Regulation of exocytosis by protein kinase C

Topical administration of GLP-1 eyedrops improves retinal ganglion cell function by facilitating presynaptic GABA release in early experimental diabetes

JOURNAL/nrgr/04.03/01300535-202602000-00048/figure1/v/2025-05-05T160104Z/r/image-tiff Diabetic retinopathy is a prominent cause of blindness in adults, with early retinal ganglion cell loss contributing to visual dysfunction or blindness. In the brain, defects in γ-aminobutyric acid synaptic transmission are associated with pathophysiological and neurodegenerative disorders, whereas glucagon-like peptide-1 has demonstrated neuroprotective effects. However, it is not yet clear whether diabetes causes alterations in inhibitory input to retinal ganglion cells and whether and how glucagon-like peptide-1 protects against neurodegeneration in the diabetic retina through regulating inhibitory synaptic transmission to retinal ganglion cells. In the present study, we used the patch-clamp technique to record γ-aminobutyric acid subtype A receptor-mediated miniature inhibitory postsynaptic currents in retinal ganglion cells from streptozotocin-induced diabetes model rats. We found that early diabetes (4 weeks of hyperglycemia) decreased the frequency of GABAergic miniature inhibitory postsynaptic currents in retinal ganglion cells without altering their amplitude, suggesting a reduction in the spontaneous release of γ-aminobutyric acid to retinal ganglion cells. Topical administration of glucagon-like peptide-1 eyedrops over a period of 2 weeks effectively countered the hyperglycemia-induced downregulation of GABAergic mIPSC frequency, subsequently enhancing the survival of retinal ganglion cells. Concurrently, the protective effects of glucagon-like peptide-1 on retinal ganglion cells in diabetic rats were eliminated by topical administration of exendin-9-39, a specific glucagon-like peptide-1 receptor antagonist, or SR95531, a specific antagonist of the γ-aminobutyric acid subtype A receptor. Furthermore, extracellular perfusion of glucagon-like peptide-1 was found to elevate the frequencies of GABAergic miniature inhibitory postsynaptic currents in both ON- and OFF-type retinal ganglion cells. This elevation was shown to be mediated by activation of the phosphatidylinositol-phospholipase C/inositol 1,4,5-trisphosphate receptor/Ca 2+ /protein kinase C signaling pathway downstream of glucagon-like peptide-1 receptor activation. Moreover, multielectrode array recordings revealed that glucagon-like peptide-1 functionally augmented the photoresponses of ON-type retinal ganglion cells. Optomotor response tests demonstrated that diabetic rats exhibited reductions in visual acuity and contrast sensitivity that were significantly ameliorated by topical administration of glucagon-like peptide-1. These results suggest that glucagon-like peptide-1 facilitates the release of γ-aminobutyric acid onto retinal ganglion cells through the activation of glucagon-like peptide-1 receptor, leading to the de-excitation of retinal ganglion cell circuits and the inhibition of excitotoxic processes associated with diabetic retinopathy. Collectively, our findings indicate that the γ-aminobutyric acid system has potential as a therapeutic target for mitigating early-stage diabetic retinopathy. Furthermore, the topical administration of glucagon-like peptide-1 eyedrops represents a non-invasive and effective treatment approach for managing early-stage diabetic retinopathy.

Anionic Species Regulate Chemical Storage in Nanometer Vesicles and Amperometrically Detected Exocytotic Dynamics

Hofmeister effects have often been ignored in living organisms, although they affect the activity and functions of biological molecules. Herein, amperometry has been applied to show that the vesicular content, dynamics of exocytosis and vesicles opening, depend on the anionic species treatment. Compared to 100 μM Cl^- treated chromaffin cells, a similar number of catecholamine molecules is released after chaotropic anions (ClO₄^- and SCN^-) treatment, even though the vesicular catecholamine content significantly increases, suggesting a lower release fraction. In addition, there are opposite effects on the dynamics of vesicles release (shorter duration) and vesicle opening (longer duration) for chaotropic anions treated cells. Our results show anion-dependent vesicle release, vesicle opening, and vesicular content, providing understanding of the pharmacological and pathological processes induced by inorganic ions.

Natural Product Isoliquiritigenin Activates GABAB Receptors to Decrease Voltage-Gate Ca2+ Channels and Glutamate Release in Rat Cerebrocortical Nerve Terminals

Reduction in glutamate release is a key mechanism for neuroprotection and we investigated the effect of isoliquiritigenin (ISL), an active ingredient of Glycyrrhiza with neuroprotective activities, on glutamate release in rat cerebrocortical nerve terminals (synaptosomes). ISL produced a concentration-dependent inhibition of glutamate release and reduced the intraterminal [Ca²⁺] increase. The inhibition of glutamate release by ISL was prevented after removing extracellular Ca²⁺ or blocking P/Q-type Ca²⁺ channels. This inhibition was mediated through the γ-aminobutyric acid type B (GABA_B) receptors because ISL was unable to inhibit glutamate release in the presence of baclofen (an GABA_B agonist) or CGP3548 (an GABA_B antagonist) and docking data revealed that ISL interacted with GABA_B receptors. Furthermore, the ISL inhibition of glutamate release was abolished through the inhibition of G_i/o-mediated responses or G_βγ subunits, but not by 8-bromoadenosine 3′,5′-cyclic monophosphate or adenylate cyclase inhibition. The ISL inhibition of glutamate release was also abolished through the inhibition of protein kinase C (PKC), and ISL decreased the phosphorylation of PKC. Thus, we inferred that ISL, through GABA_B receptor activation and G_βγ-coupled inhibition of P/Q-type Ca²⁺ channels, suppressed the PKC phosphorylation to cause a decrease in evoked glutamate release at rat cerebrocortical nerve terminals.

M1 and M2 mAChRs activate PDK1 and regulate PKC βI and ε and the exocytotic apparatus at the NMJ

Neuromuscular junctions (NMJ) regulate cholinergic exocytosis through the M₁ and M₂ muscarinic acetylcholine autoreceptors (mAChR), involving the crosstalk between receptors and downstream pathways. Protein kinase C (PKC) regulates neurotransmission but how it associates with the mAChRs remains unknown. Here, we investigate whether mAChRs recruit the classical PKCβI and the novel PKCε isoforms and modulate their priming by PDK1, translocation and activity on neurosecretion targets. We show that each M₁ and M₂ mAChR activates the master kinase PDK1 and promotes a particular priming of the presynaptic PKCβI and ε isoforms. M₁ recruits both primed-PKCs to the membrane and promotes Munc18-1, SNAP-25, and MARCKS phosphorylation. In contrast, M₂ downregulates PKCε through a PKA-dependent pathway, which inhibits Munc18-1 synthesis and PKC phosphorylation. In summary, our results discover a co-dependent balance between muscarinic autoreceptors which orchestrates the presynaptic PKC and their action on ACh release SNARE-SM mechanism. Altogether, this molecular signaling explains previous functional studies at the NMJ and guide toward potential therapeutic targets.

Imaging the rapid yet transient accumulation of regulatory lipids, lipid kinases, and protein kinases during membrane fusion, at sites of exocytosis of MMP-9 in MCF-7 cells

Background

The regulation of exocytosis is physiologically vital in cells and requires a variety of distinct proteins and lipids that facilitate efficient, fast, and timely release of secretory vesicle cargo. Growing evidence suggests that regulatory lipids act as important lipid signals and regulate various biological processes including exocytosis. Though functional roles of many of these regulatory lipids has been linked to exocytosis, the dynamic behavior of these lipids during membrane fusion at sites of exocytosis in cell culture remains unknown.

