Calcium shares the limelight in stimulus-secretion coupling

The diagnostic significance of the induced acrosome reaction

In trying to discern potential causes of male infertility, little investigation has been done to determine if there is predictive value in assessing acrosome reaction inducibility of human spermatozoa for clinical interpretation and application. The purpose of this review is to outline what is currently known in terms of the diagnostic significance of the induced acrosome reaction and to discuss what might be anticipated for the future. In constructing this review, every effort has been made to provide a comprehensive literature review for the reader and a number of review articles have been cited to serve as sources for additional related material. Regarding the aforementioned, it is essential to qualify that, with the exception of the Section entitled: ‘General characteristics of capacitation and the acrosome reaction’, only characteristics, properties and areas relevant to human spermatozoa will be addressed. Thus, in total, this review will: (1) provide a general description of capacitation and the acrosome reaction; (2) briefly characterize the acrosome and its properties; (3) address the signalling mechanism(s) by which the acrosome reaction occurs; (4) outline various methods for detecting the acrosome reaction; (5) definein vitroculture conditions that facilitate the acrosome reaction; (6) define agents, i.e., biological and chemical, that induce the reaction; (7) discuss the association of the acrosome reaction with fertilization; (8) discuss the clinical implications of the induced acrosome reaction; (9) discuss the relevance of sperm function assays in general; and lastly, (10) discuss future application of acrosome reaction assessment for the clinical diagnosis of male infertility.

Calcium: an important second messenger in mast cells

Recently there has been considerable controversy over the mechanism(s) by which intracellular Ca2+ is elevated when receptors for IgE on the surface of mast cells are aggregated by antigen. The central role played by calcium in the initiation of secretion from these cells has also been called into question. In a mast cell line which has been widely used to study stimulus-secretion coupling in non-excitable cells it is now clear that calcium is indeed important in the physiological response of the cells but that other intracellular messengers are also involved. In addition it has been shown that while the increase in intracellular Ca2+ probably originates from intracellular stores it can only be sustained by the influx of calcium across the plasma membrane. The nature of the Ca2+ permeability pathway has yet to be elucidated although a number of candidates for the calcium channel in mast cells have been proposed. Significant oscillations and spatial gradients of Ca2+ are often seen when the responses of individual antigen-stimulated cells are measured using digital imaging microscopy. The complexity of these responses highlights the importance of single-cell measurements in elucidating the relationship between IgE receptor activation, Ca2+ movements and exocytosis.

Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Hyphal tip growth is a complex process involving finely regulated interactions between the synthesis and expansion of cell wall and plasma membrane, diverse intracellular movements, and turgor regulation. F-actin is a major regulator and integrator of these processes. It directly contributes to (a) tip morphogenesis, most likely by participation in an apical membrane skeleton that reinforces the apical plasma membrane, (b) the transport and exocytosis of vesicles that contribute plasma membrane and cell wall material to the hyphal tips, (c) the localization of plasma membrane proteins in the tips, and (d) cytoplasmic and organelle migration and positioning. The pattern of reorganization of F-actin prior to formation of new tips during branch initiation also indicates a critical role in early stages of assembly of the tip apparatus. One of the universal characteristics of all critically examined tip-growing cells, including fungal hyphae, is the obligatory presence of a tip-high gradient of cytoplasmic Ca2+ that probably regulates both actin and nonactin components of the apparatus, and the formation of which may also initiate new tips. This review discusses the diversity of evidence behind these concepts.

Flow sensing in mycelial fungi

Hyphae of a variety of mycelial fungi were shown to reorient their direction of growth to grow upstream in a continuous flowing current of liquid medium. Among those tested fungi, Aspergillus niger hyphae was most polarized towards the upstream. Hyphae of A. niger which had been attached to poly-L-lysine coated plates on the base of a continuous-culture flow cell were exposed to a current of medium of flow rates in the range of 0-6400 microns s-1 for 1-4 h. Hyphal tips quickly reoriented towards the upstream of the flowing medium and became aligned parallel to the flow axis as they grew larger. At flow rates above 1600 microns s-1, however, the percentage of polarization of hyphae reached a constant level. With other fungi, germ tube emergence was not polarized but hyphae and branches grew towards the upstream. There was no detectable difference in the pH and oxygen concentration of the flowing medium between inlet and outlet of the flow cell. This result suggests that hyphae of mycelial fungi sense transient flow of liquid medium over their surface and reorient their direction to grow upstream.

