1b,2)), are formed by the assembly of GABA(B1a) and GABA(B1b) subunits with GABA(B2) subunits. The GABA(B1b) subunit is a shorter isoform of the GABA(B1a) subunit lacking two N-terminal protein interaction motifs, the sushi domains. Selectively GABA(B1a) protein traffics into the axons of glutamatergic neurons, whereas both the GABA(B1a) and GABA(B1b) proteins traffic into the dendrites. The mechanism(s) and targeting signal(s) responsible for the selective trafficking of GABA(B1a) protein into axons are unknown. Here, we provide evidence that the sushi domains are axonal targeting signals that redirect GABA(B1a) protein from its default dendritic localization to axons. Specifically, we show that mutations in the sushi domains preventing protein interactions preclude axonal localization of GABA(B1a). When fused to CD8alpha, the sushi domains polarize this uniformly distributed protein to axons. Likewise, when fused to mGluR1a the sushi domains redirect this somatodendritic protein to axons, showing that the sushi domains can override dendritic targeting information in a heterologous protein. Cell surface expression of the sushi domains is not required for axonal localization of GABA(B1a). Altogether, our findings are consistent with the sushi domains functioning as axonal targeting signals by interacting with axonally bound proteins along intracellular sorting pathways. Our data provide a mechanistic explanation for the selective trafficking of GABA(B(1a,2)) receptors into axons while at the same time identifying a well defined axonal delivery module that can be used as an experimental tool.

TRPV1 acts as a synaptic protein and regulates vesicle recycling

Electrophysiological studies demonstrate that transient receptor potential vanilloid subtype 1 (TRPV1) is involved in neuronal transmission. Although it is expressed in the peripheral as well as the central nervous system, the questions remain whether TRPV1 is present in synaptic structures and whether it is involved in synaptic processes. In the present study we gathered evidence that TRPV1 can be detected in spines of cortical neurons, that it colocalizes with both pre- and postsynaptic proteins, and that it regulates spine morphology. Moreover, TRPV1 is also present in biochemically prepared synaptosomes endogenously. In F11 cells, a cell line derived from dorsal-root-ganglion neurons, TRPV1 is enriched in the tips of elongated filopodia and also at sites of cell-cell contact. In addition, we also detected TRPV1 in synaptic transport vesicles, and in transport packets within filopodia and neurites. Using FM4-64 dye, we demonstrate that recycling and/or fusion of these vesicles can be rapidly modulated by TRPV1 activation, leading to rapid reorganization of filopodial structure. These data suggest that TRPV1 is involved in processes such as neuronal network formation, synapse modulation and release of synaptic transmitters.

Kinesin-1 plays a role in transport of SNAP-25 to the plasma membrane

The cellular molecular motor kinesin-1 mediates the microtubule-dependent transport of a range of cargo. We have previously identified an interaction between the cargo-binding domain of kinesin-1 heavy chain KIF5B and the membrane-associated SNARE proteins SNAP-25 and SNAP-23. In this study we further defined the minimal SNAP-25 binding domain in KIF5B to residues 874-894. Overexpression of a fragment of KIF5B (residues 594-910) resulted in significant colocalization with SNAP-25 with resulting blockage of the trafficking of SNAP-25 to the periphery of cells. This indicates that kinesin-1 facilitates the transport of SNAP-25 containing vesicles as a prerequisite to SNAP-25 driven membrane fusion events.

