Role of PI(4,5)P(2) in vesicle exocytosis and membrane fusion

MTM1-mediated production of phosphatidylinositol 5-phosphate fuels the formation of podosome-like protrusions regulating myoblast fusion

Myogenesis is a multistep process that requires a spatiotemporal regulation of cell events resulting finally in myoblast fusion into multinucleated myotubes. Most major insights into the mechanisms underlying fusion seem to be conserved from insects to mammals and include the formation of podosome-like protrusions (PLPs) that exert a driving force toward the founder cell. However, the machinery that governs this process remains poorly understood. In this study, we demonstrate that MTM1 is the main enzyme responsible for the production of phosphatidylinositol 5-phosphate, which in turn fuels PI5P 4-kinase α to produce a minor and functional pool of phosphatidylinositol 4,5-bisphosphate that concentrates in PLPs containing the scaffolding protein Tks5, Dynamin-2, and the fusogenic protein Myomaker. Collectively, our data reveal a functional crosstalk between a PI-phosphatase and a PI-kinase in the regulation of PLP formation.

PIP2 Is An Electrostatic Catalyst for Vesicle Fusion by Lowering the Hydration Energy: Arresting Vesicle Fusion by Masking PIP2

Lipids are key factors in regulating membrane fusion. Lipids are not only structural components to form membranes but also active catalysts for vesicle fusion and neurotransmitter release, which are driven by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. SNARE proteins seem to be partially assembled before fusion, but the mechanisms that arrest vesicle fusion before Ca2+ influx are still not clear. Here, we show that phosphatidylinositol 4,5-bisphosphate (PIP2) electrostatically triggers vesicle fusion as an electrostatic catalyst by lowering the hydration energy and that a myristoylated alanine-rich C-kinase substrate (MARCKS), a PIP2-binding protein, arrests vesicle fusion in a vesicle docking state where the SNARE complex is partially assembled. Vesicle-mimicking liposomes fail to reproduce vesicle fusion arrest by masking PIP2, indicating that native vesicles are essential for the reconstitution of physiological vesicle fusion. PIP2 attracts cations to repel water molecules from membranes, thus lowering the hydration energy barrier.

MARCKS and PI(4,5)P2 reciprocally regulate actin-based dendritic spine morphology

Myristoylated, alanine-rich C-kinase substrate (MARCKS) is an F-actin and phospholipid binding protein implicated in numerous cellular activities, including the regulation of morphology in neuronal dendrites and dendritic spines. MARCKS contains a lysine-rich effector domain that mediates its binding to plasma membrane phosphatidylinositol-4,5-biphosphate (PI(4,5)P2) in a manner controlled by PKC and calcium/calmodulin. In neurons, manipulations of MARCKS concentration and membrane targeting strongly affect the numbers, shapes, and F-actin properties of dendritic spines, but the mechanisms remain unclear. Here, we tested the hypothesis that the effects of MARCKS on dendritic spine morphology are due to its capacity to regulate the availability of plasma membrane PI(4,5)P2. We observed that the concentration of free PI(4,5)P2 on the dendritic plasma membrane was inversely proportional to the concentration of MARCKS. Endogenous PI(4,5)P2 levels were increased or decreased, respectively, by acutely overexpressing either phosphatidylinositol-4-phosphate 5-kinase (PIP5K) or inositol polyphosphate 5-phosphatase (5ptase). PIP5K, like MARCKS depletion, induced severe spine shrinkage; 5ptase, like constitutively membrane-bound MARCKS, induced aberrant spine elongation. These phenotypes involved changes in actin properties driven by the F-actin severing protein cofilin. Collectively, these findings support a model in which neuronal activity regulates actin-dependent spine morphology through antagonistic interactions of MARCKS and PI(4,5)P2.

