Structures, alternative splicing, and neurexin binding of multiple neuroligins

Structure function and splice site analysis of the synaptogenic activity of the neurexin-1 beta LNS domain

Recent findings suggest that the neurexin-neuroligin link promotes both GABAergic and glutamatergic synaptogenesis, but the mechanism by which neurexins influence the clustering of appropriate neuroligins and postsynaptic differentiation remains unclear. Previous studies suggested that the presence or absence of alternatively spliced residues at splice site 4 (S4) in the neurexin LNS domain may regulate neurexin function. We demonstrate that addition of the S4 insert selectively reduces the ability of neurexin-1beta to cluster neuroligin-1/3/4 and glutamatergic postsynaptic proteins, although clustering of neuroligin-2 and GABAergic postsynaptic proteins remain strong. Furthermore, addition of the S4 insert decreases the binding affinity of neurexin-1beta to neuroligins-1 and -4 but has little effect on binding to neuroligins-2 and -3. Additional structure-function studies reveal the neurexin binding interface mediating synaptogenic activity to be composed primarily of residues in the beta2beta3, beta6beta7, and beta10beta11 loops on one rim of the LNS domain beta sandwich. Mutation of two predicted Ca(2+)-binding residues disrupts postsynaptic protein clustering and binding to neuroligins, consistent with previous findings that neurexin-neuroligin binding is Ca2+ dependent. Glutamatergic postsynaptic clustering was more readily disrupted by the mutagenesis than GABAergic postsynaptic protein clustering. Perhaps neurexins-neuroligins, or neurexin-1beta at least, is most important for GABA synapse formation or controlling the balance of GABA and glutamate synapses. These results suggest that differential neurexin-neuroligin binding affinities and splice variations may play an instructive role in postsynaptic differentiation.

Genome-wide P-element screen for Drosophila synaptogenesis mutants

A molecular understanding of synaptogenesis is a critical step toward the goal of understanding how brains "wire themselves up," and then "rewire" during development and experience. Recent genomic and molecular advances have made it possible to study synaptogenesis on a genomic scale. Here, we describe the results of a screen for genes involved in formation and development of the glutamatergic Drosophila neuromuscular junction (NMJ). We screened 2185 P-element transposon mutants representing insertions in approximately 16% of the entire Drosophila genome. We first identified recessive lethal mutants, based on the hypothesis that mutations causing severe disruptions in synaptogenesis are likely to be lethal. Two hundred twenty (10%) of all insertions were homozygous lethal. Two hundred five (93%) of these lethal mutants developed at least through late embryogenesis and formed neuromusculature. We examined embryonic/larval NMJs in 202 of these homozygous mutants using immunocytochemistry and confocal microscopy. We identified and classified 88 mutants with altered NMJ morphology. Insertion loci in these mutants encode several different types of proteins, including ATP- and GTPases, cytoskeletal regulators, cell adhesion molecules, kinases, phosphatases, RNA regulators, regulators of protein formation, transcription factors, and transporters. Thirteen percent of insertions are in genes that encode proteins of novel or unknown function. Complementation tests and RT-PCR assays suggest that approximately 51% of the insertion lines carry background mutations. Our results reveal that synaptogenesis requires the coordinated action of many different types of proteins--perhaps as much as 44% of the entire genome--and that transposon mutageneses carry important caveats that must be respected when interpreting results generated using this method.

A Mutation Linked with Autism Reveals a Common Mechanism of Endoplasmic Reticulum Retention for the α,β-Hydrolase Fold Protein Family

A mutation linked to autistic spectrum disorders encodes an Arg to Cys replacement in the C-terminal portion of the extracellular domain of neuroligin-3. The solvent-exposed Cys causes virtually complete retention of the protein in the endoplasmic reticulum when the protein is expressed in transfected cells. An identical Cys substitution was reported for butyrylcholinesterase through genotyping patients with post-succinylcholine apnea. Neuroligin, butyrylcholinesterase, and acetylcholinesterase are members of the alpha,beta-hydrolase fold family of proteins sharing sequence similarity and common tertiary structures. Although these proteins have distinct oligomeric assemblies and cellular dispositions, homologous Arg residues in neuroligin-3 (Arg-451), in butyrylcholinesterase (Arg-386), and in acetylcholinesterase (Arg-395) are conserved in all studied mammalian species. To examine whether an homologous Arg to Cys mutation affects related proteins similarly despite their differing capacities to oligomerize, we inserted homologous mutations in the acetylcholinesterase and butyrylcholinesterase cDNAs. Using confocal fluorescence microscopy and analysis of oligosaccharide processing, we find that the homologous Arg to Cys mutation also results in endoplasmic reticulum retention of the two cholinesterases. Small quantities of mutated acetylcholinesterase exported from the cell retain activity but show a greater K(m), a much smaller k(cat), and altered substrate inhibition. The nascent proteins associate with chaperones during processing, but the mutation presumably restricts processing through the endoplasmic reticulum and Golgi apparatus, because of local protein misfolding and inability to oligomerize. The mutation may alter the capacity of these proteins to dissociate from their chaperone prior to oligomerization and processing for export.

