SNAP-25 requirement for dendritic growth of hippocampal neurons

Delivering synaptic protein mRNAs via extracellular vesicles ameliorates cognitive impairment in a mouse model of Alzheimer's disease

BACKGROUND

Synaptic dysfunction with reduced synaptic protein levels is a core feature of Alzheimer's disease (AD). Synaptic proteins play a central role in memory processing, learning, and AD pathogenesis. Evidence suggests that synaptic proteins in plasma neuronal-derived extracellular vesicles (EVs) are reduced in patients with AD. However, it remains unclear whether levels of synaptic proteins in EVs are associated with hippocampal atrophy of AD and whether upregulating the expression of these synaptic proteins has a beneficial effect on AD.

METHODS

In this study, we included 57 patients with AD and 56 healthy controls. We evaluated their brain atrophy through magnetic resonance imaging using the medial temporal lobe atrophy score. We measured the levels of four synaptic proteins, including synaptosome-associated protein 25 (SNAP25), growth-associated protein 43 (GAP43), neurogranin, and synaptotagmin 1 in both plasma neuronal-derived EVs and cerebrospinal fluid (CSF). We further examined the association of synaptic protein levels with brain atrophy. We also evaluated the levels of these synaptic proteins in the brains of 5×FAD mice. Then, we loaded rabies virus glycoprotein-engineered EVs with messenger RNAs (mRNAs) encoding GAP43 and SNAP25 and administered these EVs to 5×FAD mice. After treatment, synaptic proteins, dendritic density, and cognitive function were evaluated.

RESULTS

The results showed that GAP43, SNAP25, neurogranin, and synaptotagmin 1 were decreased in neuronal-derived EVs but increased in CSF in patients with AD, and the changes corresponded to the severity of brain atrophy. GAP43 and SNAP25 were decreased in the brains of 5×FAD mice. The engineered EVs efficiently and stably delivered these synaptic proteins to the brain, where synaptic protein levels were markedly upregulated. Upregulation of synaptic protein expression could ameliorate cognitive impairment in AD by promoting dendritic density. This marks the first successful delivery of synaptic protein mRNAs via EVs in AD mice, yielding remarkable therapeutic effects.

CONCLUSIONS

Synaptic proteins are closely related to AD processes. Delivery of synaptic protein mRNAs via EVs stands as a promising effective precision treatment strategy for AD, which significantly advances the current understanding of therapeutic approaches for the disease.

Regulation of cargo exocytosis by a Reps1-Ralbp1-RalA module

Surface levels of membrane proteins are determined by a dynamic balance between exocytosis-mediated surface delivery and endocytosis-dependent retrieval from the cell surface. Imbalances in surface protein levels perturb surface protein homeostasis and cause major forms of human disease such as type 2 diabetes and neurological disorders. Here, we found a Reps1-Ralbp1-RalA module in the exocytic pathway broadly regulating surface protein levels. Reps1 and Ralbp1 form a binary complex that recognizes RalA, a vesicle-bound small guanosine triphosphatases (GTPase) promoting exocytosis through interacting with the exocyst complex. RalA binding results in Reps1 release and formation of a Ralbp1-RalA binary complex. Ralbp1 selectively recognizes GTP-bound RalA but is not a RalA effector. Instead, Ralbp1 binding maintains RalA in an active GTP-bound state. These studies uncovered a segment in the exocytic pathway and, more broadly, revealed a previously unrecognized regulatory mechanism for small GTPases, GTP state stabilization.

Family-based association study identifies SNAP25 as a susceptibility gene for autism in the Han Chinese population

Autism is a neurodevelopmental disorder with high heritability. Synaptosome associated protein 25 (SNAP25) encodes a presynaptic membrane-binding protein. It plays a crucial role in neurotransmission and may be involved in the pathogenesis of autism. However, the association between SNAP25 and autism in the Han Chinese population remains unclear. To investigate whether single nucleotide polymorphisms (SNPs) in SNAP25 contribute to the risk of autism, we performed a family-based association study of 14 tagSNPs in SNAP25 in 640 Han Chinese autism trios. Our results demonstrated that rs363018 in SNAP25 was significantly associated with autism under both additive (A > G, Z = 3.144, P = .0017) and recessive models (A > G, Z = 3.055, P = .0023) after Bonferroni correction (P < .0036). An additional SNP, rs8636, was nominally associated with autism under the recessive model (C > T, Z = 1.972, P = .0487). Haplotype-based association test revealed that haplotypes A-T (Z = 2.038, P = .0415) and G-T (Z = -3.114, P = .0018) of rs363018-rs362582 were significantly associated with autism after the permutation test (P = .0158). These findings suggest that SNAP25 may represent a susceptibility gene for autism in the Han Chinese population.

Developmental Time Course of SNAP-25 Isoforms Regulate Hippocampal Long-Term Synaptic Plasticity and Hippocampus-Dependent Learning

SNAP-25 is essential to activity-dependent vesicle fusion and neurotransmitter release in the nervous system. During early development and adulthood, SNAP-25 appears to have differential influences on short- and long-term synaptic plasticity. The involvement of SNAP-25 in these processes may be different at hippocampal and neocortical synapses because of the presence of two different splice variants, which are developmentally regulated. We show here that the isoform SNAP-25a, which is expressed first developmentally in rodent brain, contributes to developmental regulation of the expression of both long-term depression (LTD) and long-term potentiation (LTP) at Schaffer collateral-CA1 synapses in the hippocampus. In one month old mice lacking the developmentally later expressed isoform SNAP-25b, Schaffer collateral-CA1 synapses showed faster release kinetics, decreased LTP and enhanced LTD. By four months of age, SNAP-25b-deficient mice appeared to have compensated for the lack of the adult SNAP-25b isoform, now exhibiting larger LTP and no differences in LTD compared to wild type mice. Interestingly, learning a hippocampus-dependent task reversed the reductions in LTP, but not LTD, seen at one month of age. In four month old adult mice, learning prevented the compensatory up-regulation of LTD that we observed prior to training. These findings support the hypothesis that SNAP-25b promotes stronger LTP and weakens LTD at Schaffer collateral-CA1 synapses in young mice, and suggest that compensatory mechanisms can reverse alterations in synaptic plasticity associated with a lack of SNAP-25b, once mice reach adulthood.

Synaptic structure and alterations in the hippocampus in neonatal rats exposed to lipopolysaccharide

Synaptic structure integrity plays a key role in learning and memory. Previous studies have shown that there is cognitive dysfunction in septic neonates in later life. In this study, intraperitoneal injection of lipopolysaccharide (LPS) in the developing rats was used as a sepsis model to determine whether hippocampal synapses would be affected. Expression of synaptophysin (Syn), synaptosomal associated protein of 25 kD (SNAP-25), and N-methyl d-aspartate receptor (NMDAR) in the hippocampus in septic brain were significantly reduced. Consistent with this, the number of dendritic spines associated with the pyramidal neurons in the CA1 region of hippocampus at 28d after LPS administration was decreased. Additionally, the number of synapse and synaptic vesicles were reduced and appeared swollen. The number of neurons in the CA1 and CA3 of hippocampus at 14, and 28d after LPS injection remained unchanged. Coupled with the above was upregulated expression of interleukin-1β (IL-1β), IL-1 receptor 1 (IL-R1), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and inducible nitric oxide synthase (iNOS) at 1-3d after LPS injection. IL-1β expression was specifically detected in activated microglia. The plasma corticosterone (CORT) concentration in the LPS treatment rats was increased; but the glucocorticoid receptor (GR) expression in the hippocampus was decreased. We conclude that LPS injection in neonatal rats can cause synaptic disruption in the hippocampus which may be attributed to inflammatory response due to excess production of proinflammatory cytokines e.g., IL-1β derived from activated microglia. The significance of increased plasma CORT concentration and decreased GR expression in the hippocampus is discussed.

