A novel seizure-induced synaptotagmin gene identified by differential display

Phosphoproteome Analysis Identifies a Synaptotagmin-1-Associated Complex Involved in Ischemic Neuron Injury

Cerebral stroke is one of the leading causes of death in adults worldwide. However, the molecular mechanisms of stroke-induced neuron injury are not fully understood. Here, we obtained phosphoproteomic and proteomic profiles of the acute ischemic hippocampus by LC–MS/MS analysis. Quantitative phosphoproteomic analyses revealed that the dysregulated phosphoproteins were involved in synaptic components and neurotransmission. We further demonstrated that phosphorylation of Synaptotagmin-1 (Syt1) at the Thr112 site in cultured hippocampal neurons aggravated oxygen-glucose deprivation–induced neuronal injury. Immature neurons with low expression of Syt1 exhibit slight neuronal injury in a cerebral ischemia model. Administration of the Tat-Syt1^T112A peptide protects neurons against cerebral ischemia-induced injury in vitro and in vivo. Surprisingly, potassium voltage-gated channel subfamily KQT member 2 (Kcnq2) interacted with Syt1 and Annexin A6 (Anxa6) and alleviated Syt1-mediated neuronal injury upon oxygen-glucose deprivation treatment. These results reveal a mechanism underlying neuronal injury and may provide new targets for neuroprotection after acute cerebral ischemia onset.

•

Established the phosphoproteome profiles of acute cerebral ischemic hippocampus.

•

Phosphoproteomic profile reveals phosphorylation of Syt1 and Kcnq2, which are upregulated.

•

Phosphorylation of Syt1 aggravates neuron injury, which is relieved by Tat-Syt1^T112A.

•

Kcnq2 interacts with Syt1 and Anxa6 and alleviates Syt1-mediated neuronal injury.

The diversity of synaptotagmin isoforms

The synaptotagmin family of molecules is known for regulating calcium-dependent membrane fusion events. Mice and humans express 17 synaptotagmin isoforms, where most studies have focused on isoforms 1, 2, and 7, which are involved in synaptic vesicle exocytosis. Recent work has highlighted how brain function relies on additional isoforms, with roles in postsynaptic receptor endocytosis, vesicle trafficking, membrane repair, synaptic plasticity, and protection against neurodegeneration, for example, in addition to the traditional concept of synaptotagmin-mediated neurotransmitter release - in neurons as well as glia, and at different timepoints. In fact, it is not uncommon for the same isoform to feature several splice isoforms, form homo- and heterodimers, and function in different subcellular locations and cell types. This review aims to highlight the diversity of synaptotagmins, offers a concise summary of key findings on all isoforms, and discusses different ways of grouping these.

Genome-wide analysis and expression profiles of NTMC2 family genes in Oryza sativa

N-terminal-TM-C2 domain proteins (NTMC2), which share domain architecture and sequence similarity to synaptotagmins (Syts) in mammals and FAM62 (extended Syts) in metazoans, form a small gene family in plants. Previous studies showed that the Arabidopsis thaliana NTMC2 type 1.1 protein (NTMC2T1.1, named AtSyt1) possesses calcium- and membrane-binding activities that allow it to function in a plasma membrane repair pathway induced by stress. However, we lack understanding of the diverse biological roles of plant NTMC2 family genes. In this study, a total of 13 OsNTMC2 genes was identified through a comprehensive bioinformatics analysis of the rice (Oryza sativa L.) genome and classified into six OsNTMC2 groups (OsNTMC2T1 to OsNTMC2T6) based on phylogeny and motif constitution. OsNTMC2T1 to OsNTMC2T3 have two calcium-binding domains (C2A and C2B), but OsNTMC2T4 to OsNTMC2T6 have single C2 domain. The expression profiles of OsNTMC2 genes were analysed at different stages of vegetative and reproductive development. This analysis revealed that at least one OsNTMC2 gene was abundantly expressed at each stage of development. These results should facilitate research on this gene family and provide new insights elucidating their functions in higher plants.

Identification of Synaptotagmin 10 as Effector of NPAS4-Mediated Protection from Excitotoxic Neurodegeneration

Neuronal degeneration represents a pathogenetic hallmark after different brain insults, such as ischemia and status epilepticus (SE). Excessive release of glutamate triggered by pathophysiologic synaptic activity has been put forward as key mechanism in this context. In response to pathophysiologic synaptic activity, multiple signaling cascades are activated that ultimately initiate expression of specific sets of genes, which may decide between neuronal survival versus death. Recently, a core set of genes ["activity-regulated inhibitor of death" (AID) genes] including the transcription factor (TF) NPAS4 (neuronal PAS domain protein 4) has been found to provide activity-induced protection against neuronal death caused by excitotoxic stimulation. However, the downstream targets of AID action mediating neuroprotection remained so far unknown. Here, we have identified synaptotagmin 10 (Syt10), a vesicular Ca(2+) sensor, as the first neuroprotective effector protein downstream of the TF NPAS4. The expression of Syt10 is strongly upregulated by pathophysiologic synaptic activity after kainic acid (KA) exposure and its absence renders mouse hippocampal neurons highly susceptible to excitotoxic insults. We found NPAS4 as critical for the increase in Syt10 levels and in turn the ability of NPAS4 to confer neuroprotection against KA-induced excitotoxicity to be severely diminished in Syt10 knock-out neurons. In summary, our results point to an important role for signaling of the NPAS4-Syt10 pathway in the neuronal response to strong synaptic activity as a consequence of excitotoxic insults.

SIGNIFICANCE STATEMENT

Aberrant synaptic activity is observed in many neurological disorders and has been suggested as an important factor contributing to the pathophysiology. Intriguingly, pathophysiologic activity can also trigger signaling cascades mediating potentially compensatory neuroprotection against excitotoxic insult. Here, we identify a new neuroprotective signaling cascade involving the activity-induced transcriptional regulator NPAS4 and the vesicular Ca(2+)-sensor protein synaptotagmin 10 (Syt10). Syt10 is required for NPAS4 to protect hippocampal neurons against excitotoxic cell death. NPAS4 in turn controls the activity of the Syt10 gene, which is strongly induced by pathophysiologic activity. Our results uncover an entirely unexpected, novel function of Syt10 underlying the response of neurons to pathophysiologic activity and provide new therapeutic perspectives for neurological disorders.

Up-regulation of Synaptotagmin IV within amyloid plaque-associated dystrophic neurons in Tg2576 mouse model of Alzheimer's disease

AIM

To investigate the involvement of the vesicular membrane trafficking regulator Synaptotagmin IV (Syt IV) in Alzheimer's disease pathogenesis and to define the cell types containing increased levels of Syt IV in the β-amyloid plaque vicinity.

METHODS

Syt IV protein levels in wild type (WT) and Tg2576 mice cortex were determined by Western blot analysis and immunohistochemistry. Co-localization studies using double immunofluorescence staining for Syt IV and markers for astrocytes (glial fibrillary acidic protein), microglia (major histocompatibility complex class II), neurons (neuronal specific nuclear protein), and neurites (neurofilaments) were performed in WT and Tg2576 mouse cerebral cortex.

