Developmental regulation of synaptotagmin I, II, III, and IV mRNAs in the rat CNS

Conserved expression of the zebrafish syt4 gene in GABAergic neurons in the cerebellum of adult fishes revealed by mammalian SYT4 immunoreactive-like signals

Synaptotagmin 4 (syt4) belongs to the synaptotagmin protein family, which has 17 and 28 family members in human and zebrafish, respectively. In zebrafish and rodents, syt4 is known to express abundantly in the entire central nervous system in the early developmental stages. In adult rodents, the gene expression shifts to be predominant in the cerebellum, mostly in Purkinje cells, a type of GABAergic neurons. However, there is no report of the expression pattern of syt4 in the adult zebrafish brain. Therefore, we hypothesize that the expression of syt4 is conserved in adult zebrafish and is specific to the GABAergic neurons, likely Purkinje cells, in the cerebellum. To examine the hypothesis, we first show that only one copy of syt4 gene remains in the zebrafish genome, and it is orthologous to the gene in other vertebrates. We further observe mammalian SYT4 antibody immunoreactive-like (mSYT4-ir) signals in several structures in the hindbrain including the medial divisions of the valvula cerebelli and the corpus cerebelli. In addition, our observations indicate the presence of mSYT4-ir signals in GABAergic neurons, most notably in the Purkinje cell layer of the molecular layer in the aforementioned structures. Conversely, mSYT4-ir signals are not observed in glutamatergic or cholinergic neurons. Therefore, we deduce that the syt4 gene in zebrafish exhibits a homologous expression pattern to those of previously studied vertebrate species, which is revealed by the positive immunoreactive-like signals of mammalian SYT4 antibodies.

Establishment of transgenic fluorescent mice for labeling synapses and screening synaptogenic adhesion molecules

Synapse is the fundamental structure for neurons to transmit information between cells. The proper synapse formation is crucial for developing neural circuits and cognitive functions of the brain. The aberrant synapse formation has been proved to cause many neurological disorders, including autism spectrum disorders and intellectual disability. Synaptic cell adhesion molecules (CAMs) are thought to play a major role in achieving mechanistic cell-cell recognition and initiating synapse formation via trans-synaptic interactions. Due to the diversity of synapses in different brain areas, circuits and neurons, although many synaptic CAMs, such as Neurexins (NRXNs), Neuroligins (NLGNs), Synaptic cell adhesion molecules (SynCAMs), Leucine-rich-repeat transmembrane neuronal proteins (LRRTMs), and SLIT and NTRK-like protein (SLITRKs) have been identified as synaptogenic molecules, how these molecules determine specific synapse formation and whether other molecules driving synapse formation remain undiscovered are unclear. Here, to provide a tool for synapse labeling and synaptic CAMs screening by artificial synapse formation (ASF) assay, we generated synaptotagmin-1-tdTomato (Syt1-tdTomato) transgenic mice by inserting the tdTomato-fused synaptotagmin-1 coding sequence into the genome of C57BL/6J mice. In the brain of Syt1-tdTomato transgenic mice, the tdTomato-fused synaptotagmin-1 (SYT1-tdTomato) signals were widely observed in different areas and overlapped with synapsin-1, a widely-used synaptic marker. In the olfactory bulb, the SYT1-tdTomato signals are highly enriched in the glomerulus. In the cultured hippocampal neurons, the SYT1-tdTomato signals showed colocalization with several synaptic markers. Compared to the wild-type (WT) mouse neurons, cultured hippocampal neurons from Syt1-tdTomato transgenic mice presented normal synaptic neurotransmission. In ASF assays, neurons from Syt1-tdTomato transgenic mice could form synaptic connections with HEK293T cells expressing NLGN2, LRRTM2, and SLITRK2 without immunostaining. Therefore, our work suggested that the Syt1-tdTomato transgenic mice with the ability to label synapses by tdTomato, and it will be a convenient tool for screening synaptogenic molecules.

Implication of synaptotagmins 4 and 7 in activity-dependent somatodendritic dopamine release in the ventral midbrain

Dopamine (DA) neurons can release DA not just from axon terminals, but also from their somatodendritic (STD) compartment through a mechanism that is still incompletely understood. Using voltammetry in mouse mesencephalic brain slices, we find that STD DA release has low capacity and shows a calcium sensitivity that is comparable to that of axonal release. We find that the molecular mechanism of STD DA release differs from axonal release with regard to the implication of synaptotagmin (Syt) calcium sensors. While individual constitutive knockout of Syt4 or Syt7 is not sufficient to reduce STD DA release, the removal of both isoforms reduces this release by approximately 50%, leaving axonal release unimpaired. Our work unveils clear differences in the mechanisms of STD and axonal DA release.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

Biallelic loss of function variants in SYT2 cause a treatable congenital onset presynaptic myasthenic syndrome

Synaptotagmins are integral synaptic vesicle membrane proteins that function as calcium sensors and regulate neurotransmitter release at the presynaptic nerve terminal. Synaptotagmin-2 (SYT2), is the major isoform expressed at the neuromuscular junction. Recently, dominant missense variants in SYT2 have been reported as a rare cause of distal motor neuropathy and myasthenic syndrome, manifesting with stable or slowly progressive distal weakness of variable severity along with presynaptic NMJ impairment. These variants are thought to have a dominant-negative effect on synaptic vesicle exocytosis, although the precise pathomechanism remains to be elucidated. Here we report seven patients of five families, with biallelic loss of function variants in SYT2, clinically manifesting with a remarkably consistent phenotype of severe congenital onset hypotonia and weakness, with variable degrees of respiratory involvement. Electrodiagnostic findings were consistent with a presynaptic congenital myasthenic syndrome (CMS) in some. Treatment with an acetylcholinesterase inhibitor pursued in three patients showed clinical improvement with increased strength and function. This series further establishes SYT2 as a CMS-disease gene and expands its clinical and genetic spectrum to include recessive loss-of-function variants, manifesting as a severe congenital onset presynaptic CMS with potential treatment implications.

Identification of differentially expressed microRNAs and their target genes in the hippocampal tissues of Fmr1 knockout mice

Fragile X syndrome (FXS) is one of the most common forms of inherited mental retardation; it is usually associated with the transcriptional silencing of the Fmr1 gene and loss of its encoded protein, the fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein and participates in regulating the development of dendritic spines and synaptic plasticity. To uncover the possible role of microRNAs (miRNAs) in FXS and their relationship with FMRP, we used microarray analysis to investigate the miRNA expression profiles in the hippocampal tissues of Fmr1 knockout (Fmr1-KO) mice and wild type (WT) mice. A total of 75 differentially expressed miRNAs were identified, of which 58 were significantly upregulated and no miRNAs were significantly downregulated in Fmr1-KO mice. Quantitative real-time PCR (qRT-PCR) analysis was applied to validate the expression of 7 upregulated miRNAs; results indicated that the levels of only miR-449a and miR-720 were significantly upregulated. We further used bioinformatics software and databases to predict the target genes of these two miRNAs. The genes were related to dendritic spine development and synaptic plasticity; the qRT-PCR and western blotting results showed that cyclin-dependent kinase 5 (CDK5) and synaptotagmin 1 (SYT1) were differentially expressed in the Fmr1-KO mice and WT mice. In conclusion, this study evidenced diverse changes in the expression of miRNAs, and validated the miRNAs and their targeted genes in Fmr1-KO mice. Although further studies are required to better understand the function of miRNAs in FXS, the present research highlights a potential role of miRNAs in the pathogenesis of FXS.

