Localization of phosphatidylinositol 4,5-P(2) important in exocytosis and a quantitative analysis of chromaffin granule motion adjacent to the plasma membrane

A molecular mechanism for calcium-mediated synaptotagmin-triggered exocytosis

The regulated exocytotic release of neurotransmitter and hormones is accomplished by a complex protein machinery consisting in its core of SNARE proteins and the calcium sensor synaptotagmin-1. We propose a mechanism where the lipid membrane is intimately involved in coupling calcium sensing to release. We demonstrate that fusion of dense core vesicles, derived from rat PC12 cells is strongly linked to the angle between the cytoplasmic domain of the SNARE complex and the plane of the target membrane. We propose that, as this tilt angle increases, force is exerted on the SNARE transmembrane domains to drive the merger of the two bilayers. The tilt angle dramatically increases upon calcium-mediated binding of synaptotagmin to membranes, strongly depends on the surface electrostatics of the membrane, and is strictly coupled to lipid order of the target membrane.

The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis

MacDougall et al. review the structure and function of the calcium sensor synaptotagmin-7 in exocytosis.

Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.

Reconstitution of calcium-mediated exocytosis of dense-core vesicles

Regulated exocytosis is a process by which neurotransmitters, hormones, and secretory proteins are released from the cell in response to elevated levels of calcium. In cells, secretory vesicles are targeted to the plasma membrane, where they dock, undergo priming, and then fuse with the plasma membrane in response to calcium. The specific roles of essential proteins and how calcium regulates progression through these sequential steps are currently incompletely resolved. We have used purified neuroendocrine dense-core vesicles and artificial membranes to reconstruct in vitro the serial events that mimic SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-dependent membrane docking and fusion during exocytosis. Calcium recruits these vesicles to the target membrane aided by the protein CAPS (calcium-dependent activator protein for secretion), whereas synaptotagmin catalyzes calcium-dependent fusion; both processes are dependent on phosphatidylinositol 4,5-bisphosphate. The soluble proteins Munc18 and complexin-1 are necessary to arrest vesicles in a docked state in the absence of calcium, whereas CAPS and/or Munc13 are involved in priming the system for an efficient fusion reaction.

Imaging the recruitment and loss of proteins and lipids at single sites of calcium-triggered exocytosis

Imaging of exocytic and endocytic proteins shows which are present at exocytic sites before, during, and after exocytosis in living cells. Rab proteins and SNARE modulators are lost, and dynamin, PIP2, and BAR-domain proteins are rapidly and transiently recruited, where they may modulate the nascent fusion pore.

How and when the dozens of molecules that control exocytosis assemble in living cells to regulate the fusion of a vesicle with the plasma membrane is unknown. Here we image with two-color total internal reflection fluorescence microscopy the local changes of 27 proteins at single dense-core vesicles undergoing calcium-triggered fusion. We identify two broad dynamic behaviors of exocytic molecules. First, proteins enriched at exocytic sites are associated with DCVs long before exocytosis, and near the time of membrane fusion, they diffuse away. These proteins include Rab3 and Rab27, rabphilin3a, munc18a, tomosyn, and CAPS. Second, we observe a group of classical endocytic proteins and lipids, including dynamins, amphiphysin, syndapin, endophilin, and PIP2, which are rapidly and transiently recruited to the exocytic site near the time of membrane fusion. Dynamin mutants unable to bind amphiphysin were not recruited, indicating that amphiphysin is involved in localizing dynamin to the fusion site. Expression of mutant dynamins and knockdown of endogenous dynamin altered the rate of cargo release from single vesicles. Our data reveal the dynamics of many key proteins involved in exocytosis and identify a rapidly recruited dynamin/PIP2/BAR assembly that regulates the exocytic fusion pore of dense-core vesicles in cultured endocrine beta cells.

