Secretion and exocytosis

The homology of p36 with lipocortin II and its ability to inhibit pancreatic phospholipase A2 suggests a possible but unproven role as inhibitor of intracellular phospholipase A2 [86]. Phospholipase A2 activity provides one of the many means of arachidonate production which would have important consequences to the activation of secretory response and membrane fusion events. The observation that the 40 kDa protein kinase C substrate, whose phosphorylation state changes dramatically upon platelet activation, may partially cross-react with a polyclonal antibody raised to rat renal lipocortin, is therefore interesting [120]. An alternate sug- 

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Figure 6. Diagram illustrating the modern-day functioning of the endomembrane system in secretion, exocytosis and endocytosis. Secretion and exocytosis allow delivery of products of synthesis to contiguous membrane compartments and eventually their release to the cell s exterior. Endocytosis provides a means whereby insoluble materials, large particles or even cells or cell parts, may be engulfed to gain entry into the cell s interior. Endocytosis also provides a mechanism of membrane removal from the cell surface to compensate for excess membrane delivered during secretion in nonexpanding cells and for membrane replacement and renewal.

Fig. 11.4. Model for cholinergic signalling in the intestinal mucosa, providing a possible rationale for AChE secretion by parasitic nematodes. ACh released from enteric cholinergic motor neurons stimulates chloride secretion, mucus secretion and Paneth cell exocytosis through muscarinic receptors. Secretory responses may be modulated by mast cell mediators, either directly or via the induction of neural reflex programmes. The role of muscarinic receptor-positive cells in the lamina propria of rats infected with N. brasiliensis is undetermined, as are potential mechanisms of trans-epithelial transport of the enzymes. Adapted from Cooke (1984).

Defining the biochemical mechanisms that couple stimulation at the cell surface to secretion by exocytosis is a major focus in biological research. In the mast cell, the steps by which immunologic secretagogues (for example, specific antigen, anti-IgE antibodies) activate a secretory response have been extensively studied and there are several reviews covering this area [ 12-14, 16, 121, 177],... 

Clearly, the most studied aspect of stimulus-secretion coupling is the requirement for calcium. That Ca ions are essential for many cellular processes has been known since the days of Sidney Ringer [178], The eloquent studies of Douglas and his collaborators [179, 180] and others [181] firmly established the necessity for Ca in exocytotic secretion and set forth the notion that an increase in the level of free intracellular Ca, [Ca2+]j was responsible for initiating exocytosis. Evidence for increases in the level of intracellular free Ca as prerequisite for initiating exocytosis by cell surface stimulation is now available from studies using a variety of systems [181-183]. 

Secretory glycoproteins are known to move from the ER to the Golgi complex where their carbohydrate side chains are trimmed and further modified (see Palade, 1975 Tartakoff, 1980 Farquhar and Palade, 1981). In most secretory cells the proteins are then concentrated into condensing vacuoles which store the secretory proteins until they are discharged by exocytosis through a fusion reaction between the vacuolar membrane and the plasma membrane. In other secretory cells, like plasma cells, proteins are condnuously secreted and they appear to leave the Golgi complex within vesicles without being concentrated before exocytosis. 

A process similar to endocytosis occurs in the reverse direction when it is known as exocytosis (Figure 5.11). Membrane-bound vesicles in the cytosol fuse with the plasma membrane and release their contents to the outside of the cell. Both endocytosis and exocytosis are manifestations of the widespread phenomenon of vesicular transport, which not only ferries materials in and out of cells but also between organelles, e.g. from the endoplasmic reticulum to the Golgi and then to the lysosomes or to the plasma membrane for secretion (Chapter 1). Many hormones are also secreted in this way, as are neurotransmitters from one nerve into a synaptic junction that joins two nerves (Chapters 12 and 14). 

Nickerson, S. C., Smith, J. J. and Keenan, T. W. 1980A. Role of microtubules in milk secretion—action of colchicine on microtubules and exocytosis of secretory vesicles in rat mammary epithelial cells. Cell Tiss. Res. 207, 361-376. 