Inhibition of rat mitochondrial functions in vivo by 6-OHDA and reserpine

Reserpine caused a decrease in the state 3, respiratory control ratio (RCR) and ADP/O ratio in frontal cortex, striatum and liver of rats 1 h after drug administration. State 4 respiratory rate was stimulated in frontal cortex and striatum. In the liver, 6-OHDA decreased the ADP/O ratio when both pyruvate/malate and succinate were used as substrates. Reserpine induced changes in the activities of Na+K(+)-ATPase and Mg(2+)-ATPase in frontal cortex and liver 15 min and 4 h after administration of the drug. In the liver only 6-OHDA caused the depression of Mg(2+)-ATPase activity (P < 0.05). Reserpine altered the levels of K+, Na+ and Ca2+ cations in rat frontal cortex and striatum, while 6-OHDA caused a decrease in the amount of Mg2+ in liver (P < 0.05).

A calcium influx is neither strictly associated with nor necessary for exocytotic membrane fusion in Paramecium cells

Exocytosis of trichocysts in Paramecium cells was generally believed to depend on extracellular Ca, since it is accompanied by a Ca influx and not seen in the absence of Ca. However, by short term removal of Ca we showed recently that only extrusion of secretory contents, but not membrane fusion after stimulation with aminoethyldextran (AED), depends on extracellular Ca. We have now extended these studies to longer times and shown that membrane fusion is stimulated by AED even after 1 min at low Ca (< or = 30 nM). At prolonged times membrane fusion was induced by sole removal of Ca. In the presence of AED, trichocyst contents were slowly extruded followed by resealing of the fused membranes, indicating independency of endocytotic membrane fusion from extracellular Ca (though we observed aberrant resealing). Later on, Ca removal is followed by cell death. By using videomicroscopy, we further provide the first evidence that exocytosis is not necessarily accompanied by an influx of Ca in the presence of the usual high concentrations (1 mM), since local exocytosis at the rear end of the cells is not followed by ciliary reversal which is triggered by Ca influx. We conclude that a Ca influx is neither regularly associated with, nor necessary for, induction of exocytotic membrane fusion in Paramecium cells. As a source for a possible alternative intracellular liberation of calcium during exocytosis, we analyzed the subplasmalemmal alveolar sac system by electron spectroscopic imaging and found indications for Ca redistributions shortly after stimulation.

Biogenesis of secretory granules. Implications arising from the immature secretory granule in the regulated pathway of secretion

In endocrine cells the regulated secretion of hormones, peptides, enzymes and neurotransmitters into the external medium occurs when mature secretory granules fuse with the plasma membrane. Secretory granules form at the trans-Golgi network (TGN) by envelopment of the dense-core aggregate of regulated secretory proteins by a specific membrane. The secretory granules initially formed at the TGN, referred to here as immature secretory granules, are morphologically and biochemically distinct from mature secretory granules. The functional similarities and differences between the immature secretory granule and the mature secretory granule, and the events involved in the maturation of the secretory granules are briefly discussed.

Trypsin inhibitors prevent the progesterone-initiated increase in intracellular calcium required for the human sperm acrosome reaction

Inhibitors of trypsin-like enzymes, benzamidine hydrochloride and 4'-acetamidophenyl 4-guanidinobenzoate (also an inhibitor of other serine proteases), were tested for their effects on the acrosome reaction (AR) of human sperm initiated by progesterone or the calcium ionophore ionomycin. The AR was assayed by indirect immunofluorescence and transmission electron microscopy. The trypsin inhibitors, when added 10 min prior to stimulation by progesterone, significantly inhibited the AR in comparison with progesterone treatment alone. Transmission electron microscopic examination of the sperm after progesterone treatment indicated that the inhibitors blocked the membrane fusion events of the AR. By contrast, when ionomycin (at final concentrations of 3 microM) was added to sperm preincubated in inhibitors, sperm underwent morphologically normal AR, acrosomal matrix loss was not inhibited, and the percentage of acrosome-reacted sperm was the same as that obtained in the absence of inhibitors. Using the cell calcium indicator fura-2, we further demonstrated that both trypsin inhibitors prevented the progesterone-stimulated rise in intracellular Ca2+ ([Ca2+]int) required for the AR, but did not affect [Ca2+]int in unstimulated sperm. These results suggest that sperm trypsin-like activity may be directly or indirectly involved in increasing sperm [Ca2+]int during stimulation by progesterone.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

