Synaptotagmin 13: structure and expression of a novel synaptotagmin

Hypoosmotic stress shifts transcription of circadian genes

Cells respond to hypoosmotic stress by initial swelling followed by intracellular increases in the number of osmolytes and initiation of gene transcription that allow cells to adapt to the stress. Here, we have studied the genes that change expression under mild hypoosmotic stress for 12 and 24 h in rat cultured smooth muscle cells (WKO-3M22). We find shifts in the transcription of many genes, several of which are associated with circadian rhythm, such as per1, nr1d1, per2, dbp, and Ciart. To determine whether there is a connection between osmotic stress and circadian rhythm, we first subjected cells to hypoosmotic stress for 12 h, and find that Bmal1, a transcription factor whose nuclear localization promotes transit through the cell cycle, localizes to the cytoplasm, which may connect osmotic stress to cell cycle. Bmal1 nuclear localization recovers after 24 h and cell cycle resumes even though the osmotic stress remains elevated. We hypothesized that osmotic force is transmitted into the cell by deforming caveolae membrane domains releasing one of its structural proteins, cavin-1, which can travel to the nucleus and affect gene transcription. In support of this idea, we find that Bmal1 localization becomes independent of osmotic stress with cavin-1 downregulation, and Bmal1 localization is independent of osmotic stress in a cell line with low caveolae expression. These studies indicate that osmotic stress transiently arrests circadian rhythm and cell-cycle progression through caveolae deformation.

Preclinical toxicological assessment of amido-bridged nucleic acid-modified antisense oligonucleotides targeting synaptotagmin XIII for intra-abdominal treatment of peritoneal metastasis of gastric cancer

BACKGROUND

Peritoneal metastasis of gastric cancer is closely associated with dismal prognosis. In previous preclinical proof-of-concept studies, an amido-bridged nucleic acid (AmNA)-modified antisense oligonucleotide (ASO), designated ASO-4733 that targets the gene encoding synaptotagmin XIII (SYT13), inhibited cellular functions required for the formation of peritoneal metastasis of gastric cancer cells. ASO-4733 achieved therapeutic effects when intra-abdominally administered to mouse xenograft models. Here, we conducted an analysis of Syt13-deficient mice to determine the pharmacokinetics and toxicity of intra-abdominal administration of ASO-4733.

METHODS

The effects of Syt13-deficiency in mice were determined. Good Laboratory Practice toxicity tests and the toxicokinetics of intra-abdominal administration of ASO-4733 were conducted in cynomolgus monkeys and rats. The pharmacokinetics of ASO-4733 administered intravenously or intra-abdominally to rats were investigated.

RESULTS

Syt13-deficient mice exhibited normal reproduction, organ functions, and motor functions. Weekly intra-abdominal administration of ASO-4733 (125 mg/kg), corresponding to a 50-fold increase of the estimated clinical dose for 4 weeks, was well tolerated by cynomolgus monkeys. In rats, off-target toxicity (not attributable to hybridization) was observed after weekly intra-abdominal administration of ASO-4733. Blood concentrations of ASO-4733 were lower and rose more slowly after intra-abdominal administration compared with intravenous administration.

CONCLUSIONS

The preclinical profile of intra-abdominal administration of ASO-4733 demonstrated its suitability for entry into clinical trials of patients with peritoneal metastasis of gastric cancer.

Proper Frequency of Perinatal Retinal Waves Is Essential for the Precise Wiring of Visual Axons in Nonimage-Forming Nuclei

The development of the visual system is a complex and multistep process characterized by the precise wiring of retinal ganglion cell (RGC) axon terminals with their corresponding neurons in the visual nuclei of the brain. Upon reaching primary image-forming nuclei (IFN), such as the superior colliculus and the lateral geniculate nucleus, RGC axons undergo extensive arborization that refines over the first few postnatal weeks. The molecular mechanisms driving this activity-dependent remodeling process, which is influenced by waves of spontaneous activity in the developing retina, are still not well understood. In this study, by manipulating the activity of RGCs in mice from either sex and analyzing their transcriptomic profiles before eye-opening, we identified the Type I membrane protein synaptotagmin 13 (Syt13) as involved in spontaneous activity-dependent remodeling. Using these mice, we also explored the impact of spontaneous retinal activity on the development of other RGC recipient targets such as nonimage-forming (NIF) nuclei and demonstrated that proper frequency and duration of retinal waves occurring prior to visual experience are essential for shaping the connectivity of the NIF circuit. Together, these findings contribute to a deeper understanding of the molecular and physiological mechanisms governing activity-dependent axon refinement during the assembly of the visual circuit.

