Spatial changes in calcium signaling during the establishment of neuronal polarity and synaptogenesis

Activity Dependent and Independent Determinants of Synaptic Size Diversity

The extraordinary diversity of excitatory synapse sizes is commonly attributed to activity-dependent processes that drive synaptic growth and diminution. Recent studies also point to activity-independent size fluctuations, possibly driven by innate synaptic molecule dynamics, as important generators of size diversity. To examine the contributions of activity-dependent and independent processes to excitatory synapse size diversity, we studied glutamatergic synapse size dynamics and diversification in cultured rat cortical neurons (both sexes), silenced from plating. We found that in networks with no history of activity whatsoever, synaptic size diversity was no less extensive than that observed in spontaneously active networks. Synapses in silenced networks were larger, size distributions were broader, yet these were rightward-skewed and similar in shape when scaled by mean synaptic size. Silencing reduced the magnitude of size fluctuations and weakened constraints on size distributions, yet these were sufficient to explain synaptic size diversity in silenced networks. Model-based exploration followed by experimental testing indicated that silencing-associated changes in innate molecular dynamics and fluctuation characteristics might negatively impact synaptic persistence, resulting in reduced synaptic numbers. This, in turn, would increase synaptic molecule availability, promote synaptic enlargement, and ultimately alter fluctuation characteristics. These findings suggest that activity-independent size fluctuations are sufficient to fully diversify glutamatergic synaptic sizes, with activity-dependent processes primarily setting the scale rather than the shape of size distributions. Moreover, they point to reciprocal relationships between synaptic size fluctuations, size distributions, and synaptic numbers mediated by the innate dynamics of synaptic molecules as they move in, out, and between synapses.SIGNIFICANCE STATEMENT Sizes of glutamatergic synapses vary tremendously, even when formed on the same neuron. This diversity is commonly thought to reflect the outcome of activity-dependent forms of synaptic plasticity, yet activity-independent processes might also play some part. Here we show that in neurons with no history of activity whatsoever, synaptic sizes are no less diverse. We show that this diversity is the product of activity-independent size fluctuations, which are sufficient to generate a full repertoire of synaptic sizes at correct proportions. By combining modeling and experimentation we expose reciprocal relationships between size fluctuations, synaptic sizes and synaptic counts, and show how these phenomena might be connected through the dynamics of synaptic molecules as they move in, out, and between synapses.

A Microfluidic Human Model of Blood-Brain Barrier Employing Primary Human Astrocytes

The neurovascular unit (NVU) is the most important biological barrier between vascular districts and central nervous system (CNS) parenchyma, which maintains brain homeostasis, protects the CNS from pathogens penetration, and mediates neuroimmune communication. T lymphocytes migration across the blood-brain barrier is heavily affected in different brain diseases, representing a major target for novel drug development. In vitro models of NVU could represent a primary tool to investigate the molecular events occurring at this interface. To move toward the establishment of personalized therapies, a patient-related NVU-model is set, incorporating human primary astrocytes integrated into a microfluidic platform. The model is morphologically and functionally characterized, proving to be an advantageous tool to investigate human T lymphocytes transmigration and thus the efficacy of potential novel drugs affecting this process.

Maternal Immune Activation Delays Excitatory-to-Inhibitory Gamma-Aminobutyric Acid Switch in Offspring

BACKGROUND

The association between maternal infection and neurodevelopmental defects in progeny is well established, although the biological mechanisms and the pathogenic trajectories involved have not been defined.

METHODS

Pregnant dams were injected intraperitoneally at gestational day 9 with polyinosinic:polycytidylic acid. Neuronal development was assessed by means of electrophysiological, optical, and biochemical analyses.

RESULTS

Prenatal exposure to polyinosinic:polycytidylic acid causes an imbalanced expression of the Na+-K+-2Cl- cotransporter 1 and the K+-Cl- cotransporter 2 (KCC2). This results in delayed gamma-aminobutyric acid switch and higher susceptibility to seizures, which endures up to adulthood. Chromatin immunoprecipitation experiments reveal increased binding of the repressor factor RE1-silencing transcription (also known as neuron-restrictive silencer factor) to position 509 of the KCC2 promoter that leads to downregulation of KCC2 transcription in prenatally exposed offspring. Interleukin-1 receptor type I knockout mice, which display braked immune response and no brain cytokine elevation upon maternal immune activation, do not display KCC2/Na+-K+-2Cl- cotransporter 1 imbalance when implanted in a wild-type dam and prenatally exposed. Notably, pretreatment of pregnant dams with magnesium sulfate is sufficient to prevent the early inflammatory state and the delay in excitatory-to-inhibitory switch associated to maternal immune activation.

CONCLUSIONS

We provide evidence that maternal immune activation hits a key neurodevelopmental process, the excitatory-to-inhibitory gamma-aminobutyric acid switch; defects in this switch have been unequivocally linked to diseases such as autism spectrum disorder or epilepsy. These data open the avenue for a safe pharmacological treatment that may prevent the neurodevelopmental defects caused by prenatal immune activation in a specific pregnancy time window.

Lack of IL-1R8 in neurons causes hyperactivation of IL-1 receptor pathway and induces MECP2-dependent synaptic defects

Inflammation modifies risk and/or severity of a variety of brain diseases through still elusive molecular mechanisms. Here we show that hyperactivation of the interleukin 1 pathway, through either ablation of the interleukin 1 receptor 8 (IL-1R8, also known as SIGIRR or Tir8) or activation of IL-1R, leads to up-regulation of the mTOR pathway and increased levels of the epigenetic regulator MeCP2, bringing to disruption of dendritic spine morphology, synaptic plasticity and plasticity-related gene expression. Genetic correction of MeCP2 levels in IL-1R8 KO neurons rescues the synaptic defects. Pharmacological inhibition of IL-1R activation by Anakinra corrects transcriptional changes, restores MeCP2 levels and spine plasticity and ameliorates cognitive defects in IL-1R8 KO mice. By linking for the first time neuronal MeCP2, a key player in brain development, to immune activation and demonstrating that synaptic defects can be pharmacologically reversed, these data open the possibility for novel treatments of neurological diseases through the immune system modulation.

DOI:http://dx.doi.org/10.7554/eLife.21735.001

Errors that occur while the brain is developing can lead to conditions such as autism and schizophrenia. They can also lead to rare disorders like Rett syndrome and MeCP2 duplication syndromes, which are characterized by severe cognitive and physical disabilities. Many people with these neurodevelopmental disorders have mutations in genes that encode proteins found at synapses, which are the junctions between neurons where the cells exchange information with one another. However, not everyone with these mutations develops a neurodevelopmental disorder, which indicates that other, non-genetic factors also play a part.

One of the main non-genetic factors that can influence the risk and severity of neurodevelopmental disorders is inflammation of the brain. Inflammation is a normal part of the body’s immune response to threats such as invading microorganisms or tissue damage. However, abnormal activation of the immune system in early life can trigger excessive inflammation. This increases the risk of a neurodevelopmental disorder, but it is not clear exactly how it does so.