RESULTS

Western blot analysis showed higher Syt IV levels in Tg2576 mice cortex than in WT cortex. Syt IV was found only in neurons. In plaque vicinity, Syt IV was up-regulated in dystrophic neurons. The Syt IV signal was not up-regulated in the neurons of Tg2576 mice cortex without plaques (resembling the pre-symptomatic conditions).

CONCLUSIONS

Syt IV up-regulation within dystrophic neurons probably reflects disrupted vesicular transport or/and impaired protein degradation occurring in Alzheimer's disease and is probably a consequence but not the cause of neuronal degeneration. Hence, Syt IV up-regulation and/or its accumulation in dystrophic neurons may have adverse effects on the survival of the affected neuron.

Insights into food preference in hybrid F1 of Siniperca chuatsi (♀) × Siniperca scherzeri (♂) mandarin fish through transcriptome analysis

BACKGROUND

As economically relevant traits, feeding behavior and food preference domestication determine production cost and profitability. Although there are intensive research efforts on feeding behavior and food intake, little is known about food preference. Mandarin fish accept only live prey fish and refuse dead prey fish or artificial diets. Very little is currently known about the genes regulating this unique food preference.

RESULTS

Using transcriptome sequencing and digital gene expression profiling, we identified 1,986 and 4,526 differentially expressed genes in feeders and nonfeeders of dead prey fish, respectively. Up-regulation of Crbp, Rgr and Rdh8, and down-regulation of Gc expression, consistent with greater visual ability in feeders, could promote positive phototaxis. Altered expressions of period, casein kinase and Rev-erbα might reset circadian phase. Down-regulation of orexigenic and up-regulation of anorexigenic genes in feeders were associated with lower appetite. The mRNA levels of Creb, c-fos, C/EBP, zif268, Bdnf and Syt were dramatically decreased in feeders, which might result in significant deficiency in memory retention of its natural food preference (live prey fish). There were roughly 100 times more potential SNPs in feeders than in nonfeeders.

CONCLUSIONS

In summary, differential expression in the genes identified shed new light on why mandarin fish only feed on live prey fish, with pathways regulating retinal photosensitivity, circadian rhythm, appetite control, learning and memory involved. We also found dramatic difference in SNP abundance in feeders vs nonfeeders. These differences together might account for the different food preferences. Elucidating the genes regulating the unique food preference (live prey fish) in mandarin fish could lead to a better understanding of mechanisms controlling food preference in animals, including mammals.

Regulation of postsynaptic retrograde signaling by presynaptic exosome release

Retrograde signals from postsynaptic targets are critical during development and plasticity of synaptic connections. These signals serve to adjust the activity of presynaptic cells according to postsynaptic cell outputs and to maintain synaptic function within a dynamic range. Despite their importance, the mechanisms that trigger the release of retrograde signals and the role of presynaptic cells in this signaling event are unknown. Here we show that a retrograde signal mediated by Synaptotagmin 4 (Syt4) is transmitted to the postsynaptic cell through anterograde delivery of Syt4 via exosomes. Thus, by transferring an essential component of retrograde signaling through exosomes, presynaptic cells enable retrograde signaling.

Amphiphysin I but not dynamin I nor synaptojanin mRNA expression increased after repeated methamphetamine administration in the rat cerebrum and cerebellum

Dopamine increases/decreases synaptic vesicle recycling and in schizophrenia the proteins/mRNA is decreased. We isolated cDNA clone, similar to amphiphysin 1 (vesicle protein) mRNA from the neocortex of rats injected repeatedly with methamphetamine using polymerase chain reaction (PCR) differential display. This clone is highly homologous to the 3' region of the human amphiphysin gene. PCR extension study using a primer specific for the rat amphiphysin 1 gene and a primer located within the clone revealed that it is the 3' UTR region of the rat amphiphysin 1 gene. Furthermore, in situ hybridization revealed that amphiphysin 1 mRNA is expressed in the cerebrum, medial thalamus, hippocampus and cerebellum. In the cerebellum, amphiphysin mRNA expression was confined to upper granule cell layer. Repeated methamphetamine administration increased amphiphysin I mRNA expression in both anterior part of the cerebrum, and the cerebellum. However, the repeated administration did not alter mRNA expression of the other vesicle proteins, synaptotagmin I, synapsin I, synaptojanin and dynamin I, we conclude that the repeated administration selectively increased amphiphysin 1 mRNA expression. Thus, amphiphysin 1 does not work as synaptic recycling, but it is suggested, as a part of pathogenesis of brain tissue injury (under Ca²⁺ and Mg²⁺ devoid environment) in repeated methamphetamine-injected states, the gene regulate actin-asssembly, learning, cell stress signaling and cell polarity.

Axonal and dendritic synaptotagmin isoforms revealed by a pHluorin-syt functional screen

The synaptotagmins (syts) are a family of molecules that regulate membrane fusion. There are 17 mammalian syt isoforms, most of which are expressed in the brain. However, little is known regarding the subcellular location and function of the majority of these syts in neurons, largely due to a lack of isoform-specific antibodies. Here we generated pHluorin-syt constructs harboring a luminal domain pH sensor, which reports localization, pH of organelles to which syts are targeted, and the kinetics and sites of exocytosis and endocytosis. Of interest, only syt-1 and 2 are targeted to synaptic vesicles, whereas other isoforms selectively recycle in dendrites (syt-3 and 11), axons (syt-5, 7, 10, and 17), or both axons and dendrites (syt-4, 6, 9, and 12), where they undergo exocytosis and endocytosis with distinctive kinetics. Hence most syt isoforms localize to distinct secretory organelles in both axons and dendrites and may regulate neuropeptide/neurotrophin release to modulate neuronal function.

Activity-dependent IGF-1 exocytosis is controlled by the Ca(2+)-sensor synaptotagmin-10

Synaptotagmins Syt1, Syt2, Syt7, and Syt9 act as Ca(2+)-sensors for synaptic and neuroendocrine exocytosis, but the function of other synaptotagmins remains unknown. Here, we show that olfactory bulb neurons secrete IGF-1 by an activity-dependent pathway of exocytosis, and that Syt10 functions as the Ca(2+)-sensor that triggers IGF-1 exocytosis in these neurons. Deletion of Syt10 impaired activity-dependent IGF-1 secretion in olfactory bulb neurons, resulting in smaller neurons and an overall decrease in synapse numbers. Exogenous IGF-1 completely reversed the Syt10 knockout phenotype. Syt10 colocalized with IGF-1 in somatodendritic vesicles of olfactory bulb neurons, and Ca(2+)-binding to Syt10 caused these vesicles to undergo exocytosis, thereby secreting IGF-1. Thus, Syt10 controls a previously unrecognized pathway of Ca(2+)-dependent exocytosis that is spatially and temporally distinct from Ca(2+)-dependent synaptic vesicle exocytosis controlled by Syt1. Our findings thereby reveal that two different synaptotagmins can regulate functionally distinct Ca(2+)-dependent membrane fusion reactions in the same neuron.