Munc13-3 Is Required for the Developmental Localization of Ca2+ Channels to Active Zones and the Nanopositioning of Cav2.1 Near Release Sensors

Spatial relationships between Ca_v channels and release sensors at active zones (AZs) are a major determinant of synaptic fidelity. They are regulated developmentally, but the underlying molecular mechanisms are largely unclear. Here, we show that Munc13-3 regulates the density of Ca_v2.1 and Ca_v2.2 channels, alters the localization of Ca_v2.1, and is required for the development of tight, nanodomain coupling at parallel-fiber AZs. We combined EGTA application and Ca²⁺-channel pharmacology in electrophysiological and two-photon Ca²⁺ imaging experiments with quantitative freeze-fracture immunoelectron microscopy and mathematical modeling. We found that a normally occurring developmental shift from release being dominated by Ca²⁺ influx through Ca_v2.1 and Ca_v2.2 channels with domain overlap and loose coupling (microdomains) to a nanodomain Ca_v2.1 to sensor coupling is impaired in Munc13-3-deficient synapses. Thus, at AZs lacking Munc13-3, release remained triggered by Ca_v2.1 and Ca_v2.2 microdomains, suggesting a critical role of Munc13-3 in the formation of release sites with calcium channel nanodomains.

Synaptotagmin Ca2+ Sensors and Their Spatial Coupling to Presynaptic Cav Channels in Central Cortical Synapses

Ca²⁺ concentrations drop rapidly over a distance of a few tens of nanometers from an open voltage-gated Ca²⁺ channel (Ca_v), thereby, generating a spatially steep and temporally short-lived Ca²⁺ gradient that triggers exocytosis of a neurotransmitter filled synaptic vesicle. These non-steady state conditions make the Ca²⁺-binding kinetics of the Ca²⁺ sensors for release and their spatial coupling to the Ca_vs important parameters of synaptic efficacy. In the mammalian central nervous system, the main release sensors linking action potential mediated Ca²⁺ influx to synchronous release are Synaptotagmin (Syt) 1 and 2. We review here quantitative work focusing on the Ca²⁺ kinetics of Syt2-mediated release. At present similar quantitative detail is lacking for Syt1-mediated release. In addition to triggering release, Ca²⁺ remaining bound to Syt after the first of two successive high-frequency activations was found to be capable of facilitating release during the second activation. More recently, the Ca²⁺ sensor Syt7 was identified as additional facilitation sensor. We further review how several recent functional studies provided quantitative insights into the spatial topographical relationships between Syts and Ca_vs and identified mechanisms regulating the sensor-to-channel coupling distances at presynaptic active zones. Most synapses analyzed in matured cortical structures were found to operate at tight, nanodomain coupling. For fast signaling synapses a developmental switch from loose, microdomain to tight, nanodomain coupling was found. The protein Septin5 has been known for some time as a developmentally down-regulated “inhibitor” of tight coupling, while Munc13-3 was found only recently to function as a developmentally up-regulated mediator of tight coupling. On the other hand, a highly plastic synapse was found to operate at loose coupling in the matured hippocampus. Together these findings suggest that the coupling topography and its regulation is a specificity of the type of synapse. However, to definitely draw such conclusion our knowledge of functional active zone topographies of different types of synapses in different areas of the mammalian brain is too incomplete.

Astrocytes, neurons, synapses: a tripartite view on cortical circuit development

In the mammalian cerebral cortex neurons are arranged in specific layers and form connections both within the cortex and with other brain regions, thus forming a complex mesh of specialized synaptic connections comprising distinct circuits. The correct establishment of these connections during development is crucial for the proper function of the brain. Astrocytes, a major type of glial cell, are important regulators of synapse formation and function during development. While neurogenesis precedes astrogenesis in the cortex, neuronal synapses only begin to form after astrocytes have been generated, concurrent with neuronal branching and process elaboration. Here we provide a combined overview of the developmental processes of synapse and circuit formation in the rodent cortex, emphasizing the timeline of both neuronal and astrocytic development and maturation. We further discuss the role of astrocytes at the synapse, focusing on astrocyte-synapse contact and the role of synapse-related proteins in promoting formation of distinct cortical circuits.

Defined Culture Conditions Accelerate Small-molecule-assisted Neural Induction for the Production of Neural Progenitors from Human-induced Pluripotent Stem Cells

The use of defined conditions for derivation, maintenance, and differentiation of human-induced pluripotent stem cells (hiPSCs) provides a superior experimental platform to discover culture responses to differentiation cues and elucidate the basic requirements for cell differentiation and fate restriction. Adoption of defined systems for reprogramming, undifferentiated growth, and differentiation of hiPSCs was found to significantly influence early stage differentiation signaling requirements and temporal kinetics for the production of primitive neuroectoderm. The bone morphogenic protein receptor agonist LDN-193189 was found to be necessary and sufficient for neural induction in a monolayer system with landmark antigens paired box 6 and sex-determining region Y-box 1 appearing within 72 h. Preliminary evidence suggests this neuroepithelium was further differentiated to generate ventral spinal neural progenitors that produced electrophysiologically active neurons in vitro, maintaining viability posttransplantation in an immunocompromised host. Our findings support current developments in the field, demonstrating that adoption of defined reagents for the culture and manipulation of pluripotent stem cells is advantages in terms of simplification and acceleration of differentiation protocols, which will be critical for future clinical translation.

Developmental reduction of asynchronous GABA release from neocortical fast-spiking neurons

Delayed asynchronous release (AR) evoked by bursts of presynaptic action potentials (APs) occurs in certain types of hippocampal and neocortical inhibitory interneurons. Previous studies showed that AR provides long-lasting inhibition and desynchronizes the activity in postsynaptic cells. However, whether AR undergoes developmental change remains unknown. In this study, we performed whole-cell recording from fast-spiking (FS) interneurons and pyramidal cells (PCs) in prefrontal cortical slices obtained from juvenile and adult rats. In response to AP trains in FS neurons, AR occurred at their output synapses during both age periods, including FS autapses and FS-PC synapses; however, the AR strength was significantly weaker in adults than that in juveniles. Further experiments suggested that the reduction of AR in adult animals could be attributable to the rapid clearance of residual Ca(2+) from presynaptic terminals. Together, our results revealed that the AR strength was stronger at juvenile but weaker in adult, possibly resulting from changes in presynaptic Ca(2+) dynamics. AR changes may meet the needs of the neural network to generate different types of oscillations for cortical processing at distinct behavioral states.