The neurotoxicological effects of mastoparan Polybia-MPII at the murine neuromuscular junction: an ultrastructural and immunocytochemical study

Polybia-MPII (INWLKLGKMVIDAL-NH2), a mastoparan isolated from the crude venom of the swarming wasp Polybia paulista, was injected into the left hind limb of Swiss white mice. Between 3 h and 21 days later the mice were killed and the soleus muscles from both hind limbs were removed. Sections of the muscles were made for transmission electron microscopy and immunocytochemistry. Transmission electron microscopy showed that both the volume fraction occupied by synaptic vesicles and synaptic vesicle density was greatly reduced after exposure to Polybia-MPII, although there was no significant structural damage to the plasma membrane of the terminal boutons and mitochondria were indistinguishable from those in normal, control boutons. Immunocytochemistry revealed that in control muscles 99% of motor end plates identified by the positive labelling of acetylcholine receptors by TRITC-alpha-bungarotoxin co-labelled with anti-synaptophysin antibody, but this figure fell by 30% in muscles exposed to the toxin. These changes were transient. They were maximal at 6 h and fully reversed by 3 days. At no time was axonal labelling with anti-neurofilament antibodies affected by exposure to Polybia-MPII. We conclude that mastoparan Polybia-MPII is a minor neurotoxin and suggest that its neurotoxic activity is unlikely to be of clinical significance.

A mechanism distinct from highwire for the Drosophila ubiquitin conjugase bendless in synaptic growth and maturation

The signaling mechanisms that allow the conversion of a growth cone into a mature and stable synapse are yet to be completely understood. Ubiquitination plays key regulatory roles in synaptic development and may be involved in this process. Previous studies identified the Drosophila ubiquitin conjugase bendless (ben) to be important for central synapse formation, but the precise role it plays has not been elucidated. Our studies indicate that Ben plays a pivotal role in synaptic growth and maturation. We have determined that an incipient synapse is present with a high penetrance in ben mutants, suggesting that Ben is required for a developmental step after target recognition. We used cell-autonomous rescue experiments to show that Ben has a presynaptic role in synapse growth. We then harnessed the TARGET system to transiently express UAS (upstream activating sequence)-ben in a ben mutant background and identified a well defined critical period for Ben function in establishing a full-grown, mature synaptic terminal. We demonstrate that the protein must be present at a time point before but not during the actual growth process. We also provide phenotypic evidence demonstrating that Ben is not a part of the signal transduction pathway involving the well characterized ubiquitin ligase highwire. We conclude that Bendless functions as a novel developmental switch that permits the transition from axonal growth and incipient synapse formation to synaptic growth and maturation in the CNS.

Synaptic vesicle protein trafficking at the glutamate synapse

Expression of the integral and associated proteins of synaptic vesicles is subject to regulation over time, by region, and in response to activity. The process by which changes in protein levels and isoforms result in different properties of neurotransmitter release involves protein trafficking to the synaptic vesicle. How newly synthesized proteins are incorporated into synaptic vesicles at the presynaptic bouton is poorly understood. During synaptogenesis, synaptic vesicle proteins sort through the secretory pathway and are transported down the axon in precursor vesicles that undergo maturation to form synaptic vesicles. Changes in protein content of synaptic vesicles could involve the formation of new vesicles that either mix with the previous complement of vesicles or replace them, presumably by their degradation or inactivation. Alternatively, new proteins could individually incorporate into existing synaptic vesicles, changing their functional properties. Glutamatergic vesicles likely express many of the same integral membrane proteins and share certain common mechanisms of biogenesis, recycling, and degradation with other synaptic vesicles. However, glutamatergic vesicles are defined by their ability to package glutamate for release, a property conferred by the expression of a vesicular glutamate transporter (VGLUT). VGLUTs are subject to regional, developmental, and activity-dependent changes in expression. In addition, VGLUT isoforms differ in their trafficking, which may target them to different pathways during biogenesis or after recycling, which may in turn sort them to different vesicle pools. Emerging data indicate that differences in the association of VGLUTs and other synaptic vesicle proteins with endocytic adaptors may influence their trafficking. These observations indicate that independent regulation of synaptic vesicle protein trafficking has the potential to influence synaptic vesicle protein composition, the maintenance of synaptic vesicle pools, and the release of glutamate in response to changing physiological requirements.

