Sensitization of regulated exocytosis by protein kinase C

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.

Gonadotropin-releasing hormone signaling: An information theoretic approach

Gonadotropin-releasing hormone (GnRH) is a peptide hormone that mediates central control of reproduction, acting via G-protein coupled receptors that are primarily G_q coupled and mediate GnRH effects on the synthesis and secretion of luteinizing hormone and follicle-stimulating hormone. A great deal is known about the GnRH receptor signaling network but GnRH is secreted in short pulses and much less is known about how gonadotropes decode this pulsatile signal. Similarly, single cell measures reveal considerable cell-cell heterogeneity in responses to GnRH but the impact of this variability on signaling is largely unknown. Ordinary differential equation-based mathematical models have been used to explore the decoding of pulse dynamics and information theory-derived statistical measures are increasingly used to address the influence of cell-cell variability on the amount of information transferred by signaling pathways. Here, we describe both approaches for GnRH signaling, with emphasis on novel insights gained from the information theoretic approach and on the fundamental question of why GnRH is secreted in pulses.

Exploring Dynamics and Noise in Gonadotropin-Releasing Hormone (GnRH) Signaling

Gonadotropin-releasing hormone (GnRH) acts via G-protein coupled receptors on pituitary gonadotropes. These are G_q-coupled receptors that mediate acute effects of GnRH on the exocytotic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as the chronic regulation of their synthesis. FSH and LH control steroidogenesis and gametogenesis in the gonads so GnRH mediates control of reproduction by the central nervous system. GnRH is secreted in short pulses and the effects of GnRH on its target cells are dependent on the dynamics of these pulses. Here we provide a brief overview of the signaling network activated by GnRH with emphasis on the use of high content imaging for their examination. We also describe computational approaches that we have used to simulate GnRH signaling in order to explore dynamics, noise, and information transfer in this system.

Mathematical modeling of gonadotropin-releasing hormone signaling

Gonadotropin-releasing hormone (GnRH) acts via G-protein coupled receptors on pituitary gonadotropes to control reproduction. These are G_q-coupled receptors that mediate acute effects of GnRH on the exocytotic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as the chronic regulation of their synthesis. GnRH is secreted in short pulses and GnRH effects on its target cells are dependent upon the dynamics of these pulses. Here we overview GnRH receptors and their signaling network, placing emphasis on pulsatile signaling, and how mechanistic mathematical models and an information theoretic approach have helped further this field.

PKC enhances the capacity for secretion by rapidly recruiting covert voltage-gated Ca2+ channels to the membrane

It is unknown whether neurons can dynamically control the capacity for secretion by promptly changing the number of plasma membrane voltage-gated Ca(2+) channels. To address this, we studied peptide release from the bag cell neurons of Aplysia californica, which initiate reproduction by secreting hormone during an afterdischarge. This burst engages protein kinase C (PKC) to trigger the insertion of a covert Ca(2+) channel, Apl Cav2, alongside a basal channel, Apl Cav1. The significance of Apl Cav2 recruitment to secretion remains undetermined; therefore, we used capacitance tracking to assay secretion, along with Ca(2+) imaging and Ca(2+) current measurements, from cultured bag cell neurons under whole-cell voltage-clamp. Activating PKC with the phorbol ester, PMA, enhanced Ca(2+) entry, and potentiated stimulus-evoked secretion. This relied on channel insertion, as it was occluded by preventing Apl Cav2 engagement with prior whole-cell dialysis or the cytoskeletal toxin, latrunculin B. Channel insertion reduced the stimulus duration and/or frequency required to initiate secretion and strengthened excitation-secretion coupling, indicating that Apl Cav2 accesses peptide release more readily than Apl Cav1. The coupling of Apl Cav2 to secretion also changed with behavioral state, as Apl Cav2 failed to evoke secretion in silent neurons from reproductively inactive animals. Finally, PKC also acted secondarily to enhance prolonged exocytosis triggered by mitochondrial Ca(2+) release. Collectively, our results suggest that bag cell neurons dynamically elevate Ca(2+) channel abundance in the membrane to ensure adequate secretion during the afterdischarge.

GnRH-Induced Ca(2+) Signaling Patterns and Gonadotropin Secretion in Pituitary Gonadotrophs. Functional Adaptations to Both Ordinary and Extraordinary Physiological Demands

PITUITARY GONADOTROPHS ARE A SMALL FRACTION OF THE ANTERIOR PITUITARY POPULATION, YET THEY SYNTHESIZE GONADOTROPINS: luteinizing (LH) and follicle-stimulating (FSH), essential for gametogenesis and steroidogenesis. LH is secreted via a regulated pathway while FSH release is mostly constitutive and controlled by synthesis. Although gonadotrophs fire action potentials spontaneously, the intracellular Ca(2+) rises produced do not influence secretion, which is mainly driven by Gonadotropin-Releasing Hormone (GnRH), a decapeptide synthesized in the hypothalamus and released in a pulsatile manner into the hypophyseal portal circulation. GnRH binding to G-protein-coupled receptors triggers Ca(2+) mobilization from InsP3-sensitive intracellular pools, generating the global Ca(2+) elevations necessary for secretion. Ca(2+) signaling responses to increasing (GnRH) vary in stereotyped fashion from subthreshold to baseline spiking (oscillatory), to biphasic (spike-oscillatory or spike-plateau). This progression varies somewhat in gonadotrophs from different species and biological preparations. Both baseline spiking and biphasic GnRH-induced Ca(2+) signals control LH/FSH synthesis and exocytosis. Estradiol and testosterone regulate gonadotropin secretion through feedback mechanisms, while FSH synthesis and release are influenced by activin, inhibin, and follistatin. Adaptation to physiological events like the estrous cycle, involves changes in GnRH sensitivity and LH/FSH synthesis: in proestrus, estradiol feedback regulation abruptly changes from negative to positive, causing the pre-ovulatory LH surge. Similarly, when testosterone levels drop after orquiectomy the lack of negative feedback on pituitary and hypothalamus boosts both GnRH and LH secretion, gonadotrophs GnRH sensitivity increases, and Ca(2+) signaling patterns change. In addition, gonadotrophs proliferate and grow. These plastic changes denote a more vigorous functional adaptation in response to an extraordinary functional demand.