PLCγ1 in dopamine neurons critically regulates striatal dopamine release via VMAT2 and synapsin III

Dopamine neurons are essential for voluntary movement, reward learning, and motivation, and their dysfunction is closely linked to various psychological and neurodegenerative diseases. Hence, understanding the detailed signaling mechanisms that functionally modulate dopamine neurons is crucial for the development of better therapeutic strategies against dopamine-related disorders. Phospholipase Cγ1 (PLCγ1) is a key enzyme in intracellular signaling that regulates diverse neuronal functions in the brain. It was proposed that PLCγ1 is implicated in the development of dopaminergic neurons, while the physiological function of PLCγ1 remains to be determined. In this study, we investigated the physiological role of PLCγ1, one of the key effector enzymes in intracellular signaling, in regulating dopaminergic function in vivo. We found that cell type-specific deletion of PLCγ1 does not adversely affect the development and cellular morphology of midbrain dopamine neurons but does facilitate dopamine release from dopaminergic axon terminals in the striatum. The enhancement of dopamine release was accompanied by increased colocalization of vesicular monoamine transporter 2 (VMAT2) at dopaminergic axon terminals. Notably, dopamine neuron-specific knockout of PLCγ1 also led to heightened expression and colocalization of synapsin III, which controls the trafficking of synaptic vesicles. Furthermore, the knockdown of VMAT2 and synapsin III in dopamine neurons resulted in a significant attenuation of dopamine release, while this attenuation was less severe in PLCγ1 cKO mice. Our findings suggest that PLCγ1 in dopamine neurons could critically modulate dopamine release at axon terminals by directly or indirectly interacting with synaptic machinery, including VMAT2 and synapsin III.

Redox-Mediated Rewiring of Signalling Pathways: The Role of a Cellular Clock in Brain Health and Disease

Metazoan signalling pathways can be rewired to dampen or amplify the rate of events, such as those that occur in development and aging. Given that a linear network topology restricts the capacity to rewire signalling pathways, such scalability of the pace of biological events suggests the existence of programmable non-linear elements in the underlying signalling pathways. Here, we review the network topology of key signalling pathways with a focus on redox-sensitive proteins, including PTEN and Ras GTPase, that reshape the connectivity profile of signalling pathways in response to an altered redox state. While this network-level impact of redox is achieved by the modulation of individual redox-sensitive proteins, it is the population by these proteins of critical nodes in a network topology of signal transduction pathways that amplifies the impact of redox-mediated reprogramming. We propose that redox-mediated rewiring is essential to regulate the rate of transmission of biological signals, giving rise to a programmable cellular clock that orchestrates the pace of biological phenomena such as development and aging. We further review the evidence that an aberrant redox-mediated modulation of output of the cellular clock contributes to the emergence of pathological conditions affecting the human brain.

Non-Vesicular Lipid Transport Machinery in Leishmania donovani: Functional Implications in Host-Parasite Interaction

Eukaryotic cells have distinct membrane-enclosed organelles, each with a unique biochemical signature and specialized function. The unique identity of each organelle is greatly governed by the asymmetric distribution and regulated intracellular movement of two important biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid-transfer proteins (LTPs) plays essential roles in intra-cellular lipid trafficking and cellular lipid homeostasis, while vesicular transport regulates protein trafficking. A comparative analysis of non-vesicular lipid transport machinery in protists could enhance our understanding of parasitism and basis of eukaryotic evolution. Leishmania donovani, the trypanosomatid parasite, greatly depends on receptor-ligand mediated signalling pathways for cellular differentiation, nutrient uptake, secretion of virulence factors, and pathogenesis. Lipids, despite being important signalling molecules, have intracellular transport mechanisms that are largely unexplored in L. donovani. We have identified a repertoire of sixteen (16) potential lipid transfer protein (LTP) homologs based on a domain-based search on TriTrypDB coupled with bioinformatics analyses, which signifies the presence of well-organized lipid transport machinery in this parasite. We emphasized here their evolutionary uniqueness and conservation and discussed their potential implications for parasite biology with regards to future therapeutic targets against visceral leishmaniasis.