Postnatal differentiation of cortical interneuron signalling

Most GABAergic interneurons in the cortex are born at embryonic stages in the ganglionic eminences and migrate tangentially to their final destination. They continue, however, to differentiate and functionally integrate in the circuitry until later postnatal stages of the rodent brain. Recent investigations show that interneurons undergo marked changes in their morphological, intrinsic and synaptic properties as they mature. Action potential shape and its propagation, the period of transmitter release and the time course of the postsynaptic GABA(A) receptor-mediated conductance become faster during the first three to four postnatal weeks, resulting in a developmental switch of interneurons from slow to fast signalling units. At the same time, the nature of GABAergic signalling is classically considered to shift from depolarizing to hyperpolarizing. However, recent studies oppose this view as interneuron synapses can be shunting, excitatory or hyperpolarizing in the mature cortex, demonstrating the coexistence of diverse developmental rules for the emerging effects of GABAergic synapses. Thus, mature interneuron signalling comes in many forms and is apparently optimized to the network in which the neurons are embedded.

Neuronal differentiation by TAp73 is mediated by microRNA-34a regulation of synaptic protein targets

The p53-family member TAp73 is a transcription factor that plays a key role in many biological processes. Here, we show that p73 drives the expression of microRNA (miR)-34a, but not miR-34b and -c, by acting on specific binding sites on the miR-34a promoter. Expression of miR-34a is modulated in parallel with that of TAp73 during in vitro differentiation of neuroblastoma cells and cortical neurons. Retinoid-driven neuroblastoma differentiation is inhibited by knockdown of either p73 or miR-34a. Transcript expression of miR-34a is significantly reduced in vivo both in the cortex and hippocampus of p73(-/-) mice; miR-34a and TAp73 expression also increase during postnatal development of the brain and cerebellum when synaptogenesis occurs. Accordingly, overexpression or silencing of miR-34a inversely modulates expression of synaptic targets, including synaptotagmin-1 and syntaxin-1A. Notably, the axis TAp73/miR-34a/synaptotagmin-1 is conserved in brains from Alzheimer's patients. These data reinforce a role for TAp73 in neuronal development.

Calmodulin suppresses synaptotagmin-2 transcription in cortical neurons

Calmodulin (CaM) is a ubiquitous Ca(2+) sensor protein that plays a pivotal role in regulating innumerable neuronal functions, including synaptic transmission. In cortical neurons, most neurotransmitter release is triggered by Ca(2+) binding to synaptotagmin-1; however, a second delayed phase of release, referred to as asynchronous release, is triggered by Ca(2+) binding to an unidentified secondary Ca(2+) sensor. To test whether CaM could be the enigmatic Ca(2+) sensor for asynchronous release, we now use in cultured neurons short hairpin RNAs that suppress expression of ∼70% of all neuronal CaM isoforms. Surprisingly, we found that in synaptotagmin-1 knock-out neurons, the CaM knockdown caused a paradoxical rescue of synchronous release, instead of a block of asynchronous release. Gene and protein expression studies revealed that both in wild-type and in synaptotagmin-1 knock-out neurons, the CaM knockdown altered expression of >200 genes, including that encoding synaptotagmin-2. Synaptotagmin-2 expression was increased several-fold by the CaM knockdown, which accounted for the paradoxical rescue of synchronous release in synaptotagmin-1 knock-out neurons by the CaM knockdown. Interestingly, the CaM knockdown primarily activated genes that are preferentially expressed in caudal brain regions, whereas it repressed genes in rostral brain regions. Consistent with this correlation, quantifications of protein levels in adult mice uncovered an inverse relationship of CaM and synaptotagmin-2 levels in mouse forebrain, brain stem, and spinal cord. Finally, we employed molecular replacement experiments using a knockdown rescue approach to show that Ca(2+) binding to the C-lobe but not the N-lobe of CaM is required for suppression of synaptotagmin-2 expression in cortical neurons. Our data describe a previously unknown, Ca(2+)/CaM-dependent regulatory pathway that controls the expression of synaptic proteins in the rostral-caudal neuraxis.

Functional polarity in neurons: what can we learn from studying an exception?

Dendrites and axons typically handle very different aspects of neuronal signaling. However, many of the functional distinctions between these two types of processes are absent in neurons with release-competent dendrites. This raises fundamental questions about the molecular mechanisms that promote and permit functional specialization, and suggests that the 'exceptional' case of presynaptic dendrites may provide important clues on how neuronal polarity is established. To help stimulate thinking on this new front, we summarize some key aspects of the physiology of dendritic neurotransmitter release, together with recent work on the molecular basis of neuronal polarity.

A genomic analysis of transcytosis in the pea aphid, Acyrthosiphon pisum, a mechanism involved in virus transmission

Aphids are the primary vectors of plant viruses. Transmission can occur via attachment to the cuticle lining of the insect (non-circulative transmission) or after internalization in the insect cells with or without replication (circulative transmission). In this paper, we have focused on the circulative and non-propagative mode during which virions enter the cell following receptor-mediated endocytosis, are transported across the cell in vesicles and released by exocytosis without replicating. The correct uptake, transport and delivery of the vesicles cargo relies on the participation of proteins from different families which have been identified in the Acyrthosiphon pisum genome. Assemblage of this annotated dataset provides a useful basis to improve our understanding of the molecules and mechanisms involved in virus transmission by A. pisum and other aphid species.

Collagen XIX is expressed by interneurons and contributes to the formation of hippocampal synapses

Extracellular matrix (ECM) molecules contribute to the formation and maintenance of synapses in the mammalian nervous system. We previously discovered a family of nonfibrillar collagens that organize synaptic differentiation at the neuromuscular junction (NMJ). Although many NMJ-organizing cues contribute to central nervous system (CNS) synaptogenesis, whether similar roles for collagens exist at central synapses remained unclear. In the present study we discovered that col19a1, the gene encoding nonfibrillar collagen XIX, is expressed by subsets of hippocampal neurons. Colocalization with the interneuron-specific enzyme glutamate decarboxylase 67 (Gad67), but not other cell-type-specific markers, suggests that hippocampal expression of col19a1 is restricted to interneurons. However, not all hippocampal interneurons express col19a1 mRNA; subsets of neuropeptide Y (NPY)-, somatostatin (Som)-, and calbindin (Calb)-immunoreactive interneurons express col19a1, but those containing parvalbumin (Parv) or calretinin (Calr) do not. To assess whether collagen XIX is required for the normal formation of hippocampal synapses, we examined synaptic morphology and composition in targeted mouse mutants lacking collagen XIX. We show here that subsets of synaptotagmin 2 (Syt2)-containing hippocampal nerve terminals appear malformed in the absence of collagen XIX. The presence of Syt2 in inhibitory hippocampal synapses, the altered distribution of Gad67 in collagen XIX-deficient subiculum, and abnormal levels of gephyrin in collagen XIX-deficient hippocampal extracts all suggest inhibitory synapses are affected by the loss of collagen XIX. Together, these data not only reveal that collagen XIX is expressed by central neurons, but show for the first time that a nonfibrillar collagen is necessary for the formation of hippocampal synapses.

