Neuroligin 1: a splice site-specific ligand for beta-neurexins

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

RIBEYE, a component of synaptic ribbons: a protein's journey through evolution provides insight into synaptic ribbon function

Photoreceptor cells utilize ribbon synapses to transmit sensory signals at high resolution. Ribbon synapses release neurotransmitters tonically, with a high release rate made possible by continuous docking of synaptic vesicles on presynaptic ribbons. We have partially purified synaptic ribbons from retina and identified a major protein component called RIBEYE. RIBEYE is composed of a unique A domain specific for ribbons, and a B domain identical with CtBP2, a transcriptional repressor that in turn is related to 2-hydroxyacid dehydrogenases. The A domain mediates assembly of RIBEYE into large structures, whereas the B domain binds NAD(+) with high affinity, similar to 2-hydroxyacid dehydrogenases. Our results define a unique component of synaptic ribbons and suggest that RIBEYE evolved in vertebrates under utilization of a preexisting protein to build a unique scaffold for a specialized synapse.

Cholinesterases modulate cell adhesion in human neuroblastoma cells in vitro

Cholinesterases are expressed non-synaptically during embryonic development, neoplasia and neurodegeneration. We have investigated the effects of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) and, conversely, anti-AChE and -BChE antibodies and inhibitors on cell adhesion and neurite outgrowth in human neuroblastoma cells. Analysis of cholinesterase levels and isoforms in undifferentiated and differentiated cells indicated a significant rise in AChE levels on differentiation. This increase was related to both cell-associated and secreted enzyme, and was predominantly the G4 isoform. BChE levels and isoforms, on the other hand, showed no significant variation. Coating the tissue culture plate with AChE stimulated neurite outgrowth, while BChE had an anti-adhesive effect. Cell adhesion was affected by the BChE inhibitor, ethopropazine, and the AChE peripheral site inhibitor, BW284c51, but not by eserine which binds to the active site. This indicates that the adhesion function is non-cholinergic, a finding supported by the lack of effect of AE-2, a monoclonal antibody that inhibits AChE, on cell adhesion. Four out of a panel of nine anti-AChE antibodies inhibited adhesion to varying degrees. Of these antibodies, two are catalytic, with epitopes associated with the peripheral anionic site of AChE, and the remaining two have epitopes overlapping this site. Neither of the two anti-BChE antibodies used had any effect on adhesion. These results indicate the importance of AChE in neuroblastoma cell adhesion and neurite outgrowth, and suggest that the peripheral anionic site may be involved in these processes.

Postsynaptic organization and regulation of excitatory synapses

Dynamic regulation of synaptic efficacy is one of the mechanisms thought to underlie learning and memory. Many of the observed changes in efficacy, such as long-term potentiation and long-term depression, result from the functional alteration of excitatory neurotransmission mediated by postsynaptic glutamate receptors. These changes may result from the modulation of the receptors themselves and from regulation of protein networks associated with glutamate receptors. Understanding the interactions in this synaptic complex will yield invaluable insight into the molecular basis of synaptic function. This review focuses on the molecular organization of excitatory synapses and the processes involved in the dynamic regulation of glutamate receptors.

Synapse formation: if it looks like a duck and quacks like a duck

Neuroligin and neurexin form an intercellular adhesion complex sufficient to trigger formation of functional presynaptic elements in vitro. This single molecular interaction appears to initiate clustering of synaptic vesicles, assembly of vesicle-release machinery and morphological changes at the presynaptic membrane.

Amalgam is a ligand for the transmembrane receptor neurotactin and is required for neurotactin-mediated cell adhesion and axon fasciculation in Drosophila

Neurotactin (NRT), a member of the cholinesterase-homologous protein family, is a heterophilic cell adhesion molecule that is required for proper axon guidance during Drosophila development. In this study, we identify amalgam (AMA), a member of the immunoglobulin superfamily, as a ligand for the NRT receptor. Using transfected Schneider 2 cells and embryonic primary cultures, we demonstrate that AMA is a secreted protein. Furthermore, AMA is necessary for NRT-expressing cells both to aggregate with themselves and to associate with embryonic primary culture cells. Aggregation assays performed with truncated NRT molecules reveal that the integrity of the cholinesterase-like extracellular domain was not required either for AMA binding or for adhesion, with only amino acids 347-482 of the extracellular domain being necessary for both activities. Moreover, the NRT cytoplasmic domain is required for NRT-mediated adhesion, although not for AMA binding. Using an ama-deficient stock, we find that ama function is not essential for viability. Pupae deficient for ama do exhibit defasciculation defects of the ocellar nerves similar to those found in nrt mutants.

Neuroligin expressed in nonneuronal cells triggers presynaptic development in contacting axons

Most neurons form synapses exclusively with other neurons, but little is known about the molecular mechanisms mediating synaptogenesis in the central nervous system. Using an in vitro system, we demonstrate that neuroligin-1 and -2, postsynaptically localized proteins, can trigger the de novo formation of presynaptic structure. Nonneuronal cells engineered to express neuroligins induce morphological and functional presynaptic differentiation in contacting axons. This activity can be inhibited by addition of a soluble version of beta-neurexin, a receptor for neuroligin. Furthermore, addition of soluble beta-neurexin to a coculture of defined pre- and postsynaptic CNS neurons inhibits synaptic vesicle clustering in axons contacting target neurons. Our results suggest that neuroligins are part of the machinery employed during the formation and remodeling of CNS synapses.

Expression of neurexin Ialpha splice variants in sympathetic neurons: selective changes during differentiation and in response to neurotrophins

Neurexins are a surprisingly diverse group of alternatively spliced proteins possibly involved in neural cell recognition processes. We find neurexin Ialpha and its splice variants highly conserved between mammals and birds. In vivo, neurexin Ialpha is expressed in sympathetic neurons during target innervation and relative expression levels of splice variants change with development. In vitro, no such changes are observed in the absence of growth factors, indicating that interactions with the environment are required to modify the splicing pattern. Specific alterations in splice variant expression are induced in vitro by neurotrophins. Expression patterns of splice variants in vivo and neurotrophin-induced regulation without changes in cell composition in vitro demonstrate that neurexin splice variant expression varies during differentiation of individual neurons. Our data suggest that changes in neurexin splice variants contribute to alterations of neuronal cell surface properties during target innervation.

The structure and expression of the human neuroligin-3 gene

The neuroligins are a family of proteins that are thought to mediate cell to cell interactions between neurons. During the sequencing at an Xq13 locus associated with a mental retardation syndrome in some studies, we discovered a portion of the human orthologue of the rat neuroligin-3 gene. We now report the structure and the expression of that gene. The gene spans approximately 30kb and contains eight exons. Unlike the rat gene, it codes for at least two mRNAs and at least one of which is expressed outside the CNS. Interestingly, the putative promoter for the gene overlaps the last exon of the neighboring HOPA gene and is located less than 1kb from an OPA element in which a polymorphism associated with mental retardation is found. These findings suggest a possible role for the neuroligin gene in mental retardation and that the role of the gene in humans may differ from its role in rats.

