Neuroligation: building synapses around the neurexin-neuroligin link

Mass spectrometry-based proteomic characterization of the middle-aged mouse brain for animal model research of neuromuscular diseases

Neuromuscular diseases with primary muscle wasting symptoms may also display multi-systemic changes in the body and exhibit secondary pathophysiological alterations in various non-muscle tissues. In some cases, this includes proteome-wide alterations and/or adaptations in the central nervous system. Thus, in order to provide an improved bioanalytical basis for the comprehensive evaluation of animal models that are routinely used in muscle research, this report describes the mass spectrometry-based proteomic characterization of the mouse brain. Crude tissue extracts were examined by bottom-up proteomics and detected 4558 distinct protein species. The detailed analysis of the brain proteome revealed the presence of abundant cellular proteoforms in the neuronal cytoskeleton, as well as various brain region enriched proteins, including markers of the cerebral cortex, cerebellum, hippocampus and the olfactory bulb. Neuroproteomic markers of specific cell types in the brain were identified in association with various types of neurons and glia cells. Markers of subcellular structures were established for the plasmalemma, nucleus, endoplasmic reticulum, mitochondria and other crucial organelles, as well as synaptic components that are involved in presynaptic vesicle docking, neurotransmitter release and synapse remodelling.

Erythropoietin prevents the effect of chronic restraint stress on the number of hippocampal CA3c dendritic terminals-relation to expression of genes involved in synaptic plasticity, angiogenesis, inflammation, and oxidative stress in male rats

Stress-induced allostatic load affects a variety of biological processes including synaptic plasticity, angiogenesis, oxidative stress, and inflammation in the brain, especially in the hippocampus. Erythropoietin (EPO) is a pleiotropic cytokine that has shown promising neuroprotective effects. Recombinant human EPO is currently highlighted as a new candidate treatment for cognitive impairment in neuropsychiatric disorders. Because EPO enhances synaptic plasticity, attenuates oxidative stress, and inhibits generation of proinflammatory cytokines, EPO may be able to modulate the effects of stress-induced allostatic load at the molecular level. The aim of this study was therefore to investigate how EPO and repeated restraint stress, separately and combined, influence (i) behavior in the novelty-suppressed feeding test of depression/anxiety-related behavior; (ii) mRNA levels of genes encoding proteins involved in synaptic plasticity, angiogenesis, oxidative stress, and inflammation; and (iii) remodeling of the dendritic structure of the CA3c area of the hippocampus in male rats. As expected, chronic restraint stress lowered the number of CA3c apical dendritic terminals, and EPO treatment reversed this effect. Interestingly, these effects seemed to be mechanistically distinct, as stress and EPO had differential effects on gene expression. While chronic restraint stress lowered the expression of spinophilin, tumor necrosis factor α, and heat shock protein 72, EPO increased expression of hypoxia-inducible factor-2α and lowered the expression of vascular endothelial growth factor in hippocampus. These findings indicate that the effects of treatment with EPO follow different molecular pathways and do not directly counteract the effects of stress in the hippocampus.

Neuropathic Allodynia Involves Spinal Neurexin-1β-dependent Neuroligin-1/Postsynaptic Density-95/NR2B Cascade in Rats

BACKGROUND

Neuroligin-1 (NL1) forms a complex with the presynaptic neurexin-1β (Nrx1b), regulating clustering of N-methyl-D-aspartate receptors with postsynaptic density-95 (PSD-95) to underlie learning-/memory-associated plasticity. Pain-related spinal neuroplasticity shares several common features with learning-/memory-associated plasticity. The authors thereby investigated the potential involvement of NL1-related mechanism in spinal nerve ligation (SNL)-associated allodynia.

METHODS

In 626 adult male Sprague-Dawley rats, the withdrawal threshold and NL1, PSD-95, phosphorylated NR2B (pNR2B) expressions, interactions, and locations in dorsal horn (L4 to L5) were compared between the sham operation and SNL groups. A recombinant Nrx1b Fc chimera (Nrx1b Fc, 10 μg, 10 μl, i.t., bolus), antisense small-interfering RNA targeting to NL1 (10 μg, 10 μl, i.t., daily for 4 days), or NR2B antagonist (Ro 25-6981; 1 μM, 10 μl, i.t., bolus) were administered to SNL animals to elucidate possible cascades involved.

RESULTS

SNL-induced allodynia failed to affect NL1 or PSD-95 expression. However, pNR2B expression (mean ± SD from 13.1 ± 2.87 to 23.1 ± 2.52, n = 6) and coexpression of NL1-PSD-95, pNR2B-PSD-95, and NL1-total NR2B were enhanced by SNL (from 10.7 ± 2.27 to 22.2 ± 3.94, 11.5 ± 2.15 to 23.8 ± 3.32, and 8.9 ± 1.83 to 14.9 ± 2.27 at day 7, n = 6). Furthermore, neuron-localized pNR2B PSD-95-pNR2B double-labeled and NL1/PSD-95/pNR2B triple-labeled immunofluorescence in the ipsilateral dorsal horn was all prevented by Nrx1b Fc and NL1-targeted small-interfering RNA designed to block and prevent NL1 expression. Without affecting NL1-PSD-95 coupling, Ro 25-6981 decreased the SNL-induced PSD-95-pNR2B coprecipitation (from 18.7 ± 1.80 to 14.7 ± 2.36 at day 7, n = 6).

CONCLUSION

SNL-induced allodynia, which is mediated by the spinal NL1/PSD-95/pNR2B cascade, can be prevented by blockade of transsynaptic Nrx1b-NL1 interactions.

