Cartography of neurexins: more than 1000 isoforms generated by alternative splicing and expressed in distinct subsets of neurons

Studies on protocadherin-2 expression in the human fetal central nervous system

Protocadherin (Pcad) is a group of molecules obtained by polymerase chain reaction (PCR) utilizing the sequence that is well preserved in the extracellular domain of cadherin. Sano et al. analyzed Pcad (PC42,43) that had been cloned from rats, and found that it basically had homology to cadherin, but contained more than six cadherin repeats with a completely different intracellular domains (Sano et al. 1993). In the present study, of the Pcad (Pcad-1,2) cloned from a human cDNA library, as-yet-unspecified Pcad-2 was analyzed for expression in the human fetal central nervous system (CNS). Northern blot analysis of adult human tissue showed that Pcad-2 was expressed in the brain and the placenta, and that Pcad-2 mRNA was expressed in actively dividing neural tumor cell lines. Monoclonal antibodies against Pcad-2 were then made, and the CNS of fetuses were immunohistochemically stained. The expression was hardly visible at the 6th week of pregnancy, and began to become visible along the nerve fiber in the brain stem at the 8th week, and spread over the entire brain at the 11th week. At the 18th week, however, expression in the nerve fascicles, which had been visible by that time, was no longer visible or had decreased. These results suggest that Pcad-2 appears relatively early in the critical stage of development of the fetal CNS, and is involved in the induction, fasciculation, and extension of axons.

Regulation of alternative RNA splicing by exon definition and exon sequences in viral and mammalian gene expression

Intron removal from a pre-mRNA by RNA splicing was once thought to be controlled mainly by intron splicing signals. However, viral and other eukaryotic RNA exon sequences have recently been found to regulate RNA splicing, polyadenylation, export, and nonsense-mediated RNA decay in addition to their coding function. Regulation of alternative RNA splicing by exon sequences is largely attributable to the presence of two major cis-acting elements in the regulated exons, the exonic splicing enhancer (ESE) and the suppressor or silencer (ESS). Two types of ESEs have been verified from more than 50 genes or exons: purine-rich ESEs, which are the more common, and non-purine-rich ESEs. In contrast, the sequences of ESSs identified in approximately 20 genes or exons are highly diverse and show little similarity to each other. Through interactions with cellular splicing factors, an ESE or ESS determines whether or not a regulated splice site, usually an upstream 3' splice site, will be used for RNA splicing. However, how these elements function precisely in selecting a regulated splice site is only partially understood. The balance between positive and negative regulation of splice site selection likely depends on the cis-element's identity and changes in cellular splicing factors under physiological or pathological conditions.

Proteomics in the chicken: tools for understanding immune responses to avian diseases

The entire chicken genome sequence will be available by the time this review is in press. Chickens will be the first production animal species to enter the "postgenomic era." This fundamental structural genomics achievement allows, for the first time, complete functional genomics approaches for understanding the molecular basis of chicken normo- and pathophysiology. The functional genomics paradigm, which contrasts with classical functional genetic investigations of one gene (or few) in isolation, is the systematic holistic genetic analyses of biological systems in defined contexts. Context-dependent gene interactions are the fundamental mechanics of all life. Functional genomics uses high-throughput large-scale experimental methods combined with statistical and computational analyses. Projects with expressed sequence tags in chickens have already allowed the creation of cDNA microarrays for large-scale context-dependant mRNA analysis (transcriptomics). However, proteins are the functional units of almost all biological processes, and protein expression very often bears no correlation to mRNA expression. Proteomics, a discipline within functional genomics, is the context-defined analysis of complete complements of proteins. Proteomics bridges the "sequence-to-phenotype gap;" it complements structural and other functional genomics approaches. Proteomics requires high capital investment but has ubiquitous biological applications. Although currently the fastest-growing human biomedical discipline, new paradigms may need to be established for production animal proteomics research. The prospective promise and potential pitfalls of using proteomics approaches to improve poultry pathogen control will be specifically highlighted. The first stage of our recently established proteomics program is global protein profiling to identify differentially expressed proteins in the context of the commercially important pathogens. Our trials and tribulations in establishing our proteomics program, as well some of our initial data to understand chicken immune system function, will be discussed.