A splice code for trans-synaptic cell adhesion mediated by binding of neuroligin 1 to alpha- and beta-neurexins

Previous studies suggested that postsynaptic neuroligins form a trans-synaptic complex with presynaptic beta-neurexins, but not with presynaptic alpha-neurexins. Unexpectedly, we now find that neuroligins also bind alpha-neurexins and that alpha- and beta-neurexin binding by neuroligin 1 is regulated by alternative splicing of neuroligin 1 (at splice site B) and of neurexins (at splice site 4). In neuroligin 1, splice site B is a master switch that determines alpha-neurexin binding but leaves beta-neurexin binding largely unaffected, whereas alternative splicing of neurexins modulates neuroligin binding. Moreover, neuroligin 1 splice variants with distinct neurexin binding properties differentially regulate synaptogenesis: neuroligin 1 that binds only beta-neurexins potently stimulates synapse formation, whereas neuroligin 1 that binds to both alpha- and beta-neurexins more effectively promotes synapse expansion. These findings suggest that neuroligin binding to alpha- and beta-neurexins mediates trans-synaptic cell adhesion but has distinct effects on synapse formation, indicating that expression of different neuroligin and neurexin isoforms specifies a trans-synaptic signaling code.

Cell-cell interactions in synaptogenesis

Synaptogenesis is a finely organized process, intriguing in its precise temporal and spatial resolution. It occurs as the dendrite of a postsynaptic neuron and an incoming axon communicate at defined sites to establish a stable synapse together. The molecular cues that guide synaptogenesis are now beginning to be identified, and cell surface interactions at synaptic sites participate prominently in the key steps. Interactions include trans-synaptic adhesion of pre- and post-synaptic neurons but also binding to non-neuronal neighboring cells and the extracellular matrix. These signals recruit scaffolding molecules, other adhesion molecules, and neurotransmitter receptors to bring together the key components of functional synapses. Recent progress provides stimulating insights into the role of adhesion and signaling molecules in the formation and function of synaptic specializations.

alpha-Neurexins are required for efficient transmitter release and synaptic homeostasis at the mouse neuromuscular junction

Neurotransmission at chemical synapses of the brain involves alpha-neurexins, neuron-specific cell-surface molecules that are encoded by three genes in mammals. Deletion of alpha-neurexins in mice previously demonstrated an essential function, leading to early postnatal death of many double-knockout mice and all triple mutants. Neurotransmitter release at central synapses of newborn knockouts was severely reduced, a function of alpha-neurexins that requires their extracellular sequences. Here, we investigated the role of alpha-neurexins at neuromuscular junctions, presynaptic terminals that lack a neuronal postsynaptic partner, addressing an important question because the function of neurexins was hypothesized to involve cell-adhesion complexes between neurons. Using systems physiology, morphological analyses and electrophysiological recordings, we show that quantal content, i.e. the number of acetylcholine quanta released per nerve impulse from motor nerve terminals, and frequency of spontaneous miniature endplate potentials at the slow-twitch soleus muscle are reduced in adult alpha-neurexin double-knockouts, consistent with earlier data on central synapses. However, the same parameters at diaphragm muscle neuromuscular junctions showed no difference in basal neurotransmission. To reconcile these observations, we tested the capability of control and alpha-neurexin-deficient diaphragm neuromuscular junctions to compensate for an experimental reduction of postsynaptic acetylcholine receptors by a compensatory increase of presynaptic release: Knockout neuromuscular junctions produced significantly less upregulation of quantal content than synapses from control mice. Our data suggest that alpha-neurexins are required for efficient neurotransmitter release at neuromuscular junctions, and that they may perform a role in the molecular mechanism of synaptic homeostasis at these peripheral synapses.

Neuroligins and neurexins: linking cell adhesion, synapse formation and cognitive function

Cell adhesion represents the most direct way of coordinating synaptic connectivity in the brain. Recent evidence highlights the importance of a trans-synaptic interaction between postsynaptic neuroligins and presynaptic neurexins. These transmembrane molecules bind each other extracellularly to promote adhesion between dendrites and axons. This signals the recruitment of presynaptic and postsynaptic molecules to form a functional synapse. Remarkably, neuroligins alone can induce the formation of fully functional presynaptic terminals in contacting axons. Conversely, neurexins alone can induce postsynaptic differentiation and clustering of receptors in dendrites. Therefore, the neuroligin-neurexin interaction has the unique ability to act as a bi-directional trigger of synapse formation. Here, we review several recent studies that offer clues as to how these proteins form synapses and how they might function in the brain to establish and modify neuronal network properties and cognition.

Expression patterns of neurexin-1 and neuroligins in brain and retina of the chick embryo: Neuroligin-3 is absent in retina

Neuroligins (NLs) constitute a family of cell-surface proteins that interact with neurexins (beta-Nxs), another class of neuronal cell-surface proteins, one of each class functioning together in synapse formation. The localization of the various neurexins and neuroligins, however, has not yet been clarified in chicken. Therefore, we studied the expression patterns of neurexin-1 (Nx-1) and neuroligin-1 and -3 during embryonic development of the chick retina and brain by reverse-transcriptase polymerase chain reaction (RT-PCR) and in situ hybridization (ISH). While neurexin-1 increased continuously in both brain and retina, the expression of both neuroligins was more variable. As shown by ISH, Nx-1 is expressed in the inner half retina along with differentiation of ganglion and amacrine cells. Transcripts of NL-1 were detected as early as day 4 and increased with the maturation of the different brain regions. In different brain regions, NL-1 showed a different time regulation. Remarkably, neuroligin-3 was entirely absent in retina. This study indicates that synaptogenetic processes in brain and retina use different molecular machineries, whereby the neuroligins might represent the more distinctly regulated part of the neurexin-neuroligin complexes. Noticeably, NL-3 does not seem to be involved in the making of retinal synapses.