Effects of mood-stabilizing drugs on dendritic outgrowth and synaptic protein levels in primary hippocampal neurons

OBJECTIVES

Mood-stabilizing drugs, such as lithium (Li) and valproate (VPA), are widely used for the treatment of bipolar disorder, a disease marked by recurrent episodes of mania and depression. Growing evidence suggests that Li exerts neurotrophic and neuroprotective effects, leading to an increase in neural plasticity. The present study investigated whether other mood-stabilizing drugs produce similar effects in primary hippocampal neurons.

METHODS

The effects of the mood-stabilizing drugs Li, VPA, carbamazepine (CBZ), and lamotrigine (LTG) on hippocampal dendritic outgrowth were examined. Western blotting analysis was used to measure the expression of synaptic proteins - that is, brain-derived neurotrophic factor (BDNF), postsynaptic density protein-95 (PSD-95), neuroligin 1 (NLG1), β-neurexin, and synaptophysin (SYP). To determine neuroprotective effects, we used a B27-deprivation cytotoxicity model which causes hippocampal cell death upon removal of B27 from the culture medium.

RESULTS

Li (0.5-2.0 mM), VPA (0.5-2.0 mM), CBZ (0.01-0.10 mM), and LTG (0.01-0.10 mM) significantly increased dendritic outgrowth. The neurotrophic effect of Li and VPA was blocked by inhibition of phosphatidylinositol 3-kinase, extracellular signal-regulated kinase, and protein kinase A signaling; the effects of CBZ and LTG were not affected by inhibition of these signaling pathways. Li, VPA, and CBZ prevented B27 deprivation-induced decreases in BDNF, PSD-95, NLG1, β-neurexin, and SYP levels, whereas LTG did not.

CONCLUSIONS

These results suggest that Li, VPA, CBZ, and LTG exert neurotrophic effects by promoting dendritic outgrowth; however, the mechanism of action differs. Furthermore, certain mood-stabilizing drugs may exert neuroprotective effects by enhancing synaptic protein levels against cytotoxicity in hippocampal cultures.

Analysis of splice variants for the C. elegans orthologue of human neuroligin reveals a developmentally regulated transcript

Neuroligins are synaptic adhesion molecules and important determinants of synaptic function. They are expressed at postsynaptic sites and involved in synaptic organization through key extracellular and intracellular protein interactions. They undergo trans-synaptic interaction with presynaptic neurexins. Distinct neuroligins use differences in their intracellular domains to selectively recruit synaptic scaffolds and this plays an important role in how they encode specialization of synaptic function. Several levels of regulation including gene expression, splicing, protein translation and processing regulate the expression of neuroligin function. We have used in silico and cDNA analyses to investigate the mRNA splicing of the Caenorhabditis elegans orthologue nlg-1. Transcript analysis highlights the potential for gene regulation with respect to both temporal expression and splicing. We found nlg-1 splice variants with all the predicted exons are a minor species relative to major splice variants lacking exons 13 and 14, or 14 alone. These major alternatively spliced variants change the intracellular domain of the gene product NLG-1. Interestingly, exon 14 encodes a cassette with two distinct potential functional domains. One is a polyproline SH3 binding domain and the other has homology to a region encoding the binding site for the scaffolding protein gephyrin in mammalian neuroligins. This suggests differential splicing impacts on NLG-1 competence to recruit intracellular binding partners. This may have developmental relevance as nlg-1 exon 14 containing transcripts are selectively expressed in L2-L3 larvae. These results highlight a developmental regulation of C. elegans nlg-1 that could play a key role in the assembly of synaptic protein complexes during the early stages of nervous system development.

Overexpression of the cell adhesion protein neuroligin-1 induces learning deficits and impairs synaptic plasticity by altering the ratio of excitation to inhibition in the hippocampus

Trans-synaptic cell-adhesion molecules have been implicated in regulating CNS synaptogenesis. Among these, the Neuroligin (NL) family (NLs 1-4) of postsynaptic adhesion proteins has been shown to promote the development and specification of excitatory versus inhibitory synapses. NLs form a heterophilic complex with the presynaptic transmembrane protein Neurexin (NRX). A differential association of NLs with postsynaptic scaffolding proteins and NRX isoforms has been suggested to regulate the ratio of excitatory to inhibitory synapses (E/I ratio). Using transgenic mice, we have tested this hypothesis by overexpressing NL1 in vivo to determine whether the relative levels of these cell adhesion molecules may influence synapse maturation, long-term potentiation (LTP), and/or learning. We found that NL1-overexpressing mice show significant deficits in memory acquisition, but not in memory retrieval. Golgi and electron microscopy analysis revealed changes in synapse morphology indicative of increased maturation of excitatory synapses. In parallel, electrophysiological examination indicated a shift in the synaptic activity toward increased excitation as well as impairment in LTP induction. Our results demonstrate that altered balance in the expression of molecules necessary for synapse specification and development (such as NL1) can lead to defects in memory formation and synaptic plasticity and outline the importance of rigidly controlled synaptic maturation processes.

Altered neuroligin expression is involved in social deficits in a mouse model of the fragile X syndrome

The fragile X syndrome (FXS) is the most common form of inherited mental retardation. Caused by a transcriptional silencing of the fragile X mental retardation protein (FMRP), a mRNA binding protein itself, misregulated translation is thought to be the leading cause of the fragile X syndrome. Interestingly, recent results indicated several neuroligin interacting proteins to be affected by this misregulation, including neurexin1 and PSD95, which have also been implicated in autism spectrum disorders. Using co-immunoprecipitation assays and RT-PCR, FMRP is shown to interact with neuroligin1- and 2-mRNA, while no interaction with neuroligin3-mRNA is observed. In line with FMRP's role in translation regulation, Western blot as well as immunohistochemistry analysis reveal changes in protein expression levels suggesting impaired synaptic function. As increasing evidence indicates neuroligin expression to be critical for synapse maturation and function, consequences of impaired neuroligin1 expression in FXS are assessed by overexpressing HA-neuroligin1 in FMR1-/- mice, a model for FXS. Behavioural assessments demonstrate that enhanced neuroligin1 expression improves social behaviour in FMR1-/- mice, whereas no positive effect on learning and memory is seen. These results provide for the first time evidence for an involvement of a neuroligin-neurexin protein network in core symptoms of FXS.

