Plasmalemmal phosphatidylinositol-4,5-bisphosphate level regulates the releasable vesicle pool size in chromaffin cells

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.

The type B phosphatidylinositol-4-phosphate 5-kinase 3 is essential for root hair formation in Arabidopsis thaliana

Root hairs are extensions of root epidermal cells and a model system for directional tip growth of plant cells. A previously uncharacterized Arabidopsis thaliana phosphatidylinositol-4-phosphate 5-kinase gene (PIP5K3) was identified and found to be expressed in the root cortex, epidermal cells, and root hairs. Recombinant PIP5K3 protein was catalytically active and converted phosphatidylinositol-4-phosphate to phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2]. Arabidopsis mutant plants homozygous for T-DNA-disrupted PIP5K3 alleles were compromised in root hair formation, a phenotype complemented by expression of wild-type PIP5K3 cDNA under the control of a 1500-bp PIP5K3 promoter fragment. Root hair-specific PIP5K3 overexpression resulted in root hair deformation and loss of cell polarity with increasing accumulation of PIP5K3 transcript. Using reestablishment of root hair formation in T-DNA mutants as a bioassay for physiological functionality of engineered PIP5K3 variants, catalytic activity was found to be essential for physiological function, indicating that PtdIns(4,5)P2 formation is required for root hair development. An N-terminal domain containing membrane occupation and recognition nexus repeats, which is not required for catalytic activity, was found to be essential for the establishment of root hair growth. Fluorescence-tagged PIP5K3 localized to the periphery of the apical region of root hair cells, possibly associating with the plasma membrane and/or exocytotic vesicles. Transient heterologous expression of full-length PIP5K3 in tobacco (Nicotiana tabacum) pollen tubes increased plasma membrane association of a PtdIns(4,5)P2-specific reporter in these tip-growing cells. The data demonstrate that root hair development requires PIP5K3-dependent PtdIns(4,5)P2 production in the apical region of root hair cells.

Unrestricted diffusion of exogenous and endogenous PIP(2 )in baby hamster kidney and Chinese hamster ovary cell plasmalemma

We used two approaches to characterize the lateral mobility of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in the plasmalemma of baby hamster kidney and Chinese hamster ovary fibroblasts. First, nitrobenzoxadiazole-labeled C6-phosphatidylcholine and C16-PIP(2) were incorporated into plasma membrane "lawns" ( approximately 20 x 30 microm) from these cells and into the outer monolayer of intact cells. Diffusion coefficients determined by fluorescence recovery after photobleaching were similar for the two lipids and were higher in lawns, approximately 0.3 microm(2)/s, than on the cell surface, approximately 0.1 microm(2)/s. For membrane lawns, the fractional recoveries (75-90%) were close to those expected from the fraction of total membrane bleached, and labeling by the probes was several times greater than for intact cells. Second, we analyzed cells expressing M1 muscarinic receptors and green fluorescent protein fused with PIP(2)-binding pleckstrin-homology domains, Tubby domains or diacylglycerol (DAG)-binding C1 domains. On-cell gigaseal patches were formed with pipette tips >5 microm in diameter. When the agonist carbachol (0.3 mM: ) was applied either within or outside of the pipette, lipid signals crossed the pipette barrier rapidly in both directions and membrane blebbing occurred on both membrane sides. Accurate simulations of lipid gradients required diffusion coefficients >1 microm(2)/s. Exogenous DAG also crossed the pipette barrier rapidly. In summary, we found no evidence for restricted diffusion of signaling lipids in these cells. The lower mobility and incorporation of phospholipid at the extracellular leaflet may reflect a more ordered and condensed extracellular monolayer, as expected from previous studies.

Secretory Carrier Membrane Protein SCAMP2 and Phosphatidylinositol 4,5-Bisphosphate Interactions in the Regulation of Dense Core Vesicle Exocytosis

