Mechanism Of Exocytosis Research Articles

Prelysosomal Compartments in the Unconventional Secretion of Amyloidogenic Seeds.

A mechanistic link between neuron-to-neuron transmission of secreted amyloid and propagation of protein malconformation cytopathology and disease has recently been uncovered in animal models. An enormous interest in the unconventional secretion of amyloids from neurons has followed. Amphisomes and late endosomes are the penultimate maturation products of the autophagosomal and endosomal pathways, respectively, and normally fuse with lysosomes for degradation. However, under conditions of perturbed membrane trafficking and/or lysosomal deficiency, prelysosomal compartments may instead fuse with the plasma membrane to release any contained amyloid. After a brief introduction to the endosomal and autophagosomal pathways, we discuss the evidence for autophagosomal secretion (exophagy) of amyloids, with a comparative emphasis on Aβ1–42 and α-synuclein, as luminal and cytosolic amyloids, respectively. The ESCRT-mediated import of cytosolic amyloid into late endosomal exosomes, a known vehicle of transmission of macromolecules between cells, is also reviewed. Finally, mechanisms of lysosomal dysfunction, deficiency, and exocytosis are exemplified in the context of genetically identified risk factors, mainly for Parkinson’s disease. Exocytosis of prelysosomal or lysosomal organelles is a last resort for clearance of cytotoxic material and alleviates cytopathy. However, they also represent a vehicle for the concentration, posttranslational modification, and secretion of amyloid seeds.

Cellular interactions of a lipid-based nanocarrier model with human keratinocytes: Unravelling transport mechanisms.

Transdermal drug delivery is a challenging and growing area of clinical application. Lipid nanoparticles such as nanostructured lipid carriers (NLCs) have gained wide interest for transdermal drug delivery. However these nanocarriers' interactions with epidermal epithelial barrier are yet unknown. Unveiling the mechanisms involved in NLCs transport across the epidermal epithelial monolayers will contribute with valuable information to achieve enhanced skin permeability, superior bioavailability and consequently improved therapeutic effect. With our present work we could certainly provide researchers and clinicians guidance for the design of optimized transdermal delivery systems, based on the nanomaterials and biological interactions.

Munc13-4 functions as a Ca2+ sensor for homotypic secretory granule fusion to generate endosomal exocytic vacuoles.

Munc13-4 is a Ca2+-dependent SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)- and phospholipid-binding protein that localizes to and primes secretory granules (SGs) for Ca2+-evoked secretion in various secretory cells. Studies in mast cell-like RBL-2H3 cells provide direct evidence that Munc13-4 with its two Ca2+-binding C2 domains functions as a Ca2+ sensor for SG exocytosis. Unexpectedly, Ca2+ stimulation also generated large (>2.4 μm in diameter) Munc13-4+/Rab7+/Rab11+ endosomal vacuoles. Vacuole generation involved the homotypic fusion of Munc13-4+/Rab7+ SGs, followed by a merge with Rab11+ endosomes, and depended on Ca2+ binding to Munc13-4. Munc13-4 promoted the Ca2+-stimulated fusion of VAMP8-containing liposomes with liposomes containing exocytic or endosomal Q-SNAREs and directly interacted with late endosomal SNARE complexes. Thus Munc13-4 is a tethering/priming factor and Ca2+ sensor for both heterotypic SG-plasma membrane and homotypic SG-SG fusion. Total internal reflection fluorescence microscopy imaging revealed that vacuoles were exocytic and mediated secretion of β-hexosaminidase and cytokines accompanied by Munc13-4 diffusion onto the plasma membrane. The results provide new molecular insights into the mechanism of multigranular compound exocytosis commonly observed in various secretory cells.

Defective Autophagy, Mitochondrial Clearance and Lipophagy in Niemann-Pick Type B Lymphocytes.

