Resolution of regulated secretion into sequential MgATP-dependent and calcium-dependent stages mediated by distinct cytosolic proteins

ATP-dependent inositide phosphorylation required for Ca(2+)-activated secretion

Regulated fusion of secretory granules with the plasma membrane in secretory cells requires ATP, Ca2+ and cytosolic as well as membrane proteins. ATP-dependent steps in Ca(2+)-activated secretion from PC12 cells require three cytosolic PEP proteins (priming in exocytosis proteins, PEP1-3), the identity of which will provide insights into the required ATP-using reactions. PEP3 was recently identified as phosphatidylinositol transfer protein (PtdInsTP), and here we report that PEP1 consists of the type I phosphatidylinositol-4-phosphate 5-kinase (PtdInsP5K). The roles of PEP3/PtdInsTP and PEP1/PtdInsP5K in sequential phosphoinositide recruitment and phosphorylation explains their synergistic activity in ATP-dependent priming. Moreover, inhibition of Ca(2+)-activated secretion by PtdIns(4,5)P2-specific antibodies and phospholipase C implies that 5-phosphorylated inositides play a novel, necessary role in the regulated secretory pathway. The results indicate that lipid kinase-mediated phosphorylation is an important basis for ATP use in the exocytotic 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.

Delay in granular fusion evoked by repetitive cytosolic Ca2+ spikes in mouse pancreatic acinar cells

Patch-clamp whole-cell recording in combination with a phase-sensitive detection method was applied to single, enzymatically isolated, mouse pancreatic acinar cells. Either muscarinic stimulation with a low concentration of ACh (50 nM) or cell infusion of inositol 1,4,5-trisphosphate (InsP3) induced repetitive spike-like increases of membrane capacitance (delta C), membrane conductance (delta G) and membrane current (delta I). Cellular perfusion of InsP3, 10 microM in patch-pipettes, induced baseline spikes in delta C and delta G, resembling those evoked by ACh. The result indicates that exocytotic granular fusion is primarily triggered by the InsP3-induced repetitive rise of [Ca2+]i. The ACh-induced delta C took off almost synchronously with delta G with an apparent delay of less than 40 ms in the initial spike response. This delay of delta C, however, becomes longer by a factor of 7-12 during repetitive Ca2+ spike cycles. Concomitantly a faster decrease in delta C spikes than delta G spikes was observed during the cycles. Two explanations are proposed. First, the Ca2+ sensitivity of granular fusion decreases during the repetitive Ca2+ spikes. This might be due to gradual washout of low molecular components responsible for exocytosis under the whole-cell recording condition. Second, the pool of immediately releasable or of primed zymogen granules is easily exhausted or desensitized during the Ca2+ spike cycles, and has to be supplied from newly primed or sensitized resources. The progressive delay in delta C during the spike cycle is interpreted as a delay in the process of supplying fusible granules.

Transport via the regulated secretory pathway in semi-intact PC12 cells: role of intra-cisternal calcium and pH in the transport and sorting of secretogranin II

To gain insight into the mechanisms governing protein sorting, we have developed a system that reconstitutes both the formation of immature secretory granules and their fusion with the plasma membrane. Semi-intact PC12 cells were incubated with ATP and cytosol for 15 min to allow immature granules to form, and then in a buffer containing 30 microM [Ca2+]free to induce exocytosis. Transport via the regulated pathway, as assayed by the release of secretogranin II (SgII) labeled in the TGN, was inhibited by depletion of ATP, or by the inclusion of 100 microM GTP gamma S, 50 microM AlF3-5 or 5 micrograms/ml BFA. When added after immature granules had formed, GTP gamma S stimulated rather than inhibited exocytosis. Thus, exocytosis of immature granules in this system resembles the characteristics of fully matured granules. Transport of SgII via the regulated pathway occurred at a fourfold higher efficiency than glycosaminoglycan chains, indicating that SgII is sorted to some extent upon exit from the TGN. Addition of A23187 to release Ca2+ from the TGN had no significant effect on sorting of SgII into immature granules. In contrast, depletion of lumenal calcium inhibited the endoproteolytic cleavage of POMC and proinsulin. These results establish the importance of intra-cisternal Ca2+ in prohormone processing, but raise the question whether lumenal calcium is required for proper sorting of SgII into immature granules. Disruption of organelle pH gradients with an ionophore or a weak base resulted in the inhibition of transport via both the constitutive and the regulated pathways.

