A new method for cell permeabilization reveals a cytosolic protein requirement for Ca2+-activated secretion in GH3 pituitary cells

Spatial and Temporal Aspects of Exocytosis Studied on the Isolated Plasma Membranes

Exocytosis of large-dense core vesicles in neuroendocrine cells is a highly regulated, calcium-dependent process, mediated by networks of interrelated proteins and lipids. Here, I describe experimental procedures for studies of selective spatial and temporal aspects of exocytosis at the plasma membrane, or in its proximity, using adrenal chromaffin cells. The assay utilizes primary cells subjected to a brief ultrasonic pulse, resulting in the formation of thin, flat inside-out plasma membranes with attached secretory vesicles and elements of cell cytoskeleton. In this model, secretion of plasma membrane-attached secretory vesicles was found to be dependent on calcium and sensitive to clostridial neurotoxins. Depending on the probe selected for secretory vesicle cargo, protein, and/or lipid detection, this simple assay is versatile, fast and inexpensive, and offers excellent spatial resolution.

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.

The Vesicle Priming Factor CAPS Functions as a Homodimer via C2 Domain Interactions to Promote Regulated Vesicle Exocytosis

Neurotransmitters and peptide hormones are secreted by regulated vesicle exocytosis. CAPS (also known as CADPS) is a 145-kDa cytosolic and peripheral membrane protein required for vesicle docking and priming steps that precede Ca2+-triggered vesicle exocytosis. CAPS binds phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and SNARE proteins and is proposed to promote SNARE protein complex assembly for vesicle docking and priming. We characterized purified soluble CAPS as mainly monomer in equilibrium with small amounts of dimer. However, the active form of CAPS bound to PC12 cell membranes or to liposomes containing PI(4,5)P2 and Q-SNARE proteins was mainly dimer. CAPS dimer formation required its C2 domain based on mutation or deletion studies. Moreover, C2 domain mutations or deletions resulted in a loss of CAPS function in regulated vesicle exocytosis, indicating that dimerization is essential for CAPS function. Comparison of the CAPS C2 domain to a structurally defined Munc13-1 C2A domain dimer revealed conserved residues involved in CAPS dimerization. We conclude that CAPS functions as a C2 domain-mediated dimer in regulated vesicle exocytosis. The unique tandem C2-PH domain of CAPS may serve as a PI(4,5)P2-triggered switch for dimerization. CAPS dimerization may be coupled to oligomeric SNARE complex assembly for vesicle docking and priming.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

RNAi screen identifies a role for adaptor protein AP-3 in sorting to the regulated secretory pathway

AP-3 concentrates proteins within large dense-core vesicles to promote regulated exocytosis.

The regulated release of proteins depends on their inclusion within large dense-core vesicles (LDCVs) capable of regulated exocytosis. LDCVs form at the trans-Golgi network (TGN), but the mechanism for protein sorting to this regulated secretory pathway (RSP) and the cytosolic machinery involved in this process have remained poorly understood. Using an RNA interference screen in Drosophila melanogaster S2 cells, we now identify a small number of genes, including several subunits of the heterotetrameric adaptor protein AP-3, which are required for sorting to the RSP. In mammalian neuroendocrine cells, loss of AP-3 dysregulates exocytosis due to a primary defect in LDCV formation. Previous work implicated AP-3 in the endocytic pathway, but we find that AP-3 promotes sorting to the RSP within the biosynthetic pathway at the level of the TGN. Although vesicles with a dense core still form in the absence of AP-3, they contain substantially less synaptotagmin 1, indicating that AP-3 concentrates the proteins required for regulated exocytosis.

The Ca(2+)-dependent activator protein for secretion CAPS: do I dock or do I prime?

The "Ca(2+)-dependent activator protein for secretion" (CAPS) is a protein which reconstitutes regulated secretion in permeabilized neuroendocrine cells. It is generally accepted that CAPS plays an important role in the release of the contents of dense core vesicles in the nervous system as well as in a variety of other secretory tissues. At which step in the exocytotic process CAPS functions as well as its role in the fusion of synaptic vesicles is still under dispute. A recent growth spurt in the CAPS field has been fueled by genetic approaches in Caenorhabditis elegans and Drosophila as well as the application of knockout and knockdown approaches in mouse cells and in cell lines, respectively. We have attempted to review the body of work that established CAPS as an important regulator of secretion and to describe new information that has furthered our understanding of how CAPS may function. We discuss the conclusions, point out areas where controversy remains, and suggest directions for future experiments.