The dynamics of somatic exocytosis in monoaminergic neurons

Some monoaminergic neurons can release neurotransmitters by exocytosis from their cell bodies. The amount of monoamine released by somatic exocytosis can be comparable to that released by synaptic exocytosis, though neither the underlying mechanisms nor the functional significance of somatic exocytosis are well understood. A detailed examination of these characteristics may provide new routes for therapeutic intervention in mood disorders, substance addiction, and neurodegenerative diseases. The relatively large size of the cell body provides a unique opportunity to understand the mechanism of this mode of neuronal exocytosis in microscopic detail. Here we used three photon and total internal reflection fluorescence microscopy to focus on the dynamics of the pre-exocytotic events and explore the nature of somatic vesicle storage, transport, and docking at the membrane of serotonergic neurons from raphe nuclei of the rat brain. We find that the vesicles (or unresolved vesicular clusters) are quiescent (mean square displacement, MSD ∼0.04 μm(2)/s) before depolarization, and they move minimally (<1 μm) from their locations over a time-scale of minutes. However, within minutes of depolarization, the vesicles become more dynamic (MSD ∼0.3 μm(2)/s), and display larger range (several μm) motions, though without any clear directionality. Docking and subsequent exocytosis at the membrane happen at a timescale (∼25 ms) that is slower than most synaptic exocytosis processes, but faster than almost all somatic exocytosis processes observed in endocrine cells. We conclude that, (A) depolarization causes de-tethering of the neurotransmitter vesicles from their storage locations, and this constitutes a critical event in somatic exocytosis; (B) their subsequent transport kinetics can be described by a process of constrained diffusion, and (C) the pre-exocytosis kinetics at the membrane is faster than most other somatic exocytosis processes reported so far.

Photoperiodic modulation of the suppressive actions of prolactin and dopamine on the pituitary gonadotropin responses to gonadotropin-releasing hormone in sheep

In a variety of species, the LH-secretory response to gonadotropin-releasing hormone (GnRH) is completely suppressed by the combined actions of prolactin (PRL) and dopamine (DA). In sheep, this effect is only observed under long days (nonbreeding season [NBS]). To investigate the level at which these mechanisms operate, we assessed the effects of PRL and bromocriptine (Br), a DA agonist, on the gonadotropin-secretory and mRNA responses to GnRH in pituitary cell cultures throughout the ovine annual reproductive cycle. As expected, the LH-secretory response to GnRH was only abolished during the NBS following combined PRL and Br application. Conversely, the LHB subunit response to GnRH was reduced during both the BS and NBS by the combined treatment and Br alone. Similar results were obtained in pars distalis-only cultures, indicating that the effects are pars tuberalis (PT)- independent. Further signaling studies revealed that PRL and Br alter the LH response to GnRH via convergence at the level of PLC and PKC. Results for FSH generally reflected those for LH, except during the BS where removal of the PT allowed PRL and Br to suppress the FSH-secretory response to GnRH. These data show that suppression of the LH-secretory response to GnRH by PRL and DA is accompanied by changes in mRNA synthesis, and that the photoperiodic modulation of this inhibition operates primarily at the level of LH release through alterations in PKC and PLC. Furthermore, the suppressive effects of PRL and DA on the secretion of FSH are photoperiodically regulated in a PT-dependent manner.

Coordination of calcium signals by pituitary endocrine cells in situ

The pulsatile secretion of hormones from the mammalian pituitary gland drives a wide range of homeostatic responses by dynamically altering the functional set-point of effector tissues. To accomplish this, endocrine cell populations residing within the intact pituitary display large-scale changes in coordinated calcium-spiking activity in response to various hypothalamic and peripheral inputs. Although the pituitary gland is structurally compartmentalized into specific and intermingled endocrine cell networks, providing a clear morphological basis for such coordinated activity, the mechanisms which facilitate the timely propagation of information between cells in situ remain largely unexplored. Therefore, the aim of the current review is to highlight the range of signalling modalities known to be employed by endocrine cells to coordinate intracellular calcium rises, and discuss how these mechanisms are integrated at the population level to orchestrate cell function and tissue output.

Mucin secretion induced by titanium dioxide nanoparticles

Nanoparticle (NP) exposure has been closely associated with the exacerbation and pathophysiology of many respiratory diseases such as Chronic Obstructive Pulmonary Disease (COPD) and asthma. Mucus hypersecretion and accumulation in the airway are major clinical manifestations commonly found in these diseases. Among a broad spectrum of NPs, titanium dioxide (TiO(2)), one of the PM10 components, is widely utilized in the nanoindustry for manufacturing and processing of various commercial products. Although TiO(2) NPs have been shown to induce cellular nanotoxicity and emphysema-like symptoms, whether TiO(2) NPs can directly induce mucus secretion from airway cells is currently unknown. Herein, we showed that TiO(2) NPs (<75 nm) can directly stimulate mucin secretion from human bronchial ChaGo-K1 epithelial cells via a Ca(2+) signaling mediated pathway. The amount of mucin secreted was quantified with enzyme-linked lectin assay (ELLA). The corresponding changes in cytosolic Ca(2+) concentration were monitored with Rhod-2, a fluorescent Ca(2+) dye. We found that TiO(2) NP-evoked mucin secretion was a function of increasing intracellular Ca(2+) concentration resulting from an extracellular Ca(2+) influx via membrane Ca(2+) channels and cytosolic ER Ca(2+) release. The calcium-induced calcium release (CICR) mechanism played a major role in further amplifying the intracellular Ca(2+) signal and in sustaining a cytosolic Ca(2+) increase. This study provides a potential mechanistic link between airborne NPs and the pathoetiology of pulmonary diseases involving mucus hypersecretion.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Cyclic AMP potentiates Ca2+-dependent exocytosis in pancreatic duct epithelial cells

Exocytosis is evoked by intracellular signals, including Ca²⁺ and protein kinases. We determined how such signals interact to promote exocytosis in exocrine pancreatic duct epithelial cells (PDECs). Exocytosis, detected using carbon-fiber microamperometry, was stimulated by [Ca²⁺]_i increases induced either through Ca²⁺ influx using ionomycin or by activation of P2Y2 or protease-activated receptor 2 receptors. In each case, the exocytosis was strongly potentiated when cyclic AMP (cAMP) was elevated either by activating adenylyl cyclase with forskolin or by activating the endogenous vasoactive intestinal peptide receptor. This potentiation was completely inhibited by H-89 and partially blocked by Rp-8-Br-cAMPS, inhibitors of protein kinase A. Optical monitoring of fluorescently labeled secretory granules showed slow migration toward the plasma membrane during Ca²⁺ elevations. Neither this Ca²⁺-dependent granule movement nor the number of granules found near the plasma membrane were detectably changed by raising cAMP, suggesting that cAMP potentiates Ca²⁺-dependent exocytosis at a later stage. A kinetic model was made of the exocytosis stimulated by UTP, trypsin, and Ca²⁺ ionophores with and without cAMP increase. In the model, without a cAMP rise, receptor activation stimulates exocytosis both by Ca²⁺ elevation and by the action of another messenger(s). With cAMP elevation the docking/priming step for secretory granules was accelerated, augmenting the releasable granule pool size, and the Ca²⁺ sensitivity of the final fusion step was increased, augmenting the rate of exocytosis. Presumably both cAMP actions require cAMP-dependent phosphorylation of target proteins. cAMP-dependent potentiation of Ca²⁺-induced exocytosis has physiological implications for mucin secretion and, possibly, for membrane protein insertion in the pancreatic duct. In addition, mechanisms underlying this potentiation of slow exocytosis may also exist in other cell systems.