Niemann-Pick disease type A (NP-A) and type B (NP-B) are lysosomal storage diseases (LSDs) caused by sphingomyelin accumulation in lysosomes relying on reduced or absent acid sphingomyelinase. A considerable body of evidence suggests that lysosomal storage in many LSD impairs autophagy, resulting in the accumulation of poly-ubiquitinated proteins and dysfunctional mitochondria, ultimately leading to cell death. Here we test this hypothesis in a cellular model of Niemann-Pick disease type B, in which autophagy has never been studied. The basal autophagic pathway was first examined in order to evaluate its functionality using several autophagy-modulating substances such as rapamycin and nocodazole. We found that human NP-B B lymphocytes display considerable alteration in their autophagic vacuole accumulation and mitochondrial fragmentation, as well as mitophagy induction (for damaged mitochondria clearance). Furthermore, lipid traceability of intra and extra-cellular environments shows lipid accumulation in NP-B B lymphocytes and also reveals their peculiar trafficking/management, culminating in lipid microparticle extrusion (by lysosomal exocytosis mechanisms) or lipophagy. All of these features point to the presence of a deep autophagy/mitophagy alteration revealing autophagic stress and defective mitochondrial clearance. Hence, rapamycin might be used to regulate autophagy/mitophagy (at least in part) and to contribute to the clearance of lysosomal aberrant lipid storage.

Estimating the dissipative factors of synaptic exocytosis in Drosophila using a filter based reverse engineering method

The SNARE-regulated exocytosis is an important molecular level mechanism for the persistent inter-neuronal communication via plasticity of neurons. Without considering the dissipation of synaptic vesicle, the explanation of the inter-neuronal communication via plasticity of neurons in existing models cannot fully account for the persistence of neuronal plasticity. In this paper the dissipative factors CPX and SYT corresponding to unbinding neurotransmitters fused with the membrane are estimated using a reverse engineering method where the dynamic flux is embedded in the filtering model of synaptic transmission. With the CPX and SYT in the corresponding regulation mechanism of exocytosis being estimated, the dissipation function of synaptic vesicle for the persistence of neuronal plasticity is explicitly expressed.

Crucial Role of P2X7 Receptor in Regulating Exocytosis of Single-Walled Carbon Nanotubes in Macrophages.

Exocytosis of single-walled carbon nanotubes (SWCNTs) determines therapeutic efficiency and toxicity of nanoproducts but its underlying mechanism remains elusive. In this study, it is found that the exocytosis mechanism of SWCNTs is mediated mainly through the activation of P2X7 receptor (P2X7 R), an ATP-gated membrane receptor highly expressed in macrophages. Inhibition of P2X7 R signaling by either a specific inhibitor (oxidized ATP) or small interfering RNA targeting P2X7 R largely prevents the exocytosis of SWCNTs from Raw264.7 cells, resulting in significant accumulation of SWCNTs within cells. In contrast, activation of P2X7 R with ATP promotes exocytosis of SWCNTs. Specifically, it is elucidated that internalized SWCNTs are accumulated in lysosomes and induce transitional release of ATP into extracellular space, which further activates P2X7 R, leading to the influx of calcium ions, phosphorylation of protein kinase C, ERK1/2, p38, and JNK, as well as alkalization of lysosomes. SWCNTs exposure also induces microtubules reorganization that facilitates the secretion of SWCNTs-containing lysosomes. It is also found that P2X7 R simultaneously mediates secretion of IL-1β from Raw264.7 cells during the process of SWCNTs exocytosis. The combined data reveals that P2X7 R-mediated pathway is the predominant molecular mechanism for exocytosis of SWCNTs in Raw264.7 cells. Moreover, SWCNT-induced inflammation is closely coupled with the exocytosis of SWCNTs through P2X7 R.

The 18th International Symposia on Chromaffin Cell Biology

The International Symposia on Chromaffin Cell Biology (ISCCB) have a very long history. In this issue of The Journal of Neurochemistry we have compiled a number of review articles that encompass some of the work presented during the sessions of the ISCCB meeting held in Australia in August 2015, covering aspects of chromaffin cell biology, the mechanisms of secretory vesicle biogenesis and exocytosis, and the physiological consequences of these mechanisms. This is the Preface for the Mini Review Series: “ISCCB Meeting on Chromaffin Cells”.

Purinergic signalling underlies transforming growth factor-β-mediated bladder afferent nerve hyperexcitability.