Direct interaction of endogenous Kv channels with syntaxin enhances exocytosis by neuroendocrine cells

K⁺ efflux through voltage-gated K⁺ (Kv) channels can attenuate the release of neurotransmitters, neuropeptides and hormones by hyperpolarizing the membrane potential and attenuating Ca²⁺ influx. Notably, direct interaction between Kv2.1 channels overexpressed in PC12 cells and syntaxin has recently been shown to facilitate dense core vesicle (DCV)-mediated release. Here, we focus on endogenous Kv2.1 channels and show that disruption of their interaction with native syntaxin after ATP-dependent priming of the vesicles by Kv2.1 syntaxin–binding peptides inhibits Ca²⁺ -triggered exocytosis of DCVs from cracked PC12 cells in a specific and dose-dependent manner. The inhibition cannot simply be explained by the impairment of the interaction of syntaxin with its SNARE cognates. Thus, direct association between endogenous Kv2.1 and syntaxin enhances exocytosis and in combination with the Kv2.1 inhibitory effect to hyperpolarize the membrane potential, could contribute to the known activity dependence of DCV release in neuroendocrine cells and in dendrites where Kv2.1 commonly expresses and influences release.

Phosphatidylinositol 3-kinase C2alpha is essential for ATP-dependent priming of neurosecretory granule exocytosis

Neurotransmitter release and hormonal secretion are highly regulated processes culminating in the calcium-dependent fusion of secretory vesicles with the plasma membrane. Here, we have identified a role for phosphatidylinositol 3-kinase C2alpha (PI3K-C2alpha) and its main catalytic product, PtdIns3P, in regulated exocytosis. In neuroendocrine cells, PI3K-C2alpha is present on a subpopulation of mature secretory granules. Impairment of PI3K-C2alpha function specifically inhibits the ATP-dependent priming phase of exocytosis. Overexpression of wild-type PI3K-C2alpha enhanced secretion, whereas transfection of PC12 cells with a catalytically inactive PI3K-C2alpha mutant or a 2xFYVE domain sequestering PtdIns3P abolished secretion. Based on these results, we propose that production of PtdIns3P by PI3K-C2alpha is required for acquisition of fusion competence in neurosecretion.

Distinct signalling particles containing ERK/MEK and B-Raf in PC12 cells

Although several multiprotein complexes containing MAPKs (mitogen-activated protein kinases) have been identified using overexpression of kinases and scaffold proteins, the components of the complexes and their physical properties at endogenous expression levels have not been defined. We characterized a large protein complex containing a nerve-growth-factor-activated ERK (extracellular-signal-regulated kinase) and MEK (MAPK/ERK kinase) in rat pheochromocytoma (PC12) cells. This protein complex fractionated into a high-speed pellet and was resistant to non-ionic detergent treatments that solubilized membranes. Disruption of protein-protein interactions by treatment with high salt was required to facilitate immunoprecipitation of active ERK1 and co-precipitation of MEK1. Microtubule fragments were also present in the detergent-resistant high-speed pellet, and some kinases were bound to them, especially ERK1b (an alternatively spliced isoform of ERK1), which showed a strong preference for binding microtubules. The large protein complex containing ERK1 and MEK1 was resolved by velocity sedimentation from fragments of microtubules; however, it did not contain other scaffolding components known to bind ERK and MEK. B-Raf was also present in a distinct detergent-resistant, microtubule-independent protein complex slightly larger than that containing ERK and MEK. We conclude that there are two independent nerve growth factor-regulated 'signalling particles' with an estimated size of 60-75 S, one containing ERK1 and MEK1 and the other containing B-Raf. These signalling particles may have a role in the temporal and spatial regulation of kinase activity inside cells.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Role of CAPS in dense-core vesicle exocytosis

Calcium-dependent activator protein for secretion (CAPS) was initially identified in brain cytosol based on its ability to reconstitute calcium-triggered dense-core vesicle (DCV) exocytosis in permeable cell lines (PC12) of adrenal chromaffin origin. Current evidence indicates that CAPS functions selectively in DCV exocytosis by interacting with DCVs, the plasma membrane, and protein components of the fusion machinery. To further delineate the role of CAPS in endocrine and neural secretion, the tissue distribution of CAPS was determined. Immunoreactive CAPS I localized exclusively to neural and endocrine tissues including adrenal medulla, pancreatic islets, anterior pituitary, thyroid parafollicular C cells, gastrointestinal G cells, renal juxtaglomerular cells, and gray matter throughout the central nervous system. The results are consistent with a widespread functional role of CAPS in the regulated exocytosis of DCVs in the nervous and endocrine systems.