Positive association of neuroligin-4 gene with nonspecific mental retardation in the Qinba Mountains Region of China

OBJECTIVE

Neuroligin-4 is essential for proper brain function. Some studies indicate a close relationship between neuroligin-4 and several human psychiatric conditions.

METHODS

The case-control method was used to study the association between nonspecific mental retardation (NSMR) and genetic variants of neuroligin-4 gene (NLGN4). Five single nucleotide polymorphisms (SNPs: rs5916271, rs7049300, rs6638575, rs3810686, and rs1882260) were genotyped by PCR-RFLP/SSCP method in the NLGN4.

RESULTS

Individual SNP analysis shows significant differences at SNPs rs3810686 and rs1882260 for allele frequency when NSMR cases and controls [odds ratio (OR)=1.589, 95% confidence interval (CI)=1.035-2.438, chi2=4.53, df=1, P=0.033; OR=2.050, 95% CI=1.211-3.470, chi2=7.38, df=1, P=0.007, respectively] were compared. Further haplotype analysis indicates that there are two haplotype sets, rs3810686-rs1882260 and rs6638575-rs3810686-rs1882260, which show statistical differences between NSMR cases and controls (chi2=6.79, df=2, global P=0.034; chi2=9.29, df=2, global P=0.0096, respectively).

CONCLUSION

The results suggest a positive association between the genetic variants of the NLGN4 and NSMR in the Chinese children from Qinba Mountains Region.

Crystal structure of the second LNS/LG domain from neurexin 1alpha: Ca2+ binding and the effects of alternative splicing

Neurexins mediate protein interactions at the synapse, playing an essential role in synaptic function. Extracellular domains of neurexins, and their fragments, bind a distinct profile of different proteins regulated by alternative splicing and Ca2+. The crystal structure of n1alpha_LNS#2 (the second LNS/LG domain of bovine neurexin 1alpha) reveals large structural differences compared with n1alpha_LNS#6 (or n1beta_LNS), the only other LNS/LG domain for which a structure has been determined. The differences overlap the so-called hyper-variable surface, the putative protein interaction surface that is reshaped as a result of alternative splicing. A Ca2+-binding site is revealed at the center of the hyper-variable surface next to splice insertion sites. Isothermal titration calorimetry indicates that the Ca2+-binding site in n1alpha_LNS#2 has low affinity (Kd approximately 400 microm). Ca2+ binding ceases to be measurable when an 8- or 15-residue splice insert is present at the splice site SS#2 indicating that alternative splicing can affect Ca2+-binding sites of neurexin LNS/LG domains. Our studies initiate a framework for the putative protein interaction sites of neurexin LNS/LG domains. This framework is essential to understand how incorporation of alternative splice inserts expands the information from a limited set of neurexin genes to produce a large array of synaptic adhesion molecules with potentially very different synaptic function.

Dissection of Synapse Induction by Neuroligins

To study synapse formation by neuroligins, we co-cultured hippocampal neurons with COS cells expressing wild type and mutant neuroligins. The large size of COS cells makes it possible to test the effect of neuroligins presented over an extended surface area. We found that a uniform lawn of wild type neuroligins displayed on the cell surface triggers the formation of hundreds of uniformly sized, individual synaptic contacts that are labeled with neurexin antibodies. Electron microscopy revealed that these artificial synapses contain a presynaptic active zone with docked vesicles and often feature a postsynaptic density. Neuroligins 1, 2, and 3 were active in this assay. Mutations in two surface loops of neuroligin 1 abolished neuroligin binding to neurexin 1beta, a presumptive presynaptic binding partner for postsynaptic neuroligins, and blocked synapse formation. An analysis of mutant neuroligins with an amino acid substitution that corresponds to a mutation described in patients with an autistic syndrome confirmed previous reports that these mutant neuroligins have a compromised capacity to be transported to the cell surface. Nevertheless, the small percentage of mutant neuroligins that reached the cell surface still induced synapse formation. Viewed together, our data suggest that neuroligins generally promote artificial synapse formation in a manner that is associated with beta-neurexin binding and results in morphologically well differentiated synapses and that a neuroligin mutation found in autism spectrum disorders impairs cell-surface transport but does not completely abolish synapse formation activity.

Expression of three gene families encoding cell-cell communication molecules in the prepubertal nonhuman primate hypothalamus