Botulinum neurotoxin A promotes functional recovery after peripheral nerve injury by increasing regeneration of myelinated fibers

The injection of safe doses of botulinum neurotoxin A (BoNT/A) have been reported to be useful for the treatment of neuropathic pain, but it is still unknown how functional recovery is induced after peripheral nerve injury. We evaluated the effects of intranerve application of BoNT/A, on regeneration and sensorimotor functional recovery in partial and complete peripheral nerve injuries in the mouse. After sciatic nerve crush (SNC) and intranerve delivery of BoNT/A (15pg), axonal regeneration was measured by nerve pinch test at different days. Regeneration of myelinated and unmyelinated fibers was assessed by immunohistochemical double labeling for NF200/GAP43 and CGRP/GAP43. S100 was used as Schwann cells marker. Medial footpad skin reinnervation was assessed by PGP staining. Motor functions were assessed by means of nerve conduction tests. In other mice groups, nerve conduction tests were performed also after chronic constriction injury (CCI) of the sciatic nerve and intraplantar injection of BoNT/A (15pg). In SNC mice, BoNT/A increased the rate of axonal regeneration. The advantage of regrowing myelinated axons after BoNT/A injection was evidenced by longer NF200+ nerve profiles and confirmed by nerve histology. We observed also a higher expression of S100 in the distal portion of BoNT/A-injected regenerated nerves. In CCI mice, BoNT/A induced an increase in reinnervation of gastrocnemius and plantar muscles. These results show that a low dose of BoNT/A, insufficient to produce muscular dysfunction, conversely speeds up sensorimotor recovery by stimulating myelinated axonal regeneration, and points out its application as a multipotent treatment for peripheral neuropathies.

Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity

During development, neurons progress through rapid yet stereotypical shape changes to achieve proper neuronal connectivity. This morphological progression requires carefully orchestrated plasma membrane expansion, insertion of membrane components including receptors for extracellular cues into the plasma membrane and removal and trafficking of membrane materials and proteins to specific locations. This review outlines the cellular machinery of membrane trafficking that play an integral role in neuronal cell shape change and function from initial neurite formation to pathway navigation and synaptogenesis.

Botulinum toxin A, brain and pain

Botulinum neurotoxin type A (BoNT/A) is one of the most potent toxins known and a potential biological threat. At the same time, it is among the most widely used therapeutic proteins used yearly by millions of people, especially for cosmetic purposes. Currently, its clinical use in certain types of pain is increasing, and its long-term duration of effects represents a special clinical value. Efficacy of BoNT/A in different types of pain has been found in numerous clinical trials and case reports, as well as in animal pain models. However, sites and mechanisms of BoNT/A actions involved in nociception are a matter of controversy. In analogy with well known neuroparalytic effects in peripheral cholinergic synapses, presently dominant opinion is that BoNT/A exerts pain reduction by inhibiting peripheral neurotransmitter/inflammatory mediator release from sensory nerves. On the other hand, growing number of behavioral and immunohistochemical studies demonstrated the requirement of axonal transport for BoNT/A's antinociceptive action. In addition, toxin's enzymatic activity in central sensory regions was clearly identified after its peripheral application. Apart from general pharmacology, this review summarizes the clinical and experimental evidence for BoNT/A antinociceptive activity and compares the data in favor of peripheral vs. central site and mechanism of action. Based on literature review and published results from our laboratory we propose that the hypothesis of peripheral site of BoNT/A action is not sufficient to explain the experimental data collected up to now.

Synaptosomal-associated protein 25 mutation induces immaturity of the dentate granule cells of adult mice

BACKGROUND

Synaptosomal-associated protein, 25 kDa (SNAP-25) regulates the exocytosis of neurotransmitters. Growing evidence suggests that SNAP-25 is involved in neuropsychiatric disorders, such as schizophrenia, attention-deficit/hyperactivity disorder, and epilepsy. Recently, increases in anxiety-related behaviors and epilepsy have been observed in SNAP-25 knock-in (KI) mice, which have a single amino acid substitution of Ala for Ser187. However, the molecular and cellular mechanisms underlying the abnormalities in this mutant remain unknown.

RESULTS

In this study, we found that a significant number of dentate gyrus (DG) granule cells was histologically and electrophysiologically similar to immature DG neurons in the dentate gyrus of the adult mutants, a phenomenon termed the "immature DG" (iDG). SNAP-25 KI mice and other mice possessing the iDG phenotype, i.e., alpha-calcium/calmodulin-dependent protein kinase II heterozygous mice, Schnurri-2 knockout mice, and mice treated with the antidepressant fluoxetine, showed similar molecular expression patterns, with over 100 genes similarly altered. A working memory deficit was also identified in mutant mice during a spontaneous forced alternation task using a modified T-maze, a behavioral task known to be dependent on hippocampal function. Chronic treatments with the antiepileptic drug valproate abolished the iDG phenotype and the working memory deficit in mutants.

CONCLUSIONS

These findings suggest that the substitution of Ala for Ser187 in SNAP-25 induces the iDG phenotype, which can also be caused by epilepsy, and led to a severe working memory deficit. In addition, the iDG phenotype in adulthood is likely an endophenotype for at least a part of some common psychiatric disorders.

miR-153 regulates SNAP-25, synaptic transmission, and neuronal development

SNAP-25 is a core component of the trimeric SNARE complex mediating vesicle exocytosis during membrane addition for neuronal growth, neuropeptide/growth factor secretion, and neurotransmitter release during synaptic transmission. Here, we report a novel microRNA mechanism of SNAP-25 regulation controlling motor neuron development, neurosecretion, synaptic activity, and movement in zebrafish. Loss of miR-153 causes overexpression of SNAP-25 and consequent hyperactive movement in early zebrafish embryos. Conversely, overexpression of miR-153 causes SNAP-25 down regulation resulting in near complete paralysis, mimicking the effects of treatment with Botulinum neurotoxin. miR-153-dependent changes in synaptic activity at the neuromuscular junction are consistent with the observed movement defects. Underlying the movement defects, perturbation of miR-153 function causes dramatic developmental changes in motor neuron patterning and branching. Together, our results indicate that precise control of SNAP-25 expression by miR-153 is critically important for proper neuronal patterning as well as neurotransmission.