In vitro assays of vesicular transport

Movement of proteins and lipids between the various compartments of eukaryotic cells is fundamental to the maintenance of cellular homeostasis, and an understanding of the molecular mechanisms that govern these processes remains a key goal of cell biological research. This aim has been greatly facilitated by the development of assays that recapitulate specific events in vitro. In the following article we provide an overview of some of the currently used assays that measure the movement of proteins within the exocytic and endocytic pathways, and provide a starting point for those wishing to establish their own systems to study other vesicular transport steps.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

Biosynthesis and packaging of carboxypeptidase D into nascent secretory vesicles in pituitary cell lines

Metallocarboxypeptidase D (CPD) is a membrane-bound trans-Golgi network (TGN) protein. In AtT-20 cells, CPD is initially produced as a 170-kDa endoglycosidase H-sensitive glycoprotein. Within 30 min of chase, the CPD increases to 180 kDa and is resistant to endoglycosidase H as a result of carbohydrate maturation. CPD also undergoes an activation step required for binding to a substrate affinity resin. Blocking the protein exit from the endoplasmic reticulum inhibits the increase in molecular mass but not the step required for affinity column binding, suggesting that enzyme activation precedes carbohydrate maturation and that these reactions occur in distinct intracellular compartments. Only the higher molecular weight mature CPD enters nascent secretory vesicles, which bud from the TGN of permeabilized AtT-20 and GH3 cells. The budding efficiency of CPD into vesicles is 2-3-fold lower than that of endogenous proopiomelanocortin in AtT-20 cells or prolactin in GH3 cells. In contrast, the packaging of a truncated form of CPD, which lacks the cytoplasmic tail and transmembrane domain, was similar to that of proopiomelanocortin. Taken together, the results support the proposal that CPD functions in the TGN in the processing of proteins that transit the secretory pathway and that the C-terminal region plays a major role in TGN retention.

Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal

Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.

Ca2+ and protein kinase C activation of mucin granule exocytosis in permeabilized SPOC1 cells

Mucin secretion by airway goblet cells is under the control of apical P2Y2, phospholipase C-coupled purinergic receptors. In SPOC1 cells, the mobilization of intracellular Ca2+ by ionomycin or the activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) stimulates mucin secretion in a fully additive fashion [L. H. Abdullah, J. D. Conway, J. A. Cohn, and C. W. Davis. Am. J. Physiol. 273 (Lung Cell. Mol. Physiol. 17): L201-L210, 1997]. This apparent independence between PKC and Ca2+ in the stimulation of mucin secretion was tested in streptolysin O-permeabilized SPOC1 cells. These cells were fully competent to secrete mucin when Ca2+ was elevated from 100 nM to 3.1 microM for 2 min following permeabilization; the Ca2+ EC50 was 2.29 +/- 0.07 microM. Permeabilized SPOC1 cells were exposed to PMA or 4alpha-phorbol at Ca2+ activities ranging from 10 nM to 10 microM. PMA, but not 4alpha-phorbol, increased mucin release at all Ca2+ activities tested: at 10 nM Ca2+ mucin release was 2.1-fold greater than control and at 4.7 microM Ca2+ mucin release was maximal (3.6-fold increase). PMA stimulated 27% more mucin release at 4.7 microM than at 10 nM Ca2+. Hence, SPOC1 cells possess Ca2+-insensitive, PKC-dependent, and Ca2+-dependent PKC-potentiated pathways for mucin granule exocytosis.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Amyloid beta peptide formation in cell-free preparations. Regulation by protein kinase C, calmodulin, and calcineurin