Purification and characterization of a novel GTP-binding protein with a Mr value of 24,000 from rat liver

About 15% of the total GTP-binding proteins (G proteins) of rat liver homogenate was found in the microsomes-Golgi complex fraction. From this fraction, we purified to near homogeneity and characterized a G protein with a Mr value of 24,000 (24K G). 24K G specifically bound guanosine 5'-(3-Q-thio) triphosphate (GTP gamma S), GTP and GDP with a Kd value for GTP gamma S of about 30 nM. 24K G bound maximally about 0.7 mol of GTP gamma S/mol of protein. 24K G hydrolyzed GTP to liberate Pi with a turnover number of about 0.008 min-1. 24K G was not copurified with the beta gamma subunit of heterotrimeric G proteins. The partial amino acid sequences of 24K G revealed that this protein was a novel small G protein.

Rat pancreatic acini permeabilised with streptolysin O secrete amylase at Ca2+ concentrations in the micromolar range, when provided with ATP and GTP gamma S

The aim of this study was to define the conditions required for exocytosis in pancreatic acini permeabilised with the bacterial toxin streptolysin O. Treatment of a suspension of acini with streptolysin O caused the release of both the cytoplasmic enzyme lactate dehydrogenase and the zymogen granule enzyme amylase. The release of amylase occurred more quickly than that of lactate dehydrogenase and was smaller in magnitude. In addition, a component of amylase release occurred only in the presence of Ca2+ (at concentrations in the micromolar range), ATP and GTP gamma S. We conclude that this component represents an exocytotic event, but that the release of lactate dehydrogenase occurs through toxin-generated lesions. The concentrations of Ca2+, ATP and GTP gamma S causing half-maximal exocytosis were 0.7 microM, 0.2 mM and 10 microM, respectively. This system should permit a study of the mechanisms underlying regulated exocytosis in this cell type.

Small GTP-binding protein associated with Golgi cisternae

Eukaryotic cells seem to use GTP hydrolysis to regulate vesicular traffic in exocytosis and endocytosis. The best evidence for this comes from studies on the yeast Saccharomyces cerevisiae that have identified two small Ras-related GTP-binding proteins, Sec4p and Ypt1p, which control distinct stages of the secretory pathway. In mammalian cells the effects of a non-hydrolysable GTP analogue, GTP-gamma S, on different transport events have suggested that they also have proteins functionally related to yeast Sec4p and Ypt1p. The rab genes have recently been cloned and sequenced for rat and human and their proteins have highly conserved domains in common with Sec4p and Ypt1p (including a putative effector binding site). They are therefore good candidates for GTP-binding proteins involved in intracellular transport in mammalian cells. One of the Rab proteins (Rab1p) is the mammalian counterpart of Ypt1p (ref. 13). Here we report the localization of the protein Rab6p to the Golgi apparatus in several cell types. By immunolabelling and electron microscopy, Rab6p appears to be concentrated predominantly on the medial and trans cisternae and distributed over their entire surface.