The role of glutamate and nitric oxide in the reproductive neuroendocrine system

The preovulatory surge of gonadotropin releasing hormone (GnRH) is essential for mammalian reproduction. Recent work has implicated the neurotransmitters glutamate and nitric oxide as having a key role in this process. Large concentrations of glutamate are found in several hypothalamic nuclei known to be important for GnRH release and glutamate receptors are also located in these key hypothalamic nuclei. Administration of glutamate agonists stimulate GnRH and LH release, while glutamate receptor antagonists attenuate the steroid-induced and preovulatory LH surge. Glutamate has also been implicated in the critical processes of puberty, hormone pulsatility, and sexual behavior. Glutamate is believed to elicit many of these effects by activating the release of the gaseous neurotransmitter, nitric oxide (NO). NO potently stimulates GnRH by activating a heme containing enzyme, guanylate cyclase, which in turn leads to increased production of cGMP and GnRH release. Recent work has focused on identifying anchoring and (or) clustering proteins that target glutamate receptors to the synapse and couple the glutamate-NO neurotransmission system. The present review will discuss these new findings, as well as the role of glutamate and nitric oxide in important mammalian reproductive events, with a focus on the hypothalamic control of preovulatory GnRH release.

Key words: glutamate, nitric oxide, GnRH, postsynaptic density, hypothalamus.

Comparative genomics of the eukaryotes

A comparative analysis of the genomes of Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae-and the proteins they are predicted to encode-was undertaken in the context of cellular, developmental, and evolutionary processes. The nonredundant protein sets of flies and worms are similar in size and are only twice that of yeast, but different gene families are expanded in each genome, and the multidomain proteins and signaling pathways of the fly and worm are far more complex than those of yeast. The fly has orthologs to 177 of the 289 human disease genes examined and provides the foundation for rapid analysis of some of the basic processes involved in human disease.

Synaptogenesis and myopathy under acetylcholinesterase overexpression

Environmental, congenital, and acquired immunological insults perturbing neuromuscular junction (NMJ) activity may induce a variety of debilitating neuromuscular pathologies. However, the molecular elements linking NMJ dysfunction to long-term myopathies are unknown. Here, we report dramatically elevated levels of mRNA encoding c-Fos and the "readthrough" (R) variant of acetylcholinesterase (AChE) in muscles of transgenic mice overexpressing synaptic (S) AChE in motoneurons and in control mice treated with the irreversible cholinesterase inhibitor diisopropylfluorophosphonate (DFP). Tongue muscles from DFP-treated and AChE-S transgenic mice displayed exaggerated neurite branching and disorganized, wasting fibers. Moreover, diaphragm muscles from both transgenic and DFP-treated mice exhibited NMJ proliferation. 2'-O-methyl-protected antisense oligonucleotides targeted to AChE mRNA suppressed feedback upregulation of AChE and ameliorated DFP-induced NMJ proliferation. Our findings demonstrate common transcriptional responses to cholinergic NMJ stress of diverse origin, and implicate deregulated AChE expression in excessive neurite outgrowth, uncontrolled synaptogenesis, and myopathology.

Molecular modification of N-cadherin in response to synaptic activity

The relationship between adhesive interactions across the synaptic cleft and synaptic function has remained elusive. At certain CNS synapses, pre- to postsynaptic adhesion is mediated at least in part by neural (N-) cadherin. Here, we demonstrate that upon depolarization of hippocampal neurons in culture by K+ treatment, or application of NMDA or alpha-latrotoxin, synaptic N-cadherin dimerizes and becomes markedly protease resistant. These properties are indices of strong, stable, enhanced cadherin-mediated intercellular adhesion. N-cadherin retained protease resistance for at least 2 hr after recovery, while other surface molecules, including other cadherins, were completely degraded. The acquisition of protease resistance and dimerization of N-cadherin is not dependent on new protein synthesis, nor is it accompanied by internalization of N-cadherin. By immunocytochemistry, we found that high K+ selectively induces surface dispersion of N-cadherin, which, after recovery, returns to synaptic puncta. N-cadherin dispersion under K+ treatment parallels the rapid expansion of the presynaptic membrane consequent to the massive vesicle fusion that occurs with this type of depolarization. In contrast, with NMDA application, N-cadherin does not disperse but does acquire enhanced protease resistance and dimerizes. Our data strongly suggest that synaptic adhesion is dynamically and locally controlled, and modulated by synaptic activity.

Role of acetylcholinesterase in the development of axon tracts within the embryonic vertebrate brain

In the developing vertebrate brain, acetylcholinesterase (AChE) expression coincides temporally with axon tract formation. Although AChE promotes neurite outgrowth in vitro, the role of this molecule in the development of axon tracts in vivo is unknown. To address this question, we examined the effects of the AChE inhibitor, BW284C51, on the formation of the early scaffold of axon tracts in the embryonic Xenopus brain. In exposed Xenopus brain preparations, axons elongate and establish a normal topography of axon tracts. However, when brains were exposed to BW284C51, the thickness of the major longitudinal axon tract, the tract of the post-optic commissure decreased in a dose-dependent manner. When BW284C51 was removed from the culture media axon tract development returned to normal within 5 h. These findings provide the first evidence for a non-classical role of AChE in the initial formation of axon tracts within the developing vertebrate brain.

Carboxyl/cholinesterases: a case study of the evolution of a successful multigene family

The evolution of organismal diversity among the Metazoa is dependent on the proliferation of genes and diversification of functions in multigene families. Here we analyse these processes for one highly successful family, the carboxyl/cholinesterases. One key to the expansion of the functional niche of this group of enzymes is associated with versatile substrate binding and catalytic machinery. Qualitatively new functions can be obtained by substitution of one or a very few amino acids. This crudely adapted new functionality is then refined rapidly by a pulse of change elsewhere in the molecule; in one case about 13% amino acid divergence occurred in 5-10 million years. Furthermore, we postulate that the versatility of the substrate binding motifs underpins the recruitment of several family members to additional noncatalytic signal transduction functions.

Acetylcholinesterase of Schistosoma mansoni--functional correlates. Contributed in honor of Professor Hans Neurath's 90th birthday

Acetylcholinesterase (AChE) is an enzyme broadly distributed in many species, including parasites. It occurs in multiple molecular forms that differ in their quaternary structure and mode of anchoring to the cell surface. This review summarizes biochemical and immunological investigations carried out in our laboratories on AChE of the helmint, Schistosoma mansoni. AChE appears in S. mansoni in two principal molecular forms, both globular, with sedimentation coefficients of approximately 6.5 and 8 S. On the basis of their substrate specificity and sensitivity to inhibitors, both are "true" acetylcholinesterases. Approximately half of the AChE activity of S. mansoni is located on the outer surface of the parasite, attached to the tegumental membrane via a covalently attached glycosylphosphatidylinositol anchor. The remainder is located within the parasite, mainly associated with muscle tissue. Whereas the internal enzyme is most likely involved in termination of neurotransmission at cholinergic synapses, the role of the surface enzyme remains to be established; there are, however, indications that it is involved in signal transduction. The two forms of AChE differ in their heparin-binding properties, only the internal 8 S form of the AChE being retained on a heparin column. The two forms differ also in their immunological specificity, since they are selectively recognized by different monoclonal antibodies. Polyclonal antibodies raised against S. mansoni AChE purified by affinity chromatography are specific for the parasite AChE, reacting with both molecular forms, but do not recognize AChE from other species. They interact with the surface-localized enzyme on the intact organism, and produce almost total complement-dependent killing of the parasite. S. mansoni AChE is thus demonstrated to be a functional protein, involved in multifaceted activities, which can serve as a suitable candidate for diagnostic purposes, vaccine development, and drug design.