Secretory carrier membrane protein 2 (SCAMP2) functions in late steps of membrane fusion in calcium-dependent granule exocytosis. A basic/hydrophobic peptide segment within SCAMP2 (SCAMP2 E: CWYRPIYKAFR) has been implicated in this function and shown to bind and sequester phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2 or PIP2] within membranes through an electrostatic mechanism. We now show that alanine substitution of tryptophan W2 within SCAMP2 E substantially weakens peptide binding to negatively charged liposomes; other substitutions for arginine R4 and lysine K8 have only limited effects on binding. Electron paramagnetic resonance analysis of liposomes containing spin-labeled PIP2 shows that R4 but not K8 is critical for SCAMP E binding to PIP2. The interfacial locations of SCAMP E and its structural variants within lipid bicelles measured by oxygen enhancement of nuclear relaxation are all similar. Corresponding point mutations within full-length SCAMP2 (SC2-R204A, SC2-K208A, and SC2-W202A) have been analyzed for biological effects on dense core vesicle exocytosis in neuroendocrine PC12 cells. With the same level of overexpression, SC2-R204A but not SC2-K208A inhibited secretion of cotransfected human growth hormone and of noradrenalin. Inhibition by SC2-R204A was the same as or greater than previously observed for SC2-W202A. Analysis of noradrenalin secretion by amperometry showed that inhibitory mutants of SCAMP2 decrease the probability of fusion pore opening and the stability of initially opened but not yet expanded fusion pores. The strong correlation between SCAMP2 E interactions with PIP2 and inhibition of exocytosis, particularly by SC2-R204A, led us to propose that SCAMP2 interaction with PIP2 within the membrane interface regulates fusion pore formation during exocytosis.

Dual control of cardiac Na+ Ca2+ exchange by PIP(2): electrophysiological analysis of direct and indirect mechanisms

Cardiac Na(+)-Ca(2+) exchange (NCX1) inactivates in excised membrane patches when cytoplasmic Ca(2+) is removed or cytoplasmic Na(+) is increased. Exogenous phosphatidylinositol-4,5-bis-phosphate (PIP(2)) can ablate both inactivation mechanisms, while it has no effect on inward exchange current in the absence of cytoplasmic Na(+). To probe PIP(2) effects in intact cells, we manipulated PIP(2) metabolism by several means. First, we used cell lines with M1 (muscarinic) receptors that couple to phospholipase C's (PLCs). As expected, outward NCX1 current (i.e. Ca(2+) influx) can be strongly inhibited when M1 agonists induce PIP(2) depletion. However, inward currents (i.e. Ca(2+) extrusion) without cytoplasmic Na(+) can be increased markedly in parallel with an increase of cell capacitance (i.e. membrane area). Similar effects are incurred by cytoplasmic perfusion of GTPgammaS or the actin cytoskeleton disruptor latrunculin, even in the presence of non-hydrolysable ATP (AMP-PNP). Thus, G-protein signalling may increase NCX1 currents by destabilizing membrane cytoskeleton-PIP(2) interactions. Second, to increase PIP(2) we directly perfused PIP(2) into cells. Outward NCX1 currents increase as expected. But over minutes currents decline substantially, and cell capacitance usually decreases in parallel. Third, using BHK cells with stable NCX1 expression, we increased PIP(2) by transient expression of a phosphatidylinositol-4-phosphate-5-kinase (hPIP5KIbeta) and a PI4-kinase (PI4KIIalpha). NCX1 current densities were decreased by > 80 and 40%, respectively. Fourth, we generated transgenic mice with 10-fold cardiac-specific overexpression of PI4KIIalpha. This wortmannin-insensitive PI4KIIalpha was chosen because basal cardiac phosphoinositides are nearly insensitive to wortmannin, and surface membrane PI4-kinase activity, defined functionally in excised patches, is not blocked by wortmannin. Both phosphatidylinositol-4-phosphate (PIP) and PIP(2) were increased significantly, while NCX1 current densities were decreased by 78% with no loss of NCX1 expression. Most mice developed cardiac hypertrophy, and immunohistochemical analysis suggests that NCX1 is redistributed away from the outer sarcolemma. Cholera toxin uptake was increased 3-fold, suggesting that clathrin-independent endocytosis is enhanced. We conclude that direct effects of PIP(2) to activate NCX1 can be strongly modulated by opposing mechanisms in intact cells that probably involve membrane cytoskeleton remodelling and membrane trafficking.

Local PIP(2) signals: when, where, and how?