The Arg473Cys-neuroligin-1 mutation modulates NMDA mediated synaptic transmission and receptor distribution in hippocampal neurons

Synapses mediate communication between neurons, thus playing a fundamental role in information processing in the CNS. Neuroligins form a family of heterophilic synaptic cell adhesion molecules, and neuroligin 1 (NL1) has been shown to be involved in the formation of excitatory synapses and have been suggested to associate indirectly with NMDA receptors by common binding to PSD95. A mutation in neuroligin 3 (Arg451Cys-NL3, human sequence numbering) identified in autistic patients is associated with altered spine density and has reduced binding capacity for its presynaptic partner beta-neurexin. Here, we investigated the role of NL1 and the homologous NL1 mutation Arg473Cys-NL1 (R473C-NL1) in excitatory synaptic transmission and NMDA receptor distribution. We demonstrate that R473C-NL1, when expressed in cultured hippocampal neurons, can induce a dramatic increase in NMDA current amplitude and that this change is accompanied by NMDA receptor clustering in the postsynaptic cell.

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.

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.

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.

Progress from the postsynaptic side: signaling in synaptic differentiation

The signals mediating synaptic differentiation are critical for our understanding of synapse formation and nervous system development. Recent progress highlights the roles of membrane-bound and soluble signaling pathways in this process. The postsynaptic adhesion molecules neuroligin-1, neuroligin-2, and SynCAM1 provide target-derived signals leading to the assembly of presynaptic terminals. In the reverse direction, neuroligins also relay signals into postsynaptic sites through interactions with their presynaptic partners, the beta-neurexins. This adhesion-based system helps to determine the neurotransmitter specificity of postsynaptic specializations. In addition, soluble molecules are released from target neurons and initiate presynaptic differentiation, as demonstrated for the fibroblast growth factor FGF-22. This Perspective summarizes new insights into early synaptic differentiation signals and discusses underlying principles.

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.

Neurexins induce differentiation of GABA and glutamate postsynaptic specializations via neuroligins

Formation of synaptic connections requires alignment of neurotransmitter receptors on postsynaptic dendrites opposite matching transmitter release sites on presynaptic axons. beta-neurexins and neuroligins form a trans-synaptic link at glutamate synapses. We show here that neurexin alone is sufficient to induce glutamate postsynaptic differentiation in contacting dendrites. Surprisingly, neurexin also induces GABA postsynaptic differentiation. Conversely, neuroligins induce presynaptic differentiation in both glutamate and GABA axons. Whereas neuroligins-1, -3, and -4 localize to glutamate postsynaptic sites, neuroligin-2 localizes primarily to GABA synapses. Direct aggregation of neuroligins reveals a linkage of neuroligin-2 to GABA and glutamate postsynaptic proteins, but the other neuroligins only to glutamate postsynaptic proteins. Furthermore, mislocalized expression of neuroligin-2 disperses postsynaptic proteins and disrupts synaptic transmission. Our findings indicate that the neurexin-neuroligin link is a core component mediating both GABAergic and glutamatergic synaptogenesis, and differences in isoform localization and binding affinities may contribute to appropriate differentiation and specificity.

Synaptic targeting of neuroligin is independent of neurexin and SAP90/PSD95 binding

Synaptic cell adhesion and synaptogenesis are thought to involve the interaction of neuroligin, a postsynaptic transmembrane protein, with its presynaptic ligand neurexin. Neuroligin also interacts with SAP90/PSD95, a multidomain scaffolding protein thought to recruit proteins to postsynaptic sites. Using expression of GFP-tagged versions of neuroligin in cultured hippocampal neurons, we find that neuroligin is targeted to synapses via intracellular sequences distinct from its SAP90/PSD95 binding site. A neuroligin mutant lacking the intracellular domain fails to target to synapses. These data indicate that postsynaptic targeting of neuroligin does not rely on the scaffolding action of SAP90/PSD95 and is not induced by binding to presynaptic neurexin. Neuroligin is rather targeted to synapses via a postsynaptic mechanism, which may precede and be necessary for subsequent recruitment of neurexin and other neuroligin interactors such as SAP90/PSD95, suggesting a pivotal position for neuroligin in a putative hierarchy of interactions assembling or stabilizing 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.

Genética do autismo

O autismo é uma doença neuropsiquiátrica com profundas conseqüências sociofamilares. Inúmeros trabalhos investigaram pacientes e famílias com metodologia genético-clínica, citogenética e biologia molecular. Os resultados destes trabalhos apontam para um modelo multiloci com interação epistática associado à etiologia do autismo.

Alternative Splice Variants Encoding Unstable Protein Domains Exist in the Human Brain

Alternative splicing has been recognized as a major mechanism by which protein diversity is increased without significantly increasing genome size in animals and has crucial medical implications, as many alternative splice variants are known to cause diseases. Despite the importance of knowing what structural changes alternative splicing introduces to the encoded proteins for the consideration of its significance, the problem has not been adequately explored. Therefore, we systematically examined the structures of the proteins encoded by the alternative splice variants in the HUGE protein database derived from long (>4 kb) human brain cDNAs. Limiting our analyses to reliable alternative splice junctions, we found alternative splice junctions to have a slight tendency to avoid the interior of SCOP domains and a strong statistically significant tendency to coincide with SCOP domain boundaries. These findings reflect the occurrence of some alternative splicing events that utilize protein structural units as a cassette. However, 50 cases were identified in which SCOP domains are disrupted in the middle by alternative splicing. In six of the cases, insertions are introduced at the molecular surface, presumably affecting protein functions, while in 11 of the cases alternatively spliced variants were found to encode pairs of stable and unstable proteins. The mRNAs encoding such unstable proteins are much less abundant than those encoding stable proteins and tend not to have corresponding mRNAs in non-primate species. We propose that most unstable proteins encoded by alternative splice variants lack normal functions and are an evolutionary dead-end.