The molecular machinery for fast and slow neurosecretion

Recent studies indicate that the molecular machinery for synaptic vesicle docking and fusion consists of a triad of botulinum/tetanus neurotoxin substrates (synaptobrevin, syntaxin, SNAP-25) that are homologues of proteins required for constitutive secretion. Proposed low-affinity Ca2+ sensors that regulate exocytosis remain to be identified, although recent studies on synaptotagmin suggest that it, along with other proteins, could play this role. Regulated peptide secretion from dense-core granules has been found to utilize a similar machinery for docking/fusion, and recent studies indicate that this pathway involves a pre-docking step that is regulated by a higher affinity Ca2+ sensor.

Role of protein phosphorylation in the degranulation of electropermeabilized human neutrophils

We have studied the role of protein phosphorylation in the degranulation response of human neutrophils by measuring the effect of ATP depletion and the addition of the protein kinase inhibitor staurosporine in electropermeabilized human neutrophils, activated with Ca2+ and/or GTP-gamma-S. Our studies were carried out in the presence of cytochalasin B to prevent inhibitory effects of actin polymerization on the degranulation response. It was found that protein phosphorylation plays an important role in the degranulation response in cells stimulated with the single stimuli Ca2+ or GTP-gamma-S. However, in neutrophils stimulated with the combination of these activators degranulation can occur without the apparent need for protein phosphorylation, albeit with a slower rate than with protein phosphorylation.

Mechanisms of vesicle docking and fusion: insights from the nervous system

Upon stimulation of nerve cells, synaptic vesicles fuse with the presynaptic plasma membrane to release neurotransmitter. The biochemical pathway responsible for synaptic-vesicle docking and fusion is now being elucidated. Many of the proteins implicated in this process have homologs elsewhere in the cell. The docking and fusion mechanism discussed in this review may account for the specificity of vesicular trafficking throughout both regulated and constitutive secretory pathways.

Botulinum A and the light chain of tetanus toxins inhibit distinct stages of Mg.ATP-dependent catecholamine exocytosis from permeabilised chromaffin cells

Susceptibilities of Mg.ATP-independent and Mg.ATP-requiring components of catecholamine secretion from digitonin-permeabilised chromaffin cells to inhibition by Clostridial botulinum type A and tetanus toxins were investigated. These toxins are Zn(2+)-dependent proteases which specifically cleave the 25-kDa synaptosomal-associated protein (SNAP-25) and vesicle-associated membrane protein (VAMP) II, respectively. When applied to permeabilised chromaffin cells they rapidly inhibited secretion in the presence of Mg.ATP but the catecholamine released in the absence of Mg.ATP, thought to represent fusion of primed granules, was not perturbed. The toxins can exert their effects per se in the absence of the nucleotide complex; therefore, Mg.ATP-requiring steps of secretion are implicated as roles for their targets. Primed release was lost rapidly after permeabilisation of the cells but could be maintained by including Mg.ATP during the incubation before stimulating release with Ca2+. This ability of Mg.ATP to maintain primed release was only partially inhibited by botulinum neurotoxin A whereas it was abolished by tetanus toxin, consistent with the distinct substrates for these toxins. This study reveals a component of release within which these proteins are either resistant to cleavage by these toxins or in such a position that degradation can no longer prevent granule fusion. Differences in the steps of release at which these toxins can affect inhibition are also revealed.

Molecular correlates of synaptic vesicle docking and fusion

The mechanisms responsible for neurotransmitter release at the synapse have been extensively studied using biochemical, genetic, and cell biological approaches. Several significant advances have recently contributed towards an improved understanding of the molecular details of both synaptic vesicle docking and fusion, and of the general process of vesicle-mediated membrane trafficking.

Exocytosis in chromaffin cells: evidence for a MgATP-independent step that requires a pertussis toxin-sensitive GTP-binding protein