Exploratory behaviour in NO-dependent cyclase mutants of Drosophila shows defects in coincident neuronal signalling

Background

Drosophila flies explore the environment very efficiently in order to colonize it. They explore collectively, not individually, so that when a few land on a food spot, they attract the others by signs. This behaviour leads to aggregation of individuals and optimizes the screening of mates and egg-laying on the most favourable food spots.

Results

Flies perform cycles of exploration/aggregation depending on the resources of the environment. This behavioural ecology constitutes an excellent model for analyzing simultaneous processing of neurosensory information. We reasoned that the decision of flies to land somewhere in order to achieve aggregation is based on simultaneous integration of signals (visual, olfactory, acoustic) during their flight. On the basis of what flies do in nature, we designed laboratory tests to analyze the phenomenon of neuronal coincidence. We screened many mutants of genes involved in neuronal metabolism and the synaptic machinery.

Conclusion

Mutants of NO-dependent cyclase show a specifically-marked behaviour phenotype, but on the other hand they are associated with moderate biochemical defects. We show that these mutants present errors in integrative and/or coincident processing of signals, which are not reducible to the functions of the peripheral sensory cells.

Syntabulin-kinesin-1 family member 5B-mediated axonal transport contributes to activity-dependent presynaptic assembly

The mechanism by which microtubule-based axonal transport regulates activity-dependent presynaptic plasticity in developing neurons remains mostly unknown. Our previous studies established that syntabulin is an adaptor capable of conjoining the kinesin family member 5B (KIF5B) motor and syntaxin-1. We now report that the complex of syntaxin-1-syntabulin-KIF5B mediates axonal transport of the active zone (AZ) components essential for presynaptic assembly. Syntabulin associates with AZ precursor carriers and colocalizes and comigrates with green fluorescent protein (GFP)-Bassoon-labeled AZ transport cargos within developing axons. Knock-down of syntabulin or disruption of the syntaxin-1-syntabulin-KIF5B complex impairs the anterograde transport of GFP-Bassoon out of the soma and reduces the axonal densities of synaptic vesicle (SV) clusters and FM4-64 [N-(3-triethylammoniumpropyl)-4-(p-dibutylaminostyryl)pyridinium, dibromide] loading. Furthermore, syntabulin loss of function results in a reduction in both the amplitude of postsynaptic currents and the frequency of asynchronous quantal events, and abolishes the activity-induced recruitment of new GFP-Bassoon into the axons and subsequent coclustering with SVs. Consequently, syntabulin loss of function blocks the formation of new presynaptic boutons during activity-dependent synaptic plasticity in developing neurons. These studies establish that a kinesin motor-adaptor complex is critical for the anterograde axonal transport of AZ components, thus contributing to activity-dependent presynaptic assembly during neuronal development.

A Drosophila kinesin required for synaptic bouton formation and synaptic vesicle transport

The morphological transition of growth cones to synaptic boutons characterizes synaptogenesis. Here we have isolated mutations in immaculate connections (imac; CG8566), a previously uncharacterized Drosophila gene encoding a member of the Kinesin-3 family. Whereas earlier studies in Drosophila implicated Kinesin-1 in transporting synaptic vesicle precursors, we find that Imac is essential for this transport. An unexpected feature of imac mutants is the failure of synaptic boutons to form. Motor neurons lacking imac properly target to muscles but remain within target fields as thin processes, a structure that is distinct from either growth cones or mature terminals. Few active zones form at these endings. We show that the arrest of synaptogenesis is not a secondary consequence of the absence of transmission. Our data thus indicate that Imac transports components required for synaptic maturation and provide insight into presynaptic maturation as a process that can be differentiated from axon outgrowth and targeting.