Developmental regulation of metabotropic glutamate receptor 1 splice variants in olfactory bulb mitral cells

Alternative splicing of the metabotropic glutamate receptor 1 (mGluR1) receptor gene generates two major receptor isoforms, mGluR1a and mGluR1b, differing in intracellular function and distribution. However, little is known on the expression profiles of these variants during development. We examined the mRNA expression profile of mGluR1a/b in microdissected layers and acutely isolated mitral cells in the developing mouse olfactory bulb. This analysis showed that the two mGluR1 variants are differentially regulated within each bulb layer. During the first postnatal week, the mGluR1a isoform replaces GluR1b in the microdissected mitral cell layer (MCL) and in isolated identified mitral cells, coinciding with a developmental epoch of mitral cell dendritic reorganization. Although mGluR1a mRNA is expressed at high levels in both the adult external plexiform layer (EPL) and MCL, Western blotting analysis reveals a marked reduction of the mGluR1a protein in the MCL, where mitral cell bodies are located, and strong labeling in the EPL, which contains mitral cell dendrites. This suggests that there is increased dendritic trafficking efficiency of the receptor in adult. The temporal and spatial shift in mGluR1b/a expression suggests distinct roles of the mGluR1 isoforms, with mGluR1b potentially involved in the early mitral cell maturation and mGluR1a in dendritic and synapse function.

Developmental alterations in the functional properties of excitatory neocortical synapses

In the neocortex, most excitatory, glutamatergic synapses are established during the first 4-5 weeks after birth. During this period profound changes in the properties of synaptic transmission occur. Excitatory postsynaptic potentials (EPSPs) at immature synaptic connections are profoundly and progressively reduced in response to moderate to high frequency (5-100 Hz) stimulation. With maturation, this frequency-dependent depression becomes progressively weaker and may eventually transform into a weak to moderate EPSP facilitation. In parallel to changes in the short-term plasticity, a reduction in the synaptic reliability occurs at most glutamatergic neocortical synapses: immature synapses show a high probability of neurotransmitter release as indicated by their low failure rate and small EPSP amplitude variation. This high reliability is reduced in mature synapses, which show considerably higher failure rates and more variable EPSP amplitudes. During early neocortical development synaptic vesicle pools are not yet fully differentiated and their replenishment may be slow, thus resulting in EPSP amplitude depression. The decrease in the probability of neurotransmitter release may be the result of an altered Ca(2+) control in the presynaptic terminal with a reduced Ca(2+) influx and/or a higher Ca(2+) buffering capacity. This may lead to a lower synaptic reliability and a weaker short-term synaptic depression with maturation.

Polyglutamine-expanded ataxin-3 causes cerebellar dysfunction of SCA3 transgenic mice by inducing transcriptional dysregulation

In the present study, we prepared a SCA3 animal model by generating transgenic mice expressing polyglutamine-expanded ataxin-3-Q79. Ataxin-3-Q79 was expressed in brain areas implicated in SCA3 neurodegeneration, including cerebellum, pontine nucleus and substantia nigra. Ataxin-3-Q79 transgenic mice displayed motor dysfunction with an onset age of 5-6 months, and neurological symptoms deteriorated in the following months. A prominent neuronal loss was not found in the cerebellum of 10 to 11-month-old ataxin-3-Q79 mice displaying pronounced ataxic symptoms, suggesting that instead of neuronal demise, ataxin-3-Q79 causes neuronal dysfunction of the cerebellum and resulting ataxia. To test the involvement of transcriptional dysregulation in ataxin-3-Q79-induced cerebellar malfunction, microarray analysis and real-time RT-PCR assays were performed to identify altered cerebellar mRNA expressions of ataxin-3-Q79 mice. Compared to non-transgenic mice or mice expressing wild-type ataxin-3-Q22, 10 to 11-month-old ataxin-3-Q79 mice exhibited downregulated mRNA expressions of proteins involved in glutamatergic neurotransmission, intracellular calcium signaling/mobilization or MAP kinase pathways, GABA(A/B) receptor subunits, heat shock proteins and transcription factor regulating neuronal survival and differentiation. Upregulated expressions of Bax, cyclin D1 and CDK5-p39, which may mediate neuronal death, were also observed in ataxin-3-Q79 transgenic mice. The involvement of transcriptional abnormality in initiating the pathological process of SCA3 was indicated by the finding that 4 to 5-month-old ataxin-3-Q79 mice, which did not display neurological phenotype, exhibited downregulated mRNA levels of genes involved in glutamatergic signaling and signal transduction. Our study suggests that polyglutamine-expanded ataxin-3 causes cerebellar dysfunction and ataxia by disrupting the normal pattern of gene transcriptions.

Antisense in vivo knockdown of synaptotagmin I by HVJ-liposome mediated gene transfer attenuates ischemic brain damage in neonatal rats

Synaptic release of the excitatory amino acid glutamate is considered as an important mechanism in the pathogenesis of ischemic brain damage in neonates. Synaptotagmin I is one of exocytosis-related proteins at nerve terminals and considered to accelerate the exocytosis of synaptic vesicles by promoting fusion between the vesicles and plasma membrane. To test the possibility that antisense in vivo knockdown of synaptotagmin I modulates the exocytotic release of glutamate, thus suppressing the excitotoxic intracellular processes leading to neuronal death following ischemia in the neonatal brain, we injected antisense oligodeoxynucleotides (ODNs) targeting synaptotagmin I (0.3 (AS), 0.15 (0.5 AS), or 0.03 microg (0.1 AS), or vehicle) into the lateral ventricles of 7-day-old rats by using a hemagglutinating virus of Japan (HVJ)-liposome mediated gene transfer technique. At 10 days of age, these rats were subjected to an electrical coagulation of the right external and internal carotid arteries, then the insertion of a solid nylon thread into the right common carotid artery toward the ascending aorta up to 10-12 mm from the upper edge of the sternocleidomastoid muscle. Cerebral ischemia was induced by clamping the left external and internal carotid arteries with a clip, and ended by removing the clip 2h later. Twenty-four hours after the end of ischemia, the extent of ischemic brain damage was neuropathologically and quantitatively evaluated in the neocortex and striatum. While the relative volume of damage in the cerebral cortex and striatum of the vehicle group was extended to 40% and 13.7%, respectively, that in the AS group was significantly reduced to 4.8% and 0.6%. In the 0.5 AS group, the relative volume of ischemic damage in the cerebral cortex and striatum was reduced to 20.5% and 15.4%, respectively, and the difference between the 0.5 AS group and vehicle group was statistically significant in the neocortex, but not in the striatum. These results indicated that antisense in vivo knockdown of synaptotagmin I successfully attenuated ischemic brain damage in neonatal rats and that the effect was dose-dependent. It was also suggested that this treatment was more effective in the neocortex than in the striatum in neonatal rats.

Influence of binocular competition on the expression profiles of CRMP2, CRMP4, Dyn I, and Syt I in developing cat visual cortex

The visual cortex is vulnerable to changes in visual input, especially during the critical period when numerous molecules drive the refinement of the circuitry. From a list of potential actors identified in a recent proteomics study, we selected 2 collapsin response mediator proteins (CRMP2/CRMP4) and 2 synaptic proteins, Dynamin I (Dyn I) and Synaptotagmin I (Syt I), for in-depth analysis of their developmental expression profile in cat visual cortex. CRMP2 and CRMP4 levels were high early in life and clearly declined toward adulthood. In contrast, Dyn I expression levels progressively augmented during maturation. Syt I showed low levels at eye opening and in adults, high levels around the peak of the critical period, and maximal levels at juvenile age. We further determined a role for each molecule in ocular dominance plasticity. CRMP2 and Syt I levels decreased in area 17 upon monocular deprivation, whereas CRMP4 and Dyn I levels remained unaffected. In contrast, binocular removal of pattern vision had no influence on CRMP2 and Syt I expression in kitten area 17. This study illustrates that not the loss of quality of vision through visual deprivation, but disruption of normal binocular visual experience is crucial to induce the observed molecular changes.