We have previously described that mastoparan, an amphiphilic tetradecapeptide that activates heterotrimeric G-proteins, inhibits Ca(2+)-induced MgATP-dependent secretion from streptolysin-O-permeabilized chromaffin cells [Vitale, Mukai, Rouot, Thiersé, Aunis and Bader (1993) J. Biol. Chem. 268, 14715-14723]. Our observations suggest the involvement of an inhibitory G(o)-protein, possibly located on the membrane of secretory granules, in the final stages of the exocytotic pathway in chromaffin cells. Here, we demonstrate that mastoparan is also able to stimulate the Ca(2+)-dependent secretion of catecholamines in the absence of MgATP in the medium. This MgATP-independent secretion is totally blocked by tetanus toxin, a potent inhibitor of exocytosis in all neurosecretory cells so far investigated, suggesting that the mastoparan target is a component of the exocytotic machinery. Mas17, a mastoparan analogue inactive on G-proteins, had no effect on catecholamine secretion whereas both Mas7, a highly active analogue of mastoparan, and AlF4-, which selectively activates trimeric G-proteins, triggered MgATP-independent secretion. Non-hydrolysable GTP analogues (GTP[S] and p[NH]ppG) mimicked the dual effects of mastoparan on secretion: they inhibited exocytosis in the presence of MgATP and stimulated MgATP-independent secretion. The different potencies displayed by these two analogues suggest the involvement of two distinct G-proteins. Accordingly, the mastoparan-induced MgATP-independent secretion is highly sensitive to pertussis toxin (PTX) whereas the inhibition by mastoparan of secretion in the presence of MgATP is resistant to PTX treatment. When permeabilized cells were incubated with mastoparan, the release of arachidonic acid increased in a PTX-sensitive manner. 7,7-Dimethyl-5,8-eicosadienoic acid, a potent inhibitor of intracellular phospholipase A2, inhibited both the arachidonate release and the MgATP-independent catecholamine secretion evoked by mastoparan. In contrast, neomycin, an inhibitor of phospholipase C, had no significant effect on either the release of arachidonic acid or the secretion of catecholamines provoked by mastoparan. We conclude that two distinct heterotrimeric G-proteins act in series in the exocytotic pathway in chromaffin cells: one controls an ATP-dependent priming step through an effector pathway that remains to be determined, and the second is involved in a late Ca(2+)-dependent step which does not require MgATP but possibly involves the generation of arachidonic acid.

Prohormone processing in permeabilized cells: endoproteolytic cleavage of prosomatostatin in the trans-Golgi network

Many peptide hormones are synthesized as larger precursors which undergo endoproteolytic cleavage at paired basic residues to generate a bioactive molecule. Morphological evidence has implicated either the trans-Golgi network (TGN) or immature secretory granules as the site of prohormone cleavage. To identify the site where prohormone cleavage is initiated, we have used retrovirally infected rat anterior pituitary GH3 cells which express high levels of prosomatostatin, proSRIF, (Stoller TJ, Shields D (1988) J Cell Biol 107, 2087-2095). By incubating these cells at 20 degrees C, a temperature that prevents exit from the Golgi apparatus, proSRIF accumulated quantitatively in the TGN and no proteolytic processing was evident. Following the 20 degrees C block, the cells were permeabilized and proSRIF processing determined. Cleavage of proSRIF to the mature hormone was approximately 35-50% efficient, required incubation at 37 degrees C and ATP hydrolysis, but was independent of GTP or cytosol. The in vitro ATP-dependent proSRIF processing was inhibited by inclusion of chloroquine, a weak base, CCCP, a protonophore, or by pre-incubating the permeabilized cells with low concentrations of N-ethylmaleimide or bafilomycin, both inhibitors of vacuolar-type ATP-dependent proton pumps. These data suggest that ATP is required for generation of an acidic pH in the lumen of the TGN which is necessary for the activity of prohormone processing enzymes. By exploiting a permeabilized cell system, we have demonstrated that proSRIF cleavage is initiated in the TGN, in a reaction which is facilitated by a Golgi-associated vacuolar type ATPase.

Phosphatidylinositol transfer protein required for ATP-dependent priming of Ca(2+)-activated secretion

Elucidation of the reactions responsible for the calcium-regulated fusion of secretory granules with the plasma membrane in secretory cells would be facilitated by the identification of participant proteins having known biochemical activities. The successful characterization of cytosolic and vesicle proteins that may function in calcium-regulated secretion has not yet revealed the molecular events underlying this process. Regulated secretion consists of sequential priming and triggering steps which depend on ATP and Ca2+, respectively, and require distinct cytosolic proteins. Characterization of priming-specific factors (PEP proteins) should enable the ATP-requiring reactions to be identified. Here we show that one of the mammalian priming factors (PEP3) is identical to phosphatidylinositol transfer protein (PITP). The physiological role of PITP was previously unknown. We also find that SEC14p, the yeast phosphatidylinositol transfer protein which is essential for constitutive secretion, can substitute for PEP3/PITP in priming. Our results indicate that a role for phospholipid transfer proteins is conserved in the constitutive and regulated secretory pathways.