Inhibition of protein kinase C phosphorylation of hepatitis B virus capsids inhibits virion formation and causes intracellular capsid accumulation

Capsids of hepatitis B virus and other hepadnaviruses contain a cellular protein kinase, which phosphorylates the capsid protein. Some phosphorylation sites are shown to be essential for distinct steps of viral replication as pregenome packaging or plus strand DNA synthesis. Although different protein kinases have been reported to phosphorylate the capsid protein, varying experimental approaches do not allow direct comparison. Furthermore, the activity of a specific protein kinase has not yet been correlated to steps in the hepadnaviral life cycle. In this study we show that capsids from various sources encapsidate active protein kinase Calpha, irrespective of hepatitis B virus genotype and host cell. Treatment of a virion expressing cell line with a pseudosubstrate inhibitor showed that inhibition of protein kinase C phosphorylation did not affect genome maturation but resulted in capsid accumulation and inhibited virion release to the medium. Our results imply that different protein kinases have distinct functions within the hepadnaviral life cycle.

Estradiol-17beta inhibits gonadotropin-releasing hormone-induced Ca2+ in gonadotropes to regulate negative feedback on luteinizing hormone release

In pituitary gonadotropes, estrogens have biphasic actions to cause an initial negative feedback followed by a positive feedback on LH secretion, but the mechanisms involved are not clearly understood. To investigate the feedback effects of estrogen, we used mixed ovine pituitary cell cultures (48-72 h), which were treated with 10(-9) M estradiol-17beta (E(2)) or vehicle followed by a pulse of 10(-9) M GnRH. Medium was collected for LH assay and cells extracted to determine activation of MAPK (phosphorylated ERK-1/2). E(2) treatment for 5 min reduced GnRH-induced LH release and caused phosphorylation of ERK-1/2. E(2) alone also caused phosphorylation of ERK-1/2, similar to the response evoked by GnRH alone. GnRH increased cytoplasmic intracellular free calcium concentration ([Ca(2+)](i)) and this was abolished by 2 min pretreatment with E(2) or E-bovine serum albumen conjugate. Blockade of Ca(2+) channels with nifedipine had no effect on the initial peak of GnRH-induced increase in [Ca(2+)](i) but reduced its duration by 27 +/- 6%. Depletion of intracellular Ca(2+) stores with thapsigargin prevented GnRH-induced increase in [Ca(2+)](i). Thapsigargin (10(-7) M) or nifedipine (10(-5) M) pretreatment (15 min) of cells lowered GnRH-induced LH secretion by 30 +/- 6 and 50% +/- 4%, respectively. We conclude that inhibition of the GnRH-induced increase in [Ca(2+)](i) in gonadotropes by E(2) is a likely mechanism for the negative feedback effect of E(2) on LH secretion involving a rapid nongenomic effect of E(2). Activation of the MAPK pathway by E(2) may be the mechanism for the time-delayed positive feedback effect on LH secretion at the level of the gonadotrope.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Synaptotagmin IV regulates dense core vesicle (DCV) release in LbetaT2 cells

Synaptotagmins (Syts) are calcium-binding proteins which are conserved from nematodes to humans. Fifteen Syts have been identified in mammalian species. Syt I is recognized as a Ca(2+) sensor for the synchronized release of synaptic vesicles in some types of neurons, but its role in the secretion of dense core vesicles (DCVs) remains unclear. The function of Syt IV is of particular interest because it is rapidly up-regulated by chronic depolarization and seizures. Using RNAi-mediated gene silencing, we have explored the role of Syt I and IV on secretion in a pituitary gonadotrope cell line. Downregulation of Syt IV clearly reduced Ca(2+)-triggered exocytosis of dense core vesicles (DCVs) in LbetaT2 cells. Syt I silencing, however, had no effect on vesicular release.

Dopamine inhibits basal prolactin release in pituitary lactotrophs through pertussis toxin-sensitive and -insensitive signaling pathways

Dopamine D2 receptors signal through the pertussis toxin (PTX)-sensitive G(i/o) and PTX-insensitive G(z) proteins, as well as through a G protein-independent, beta-arrestin/glycogen synthase kinase-3-dependent pathway. Activation of these receptors in pituitary lactotrophs leads to inhibition of prolactin (PRL) release. It has been suggested that this inhibition occurs through the G(i/o)-alpha protein-mediated inhibition of cAMP production and/or G(i/o)-betagamma dimer-mediated activation of inward rectifier K(+) channels and inhibition of voltage-gated Ca(2+) channels. Here we show that the dopamine agonist-induced inhibition of spontaneous Ca(2+) influx and release of prestored PRL was preserved when cAMP levels were elevated by forskolin treatment. We further observed that dopamine agonists inhibited both spontaneous and depolarization-induced Ca(2+) influx in untreated but not in PTX-treated cells. This inhibition was also observed in cells with blocked inward rectifier K(+) channels, suggesting that the dopamine effect on voltage-gated Ca(2+) channel gating is sufficient to inhibit spontaneous Ca(2+) influx. However, agonist-induced inhibition of PRL release was only partially relieved in PTX-treated cells, indicating that dopamine receptors also inhibit exocytosis downstream of voltage-gated Ca(2+) influx. The PTX-insensitive step in agonist-induced inhibition of PRL release was not affected by the addition of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and lithium, an inhibitor of glycogen synthase kinase-3, but was attenuated in the presence of phorbol 12-myristate 13-acetate, which inhibits G(z) signaling pathway in a protein kinase C-dependent manner. Thus, dopamine inhibits basal PRL release by blocking voltage-gated Ca(2+) influx through the PTX-sensitive signaling pathway and by desensitizing Ca(2+) secretion coupling through the PTX-insensitive and protein kinase C-sensitive signaling pathway.