Caspr2, a new member of the neurexin superfamily, is localized at the juxtaparanodes of myelinated axons and associates with K+ channels

Rapid conduction in myelinated axons depends on the generation of specialized subcellular domains to which different sets of ion channels are localized. Here, we describe the identification of Caspr2, a mammalian homolog of Drosophila Neurexin IV (Nrx-IV), and show that this neurexin-like protein and the closely related molecule Caspr/Paranodin demarcate distinct subdomains in myelinated axons. While contactin-associated protein (Caspr) is present at the paranodal junctions, Caspr2 is precisely colocalized with Shaker-like K+ channels in the juxtaparanodal region. We further show that Caspr2 specifically associates with Kv1.1, Kv1.2, and their Kvbeta2 subunit. This association involves the C-terminal sequence of Caspr2, which contains a putative PDZ binding site. These results suggest a role for Caspr family members in the local differentiation of the axon into distinct functional subdomains.

Signaling pathways are focused at specialized regions of the plasma membrane by scaffolding proteins of the MAGUK family

The MAGUKs (membrane-associated guanylate kinase homologs) are a family of proteins that act as molecular scaffolds for signaling pathway components at the plasma membrane of animal cells. They are localized in and required for the formation of several types of cell junctions, including epithelial tight and septate junctions as well as synaptic and neuromuscular junctions. They are also localized at the plasma membrane of other cell types, including erythrocytes, where they contribute to cell shape maintenance. MAGUKs function mainly by binding directly to the cytoplasmic termini of transmembrane proteins as well as to other signal transduction proteins. They appear to hold together elements of individual signaling pathways, thereby contributing to the efficiency and specificity of signaling interactions while simultaneously maintaining the structural specializations of the plasma membrane. BioEssays 1999;21:912-921.

The structure of the ligand-binding domain of neurexin Ibeta: regulation of LNS domain function by alternative splicing

Neurexins are expressed in hundreds of isoforms on the neuronal cell surface, where they may function as cell recognition molecules. Neurexins contain LNS domains, folding units found in many proteins like the G domain of laminin A, agrin, and slit. The crystal structure of neurexin Ibeta, a single LNS domain, reveals two seven-stranded beta sheets forming a jelly roll fold with unexpected structural similarity to lectins. The LNS domains of neurexin and agrin undergo alternative splicing that modulates their affinity for protein ligands in a neuron-specific manner. These splice sites are localized within loops at one edge of the jelly roll, suggesting a distinct protein interaction surface in LNS domains that is regulated by alternative splicing.

Human carboxylesterases in term placentae: enzymatic characterization, molecular cloning and evidence for the existence of multiple forms

The placenta is a temporary organ that is known to metabolize numerous endogenous and xenobiotic compounds. Carboxylesterases represent a family of enzymes which hydrolyse a variety of esters, amides and thioesters. Many studies have demonstrated that carboxylesterases are widely distributed among mammalian tissues, but little is known about these enzymes in the placenta. The present study was conducted to establish the kinetic parameters of placental carboxylesterases toward several p -nitrophenol and 1-naphthol esters, and to establish the molecular basis for these enzymes in the placenta. The enzymatic rate of the hydrolysis of 1-naphthylacetate and carboxylic esters of p -nitrophenol as a function of substrate concentration (0.01-1.00 m m) was examined with human placental microsomes pooled from six placentae. Data from these studies yielded a linear Lineweaver-Burk plot with each substrate examined. K(m)values for these substrates ranged from 92 to 370 microm, and V(max)values ranged from 85 to 170 nmol/mg/min. These results suggest that each substrate is hydrolysed by a single enzyme, or enzymes that are kinetically indistinguishable, or that one of them is dominant. Microsomes from all individual placentae contained esterase activity toward all four substrates, and exhibited a one- to three-fold variation. The activity toward p -nitrophenylacetate correlated well with the activity toward 1-naphthylacetate (r(2)=0.957). In contrast, the activity toward p -nitrophenylbutyrate correlated poorly with the activity toward 1-naphthylacetate (r(2)=0.121). These results suggest that placental microsomes have more than one carboxylesterase activity. Screening of a placental cDNA library with gene-trapping hybridization resulted in the isolation of three distinct cDNAs, designated PCE-1, PCE-2 and PCE-3. PCE-1 and PCE-2 have a significant sequence identity (approx 99 per cent) with liver carboxylesterases hCE and hCE-2, respectively. PCE-3 has a 96 per cent sequence identity with hCE but only at the first 874 nucleotide of the 5' end. The rest of the 1396 nucleotides of the 3' end exhibit no significant sequence identity with any known mammalian carboxylesterases. A probe derived from the 3' end of PCE-3 detected an approx 2.2 kb messenger transcript, the size of a regular carboxylesterase. However, the entire PCE-3 cDNA has multiple internal stop codons and encodes only 269 amino acids; half the size of a regular carboxylesterase. Northern blotting experiments detected the transcripts coding for PCE-1, PCE-2 or PCE-3 in all placentae, and the levels of these messengers showed an approx six-fold individual variation. Placenta 6 had the highest activity toward all four substrates, and highest levels of the messengers for PCE-1, PCE-2 and PCE-3. In contrast, placenta 1 had relatively high levels of messengers for PCE-1 and PCE-2, but the activity toward these four substrates was only moderate. These results suggest that a discrepancy between the messenger level and the enzyme protein exists or that there are other as yet unidentified carboxylesterase(s) in the placenta which contribute to the hydrolytic activity. Carboxylesterases are known to involve the detoxication and metabolic activation of various drugs, environmental toxicants and carcinogens. Therefore, placental carboxylesterases have both pharmacological and toxicological significance in the development of the fetus.

Structural roles of acetylcholinesterase variants in biology and pathology

Apart from its catalytic function in hydrolyzing acetylcholine, acetylcholinesterase (AChE) affects cell proliferation, differentiation and responses to various insults, including stress. These responses are at least in part specific to the three C-terminal variants of AChE which are produced by alternative splicing of the single ACHE gene. 'Synaptic' AChE-S constitutes the principal multimeric enzyme in brain and muscle; soluble, monomeric 'readthrough' AChE-R appears in embryonic and tumor cells and is induced under psychological, chemical and physical stress; and glypiated dimers of erythrocytic AChE-E associate with red blood cell membranes. We postulate that the homology of AChE to the cell adhesion proteins, gliotactin, glutactin and the neurexins, which have more established functions in nervous system development, is the basis of its morphogenic functions. Competition between AChE variants and their homologs on interactions with the corresponding protein partners would inevitably modify cellular signaling. This can explain why AChE-S exerts process extension from cultured amphibian, avian and mammalian glia and neurons in a manner that is C-terminus-dependent, refractory to several active site inhibitors and, in certain cases, redundant to the function of AChE-like proteins. Structural functions of AChE variants can explain their proliferative and developmental roles in blood, bone, retinal and neuronal cells. Moreover, the association of AChE excess with amyloid plaques in the degenerating human brain and with progressive cognitive and neuromotor deficiencies observed in AChE-transgenic animal models most likely reflects the combined contributions of catalytic and structural roles.

