Synaptotagmin I and IX function redundantly in controlling fusion pore of large dense core vesicles

A new GRAB sensor reveals differences in the dynamics and molecular regulation between neuropeptide and neurotransmitter release

The co-existence and co-transmission of neuropeptides and small molecule neurotransmitters in the same neuron is a fundamental aspect of almost all neurons across various species. However, the differences regarding their in vivo spatiotemporal dynamics and underlying molecular regulation remain poorly understood. Here, we developed a GPCR-activation-based (GRAB) sensor for detecting short neuropeptide F (sNPF) with high sensitivity and spatiotemporal resolution. Furthermore, we explore the differences of in vivo dynamics and molecular regulation between sNPF and acetylcholine (ACh) from the same neurons. Interestingly, the release of sNPF and ACh shows different spatiotemporal dynamics. Notably, we found that distinct synaptotagmins (Syt) are involved in these two processes, as Syt7 and Sytα for sNPF release, while Syt1 for ACh release. Thus, this new GRAB sensor provides a powerful tool for studying neuropeptide release and providing new insights into the distinct release dynamics and molecular regulation between neuropeptides and small molecule neurotransmitters.

Current and emerging methods for probing neuropeptide transmission

Neuropeptides comprise the most diverse category of neurochemicals in the brain, playing critical roles in a wide range of physiological and pathophysiological processes. Monitoring neuropeptides with high spatial and temporal resolution is essential for understanding how peptidergic transmission is regulated throughout the central nervous system. In this review, we provide an overview of current non-optical and optical approaches used to detect neuropeptides, including their design principles, intrinsic properties, and potential limitations. We also highlight the advantages of using G protein‒coupled receptor (GPCR) activation‒based (GRAB) sensors to monitor neuropeptides in vivo with high sensitivity, good specificity, and high spatiotemporal resolution. Finally, we present a promising outlook regarding the development and optimization of new GRAB neuropeptide sensors, as well as their potential applications.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

Imaging neuropeptide release at synapses with a genetically engineered reporter

Research on neuropeptide function has advanced rapidly, yet there is still no spatio-temporally resolved method to measure the release of neuropeptides in vivo. Here we introduce Neuropeptide Release Reporters (NPRRs): novel genetically-encoded sensors with high temporal resolution and genetic specificity. Using the Drosophila larval neuromuscular junction (NMJ) as a model, we provide evidence that NPRRs recapitulate the trafficking and packaging of native neuropeptides, and report stimulation-evoked neuropeptide release events as real-time changes in fluorescence intensity, with sub-second temporal resolution.

The fusion pore, 60 years after the first cartoon

Neurotransmitter release occurs in the form of quantal events by fusion of secretory vesicles with the plasma membrane, and begins with the formation of a fusion pore that has a conductance similar to that of a large ion channel or gap junction. In this review, we propose mechanisms of fusion pore formation and discuss their implications for fusion pore structure and function. Accumulating evidence indicates a direct role of soluble N-ethylmaleimide-sensitive-factor attachment receptor proteins in the opening of fusion pores. Fusion pores are likely neither protein channels nor purely lipid, but are of proteolipidic composition. Future perspectives to gain better insight into the molecular structure of fusion pores are discussed.

Confocal Imaging of Neuropeptide Y-pHluorin: A Technique to Visualize Insulin Granule Exocytosis in Intact Murine and Human Islets

Insulin secretion plays a central role in glucose homeostasis under normal physiological conditions as well as in disease. Current approaches to study insulin granule exocytosis either use electrophysiology or microscopy coupled to the expression of fluorescent reporters. However most of these techniques have been optimized for clonal cell lines or require dissociating pancreatic islets. In contrast, the method presented here allows for real time visualization of insulin granule exocytosis in intact pancreatic islets. In this protocol, we first describe the viral infection of isolated pancreatic islets with adenovirus that encodes a pH-sensitive green fluorescent protein (GFP), pHluorin, coupled to neuropeptide Y (NPY). Second, we describe the confocal imaging of islets five days after viral infection and how to monitor the insulin granule secretion. Briefly, the infected islets are placed on a coverslip on an imaging chamber and imaged under an upright laser-scanning confocal microscope while being continuously perfused with extracellular solution containing various stimuli. Confocal images spanning 50 µm of the islet are acquired as time-lapse recordings using a fast-resonant scanner. The fusion of insulin granules with the plasma membrane can be followed over time. This procedure also allows for testing a battery of stimuli in a single experiment, is compatible with both mouse and human islets, and can be combined with various dyes for functional imaging (e.g., membrane potential or cytosolic calcium dyes).