Phosphorylation of SNAP-25 at Ser187 mediates enhancement of exocytosis by a phorbol ester in INS-1 cells

Activation of diacylglycerol (DAG) signaling pathways with phorbol esters dramatically enhances Ca2+-triggered exocytosis from both endocrine cells and neurons, however the relevant targets of DAG are controversial. A possible effector mechanism for this signaling pathway is phosphorylation of SNAP-25 (25 kDa synaptosome-associated protein) at Ser187 by PKC. Here, we investigated the role of Ser187 in the enhancement of exocytosis by the phorbol ester PMA (phorbol 12-myristate 13-acetate). We used patch-clamp measurements of membrane capacitance together with photorelease of caged-Ca2+ and membrane depolarization to study exocytosis. Expression of the nonphosphorylatable S187C SNAP-25 mutant did not attenuate the enhancement of exocytosis by PMA in either bovine chromaffin cells or the INS-1 insulin-secreting cell line. To test the effects of Ser187 mutations under conditions in which the endogenous SNAP-25 is disabled, we expressed botulinum toxin serotype E to cleave SNAP-25 in INS-1 cells. Coexpression of a toxin-resistant mutant (TR), but not wild-type SNAP-25, was able to rescue PMA-modulated exocytosis. Coexpression of the toxin with the TR-S187C SNAP-25 mutant was able to completely block the enhancement of exocytosis by PMA in response to photoelevation of [Ca2+]i to low microM levels or to a depolarizing train. The phospho-mimetic S187E mutation enhanced the small, fast burst of exocytosis evoked by photelevation of Ca2+, but, like PMA, had smaller effects on exocytosis evoked by a depolarizing train. This work supports the hypothesis that phosphorylation of Ser187 of SNAP-25 by PKC is a key step in the enhancement of exocytosis by DAG.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Phosphomimetic mutation of Ser-187 of SNAP-25 increases both syntaxin binding and highly Ca2+-sensitive exocytosis

The phosphorylation targets that mediate the enhancement of exocytosis by PKC are unknown. PKC phosporylates the SNARE protein SNAP-25 at Ser-187. We expressed mutants of SNAP-25 using the Semliki Forest Virus system in bovine adrenal chromaffin cells and then directly measured the Ca²⁺ dependence of exocytosis using photorelease of caged Ca²⁺ together with patch-clamp capacitance measurements. A flash of UV light used to elevate [Ca²⁺]_i to several μM and release the highly Ca²⁺-sensitive pool (HCSP) of vesicles was followed by a train of depolarizing pulses to elicit exocytosis from the less Ca²⁺-sensitive readily releasable pool (RRP) of vesicles. Carbon fiber amperometry confirmed that the amount and kinetics of catecholamine release from individual granules were similar for the two phases of exocytosis. Mimicking PKC phosphorylation with expression of the S187E SNAP-25 mutant resulted in an approximately threefold increase in the HCSP, whereas the response to depolarization increased only 1.5-fold. The phosphomimetic S187D mutation resulted in an ∼1.5-fold increase in the HCSP but a 30% smaller response to depolarization. In vitro binding assays with recombinant SNARE proteins were performed to examine shifts in protein–protein binding that may promote the highly Ca²⁺-sensitive state. The S187E mutant exhibited increased binding to syntaxin but decreased Ca²⁺-independent binding to synaptotagmin I. Mimicking phosphorylation of the putative PKA phosphorylation site of SNAP-25 with the T138E mutation decreased binding to both syntaxin and synaptotagmin I in vitro. Expressing the T138E/ S187E double mutant in chromaffin cells demonstrated that enhancing the size of the HCSP correlates with an increase in SNAP-25 binding to syntaxin in vitro, but not with Ca²⁺-independent binding of SNAP-25 to synaptotagmin I. Our results support the hypothesis that exocytosis triggered by lower Ca²⁺ concentrations (from the HCSP) occurs by different molecular mechanisms than exocytosis triggered by higher Ca²⁺ levels.

Interdependence of PKC-Dependent and PKC-Independent Pathways for Presynaptic Plasticity

Diacylglycerol (DAG) is a prominent endogenous modulator of synaptic transmission. Recent studies proposed two apparently incompatible pathways, via protein kinase C (PKC) and via Munc13. Here we show how these two pathways converge. First, we confirm that DAG analogs indeed continue to potentiate transmission after PKC inhibition (the Munc13 pathway), but only in neurons that previously experienced DAG analogs, before PKC inhibition started. Second, we identify an essential PKC pathway by expressing a PKC-insensitive Munc18-1 mutant in munc18-1 null mutant neurons. This mutant supported basic transmission, but not DAG-induced potentiation and vesicle redistribution. Moreover, synaptic depression was increased, but not Ca2+-independent release evoked by hypertonic solutions. These data show that activation of both PKC-dependent and -independent pathways (via Munc13) are required for DAG-induced potentiation. Munc18-1 is an essential downstream target in the PKC pathway. This pathway is of general importance for presynaptic plasticity.

Heterogeneity of the Ca2+ sensitivity of secretion in a pituitary gonadotrope cell line and its modulation by protein kinase C and Ca2+

Modulation of the Ca2+ sensitivity and cooperativity of secretion is an important means of regulating neurotransmission and hormone secretion. Employing high-time resolution measurement of membrane capacitance (Cm) stimulated by step-like or ramp [Ca2+]i elevation, we have identified the co-existence of both a high and low Ca2+-sensitive exocytosis in an immortal pituitary gonadotrope cell line, LbetaT2. Ramp [Ca2+]i generated by slow uncaging elicited a biphasic C(m) response. The first phase of response, which represents a highly Ca2+-sensitive pool (HCSP) of vesicles, began to secrete at low [Ca2+]i concentration (<1 microM) with low Ca2+ cooperativity. In contrast, the second phase, which represents a lowly Ca2+-sensitive pool (LCSP) of vesicles, only exocytozed at higher [Ca2+]i (>5 microM) and displayed a steep Ca2+ cooperativity. The co-existence of vesicle populations with different Ca2+ sensitivities was further confirmed by flash photolysis stimuli. The size of the HCSP was approximately 30 fF under resting conditions, but was dramatically increased (approximately threefold) by application of phorbol-12-myristate-13-acetate (PMA, an activator of protein kinase C). Forskolin (an activator of protein kinase A), however, exerted no significant effect on the size of both HCSP and LCSP. GnRH (gonadotropin releasing hormone) augmented the size of both pools to a larger extent (5- and 1.7-fold increase for HCSP and LCSP, respectively). The heterogeneity of Ca2+ sensitivity from different pools of vesicles and its differential modulation by intracellular signals suggests that LbetaT2 cells are an ideal model to further unravel the mechanism underlying the modulation of Ca2+-sensing machineries for exocytosis.