A collection of cDNAs enriched in upper cortical layers of the embryonic mouse brain

In an attempt to elucidate the molecular basis of neuronal migration and corticogenesis, we performed subtractive hybridization of mRNAs from the upper cortical layers (layer I and upper cortical plate) against mRNAs from the remaining cerebral cortex at E15-E16. We obtained a collection of subtracted cDNA clones and analyzed their 3' UTR sequences, 47% of which correspond to EST sequences, and may represent novel products. Among the cloned sequences, we identified gene products that have not been reported in brain or in the cerebral cortex before. We examined the expression pattern of 39 subtracted clones, which was enriched in the upper layers of the cerebral cortex at embryonic stages. The expression of most clones is developmentally regulated, and especially high in embryonic and early postnatal stages. Four of the unknown clones were studied in more detail and identified as a new member of the tetraspanin superfamily, a putative RNA binding protein, a specific product of the adult dentate gyrus and a protein containing a beta-catenin repeat. We thus cloned a collection of subtracted cDNAs coding for protein products that may be involved in the development of the cerebral cortex.

Watson and Crick 50 years on. From double helix to pharmacogenomics

The second half of the 20th century has seen quantum leaps in our understanding of molecular biology. The technological advances, which facilitated the recent successful completion of the Human Genome Project, have provided the tools for deciphering the complexity of the human condition. At present, the function of only 50% of genes is known. However, as understanding of the human genome improves, a plethora of gene targets for treating disease will be uncovered - leading to therapies which will be considered revolutionary. Genome related science has begun to impact almost every facet of medicine including anaesthesia and intensive care. Better understanding of interindividual differences will enable better prediction of illness susceptibility as well as response to treatment. These insights will permit therapies to be tailored to individuals or racial groups. At present, there is only rudimentary knowledge of factors controlling gene regulation, but in the future, better understanding of gene-environment interactions and gene expression will enable pharmaceutical companies to develop new therapies and permit clinicians to optimise their effects, without recourse to current laborious testing regimens. As genomic science progresses, new ethical, legal, social and philosophical dilemmas will also continue to emerge.

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.

The complexity of alternative splicing of hagoromo mRNAs is increased in an explosively speciated lineage in East African cichlids

The adaptive radiation of cichlid fishes in the lakes of East Africa is a prime example of speciation. The choice of cichlid mates on the basis of a variety of coloration represents a potential basis for speciation that led to adaptive radiation. Here, we characterize the cichlid homolog of the zebrafish hagoromo (hag) gene that was recently cloned and characterized from a pigmentation mutant. Although only one hag mRNA was reported in zebrafish, cichlids express nine different hag mRNAs resulting from alternative splicing. The hag mRNAs are expressed between the myotome and the epidermis where pigment cells are located, suggesting the cichlid hag gene is involved in pigmentation. The hag mRNA splicing pattern does not fluctuate among individuals from each of two species, suggesting that alternative splice site choice is fixed within species. Furthermore, cichlids in lineages that underwent explosive speciation expressed a greater variety of hag mRNAs than those in lineages that did not undergo such a degree of speciation, suggesting that species in the explosively speciated lineage acquired a complex regulatory mechanism of alternative splicing over a very short evolutionary period. Here, we provide an example in which alternative splicing may play a role in mate choice, leading to cichlid speciation through diversification of gene function by production of multiple mRNAs from a single gene.

ORESTES are enriched in rare exon usage variants affecting the encoded proteins

A significant fraction of the variability found in the human transcriptome is due to alternative splicing, including alternative exon usage (AEU), intron retention and use of cryptic splice sites. We present a comparison of a large-scale analysis of AEU in the human transcriptome through genome mapping of Open Reading Frame ESTs (ORESTES) and conventional ESTs. It is shown here that ORESTES probe low abundant messages more efficiently. In addition, most of the variants detected by ORESTES affect the structure of the corresponding proteins.