Amyloid beta peptide (Abeta) is a short peptide that is the major constituent of the amyloid plaques and cerebrovascular amyloid deposits found in Alzheimer's disease. The lack of availability of a cell-free system in which to study Abeta formation has limited our understanding of the molecular mechanisms involved in its production. We report here the reconstitution of such a cell-free system. The reconstituted Abeta formation was temperature-dependent and required ATP. Preincubation with purified protein kinase C (PKC) induced a pronounced inhibition of Abeta formation, similar to that observed in intact cells upon stimulation of PKC. The calmodulin antagonists W-7 and trifluoperazine inhibited Abeta formation and enhanced the action of PKC in both the cell-free system and intact cells. A role for the calcium/calmodulin-activated protein phosphatase calcineurin in the regulation of Abeta formation was demonstrated using a specific peptide inhibitor of calcineurin in vitro as well as cyclosporin A, a cell-permeant inhibitor of calcineurin, in intact cells. Our results suggest that a single substrate might mediate opposing actions of PKC and calcineurin in the regulation of Abeta formation.

Prosomatostatin processing in permeabilized cells. Calcium is required for prohormone cleavage but not formation of nascent secretory vesicles

Our laboratory has been using a permeabilized cell system derived from rat anterior pituitary GH3 cells expressing prosomatostatin (pro-SRIF) to study prohormone processing and nascent secretory vesicle formation in vitro. Because calcium is necessary for prohormone processing enzyme activity, secretory granule fusion with the plasma membrane, and possibly sorting to the regulated pathway, we treated permeabilized cells with the calcium ionophore A23187 to determine the role of calcium in pro-SRIF cleavage and nascent vesicle formation from the trans-Golgi network (TGN). Here we demonstrate that pro-SRIF cleavage was markedly inhibited when lumenal free calcium was chelated with EGTA in the presence of A23187. Surprisingly, submillimolar free calcium (approximately 15 microM) was sufficient to maintain prohormone cleavage efficiency, a value far lower than that estimated for total calcium levels in the TGN and secretory granules. Experiments using both A23187 and the protonophore CCCP revealed that free calcium is absolutely required for efficient pro-SRIF cleavage, even at the optimal pH of 6.1. Secretory vesicle formation by contrast was not inhibited by calcium chelation but rather by millimolar extralumenal free calcium. Together, these observations demonstrate that pro-SRIF processing and budding of nascent secretory vesicles from the TGN can be uncoupled and therefore have distinct biochemical requirements. Interestingly, our data using intact GH3 cells demonstrate that basal secretion of SRIF-related material is largely calcium-dependent and therefore cannot be equated with constitutive pathway secretion. These results underscore the importance of determining calcium requirements before assigning a secretion event to either the constitutive or regulated secretory pathway.

The biochemistry of neurotransmitter secretion

The progress that has resulted from the convergence of biochemistry with yeast genetics has accelerated the pace at which the molecular events of membrane transport are being elucidated. Future research will focus not only on testing the proposed sequence of protein-protein interactions but also on identifying how calcium regulation is imposed on this system. As our understanding of the basic mechanisms of neurosecretion increases, attention will undoubtedly shift to how the molecules of release are modified to produce changes in synaptic efficacy.

Nerve growth factor induces sensitivity to botulinum neurotoxin type A in norepinephrine-secreting PC12 cells

Inhibition of Ca(2+)-activated norepinephrine secretion by the botulinum neurotoxin (NT) serotypes A and E was examined in permeabilized PC12 cells. The dichain type E NT reduced with dithiothreitol (DTT) completely inhibited secretion whereas the dichain type A NT reduced with DTT exhibited incomplete inhibitory activity. In contrast, Ca(2+)-activated secretion in PC12 cells treated with nerve growth factor (NGF) was completely inhibited by reduced type A NT. The NGF-treated PC12 cells retained a sensitivity to the type E NT similar to that of untreated PC12 cells. These results indicate that the intracellular mechanisms of inhibition of the types E and A NTs are distinct. NGF appears to either induce the expression of a component selectively required for type A NT sensitivity, or otherwise modifies the secretory apparatus to acquire type A NT sensitivity.

