Synaptotagmin isoforms confer distinct activation kinetics and dynamics to chromaffin cell granules

Membrane lipids couple synaptotagmin to SNARE-mediated granule fusion in insulin-secreting cells

Insulin secretion depends on the Ca2+-regulated fusion of granules with the plasma membrane. A recent model of Ca2+-triggered exocytosis in secretory cells proposes that lipids in the plasma membrane couple the calcium sensor Syt1 to the membrane fusion machinery (Kiessling et al., 2018). Specifically, Ca2+-mediated binding of Syt1's C2 domains to the cell membrane shifts the membrane-anchored SNARE syntaxin-1a to a more fusogenic conformation, straightening its juxtamembrane linker. To test this model in live cells and extend it to insulin secretion, we enriched INS1 cells with a panel of lipids with different acyl chain compositions. Fluorescence lifetime measurements demonstrate that cells with more disordered membranes show an increase in fusion efficiency, and vice versa. Experiments with granules purified from INS1 cells and recombinant SNARE proteins reconstituted in supported membranes confirmed that lipid acyl chain composition determines SNARE conformation and that lipid disordering correlates with increased fusion. Addition of Syt1's C2AB domains significantly decreased lipid order in target membranes and increased SNARE-mediated fusion probability. Strikingly, Syt's action on both fusion and lipid order could be partially bypassed by artificially increasing unsaturated phosphatidylserines in the target membrane. Thus, plasma membrane lipids actively participate in coupling Ca2+/synaptotagmin-sensing to the SNARE fusion machinery in cells.

Synaptotagmin-7 mediates cardiac hypertrophy by targeting autophagy

Sustained cardiac hypertrophy damages the heart and weakens cardiac function, often leading to heart failure and even death. Pathological cardiac hypertrophy has become a central therapeutic target for many heart diseases including heart failure. However, the underlying mechanisms of cardiac hypertrophy, especially the involvement of autophagy program, are still ill-understood. Synaptotagmin-7 (Syt7), a multifunctional and high-affinity calcium sensor, plays a pivotal role in asynchronous neurotransmitter release, synaptic facilitation, and vesicle pool regulation during synaptic transmission. However, little is known about whether Syt7 is expressed in the myocardium and involved in the pathogenesis of heart diseases. Here we showed that Syt7 was significantly upregulated in Ang II-treated hearts and cardiomyocytes. Homozygous syt7 knockout (syt7-/-) mice exhibited significantly attenuated cardiac hypertrophy and fibrosis and improved cardiac function. We further found that Syt7 exerted a pro-hypertrophic effect by suppressing the autophagy process. In exploring the upstream mechanisms, microRNA (miR)-93 was identified to participate in the regulation of Syt7 expression. miR-93 protected hearts against Ang II-induced hypertrophy through targeting Syt7-autophagy pathway. In summary, our data reveal a new cardiac hypertrophy regulator and a novel hypertrophy regulating model composed of miR-93, Syt7 and autophagy program. These molecules may serve as potential therapeutic targets in the treatment of cardiac hypertrophy and heart failure.

Membrane transformations of fusion and budding

Membrane fusion and budding mediate fundamental processes like intracellular trafficking, exocytosis, and endocytosis. Fusion is thought to open a nanometer-range pore that may subsequently close or dilate irreversibly, whereas budding transforms flat membranes into vesicles. Reviewing recent breakthroughs in real-time visualization of membrane transformations well exceeding this classical view, we synthesize a new model and describe its underlying mechanistic principles and functions. Fusion involves hemi-to-full fusion, pore expansion, constriction and/or closure while fusing vesicles may shrink, enlarge, or receive another vesicle fusion; endocytosis follows exocytosis primarily by closing Ω-shaped profiles pre-formed through the flat-to-Λ-to-Ω-shape transition or formed via fusion. Calcium/SNARE-dependent fusion machinery, cytoskeleton-dependent membrane tension, osmotic pressure, calcium/dynamin-dependent fission machinery, and actin/dynamin-dependent force machinery work together to generate fusion and budding modes differing in pore status, vesicle size, speed and quantity, controls release probability, synchronization and content release rates/amounts, and underlies exo-endocytosis coupling to maintain membrane homeostasis. These transformations, underlying mechanisms, and functions may be conserved for fusion and budding in general.