Alternative splicing of lola generates 19 transcription factors controlling axon guidance in Drosophila

The Drosophila melanogaster transcription factor Lola (longitudinals lacking) is a pivotal regulator of neural wiring that sets the precise expression levels of proteins that execute specific axon guidance decisions. Lola has a zinc finger DNA binding domain and a BTB (for Broad-complex, Tramtrack and Bric a brac) dimerization motif. We now show that alternative splicing of the lola gene creates a family of 19 transcription factors. All lola isoforms share a common dimerization domain, but 17 have their own unique DNA-binding domains. Seven of these 17 isoforms are present in the distantly-related Dipteran Anopheles gambiae, suggesting that the properties of specific isoforms are likely to be critical to lola function. Analysis of the expression patterns of individual splice variants and of the phenotypes of mutants lacking single isoforms supports this idea and establishes that the alternative forms of lola are responsible for different functions of this gene. Thus, in this system, the alternative splicing of a key transcription factor helps to explain how a small genome encodes all the information that is necessary to specify the enormous diversity of axonal trajectories.

Alpha-neurexins couple Ca2+ channels to synaptic vesicle exocytosis

Synapses are specialized intercellular junctions in which cell adhesion molecules connect the presynaptic machinery for neurotransmitter release to the postsynaptic machinery for receptor signalling. Neurotransmitter release requires the presynaptic co-assembly of Ca2+ channels with the secretory apparatus, but little is known about how synaptic components are organized. Alpha-neurexins, a family of >1,000 presynaptic cell-surface proteins encoded by three genes, link the pre- and postsynaptic compartments of synapses by binding extracellularly to postsynaptic cell adhesion molecules and intracellularly to presynaptic PDZ domain proteins. Using triple-knockout mice, we show that alpha-neurexins are not required for synapse formation, but are essential for Ca2+-triggered neurotransmitter release. Neurotransmitter release is impaired because synaptic Ca2+ channel function is markedly reduced, although the number of cell-surface Ca2+ channels appears normal. These data suggest that alpha-neurexins organize presynaptic terminals by functionally coupling Ca2+ channels to the presynaptic machinery.

Proteomics: the first decade and beyond

Proteomics is the systematic study of the many and diverse properties of proteins in a parallel manner with the aim of providing detailed descriptions of the structure, function and control of biological systems in health and disease. Advances in methods and technologies have catalyzed an expansion of the scope of biological studies from the reductionist biochemical analysis of single proteins to proteome-wide measurements. Proteomics and other complementary analysis methods are essential components of the emerging 'systems biology' approach that seeks to comprehensively describe biological systems through integration of diverse types of data and, in the future, to ultimately allow computational simulations of complex biological systems.

Aberrant transcription of unrearranged T-cell receptor ? gene in mouse brain

The nervous system and the immune system share several functional molecules involved in various cell-cell interaction events. In this study, we used in situ hybridization to identify immune molecules that are expressed by a restricted population of neurons in the mouse brain and found that mRNA for the beta subunit of T-cell receptor (TCRbeta) was predominantly and strongly localized to neurons in deep layers of the cerebral neocortex and weakly expressed in the thalamus. Developmentally, TCRbeta mRNA expression started at embryonic day 15 in the thalamic nuclei and at postnatal day 1 in the cerebral neocortex. The level of TCRbeta mRNA in the neocortex subsequently increased until postnatal day 21, and it remained high in the adult. Detailed analysis revealed that only the Cbeta2 segment of TCRbeta, not the Cbeta1 or Vbeta segments, was expressed by the brain neurons. By the 5' rapid amplification of cDNA ends method, we determined a brain-specific transcription start site in the Jbeta2 region locus, not in the Vbeta region locus. Furthermore, we confirmed that the aberrant transcription around the Jbeta2 region took place only in neurons and lymphocytes in transgenic mice. These results demonstrate that the transcriptional machinery for unrearranged TCRbeta expression is shared by the nervous and immune systems and raise a possibility of gene rearrangement in neurons under certain circumstances.

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.

Quantification of splice variants using molecular beacon or scorpion primers

Uncovering the relationship between the generation of alternative transcripts and cellular processes is of crucial importance in the exploration of a gene's biology. The description and quantification of the spatiotemporal splicing pattern can be one method to select the most interesting transcripts for future studies. Fluorescence-based real-time quantitative RT-PCR has recently revolutionized the possibilities for transcriptional quantification studies. In this report, Molecular Beacon and Scorpion probes have been tested as new possibilities for determining the expression level of alternative transcripts. We validated these systems by analyzing alternative splicing of exons 6, 15, and 16 of the calpain 3 gene with tissues containing large variation in the ratio of the different transcripts. We determined conditions that demonstrated that boundary probes are useful tools and good alternatives to boundary primers, when developing a system to quantify specific transcripts. We suggest that the choice of a quantification system should depend in part on the structure and base composition of the gene and may have to be determined experimentally.