The role of protein kinase C in histamine secretion from mast cells

Receptor activation on the cell surface is coupled through a guanine nucleotide regulatory protein to polyphosphoinositide phosphodiesterase. The activation of this enzyme catalyses the hydrolysis of phosphatidylinositol biphosphate. One of the products of this hydrolysis is diacylglycerol, which activates protein kinase C. It can also be activated by tumour-promoting phorbol esters. The synthetic diacylglycerol, 1-oleoyl-2-acetyl-rac-glycerol (OAG) and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) have been used to stimulate protein kinase C in a pure population of rat peritoneal mast cells. Both of them caused histamine release, but the rate of release with TPA or OAG alone was slow. The release was inhibited by blocking the oxidative energy metabolism with antimycin A, and was associated with progressive exocytosis, showing that it is a secretory process. Studies on the interaction between the stimulation of protein kinase C by OAG/TPA and the secretagogues showed a dual effect, both potentiation and inhibition. Antigen (in sensitized cells) and compound 48/80 showed this pattern of response. With the calcium ionophore, A23187, potentiation was the dominant effect, although some inhibition could be shown with TPA. This is possibly related to the large calcium influx which causes translocation of protein kinase C to the membranes and enhances its activity. The potentiation suggests that protein kinase C is involved in the secretion process by the secretagogues, while the inhibition reflects a regulatory function, which is apparently exerted through an inhibition of phosphatidylinositol breakdown. Calcium uptake was enhanced by both TPA and OAG. Protein kinase C may thus contribute to the replenishment of the intracellular calcium stores after the secretory response.

Evidence that receptor-linked G protein inhibits exocytosis by a post-second-messenger mechanism in AtT-20 cells

In AtT-20 cells somatostatin inhibits the secretion of adrenocorticotropic hormone (ACTH) through the activation of GTP binding proteins (G proteins) linked to second messengers such as calcium and cyclic AMP (cAMP). Recently, it has been proposed that there may be G proteins that regulate directly the exocytotic machinery. We have investigated whether somatostatin could inhibit secretion at a step distal to second messengers through a GTP binding protein. For these studies two experimental paradigms were used: (1) intact cells stimulated by calcium ionophores and (2) digitonin-permeabilized cells exposed to buffers of increasing Ca2+ concentrations. Somatostatin inhibited by 70% the ACTH release caused by the calcium ionophore ionomycin without modifying the ionophore-induced elevation in cytosolic [Ca2+]. This effect was cAMP independent because (1) it was observed in the presence of high concentrations of membrane-permeant cAMP analogues, and (2) it was not accompanied by a change in cAMP levels. The effect was also independent of the levels of activators of protein kinase C because it could be produced in the presence of high concentrations of phorbol esters. The action of somatostatin was prevented by pertussis toxin. In digitonin-permeabilized AtT-20 cells somatostatin inhibited release induced by calcium buffers in a GTP-dependent manner. These two observations indicate the involvement of a G protein. It is proposed that a G protein coupled to somatostatin receptors inhibits the intracellular machinery of secretion at a step distal to second messengers, perhaps at the exocytotic site.

Synergistic effects of calcium-mobilizing agents and adenosine on histamine release from rat peritoneal mast cells

1. Adenosine and its metabolically stable analogue N-ethyl-carboxamidoadenosine (NECA) enhance histamine release from rat peritoneal mast cells when these are stimulated by calcium-mobilizing agents. NECA and adenosine shift the concentration-response curve of the calcium ionophore A23187 to lower concentrations. 2. The potencies of NECA or adenosine in enhancing A23187-induced histamine release are dependent on the level of stimulated release in the absence of adenosine analogues. At high levels of release their potencies are up to 20 times higher than at low levels. Consequently, averaged concentration-response curves of adenosine and NECA for enhancing histamine release are shallow. 3. The adenosine transport blocker S-(p-nitrobenzyl)-6-thioinosine (NBTI) has no effect by itself at low levels of stimulated histamine release, but abolishes the enhancing effect of adenosine. At high levels of release, however, NBTI alone enhances the release of histamine. 4. It is concluded that adenosine and calcium reciprocally enhance the sensitivity of the secretory processes to the effects of the other agent. The levels of intracellular adenosine obtained by trapping adenosine inside stimulated mast cells are sufficient to enhance histamine release substantially, suggesting that this effect may play a physiological and pathophysiological role.