PACAP and acetylcholine cause distinct Ca2+ signals and secretory responses in chromaffin cells

The adrenomedullary chromaffin cell transduces chemical messages into outputs that regulate end organ function throughout the periphery. At least two important neurotransmitters are released by innervating preganglionic neurons to stimulate exocytosis in the chromaffin cell-acetylcholine (ACh) and pituitary adenylate cyclase activating polypeptide (PACAP). Although PACAP is widely acknowledged as an important secretagogue in this system, the pathway coupling PACAP stimulation to chromaffin cell secretion is poorly understood. The goal of this study is to address this knowledge gap. Here, it is shown that PACAP activates a Gαs-coupled pathway that must signal through phospholipase C ε (PLCε) to drive Ca2+ entry and exocytosis. PACAP stimulation causes a complex pattern of Ca2+ signals in chromaffin cells, leading to a sustained secretory response that is kinetically distinct from the form stimulated by ACh. Exocytosis caused by PACAP is associated with slower release of peptide cargo than exocytosis stimulated by ACh. Importantly, only the secretory response to PACAP, not ACh, is eliminated in cells lacking PLCε expression. The data show that ACh and PACAP, acting through distinct signaling pathways, enable nuanced and variable secretory outputs from chromaffin cells.

Synaptotagmins 1 and 7 in vesicle release from rods of mouse retina

Synaptotagmins are the primary Ca2+ sensors for synaptic exocytosis. Previous work suggested synaptotagmin-1 (Syt1) mediates evoked vesicle release from cone photoreceptor cells in the vertebrate retina whereas release from rods may involve another sensor in addition to Syt1. We found immunohistochemical evidence for syntaptotagmin-7 (Syt7) in mouse rod terminals and so performed electroretinograms (ERG) and single-cell recordings using mice in which Syt1 and/or Syt7 were conditionally removed from rods and/or cones. Synaptic release was measured in mouse rods by recording presynaptic anion currents activated during glutamate re-uptake and from exocytotic membrane capacitance changes. Deleting Syt1 from rods reduced glutamate release evoked by short depolarizing steps but not long steps whereas deleting Syt7 from rods reduced release evoked by long but not short steps. Deleting both sensors completely abolished depolarization-evoked release from rods. Effects of various intracellular Ca2+ buffers showed that Syt1-mediated release from rods involves vesicles close to ribbon-associated Ca2+ channels whereas Syt7-mediated release evoked by longer steps involves more distant release sites. Spontaneous release from rods was unaffected by eliminating Syt7. While whole animal knockout of Syt7 slightly reduced ERG b-waves and oscillatory potentials, selective elimination of Syt7 from rods had no effect on ERGs. Furthermore, eliminating Syt1 from rods and cones abolished ERG b-waves and additional elimination of Syt7 had no further effect. These results show that while Syt7 contributes to slow non-ribbon release from rods, Syt1 is the principal sensor shaping rod and cone inputs to bipolar cells in response to light flashes.

A two-subpopulation model that reflects heterogeneity of large dense core vesicles in exocytosis

Exocytosis of large dense core vesicles is responsible for hormone secretion in neuroendocrine cells. The population of primed vesicles ready to release upon cell excitation demonstrates large heterogeneity. However, there are currently no models that clearly reflect such heterogeneity. Here, we develop a novel model based on single vesicle release events from amperometry recordings of PC12 cells using carbon fiber microelectrode. In this model, releasable vesicles can be grouped into two subpopulations, namely, SP1 and SP2. SP1 vesicles replenish quickly, with kinetics of ~0.0368 s^-1, but likely undergo slow fusion pore expansion (amperometric signals rise at ~2.5 pA/ms), while SP2 vesicles demonstrate slow replenishment (kinetics of ~0.0048 s^-1) but prefer fast dilation of fusion pore, with an amperometric signal rising rate of ~9.1 pA/ms. Phorbol ester enlarges the size of SP2 partially via activation of protein kinase C and conveys SP1 vesicles into SP2. Inhibition of Rho GTPase-dependent actin rearrangement almost completely depletes SP2. We also propose that the phorbol ester-sensitive vesicle subpopulation (SP2) is analogous to the subset of superprimed synaptic vesicles in neurons. This model provides a meticulous description of the architecture of the readily releasable vesicle pool and elucidates the heterogeneity of the vesicle priming mechanism.