Tomasoni et al. set out to test whether the missing link between inflammation and neurodevelopmental disorders might be damage to synapses. The experiments revealed that genetically modified mice with inflammation of the brain have abnormal synapses and are unable to learn properly. These mutant mice also have excessive levels of a protein that influences how synapses function called MeCP2, which is missing in the brains of people with Rett syndrome and abnormally increased in brains of patients affected by MeCP2 Duplication Syndrome. This is thus the first evidence that directly links inflammation of the brain to a synapse protein implicated in a disorder of brain development.

Tomasoni et al. also found that a drug called anakinra – which is used to treat an inflammatory disease called rheumatoid arthritis – reduced levels of MeCP2 in the mutant mice and improved their performance in cognitive tasks. Together, these results raise the possibility that anti-inflammatory medications may be beneficial in the treatment of neurodevelopment disorders.

DOI:http://dx.doi.org/10.7554/eLife.21735.002

Sphingosine-1-Phosphate (S1P) Impacts Presynaptic Functions by Regulating Synapsin I Localization in the Presynaptic Compartment

Growing evidence indicates that sphingosine-1-P (S1P) upregulates glutamate secretion in hippocampal neurons. However, the molecular mechanisms through which S1P enhances excitatory activity remain largely undefined. The aim of this study was to identify presynaptic targets of S1P action controlling exocytosis. Confocal analysis of rat hippocampal neurons showed that S1P applied at nanomolar concentration alters the distribution of Synapsin I (SynI), a presynaptic phosphoprotein that controls the availability of synaptic vesicles for exocytosis. S1P induced SynI relocation to extrasynaptic regions of mature neurons, as well as SynI dispersion from synaptic vesicle clusters present at axonal growth cones of developing neurons. S1P-induced SynI relocation occurred in a Ca(2+)-independent but ERK-dependent manner, likely through the activation of S1P3 receptors, as it was prevented by the S1P3 receptor selective antagonist CAY1044 and in neurons in which S1P3 receptor was silenced. Our recent evidence indicates that microvesicles (MVs) released by microglia enhance the metabolism of endogenous sphingolipids in neurons and stimulate excitatory transmission. We therefore investigated whether MVs affect SynI distribution and whether endogenous S1P could be involved in the process. Analysis of SynI immunoreactivity showed that exposure to microglial MVs induces SynI mobilization at presynaptic sites and growth cones, whereas the use of inhibitors of sphingolipid cascade identified S1P as the sphingolipid mediating SynI redistribution. Our data represent the first demonstration that S1P induces SynI mobilization from synapses, thereby indicating the phosphoprotein as a novel target through which S1P controls exocytosis.

SIGNIFICANCE STATEMENT

Growing evidence indicates that the bioactive lipid sphingosine and its metabolite sphingosine-1-P (S1P) stimulate excitatory transmission. While it has been recently clarified that sphingosine influences directly the exocytotic machinery by activating the synaptic vesicle protein VAMP2 to form SNARE fusion complexes, the molecular mechanism by which S1P promotes neurotransmission remained largely undefined. In this study, we identify Synapsin I, a presynaptic phosphoprotein involved in the control of availability of synaptic vesicles for exocytosis, as the key target of S1P action. In addition, we provide evidence that S1P can be produced at mature axon terminals as well as at immature growth cones in response to microglia-derived signals, which may be important to stabilize nascent synapses and to restore or potentiate transmission.

SNAP-25 regulates spine formation through postsynaptic binding to p140Cap

Synaptosomal-associated protein of 25 kDa (SNAP-25) is a member of the Soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNARE) protein family, required for exocytosis of synaptic vesicles and regulation of diverse ion channels. Here, we show that acute reduction of SNAP-25 expression leads to an immature phenotype of dendritic spines that are, consistently, less functional. Conversely, over-expression of SNAP-25 results in an increase in the density of mature, Postsynaptic Density protein 95 (PSD-95)-positive spines. The regulation of spine morphogenesis by SNAP-25 depends on the protein's ability to bind both the plasma membrane and the adaptor protein p140Cap, a key protein regulating actin cytoskeleton and spine formation. We propose that SNAP-25 allows the organization of the molecular apparatus needed for spine formation by recruiting and stabilizing p140Cap.

Nitric oxide synthase mediates PC12 differentiation induced by the surface topography of nanostructured TiO2

Background

Substrate nanoscale topography influences cell proliferation and differentiation through mechanisms that are at present poorly understood. In particular the molecular mechanism through which cells 'sense’ and adapt to the substrate and activate specific intracellular signals, influencing cells survival and behavior, remains to be clarified.

Results

To characterize these processes at the molecular level we studied the differentiation of PC12 cells on nanostructured TiO₂ films obtained by supersonic cluster beam deposition.

Our findings indicate that, in PC12 cells grown without Nerve Growth Factor (NGF), the roughness of nanostructured TiO₂ triggers neuritogenesis by activating the expression of nitric oxide synthase (NOS) and the phospho-extracellular signal-regulated kinase 1/2 (pERK1/2) signaling. Differentiation is associated with an increase in protein nitration as observed in PC12 cells grown on flat surfaces in the presence of NGF. We demonstrate that cell differentiation and protein nitration induced by topography are not specific for PC12 cells but can be regarded as generalized effects produced by the substrate on different neuronal-like cell types, as shown by growing the human neuroblastoma SH-SY5Y cell line on nanostructured TiO₂.

Conclusion

Our data provide the evidence that the nitric oxide (NO) signal cascade is involved in the differentiation process induced by nanotopography, adding new information on the mechanism and proteins involved in the neuritogenesis triggered by the surface properties.

Endogenous SNAP-25 regulates native voltage-gated calcium channels in glutamatergic neurons

In addition to its primary role as a fundamental component of the SNARE complex, SNAP-25 also modulates voltage-gated calcium channels (VGCCs) in various overexpression systems. Although these studies suggest a potential negative regulatory role of SNAP-25 on VGCC activity, the effects of endogenous SNAP-25 on native VGCC function in neurons are unclear. In the present study, we investigated the VGCC properties of cultured glutamatergic and GABAergic rat hippocampal neurons. Glutamatergic currents were dominated by P/Q-type channels, whereas GABAergic cells had a dominant L-type component. Also, glutamatergic VGCC current densities were significantly lower with enhanced inactivation rates and shifts in the voltage dependence of activation and inactivation curves compared with GABAergic cells. Silencing endogenous SNAP-25 in glutamatergic neurons did not alter P/Q-type channel expression or localization but led to increased VGCC current density without changes in the VGCC subtype proportions. Isolation of the P/Q-type component indicated that increased current in the absence of SNAP-25 was correlated with a large depolarizing shift in the voltage dependence of inactivation. Overexpressing SNAP-25 in GABAergic neurons reduced current density without affecting the VGCC subtype proportion. Accordingly, VGCC current densities in glutamatergic neurons from Snap-25(+/-) mice were significantly elevated compared with wild type glutamatergic neurons. Overall, this study demonstrates that endogenous SNAP-25 negatively regulates native VGCCs in glutamatergic neurons which could have important implications for neurological diseases associated with altered SNAP-25 expression.