Synaptotagmin 13 Could Drive the Progression of Esophageal Squamous Cell Carcinoma Through Upregulating ACRV1

SYT13 is one of the atypical members of the synaptotagmin (SYT) family whose function has attracted considerable attention in recent years. Although SYT13 has been studied in several types of human cancers, such as lung cancer, its role in esophageal squamous cell carcinoma (ESCC) is still unclear. It was demonstrated that SYT13 is significantly upregulated in ESCC tissues compared with normal ones and correlated with higher degree of malignancy. Knockdown of SYT13 could inhibit ESCC cell proliferation and migration, while promoting cell apoptosis. Meanwhile, ESCC cells with relatively lower SYT13 expression grew slower in vivo and finally formed smaller xenografts. Furthermore, acrosomal vesicular protein 1 was identified as a potential downstream target of SYT13, which regulates cell phenotypes of ESCC cells in cooperation with SYT13. All the in vitro and in vivo results in this study identified that SYT13 silencing could be an effective strategy to inhibit the development of ESCC, which could be considered as a promising therapeutic target in the treatment of ESCC.

Heterozygous knockout of Synaptotagmin13 phenocopies ALS features and TP53 activation in human motor neurons

Spinal motor neurons (MNs) represent a highly vulnerable cellular population, which is affected in fatal neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). In this study, we show that the heterozygous loss of SYT13 is sufficient to trigger a neurodegenerative phenotype resembling those observed in ALS and SMA. SYT13+/- hiPSC-derived MNs displayed a progressive manifestation of typical neurodegenerative hallmarks such as loss of synaptic contacts and accumulation of aberrant aggregates. Moreover, analysis of the SYT13+/- transcriptome revealed a significant impairment in biological mechanisms involved in motoneuron specification and spinal cord differentiation. This transcriptional portrait also strikingly correlated with ALS signatures, displaying a significant convergence toward the expression of pro-apoptotic and pro-inflammatory genes, which are controlled by the transcription factor TP53. Our data show for the first time that the heterozygous loss of a single member of the synaptotagmin family, SYT13, is sufficient to trigger a series of abnormal alterations leading to MN sufferance, thus revealing novel insights into the selective vulnerability of this cell population.

Role of synaptotagmin 13 (SYT13) in promoting breast cancer and signaling pathways

PURPOSE

Breast cancer is one of the leading causes of tumor death worldwide in female, and the five-year overall survival of breast cancer patients remains poor. It is an urgent need to seek novel target for its treatment. Synaptotagmin 13 (SYT13) is a synaptic vesicle transporting protein that regulates the malignant phenotypes of various cancers. However, its role in breast cancer is still unclear. The current study aimed to investigate the effects of SYT13 on the progression of breast cancer.

METHODS

Twenty-five pairs of breast cancer tissues and non-tumor tissues were obtained to assess the expression of SYT13. We manually modified the expression of SYT13 in MCF-7 and MDA-MB-231 cells. CCK-8 assay, EdU staining, and cell cycle analysis were carried out to measure the proliferated ability of cells. Annexin V/PI and TUNEL assays were used to detect the apoptotic ability of cells. Wound healing and transwell assays were employed to evaluate the migrated and invasive ability of breast cancer cells.

RESULTS

The results revealed that the mRNA and protein levels of SYT13 were higher in breast cancer tissues and cell lines. Knockdown of SYT13 inhibited the cell proliferation and induced cell cycle arrest in G1 phase of MCF-7 cells by downregulating cyclin D1 and CDK4, as well as upregulating p21. The migration and invasion of MCF-7 cells were repressed by the loss of SYT13 via the gain of E-cadherin and the loss of vimentin. Overexpression of SYT13 in MDA-MB-231 cells led to the opposite effects. Silencing of SYT13 induced the apoptosis ability of MCF-7 cells by the upregulation of bax and the downregulation of bcl-2. Moreover, we found that SYT13 depletion suppressed the FAK/AKT signaling pathway. PF573228 (a FAK inhibitor) and MK2206 (an AKT inhibitor) reversed the SYT13 overexpression-induced promotion of proliferation, migration, and invasion of MDA-MB-231 cells.

CONCLUSION

The results indicated that SYT13 promoted the malignant phenotypes of breast cancer cells by the activation of FAK/AKT signaling pathway.