Methods

Total internal reflection fluorescence microscopy (TIRF) was used to observe the spatial organization and temporal dynamics (i.e. spatial positioning and timing patterns) of several lipids, and accessory proteins, like lipid kinases and protein kinases, in the form of protein kinase C (PRKC) associated with sites of exocytosis of matrix metalloproteinase-9 (MMP-9) in living MCF-7 cancer cells.

Results

Following stimulation with phorbol myristate acetate (PMA) to promote exocytosis, a transient accumulation of several distinct regulatory lipids, lipid kinases, and protein kinases at exocytic sites was observed. This transient accumulation centered at the time of membrane fusion is followed by a rapid diffusion away from the fusion sites. Additionally, the synthesis of these regulatory lipids, degradation of these lipids, and the downstream effectors activated by these lipids, are also achieved by the recruitment and accumulation of key enzymes at exocytic sites (during the moment of cargo release). This includes key enzymes like lipid kinases, protein kinases, and phospholipases that facilitate membrane fusion and exocytosis of MMP-9.

Conclusions

This work suggests that these regulatory lipids and associated effector proteins are locally synthesized and/or recruited to sites of exocytosis, during membrane fusion and cargo release. More importantly, their enrichment at fusion sites serves as an important spatial and temporal organizing “element” defining individual exocytic sites.

Exogenous carbon monoxide suppresses LPS-Induced platelet SNAREs complex assembly and α-granule exocytosis via integrin αIIbβ3-Mediated PKCθ/Munc18a pathway

Activation of coagulation occurs in sepsis and contributes to the development of thrombosis. Platelet α-granule exocytosis plays an important role in septic coagulation abnormalities. The present study aimed to investigate the effects and the underlying mechanisms of exogenous carbon monoxide, carbon monoxide-releasing molecules II (CORM-2)-liberated CO, on suppressing platelet α-granule exocytosis in sepsis. It was shown that CORM-2 weakened α-granule membrane fusion with platelet plasma membrane and attenuated α-granule contents exocytosis in LPS-Induced platelet. Further studies revealed that CORM-2 suppressed the expression of integrin α_IIbβ₃ in platelets stimulated by LPS. This was accompanied by a decrease in production and phosphorylation of PKCθ and Munc18a, SNARE complex assembly and subsequently platelet α-granule exocytosis. Taken together, we suggested that the potential mechanism of suppressive effect of CORM-2 on LPS-induced platelet SNAREs complex assembly and α-Granule Exocytosis might involve integrin α_IIbβ₃-mediated PKCθ/Munc18a pathway activation.

GPCR regulation of secretion

Modulation of neurotransmitter exocytosis by activated Gi/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.

Lycopene depresses glutamate release through inhibition of voltage-dependent Ca2+ entry and protein kinase C in rat cerebrocortical nerve terminals

Lycopene is a natural dietary carotenoid that was reported to exhibit a neuroprotective profile. Considering that excitotoxicity and cell death induced by glutamate are involved in many brain disorders, the effect of lycopene on glutamate release in rat cerebrocortical nerve terminals and the possible mechanism involved in such effect was investigated. We observed here that lycopene inhibited 4-aminopyridine (4-AP)-evoked glutamate release and intrasynaptosomal Ca²⁺ concentration elevation. The inhibitory effect of lycopene on 4-AP-evoked glutamate release was markedly reduced in the presence of the Ca_v2.2 (N-type) and Ca_v2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC, but was insensitive to the intracellular Ca²⁺-release inhibitors dantrolene and CGP37157. Furthermore, in the presence of the protein kinase C inhibitors GF109203X and Go6976, the action of lycopene on evoked glutamate release was prevented. These results are the first to suggest that lycopene inhibits glutamate release from rat cortical synaptosomes by suppressing presynaptic Ca²⁺ entry and protein kinase C activity.

VPAC1 and VPAC2 receptor activation on GABA release from hippocampal nerve terminals involve several different signalling pathways

BACKGROUND AND PURPOSE

Vasoactive intestinal peptide (VIP) is an important modulator of hippocampal synaptic transmission that influences both GABAergic synaptic transmission and glutamatergic cell excitability through activation of VPAC₁ and VPAC₂ receptors. Presynaptic enhancement of GABA release contributes to VIP modulation of hippocampal synaptic transmission.

EXPERIMENTAL APPROACH

We investigated which VIP receptors and coupled transduction pathways were involved in VIP enhancement of K⁺ -evoked [³ H]-GABA release from isolated nerve terminals of rat hippocampus.

KEY RESULTS

VIP enhancement of [³ H]-GABA release was potentiated in the presence of the VPAC₁ receptor antagonist PG 97-269 but converted into an inhibition in the presence of the VPAC₂ receptor antagonist PG 99-465, suggesting that activation of VPAC₁ receptors inhibits and activation of VPAC₂ receptors enhances, GABA release. A VPAC₁ receptor agonist inhibited exocytotic voltage-gated calcium channel (VGCC)-dependent [³ H]-GABA release through activation of protein G_i/o , an effect also dependent on PKC activity. A VPAC₂ receptor agonist enhanced both exocytotic VGCC-dependent release through protein G_s -dependent, PKA-dependent and PKC-dependent mechanisms and GABA transporter 1-mediated [³ H]-GABA release through a G_s protein-dependent and PKC-dependent mechanism.

CONCLUSIONS AND IMPLICATIONS

Our results show that VPAC₁ and VPAC₂ VIP receptors have opposing actions on GABA release from hippocampal nerve terminals through activation of different transduction pathways. As VPAC₁ and VPAC₂ receptors are located in different layers of Ammon's horn, our results suggest that these VIP receptors underlie different modulation of synaptic transmission to pyramidal cell dendrites and cell bodies, with important consequences for their possible therapeutic application in the treatment of epilepsy.

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

SNAP-25 phosphorylation at Ser187 regulates synaptic facilitation and short-term plasticity in an age-dependent manner

Neurotransmitter release is mediated by the SNARE complex, but the role of its phosphorylation has scarcely been elucidated. Although PKC activators are known to facilitate synaptic transmission, there has been a heated debate on whether PKC mediates facilitation of neurotransmitter release through phosphorylation. One of the SNARE proteins, SNAP-25, is phosphorylated at the residue serine-187 by PKC, but its physiological significance has been unclear. To examine these issues, we analyzed mutant mice lacking the phosphorylation of SNAP-25 serine-187 and found that they exhibited reduced release probability and enhanced presynaptic short-term plasticity, suggesting that not only the release process, but also the dynamics of synaptic vesicles was regulated by the phosphorylation. Furthermore, it has been known that the release probability changes with development, but the precise mechanism has been unclear, and we found that developmental changes in release probability of neurotransmitters were regulated by the phosphorylation. These results indicate that SNAP-25 phosphorylation developmentally facilitates neurotransmitter release but strongly inhibits presynaptic short-term plasticity via modification of the dynamics of synaptic vesicles in presynaptic terminals.