Eps8 regulates axonal filopodia in hippocampal neurons in response to brain-derived neurotrophic factor (BDNF)

A novel signaling cascade controlling actin polymerization in response to extracellular signals regulates filopodia formation and likely also neuronal synapse formation.

The regulation of filopodia plays a crucial role during neuronal development and synaptogenesis. Axonal filopodia, which are known to originate presynaptic specializations, are regulated in response to neurotrophic factors. The structural components of filopodia are actin filaments, whose dynamics and organization are controlled by ensembles of actin-binding proteins. How neurotrophic factors regulate these latter proteins remains, however, poorly defined. Here, using a combination of mouse genetic, biochemical, and cell biological assays, we show that genetic removal of Eps8, an actin-binding and regulatory protein enriched in the growth cones and developing processes of neurons, significantly augments the number and density of vasodilator-stimulated phosphoprotein (VASP)-dependent axonal filopodia. The reintroduction of Eps8 wild type (WT), but not an Eps8 capping-defective mutant, into primary hippocampal neurons restored axonal filopodia to WT levels. We further show that the actin barbed-end capping activity of Eps8 is inhibited by brain-derived neurotrophic factor (BDNF) treatment through MAPK-dependent phosphorylation of Eps8 residues S624 and T628. Additionally, an Eps8 mutant, impaired in the MAPK target sites (S624A/T628A), displays increased association to actin-rich structures, is resistant to BDNF-mediated release from microfilaments, and inhibits BDNF-induced filopodia. The opposite is observed for a phosphomimetic Eps8 (S624E/T628E) mutant. Thus, collectively, our data identify Eps8 as a critical capping protein in the regulation of axonal filopodia and delineate a molecular pathway by which BDNF, through MAPK-dependent phosphorylation of Eps8, stimulates axonal filopodia formation, a process with crucial impacts on neuronal development and synapse formation.

Neurons communicate with each other via specialized cell–cell junctions called synapses. The proper formation of synapses (“synaptogenesis”) is crucial to the development of the nervous system, but the molecular pathways that regulate this process are not fully understood. External cues, such as brain-derived neurotrophic factor (BDNF), trigger synaptogenesis by promoting the formation of axonal filopodia, thin extensions projecting outward from a growing axon. Filopodia are formed by elongation of actin filaments, a process that is regulated by a complex set of actin-binding proteins. Here, we reveal a novel molecular circuit underlying BDNF-stimulated filopodia formation through the regulated inhibition of actin-capping factor activity. We show that the actin-capping protein Eps8 down-regulates axonal filopodia formation in neurons in the absence of neurotrophic factors. In contrast, in the presence of BDNF, the kinase MAPK becomes activated and phosphorylates Eps8, leading to inhibition of its actin-capping function and stimulation of filopodia formation. Our study, therefore, identifies actin-capping factor inhibition as a critical step in axonal filopodia formation and likely in new synapse formation.

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.

A novel pathway for presynaptic mitogen-activated kinase activation via AMPA receptors

AMPA-type glutamate receptors play a key role in mediating postsynaptic responses of excitatory neurotransmitters. It is now well accepted that AMPA receptors are also present at the presynapse, where they are thought to modulate neurotransmitter release. However, the mechanisms through which they control synaptic vesicle traffic have remained elusive. We used cultured hippocampal neurons and growth cone particles prepared from fetal rat brain to investigate the functional role of presynaptic AMPA receptors. We show here that stimulation of presynaptic AMPA receptors induces activation of mitogen-activated protein kinase (MAPK) through a nonreceptor tyrosine kinase-dependent and Na+/Ca2+-independent mechanism. This pathway is activated predominantly in axonal growth cones compared with the somatodendritic compartment. After stimulation of presynaptic AMPA receptors, synapsin I is phosphorylated at MAPK-specific sites. These events are paralleled by a prominent increase in evoked synaptic vesicle recycling that is blocked by the specific MAPK inhibitor 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one. Similarly, in synaptosomes isolated from adult brain, AMPA stimulation induces MAPK activation and phosphorylation of synapsin I at MAPK-dependent sites and enhances significantly synaptic vesicle recycling. These results reveal a novel pathway for activation of presynaptic MAPK and suggest a role of this pathway in the regulation of short-term presynaptic plasticity.

Quantification of synapse formation and maintenance in vivo in the absence of synaptic release

Outgrowing axons in the developing nervous system secrete neurotransmitters and neuromodulatory substances, which is considered to stimulate synaptogenesis. However, some synapses develop independent of presynaptic secretion. To investigate the role of secretion in synapse formation and maintenance in vivo, we quantified synapses and their morphology in the neocortical marginal zone of munc18-1 deficient mice which lack both evoked and spontaneous secretion [Science 287 (2000) 864]. Histochemical analyses at embryonic day 18 (E18) showed that the overall organization of the neocortex and the number of cells were similar in mutants and controls. Western blot analysis revealed equal concentrations of pre- and post-synaptic marker proteins in mutants and controls and immunocytochemical analyses indicated that these markers were targeted to the neuropil of the synaptic layer in the mutant neocortex. Electron microscopy revealed that at E16 immature synapses had formed both in mutants and controls. These synapses had a similar synapse diameter, active zone length and contained similar amounts of synaptic vesicles, which were immuno-positive for two synaptic vesicle markers. However, these synapses were three times less abundant in the mutant. Two days later, E18, synapses in the controls had more total and docked vesicles, but not in the mutant. Furthermore, synapses were now five times less abundant in the mutant. In both mutant and controls, synapse-like structures were observed with irregular shaped vesicles on both sides of the synaptic cleft. These 'multivesicular structures' were immuno-positive for synaptic vesicle markers and were four times more abundant in the mutant. We conclude that in the absence of presynaptic secretion immature synapses with a normal morphology form, but fewer in number. These secretion-deficient synapses might fail to mature and instead give rise to multivesicular structures. These two observations suggest that secretion of neurotransmitters and neuromodulatory substances is required for synapse maintenance, not for synaptogenesis. Multivesicular structures may develop out of unstable synapses.

Ethanol withdrawal influences survival and morphology of developing rat hippocampal neurons in vitro

BACKGROUND

Previous studies in this laboratory have shown that, like their counterparts in vivo, fetal rat hippocampal pyramidal neurons in culture develop abnormally small dendritic arbors when exposed to ethanol. This study asked whether ethanol's inhibitory effects on dendritic development differ when the duration of ethanol exposure and timing of withdrawal are varied to correspond with early versus later stages of development and whether ethanol withdrawal influences survival of these neurons.

METHODS

We compared neurons exposed continuously for 6 or 14 days to ethanol (70 mM) with neurons transferred from ethanol-containing medium to control medium either 1 day after adding ethanol (before dendrites elongated) or 6 days after adding ethanol (after dendrites began elongating). We then performed morphometric and cell density analyses at 6 and 14 days using digital images of neurons immunostained with microtubule-associated protein 2 (MAP2) to visualize dendrites.