Storage and release of neurotransmitters

Because synaptic vesicles and secretory granules are simple in composition and easy to purify, many of their protein components have been identified and often sequenced. Attempts are underway to link the small number of membrane proteins to the small number of functions the vesicles perform. The discovery of sequence homologies has helped greatly with this. In addition, techniques that have begun to prove successful involve microinjection, identification of proteins that bind synaptic vesicle proteins, DNA transfection into cells and oocytes, and more recently, in vitro reconstitution of exocytosis, endocytosis, and vesicle biogenesis. Advances in the latter areas have been strongly influenced by the breakthroughs in our knowledge of membrane traffic in nonneuronal cells. The budding reactions involved in making synaptic vesicles and secretory granules resemble in many ways the generation of carrier vesicles from the ER and the Golgi complex. Finally, exocytosis in neurons may closely resemble fusion of carrier vesicles with target organelles in nonneuronal cells, using complexes of peripheral membrane proteins, GTP hydrolysis, and integral membrane proteins with fusogenic domains. The usefulness of in vitro reconstitution, reverse genetics, and the parallels with better understood systems compensates in part for a major weakness in the field, namely the difficulty in obtaining viable mutants that are defective in the storage and release of secretory vesicle content.

Sorting of chromogranin B into immature secretory granules in pheochromocytoma (PC12) cells

Two tyrosine sulfated proteins, chromogranin B and secretogranin II, are targeted into regulated secretory granules, which are stored until stimulation causes them to fuse with the plasma membrane. In PC12 cells, [35S]-sulfate labels chromogranin B, secretogranin II and proteoglycans just before they are sorted into the regulated and constitutive secretory pathways. Chromogranin B was secreted by both the regulated and constitutive secretory pathways. In contrast, very little secretogranin II was secreted constitutively. Formation of regulated and constitutive secretory vesicles from the trans Golgi network was reconstituted in vitro. Mechanically permeabilized PC12 cells released regulated and constitutive vesicles containing sulfated markers. At early chase times, chromogranin B was predominantly released from permeabilized cells in small vesicles that comigrated with constitutive secretory vesicles containing proteoglycans. At intermediate chase times the vesicles containing chromogranin B resembled immature secretory granules containing secretogranin II. At very long chase times chromogranin B and secretogranin II were found in mature secretory granules that did not escape the permeabilized cells in vitro. We conclude that chromogranin B is sorted into immature regulated secretory granules with lower efficiency than secretogranin II.

A novel 145 kd brain cytosolic protein reconstitutes Ca(2+)-regulated secretion in permeable neuroendocrine cells

The regulated secretory pathway is activated by elevated cytoplasmic Ca2+; however, the components mediating Ca2+ regulation have not been identified. In semi-intact neuroendocrine cells, Ca(2+)-activated secretion is ATP- and cytosol protein-dependent. We have identified a novel brain protein, p145, as a cytosolic factor that reconstitutes Ca(2+)-activated secretion in two neuroendocrine cell types. The protein is a dimer of 145 kd subunits, exhibits Ca(2+)-dependent interaction with a hydrophobic matrix, and binds phospholipid vesicles, suggesting a membrane-associated function. A p145-specific antibody inhibits the reconstitution of Ca(2+)-activated secretion by cytosol, indicating an essential role for p145. The restricted expression of p145 in tissues exhibiting a regulated secretory pathway suggests a key role for this protein in the transduction of Ca2+ signals into vectorial membrane fusion events.

Exo1 and Exo2 proteins stimulate calcium-dependent exocytosis in permeabilized adrenal chromaffin cells

In many cell types an increase in cytosolic calcium is the main signal for the exocytotic release of stored secretory components such as hormones and neurotransmitters. The site of action of calcium in exocytosis is not known, neither are the participating molecules. In the case of the intracellular membrane fusions that occur during transport through early stages of the secretory pathway, several cytosolic and peripheral membrane proteins are necessary. Permeabilized cells have been useful in understanding the requirements for calcium and nucleotides in regulated exocytosis and under certain conditions there is leakage of soluble protein components and run-down of the exocytotic response. This system can be used to identify the soluble proteins involved in exocytosis, one candidate in chromaffin cells being annexin II (calpactin). Here we use this assay to identify two other cytosolic protein factors that regulate exocytosis in permeabilized adrenal chromaffin cells, which we term Exo1 and Exo2. Exo1 from brain cytosol resolves on electrophoresis in SDS-polyacrylamide gels as a group of polypeptides of relative molecular mass approximately 30,000 and shares sequence homology with the 14-3-3 family of proteins. The ability of Exo1 to reactivate exocytosis is potentiated by protein kinase C activation and therefore Exo1 may influence the protein kinase C-mediated control of Ca(2+)-dependent exocytosis.