PIP(2) is a minor phospholipid that modulates multiple cellular processes. However, its abundance by mass, like diacylglycerol, is still 20 to 100 times greater than the master phospholipid second messenger, PIP(3). Therefore, it is a case-by-case question whether PIP(2) is acting more like GTP, in being a cofactor in regulatory processes, or whether it is being used as a true second messenger. Analysis of signaling mechanisms in primary cells is essential to answer this question, as overexpression studies will naturally generate false positives. In connection with the possible messenger function of PIP(2), a second question arises as to how and if PIP(2) metabolism and signaling may be limited in space. This review summarizes succinctly the notable cases in which PIP(2) is proposed to function in a localized way and the different mechanistic models that may allow it to function locally. In general, drastic restrictions of PIP(2) diffusion are required. It is speculated that molecular PIP(2) signaling may be possible in the absence of PIP(2) gradients via ternary complexes between PIP(2) and two protein partners. That PIP(2) synthesis and hydrolysis might be locally dependent on protein-protein interactions, and direct lipid "hand-off" is suggested by multiple results.

Regulation of the actin cytoskeleton by phosphatidylinositol 4-phosphate 5 kinases

Phosphatidylinositol (4,5)-bisphosphate (PIP(2)) is an important lipid mediator that has multiple regulatory functions. There is now increasing evidence that the phosphatidylinositol 4-phosphate 5 kinases (PIP5Ks), which synthesize PIP(2), are regulated spatially and temporally and that they have isoform-specific functions and regulations. This review will summarize the highlights of recent developments in understanding how the three major PIP5K isoforms regulate the actin cytoskeleton and other important cellular processes.

Pacemaking by HCN channels requires interaction with phosphoinositides

Hyperpolarization-activated, cyclic-nucleotide-gated (HCN) channels mediate the depolarizing cation current (termed I(h) or I(f)) that initiates spontaneous rhythmic activity in heart and brain. This function critically depends on the reliable opening of HCN channels in the subthreshold voltage-range. Here we show that activation of HCN channels at physiologically relevant voltages requires interaction with phosphoinositides such as phosphatidylinositol-4,5-bisphosphate (PIP(2)). PIP(2) acts as a ligand that allosterically opens HCN channels by shifting voltage-dependent channel activation approximately 20 mV toward depolarized potentials. Allosteric gating by PIP(2) occurs in all HCN subtypes and is independent of the action of cyclic nucleotides. In CNS neurons and cardiomyocytes, enzymatic degradation of phospholipids results in reduced channel activation and slowing of the spontaneous firing rate. These results demonstrate that gating by phospholipids is essential for the pacemaking activity of HCN channels in cardiac and neuronal rhythmogenesis.

Phosphoinositides in cell regulation and membrane dynamics

Inositol phospholipids have long been known to have an important regulatory role in cell physiology. The repertoire of cellular processes known to be directly or indirectly controlled by this class of lipids has now dramatically expanded. Through interactions mediated by their headgroups, which can be reversibly phosphorylated to generate seven species, phosphoinositides play a fundamental part in controlling membrane-cytosol interfaces. These lipids mediate acute responses, but also act as constitutive signals that help define organelle identity. Their functions, besides classical signal transduction at the cell surface, include regulation of membrane traffic, the cytoskeleton, nuclear events and the permeability and transport functions of membranes.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

Localization of synaptic proteins involved in neurosecretion in different membrane microdomains

A number of proteins and signalling molecules modulate voltage-gated calcium channel activity and neurosecretion. As recent findings have indicated the presence of Ca(v)2.1 (P/Q-type) channels and soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptors (SNAREs) in the cholesterol-enriched microdomains of neuroendocrine and neuronal cells, we investigated whether molecules known to modulate neurosecretion, such as the heterotrimeric G proteins and neuronal calcium sensor-1 (NCS-1), are also localized in these microdomains. After immuno-isolation, flotation gradients from Triton X-100-treated synaptosomal membranes revealed the presence of different detergent-resistant membranes (DRMs) containing proteins of the exocytic machinery (Ca(v)2.1 channels and SNAREs) or NCS-1; both DRM subtypes contained aliquots of heterotrimeric G protein subunits and phosphatidylinositol-4,5-bisphosphate. In line with the biochemical data, confocal imaging of immunolabelled membrane sheets revealed the localization of SNARE proteins and NCS-1 in different dot-like structures. This distribution was largely impaired by treatment with methyl-beta-cyclodextrin, thus suggesting the localization of all three proteins in cholesterol-dependent domains. Finally, bradykinin (which is known to activate the NCS-1 pathway) caused a significant increase in NCS-1 in the DRMs. These findings suggest that different membrane microdomains are involved in the spatial organization of the complex molecular network that converges on calcium channels and the secretory machinery.