RESULTS

Continuous exposure to ethanol decreased the length and number of dendrites formed but had no effect on neuron survival compared with controls without ethanol. Dendritic length was less inhibited when ethanol was withdrawn after 1 day, but the number of dendrites per cell was unchanged compared with neurons continuously exposed to ethanol. Withdrawal from ethanol at 1 day slightly enhanced the survival of neurons assessed at 14 days compared with neurons in control medium and with neurons exposed continuously to ethanol. In contrast, withdrawal from ethanol at 6 days severely decreased the number of neurons at 14 days.

CONCLUSIONS

These results suggest that dendrites can achieve normal length when ethanol exposure is limited to only 1 day and withdrawal occurs before dendrites begin elongating. However, a persistent reduction in dendrite number results in smaller overall dendritic arbor size. Although continuous exposure to ethanol has little effect on neuron survival in these cultures, and exposure limited to 1 day followed by withdrawal can be neuroprotective against cell death associated with increased time in culture, longer exposure before withdrawal can trigger cell death.

Regulated delivery of AMPA receptor subunits to the presynaptic membrane

In recent years, a role for AMPA receptors as modulators of presynaptic functions has emerged. We have investigated the presence of AMPA receptor subunits and the possible dynamic control of their surface exposure at the presynaptic membrane. We demonstrate that the AMPA receptor subunits GluR1 and GluR2 are expressed and organized in functional receptors in axonal growth cones of hippocampal neurons. AMPA receptors are actively internalized upon activation and recruited to the surface upon depolarization. Pretreatment of cultures with botulinum toxin E or tetanus toxin prevents the receptor insertion into the plasma membrane, whereas treatment with alpha-latrotoxin enhances the surface exposure of GluR2, both in growth cones of cultured neurons and in brain synaptosomes. Purification of small synaptic vesicles through controlled-pore glass chromatography, revealed that both GluR2 and GluR1, but not the GluR2 interacting protein GRIP, copurify with synaptic vesicles. These data indicate that, at steady state, a major pool of AMPA receptor subunits reside in synaptic vesicle membranes and can be recruited to the presynaptic membrane as functional receptors in response to depolarization.

Morphologic and neurotoxic effects of ethanol vary with timing of exposure in vitro

Results of investigations with animal models of fetal alcohol syndrome (FAS) seem to indicate that neuronal vulnerability to ethanol-induced cell death may be correlated with specific developmental events. In the present study, we sought to test this observation in a cell culture model of neuronal development in which morphogenesis as well as survival could be assessed. Using embryonic rat hippocampal pyramidal neurons in primary cultures, we compared the sensitivity of neurons to ethanol added, at 400 mg/dl, to the medium at different times relative to the development of axons and dendrites. Quantitative morphometric analysis was performed by using phase contrast at 12 h (0.5 day) and 24 h (1 day), or fluorescence microscopy after microtubule-associated protein-2 (MAP2) immunostaining at 6 and 14 days. Survival was assessed by counting the number of neurons per unit area of the substrate at 14 days. Addition of ethanol 1 day after plating, when most neurons had developed an axon, had no effect on survival up to 14 days in vitro, but resulted in significantly shorter, less branched dendrites than observed when ethanol was added 2 h after plating. Despite the shorter duration of ethanol exposure, the addition of ethanol on day 6, after rapid growth of dendrites and synapses had begun, resulted in loss of all but about one third of the neurons by 14 days. This supports the suggestion that increased neuronal vulnerability to the morphoregulatory effects of ethanol is correlated with the establishment of polarity, but that the sensitivity of neurons to the cytotoxic effects of ethanol occurs later, when dendrites and synapses are rapidly forming.

Rapid recruitment of NMDA receptor transport packets to nascent synapses

Although many of the molecules involved in synaptogenesis have been identified, the sequence and kinetics of synapse assembly in the central nervous system (CNS) remain largely unknown. We used simultaneous time-lapse imaging of fluorescent glutamate receptor subunits and presynaptic proteins in rat cortical neurons in vitro to determine the dynamics and time course of N-methyl-D-aspartate receptor (NMDAR) recruitment to nascent synapses. We found that both NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits are present in mobile transport packets in neurons before and during synaptogenesis. NMDAR transport packets are more mobile than AMPAR subunits, moving along microtubules at about 4 microm/min, and are recruited to sites of axodendritic contact within minutes. Whereas NMDAR recruitment to new synapses can be either concurrent with or independent of the protein PSD-95, AMPARs are recruited with a slower time course. Thus, glutamatergic synapses can form rapidly by the sequential delivery of modular transport packets containing glutamate receptors.

GABAergic terminals are required for postsynaptic clustering of dystrophin but not of GABA(A) receptors and gephyrin

In rat hippocampal cultures, we show by multilabeling immunocytochemistry that pyramidal cells, which receive little or no GABAergic input, mistarget alpha2-GABA(A) receptors and gephyrin to glutamatergic terminals. This mismatch does not occur in neurons innervated by numerous GABAergic terminals. A similar phenomenon has been reported for isolated autaptic hippocampal neurons (Rao et al., 2000). GABAergic synapses typically form multiple release sites apposed to GABA(A) receptor and gephyrin clusters. Remarkably, dystrophin, a protein highly abundant in skeletal muscle membranes, is extensively colocalized with alpha2-GABA(A) receptors exclusively opposite GABAergic terminals. In addition, selective apposition of syntrophin and beta-dystroglycan to GABAergic presynaptic terminals suggests that the entire dystrophin-associated protein complex (DPC) clusters at GABAergic synapses. In contrast to gephyrin and GABA(A) receptors, DPC proteins are not mistargeted to glutamatergic synapses, indicating independent clustering mechanisms. This was confirmed in hippocampal neurons cultured from GABA(A) receptor gamma2 subunit-deficient mice. Clustering of GABA(A) receptor and gephyrin in these neurons was strongly impaired, whereas clustering of dystrophin and associated proteins was unaffected by the absence of the gamma2 subunit. Our results indicate that accumulation of dystrophin and DPC proteins at GABAergic synapses occurs independently of postsynaptic GABA(A) receptors and gephyrin. We suggest that selective signaling from GABAergic terminals contributes to postsynaptic clustering of dystrophin.