Endothelial cell-specific molecule 1 drives cervical cancer progression

The expression, biological functions and underlying molecular mechanisms of endothelial cell-specific molecule 1 (ESM1) in human cervical cancer remain unclear. Bioinformatics analysis revealed that ESM1 expression was significantly elevated in human cervical cancer tissues, correlating with patients' poor prognosis. Moreover, ESM1 mRNA and protein upregulation was detected in local cervical cancer tissues and various cervical cancer cells. In established and primary cervical cancer cells, ESM1 shRNA or CRISPR/Cas9-induced ESM1 KO hindered cell proliferation, cell cycle progression, in vitro cell migration and invasion, and induced significant apoptosis. Whereas ESM1 overexpression by a lentiviral construct accelerated proliferation and migration of cervical cancer cells. Further bioinformatics studies and RNA sequencing data discovered that ESM1-assocaited differentially expressed genes (DEGs) were enriched in PI3K-Akt and epithelial-mesenchymal transition (EMT) cascades. Indeed, PI3K-Akt cascade and expression of EMT-promoting proteins were decreased after ESM1 silencing in cervical cancer cells, but increased following ESM1 overexpression. Further studies demonstrated that SYT13 (synaptotagmin 13) could be a primary target gene of ESM1. SYT13 silencing potently inhibited ESM1-overexpression-induced PI3K-Akt cascade activation and cervical cancer cell migration/invasion. In vivo, ESM1 knockout hindered SiHa cervical cancer xenograft growth in mice. In ESM1-knockout xenografts tissues, PI3K-Akt inhibition, EMT-promoting proteins downregulation and apoptosis activation were detected. In conclusion, overexpressed ESM1 is important for cervical cancer growth in vitro and in vivo, possibly by promoting PI3K-Akt activation and EMT progression. ESM1 represents as a promising diagnostic marker and potential therapeutic target of cervical cancer.

Potential roles of synaptotagmin family members in cancers: Recent advances and prospects

With the continuous development of bioinformatics and public database, more and more genes that play a role in cancers have been discovered. Synaptotagmins (SYTs) are abundant, evolutionarily conserved integral membrane proteins composed of a short N-terminus, a variable linker domain, a single transmembrane domain, and two C2 domains, and they constitute a family of 17 isoforms. The synaptotagmin family members are known to regulate calcium-dependent membrane fusion events. Some SYTs play roles in hormone secretion or neurotransmitter release or both, and much evidence supports SYTs as Ca²⁺ sensors of exocytosis. Since 5 years ago, an increasing number of studies have found that SYTs also played important roles in the occurrence and development of lung cancer, gastric cancer, colon cancer, and other cancers. Down-regulation of SYTs inhibited cell proliferation, migration, and invasion of cancer cells, but promoted cell apoptosis. Growth of peritoneal nodules is inhibited and survival is prolonged in mice administrated with siSYTs intraperitoneally. Therefore, most studies have found SYTs serve as an oncogene after overexpression and may become potential prognostic biomarkers for multiple cancers. This article provides an overview of recent studies that focus on SYT family members’ roles in cancers and highlights the advances that have been achieved.

Synaptotagmin-13 orchestrates pancreatic endocrine cell egression and islet morphogenesis

During pancreas development endocrine cells leave the ductal epithelium to form the islets of Langerhans, but the morphogenetic mechanisms are incompletely understood. Here, we identify the Ca2+-independent atypical Synaptotagmin-13 (Syt13) as a key regulator of endocrine cell egression and islet formation. We detect specific upregulation of the Syt13 gene and encoded protein in endocrine precursors and the respective lineage during islet formation. The Syt13 protein is localized to the apical membrane of endocrine precursors and to the front domain of egressing endocrine cells, marking a previously unidentified apical-basal to front-rear repolarization during endocrine precursor cell egression. Knockout of Syt13 impairs endocrine cell egression and skews the α-to-β-cell ratio. Mechanistically, Syt13 is a vesicle trafficking protein, transported via the microtubule cytoskeleton, and interacts with phosphatidylinositol phospholipids for polarized localization. By internalizing a subset of plasma membrane proteins at the front domain, including α6β4 integrins, Syt13 modulates cell-matrix adhesion and allows efficient endocrine cell egression. Altogether, these findings uncover an unexpected role for Syt13 as a morphogenetic driver of endocrinogenesis and islet formation.

Synaptotagmin-13 Is a Neuroendocrine Marker in Brain, Intestine and Pancreas

Synaptotagmin-13 (Syt13) is an atypical member of the vesicle trafficking synaptotagmin protein family. The expression pattern and the biological function of this Ca²⁺-independent protein are not well resolved. Here, we have generated a novel Syt13-Venus fusion (Syt13-VF) fluorescence reporter allele to track and isolate tissues and cells expressing Syt13 protein. The reporter allele is regulated by endogenous cis-regulatory elements of Syt13 and the fusion protein follows an identical expression pattern of the endogenous Syt13 protein. The homozygous reporter mice are viable and fertile. We identify the expression of the Syt13-VF reporter in different regions of the brain with high expression in tyrosine hydroxylase (TH)-expressing and oxytocin-producing neuroendocrine cells. Moreover, Syt13-VF is highly restricted to all enteroendocrine cells in the adult intestine that can be traced in live imaging. Finally, Syt13-VF protein is expressed in the pancreatic endocrine lineage, allowing their specific isolation by flow sorting. These findings demonstrate high expression levels of Syt13 in the endocrine lineages in three major organs harboring these secretory cells. Collectively, the Syt13-VF reporter mouse line provides a unique and reliable tool to dissect the spatio-temporal expression pattern of Syt13 and enables isolation of Syt13-expressing cells that will aid in deciphering the molecular functions of this protein in the neuroendocrine system.