A Plethora of Functions Condensed into Tiny Phospholipids: The Story of PI4P and PI(4,5)P2

Phosphoinositides (PIs) are small, phosphorylated lipids that serve many functions in the cell. They regulate endo- and exocytosis, vesicular trafficking, actin reorganization, and cell mobility, and they act as signaling molecules. The most abundant PIs in the cell are phosphatidylinositol-4-monophosphate (PI4P) and phosphatidylinositol-4,5-bisphosphate [PI(4,5)P₂]. PI4P is mostly localized at the Golgi apparatus where it regulates the anterograde trafficking from the Golgi apparatus to the plasma membrane (PM), but it also localizes at the PM. On the other hand, the main localization site of PI(4,5)P₂ is the PM where it regulates the formation of endocytic vesicles. The levels of PIs are regulated by many kinases and phosphatases. Four main kinases phosphorylate the precursor molecule phosphatidylinositol into PI4P, divided into two classes (PI4KIIα, PI4KIIβ, PI4KIIIα, and PI4KIIIβ), and three main kinases phosphorylate PI4P to form PI(4,5)P₂ (PI4P5KIα, PI4P5KIβ, and PI4P5KIγ). In this review, we discuss the localization and function of the kinases that produce PI4P and PI(4,5)P₂, as well as the localization and function of their product molecules with an overview of tools for the detection of these PIs.

Talin and kindlin cooperate to control the density of integrin clusters

Focal adhesions are composed of transmembrane integrins, linking the extracellular matrix to the actomyosin cytoskeleton, via cytoplasmic proteins. Adhesion depends on the activation of integrins. Talin and kindlin proteins are intracellular activators of integrins that bind to β-integrin cytoplasmic tails. Integrin activation and clustering through extracellular ligands guide the organization of adhesion complexes. However, the roles of talin and kindlin in this process are poorly understood. To determine the contribution of talin, kindlin, lipids and actomyosin in integrin clustering, we used a biomimetic in vitro system, made of giant unilamellar vesicles, containing transmembrane integrins (herein αIIbβ3), with purified talin (talin-1), kindlin (kindlin-2, also known as FERMT2) and actomyosin. Here, we show that talin and kindlin individually have the ability to cluster integrins. Talin and kindlin synergize to induce the formation of larger integrin clusters containing the three proteins. Comparison of protein density reveals that kindlin increases talin and integrin density, whereas talin does not affect kindlin and integrin density. Finally, kindlin increases integrin-talin-actomyosin coupling. Our study unambiguously demonstrates how kindlin and talin cooperate to induce integrin clustering, which is a major parameter for cell adhesion.

Bioinspired Membrane Interfaces: Controlling Actomyosin Architecture and Contractility

The creation of biologically inspired artificial lipid bilayers on planar supports provides a unique platform to study membrane-confined processes in a well-controlled setting. At the plasma membrane of mammalian cells, the linkage of the filamentous (F)-actin network is of pivotal importance leading to cell-specific and dynamic F-actin architectures, which are essential for the cell's shape, mechanical resilience, and biological function. These networks are established through the coordinated action of diverse actin-binding proteins and the presence of the plasma membrane. Here, we established phosphatidylinositol-4,5-bisphosphate (PtdIns[4,5]P₂)-doped supported planar lipid bilayers to which contractile actomyosin networks were bound via the membrane-actin linker ezrin. This membrane system, amenable to high-resolution fluorescence microscopy, enabled us to analyze the connectivity and contractility of the actomyosin network. We found that the network architecture and dynamics are not only a function of the PtdIns[4,5]P₂ concentration but also depend on the presence of negatively charged phosphatidylserine (PS). PS drives the attached network into a regime, where low but physiologically relevant connectivity to the membrane results in strong contractility of the actomyosin network, emphasizing the importance of the lipid composition of the membrane interface.

Local PI(4,5)P2 signaling inhibits fusion pore expansion during exocytosis

Phosphatidylinositol(4,5)bisphosphate (PI(4,5)P2) is an important signaling phospholipid that is required for regulated exocytosis and some forms of endocytosis. The two processes share a topologically similar pore structure that connects the vesicle lumen with the outside. Widening of the fusion pore during exocytosis leads to cargo release, while its closure initiates kiss&run or cavicapture endocytosis. We show here, using live-cell total internal reflection fluorescence (TIRF) microscopy of insulin granule exocytosis, that transient accumulation of PI(4,5)P2 at the release site recruits components of the endocytic fission machinery and stalls the late fusion pore expansion that is required for peptide release. The absence of clathrin differentiates this mechanism from clathrin-mediated endocytosis. Knockdown of phosphatidylinositol-phosphate-5-kinase-1c or optogenetic recruitment of 5-phosphatase reduces PI(4,5)P2 transients and accelerates fusion pore expansion, suggesting that acute PI(4,5)P2 synthesis is involved. Thus, local phospholipid signaling inhibits fusion pore expansion and peptide release through an unconventional endocytic mechanism.