Glucose-induced [Ca2+]i oscillations in β cells are composed of trains of spikes within a subplasmalemmal microdomain

Electrical activity and oscillations of cytosolic Ca²⁺ concentrations ([Ca²⁺]_i) that trigger insulin release in response to glucose are key functions of pancreatic β cells. Although oscillatory Ca²⁺ signals have been intensively studied in β cells, their lower frequency did not match that of electrical activity. In addition, the measured peak [Ca²⁺]_i did not reach levels that are typically required by synaptotagmins to elicit the release of insulin-containing vesicles in live-cell experiments. We therefore sought to resolve the Ca²⁺ dynamics in the subplasmalemmal microdomain that is critical for triggering fast exocytosis. Applying total internal reflection fluorescence (TIRF) microscopy in insulin-producing INS-1E and primary mouse β cells, we resolved extraordinary fast trains of Ca²⁺ spiking (frequency > 3 s^-1) in response to glucose exposure. Using a low-affinity [Ca²⁺]_i indicator dye, we provide experimental evidence that Ca²⁺ spikes reach low micromolar apparent concentrations in the vicinity of the plasma membrane_. Analysis of Ca²⁺ spikes evoked by repeated depolarization for 10 ms closely matched the Ca²⁺ dynamics observed upon glucose application. To our knowledge, this is the first study that experimentally demonstrates Ca²⁺ spikes in β cells with velocities that resemble those of bursting or continuously appearing trains of action potentials (APs) in non-patched cells.

Mitochondrial hydrogen peroxide positively regulates neuropeptide secretion during diet-induced activation of the oxidative stress response

Mitochondria play a pivotal role in the generation of signals coupling metabolism with neurotransmitter release, but a role for mitochondrial-produced ROS in regulating neurosecretion has not been described. Here we show that endogenously produced hydrogen peroxide originating from axonal mitochondria (mtH2O2) functions as a signaling cue to selectively regulate the secretion of a FMRFamide-related neuropeptide (FLP-1) from a pair of interneurons (AIY) in C. elegans. We show that pharmacological or genetic manipulations that increase mtH2O2 levels lead to increased FLP-1 secretion that is dependent upon ROS dismutation, mitochondrial calcium influx, and cysteine sulfenylation of the calcium-independent PKC family member PKC-1. mtH2O2-induced FLP-1 secretion activates the oxidative stress response transcription factor SKN-1/Nrf2 in distal tissues and protects animals from ROS-mediated toxicity. mtH2O2 levels in AIY neurons, FLP-1 secretion and SKN-1 activity are rapidly and reversibly regulated by exposing animals to different bacterial food sources. These results reveal a previously unreported role for mtH2O2 in linking diet-induced changes in mitochondrial homeostasis with neuropeptide secretion.

Synaptotagmin-7 places dense-core vesicles at the cell membrane to promote Munc13-2- and Ca2+-dependent priming

Synaptotagmins confer calcium-dependence to the exocytosis of secretory vesicles, but how coexpressed synaptotagmins interact remains unclear. We find that synaptotagmin-1 and synaptotagmin-7 when present alone act as standalone fast and slow Ca²⁺-sensors for vesicle fusion in mouse chromaffin cells. When present together, synaptotagmin-1 and synaptotagmin-7 are found in largely non-overlapping clusters on dense-core vesicles. Synaptotagmin-7 stimulates Ca²⁺-dependent vesicle priming and inhibits depriming, and it promotes ubMunc13-2- and phorbolester-dependent priming, especially at low resting calcium concentrations. The priming effect of synaptotagmin-7 increases the number of vesicles fusing via synaptotagmin-1, while negatively affecting their fusion speed, indicating both synergistic and competitive interactions between synaptotagmins. Synaptotagmin-7 places vesicles in close membrane apposition (<6 nm); without it, vesicles accumulate out of reach of the fusion complex (20-40 nm). We suggest that a synaptotagmin-7-dependent movement toward the membrane is involved in Munc13-2/phorbolester/Ca²⁺-dependent priming as a prelude to fast and slow exocytosis triggering.