A genetic variant in a homocysteine metabolic gene that increases the risk of congenital cardiac septal defects in Han Chinese populations

Elevated homocysteine levels are known to be a risk factor for congenital cardiac septal defects (CCSDs), but the mechanism underlying this effect is unknown. The genetic variants that were significantly associated with circulating homocysteine concentrations have been systematically identified through the genome-wide association studies of one-carbon core metabolites. To examine the role of the genome-wide significant homocysteine related variants in the occurrence of CCSDs, we investigated the association between these variants and CCSDs in Han Chinese populations. Five variants of the genome-wide significant homocysteine-related genes were selected for analysis in two stages of case-controlled studies with a total of 904 CCSD patients and 997 controls. SYT9 expression was detected in human cardiovascular tissue using qRT-PCR. The intronic variant rs11041321 of the SYT9 gene was associated with an increased risk of developing CCSDs in both the separate and combined case-controlled studies. Combined samples from the two stage cohorts had a significant elevation in CCSD risk for the T allele (OR = 1.43, P = 2.6 × 10^-6 ), CT genotype and TT genotype (CT: OR = 1.30, TT: OR = 2.21; P = 1 × 10^-4 ) compared with the wild-type C allele and CC genotype, respectively. The risky T allele carriers exhibited decreased SYT9 mRNA expression, compared with wild-type C allele carriers. The intronic SYT9 variant rs11041321, which exhibits a significant genome-wide association with circulating homocysteine, was associated with the occurrence of CCSDs. This finding helps to characterize the unexpected role of SYT9 in homocysteine metabolism and the development of CCSDs, which further highlighted the interplay of diet, genetics, and human birth defects. © 2017 IUBMB Life, 69(9):700-705, 2017.

Pharmacological Bypass of Cockayne Syndrome B Function in Neuronal Differentiation

Cockayne syndrome (CS) is a severe neurodevelopmental disorder characterized by growth abnormalities, premature aging, and photosensitivity. Mutation of Cockayne syndrome B (CSB) affects neuronal gene expression and differentiation, so we attempted to bypass its function by expressing downstream target genes. Intriguingly, ectopic expression of Synaptotagmin 9 (SYT9), a key component of the machinery controlling neurotrophin release, bypasses the need for CSB in neuritogenesis. Importantly, brain-derived neurotrophic factor (BDNF), a neurotrophin implicated in neuronal differentiation and synaptic modulation, and pharmacological mimics such as 7,8-dihydroxyflavone and amitriptyline can compensate for CSB deficiency in cell models of neuronal differentiation as well. SYT9 and BDNF are downregulated in CS patient brain tissue, further indicating that sub-optimal neurotrophin signaling underlies neurological defects in CS. In addition to shedding light on cellular mechanisms underlying CS and pointing to future avenues for pharmacological intervention, these data suggest an important role for SYT9 in neuronal differentiation.

Spatiotemporal detection and analysis of exocytosis reveal fusion "hotspots" organized by the cytoskeleton in endocrine cells

Total internal reflection fluorescence microscope has often been used to study the molecular mechanisms underlying vesicle exocytosis. However, the spatial occurrence of the fusion events within a single cell is not frequently explored due to the lack of sensitive and accurate computer-assisted programs to analyze large image data sets. Here, we have developed an image analysis platform for the nonbiased identification of different types of vesicle fusion events with high accuracy in different cell types. By performing spatiotemporal analysis of stimulus-evoked exocytosis in insulin-secreting INS-1 cells, we statistically prove that individual vesicle fusion events are clustered at hotspots. This spatial pattern disappears upon the disruption of either the actin or the microtubule network; this disruption also severely inhibits evoked exocytosis. By demonstrating that newcomer vesicles are delivered from the cell interior to the surface membrane for exocytosis, we highlight a previously unappreciated mechanism in which the cytoskeleton-dependent transportation of secretory vesicles organizes exocytosis hotspots in endocrine cells.