Selective stimulation of in situ intermediary metabolism by free calcium in permeabilized rat adipocytes

The hypothesis that ionized calcium [Ca2+]i may stimulate in situ rat adipocyte intermediary metabolism distal to glucose transport was tested. A metabolically active porous adipocyte model was employed in which pathway metabolism is exclusively pore-dependent using glucose 6-phosphate (G6P) as substrate. Cellular [Ca2+]i was, furthermore, directly adjusted to between 0-2.5 microM via the membrane pores. Three metabolic fluxes were examined, (1) glycolysis-Krebs ([6-14C]G6P oxidation), (2) glycolysis to lactate ([U-14C]G6P to [14C]lactate) and (3) pentose pathway ([1-14C]G6P oxidation). Glycolysis-Krebs oxidation was was found to be selectively (33% above basal P less than 0.001) stimulated by 0.625 microM free calcium. In contrast, there was no effect of [Ca2+]i on the other, exclusively cytoplasmic, pathways. The stimulation of glycolysis-Krebs by [Ca2+]i was inhibited by a mitochondrial calcium channel blocker (Ruthenium red) and persisted over a range of ATP/ADP ratios. Separate studies demonstrated that 2-[1-14C]ketoglutarate oxidation was also calcium-stimulated in the porous adipocytes (160% over baseline at 1 microM [Ca2+]i). These studies thus demonstrate that physiologically relevant increments in porous adipocyte [Ca2+]i enhance overall in situ glycolytic-Krebs pathway oxidation by a mechanism which entails mitochondrial calcium uptake. Methodologically, this metabolically active porous adipocyte model presents a novel experimental approach to investigations regarding the effects of ionized calcium on intermediary metabolism beyond glucose transport.

Botulinum neurotoxin light chain inhibits norepinephrine secretion in PC12 cells at an intracellular membranous or cytoskeletal site

Botulinum neurotoxin (NT) is a potent inhibitor of neurotransmitter secretion, but its intracellular mechanism and site of action are unknown. In this study, the intracellular action of NT was investigated by rendering the secretory apparatus of PC12 cells accessible to macromolecules by a recently described "cell cracking" procedure. Soluble cytoplasmic factors were depleted from permeabilized cells by washing to generate cell "ghosts" which retained cellular structural components and intracellular organelles (including secretory granules). The PC12 cell ghosts exhibited Ca(2+)-activated [3H]norepinephrine release which was enhanced by cytosolic proteins and MgATP. PC12 cell ghosts provide the opportunity to distinguish the intracellular action of NT on soluble cytoplasmic components versus structural cellular components. The 150-kDa NT and the 50-kDa light chain of serotypes E and B, and to a lesser extent type A, inhibited Ca(2+)-activated [3H]norepinephrine release in PC12 ghosts, but not in intact PC12 cells. The 100-kDa heavy chain had no effect. This indicates that NT acts at an intracellular site in these cells permeabilized by "cell cracking." The inhibition of secretion by NT was rapid and irreversible under the incubation conditions used. NT inhibition of [3H]-norepinephrine release from PC12 ghosts occurred in the absence of cytosolic proteins and MgATP and was not reversed by the addition of cytosolic proteins and MgATP, indicating that NT acts at an intracellular membranous or cytoskeletal site.

Calcium-dependent release of accumulated glutamate from synaptic vesicles within permeabilized nerve terminals

We have studied glutamate release from synaptic vesicles in permeabilized synaptosomes, which were preloaded with [3H]glutamate in an ATP-dependent manner. The release was found to be calcium-dependent and to require a heat-labile cytosolic macromolecule factor for maximum activity. Maximal release occurred at 5 microM free Ca2+ and within 5 min. Of the other divalent cations tested, only barium stimulated release of vesicular glutamate. The release was inhibited by N-ethylmaleimide. These results are characteristic of exocytotic release of monoamines and peptides observed in endocrine systems, and constitute direct evidence for the notion that calcium-dependent release of glutamate originates from the vesicular pool.