A comparison between exocytic control mechanisms in adrenal chromaffin cells and a glutamatergic synapse

It has been known since the work of Katz and collaborators in the early 1950s that an increase in intracellular Ca(++) concentration ([Ca(++)]) is the immediate trigger for neurotransmitter release. Later work has shown that, next to Ca(++), many other signaling pathways, particularly via cyclic AMP, modulate the release of both neurotransmitters and hormones. However, regulated secretion is a multistep process and the signaling mechanisms involved act at many stages. Biochemical and traditional electrophysiological techniques very often cannot distinguish whether a change in secretion is caused by regulation of ion channels, vesicle trafficking, or the exocytic process itself. My laboratory has made an effort to dissect the stimulus secretion pathway by developing assays in chromaffin cells (for catecholamine release) and at a glutamatergic central nervous synapse (the calyx of Held, a component of the auditory pathway), which permit the study of secretion in single cells under voltage clamp conditions. This enables us to clearly distinguish between consequences of changes in electrical signaling, from those regarding the process of vesicle recruitment or the process of exocytosis.

Phosphoinositide regulation of neuroexocytosis: adding to the complexity

Exocytosis of neurotransmitter containing vesicles supports neuronal communication. The importance of molecular interactions involving specific lipids has become progressively more evident and the lipid composition of both the synaptic vesicle and the pre-synaptic plasma membrane at the active zone has significant functional consequences for neurotransmitter release. Several classes of lipids have been implicated in exocytosis including polyunsaturated fatty acids and phosphoinositides. This minireview will focus on recent developments regarding the role of phosphoinositides in neurosecretion.

Monitoring the diffusion of single heterotrimeric G proteins in supported cell-membrane sheets reveals their partitioning into microdomains

Supported cell-membrane sheets are promising in vitro systems to investigate the properties of membranes with native protein/lipid composition, in particular their sub-compartmentalization and the differential localization of proteins associated to them. While such studies are usually performed using static microscopy techniques, we demonstrate here the potential offered by dynamic diffusion measurements. Whereas the overall fluidity of the lipid bilayer was preserved, the preparation of the membrane sheets led to the selective immobilization of extracellular and transmembrane (TM) glycosylated proteins and the anchored proteins/lipids associated with them. Taking advantage of this, we investigated the association of the G protein Gq with TM proteins, in particular G-protein coupled receptors (GPCRs), by monitoring the changes in diffusion occurring after preparation of the supported membranes. Two fluorescently tagged Galphaq proteins were constructed, which remained either mostly monomeric in the plasma membrane or associated with Gbetagamma in heterotrimers. While both constructs diffused similarly in living cells, the preparation of the supported membranes led to the selective immobilization of the heterotrimers with minimal changes of the diffusion of the monomeric Galphaq. The diverse mobility of monomeric and heterotrimeric Galphaq was a result of their different lipid anchors as demonstrated by monitoring the diffusion of the corresponding anchors alone. We propose that the immobilization of the heterotrimer was caused by its partitioning inside membrane microdomains surrounding GPCRs.

Opening of holes in liposomal membranes is induced by proteins possessing the FERM domain

The destabilization of vesicles caused by interactions between lipid bilayers and proteins was studied by direct, real-time observation using high-intensity dark-field microscopy. We previously reported that talin, a cytoskeletal submembranous protein, can reversibly open stable large holes in giant liposomes made of neutral and acidic phospholipids. Talin and other proteins belonging to the band 4.1 superfamily have the FERM domain at their N-terminal and interact with lipid membranes via that domain. Here, we observed that band 4.1, ezrin and moesin, members of the band 4.1 superfamily, are also able to open stable holes in liposomes. However, truncation of their C-terminal domains, which can interact with the N-terminal FERM domain, impaired their hole opening activities. Oligomeric states of ezrin affected the capability of the membrane hole formation. Phosphatidylinositol bisphosphate (PIP2), which binds to the FERM domain and disrupts the interaction between the N and C termini of the band 4.1 superfamily, down-regulates their membrane opening activity. These results suggest that the intermolecular interaction plays a key role in the observed membrane hole formation.