Amido-Bridged Nucleic Acid-Modified Antisense Oligonucleotides Targeting SYT13 to Treat Peritoneal Metastasis of Gastric Cancer

Patients with peritoneal metastasis of gastric cancer have dismal prognosis, mainly because of inefficient systemic delivery of drugs to peritoneal tumors. We aimed to develop an intraperitoneal treatment strategy using amido-bridged nucleic acid (AmNA)-modified antisense oligonucleotides (ASOs) targeting synaptotagmin XIII (SYT13) and to identify the function of SYT13 in gastric cancer cells. We screened 71 candidate oligonucleotide sequences according to SYT13-knockdown efficacy, in vitro activity, and off-target effects. We evaluated the effects of SYT13 knockdown on cellular functions and signaling pathways, as well as the effects of intraperitoneal administration to mice of AmNA-modified anti-SYT13 ASOs. We selected the ASOs (designated hSYT13-4378 and hSYT13-4733) with the highest knockdown efficiencies and lowest off-target effects and determined their abilities to inhibit cellular functions associated with the metastatic potential of gastric cancer cells. We found that SYT13 interfered with focal adhesion kinase (FAK)-mediated intracellular signals. Intraperitoneal administration of hSYT13-4378 and hSYT13-4733 in a mouse xenograft model of metastasis inhibited the formation of peritoneal nodules and significantly increased survival. Reversible, dose- and sequence-dependent liver damage was induced by ASO treatment without causing abnormal morphological and histological changes in the brain. Intra-abdominal administration of AmNA-modified anti-SYT13 ASOs represents a promising strategy for treating peritoneal metastasis of gastric cancer.

Silencing of synaptotagmin 13 inhibits tumor growth through suppressing proliferation and promoting apoptosis of colorectal cancer cells

The treatment of colorectal cancer is currently hampered by the lack of early detection technology. The identification of molecular biomarkers for colorectal cancer is crucial for improving prognosis. Synaptotagmin (SYT) 13 has been reported to be associated with several human tumors, but its role in colorectal cancer remains elusive. In the present study, immunohistochemistry was utilized to detect the expression of SYT13 in colorectal cancer tissues and cells. MTT, colony formation, wound healing and Transwell assays were conducted to evaluate the effect of SYT13 knockdown on the biological behavior of RKO and HCT116 cells. Cell apoptosis and cell cycle profiles were detected by FACS. A mouse xenograft model was constructed to investigate the effect of SYT13 on colorectal cancer in vivo. The results indicated that SYT13 was upregulated in colorectal tumor tissues compared with paracancerous tissues. Silencing of SYT13 inhibited the proliferation, colony formation, migration and invasion ability of RKO and HCT116 cells. Moreover, SYT13 knockdown arrested the cell cycle in the G2 phase, thus inducing cell apoptosis. The in vivo experiments also demonstrated the inhibitory effect of SYT13 on tumor growth. In conclusion, the present study demonstrated that SYT13 may act as a promoter in the development and progression of colorectal cancer and, therefore, may be of value as a target for the development of novel treatment strategies.

The levels of SYT13 and CEA mRNAs in peritoneal lavages predict the peritoneal recurrence of gastric cancer

BACKGROUND

Although peritoneal lavage cytology often serves as a sensitive method to detect free cancer cells in the abdominal cavity, some patients experience peritoneal recurrence despite negative cytology. The aim of this study was to evaluate mRNAs in peritoneal lavage fluid as potential markers for predicting the peritoneal recurrence of gastric cancer (GC).

METHODS

Peritoneal lavage fluid samples were obtained during surgery conducted on 187 patients with GC and from 30 patients with non-malignant disease (controls). The mRNA levels of nine candidate markers were quantified, and analysis of a receiver-operating characteristic curve compared their accuracies. The cutoff was defined as the highest value of the controls.

RESULTS

Synaptotagmin XIII (SYT13) and carcinoembryonic antigen (CEA) mRNA levels were analyzed further. SYT13 levels were significantly associated with shorter peritoneal recurrence-free survival (PRFS) and overall survival. Among patients with negative peritoneal lavage cytology, those positive for either SYT13 or CEA mRNA experienced significantly shorter peritoneal recurrence-free survival compared with those with negative fluid (hazards ratio [HR] 4.21, P = 0.0114; HR 3.53; P = 0.0426, respectively). Univariate analysis revealed that SYT13 and CEA mRNA levels were significant predictors of peritoneal recurrence. Positive levels of both SYT13 and CEA mRNA demonstrated the highest HR for peritoneal recurrence (HR 12.27, P = 0.0064). Multivariable analysis revealed that SYT13 positivity was a significant independent prognostic factor (HR 3.69; 95% confidence interval, 1.18-12.74; P = 0.0246).