Association of neuronal calcium channels with modular adaptor proteins

Presynaptic voltage-gated calcium (Ca(2+)) channels mediate Ca(2+) influx into the presynaptic terminal that triggers synaptic vesicle fusion and neurotransmitter release. The immediate proximity of Ca(2+) channels to the synaptic vesicle release apparatus is critical for rapid and efficient synaptic transmission. In a series of biochemical experiments, we demonstrate a specific association of the cytosolic carboxyl terminus of the N-type Ca(2+) channel pore-forming alpha(1B) subunit with the modular adaptor proteins Mint1 and CASK. The carboxyl termini of alpha(1B) bind to the first PDZ domain of Mint1 (Mint1-1). The proline-rich region present in the carboxyl termini of alpha(1B) binds to the SH3 domain of CASK. Mint1-1 is specific for the E/D-X-W-C/S-COOH consensus, which defines a novel class of PDZ domains (class III). The Mint1-1 PDZ domain-binding motif is present only in the "long" carboxyl-terminal splice variants of N-type (alpha(1B)) and P/Q-type (alpha(1A)) Ca(2+) channels, but not in R-type (alpha(1E)) or L-type (alpha(1C)) Ca(2+) channels. Our results directly link presynaptic Ca(2+) channels to a macromolecular complex formed by modular adaptor proteins at synaptic junction and advance our understanding of coupling between cell adhesion and synaptic vesicle exocytosis.

The adhesion function on acetylcholinesterase is located at the peripheral anionic site

There is accumulating evidence that acetylcholinesterase has secondary noncholinergic functions, related to adhesion, differentiation, and the deposition of beta-amyloid in Alzheimer's disease. We have observed that the specific acetylcholinesterase peripheral anionic site inhibitors, BW284c51 and propidium iodide, abrogated cell-substrate adhesion in three human neuroblastoma cell lines. The active-site inhibitors, eserine and edrophonium, in contrast, had no effect. Certain anti-AChE antibodies were also shown to inhibit adhesion. Of these, the most effective were a monoclonal (E8) and a polyclonal having cholinesterase-like catalytic activity. These were raised against an acetylcholinesterase-inhibitor complex, implying that the epitope is associated with active-site structures. Two other monoclonal antibodies (E62A1 and E65E8) partially inhibited adhesion. The epitopes of these antibodies have been shown to overlap the peripheral anionic site of acetylcholinesterase. Competition ELISA between the monoclonal antibodies and inhibitors indicated competition between E8, E62A1, and E65E8 and the peripheral-site inhibitors BW284c51 and propidium, but not with the active-site inhibitors eserine and edrophonium. Fluorescence titration between antibodies and propidium confirmed these results. We conclude that the adhesion function of acetylcholinesterase is located at the peripheral anionic site. This has implications, not only for our understanding of neural development and its disorders, but also for the treatment of neuroblastoma, the leukemias, and Alzheimer's disease.

Proteolytic fragments of Alzheimer's disease-associated presenilin 1 are present in synaptic organelles and growth cone membranes of rat brain

Previous studies have demonstrated the molecular linkage of three causative genes for early-onset Alzheimer's disease: the presenilin 1 gene on chromosome 14, the presenilin 2 gene on chromosome 1, and the amyloid precursor protein gene on chromosome 21. In the present study, we have investigated the distributions of the approximately 20-kDa C-terminal and approximately 30-kDa N-terminal fragments of presenilin 1 and the amyloid precursor protein in rat brain and compared them with the distribution of several marker proteins. The fragments of presenilin 1 are present in synaptic plasma membranes, neurite growth cone membranes, and small synaptic vesicles of rat brain. Both proteolytic fragments are coenriched in the corresponding tissue fractions. Based on this observation, it seems likely that N- and C-terminal presenilin 1 fragments form a functional unit while remaining associated. In contrast to a predominant subcellular localization of presenilin 1 to the endoplasmic reticulum and Golgi apparatus in different cell lines, our results indicate that rat brain presenilin 1 fragments exit from these biosynthetic compartments to reach synaptic organelles in neurons.

Organization and regulation of proteins at synapses

The organization and regulation of synaptic connections in the mammalian nervous system entail complicated and co-ordinated molecular and cellular processes. The unveiling of various protein-protein interactions and their functional consequences at synapses have led to a greater understanding of the process of synapse formation and the modulation of synaptic transmission. Recent studies indicate that the major excitatory neurotransmitter receptors in the brain, the glutamate receptors, are associated with many different molecules that are involved in the formation of elaborate synaptic cytoskeletal networks and signal transduction cascades. These complex protein networks may play critical roles in the regulation of neurotransmitter receptor function and the efficacy of synaptic transmission.

Osteoblast-derived acetylcholinesterase: a novel mediator of cell-matrix interactions in bone?

The adhesive interactions that occur between bone cells and the developing matrix during bone formation help guide coupled remodeling and the maintenance of bone mass. Here, we provide evidence that acetylcholinesterase (AChE) is a novel osteoblast-derived mediator of cell-matrix interactions in bone. These findings complement an increasing body of evidence which suggests that AChE, in addition to its role in terminating cholinergic signaling, may be instrumental in regulating cellular differentiation and adhesion. We have shown, using RT-PCR, that osteosarcoma cell lines and primary cultures of osteoblasts express AChE mRNA. Expression appeared to be differentiation-dependent, and restricted to AChE splice variants containing the T subunit (exon 6). Immunofluorescent localization demonstrated that these osteoblastic cells expressed protein for AChE with an intracellular vesicular distribution. Immunohistochemistry on tissue sections confirmed AChE expression by osteoblasts in vivo, and revealed the presence of AChE along cement lines, also identified by enzyme histochemistry. In vitro functional studies indicated that osteoblast-like cells adhered specifically to and spread on AChE substrates, but did not interact with butyrylcholinesterase, a closely related protein. Our evidence strongly implicates AChE as a novel bone matrix protein, capable of mediating cell-matrix interactions, and as such may be a principal participant in organized bone formation and the regulation of remodeling.

Neurexins are functional alpha-latrotoxin receptors

Alpha-latrotoxin is a potent neurotoxin that triggers synaptic exocytosis. Surprisingly, two distinct neuronal receptors for alpha-latrotoxin have been described: CIRL/latrophilin 1 (CL1) and neurexin-1alpha. Alpha-latrotoxin is thought to trigger exocytosis by binding to CL1, while the role of neurexin 1alpha is uncertain. Using PC12 cells, we now demonstrate that neurexins indeed function as alpha-latrotoxin receptors that are at least as potent as CL1. Both alpha- and beta-neurexins represent autonomous alpha-latrotoxin receptors that are regulated by alternative splicing. Similar to CL1, truncated neurexins without intracellular sequences are fully active; therefore, neurexins and CL1 recruit alpha-latrotoxin but are not themselves involved in exocytosis. Thus, alpha-latrotoxin is unique among neurotoxins, because it utilizes two unrelated receptors, probably to amplify recruitment of alpha-latrotoxin to active sites.