The sensory components of the urinary bladder are responsible for the transduction of bladder filling and are often impaired with neurological injury or disease. Elevated extracellular ATP contributes, in part, to bladder afferent nerve hyperexcitability during urinary bladder inflammation or irritation. Transforming growth factor-β1 (TGF-β1) may stimulate ATP release from the urothelium through vesicular exocytosis mechanisms with minimal contribution from pannexin-1 channels to increase bladder afferent nerve discharge. Bladder afferent nerve hyperexcitability and urothelial ATP release with CYP-induced cystitis is decreased with TGF-β inhibition. These results establish a causal link between an inflammatory mediator, TGF-β, and intrinsic signalling mechanisms of the urothelium that may contribute to the altered sensory processing of bladder filling. The afferent limb of the micturition reflex is often compromised following bladder injury, disease and inflammatory conditions. We have previously demonstrated that transforming growth factor-β (TGF-β) signalling contributes to increased voiding frequency and decreased bladder capacity with cystitis. Despite the functional presence of TGF-β in bladder inflammation, the precise mechanisms of TGF-β mediating bladder dysfunction are not yet known. Thus, the present studies investigated the sensory components of the urinary bladder that may underlie the pathophysiology of aberrant TGF-β activation. We utilized bladder-pelvic nerve preparations to characterize bladder afferent nerve discharge and the mechanisms of urothelial ATP release with distention. Our findings indicate that bladder afferent nerve discharge is sensitive to elevated extracellular ATP during pathological conditions of urinary bladder inflammation or irritation. We determined that TGF-β1 may increase bladder afferent nerve excitability by stimulating ATP release from the urothelium via vesicular exocytosis mechanisms with minimal contribution from pannexin-1 channels. Furthermore, blocking aberrant TGF-β signalling in cyclophosphamide-induced cystitis with TβR-1 inhibition decreased afferent nerve hyperexcitability with a concomitant decrease in urothelial ATP release. Taken together, these results establish a role for purinergic signalling mechanisms in TGF-β-mediated bladder afferent nerve activation that may ultimately facilitate increased voiding frequency. The synergy between intrinsic urinary bladder signalling mechanisms and an inflammatory mediator provides novel insight into bladder dysfunction and supports new avenues for therapeutic intervention.

Phosphorylation of syntaxin-3 at Thr 14 negatively regulates exocytosis in RBL-2H3 mast cells.

Recent studies have revealed that soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins interact with each other, forming a SNARE complex that induces exocytosis in mast cells. Previously, we reported that syntaxin-3, a SNARE protein, regulates mast cell exocytosis and is constantly phosphorylated. In this study, we tried to identify the amino acid residue that is phosphorylated in mast cells, and to elucidate the regulatory mechanism of exocytosis by phosphorylation in syntaxin-3. We found that Thr 14 of syntaxin-3 was a phosphorylation site in mast cells. In addition, the overexpression of a constitutively dephosphorylated syntaxin-3 (T14A) mutant enhanced mast cell exocytosis. We also showed that the phosphomimetic mutation of syntaxin-3 at Thr 14 (T14E) induced structural changes in syntaxin-3, and this mutation inhibited binding of syntaxin-3 to Munc18-2. These results suggest that phosphorylated syntaxin-3 at Thr 14 negatively regulates mast cell exocytosis by impairing the interaction between syntaxin-3 and Munc18-2.

Editorial: Determinants of Synaptic Information Transfer: From Ca(2+) Binding Proteins to Ca(2+) Signaling Domains.

Editorial: Determinants of Synaptic Information Transfer: From Ca(2+) Binding Proteins to Ca(2+) Signaling Domains.

Gliotransmitter Release from Astrocytes: Functional, Developmental, and Pathological Implications in the Brain.