CONCLUSIONS

Combined measurement of SYT13 and CEA mRNA levels in peritoneal lavage fluid could serve as a promising approach to predict peritoneal recurrence of GC.

Synaptotagmin XIII expression and peritoneal metastasis in gastric cancer

BACKGROUND

Peritoneal metastasis is a frequent cause of death in patients with gastric cancer. The aim of this study was to identify molecules responsible for mediating peritoneal metastasis of gastric cancer.

METHODS

Transcriptome and bioinformatics analyses were conducted to identify molecules associated with peritoneal metastasis. The therapeutic effects of intraperitoneally administered small interfering (si) RNA were evaluated using mouse xenograft models. Expression of mRNA and protein was determined in gastric tissues from patients with gastric cancer.

RESULTS

Synaptotagmin XIII (SYT13) was expressed at significantly higher levels in patients with peritoneal recurrence, but not in those with hepatic or distant lymph node recurrence. Inhibition of SYT13 expression in a gastric cancer cell line transfected with SYT13-specific siRNA (siSYT13) was associated with decreased invasion and migration ability of the cells, but not with proliferation and apoptosis. Intraperitoneal administration of siSYT13 significantly inhibited the growth of peritoneal nodules and prolonged survival in mice. In an analysis of 200 patients with gastric cancer, SYT13 expression in primary gastric cancer tissues was significantly greater in patients with peritoneal recurrence or metastasis. A high level of SYT13 expression in primary gastric cancer tissues was an independent risk factor for peritoneal recurrence.

CONCLUSION

SYT13 expression in gastric cancer is associated with perioneal metatases and is a potential target for treatment.

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.

Synaptotagmin 11 interacts with components of the RNA-induced silencing complex RISC in clonal pancreatic β-cells

Synaptotagmins are two C2 domain-containing transmembrane proteins. The function of calcium-sensitive members in the regulation of post-Golgi traffic has been well established whereas little is known about the calcium-insensitive isoforms constituting half of the protein family. Novel binding partners of synaptotagmin 11 were identified in β-cells. A number of them had been assigned previously to ER/Golgi derived-vesicles or linked to RNA synthesis, translation and processing. Whereas the C2A domain interacted with the Q-SNARE Vti1a, the C2B domain of syt11 interacted with the SND1, Ago2 and FMRP, components of the RNA-induced silencing complex (RISC). Binding to SND was direct via its N-terminal tandem repeats. Our data indicate that syt11 may provide a link between gene regulation by microRNAs and membrane traffic.

Acyl-chain correlation in membrane fusion intermediates: x-ray diffraction from the rhombohedral lipid phase

We have studied the acyl-chain conformation in stalk phases of model membranes by x-ray diffraction from oriented samples. As an equilibrium lipid phase induced by dehydration, the stalk or rhombohedral phase exhibits lipidic passages (stalks) between adjacent bilayers, representing a presumed intermediate state in membrane fusion. From the detailed analysis of the acyl-chain correlation peak, we deduce the structural parameters of the acyl-chain fluid above, at, and below the transition from the lamellar to rhombohedral state, at the molecular level.

Altered expression of synaptotagmin 13 mRNA in adult mouse brain after contextual fear conditioning

Contextual fear memory processing requires coordinated changes in neuronal activity and molecular networks within brain. A large number of fear memory-related genes, however, still remain to be identified. Synaptotagmin 13 (Syt13), an atypical member of synaptotagmin family, is highly expressed in brain, but its functional roles within brain have not yet been clarified. Here, we report that the expression of Syt13 mRNA in adult mouse brain was altered following contextual fear conditioning. C57BL/6 mice were exposed to a novel context and stimulated by strong electrical footshock according to a contextual fear conditioning protocol. After 24 h, the mice were re-exposed to the context without electrical footshock for the retrieval of contextual fear memory. To investigate the relationship between Syt13 and contextual fear memory, we carried out in situ hybridization and analyzed gene expression patterns for Syt13 at four groups representing temporal changes in brain activity during contextual fear memory formation. Contextual fear conditioning test induced significant changes in mRNA levels for Syt13 within various brain regions, including lateral amygdala, somatosensory cortex, piriform cortex, habenula, thalamus, and hypothalamus, during both acquisition and retrieval sessions. Our data suggest that Syt13 may be involved in the process of contextual fear memory.