Neuroligin 1 is a postsynaptic cell-adhesion molecule of excitatory synapses

At the synapse, presynaptic membranes specialized for vesicular traffic are linked to postsynaptic membranes specialized for signal transduction. The mechanisms that connect pre- and postsynaptic membranes into synaptic junctions are unknown. Neuroligins and beta-neurexins are neuronal cell-surface proteins that bind to each other and form asymmetric intercellular junctions. To test whether the neuroligin/beta-neurexin junction is related to synapses, we generated and characterized monoclonal antibodies to neuroligin 1. With these antibodies, we show that neuroligin 1 is synaptic. The neuronal localization, subcellular distribution, and developmental expression of neuroligin 1 are similar to those of the postsynaptic marker proteins PSD-95 and NMDA-R1 receptor. Quantitative immunogold electron microscopy demonstrated that neuroligin 1 is clustered in synaptic clefts and postsynaptic densities. Double immunofluorescence labeling revealed that neuroligin 1 colocalizes with glutamatergic but not gamma-aminobutyric acid (GABA)ergic synapses. Thus neuroligin 1 is a synaptic cell-adhesion molecule that is enriched in postsynaptic densities where it may recruit receptors, channels, and signal-transduction molecules to synaptic sites of cell adhesion. In addition, the neuroligin/beta-neurexin junction may be involved in the specification of excitatory synapses.

Neurexophilin binding to alpha-neurexins. A single LNS domain functions as an independently folding ligand-binding unit

alpha-Neurexins (Ialpha, IIalpha, and IIIalpha) are receptor-like proteins expressed in hundreds of isoforms on the neuronal cell surface. The extracellular domains of alpha-neurexins are composed of six LNS repeats, named after homologous sequences in the Laminin A G domain, Neurexins, and Sex hormone-binding globulin, with three interspersed epidermal growth factor-like domains. Purification of neurexin Ialpha revealed that it is tightly complexed to a secreted glycoprotein called neurexophilin 1. Neurexophilin 1 is a member of a family of at least four genes and resembles a neuropeptide, suggesting a function as an endogenous ligand for alpha-neurexins. We have now used recombinant proteins and knockout mice to investigate which isoforms and domains of different neurexins and neurexophilins interact with each other. We show that neurexophilins 1 and 3 but not 4 (neurexophilin 2 is not expressed in rodents) bind to a single individual LNS domain, the second overall LNS domain in all three alpha-neurexins. Although this domain is alternatively spliced, all splice variants bind, suggesting that alternative splicing does not regulate binding. Using homologous recombination to disrupt the neurexophilin 1 gene, we generated mutant mice that do not express detectable neurexophilin 1 mRNA. Mice lacking neurexophilin 1 are viable with no obvious morbidity or mortality. However, homozygous mutant mice exhibit male sterility, probably because homologous recombination resulted in the co-insertion into the neurexophilin gene of herpes simplex virus thymidine kinase, which is known to cause male sterility. In the neurexophilin 1 knockout mice, neurexin Ialpha is complexed with neurexophilin 3 but not neurexophilin 4, suggesting that neurexophilin 1 is redundant with neurexophilin 3 and that neurexophilins 1 and 3 but not 4 bind to neurexins. This hypothesis was confirmed using expression experiments. Our data reveal that the six LNS and three epidermal growth factor domains of neurexins are independently folding ligand-binding domains that may interact with distinct targets. The results support the notion that neurexophilins represent a family of extracellular signaling molecules that interact with multiple receptors including all three alpha-neurexins.

Functional redundancy of acetylcholinesterase and neuroligin in mammalian neuritogenesis

Accumulated evidence attributes noncatalytic morphogenic activitie(s) to acetylcholinesterase (AChE). Despite sequence homologies, functional overlaps between AChE and catalytically inactive AChE-like cell surface adhesion proteins have been demonstrated only for the Drosophila protein neurotactin. Furthermore, no mechanism had been proposed to enable signal transduction by AChE, an extracellular enzyme. Here, we report impaired neurite outgrowth and loss of neurexin Ialpha mRNA under antisense suppression of AChE in PC12 cells (AS-ACHE cells). Neurite growth was partially rescued by addition of recombinant AChE to the solid substrate or by transfection with various catalytically active and inactive AChE variants. Moreover, overexpression of the homologous neurexin I ligand, neuroligin-1, restored both neurite extension and expression of neurexin Ialpha. Differential PCR display revealed expression of a novel gene, nitzin, in AS-ACHE cells. Nitzin displays 42% homology to the band 4.1 protein superfamily capable of linking integral membrane proteins to the cytoskeleton. Nitzin mRNA is high throughout the developing nervous system, is partially colocalized with AChE, and increases in rescued AS-ACHE cells. Our findings demonstrate redundant neurite growth-promoting activities for AChE and neuroligin and implicate interactions of AChE-like proteins and neurexins as potential mediators of cytoarchitectural changes supporting neuritogenesis.

A tripartite protein complex with the potential to couple synaptic vesicle exocytosis to cell adhesion in brain

We identify a complex of three proteins in brain that has the potential to couple synaptic vesicle exocytosis to neuronal cell adhesion. The three proteins are: (1) CASK, a protein related to MAGUKs (membrane-associated guanylate kinases); (2) Mint1, a putative vesicular trafficking protein; and (3) Veli1, -2, and -3, vertebrate homologs of C. elegans LIN-7. CASK, Mint1, and Velis form a tight, salt-resistant complex that can be readily isolated. CASK, Mint1, and Velis contain PDZ domains in addition to other modules. However, no PDZ domains are involved in complex formation, leaving them free to recruit cell adhesion molecules, receptors, and channels to the complex. We propose that the tripartite complex acts as a nucleation site for the assembly of proteins involved in synaptic vesicle exocytosis and synaptic junctions.

A novel multiple PDZ domain-containing molecule interacting with N-methyl-D-aspartate receptors and neuronal cell adhesion proteins

At synaptic junctions, pre- and postsynaptic membranes are connected by cell adhesion and have distinct structures for specialized functions. The presynaptic membranes have a machinery for fast neurotransmitter release, and the postsynaptic membranes have clusters of neurotransmitter receptors. The molecular mechanism of the assembly of synaptic junctions is not yet clear. Pioneering studies identified postsynaptic density (PSD)-95/SAP90 as a prototypic synaptic scaffolding protein to maintain the structure of synaptic junctions. PSD-95/SAP90 belongs to a family of membrane-associated guanylate kinases and binds N-methyl-D-aspartate receptors, potassium channels, and neuroligins through the PDZ domains and GKAP/SAPAP/DAP through the guanylate kinase (GK) domain. We performed here a yeast two-hybrid screening for SAPAP-interacting molecules and identified a novel protein that has an inverse structure of membrane-associated guanylate kinases with an NH2-terminal GK-like domain followed by two WW and five PDZ domains. It binds SAPAP through the GK-like domain and NMDA receptors and neuroligins through the PDZ domains. We named this protein S-SCAM (synaptic scaffolding molecule) because S-SCAM may assemble receptors and cell adhesion proteins at synaptic junctions.