Distinct insulin granule subpopulations implicated in the secretory pathology of diabetes types 1 and 2

Insulin secretion from β-cells is reduced at the onset of type-1 and during type-2 diabetes. Although inflammation and metabolic dysfunction of β-cells elicit secretory defects associated with type-1 or type-2 diabetes, accompanying changes to insulin granules have not been established. To address this, we performed detailed functional analyses of insulin granules purified from cells subjected to model treatments that mimic type-1 and type-2 diabetic conditions and discovered striking shifts in calcium affinities and fusion characteristics. We show that this behavior is correlated with two subpopulations of insulin granules whose relative abundance is differentially shifted depending on diabetic model condition. The two types of granules have different release characteristics, distinct lipid and protein compositions, and package different secretory contents alongside insulin. This complexity of β-cell secretory physiology establishes a direct link between granule subpopulation and type of diabetes and leads to a revised model of secretory changes in the diabetogenic process.

Synaptotagmin-7 enhances calcium-sensing of chromaffin cell granules and slows discharge of granule cargos

Synaptotagmin-7 (Syt-7) is one of two major calcium sensors for exocytosis in adrenal chromaffin cells, the other being synaptotagmin-1 (Syt-1). Despite a broad appreciation for the importance of Syt-7, questions remain as to its localization, function in mediating discharge of dense core granule cargos, and role in triggering release in response to physiological stimulation. These questions were addressed using two distinct experimental preparations-mouse chromaffin cells lacking endogenous Syt-7 (KO cells) and a reconstituted system employing cell-derived granules expressing either Syt-7 or Syt-1. First, using immunofluorescence imaging and subcellular fractionation, it is shown that Syt-7 is widely distributed in organelles, including dense core granules. Total internal reflection fluorescence (TIRF) imaging demonstrates that the kinetics and probability of granule fusion in Syt-7 KO cells stimulated by a native secretagogue, acetylcholine, are markedly lower than in WT cells. When fusion is observed, fluorescent cargo proteins are discharged more rapidly when only Syt-1 is available to facilitate release. To determine the extent to which the aforementioned results are attributable purely to Syt-7, granules expressing only Syt-7 or Syt-1 were triggered to fuse on planar supported bilayers bearing plasma membrane SNARE proteins. Here, as in cells, Syt-7 confers substantially greater calcium sensitivity to granule fusion than Syt-1 and slows the rate at which cargos are released. Overall, this study demonstrates that by virtue of its high affinity for calcium and effects on fusion pore expansion, Syt-7 plays a central role in regulating secretory output from adrenal chromaffin cells.

SNARE-Mediated Exocytosis in Neuronal Development

The formation of the nervous system involves establishing complex networks of synaptic connections between proper partners. This developmental undertaking requires the rapid expansion of the plasma membrane surface area as neurons grow and polarize, extending axons through the extracellular environment. Critical to the expansion of the plasma membrane and addition of plasma membrane material is exocytic vesicle fusion, a regulated mechanism driven by soluble N-ethylmaleimide-sensitive factor attachment proteins receptors (SNAREs). Since their discovery, SNAREs have been implicated in several critical neuronal functions involving exocytic fusion in addition to synaptic transmission, including neurite initiation and outgrowth, axon specification, axon extension, and synaptogenesis. Decades of research have uncovered a rich variety of SNARE expression and function. The basis of SNARE-mediated fusion, the opening of a fusion pore, remains an enigmatic event, despite an incredible amount of research, as fusion is not only heterogeneous but also spatially small and temporally fast. Multiple modes of exocytosis have been proposed, with full-vesicle fusion (FFV) and kiss-and-run (KNR) being the best described. Whereas most in vitro work has reconstituted fusion using VAMP-2, SNAP-25, and syntaxin-1; there is much to learn regarding the behaviors of distinct SNARE complexes. In the past few years, robust heterogeneity in the kinetics and fate of the fusion pore that varies by cell type have been uncovered, suggesting a paradigm shift in how the modes of exocytosis are viewed is warranted. Here, we explore both classic and recent work uncovering the variety of SNAREs and their importance in the development of neurons, as well as historical and newly proposed modes of exocytosis, their regulation, and proteins involved in the regulation of fusion kinetics.