Phosphoinositides and vesicular membrane traffic

Phosphoinositide lipids were initially discovered as precursors for specific second messengers involved in signal transduction, but have now taken the center stage in controlling many essential processes at virtually every cellular membrane. In particular, phosphoinositides play a critical role in regulating membrane dynamics and vesicular transport. The unique distribution of certain phosphoinositides at specific intracellular membranes makes these molecules uniquely suited to direct organelle-specific trafficking reactions. In this regulatory role, phosphoinositides cooperate specifically with small GTPases from the Arf and Rab families. This review will summarize recent progress in the study of phosphoinositides in membrane trafficking and organellar organization and highlight the particular relevance of these signaling pathways in disease. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

New insights into the role of the cortical cytoskeleton in exocytosis from neuroendocrine cells

The cortical cytoskeleton is a dense network of filamentous actin (F-actin) that participates in the events associated with secretion from neuroendocrine cells. This filamentous web traps secretory vesicles, acting as a retention system that blocks the access of vesicles to secretory sites during the resting state, and it mediates their active directional transport during stimulation. The changes in the cortical cytoskeleton that drive this functional transformation have been well documented, particularly in cultured chromaffin cells. At the biochemical level, alterations in F-actin are governed by the activity of molecular motors like myosins II and V and by other calcium-dependent proteins that influence the polymerization and cross-linking of F-actin structures. In addition to modulating vesicle transport, the F-actin cortical network and its associated motor proteins also influence the late phases of the secretory process, including membrane fusion and the release of active substances through the exocytotic fusion pore. Here, we discuss the potential interactions between the F-actin cortical web and proteins such as SNAREs during secretion. We also discuss the role of the cytoskeleton in organizing the molecular elements required to sustain regulated exocytosis, forming a molecular structure that foments the efficient release of neurotransmitters and hormones.

The role of TRPA1 in visceral inflammation and pain

Despite significant progress in our understanding of the cellular and molecular mechanisms underlying sensory transduction and nociception, clinical pain management remains a considerable challenge in health care and basic research. The identification of the superfamily of transient receptor potential (TRP) cation channels, particularly TRPV1 and TRPA1, has shed light on the molecular basis of pain signaling during inflammatory conditions. TRPV1 and TRPA1 are considered as potential targets in the treatment of inflammatory pain because of their ability to be activated by nociceptive signals and sensitized by pro-inflammatory mediators. Notably, TRPA1 is expressed in visceral afferent neurons and is known to participate in inflammatory responses and the establishment of hypersensitivity. This review summarizes the current knowledge of the role of TRPA1 in sensory transduction, particularly in the context of visceral inflammation and pain in the gastrointestinal and urinary tracts.

S100A10-mediated translocation of annexin-A2 to SNARE proteins in adrenergic chromaffin cells undergoing exocytosis

In neuroendocrine cells, annexin-A2 is implicated as a promoter of monosialotetrahexosylganglioside (GM1)-containing lipid microdomains that are required for calcium-regulated exocytosis. As soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) require a specific lipid environment to mediate granule docking and fusion, we investigated whether annexin-A2-induced lipid microdomains might be linked to the SNAREs present at the plasma membrane. Stimulation of adrenergic chromaffin cells induces the translocation of cytosolic annexin-A2 to the plasma membrane, where it colocalizes with SNAP-25 and S100A10. Cross-linking experiments performed in stimulated chromaffin cells indicate that annexin-A2 directly interacts with S100A10 to form a tetramer at the plasma membrane. Here, we demonstrate that S100A10 can interact with vesicle-associated membrane protein 2 (VAMP2) and show that VAMP2 is present at the plasma membrane in resting adrenergic chromaffin cells. Tetanus toxin that cleaves VAMP2 solubilizes S100A10 from the plasma membrane and inhibits the translocation of annexin-A2 to the plasma membrane. Immunogold labelling of plasma membrane sheets combined with spatial point pattern analysis confirmed that S100A10 is present in VAMP2 microdomains at the plasma membrane and that annexin-A2 is observed close to S100A10 and to syntaxin in stimulated chromaffin cells. In addition, these results showed that the formation of phosphatidylinositol (4,5)-bisphosphate (PIP(2)) microdomains colocalized with S100A10 in the vicinity of docked granules, suggesting a functional interplay between annexin-A2-mediated lipid microdomains and SNAREs during exocytosis.