CIPP, a novel multivalent PDZ domain protein, selectively interacts with Kir4.0 family members, NMDA receptor subunits, neurexins, and neuroligins

We report a novel multivalent PDZ domain protein, CIPP (for channel-interacting PDZ domain protein), which is expressed exclusively in brain and kidney. Within the brain, the highest CIPP mRNA levels were found in neurons of the cerebellum, inferior colliculus, vestibular nucleus, facial nucleus, and thalamus. Furthermore, we identified the inward rectifier K+ (Kir) channel, Kir4.1 (also called "Kir1.2"), as a cellular CIPP ligand. Among several other Kir channels tested, only the closely related Kir4.2 (or "Kir1.3") also interacted with CIPP. In addition, specific PDZ domains within CIPP associated selectively with the C-termini of N-methyl-D-aspartate subtypes of glutamate receptors, as well as neurexins and neuroligins, cell surface molecules enriched in synaptic membranes. Thus, CIPP may serve as a scaffold that brings structurally diverse but functionally connected proteins into close proximity at the synapse. The functional consequences of CIPP expression on Kir4.1 channels were studied using whole-cell voltage clamp techniques in Kir4.1 transfected COS-7 cells. On average, Kir4.1 current densities were doubled by cotransfection with CIPP.

Neurexophilins form a conserved family of neuropeptide-like glycoproteins

Neurexophilin was discovered as a neuronal glycoprotein that is copurified with neurexin Ialpha during affinity chromatography on immobilized alpha-latrotoxin (Petrenko et al., 1996). We have now investigated how neurexophilin interacts with neurexins, whether it is post-translationally processed by site-specific cleavage similar to neuropeptides, and whether related neuropeptide-like proteins are expressed in brain. Our data show that mammalian brains contain four genes for neurexophilins the products of which share a common structure composed of five domains: an N-terminal signal peptide, a variable N-terminal domain, a highly conserved central domain that is N-glycosylated, a short linker region, and a conserved C-terminal domain that is cysteine-rich. When expressed in pheochromocytoma (PC12) cells with a replication-deficient adenovirus, neurexophilin 1 was rapidly N-glycosylated and then slowly processed to a smaller mature form, probably by endoproteolytic cleavage. Similar expression experiments in other neuron-like cells and in fibroblastic cells revealed that N-glycosylation of neurexophilin 1 occurred in all cell types tested, whereas proteolytic processing was observed only in neuron-like cells. Finally, only recombinant neurexin Ialpha and IIIalpha but not neurexin Ibeta interacted with neurexophilin 1 and were preferentially bound to the processed mature form of neurexophilin. Together our data demonstrate that neurexophilins form a family of related glycoproteins that are proteolytically processed after synthesis and bind to alpha-neurexins. The structure and characteristics of neurexophilins indicate that they function as neuropeptides that may signal via alpha-neurexins.

Developmental expression of acetyl- and butyrylcholinesterase in the rat: enzyme and mRNA levels in embryonic dorsal root ganglia

Dorsal root ganglia (DRG) in the adult rat contain acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), enzymes implicated in neural morphogenesis. We used quantitative histochemistry, reverse transcription-PCR (RT-PCR), and in situ hybridization histochemistry to study cholinesterase expression during embryogenesis. Longitudinal sections of rat embryos, embryonic day 9 (E9), E11-E17, and E19, were studied by video microscopy of the stained enzyme reaction products. Both enzymes were detectable in the early DRG (E11-E12), with BChE being most prominent. There was a spatiotemporal change in expression of each cholinesterase within the DRG. From E13 on, AChE expression predominated, especially in the neuronal cell bodies, while BChE was more highly expressed in the surrounding neuropil and the ganglionic roots. This distribution resembled the pattern in adult DRG. AChE mRNA levels, as determined by RT-PCR from DRG collected at days E12-E17, and E19, varied in parallel with the intensity of enzyme stain in the DRG. Overall, these results demonstrate temporally regulated ganglionic expression of cholinesterases, which may be important in the development of the sensory nervous system.

Oligomerization of the extracellular domain of Boss enhances its binding to the Sevenless receptor and its antagonistic effect on R7 induction

In the developing compound eye of Drosophila, neuronal differentiation of the R7 photoreceptor cell is induced by the interaction of the receptor tyrosine kinase Sevenless with its ligand Bride of sevenless (Boss), which is expressed on the neighboring R8 cell. Boss is an unusual ligand of a receptor tyrosine kinase: it is composed of a large extracellular domain, a transmembrane domain with seven membrane-spanning segments and a cytoplasmic tail. Expression of a monomeric, secreted form of the extracellular domain of Boss is not sufficient for Sevenless activation, and instead acts as a weak antagonist. Because oligomerization appears to be a critical step in the activation of receptor tyrosine kinases, we used oligomerized forms of the Boss extracellular domain to test their ability to bind to Sevenless in vivo and restore R7 induction in vivo. Oligomerization was achieved by fusion to the leucine zipper of the yeast transcription factor GCN4 or to the tetramerization helix of Lac repressor. Binding of these multivalent proteins to Sevenless could be detected in vitro by immunoprecipitation of cross-linked ligand/receptor complexes and in vivo by receptor-dependent ligand localization. However, neither R8-specific or ubiquitous expression of multivalent Exboss ligands rescued the boss phenotype. Instead, these ligands acted as competitive inhibitors for wild-type Boss protein and thereby suppressed R7 induction. Therefore the role of the transmembrane or cytoplasmic domains of Boss in the activation of the Sev receptor cannot be replaced by oligomerization.

Acetylcholinesterase enhances neurite growth and synapse development through alternative contributions of its hydrolytic capacity, core protein, and variable C termini

Accumulated indirect evidence suggests nerve growth-promoting activities for acetylcholinesterase (AChE). To determine unequivocally whether such activities exist, whether they are related to the capacities of this enzyme to hydrolyze acetylcholine and enhance synapse development, and whether they are associated with alternative splicing variants of AChEmRNA, we used four recombinant human AChEDNA vectors. When Xenopus laevis embryos were injected with a vector expressing the synapse-characteristic human AChE-E6, which contains the exon 6-encoded C terminus, cultured spinal neurons expressing this enzyme grew threefold faster than co-cultured control neurons. Similar enhancement occurred in neurons expressing an insertion-inactivated human AChE-E6-IN protein, containing the same C terminus, and displaying indistinguishable immunochemical and electrophoretic migration properties from AChE-E6, but incapable of hydrolyzing acetylcholine. In contrast, the nonsynaptic secretory human AChE-I4, which contains the pseudointron 4-derived C terminus, did not affect neurite growth. Moreover, no growth promotion occurred in neurons expressing the catalytically active C-terminally truncated human AChE-E4, demonstrating a dominant role for the E6-derived C terminus in neurite extension. Also, AChE-E6 was the only active enzyme variant to be associated with Xenopus membranes. However, postsynaptic length measurements demonstrated that both AChE-E6 and AChE-E4 enhanced the development of neuromuscular junctions in vivo, unlike the catalytically inert AChE-E6-IN and the nonsynaptic AChE-I4. These findings demonstrate an evolutionarily conserved synaptogenic activity for AChE that depends on its hydrolytic capacity but not on its membrane association. Moreover, this synaptogenic effect differs from the growth-promoting activity of AChE, which is unrelated to its hydrolytic capacity yet depends on its exon 6-mediated membrane association.