Mouse acetylcholinesterase interacts in yeast with the extracellular matrix component laminin-1β

Acetylcholinesterase (AChE) is likely to have roles other than the hydrolysis of acetylcholine, e.g., related to developmental processes like neurite outgrowth, differentiation and adhesion. Here, we investigated whether AChE can function as a heterophilic cell adhesion molecule and searched for proteins interacting with it. Using the yeast two-hybrid method and a mouse brain cDNA library, we have identified an interaction between a partial cDNA encoding the globular domain IV of laminin chain beta1 and the amino acids 240-503 of mouse AChE. Biochemical co-immunoprecipitation assays confirmed the genetic results. We suggest that AChE, by interacting with laminin-1, is able to exert changes in adhesion signaling pathways.

The Arg451Cys-neuroligin-3 mutation associated with autism reveals a defect in protein processing

The neuroligins are a family of postsynaptic transmembrane proteins that associate with presynaptic partners, the beta-neurexins. Neurexins and neuroligins play a critical role in initiating formation and differentiation of synaptic junctions. A recent study reported that a mutation of neuroligin-3 (NL3), an X-linked gene, was found in siblings with autistic spectrum disorder in which two affected brothers had a point mutation that substituted a Cys for Arg451. To characterize the mutation at the biochemical level, we analyzed expression and activity of the mutated protein. Mass spectrometry comparison of the disulfide bonding pattern between the native and the mutated proteins indicates the absence of aberrant disulfide bonding, suggesting that the secondary structure of the mutated protein is conserved. However, the mutation separately affects protein expression and activity. The Cys mutation causes defective neuroligin trafficking, leading to retention of the protein in the endoplasmic reticulum. This, in turn, decreases the delivery of NL3 to the cell surface. Also, the small fraction of protein that reaches the cell membrane lacks or has markedly diminished beta-neurexin-1 (NX1beta) binding activity. Other substitutions for Arg451 allow for normal cellular expression but diminished affinity for NX1beta. Our findings reveal a cellular phenotype and loss of function for a congenital mutation associated with autistic spectrum disorders.

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.

Disorder-associated mutations lead to functional inactivation of neuroligins

Autism is a neuro-developmental syndrome that affects 0.1-0.5% of the population. It has been proposed that alterations in neuronal circuitry and/or neuronal signaling are responsible for the behavioral and cognitive aberrations in autism patients. However, the cellular basis of such alterations is unknown. Recently, point mutations in a family of neuronal cell adhesion molecules called neuroligins have been linked to autism-spectrum disorders and mental retardation. We investigated the consequences of these disease-associated mutations on neuroligin function. We demonstrate that the point mutation at arginine 451 and a nonsense mutation at aspartate 396 of neuroligin-3 and -4 (NL3 and NL4), respectively, result in intracellular retention of the mutant proteins. Over-expression of wild-type NL3 and NL4 proteins in hippocampal neurons stimulates the formation of presynaptic terminals, whereas the disease-associated mutations result in a loss of this synaptic function. Our findings suggest that the previously identified mutations in neuroligin genes are likely to be relevant for the neuro-developmental defects in autism-spectrum disorders and mental retardation since they impair the function of a synaptic cell adhesion molecule.

Postsynaptic N-methyl-D-aspartate receptor function requires alpha-neurexins

Alpha-neurexins are neuron-specific cell-surface molecules that are essential for the functional organization of presynaptic Ca2+ channels and release sites. We have now examined postsynaptic glutamate receptor function in alpha-neurexin knockout (KO) mice by using whole-cell recordings in cultured neocortical slices. Unexpectedly, we find that alpha-neurexins are required for normal activity of N-methyl-D-aspartate (NMDA)- but not alpha-amino-3-hydroxy-5-methyl-4-isoxyzolepropionic acid (AMPA)-type glutamate receptors. In alpha-neurexin-deficient mice, the ratio of NMDA- to AMPA-receptor currents, recorded as evoked synaptic responses, was diminished approximately 50%. Furthermore, the NMDA-receptor-dependent component of spontaneous synaptic miniature responses was reduced approximately 50%, whereas the AMPA-receptor-dependent component was unaffected. No alterations in the levels of NMDA- or AMPA-receptor proteins were detected. These results suggest that alpha-neurexins are required to maintain normal postsynaptic NMDA-receptor function. The decrease in NMDA-receptor activity in alpha-neurexin-deficient synapses could be due to a transsynaptic effect on the postsynaptic neuron (i.e., alpha-neurexins on the presynaptic inputs guide postsynaptic NMDA-receptor function) or to a cell-autonomous postsynaptic effect of alpha-neurexins on NMDA-receptor activity. To distinguish between these two possibilities, we cocultured WT GFP-labeled neurons with neocortical slices from alpha-neurexin-deficient or control mice. No difference was found between WT neurons innervated by inputs that contained or lacked alpha-neurexins, indicating that the absence of presynaptic alpha-neurexins alone does not depress postsynaptic NMDA-receptor function. Our data suggest that, in addition to the previously described presynaptic impairments, loss of alpha-neurexins induces postsynaptic changes by a cell-autonomous mechanism.