Rat and Drosophila synaptotagmin 4 have opposite effects during SNARE-catalyzed membrane fusion

Synaptotagmins (Syt) are a large family of proteins that regulate membrane traffic in neurons and other cell types. One isoform that has received considerable attention is SYT4, with apparently contradictory reports concerning the function of this isoform in fruit flies and mice. SYT4 was reported to function as a negative regulator of neurotrophin secretion in mouse neurons and as a positive regulator of secretion of a yet to be identified growth factor from muscle cells in flies. Here, we have directly compared the biochemical and functional properties of rat and fly SYT4. We report that rat SYT4 inhibited SNARE-catalyzed membrane fusion in both the absence and presence of Ca²⁺. In marked contrast, fly SYT4 stimulated SNARE-mediated membrane fusion in response to Ca²⁺. Analysis of chimeric molecules, isolated C2 domains, and point mutants revealed that the C2B domain of the fly protein senses Ca²⁺ and is sufficient to stimulate fusion. Rat SYT4 was able to stimulate fusion in response to Ca²⁺ when the conserved Asp-to-Ser Ca²⁺ ligand substitution in its C2A domain was reversed. In summary, rat SYT4 serves as an inhibitory isoform, whereas fly SYT4 is a bona fide Ca²⁺ sensor capable of coupling Ca²⁺ to membrane fusion.

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.

Synaptotagmins in neurodegeneration

Synaptotagmins (Syts) are transmembrane proteins involved in the regulation of membrane trafficking. Here, we summarize literature data that provide growing evidence that several Syts are involved in the pathophysiological mechanisms of temporal lobe epilepsy and Parkinson's disease, as well as few reports related to brain ischemia and Alzheimer's disease (AD). We also report new data from our laboratories, showing changes of the expression of several Syts in Tg2576 mouse model of AD that may be related to neuroinflammation surrounding the beta-amyloid plaques. Furthermore, we demonstrate N-methyl-D-aspartate receptor-mediated upregulation of Syt 4 mRNA in a model of excitotoxic striatal lesion induced by unilateral striatal injection of quinolinic acid, associating the upregulation of Syt 4 with mechanisms of excitotoxicity. We propose that pharmacological manipulation of Syt expression in animal models of neurodegeneration should be further explored, as it may help to clarify the role of individual Syt isoforms in the regulation of membrane trafficking in neurodegeneration.

Analysis of the synaptotagmin family during reconstituted membrane fusion. Uncovering a class of inhibitory isoforms

Ca(2+)-triggered exocytosis in neurons and neuroendocrine cells is regulated by the Ca(2+)-binding protein synaptotagmin (syt) I. Sixteen additional isoforms of syt have been identified, but little is known concerning their biochemical or functional properties. Here, we assessed the abilities of fourteen syt isoforms to directly regulate SNARE (soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor)-catalyzed membrane fusion. One group of isoforms stimulated neuronal SNARE-mediated fusion in response to Ca(2+), while another set inhibited SNARE catalyzed fusion in both the absence and presence of Ca(2+). Biochemical analysis revealed a strong correlation between the ability of syt isoforms to bind 1,2-dioleoyl phosphatidylserine (PS) and t-SNAREs in a Ca(2+)-promoted manner with their abilities to enhance fusion, further establishing PS and SNAREs as critical effectors for syt action. The ability of syt I to efficiently stimulate fusion was specific for certain SNARE pairs, suggesting that syts might contribute to the specificity of intracellular membrane fusion reactions. Finally, a subset of inhibitory syts down-regulated the ability of syt I to activate fusion, demonstrating that syt isoforms can modulate the function of each other.

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.

Molecular responses to acidosis of central chemosensitive neurons in brain

Significant advances have been made in understanding how neurons sense and respond to acidosis at the cellular level. Decrease in pH of the cerebrospinal fluid followed by hypercapnia (increased arterial CO2) is monitored by the chemosensory neurons of the medulla oblongata. Then the intracellular signalling pathways are activated to regulate specific gene expression, which leads to a hyperventilatory response. However, little is known about molecular details of such cellular responses. Recent studies have identified several transcription factors such as c-Jun, Fos and small Maf proteins that may play critical roles in the brain adaptation to hypercapnia. Hypercapnic stimulation also activates c-Jun NH2-terminal kinase (JNK) cascade via influx of extracellular Ca2+ through voltage-gated Ca2+ channels. In addition, several transmembrane proteins including Rhombex-29 (rhombencephalic expression protein-29 kDa) and Past-A (proton-associated sugar transporter-A) have been implicated in regulation of H+ sensitivity and brain acidosis-mediated energy metabolism, respectively. This review discusses current knowledge on the signalling mechanisms and molecular basis of neuronal adaptation during acidosis.

Synaptotagmin VI and VIII and Syntaxin 2 Are Essential for the Mouse Sperm Acrosome Reaction

The sperm acrosome is a large secretory granule that undergoes calcium-stimulated exocytosis by a mechanism analogous to neuronal secretion. In neurons the core SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, composed of syntaxin (Stx), SNAP-25, and VAMP2, mediates vesicle fusion, whereas calcium regulation is thought to be accomplished by the synaptotagmin (Syt) family, some of which exhibit calcium-dependent binding to syntaxin and SNAP-25. Sperm express Syt VI and VIII and Stx2, which are co-localized to the acrosomal compartment where they might mediate exocytosis in response to calcium influx. Therefore, we examined the calcium dependence and isoform-specific interaction of Syt and Stx. We found that Stx2 binds to Syt I, VI, and VIII in a calcium-dependent manner with EC(50) values of 175, 233, and 96 mum calcium, respectively. We also determined that the EC(50) for calcium of the acrosome reaction in streptolysin O-permeabilized sperm is 87 mum, which closely coincides with the calcium sensitivity of Stx2 and Syt VIII interaction. Consistent with this is the greater potency of recombinant Syt VIII, VI, and Stx2 compared with other isoforms in inhibiting the acrosome reaction in streptolysin O-permeabilized sperm. Similarly, introduction of Syt VIII-specific antibodies was equally effective in inhibiting the acrosome fusion. Taken together, our data suggest a critical role for Syt VIII and Stx2 in membrane fusion and acrosome reaction in the sperm.

Synaptotagmin gene content of the sequenced genomes

Background

Synaptotagmins exist as a large gene family in mammals. There is much interest in the function of certain family members which act crucially in the regulated synaptic vesicle exocytosis required for efficient neurotransmission. Knowledge of the functions of other family members is relatively poor and the presence of Synaptotagmin genes in plants indicates a role for the family as a whole which is wider than neurotransmission. Identification of the Synaptotagmin genes within completely sequenced genomes can provide the entire Synaptotagmin gene complement of each sequenced organism. Defining the detailed structures of all the Synaptotagmin genes and their encoded products can provide a useful resource for functional studies and a deeper understanding of the evolution of the gene family. The current rapid increase in the number of sequenced genomes from different branches of the tree of life, together with the public deposition of evolutionarily diverse transcript sequences make such studies worthwhile.