Stabilization of hyperdynamic microtubules is neuroprotective in amyotrophic lateral sclerosis

Mutations in copper/zinc superoxide dismutase 1 (SOD1), a genetic cause of human amyotrophic lateral sclerosis, trigger motoneuron death through unknown toxic mechanisms. We report that transgenic SOD1G93A mice exhibit striking and progressive changes in neuronal microtubule dynamics from an early age, associated with impaired axonal transport. Pharmacologic administration of a microtubule-modulating agent alone or in combination with a neuroprotective drug to symptomatic SOD1G93A mice reduced microtubule turnover, preserved spinal cord neurons, normalized axonal transport kinetics, and delayed the onset of symptoms, while prolonging life by up to 26%. The degree of reduction of microtubule turnover was highly predictive of clinical responses to different treatments. These data are consistent with the hypothesis that hyperdynamic microtubules impair axonal transport and accelerate motor neuron degeneration in amyotrophic lateral sclerosis. Measurement of microtubule dynamics in vivo provides a sensitive biomarker of disease activity and therapeutic response and represents a new pharmacologic target in neurodegenerative disorders.

Cochlear damage induces GAP-43 expression in cholinergic synapses of the cochlear nucleus in the adult rat: a light and electron microscopic study

Recent studies suggest a potential for activity-dependent reconstruction in the adult mammalian brainstem that exceeds previous expectations. We found that a unilateral cochlear lesion led within 1 week to a rise of choline acetyltransferase (ChAT) immunoreactivity in the ventral cochlear nucleus of the affected side, matching the lesion-induced expression of growth-associated protein 43 (GAP-43) previously described. The rise of both ChAT and GAP-43 immunoreactivity was reflected in the average density of the staining. Moreover, the number of light-microscopically identifiable boutons increased in both stains. GAP-43-positive boutons could, by distinct ultrastructural features, regularly be identified as presynaptic endings. However, GAP-43 immunoreactivity was not only found in presynaptic endings with a classical morphology, but also in profiles that suggest morphological dynamic structures by showing filopodia, assemblages of pleomorphic vesicles, large vesicles (diameter up to 200 nm) fusing with the presynaptic plasma membrane close to synaptic contacts, small dense-core vesicles (diameter about 80 nm) and presynaptic ribosomes. Moreover, we observed perforated synapses as well as GAP-43 immunoreactivity condensed in rafts, both indicative of growing or changing neuronal connections. Classical and untypical ultrastructural profiles that contained GAP-43 also contained ChAT. We conclude that there is extensive deafness-induced GAP-43-mediated synaptic plasticity in the cochlear nucleus, and that this plasticity is predominantly, if not exclusively, based on cholinergic afferents.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

Dense core vesicles resemble active-zone transport vesicles and are diminished following synaptogenesis in mature hippocampal slices

Large dense core vesicles (approximately 100 nm) contain neuroactive peptides and other co-transmitters. Smaller dense core vesicles (approximately 80 nm) are known to contain components of the presynaptic active zone and thought to transport and deliver these components during developmental synaptogenesis. It is not known whether excitatory axons in area CA1 contain such dense core vesicles, and whether they contribute to synaptic plasticity of mature hippocampus. Serial section electron microscopy was used to identify dense core vesicles in presynaptic axons in s. radiatum of area CA1 in adult rat hippocampus. Comparisons were made among perfusion-fixed hippocampus and hippocampal slices that undergo synaptogenesis during recovery in vitro. Dense core vesicles occurred in 26.1+/-3.6% of axonal boutons in perfusion fixed hippocampus, and in only 17.6+/-4.5% of axonal boutons in hippocampal slices (P<0.01). Most of the dense core vesicle positive boutons contained only one dense core vesicle, and no reconstructed axonal bouton had more than a total of 10 dense core vesicles in either condition. Overall the dense core vesicles had average diameters of 79+/-11 nm. These small dense core vesicles were usually located near nonsynaptic membranes and rarely occurred near the edge of a presynaptic active zone. Their size, low frequency, locations, and decrease following recuperative synaptogenesis in slices are novel findings that merit further study with respect to small dense core vesicle content and possible contributions to synapse assembly and plasticity in the mature hippocampus.

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.