Diverse roles of Synaptotagmin-7 in regulating vesicle fusion

Synaptotagmin 7 (Syt7) is a multifunctional calcium sensor expressed throughout the body. Its high calcium affinity makes it well suited to act in processes triggered by modest calcium increases within cells. In synaptic transmission, Syt7 has been shown to mediate asynchronous neurotransmitter release, facilitation, and vesicle replenishment. In this review we provide an update on recent developments, and the newly emerging roles of Syt7 in frequency invariant synaptic transmission and in suppressing spontaneous release. Additionally, we discuss Syt7's regulation of membrane fusion in non-neuronal cells, and its involvement in disease. How such diversity of functions is regulated remains an open question. We discuss several potential factors including temperature, presynaptic calcium signals, the localization of Syt7, and its interaction with other Syt isoforms.

Vesicle Shrinking and Enlargement Play Opposing Roles in the Release of Exocytotic Contents

For decades, two fusion modes were thought to control hormone and transmitter release essential to life; one facilitates release via fusion pore dilation and flattening (full collapse), and the other limits release by closing a narrow fusion pore (kiss-and-run). Using super-resolution stimulated emission depletion (STED) microscopy to visualize fusion modes of dense-core vesicles in neuroendocrine cells, we find that facilitation of release is mediated not by full collapse but by shrink fusion, in which the Ω-profile generated by vesicle fusion shrinks but maintains a large non-dilating pore. We discover that the physiological osmotic pressure of a cell squeezes, but does not dilate, the Ω-profile, which explains why shrink fusion prevails over full collapse. Instead of kiss-and-run, enlarge fusion, in which Ω-profiles grow while maintaining a narrow pore, slows down release. Shrink and enlarge fusion may thus account for diverse hormone and transmitter release kinetics observed in secretory cells, previously interpreted within the full-collapse/kiss-and-run framework.

Resolving kinetic intermediates during the regulated assembly and disassembly of fusion pores

The opening of a fusion pore during exocytosis creates the first aqueous connection between the lumen of a vesicle and the extracellular space. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate the formation of these dynamic structures, and their kinetic transitions are tightly regulated by accessory proteins at the synapse. Here, we utilize two single molecule approaches, nanodisc-based planar bilayer electrophysiology and single-molecule FRET, to address the relationship between SNARE complex assembly and rapid (micro-millisecond) fusion pore transitions, and to define the role of accessory proteins. Synaptotagmin (syt) 1, a major Ca²⁺-sensor for synaptic vesicle exocytosis, drove the formation of an intermediate: committed trans-SNARE complexes that form large, stable pores. Once open, these pores could only be closed by the action of the ATPase, NSF. Time-resolved measurements revealed that NSF-mediated pore closure occurred via a complex ‘stuttering’ mechanism. This simplified system thus reveals the dynamic formation and dissolution of fusion pores.

SNAREs mediate the formation of a fusion pore during exocytosis which connects the lumen of a vesicle with the extracellular space. Here, authors use single molecule approaches to define the role of synaptotagmin 1 and NSF in synaptic pore formation and dissolution.

Spatiotemporal organization and protein dynamics involved in regulated exocytosis of MMP-9 in breast cancer cells

This paper describes the dynamics of proteins and lipids during exocytosis of MMP-9 from cancer cells in real time using fluorescence microscopy. Stephens et al. find that core exocytic proteins, accessory proteins, and lipids are involved at sites of secretory vesicle fusion.