GABAergic terminals are required for postsynaptic clustering of dystrophin but not of GABA(A) receptors and gephyrin

In rat hippocampal cultures, we show by multilabeling immunocytochemistry that pyramidal cells, which receive little or no GABAergic input, mistarget alpha2-GABA(A) receptors and gephyrin to glutamatergic terminals. This mismatch does not occur in neurons innervated by numerous GABAergic terminals. A similar phenomenon has been reported for isolated autaptic hippocampal neurons (Rao et al., 2000). GABAergic synapses typically form multiple release sites apposed to GABA(A) receptor and gephyrin clusters. Remarkably, dystrophin, a protein highly abundant in skeletal muscle membranes, is extensively colocalized with alpha2-GABA(A) receptors exclusively opposite GABAergic terminals. In addition, selective apposition of syntrophin and beta-dystroglycan to GABAergic presynaptic terminals suggests that the entire dystrophin-associated protein complex (DPC) clusters at GABAergic synapses. In contrast to gephyrin and GABA(A) receptors, DPC proteins are not mistargeted to glutamatergic synapses, indicating independent clustering mechanisms. This was confirmed in hippocampal neurons cultured from GABA(A) receptor gamma2 subunit-deficient mice. Clustering of GABA(A) receptor and gephyrin in these neurons was strongly impaired, whereas clustering of dystrophin and associated proteins was unaffected by the absence of the gamma2 subunit. Our results indicate that accumulation of dystrophin and DPC proteins at GABAergic synapses occurs independently of postsynaptic GABA(A) receptors and gephyrin. We suggest that selective signaling from GABAergic terminals contributes to postsynaptic clustering of dystrophin.

Glutamate regulates actin-based motility in axonal filopodia

The dynamics of axonal arbors during synaptogenesis and their plasticity in the adult nervous system remain poorly understood. Axonal filopodia, which emerge from the shaft of axonal branches and contain small synaptic vesicle clusters, initiate synaptic formation. We found that the movement of axonal filopodia is strongly inhibited by the neurotransmitter glutamate. This inhibitory effect is local, requires extracellular Ca2+, and can be blocked by CNQX treatment but not by NMDA, implicating axonal AMPA/kainate glutamate receptors. Transport and exo-endocytic recycling of synaptic vesicle packages in filopodia are not affected. These results reveal that the effect of glutamate on axonal filopodia is similar to its previously described effect on dendritic spines. Our results raise the possibility that axonal ionotropic glutamate receptors are also involved in synaptic plasticity in the adult nervous system.

Bidirectional regulation of neurite elaboration by alternatively spliced metabotropic glutamate receptor 5 (mGluR5) isoforms

Alternative splicing in the mGluR5 gene generates two different receptor isoforms, of which expression is developmentally regulated. However, little is known about the functional significance of mGluR5 splice variants. We have examined the functional coupling, subcellular targeting, and effect on neuronal differentiation of epitope-tagged mGluR5 isoforms by expression in neuroblastoma NG108-15 cells. We found that both mGluR5 splice variants give rise to comparable [Ca2+]i transients and have similar pharmacological profile. Tagged receptors were shown by immunofluorescence to be inserted in the plasma membrane. In undifferentiated cells the subcellular localization of the two mGluR5 isoforms was partially segregated, whereas in differentiated cells the labeling largely redistributed to the newly formed neurites. Interestingly, we demonstrate that mGluR5 splice variants dramatically influence the formation and maturation of neurites; mGluR5a hinders the acquisition of mature neuronal traits and mGluR5b fosters the elaboration and extension of neurites. These effects are partly inhibited by MPEP.

Molecular heterogeneity of central synapses: afferent and target regulation

Electrophysiological recordings show a functional spectrum even within a single class of synapse, with individual synapses ranging widely in fundamental properties, including release probability, unitary response and effects of previous stimulation on subsequent response. Molecular and cellular biological approaches have shown a corresponding diversity in the complement of ion channels, receptors, scaffolds and signal transducing proteins that make up individual synapses. Indeed, we believe that each individual synapse is unique, a function of presynaptic cell type, postsynaptic cell type, environment, developmental stage and history of activity. We review here the molecular diversity of glutamatergic and GABAergic synapses in the mammalian brain in the context of potential cell biological mechanisms that may explain how individual cells develop and maintain such a mosaic of synaptic connections.

Neurotrophins act at presynaptic terminals to activate synapses among cultured hippocampal neurons

We have recently demonstrated that embryonic E16 hippocampal neurons grown in cultures are unable to form fast synaptic connections unless treated with BDNF or NT-3. This experimental system offers an opportunity to define the roles of neurotrophins in processes leading to formation of functional synaptic connections. We have used ultrastructural and electrophysiological methods to explore the cellular locations underlying neurotrophin action on synaptic maturation. The rate of spontaneous miniature excitatory postsynaptic currents (mEPSCs) evoked by hyperosmotic stimulation was 7-16-fold higher in neurotrophin-treated cells than in controls. In addition, the potent neurotransmitter-releasing drug alpha-latrotoxin was virtually ineffective in the control cells while it stimulated synaptic events in neurotrophin-treated cells. Likewise, the membrane-bound dye FM1-43 was taken up by terminals in neurotrophin-treated cultures five-fold more than in controls. Both the total number and the number of docked synaptic vesicles were increased by neurotrophin treatment. Activation of synaptic responses by neurotrophins occurred even when postsynaptic glutamate receptors and action potential discharges were pharmacologically blocked. These results are consistent with a presynaptic locus of action of neurotrophins to increase synaptic vesicle density which is critical for rapid synaptic transmission. They also suggest that neurotrophins can activate synapses in the absence of pre- and postsynaptic neuronal activity.

Different localizations and functions of L-type and N-type calcium channels during development of hippocampal neurons

Using immunocytochemical assays and patch-clamp and calcium-imaging recordings, we demonstrate that L-type and N-type calcium channels have distinct patterns of expression and distribution and play different functional roles during hippocampal neuron differentiation. L-type channels, which support the depolarization-induced calcium influx in neurons from the very early developmental stages, are functionally restricted to the somatodendritic compartment throughout neuronal development and play a crucial role in supporting neurite outgrowth at early developmental stages. N-type channels, which start contributing at later neuronal differentiation stages (3-4 DIV), are also functionally expressed in the axons of immature neurons. At this developmental stage preceding synaptogenesis, N-type (but not L-type) channels are involved in controlling synaptic vesicle recycling. It is only at later developmental stages (10-12 DIV), when the neurons have established a clear axodendritic polarity and form synaptic contacts, that N-type channels are progressively excluded from the axon. Electrophysiological recordings of single neurons growing in microislands revealed that synaptic maturation coincides with a progressive increase in N-type channels in the somatodendritic region and a progressive decrease in the N-type channels supporting glutamate release from the presynaptic terminal. These results indicate that L-type and N-type calcium channels undergo dynamic, developmentally regulated rearrangements in regional distribution and function and also suggest that different mechanisms may be involved in the sorting and/or stabilization of these two types of channels in different plasma membrane domains during neuronal differentiation.