Transsynaptic and glial-neuronal communication are important components of the mechanism underlying the pubertal activation of luteinizing hormone-releasing hormone (LHRH) secretion. The molecules required for the architectural organization of these cell-cell interactions have not been identified. We now show that the hypothalamus of the prepubertal female rhesus monkey expresses a multiplicity of genes encoding three families of adhesion/signalling proteins involved in the structural definition of both neurone-to-neurone and bi-directional neurone-glia communication. These include the neurexin/neuroligin (NRX/NRL) and protocadherin-alpha (PCDHalpha) families of synaptic specifiers/adhesion molecules, and key components of the contactin-dependent neuronal-glial adhesiveness complex, including contactin/F3 itself, the contactin-associated protein-1 (CASPR1), and the glial receptor protein tyrosine phosphatase beta. Prominently expressed among members of the NRX family is the neurexin isoform involved in the specification of glutamatergic synapses. Although NRXs, PCDHalphas and CASPR1 transcripts are mostly detected in neurones, the topography of expression appears different. NRX1 mRNA-containing neurones are scattered throughout the hypothalamus, PCDHalpha mRNA transcripts appear more abundant in neurones of the arcuate nucleus and periventricular region, and neurones positive for CASPR1 mRNA exhibit a particularly striking distribution pattern that delineates the hypothalamus. Examination of LHRH neurones, using the LHRH-secreting cell line GT1-7, showed that these cells contain transcripts encoding NRXs and one of their ligands (NRL1), at least one PCDHalpha (CNR-8/PCDHalpha10), and the CASPR1/contactin complex. The results indicate that the prepubertal female monkey hypothalamus contains a plethora of adhesion/signalling molecules with different but complementary functions, and that an LHRH neuronal cell line expresses key components of this structural complex. The presence of such cell-cell communication machinery in the neuroendocrine brain suggests an integrated participation of their individual components in the central control of female sexual development.

Neuroligins mediate excitatory and inhibitory synapse formation: involvement of PSD-95 and neurexin-1beta in neuroligin-induced synaptic specificity

The balance between excitatory and inhibitory synapses is a tightly regulated process that requires differential recruitment of proteins that dictate the specificity of newly formed contacts. However, factors that control this process remain unidentified. Here we show that members of the neuroligin (NLG) family, including NLG1, NLG2, and NLG3, drive the formation of both excitatory and inhibitory presynaptic contacts. The enrichment of endogenous NLG1 at excitatory contacts and NLG2 at inhibitory synapses supports an important in vivo role for these proteins in the development of both types of contacts. Immunocytochemical and electrophysiological analysis showed that the effects on excitatory and inhibitory synapses can be blocked by treatment with a fusion protein containing the extracellular domain of neurexin-1beta. We also found that overexpression of PSD-95, a postsynaptic binding partner of NLGs, resulted in a shift in the distribution of NLG2 from inhibitory to excitatory synapses. These findings reveal a critical role for NLGs and their synaptic partners in controlling the number of inhibitory and excitatory synapses. Furthermore, relative levels of PSD-95 alter the ratio of excitatory to inhibitory synaptic contacts by sequestering members of the NLG family to excitatory synapses.

Selective capability of SynCAM and neuroligin for functional synapse assembly

Synaptic cell adhesion is central for synapse formation and function. Recently, the synaptic cell adhesion molecules neuroligin 1 (NL1) and SynCAM were shown to induce presynaptic differentiation in cocultured neurons when expressed in a non-neuronal cell. However, it is uncertain how similar the resulting artificial synapses are to regular synapses. Are these molecules isofunctional, or do all neuronal cell adhesion molecules nonspecifically activate synapse formation? To address these questions, we analyzed the properties of artificial synapses induced by NL1 and SynCAM, compared the actions of these molecules with those of other neuronal cell adhesion molecules, and examined the functional effects of NL1 and SynCAM overexpression in neurons. We found that only NL1 and SynCAM specifically induced presynaptic differentiation in cocultured neurons. The induced nerve terminals were capable of both spontaneous and evoked neurotransmitter release, suggesting that a full secretory apparatus was assembled. By all measures, SynCAM- and NL1-induced artificial synapses were identical. Overexpression in neurons demonstrated that only SynCAM, but not NL1, increased synaptic function in immature developing excitatory neurons after 8 d in vitro. Tests of chimeric molecules revealed that the dominant-positive effect of SynCAM on synaptic function in developing neurons was mediated by its intracellular cytoplasmic tail. Interestingly, morphological analysis of neurons overexpressing SynCAM or NL1 showed the opposite of the predictions from electrophysiological results. In this case, only NL1 increased the synapse number, suggesting a role for NL1 in morphological synapse induction. These results suggest that both NL1 and SynCAM act similarly and specifically in artificial synapse induction but that this process does not reflect a shared physiological function of these molecules.

Ethanol-response genes and their regulation analyzed by a microarray and comparative genomic approach in the nematode Caenorhabditis elegans

The nematode shows responses to acute ethanol exposure that are similar to those observed in humans, mice, and Drosophila, namely hyperactivity followed by uncoordination and sedation. We used in this report the nematode Caenorhabditis elegans as a model system to identify and characterize the genes that are affected by ethanol exposure and to link those genes functionally into an ethanol-induced gene network. By analyzing the expression profiles of all C. elegans ORFs using microarrays, we identified 230 genes affected by ethanol. While the ethanol response of some of the identified genes was significant at early time points, that of the majority was at late time points, indicating that the genes in the latter case might represent the physiological consequence of the ethanol exposure. We further characterized the early response genes that may represent those involved directly in the ethanol response. These genes included many heat shock protein genes, indicating that high concentration of ethanol acts as a strong stress to the animal. Interestingly, we identified two non-heat-shock protein genes that were specifically responsive to ethanol. glr-2 was the only glutamate receptor gene to be induced by ethanol. T28C12.4, which encodes a protein with limited homology to human neuroligin, was also specific to ethanol stress. Finally, by analyzing the promoter regions of the early response genes, we identified a regulatory element, TCTGCGTCTCT, that was necessary for the expression of subsets of ethanol response genes.

