Lysosomes Are the Major Vesicular Compartment Undergoing Ca2+-Regulated Exocytosis from Cortical Astrocytes

Anaphylaxis is a notorious type 2 immune response which may result in a systemic response and lead to death. A precondition for the unfolding of the anaphylactic shock is the secretion of inflammatory mediators from mast cells in response to an allergen, mostly through activation of the cells via the IgE-dependent pathway. While mast cells are specialized secretory cells that can secrete through a variety of exocytic modes, the most predominant mode exerted by the mast cell during anaphylaxis is compound exocytosis—a specialized form of regulated exocytosis where secretory granules fuse to one another. Here, we review the modes of regulated exocytosis in the mast cell and focus on compound exocytosis. We review historical landmarks in the research of compound exocytosis in mast cells and the methods available for investigating compound exocytosis. We also review the molecular mechanisms reported to underlie compound exocytosis in mast cells and expand further with reviewing key findings from other cell types. Finally, we discuss the possible reasons for the mast cell to utilize compound exocytosis during anaphylaxis, the conflicting evidence in different mast cell models, and the open questions in the field which remain to be answered.

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Synaptic Transmission in the Immune System

e-Neuroforum ◽

10.1515/nf-2016-a052 ◽

2017 ◽

Vol 23 (4) ◽

Author(s):

Jens Rettig ◽

David R. Stevens

Keyword(s):

Nervous System ◽

Immune System ◽

Synaptic Transmission ◽

Molecular Mechanisms ◽

Secretory Granules ◽

Neuroendocrine Cells ◽

Regulated Exocytosis ◽

Protein Receptor ◽

Immunological Synapses ◽

Immunological Diseases

AbstractThe release of neurotransmitters at synapses belongs to the most important processes in the central nervous system. In the last decades much has been learned about the molecular mechanisms which form the basis for this fundamental process. Highly regulated exocytosis, based on the SNARE (soluble N-ethylmaleimide-sensitive attachment protein receptor) complex and its regulatory molecules is the signature specialization of the nervous system and is shared by neurons and neuroendocrine cells. Cells of the immune system use a similar mechanism to release cytotoxic materials from secretory granules at contacts with virally or bacterially infected cells or cancer cells, in order to remove these threats. These contact zones have been termed immunological synapses in reference to the highly specific targeted exocytosis of effector molecules. Recent findings indicate that mutations in SNARE or SNARE-interacting proteins are the basis of a number of devastating immunological diseases. While SNARE complexes are ubiquitous and mediate a wide variety of membrane fusion events it is surprising that in many cases the SNARE proteins involved in immunological synapses are the same molecules which mediate regulated exocytosis of transmitters and hormones in neurons and neuroendocrine cells. These similarities raise the possibility that results obtained at immunological synapses may be applicable, in particular in the area of presynaptic function, to neuronal synapses. Since immunological synapses (IS) are assembled and disassembled in about a half an hour, the use of immune cells isolated from human blood allows not only the study of the molecular mechanisms of synaptic transmission in human cells, but is particularly suited to the examination of the assembly and disassembly of these “synapses” via live imaging. In this overview we discuss areas of similarity between synapses of the nervous and immune systems and in the process will refer to results of our experiments of the last few years.

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Redistribution of a rab3-like GTP-binding protein from secretory granules to the Golgi complex in pancreatic acinar cells during regulated exocytosis

The Journal of Cell Biology ◽

10.1083/jcb.124.1.43 ◽

1994 ◽

Vol 124 (1) ◽

pp. 43-53 ◽

Cited By ~ 54

Author(s):

BP Jena ◽

FD Gumkowski ◽

EM Konieczko ◽

GF von Mollard ◽

R Jahn ◽

...

Keyword(s):

Plasma Membrane ◽

Golgi Complex ◽

Binding Protein ◽

Secretory Granules ◽

Acinar Cells ◽

Pancreatic Acinar Cells ◽

Apical Plasma Membrane ◽

Gtp Binding Protein ◽

Regulated Exocytosis ◽

Gtp Binding

Regulated secretion from pancreatic acinar cells occurs by exocytosis of zymogen granules (ZG) at the apical plasmalemma. ZGs originate from the TGN and undergo prolonged maturation and condensation. After exocytosis, the zymogen granule membrane (ZGM) is retrieved from the plasma membrane and ultimately reaches the TGN. In this study, we analyzed the fate of a low M(r) GTP-binding protein during induced exocytosis and membrane retrieval using immunoblots as well as light and electron microscopic immunocytochemistry. This 27-kD protein, identified by a monoclonal antibody that recognizes rab3A and B, may be a novel rab3 isoform. In resting acinar cells, the rab3-like protein was detected primarily on the cytoplasmic face of ZGs, with little labeling of the Golgi complex and no significant labeling of the apical plasmalemma or any other intracellular membranes. Stimulation of pancreatic lobules in vitro by carbamylcholine for 15 min, resulted in massive exocytosis that led to a near doubling of the area of the apical plasma membrane. However, no relocation of the rab3-like protein to the apical plasmalemma was seen. After 3 h of induced exocytosis, during which time approximately 90% of the ZGs is released, the rab3-like protein appeared to translocate to small vesicles and newly forming secretory granules in the TGN. No significant increase of the rab3-like protein was found in the cytosolic fraction at any time during stimulation. Since the protein is not detected on the apical plasmalemma after stimulation, we conclude that recycling may involve a membrane dissociation-association cycle that accompanies regulated exocytosis.

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C2C12 myocytes lack an insulin-responsive vesicular compartment despite dexamethasone-induced GLUT4 expression

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00092.2002 ◽

2002 ◽

Vol 283 (3) ◽

pp. E514-E524 ◽

Cited By ~ 38

Author(s):

Lori L. Tortorella ◽

Paul F. Pilch

Keyword(s):

Glucose Transporter ◽

Fold Increase ◽

Snare Proteins ◽

Vesicular Traffic ◽

Glut4 Expression ◽

Protein Receptor ◽

Syntaxin 4 ◽

Insulin Dependent ◽

Glucose Transporter Glut4 ◽

Vesicular Compartment

Insulin regulates the uptake of glucose into skeletal muscle and adipocytes by redistributing the tissue-specific glucose transporter GLUT4 from intracellular vesicles to the cell surface. To date, GLUT4 is the only protein involved in insulin-regulated vesicular traffic that has this tissue distribution, thus raising the possibility that its expression alone may allow formation of an insulin-responsive vesicular compartment. We show here that treatment of differentiating C2C12myoblasts with dexamethasone, acting via the glucocorticoid receptor, causes a ≥10-fold increase in GLUT4 expression but results in no significant change in insulin-stimulated glucose transport. Signaling from the insulin receptor to its target, Akt2, and expression of the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor, or SNARE, proteins syntaxin 4 and vesicle-associated membrane protein are normal in dexamethasone-treated C2C12 cells. However, these cells show no insulin-dependent trafficking of the insulin-responsive aminopeptidase or the transferrin receptor, respective markers for intracellular GLUT4-rich compartments and endosomes that are insulin responsive in mature muscle and adipose cells. Therefore, these data support the hypothesis that GLUT4 expression by itself is insufficient to establish an insulin-sensitive vesicular compartment.

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