The GLT-1 and GLAST glutamate transporters are expressed on morphologically distinct astrocytes and regulated by neuronal activity in primary hippocampal cocultures

The GLT-1 and GLAST astroglial transporters are the glutamate transporters mainly involved in maintaining physiological extracellular glutamate concentrations. Defects in neurotransmitter glutamate transport may represent an important component of glutamate-induced neurodegenerative disorders (such as amyotrophic lateral sclerosis) and CNS insults (ischemia and epilepsy). We characterized the protein expression of GLT-1 and GLAST in primary astrocyte-neuron cocultures derived from rat hippocampal tissues during neuron differentiation/maturation. GLT-1 and GLAST are expressed by morphologically distinct glial fibrillary acidic protein-positive astrocytes, and their expression correlates with the status of neuron differentiation/maturation and activity. Up-regulation of the transporters paralleled the content of the synaptophysin synaptic vesicle marker p38, and down-regulation was a consequence of glutamate-induced neuronal death or the reduction of synaptic activity. Finally, soluble factors in neuronal-conditioned media prevented the down-regulation of the GLT-1 and GLAST proteins. Although other mechanisms may participate in regulating GLT-1 and GLAST in the CNS, our data indicate that soluble factors dependent on neuronal activity play a major regulating role in hippocampal cocultures.

Increased exocytotic capability of rat cerebellar granule neurons cultured under depolarizing conditions

To obtain insights into the mechanisms underlying activity-dependent survival of neurons, we surveyed various indices of cellular activity in rat cerebellar granule neurons cultured under conditions advantageous and disadvantageous for survival. Previously, we reported that the turnover of Ca2+ (both influx and efflux) is activated in raised K+-cultures (survival condition), although the cytoplasmic Ca2+ concentration is not affected. We also reported that endocytotic activity was high in the high K+-cultures. In the present study, we used the release of FM1-43 dye [N-(3-triethylammoniumpropyl)-4-(4-dibutylamino)styryl)py ridium bromide] to determine the exocytotic capabilities of neurons cultured in normal K+ (death condition), high K+ (survival condition) and brain-derived neurotrophic factor-supplemented (survival condition) media. The FM1-43 releases triggered by K+-induced depolarization and glutamate exposure were significantly higher in the high K+-cultures than in normal K+-cultures. Interestingly, the neurons whose survival was supported by brain-derived neurotrophic factor did not show high exocytotic capability, indicating that the high exocytotic capability is not a mere result of viability. However, the number of synaptic sites per cell (as monitored by synaptophysin immunopositivity) was unaffected by culture conditions. The present results suggest that an enhanced exocytotic activity supported by a strengthened exocytotic capability may underlie the high viability of rat cerebellar granule neurons cultured under depolarizing conditions.

Development of neuron-neuron synapses

Our understanding of neuronal synapse development has advanced in recent years. The development of glycinergic synapses appears to depend on gephyrin and glycine receptor activity. Molecular characterization of the structure and development of glutamatergic synapses is in progress, but the underlying mechanisms remain unclear. Activity-dependent mechanisms and specific molecules that regulate the morphological development of dendritic spines have recently been identified.

N-cadherin redistribution during synaptogenesis in hippocampal neurons

Cadherins are homophilic adhesion molecules that, together with their intracellular binding partners the catenins, mediate adhesion and signaling at a variety of intercellular junctions. This study shows that neural (N)-cadherin and beta-catenin, an intracellular binding partner for the classic cadherins, are present in axons and dendrites before synapse formation and then cluster at developing synapses between hippocampal neurons. N-cadherin is expressed initially at all synaptic sites but rapidly becomes restricted to a subpopulation of excitatory synaptic sites. Sites of GABAergic, inhibitory synapses in mature cultures therefore lack N-cadherin but are associated with clusters of beta-catenin, implying that they contain a different classic cadherin. These findings indicate that N-cadherin adhesion may stabilize early synapses that can then be remodeled to express a different cadherin and that cadherins systematically differentiate between functionally (excitatory and inhibitory) and spatially distinct synaptic sites on single neurons. These results suggest that differential cadherin expression may orchestrate the point-to-point specificity displayed by developing synapses.

N-cadherin redistribution during synaptogenesis in hippocampal neurons

Cadherins are homophilic adhesion molecules that, together with their intracellular binding partners the catenins, mediate adhesion and signaling at a variety of intercellular junctions. This study shows that neural (N)-cadherin and beta-catenin, an intracellular binding partner for the classic cadherins, are present in axons and dendrites before synapse formation and then cluster at developing synapses between hippocampal neurons. N-cadherin is expressed initially at all synaptic sites but rapidly becomes restricted to a subpopulation of excitatory synaptic sites. Sites of GABAergic, inhibitory synapses in mature cultures therefore lack N-cadherin but are associated with clusters of beta-catenin, implying that they contain a different classic cadherin. These findings indicate that N-cadherin adhesion may stabilize early synapses that can then be remodeled to express a different cadherin and that cadherins systematically differentiate between functionally (excitatory and inhibitory) and spatially distinct synaptic sites on single neurons. These results suggest that differential cadherin expression may orchestrate the point-to-point specificity displayed by developing synapses.

Activity-dependent regulation of dendritic BC1 RNA in hippocampal neurons in culture

Several neuronal RNAs have been identified in dendrites, and it has been suggested that the dendritic location of these RNAs may be relevant to the spatiotemporal regulation of mosaic postsynaptic protein repertoires through transsynaptic activity. Such regulation would require that dendritic RNAs themselves, or at least some of them, be subject to physiological control. We have therefore examined the functional regulation of somatodendritic expression levels of dendritic BC1 RNA in hippocampal neurons in culture. BC1 RNA, an RNA polymerase III transcript that is a component of a ribonucleoprotein particle, became first detectable in somatodendritic domains of developing hippocampal neurons at times of initial synapse formation. BC1 RNA was identified only in such neurons that had established synapses on cell bodies and/or developing dendritic arbors. When synaptic contact formation was initiated later in low-density cultures, BC1 expression was coordinately delayed. Inhibition of neuronal activity in hippocampal neurons resulted in a substantial but reversible reduction of somatodendritic BC1 expression. We conclude that expression of BC1 RNA in somatic and dendritic domains of hippocampal neurons is regulated in development, and is dependent upon neuronal activity. These results establish (for the first time to our knowledge) that an RNA polymerase III transcript can be subject to control through physiological activity in nerve cells.

Depolarization stimulates lamellipodia formation and axonal but not dendritic branching in cultured rat cerebral cortex neurons

Electric activity is known to have profound effects on growth cone morphology and neurite outgrowth, but the nature of the response varies strongly between neurons derived from different species or brain areas. To establish the role of electric activity in neurite outgrowth and neuronal morphogenesis of rat cerebral cortex neurons, cultured neurons were depolarized for up to 72 h and quantitatively analyzed for changes in axonal and dendritic morphology. Depolarization with 25 mM potassium chloride induced a rapid increase in lamellipodia in almost all growth cones and along both axons and dendrites. Lamellipodia formation was dependent on an influx of extracellular calcium through L-type voltage-sensitive calcium channels. Prolonged depolarization for 24 h induced an increase in total axonal length, mainly due to an increase in branching. After three days of depolarization axonal outgrowth was largely the same as in control neurons, suggesting accommodation of the growth cones to chronic depolarization. Dendrites showed very little change during the first three days in culture, and dendritic length or branching were not affected by depolarization. Thus, in early cerebral cortex neurons depolarization specifically stimulates axonal outgrowth through increased branching. This increase in branching may be a consequence of the earlier increase in lamellipodia formation. In contrast, early dendrites seem to be unable to translate the increase in lamellipodia into changes in outgrowth or branching. This difference between axons and dendrites could be due to differences in the stabilization of the tubulin cytoskeleton.