The Tim-3-galectin-9 Secretory Pathway is Involved in the Immune Escape of Human Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) is a severe and often fatal systemic malignancy. Malignant cells are capable of escaping host immune surveillance by inactivating cytotoxic lymphoid cells. In this work we discovered a fundamental molecular pathway, which includes ligand-dependent activation of ectopically expressed latrophilin 1 and possibly other G-protein coupled receptors leading to increased translation and exocytosis of the immune receptor Tim-3 and its ligand galectin-9. This occurs in a protein kinase C and mTOR (mammalian target of rapamycin)-dependent manner. Tim-3 participates in galectin-9 secretion and is also released in a free soluble form. Galectin-9 impairs the anti-cancer activity of cytotoxic lymphoid cells including natural killer (NK) cells. Soluble Tim-3 prevents secretion of interleukin-2 (IL-2) required for the activation of cytotoxic lymphoid cells. These results were validated in ex vivo experiments using primary samples from AML patients. This pathway provides reliable targets for both highly specific diagnosis and immune therapy of AML.

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

The control of insulin release from pancreatic beta cells helps ensure proper blood glucose level, which is critical for human health. Protein kinase C has been shown to be one key control mechanism for this process. After glucose stimulation, calcium influx into beta cells triggers exocytosis of insulin-containing dense-core granules and activates protein kinase C via calcium-dependent phospholipase C-mediated generation of diacylglycerol. Activated protein kinase C potentiates insulin release by enhancing the calcium sensitivity of exocytosis, likely by affecting two main pathways that could be linked: (1) the reorganization of the cortical actin network, and (2) the direct phosphorylation of critical exocytotic proteins such as munc18, SNAP25, and synaptotagmin. Here, we review what is currently known about the molecular mechanisms of protein kinase C action on each of these pathways and how these effects relate to the control of insulin release by exocytosis. We identify remaining challenges in the field and suggest how these challenges might be addressed to advance our understanding of the regulation of insulin release in health and disease.

Host Serine/Threonine Kinases mTOR and Protein Kinase C-α Promote InlB-Mediated Entry of Listeria monocytogenes

The bacterial pathogen Listeria monocytogenes causes foodborne illnesses resulting in gastroenteritis, meningitis, or abortion. Listeria induces its internalization into some human cells through interaction of the bacterial surface protein InlB with the host receptor tyrosine kinase Met. InlB-dependent entry requires localized polymerization of the host actin cytoskeleton. The signal transduction pathways that act downstream of Met to regulate actin filament assembly or other processes during Listeria uptake remain incompletely characterized. Here, we demonstrate important roles for the human serine/threonine kinases mTOR and protein kinase C-α (PKC-α) in InlB-dependent entry. Experiments involving RNA interference (RNAi) indicated that two multiprotein complexes containing mTOR, mTORC1 and mTORC2, are each needed for efficient internalization of Listeria into cells of the human cell line HeLa. InlB stimulated Met-dependent phosphorylation of mTORC1 or mTORC2 substrates, demonstrating activation of both mTOR-containing complexes. RNAi studies indicated that the mTORC1 effectors 4E-BP1 and hypoxia-inducible factor 1α (HIF-1α) and the mTORC2 substrate PKC-α each control Listeria uptake. Genetic or pharmacological inhibition of PKC-α reduced the internalization of Listeria and the accumulation of actin filaments that normally accompanies InlB-mediated entry. Collectively, our results identify mTOR and PKC-α to be host factors exploited by Listeria to promote infection. PKC-α controls Listeria entry, at least in part, by regulating the actin cytoskeleton downstream of the Met receptor.

Myosin phosphatase and RhoA-activated kinase modulate neurotransmitter release by regulating SNAP-25 of SNARE complex

Reversible phosphorylation of neuronal proteins plays an important role in the regulation of neurotransmitter release. Myosin phosphatase holoenzyme (MP) consists of a protein phosphatase-1 (PP1) catalytic subunit (PP1c) and a regulatory subunit, termed myosin phosphatase targeting subunit (MYPT1). The primary function of MP is to regulate the phosphorylation level of contractile proteins; however, recent studies have shown that MP is localized to neurons, and is also involved in the mediation of neuronal processes. Our goal was to investigate the effect of RhoA-activated kinase (ROK) and MP on the phosphorylation of one potential neuronal substrate, the synaptosomal-associated protein of 25 kDa (SNAP-25). SNAP-25 is a member of the SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) complex, along with synaptobrevin and syntaxin, and the primary role of SNAP25 is to mediate vesicle fusion. We showed that MYPT1 interacts with SNAP-25, as revealed by immunoprecipitation and surface plasmon resonance based binding studies. Mass spectrometry analysis and in vitro phosphorylation/dephosphorylation assays demonstrated that ROK phosphorylates, while MP dephosphorylates, SNAP-25 at Thr138. Silencing MYPT1 in B50 neuroblastoma cells increased phosphorylation of SNAP-25 at Thr138. Inhibition of PP1 with tautomycetin increased, whereas inhibition of ROK by H1152, decreased the phosphorylation of SNAP-25 at Thr138 in B50 cells, in cortical synaptosomes, and in brain slices. In response to the transduction of the MP inhibitor, kinase-enhanced PP1 inhibitor (KEPI), into synaptosomes, an increase in phosphorylation of SNAP-25 and a decrease in the extent of neurotransmitter release were detected. The interaction between SNAP-25 and syntaxin increased with decreasing phosphorylation of SNAP-25 at Thr138, upon inhibition of ROK. Our data suggest that ROK/MP play a crucial role in vesicle trafficking, fusion, and neurotransmitter release by oppositely regulating the phosphorylation of SNAP-25 at Thr138.

Native metabotropic glutamate receptor 4 depresses synaptic transmission through an unusual Gαq transduction pathway

In cerebellar cortex, mGlu₄ receptors located on parallel fibers play an essential role in normal motor function, but the molecular mechanisms involved are not yet completely understood. Using a strategy combining biochemical and electrophysiological approaches in the rodent cerebellum, we demonstrate that presynaptic mGlu₄ receptors control synaptic transmission through an atypical activation of Gα_q proteins. First, the Gα_q subunit, PLC and PKC signaling proteins present in cerebellar extracts are retained on affinity chromatography columns grafted with different sequences of the cytoplasmic domain of mGlu₄ receptor. The i2 loop and the C terminal domain were used as baits, two domains that are known to play a pivotal role in coupling selectivity and efficacy. Second, in situ proximity ligation assays show that native mGlu₄ receptors and Gα_q subunits are in close physical proximity in cerebellar cortical slices. Finally, electrophysiological experiments demonstrate that the molecular mechanisms underlying mGlu₄ receptor-mediated inhibition of transmitter release at cerebellar Parallel Fiber (PF) - Molecular Layer Interneuron (MLI) synapses involves the Gα_q-PLC signaling pathway. Taken together, our results provide compelling evidence that, in the rodent cerebellar cortex, mGlu₄ receptors act by coupling to the Gα_q protein and PLC effector system to reduce glutamate synaptic transmission.