Stimulation of phosphatidylinositol kinase type I-mediated phosphatidylinositol (4,5)-bisphosphate synthesis by AP-2mu-cargo complexes

Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P(2)] is an important factor for a variety of cellular functions ranging from cell signaling to actin cytoskeletal dynamics and endocytic membrane traffic. Here, we have identified the clathrin adaptor complex AP-2 as a regulator of phosphatidylinositol 4-phosphate 5-kinase (PIPK)-mediated PI(4,5)P(2) synthesis. AP-2 directly interacts with the kinase core domain of type I PIPK isozymes via its mu2-subunit in vitro and in native protein extracts. Endocytic cargo protein binding to mu2 leads to a potent stimulation of PIPK activity. These data thus identify a positive feedback loop consisting of endocytic cargo proteins, AP-2mu, and PIPK type I which may provide a specific pool of PI(4,5)P(2) dedicated to clathrin/AP-2-dependent receptor internalization.

Elimination of plasma membrane phosphatidylinositol (4,5)-bisphosphate is required for exocytosis from mast cells

The inositol lipid phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] is involved in a myriad of cellular processes, including the regulation of exocytosis and endocytosis. In this paper, we address the role of PtdIns(4,5)P2 in compound exocytosis from rat peritoneal mast cells. This process involves granule-plasma membrane fusion as well as homotypic granule membrane fusion and occurs without any immediate compensatory endocytosis. Using a novel quantitative immunofluorescence technique, we report that plasma membrane PtdIns(4,5)P2 becomes transiently depleted upon activation of exocytosis, and is not detected on the membranes of fusing granules. Depletion is caused by phospholipase C activity, and is mandatory for exocytosis. Although phospholipase C is required for Ca2+ release from internal stores, the majority of the requirement for PtdIns(4,5)P2 hydrolysis occurs downstream of Ca2+ signalling - as shown in permeabilised cells, where the inositol (1,4,5)-trisphosphate-Ca2+ pathway is bypassed. Neither generation of the PtdIns(4,5)P2 metabolite, diacylglycerol (DAG) or simple removal and/or sequestration of PtdIns(4,5)P2 are sufficient for exocytosis to occur. However, treatment of permeabilised cells with DAG induces a small potentiation of exocytosis, indicating that it may be required. We propose that a cycle of PtdIns(4,5)P2 synthesis and breakdown is crucial for exocytosis to occur in mast cells, and may have a more general role in all professional secretory cells.

Functional assay of effectors of ADP ribosylation factor 6 during clathrin/AP-2 coat recruitment to membranes

Arf proteins play pivotal roles in membrane traffic, cell signaling, and actin cytoskeletal rearrangements. We describe here methods to functionally analyze interacting partner proteins of recombinantly produced N-myristoylated Arf6. Combined evidence from affinity purification and chemical crosslinking experiments, in vitro recruitment assays, and the analysis of lipid kinase activities indicates that Arf6-GTP facilitates clathrin/AP-2 recruitment to synaptic membranes by direct binding and activation of the brain-specific phosphatidylinositol 4-phosphate 5-kinase type Igamma (PIPK Igamma). These methods shall help to mechanistically dissect the role of Arf6 in regulating exo-endocytic vesicle cycling at synapses and in related membrane trafficking events.

Phosphoinositide regulation of clathrin-mediated endocytosis

Endocytosis of transmembrane receptors largely occurs via clathrin-coated vesicles that bud from the plasma membrane and deliver their cargo to the endosomal system for recycling or degradation. PIs (phosphoinositides) control the timing and localization of endocytic membrane trafficking by recruiting adaptors and other components of the transport machinery, thereby being part of a coincidence detection system in adaptor-mediated vesicle transport. Activation of organelle- and substrate-specific PI kinases by small GTPases such as Arf (ADP-ribosylation factor) and other factors may result in local changes of PI content, thereby regulating activity-dependent endocytic events including the recycling of synaptic vesicle membranes at nerve terminals. One such example is the PtdIns(4)P 5-kinase-mediated formation of PI(4,5)P2 [PtdIns(4,5)P2], which is required for the exo- and endo-cytic cycling of presynaptic vesicles and secretory granules. Over the last few years, protein X-ray crystallography in combination with biochemical and cell biological assays has been used to investigate the structure and function of many PI-binding proteins, including protein components of the endocytic machinery. These studies have provided molecular insights into the mechanisms by which PI(4,5)P2 recruits and activates adaptor proteins and their binding partners. In this mini-review, I will discuss the pathways of PI(4,5)P2 formation and its interactions with endocytic trafficking adaptors.