A balance between excitatory and inhibitory synapses is controlled by PSD-95 and neuroligin

Factors that control differentiation of presynaptic and postsynaptic elements into excitatory or inhibitory synapses are poorly defined. Here we show that the postsynaptic density (PSD) proteins PSD-95 and neuroligin-1 (NLG) are critical for dictating the ratio of excitatory-to-inhibitory synaptic contacts. Exogenous NLG increased both excitatory and inhibitory presynaptic contacts and the frequency of miniature excitatory and inhibitory synaptic currents. In contrast, PSD-95 overexpression enhanced excitatory synapse size and miniature frequency, but reduced the number of inhibitory synaptic contacts. Introduction of PSD-95 with NLG augmented synaptic clustering of NLG and abolished NLG effects on inhibitory synapses. Interfering with endogenous PSD-95 expression alone was sufficient to reduce the ratio of excitatory-to-inhibitory synapses. These findings elucidate a mechanism by which the amounts of specific elements critical for synapse formation control the ratio of excitatory-to-inhibitory synaptic input.

Molecular genetics of autism spectrum disorder

We are on the brink of exciting discoveries into the molecular genetic underpinnings of autism spectrum disorder. Overwhelming evidence of genetic involvement coupled with increased societal attention to the disorder has drawn in more researchers and more research funding. Autism is a strongly genetic yet strikingly complex disorder, in which evidence from different cases supports chromosomal disorders, rare single gene mutations, and multiplicative effects of common gene variants. With more and more interesting yet sometimes divergent findings emerging every year, it is tempting to view these initial molecular studies as so much noise, but the data have also started to coalesce in certain areas. In particular, recent studies in families with autism spectrum disorder have identified uncommon occurrences of a novel genetic syndrome caused by disruptions of the NLGN4 gene on chromosome Xp22. Previous work had identified another uncommon syndrome that is caused by maternal duplications of the chromosome 15q11-13 region. We highlight other converging findings, point toward those areas most likely to yield results, and emphasize the contributions of multiple approaches to identifying the genes of interest.

X-linked mental retardation and autism are associated with a mutation in the NLGN4 gene, a member of the neuroligin family

A large French family including members affected by nonspecific X-linked mental retardation, with or without autism or pervasive developmental disorder in affected male patients, has been found to have a 2-base-pair deletion in the Neuroligin 4 gene (NLGN4) located at Xp22.33. This mutation leads to a premature stop codon in the middle of the sequence of the normal protein and is thought to suppress the transmembrane domain and sequences important for the dimerization of neuroligins that are required for proper cell-cell interaction through binding to beta-neurexins. As the neuroligins are mostly enriched at excitatory synapses, these results suggest that a defect in synaptogenesis may lead to deficits in cognitive development and communication processes. The fact that the deletion was present in both autistic and nonautistic mentally retarded males suggests that the NLGN4 gene is not only involved in autism, as previously described, but also in mental retardation, indicating that some types of autistic disorder and mental retardation may have common genetic origins.

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

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

Functional excitatory synapses in HEK293 cells expressing neuroligin and glutamate receptors

The discovery that neuroligin is a key protein involved in synapse formation offers the unprecedented opportunity to induce functional synapses between neurons and heterologous cells. We took this opportunity recording for the first-time synaptic currents in human embryonic kidney 293 (HEK293) cells transfected with neuroligin and the N-methyl-d-aspartate or AMPA receptor subunits in a co-culture with rat cerebellar granule cells. These currents were similar to synaptic currents recorded in neurons, and their decay kinetics was determined by the postsynaptic subunit combination. Although neuroligin expression was sufficient to detect functional synapses, cotransfection of HEK293 cells with Postsynaptic density-95/synapse-associated protein-90 (PSD-95) significantly increased current frequency. Our results support the central role of neuroligin in the formation of CNS synapses, validate the proposal that PSD-95 allows synaptic maturation, and provide a unique experimental model to study how molecular components determine functional properties of excitatory synapses.

Levels of the synaptic protein X11 alpha/mint1 are increased in hippocampus of rats with epilepsy

X11 alpha or Mint1 is a protein containing an N-terminal sequence, which binds to Munc-18 protein, a middle phosphotyrosine-binding domain (PTB) and two C-terminal PDZ (Post-synaptic density/Discs large/Zone Occludens-1) domains. The PDZ domains, which mediate protein-protein interactions have been shown to be involved in the organization of synaptic signaling pathways. Mint1 plays an important role in vesicle synaptic transport toward the active zone at the pre-synaptic site, and also participates in the transport of NR2B subunit of the NMDA receptor, to the post-synaptic site. To investigate the participation and distribution of this protein in the hippocampal subfield of rats submitted to the pilocarpine model of epilepsy, Mint1 was analyzed using Western blotting and immunohistochemistry. Animals of 5 h of status epilepticus showed decreased levels of this protein in the hippocampus when compared to the control animals. In contrast, animals from seizure-free period (silent group) and during spontaneous seizures phase (chronic group) showed increased Mint1 immunostaining in all hippocampal subfields, mainly in the dentate gyrus, when compared to the control group. The blotting confirmed the results obtained by immunohistochemistry. The present work suggests that Mint1 may be related to hippocampal plasticity during epileptogenesis in the pilocarpine model of temporal lobe epilepsy.

The intracellular domain of theDrosophila cholinesterase-like neural adhesion protein, gliotactin, is natively unfolded

Drosophila gliotactin (Gli) is a 109-kDa transmembrane, cholinesterase-like adhesion molecule (CLAM), expressed in peripheral glia, that is crucial for formation of the blood-nerve barrier. The intracellular portion (Gli-cyt) was cloned and expressed in the cytosolic fraction of Escherichia coli BLR(DE3) at 45 mg/L and purified by Ni-NTA (nitrilotriacetic acid) chromatography. Although migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), under denaturing conditions, was unusually slow, molecular weight determination by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) confirmed that the product was consistent with its theoretical size. Gel filtration chromatography yielded an anomalously large Stokes radius, suggesting a fully unfolded conformation. Circular dichroism (CD) spectroscopy demonstrated that Gli-cyt was >50% unfolded, further suggesting a nonglobular conformation. Finally, 1D-(1)H NMR conclusively demonstrated that Gli-cyt possesses an extended unfolded structure. In addition, Gli-cyt was shown to possess charge and hydrophobic properties characteristic of natively unfolded proteins (i.e., proteins that, when purified, are intrinsically disordered under physiologic conditions in vitro).