Autocrine Signaling Underlies Fast Repetitive Plasma Membrane Translocation of Conventional and Novel Protein Kinase C Isoforms in β Cells

PKC signaling has been implicated in the regulation of many cell functions, including metabolism, cell death, proliferation, and secretion. Activation of conventional and novel PKC isoforms is associated with their Ca²⁺- and/or diacylglycerol (DAG)-dependent translocation to the plasma membrane. In β cells, exocytosis of insulin granules evokes brief (<10 s) local DAG elevations (“spiking”) at the plasma membrane because of autocrine activation of P2Y₁ purinoceptors by ATP co-released with insulin. Using total internal reflection microscopy, fluorescent protein-tagged PKCs, and signaling biosensors, we investigated whether DAG spiking causes membrane recruitment of PKCs and whether different classes of PKCs show characteristic responses. Glucose stimulation of MIN6 cells triggered DAG spiking with concomitant repetitive translocation of the novel isoforms PKCδ, PKCϵ, and PKCη. The conventional PKCα, PKCβI, and PKCβII isoforms showed a more complex pattern with both rapid and slow translocation. K⁺ depolarization-induced PKCϵ translocation entirely mirrored DAG spiking, whereas PKCβI translocation showed a sustained component, reflecting the subplasma membrane Ca²⁺ concentration ([Ca²⁺]_pm), with additional effect during DAG spikes. Interference with DAG spiking by purinoceptor inhibition prevented intermittent translocation of PKCs and reduced insulin secretion but did not affect [Ca²⁺]_pm elevation or sustained PKCβI translocation. The muscarinic agonist carbachol induced pronounced transient PKCβI translocation and sustained recruitment of PKCϵ. When rise of [Ca²⁺]_pm was prevented, the carbachol-induced DAG and PKCϵ responses were somewhat reduced, but PKCβI translocation was completely abolished. We conclude that exocytosis-induced DAG spikes efficiently recruit both conventional and novel PKCs to the β cell plasma membrane. PKC signaling is thus implicated in autocrine regulation of β cell function.

Molecular biology of thermosensory transduction in C. elegans

As the environmental temperature prominently influences diverse biological aspects of the animals, thermosensation and the subsequent information processing in the nervous system has attracted much attention in biology. Thermotaxis in the nematode Caenorhabditis elegans is an ideal behavioral paradigm by which to address the molecular mechanism underlying thermosensory transduction. Molecular genetic analysis in combination with other physiological and behavioral studies revealed that sensation of ambient temperature is mediated mainly by cyclic guanosine monophosphate (cGMP) signaling in thermosensory neurons. The information of the previously perceived temperature is also stored within the thermosensory neurons, and the consequence of the comparison between the past and the present temperature is conveyed to the downstream interneurons to further regulate the motor-circuits that encode the locomotion.

TRPV1

TRPV1 is a well-characterised channel expressed by a subset of peripheral sensory neurons involved in pain sensation and also at a number of other neuronal and non-neuronal sites in the mammalian body. Functionally, TRPV1 acts as a sensor for noxious heat (greater than ~42 °C). It can also be activated by some endogenous lipid-derived molecules, acidic solutions (pH < 6.5) and some pungent chemicals and food ingredients such as capsaicin, as well as by toxins such as resiniferatoxin and vanillotoxins. Structurally, TRPV1 subunits have six transmembrane (TM) domains with intracellular N- (containing 6 ankyrin-like repeats) and C-termini and a pore region between TM5 and TM6 containing sites that are important for channel activation and ion selectivity. The N- and C- termini have residues and regions that are sites for phosphorylation/dephosphorylation and PI(4,5)P2 binding, which regulate TRPV1 sensitivity and membrane insertion. The channel has several interacting proteins, some of which (e.g. AKAP79/150) are important for TRPV1 phosphorylation. Four TRPV1 subunits form a non-selective, outwardly rectifying ion channel permeable to monovalent and divalent cations with a single-channel conductance of 50-100 pS. TRPV1 channel kinetics reveal multiple open and closed states, and several models for channel activation by voltage, ligand binding and temperature have been proposed. Studies with TRPV1 agonists and antagonists and Trpv1 (-/-) mice have suggested a role for TRPV1 in pain, thermoregulation and osmoregulation, as well as in cough and overactive bladder. TRPV1 antagonists have advanced to clinical trials where findings of drug-induced hyperthermia and loss of heat sensitivity have raised questions about the viability of this therapeutic approach.

Contribution of PKC to the maintenance of 5-HT-induced short-term facilitation at sensorimotor synapses of Aplysia

Aplysia sensorimotor synapses provide a useful model system for analyzing molecular processes that contribute to heterosynaptic plasticity. For example, previous studies demonstrated that multiple kinase cascades contribute to serotonin (5-HT)-induced short-term synaptic facilitation (STF), including protein kinase A (PKA) and protein kinase C (PKC). Moreover, the contribution of each kinase is believed to depend on the state of the synapse (e.g., depressed or nondepressed) and the time after application of 5-HT. Here, a previously unappreciated role for PKC-dependent processes was revealed to underlie the maintenance of STF at relatively nondepressed synapses. This PKC dependence was revealed when the synapse was stimulated repeatedly after application of 5-HT. The contributions of the PKA and PKC pathways were examined by blocking adenylyl cyclase-coupled 5-HT receptors with methiothepin and by blocking PKC with chelerythrine. STF was assessed 20 s after 5-HT application. The effects of PKC were consistent with enhanced mobilization of transmitter, as assessed by application of hypertonic sucrose solutions to measure the readily releasable pool of vesicles and recovery of the readily releasable pool after depletion. A computational model of transmitter release demonstrated that a PKC-dependent mobilization process was sufficient to explain the maintenance of STF at nondepressed synapses and the facilitation of depressed synapses.

Orexin A inhibits propofol-induced neurite retraction by a phospholipase D/protein kinase Cε-dependent mechanism in neurons

Background

The intravenous anaesthetic propofol retracts neurites and reverses the transport of vesicles in rat cortical neurons. Orexin A (OA) is an endogenous neuropeptide regulating wakefulness and may counterbalance anaesthesia. We aim to investigate if OA interacts with anaesthetics by inhibition of the propofol-induced neurite retraction.

Methods

In primary cortical cell cultures from newborn rats’ brains, live cell light microscopy was used to measure neurite retraction after propofol (2 µM) treatment with or without OA (10 nM) application. The intracellular signalling involved was tested using a protein kinase C (PKC) activator [phorbol 12-myristate 13-acetate (PMA)] and inhibitors of Rho-kinase (HA-1077), phospholipase D (PLD) [5-fluoro-2-indolyl des-chlorohalopemide (FIPI)], PKC (staurosporine), and a PKCε translocation inhibitor peptide. Changes in PKCε Ser⁷²⁹ phosphorylation were detected with Western blot.

Results

The neurite retraction induced by propofol is blocked by Rho-kinase and PMA. OA blocks neurite retraction induced by propofol, and this inhibitory effect could be prevented by FIPI, staurosporine and PKCε translocation inhibitor peptide. OA increases via PLD and propofol decreases PKCε Ser⁷²⁹ phosphorylation, a crucial step in the activation of PKCε.

Conclusions

Rho-kinase is essential for propofol-induced neurite retraction in cortical neuronal cells. Activation of PKC inhibits neurite retraction caused by propofol. OA blocks propofol-induced neurite retraction by a PLD/PKCε-mediated pathway, and PKCε maybe the key enzyme where the wakefulness and anaesthesia signal pathways converge.