Altered regulation of exocytosis is an important mechanism controlling many diseases, including cancer. Defects in exocytosis have been implicated in many cancer cell types and are generally attributed to mutations in cellular transport, trafficking, and assembly of machinery necessary for exocytosis of secretory vesicle cargo. In these cancers, up-regulation of trafficking and secretion of matrix metalloproteinase-9 (MMP-9), a proteolytic enzyme, is responsible for degrading the extracellular matrix, a necessary step in tumor progression. Using TIRF microscopy, we identified proteins associated with secretory vesicles containing MMP-9 and imaged the local dynamics of these proteins at fusion sites during regulated exocytosis of MMP-9 from MCF-7 breast cancer cells. We found that many regulators of exocytosis, including several Rab GTPases, Rab effector proteins, and SNARE/SNARE modulator proteins, are stably assembled on docked secretory vesicles before exocytosis. At the moment of fusion, many of these components are quickly lost from the vesicle, while several endocytic proteins and lipids are simultaneously recruited to exocytic sites at precisely that moment. Our findings provide insight into the dynamic behavior of key core exocytic proteins, accessory proteins, lipids, and some endocytic proteins at single sites of secretory vesicle fusion in breast cancer cells.

Chromaffin Cells of the Adrenal Medulla: Physiology, Pharmacology, and Disease

Chromaffin cells (CCs) of the adrenal gland and the sympathetic nervous system produce the catecholamines (epinephrine and norepinephrine; EPI and NE) needed to coordinate the bodily "fight-or-flight" response to fear, stress, exercise, or conflict. EPI and NE release from CCs is regulated both neurogenically by splanchnic nerve fibers and nonneurogenically by hormones (histamine, corticosteroids, angiotensin, and others) and paracrine messengers [EPI, NE, adenosine triphosphate, opioids, γ-aminobutyric acid (GABA), etc.]. The "stimulus-secretion" coupling of CCs is a Ca2+ -dependent process regulated by Ca2+ entry through voltage-gated Ca2+ channels, Ca2+ pumps, and exchangers and intracellular organelles (RE and mitochondria) and diffusible buffers that provide both Ca2+ -homeostasis and Ca2+ -signaling that ultimately trigger exocytosis. CCs also express Na+ and K+ channels and ionotropic (nAChR and GABAA ) and metabotropic receptors (mACh, PACAP, β-AR, 5-HT, histamine, angiotensin, and others) that make CCs excitable and responsive to autocrine and paracrine stimuli. To maintain high rates of E/NE secretion during stressful conditions, CCs possess a large number of secretory chromaffin granules (CGs) and members of the soluble NSF-attachment receptor complex protein family that allow docking, fusion, and exocytosis of CGs at the cell membrane, and their recycling. This article attempts to provide an updated account of well-established features of the molecular processes regulating CC function, and a survey of the as-yet-unsolved but important questions relating to CC function and dysfunction that have been the subject of intense research over the past 15 years. Examples of CCs as a model system to understand the molecular mechanisms associated with neurodegenerative diseases are also provided. Published 2019. Compr Physiol 9:1443-1502, 2019.

In vitro fusion of single synaptic and dense core vesicles reproduces key physiological properties

Regulated exocytosis of synaptic vesicles is substantially faster than of endocrine dense core vesicles despite similar molecular machineries. The reasons for this difference are unknown and could be due to different regulatory proteins, different spatial arrangements, different vesicle sizes, or other factors. To address these questions, we take a reconstitution approach and compare regulated SNARE-mediated fusion of purified synaptic and dense core chromaffin and insulin vesicles using a single vesicle-supported membrane fusion assay. In all cases, Munc18 and complexin are required to restrict fusion in the absence of calcium. Calcium triggers fusion of all docked vesicles. Munc13 (C1C2MUN domain) is required for synaptic and enhanced insulin vesicle fusion, but not for chromaffin vesicles, correlating inversely with the presence of CAPS protein on purified vesicles. Striking disparities in calcium-triggered fusion rates are observed, increasing with curvature with time constants 0.23 s (synaptic vesicles), 3.3 s (chromaffin vesicles), and 9.1 s (insulin vesicles) and correlating with rate differences in cells.