In Chapter 11 the effects of binding of hormones to cell surface receptors have been emphasized. Equally important are the mechanisms that control the secretion of hormones. The topic of exocytosis has been considered briefly in Chapter 8, Section C,6 and aspects of the Golgi in Fig. 20-8 and associated text. Both hormones and neurotransmitters are secreted by exocytosis of vesicles. Cells have two pathways for secretion.386 387 The constitutive pathway is utilized for continuous secretion of membrane constituents, enzymes, growth factors, viral proteins, and components of the extracellular matrix. This pathway carries small vesicles that originate in the trans-Golgi network (TGN Fig. 20-8). The regulated pathway is utilized for secretion of hormones and neurotransmitters in response to chemical, electrical, or other stimuli. 

Neuropeptides are secreted by exocytosis of large dense-core vesicles (LDCVs) outside of synapses (Figure 1 Salio et al., 2006). LDCVs undergo exocytosis in all parts of a neuron, most often in axon terminals and dendrites. Monoamines are often co-stored with neuropeptides in LDCVs and co-secreted with them upon exocytosis. For all intents and purposes, LDCV-mediated secretion resembles hormone secretion in endocrine cells. 

Verdugo P. (1990). Goblet cells secretion and mucogenesis. Annu Rev Physiol 52, 157-176 Verdugo P. (1991). Mucin exocytosis. Am Rev Respir Dis 144, S33-S37 Vinall L.E., Pratt W.S. and Swallow D.M. (2000). Detection of mucin gene polymorphism. Methods Mol Biol 125, 337-350... 

Such electrodes have been used to examine insulin secretion from single pancreatic P cells. A stimulant is introduced to contact a single P cell adhering to the bottom of a petri dish. The microelectrode is brought into contact with the cell. The result (representing insulin secretion) is shown in Fig 14.45. The peaks shown are Ca2+ dependent, and this is characteristic of an exocytotic process (Section 14.10.1). The area under the peak represents 360,000 insulin molecules. The results show that the spikes correspond to the ejection of packets of insulin secreted in exocytosis. 

These results implicate membrane-specific functions for signaling and exocytosis that allow secretory vesicles to produce, store, and secrete active neuropeptides for the control of physiological functions. 

Cockcroft, S., Howell, T.W. and Gomperts, B.D. (1987). Two G proteins act in series to control stimulus-secretion coupling in mast cells use of neomycin to distinguish between G-proteins controlling polyphosphoinositide phosphodiesterase and exocytosis. J. Cell Biol., 105, 2745-2750. 

Permeabilized secretory cells are widely used to study the final events during secretion by exocytosis. Convenient cellular models are bovine adrenal chromaffin cells in short term culture and the rat pheochromocytoma cell line PC 12. Both cell types take up labeled catecholamines and store them in secretory vesicles, from which they can be released upon stimulation. The released catecholamines can be detected in the supernatant. After permeabilization of the plasma membrane, release of catecholamines can be triggered by micromolar concentrations of Ca. ... 

Fig. 20. Hypothetical model of how insulin secretion is regulated. The most important event is the depolarization of the B-cell which causes Ca"+ influx along L-type Ca2+ channels and subsequent increase in cytosolic Ca"+. Depolarization is produced by nutrient (glucose) metabolism via an increase in B-cell ATP and/or ATP/ADP ratio which closes KAXP channels. Also, sulphonylureas, at a distinct location, close KATP channels. The increase in [Ca2+]j activates CaCaMK. Ca2+ uptake appears to be modulated by nutrient metabolism (redox state of NAD(P)H and GSH). Insulin release in response to depolarization is also modulated by factors affecting PLC and adenylate cyclase. Here, production of IP3 leads to release of stored Ca2+ from the endoplasmic reticiulum. DAG activates PKC whereas cAMP activates PKA. CaMK, PKC and PKA cause protein phosphorylations which finally cause granule movement and exocytosis. But there will also be other effects of phosphorylations related to stimulus-secretion coupling, e.g. a possible interaction with voltage-dependent Ca2+ channels.

C. Amatore, S. Arbault, M. Guille and F. Lemaitre, Electrochemical monitoring of single cell secretion Vesicular exocytosis and oxidative stress, Chemical Reviews, 108(7), 2585-2621 (2008). 

Neurotransmitters are released by exocytosis, a process in which neurotransmitter-filled synaptic vesicles fuse with the axonal membrane, releasing their contents into the synaptic cleft. The exocytosis of neurotransmitters from synaptic vesicles involves vesicle-targeting and fusion events similar to those that occur during the intracellular transport of secreted and plasma-membrane proteins (Chapter 17). Two features... 

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