Measuring Ca2+-induced structural changes in lipid monolayers: implications for synaptic vesicle exocytosis

Synaptic vesicles (SVs) are small, membrane-bound organelles that are found in the synaptic terminal of neurons. Although tremendous progress has been made in understanding the protein machinery that drives fusion of SVs with the presynaptic membrane, little progress has been made in understanding changes in the membrane structure that accompany this process. We used lipid monolayers of defined composition to mimic biological membranes, which were probed by x-ray reflectivity and grazing incidence x-ray diffraction. These techniques allowed us to successfully monitor structural changes in the membranes at molecular level, both in response to injection of SVs in the subphase below the monolayer, as well as to physiological cues involved in neurotransmitter release, such as increases in the concentration of the membrane lipid PIP(2), or addition of physiological levels of Ca(2+). Such structural changes may well modulate vesicle fusion in vivo.

Transcriptional inhibition of progressive renal disease by gene silencing pyrrole-imidazole polyamide targeting of the transforming growth factor-β1 promoter

Pyrrole-imidazole (PI) polyamides are small synthetic molecules that recognize and attach to the minor groove of DNA, thereby inhibiting gene transcription by blocking transcription factor binding. These derivatives can act as gene silencers inhibiting target gene expression under stimulatory conditions such as disease. To evaluate PI polyamides as treatments for the progression of renal diseases, we examined morphological effects, pharmacological properties, and the specificity of PI polyamides targeted to the transforming growth factor (TGF)-β1 promoter during salt-induced hypertensive nephrosclerosis in Dahl salt-sensitive rats. The targeted PI polyamide markedly reduced glomerulosclerosis and interstitial fibrosis without side effects. PI polyamide significantly decreased expression of TGF-β1 and extracellular matrix in the renal cortex. Microarray analysis found that only 3% of the transcripts were affected by PI polyamide, but this included decreased expression of extracellular matrix, TGF-β1-related cytokines, angiogenic, and cell stabilizing factors, proteinases, and renal injury-related factors. Thus, targeted PI polyamides are potential gene silencers for diseases not treatable by current remedies.

Calcium-sensing beyond neurotransmitters: functions of synaptotagmins in neuroendocrine and endocrine secretion

Neurotransmitters, neuropeptides and hormones are released through the regulated exocytosis of SVs (synaptic vesicles) and LDCVs (large dense-core vesicles), a process that is controlled by calcium. Synaptotagmins are a family of type 1 membrane proteins that share a common domain structure. Most synaptotagmins are located in brain and endocrine cells, and some of these synaptotagmins bind to phospholipids and calcium at levels that trigger regulated exocytosis of SVs and LDCVs. This led to the proposed synaptotagmin-calcium-sensor paradigm, that is, members of the synaptotagmin family function as calcium sensors for the regulated exocytosis of neurotransmitters, neuropeptides and hormones. Here, we provide an overview of the synaptotagmin family, and review the recent mouse genetic studies aimed at understanding the functions of synaptotagmins in neurotransmission and endocrine-hormone secretion. Also, we discuss potential roles of synaptotagmins in non-traditional endocrine systems.

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.

High metal concentrations are required for self-association of synaptotagmin II

Several members of the synaptotagmin (syt) family of vesicle proteins have been proposed to act as Ca2+ sensors on synaptic vesicles. The mechanism by which calcium activates this class of proteins has been the subject of controversy, yet relatively few detailed biophysical studies have been reported on how isoforms other than syt I respond to divalent metal ions. Here, we report a series of studies on the response of syt II to a wide range of metal ions. Analytical ultracentrifugation studies demonstrate that Ca2+ induces protein dimerization upon exposure to 5 mM Ca2+. Whereas Ba2+, Mg2+, or Sr2+ do not potentiate self-association as strongly as Ca2+, Pb2+ triggers self-association of syt II at concentrations as low as 10 microM. Partial proteolysis studies suggest that the various divalent metals cause different changes in the conformation of the protein. The high calcium concentrations required for self-association of syt II suggest that the oligomerized state of this protein is not a critical intermediate in vesicle fusion; however, low-affinity calcium sites on syt II may play a critical role in buffering calcium at the presynaptic active zone. In addition, the high propensity of lead to oligomerize syt II offers a possible molecular explanation for how lead interferes with calcium-evoked neurotransmitter release.

Molecular profiling reveals synaptic release machinery in Merkel cells

Merkel cell-neurite complexes are somatosensory receptors that initiate the perception of gentle touch. The role of epidermal Merkel cells within these complexes is disputed. To ask whether Merkel cells are genetically programmed to be excitable cells that may participate in touch reception, we purified Merkel cells from touch domes and used DNA microarrays to compare gene expression in Merkel cells and other epidermal cells. We identified 362 Merkel-cell-enriched transcripts, including neuronal transcription factors, presynaptic molecules, and ion-channel subunits. Antibody staining of skin sections showed that Merkel cells are immunoreactive for presynaptic proteins, including piccolo, Rab3C, vesicular glutamate transporter 2, and cholecystokinin 26-33. These data indicate that Merkel cells are poised to release glutamate and neuropeptides. Finally, by using Ca(2+) imaging, we discovered that Merkel cells have L- and P/Q-type voltage-gated Ca(2+) channels, which have been shown to trigger vesicle release at synapses. These results demonstrate that Merkel cells are excitable cells and suggest that they release neurotransmitters to shape touch sensitivity.