Type VI adenylyl cyclase regulates neurite extension by binding to Snapin and Snap25

3'-5'-Cyclic AMP (cAMP) is an important second messenger which regulates neurite outgrowth. We demonstrate here that type VI adenylyl cyclase (AC6), an enzyme which catalyzes cAMP synthesis, regulates neurite outgrowth by direct interaction with a binding protein (Snapin) of Snap25 at the N terminus of AC6 (AC6-N). We first showed that AC6 expression increased during postnatal brain development. In primary hippocampal neurons and Neuro2A cells, elevated AC6 expression suppressed neurite outgrowth, whereas the downregulation or genetic removal of AC6 promoted neurite extension. An AC6 variant (AC6-N5) that contains the N terminus of AC5 had no effect, indicating the importance of AC6-N. The downregulation of endogenous Snapin or the overexpression of a Snapin mutant (Snap(Δ33-51)) that does not bind to AC6, or another Snapin mutant (Snapin(S50A)) that does not interact with Snap25, reversed the inhibitory effect of AC6. Pulldown assays and immunoprecipitation-AC assays revealed that the complex formation of AC6, Snapin, and Snap25 is dependent on AC6-N and the phosphorylation of Snapin. The overexpression of Snap25 completely reversed the action of AC6. Collectively, in addition to cAMP production, AC6 plays a complex role in modulating neurite outgrowth by redistributing localization of the SNARE apparatus via its interaction with Snapin.

Differential expression of SNAP-25 family proteins in the mouse brain

Soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP)-25 is a neuronal SNARE protein essential for neurotransmitter release from presynaptic terminals. Three palmitoylated SNAP-25 family proteins: SNAP-25a, SNAP-25b, and SNAP-23, are expressed in the brain, but little is known about their distributions and functions. In the present study, we generated specific antibodies to distinguish these three homologous proteins. Immunoblot and immunohistochemical analyses revealed that SNAP-25b was distributed in synapse-enriched regions throughout almost the entire brain, whereas SNAP-25a and SNAP-23 were expressed in relatively specific brain regions with partially complementary expression patterns. SNAP-25a and SNAP-25b, but not SNAP-23, were also present in the axoplasm of nerve fibers. The intracellular localization was also different, and although SNAP-25b and SNAP-23 were found primarily in membrane and lipid raft-enriched fractions of mouse brain homogenates, a substantial amount of SNAP-25a was recovered in soluble fractions. In PC12 cells, SNAP-25b was localized to the plasma membrane, but SNAP-25a and SNAP-23 were distributed throughout the cytoplasm. The expression and distribution of these three proteins were also differentially regulated in the early postnatal period. These results indicate that the three SNAP-25 family proteins display a differential distribution in the brain as well as in neuronal cells, and possibly play distinct roles.

Munc18 and Munc13 regulate early neurite outgrowth

BACKGROUND INFORMATION

During development, growth cones of outgrowing neurons express proteins involved in vesicular secretion, such as SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins, Munc13 and Munc18. Vesicles are known to fuse in growth cones prior to synapse formation, which may contribute to outgrowth.

RESULTS

We tested this possibility in dissociated cell cultures and organotypic slice cultures of two release-deficient mice (Munc18-1 null and Munc13-1/2 double null). Both types of release-deficient neurons have a decreased outgrowth speed and therefore have a smaller total neurite length during early development [DIV1-4 (day in vitro 1-4)]. In addition, more filopodia per growth cone were observed in Munc18-1 null, but not WT (wild-type) or Munc13-1/2 double null neurons. The smaller total neurite length during early development was no longer observed after synaptogenesis (DIV14-23).

CONCLUSION

These data suggest that the inability of vesicle fusion in the growth cone affects outgrowth during the initial phases when outgrowth speed is high, but not during/after synaptogenesis. Overall, the outgrowth speed is probably not rate-limiting during neuronal network formation, at least in vitro. In addition, Munc18, but not Munc13, regulates growth cone filopodia, potentially via its previously observed effect on filamentous actin.

Opposing functions of two sub-domains of the SNARE-complex in neurotransmission

The SNARE-complex consisting of synaptobrevin-2/VAMP-2, SNAP-25 and syntaxin-1 is essential for evoked neurotransmission and also involved in spontaneous release. Here, we used cultured autaptic hippocampal neurons from Snap-25 null mice rescued with mutants challenging the C-terminal, N-terminal and middle domains of the SNARE-bundle to dissect out the involvement of these domains in neurotransmission. We report that the stabilities of two different sub-domains of the SNARE-bundle have opposing functions in setting the probability for both spontaneous and evoked neurotransmission. Destabilizing the C-terminal end of the SNARE-bundle abolishes spontaneous neurotransmitter release and reduces evoked release probability, indicating that the C-terminal end promotes both modes of release. In contrast, destabilizing the middle or deleting the N-terminal end of the SNARE-bundle increases both spontaneous and evoked release probabilities. In both cases, spontaneous release was affected more than evoked neurotransmission. In addition, the N-terminal deletion delays vesicle priming after a high-frequency train. We propose that the stability of N-terminal two-thirds of the SNARE-bundle has a function for vesicle priming and limiting spontaneous release.

Action of botulinum neurotoxins in the central nervous system: Antiepileptic effects

Botulinum neurotoxins (BoNTs) are metalloproteases which act on nerve terminals and cause a long-lasting inhibition of neurotransmitter release. BoNTs act by cleaving core proteins of the neurotransmitter release machinery, namely the SNARE (soluble NSF-attachment receptors) proteins. The action of BoNTs in the peripheral nervous system (PNS) has been extensively documented, and knowledge gained in this field laid the foundations for the use of BoNTs in human disorders characterized by hyperfunction of peripheral nerve terminals. Much less is known about the action of BoNTs on the central nervous system (CNS). In vitro studies have demonstrated that BoNTs can affect the release of several neurotransmitters from central neurons. Recent studies have provided the first characterization of the effects of BoNT/E on CNS neurons in vivo. It has been shown that BoNT/E injected into the rat hippocampus inhibits glutamate release and blocks spike activity of pyramidal neurons. Intrahippocampal injection of BoNT/E resulted in significant inhibition of seizure activity in experimental models of epilepsy, suggesting a potential therapeutic use of BoNTs in the CNS.

Plasma membrane expansion: a neuron's Herculean task

The formation of axons and dendrites and maintenance of the neuron's vastly expanded surface require the continuous addition of new membrane. This is achieved by membrane synthesis through the secretory pathway followed by regulated vesicle fusion with the plasma membrane, typically in the distal neurite. However, it is far from simple: multiple distinct membrane carriers are used to target specific membrane domains, dendrites seem to operate semi-autonomously from the rest of the neuron, and exocytosis for membrane expansion is different from that for release of synaptic vesicles. Current knowledge of this process and its implications for neuronal development, function and repair are reviewed.

Vasoactive intestinal peptide-induced neuritogenesis in neuroblastoma SH-SY5Y cells involves SNAP-25

Vasoactive intestinal peptide (VIP) is a neuropeptide known to regulate proliferation and differentiation in normal and tumoral cells. We previously reported that VIP induced neuritogenesis in human neuroblastoma SH-SY5Y cells cultured in serum-free medium. This neuritogenesis was associated with a regulated expression of neuronal cytoskeleton markers. To further characterize the neuroblastic cell differentiation induced by VIP in human SH-SY5Y cells, we investigated expression of synaptosomal-associated protein of 25 kDa (SNAP-25), a protein implicated in exocytosis associated with different processes, including neurite outgrowth. Western immunoblotting and real-time RT-PCR analyses revealed that VIP increased expression of the SNAP-25 protein and the level of both SNAP-25a and SNAP-25b mRNA isoforms. Immunofluorescence experiments indicated that SNAP-25 was mainly located in neurites and at the plasma membrane in SH-SY5Y cells treated with VIP. RNA interference experiments demonstrated that SNAP-25 was involved in VIP-induced neuritogenesis. In conclusion, SNAP-25 is up-regulated and implicated in neuritogenesis in human neuroblastoma SH-SY5Y cells treated with the neuropeptide VIP.