Results

I have compiled a detailed list of the Synaptotagmin genes of Caenorhabditis, Anopheles, Drosophila, Ciona, Danio, Fugu, Mus, Homo, Arabidopsis and Oryza by examining genomic and transcript sequences from public sequence databases together with some transcript sequences obtained by cDNA library screening and RT-PCR. I have compared all of the genes and investigated the relationship between plant Synaptotagmins and their non-Synaptotagmin counterparts.

Conclusions

I have identified and compared 98 Synaptotagmin genes from 10 sequenced genomes. Detailed comparison of transcript sequences reveals abundant and complex variation in Synaptotagmin gene expression and indicates the presence of Synaptotagmin genes in all animals and land plants. Amino acid sequence comparisons indicate patterns of conservation and diversity in function. Phylogenetic analysis shows the origin of Synaptotagmins in multicellular eukaryotes and their great diversification in animals. Synaptotagmins occur in land plants and animals in combinations of 4–16 in different species. The detailed delineation of the Synaptotagmin genes presented here, will allow easier identification of Synaptotagmins in future. Since the functional roles of many of these genes are unknown, this gene collection provides a useful resource for future studies.

The loss of circadian PAR bZip transcription factors results in epilepsy

DBP (albumin D-site-binding protein), HLF (hepatic leukemia factor), and TEF (thyrotroph embryonic factor) are the three members of the PAR bZip (proline and acidic amino acid-rich basic leucine zipper) transcription factor family. All three of these transcriptional regulatory proteins accumulate with robust circadian rhythms in tissues with high amplitudes of clock gene expression, such as the suprachiasmatic nucleus (SCN) and the liver. However, they are expressed at nearly invariable levels in most brain regions, in which clock gene expression only cycles with low amplitude. Here we show that mice deficient for all three PAR bZip proteins are highly susceptible to generalized spontaneous and audiogenic epilepsies that frequently are lethal. Transcriptome profiling revealed pyridoxal kinase (Pdxk) as a target gene of PAR bZip proteins in both liver and brain. Pyridoxal kinase converts vitamin B6 derivatives into pyridoxal phosphate (PLP), the coenzyme of many enzymes involved in amino acid and neurotransmitter metabolism. PAR bZip-deficient mice show decreased brain levels of PLP, serotonin, and dopamine, and such changes have previously been reported to cause epilepsies in other systems. Hence, the expression of some clock-controlled genes, such as Pdxk, may have to remain within narrow limits in the brain. This could explain why the circadian oscillator has evolved to generate only low-amplitude cycles in most brain regions.

Past-A, a novel proton-associated sugar transporter, regulates glucose homeostasis in the brain

The ventral medullary surface (VMS) of the medulla oblongata is known to be the site of the central chemosensitive neurons in mammals. These neurons sense excess H+/CO2 dissolved in the CSF and induce hyperventilation. To elucidate the mechanism of neuronal cell adaptation to changes of H+/CO2, we screened for hypercapnia-induced genes in the VMS. Here, we report cloning and characterization of a novel gene called proton-associated sugar transporter-A (Past-A), which is induced in the brain after hypercapnia and mediates glucose uptake along the pH gradient. Past-A comprises 751 amino acid residues containing 12 membrane-spanning helices, several conserved sugar transport motifs, three proline-rich regions, and leucine repeats. Past-A transcript was expressed predominantly in the brain. Moreover, the Past-A-immunoreactive neural cells were found in the VMS of the medulla oblongata, and the number of immunoreactive cells was increased by hypercapnic stimulation. Transient transfection of Past-A in COS-7 cells leads to the expression of a membrane-associated 82 kDa protein that possesses a glucose transport activity. The acidification of extracellular medium facilitated glucose uptake, whereas the addition of carbonyl cyanide m-chlorophenylhydrazone, a protonophore, inhibited glucose import. Together, our results indicate that Past-A is a brain-specific glucose transporter that may represent an adaptation mechanism regulating sugar homeostasis in neuronal cells after hypercapnia.

Past-A, a novel proton-associated sugar transporter, regulates glucose homeostasis in the brain

The ventral medullary surface (VMS) of the medulla oblongata is known to be the site of the central chemosensitive neurons in mammals. These neurons sense excess H+/CO2 dissolved in the CSF and induce hyperventilation. To elucidate the mechanism of neuronal cell adaptation to changes of H+/CO2, we screened for hypercapnia-induced genes in the VMS. Here, we report cloning and characterization of a novel gene called proton-associated sugar transporter-A (Past-A), which is induced in the brain after hypercapnia and mediates glucose uptake along the pH gradient. Past-A comprises 751 amino acid residues containing 12 membrane-spanning helices, several conserved sugar transport motifs, three proline-rich regions, and leucine repeats. Past-A transcript was expressed predominantly in the brain. Moreover, the Past-A-immunoreactive neural cells were found in the VMS of the medulla oblongata, and the number of immunoreactive cells was increased by hypercapnic stimulation. Transient transfection of Past-A in COS-7 cells leads to the expression of a membrane-associated 82 kDa protein that possesses a glucose transport activity. The acidification of extracellular medium facilitated glucose uptake, whereas the addition of carbonyl cyanide m-chlorophenylhydrazone, a protonophore, inhibited glucose import. Together, our results indicate that Past-A is a brain-specific glucose transporter that may represent an adaptation mechanism regulating sugar homeostasis in neuronal cells after hypercapnia.

Generation of kidney transcriptomes using serial analysis of gene expression

Chronic renal disease initiation and progression remain incompletely understood. Genomewide expression monitoring should clarify the mechanisms which cause progressive renal disease by determining how clusters of genes coordinately change their activity. Serial analysis of gene expression (SAGE) is a technique of expression profiling which permits simultaneous and quantitative analysis of 9- to 13-bp sequence tags that correspond to unique mRNAs. Key principles of the technique are use of PCR in a manner to minimize distortion and serial concatenation of tags which facilitates sequencing and permits identification of many expressed genes in a single cDNA molecule. Tags are extracted from many concatenated sequences, counted using software, and identified by comparison with existing gene databases. In aggregate, gene expression profiles generated from a tag library comprise a transcriptome which represents a comprehensive and quantitative profile of genes expressed at the time of analysis. These global snapshots of gene expression patterns can better define basic cell biology and provide insights into disease pathogenesis by simultaneously determining the net consequences of gene-gene and gene-environment interactions on expression of thousands of genes. Rather than applying a priori assumptions (i.e., hypothesis testing), transcriptome analysis is hypothesis generating and requires no prior knowledge of gene expression. SAGE kidney transcriptomes, from normal animals and animals with progressive kidney disease, are being produced and can be analyzed for novel pathogenetic mechanisms. The use of SAGE and other genomic and proteomic tools should result in a better understanding of kidney disease pathogenesis and in identification of new therapeutic targets.