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.

Discrete gene sets depend on POU domain transcription factor Brn3b/Brn-3.2/POU4f2 for their expression in the mouse embryonic retina

Brn3b/Brn-3.2/POU4f2 is a POU domain transcription factor that is essential for retinal ganglion cell (RGC) differentiation, axonal outgrowth and survival. Our goal was to establish a link between Brn3b and the downstream events leading to RGC differentiation. We sought to determine both the number and types of genes that depend on Brn3b for their expression. RNA probes from wild-type and Brn3b(-/-) E14.5, E16.5 and E18.5 mouse retinas were hybridized to a microarray containing 18,816 retina-expressed cDNAs. At E14.5, we identified 87 genes whose expression was significantly altered in the absence of Brn3b and verified the results by real-time PCR and in situ hybridization. These genes fell into discrete sets that encoded transcription factors, proteins associated with neuron integrity and function, and secreted signaling molecules. We found that Brn3b influenced gene expression in non RGCs of the retina by controlling the expression of secreted signaling molecules such as sonic hedgehog and myostatin/Gdf8. At later developmental stages, additional alterations in gene expression were secondary consequences of aberrant RGC differentiation caused by the absence of Brn3b. Our results demonstrate that a small but crucial fraction of the RGC transcriptome is dependent on Brn3b. The Brn3b-dependent gene sets therefore provide a unique molecular signature for the developing retina.

Presynaptic frequency- and pattern-dependent filtering

Dual intracellular recordings from pairs of synaptically connected neurones have demonstrated that the frequency-dependent pattern of transmitter release varies dramatically between different classes of connections. Somewhat surprisingly, these patterns are not determined by the class of neurone supplying the axon alone, but to a large degree by the class of postsynaptic neurone. A wide range of presynaptic mechanisms, some that depress the release of transmitter and others that enhance release have been identified. It is the selective expression of these different mechanisms that determines the unique frequency- and pattern-dependent properties of each class of connection. Although the molecular interactions underlying these several mechanisms have yet to be fully identified, the wealth and complexity of the protein-protein and protein-lipid interactions that have been shown to control the release of transmitter suggest many ways in which the properties of a synapse may be tuned to respond to particular patterns and frequencies.

Molecular characterization of prostatic small-cell neuroendocrine carcinoma

OBJECTIVES

A subset of prostate carcinomas is composed predominantly, even exclusively, of neuroendocrine (NE) cells. In this report, we sought to characterize the gene expression profile of a prostate small cell NE carcinoma by assessing the diversity and abundance of transcripts in the LuCaP 49 prostate small cell carcinoma xenograft.

METHODS

We constructed a cDNA library (PRCA3) from the LuCap 49 prostate small cell xenograft. Single pass DNA sequencing of randomly selected cDNA clones followed by sequence assembly and annotation produced a library of Expressed Sequence Tags (ESTs) representing the LuCaP 49 transcriptome. Comparative sequence analysis with ESTs derived from prostate adenocarcinoma libraries was performed using statistical algorithms designed to identify differentially expressed sequences. Putative NE cell-specific genes were further examined by Northern analysis.

RESULTS

Sequence assembly and analysis identified 1,447 distinct genes expressed in the LuCaP 49 cDNA library. These include cDNAs encoding the NE markers secretogranin (SCG2), CD24, and ENO2. Northern analysis revealed that three additional genes, ASCL1, INA, and SV2B are expressed in LuCaP 49 but not in various prostate cancer cell lines or xenografts. Fifteen genes were identified with a statistical probability (P > 0.9) of being up-regulated in LuCaP 49 small cell carcinoma relative to prostate adenocarcinoma (two primary prostate adenocarcinomas and the LNCaP prostate adenocarcinoma cell line).

CONCLUSIONS

Prostate small cell carcinoma expresses a diverse repertoire of genes that reflect characteristics of their NE cell of origin. ASCL1, INA, and SV2B are potential molecular markers for small cell NE tumors and NE cells of the prostate. This small cell NE carcinoma gene expression profile may yield insights into the development, progression, and treatment of subtypes of prostate cancer.