Unraveling the mechanisms of calcium-dependent secretion

Ca2+-dependent secretion is a process by which important signaling molecules that are produced within a cell-including proteins and neurotransmitters-are expelled to the extracellular environment. The cellular mechanism that underlies secretion is referred to as exocytosis. Many years of work have revealed that exocytosis in neurons and neuroendocrine cells is tightly coupled to Ca2+ and orchestrated by a series of protein-protein/protein-lipid interactions. Here, we highlight landmark discoveries that have informed our current understanding of the process. We focus principally on reductionist studies performed using powerful model secretory systems and cell-free reconstitution assays. In recent years, molecular cloning and genetics have implicated the involvement of a sizeable number of proteins in exocytosis. We expect reductionist approaches will be central to attempts to resolve their roles. The Journal of General Physiology will continue to be an outlet for much of this work, befitting its tradition of publishing strongly mechanistic, basic research.

Synaptotagmin 5 regulates Ca2+-dependent Weibel-Palade body exocytosis in human endothelial cells

Elevations of intracellular free Ca²⁺ concentration ([Ca²⁺]_i) are a potent trigger for Weibel-Palade body (WPB) exocytosis and secretion of von Willebrand factor (VWF) from endothelial cells; however, the identity of WPB-associated Ca²⁺-sensors involved in transducing acute increases in [Ca²⁺]_i into granule exocytosis remains unknown. Here, we show that synaptotagmin 5 (SYT5) is expressed in human umbilical vein endothelial cells (HUVECs) and is recruited to WPBs to regulate Ca²⁺-driven WPB exocytosis. Western blot analysis of HUVECs identified SYT5 protein, and exogenously expressed SYT5-mEGFP localised almost exclusively to WPBs. shRNA-mediated knockdown of endogenous SYT5 (shSYT5) reduced the rate and extent of histamine-evoked WPB exocytosis and reduced secretion of the WPB cargo VWF-propeptide (VWFpp). The shSYT5-mediated reduction in histamine-evoked WPB exocytosis was prevented by expression of shRNA-resistant SYT5-mCherry. Overexpression of SYT5-EGFP increased the rate and extent of histamine-evoked WPB exocytosis, and increased secretion of VWFpp. Expression of a Ca²⁺-binding defective SYT5 mutant (SYT5-Asp197Ser-EGFP) mimicked depletion of endogenous SYT5. We identify SYT5 as a WPB-associated Ca²⁺ sensor regulating Ca²⁺-dependent secretion of stored mediators from vascular endothelial cells.

A simple supported tubulated bilayer system for evaluating protein-mediated membrane remodeling

Fusion and fission of cellular membranes involve dramatic, protein-mediated changes in membrane curvature. Many of the experimental methods useful for investigating curvature sensing or generation require specialized equipment. We have developed a system based on supported lipid bilayers (SLBs) in which lipid tubules are simple to produce and several types of membrane remodeling events can be readily imaged using widely available instrumentation (e.g., tubule fission and/or membrane budding). Briefly, high ionic strength during lipid bilayer deposition results in incorporation of excess lipids in the SLB. After sequentially washing with water and physiological ionic strength buffer solutions, lipid tubules form spontaneously. We find that tubule formation results from solution-dependent spreading of the SLB; washing from water into physiological ionic strength buffer solution leads to expansion of the bilayer and formation of tubules. Conversely, washing from physiological buffer into water results in contraction of the membrane and loss of tubules. We demonstrate the utility of these supported tubulated bilayers, termed "STuBs," with an investigation of Sar1B, a small Ras family G-protein known to influence membrane curvature. The addition of Sar1B to STuBs results in dramatic changes in tubule topology and eventual tubule fission. Overall, STuBs are a simple experimental system, useful for monitoring protein-mediated effects on membrane topology in real time, under physiologically relevant conditions.