Mechanisms of Synapse Assembly and Disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.

Synaptogenesis in the CNS: an odyssey from wiring together to firing together

To acquire a better comprehension of nervous system function, it is imperative to understand how synapses are assembled during development and subsequently altered throughout life. Despite recent advances in the fields of neurodevelopment and synaptic plasticity, relatively little is known about the mechanisms that guide synapse formation in the central nervous system (CNS). Although many structural components of the synaptic machinery are pre-assembled prior to the arrival of growth cones at the site of their potential targets, innumerable changes, central to the proper wiring of the brain, must subsequently take place through contact-mediated cell-cell communications. Identification of such signalling molecules and a characterization of various events underlying synaptogenesis are pivotal to our understanding of how a brain cell completes its odyssey from "wiring together to firing together". Here we attempt to provide a comprehensive overview that pertains directly to the cellular and molecular mechanisms of selection, formation and refinement of synapses during the development of the CNS in both vertebrates and invertebrates.

Characterization of the interaction of a recombinant soluble neuroligin-1 with neurexin-1beta

Neuroligins, proteins of the alpha/beta-hydrolase fold family, are found as postsynaptic transmembrane proteins whose extracellular domain associates with presynaptic partners, proteins of the neurexin family. To characterize the molecular basis of neuroligin interaction with neurexin-beta, we expressed five soluble and exportable forms of neuroligin-1 from recombinant DNA sources, by truncating the protein before the transmembrane span near its carboxyl terminus. The extracellular domain of functional neuroligin-1 associates as a dimer when analyzed by sedimentation equilibrium. By surface plasmon resonance, we established that soluble neuroligins-1 bind neurexin-1beta, but the homologous alpha/beta-hydrolase fold protein, acetylcholinesterase, failed to associate with the neurexins. Neuroligin-1 has a unique N-linked glycosylation pattern in the neuroligin family, and glycosylation and its processing modify neuroligin activity. Incomplete processing of the protein and enzymatic removal of the oligosaccharides chain or the terminal sialic acids from neuroligin-1 enhance its activity, whereas deglycosylation of neurexin-1beta did not alter its association capacity. In particular, the N-linked glycosylation at position 303 appears to be a major determinant in modifying the association with neurexin-1beta. We show here that glycosylation processing of neuroligin, in addition to mRNA splicing and gene selection, contributes to the specificity of the neurexin-beta/neuroligin-1 association.

Narp and NP1 form heterocomplexes that function in developmental and activity-dependent synaptic plasticity

Narp is a neuronal immediate early gene that plays a role in excitatory synaptogenesis. Here, we report that native Narp in brain is part of a pentraxin complex that includes NP1. These proteins are covalently linked by disulfide bonds into highly organized complexes, and their relative ratio in the complex is dynamically dependent upon the neuron's activity history and developmental stage. Complex formation is dependent on their distinct N-terminal coiled-coil domains, while their closely homologous C-terminal pentraxin domains mediate association with AMPA-type glutamate receptors. Narp is substantially more effective in assays of cell surface cluster formation, coclustering of AMPA receptors, and excitatory synaptogenesis, yet their combined expression results in supraadditive effects. These studies support a model in which Narp can regulate the latent synaptogenic activity of NP1 by forming mixed pentraxin assemblies. This mechanism appears to contribute to both activity-independent and activity-dependent excitatory synaptogenesis.

Structure-stability-function relationships of dendritic spines

Dendritic spines, which receive most of the excitatory synaptic input in the cerebral cortex, are heterogeneous with regard to their structure, stability and function. Spines with large heads are stable, express large numbers of AMPA-type glutamate receptors, and contribute to strong synaptic connections. By contrast, spines with small heads are motile and unstable and contribute to weak or silent synaptic connections. Their structure-stability-function relationships suggest that large and small spines are "memory spines" and "learning spines", respectively. Given that turnover of glutamate receptors is rapid, spine structure and the underlying organization of the actin cytoskeleton are likely to be major determinants of fast synaptic transmission and, therefore, are likely to provide a physical basis for memory in cortical neuronal networks. Characterization of supramolecular complexes responsible for synaptic memory and learning is key to the understanding of brain function and disease.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

Synaptic cell adhesion goes functional

A growing number of candidate genes has been implicated in linking the two sides of a synapse but definitive proof of a specific role for many of them is still scarce. Exploiting the vast amount of sequence data, a novel family of homophilic synaptic cell-adhesion molecules (SynCAMs) has now been identified in mice. SynCAMs are evolutionarily related to invertebrate immunoglobulin-like (Ig)-domain proteins and they promote the formation and differentiation of functional synapses in vitro.

The enigma of transmitter-selective receptor accumulation at developing inhibitory synapses

The control of synaptic inhibition is crucial for normal brain function. More than 20 years ago, glycine and gamma-aminobutyric acid (GABA) were shown to be the two major inhibitory neurotransmitters. They can be released independently from different terminals or co-released from the same terminal to activate postsynaptic glycine and GABA(A) receptors. The anchoring protein gephyrin is involved in the postsynaptic accumulation of both glycine and GABA(A) receptors. In lower brain regions, both receptors can be concentrated in synapses, whereas in higher brain regions, glycine receptors are mostly excluded from postsynaptic sites. The activation of glycine and/or GABA(A) receptors determines the strength and precise timing of inhibition. Therefore, tight regulation of postsynaptic glycine versus GABA(A) receptor localization is crucial for optimizing synaptic inhibition in neurons. This review focuses on recent findings and discusses questions concerning the specificity of postsynaptic inhibitory neurotransmitter receptor accumulation during inhibitory synapse formation and development.