Rac1 and Rac2 control distinct events during antigen-stimulated mast cell exocytosis

The release of preformed mediators from immune cells is through a process described as exocytosis. In mast cells, exocytosis is regulated by several coordinated intracellular signaling pathways. Here, we investigated the role of the hematopoietic-specific Rho GTPase, Rac2, and the ubiquitously expressed Rac1, in controlling mast cell exocytosis. These two isoforms showed equivalent levels of expression in mouse BMMCs. Although Rac1 and Rac2 share 92% sequence identity, they were not functionally redundant, as Rac2-/- BMMCs were defective in exocytosis, even though Rac1 levels were unaffected. Antigen-stimulated WT mast cells underwent a series of morphological transitions: initial flattening, followed by actin-mediated peripheral membrane ruffling and calcium influx, which preceded exocytosis. Whereas membrane ruffling was unaffected in Rac2-/- BMMCs, calcium influx was decreased significantly. Calcium influx was studied further by examining SOCE. In Rac2-/- BMMCs, the activation of PLCγ1 and calcium release from intracellular stores occurred normally; however, activation of plasma membrane calcium channels was defective, shown by the lack of extracellular calcium influx and a reduction of YFP-STIM1 puncta at the plasma membrane. Additionally, we used the small molecule Rac inhibitor, EHT 1864, to target Rac signaling acutely in WT BMMCs. EHT 1864 blocked exocytosis and membrane ruffling completely in conjunction with exocytosis. Our findings suggest that antigen-stimulated membrane ruffling in mast cells is a Rac1-mediated process, as this persisted in the absence of Rac2. Therefore, we define distinct modes of Rac-regulated mast cell exocytosis: Rac2-mediated calcium influx and Rac1-mediated membrane ruffling.

Neurodegenerative changes initiated by presynaptic dysfunction

α-Synucleinopathies are a subgroup of neurodegenerative diseases including dementia with Lewy bodies (DLB) and Parkinson’s disease (PD). Pathologically, these disorders can be characterized by the presence of intraneuronal aggregates composed mainly of α-synuclein (αSyn), which are called Lewy bodies and Lewy neurites. Recent report showed that more than 90% of αSyn aggregates are present in the form of very small deposits in presynaptic terminals of the affected neurons in DLB. However, the mechanisms responsible for presynaptic accumulation of abnormal αSyn remain unclear. In this article, we review recent findings on the involvement of presynaptic dysfunction in the initiation of neuronal dysfunctional changes. This review highlights that the presynaptic failure can be a potential trigger of the dying-back neuronal death in neurodegenerative diseases.

Neuron-specific expression of tomosyn1 in the mouse hippocampal dentate gyrus impairs spatial learning and memory

Tomosyn, a syntaxin-binding protein, is known to inhibit vesicle priming and synaptic transmission via interference with the formation of SNARE complexes. Using a lentiviral vector, we specifically overexpressed tomosyn1 in hippocampal dentate gyrus neurons in adult mice. Mice were then subjected to spatial learning and memory tasks and electrophysiological measurements from hippocampal slices. Tomosyn1-overexpression significantly impaired hippocampus-dependent spatial memory while tested in the Morris water maze. Further, tomosyn1-overexpressing mice utilize swimming strategies of lesser cognitive ability in the Morris water maze compared with control mice. Electrophysiological measurements at mossy fiber-CA3 synapses revealed impaired paired-pulse facilitation in the mossy fiber of tomosyn1-overexpressing mice. This study provides evidence for novel roles for tomosyn1 in hippocampus-dependent spatial learning and memory, potentially via decreased synaptic transmission in mossy fiber-CA3 synapses. Moreover, it provides new insight regarding the role of the hippocampal dentate gyrus and mossy fiber-CA3 synapses in swimming strategy preference, and in learning and memory.

P2Y₁ receptor-dependent diacylglycerol signaling microdomains in β cells promote insulin secretion

Diacylglycerol (DAG) controls numerous cell functions by regulating the localization of C1-domain-containing proteins, including protein kinase C (PKC), but little is known about the spatiotemporal dynamics of the lipid. Here, we explored plasma membrane DAG dynamics in pancreatic β cells and determined whether DAG signaling is involved in secretagogue-induced pulsatile release of insulin. Single MIN6 cells, primary mouse β cells, and human β cells within intact islets were transfected with translocation biosensors for DAG, PKC activity, or insulin secretion and imaged with total internal reflection fluorescence microscopy. Muscarinic receptor stimulation triggered stable, homogenous DAG elevations, whereas glucose induced short-lived (7.1 ± 0.4 s) but high-amplitude elevations (up to 109 ± 10% fluorescence increase) in spatially confined membrane regions. The spiking was mimicked by membrane depolarization and suppressed after inhibition of exocytosis or of purinergic P2Y₁, but not P2X receptors, reflecting involvement of autocrine purinoceptor activation after exocytotic release of ATP. Each DAG spike caused local PKC activation with resulting dissociation of its substrate protein MARCKS from the plasma membrane. Inhibition of spiking reduced glucose-induced pulsatile insulin secretion. Thus, stimulus-specific DAG signaling patterns appear in the plasma membrane, including distinct microdomains, which have implications for the kinetic control of exocytosis and other membrane-associated processes.

Phosphorylation by protein kinase Cα regulates RalB small GTPase protein activation, subcellular localization, and effector utilization

Ras-like (Ral) small GTPases are regulated downstream of Ras and the noncanonical Ral guanine nucleotide exchange factor (RalGEF) effector pathway. Despite RalA and RalB sharing 82% sequence identity and utilization of shared effector proteins, their roles in normal and neoplastic cell growth have been shown to be highly distinct. Here, we determined that RalB function is regulated by protein kinase Cα (PKCα) phosphorylation. We found that RalB phosphorylation on Ser-198 in the C-terminal membrane targeting sequence resulted in enhanced RalB endomembrane accumulation and decreased RalB association with its effector, the exocyst component Sec5. Additionally, RalB phosphorylation regulated vesicular trafficking and membrane fusion by regulating v- and t-SNARE interactions. RalB phosphorylation regulated vesicular traffic of α5-integrin to the cell surface and cell attachment to fibronectin. In summary, our data suggest that phosphorylation by PKCα is critical for RalB-mediated vesicle trafficking and exocytosis.

A single amino acid mutation in SNAP-25 induces anxiety-related behavior in mouse

Synaptosomal-associated protein of 25 kDa (SNAP-25) is a presynaptic protein essential for neurotransmitter release. Previously, we demonstrate that protein kinase C (PKC) phosphorylates Ser¹⁸⁷ of SNAP-25, and enhances neurotransmitter release by recruiting secretory vesicles near to the plasma membrane. As PKC is abundant in the brain and SNAP-25 is essential for synaptic transmission, SNAP-25 phosphorylation is likely to play a crucial role in the central nervous system. We therefore generated a mutant mouse, substituting Ser¹⁸⁷ of SNAP-25 with Ala using “knock-in” technology. The most striking effect of the mutation was observed in their behavior. The homozygous mutant mice froze readily in response to environmental change, and showed strong anxiety-related behavior in general activity and light and dark preference tests. In addition, the mutant mice sometimes exhibited spontaneously occurring convulsive seizures. Microdialysis measurements revealed that serotonin and dopamine release were markedly reduced in amygdala. These results clearly indicate that PKC-dependent SNAP-25 phosphorylation plays a critical role in the regulation of emotional behavior as well as the suppression of epileptic seizures, and the lack of enhancement of monoamine release is one of the possible mechanisms underlying these defects.