Interactions of intracellular mediators of amylase secretion in permeabilized pancreatic acini

Mouse pancreatic acini were permeabilized with streptolysin O to investigate amylase secretion stimulated by various intracellular mediators and the kinetics of secretion as a function of temperature. Amylase secretion was temperature dependent in that the initial rate of Ca2(+)-stimulated secretion increased with increasing temperature. In addition, there was no enhancement of Ca2(+)-stimulated secretion by GTP[gamma S] at 14 degrees C, while enhancement was maximal at 30 degrees C. GTP[gamma S]-mediated enhancement of secretion at a given temperature was mostly due to sustained secretion with a small increase in secretory rate. At 30 degrees C Ca2(+)-stimulated secretion was also enhanced by cAMP and phorbol ester (TPA) to similar extents as by GTP[gamma S]. The maximally effective concentration of cAMP was 1-10 microM in the presence of 0.1 mM isobutylmethylxanthine. The enhancements of Ca2(+)-stimulated amylase secretion by all combinations of cAMP (100 microM plus 0.1 mM isobutylmethylxanthine), TPA (1 microM), and GTP[gamma S] (30 microM) were fully additive. In Ca2(+)-free buffer, cAMP, TPA or GTP[gamma S] individually had no effect on amylase secretion. Together, TPA and GTP[gamma S] stimulated Ca2(+)-independent secretion, which was 187 +/- 38% of basal. Cyclic AMP together with TPA and GTP[gamma S] in the absence of Ca2+ stimulated 329 +/- 30% of basal secretion. Ca2(+)-stimulated amylase secretion was decreased about 50% by metabolic inhibition, while the enhancement by cAMP, TPA or GTP[gamma S] was totally blocked by metabolic inhibitors. These data demonstrate that amylase secretion in the acinar cell is mediated by multiple intracellular pathways which act in parallel and probably converge at a distal step in the exocytotic process.

Endothelial cells reestablish functional integrity after reversible permeabilization

Permeabilization is an important tool in cell biology that allows manipulation of intracellular mechanisms by introduction of probes and regulatory molecules into the cell cytoplasm. We found that incubation of endothelial cells (ECs) with glass beads resulted in nonspecific permeabilization of human and bovine ECs without removal of the cells from monolayer culture. This poration of the plasma membrane allowed the introduction of macromolecules (dextrans less than or equal to 152 kd and immunoglobulins) as well as small, charged molecules (Lucifer Yellow). We found that nonspecific permeabilization of the EC was transient and defined the conditions under which integrity of the plasma membrane was reestablished. This process was dependent on time, temperature, and the presence of extracellular calcium. We also demonstrated that permeabilized ECs regain functional characteristics. This was defined by a number of criteria, including the ability to rapidly reestablish confluent monolayer morphology, to extrude Lucifer Yellow, to adhere and spread after passage, and to synthesize biologically active molecules after stimulation with a receptor-mediated agonist. Thus, transiently porated ECs loaded with appropriate probes can be used in studies of regulatory mechanisms while remaining in monolayer culture, a condition in which many phenotypic features are similar to in situ endothelium. Furthermore, the porated EC may be a useful model for defining the mechanisms that influence the repair of endothelial plasma membrane injury.

Ca2(+)-independent secretory mechanism of thyrotropin-releasing hormone (TRH) involves protein kinase C in rat pituitary cells

Thyrotropin-releasing hormone (TRH) stimulates biphasic prolactin (PRL) secretion from rat pituitary GH3 cells. The pretreatment of cells with EGTA (100 microM) plus arachidonic acid (15 microM), a condition which decreased TRH-responsive intracellular Ca2+ pools, eliminated the activity of TRH on burst PRL secretion (2 min) but did not alter that on sustained PRL secretion (30 min). However, the treatment of cells with EGTA, arachidonic acid and H-7 (300 microM), a potent inhibitor of protein kinase C (PKC), almost completely suppressed the activity of TRH for sustained PRL secretion. In cells down-modulated for PKC, TRH abolished this Ca2(+)-independent sustained PRL secretion. These results suggest that TRH acts through a separate, Ca2(+)-independent secretory mechanism, besides by modulating the Ca2(+)-dependent mechanism and that PKC is involved in this Ca2(+)-independent secretory pathway.