Freeze-frame inhibitor captures acetylcholinesterase in a unique conformation

The 1,3-dipolar cycloaddition reaction between unactivated azides and acetylenes proceeds exceedingly slowly at room temperature. However, considerable rate acceleration is observed when this reaction occurs inside the active center gorge of acetylcholinesterase (AChE) between certain azide and acetylene reactants, attached via methylene chains to specific inhibitor moieties selective for the active center and peripheral site of the enzyme. AChE catalyzes the formation of its own inhibitor in a highly selective fashion: only a single syn1-triazole regioisomer with defined substitution positions and linker distances is generated from a series of reagent combinations. Inhibition measurements revealed this syn1-triazole isomer to be the highest affinity reversible organic inhibitor of AChE with association rate constants near the diffusion limit. The corresponding anti1 isomer, not formed by the enzyme, proved to be a respectable but weaker inhibitor. The crystal structures of the syn1- and anti1-mouse AChE complexes at 2.45- to 2.65-A resolution reveal not only substantial binding contributions from the triazole moieties, but also that binding of the syn1 isomer induces large and unprecedented enzyme conformational changes not observed in the anti1 complex nor predicted from structures of the apoenzyme and complexes with the precursor reactants. Hence, the freeze-frame reaction offers both a strategically original approach for drug discovery and a means for kinetically controlled capture, as a high-affinity complex between the enzyme and its self-created inhibitor, of a highly reactive minor abundance conformer of a fluctuating protein template.

Structural Characterization of Recombinant Soluble Rat Neuroligin 1: Mapping of Secondary Structure and Glycosylation by Mass Spectrometry

Neuroligins (NLs) are a family of transmembrane proteins that function in synapse formation and/or remodeling by interacting with beta-neurexins (beta-NXs) to form heterophilic cell adhesions. The large N-terminal extracellular domain of NLs, required for beta-NX interactions, has sequence homology to the alpha/beta hydrolase fold superfamily of proteins. By peptide mapping and mass spectrometric analysis of a soluble recombinant form of NL1, several structural features of the extracellular domain have been established. Of the nine cysteine residues in NL1, eight are shown to form intramolecular disulfide bonds. Disulfide pairings of Cys 117 to Cys 153 and Cys 342 to Cys 353 are consistent with disulfide linkages that are conserved among the family of alpha/beta hydrolase proteins. The disulfide bond between Cys 172 and Cys 181 occurs within a region of the protein encoded by an alternatively spliced exon. The disulfide pairing of Cys 512 and Cys 546 in NL1 yields a structural motif unique to the NLs, since these residues are highly conserved. The potential N-glycosylation sequons in NL1 at Asn 109, Asn 303, Asn 343, and Asn 547 are shown occupied by carbohydrate. An additional consensus sequence for N-glycosylation at Asn 662 is likely occupied. Analysis of N-linked oligosaccharide content by mass matching paradigms reveals significant microheterogeneous populations of complex glycosyl moieties. In addition, O-linked glycosylation is observed in the predicted stalk region of NL1, prior to the transmembrane spanning domain. From predictions based on sequence homology of NL1 to acetylcholinesterase and the molecular features of NL1 established from mass spectrometric analysis, a novel topology model for NL three-dimensional structure has been constructed.

CASK and Dlg form a PDZ protein complex at the mammalian neuromuscular junction

Membrane-associated guanylate kinases (MAGUKs) are modular adapter proteins that serve as scaffolding molecules and anchor channels and receptors via their PDZ (PSD-95, Dlg, Zo-1) domains. Calcium, calmodulin-associated serine/threonine kinase (CASK) is a MAGUK that is critical at synapses in the central nervous system and at cell-cell junctions because of its interactions with channels, receptors, and structural proteins. We show via confocal microscopy that CASK and another MAGUK, Discs Large (Dlg), are present at the mammalian neuromuscular junction in skeletal muscle. Immunoprecipitation data from mouse muscle show that CASK associates with Dlg, providing evidence of a MAGUK protein complex at this synapse. These data indicate that CASK and Dlg may act as a scaffold for organizing receptors and channels at the postsynaptic membrane of the neuromuscular junction.

Neuroligin 2 is exclusively localized to inhibitory synapses

Neuroligins are cell adhesion proteins that are thought to instruct the formation and alignment of synaptic specializations. The three known rodent neuroligin isoforms share homologous extracellular acetylcholinesterase-like domains that bridge the synaptic cleft and bind beta-neurexins. All neuroligins have identical intracellular C-terminal motifs that bind to PDZ domains of various target proteins. Neuroligin 1 is specifically localized to glutamatergic postsynaptic specializations. We show here that neuroligin 2 is exclusively localized to inhibitory synapses in rat brain and dissociated neurons. In immature neurons, neuroligin 2 is found at synapses and also at GABAA receptor aggregates that are not facing presynaptic termini, indicating that postsynaptic mechanisms lead to synaptic recruitment of neuroligin 2. Our findings identify neuroligin 2 as a new cell adhesion protein specific for inhibitory synapses and open new avenues for identifiying the constituents of this unique type of postsynaptic specialization.