Munc18-1 and Munc18-2 proteins modulate beta-cell Ca2+ sensitivity and kinetics of insulin exocytosis differently

Fast neurotransmission and slower hormone release share the same core fusion machinery consisting of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. In evoked neurotransmission, interactions between SNAREs and the Munc18-1 protein, a member of the Sec1/Munc18 (SM) protein family, are essential for exocytosis, whereas other SM proteins are dispensable. To address if the exclusivity of Munc18-1 demonstrated in neuroexocytosis also applied to fast insulin secretion, we characterized the presence and function of Munc18-1 and its closest homologue Munc18-2 in β-cell stimulus-secretion coupling. We show that pancreatic β-cells express both Munc18-1 and Munc18-2. The two Munc18 homologues exhibit different subcellular localization, and only Munc18-1 redistributes in response to glucose stimulation. However, both Munc18-1 and Munc18-2 augment glucose-stimulated hormone release. Ramp-like photorelease of caged Ca(2+) and high resolution whole-cell patch clamp recordings show that Munc18-1 and Munc18-2 overexpression shift the Ca(2+) sensitivity of the fastest phase of insulin exocytosis differently. In addition, we reveal that Ca(2+) sensitivity of exocytosis in β-cells depends on the phosphorylation status of the Munc18 proteins. Even though Munc18-1 emerges as the key SM-protein determining the Ca(2+) threshold for triggering secretory activity in a stimulated β-cell, Munc18-2 has the ability to increase Ca(2+) sensitivity and thus mediates the release of fusion-competent granules requiring a lower cytoplasmic-free Ca(2+) concentration, [Ca(2+)](i)(.) Hence, Munc18-1 and Munc18-2 display distinct subcellular compartmentalization and can coordinate the insulin exocytotic process differently as a consequence of the actual [Ca(2+)](i).

Regulation of purinergic signaling in biliary epithelial cells by exocytosis of SLC17A9-dependent ATP-enriched vesicles

ATP in bile is a potent secretogogue, stimulating biliary epithelial cell (BEC) secretion through binding apical purinergic receptors. In response to mechanosensitive stimuli, BECs release ATP into bile, although the cellular basis of ATP release is unknown. The aims of this study in human and mouse BECs were to determine whether ATP release occurs via exocytosis of ATP-enriched vesicles and to elucidate the potential role of the vesicular nucleotide transporter SLC17A9 in purinergic signaling. Dynamic, multiscale, live cell imaging (confocal and total internal reflection fluorescence microscopy and a luminescence detection system with a high sensitivity charge-coupled device camera) was utilized to detect vesicular ATP release from cell populations, single cells, and the submembrane space of a single cell. In response to increases in cell volume, BECs release ATP, which was dependent on intact microtubules and vesicular trafficking pathways. ATP release occurred as stochastic point source bursts of luminescence consistent with exocytic events. Parallel studies identified ATP-enriched vesicles ranging in size from 0.4 to 1 μm that underwent fusion and release in response to increases in cell volume in a protein kinase C-dependent manner. Present in all models, SLC17A9 contributed to ATP vesicle formation and regulated ATP release. The findings are consistent with the existence of an SLC17A9-dependent ATP-enriched vesicular pool in biliary epithelium that undergoes regulated exocytosis to initiate purinergic signaling.

Exocyst function is regulated by effector phosphorylation

The exocyst complex tethers vesicles at sites of fusion through interactions with small GTPases. The G protein RalA resides on Glut4 vesicles, and binds to the exocyst after activation by insulin, but must then disengage to ensure continuous exocytosis. Here we report that, after recognition of the exocyst by activated RalA, disengagement occurs through phosphorylation of its effector Sec5, rather than RalA inactivation. Sec5 undergoes phosphorylation in the G-protein binding domain, allosterically reducing RalA interaction. The phosphorylation event is catalysed by protein kinase C and is reversed by an exocyst-associated phosphatase. Introduction of Sec5 bearing mutations of the phosphorylation site to either alanine or aspartate disrupts insulin-stimulated Glut4 exocytosis, as well as other trafficking processes in polarized epithelial cells and during development of zebrafish embryos. The exocyst thus serves as a 'gatekeeper' for exocytic vesicles through a circuit of engagement, disengagement and re-engagement with G proteins.

Dynamic conformational changes in munc18 prevent syntaxin binding

The Sec1/munc18 protein family is essential for vesicle fusion in eukaryotic cells via binding to SNARE proteins. Protein kinase C modulates these interactions by phosphorylating munc18a thereby reducing its affinity to one of the central SNARE members, syntaxin-1a. The established hypothesis is that the reduced affinity of the phosphorylated munc18a to syntaxin-1a is a result of local electrostatic repulsion between the two proteins, which interferes with their compatibility. The current study challenges this paradigm and offers a novel mechanistic explanation by revealing a syntaxin-non-binding conformation of munc18a that is induced by the phosphomimetic mutations. In the present study, using molecular dynamics simulations, we explored the dynamics of the wild-type munc18a versus phosphomimetic mutant munc18a. We focused on the structural changes that occur in the cavity between domains 3a and 1, which serves as the main syntaxin-binding site. The results of the simulations suggest that the free wild-type munc18a exhibits a dynamic equilibrium between several conformations differing in the size of its cavity (the main syntaxin-binding site). The flexibility of the cavity's size might facilitate the binding or unbinding of syntaxin. In silico insertion of phosphomimetic mutations into the munc18a structure induces the formation of a conformation where the syntaxin-binding area is rigid and blocked as a result of interactions between residues located on both sides of the cavity. Therefore, we suggest that the reduced affinity of the phosphomimetic mutant/phosphorylated munc18a is a result of the closed-cavity conformation, which makes syntaxin binding energetically and sterically unfavorable. The current study demonstrates the potential of phosphoryalation, an essential biological process, to serve as a driving force for dramatic conformational changes of proteins modulating their affinity to target proteins.

Protein phosphorylation plays a significant regulatory role in multi-component systems engaged in signal transduction or coordination of cellular processes, by activating or deactivating proteins. The potential of phosphorylation to induce substantial conformational changes in proteins, thereby changing their affinity to target proteins, has already been shown but the dynamics of the process is not fully elucidated. In the present study, we investigated, by molecular dynamics simulations, the dynamic conformational changes in munc18a, a protein that is crucial for neurotransmitter release and interacts tightly with the SNARE syntaxin-1. We further investigated the conformational changes that occur in munc18a when it is phosphorylated, reducing its affinity to syntaxin-1a. The results of the simulations suggest that there is a conformational flexibility of the syntaxin-unbounded munc18a that allows changes in the shape of the syntaxin-1a binding cavity. In silico insertion of phosphomimetic mutations into munc18a led to a reduction in the flexibility and closure of the syntaxin-binding site. We suggest that the reduced affinity of phosphorylated munc18a to syntaxin-1a stems from the difficulty of syntaxin-1a to bind to the munc18a closed-cavity conformation, induced by the PKC phosphorylation of munc18a.

Multigranular exocytosis of Weibel-Palade bodies in vascular endothelial cells

Regulated exocytosis of Weibel-Palade bodies (WPBs) is a pivotal mechanism via which vascular endothelial cells initiate repair in response to injury and inflammation. Several pathways have been proposed to enable differential release of bioactive molecules from WPBs under different pathophysiologic conditions. Due to the complexity, many aspects of WPB biogenesis and exocytosis are still poorly understood. Herein, we have investigated the regulated exocytosis of the major WPB constituent, von Willebrand Factor (VWF), which upon its release forms strings of up to several millimeters long that capture circulating platelets and thereby initiate the formation of a haemostatic plug. Using correlative, fluorescence, and electron microscopic imaging techniques, we provide evidence that multigranular exocytosis is an important pathway for VWF release in secretagogue-challenged human umbilical vein endothelial cells. A novel membrane-delimited structure (secretory pod) was identified as the site of WPB coalescence and VWF exocytosis. Clathrin-coated profiles present on the secretory pods suggested remodeling via compensatory membrane retrieval. Small, 30- to 40-nm cytoplasmic vesicles (nanovesicles) mediated the fusion of WPBs with secretory pods. Multigranular exocytosis may facilitate VWF string formation by pooling the content of multiple WPBs. In addition, it may provide a novel mechanism for the differential release of WPB cargo.