Synaptotagmin V is targeted to dense-core vesicles that undergo calcium-dependent exocytosis in PC12 cells

Synaptotagmins (Syts) III, V, VI, and X are classified as a subclass of Syt, based on their sequence similarities and biochemical properties (Ibata, K., Fukuda, M., and Mikoshiba, K. (1998) J. Biol. Chem. 273, 12267-12273; Fukuda, M., Kanno, E., and Mikoshiba, K. (1999) J. Biol. Chem. 274, 31421-31427). Although they have been suggested to be involved in vesicular trafficking, as in the role of the Syt I isoform in synaptic vesicle exocytosis, their exact functions remain to be clarified, and even their precise subcellular localization is still a matter of controversy. In this study, we established rat pheochromocytoma (PC12) cell lines that stably express Syts III-, V-, VI-, and X-GFP (green fluorescence protein) fusion proteins, respectively, to determine their precise subcellular localizations. Surprisingly, Syts III-, V-, VI-, and X-GFP proteins were found to be targeted to specific organelles: Syt III-GFP to near the plasma membrane, Syt V-GFP to dense-core vesicles, Syt VI-GFP to endoplasmic reticulum-like structures, and Syt X-GFP to vesicles (other than dense-core vesicles) present in cytoplasm. We showed that Syt V-containing vesicles at the neurites of PC12 cells were processed to exocytosis in a Ca2+-dependent manner. Immunohistochemical analysis further showed that endogenous Syt V was also localized on dense-core vesicles in the mouse brain and specifically expressed in glucagon-positive alpha-cells in mouse pancreatic islets, but not in beta- or delta-cells. Based on these results, we propose that Syt V is a dense-core vesicle-specific Syt isoform that controls a specific type of Ca2+-regulated secretion.

Synaptotagmin VIII is localized to the mouse sperm head and may function in acrosomal exocytosis

The acrosome is a large secretory granule that undergoes exocytosis when receptors on the sperm surface bind ligands in the egg extracellular matrix. Acrosomal exocytosis resembles stimulated secretion in neurons in that it is triggered by a rise in intracellular Ca(2+). Synaptotagmins (Syt) comprise proteins thought to transduce this Ca(2+) signal to the fusion machinery. In this study, we showed that Syt VIII is present in spermatogenic cDNA libraries. Antiserum raised against a Syt VIII-specific peptide, which recognizes Syt VIII but does not cross-react with other Syt isoforms, labeled a single prominent band on Western immunoblots of mouse sperm homogenate. Syt VIII was restricted to the sperm membrane fraction enriched in markers associated with the mouse sperm head. Fluorescent immunocytochemistry on intact mouse sperm showed that Syt VIII is localized to the acrosomal crescent and is lost upon acrosome reaction. Moreover, the amount of Syt VIII remaining with the sperm decreased proportionately with the extent of acrosome-reacted sperm. Thus, Syt VIII is a candidate for the Ca(2+) sensor that regulates acrosomal exocytosis in mammalian sperm.

Genomic analysis of synaptotagmin genes

I used TBLASTn to probe DNA sequence databases with a consensus peptide sequence corresponding to the most highly conserved region of the rodent synaptotagmin (Syt) gene family, which is within the C2B domain. I found human homologues for all known rodent genes, and found six further human genomic loci which encode potential family members. I found eight potential family members in Caenorhabditis elegans, six in Drosophila melanogaster, and four in Arabidopsis thaliana. The C. elegans Syt1 homologue uniquely encodes two alternative C2B exons, one or the other of which is expressed at a time. Comparison of the genomic structures of the Syt genes makes clear the different phylogenies of the different subgroups. Knowledge of the genomic structures will aid the systematic investigation of alternative splicing in Syt genes.

The AP2 binding site of synaptotagmin 1 is not an internalization signal but a regulator of endocytosis

One characteristic linking members of the synaptotagmin family to endocytosis is their ability to bind the heterotetrameric AP2 complex via their C2B domain. By using CD4/synaptotagmin 1 chimeras, we found that the internalization signal of synaptotagmin 1 lies at the extreme COOH-terminus of the protein and can function in the absence of the C2B domain that contains the AP2 binding site. However, although not essential for internalization, the C2B domain of synaptotagmin 1 appeared to control the recognition of the internalization motif. By mutagenesis, two sites have been identified that modify regulation by the C2B domain in the neuroendocrine PC12 cell line. Mutation of a dilysine motif in the beta sandwich core of the domain eliminates endocytosis. This site is known to be a site of protein-protein interaction. Mutations in the calcium binding region, or in its close proximity, also affect internalization in PC12 cells. In fibroblasts, the C2B domain inhibits the COOH-terminal internalization signal, resulting in an absence of internalization in those cells. Thus, internalization of synaptotagmin 1 is controlled by the presence of a latent internalization signal in the COOH-terminal region and a regulatory region in the C2B domain. We propose that internalization of synaptotagmin 1 is regulated in this way to allow it to couple the processes of endocytosis and calcium-mediated exocytosis in cells of the neuroendocrine lineage.

Synaptotagmin VII as a plasma membrane Ca(2+) sensor in exocytosis

Synaptotagmins I and II are Ca(2+) binding proteins of synaptic vesicles essential for fast Ca(2+)-triggered neurotransmitter release. However, central synapses and neuroendocrine cells lacking these synaptotagmins still exhibit Ca(2+)-evoked exocytosis. We now propose that synaptotagmin VII functions as a plasma membrane Ca(2+) sensor in synaptic exocytosis complementary to vesicular synaptotagmins. We show that alternatively spliced forms of synaptotagmin VII are expressed in a developmentally regulated pattern in brain and are concentrated in presynaptic active zones of central synapses. In neuroendocrine PC12 cells, the C(2)A and C(2)B domains of synaptotagmin VII are potent inhibitors of Ca(2+)-dependent exocytosis, but only when they bind Ca(2+). Our data suggest that in synaptic vesicle exocytosis, distinct synaptotagmins function as independent Ca(2+) sensors on the two fusion partners, the plasma membrane (synaptotagmin VII) versus synaptic vesicles (synaptotagmins I and II).