Acute stress regulation of neuroplasticity genes in mouse hippocampus CA3 area--possible novel signalling pathways

Stress exposure can lead to the precipitation of psychiatric disorders in susceptible individuals, but the molecular underpinnings are incompletely understood. We used forced swimming in mice to reveal stress-regulated genes in the CA3 area of the hippocampus. To determine changes in the transcriptional profile 4 h and 8 h after stress exposure microarrays were used in the two mouse strains C57BL/6J and DBA/2J, which are known for their differential stress response. We discovered a surprisingly distinct set of regulated genes for each strain and followed selected ones by in situ hybridisation. Our results support the concept of a phased transcriptional reaction to stress. Moreover, we suggest novel stress-elicited pathways, which comprise a number of genes involved in the regulation of neuronal plasticity. Furthermore, we focused in particular on dihydropyrimidinase like 2, to which we provide evidence for its regulation by NeuroD, an important factor for neuronal activity-dependent dendritic morphogenesis.

Thyroid hormone regulates the expression of SNAP-25 during rat brain development

Thyroid hormones are major regulators of postnatal brain development. Thyroid hormones act through nuclear receptors to modulate the expression of specific genes in the brain. We have used microarray analysis to identify novel responsive genes in 14-day-old hypothyroid rat brains, and discovered that synaptosomal-associated protein of 25 kDa (SNAP-25) was one of the thyroid hormone-responsive genes. SNAP-25 is a presynaptic plasma membrane protein and an integral component of the vesicle docking and fusion machinery mediating secretion of neurotransmitters and is required for neuritic outgrowth and synaptogenesis. Using microarray analysis we have shown that SNAP-25 was down-regulated in the hypothyroid rat brain compared with the age-matched controls. Real-time RT-PCR and western blotting analysis confirmed that SNAP-25 mRNA and protein levels decreased significantly in the developing hypothyroid rat brain. Our data suggest that in the developing rat brain, SNAP-25 expression is regulated by thyroid hormone, and thyroid hormone deficiency can cause decreased expression of SNAP-25 and this may on some level account for the impaired brain development seen in hypothyroidism.

Differential abilities of SNAP-25 homologs to support neuronal function

The SNAP receptor (SNARE) complex, consisting of synaptosome-associated protein of 25 kDa (SNAP-25), synaptobrevin-2, and syntaxin-1, is involved in synaptic vesicles exocytosis. In addition, SNAP-25 has been implicated in constitutive exocytosis processes required for neurite outgrowth. However, at least three isoforms of SNAP-25 have been reported from neurons: SNAP-23, which is also present in non-neuronal cells, and the two alternative splice variants SNAP-25a and SNAP-25b. Here, we studied the differential ability of these isoforms to support the functions previously broadly ascribed to "SNAP-25." We studied the rescue of snap-25 null neurons in culture with different SNAP-25 homologs. We find that deletion of SNAP-25 leads to strongly reduced neuron survival, and, in the few surviving cells, impaired arborization, reduced spontaneous release, and complete arrest of evoked release. Lentiviral expression of SNAP-25a, SNAP-25b, or SNAP-23 rescued neuronal survival, arborization, amplitude, and frequency of spontaneous events. Also evoked release was rescued by all isoforms, but synchronous release required SNAP-25a/b in both glutamatergic and GABAergic neurons. SNAP-23 supported asynchronous release only, reminiscent of synaptotagmin-1 null neurons. SNAP-25b was superior to SNAP-25a in vesicle priming, resembling the shift to larger releasable vesicle pools that accompanies synaptic maturation. These data demonstrate a differential ability of SNAP-25b, SNAP-25a, and SNAP-23 to support neuronal function.

Neurexin Ibeta and neuroligin are localized on opposite membranes in mature central synapses

Synaptogenesis requires formation of trans-synaptic complexes between neuronal cell-adhesion receptors. Heterophilic receptor pairs, such as neurexin Ibeta and neuroligin, can mediate distinct intracellular signals and form different cytoplasmic scaffolds in the pre- and post-synaptic neuron, and may be particularly important for synaptogenesis. However, the functions of neurexin and neuroligin depend on their distribution in the synapse. Neuroligin has been experimentally assigned to the post-synaptic membrane, while the localization of neurexin remains unclear. To study the subcellular distribution of neurexin Ibeta and neuroligin in mature cerebrocortical synapses, we have developed a novel method for the physical separation of junctional membranes and their direct analysis by western blotting. Using urea and dithiothreitol, we disrupted trans-synaptic protein links, without dissolving the lipid phase, and fractionated the pre- and post-synaptic membranes. The purity of these fractions was validated by electron microscopy and western blotting using multiple synaptic markers. A quantitative analysis has confirmed that neuroligin is localized strictly in the post-synaptic membrane. We have also demonstrated that neurexin Ibeta is largely (96%) pre-synaptic. Thus, neurexin Ibeta and neuroligin normally form trans-synaptic complexes and can transduce bidirectional signals.

ARF6 in the nervous system

Actin cytoskeleton dynamics and membrane trafficking are tightly connected and are among the most important driving forces of neuronal development, basic synaptic transmission events, and synaptic plasticity. One group of proteins involved in coordination of these two processes is the family of ADP ribosylation factors (ARFs) regulating actin dynamics, lipid modification and membrane trafficking. ARF6 is the only member of the ARF family that can simultaneously regulate actin cytoskeleton changes and membrane exchange between plasma membrane and endocytic compartments. The presence of ARF6 and its guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) in the brain, as well as its capability to regulate several aspects of neuronal development and synaptic plasticity, has been recently demonstrated. The main purpose of this review is to present the current knowledge about how ARF6 can influence morphological processes crucial for proper formation of the neuronal circuits in the brain, including dendrite and axon differentiation, development of dendritic arbor complexity and dendritic spine formation. Potential effects of ARF6 on synaptic events resulting from its ability to control exo- and endocytosis will be also discussed.

The SNAP-25 gene is associated with cognitive ability: evidence from a family-based study in two independent Dutch cohorts

The synaptosomal-associated protein of 25 kDa (SNAP-25) gene plays an integral role in synaptic transmission, and is differentially expressed in the mammalian brain in the neocortex, hippocampus, anterior thalamic nuclei, substantia nigra and cerebellar granular cells. Recent studies have suggested a possible involvement of SNAP-25 in learning and memory, both of which are key components of human intelligence. In addition, the SNAP-25 gene lies in a linkage area implicated previously in human intelligence. In two independent family-based Dutch samples of 391 (mean age 12.4 years) and 276 (mean age 37.3 years) subjects, respectively, we genotyped 12 single-nucleotide polymorphisms (SNPs) in the SNAP-25 gene on 20p12-20p11.2. From all individuals, standardized intelligence measures were available. Using a family-based association test, a strong association was found between three SNPs in the SNAP-25 gene and intelligence, two of which showed association in both independent samples. The strongest, replicated association was found between SNP rs363050 and performance IQ (PIQ), where the A allele was associated with an increase of 2.84 PIQ points (P=0.0002). Variance in this SNP accounts for 3.4% of the phenotypic variance in PIQ.