Astrocytes comprise a large population of cells in the brain and are important partners to neighboring neurons, vascular cells, and other glial cells. Astrocytes not only form a scaffold for other cells, but also extend foot processes around the capillaries to maintain the blood–brain barrier. Thus, environmental chemicals that exist in the blood stream could have potentially harmful effects on the physiological function of astrocytes. Although astrocytes are not electrically excitable, they have been shown to function as active participants in the development of neural circuits and synaptic activity. Astrocytes respond to neurotransmitters and contribute to synaptic information processing by releasing chemical transmitters called “gliotransmitters.” State-of-the-art optical imaging techniques enable us to clarify how neurotransmitters elicit the release of various gliotransmitters, including glutamate, D-serine, and ATP. Moreover, recent studies have demonstrated that the disruption of gliotransmission results in neuronal dysfunction and abnormal behaviors in animal models. In this review, we focus on the latest technical approaches to clarify the molecular mechanisms of gliotransmitter exocytosis, and discuss the possibility that exposure to environmental chemicals could alter gliotransmission and cause neurodevelopmental disorders.

Botulismo: Revisão dos aspectos toxicológicos e perspectivas terapêuticas. (Parte I)

The food poisoning is one of significant public health problems, which high incidence even in developed countries. Although rare, botulism is among the most severe and fatal poisonings existing because the great power of the toxin, diagnostic difficulty and short therapeutic margin of antidotes used. With the knowledge of the biochemical mechanisms of vesicular exocytosis, including the composition of the machinery exocytotic SNARE (Soluble N-ethylmaleimide-sensitive factor [NSF] attacchment receptor) and its corresponding set of proteins involved in exocytotic steps, it became possible to elucidate the mechanism toxic action of botulinum toxin, and direct studies of potential therapeutic agents for botulism. In this work, we will review on botulism, discuss current information about the mechanism of exocytosis by linking them to new toxicological data of botulinum neurotoxin; in the Part II, we’ll describe recent research, advances and perspectives in the production of antidotes with greater therapeutic margin in the context of botulism.

AD genetic risk factors and tau spreading.

Development of tau pathology is associated with progressive neuronal loss and cognitive decline. In the brains of Alzheimer's disease (AD) patients, tau pathology propagates according to an anatomically defined pattern with relatively uniform distribution, and contributes to cognitive decline in age-associated tauopathy (Braak and Braak, 1991; Saito et al., 2004). Recently, it has been revealed that tau, which is an intracellular protein, can appear in the extracellular space, likely due to an exocytosis mechanism. Such extracellular tau could then be internalized into neighboring cells in at least two different ways depending on its aggregation state. In the case of soluble monomeric or small oligomeric tau protein, the endocytosis appears to be clathrin dependent (reviewed in Rubinsztein, 2006). In contrast, larger aggregates of tau could bind heparin in the extracellular matrix and be internalized through macropinocytosis (Holmes et al., 2014). As a result of exocytosis and endocytosis, the spreading of tau can occur in various neurodegenerative diseases (tauopathies) including AD. In this opinion article we have focused on the endocytosis mechanism. Several genetic risk factors have been associated with a higher probability of developing sporadic Alzheimer's disease (SAD). The Alzheimer Association (http://www.alzforum.org/) has ranked the top six risk genes, shown in Table ​Table1,1, based on genome-wide association studies (GWAS). Table 1 The top six AD risk genes that interact with tau.

The molecular requirements for the G protein betagamma‐SNARE interaction

Inhibitory G‐protein coupled receptors (GPCRs) can attenuate the secretion of hormones and neurotransmitters in an Ca2+‐dependent fashion. It has been shown that one mechanism for this inhibition involves the direct interaction of G‐protein βγ dimers (Gβγ) with the soluble N‐ethylmaleimide sensitive attachment protein receptor (SNARE) proteins syntaxin1A (Stx1A) and SNAP‐25. We have previously shown that Gβγ competes with the calcium sensor synaptotagmin 1 (Syt1) for binding sites upon SNARE proteins in a Ca2+ dependent manner. The goal of this study is to determine the individual residues required for the Gβγ‐SNARE interaction. To investigate the molecular basis for this interaction, prior studies performed by our group utilizing a “peptide mapping” approach have shown that both the N‐terminal and the C‐terminal domains of SNAP‐25 are essential for the interaction of Gβγ with SNAP‐25. Here, utilizing a similar approach, we demonstrate that the N‐terminal Habc domain is essential for the interaction of Gβγ with Stx1A. The interaction is mediated through two positively‐charged residues: K55 and R125. Mutation of these residues to Ala abolishes Gβγ binding in vitro, while retaining the ability to bind Syt1. We anticipate that these studies will enhance our understanding of the GPCR‐mediated regulatory mechanisms of exocytosis.