Pollen tube tip growth

This review considers pollen tube growth with regard to current information on pollen tube cytoplasm, wall structure and calcium ion interactions with pollen tubes. Pollen tubes have a marked cytoplasmic Polarity with a number of distinct zones along the tube, each with a characteristic complement of cytoplasmic and nuclear structures. The cytoplasmic structures are characteristic of secretory cells with extensive endoplasmic reticulum cisternae and numerous dictyosomes. The dictyosomes produce secretory vesicles that are mainly directed to the extending tip of the tube, where they provide new plasma membrane and wall components. The rates of secretory vesicle production and delivery have been estimated, allowing quantitative assessments of the rate of delivery of materials to the tip. Pollen tubes contain cytoskeletal components, with microtubules and microfilament strands lying axially in the main tube and diffuse microfilament strands at the tip. The tube wall consists of an outer fibrous layer containing pectins and an inner, more homogeneous layer containing callose and cellulose-like microfibrils, possessing both β-1,4 and β-1,3 linkages. Protein is also present in the wall. The tube tip lacks the inner callosic wall. This type of structure is considered to be different from that of elongating sporophyte tissue cells which are enclosed by a wall containing layers of cellulose microfibrils. Calcium ions are required for pollen tube growth and, in at least some species, act as a chemotropic agent. High concentrations of calcium ions in the external medium inhibit growth. Pollen tubes contain some calcium ions bound to the cell wall and larger amounts located intracellularly, which enter the tube at the tip. This intracellular calcium is present as ions that exist freely within the cytoplasmic Matrix and as ions bound to membrane systems. The highest concentrations in both of these pools are found at the tip and in both they decline towards the base. The structure of the tip and the activity involved in providing components for plasma membrane and Wall assembly provide a basis for considering possible mechanisms of tip growth. Two hypotheses to account for the regulation of tip extension are considered, cell wall control and cytoskeletal control. In the cell wall hypothesis, control depends on an interaction between internal turgor pressure and a plastic cell wall. The mechanical properties of the wall are assumed to be partly dependent on the availability of external calcium ions to crosslink acidic pectin chains. According to this hypothesis, high external calcium ion concentrations cause cessation of tip growth due to increased mechanical resistance of the tip wall. Various observations on plant cell-wall interactions with calcium ions and on experimentally-treated pollen tubes provide evidence that does not support this hypothesis. The cytoskeletal control hypothesis of tip growth depends on the internal tip cytoskeleton to contain the tube tip cytoplasm against the internal turgor pressure during cell wall assembly. The activities and mechanical properties of the cytoskeleton are assumed to depend on the availability of external calcium ions. High external concentrations are believed to cause a state of rigor in the cytoskeleton and hence a cessation of tip growth. Some experimental evidence is presented which suggests that the effects of excess calcium ions are on intracellular processes, and not extracellular ones. The mitochondrial zone behind the tip is believed to maintain the tip calcium ion concentration at an optimal level for growth. Some comparisons are made between tip growth in pollen tubes and that in other tip growing cells. CONTENTS Summary 323 I. Introduction 324 II. Cytoplasm 326 III. Wall structure 332 IV. Calcium 335 V. Tip growth 339 VI. Conclusions 350 References 351.

Mechanism of action of bombesin on amylase secretion. Evidence for a Ca2+-independent pathway

The mode of action of bombesin on amylase secretion was investigated in rat pancreatic acini. Bombesin induced a dose-dependent increase in inositol 1,4,5-trisphosphate and cytosolic free Ca2+. The threshold concentration capable of inducing both effects was 0.1 nM and the half-maximal dose of the peptide for Ca2+ mobilization was approximately 0.6 nM. By contrast, amylase release was approximately 30 times more sensitive than inositol 1,4,5-trisphosphate production and Ca2+ mobilization to bombesin action, with 1 pM being the first stimulatory concentration and a half-maximal effect at approximately 20 pM. The ability of low bombesin doses to trigger enzyme secretion was unaffected by chelation of extracellular Ca2+ with EGTA. In order to test whether the stimulation of amylase release was truly a Ca2+-independent response, the intracellular Ca2+ stores were depleted by pretreating acini with EGTA plus ionomycin, the Ca2+ ionophore. Under these conditions bombesin was still capable of eliciting a significant twofold enhancement of the secretory activity. These results indicate that bombesin, an agonist thought to activate secretion mainly through mobilization of Ca2+ from intracellular stores, elicits amylase release at low concentrations, independently of a concomitant rise in cytosolic free Ca2+. The relevance of these findings to the physiological regulation of pancreatic exocrine secretion is discussed.