Envenoming bites by kraits: the biological basis of treatment-resistant neuromuscular paralysis

Beta-bungarotoxin, a neurotoxic phospholipase A2 is a major fraction of the venom of kraits. The toxin was inoculated into one hind limb of young adult rats. The inoculated hind limb was paralysed within 3 h, and remained paralysed for 2 days. The paralysis was associated with the loss of synaptic vesicles from motor nerve terminal boutons, a decline in immunoreactivity of synaptophysin, SNAP-25 and syntaxin, a loss of muscle mass and the upregulation of NaV(1.5) mRNA and protein. Between 3 and 6 h after the inoculation of toxin, some nerve terminal boutons exhibited clear signs of degeneration. Others appeared to be in the process of withdrawing from the synaptic cleft and some boutons were fully enwrapped in terminal Schwann cell processes. By 12 h all muscle fibres were denervated. Re-innervation began at 3 days with the appearance of regenerating nerve terminals, a return of neuromuscular function in some muscles and a progressive increase in the immunoreactivity of synaptophysin, SNAP-25 and syntaxin. Full recovery occurred at 7 days. The data were compared with recently published clinical data on envenoming bites by kraits and by extrapolation we suggest that the acute, reversible denervation caused by beta-bungarotoxin is a credible explanation for the clinically important, profound treatment-resistant neuromuscular paralysis seen in human subjects bitten by these animals.

Syntabulin is a microtubule-associated protein implicated in syntaxin transport in neurons

Different types of cargo vesicles containing presynaptic proteins are transported from the nerve cell body to the nerve terminal, and participate in the formation of active zones. However, the identity of the membranous cargoes and the nature of the motor-cargo interactions remain unsolved. Here, we report the identification of a syntaxin-1-binding protein named syntabulin. Syntabulin attaches syntaxin-containing vesicles to microtubules and migrates with syntaxin within the processes of hippocampal neurons. Knock-down of syntabulin expression with targeted small interfering RNAs (siRNAs) or interference with the syntabulin-syntaxin interaction inhibit attachment of syntaxin-cargo vesicles to microtubules and reduce syntaxin-1 distribution in neuronal processes. Furthermore, conventional kinesin I heavy chain binds to syntabulin and associates with syntabulin-linked syntaxin vesicles in vivo. These findings suggest that syntabulin functions as a linker molecule that attaches syntaxin-cargo vesicles to kinesin I, enabling the transport of syntaxin-1 to neuronal processes.

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.

Transport of a synaptotagmin–YFP fusion protein in sympathetic neurons during early neurite outgrowth in vitro after transfection in vivo

Developing neurons are engaged in neurite outgrowth as well as the synthesis and transport of proteins involved in synaptic transmission. Very little is known about when transport is established in these rudimentary neurites. We used a novel technique to visualize protein transport during the early hours of neurite outgrowth in culture. Recombinant adenoviruses were used to express a synaptotagmin-YFP fusion protein in the superior cervical ganglia of neonatal rats in vivo and protein transport was examined in neuronal cultures established from the superior cervical ganglions (SCGs). We find that, as early as 4h in culture, synaptotagmin-YFP was present in the cytoplasm, lamellipodia, filopodia and growth cones. Protein expression appeared punctate in neurites at 8h in vitro and is consistent with a vesicular localization. These results indicate that the machinery to transport synapse-specific proteins is functional in rudimentary neurites at this time and indicates that this technique can be used to study early neuronal development.

Visualization of vesicle transport along and between distinct pathways in neurites of living cells

Trafficking of secretory vesicles along neurites of PC12 cells was visualized by 2D and 3D real-time imaging using fluorescence microscopy. Vesicle motion along distinct pathways was directly seen. From an overlay of individual pathways, the underlying cytoskeletal filament could be imaged at a subwavelength resolution. Continuous vesicle transport was interrupted by periods of diffusive motion with concomitant pathway changes. Statistical analysis shows that such interruptions were distributed stochastically along the filament, indicating a limited processivity of motor proteins also in a cellular context. Periods of diffusive motion facilitated the interaction with actively transported vesicles. Frequent associations and dissociations of vesicles have been observed consistently, pointing to a functional relevance of vesicle cotransport.