Gene expression analysis of the pro-oestrous-stage rat uterus reveals neuroligin 2 as a novel steroid-regulated gene

In the present study, differential gene expression in the uteri of ovariectomised (OVX) and pro-oestrous rats (OVX v. pro-oestrus pair) was investigated using cDNA expression array analysis. Differential uterine gene expression in OVX rats and progesterone (P4)-injected OVX rats (OVX v. OVX + P4 pair) was also examined. The uterine gene expression profiles of these two sets of animals were also compared for the effects of P4 treatment. RNA samples were extracted from uterine tissues and reverse transcribed in the presence of [α32P]-dATP. Membrane sets of rat arrays were hybridised with cDNA probe sets. Northern blot analysis was used to validate the relative gene expression patterns obtained from the cDNA array. Of the 1176 cDNAs examined, 23 genes showed significant (>two-fold) changes in expression in the OVX v. pro-oestrus pair. Twenty of these genes were upregulated during pro-oestrus compared with their expression in the OVX rat uterus. In the OVX v. OVX + P4 pair, 22 genes showed significant (>two-fold) changes in gene expression. Twenty of these genes were upregulated in the OVX + P4 animals. The genes for nuclear factor I–XI, afadin, neuroligin 2, semaphorin Z, calpain 4, cyclase-associated protein homologue, thymosin β-4X and p8 were significantly upregulated in the uteri of the pro-oestrus and OVX + P4 rats of both experimental pairs compared with the OVX rat uteri. These genes appear to be under the control of P4. One of the most interesting findings of the present study is the unexpected and marked expression of the neuroligin 2 gene in the rat uterus. This gene is expressed at high levels in the central nervous system and acts as a nerve cell adhesion factor. According to Northern blot analysis, neuroligin 2 gene expression was higher during the pro-oestrus and metoestrus stages than during the oestrus and dioestrus stages of the oestrous cycle. In addition, neuroligin 2 mRNA levels were increased by both 17β-oestradiol (E2) and P4, although P4 administration upregulated gene expression to a greater extent than injection of E2. These results indicate that neuroligin 2 gene expression in the rat uterus is under the control of both E2 and P4, which are secreted periodically during the oestrous cycle.

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.

Cell-cell signaling during synapse formation in the CNS

Synapses join individual nerve cells into a functional network. Specific cell-cell signaling events regulate synapse formation during development and thereby generate a highly reproducible connectivity pattern. The accuracy of this process is fundamental for normal brain function, and aberrant connectivity leads to nervous system disorders. However, despite the overall precision with which neuronal circuits are formed, individual synapses and synaptic networks are also plastic and can readily adapt to external stimuli or perturbations. In recent studies, several trans-synaptic signaling systems have been identified that can mediate various aspects of synaptic differentiation in the central nervous system. It appears that these individual pathways functionally cooperate, thereby generating robustness and flexibility, which ensure normal nervous system function.

Making connections: cholinesterase-domain proteins in the CNS

Recent studies have highlighted novel functions of a group of cell adhesion molecules during nervous system development. Members of this protein family are characterized by an extracellular domain with sequence homology to cholinesterases and include the neuroligins, synaptic cell adhesion molecules recently implicated in autism, and neurotactin, a cell surface receptor involved in axonal pathfinding. Although these proteins have a structural organization similar to the enzyme acetylcholinesterase, the cholinesterase domain lacks enzymatic activity and functions as a protein-protein interaction motif. This protein family provides a striking example of how the function of a catalytically active domain has evolved to mediate receptor-ligand interactions that regulate morphogenetic processes during development of the nervous system.

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.

Neurexin mediates the assembly of presynaptic terminals

Neurexins are a large family of proteins that act as neuronal cell-surface receptors. The function and localization of the various neurexins, however, have not yet been clarified. Beta-neurexins are candidate receptors for neuroligin-1, a postsynaptic membrane protein that can trigger synapse formation at axon contacts. Here we report that neurexins are concentrated at synapses and that purified neuroligin is sufficient to cluster neurexin and to induce presynaptic differentiation. Oligomerization of neuroligin is required for its function, and we find that beta-neurexin clustering is sufficient to trigger the recruitment of synaptic vesicles through interactions that require the cytoplasmic domain of neurexin. We propose a two-step model in which postsynaptic neuroligin multimers initially cluster axonal neurexins. In response to this clustering, neurexins nucleate the assembly of a cytoplasmic scaffold to which the exocytotic apparatus is recruited.

Sarin (nerve agent GB)-induced differential expression of mRNA coding for the acetylcholinesterase gene in the rat central nervous system

We carried out a time-course study on the effects of a single intramuscular (i.m.) dose (0.5x LD(50)) of sarin (O-isopropyl methylphosphonofluoridate), also known as nerve agent GB, on the mRNA expression of acetylcholinesterase (AChE) in the brain of male Sprague-Dawley rats. Sarin inactivates the enzyme AChE which is responsible for the breakdown of the neurotransmitter acetylcholine (ACh), leading to its accumulation at ACh receptors and overstimulation of the cholinergic system. Rats were treated with 50 microg/kg of sarin (0.5x LD(50)) in 1 mL saline/kg and terminated at the following time points: 1 and 2 hr and 1, 3, and 7 days post-treatment. Control rats were treated with normal saline. Total RNA was extracted, and northern blots were hybridized with cDNA probes for AChE and 28S RNA (control). Poly-A RNA from both treated and control cortex was used for reverse transcription-polymerase chain reaction (RT-PCR)-based verification of the data from the northern blots. The results obtained indicate that a single (i.m.) dose of sarin (0.5x LD(50)) produced differential induction and persistence of AChE mRNA levels in different regions of the brain. Immediate induction of AChE transcripts was noted in the brainstem (126+/-6%), cortex (149+/-4%), midbrain (153+/-5%), and cerebellum (234+/-2%) at 1 hr. The AChE expression level, however, increased over time and remained elevated after a decline at 1 day in the previously shown more susceptible brainstem. The transcript levels remained elevated at a later time point (3 days) in the midbrain, after a dramatic decline at day 1 (110+/-2%). In the cortex, transcript levels came down to control values by day 1. The cerebellum also showed a decline of the elevated levels observed at 2 hr (275+/-2%) to control values by day 1. RT-PCR analysis of the AChE transcript at 30 min in the cortex showed an induction to 213+/-3% of the control level, confirming the expression pattern obtained by the northern blot data. The immediate induction followed by the complex pattern of the AChE mRNA time-course in the CNS may indicate that the activation of both cholinergic-related and unrelated functions of the gene plays an important role in the pathological manifestations of sarin-induced neurotoxicity.