Relating structure and function of inner hair cell ribbon synapses.

In the mammalian cochlea, sound is encoded at synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs). Each SGN receives input from a single IHC ribbon-type active zone (AZ) and yet SGNs indefatigably spike up to hundreds of Hz to encode acoustic stimuli with submillisecond precision. Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands. However, we are only beginning to understand key features such as stimulus–secretion coupling, exocytosis mechanisms, exo–endocytosis coupling, modes of endocytosis and vesicle reformation, as well as replenishment of the readily releasable pool. Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses. Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

Cellular mechanisms of alpha herpesvirus egress: live cell fluorescence microscopy of pseudorabies virus exocytosis.

Egress of newly assembled herpesvirus particles from infected cells is a highly dynamic process involving the host secretory pathway working in concert with viral components. To elucidate the location, dynamics, and molecular mechanisms of alpha herpesvirus egress, we developed a live-cell fluorescence microscopy method to visualize the final transport and exocytosis of pseudorabies virus (PRV) particles in non-polarized epithelial cells. This method is based on total internal reflection fluorescence (TIRF) microscopy to selectively image fluorescent virus particles near the plasma membrane, and takes advantage of a virus-encoded pH-sensitive probe to visualize the precise moment and location of particle exocytosis. We performed single-particle tracking and mean squared displacement analysis to characterize particle motion, and imaged a panel of cellular proteins to identify those spatially and dynamically associated with viral exocytosis. Based on our data, individual virus particles travel to the plasma membrane inside small, acidified secretory vesicles. Rab GTPases, Rab6a, Rab8a, and Rab11a, key regulators of the plasma membrane-directed secretory pathway, are present on the virus secretory vesicle. These vesicles undergo fast, directional transport directly to the site of exocytosis, which is most frequently near patches of LL5β, part of a complex that anchors microtubules to the plasma membrane. Vesicles are tightly docked at the site of exocytosis for several seconds, and membrane fusion occurs, displacing the virion a small distance across the plasma membrane. After exocytosis, particles remain tightly confined on the outer cell surface. Based on recent reports in the cell biological and alpha herpesvirus literature, combined with our spatial and dynamic data on viral egress, we propose an integrated model that links together the intracellular transport pathways and exocytosis mechanisms that mediate alpha herpesvirus egress.

NOX2-Derived ROS-Mediated Surface Translocation of BLT1 Is Essential for Exocytosis in Human Eosinophils Induced by LTB4

Background: Leukotriene B₄ (LTB₄) is a proinflammatory lipid mediator that elicits eosinophil exocytosis, leading to allergic inflammation. However, the detailed intracellular signaling mechanisms of eosinophil exocytosis induced by LTB₄ are poorly understood. Herein, we report that NADPH oxidase (NOX)2-derived reactive oxygen species (ROS)-mediated BLT1 migration to the cell surface is required for exocytosis in human eosinophils induced by LTB₄. Methods: Peripheral blood eosinophils were purified and stimulated for up to 60 min with LTB₄. The signaling role of NOX2-derived ROS in BLT1-dependent exocytosis in LTB₄-stimulated eosinophils was investigated. Results: Stimulating eosinophils with LTB₄ induced intracellular ROS production and surface upregulation of the exocytosis marker protein CD63 via BLT1-mediated signaling. LTB₄ induced p47^phox phosphorylation and 91^phox expression required for NOX2 activation in a BLT1-dependent manner. Pretreatment with NOX2 inhibitors, but not mitochondria inhibitor, prevented LTB₄-induced ROS generation and exocytosis. At 30 min after stimulation with LTB₄, BLT1 expression at the cell surface was upregulated. LTB₄-triggered surface upregulation of BLT1 was also blocked by inhibition of ROS generation with NOX2 inhibitors. Moreover, stimulation for 30 min with LTB₄ resulted in the interaction of BLT1 with NOX2 by immunoprecipitation. LTB₄-induced ROS generation, surface upregulation of BLT1 and exocytosis was also inhibited by pretreatment with a lipid raft disruptor, protein kinase C inhibitor, or Src kinase inhibitor. Conclusion: These results suggest that NOX2-derived ROS-mediated BLT1 trafficking to the cell surface plays a key role in the exocytosis of human eosinophils induced by LTB₄.