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.

Two distinct mechanisms target membrane proteins to the axonal surface

We have investigated the trafficking of two endogenous axonal membrane proteins, VAMP2 and NgCAM, in order to elucidate the cellular events that underlie their polarization. We found that VAMP2 is delivered to the surface of both axons and dendrites, but preferentially endocytosed from the dendritic membrane. A mutation in the cytoplasmic domain of VAMP2 that inhibits endocytosis abolished its axonal polarization. In contrast, the targeting of NgCAM depends on sequences in its ectodomain, which mediate its sorting into carriers that preferentially deliver their cargo proteins to the axonal membrane. These observations show that neurons use two distinct mechanisms to polarize proteins to the axonal domain: selective retention in the case of VAMP2, selective delivery in the case of NgCAM.

The developing synapse: construction and modulation of synaptic structures and circuits

Synapse formation and stabilization in the vertebrate central nervous system is a dynamic process, requiring bi-directional communication between pre- and postsynaptic partners. Numerous mechanisms coordinate where and when synapses are made in the developing brain. This review discusses cellular and activity-dependent mechanisms that control the development of synaptic connectivity.

Cast: a novel protein of the cytomatrix at the active zone of synapses that forms a ternary complex with RIM1 and munc13-1

The cytomatrix at the active zone (CAZ) has been implicated in defining the site of Ca2+-dependent exocytosis of neurotransmitter. We have identified here a novel CAZ protein of approximately 120 kD from rat brain and named it CAST (CAZ-associated structural protein). CAST had no transmembrane segment, but had four coiled-coil domains and a putative COOH-terminal consensus motif for binding to PDZ domains. CAST was localized at the CAZ of conventional synapses of mouse brain. CAST bound directly RIM1 and indirectly Munc13-1, presumably through RIM1, forming a ternary complex. RIM1 and Munc13-1 are CAZ proteins implicated in Ca2+-dependent exocytosis of neurotansmitters. Bassoon, another CAZ protein, was also associated with this ternary complex. These results suggest that a network of protein-protein interactions among the CAZ proteins exists at the CAZ. At the early stages of synapse formation, CAST was expressed and partly colocalized with bassoon in the axon shaft and the growth cone. The vesicles immunoisolated by antibassoon antibody-coupled beads contained not only bassoon but also CAST and RIM1. These results suggest that these CAZ proteins are at least partly transported on the same vesicles during synapse formation.

Molecular mechanisms of CNS synaptogenesis

Synapses of the mammalian CNS are asymmetric sites of cell-cell adhesion between nerve cells. They are designed to mediate the rapid and efficient transmission of signals from the presynaptic bouton of one neuron to the postsynaptic plasma membrane of a second neuron. Significant progress has been made in the characterization of the structural, functional and developmental assembly of CNS synapses. Recent progress has been made in understanding the molecular and cellular mechanisms that underlie synaptogenesis, in particular that of glutamatergic synapses of the CNS.

Alterations in transmission, vesicle dynamics, and transmitter release machinery at NCAM-deficient neuromuscular junctions

Although functional neuromuscular junctions (NMJs) form in NCAM-deficient mice, they exhibit multiple alterations in presynaptic organization and function. Profound depression and unusual periodic total transmission failures with repetitive stimulation point to a defect in vesicle mobilization/cycling, and these defects were mimicked in (+/+) NMJs by inhibitors of myosin light chain kinase, known to affect vesicle mobilization. Two separate release mechanisms, utilizing different endocytic machinery and Ca(2+) channels, were shown to coexist in (-/-) terminals, with the mature process targeted to presynaptic membrane opposed to muscle, and an abnormally retained immature process targeted to the remainder of the presynaptic terminal and axon. Thus, NCAM plays a critical and heretofore unsuspected role in the molecular organization of the presynaptic NMJ.