SNAP-25 in hippocampal CA3 region is required for long-term memory formation

SNAP-25 is a synaptosomal protein of 25 kDa, a key component of synaptic vesicle-docking/fusion machinery, and plays a critical role in exocytosis and neurotransmitter release. We previously reported that SNAP-25 in the hippocampal CA1 region is involved in consolidation of contextual fear memory and water-maze spatial memory (Hou et al. European J Neuroscience, 20: 1593-1603, 2004). SNAP-25 is expressed not only in the CA1 region, but also in the CA3 region, and the SNAP-25 mRNA level in the CA3 region is higher than in the CA1 region. Here, we provide evidence that SNAP-25 in the CA3 region is also involved in learning/memory. Intra-CA3 infusion of SNAP-25 antisense oligonucleotide impaired both long-term contextual fear memory and water-maze spatial memory, with short-term memory intact. Furthermore, the SNAP-25 antisense oligonucleotide suppressed the long-term potentiation (LTP) of field excitatory post-synaptic potential (fEPSP) in the mossy-fiber pathway (DG-CA3 pathway), with no effect on paired-pulse facilitation of the fEPSP. These results are consistent with the notion that SNAP-25 in the hippocampal CA3 region is required for long-term memory formation.

A role for synaptotagmin VII-regulated exocytosis of lysosomes in neurite outgrowth from primary sympathetic neurons

Neurite outgrowth is mediated by the exocytosis of intracellular vesicles at the tips of elongating neuronal processes. The lysosomal vesicle-associated soluble N-ethylmaleimide-sensitive factor attachment protein receptor tetanus neurotoxin insensitive vesicle-associated membrane protein (TI-VAMP)/VAMP7 was previously implicated in membrane fusion events mediating neurite outgrowth, but the participation of lysosomes in this exocytic process has remained unclear. Here, we show that VAMP7 and the lysosomal glycoprotein Lamp1 extensively colocalize in vesicles present throughout the soma and neurite outgrowths of primary sympathetic neurons. Synaptotagmin VII (Syt VII), a Ca(2+)-sensing synaptotagmin isoform previously shown to interact with VAMP7 during lysosomal exocytosis in fibroblasts, was detected on a subset of these lysosomal glycoprotein 1 (Lamp1)/VAMP7-positive neuronal vesicles. Ionophore-stimulated exocytosis triggered exposure of the luminal domains of both Lamp1 and Syt VII at overlapping sites on the neuronal surface, indicating that the Syt VII-containing lysosomal compartments fuse with the plasma membrane in response to [Ca2+]i elevation. To determine whether Syt VII was required for the exocytic events mediating neurite extension, we followed the development of superior cervical ganglion neurons explanted from Syt VII-deficient mice. The results revealed a marked defect in neurite outgrowth and arborization, suggesting that Ca(2+)-dependent, Syt VII-regulated exocytosis of late endosomes/lysosomes plays a role in the addition of new membrane to developing neurite extensions.

A second SNARE role for exocytic SNAP25 in endosome fusion

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins play key roles in membrane fusion, but their sorting to specific membranes is poorly understood. Moreover, individual SNARE proteins can function in multiple membrane fusion events dependent upon their trafficking itinerary. Synaptosome-associated protein of 25 kDa (SNAP25) is a plasma membrane Q (containing glutamate)-SNARE essential for Ca2+-dependent secretory vesicle-plasma membrane fusion in neuroendocrine cells. However, a substantial intracellular pool of SNAP25 is maintained by endocytosis. To assess the role of endosomal SNAP25, we expressed botulinum neurotoxin E (BoNT E) light chain in PC12 cells, which specifically cleaves SNAP25. BoNT E expression altered the intracellular distribution of SNAP25, shifting it from a perinuclear recycling endosome to sorting endosomes, which indicates that SNAP25 is required for its own endocytic trafficking. The trafficking of syntaxin 13 and endocytosed cargo was similarly disrupted by BoNT E expression as was an endosomal SNARE complex comprised of SNAP25/syntaxin 13/vesicle-associated membrane protein 2. The small-interfering RNA-mediated down-regulation of SNAP25 exerted effects similar to those of BoNT E expression. Our results indicate that SNAP25 has a second function as an endosomal Q-SNARE in trafficking from the sorting endosome to the recycling endosome and that BoNT E has effects linked to disruption of the endosome recycling pathway.

Cdc42 and actin control polarized expression of TI-VAMP vesicles to neuronal growth cones and their fusion with the plasma membrane

Tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP)-mediated fusion of intracellular vesicles with the plasma membrane is crucial for neurite outgrowth, a pathway not requiring synaptobrevin-dependent exocytosis. Yet, it is not known how the TI-VAMP membrane trafficking pathway is regulated or how it is coordinated with cytoskeletal dynamics within the growth cone that guide neurite outgrowth. Here, we demonstrate that TI-VAMP, but not synaptobrevin 2, concentrates in the peripheral, F-actin-rich region of the growth cones of hippocampal neurons in primary culture. Its accumulation correlates with and depends upon the presence of F-actin. Moreover, acute stimulation of actin remodeling by homophilic activation of the adhesion molecule L1 induces a site-directed, actin-dependent recruitment of the TI-VAMP compartment. Expression of a dominant-positive mutant of Cdc42, a key regulator of cell polarity, stimulates formation of F-actin- and TI-VAMP-rich filopodia outside the growth cone. Furthermore, we report that Cdc42 activates exocytosis of pHLuorin tagged TI-VAMP in an actin-dependent manner. Collectively, our data suggest that Cdc42 and regulated assembly of the F-actin network control the accumulation and exocytosis of TI-VAMP-containing membrane vesicles in growth cones to coordinate membrane trafficking and actin remodeling during neurite outgrowth.

Synapse composition and organization following chronic activity blockade in cultured hippocampal neurons

Activity plays multiple roles in the expression of synaptic plasticity, and has been shown to regulate the localization of both neurotransmitter receptors and downstream signaling machinery. However, the role of activity in central synapse formation and organization is incompletely understood. Some studies indicate that synapse formation can occur in the absence of synaptic activity, while others indicate that activity is required for synapse maintenance and receptor recruitment. In addition, the effects of long-term blockade of transmission generally, rather than blockade of specific receptors, on postsynaptic protein complement has been poorly characterized. In order to address the role of activity in synapse formation and postsynaptic specialization, we used tetanus toxin to chronically cleave VAMP2 and inhibit SNARE-mediated neurotransmitter release in cultured hippocampal neurons. Although these neurons are deficient in synaptic release, they are of normal size and morphology. In addition, both excitatory and inhibitory synapses form along their processes with normal density. These synapses have a remarkably similar cellular and molecular organization compared to controls, and are capable of recruiting postsynaptic scaffolding proteins, GABA, and glutamate receptors. Subcellular enrichment of synaptic proteins into specialized domains also appears intact. These data indicate that global activity inhibition is insufficient to disrupt central synapse formation or organization.