Mutations of the X-linked genes encoding neuroligins NLGN3 and NLGN4 are associated with autism

Many studies have supported a genetic etiology for autism. Here we report mutations in two X-linked genes encoding neuroligins NLGN3 and NLGN4 in siblings with autism-spectrum disorders. These mutations affect cell-adhesion molecules localized at the synapse and suggest that a defect of synaptogenesis may predispose to autism.

Acetylcholinesterase promotes neurite elongation, synapse formation, and surface expression of AMPA receptors in hippocampal neurones

Here we show that chronic application of low concentrations (0.01-0.05 U/ml) or a single application of 1-5 U/ml acetylcholinesterase (AChE) promotes the extension of neuronal processes, synapse formation, and alpha-amino-3-hydroxy-5-methylisoxazolepropionate receptor (AMPAR) surface expression in both embryonic and postnatal hippocampal cultures. The total number of AMPARs was unchanged but the proportion of receptors that were surface-expressed, predominantly at synapses, was approximately doubled following AChE treatment. Blockade of the peripheral anionic site of endogenous AChE in the cultures dramatically reduced neurite outgrowth but did not alter the appearance of synaptic markers SV2a and PSD95. These results indicate that AChE is necessary for normal dendrite and axon formation in hippocampal neurones and suggest that it may also play a role in excitatory synapse development, plasticity, and remodelling.

Identification and characterization of heart-specific splicing of human neurexin 3 mRNA (NRXN3)

Three neurexin (NRXN) genes are known in humans, each transcribed from two promoters and extensively spliced at five canonical positions, thus generating thousands of isoforms. For NRXN3, only neuronal expression was reported so far. We reported here on the expression of NRXN3 in additional tissues (lung, pancreas, heart, placenta, liver, and kidney) and on the identification and characterization of heart-specific splicing variants of NRXN3. Cardiac isoforms of NRXN3 probably participate in a complex involving dystroglycan and proteins of extracellular matrix, involved in intercellular connections.

Identification and characterization of Pkhd1, the mouse orthologue of the human ARPKD gene

PKHD1, the gene mutated in human autosomal recessive polycystic kidney disease has recently been identified. Its translation products are predicted to belong to a superfamily of proteins involved in the regulation of cellular adhesion and repulsion. One notable aspect of the gene is its unusually complex pattern of splicing. This study shows that mouse Pkhd1 and its translation products have very similar properties to its human orthologue. Mouse Pkhd1 extends over approximately 500 kb of genomic DNA, includes a minimum of 68 nonoverlapping exons, and exhibits a complex pattern of splicing. The longest ORF encodes a protein of 4059aa predicted to have an N-terminal signal peptide, multiple IPTs and PbH1 repeats, a single transmembrane span (TM), and a short cytoplasmic C-terminus. Although the protein sequence is generally well conserved (approximately 73% average identity), the C-termini share only 55% identity. The pattern of Pkhd1 expression by in situ hybridization was also examined in developing and adult mouse tissues over a range of ages (E12.5 to 3 mo postnatal). High levels of expression were present in renal and biliary tubular structures at all time points examined. Prominent Pkhd1 signals were also found in a number of other organs and tissues. Tissue-specific differences in transcript expression were revealed through the use of single exon probes. These data show that key features of human PKHD1 are highly conserved in the mouse and suggest that the complicated pattern of splicing is likely to be functionally important.

Structure and evolution of neurexin genes: insight into the mechanism of alternative splicing

Neurexins are neuron-specific vertebrate proteins with hundreds of differentially spliced isoforms that may function in synapse organization. We now show that Drosophila melanogaster and Caenorhabditis elegans express a single gene encoding only an alpha-neurexin, whereas humans and mice express three genes, each of which encodes alpha- and beta-neurexins transcribed from separate promoters. The neurexin genes are very large (up to 1.62 Mb), with the neurexin-3 gene occupying almost 2% of human chromosome 14. Although invertebrate and vertebrate neurexins exhibit a high degree of evolutionary conservation, only vertebrate neurexins are subject to extensive alternative splicing that uses mechanisms ranging from strings of mini-exons to multiple alternative splice donor and acceptor sites. Consistent with their proposed role in synapse specification, neurexins thus have evolved from relatively simple genes in invertebrates to diversified genes in vertebrates with multiple promoters and extensive alternative splicing.