Nociceptive signals induce trafficking of TRPA1 to the plasma membrane

Transient receptor potential A1 (TRPA1) ion channel senses a variety of noxious stimuli and is involved in nociception. Many TRPA1 agonists covalently modify the channel, which can lead to desensitization. The fate of modified TRPA1 and the mechanism of preserving its response to subsequent stimuli are not understood. Moreover, inflammatory signals sensitize TRPA1 by involving protein kinase A (PKA) and phospholipase C (PLC) through unknown means. We show that TRPA1-mediated nocifensive behavior can be sensitized in vivo via PKA/PLC signaling and by activating TRPA1 with the ligand mustard oil (MO). Interestingly, both stimuli increased TRPA1 membrane levels in vitro. Tetanus toxin attenuated the response to the second of two pulses of MO in neurons, suggesting that vesicle fusion increases functional surface TRPA1. Capacitance recordings suggest that MO can induce exocytosis. We propose that TRPA1 translocation to the membrane might represent one of the mechanisms controlling TRPA1 functionality upon acute activation or inflammatory signals.

Studying individual events in biology

Studying the properties of individual events and molecules offers a host of advantages over taking only macroscopic measurements of populations. Here we review such advantages, as well as some pitfalls, focusing on examples from biological imaging. Examples include single proteins, their interactions in cells, organelles, and their interactions both with each other and with parts of the cell. Additionally, we discuss constraints that limit the study of single events, along with the criteria that must be fulfilled to determine whether single molecules or events are being detected.

Temperature-dependent differences between readily releasable and reserve pool vesicles in chromaffin cells

Statistical differences between amperometric traces recorded from chromaffin cells using K(+) and Ba(2+) secretagogues support the assertion that readily releasable pool (RRP) and reserve pool (RP) vesicles can be probed with pool-specific secretagogues. Release from the RRP was evoked by K(+) while release from the RP was evoked by Ba(2+). Similar temperature-dependent changes in spike area and half-width for both pools suggest that the content of RRP and RP vesicles is similar and packaged in the same way. Differences between the vesicle pools were revealed in the temperature dependence of spike frequency. While the burst spike frequency of the RRP, which is comprised of pre-docked and primed vesicles, increased 2.8% per degrees C, the RP spike frequency increased 12% per degrees C. This difference is attributed to a temperature-dependent mobilization of the RP. Furthermore, the RP exhibited more foot events at room temperature than the RRP but this difference was not apparent at 37 degrees C. This trend suggests that RP vesicle membranes have a compromised surface tension compared to RRP vesicles. Collectively, the changes of release characteristics with temperature reveal distinctions between the RRP and the RP.

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.

Differential properties of GTP- and Ca(2+)-stimulated exocytosis from large dense core vesicles

Many cells utilize a GTP-dependent pathway to trigger exocytosis in addition to Ca(2+)-triggered exocytosis. However, little is known about the mechanism by which GTP triggers exocytosis independent of Ca(2+). We used dual-color evanescent field microscopy to compare the motion and fusion of large dense core vesicles stimulated by either mastoparan (Mas) in Ca(2+)-free conditions or high K(+) in the presence of Ca(2+). We demonstrate that Mas is hardly effective in triggering the fusion of the predocked vesicles but predominantly mobilizes cytosolic vesicles. In contrast, Ca(2+)-dependent exocytosis is largely due to predocked vesicles. Fusion kinetics analysis and carbon-fiber amperometry reveal that Mas induces a brief 'kiss-and-run' fusion and releases only a small amount of the cargo, whereas Ca(2+) stimulates a more persistent opening of the fusion pore and larger release of the contents. Furthermore, we show that Mas-released vesicles require a much shorter time to reach fusion competence once they approach the plasma membrane. Our data suggest the involvement of different mechanisms not only in triggering and fusion but also in the docking and priming process for Ca(2+)- and GTP-dependent exocytosis.