A spontaneous recurrent seizure-related Rattus NSF gene identified by linker capture subtraction

Spontaneous recurrent seizures (SRS) are the major clinical characteristic of epilepsy. In this study, using a SRS-behavior test combined with linker capture subtraction (LCS) to identify genes altered in their expression in response to a single kainic acid (KA)-induced SRS at 3 weeks in the rat hippocampal formation. Dot blot analysis of the differentially expressed cDNA fragments with LCS showed the down-regulation of one cDNA related to SRS, which was designated epilepsy-related gene 1 (ERG1). Northern blot analysis showed that ERG1 mRNA was reduced by KA administration with and without SRS, but more so with SRS. This differential expression had also been confirmed by in situ hybridization, which showed that ERG1 mRNA was down-regulated in the dorsal dentate granule cells (dDGCs) of the hippocampal formation, but remarkable up-regulated in the amygdalohippocampal area (AHi), posteromedial cortical amygdaloid nucleus (PMCo) and perirhinal cortex (PRh). The complete cDNA of ERG1 was cloned, sequenced (AF142097). It encodes a Rattus homologue of N-ethylmaleimide-sensitive fusion protein (NSF), which is an ATPase that plays a key role in mediating docking and/or fusion of transport vesicles in the multi-step pathways of vesicular transport. Sequence analysis revealed that ERG1 has high sequence similarity with the cDNA of the Mus musculus suppressor of K(+) transport growth defect (SKD2), N-ethylmaleimide(NEM)-sensitive fusion protein of Chinese hamster and human NEM-sensitive factor (HSU03985).

Synaptotagmin 13: structure and expression of a novel synaptotagmin

Synaptotagmins represent a family of putative vesicular trafficking proteins. With synaptotagmin 13, we have now identified a novel synaptotagmin, making this one of the largest families of trafficking proteins. Similar to synaptotagmins 3, 4, 6, 7, 9, and 11, synaptotagmin 13 is expressed at highest levels in brain but is also detectable at lower levels in non-neuronal tissues. Synaptotagmin 13 is composed of the canonical domains of synaptotagmins that include an N-terminal transmembrane region and two C-terminal cytoplasmic C2-domains (C2A- and C2B-domain) and a connecting sequence between the transmembrane region and the C2-domains. Different from most other synaptotagmins, however, synaptotagmin 13 does not have an N-terminal sequence preceding the transmembrane region, and features an unusually long connecting sequence that is proline-rich. Furthermore, the C2-domains of synaptotagmin are degenerate and lack almost all of the residues involved in Ca2+ binding, suggesting that synaptotagmin 13 is not a Ca2+-binding protein unlike most other synaptotagmins. Our data demonstrate that synaptotagmins represent a larger and more complex gene family than previously envisioned.

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.

The genomic action potential

Neurons compute in part by integrating, on a time scale of milliseconds, many synaptic inputs and generating a digital output-the "action potential" of classic electrophysiology. Recent discoveries indicate that neurons also perform a second, much slower, integration operating on a time scale of minutes or even hours. The output of this slower integration involves a pulse of gene expression which may be likened to the electrophysiological action potential. Its function, however, is not directed toward immediate transmission of a synaptic signal but rather toward the experience-dependent modification of the underlying synaptic circuitry. Commonly termed the "immediate early gene" (IEG) response, this phenomenon is often assumed to be a necessary component of a linear, deterministic cascade of memory consolidation. Critical review of the large literature describing the phenomenon, however, leads to an alternative model of IEG function in the brain. In this alternative, IEG activation is not directed at the consolidation of memories of a specific inducing event; instead, it sets the overall gain or efficiency of memory formation and directs it to circuits engaged by behaviorally significant contexts. The net result is a sharpening of the selectivity of memory formation, a recruitment of temporally correlated associations, and an ultimate enhancement of long-term memory retrieval.

Distinct self-oligomerization activities of synaptotagmin family. Unique calcium-dependent oligomerization properties of synaptotagmin VII

Synaptotagmins constitute a large protein family, characterized by one transmembrane region and two C2 domains, and can be classified into several subclasses based on phylogenetic relationships and biochemical activities (Fukuda, M., Kanno, E., and Mikoshiba, K. (1999) J. Biol. Chem. 274, 31421-31427). Synaptotagmin I (Syt I), a possible Ca(2+) sensor for neurotransmitter release, showed both Ca(2+)-dependent (via the C2 domain) and -independent (via the NH(2)-terminal domain) self-oligomerization, which are thought to be important for synaptic vesicle exocytosis. However, little is known about the relationship between these two interactions and the Ca(2+)-dependent oligomerization properties of other synaptotagmin isoforms. In this study, we first examined the Ca(2+)-dependent self-oligomerization properties of synaptotagmin family by co-expression of T7- and FLAG-tagged Syts (full-length or cytoplasmic domain) in COS-7 cells. We found that Syt VII is a unique class of synaptotagmins that only showed robust Ca(2+)-dependent self-oligomerization at the cytoplasmic domain with EC(50) values of about 150 micrometer Ca(2+). In addition, Syt VII preferentially interacted with the previously described subclass of Syts (V, VI, and X) in a Ca(2+)-dependent manner. Co-expression of full-length and cytoplasmic portion of Syts VII (or II) indicate that Syt VII cytoplasmic domain oligomerizes in a Ca(2+)-dependent manner without being tethered at the NH(2)-terminal domain, whereas Ca(2+)-dependent self-oligomerization at the cytoplasmic domain of other isoforms (e.g. Syt II) occurs only when the two molecules are tethered at the NH(2)-terminal domain.

Use of serial analysis of gene expression to generate kidney expression libraries

Chronic renal disease initiation and progression remain incompletely understood. Genome-wide expression monitoring should clarify mechanisms that cause progressive renal disease by determining how clusters of genes coordinately change their activity. Serial analysis of gene expression (SAGE) is a technique of expression profiling, which permits simultaneous, comparative, and quantitative analysis of gene-specific, 9- to 13-bp sequence tags. Using SAGE, we have constructed a tag expression library from ROP-+/+ mouse kidney. Tag sequences were sorted by abundance, and identity was determined by sequence homology searching. Analyses of 3,868 tags yielded 1,453 unique kidney transcripts. Forty-two percent of these transcripts matched mRNA sequence entries with known function, 35% of the transcripts corresponded to expressed sequence tag (EST) entries or cloned genes, whose function has not been established, and 23% represented unidentified genes. Previously characterized transcripts were clustered into functional groups, and those encoding metabolic enzymes, plasma membrane proteins (transporters/receptors), and ribosomal proteins were most abundant (39, 14, and 12% of known transcripts, respectively). The most common, kidney-specific transcripts were kidney androgen-regulated protein (4% of all transcripts), sodium-phosphate cotransporter (0.3%), renal cytochrome P-450 (0.3%), parathyroid hormone receptor (0.1%), and kidney-specific cadherin (0.1%). Comprehensively characterizing and contrasting gene expression patterns in normal and diseased kidneys will provide an alternative strategy to identify candidate pathways, which regulate nephropathy susceptibility and progression, and novel targets for therapeutic intervention.

The salt-inducible kinase, SIK, is induced by depolarization in brain

Membrane depolarization of neurons is thought to lead to changes in gene expression that modulate neuronal plasticity. We used representational difference analysis to identify a group of cDNAs that are induced by membrane depolarization or by forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. One of these genes, SIK (salt-inducible kinase), is a member of the sucrose-nonfermenting 1 protein kinase/AMP-activated protein kinase protein kinase family that was also recently identified from the adrenal gland of rats treated with high-salt diets. SIK mRNA is induced up to eightfold in specific regions of the hippocampus and cortex in rats, following systemic kainic acid administration and seizure induction.