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.

Caspr3 and caspr4, two novel members of the caspr family are expressed in the nervous system and interact with PDZ domains

The NCP family of cell-recognition molecules represents a distinct subgroup of the neurexins that includes Caspr and Caspr2, as well as Drosophila Neurexin-IV and axotactin. Here, we report the identification of Caspr3 and Caspr4, two new NCPs expressed in nervous system. Caspr3 was detected along axons in the corpus callosum, spinal cord, basket cells in the cerebellum and in peripheral nerves, as well as in oligodendrocytes. In contrast, expression of Caspr4 was more restricted to specific neuronal subpopulations in the olfactory bulb, hippocampus, deep cerebellar nuclei, and the substantia nigra. Similar to the neurexins, the cytoplasmic tails of Caspr3 and Caspr4 interacted differentially with PDZ domain-containing proteins of the CASK/Lin2-Veli/Lin7-Mint1/Lin10 complex. The structural organization and distinct cellular distribution of Caspr3 and Caspr4 suggest a potential role of these proteins in cell recognition within the nervous system.

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.

A molecular view on paranodal junctions of myelinated fibers

The axoglial paranodal junctions, flanking the Ranvier nodes, are specialized adhesion sites between the axolemma and myelinating glial cells. Unraveling the molecular composition of paranodal junctions is crucial for understanding the mechanisms involved in the regulation of myelination, and positioning and segregation of the voltage-gated Na+ and K+ channels, essential for the generation and conduction of action potentials. Paranodin/Caspr was the first neuronal transmembrane glycoprotein identified at the paranodal junctions. Paranodin/Caspr is associated on the axonal membrane with contactin/F3, a glycosylphosphatidylinositol-anchored protein, essential for its correct targeting. The extra and intracellular regions of paranodin encompass multiple domains which can be involved in protein-protein interactions with other axonal proteins and glial proteins. Thus, paranodin plays a central role in the assembly of multiprotein complexes necessary for the formation and maintenance of paranodal junctions.

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.

The cadherin-related neuronal receptor family: a novel diversified cadherin family at the synapse

The cadherin-related neuronal receptor (CNR) family has been identified as a receptor family that cooperates with Fyn, a member of the Src family of tyrosine kinases. The CNR family is composed of 14 members in mice and 15 members in humans. The mRNAs of CNRs are highly expressed in the brain and CNR1 protein is localized at synaptic junctions. Hence CNR family proteins are synaptic cadherins. The unique structure of CNR family cDNAs, which is characterized by complete DNA sequence identity among their 3'-termini including a part of the coding region, prompted us to investigate the genomic organization of this family. The genomic organization of CNRs is divided into 'variable' and 'constant' region exons, analogous to immunoglobulin and T cell receptor gene clusters. This organization raised the possibility that the CNR gene cluster may undergo somatic DNA rearrangement or trans-splicing and produce diversified gene products. Although it is not yet clear that the CNR gene cluster in the neuronal genomic DNA is somatically changed, a recent study suggested the occurrence of trans-transcripts and accumulation of somatic mutations in CNR transcripts (Genes Cells 6 (2001) 151). These results suggested that the proteins from the CNR gene cluster are enormously diversified by unique mechanisms. The localization of CNR1 protein at the synapse and the diversity of CNRs led us to the hypothesis that gene regulation of the CNR family dictates the formation and reorganization of synaptic connections in the nervous system.

Alternative RNA splicing in the nervous system

Tissue-specific alternative splicing profoundly effects animal physiology, development and disease, and this is nowhere more evident than in the nervous system. Alternative splicing is a versatile form of genetic control whereby a common pre-mRNA is processed into multiple mRNA isoforms differing in their precise combination of exon sequences. In the nervous system, thousands of alternatively spliced mRNAs are translated into their protein counterparts where specific isoforms play roles in learning and memory, neuronal cell recognition, neurotransmission, ion channel function, and receptor specificity. The essential nature of this process is underscored by the finding that its misregulation is a common characteristic of human disease. This review highlights the current views of the biological phenomenon of alternative splicing, and describes evidence for its intricate underlying biochemical mechanisms. The roles of RNA binding proteins and their tissue-specific properties are discussed. Why does alternative splicing occur in cosmic proportions in the nervous system? How does it affect integrated cellular functions? How are region-specific, cell-specific and developmental differences in splicing directed? How are the control mechanisms that operate in the nervous system distinct from those of other tissues? Although there are many unanswered questions, substantial progress has been made in showing that alternative splicing is of major importance in generating proteomic diversity, and in modulating protein activities in a temporal and spatial manner. The relevance of alternative splicing to diseases of the nervous system is also discussed.