Isolation and characterization of human intestinal mast cells.

Mast cells are granulated immune cells typically located at barrier sites of the body, such as the skin and the mucosa of the respiratory, urogenital, and gastrointestinal tract. They are well known for their capacity to participate in the orchestration of inflammatory and immune responses by releasing a broad array of mediators as a consequence of IgE-dependent and IgE-independent activation. Mast cells derive from myeloid progenitors, but in contrast to other myeloid cells, they leave the bone marrow in an immature state; therefore, mast cells are not visible in the blood under normal conditions. For full maturation, the tissue environment is necessary. Thus, mature mast cells can be only isolated from tissue such as skin or mucosal sites, which makes mast cell isolation complicated. This chapter describes methods to isolate, purify, and culture mast cells from the human intestinal mucosa. Human mucosal mast cells can be used to characterize their mediators and to study the mechanisms of human mast cell activation, signal transduction, and exocytosis in response to specific stimuli.

Uniquantal Release through a Dynamic Fusion Pore Is a Candidate Mechanism of Hair Cell Exocytosis

The mechanisms underlying the large amplitudes and heterogeneity of excitatory postsynaptic currents (EPSCs) at inner hair cell (IHC) ribbon synapses are unknown. Based on electrophysiology, electron and superresolution light microscopy, and modeling, we propose that uniquantal exocytosis shaped by a dynamic fusion pore is a candidate neurotransmitter release mechanism in IHCs. Modeling indicated that the extended postsynaptic AMPA receptor clusters enable large uniquantal EPSCs. Recorded multiphasic EPSCs were triggered by similar glutamate amounts as monophasic ones and were consistent with progressive vesicle emptying during pore flickering. The fraction of multiphasic EPSCs decreased in absence of Ca(2+) influx and upon application of the Ca(2+) channel modulator BayK8644. Our experiments and modeling did not support the two most popular multiquantal release interpretations of EPSC heterogeneity: (1) Ca(2+)-synchronized exocytosis of multiple vesicles and (2) compound exocytosis fueled by vesicle-to-vesicle fusion. We propose that IHC synapses efficiently use uniquantal glutamate release for achieving high information transmission rates.

The effects of hypokalaemia on the hormone exocytosis in adenohypophysis and prolactinoma cell culture model systems.

The extracellular ion milieu determines the exocytosis mechanism that is coupled to spontaneous electrical activity. The K(+) ion plays crucial role in this mechanism: as the potassium current is associated with membrane hyperpolarization and hormone release through protein cascade activation. The primary aim of this study was to investigate the response mechanisms of normal adenohypophysis and adenohypophyseal prolactinoma cell populations at different extracellular K(+) levels with an otherwise isoionic milieu of all other essential ions. We focused on prolactin (PRL) and adrenocorticotrophic hormone (ACTH) release.In our experimental study, female Wistar rats (n=20) were treated with estrone-acetate (150 μg/kg b.w./week) for 6 months to induce prolactinomas in the adenohypophysis. Primary, monolayer cell cultures were prepared by enzymatic and mechanical digestion. PRL and ACTH hormone presence was measured by radioimmunoassay or immuno-chemiluminescence assay. Immunocytochemistry was used to assess the apoptotic cells.Differences between the effects of hypokalaemia on normal adenohypophysis cultures and prolactinoma cell populations were investigated. Significant alteration (p<0.001, n=10) in hormone exocytosis was detected in K(+) treated adenohypophyseal and prolactinoma cell cultures compared to untreated groups. Immunocyto-chemistry showed that Bcl-2 expression was reduced under hypokalaemic conditions.The decrease in hormone exocytosis was tightly correlated to the extracellular K(+) in both cell types, leading to the conclusion that external K(+) may be the major factor for the inhibition of hormone release. The significant increase in hormone content in supernatant media suggests that hypokalaemia may play important role in apoptosis.