Neurabin-II/spinophilin. An actin filament-binding protein with one pdz domain localized at cadherin-based cell-cell adhesion sites

In a preceding paper, we reported a novel actin filament (F-actin)-binding protein, named neurabin, which was specifically expressed in neural tissue and implicated in neurite formation. We purified from rat brain another F-actin-binding protein, which had a domain organization similar to that of neurabin but was ubiquitously expressed, and named it neurabin-II. The original neurabin, renamed neurabin-I, had 1095 amino acids and a calculated Mr of 122,729, whereas neurabin-II had 817 amino acids and a calculated Mr of 89, 642. Both neurabin-I and -II had one F-actin-binding domain at the N-terminal region, one PDZ domain at the middle region, a domain known to interact with transmembrane proteins, and domains predicted to form coiled-coil structures at the C-terminal region. Both neurabin-I and -II bound along the sides of F-actin and showed F-actin-cross-linking activity. The subcellular distribution analysis indicated that neurabin-II was enriched at the postsynaptic density fraction in rat brain and the adherens junction fraction in rat liver. Immunofluorescence microscopic analysis revealed that neurabin-II was highly concentrated at the synapse in primary cultured rat hippocampal neurons and at the cadherin-based cell-cell adhesion sites in Madin-Darby canine kidney cells. Neurabin-II turned out to be the same as a recently reported protein phosphatase 1-binding protein named spinophilin. These results suggest that neurabin-II/spinophilin plays an important role in linking the actin cytoskeleton to the plasma membrane.

Neurotactin functions in concert with other identified CAMs in growth cone guidance in Drosophila

We have isolated and characterized mutations in Drosophila neurotactin, a gene that encodes a cell adhesion protein widely expressed during neural development. Analysis of both loss and gain of gene function conditions during embryonic and postembryonic development revealed specific requirements for neurotactin during axon outgrowth, fasciculation, and guidance. Furthermore, embryos of some double mutant combinations of neurotactin and other genes encoding adhesion/signaling molecules, including neuroglian, derailed, and kekkon1, displayed phenotypic synergy. This result provides evidence for functional cooperativity in vivo between the adhesion and signaling pathways controlled by neurotactin and the other three genes.

Neurexin I alpha is a major alpha-latrotoxin receptor that cooperates in alpha-latrotoxin action

alpha-Latrotoxin is a potent neurotoxin from black widow spider venom that binds to presynaptic receptors and causes massive neurotransmitter release. A surprising finding was the biochemical description of two distinct cell surface proteins that bind alpha-latrotoxin with nanomolar affinities; Neurexin I alpha binds alpha-latrotoxin in a Ca(2+)-dependent manner, and CIRL/latrophilin binds in a Ca(2+)-independent manner. We have now generated and analyzed mice that lack neurexin I alpha to test its importance in alpha-latrotoxin action. alpha-Latrotoxin binding to brain membranes from mutant mice was decreased by almost 50% compared with wild type membranes; the decrease was almost entirely due to a loss of Ca(2+)-dependent alpha-latrotoxin binding sites. In cultured hippocampal neurons, alpha-latrotoxin was still capable of activating neurotransmission in the absence of neurexin I alpha. Direct measurements of [3H]glutamate release from synaptosomes, however, showed a major decrease in the amount of release triggered by alpha-latrotoxin in the presence of Ca2+. Thus neurexin I alpha is not essential for alpha-latrotoxin action but contributes to alpha-latrotoxin action when Ca2+ is present. Viewed as a whole, our results show that mice contain two distinct types of alpha-latrotoxin receptors with similar affinities and abundance but different properties and functions. The action of alpha-latrotoxin may therefore be mediated by independent parallel pathways, of which the CIRL/latrophilin pathway is sufficient for neurotransmitter release, whereas the neurexin I alpha pathway contributes to the Ca(2+)-dependent action of alpha-latrotoxin.

Neurexins: three genes and 1001 products

The human brain has approximately 10(12) neurons, three orders of magnitude more than there are basepairs in the human genome. Each neuron is connected to other neurons by thousands of synapses, creating a dense network of communicating neurons. Cell-recognition events between neurons at, and outside of synapses, are likely to guide the development and maintenance of the complex network formed by neurons. However, little is known about which proteins are important for neuronal cell recognition. Neurexins, a family of polymorphic cell-surface proteins, might mediate some of these cell recognition events. Thousands of neurexin isoforms are generated from three genes by usage of alternative promoters and alternative splicing. These isoforms are displayed on the neuronal cell surface, with different classes of neurons expressing distinct combinations of isoforms. Neurexins probably have a multitude of ligands, some of which interact only with subsets of neurexin isoforms. This review describes the properties of the neurexin protein family and their potential roles in neuronal cell adhesion and intercellular signaling.

The axonal membrane protein Caspr, a homologue of neurexin IV, is a component of the septate-like paranodal junctions that assemble during myelination

We have investigated the potential role of contactin and contactin-associated protein (Caspr) in the axonal-glial interactions of myelination. In the nervous system, contactin is expressed by neurons, oligodendrocytes, and their progenitors, but not by Schwann cells. Expression of Caspr, a homologue of Neurexin IV, is restricted to neurons. Both contactin and Caspr are uniformly expressed at high levels on the surface of unensheathed neurites and are downregulated during myelination in vitro and in vivo. Contactin is downregulated along the entire myelinated nerve fiber. In contrast, Caspr expression initially remains elevated along segments of neurites associated with nascent myelin sheaths. With further maturation, Caspr is downregulated in the internode and becomes strikingly concentrated in the paranodal regions of the axon, suggesting that it redistributes from the internode to these sites. Caspr expression is similarly restricted to the paranodes of mature myelinated axons in the peripheral and central nervous systems; it is more diffusely and persistently expressed in gray matter and on unmyelinated axons. Immunoelectron microscopy demonstrated that Caspr is localized to the septate-like junctions that form between axons and the paranodal loops of myelinating cells. Caspr is poorly extracted by nonionic detergents, suggesting that it is associated with the axon cytoskeleton at these junctions. These results indicate that contactin and Caspr function independently during myelination and that their expression is regulated by glial ensheathment. They strongly implicate Caspr as a major transmembrane component of the paranodal junctions, whose molecular composition has previously been unknown, and suggest its role in the reciprocal signaling between axons and glia.