AMPA receptor potentiation by acetylcholinesterase is age-dependently upregulated at synaptogenesis sites of the rat brain

We have used radioligand binding to synaptic membranes from distinct rat brain regions and quantitative autoradiography to investigate the postnatal evolution of acetylcholinesterase (AChE)-evoked up-regulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in CNS areas undergoing synaptogenesis. Incubation of synaptosomal membranes or brain sections with purified AChE caused a developmentally modulated enhancement in the binding of [3H]-(S)-AMPA and the specific AMPA receptor ligand [3H]-(S)-5-fluorowillardiine, but did not modify binding to kainate neither N-methyl-D-aspartate receptors. In all postnatal ages investigated (4, 7, 14, 20, 27, 40 days-old and adult rats), AChE effect on binding was concentration-dependent and blocked by propidium, BW 284c51, diisopropylfluorophosphonate and eserine, therefore requiring indemnity of both peripheral and active sites of the enzyme. AChE-mediated enhancement of [3H]-fluorowillardiine binding was measurable in all major CNS areas, but displayed remarkable anatomical selectivity and developmental regulation. Autoradiograph densitometry exhibited distinct temporal profiles and peaks of treated/control binding ratios for different cortices, cortical layers, and nuclei. Within the parietal, occipital and temporal neocortices, hippocampal CA1 field and cerebellum, AChE-potentiated binding ratios peaked in chronological correspondence with synaptogenesis periods of the respective AMPA-receptor containing targets. This modulation of AMPA receptors by AChE is a molecular mechanism able to transduce localized neural activity into durable modifications of synaptic molecular structure and function. It might also contribute to AChE-mediated neurotoxicity, as postulated in Alzheimer's disease and other CNS disorders.

Diversity of the cadherin-related neuronal receptor/protocadherin family and possible DNA rearrangement in the brain

Both the brain and the immune systems are complex. The complexity is generated by enormously diversified single cells. In the immune system, extensive cell death, gene regulation of immunoglobulin (Ig) and T-cell receptor (TCR) gene expression, and somatic rearrangement and mutations are known to generate an enormous diversity of lymphocytes. In this process, double-strand DNA breaks (DSBs) and DSB repair play significant roles. These processes at a DNA level are also physiologically significant in the nervous system during neurogenesis, and chromosomal variations have been detected in the nucleus of differentiated neurones. In another parallel with the immune system, cadherin-related neuronal receptors (CNRs) are diversified synaptic proteins. The CNR genes belong to protocadherin (Pcdh) gene clusters. Genomic organizations of CNR/Pcdh genes are similar to that of the Ig and TCR genes. Somatic mutations in and combinatorial gene regulation of CNR/Pcdh transcripts during neurogenesis have been reported. This review focuses on the diversity of the CNR/Pcdh genes and possible DNA diversification in the nervous system.

Differing mechanisms for glutamate receptor aggregation on dendritic spines and shafts in cultured hippocampal neurons

We have explored the ability of axons from spinal and hippocampal neurons to aggregate NMDA- and AMPA-type glutamate receptors on each other as a way of exploring the molecular differences between their presynaptic elements. Spinal axons, which normally cluster only AMPA-type glutamate receptors on other spinal neurons, cluster both AMPA- and NMDA-type glutamate receptors on the dendritic shafts of hippocampal interneurons but are ineffective at clustering either subtype of glutamate receptor on the dendritic spines of hippocampal pyramidal neurons. Conversely, hippocampal axons appear to be multipotent, capable of clustering both AMPA- and NMDA-type glutamate receptors on hippocampal interneurons and pyramidal cells. The secretion of the neuronal activity-regulated pentraxin (Narp) by hippocampal axons is restricted to contacts with interneurons. Exogenous application of Narp to cultured hippocampal neurons results in clusters of both NMDA- and AMPA-type glutamate receptors on hippocampal interneurons but not hippocampal pyramidal neurons. Because Narp displays no ability to directly aggregate NMDA receptors, we propose that Narp aggregates NMDA receptors in hippocampal interneurons indirectly through cytoplasmic coupling to synaptic AMPA receptors. Furthermore, our data suggest the existence of a novel molecule(s), capable of forming excitatory synapses on dendritic spines.

Synaptic targeting of N-type calcium channels in hippocampal neurons

N-type calcium (Ca2+) channels play a critical role in synaptic function, but the mechanisms responsible for their targeting in neurons are poorly understood. N-type channels are formed by an alpha(1B) (Ca(V)2.2) pore-forming subunit associated with beta and alpha2delta auxiliary subunits. By expressing epitope-tagged recombinant alpha1B subunits in rat hippocampal neuronal cultures, we demonstrate here that synaptic targeting of N-type channels depends on neuronal contacts and synapse formation. We also establish that the C-terminal 163 aa (2177-2339) of the alpha1B-1 (Ca(V)2.2a) splice variant contain sequences that are both necessary and sufficient for synaptic targeting. By site-directed mutagenesis, we demonstrate that postsynaptic density-95/discs large/zona occludens-1 and Src homology 3 domain-binding motifs located within this region of the alpha1B subunit (Maximov et al., 1999) act as synergistic synaptic targeting signals. We also show that the recombinant modular adaptor proteins Mint1 and CASK colocalize with N-type channels in synapses. We found that the alpha1B-2 (Ca(V)2.2b) splice variant is restricted to soma and dendrites and postulated that somatodendritic and axonal/presynaptic isoforms of N-type channels are generated via alternative splicing of alpha1B C termini. These data lead us to propose that during synaptogenesis, the alpha1B-1 (Ca(V)2.2a) splice variant of the N-type Ca2+ channel pore-forming subunit is recruited to presynaptic locations by means of interactions with modular adaptor proteins Mint1 and CASK. Our results provide a novel insight into the molecular mechanisms responsible for targeting of Ca2+ channels and other synaptic proteins in neurons.