Synaptotagmin I and II are present in distinct subsets of central synapses

Synaptotagmin 1 and 2 (syt 1, syt 2) are synaptic vesicle-associated membrane proteins that act as calcium sensors for fast neurotransmitter release from presynaptic nerve terminals. Here we show that widely used monoclonal antibodies, mab 48 and znp-1, stain nerve terminals in multiple species and, in mouse, recognize syt 1 and syt 2, respectively. With these antibodies, we examined the synaptic localization of these synaptotagmin isoforms in the mouse central nervous system. Syt 1 and syt 2 are localized predominantly to different subsets of synapses in retina, hippocampus, cerebellum, and median nucleus of the trapezoid body (MNTB). In the MNTB, syt 1 and syt 2 are present in different presynaptic terminals on the same postsynaptic principal neuron. In retina, horizontal and OFF-bipolar cell terminals contain syt 2, whereas most other terminals contain syt 1. Syt 1 localization in the immature retina resembles that seen in adult; however, syt 2 localization appears strikingly different at perinatal ages and continues to change dramatically prior to eye opening. For example, starburst amacrine cells, which lack syt 2 in adult retina, transiently express syt 2 during the first 2 postnatal weeks. In addition to differences in spatial and temporal distribution, species-specific differences in synaptotagmin localization were observed in retina and cerebellum. The cell-, temporal-, and species-specific expression of synaptotagmin isoforms suggests that each may have distinct functions in neurotransmitter release.

Identification of novel striatal genes by expression profiling in adult mouse brain

Large-scale transcriptome analysis in the brain is a powerful approach to identify novel genes of potential interest toward understanding cerebral organization and function. We utilized the microarray technology to measure expression levels of about 24,000 genes and expressed sequence tags in mouse hippocampus, frontal cortex and striatum. Using expression profile obtained from whole brain as a reference, we categorized the genes into groups of genes either enriched in, or restricted to, one of the three areas of interest. We found enriched genes for each target area. Further, we identified 14 genes in the category of genes restricted to the striatum, among which were the orphan G protein-coupled receptor GPR88 and retinoic acid receptor-beta. These two genes were already reported to be selectively expressed in the striatum, thus validating our experimental approach. We selected 6 striatal-restricted genes, as well as 10 striatal-enriched candidates, that were previously undescribed. We analyzed their expression by in situ hybridization analysis in the brain, and quantitative RT-PCR in both brain and peripheral organs. Two of these unknown genes displayed a notable expression pattern. The striatal-restricted gene H3076B11 shows uniform expression throughout and uniquely in the striatum, representing a genuine striatal marker. The striatal-enriched gene 4833421E05Rik is preferentially expressed in the rostral striatum, and is also abundant in kidney, liver and lung. These two genes may contribute to some of the many striatal-controlled behaviors, including initiation of movement, habit formation, or reward and motivation.

Singing, but not seizure, induces synaptotagmin IV in zebra finch song circuit nuclei

Synaptotagmins are a family of proteins that function in membrane fusion events, including synaptic vesicle exocytosis. Within this family, synaptotagmin IV (Syt IV) is unique in being a depolarization-induced immediate early gene (IEG). Experimental perturbation of Syt IV modulates neurotransmitter release in mice, flies, and PC12 cells, and modulates learning in mice. Despite these features, induction of Syt IV expression by a natural behavior has not been previously reported. We used the zebra finch, a songbird species, to investigate Syt IV because song is a naturally learned behavior whose neuroanatomical basis is largely identified. We observed that, similar to rodents, Syt IV is inducible in songbirds. This induction was selective and depended on the nature of neuronal depolarization. Generalized seizures caused by the GABA(A) receptor antagonist, metrazole, induced the IEG, ZENK, in zebra finch brain. However, these same seizures failed to induce Syt IV in song control areas. In contrast, when nontreated birds sang, three song control areas showed striking Syt IV induction. Further, this induction appeared sensitive to the social context in which song was sung. Together, these data suggest that neural activity during singing can drive Syt IV expression within song circuitry whereas generalized seizure activity fails to do so even though song control areas are depolarized. Our findings indicate that, within this neural circuit for a procedurally learned sensorimotor behavior, Syt IV is selective and requires precisely patterned neural activity and/or neuromodulation associated with singing.

The application of proteomics and genomics to the study of age-related neurodegeneration and neuroprotection

The present study aimed to acquire more information on aging-related alterations, using proteomic and genomic analyses of hippocampus from young (8 months) and old (27 months) rats. In the old rats, the proteomic analysis identified changes in proteins related to the iron-mediated oxidative stress (OS) pathway, including reduction in antioxidant enzymes (e.g., peroxiredoxin, cytochrome c oxidase) and induction of ferritin. Furthermore, the neurofilament light peptide, associated with neurodegenerative processes, was enhanced and binding/ chaperone proteins were altered in old vs. young rats. At the genes levels, significant molecular changes related to neurodegeneration were identified in aged rat hippocampus. Thus, the effects of the potent neuroprotective compounds, the anti-Parkinson drug, rasagiline and the anti-Alzheimer drug, ladostigil (1 mg/kg, for 30 days) on gene expression in the hippocampus were further investigated. Both drugs reversed the effect of aging on the expression of various mitochondrial and key regulator genes involved in neurodegeneration, cell survival, synaptogenesis, oxidation, and metabolism. These results support the hypothesis that OS and mitochondrial dysfunction may play a pivotal role in aging and age-associated neurodegenerative diseases, and can serve as potential clinical targets for future therapy.

Age- and experience-dependent expression of Dynamin I and Synaptotagmin I in cat visual system

Dynamin I (Dyn I) and Synaptotagmin I (Syt I) are essential for endocytosis-exocytosis processes, thus for neurotransmission. Despite their related function at presynaptic terminals, Dyn I and Syt I displayed opposite expression patterns during visual cortex maturation in the cat. Dyn I was more abundantly expressed in adults, while Syt I exhibited higher levels in kittens of postnatal day 30 (P30). In area 17 this developmental difference was most obvious in layers II/III. Layer VI displayed a strong hybridization signal for both molecules, independent of age. In addition, Syt I levels were higher in posterior compared to anterior area 17 in adult subjects. Moreover, in higher-order visual areas Syt I was unevenly distributed over the cortical layers, thereby setting clear areal boundaries in mature cortex. In contrast, Dyn I was rather homogeneously distributed over extrastriate areas at both ages. Both molecules thus demonstrated a widespread but different distribution and an opposite temporal expression pattern during visual system development. Notably, monocular deprivation during the critical period of ocular dominance plasticity significantly decreased Syt I expression levels in area 17 ipsilateral to the deprived eye, while no effect was observed on Dyn I expression. We therefore conclude that visual experience induces changes in Syt I expression that may reflect changes in constitutive exocytosis involved in postnatal structural refinements of the visual cortex. On the other hand, the spatial and temporal expression patterns of Dyn I correlate with the establishment and maintenance of the mature neuronal structure rather than neurite remodeling.