The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis

MacDougall et al. review the structure and function of the calcium sensor synaptotagmin-7 in exocytosis.

Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.

The synaptotagmin C2B domain calcium-binding loops modulate the rate of fusion pore expansion

In chromaffin cells, the kinetics of fusion pore expansion vary depending on which synaptotagmin isoform (Syt-1 or Syt-7) drives release. Our recent studies have shown that fusion pores of granules harboring Syt-1 expand more rapidly than those harboring Syt-7. Here we sought to define the structural specificity of synaptotagmin action at the fusion pore by manipulating the Ca2+-binding C2B module. We generated a chimeric Syt-1 in which its C2B Ca2+-binding loops had been exchanged for those of Syt-7. Fusion pores of granules harboring a Syt-1 C2B chimera with all three Ca2+-binding loops of Syt-7 (Syt-1:7C2B123) exhibited slower rates of fusion pore expansion and neuropeptide cargo release relative to WT Syt-1. After fusion, this chimera also dispersed more slowly from fusion sites than WT protein. We speculate that the Syt-1:7 C2B123 and WT Syt-1 are likely to differ in their interactions with Ca2+ and membranes. Subsequent in vitro and in silico data demonstrated that the chimera exhibits a higher affinity for phospholipids than WT Syt-1. We conclude that the affinity of synaptotagmin for the plasma membrane, and the rate at which it releases the membrane, contribute in important ways to the rate of fusion pore expansion.

Kiss-and-Run Is a Significant Contributor to Synaptic Exocytosis and Endocytosis in Photoreceptors

Accompanying sustained release in darkness, rod and cone photoreceptors exhibit rapid endocytosis of synaptic vesicles. Membrane capacitance measurements indicated that rapid endocytosis retrieves at least 70% of the exocytotic membrane increase. One mechanism for rapid endocytosis is kiss-and-run fusion where vesicles briefly contact the plasma membrane through a small fusion pore. Release can also occur by full-collapse in which vesicles merge completely with the plasma membrane. We assessed relative contributions of full-collapse and kiss-and-run in salamander photoreceptors using optical techniques to measure endocytosis and exocytosis of large vs. small dye molecules. Incubation with small dyes (SR101, 1 nm; 3-kDa dextran-conjugated Texas Red, 2.3 nm) loaded rod and cone synaptic terminals much more readily than larger dyes (10-kDa Texas Red, 4.6 nm; 10-kDa pHrodo, 4.6 nm; 70-kDa Texas Red, 12 nm) consistent with significant uptake through 2.3–4.6 nm fusion pores. By using total internal reflection fluorescence microscopy (TIRFM) to image individual vesicles, when rods were incubated simultaneously with Texas Red and AlexaFluor-488 dyes conjugated to either 3-kDa or 10-kDa dextran, more vesicles loaded small molecules than large molecules. Using TIRFM to detect release by the disappearance of dye-loaded vesicles, we found that SR101 and 3-kDa Texas Red were released from individual vesicles more readily than 10-kDa and 70-kDa Texas Red. Although 10-kDa pHrodo was endocytosed poorly like other large dyes, the fraction of release events was similar to SR101 and 3-kDa Texas Red. We hypothesize that while 10-kDa pHrodo may not exit through a fusion pore, release of intravesicular protons can promote detection of fusion events by rapidly quenching fluorescence of this pH-sensitive dye. Assuming that large molecules can only be released by full-collapse whereas small molecules can be released by both modes, our results indicate that 50%–70% of release from rods involves kiss-and-run with 2.3–4.6 nm fusion pores. Rapid retrieval of vesicles by kiss-and-run may limit membrane disruption of release site function during ongoing release at photoreceptor ribbon synapses.

Chemistry in a vesicle

Slow fusion pore expansion could retain molecules within vesicles, enabling a chemical reaction that modifies secreted products.

Different strokes for different synaptotagmins

JGP study shows how synaptotagmins 1 and 7 affect chromaffin cell granule fusion.