BMP growth factors and Phox2 transcription factors can induce synaptotagmin I and neurexin I during sympathetic neuron development

Synaptotagmin I and neurexin I mRNAs, coding for proteins involved in neurotransmitter secretion, become detectable in primary sympathetic ganglia shortly after initial induction of the noradrenergic transmitter phenotype. To test whether the induction of these more general neuronal genes is mediated by signals known to initiate noradrenergic differentiation in a neuronal subpopulation, we examined their expression in noradrenergic neurons induced by ectopic overexpression of growth and transcription factors. Overexpression of BMP4 or Phox2a in vivo results in synaptotagmin I and neurexin I expression in ectopically located noradrenergic cells. In vitro, BMP4 initiates synaptotagmin I and neurexin I expression in addition to tyrosine hydroxylase induction. Thus, the induction of synaptotagmin I and neurexin I, which are expressed in a large number of different neuron populations, can be accomplished by growth and transcription factors available only to a subset of neurons. These findings suggest that the initial expression of proteins involved in neurotransmitter secretion is regulated by different signals in different neuron populations.

Retrograde signaling at central synapses

Transcellular retrograde signaling from the postsynaptic target cell to the presynaptic neuron plays critical roles in the formation, maturation, and plasticity of synaptic connections. We here review recent progress in our understanding of the retrograde signaling at developing central synapses. Three forms of potential retrograde signals-membrane-permeant factors, membrane-bound factors, and secreted factors-have been implicated at both developing and mature synapses. Although many of these signals may be active constitutively, retrograde factors produced in association with activity-dependent synaptic plasticity, e.g., long-term potentiation and long-term depression, are of particular interest, because they may induce modification of neuronal excitability and synaptic transmission, functions directly related to the processing and storage of information in the nervous system.

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.

Extensive repertoire of membrane-bound and soluble dendritic cell-specific ICAM-3-grabbing nonintegrin 1 (DC-SIGN1) and DC-SIGN2 isoforms. Inter-individual variation in expression of DC-SIGN transcripts

Expression in dendritic cells (DCs) of DC-SIGN, a type II membrane protein with a C-type lectin ectodomain, is thought to play an important role in establishing the initial contact between DCs and resting T cells. DC-SIGN is also a unique type of human immunodeficiency virus-1 (HIV-1) attachment factor and promotes efficient infection in trans of cells that express CD4 and chemokine receptors. We have identified another gene, designated here as DC-SIGN2, that exhibits high sequence homology with DC-SIGN. Here we demonstrate that alternative splicing of DC-SIGN1 (original version) and DC-SIGN2 pre-mRNA generates a large repertoire of DC-SIGN-like transcripts that are predicted to encode membrane-associated and soluble isoforms. The range of DC-SIGN1 mRNA expression was significantly broader than previously reported and included THP-1 monocytic cells, placenta, and peripheral blood mononuclear cells (PBMCs), and there was cell maturation/activation-induced differences in mRNA expression levels. Immunostaining of term placenta with a DC-SIGN1-specific antiserum showed that DC-SIGN1 is expressed on endothelial cells and CC chemokine receptor 5 (CCR5)-positive macrophage-like cells in the villi. DC-SIGN2 mRNA expression was high in the placenta and not detectable in PBMCs. In DCs, the expression of DC-SIGN2 transcripts was significantly lower than that of DC-SIGN1. Notably, there was significant inter-individual heterogeneity in the repertoire of DC-SIGN1 and DC-SIGN2 transcripts expressed. The genes for DC-SIGN1, DC-SIGN2, and CD23, another Type II lectin, colocalize to an approximately 85 kilobase pair region on chromosome 19p13.3, forming a cluster of related genes that undergo highly complex alternative splicing events. The molecular diversity of DC-SIGN-1 and -2 is reminiscent of that observed for certain other adhesive cell surface proteins involved in cell-cell connectivity. The generation of this large collection of polymorphic cell surface and soluble variants that exhibit inter-individual variation in expression levels has important implications for the pathogenesis of HIV-1 infection, as well as for the molecular code required to establish complex interactions between antigen-presenting cells and T cells, i.e. the immunological synapse.