Binding properties of neuroligin 1 and neurexin 1beta reveal function as heterophilic cell adhesion molecules

beta-Neurexins and neuroligins are plasma membrane proteins that are displayed on the neuronal cell surface. We have now investigated the interaction of neurexin 1beta with neuroligin 1 to evaluate their potential to function as heterophilic cell adhesion molecules. Using detergent-solubilized neuroligins and secreted neurexin 1beta-IgG fusion protein, we observed binding of these proteins to each other only in the presence of Ca2+ and in no other divalent cation tested. Only neurexin 1beta lacking an insert in splice site 4 bound neuroligins, whereas neurexin 1beta containing an insert was inactive. Half-maximal binding required 1-3 microM free Ca2+, which probably acts by binding to neuroligin 1 but not to neurexin 1beta. To determine if neurexin 1beta and neuroligin 1 can also interact with each other when present in a native membrane environment on the cell surface, we generated transfected cell lines expressing neuroligin 1 and neurexin 1beta. Upon mixing different cell populations, we found that cells aggregate only if cells expressing neurexin 1beta are mixed with cells expressing neuroligin 1. Aggregation was dependent on Ca2+ and was inhibited by the addition of soluble neurexin 1beta lacking an insert in splice site 4 but not by the addition of neurexin 1beta containing an insert in splice site 4. We conclude that neurexin 1beta and neuroligin 1 (and, by extension, other beta-neurexins and neuroligins) function as heterophilic cell adhesion molecules in a Ca2+-dependent reaction that is regulated by alternative splicing of beta-neurexins.

The brain as a symbol-processing machine

The knowledge accumulated about the biochemistry of the synapsis in the last decades completely changes the notion of brain processing founded exclusively over an electrical mechanism, toward that supported by a complex chemical message exchange occurring both locally, at the synaptic site, as well as at other localities, depending on the solubility of the involved chemical substances in the extracellular compartment. These biochemical transactions support a rich symbolic processing of the information both encoded by the genes and provided by actual data collected from the surrounding environment, by means of either special molecular or cellular receptor systems. In this processing, molecules play the role of symbols and chemical affinity shared by them specifies the syntax for symbol manipulation in order to process and to produce chemical messages. In this context, neurons are conceived as message-exchanging agents. Chemical strings are produced and stored at defined places, and ionic currents are used to speed up message delivery. Synaptic transactions can no longer be assumed to correspond to a simple process of propagating numbers powered by a factor measuring the presynaptic capacity to influence the postsynaptic electrical activity, but they must be modeled by more powerful formal tools supporting both numerical and symbolic calculations. It is proposed here that formal language theory is the adequate mathematical tool to handle such symbolic processing. The purpose of the present review is therefore: (a) to discuss the relevant and recent literature about trophic factors, signal transduction mechanisms, neuromodulators and neurotransmitters in order (b) to point out the common features of these correlated processes; and (c) to show how they may be organized into a formal model supported by the theory of fuzzy formal languages (d) to model the brain as a distributed intelligent problem solver.

Binding of neuroligins to PSD-95

PSD-95 is a component of postsynaptic densities in central synapses. It contains three PDZ domains that localize N-methyl-D-aspartate receptor subunit 2 (NMDA2 receptor) and K+ channels to synapses. In mouse forebrain, PSD-95 bound to the cytoplasmic COOH-termini of neuroligins, which are neuronal cell adhesion molecules that interact with beta-neurexins and form intercellular junctions. Neuroligins bind to the third PDZ domain of PSD-95, whereas NMDA2 receptors and K+ channels interact with the first and second PDZ domains. Thus different PDZ domains of PSD-95 are specialized for distinct functions. PSD-95 may recruit ion channels and neurotransmitter receptors to intercellular junctions formed between neurons by neuroligins and beta-neurexins.

Afferent innervation influences the development of dendritic branches and spines via both activity-dependent and non-activity-dependent mechanisms

The present investigation uses an in vitro co-culture system to study the role of afferent innervation in early development and differentiation of hippocampal neurons. Our experiments indicate that the formation of two key morphological features, dendritic branches and dendritic spines, is induced by afferent innervation. Hippocampal neurons develop multiple dendritic branches and spines only when extensively innervated by living axonal afferents. No morphological changes occurred when hippocampal neurons were plated on other cell surfaces such as fixed axons or astrocytes. Furthermore, afferents exerted their effect locally on individual dendrites that they contacted. When one portion of the dendritic arbor of a neuron was contacted by afferents and the other portion was not, morphological effects were restricted to the innervated dendrites. Innervation of some of the dendrites on a neuron did not produce global effects throughout the neuron. Afferent-induced dendritic branching is independent of activity, since branch induction was unaffected by chronic application of TTX or glutamate receptor blockers. In contrast, the formation of dendritic spines is influenced by activity. The number of developing spines was reduced when TTX or a cocktail of three glutamate receptor blockers was applied. Blockade of individual AMPA, NMDA, or metabotropic glutamate receptors did not affect the number of spines. These results, taken together, demonstrate that afferents can have a prominent influence on the development of postsynaptic target cells via both activity-dependent and non-activity-dependent mechanisms, indicating the presence of multiple signals. Accordingly, this suggests an important interplay between pre- and postsynaptic elements early in development.

Rim is a putative Rab3 effector in regulating synaptic-vesicle fusion

Rab3 is a neuronal GTP-binding protein that regulates fusion of synaptic vesicles and is essential for long-term potentiation of hippocampal mossy fibre synapses. More than thirty Rab GTP-binding proteins are known to function in diverse membrane transport pathways, although their mechanisms of action are unclear. We have now identified a putative Rab3-effector protein called Rim. Rim is composed of an amino-terminal zinc-finger motif and carboxy-terminal PDZ and C2 domains. It binds only to GTP (but not to GDP)-complexed Rab3, and interacts with no other Rab protein tested. There is enrichment of Rab3 and Rim in neurons, where they have complementary distributions. Rab3 is found only on synaptic vesicles, whereas Rim is localized to presynaptic active zones in conventional synapses, and to presynaptic ribbons in ribbon synapses. Transfection of PC12 cells with the amino-terminal domains of Rim greatly enhances regulated exocytosis in a Rab3-dependent manner. We propose that Rim serves as a Rab3-dependent regulator of synaptic-vesicle fusion by forming a GTP-dependent complex between synaptic plasma membranes and docked synaptic vesicles.

Acetylcholinesterase-transgenic mice display embryonic modulations in spinal cord choline acetyltransferase and neurexin Ibeta gene expression followed by late-onset neuromotor deterioration

To explore the possibility that overproduction of neuronal acetylcholinesterase (AChE) confers changes in both cholinergic and morphogenic intercellular interactions, we studied developmental responses to neuronal AChE overexpression in motoneurons and neuromuscular junctions of AChE-transgenic mice. Perikarya of spinal cord motoneurons were consistently enlarged from embryonic through adult stages in AChE-transgenic mice. Atypical motoneuron development was accompanied by premature enhancement in the embryonic spinal cord expression of choline acetyltransferase mRNA, encoding the acetylcholine-synthesizing enzyme choline acetyltransferase. In contrast, the mRNA encoding for neurexin-Ibeta, the heterophilic ligand of the AChE-homologous neuronal cell surface protein neuroligin, was drastically lower in embryonic transgenic spinal cord than in controls. Postnatal cessation of these dual transcriptional responses was followed by late-onset deterioration in neuromotor performance that was associated with gross aberrations in neuromuscular ultrastructure and with pronounced amyotrophy. These findings demonstrate embryonic feedback mechanisms to neuronal AChE overexpression that are attributable to both cholinergic and cell-cell interaction pathways, suggesting that embryonic neurexin Ibeta expression is concerted in vivo with AChE levels and indicating that postnatal changes in neuronal AChE-associated proteins may be involved in late-onset neuromotor pathologies.