Cellular calcium and atherosclerosis: a brief review

Evidence for and against the theory that cell calcium is causally involved in the pathogenesis of atherosclerosis is presented and evaluated. In particular, it is argued that: (1) arterial calcium is increased in atherosclerosis; (2) this increase in tissue calcium content is largely intracellular; (3) this increased intracellular calcium content is caused by increased plasma membrane calcium permeability; (4) the increased calcium content is causally related to atherogenesis; (5) many of the cell physiological, cell biological, biochemical, and molecular biological processes, known to function abnormally in atherosclerosis, are also known to be calcium regulated; and (6) these processes are activated or inactivated in atherosclerosis in a manner consistent with increased cell calcium. It is concluded that the calcium-atherogenesis hypothesis has the potential to unify macroscopic clinical risk factors in terms of intracellular mechanisms that are controlled by cell calcium, and that this hypothesis deserves further experimental tests.

Effect of the genetic background and specific mutation on adenylate cyclase activity in obesity syndromes

Adenylate cyclase activity and its modulation by guanine nucleotides and isoproterenol were assessed in adipocyte membranes of mice with mutations causing different genetic obesity syndromes. The object was to determine whether the defect in inhibitory modulation observed in the obese (ob/ob) mouse was also present in the diabetes (db/db) mouse. The data show that adipocyte adenylate cyclase in both the ob/ob and the db/db mouse is resistant to activation by isoproterenol. The response to guanosine triphosphate (GTP) differed between the two mutants, such that an inhibitory phase was visible in the db/db but not in the ob/ob membranes. Moreover, pertussis toxin attenuated the inhibitory effect of GTP and significantly stimulated cyclase activity in the db/db but not in the ob/ob membranes. The data show that the two mutations affect the expression of adenylate cyclase activity via different mechanisms.

An influx of extracellular calcium is required for initiation of the human sperm acrosome reaction induced by human follicular fluid

The role of Ca2+ in the human sperm acrosome reaction was investigated using the fluorescent calcium indicator fura-2. Previous experiments have shown that a Sephadex G-75 column fraction of human follicular fluid can stimulate the human sperm acrosome reaction [Suarez SS, Wolf DP, Meizel S (1986): Gamete Res 14:107-121]. Using fura-2, we demonstrated that this Sephadex G-75 fraction also stimulates a rapid, transient increase in intracellular free Ca2+. This Ca2+ transient is blocked either by chelation of extracellular calcium or by addition of the Ca2+ antagonist La3+. We have also been able to stimulate the acrosome reaction in human sperm without significant loss of motility, using the divalent cation ionophore ionomycin. Acrosome reactions stimulated by whole follicular fluid, the G-75 fraction, or ionomycin are all blocked by removal of extracellular Ca2+. These results strongly suggest that an influx of extracellular Ca2+ is responsible for initiating the acrosome reaction in human sperm treated with human follicular fluid. This is the first demonstration in mammalian sperm that a potentially physiological stimulus can cause an increase in intracellular Ca2+ concomitant with the acrosome reaction.

A GTP-binding protein required for secretion rapidly associates with secretory vesicles and the plasma membrane in yeast

SEC4, one of the 10 genes involved in the final stage of the yeast secretory pathway, encodes a ras-like, GTP-binding protein. In wild-type cells, Sec4 protein is located on the cytoplasmic face of both the plasma membrane and the secretory vesicles in transit to the cell surface. In all post-Golgi blocked sec mutants, Sec4p is predominantly associated with the secretory vesicles that accumulate as a result of the secretory block. Sec4p is synthesized as a soluble protein that rapidly (t1/2 less than or equal to 1 min) and tightly associates with secretory vesicles and the plasma membrane by virtue of a conformational change of a covalent modification. These data suggest that Sec4p may function as a "G" protein on the vesicle surface to transduce an intracellular signal needed to regulate transport between the Golgi apparatus and the plasma membrane.