A stoichiometric complex of neurexins and dystroglycan in brain

In nonneuronal cells, the cell surface protein dystroglycan links the intracellular cytoskeleton (via dystrophin or utrophin) to the extracellular matrix (via laminin, agrin, or perlecan). Impairment of this linkage is instrumental in the pathogenesis of muscular dystrophies. In brain, dystroglycan and dystrophin are expressed on neurons and astrocytes, and some muscular dystrophies cause cognitive dysfunction; however, no extracellular binding partner for neuronal dystroglycan is known. Regular components of the extracellular matrix, such as laminin, agrin, and perlecan, are not abundant in brain except in the perivascular space that is contacted by astrocytes but not by neurons, suggesting that other ligands for neuronal dystroglycan must exist. We have now identified alpha- and beta-neurexins, polymorphic neuron-specific cell surface proteins, as neuronal dystroglycan receptors. The extracellular sequences of alpha- and beta-neurexins are largely composed of laminin-neurexin-sex hormone-binding globulin (LNS)/laminin G domains, which are also found in laminin, agrin, and perlecan, that are dystroglycan ligands. Dystroglycan binds specifically to a subset of the LNS domains of neurexins in a tight interaction that requires glycosylation of dystroglycan and is regulated by alternative splicing of neurexins. Neurexins are receptors for the excitatory neurotoxin alpha-latrotoxin; this toxin competes with dystroglycan for binding, suggesting overlapping binding sites on neurexins for dystroglycan and alpha-latrotoxin. Our data indicate that dystroglycan is a physiological ligand for neurexins and that neurexins' tightly regulated interaction could mediate cell adhesion between brain cells.

Alternative splicing: increasing diversity in the proteomic world

How can the genome of Drosophila melanogaster contain fewer genes than the undoubtedly simpler organism Caenorhabditis elegans? The answer must lie within their proteomes. It is becoming clear that alternative splicing has an extremely important role in expanding protein diversity and might therefore partially underlie the apparent discrepancy between gene number and organismal complexity. Alternative splicing can generate more transcripts from a single gene than the number of genes in an entire genome. However, for the vast majority of alternative splicing events, the functional significance is unknown. Developing a full catalog of alternatively spliced transcripts and determining each of their functions will be a major challenge of the upcoming proteomic era.

alpha-latrotoxin forms calcium-permeable membrane pores via interactions with latrophilin or neurexin

In order to explore the mechanisms by which alpha-latrotoxin activates neurotransmitter release, we have characterized its effects by patch-clamp methods on cells heterologously expressing its receptors, latrophilin-1 or neurexin-Ialpha. Application of alpha-latrotoxin (1 nM) to cells expressing rat latrophilin or neurexin, but not mock-transfected cells, induced a cationic conductance. In cells expressing latrophilin, current development was slow in the absence of divalent cations, but was accelerated by Ca2+ or Mg2+. In cells expressing neurexin, alpha-latrotoxin did not elicit currents in the absence of Ca2+. The toxin-induced conductance was rectifying, persistent, permeable to monovalent and divalent cations, but blocked by La3+. Single-channel recording revealed a permanently open state, with the same unitary conductance irrespective of whether cells expressed latrophilin or neurexin. Therefore, while pore formation displayed differences consistent with the reported properties of alpha-latrotoxin binding to latrophilin and neurexin, the pores induced by alpha-latrotoxin had identical properties. These results suggest that after anchoring to either of its nerve terminal receptors, alpha-latrotoxin inserts into the membrane and constitutes a single type of transmembrane ion pore.

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.