Differential appearance of dynamin in constitutive and regulated exo-endocytosis: a single-cell multiplex RT-PCR study

Neurons in the central nervous system establish, via their axons and dendrites, an extended network that allows synaptic transmission. During developmental maturation and process outgrowth, membrane turnover is necessary for the enlargement and subsequent growth of axons and dendrites from the perikarya to the target cell (constitutive exocytosis/endocytosis). After targeting and synapse formation, small synaptic vesicles are needed for the quantal release of neurotransmitters from the presynaptic terminal with subsequent recycling by regulated exocytosis/endocytosis. An investigation of the onset of the appearance of mRNA and protein in dissociated cultures of neurons from mouse hippocampus or from chick retina has shown an early abundance of proteins involved in exocytosis, such as syntaxin 1, SNAP-25, and synaptotagmin 1, whereas dynamin 1, a protein necessary for clathrin-mediated endocytosis, can be detected only after neurons have established contacts with neighboring cells. The results reveal that constitutive membrane incorporation and regulated synaptic transmitter release is mediated by the same neuronal proteins. Moreover, the data exclude that dynamin 1 takes part in constitutive recycling before synapse formation, but dynamin 2 is present at this stage. Thus, dynamin 2 may be the constitutive counterpart of dynamin 1 in growing neurons. Synapse establishment is linked to an upregulation of dynamin 1 and thereby represents the beginning of the regulated recycling of membranes back into the presynaptic terminal.

Axonal sorting of Kir3.3 defines a GABA-containing neuron in the CA3 region of rodent hippocampus

Hippocampal interneurons comprise a heterogeneous group of locally acting GABAergic neurons. In addition to their variability in cotransmitter content and receptor profile, they express a variety of potassium channels that specify their individual properties. Here we describe a new type of large GABA-containing neuron in rodent hippocampus that is characterized by an axonal sorting of the potassium channel Kir3.3. The parent cell bodies of the Kir3.3-positive axons are located in CA3, as assessed by primary cultures derived from hippocampal subareas. At postnatal day 14 these neurons appear at the border between stratum oriens and stratum pyramidale of CA3, from where their axons pass through stratum pyramidale to join the mossy fiber tract. In adult hippocampus, high levels of Kir3.3 channel protein exist in axons that run with the mossy fiber tract. Kir3.3 and the vesicular GABA transporter could be identified in subpopulations of large synaptic terminals that probably derive from Kir3.3 neurons. Axonal sorting of Kir3.3 appears to be typical of a group of large inhibitory neurons, including Purkinje cells and a novel type of interneuron in CA3. Kir3.3 neurons might modulate the activity of CA3 circuitries and consequently memory processing in the hippocampus.

The cholesterol-lowering drug probucol increases apolipoprotein E production in the hippocampus of aged rats: implications for Alzheimer's disease

Several recent epidemiological studies have proposed that cholesterol-lowering drug Statin may provide protection against Alzheimer's disease (AD). Probucol is a non-Statin cholesterol-lowering drug and a potent inducer of apolipoprotein E (apoE) production in peripheral circulation. A recent clinical study using Probucol in elderly AD subjects revealed a concomitant stabilisation of cognitive symptoms and significant increases in apoE levels in the cerebral spinal fluid in these patients. To gain insight into the mechanisms underlying these effects, we treated a cohort of aged male rats (26-month-old) with oral dose of Probucol for 30 days. Specifically, we examined the effects of Probucol on apoE production and its receptors (low density lipoprotein receptor [LDLr] and low density lipoprotein receptor-related protein [LRP]), astroglial marker of cell damage (glial fibrillary acidic protein [GFAP]), markers of neuronal synaptic plasticity and integrity (synaptosomal associated protein of 25 kDa [SNAP-25] and synaptophysin) as well as cholesterol biosynthesis (3-hydroxy-3-methylglutaryl coenzyme A reductase [HMGCoAr]) in the hippocampus. We report that Probucol induces the production of apoE and one of its main receptors, LRP, increases HMGCoAr (rate-limiting enzyme in cholesterol synthesis), substantially attenuates age-related increases in glial activation, and induces production of synaptic marker SNAP-25, a molecule commonly associated with synaptogenesis and dendritic remodeling. These findings suggest that Probucol could promote neural and synaptic plasticity to counteract the synaptic deterioration associated with brain aging through an apoE/LRP-mediated system. Consistent with the beneficial effects of other cholesterol-lowering drugs such as the Statin, Probucol could also offers additional benefits based on apoE neurobiology.

Tetanus neurotoxin-insensitive vesicle-associated membrane protein localizes to a presynaptic membrane compartment in selected terminal subsets of the rat brain

Tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) is a vesicular soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptor (SNARE) that has been implicated in neurite outgrowth. It has previously been reported that TI-VAMP is localised in the somatodendritic compartment of neurons indicating a role in membrane fusion events within dendrites. Using a newly produced monoclonal antibody to TI-VAMP that improves signal/noise immunodetection, we report that TI-VAMP is also present in subsets of axon terminals of the adult rat brain. Four distinctive populations of labelled axon terminals were identified: 1) the hippocampal mossy fibres of the dentate gyrus and of CA3, 2) the striatal peridendritic terminal plexuses in the globus pallidus (GP), substantia nigra pars reticulata (SNr), 3) peridendritic plexuses in the central nucleus of the amygdala, and 4) the primary sensory afferents in the dorsal horn of the spinal cord. The presynaptic localisation of TI-VAMP in these locations was demonstrated by co-localisation with synaptophysin. Ultrastructural studies showed TI-VAMP labelling over synaptic vesicles in the mossy fibres, whereas it was localised in tubulo-vesicular structures and multivesicular bodies in the pyramidal cell dendrites. The presynaptic localisation of TI-VAMP occurred by P15, so relatively late during development. In contrast, dendritic labelling was most prominent during the early post-natal period. Co-localisation with markers of neurotransmitters showed that TI-VAMP-positive terminals are GABAergic in the GP and SNr and glutamatergic in the mossy fibre system and in the dorsal root afferents. Most of these terminals are known to co-localise with neuropeptides. We found met-enkephalin-immunoreactivity in a sizeable fraction of the TI-VAMP positive terminals in the GP, amygdala, and dorsal horn, as well as in a few mossy fibre terminals. The function of TI-VAMP in subsets of mature axon terminals remains to be elucidated; it could participate in the exocytotic molecular machinery and/or be implicated in particular growth properties of the mature axon terminals. Thus, the presence of TI-VAMP in the mossy fibres may correspond to the high degree of plasticity that characterises this pathway throughout adult life.

Neuronal Polarity and Trafficking

Among the most morphologically complex cells, neurons are masters of membrane specialization. Nowhere is this more striking than in the division of cellular labor between the axon and the dendrites. In morphology, signaling properties, cytoskeletal organization, and physiological function, axons and dendrites (or more properly, the somatodendritic compartment) are radically different. Such polarization of neurons into domains specialized for either receiving (dendrites) or transmitting (axons) cellular signals provides the underpinning for all neural circuitry. The initial specification of axonal and dendritic identity occurs early in neuronal life, persists for decades, and is manifested by the presence of very different sets of cell surface proteins. Yet, how neuronal polarity is established, how distinct axonal and somatodendritic domains are maintained, and how integral membrane proteins are directed to dendrites or accumulate in axons remain enduring and formidable questions in neuronal cell biology.