The yeast GTP-binding YPT1 protein and a mammalian counterpart are associated with the secretion machinery

A yeast GTP-binding protein, the YPT1 gene product, has been found to function early in the secretion pathway. The ypt1-1 mutation causes a phenotype reminiscent of early secretion-defective mutants, including accumulation of membranes and vesicles as well as a partial defect in secretion and incomplete glycosylation of invertase. Immunofluorescence localization studies using affinity-purified antibody directed against the YPT1 protein showed punctate staining of the cytoplasm of growing yeast cells and very intense staining of small buds, where membrane growth and secretion are most active. The punctate cytoplasmic staining is changed in a mutant (sec7) under conditions that cause aberrant Golgi structures to accumulate. The pattern of immunofluorescence obtained when mouse cells were stained with the antibody coincided closely with the pattern observed with wheat germ agglutinin, suggesting that a mammalian counterpart of the yeast YPT1 protein is located in the Golgi apparatus. These results are interpreted as suggesting that GTP-binding proteins may act to direct intracellular vesicle traffic.

Dual regulation of ACTH secretion by guanine nucleotides in permeabilized AtT-20 cells

1. We have examined the effects of guanine nucleotides on ACTH secretion from digitonin-permeabilized AtT-20 cells, with the aim of analyzing the involvement of GTP-binding proteins (G proteins) in the secretory process. 2. AtT-20 cells permeabilized with 20 microM digitonin displayed calcium-dependent secretion. The EC50 of calcium was approximately 2 microM and the maximal stimulation was 350% of basal release. 3. Nonhydrolyzable guanine nucleotides also stimulated ACTH release, in a virtually Ca2+-free medium. The EC50 of guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) was approximately 15 microM and the maximal stimulation was approximately 230% of basal release. The effects of calcium and guanine nucleotides were not additive. 4. In the presence of the inhibitory hormone, somatostatin guanine nucleotides inhibited the calcium-stimulated secretion. 5. Both the stimulatory and the inhibitory effects on secretion of guanine nucleotides were independent of changes in cyclic AMP (cAMP) and calcium. It is suggested that G proteins influence an unknown step in the secretion process, which would be near or at the exocytotic site. 6. The results can be explained by assuming the existence of two types of G proteins, one with stimulatory effects on exocytotic release (GeS) and another with inhibitory effects (GeI).

Secretory responses of rat peritoneal mast cells to high intracellular calcium

The patch-clamp technique was used to investigate the secretory responses of rat peritoneal mast cells at various intracellular calcium concentrations ([Ca2+]i). When Calcium was introduced into the cell with pipette-loaded dibromo-BAPTA, elevation of [Ca2+]i into the range 1-10 microM induced membrane capacitance increases indicative of exocytosis in a concentration-dependent manner. At higher concentrations a decrease of the response was observed. Cells that were exposed to micromolar [Ca2+]i underwent morphological alterations resulting in swelling, which is indicative of cytoskeletal alterations. The presence of dibromo-BAPTA (4 mM) strongly inhibited secretion induced by GTP-gamma-S, thus hampering the contribution of G-protein-mediated stimulation. Application of the Ca2+ ionophore ionomycin resulted in transient increases in [Ca2+]i which were parallelled by Ca2+-dependent secretion. Effective buffering of the cytosolic calcium level below 1 microM abolished the secretory response. Our results show that an increase in [Ca2+]i can trigger secretion, but only if it is high and sustained. During physiological stimulation, however, secretion proceeds at [Ca2+]i below 1 microM. It is, therefore, concluded that mast cell degranulation under physiological conditions is not simply a result of an increase in [Ca2+]i, but that other second messenger systems in conjunction with calcium act synergistically in order to ensure fast and efficient secretion.

Recent advances in cystic fibrosis

Cystic fibrosis, one of the most common lethal inherited disorders in N. European and N. American populations, is characterized by the production of abnormally viscous mucous secretions in the lungs and digestive tract. The pathophysiological basis of the disease is unknown. However, during the last few years, rapid advances in molecular genetics and biochemical and physiological studies on cystic fibrosis epithelial cells have led to optimism that the cystic fibrosis defect will soon be identified. Current evidence suggests that the basic disturbance lies in altered regulation of protein secretion and electrolyte transport leading to an imbalance in composition of epithelial secretions in cystic fibrosis patients. Increasing knowledge of the mechanisms regulating production and secretion of mucins and movement of electrolytes across the cell membrane should lead to development of pharmacological manipulation(s) to correct the cellular abnormality. Ultimately, it is hoped that this will lead to the development of a rational treatment for cystic fibrosis patients.