Vesicle-associated membrane protein-2/synaptobrevin binding to synaptotagmin I promotes O-glycosylation of synaptotagmin I

Synaptotagmin I (Syt I), an evolutionarily conserved integral membrane protein of synaptic vesicles, is now known to regulate Ca2+-dependent neurotransmitter release. Syt I protein should undergo several post-translational modifications before maturation and subsequent functioning on synaptic vesicles (e.g. N-glycosylation and fatty acylation in vertebrate Syt I), because the apparent molecular weight of Syt I on synaptic vesicles (mature form, 65,000) was much higher than the calculated molecular weight (47,400) predicted from the cDNA sequences both in vertebrates and invertebrates. Common post-translational modification(s) of Syt I conserved across phylogeny, however, have never been elucidated. In the present study, I discovered that dithreonine residues (Thr-15 and Thr-16) at the intravesicular domain of mouse Syt I are post-translationally modified by a complex form of O-linked sugar (i.e. the addition of sialic acids) in PC12 cells and that the O-glycosylation of Syt I in COS-7 cells depends on the coexpression of vesicle-associated membrane protein-2 (VAMP-2)/synaptobrevin. I also showed that a transmembrane domain of Syt I directly interacts with isolated VAMP-2, but not VAMP-2, in the heterotrimeric SNARE (SNAP receptor) complex (vesicle SNARE, VAMP-2, and two target SNAREs, syntaxin IA and SNAP-25). Since di-Thr or di-Ser residues are often found at the intravesicular domain of invertebrate Syt I, and VAMP-dependent O-glycosylation was also observed in squid Syt expressed in COS-7 cells, I propose that VAMP-dependent O-glycosylation of Syt I is a common modification during evolution and may have important role(s) in synaptic vesicle trafficking.

Synaptotagmin I-DeltaC2B. A novel synaptotagmin isoform with a single C2 domain in the bovine adrenal medulla

Synaptotagmin I is a 65 kDa type 1 membrane glycoprotein found in secretory organelles that plays a key role in regulated exocytosis. We have characterised two forms (long and short) of synaptotagmin I that are present in the bovine adrenal medulla. The long form is a type I integral membrane protein which has two cytoplasmic C2 domains and corresponds to the previously characterised full-length synaptotagmin I isoform. The short-form synaptotagmin I-DeltaC2B has the same structure in the lumenal and transmembrane sequences, but synaptotagmin I-DeltaC2B is truncated such that it only has a single cytoplasmic C2 domain. Analysis of synaptotagmin I-DeltaC2B expression indicates that synaptotagmin I-DeltaC2B is preferentially expressed in the bovine adrenal medulla. However, it is absent from the dense core chromaffin granules. Furthermore, when expressed in the rat pheochromocytoma cell line PC12 bovine synaptotagmin I-DeltaC2B is largely absent from dense core granules and synaptic-like microvesicles. Instead, indirect immunofluorescence microscopy reveals the intracellular location of synaptotagmin I-DeltaC2B to be the plasma membrane.

Mechanism of the SDS-resistant synaptotagmin clustering mediated by the cysteine cluster at the interface between the transmembrane and spacer domains

Synaptotagmin I (Syt I), a proposed major Ca(2+) sensor in the central nervous system, has been hypothesized as functioning in an oligomerized state during neurotransmitter release. We previously showed that Syts I, II, VII, and VIII form a stable SDS-resistant, beta-mercaptoethanol-insensitive, and Ca(2+)-independent oligomer surrounding the transmembrane domain (Fukuda, M., and Mikoshiba, K. (2000) J. Biol. Chem. 275, 28180-28185), but little is known about the molecular mechanism of the Ca(2+)-independent oligomerization by the synaptotagmin family. In this study, we analyzed the Ca(2+)-independent oligomerization properties of Syt I and found that it shows two distinct forms of self-oligomerization activity: stable SDS-resistant self-oligomerization activity and relatively unstable SDS-sensitive self-oligomerization activity. The former was found to be mediated by a post-translationally modified (i.e. fatty-acylated) cysteine (Cys) cluster (Cys-74, Cys-75, Cys-77, Cys-79, and Cys-82) at the interface between the transmembrane and spacer domains of Syt I. We also show that the number of Cys residues at the interface between the transmembrane and spacer domains determines the SDS- resistant oligomerizing capacity of each synaptotagmin isoform: Syt II, which contains seven Cys residues, showed the strongest SDS-resistant oligomerizing activity in the synaptotagmin family, whereas Syt XII, which has no Cys residues, did not form any SDS-resistant oligomers. The latter SDS-sensitive self-oligomerization of Syt I is mediated by the spacer domain, because deletion of the whole spacer domain, including the Cys cluster, abolished it, whereas a Syt I(CA) mutant carrying Cys to Ala substitutions still exhibited self-oligomerization. Based on these results, we propose that the oligomerization of the synaptotagmin family is regulated by two distinct mechanisms: the stable SDS-resistant oligomerization is mediated by the modified Cys cluster, whereas the relatively unstable (SDS-sensitive) oligomerization is mediated by the environment of the spacer domain.

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).