Cross talk between tetanus neurotoxin-insensitive vesicle-associated membrane protein-mediated transport and L1-mediated adhesion

The membrane-trafficking pathway mediated by tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) in neurons is still unknown. We show herein that TI-VAMP expression is necessary for neurite outgrowth in PC12 cells and hippocampal neurons in culture. TI-VAMP interacts with plasma membrane and endosomal target soluble N-ethylmaleimide-sensitive factor attachment protein receptors, suggesting that TI-VAMP mediates a recycling pathway. L1, a cell-cell adhesion molecule involved in axonal outgrowth, colocalized with TI-VAMP in the developing brain, neurons in culture, and PC12 cells. Plasma membrane L1 was internalized into the TI-VAMP-containing compartment. Silencing of TI-VAMP resulted in reduced expression of L1 at the plasma membrane. Finally, using the extracellular domain of L1 and N-cadherin immobilized on beads, we found that the silencing of TI-VAMP led to impaired L1- but not N-cadherin-mediated adhesion. Furthermore, TI-VAMP- but not synaptobrevin 2-containing vesicles accumulated at the site of the L1 bead-cell junction. We conclude that TI-VAMP mediates the intracellular transport of L1 and that L1-mediated adhesion controls this membrane trafficking, thereby suggesting an important cross talk between membrane trafficking and cell-cell adhesion.

TUC-4b, a novel TUC family variant, regulates neurite outgrowth and associates with vesicles in the growth cone

The TUC (TOAD-64/Ulip/CRMP) proteins are homologs of UNC-33, a protein that is required for axon extension and guidance in Caenorhabditis elegans. The TUC proteins are expressed in newly born neurons in the developing nervous system and have been implicated in semaphorin signaling and neuronal polarity. Here, we identify several new variants of the TUC family, each of which is expressed during distinct periods of neural development. We cloned and characterized TUC-4b, a variant of TUC-4a that includes a unique N-terminal extension. The functional relevance of this N-terminal domain is demonstrated by the finding that overexpression of TUC-4b, but not TUC-4a, results in increased neurite length and branching. Furthermore, whereas TUC-4a is expressed throughout life, TUC-4b is expressed exclusively during embryonic development. TUC-4b is localized to SV2 (synaptic vesicle protein 2)-positive vesicles in the central domain of the growth cone, suggesting a potential role in growth cone vesicle transport. Furthermore, TUC-4b interacts with the SH3A (Src homology 3A) domain of intersectin, an endocytic-exocytic adaptor protein. Together, these data suggest that TUC-4b can regulate neurite extension and branching through a mechanism that may involve membrane transport in the growth cone.

Targeted trafficking of neurotransmitter receptors to synaptic sites

Emerging data are sheding light on the critical task for synapses to locally control the production of neurotransmitter receptors ultimately leading to receptor accumulation and modulation at postsynaptic sites. By analogy with the epithelial-cell paradigm, the postsynaptic compartment may be regarded as a polarized domain favoring the selective recruitment and retention of newly delivered receptors at synaptic sites. Targeted delivery of receptors to synaptic sites is facilitated by a local organization of the exocytic pathway, likely resulting from spatial cues triggered by the nerve. This review focuses on the various mechanisms responsible for regulation of receptor assembly and trafficking. A particular emphasis is given to the role of synaptic anchoring and scaffolding proteins in the sorting and routing of their receptor companion along the exocytic pathway. Other cellular components such as lipidic microdomains, the docking and fusion machinery, and the cytoskeleton also contribute to the dynamics of receptor trafficking at the synapse.

Differential expression of neuronal calcium sensor-1 in the developing chick retina

Neuronal calcium sensor-1 (NCS-1) is a Ca(2+) binding protein that has been implicated in the regulation of neurotransmission and synaptogenesis. In this study we investigated the developmental expression and localization of NCS-1 in the chick retina. Single- and double-labeling experiments with three-dimensional reconstruction as well as ultrastructural data of the distribution of NCS-1 suggest that this protein is also involved in axonal process outgrowth. We found an early expression of NCS-1 in ganglion cells and their axons, in amacrine, and in horizontal cells, whereas photoreceptors were immunonegative at embryonic stages. In the early posthatching days we found strong immunostaining for NCS-1 in horizontal cells and their processes in the outer plexiform layer. In contrast, synaptic vesicle protein 2 (SV2) was prominent only in photoreceptor synaptic terminals. Ultrastructural analysis confirmed that NCS-1 was localized postsynaptically in horizontal cell processes, whereas presynaptic terminals were immunonegative. However, at late posthatching days we observed that photoreceptor ribbon synapses (from rods and/or cones) also expressed NCS-1. Thus the results support the notion that NCS-1 is involved in neuronal process outgrowth and is localized in pre- and postsynaptic compartments including mature photoreceptor synapses.

Changes in mRNA for VAMPs following facial nerve transection

Vesicle-associated membrane proteins (VAMPs) are involved in synaptic vesicle recycling and exocytosis in neurons. Here we report the changes in mRNA expression for VAMPs (VAMP1, -2 and -3) in the facial motor nucleus of adult rats following axotomy by in situ hybridization. Signals for VAMP2 and -3 mRNAs in the facial nucleus were increased from 3 to 28 days after axotomy. On the contrary, VAMP1 mRNA, which was abundantly expressed in the control facial nucleus, was transiently decreased from 3 to 21 days after axotomy. Differential regulation of VAMPs may reflect distinct roles in nerve regeneration.

Calcium-dependent translocation of synaptotagmin to the plasma membrane in the dendrites of developing neurones

In neurones, the morphological complexity of the dendritic tree requires regulated growth and the appropriate targeting of membrane components. Accurate delivery of specific supplies depends on the translocation and fusion of transport vesicles. Vesicle SNAREs (soluble N-ethylmaleimide sensitive factor attachment protein receptors) and target membrane SNAREs play a central role in the correct execution of fusion events, and mediate interactions with molecules that endow the system with appropriate regulation. Synaptotagmins, a family of Ca(2+)-sensor proteins that includes neurone-specific members involved in regulating neurotransmitter exocytosis, are among the molecules that can tune the fusion mechanism. Using immunocytochemistry, confocal and electron microscopy, the localisation of synaptotagmin I in the dendrites of cultured rat hypothalamic neurones was demonstrated. Synaptotagmin labelling is concentrated at dendritic branch points, and in microprocesses. Following depolarisation, the N-terminal domain of synaptotagmin was detected at the extracellular surface of the dendritic plasma membrane. The insertion of synaptotagmin in the plasma membrane was elicited by L-type Ca(2+) channel activation and by mobilisation of the internal ryanodine-sensitive Ca(2+)stores. Furthermore, the localisation of L-type Ca(2+) channels and of ryanodine receptors, relative to the localisation of synaptotagmin in dendrites, suggests that both Ca(2+) entry and intracellular Ca(2+) stores may contribute to the fusion of dendritic transport vesicles with the membrane. Fusion is likely to involve SNAP-25 and syntaxin 1 as both proteins were also identified in dendrites. Taken together these results suggest a putative regulatory role of synaptotagmins in the membrane fusion events that contribute to shaping the dendritic tree during development.