Involvement of protein kinase C and intracellular Ca2+ in goldfish brain somatostatin-28 inhibitory action on growth hormone release in goldfish

Goldfish brain somatostatin-28 (gbSS-28) is present in brain and pituitary tissues of goldfish. We assessed whether gbSS-28 targets Ca(2+) and/or protein kinase C (PKC)-dependent signaling cascades in inhibiting growth hormone (GH) release. gbSS-28 decreased basal GH release from primary cultures of dispersed goldfish pituitary cells and intracellular free calcium levels ([Ca(2+)](i)) in goldfish somatotropes. gbSS-28 partially reduced [Ca(2+)](i) and GH responses induced by two endogeneous gonadotropin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II. Furthermore, gbSS-28 reduced GH increases and abolished [Ca(2+)](i) elevations elicited by two PKC activators, tetradecanoyl 4beta-phorbol-13-acetate and dioctanyl glycerol. The PKC inhibitors Gö6976 and Bis II abolished [Ca(2+)](i) responses to PKC activators, but only attenuated GnRH-induced increases in [Ca(2+)](i) and did not alter basal [Ca(2+)](i). In cells pretreated with Bis II, gbSS-28 further reduced basal [Ca(2+)](i). Our results suggest that gbSS-28 inhibits GnRH-induced GH release in part by attenuating PKC-mediated GnRH [Ca(2+)](i) signals. gbSS-28 reduces basal GH release also via reduction in [Ca(2+)](i) but PKC is not involved in this regard.

Mitochondrial matrix pH controls oxidative phosphorylation and metabolism-secretion coupling in INS-1E clonal beta cells

Glucose-evoked mitochondrial signals augment ATP synthesis in the pancreatic β cell. This activation of energy metabolism increases the cytosolic ATP/ADP ratio, which stimulates plasma membrane electrical activity and insulin granule exocytosis. We have recently demonstrated that matrix pH increases during nutrient stimulation of the pancreatic β cell. Here, we have tested whether mitochondrial matrix pH controls oxidative phosphorylation and metabolism-secretion coupling in the rat β-cell line INS-1E. Acidification of the mitochondrial matrix pH by nigericin blunted nutrient-dependent respiratory and ATP responses (continuously monitored in intact cells). Using electrophysiology and single cell imaging, we find that the associated defects in energy metabolism suppress glucose-stimulated plasma membrane electrical activity and cytosolic calcium transients. The same parameters were unaffected after direct stimulation of electrical activity with tolbutamide, which bypasses mitochondrial function. Furthermore, lowered matrix pH strongly inhibited sustained, but not first-phase, insulin secretion. Our results demonstrate that the matrix pH exerts a control function on oxidative phosphorylation in intact cells and that this mode of regulation is of physiological relevance for the generation of downstream signals leading to insulin granule exocytosis. We propose that matrix pH serves a novel signaling role in sustained cell activation.

PKC epsilon facilitates recovery of exocytosis after an exhausting stimulation

It has been well documented that protein kinase Cs (PKCs) play multifaceted roles in regulating exocytosis of neurotransmitters and hormones. But the isoform-specific PKC effects are still poorly elucidated mainly because of the large variety of PKC isoforms and the dubious specificity of the commonly used pharmacological agents. In the present study, based on overexpression of wild-type or dominant negative PKC epsilon, we demonstrate in neuroendocrine PC12 cells that PKC epsilon, but not PKC alpha, facilitates recovery of exocytosis after an exhausting stimulation. Specifically, PKC epsilon mediates fast recovery of the extent of exocytosis in a phosphatidylinositol biphosphate-dependent manner, likely through enhancing the rate of vesicle delivery and reorganization of cortical actin network. In addition, PKC epsilon promotes fast recovery of vesicle release kinetics that is slowed after a strong stimulation. These experimental results may suggest a PKC-dependent mechanism relevant to the short-term plasticity of exocytosis in both neurons and neuroendocrine cells.

Structural basis of protein kinase C isoform function

Protein kinase C (PKC) isoforms comprise a family of lipid-activated enzymes that have been implicated in a wide range of cellular functions. PKCs are modular enzymes comprised of a regulatory domain (that contains the membrane-targeting motifs that respond to lipid cofactors, and in the case of some PKCs calcium) and a relatively conserved catalytic domain that binds ATP and substrates. These enzymes are coexpressed and respond to similar stimulatory agonists in many cell types. However, there is growing evidence that individual PKC isoforms subserve unique (and in some cases opposing) functions in cells, at least in part as a result of isoform-specific subcellular compartmentalization patterns, protein-protein interactions, and posttranslational modifications that influence catalytic function. This review focuses on the structural basis for differences in lipid cofactor responsiveness for individual PKC isoforms, the regulatory phosphorylations that control the normal maturation, activation, signaling function, and downregulation of these enzymes, and the intra-/intermolecular interactions that control PKC isoform activation and subcellular targeting in cells. A detailed understanding of the unique molecular features that underlie isoform-specific posttranslational modification patterns, protein-protein interactions, and subcellular targeting (i.e., that impart functional specificity) should provide the basis for the design of novel PKC isoform-specific activator or inhibitor compounds that can achieve therapeutically useful changes in PKC signaling in cells.

S-nitrosylation of syntaxin 1 at Cys(145) is a regulatory switch controlling Munc18-1 binding

Exocytosis is regulated by NO in many cell types, including neurons. In the present study we show that syntaxin 1a is a substrate for S-nitrosylation and that NO disrupts the binding of Munc18-1 to the closed conformation of syntaxin 1a in vitro. In contrast, NO does not inhibit SNARE {SNAP [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein] receptor} complex formation or binding of Munc18-1 to the SNARE complex. Cys(145) of syntaxin 1a is the target of NO, as a non-nitrosylatable C145S mutant is resistant to NO and novel nitrosomimetic Cys(145) mutants mimic the effect of NO on Munc18-1 binding in vitro. Furthermore, expression of nitrosomimetic syntaxin 1a in living cells affects Munc18-1 localization and alters exocytosis release kinetics and quantal size. Molecular dynamic simulations suggest that NO regulates the syntaxin-Munc18 interaction by local rearrangement of the syntaxin linker and H3c regions. Thus S-nitrosylation of Cys(145) may be a molecular switch to disrupt Munc18-1 binding to the closed conformation of syntaxin 1a, thereby facilitating its engagement with the membrane fusion machinery.