Potassium ion- and nitric oxide-induced exocytosis from populations of hippocampal synapses during synaptic maturation in vitro

The development of mechanisms of neurotransmitter release is an important component in the formation of functional synaptic connections. Synaptic neurotransmitter release can be modulated by nitric oxide, a compound shown to have a variety of physiologic functions in the nervous system. The goal of this study was to determine whether, during synaptic maturation, nitric oxide is capable of affecting exocytosis of synaptic vesicles, and to compare its effects with those elicited by strongly depolarizing stimuli. To address these questions we examined vesicle release from large numbers of individual synapses of hippocampal neurons between five and 13 days in culture. Synaptic vesicles were labelled by uptake of the styrylpyridinium dye N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide (FM1-43) and their release was monitored by fluorescence imaging. Across populations of developing synapses, there was a good correspondence between FM1-43 staining and synapsin immunocytochemistry. A marked heterogeneity was observed in the ability to release vesicles both after potassium and nitric oxide stimulation. In less mature populations of synapses, the rate of potassium- and nitric oxide-induced exocytosis gradually increased, while at later stages nitric oxide-induced responses levelled off and potassium-induced responses continued to rise. Application of nitric oxide donors did not trigger any detectable changes in intracellular calcium. Combined immunocytochemical analysis of cultured hippocampal neurons for neuronal nitric oxide synthase and synapsin revealed that nitric oxide synthase was present within neurites of cultured hippocampal neurons, largely distributed in a bead-like pattern which partially overlapped presynaptic sites. Stimulation of the N-methyl-D-aspartate receptor while blocking propagation of action potentials with tetrodotoxin resulted in exocytosis from numerous individually resolved sites. Preincubation of neurons with an nitric oxide synthase inhibitor or addition of an nitric oxide scavenger eliminated these responses indicating a role for nitric oxide in N-methyl-D-aspartate-stimulated exocytosis. Using fluorescence imaging of individually resolved synaptic sites, we provide direct evidence for an effect of nitric oxide on vesicular neurotransmitter release in intact neurons. Nitric oxide is capable to produce this effect at all stages of synaptic development and acts independently of calcium influx. We show that nitric oxide synthase is present at synaptic sites and endogenously produced nitric oxide is sufficient to cause exocytosis. Taken together, these experiments suggest a possible role for nitric oxide in calcium-independent transmitter release in populations of synapses at all stages of maturation.

Non-synaptic localization of the glutamate transporter EAAC1 in cultured hippocampal neurons

It has been postulated for several years that the high affinity neuronal glutamate uptake system plays a role in clearing glutamate from the synaptic cleft. Four different glutamate transporter subtypes are now identified, the major neuronal one being EAAC1. To be a good candidate for the reuptake of glutamate at the synaptic cleft, EAAC1 should be properly located at synapses, either at pre- or postsynaptic sites. We have investigated the distribution of EAAC1 in primary cultures of hippocampal neurons, which represent an advantageous model for the study of synaptogenesis and synaptic specializations. We have demonstrated that EAAC1 immunoreactivity is segregated in the somatodendritic compartment of fully differentiated hippocampal neurons, where it is localized in the dendritic shaft and in the spine neck, outside the area facing the active zone. No co-localization of EAAC1 immunoreactivity with the stainings produced by typical presynaptic and postsynaptic markers was ever observed, indicating that EAAC1 is not to be considered a synaptic protein. Accordingly, the developmental pattern of expression of EAAC1 was found to be different from that of typical synaptic markers. Moreover, EAAC1 was expressed in the somatodendritic compartment of hippocampal neurons already at stages preceding the formation of synaptic contacts, and was also expressed in GABAergic interneurons with identical subcellular distribution. Taken together, these data rule against a possible role for EAAC1 in the clearance of glutamate from within the cleft and in the regulation of its time in the synapse. They suggest an unconventional non-synaptic function of this high-affinity glutamate carrier, not restricted to glutamatergic fibres.

Activity-independent segregation of excitatory and inhibitory synaptic terminals in cultured hippocampal neurons

Cultured hippocampal neurons were used as a model system to address experimentally the spatial and temporal sequence leading to the appropriate sorting of excitatory and inhibitory synaptic terminals to different cellular target domains and the role of neural activity in this process. By using antibodies against glutamic acid decarboxylase 65 (GAD65) and synaptophysin, we examined the development and segregation of GABAergic and non-GABAergic synaptic terminals on single neurons. Electron microscopy confirmed that GAD65-labeled swellings observed using light microscopy corresponded to synaptic boutons. From the time at which GABAergic terminals first appeared, they developed at a more rapid rate on neuronal somata than non-GABAergic terminals did, such that by 18 d in culture, 60% of the total boutons on somata were GABAergic. By contrast, the majority (70%) of boutons on dendrites were non-GABAergic. These data suggest that inhibitory synaptic terminals are targeted preferentially to or maintained on cell somata at the expense of excitatory terminals. Interestingly, non-GABAergic terminals were not inhibited from forming synapses on cell somata, because in the absence of GABAergic terminals they attained the same total somatic terminal density seen in the presence of GABAergic terminals. Chronic blockade of neuronal activity did not affect the differential targeting of GABAergic and non-GABAergic axons; however, it did reduce the extent of dendritic arborization. Our findings support a two-step model for synaptic segregation whereby the majority of terminals is initially targeted in an activity-independent manner to the appropriate cellular domains, but an additional developmental mechanism serves to further restrict and refine the original synaptic distribution.

Activity-independent segregation of excitatory and inhibitory synaptic terminals in cultured hippocampal neurons

Cultured hippocampal neurons were used as a model system to address experimentally the spatial and temporal sequence leading to the appropriate sorting of excitatory and inhibitory synaptic terminals to different cellular target domains and the role of neural activity in this process. By using antibodies against glutamic acid decarboxylase 65 (GAD65) and synaptophysin, we examined the development and segregation of GABAergic and non-GABAergic synaptic terminals on single neurons. Electron microscopy confirmed that GAD65-labeled swellings observed using light microscopy corresponded to synaptic boutons. From the time at which GABAergic terminals first appeared, they developed at a more rapid rate on neuronal somata than non-GABAergic terminals did, such that by 18 d in culture, 60% of the total boutons on somata were GABAergic. By contrast, the majority (70%) of boutons on dendrites were non-GABAergic. These data suggest that inhibitory synaptic terminals are targeted preferentially to or maintained on cell somata at the expense of excitatory terminals. Interestingly, non-GABAergic terminals were not inhibited from forming synapses on cell somata, because in the absence of GABAergic terminals they attained the same total somatic terminal density seen in the presence of GABAergic terminals. Chronic blockade of neuronal activity did not affect the differential targeting of GABAergic and non-GABAergic axons; however, it did reduce the extent of dendritic arborization. Our findings support a two-step model for synaptic segregation whereby the majority of terminals is initially targeted in an activity-independent manner to the appropriate cellular domains, but an additional developmental mechanism serves to further restrict and refine the original synaptic distribution.