Diacylglycerol breakdown in plasma membranes of bovine chromaffin cells is a two-step mechanism mediated by a diacylglycerol lipase and a monoacylglycerol lipase

The recently identified diacylglycerol lipase activity in membranes of chromaffin cells from bovine adrenal medulla [24] is now shown to consist of two enzymes working in series. First the predominantly saturated fatty acid in the sn-1-position is split by a diacylglycerol lipase (glycerol ester hydrolase, EC 3.1.1.34). Subsequently the resulting sn-2-monoacylglycerol is split by a monoacylglycerol lipase (glycerol-monoester acylhydrolase, EC 3.1.1.23) which prefers sn-2-arachidonoyl-monoacylglycerol to sn-2-palmitoyl-monoacylglycerol. At pH 4.0 only the diacylglycerol lipase is active, whereas the monoacylglycerol lipase is irreversibly inactivated. At pH 6.0 both enzymes are active. Pretreatment of the membranes at pH 10 leads to the selective inactivation of the diacylglycerol lipase. Both enzymes are Ca2+- and calmodulin-independent and both are partially inhibited by p-bromophenacyl bromide, however, only at relatively high concentrations of the inhibitor. Chlorpromazine inhibits the diacylglycerol lipase to about the same extent as p-bromophenacyl bromide but the monoacylglycerol lipase is less sensitive. The specific diacylglycerol lipase inhibitor RHC 80267 (1,6-di(O-(carbamoyl)cyclohexanone oxime)hexane) only interacts with the first step, i.e. the diacylglycerol lipase.

Pertussis toxin does not affect the time course of exocytosis in mast cells stimulated by intracellular application of GTP-gamma-S

Exocytosis was studied in single rat peritoneal mast cells. Granule fusion was monitored by time-resolved capacitance measurements using the patch-clamp technique. Intracellular stimulation of mast cells with 20 microM GTP-gamma-S stimulates exocytosis with a calcium-dependent time course. Secretion in response to receptor-mediated stimulation with compound 48/80 was completely abolished by treatment with pertussis toxin (IAP) at 180 ng/ml for 4 h. The time course of exocytosis in response to GTP-gamma-S remained unaffected in IAP-treated cells supporting the involvement of a second GTP-binding protein in stimulus-secretion coupling.

Characterization of phospholipase activities in chromaffin granule ghosts isolated from the bovine adrenal medulla

Highly purified chromaffin granule membranes contain high levels (100 nmol/mg protein) of long-chain free fatty acids (Husebye, E.S. and Flatmark, T. (1984) J. Biol. Chem. 259, 15272-15276), as well as lysophosphatidylcholine (268 nmol/mg protein) and lysophosphatidylethanolamine (92 nmol/mg protein). The release of saturated and unsaturated long-chain fatty acids from endogenous phospholipids was 38 and 28 nmol/mg protein per h, respectively, at 37 degrees C and pH 7.5 (alkaline pH optimum). p-Bromophenacyl bromide inhibited the release of palmitate and oleate by 88 and 65%, respectively. The deacylation of membrane phospholipids was not significantly affected by micromolar free Ca2+. Based on experiments with pancreatic phospholipase A2, stearate and arachidonate were found to be suitable markers for deacylation at the sn-1 and sn-2 positions, respectively. Experiments with exogenously added labeled phosphatidylcholines confirmed that chromaffin granule ghosts contain a phospholipase A2 activity (alkaline pH optimum). The preparations also revealed a phospholipase A1 activity (acid pH optimum). Finally, the ghosts contain a lysophospholipase activity (alkaline pH optimum), that accounts for the major part of the deacylation of membrane phospholipids, notably the release of saturated fatty acids (stearate and palmitate). It is unlikely that the high content of lysophospholipids is an artifact of the procedure by which the granule ghosts are isolated.