Deficits in memory and motor performance in synaptotagmin IV mutant mice

Synaptotagmin (Syt) IV is a synaptic vesicle protein. Syt IV expression is induced in the rat hippocampus after systemic kainic acid treatment. To examine the functional role of this protein in vivo, we derived Syt IV null [Syt IV(-/-)] mutant mice. Studies with the rotorod revealed that the Syt IV mutants have impaired motor coordination, a result consistent with constitutive Syt IV expression in the cerebellum. Because Syt IV is thought to modulate synaptic function, we also have examined Syt IV mutant mice in learning and memory tests. Our studies show that the Syt IV mutation disrupts contextual fear conditioning, a learning task sensitive to hippocampal and amygdala lesions. In contrast, cued fear conditioning is normal in the Syt IV mutants, suggesting that this mutation did not disrupt amygdala function. Conditioned taste aversion, which also depends on the amygdala, is normal in the Syt IV mutants. Consistent with the idea that the Syt IV mutation preferentially affects hippocampal function, Syt IV mutant mice also display impaired social transmission of food preference. These studies demonstrate that Syt IV is critical for brain function and suggest that the Syt IV mutation affects hippocampal-dependent learning and memory, as well as motor coordination.

Synaptotagmins in membrane traffic: which vesicles do the tagmins tag?

The aim of this review is to give a broad picture of what is actually known about the synaptotagmin family. Synaptotagmin I is an abundant synaptic vesicle and secretory granule protein in neurons and endocrine cells which plays a key role in Ca(2+)-induced exocytosis. It belongs to the large family of C2 domain-proteins as it contains two internal repeats that have homology to the C2 domain of protein kinase C. Eleven synaptotagmin genes have been described in rat and mouse. Except for synaptotagmin I, and by analogy synaptotagmin II, the functions of these proteins are unknown. In this review we focus on data obtained on the various isoforms without exhaustively discussing the role of synaptotagmin I in neurotransmission. Numerous in vitro interactions of synaptotagmin I with key components of the exocytosis-endocytosis machinery have been reported. Additional data concerning the other synaptotagmins are now becoming available and are reviewed here. Only interactions which have been described for several synaptotagmins, are mentioned. It is unlikely that a single isoform displays all of these potential interactions in vivo and probably the subcellular distribution of the protein will favor some of them and preclude others. Therefore, to discuss the putative role of the various synaptotagmins we have examined in detail published data concerning their localization.

Synaptotagmin IV is present at the Golgi and distal parts of neurites

Synaptotagmin IV (SytIV) is an immediate early gene induced by membrane depolarization in PC12 cells and in rat brain. However, little is known about the function of SytIV or the functional relationship between SytIV and SytI, because SytIV has yet to be localized. Here we show that SytIV was localized at the Golgi and distal part of neurites in nerve growth factor-differentiated PC12 cells and cultured hippocampal neurons by immunocytochemistry using an isoform-specific antibody (anti-SytIV). These SytIV signals were not colocalized well with SytI signals. Upon membrane depolarization, SytIV signals were increased at both the Golgi and distal part of neurites within several hours in both types of cells. We further show that the increase of SytIV protein levels results from protein kinase A-dependent gene up-regulation. In hippocampal neurons, SytIV was developmentally regulated. These results suggest that SytIV may play a role at the Golgi and tips of neurites during development and synaptic plasticity.

Screening of novel epilepsy-related genes and isolation and identification of cDNAs

Twenty cDNA differential fragments were isolated from the hippocampus of rats in epileptic state using mRNA differential display technique. Four fragments were sequenced and compared with the known sequences in the Genebank, which showed that ERG8, ERG11, ERG12 had no significant identity to any known sequences; ERG14 had 64%-69% identity to microtubulin-associated protein of the rat. Because the differential expression of these genes was caused by epilepsy inducer coriaria lactone (CL) and anti-epilepsy drug MK-801 and ERG8 might be a novel candidate epilepsy gene; ERG11 and ERG12 might be novel candidate anti-epilepsy genes. Since the microtubulin-associated protein is closely associated with the collateral sprouting of mossy fibers in the hippocampus of seizured rat, the high expression of ERG14 in the early stage of epilepsy might predict the growth of axon and formation of synapse.

Alternative splicing of synaptotagmins involving transmembrane exon skipping

The synaptotagmin gene family currently includes 12 members. Analysis of the three known genomic synaptotagmin sequences reveals conserved exon-intron patterns which delineate the synaptotagmin structural domains. We used expressed sequence tag, reverse transcription PCR and RNAse protection assay analysis of synaptotagmin messenger RNAs to demonstrate the occurrence of alternative splicing events involving a number of exons. Exon-skipped messages where transmembrane sequences have been removed or altered were found to be abundantly expressed by synaptotagmins 1, 4, 6 and 7. Although the expression of most synaptotagmins predominates in neural tissue, we find that by contrast, synaptotagmin 6 is more abundant in thymus.

A novel alternatively spliced variant of synaptotagmin VI lacking a transmembrane domain. Implications for distinct functions of the two isoforms

Synaptotagmins are a family of membrane proteins that are characterized by a single transmembrane region and tandem C2 domains and that are likely to regulate constitutive and/or regulated vesicle traffic. We have shown that a subclass of synaptotagmins (III, V, VI, and X) forms homo- and heterodimers through an evolutionarily conserved cysteine motif at their N termini (Fukuda, M., Kanno, E., and Mikoshiba, K. (1999) J. Biol. Chem. 274, 31421-31427). In this study, we identified a novel alternatively spliced variant of synaptotagmin (Syt) VI that lacks the N-terminal 85 amino acids including the transmembrane region (thus designated as Syt VIDeltaTM). Because it lacks the cysteine motif responsible for self-dimerization, Syt VIDeltaTM could not associate with Syt VI even in the presence of Ca(2+). Despite lacking the transmembrane region, Syt VIDeltaTM can associate with the plasma membrane through the C-terminal 29 amino acids. In adult mouse brain, two closely comigrating bands at M(r) approximately 50,000, which closely corresponded to the molecular weight of recombinant Syt VIDeltaTM, were detected by anti-Syt VI antibody. These immunoreactive bands were found in both soluble and membrane fractions of mouse brain, indicating that they are membrane-associated proteins (Syt VIDeltaTM), but not transmembrane proteins (Syt VI). Expression of Syt VI and Syt VIDeltaTM in PC12 or COS-7 cells indicated that the two molecules have a distinct subcellular distribution: Syt VIDeltaTM is present in the cytosol or is associated with the plasma membrane or internal membrane structures, whereas Syt VI is localized to the endoplasmic reticulum and/or Golgi-like perinuclear compartment. These results suggest that Syt VI and Syt VIDeltaTM may play distinct roles in vesicular trafficking.