Chaperoning of closed syntaxin-3 through Lys46 and Glu59 in domain 1 of Munc18 proteins is indispensable for mast cell exocytosis

Understanding how Munc18 proteins govern exocytosis is crucial because mutations of this protein cause severe secretion deficits in neuronal and immune cells. Munc18-2 has indispensable roles in the degranulation of mast cell, partly by binding and chaperoning a subset of syntaxin isoforms. However, the key syntaxin that, crucially, participates in the degranulation – whose levels and intracellular localization are regulated by Munc18-2 – remains unknown. Here, we demonstrate that double knockdown of Munc18-1 and Munc-2 in mast cells results in greatly reduced degranulation accompanied with strikingly compromised expression levels and localization of syntaxin-3. This phenotype is fully rescued by wild-type Munc18 proteins but not by the K46E, E59K and K46E/E59K mutants of Munc-18 domain 1, each of which exhibits completely abolished binding to 'closed' syntaxin-3. Furthermore, knockdown of syntaxin-3 strongly impairs degranulation. Collectively, our data argue that residues Lys46 and Glu59 of Munc18 proteins are indispensable for mediating the interaction between Munc18 and closed syntaxin-3, which is essential for degranulation by chaperoning syntaxin-3. Our results also indicate that the functional contribution of these residues differs between immune cell degranulation and neuronal secretion.

Phosphomimetic mutation of cysteine string protein-α increases the rate of regulated exocytosis by modulating fusion pore dynamics in PC12 cells

Background

Cysteine string protein-α (CSPα) is a chaperone to ensure protein folding. Loss of CSPα function associates with many neurological diseases. However, its function in modulating regulated exocytosis remains elusive. Although cspα-knockouts exhibit impaired synaptic transmission, overexpression of CSPα in neuroendocrine cells inhibits secretion. These seemingly conflicting results lead to a hypothesis that CSPα may undergo a modification that switches its function in regulating neurotransmitter and hormone secretion. Previous studies implied that CSPα undergoes phosphorylation at Ser¹⁰ that may influence exocytosis by altering fusion pore dynamics. However, direct evidence is missing up to date.

Methodology/Principal Findings

Using amperometry, we investigated how phosphorylation at Ser¹⁰ of CSPα (CSPα-Ser¹⁰) modulates regulated exocytosis and if this modulation involves regulating a specific kinetic step of fusion pore dynamics. The real-time exocytosis of single vesicles was detected in PC12 cells overexpressing control vector, wild-type CSPα (WT), the CSPα phosphodeficient mutant (S10A), or the CSPα phosphomimetic mutants (S10D and S10E). The shapes of amperometric signals were used to distinguish the full-fusion events (i.e., prespike feet followed by spikes) and the kiss-and-run events (i.e., square-shaped flickers). We found that the secretion rate was significantly increased in cells overexpressing S10D or S10E compared to WT or S10A. Further analysis showed that overexpression of S10D or S10E prolonged fusion pore lifetime compared to WT or S10A. The fraction of kiss-and-run events was significantly lower but the frequency of full-fusion events was higher in cells overexpressing S10D or S10E compared to WT or S10A. Advanced kinetic analysis suggests that overexpression of S10D or S10E may stabilize open fusion pores mainly by inhibiting them from closing.

Conclusions/Significance

CSPα may modulate fusion pore dynamics in a phosphorylation-dependent manner. Therefore, through changing its phosphorylated state influenced by diverse cellular signalings, CSPα may have a great capacity to modulate the rate of regulated exocytosis.