Tetramerisation of alpha-latrotoxin by divalent cations is responsible for toxin-induced non-vesicular release and contributes to the Ca(2+)-dependent vesicular exocytosis from synaptosomes

A novel procedure of alpha-latrotoxin (alpha LTX) purification has been developed. Pure alpha LTX has been demonstrated to exist as a very stable homodimer. Such dimers further assemble into tetramers, and Ca(2+), Mg(2+) or higher toxin concentrations facilitate this process. However, when the venom is treated with EDTA, purified alpha LTX loses the ability to tetramerise spontaneously; the addition of Mg(2+) or Ca(2+) restores this ability. This suggests that alphaLTX has some intrinsically bound divalent cation(s) that normally support its tetramerisation. Single-particle cryoelectron microscopy and statistical image analysis have shown that: 1) the toxin has a non-compact, branching structure; 2) the alpha LTX dimers are asymmetric; and 3) the tetramers are symmetric and have a 25 A-diameter channel in the centre. Both alpha LTX oligomers bind to the same receptors in synaptosomes and rat brain sections. To study the effects of the dimers and tetramers on norepinephrine release from rat cerebrocortical synaptosomes, we used the EDTA-treated and untreated toxin preparations. The number of tetramers present in a preparation correlates with alpha LTX pore formation, suggesting that the tetramers are the pore-forming species of alpha LTX. The toxin actions mediated by the pore include: 1) Ca(2+) entry from the extracellular milieu; and 2) passive efflux of neurotransmitters via the pore that occurs independently of Ca(2+). The Ca(2+)-dependent alpha LTX-stimulated secretion conforms to all criteria of vesicular exocytosis but also depends upon intact intracellular Ca(2+) stores and functional phospholipase C (PLC). The Ca(2+)-dependent effect of the toxin is stronger when dimeric alpha LTX is used, indicating that higher receptor occupancy leads to its stronger activation, which contributes to stimulation of neuroexocytosis. In contrast, the Ca(2+)-independent release measured biochemically represents leakage of neurotransmitters through the toxin pore. These results are discussed in relation to the previously published observations.

Targeting of synaptotagmin to neurite terminals in neuronally differentiated PC12 cells

We have investigated structural elements that determine the accumulation of synaptotagmin, a major synaptic vesicle protein, in neurite terminals of neuronally differentiated neuroendocrine pheochromocytoma PC12 cells. We performed extensive deletion and point mutagenesis of rat synaptotagmin II, expressed mutant proteins in PC12 cells differentiated by nerve growth factor (NGF) and monitored their intracellular distribution by immunofluorescence. We found a structural element located at the carboxy-terminal domain of the synaptotagmin molecule, which is necessary for its accumulation at the terminal. Using alanine-scanning mutagenesis, we have identified two amino acids in this element, tryptophan W405 and leucine L408, that are critical for correct targeting of synaptotagmin II to neurite terminals. Changing either one of them to alanine prevents the accumulation of the protein at the terminals. These amino acids are evolutionarily conserved throughout the entire synaptotagmin family and also among synaptotagmin-related proteins, suggesting that different synaptotagmins may have similar mechanisms of targeting to neuronal cell terminals.

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.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

Nova-1 regulates neuron-specific alternative splicing and is essential for neuronal viability

We have combined genetic and biochemical approaches to analyze the function of the RNA-binding protein Nova-1, the paraneoplastic opsoclonus-myoclonus ataxia (POMA) antigen. Nova-1 null mice die postnatally from a motor deficit associated with apoptotic death of spinal and brainstem neurons. Nova-1 null mice show specific splicing defects in two inhibitory receptor pre-mRNAs, glycine alpha2 exon 3A (GlyRalpha2 E3A) and GABA(A) exon gamma2L. Nova protein in brain extracts specifically bound to a previously identified GlyRalpha2 intronic (UCAUY)3 Nova target sequence, and Nova-1 acted directly on this element to increase E3A splicing in cotransfection assays. We conclude that Nova-1 binds RNA in a sequence-specific manner to regulate neuronal pre-mRNA alternative splicing; the defect in splicing in Nova-1 null mice provides a model for understanding the motor dysfunction in POMA.

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.

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.

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.