Conserved N-terminal cysteine motif is essential for homo- and heterodimer formation of synaptotagmins III, V, VI, and X

The synaptotagmins now constitute a large family of membrane proteins characterized by one transmembrane region and two C2 domains. Dimerization of synaptotagmin (Syt) I, a putative low affinity Ca(2+) sensor for neurotransmitter release, is thought to be important for expression of function during exocytosis of synaptic vesicles. However, little is known about the self-dimerization properties of other isoforms. In this study, we demonstrate that a subclass of synaptotagmins (III, V, VI, and X) (Ibata, K., Fukuda, M., and Mikoshiba, K. (1998) J. Biol. Chem. 273, 12267-12273) forms beta-mercaptoethanol-sensitive homodimers and identify three evolutionarily conserved cysteine residues at the N terminus (N-terminal cysteine motif, at amino acids 10, 21, and 33 of mouse Syt III) that are not conserved in other isoforms. Site-directed mutagenesis of these cysteine residues and co-immunoprecipitation experiments clearly indicate that the first cysteine residue is essential for the stable homodimer formation of Syt III, V, or VI, and heterodimer formation between Syts III, V, VI, and X. We also show that native Syt III from mouse brain forms a beta-mercaptoethanol-sensitive homodimer. Our results suggest that the cysteine-based heterodimerization between Syt III and Syt V, VI, or X, which have different biochemical properties, may modulate the proposed function of Syt III as a putative high affinity Ca(2+) sensor for neurotransmitter release.

Variations of synaptotagmin I, synaptotagmin IV, and synaptophysin mRNA levels in rat hippocampus during the estrous cycle

Periodic changes in ovarian steroid levels during fertility cycles affect learning both in humans and in rats in parallel with electrophysiological and morphological fluctuations in selective neuronal populations. In particular, during the estrous cycle of the female rat, hippocampal CA1 region undergoes cyclic modifications in synaptic density. To investigate the molecular mechanisms involved in synaptic remodeling during the estrous cycle, we analyzed the expression of three presynaptic markers, synaptotagmin I, synaptotagmin IV, and synaptophysin, in the female adult rat brain by in situ hybridization. Relative abundance in mRNA for these three markers was quantified at four phases of the estrous cycle: diestrus, proestrus (AM and PM), and estrus. mRNA levels for syt1 exhibited cyclic variations in pyramidal neurons of the CA3 region of hippocampus during the estrous cycle, while mRNA levels for syt4 and SYN were relatively invariant in this or other regions of the hippocampus. Because CA3 pyramidal neurons make synaptic contacts in CA1, modulation of syt1 expression in CA3 may participate in the changes in synaptic density observed in CA1 during the estrous cycle. Furthermore, both syt1 and SYN mRNA varied cyclically in layer II, but not in layer III of entorhinal cortex, while syt4 remained unchanged throughout the cycle. These data suggest that regular variations in steroid hormone levels during fertility cycles may alter the properties of several networks involved in information processing and learning and memory through altered levels of presynaptic proteins.

Regulation of synaptotagmin I phosphorylation by multiple protein kinases

Synaptotagmin I has been suggested to function as a low-affinity calcium sensor for calcium-triggered exocytosis from neurons and neuroendocrine cells. We have studied the phosphorylation of synaptotagmin I by a variety of protein kinases in vitro and in intact preparations. SyntagI, the purified, recombinant, cytoplasmic domain of rat synaptotagmin I, was an effective substrate in vitro for Ca2+/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC), and casein kinase II (caskII). Sequencing of tryptic phosphopeptides from syntagI revealed that CaMKII and PKC phosphorylated the same residue, corresponding to Thr112, whereas caskII phosphorylated two residues, corresponding to Thr125 and Thr128. Endogenous synaptotagmin I was phosphorylated on purified synaptic vesicles by all three kinases. In contrast, no phosphorylation was observed on clathrin-coated vesicles, suggesting that phosphorylation of synaptotagmin I in vivo occurs only at specific stage(s) of the synaptic vesicle life cycle. In rat brain synaptosomes and PC12 cells, K+-evoked depolarization or treatment with phorbol ester caused an increase in the phosphorylation state of synaptotagmin I at Thr112. The results suggest the possibility that the phosphorylation of synaptotagmin I by CaMKII and PKC contributes to the mechanism(s) by which these two kinases regulate neurotransmitter release.

The subcellular localizations of atypical synaptotagmins III and VI. Synaptotagmin III is enriched in synapses and synaptic plasma membranes but not in synaptic vesicles

Multiple synaptotagmins are expressed in brain, but only synaptotagmins I and II have known functions in fast, synchronous Ca2+-triggered neurotransmitter release. Synaptotagmin III was proposed to regulate other aspects of synaptic vesicle exocytosis, particularly its slow component. Such a function predicts that synaptotagmin III should be an obligatory synaptic vesicle protein, as would also be anticipated from its high homology to synaptotagmins I and II. To test this hypothesis, we studied the distribution, developmental expression, and localization of synaptotagmin III and its closest homolog, synaptotagmin VI. We find that synaptotagmins III and VI are present in all brain regions in heterogeneous distributions and that their levels increase during development in parallel with synaptogenesis. Furthermore, we show by immunocytochemistry that synaptotagmin III is concentrated in synapses, as expected. Surprisingly, however, we observed that synaptotagmin III is highly enriched in synaptic plasma membranes but not in synaptic vesicles. Synaptotagmin VI was also found to be relatively excluded from synaptic vesicles. Our data suggest that synaptotagmins III and VI perform roles in neurons that are not linked to synaptic vesicle exocytosis but to other Ca2+-related nerve terminal events, indicating that the functions of synaptotagmins are more diverse than originally thought.

Functional involvement of synaptotagmin I/II C2A domain in neurite outgrowth of chick dorsal root ganglion neuron

Synaptotagmin I or II (Syt I/II) is involved in Ca2+-regulated exocytosis of secretory vesicles, probably serving as a Ca2+-sensor via its C2A domain. Synaptotagmin is also known to be expressed in neuronal growth cone vesicles, but its functional involvement in neurite outgrowth remains largely unknown. In this study, we examined the function of Syt I/II in neurite outgrowth in cultured chick dorsal root ganglion neurons using an anti-synaptotagmin I and II C2A domain (anti-STI/II-C2A) antibody that inhibits Ca2+-regulated exocytosis. Immunoblots confirmed the high specificity of the anti-STI/II-C2A antibody and showed the expression of synaptotagmin I or II in chick dorsal root ganglion neurons. Immunocytochemistry revealed that synaptotagmin I or II is enriched at the growth cone region of chick dorsal root ganglion neurons, in both lamellipodia and filopodia. Whole or Fab-fragment of the anti-STI/II-C2A antibody loaded into dorsal root ganglion neurons by trituration significantly inhibited neurite outgrowth, whereas preimmune immunoglobulin G had no effect. These results showed that the C2A domain of synaptotagmin I or II plays a crucial role in neurite outgrowth.