Protein kinase C and clostridial neurotoxins affect discrete and related steps in the secretory pathway

1. The effects on catecholamine secretion of activation of protein kinase C and clostridial neurotoxins were examined in digitonin-permeabilized bovine adrenal chromaffin cells. 2. The enhancement by phorbol esters increased only the initial rate of secretion; later rates were unaffected. This enhancement was present over a wide range of Ca2+ concentrations and was elicited at 18 as well as at 27 degrees C. 3. Tetanus toxin inhibited both ATP-dependent and ATP-independent secretion, indicating that the tetanus toxin target is important during the final steps in the pathway. 4. Prior activation of protein kinase C by the phorbol ester 12-O-tetradecanoyl phorbol acetate rendered the primed state more sensitive to inhibition by tetanus toxin. The data indicate that a phosphorylated protein kinase C substrate is either identical to or closely associated with the tetanus toxin target protein at the final steps in the pathway. 5. The interaction between the effect of protein kinase activation and that of tetanus toxin suggests that protein kinase C activation does not stimulate a separate pathway of secretion but, rather, modulates the activity of the ongoing pathway. 6. The enhancement of secretion by protein kinase C is caused, at least in part, by a qualitative change in the characteristics of the primed state. This is indicated by the increased sensitivity of primed secretion to inhibition by tetanus toxin and a threefold increase in sensitivity of primed secretion to Ca2+. 7. Because activation of protein kinase C does not increase the later rates of secretion that are limited by ATP-dependent priming reactions, it is unlikely that enhancement of the maximal rate of secretion by TPA is due to an increased amount of the primed state. Instead, protein kinase C activation may increase the efficacy with which Ca2+ stimulates secretion at all Ca2+ concentrations.

Stimulation of secretion in permeabilized PC12 cells by adenosine 5'-[gamma-thio]triphosphate: possible involvement of nucleoside diphosphate kinase

The addition of Ca2+, adenosine 5'-[gamma-thio]triphosphate (ATP[S]) or guanosine 5'-[gamma-thio]triphosphate (GTP[S]) to digitonin-permeabilized PC12 cells stimulates noradrenaline secretion. Both ATP[S] and GTP[S] stimulate release in the absence of Ca2+. Whereas ADP and adenosine 5'-[beta gamma-imido]triphosphate inhibited ATP[S]-stimulated release, they did not inhibit Ca(2+)-stimulated release even in the absence of added ATP. This suggests that the kinase which uses ATP[S] to induce secretion may not play an essential role in Ca(2+)-stimulated release. As GTP[S]-stimulated and ATP[S]-stimulated secretions were not additive, it seemed possible that stimulation by ATP[S] might in part result from the thiophosphorylation of GDP by nucleoside-diphosphate (NDP) kinase to form GTP[S]. The following results are consistent with this possibility. (1) A low concentration of GDP increased ATP[S]-stimulated secretion, but not GTP[S]-stimulated or Ca(2+)-stimulated secretion. (2) A variety of ribo- and deoxyribo-nucleoside di- and tri-phosphates inhibited ATP[S]-stimulated secretion, but not GTP[S]-stimulated or Ca(2+)-stimulated secretion. Thus, like NDP kinase, the kinase which uses ATP[S] to cause noradrenaline release appears to have a very low specificity for ATP. (3) Incubation of permeabilized cells in a sucrose-containing buffer resulted in the preferential loss of ATP[S]-stimulated secretion and a decrease in the level of NDP kinase. The addition of rat liver NDP kinase to those depleted cells partially restored ATP[S]-stimulated secretion.

A Ca-dependent early step in the release of catecholamines from adrenal chromaffin cells

Intense stimuli, such as trains of depolarizing pulses or the caffeine-induced release of calcium from intracellular stores, readily depress the secretory response in neuroendocrine cells. Secretory responses are restored by rest periods of minutes in duration. This recovery was accelerated when the concentration of cytosolic calcium was moderately increased and probably resulted from calcium-dependent replenishment of a pool of release-ready granules. Continuously increased concentrations of calcium led the over-filling of such a pool. Subsequently, secretory responses to stronger calcium stimuli were augmented. Hormone-induced calcium transients with a plateau phase of increased concentration of calcium may enhance the secretory response in this way.