MicroRNAs: Novel Mechanism Involved in the Pathogenesis of Microwave Exposure on Rats' Hippocampus

Microwave-induced adverse health outcomes have been gaining much attention in recent years. The hippocampus is sensitive and vulnerable to microwave exposure. Studies from our group and others showed that microwave-induced structural and functional injury of hippocampus, accompanied with alteration of gene and protein expression. It has been demonstrated that microRNAs (miRNAs) were involved in the physiological and pathological processes of brain. In this study, the miRNAs expression profiles of microwave-exposed hippocampus were detected by microarray analysis and verified by real-time polymerase chain reaction (PCR). At 7 days after 30 mW/cm(2) microwave exposure, the expression of 12 miRNAs increased, while other 70 miRNAs decreased in rats' hippocampus. However, most of miRNAs restored to normal levels at 14 days after exposure, only two upregulated miRNAs and 14 downregulated miRNAs were detected. Gene transcription, neuroprotection and receptors function related target genes were predicated by miRDB, miRbase and miRanda. Moreover, these differentially expressed miRNAs were involved in brain-related signaling pathways, such as synaptic vesicle cycle, long-term depression, calcium signaling and neurotrophin signaling pathways. In conclusion, we successfully characterized the miRNA profiles in microwave-exposed hippocampus, and that will be helpful to clarify the molecular mechanism and provide potential therapeutic targets.

Actin controls the vesicular fraction of dopamine released during extended kiss and run exocytosis

The effect of latrunculin A, an inhibitor of actin cross-linking, on exocytosis in PC12 cells was investigated with single cell amperometry. This analysis strongly suggests that the actin cytoskeleton might be involved in regulating exocytosis, especially by mediating the constriction of the pore. In an extended kiss-and-run release mode, actin could actually control the fraction of neurotransmitters released by the vesicle. This scaffold appears to contribute, with the lipid membrane and the protein machinery, to the closing dynamics of the pore, in competition with other forces mediating the opening of the exocytotic channel.

Self-assembly of VPS41 promotes sorting required for biogenesis of the regulated secretory pathway

The regulated release of polypeptides has a central role in physiology, behavior, and development, but the mechanisms responsible for production of the large dense core vesicles (LDCVs) capable of regulated release have remained poorly understood. Recent work has implicated cytosolic adaptor protein AP-3 in the recruitment of LDCV membrane proteins that confer regulated release. However, AP-3 in mammals has been considered to function in the endolysosomal pathway and in the biosynthetic pathway only in yeast. We now find that the mammalian homolog of yeast VPS41, a member of the homotypic fusion and vacuole protein sorting (HOPS) complex that delivers biosynthetic cargo to the endocytic pathway in yeast, promotes LDCV formation through a common mechanism with AP-3, indicating a conserved role for these proteins in the biosynthetic pathway. VPS41 also self-assembles into a lattice, suggesting that it acts as a coat protein for AP-3 in formation of the regulated secretory pathway.

The functional significance of synaptotagmin diversity in neuroendocrine secretion

Synaptotagmins (syts) are abundant, evolutionarily conserved integral membrane proteins that play essential roles in regulated exocytosis in nervous and endocrine systems. There are at least 17 syt isoforms in mammals, all with tandem C-terminal C2 domains with highly variable capacities for Ca(2+) binding. Many syts play roles in neurotransmitter release or hormone secretion or both, and a growing body of work supports a role for some syts as Ca(2+) sensors of exocytosis. Work in many types of endocrine cells has documented the presence of a number of syt isoforms on dense-core vesicles containing various hormones. Syts can influence the kinetics of exocytotic fusion pores and the choice of release mode between kiss-and-run and full-fusion. Vesicles harboring different syt isoforms can preferentially undergo distinct modes of exocytosis with different forms of stimulation. The diverse properties of syt isoforms enable these proteins to shape Ca(2+) sensing in endocrine cells, thus contributing to the regulation of hormone release and the organization of complex endocrine functions.