Different effects on fast exocytosis induced by synaptotagmin 1 and 2 isoforms and abundance but not by phosphorylation

Synaptotagmins comprise a large protein family, of which synaptotagmin 1 (Syt1) is a Ca2+ sensor for fast exocytosis, and its close relative, synaptotagmin 2 (Syt2), is assumed to serve similar functions. Chromaffin cells express Syt1 but not Syt2. We compared secretion from chromaffin cells from Syt1 null mice overexpressing either Syt isoform. High time-resolution capacitance measurement showed that Syt1 null cells lack the exocytotic phase corresponding to the readily-releasable pool (RRP) of vesicles. Comparison with the amperometric signal confirmed that the missing phase of exocytosis consists of catecholamine-containing vesicles. Overexpression of Syt1 rescued the RRP and increased its size above wild-type values, whereas the size of the slowly releasable pool decreased, indicating that the availability of Syt1 regulates the relative size of the two releasable pools. The RRP was also rescued by Syt2 overexpression, but the kinetics of fusion was slightly slower than in cells expressing Syt1. Biochemical experiments showed that Syt2 has a slightly lower Ca2+ affinity for phospholipid binding than Syt1 because of a difference in the C2A domain. These data constitute evidence for the function of Syt1 and Syt2 as alternative, but not identical, calcium-sensors for RRP fusion. By overexpression of Syt1 mutated in the shared PKC/calcium/calmodulin-dependent kinase phosphorylation site, we show that phorbol esters act independently and upstream of Syt1 to regulate the size of the releasable pools. We conclude that exocytosis from mouse chromaffin cells can be modified by the differential expression of Syt isoforms and by Syt abundance but not by phosphorylation of Syt1.

Alpha-latrotoxin triggers extracellular Ca(2+)-dependent exocytosis and sensitizes fusion machinery in endocrine cells

alpha-Latrotoxin from the venom of black widow spider induces and augments neurotransmitter and hormone release by way of extracellular Ca(2+) influx and cellular signal transduction pathways. By using whole cell current and capacitance recording, the photolysis of caged Ca(2+), and Ca(2+) microfluorometry and amperometry, we investigated the stimulating effect and mechanism of alpha-latrotoxin on exocytosis in rat pancreatic beta cells, LbetaT2 cells and latrophilin plasmid-transfected INS-1 cells. Our data indicated that: (1) alpha-latrotoxin increased cytosolic Ca(2+) concentration through the formation of cation-permitting pores and subsequent Ca(2+) influx with the presence of extracellular Ca(2+); (2) alpha-latrotoxin stimulated exocytosis in normal bath solution and its stimulating effect on secretion was eradicated in Ca(2+)-free bath solution; and (3) alpha-latrotoxin sensitized the molecular machinery of fusion through activation of protein kinase C and increased the response of cells to Ca(2+) photolyzed by a flash of ultraviolet light. In summary, alpha-latrotoxin induced exocytosis by way of Ca(2+) influx and accelerated vesicle fusion by the sensitization of fusion machinery.

Calcium-dependent regulation of exocytosis

A rapid increase in intracellular calcium directly triggers regulated exocytosis. In addition, changes in intracellular calcium concentration can adjust the extent of exocytosis (quantal content) or the magnitude of individual release events (quantal size) in both the short- and long-term. It is generally agreed that calcium achieves this regulation via an interaction with a number of different molecular targets located at or near to the site of membrane fusion. We review here the synaptic proteins with defined calcium-binding domains and protein kinases activated by calcium, summarize what is known about their function in membrane fusion and the experimental evidence in support of their involvement in synaptic plasticity.

Endothelin-induced, Long Lasting, and Ca2+ Influx-independent Blockade of Intrinsic Secretion in Pituitary Cells by Gz Subunits

The G protein-coupled receptors in excitable cells have prominent roles in controlling Ca2+-triggered secretion by modulating voltage-gated Ca2+ influx. In pituitary lactotrophs, spontaneous voltage-gated Ca2+ influx is sufficient to maintain prolactin release high. Here we show that endothelin in picomolar concentrations can interrupt such release for several hours downstream of spontaneous and high K+-stimulated voltage-gated Ca2+ influx. This action occurred through the Gz signaling pathway; the adenylyl cyclase-signaling cascade could mediate sustained inhibition of secretion, whereas rapid inhibition also occurred at elevated cAMP levels regardless of the status of phospholipase C, tyrosine kinases, and protein kinase C. In a nanomolar concentration range, endothelin also inhibited voltage-gated Ca2+ influx through the G i/o signaling pathway. Thus, the coupling of seven-transmembrane domain endothelin receptors to Gz proteins provided a pathway that effectively blocked hormone secretion distal to Ca2+ entry, whereas the cross-coupling to G i/o proteins reinforced such inhibition by simultaneously reducing the pacemaking activity.

cAMP increases Ca2+-dependent exocytosis through both PKA and Epac2 in mouse melanotrophs from pituitary tissue slices

Cyclic AMP regulates Ca(2+)-dependent exocytosis through a classical protein kinase A (PKA)-dependent and an alternative cAMP-guanine nucleotide exchange factor (GEF)/Epac-dependent pathway in many secretory cells. Although increased cAMP is believed to double secretory output in isolated pituitary cells, the direct target(s) for cAMP action and a detailed and high-time resolved analysis of the effect of intracellular cAMP levels on the secretory activity in melanotrophs are still lacking. We investigated the effect of 200 microM cAMP on the kinetics of secretory vesicle depletion in mouse melanotrophs from fresh pituitary tissue slices. The whole-cell patch-clamp technique was used to depolarize melanotrophs and increase the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Exogenous cAMP elicited an about twofold increase in cumulative membrane capacitance change and approximately 34% increase of high-voltage activated Ca(2+) channel amplitude. cAMP-dependent mechanisms did not affect [Ca(2+)](i), since the application of forskolin failed to change [Ca(2+)](i) in melanotrophs, a phenomenon readily observed in anterior lobe. Depolarization-induced secretion resulted in two distinct kinetic components: a linear and a threshold component, both stimulated by cAMP. The linear component (ATP-independent) probably represented the exocytosis of the release-ready vesicles, whereas the threshold component was assigned to the exocytosis of secretory vesicles that required ATP-dependent reaction(s) and > 800 nM [Ca(2+)](i). The linear component was modulated by 8-pCPT-2Me-cAMP (Epac agonist), while either H-89 (PKA inhibitor) or Rp-cAMPS (the competitive antagonist of cAMP binding to PKA) completely prevented the action of cAMP on the threshold component. In line with this, 6-Phe-cAMP, (PKA agonist), increased the threshold component. From our study, we suggest that the stimulation of cAMP production by application of oestrogen, as found in pregnant mice, increases the efficacy of the hormonal output through both PKA and cAMP-GEFII/Epac2-dependent mechanisms.