Entering neurons: botulinum toxins and synaptic vesicle recycling

Botulinum toxins are metalloproteases that act inside nerve terminals and block neurotransmitter release through their cleavage of components of the exocytosis machinery. These toxins are used to treat human diseases that are characterized by hyperfunction of cholinergic terminals. Recently, evidence has accumulated that gangliosides and synaptic vesicle proteins cooperate to mediate toxin binding to the presynaptic terminal. The differential distribution of synaptic vesicle protein receptors, gangliosides and toxin substrates in distinct neuronal populations opens up the possibility of using different serotypes of botulinum toxins for the treatment of central nervous system diseases caused by altered activity of selected neuronal populations.

Cardiovascular expression of the mouse WNK1 gene during development and adulthood revealed by a BAC reporter assay

Large deletions in WNK1 are associated with inherited arterial hypertension. WNK1 encodes two types of protein: a kidney-specific isoform (KS-WNK1) lacking kinase activity and a ubiquitously expressed full-length isoform (L-WNK1) with serine threonine kinase activity. Disease is thought to result from hypermorphic mutations increasing the production of one or both isoforms. However, the pattern of L-WNK1 expression remains poorly characterized. We generated transgenic mice bearing a murine WNK1 BAC containing the nlacZ reporter gene for monitoring L-WNK1 expression during development and adulthood. We observed previously unsuspected early expression in the vessels and primitive heart during embryogenesis, consistent with the early death of WNK1(-/-) mice. The generalized cardiovascular expression observed in adulthood may also suggest a possible kidney-independent role in blood pressure regulation. The second unsuspected site of L-WNK1 expression was the granular layer and Purkinje cells of the cerebellum, suggesting a role in local ion balance or cell trafficking. In the kidney, discordance between endogenous L-WNK1 and transgene expression suggests that either cis-regulatory elements important for physiological renal expression lie outside the BAC sequence or that illegitimate interactions occur between promoters. Despite this limitation, this transgenic model is a potentially valuable tool for the analysis of spatial and temporal aspects of WNK1 expression and regulation.

Genetic analysis of synaptotagmin 2 in spontaneous and Ca2+-triggered neurotransmitter release

Synaptotagmin 2 resembles synaptotagmin 1, the Ca2+ sensor for fast neurotransmitter release in forebrain synapses, but little is known about synaptotagmin 2 function. Here, we describe a severely ataxic mouse strain that harbors a single, destabilizing amino-acid substitution (I377N) in synaptotagmin 2. In Calyx of Held synapses, this mutation causes a delay and a decrease in Ca2+-induced but not in hypertonic sucrose-induced release, suggesting that synaptotagmin 2 mediates Ca2+ triggering of evoked release in brainstem synapses. Unexpectedly, we additionally observed in synaptotagmin 2 mutant synapses a dramatic increase in spontaneous release. Synaptotagmin 1-deficient excitatory and inhibitory cortical synapses also displayed a large increase in spontaneous release, demonstrating that this effect was shared among synaptotagmins 1 and 2. Our data suggest that synaptotagmin 1 and 2 perform equivalent functions in the Ca2+ triggering of action potential-induced release and in the restriction of spontaneous release, consistent with a general role of synaptotagmins in controlling 'release slots' for synaptic vesicles at the active zone.

Differential expression of striatal synaptotagmin mRNA isoforms in hemiparkinsonian rats

Synaptotagmins (Syts) constitute a multi-gene family of 15 putative membrane trafficking proteins. The expression of some of the Syts in the brain might be dopaminergically controlled and thus affected by dopamine depletion in Parkinson's disease. We used hemiparkinsonian rats to investigate the effects of chronic striatal dopamine depletion and the acute effects of antiparkinsonic drug L-DOPA or D1 agonist (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF82958) on the levels of striatal Syt I, II, IV, VI, VII, X, XI mRNA isoforms. On the 6-hydroxydopamine (6-OHDA)-lesioned side we observed a nearly total loss of tyrosine hydroxylase (TH), synaptotagmin I, Syt IV, Syt VII and Syt XI mRNA levels in the substantia nigra compacta (SNc). In dopamine-depleted striatum we also found a significant down-regulation Syt II and up-regulation of Syt X mRNA levels that could not be reversed by the acute treatment either with L-DOPA or SKF82958. By contrast, these two drugs induced an increase of Syt IV and Syt VII mRNA levels. A time-course study revealed the highest levels of Syt IV and VII mRNAs to occur at two hours and 12 hours after the treatment with SKF82958, respectively. D1 antagonist (+/-)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390) but not D2 antagonist haloperidol prevented the L-DOPA-driven increase of Syt IV and VII mRNAs. These results imply that synaptic plasticity in response to chronic striatal dopamine depletion involves a complex pattern of changes in striatal Syt mRNA expression. The L-DOPA treatment does not reverse the changes in Syt II and Syt X gene expression, but recruits additional, D1 receptor-mediated changes in Syt IV and Syt VII gene expression. Whether these D1 receptor-mediated changes play a role in the alterations of synaptic transmission that results in the unwanted side effects of chronic L-DOPA treatment in Parkinson's disease remains to be determined.

Phosphorylation of cysteine string protein in the brain: developmental, regional and synaptic specificity

Protein phosphorylation modulates regulated exocytosis in most cells, including neurons. Cysteine string protein (CSP) has been implicated in this process because its phosphorylation on Ser10 alters its interactions with syntaxin and synaptotagmin, and because the effect of CSP overexpression on exocytosis kinetics in chromaffin cells requires phosphorylatable Ser10. To characterize CSP phosphorylation in the brain, we raised phosphospecific antibodies to Ser10. Western blotting revealed that the proportion of phosphorylated CSP (P-CSP) varies between distinct brain regions and also exhibits developmental regulation, with P-CSP highest early in development. Immunohistochemical analysis of the cerebellar cortex revealed a novel pool of P-CSP that did not colocalize with synaptic vesicle markers during early development. Strikingly, in the adult cerebellar granular layer P-CSP was highly enriched in a subset of glutamatergic synapses but undetectable in neighbouring GABA-ergic synapses. In view of the functional consequences of CSP phosphorylation, such differences could contribute to the synapse-specific regulation of neurotransmitter release.