Complexin activates exocytosis of distinct secretory vesicles controlled by different synaptotagmins

Complexins are SNARE-complex binding proteins essential for the Ca(2+)-triggered exocytosis mediated by synaptotagmin-1, -2, -7, or -9, but the possible role of complexins in other types of exocytosis controlled by other synaptotagmin isoforms remains unclear. Here we show that, in mouse olfactory bulb neurons, synaptotagmin-1 localizes to synaptic vesicles and to large dense-core secretory vesicles as reported previously, whereas synaptotagmin-10 localizes to a distinct class of peptidergic secretory vesicles containing IGF-1. Both synaptotagmin-1-dependent synaptic vesicle exocytosis and synaptotagmin-10-dependent IGF-1 exocytosis were severely impaired by knockdown of complexins, demonstrating that complexin acts as a cofactor for both synaptotagmin-1 and synaptotagmin-10 despite the functional differences between these synaptotagmins. Rescue experiments revealed that only the activating but not the clamping function of complexins was required for IGF-1 exocytosis controlled by synaptotagmin-10. Thus, our data indicate that complexins are essential for activation of multiple types of Ca(2+)-induced exocytosis that are regulated by different synaptotagmin isoforms. These results suggest that different types of regulated exocytosis are mediated by similar synaptotagmin-dependent fusion mechanisms, that particular synaptotagmin isoforms confer specificity onto different types of regulated exocytosis, and that complexins serve as universal synaptotagmin adaptors for all of these types of exocytosis independent of which synaptotagmin isoform is involved.

Effect of therapeutic mild hypothermia on the genomics of the hippocampus after moderate traumatic brain injury in rats

BACKGROUND

Traumatic brain injury (TBI), a major cause of morbidity and mortality, is a serious public health concern.

OBJECTIVE

To evaluate the effect of mild hypothermia on gene expression in the hippocampus and to try to elucidate molecular mechanisms of hypothermic neuroprotection after TBI.

METHODS

Rats were subjected to mild hypothermia (group 1: n = 3, 33 degrees C, 3H) or normothermia (group 2: n = 3; 37 degrees C, 3H) after TBI. Six genome arrays were applied to detect the gene expression profiles of ipsilateral hippocampus. Functional clustering and gene ontology analysis were then carried out. Another 20 rats were randomly assigned to 4 groups (n = 5 per group): group 3, sham-normothermia; group 4, sham-hypothermia; group 5, TBI-normothermia; and group 6, TBI-hypothermia. Real-time fluorescent quantitative reverse-transcription polymerase chain reaction was used to detect specific selected genes.

RESULTS

We found that 133 transcripts in the hypothermia group were statistically different from those in the normothermia group, including 57 transcripts that were upregulated and 76 that were downregulated after TBI (P < .01). Most of these genes were involved in various pathophysiological processes, and some were critical to cell survival. Analysis showed that 9 gene ontology categories were significantly affected by hypothermia, including the most affected categories: synapse organization and biogenesis (upregulated) and regulation of inflammatory response (downregulated). The mRNA expression of Ank3, Cmbp, Nrxn3, Tgm2, and Fcgr3 was regulated by hypothermia, TBI, or a combination of TBI and hypothermia compared with the sham-normothermia group. Their mRNA expression was significantly regulated by hypothermia in TBI groups.

CONCLUSION

Posttraumatic mild hypothermia has a significant effect on the gene expression profiles of the hippocampus, especially those genes belonging to the 9 gene ontology categories. Differential expression of those genes may be involved in the most fundamental molecular mechanisms of cerebral protection by mild hypothermia after TBI.

Release mode of large and small dense-core vesicles specified by different synaptotagmin isoforms in PC12 cells

Different synaptotagmin isoforms (syt I, VII, and IX) sort to populations of dense-core vesicles with different sizes. These isoforms differ in their sensitivities to divalent cations and trigger different modes of exocytosis. Exocytosis triggered by these isoforms also differs in its sensitivity to inhibition by another isoform, syt IV.