The presynaptic particle web: ultrastructure, composition, dissolution, and reconstitution

We report the purification of a presynaptic "particle web" consisting of approximately 50 nm pyramidally shaped particles interconnected by approximately 100 nm spaced fibrils. This is the "presynaptic grid" described in early EM studies. It is completely soluble above pH 8, but reconstitutes after dialysis against pH 6. Interestingly, reconstituted particles orient and bind PSDs asymmetrically. Mass spectrometry of purified web components reveals major proteins involved in the exocytosis of synaptic vesicles and in membrane retrieval. Our data support the idea that the CNS synaptic junction is organized by transmembrane adhesion molecules interlinked in the synaptic cleft, connected via their intracytoplasmic domains to the presynaptic web on one side and to the postsynaptic density on the other. The CNS synaptic junction may therefore be conceptualized as a complicated macromolecular scaffold that isostatically bridges two closely aligned plasma membranes.

Regional localization and developmental profile of acetylcholinesterase-evoked increases in [(3)H]-5-fluororwillardiine binding to AMPA receptors in rat brain

In addition to its role in hydrolyzing the neurotransmitter acetylcholine, the synaptically enriched enzyme acetylcholinesterase (AChE) has been reported to play an important role in the development and remodelling of neural processes and synapses. We have shown previously that AChE causes an increase in binding of the specific AMPA receptor ligand (S)-[(3)H]-5-fluorowillardiine ([(3)H]-FW) to rat brain membranes. In this study we have used quantitative autoradiography to investigate the regional distribution and age-dependence of AChE-evoked increases in the binding of [(3)H]-FW in rat brain. Pretreatment of rat brain sections with AChE caused a marked enhancement of [(3)H]-FW binding to many, but not all, brain areas. The increased [(3)H]-FW binding was blocked by the specific AChE inhibitor BW 284c51. The maximal potentiation of [(3)H]-FW binding occurred at different developmental age-points in different regions with a profile consistent with the peak periods for synaptogenesis in any given region. In addition to its effects on brain sections, AChE also strongly potentiated [(3)H]-FW binding to detergent solubilized AMPA receptors suggesting a direct action on the receptors themselves rather than an indirect effect on the plasma membrane. These findings suggest that modulation of AMPA receptors could provide one molecular mechanism for the previously reported effects of AChE on synapse formation, synaptic plasticity and neurodegeneration.

Sharpin, a novel postsynaptic density protein that directly interacts with the shank family of proteins

The Shank family of proteins (also termed CortBP, ProSAP, or Synamon) is highly enriched in the postsynaptic density (PSD) of excitatory synapses in brain. Shank contains multiple domains for protein-protein interactions, including ankyrin repeats, SH3 domain, PDZ domain, SAM domain, and an extensive proline-rich region. We have identified a novel protein, termed Sharpin, that directly interacts with the ankyrin repeats of Shank. Sharpin is enriched in the PSD and forms a complex with Shank in heterologous cells and brain. Immunostaining reveals the presence of Sharpin at excitatory synapses and its colocalization with Shank. While the C-terminal half of Sharpin interacts with Shank, the N-terminal half of Sharpin mediates homomultimerization. Considering the fact that the ankyrin repeats and the SH3 domain of Shank can be truncated by alternative splicing, these results define Sharpin as a novel PSD protein that may regulate the complexity of the Shank-based protein network in an alternative splicing-dependent manner.

Modulation of amyloid precursor protein metabolism by X11alpha /Mint-1. A deletion analysis of protein-protein interaction domains

Modulation of amyloid precursor protein (APP) metabolism plays a pivotal role in the pathogenesis of Alzheimer's disease. The phosphotyrosine-binding/protein interaction (PTB/PI) domain of X11alpha, a neuronal cytosolic adaptor protein, binds to the YENPTY sequence in the cytoplasmic carboxyl terminus of APP. This interaction prolongs the half-life of APP and inhibits Abeta40 and Abeta42 secretion. X11alpha/Mint-1 has multiple protein-protein interaction domains, a Munc-18 interaction domain (MID), a Cask/Lin-2 interaction domain (CID), a PTB/PI domain, and two PDZ domains. These X11alpha protein interaction domains may modulate its effect on APP processing. To test this hypothesis, we performed a deletion analysis of X11alpha effects on metabolism of APP(695) Swedish (K595N/M596L) (APP(sw)) by transient cotransfection of HEK 293 cells with: 1) X11alpha (X11alpha-wt, N-MID-CID-PTB-PDZ-PDZ-C), 2) amino-terminal deletion (X11alpha-DeltaN, PTB-PDZ-PDZ), 3) carboxyl-terminal deletion (X11alpha-DeltaPDZ, MID-CID-PTB), or 4) deletion of both termini (PTB domain only, PTB). The carboxyl terminus of X11alpha was required for stabilization of APP(sw) in cells. In contrast, the amino terminus of X11alpha was required to stimulate APPs secretion. X11alpha, X11alpha-DeltaN, and X11alpha-PTB, but not X11alpha-DeltaPDZ, were effective inhibitors of Abeta40 and Abeta42 secretion. These results suggest that additional protein interaction domains of X11alpha modulate various aspects of APP metabolism.