Allosteric modulation of the presynaptic Ca2+ sensor for vesicle fusion

Neurotransmitter release is triggered by an increase in the cytosolic Ca2+ concentration ([Ca2+]i), but it is unknown whether the Ca2+-sensitivity of vesicle fusion is modulated during synaptic plasticity. We investigated whether the potentiation of neurotransmitter release by phorbol esters, which target presynaptic protein kinase C (PKC)/munc-13 signalling cascades, exerts a direct effect on the Ca2+-sensitivity of vesicle fusion. Using direct presynaptic Ca2+-manipulation and Ca2+ uncaging at a giant presynaptic terminal, the calyx of Held, we show that phorbol esters potentiate transmitter release by increasing the apparent Ca2+-sensitivity of vesicle fusion. Phorbol esters potentiate Ca2+-evoked release as well as the spontaneous release rate. We explain both effects by an increased fusion 'willingness' in a new allosteric model of Ca2+-activation of vesicle fusion. In agreement with an allosteric mechanism, we observe that the classically high Ca2+ cooperativity in triggering vesicle fusion (approximately 4) is gradually reduced below 3 microM [Ca2+]i, reaching a value of <1 at basal [Ca2+]i. Our data indicate that spontaneous transmitter release close to resting [Ca2+]i is a consequence of an intrinsic property of the molecular machinery that mediates synaptic vesicle fusion.

α-Latrotoxin Modulates the Secretory Machinery via Receptor-Mediated Activation of Protein Kinase C

The hypothesis whether alpha-latrotoxin (LTX) could directly regulate the secretory machinery was tested in pancreatic beta cells using combined techniques of membrane capacitance (Cm) measurement and Ca2+ uncaging. Employing ramp increase in [Ca2+]i to stimulate exocytosis, we found that LTX lowers the Ca2+ threshold required for exocytosis without affecting the size of the readily releasable pool (RRP). The burst component of exocytosis in response to step-like [Ca2+]i increase generated by flash photolysis of caged Ca2+ was also speeded up by LTX treatment. LTX increased the maximum rate of exocytosis compared with control responses with similar postflash [Ca2+]i and shifted the Ca2+ dependence of the exocytotic machinery toward lower Ca2+ concentrations. LTXN4C, a LTX mutant which cannot form membrane pores or penetrate through the plasma membrane but has similar affinity for the receptors as the wild-type LTX, mimicked the effect of LTX. Moreover, the effects of both LTX and LTXN4C) were independent of intracellular or extracellular Ca2+ but required extracellular Mg2+. Our data propose that LTX, by binding to the membrane receptors, sensitizes the fusion machinery to Ca2+ and, hence, may permit release at low [Ca2+]i level. This sensitization is mediated by activation of protein kinase C.

A highly Ca2+-sensitive pool of granules is regulated by glucose and protein kinases in insulin-secreting INS-1 cells

We have used membrane capacitance measurements and carbon-fiber amperometry to assay exocytosis triggered by photorelease of caged Ca²⁺ to directly measure the Ca²⁺ sensitivity of exocytosis from the INS-1 insulin-secreting cell line. We find heterogeneity of the Ca²⁺ sensitivity of release in that a small proportion of granules makes up a highly Ca²⁺-sensitive pool (HCSP), whereas the bulk of granules have a lower sensitivity to Ca²⁺. A substantial HCSP remains after brief membrane depolarization, suggesting that the majority of granules with high sensitivity to Ca²⁺ are not located close to Ca²⁺ channels. The HCSP is enhanced in size by glucose, cAMP, and a phorbol ester, whereas the Ca²⁺-sensitive rate constant of exocytosis from the HCSP is unaffected by cAMP and phorbol ester. The effects of cAMP and phorbol ester on the HCSP are mediated by PKA and PKC, respectively, because they can be blocked with specific protein kinase inhibitors. The size of the HCSP can be enhanced by glucose even in the presence of high concentrations of phorbol ester or cAMP, suggesting that glucose can increase granule pool sizes independently of activation of PKA or PKC. The effects of PKA and PKC on the size of the HCSP are not additive, suggesting they converge on a common mechanism. Carbon-fiber amperometry was used to assay quantal exocytosis of serotonin (5-HT) from insulin-containing granules following preincubation of INS-1 cells with 5-HT and a precursor. The amount or kinetics of release of 5-HT from each granule is not significantly different between granules with higher or lower sensitivity to Ca²⁺, suggesting that granules in these two pools do not differ in morphology or fusion kinetics. We conclude that glucose and second messengers can modulate insulin release triggered by a high-affinity Ca²⁺ sensor that is poised to respond to modest, global elevations of [Ca²⁺]_i.

Protein kinase activation increases insulin secretion by sensitizing the secretory machinery to Ca2+

Glucose and other secretagogues are thought to activate a variety of protein kinases. This study was designed to unravel the sites of action of protein kinase A (PKA) and protein kinase C (PKC) in modulating insulin secretion. By using high time resolution measurements of membrane capacitance and flash photolysis of caged Ca²⁺, we characterize three kinetically different pools of vesicles in rat pancreatic β-cells, namely, a highly calcium-sensitive pool (HCSP), a readily releasable pool (RRP), and a reserve pool. The size of the HCSP is ∼20 fF under resting conditions, but is dramatically increased by application of either phorbol esters or forskolin. Phorbol esters and forskolin also increase the size of RRP to a lesser extent. The augmenting effect of phorbol esters or forskolin is blocked by various PKC or PKA inhibitors, indicating the involvement of these kinases. The effects of PKC and PKA on the size of the HCSP are not additive, suggesting a convergent mechanism. Using a protocol where membrane depolarization is combined with photorelease of Ca²⁺, we find that the HCSP is a distinct population of vesicles from those colocalized with Ca²⁺ channels. We propose that PKA and PKC promote insulin secretion by increasing the number of vesicles that are highly sensitive to Ca²⁺.