Many cells release multiple substances in different proportions according to the specific character of a stimulus. PC12 cells, a model neuroendocrine cell line, express multiple isoforms of the exocytotic Ca²⁺ sensor synaptotagmin. We show that these isoforms sort to populations of dense-core vesicles that differ in size. These synaptotagmins differ in their Ca²⁺ sensitivities, their preference for full fusion or kiss-and-run, and their sensitivity to inhibition by synaptotagmin IV. In PC12 cells, vesicles that harbor these different synaptotagmin isoforms can be preferentially triggered to fuse by different forms of stimulation. The mode of fusion is specified by the synaptotagmin isoform activated, and because kiss-and-run exocytosis can filter small molecules through a size-limiting fusion pore, the activation of isoforms that favor kiss-and-run will select smaller molecules over larger molecules packaged in the same vesicle. Thus synaptotagmin isoforms can provide multiple levels of control in the release of different molecules from the same cell.

Silence of Synaptotagmin VII inhibits release of dense core vesicles in PC12 cells

Synaptotagmin VII (Syt VII), which has a higher Ca(2+) affinity and slower disassembly kinetics with lipid than Syt I and Syt IX, was regarded as being uninvolved in synaptic vesicle (SV) exocytosis but instead possibly as a calcium sensor for the slower kinetic phase of dense core vesicles (DCVs) release. By using high temporal resolution capacitance and amperometry measurements, it was demonstrated that the knockdown of endogenous Syt VII attenuated the fusion of DCV with the plasma membrane, reduced the amplitude of the exocytotic burst of the Ca(2+)-triggered DCV release without affecting the slope of the sustained component, and blocked the fusion pore expansion. This suggests that Syt VII is the Ca(2+) sensor of DCV fusion machinery and is an essential factor for the establishment and maintenance of the pool size of releasable DCVs in PC12 cells.

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.

Genome-wide association study of vitamin B6, vitamin B12, folate, and homocysteine blood concentrations

The B vitamins are components of one-carbon metabolism (OCM) that contribute to DNA synthesis and methylation. Homocysteine, a by-product of OCM, has been associated with coronary heart disease, stroke and neurological disease. To investigate genetic factors that affect circulating vitamin B6, vitamin B12, folate and homocysteine, a genome-wide association analysis was conducted in the InCHIANTI (N = 1175), SardiNIA (N = 1115), and BLSA (N = 640) studies. The top loci were replicated in an independent sample of 687 participants in the Progetto Nutrizione study. Polymorphisms in the ALPL gene (rs4654748, p = 8.30 x 10(-18)) were associated with vitamin B6 and FUT2 (rs602662, [corrected] p = 2.83 x 10(-20)) with vitamin B12 serum levels. The association of MTHFR, a gene consistently associated with homocysteine, was confirmed in this meta-analysis. The ALPL gene likely influences the catabolism of vitamin B6 while FUT2 interferes with absorption of vitamin B12. These findings highlight mechanisms that affect vitamin B6, vitamin B12 and homocysteine serum levels.

Fabrication of size-controllable ultrasmall-disk electrode: monitoring single vesicle release kinetics at tiny structures with high spatio-temporal resolution

Size-controllable micron or nano-disk carbon fiber electrode (CFE) is prepared and demonstrated to be excellent for extra-cellular transmitter release detection at tiny structures and vesicle fusion kinetics analysis with high spatio-temporal resolution. An improved electrochemical etching procedure was employed, for the first time, to fabricate cylindrical fiber with controlled micron or nano-diameter. Afterwards, a facile insulation with polypropylene sheath was employed to completely insulate the whole body of the thinned fiber, and an ultrasmall-disk sensing area was finally produced by cutting of the insulated fibers. Scanning electron microscopy (SEM) was employed to characterize the ultrasmall geometry size of the fabricated electrode and to show the tight adherence of the insulation sheath on the fiber. The cut ends of the electrodes were also shown to be smooth, clean and without obvious jagged layer. The fabricated micron or nano-disk carbon electrodes show ideal steady-state voltammetric behavior with satisfying reversibility. Subsequently, the performance of the ultrasmall-disk CFE for amperometric detection of cell secretion was characterized. Results showed that, compared to the conventional micro-disk CFE, the etched small disk CFE possesses higher sensitivity due to its obviously improved signal-to-noise level, which enables minute amounts of 3000 oxidizable molecules to be detectable. The nano-disk CFE was shown to be particularly ideal for analysis of fusion kinetics, due to its avoidance of diffusion broadening of the detected spikes, which is the inherent defect of the conventional micro-CFE technique.