Endoplasmic reticulum membrane-sorting protein of lymphocytes (BAP31) is highly expressed in neurons and discrete endocrine cells

BAP31 is a transmembrane protein that associates with nascent membrane proteins in transit between endoplasmic reticulum (ER) and cis-Golgi. Its C-terminal dilysine (KKEE) motif, mediating return to the ER, is consistent with a role in early sorting of membrane proteins. An initiator caspase-binding site in the C-terminal domain of BAP31 is implicated in cytoplasmic membrane fragmentation events of apoptosis. Although BAP31 RNA is ubiquitous, the protein's anatomic localization has not been determined. To gain further insight into its possible functions, we localized BAP31 in primate tissues using monoclonal antibodies. Immunoreactivity was prominent in T- and B-lymphocytes in blood and in thymus, in cerebellar Purkinje neuron bodies and dendrites, in gonadotrophs of the anterior pituitary, ovarian thecal and follicular cells, active but not quiescent thyroid epithelium, adrenal cortex more than medulla, and proximal more than distal renal tubules. Blood vessels and skeletal muscle were nonreactive. The anatomic distribution of BAP31 and the nature of proteins identified thus far as its cargo exiting the ER, suggest an interaction with proteins assembling in macromolecular complexes en route to selected sites of exocytotic and signaling activities. Apoptotic associations in mature tissues could be physiological (lymphocytes, endocrine cells) or pathological (Purkinje neurons, renal tubules).

Linkers, packages and pathways: new concepts in axonal transport

The molecular mechanisms that generate efficient and directed transport of proteins and organelles in axons remain poorly understood. In the past year, many studies have identified specific transmembrane or scaffold proteins that might link motor proteins to their cargoes. These studies have also identified previously unsuspected pathways and raised the intriguing possibility that pre-packaged groups of functionally related proteins are transported together in the axon. Evidence suggests that fast molecular motor proteins have a role in slow axonal transport, and the axonal transport machinery has been implicated in the genesis of neurodegenerative diseases.

Recruitment of synaptic molecules during synaptogenesis

The glutamatergic synapse is the main type of excitatory synapse in the mammalian brain. The formation of each glutamatergic synapse is associated with the recruitment of numerous (probably hundreds) different molecules and their assimilation into functional assemblies. Intense research has revealed the identity of many of these molecules, provided information as to interactions they are involved in, and offered clues as to their roles in synaptic function. Recent work has also begun to shed light on fundamental mechanisms underlying molecule recruitment to developing glutamatergic synapses. Current data indicate that the formation of presynaptic active zones-sites of neurotransmitter release-may be realized by the insertion of precursor vesicles containing multiple active zone components, possibly in pre-assembled form. The assembly of the postsynaptic reception apparatus, on the other hand, seems to occur via the sequential recruitment of molecules to the postsynaptic membrane and their assimilation in situ. Several molecules and mechanisms have been identified that display a capacity for inducing pre- or postsynaptic differentiation. These exciting findings are starting to provide a rudimentary framework for understanding key processes underlying the formation of glutamatergic synaptic connections.

Principles of glutamatergic synapse formation: seeing the forest for the trees

General principles regarding glutamatergic synapse formation in the central nervous system are beginning to emerge. These principles concern the specific roles that dendrites and axons play in the induction of synaptic differentiation, the modes of presynaptic and postsynaptic assembly, the time course of synapse formation and maturation, and the roles of synaptic activity in these processes.

Retrograde signaling at central synapses

Transcellular retrograde signaling from the postsynaptic target cell to the presynaptic neuron plays critical roles in the formation, maturation, and plasticity of synaptic connections. We here review recent progress in our understanding of the retrograde signaling at developing central synapses. Three forms of potential retrograde signals-membrane-permeant factors, membrane-bound factors, and secreted factors-have been implicated at both developing and mature synapses. Although many of these signals may be active constitutively, retrograde factors produced in association with activity-dependent synaptic plasticity, e.g., long-term potentiation and long-term depression, are of particular interest, because they may induce modification of neuronal excitability and synaptic transmission, functions directly related to the processing and storage of information in the nervous system.