AKAP350, a multiply spliced protein kinase A-anchoring protein associated with centrosomes

Protein kinase A-anchoring proteins (AKAPs) localize the second messenger response to particular subcellular domains by sequestration of the type II protein kinase A. Previously, AKAP120 was identified from a rabbit gastric parietal cell cDNA library; however, a monoclonal antibody raised against AKAP120 labeled a 350-kDa band in Western blots of parietal cell cytosol. Recloning has now revealed that AKAP120 is a segment of a larger protein, AKAP350. We have now obtained a complete sequence of human gastric AKAP350 as well as partial cDNA sequences from human lung and rabbit parietal cells. The genomic region containing AKAP350 is found on chromosome 7q21 and is multiply spliced, producing at least three distinct AKAP350 isoforms as well as yotiao, a protein associated with the N-methyl-D-aspartate receptor. Rabbit parietal cell AKAP350 is missing a sequence corresponding to a single exon in the middle of the molecule located just after the yotiao homology region. Two carboxyl-terminal splice variants were also identified. Both of the major splice variants showed tissue- and cell-specific expression patterns. Immunofluorescence microscopy demonstrated that AKAP350 was associated with centrosomes in many cell types. In polarized Madin-Darby canine kidney cells, AKAP350 localized asymmetrically to one pole of the centrosome, and nocodazole did not alter its localization. During the cell cycle, AKAP350 was associated with the centrosomes as well as with the cleavage furrow during anaphase and telophase. Several epithelial cell types also demonstrated noncentrosomal pools of AKAP350, especially parietal cells, which contained multiple cytosolic immunoreactive foci throughout the cells. The localization of AKAP350 suggests that it may regulate centrosomal and noncentrosomal cytoskeletal systems in many different cell types.

Regulation of the neuron-specific exon of clathrin light chain B

Clathrin light chain B (LCB) is a major component of clathrin coated vesicles, which are structures involved in intracellular transport. A neuron-specific isoform of LCB is generated by incorporation of a single exon (EN) using an alternative splicing mechanism that reflects the special demands of neurons, such as axonal transport and synaptic neurotransmission. Here, we demonstrate that this neuron-specific exon is developmentally regulated and is excluded in non-neuronal cells because its 5' and 3' splice sites deviate from the mammalian consensus sequences. A gel retardation assay indicated the presence of a developmentally regulated factor in brain that binds to the neuronal exon. In addition, EN usage is repressed by increasing the concentration of htra2-beta1, a splice factor whose isoform expression is influenced by neuronal activity. We propose that a brain-specific factor is involved in EN recognition during development and adulthood. In addition, ubiquitously expressed splicing factors such as htra2-beta1 are involved in regulating EN expression in the adult brain.

Functional expression of alpha-latrotoxin in baculovirus system

To facilitate the study of the mechanism of alpha-latrotoxin action, it is necessary to create a biologically active recombinant toxin. Mature alpha-latrotoxin is naturally produced by post-translational cleavage, probably at two furin sites located at the N- and C-termini of the precursor. A recombinant baculovirus has now been constructed, which encodes the melittin signal peptide fused to the 130-kDa mature toxin between the furin sites. Insect cells, infected with this baculovirus, secreted recombinant alpha-latrotoxin. This was partially purified and proved indistinguishable from the natural toxin with respect to its molecular mass, immunostaining, toxicity to mice, binding to alpha-latrotoxin receptors (latrophilin or neurexin Ialpha) and electrophysiological recording in the mouse diaphragm. The successful expression of recombinant alpha-latrotoxin permits mutational analysis of the toxin.

A common protocadherin tail: multiple protocadherins share the same sequence in their cytoplasmic domains and are expressed in different regions of brain

To study the diversity of protocadherins, a rat brain cDNA library was screened using a cDNA for the cytoplasmic domain of human protocadherin Pcdh2 as a probe. The resultant clones contained three different types. One type corresponds to rat Pcdh2; the other two types are distinct from Pcdh2 but contain the same sequence in their cytoplasmic domains and part of the 3' flanking sequence. To clarify the structure of the proteins defined by the new clones, a putative entire coding sequence corresponding to one of the clones was determined. The overall structure is essentially the same as Pcdh2, indicating that the proteins defined by this clone, and probably by other clones, belong to the protocadherin family. Two PCR experiments and an RNase protection assay showed the existence of the corresponding mRNAs in rat brain preparations. Human and mouse cDNA clones with the same sequence properties were also isolated. Taken together, these results indicate that the clones are not cloning artifacts and that corresponding mRNAs are actually expressed in brains of various species. The results of in situ hybridization showed that the mRNAs corresponding to these clones were expressed in different regions in brain. Since protocadherins encoded by these mRNAs are likely to have different specificity in their interaction and share a common activity at their cytoplasmic domains, these protocadherins may provide a molecular basis, in part, to support the complex cell cell interaction in brain.