Synaptotagmin IV regulates dense core vesicle (DCV) release in LbetaT2 cells

Synaptotagmins (Syts) are calcium-binding proteins which are conserved from nematodes to humans. Fifteen Syts have been identified in mammalian species. Syt I is recognized as a Ca(2+) sensor for the synchronized release of synaptic vesicles in some types of neurons, but its role in the secretion of dense core vesicles (DCVs) remains unclear. The function of Syt IV is of particular interest because it is rapidly up-regulated by chronic depolarization and seizures. Using RNAi-mediated gene silencing, we have explored the role of Syt I and IV on secretion in a pituitary gonadotrope cell line. Downregulation of Syt IV clearly reduced Ca(2+)-triggered exocytosis of dense core vesicles (DCVs) in LbetaT2 cells. Syt I silencing, however, had no effect on vesicular release.

Synaptotagmin-1 and -7 are functionally overlapping Ca2+ sensors for exocytosis in adrenal chromaffin cells

Synaptotagmin-1, the canonical isoform of the synaptotagmin family, is a Ca(2+) sensor for fast synchronous neurotransmitter release in forebrain neurons and chromaffin cells. Even though deletion of synaptotagmin-1 abolishes fast exocytosis in chromaffin cells, it reduces overall secretion by only 20% because of the persistence of slow exocytosis. Therefore, another Ca(2+) sensor dominates release in these cells. Synaptotagmin-7 has a higher Ca(2+) affinity and slower binding kinetics than synaptotagmin-1, matching the proposed properties for the second, slower Ca(2+) sensor. Here, we examined Ca(2+)-triggered exocytosis in chromaffin cells from KO mice lacking synaptotagmin-7, and from knockin mice containing normal levels of a mutant synaptotagmin-7 whose C(2)B domain does not bind Ca(2+). In both types of mutant chromaffin cells, Ca(2+)-triggered exocytosis was decreased dramatically. Moreover, in chromaffin cells lacking both synaptotagmin-1 and -7, only a very slow release component, accounting for approximately 30% of WT exocytosis, persisted. These data establish synaptotagmin-7 as a major Ca(2+) sensor for exocytosis in chromaffin cells, which, together with synaptotagmin-1, mediates almost all of the Ca(2+) triggering of exocytosis in these cells, a surprising result, considering the lack of a role of synaptotagmin-7 in synaptic vesicle exocytosis.

Stable RNA interference of synaptotagmin I in PC12 cells results in differential regulation of transmitter release

In sympathetic neurons, it is well-established that the neurotransmitters, norepinephrine (NE), neuropeptide Y (NPY), and ATP are differentially coreleased from the same neurons. In this study, we determined whether synaptotagmin (syt) I, the primary Ca(2+) sensor for regulated release, could function as the protein that differentially regulates release of these neurotransmitters. Plasmid-based RNA interference was used to specifically and stably silence expression of syt I in a model secretory cell line. Whereas stimulated release of NPY and purines was abolished, stimulated catecholamine (CA) release was only reduced by approximately 50%. Although expression levels of tyrosine hydroxylase, the rate-limiting enzyme in the dopamine synthesis pathway, was unaffected, expression of the vesicular monoamine transporter 1 was reduced by 50%. To evaluate whether NPY and CAs are found within the same vesicles and whether syt I is found localized to each of these NPY- and CA-containing vesicles, we used immunocytochemistry to determine that syt I colocalized with large dense core vesicles, with NPY, and with CAs. Furthermore, both CAs and NPY colocalized with one another and with large dense core vesicles. Electron micrographs show that large dense core vesicles are synthesized and available for release in cells that lack syt I. These results are consistent with syt I regulating differential release of transmitters.