A conserved alpha-herpesvirus protein necessary for axonal localization of viral membrane proteins

Pseudorabies virus, an alpha-herpesvirus, is capable of infecting the nervous system and spreading between synaptically connected neurons in diverse hosts. At least three viral membrane proteins (gE, gI, and Us9) are necessary for the spread of infection from presynaptic to postsynaptic neurons (anterograde spread) in infected rodents. To understand how these proteins effect anterograde spread between neurons, we analyzed the subcellular localization of viral proteins after infection of cultured rat sympathetic neurons with wild-type or mutant viruses. After Us9-null mutant infections but not gE-null mutant infections, only a subset of the viral structural proteins had entered axons. Surprisingly, capsid and tegument proteins but not viral membrane proteins were detected in axons. The spread of Us9 missense mutants in the rodent nervous system correlated with the amount of viral membrane proteins localized to axons. We conclude that the Us9 membrane protein controls axonal localization of diverse viral membrane proteins but not that of capsid or tegument proteins. The data support a model where virion subassemblies but not complete virions are transported in the axon. Our results provide new insight into the process of virion assembly and exit from neurons that leads to directional spread of herpesviruses in the nervous system.

The dynamics of SAP90/PSD-95 recruitment to new synaptic junctions

SAP90/PSD-95 is thought to be a central organizer of the glutamatergic synapse postsynaptic reception apparatus. To assess its potential role during glutamatergic synapse formation, we used GFP-tagged SAP90/PSD-95, time lapse confocal microscopy, and cultured hippocampal neurons to determine its dynamic recruitment into new synaptic junctions. We report that new SAP90/PSD-95 clusters first appeared at new axodendritic contact sites within 20-60 min of contact establishment. SAP90/PSD-95 clustering was rapid, with kinetics that fit a single exponential with a mean time constant of approximately 23 min. Most new SAP90/PSD-95 clusters were found juxtaposed to functional presynaptic boutons as determined by labeling with FM 4-64. No evidence was found for the existence of discrete transport particles similar to those previously reported to mediate presynaptic active zone cytoskeleton assembly. Instead, we found that SAP90/PSD-95 is recruited to nascent synapses from a diffuse dendritic cytoplasmic pool. Our findings show that SAP90/PSD-95 is recruited to nascent synaptic junctions early during the assembly process and indicate that its assimilation is fundamentally different from that of presynaptic active zone components.

Sorting and transport of alpha herpesviruses in axons

The alpha herpesviruses, a subfamily of the herpesviruses, are neurotropic pathogens found associated with most mammalian species. The prototypic member of this subfamily is herpes simplex virus type 1, the causative agent of recurrent cold sores in humans. The mild nature of this disease is a testament to the complex and highly regulated life cycle of the alpha herpesviruses. The cellular mechanisms used by these viruses to disseminate infection in the nervous system are beginning to be understood. Here, we overview the life cycle of alpha herpesviruses with an emphasis on assembly and transport of viral particles in neurons.

Assembling the presynaptic active zone: a characterization of an active one precursor vesicle

The active zone is a specialized region of the presynaptic plasma membrane where synaptic vesicles dock and fuse. In this study, we have investigated the cellular mechanism underlying the transport and recruitment of the active zone protein Piccolo into nascent synapses. Our results show that Piccolo is transported to nascent synapses on an approximately 80 nm dense core granulated vesicle together with other constituents of the active zone, including Bassoon, Syntaxin, SNAP-25, and N-cadherin, as well as chromogranin B. Components of synaptic vesicles, such as VAMP 2/synaptobrevin II, synaptophysin, synaptotagmin, or proteins of the perisynaptic plasma membrane such as GABA transporter 1 (GAT1), were not present. These studies demonstrate that the presynaptic active zone is formed in part by the fusion of an active zone precursor vesicle with the presynaptic plasma membrane.