Evidence for a role of dendritic filopodia in synaptogenesis and spine formation

Axo-dendritic synaptogenesis was examined in live hippocampal cell cultures using the fluorescent dyes DiO to label dendrites and FM 4-64 to label functional presynaptic boutons. As the first functional synaptic boutons appeared in these cultures, numerous filopodia (up to 10 micron long) were observed to extend transiently (mean lifetime 9.5 min) from dendritic shafts. With progressively increasing numbers of boutons, there were coincident decreases in numbers of transient filopodia and increases in numbers of stable dendritic spines. Dendritic filopodia were observed to initiate physical contacts with nearby axons. This sometimes resulted in filopodial stabilization and formation of functional presynaptic boutons. These findings suggest that dendritic filopodia may actively initiate synaptogenic contacts with nearby (5-10 micron) axons and thereafter evolve into dendritic spines.

Neuronal differentiation in the rat hippocampus involves a stage-specific reorganization of subnuclear structure both in vivo and in vitro

Pyramidal neurons from the hippocampus undergo a well characterized programme of differentiation in vitro involving five distinct stages (1-5). While some important aspects of the dynamic organization of cell cytoplasmic structure that underlie neuronal polarization have been elucidated, little is known about corresponding changes in nuclear organization. Here we identify major changes affecting nuclear structure and gene expression during late stages of differentiation. At stage 4 a sustained increase in global transcriptional activity occurs. This is followed at stage 5 by proliferation of coiled bodies, i.e. subnuclear organelles containing splicing factors, which form a novel domain around the nucleus that we refer to as the rosette. Both the morphology and timing of rosette formation are identical in neurons in vitro and in situ in the developing hippocampus in rat brain. Long-term synaptic inhibition in vitro or growth at low density does not prevent either nuclear reorganization, enhanced transcriptional activity or the formation of pre-synaptic specializations. These data indicate that stage-specific changes in nuclear structure and function, similar to distinct rearrangements of cytoplasmic components, are pre-programmed aspects of the neuronal differentiation pathway in the hippocampus.

Expression and subcellular distribution of glutamate receptor subunits 2/3 in the developing cerebellar cortex

The expression and subcellular location of glutamate receptor subunits 2&3 was investigated in the developing postnatal cerebellum. Immunoblotting revealed that glutamate receptor subunits 2/3 is expressed in an identical pattern of immunoreactive bands of approximately 108 kDa from postnatal day zero to adult animals. Light microscopy showed that within the cerebellar cortex, GluR 2/3 immunoreactivity was essentially confined to Purkinje neurons. Strong immunostaining could be observed at postnatal days 1-3 within Purkinje cell bodies and primary dendrites. With ongoing development, the cell body and an increasingly elaborate dendritic tree was outlined by immunoreaction product. In adult animals, staining of Purkinje cell dendrites was patchy, and staining intensity of the cell body, in particular, was greatly reduced. Ultrastructural analysis revealed that during early postnatal development, immunoreaction product was localized to the cell membrane, but was not confined to postsynaptic densities. From the second postnatal week, glutamate receptor subunits 2/3 immunoreactivity was largely restricted to postsynaptic densities. These observations reveal a developmentally regulated refinement of the subcellular distribution of defining subunits of the AMPA-type glutamate receptor. The presence of membrane bond receptors prior to the formation of synapses also provides a rationale for the known transmitter-mediated modulation of Purkinje cell dendritogenesis.

Sorting signals and targeting of infectious agents through axons: an annotation to the 100 years' birth of the name "axon"

A brief review is given on mechanisms by which axons may be initiated during development and by which the polarity of neurons is maintained by selective sorting and delivery of molecules to axons and dendrites. The use of viruses as tools to study targeting of newly synthesized proteins to axons is described. Emphasis is then given to the hazards that are presented to the individual by the retrograde transport of infectious agents in axons to the brain. Borna disease virus, prions, and Listeria monocytogenes are examined briefly as examples of these mechanisms. These agents have attracted interest previously in veterinary medicine for the most part, but they may present potential and substantial threats to human health. Such infectious agents also represent a new type of virus, a new principle for disease transmission, and a new mechanism for intracellular transport, respectively.

Fluorescence imaging of local membrane electric fields during the excitation of single neurons in culture

The spatial distribution of depolarized patches of membrane during the excitation of single neurons in culture has been recorded with a high spatial resolution (1 micron2/pixel) imaging system based on a liquid-nitrogen-cooled astronomical camera mounted on an inverted microscope. Images were captured from rat nodose neurons stained with the voltage-sensitive dye RH237. Conventional intracellular microelectrode recordings were made in synchrony with the images. During an action potential the fluorescence changes occurred in localized, unevenly distributed membrane areas, which formed clusters of depolarized sites of different sizes and intensities. When fast conductances were blocked by the addition of tetrodotoxin, a reduction in the number and the intensities of the depolarized sites was observed. The blockade by tetrodotoxin of voltage-clamped neurons also reduced the number of depolarized sites, although the same depolarizing voltage step was applied. Similarly, when a voltage-clamped neuron was depolarized by a constant-amplitude voltage step, the number of depolarized sites varied according to the degree of activation of the voltage-sensitive channels, which was modified by changing the holding potential. These results suggest that the spatial patterns of depolarization observed during excitation are related to the operations of ionic channels in the membrane.

Calcium-dependent glutamate release during neuronal development and synaptogenesis: different involvement of omega-agatoxin IVA- and omega-conotoxin GVIA-sensitive channels

Hippocampal neurons maintained in primary culture recycle synaptic vesicles and express functional glutamate receptors since early stages of neuronal development. By analyzing glutamate-induced cytosolic calcium changes to sense presynaptically released neurotransmitter, we demonstrate that the ability of neurons to release glutamate in the extracellular space is temporally coincident with the property of synaptic vesicles to undergo exocytotic-endocytotic recycling. Neuronal differentiation and maturation of synaptic contacts coincide with a change in the subtype of calcium channels primarily involved in controlling neurosecretion. Whereas omega-agatoxin IVA-sensitive channels play a role in controlling neurotransmitter secretion at all stages of neuronal differentiation, omega-conotoxin GVIA-sensitive channels are primarily involved in mediating glutamate release at early developmental stages only.

Mechanisms of synaptogenesis in hippocampal neurons in primary culture

To improve our understanding of the mechanisms which regulate the formation and the functional maturation of synaptic contacts between neurons, we used hippocampal neurons maintained in primary cultures as experimental system. In this model, which offers several advantages for the study of neuronal development and synaptogenesis, we investigated some of the cellular mechanisms underlying the formation of presynaptic and postsynaptic compartments.