Late postnatal maturation of excitatory synaptic transmission permits adult-like expression of hippocampal-dependent behaviors

Sensorimotor systems in altricial animals mature incrementally during early postnatal development, with complex cognitive abilities developing late. Of prominence are cognitive processes that depend on an intact hippocampus, such as contextual-configural learning, allocentric and idiocentric navigation, and certain forms of trace conditioning. The mechanisms that regulate the delayed maturation of the hippocampus are not well understood. However, there is support for the idea that these behaviors come "on line" with the final maturation of excitatory synaptic transmission. First, by providing a timeline for the first behavioral expression of various forms of learning and memory, this study illustrates the late maturation of hippocampal-dependent cognitive abilities. Then, functional development of the hippocampus is reviewed to establish the temporal relationship between maturation of excitatory synaptic transmission and the behavioral evidence of adult-like hippocampal processing. These data suggest that, in rats, mechanisms necessary for the expression of adult-like synaptic plasticity become available at around 2 postnatal weeks of age. However, presynaptic plasticity mechanisms, likely necessary for refinement of the hippocampal network, predominate and impede information processing until the third postnatal week.

Effect of acute and chronic treatment with methamphetamine on mRNA expression of synaptotagmin IV and 25 KDa-synaptic-associated protein in the rat brain

The effects of acute and chronic administration of methamphetamine (METH) on mRNA levels of synaptotagmin IV (SytIV) and an isoform of synaptic-associated protein of 25 KDa (SNAP25a) have been investigated in rat brain using in situ hybridization. Pretreatment with 0.5 mg/kg dopamine D1 receptor antagonist (SCH23390), but not 0.5 mg/kg N-methyl-D-aspartate (NMDA) receptor antagonist (MK-801), significantly attenuated the increased SytIV mRNA levels induced by acute METH administration in the striatum and the nucleus accumbens. Pretreatment with 0.5 mg/kg SCH23390, but not 0.5 mg/kg MK-801, significantly attenuated the increased SNAP25a mRNA levels induced by acute METH administration in the striatum and the dentate gyrus of the hippocampus. In the chronic treatment experiment, the SytIV mRNA levels of the group that received chronic treatment with METH followed by a METH challenge showed an increase similar to that seen after acute METH administration. In addition, those in the striatum, nucleus accumbens, and dentate gyrus were significantly higher than those of the group that received chronic treatment with saline followed by a METH challenge. The SNAP25a mRNA levels of the group that received chronic treatment with METH followed by a saline challenge were significantly higher than those of the group that received chronic treatment with saline followed by a saline challenge in the striatum and nucleus accumbens. The results of the present study suggest that SytIV may play an important role in the synaptic plasticity underlying METH-induced neuroadaptive changes including behavioral sensitization.

[Mode of action of botulinum neurotoxin: pathological, cellular and molecular aspect]

Several bacteria of the Clostridium genus (C. botulinum) produce 150 kDa di-chainal protein toxins referred as botulinum neurotoxins or BoNTs. They associate with non-toxic companion proteins and form a complex termed botulinum toxin or BoTx. The latter is used in clinic for therapeutic purpose. BoNTs affect cholinergic nerve terminals in periphery where they block acetylcholine release, thereby causing dysautonomia and motorparalysis (i.e. botulism). The cellular action of BoNTs can be depicted according to a three steps model: binding, internalisation and intraneuronal action. The toxins heavy chain mediates binding to specific receptors followed by endocytotic internalisation of BoNT/receptor complex. BoNT receptors may comprise gangliosides and synaptic vesicle-associated proteins as synaptotagmins. Vesicle recycling induces BoNT internalisation. Upon acidification of vesicles, the light chain of the neurotoxin is translocated into the cytosol. Here, this zinc-endopeptidase cleaves one or two among three synaptic proteins (VAMP-synaptobrevin, SNAP25, and syntaxin). As the three protein targets of BoNT play major role in fusion of synaptic vesicles at the release sites, their cleavage is followed by blockage of neurotransmitter exocytosis. The duration of the paralytic effect of the BoNTs is determined by 1) the turnover of their protein target; 2) the time-life of the toxin light chain in the cytosol, and 3) the sprouting of new nerve-endings that are retracted when the poisoned nerve terminal had recovered its full functionality.

Nerve growth factor-dependent sorting of synaptotagmin IV protein to mature dense-core vesicles that undergo calcium-dependent exocytosis in PC12 cells

Synaptotagmin IV (Syt IV) is a fourth member of the Syt family and has been shown to regulate some forms of memory and learning by analysis of Syt IV null mutant mice (Ferguson, G. D., Anagnostaras, S. G., Silva, A. J., and Herschman, H. R. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 5598-5603). However, the involvement of Syt IV protein in vesicular trafficking and even its localization in secretory vesicles are still matters of controversy. Here we present several lines of evidence showing that the Syt IV protein in PC12 cells is normally localized in the Golgi or immature vesicles at the cell periphery and is sorted to fusion-competent mature dense-core vesicles in response to short nerve growth factor (NGF) stimulation. (i) In undifferentiated PC12 cells, Syt IV protein is mainly localized in the Golgi and small amounts are also present at the cell periphery, but according to the results of an immunocytochemical analysis, they do not colocalize with conventional secretory vesicle markers (Syt I, Syt IX, Rab3A, Rab27A, vesicle-associated membrane protein 2, and synaptophysin) at all. By contrast, limited colocalization of Syt IV protein with dense-core vesicle markers is found in the distal parts of the neurites of NGF-differentiated PC12 cells. (ii) Immunoelectron microscopy with highly specific anti-Syt IV antibody revealed that the Syt IV protein in undifferentiated PC12 cells is mainly present on the Golgi membranes and immature secretory vesicles, whereas after NGF stimulation Syt IV protein is also present on the mature dense-core vesicles. (iii) An N-terminal antibody-uptake experiment indicated that Syt IV-containing vesicles in the neurites of NGF-differentiated PC12 cells undergo Ca(2+)-dependent exocytosis, whereas no uptake of the anti-Syt IV-N antibody was observed in undifferentiated PC12 cells. Our results suggest that Syt IV is a stimulus (e.g. NGF)-dependent regulator for exocytosis of dense-core vesicles.

Development of two transmitter release components during the critical period for imprinting in the chick IMHV

Transmitter release at an excitatory synapse has two components, fast synchronous and slow asynchronous transmitter release. Using the whole cell recording technique, we investigated the developmental properties of neurotransmitter release, which is composed of the two components in the intermediate and medial part of the hyperstriatum ventral (IMHV) of chicks during the critical period for imprinting. Analysis of the paired-pulse responses revealed that the depression of the excitatory postsynaptic currents (EPSCs), driven mainly by fast synchronous release, was frequently observed in P0-1 chicks but not in those at P5-8. The spontaneous excitatory postsynaptic currents (sEPSCs) after the paired-pulse stimulation, which were thought to be driven by asynchronous transmitter releases, were observed more frequently in P0-1 chicks than P5-8 chicks. Furthermore, examination of Ca2+ dependency in the evoked EPSCs showed that the amplitudes in P5-8 chicks were more sensitive to reduction of the extracellular Ca2+ concentration than younger chicks. Considering that the Ca2+ dependency of EPSCs is defined by both Ca2+ sensitivity and the proportion of each type of release machineries at the release site, these results indicate that the ratio of fast synchronous to slow asynchronous transmitter release machinery changed during the critical period. These changes may play critical roles in the capacity of the avian brain to consolidate novel experience in the immediate period after hatching.