A simulation study on the Ca2+-independent but voltage-dependent exocytosis and endocytosis in dorsal root ganglion neurons

In patch-clamped somata of dorsal root ganglion (DRG) neurons, two types of secretion have been proposed: Ca(2+)-dependent secretion and Ca(2+)-independent but voltage-dependent secretion (CIVDS). The Ca(2+)-induced and the depolarization-induced membrane capacitance (C(m)) increases contribute 80 and 20% to the total C(m) increase, respectively (Zhang and Zhou in Nat Neurosci 5:425, 2002). In order to explore the mechanism of the voltage-dependent C(m) change (DeltaC(m)), we constructed a model with sequential states. The simulation with this model closely approximates all the experimental data. The model predicts that the majority of fusion events (approximately 80%) are so-called "kiss-and-run" events, which account for the fast recovery or the rapid retrieval feature of the signals. The remaining 20% are attributed to full fusion events, which account for a slow retrieval feature. On the basis of the model, one mechanism of the activity-dependent endocytosis has revealed a differential distribution of vesicles between the kiss-and-run and full fusion states at different stimulation frequencies. The quantitative model presented in this study may help us to understand the mechanism of the CIVDS and the tightly coupled endocytosis found in mammalian DRG neurons.

Functional aspects and mechanisms of TRPV1 involvement in neurogenic inflammation that leads to thermal hyperalgesia

Neurogenic inflammation is produced by overstimulation of peripheral nociceptor terminals by injury or inflammation of tissues. Excessive activity of sensory neurons produces vasodilation, plasma extravasation and hypersensitivity. Mechanistically, neurogenic inflammation is due to the release of substances from primary sensory nerve terminals that act directly or indirectly at the peripheral terminals, either activating or sensitizing nociceptors, endothelial cells and immunocytes. Notably, small-diameter sensory neurons that are sensitive to capsaicin play a key role in the generation of neurogenic inflammation. The cloning of the vanilloid receptor 1 (TRPV1) has been a breakthrough that has propelled our understanding of the molecular mechanisms involved in neurogenic inflammation. TRPV1 pivotally contributes to the integration of various stimuli and modulates nociceptor excitability, thus making it a true gateway for pain transduction. In addition, TRPV1 is the endpoint target of intracellular signalling pathways triggered by inflammatory mediators. Phosphorylation-induced potentiation of TRPV1 channel activity, along with an incremented TRPV1 surface expression are major events underlying the nociceptor activation and sensitization that leads to thermal hyperalgesia. The important contribution of TRPV1 receptor to the onset and maintenance of neurogenic inflammation has validated it as a therapeutic target for inflammatory pain management. As a result, the development of specific TRPV1 antagonists is a central focus of current drug discovery programs.

Exocytosis of lung surfactant: from the secretory vesicle to the air-liquid interface

Exocytosis is fundamental in biology and requires an orchestra of proteins and other constituents to fuse a vesicle with the plasma membrane. Although the molecular fusion machinery appears to be well conserved in evolution, the process itself varies considerably with regard to the diversity of physico-chemical and structural factors that govern the delay between stimulus and fusion, the expansion of the fusion pore, the release of vesicle content, and, finally, its extracellular dispersion. Exocytosis of surfactant is unique in many of these aspects. This review deals with the secretory pathway of pulmonary surfactant from the type II cell to the air-liquid interface, with focus on the distinct mechanisms and regulation of lamellar body (LB) fusion and release. We also discuss the fate of secreted material until it is rearranged into units that finally function to reduce the surface tension in the lung.

Biophysical basis of pituitary cell type-specific Ca2+ signaling-secretion coupling

All secretory pituitary cells exhibit spontaneous and extracellular Ca2+-dependent electrical activity. Somatotrophs and lactotrophs fire plateau-bursting action potentials, which generate Ca2+ signals of sufficient amplitude to trigger hormone release. Gonadotrophs also fire action potentials spontaneously, but as single, high-amplitude spikes with limited ability to promote Ca2+ influx and secretion. However, Ca2+ mobilization in gonadotrophs transforms single spiking into plateau-bursting-type electrical activity and triggers secretion. Patch clamp analysis revealed that somatotrophs and lactotrophs, but not gonadotrophs, express BK (big)-type Ca2+-controlled K+ channels, activation of which is closely associated with voltage-gated Ca2+ influx. Conversely, pituitary gonadotrophs express SK (small)-type Ca2+-activated K+ channels that are colocalized with intracellular Ca2+ release sites. Activation of both channels is crucial for plateau-bursting-type rhythmic electrical activity and secretion.

PKC-induced sensitization of Ca2+-dependent exocytosis is mediated by reducing the Ca2+ cooperativity in pituitary gonadotropes

The highly cooperative nature of Ca2+-dependent exocytosis is very important for the precise regulation of transmitter release. It is not known whether the number of binding sites on the Ca2+ sensor can be modulated or not. We have previously reported that protein kinase C (PKC) activation sensitizes the Ca2+ sensor for exocytosis in pituitary gonadotropes. To further unravel the underlying mechanism of how the Ca2+ sensor is modulated by protein phosphorylation, we have performed kinetic modeling of the exocytotic burst and investigated how the kinetic parameters of Ca2+-triggered fusion are affected by PKC activation. We propose that PKC sensitizes exocytosis by reducing the number of calcium binding sites on the Ca2+ sensor (from three to two) without significantly altering the Ca2+-binding kinetics. The reduction in the number of Ca2+-binding steps lowers the threshold for release and up-regulates release of fusion-competent vesicles distant from Ca2+ channels.

Inhibition of cellular responses to insulin in a rat liver cell line. A role for PKC in insulin resistance

The initial response of liver cells to insulin is mediated through exocytosis of Cl- channel-containing vesicles and a subsequent opening of plasma membrane Cl- channels. Intracellular accumulation of fatty acids leads to profound defects in metabolism, and is closely associated with insulin resistance. It is not known whether the activity of Cl- channels is altered in insulin resistance and by which mechanisms. We studied the effects of fatty acid accumulation on Cl- channel opening in a model liver cell line. Overnight treatment with amiodarone increased the fat content by approximately 2-fold, and the rates of gluconeogenesis by approximately 5-fold. The ability of insulin to suppress gluconeogenesis was markedly reduced indicating that amiodarone treatment induces insulin resistance. Western blot analysis showed that these cells express the same number of insulin receptors as control cells. However, insulin failed to activate exocytosis and Cl- channel opening. These inhibitory effects were mimicked in control cells by exposures to arachidonic acid (15 microm). Further studies demonstrated that fatty acids stimulate the PKC activity, and inhibition of PKC partially restored exocytosis and Cl- channel opening in insulin-resistant cells. Accordingly, activation of PKC with PMA in control cells potently inhibited the insulin responses. These results suggest that stimulation of PKC activity in insulin resistance contributes to the inhibition of cellular responses to insulin in liver cells.