A common exocytotic mechanism mediates axonal and dendritic outgrowth

Outgrowth of the dendrites and the axon is the basis of the establishment of the neuronal shape, and it requires addition of new membrane to both growing processes. It is not yet clear whether one or two exocytotic pathways are responsible for the respective outgrowth of axons and dendrites. We have previously shown that tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) defines a novel network of tubulovesicular structures present both at the leading edge of elongating dendrites and axons of immature hippocampal neurons developing in primary culture and that TI-VAMP is an essential protein for neurite outgrowth in PC12 cells. Here we show that the expression of the N-terminal domain of TI-VAMP inhibits the outgrowth of both dendrites and axons in neurons in primary culture. This effect is more prominent at the earliest stages of the development of neurons in vitro. Expression of the N-terminal domain deleted form of TI-VAMP has the opposite effect. This constitutively active form of TI-VAMP localizes as the endogenous protein, particularly concentrating at the leading edge of growing axons. Our results suggest that a common exocytotic mechanism that relies on TI-VAMP mediates both axonal and dendritic outgrowth in developing neurons.

Adrenal gland SNAP-25 expression is altered in thyroid hormone receptor knock-out mice

SNAP-25 is a protein in neurons and neuroendocrine cells, which is involved, together with syntaxin and VAMP, in neurotransmitter release and neurite outgrowth. Since the thyroid hormone receptors TR alpha and TR beta are essential for nervous system development, their possible role in regulating the expression of these vesicle trafficking proteins was examined by analysing SNAP-25 levels in TR alpha and TR beta knock-out mice. Immunoblotting and RT-PCR showed that SNAP-25 levels are increased in the adrenal gland, but not in cerebellum, in knock-out mice, while syntaxin-1 and VAMP-2 are unaffected in either tissue. Treatment of the pheochromocytoma-derived cell line PC12 with the thyroid hormone L-3,5,3'-triiodothyronine (T3) decreased SNAP-25 expression. Together, these data suggest that thyroid hormones exert a negative regulatory effect on SNAP-25 in adrenal medullary neuroendocrine cells.

Synaptic membrane morphology in the rat cerebral cortex during development: image analysis of freeze-etching replicas of isolated synapses and synapses in situ

Both synaptic sites in situ from rat cerebral cortex and isolated ones in synaptosomal and growth cone fractions derived from it were studied during postnatal development. Freeze-etching technique and image analysis were used to determine the size of the intramembranous particles in the pre- and postsynaptic sites. At each age investigated, the greatest mean particle size was established on the E-face of the postsynaptic sites and ranges from 6.2 nm (day 0) to nearly 10 nm (day 90). The continuous mean particle size increase from birth to maturity shows the same rate but a different rhythm for the two synaptic sites. The results indicate that fractionation do not disturb the correlation between the particle size and age thus outlining the stability of the developing synaptic membrane morphology to physical treatments.

Synaptosome-associated protein of 25 kilodaltons in oocytes and steroid-producing cells of rat and human ovary: molecular analysis and regulation by gonadotropins

The synaptosome-associated protein of 25 kDa (SNAP-25) is crucially involved in exocytosis in neurons. The aim of this study was to investigate whether it is present in the ovary. We found SNAP-25 to be expressed in nonneuronal cells of the rat and human ovary, namely in all oocytes and in steroidogenic cells, including granulosa cells (GC) of large antral follicles and luteal cells. Both isoforms, SNAP-25a and b, were found in the ovary. Oocytes obtained by laser capture microdissection were shown to express SNAP-25b, whereas SNAP-25a was found in rat GC and human luteinized GC. Immunohistochemical observations of strong SNAP-25 staining in GC of large growing antral follicles compared with absent or weak staining in small follicles suggested a role in folliculogenesis. To study a presumed regulation of SNAP-25, we used a rat GC line (GFSHR-17), which expresses FSH receptors, and luteinizing human GC, which express LH receptors. FSH elevated SNAP-25 mRNA and protein levels about fivefold within 24 h in GFSHR-17 cells. The cAMP analogue dibutyryl-cAMP (db-cAMP) mimicked this action of FSH. The effects of both db-cAMP and FSH were inhibited by the protein kinase A (PKA) inhibitor H89. In contrast, SNAP-25 protein and mRNA-levels were not altered by LH/hCG in luteinized human GC. Our results for the first time identify SNAP-25b in oocytes and SNAP-25a in steroidogenic cells of the mammalian ovary. SNAP-25a and b may be involved in different exocytotic processes in these cell types.

Developmental expression of amphiphysin in the retinotectal system of the chick: from mRNA to protein

The role of amphiphysin in clathrin-mediated endocytosis of synaptic vesicles is well established. However, it is still uncertain if the protein is also involved in developmental mechanisms, e.g. axon outgrowth and synapse formation. To investigate the developmental changes in the expression of amphiphysin we used the retinotectal system of the chick, a highly ordered and easily accessible primary neuronal pathway. Reverse transcription polymerase chain reaction (RT-PCR) of total RNA from chick retina and tectum revealed first transcripts for amphiphysin, dynamin and synaptotagmin at embryonic day 5 (E5) for both regions. Surprisingly, Western blots of the retina revealed an increase of protein expression for amphiphysin only after E11 in the retina and the tectum. Immunofluorescence for amphiphysin was not detectable before E10 in the developing chick retina, while other presynaptic proteins like synaptotagmin showed already intense signals in the inner and outer plexiform layers. Subsequently, amphiphysin immunoreactivity follows the expression of synaptotagmin and synaptic vesicle protein 2 (SV2) as seen in the retina and the tectum, and exhibits the same staining as the other proteins in the mature chick brain. Ultrastructural data revealed for the first time that amphiphysin is not only limited to conventional synapses but is also abundant in retinal ribbon terminals. Taken together our data reveal that: (i) there is a developmental delay between mRNA transcription and protein expression for key proteins involved in endocytosis; (ii) amphiphysin gets upregulated after synapse formation; and (iii) amphiphysin is present in the synaptic vesicle cycle in retinal ribbon synapses.

Expression of Kv1 potassium channels in mouse hippocampal primary cultures: development and activity-dependent regulation

Excitability and discharge behavior of neurons depends on the highly variable expression pattern of voltage-dependent potassium (Kv) channels throughout the nervous system. To learn more about distribution, development, and activity-dependent regulation of Kv channel subunit expression in the rodent hippocampus, we studied the protein expression of members of the Kv1 subfamily in mouse hippocampus in situ and in primary cultures. In adult hippocampus, Kv1 (1-6) channel alpha-subunits were present, whereas at postnatal day 2, none of these proteins could be detected in CA1-CA3 and dentate gyrus. Kv1.1 was the first channel to be observed at postnatal day 6. The delayed postnatal expression and most of the subcellular distribution observed in hippocampal sections were mimicked by cultured hippocampal neurons in which Kv channels appeared only after 10 days in vitro. This developmental upregulation was paralleled by a dramatic increase in total K(+) current, as well as an elevated GABA release in the presence of 4-aminopyridine. Thus, the developmental profile, subcellular localization, and functionality of Kv1 channels in primary culture of hippocampus closely resembles the in situ situation. Impairing secretion by clostridial neurotoxins or blocking activity by tetrodotoxin inhibited the expression of Kv1.1, Kv1.2, and Kv1.4, whereas the other Kv1 channels still appeared. This activity-dependent depression was only observed before the initial appearance of the respective channels and lost after they had been expressed. Our data show that hippocampal neurons in culture are a convenient model to study the developmental expression and regulation of Kv1 channels. The ontogenetic regulation and the activity-dependent expression of Kv1.1, Kv1.2, and Kv1.4 indicate that neuronal activity plays a crucial role for the development of the mature Kv channel pattern in hippocampal neurons.