A Central Kinase Domain of Type I Phosphatidylinositol Phosphate Kinases Is Sufficient to Prime Exocytosis

Exocytosis, a critical process for neuronal communication and hormonal regulation, involves several distinct steps including MgATP-dependent priming (which involves the synthesis of phosphatidylinositol 4,5-bisphosphate). Type I phosphatidylinositol phosphate kinases (PIPKIs) were purified biochemically as a priming factor. PIPKI consists of three domains: the N-terminal region, the central kinase domain, and the C-terminal region. Three isoforms (alpha, beta, and gamma) of PIPKI have been identified, and each is alternatively spliced at the C-terminal region. In the present study, we conducted a structure/function analysis of PIPKIs in the priming of exocytosis, and we found that recombinant PIPKIalpha and PIPKIgamma had priming activity. However, an unexpected finding of these results was that PIPKIbeta did not prime exocytosis. The N- or C-terminal region of PIPKIalpha and PIPKIgamma was not required for priming, which indicates that the central kinase domain is sufficient for this process. Alternative splicing in each isoform did not affect the isoform specificity in priming. Priming activity by isoforms is strongly correlated with their phosphatidylinositol phosphate kinase activity because PIPKIalpha and PIPKIgamma had higher kinase activity than PIPKIbeta. These results suggest that PIPKIalpha and PIPKIgamma are the critical priming factors for exocytosis; it also suggests that the levels of phosphatidylinositol phosphate kinase activity in producing phosphatidylinositol 4,5-bisphosphate specify the function of PIPKI isoforms in priming.

Phosphoinositides in Constitutive Membrane Traffic

Proteins that make, consume, and bind to phosphoinositides are important for constitutive membrane traffic. Different phosphoinositides are concentrated in different parts of the central vacuolar pathway, with phosphatidylinositol 4-phosphate predominate on Golgi, phosphatidylinositol 4,5-bisphosphate predominate at the plasma membrane, phosphatidylinositol 3-phosphate the major phosphoinositide on early endosomes, and phosphatidylinositol 3,5-bisphosphate found on late endocytic organelles. This spatial segregation may be the mechanism by which the direction of membrane traffic is controlled. Phosphoinositides increase the affinity of membranes for peripheral membrane proteins that function for sorting protein cargo or for the docking and fusion of transport vesicles. This implies that constitutive membrane traffic may be regulated by the mechanisms that control the activity of the enzymes that produce and consume phosphoinositides. Although the lipid kinases and phosphatases that function in constitutive membrane traffic are beginning to be identified, their regulation is poorly understood.

Regulation of exocytosis in adrenal chromaffin cells: focus on ARF and Rho GTPases

Neurons and neuroendocrine cells release transmitters and hormones by exocytosis, a highly regulated process in which secretory vesicles or granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Several stages have been recognized in exocytosis. After recruitment and docking at the plasma membrane, vesicles/granules enter a priming step, which is then followed by the fusion process. Cortical actin remodelling accompanies the exocytotic reaction, but the links between actin dynamics and trafficking events remain poorly understood. Here, we review the action of Rho and ADP-ribosylation factor (ARF) GTPases within the exocytotic pathway in adrenal chromaffin cells. Rho proteins are well known for their pivotal role in regulating the actin cytoskeleton. ARFs were originally identified as regulators of vesicle transport within cells. The possible interplay between these two families of GTPases and their downstream effectors provides novel insights into the mechanisms that govern exocytosis.