Stimulation of insulin release by glucose is associated with an increase in the number of docked granules in the beta-cells of rat pancreatic islets

Electron microscopy and quantitative stereological techniques were used to study the dynamics of the docked granule pool in the rat pancreatic beta-cell. The mean number of granules per beta-cell was 11,136. After equilibration in RPMI containing 5.6 mmol/l glucose, 6.4% of the granules (approximately 700) were docked at the plasma membrane (also measured as [means +/- SE] 4.3 +/- 0.6 docked granules per 10 microm of plasma membrane at the perimeter of the cell sections). After a 40-min exposure to 16.7 mmol/l glucose, 10.2% of the granules (approximately 1,060) were docked (6.4 +/- 0.8 granules per 10 microm of plasma membrane). Thus, the docked pool increased by 50% during stimulation with glucose. Islets were also exposed to 16.7 mmol/l glucose in the absence or presence of 10 micromol/l nitrendipine. In the absence and presence of nitrendipine, there were 6.1 +/- 0.7 and 6.3 +/- 0.6 granules per 10 microm of membrane, respectively. Thus, glucose increased granule docking independently of increased [Ca2+]i and exocytosis. The data suggest a limit to the number of docking sites. As the rate of docking exceeded the rate of exocytosis, docking is not rate limiting for insulin release. Only with extremely high release rates, glucose stimulation after a 4-h incubation with a high concentration of fatty acid-free BSA, was the docked granule pool reduced in size.

Hypothesis: one rate-limiting step controls the magnitude of both phases of glucose-stimulated insulin secretion

The biphasic secretory response of pancreatic beta-cells to abrupt and sustained exposure to glucose is well documented. Some of the ATP-sensitive K(+) (K(ATP)) channel-dependent mechanisms underlying the first phase of insulin release are known; the mechanisms underlying the second phase are less well known. The hypothesis we propose is that one rate-limiting step, controlling the conversion of granules in a readily releasable (RR) docked granule pool to an immediately releasable (IR) pool, is responsible for the magnitude of both phases of release. Furthermore, we propose that the K(ATP) channel-independent signaling pathway regulates this rate-limiting step. The size of the IR pool of granules that constitutes the first phase is determined under resting conditions by the forward and reverse rates of conversion of granules in the RR and IR pools. The resulting equilibrium position determines the maximum number of beta-cell granules available for release during the first phase upon exposure to glucose. At the nadir between the two phases, the IR pool has been depleted so that the rate of granule release is equal to the low forward rate for the conversion of RR to IR granules. After the nadir, the forward rate is accelerated during the rising portion of the second phase until it reaches a maximum rate at the plateau.

Regulated exocytosis contributes to protein kinase C potentiation of vanilloid receptor activity

The vanilloid receptor-1 (TRPV1) plays a key role in the perception of peripheral thermal and inflammatory pain. TRPV1 expression and channel activity are notably up-regulated by proalgesic agents. The transduction pathways involved in TRPV1 sensitization are still elusive. We have used a yeast two-hybrid screen to identify proteins that associate with the N terminus of TRPV1. We report that two vesicular proteins, Snapin and synaptotagmin IX (Syt IX), strongly interact in vitro and in vivo with the TRPV1 N-terminal domain. In primary dorsal root ganglion neurons, TRPV1 co-distributes in vesicles with Syt IX and the vesicular protein synaptobrevin. Neither Snapin nor Syt IX affected channel function, but they notably inhibited protein kinase C (PKC)-induced potentiation of TRPV1 channel activity with a potency that rivaled the blockade evoked by botulinum neurotoxin A, a potent blocker of neuronal exocytosis. Noteworthily, we found that PKC activation induced a rapid delivery of functional TRPV1 channels to the plasma membrane. Botulinum neurotoxin A blocked the TRPV1 membrane translocation induced by PKC that was activated with a phorbol ester or the metabotropic glutamate receptor mGluR5. Therefore, our results indicate that PKC signaling promotes at least in part the SNARE-dependent exocytosis of TRPV1 to the cell surface. Taken together, these findings imply that activity-dependent delivery of channels to the neuronal surface may contribute to the buildup and maintenance of thermal inflammatory hyperalgesia in peripheral nociceptor terminals.

Dopamine neurons release transmitter via a flickering fusion pore

A key question in understanding mechanisms of neurotransmitter release is whether the fusion pore of a synaptic vesicle regulates the amount of transmitter released during exocytosis. We measured dopamine release from small synaptic vesicles of rat cultured ventral midbrain neurons using carbon fiber amperometry. Our data indicate that small synaptic vesicle fusion pores flicker either once or multiple times in rapid succession, with each flicker releasing approximately 25-30% of vesicular dopamine. The incidence of events with multiple flickers was reciprocally regulated by phorbol esters and staurosporine. Thus, dopamine neurons regulate the amount of neurotransmitter released by small synaptic vesicles by controlling the number of fusion pore flickers per exocytotic event. This mode of exocytosis is a potential mechanism whereby neurons can rapidly reuse vesicles without undergoing the comparatively slow process of recycling.

Tuning exocytosis for speed: fast and slow modes

Exocytic fusion reactions triggered by Ca(2+) are widespread in neural, endocrine, exocrine, hemapoetic and perhaps all cell types. These processes exhibit tremendous variation in latencies to fusion following a Ca(2+) rise and in rates of fusion. We review reported differences for synaptic vesicle (SV) and dense-core vesicle (DCV) exocytosis and attempt to identify key features in the molecular mechanisms of docking, priming and fusion of SVs and DCVs that may account for differences in speed.

New Aspects of Neurotransmitter Releasee and Exocytosis: Regulation of Neurotransmitter Release by Phosphorylation

Synaptic transmission is conducted by neurotransmitters released from nerve terminals. Neurotransmitter release is regulated both positively and negatively by multiple mechanisms, and its regulation is believed to be one of the important mechanisms of synaptic plasticity underlying learning and memory. Various protein kinases play important roles in the regulation, and candidates for protein substrates essential for the regulation have been identified.

A highly Ca2+-sensitive pool of vesicles is regulated by protein kinase C in adrenal chromaffin cells

We have used flash photolysis of caged Ca2+ and membrane capacitance measurements to probe exocytosis in chromaffin cells at low concentrations of intracellular Ca2+ ([Ca2+]i) (<10 microM). We observed a small pool of granules that is more sensitive to [Ca2+]i than the previously described "readily releasable pool." Upon activation of PKC, this "highly Ca2+-sensitive pool" is enhanced in size to a greater extent than the readily releasable pool but is eliminated upon expression of a C-terminal deletion mutant (Delta9) of synaptosome-associated protein of 25 kDa (SNAP-25). Thus, in chromaffin cells, PKC enhances exocytosis both by increasing the number of readily releasable vesicles and by shifting vesicles to a highly Ca2+-sensitive state, enabling exocytosis at sites relatively distant from Ca2+ channels.