Prohormone processing in the trans-Golgi network: endoproteolytic cleavage of prosomatostatin and formation of nascent secretory vesicles in permeabilized cells

Many peptide hormones are synthesized as larger precursors which undergo endoproteolytic cleavage at paired basic residues to generate a bioactive molecule. Morphological evidence from several laboratories has implicated either the TGN or immature secretory granules as the site of prohormone cleavage. To identify the site where prohormone cleavage is initiated, we have used retrovirally infected rat anterior pituitary GH3 cells which express high levels of prosomatostatin (proSRIF) (Stoller, T. J., and D. Shields. J. Cell Biol. 1988. 107:2087-2095). By incubating these cells at 20 degrees C, a temperature that prevents exit from the Golgi apparatus, proSRIF accumulated quantitatively in the TGN and no proteolytic processing was evident; processing resumed upon shifting the cells back to 37 degrees C. After the 20 degrees C block, the cells were mechanically permeabilized and pro-SRIF processing determined. Cleavage of proSRIF to the mature hormone was approximately 35-50% efficient, required incubation at 37 degrees C and ATP hydrolysis, but was independent of GTP or cytosol. The in vitro ATP-dependent proSRIF processing was inhibited by inclusion of chloroquine, a weak base, CCCP, a protonophore, or by preincubating the permeabilized cells with low concentrations of N-ethylmaleimide, an inhibitor of vacuolar-type ATP-dependent proton pumps. These data suggest that: (a) proSRIF cleavage is initiated in the TGN, and (b) this reaction requires an acidic pH which is facilitated by a Golgi-associated vacuolar-type ATPase. A characteristic feature of polypeptide hormone-producing cells is their ability to store the mature hormone in dense core secretory granules. To investigate the mechanism of protein sorting to secretory granules, the budding of nascent secretory vesicles from the TGN was determined. No vesicle formation occurred at 20 degrees C; in contrast, at 37 degrees C, the budding of secretory vesicles was approximately 40% efficient and was dependent on ATP, GTP, and cytosolic factors. Vesicle formation was inhibited by GTP gamma S suggesting a role for GTP-binding proteins in this process. Vesicle budding was dependent on cytosolic factors that were tightly membrane associated and could be removed only by treating the permeabilized cells with high salt. After high salt treatment, vesicle formation was dependent on added cytosol or the dialyzed salt extract. The formation of nascent secretory vesicles contrasts with prosomatostatin processing which required only ATP for efficient cleavage. Our results demonstrate that prohormone cleavage which is initiated in the TGN, precedes vesicle formation and that processing can be uncoupled from the generation of nascent secretory vesicles.

Regulated exocytosis

Images

A low affinity Ca2+ receptor controls the final steps in peptide secretion from pituitary melanotrophs

Using flash photolysis of caged Ca2+ and the membrane capacitance to monitor exocytosis, we have studied the response of single melanotrophs to a step rise in cytosolic Ca2+ concentration ([Ca2+]i). Exocytosis begins with a rapid burst. This burst is followed by a slower phase, which is inhibited at cytosolic pH 6.2, and an ultraslow phase, which is strongly temperature sensitive. The exocytic burst starts with a delay of 6-11 ms and continues at a rate that grows steeply with [Ca2+]i and is half-maximal at [Ca2+]i = 27 microM. At least 3 Ca2+ ions are required to trigger exocytosis. The rate constant at saturating [Ca2+]i suggests that exocytosis of a dense core vesicle takes 40 ms after all Ca2+ ions have bound to their regulatory sites. If docked dense core vesicles cause the exocytic burst, they must decorate the plasma membrane at a mean density of 0.5/micron2.

Reconstitution of transmitter secretion

Molecular mechanisms involved in the various stages of transmitter secretion have been studied by perturbing the composition of secretory cells using pharmacological and biochemical agents. An emerging approach is to reconstitute individual steps or the entire sequence of secretion mechanisms in non-secretory cells by loading the cell with presynaptic components or their mRNAs.

Identification of a key domain in annexin and 14-3-3 proteins that stimulate calcium-dependent exocytosis in permeabilized adrenal chromaffin cells

Calcium-dependent secretion in digitonin-permeabilized adrenal chromaffin cells is stimulated by exogenous annexin II and 14-3-3 proteins. These proteins share a conserved domain that has been suggested to be involved in specific protein-protein interactions. We examined whether this domain was involved in secretion by using a synthetic peptide (P16) of sequence KGDYQKALLYLCGGDD corresponding to the C-terminus of annexin II. P16, but not truncated peptides, prevented the stimulation of secretion by 14-3-3 proteins and produced a partial inhibition of control secretion. These data suggest that the shared annexin/14-3-3 domain is important in the mechanisms controlling Ca(2+)-dependent secretion and may play a key role in protein-protein interactions during exocytosis.

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.

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.