Non-polarized distribution of synaptotagmin IV in neurons: evidence that synaptotagmin IV is not a synaptic vesicle protein

Synaptotagmin IV (Syt IV) expression is regulated by neuronal development and by depolarization in the brain and in neuronal cell cultures. In cultures, immunocytochemical analysis has shown that Syt IV is localized at the Golgi and at the tips of growing neurites, but little was known about associations between Syt IV and vesicles or organelles [J. Neurochem. 74 (2000) 518]. In this study we performed an electron microscopic (EM) analysis of developing mouse neocortex to determine the exact localization of Syt IV in native mouse tissues. In neurons of layer II/III, Syt IV was found to be localized in the dendrites and axons, and at the Golgi in the cell body. Some Syt IV signals were clearly associated with vesicles and/or organelles, but EM and cell fractionation studies showed no Syt IV signals at synaptic vesicles. Detection of fluorescence protein-tagged Syt IV (Syt IV-EGFP) in hippocampal neurons also showed the presence of Syt IV-EGFP vesicles or organelles in the axons and dendrites. These results suggest that Syt IV regulates non-polarized membrane trafficking in neurons, which may be involved in synaptic plasticity or neuronal development.

Genes preferentially induced by depolarization after concussive brain injury: effects of age and injury severity

Fluid percussion (FP) brain injury leads to immediate indiscriminate depolarization and massive potassium efflux from neurons. Using Northern blotting, we examined the post-FP expression of primary response/immediate early genes previously described as induced by depolarization in brain. RNA from ipsilateral and contralateral hippocampus was harvested from immature and adult rats 1 h following mild, moderate, or severe lateral fluid percussion injury and compared against age-matched sham animals. C-fos gene expression was used as a positive control and showed marked induction in both pups (6-25-fold with increasing severity) and adults (9.7-17.1-fold). Kinase-induced-by-depolarization-1 (KID-1) and salt-inducible kinase (SIK) gene expression was increased in adult (KID-1 1.5-1.6-fold; SIK 1.3-3.9-fold) but not developing rats. NGFI-b RNA was elevated after injury in both ages (pups 1.8-6.1-fold; adults 3.5-5-fold), in a pattern similar to that seen for c-fos. Secretogranin I (sec I) demonstrated no significant changes. Synaptotagmin IV (syt IV) was induced only following severe injury in the immature rats (1.4-fold). Our results reveal specific severity- and age-dependent patterns of hippocampal immediate early gene expression for these depolarization-induced genes following traumatic brain injury. Differential expression of these genes may be an important determinant of the distinct molecular responses of the brain to varying severities of trauma experienced at different ages.

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.

Synaptotagmin IV: biochemistry, genetics, behavior, and possible links to human psychiatric disease

We isolated the rat synaptotagmin IV (Syt IV) cDNA in a screen for sequences that are specifically induced in neuronal cells. The Syts are a large family of genes thought to mediate synaptic function. Syt IV is brain-specific, induced in hippocampus by depolarization, and predominantly vesicular. To assess the function role of Syt IV in vivo, we generated Syt IV(-/-) mutant mice. Syt IV (-/-) mice are viable and appear normal, indicating this gene is not essential for survival or gross development. However, Syt IV (-/-) mutants, when compared to wild-type littermates, have deficits in fine motor coordination and hippocampus-dependent memory, suggesting Syt IV has a role in normal brain function. The human Syt IV ortholog maps to a region of chromosome 18 previously associated with the human psychiatric disorders, schizophrenia and bipolar disease. These results suggest that Syt IV is required in certain types of neurons for optimal functionality, that perturbations in the levels of Syt IV can result in memory loss in mice, and that Syt IV alterations may lead to psychiatric disease in humans.

The AMPAR subunit GluR2: still front and center-stage

Abnormal influx of Ca(2+) through AMPA-type glutamate receptors (AMPARs) is thought to contribute to the neuronal death associated with a number of brain disorders. AMPARs exist as both Ca(2+)-impermeable and Ca(2+)-permeable channels. AMPARs are encoded by four genes designated GluR1 (GluR-A) through GluR4 (GluR-D). The presence of the GluR2 subunit renders heteromeric AMPA receptor assemblies Ca(2+)-impermeable. Molecular diversity of AMPARs under physiological and pathological conditions is generated by differential spatio-temporal patterns of GluR expression, by alternative RNA splicing and editing and by targeting and trafficking of receptor subunits at dendritic spines. The GluR2 gene is under transcriptional control by the RE1 element specific transcription factor, a gene silencing factor which renders it neuron-specific. GluR2 transcripts are edited by ADAR2 (double-stranded RNA-specific editase 1). AMPAR targeting and trafficking to spines are regulated by synaptic activity and are critical to synaptic plasticity. Recent studies involving animal models of transient forebrain ischemia and epilepsy show that GluR2 mRNA and GluR2 subunit expression are downregulated in vulnerable neurons prior to cell death. Ca(2+) imaging and electrical recording from individual pyramidal neurons in hippocampal slices reveal changes in AMPAR functional properties after ischemia. In slices from post-ischemia animals, CA1 neurons with robust action potentials exhibit greatly enhanced AMPA-elicited rises in intracellular Ca(2+). Excitatory postsynaptic currents in post-ischemic CA1 exhibit an enhanced Ca(2+)-dependent component that appears to be mediated by Ca(2+)-permeable AMPARs. These studies provide evidence for Ca(2+) influx through AMPARs in neurons destined to die. To examine whether acute GluR2 downregulation, even in the absence of a neurological insult, can induce neuronal death, we performed knockdown experiments in rats and gerbils with antisense oligonucleotides targeted to GluR2 mRNA. GluR2 antisense oligonucleotide induced neuronal cell death of pyramidal neurons and enhanced pathogenicity of brief ischemic episodes. These observations provide evidence for Ca(2+) influx through AMPARs in neurons destined to die and implicate Ca(2+)-permeable AMPARs in the pathogenesis of ischemia-induced neuronal death.

The human synaptotagmin IV gene defines an evolutionary break point between syntenic mouse and human chromosome regions but retains ligand inducibility and tissue specificity

Rat synaptotagmin IV (SYT IV) is a depolarization-inducible synaptic vesicle protein. SYT IV homozygous mutant mice are viable and have deficits in fine motor coordination and some forms of memory. In this study, we report the identification of a human SYT IV orthologue. The predicted amino acid sequence of the human SYT IV clone is nearly 90% identical to the rat and mouse SYT IV proteins. In addition, human SYT IV has a characteristic serine for aspartate substitution within the first C2 domain that is conserved among Drosophila, Caenorhabditis elegans, mouse, and rat SYT IV sequences. The human SYT IV gene maps to chromosome band 18q12.3, a region that defines a break point in the synteny with mouse chromosome 18 and has been implicated by associated markers in two human psychiatric disorders. In the human neuroblastoma cell line SK-N-SH, SYT IV is an immediate-early gene inducible by elevated intracellular calcium and by forskolin, an activator of adenylyl cyclase. Expression of human SYT IV mRNA is restricted to brain and is not detectable in non-neuronal tissues. Within brain, human SYT IV mRNA is most highly expressed in hippocampus, with lower levels present in amygdala and thalamus. These results suggest a role for SYT IV in human brain function and in human neurological disease.