Molecular mechanisms of CNS synaptogenesis

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

Analysis of the human neurexin genes: alternative splicing and the generation of protein diversity

The neurexins are neuronal proteins that function as cell adhesion molecules during synaptogenesis and in intercellular signaling. Although mammalian genomes contain only three neurexin genes, thousands of neurexin isoforms may be expressed through the use of two alternative promoters and alternative splicing at up to five different positions in the pre-mRNA. To begin understanding how the expression of the neurexin genes is regulated, we have determined the complete nucleotide sequence of all three human neurexin genes: NRXN1, NRXN2, and NRXN3. Unexpectedly, two of these, NRXN1 ( approximately 1.1 Mb) and NRXN3 ( approximately 1.7 Mb), are among the largest known human genes. In addition, we have identified several conserved intronic sequence elements that may participate in the regulation of alternative splicing. The sequences of these genes provide insight into the mechanisms used to generate the diversity of neurexin protein isoforms and raise several interesting questions regarding the expression mechanism of large genes.

The development of postganglionic sympathetic neurons: coordinating neuronal differentiation and diversification

The fine-tuned operation of the nervous system is accomplished by a diverse set of neurons which differ in their morphology, biochemistry and, consequently, their functional properties. The accurate interconnection between different neuron populations and their target tissues is the prerequisite for physiologically appropriate information processing. This is exemplified by the regulatory action of the autonomic nervous system in vertebrates to sustain homeostasis under changing physiological demands. For this purpose, the coordination of divergent regulatory responses is required in a multitude of tissues spread over the entire body. To meet this task, diverse neuronal populations interact at different levels. In the sympathetic system. chemical relations between preganglionic and postganglionic neurons appear to differ along the rostrocaudal axis. In addition, postganglionic neurons innervating different target tissues at a segmental level have distinct properties. Differences in their preganglionic innervation and their integrative membrane properties result in diverse activation patterns upon reflex stimulation. Moreover, postganglionic neurons differ in the transmitter molecules they employ to convey information to the target tissues. The segregation of noradrenaline and acetylcholine to different populations of postganglionic sympathetic neurons is well established. A combination of cellular and molecular approaches has begun to uncover how such a complex system may be generated during development. Growth and transcription factors involved in noradrenergic and cholinergic differentiation are characterised. Interestingly, they can also promote the expression of proteins involved in transmitter secretion. As the proteins participating in the vesicle cycle are expressed in many neuron populations, whereas the enzymes of transmitter biosynthesis are restricted to subpopulations of neurons, the findings suggest that early in neuronal development subpopulation-specific and more widely expressed neuronal properties can be commonly induced. Still, many details concerning the signals involved in the induction of the neurotransmitter synthesis and release machinery remain to be worked out. Likewise, the regulatory processes resulting in differences of electrophysiological membrane properties and the specific recognition between pre- and postganglionic neurons have to be determined. Ultimately, this will lead to an understanding at the molecular level of the development of a nervous system with diverse neuronal populations that are specifically interconnected to distinct input neurons and target tissues as required for the performance of a complex regulatory function.

Disparate cell types use a shared complex of PDZ proteins for polarized protein localization

Based on their morphology and function, epithelial cells and neurons appear to have very little in common; however, growing evidence indicates that these two disparate cell types share an underlying polarization pathway responsible for sorting proteins to specific subcellular sites. An evolutionarily conserved complex of PDZ domain-containing proteins thought to be responsible for polarized protein localization has been identified from both brain and epithelial tissue, both from mammals and from the nematode C. elegans. Some of the most recent data on PDZ proteins and the proteins with which they interact are summarized. In particular, some of the more recently proposed models for their function in cells, and the in vivo and in vitro data that support these models are focussed upon.

Glial cell development in the Drosophila embryo

Glial cells play a central role in the development and function of complex nervous systems. Drosophila is an excellent model organism for the study of mechanisms underlying neural development, and recent attention has been focused on the differentiation and function of glial cells. We now have a nearly complete description of glial cell organization in the embryo, which enables a systematic genetic analysis of glial cell development. Most glia arise from neural stem cells that originate in the neurogenic ectoderm. The bifurcation of glial and neuronal fates is under the control of the glial promoting factor glial cells missing. Differentiation is propagated through the regulation of several transcription factors. Genes have been discovered affecting the terminal differentiation of glia, including the promotion glial-neuronal interactions and the formation of the blood-nerve barrier. Other roles of glia are being explored, including their requirement for axon guidance, neuronal survival, and signaling.

alpha-Latrotoxin and its receptors: neurexins and CIRL/latrophilins

alpha-Latrotoxin, a potent neurotoxin from black widow spider venom, triggers synaptic vesicle exocytosis from presynaptic nerve terminals. alpha-Latrotoxin is a large protein toxin (120 kDa) that contains 22 ankyrin repeats. In stimulating exocytosis, alpha-latrotoxin binds to two distinct families of neuronal cell-surface receptors, neurexins and CLs (Cirl/latrophilins), which probably have a physiological function in synaptic cell adhesion. Binding of alpha-latrotoxin to these receptors does not in itself trigger exocytosis but serves to recruit the toxin to the synapse. Receptor-bound alpha-latrotoxin then inserts into the presynaptic plasma membrane to stimulate exocytosis by two distinct transmitter-specific mechanisms. Exocytosis of classical neurotransmitters (glutamate, GABA, acetylcholine) is induced in a calcium-independent manner by a direct intracellular action of alpha-latrotoxin, while exocytosis of catecholamines requires extracellular calcium. Elucidation of precisely how alpha-latrotoxin works is likely to provide major insight into how synaptic vesicle exocytosis is regulated, and how the release machineries of classical and catecholaminergic neurotransmitters differ.