ERK activation is only one role of PKC in TCR-independent cytotoxic T cell granule exocytosis

Cytotoxic T cells (CTLs) kill target cells by releasing lytic agents via regulated exocytosis. Three signals are known to be required for exocytosis: an increase in intracellular Ca(2+), activation of protein kinase C (PKC) and activation of extracellular signal regulated signal kinase (ERK). ERK activation required for exocytosis depends on activity of PKC. The simplest possibility is that the sole effect of PKC required for exocytosis is ERK activation. Testing this requires dissociating ERK and PKC activation. We did this using TCR-independent stimulation of TALL-104 human leukemic CTLs. When cells are stimulated with thapsigargin and PMA, agents that increase intracellular Ca(2+) and activate PKC, respectively, PKC-dependent ERK activation is required for lytic granule exocytosis. Expressing a constitutively active mutant MAP kinase kinase activates ERK independent of PKC. However, activating ERK without PKC does not support lytic granule exocytosis, indicating that there are multiple effects of PKC required for granule exocytosis.

Effect of purines on calcium-independent acetylcholine release at the mouse neuromuscular junction

At the mouse neuromuscular junction, activation of adenosine A(1) and P2Y receptors inhibits acetylcholine release by an effect on voltage dependent calcium channels related to spontaneous and evoked secretion. However, an effect of purines upon the neurotransmitter-releasing machinery downstream of Ca(2+) influx cannot be ruled out. An excellent tool to study neurotransmitter exocytosis in a Ca(2+)-independent step is the hypertonic response. Intracellular recordings were performed on diaphragm fibers of CF1 mice to determine the action of the specific adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentyl-adenosine (CCPA) and the P2Y(12-13) agonist 2-methylthio-adenosine 5'-diphosphate (2-MeSADP) on the hypertonic response. Both purines significantly decreased such response (peak and area under the curve), and their effect was prevented by specific antagonists of A(1) and P2Y(12-13) receptors, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and N-[2-(methylthioethyl)]-2-[3,3,3-trifluoropropyl]thio-5'-adenylic acid, monoanhydride with dichloromethylenebiphosphonic acid, tetrasodium salt (AR-C69931MX), respectively. Moreover, incubation of preparations only with the antagonists induced a higher response compared with controls, suggesting that endogenous ATP/ADP and adenosine are able to modulate the hypertonic response by activating their specific receptors. To search for the intracellular pathways involved in this effect, we studied the action of CCPA and 2-MeSADP in hypertonicity in the presence of inhibitors of several pathways. We found that the effect of CPPA was prevented by the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) while that of 2-MeSADP was occluded by the protein kinase C antagonist chelerythrine and W-7. On the other hand, the inhibitors of protein kinase A (N-(2[pbromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide, H-89) and phosphoinositide-3 kinase (PI3K) (2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one hydrochloride, LY-294002) did not modify the modulatory action in hypertonicity of both purines. Our results provide evidence that activation of A(1) and P2Y(12-13) receptors by CCPA and 2-MeSADP inhibits ACh release from mammalian motor nerve terminals through an effect on a Ca(2+)-independent step in the cascade of the exocytotic process. Since presynaptic calcium channels are intimately associated with components of the synaptic vesicle docking and fusion processes, further experiments could clarify if the actions of purines on calcium channels and on secretory machinery are related.

Effects of phorbol ester on vesicle dynamics as revealed by total internal reflection fluorescence microscopy

Exocytosis of neurotransmitter or hormone-filled vesicles is a highly dynamic process regulated by various proteins and lipids. As mainly revealed indirectly by the electrophysiological methods, exocytosis is believed to involve multiple kinetic steps in which vesicles transit from one state to another. Using total internal reflection fluorescence microscopy which enables direct visualization of individual vesicles, we developed an analytical framework to track and analyze vesicle dynamics. We demonstrated that all subplasmalemmal vesicles generally undergo constant and caged Brownian motion. And they can be classified into three populations that differ in their motion characteristics and fusion competence. Furthermore, we showed that these vesicle pools are differentially modulated by phorbol-12-myristate-13-acetate, a phorbol ester analog to endogenous diacylglycerol, through both protein-kinase-C-dependent and -independent pathways.

Myosin II contributes to fusion pore expansion during exocytosis

During exocytosis, the fusion pore expands to allow release of neurotransmitters and hormones to the extracellular space. To understand the process of synaptic transmission, it is of outstanding importance to know the properties of the fusion pore and how these properties affect the release process. Many proteins have been implicated in vesicle fusion; however, there is little evidence for proteins involved in fusion pore expansion. Myosin II has been shown to participate in the transport of vesicles and, surprisingly, in the final phases of exocytosis, affecting the kinetics of catecholamine release in adrenal chromaffin cells as measured by amperometry. Here, we have studied single vesicle exocytosis in chromaffin cells overexpressing an unphosphorylatable form (T18AS19A RLC-GFP) of myosin II that produces an inactive protein by patch amperometry. This method allows direct determination of fusion pore expansion by measuring its conductance, whereas the release of catecholamines is recorded simultaneously by amperometry. Here we demonstrated that the fusion pore is of critical importance to control the release of catecholamines during single vesicle secretion in chromaffin cells. We proved that myosin II acts as a molecular motor on the fusion pore expansion by hindering its dilation when it lacks the phosphorylation sites.

The protein kinase C (PKC) family of proteins in cytokine signaling in hematopoiesis

The members of the protein kinase C (PKC) family of proteins play important roles in signaling for various growth factors, cytokines, and hormones. Extensive work over the years has led to the identification of three major groups of PKC isoforms. These include the classic PKCs (PKCalpha, PKCbeta(I), PKCbeta(II), PKCgamma), the novel PKCs (PKCdelta, PKCepsilon, PKCeta, PKCmu, PKCtheta), and the atypical PKCs (PKCzeta, PKCiota/lambda). All these PKC subtypes have been shown to participate in the generation of signals for important cellular processes and to mediate diverse and, in some cases, opposing biologic responses. There is emerging evidence that these kinases also play key functional roles in the regulation of cell growth, apoptosis, and differentiation of hematopoietic cells. In this review, both the engagement of the various PKC members in cytokine and growth factor signaling and their role in the regulation of hematopoiesis are discussed.

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.

Enlargeosome traffic: exocytosis triggered by various signals is followed by endocytosis, membrane shedding or both

Enlargeosomes are cytoplasmic organelles discharged by regulated exocytosis, identified by immunofluorescence of their membrane marker, desmoyokin/Ahnak, but never revealed at the ultrastructural level. Among the numerous enlargeosome-positive cells, the richest and most extensively characterized are those of a PC12 clone, PC12-27, defective of classical neurosecretion. By using ultrastructural immunoperoxidase labeling of formaldehyde-fixed, Triton-X-100-permeabilized PC12-27 cells, we have now identified the enlargeosomes as small vesicles scattered in the proximity of, but never docked to, the plasma membrane. Upon stimulation, these vesicles undergo exocytosis [rapid after the Ca(2+) ionophore, ionomycin, much slower after either the phorbol ester, phorbol myristate acetate (PMA), or ATP, working through a P2Y receptor], with appearance in the plasma membrane of typical desmoyokin/Ahnak (d/A)-positive, Omega-shaped and open profiles evolving into flat patches. Postexocytic removal of the exocytized d/A-positive membrane occurs by two processes: generation of endocytic vesicles, predominant after ionomycin and ATP 100-500 microM; and shedding of membrane-bound cytoplasmic bodies, predominant after PMA and 1 mM ATP, containing little or no trace of endoplasmic reticulum, Golgi, endo/lysosomes and also of a plasma membrane marker. Depending on the stimulation, therefore, the cell-surface expansion by enlargeosome exocytosis is not always recycled but can induce release of specific membranes, possibly important in the pericellular environment.