Fatty acids add grease to exocytosis

In this issue of Chemistry & Biology, Rickman and Davletov suggest a novel mechanism for biological lipids to regulate synaptic transmission in the brain. Physiologically relevant concentrations of arachidonic acid help to dissociate a protein complex involved in exocytosis.

Presynaptic UNC-31 (CAPS) is required to activate the G alpha(s) pathway of the Caenorhabditis elegans synaptic signaling network

C. elegans mutants lacking the dense-core vesicle priming protein UNC-31 (CAPS) share highly similar phenotypes with mutants lacking a neuronal G alpha(s) pathway, including strong paralysis despite exhibiting near normal levels of steady-state acetylcholine release as indicated by drug sensitivity assays. Our genetic analysis shows that UNC-31 and neuronal G alpha(s) are different parts of the same pathway and that the UNC-31/G alpha(s) pathway is functionally distinct from the presynaptic G alpha(q) pathway with which it interacts. UNC-31 acts upstream of G alpha(s) because mutations that activate the G alpha(s) pathway confer similar levels of strongly hyperactive, coordinated locomotion in both unc-31 null and (+) backgrounds. Using cell-specific promoters, we show that both UNC-31 and the G alpha(s) pathway function in cholinergic motor neurons to regulate locomotion rate. Using immunostaining we show that UNC-31 is often concentrated at or near active zones of cholinergic motor neuron synapses. Our data suggest that presynaptic UNC-31 activity, likely acting via dense-core vesicle exocytosis, is required to locally activate the neuronal G alpha(s) pathway near synaptic active zones.

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

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

Membrane phosphoinositides control insulin secretion through their effects on ATP-sensitive K+ channel activity

ATP-sensitive K(+) channels (K(ATP) channels) of pancreatic beta-cells play key roles in glucose-stimulated insulin secretion by linking metabolic signals to cell excitability. Membrane phosphoinositides, in particular phosphatidylinositol 4,5-bisphosphates (PIP(2)), stimulate K(ATP) channels and decrease channel sensitivity to ATP inhibition; as such, they have been postulated as critical regulators of K(ATP) channels and hence of insulin secretion in beta-cells. Here, we tested this hypothesis by manipulating the interactions between K(ATP) channels and membrane phospholipids in a beta-cell line, INS-1, and assessing how the manipulations affect membrane excitability and insulin secretion. We demonstrate that disruption of channel interactions with PIP(2) by overexpressing PIP(2)-insensitive channel subunits leads to membrane depolarization and elevated basal level insulin secretion at low glucose concentrations. By contrast, facilitation of channel interactions with PIP(2) by upregulating PIP(2) levels via overexpression of a lipid kinase, phosphatidylinositol 4-phosphate 5 kinase, decreases the ATP sensitivity of endogenous K(ATP) channels by approximately 26-fold and renders INS-1 cells hyperpolarized, unable to secrete insulin properly in the face of high glucose. Our results establish an important role of the interaction between membrane phosphoinositides and K(ATP) channels in regulating insulin secretion.

Neuronal calcium sensor-1 potentiates glucose-dependent exocytosis in pancreatic beta cells through activation of phosphatidylinositol 4-kinase beta

Cytosolic free Ca2+ plays an important role in the molecular mechanisms leading to regulated insulin secretion by the pancreatic beta cell. A number of Ca2+-binding proteins have been implicated in this process. Here, we define the role of the Ca2+-binding protein neuronal Ca2+ sensor-1 (NCS-1) in insulin secretion. In pancreatic beta cells, NCS-1 increases exocytosis by promoting the priming of secretory granules for release and increasing the number of granules residing in the readily releasable pool. The effect of NCS-1 on exocytosis is mediated through an increase in phosphatidylinositol (PI) 4-kinase beta activity and the generation of phosphoinositides, specifically PI 4-phosphate and PI 4,5-bisphosphate. In turn, PI 4,5-bisphosphate controls exocytosis through the Ca2+-dependent activator protein for secretion present in beta cells. Our results provide evidence for an essential role of phosphoinositide synthesis in the regulation of glucose-induced insulin secretion by the pancreatic beta cell. We also demonstrate that NCS-1 and its downstream target, PI 4-kinase beta, are critical players in this process by virtue of their capacity to regulate the release competence of the secretory granules.