Lipids and Secretory Vesicle Exocytosis

In recent years, the number of studies implicating lipids in the regulation of synaptic vesicle exocytosis has risen considerably. It has become increasingly clear that lipids such as phosphoinositides, lysophospholipids, cholesterol, arachidonic acid and myristic acid play critical regulatory roles in the processes leading up to exocytosis. Lipids may affect membrane fusion reactions by altering the physical properties of the membrane, recruiting key regulatory proteins, concentrating proteins into exocytic "hotspots" or by modulating protein functions allosterically. Discrete changes in phosphoinositides concentration are involved in multiple trafficking events including exocytosis and endocytosis. Lipid-modifying enzymes such as the DDHD2 isoform of phospholipase A1 were recently shown to contribute to memory acquisition via dynamic modifications of the brain lipid landscape. Considering the increasing reports on neurodegenerative disorders associated with aberrant intracellular trafficking, an improved understanding of the control of lipid pathways is physiologically and clinically significant and will afford unique insights into mechanisms and therapeutic methods for neurodegenerative diseases. Consequently, this chapter will discuss the different classes of lipids, phospholipase enzymes, the evidence linking them to synaptic neurotransmitter release and how they act to regulate key steps in the multi-step process leading to neuronal communication and memory acquisition.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

DIACYLGLYCEROL KINASE 5 regulates polar tip growth of tobacco pollen tubes

Pollen tubes require a tightly regulated pectin secretion machinery to sustain the cell wall plasticity required for polar tip growth. Involved in this regulation at the apical plasma membrane are proteins and signaling molecules, including phosphoinositides and phosphatidic acid (PA). However, the contribution of diacylglycerol kinases (DGKs) is not clear. We transiently expressed tobacco DGKs in pollen tubes to identify a plasma membrane (PM)-localized isoform, and then to study its effect on pollen tube growth, pectin secretion and lipid signaling. In order to potentially downregulate DGK5 function, we overexpressed an inactive variant. Only one of eight DGKs displayed a confined localization at the apical PM. We could demonstrate its enzymatic activity and that a kinase-dead variant was inactive. Overexpression of either variant led to differential perturbations including misregulation of pectin secretion. One mode of regulation could be that DGK5-formed PA regulates phosphatidylinositol 4-phosphate 5-kinases, as overexpression of the inactive DGK5 variant not only led to a reduction of PA but also of phosphatidylinositol 4,5-bisphosphate levels and suppressed related growth phenotypes. We conclude that DGK5 is an additional player of polar tip growth that regulates pectin secretion probably in a common pathway with PI4P 5-kinases.

Activity-dependent PI4P synthesis by PI4KIIIα regulates long-term synaptic potentiation

Phosphatidylinositol 4-phosphate (PI4P) is a low abundant phospholipid with important roles in lipid transport and membrane trafficking. However, little is known of its metabolism and function in neurons. Here, we investigate its subcellular distribution and functional roles in dendrites of rodent hippocampal neurons during resting state and long-term synaptic potentiation (LTP). We show that neural activity causes dynamic reversible changes in PI4P metabolism in dendrites. Upon LTP induction, PI4KIIIα, a type III phosphatidylinositol 4-kinase, localizes to the dendritic plasma membrane (PM) in a calcium-dependent manner and causes substantial increase in the levels of PI4P. Acute inhibition of PI4KIIIα activity abolishes trafficking of the AMPA-type glutamate receptor to the PM during LTP induction, and silencing of PI4KIIIα expression in the hippocampal CA1 region causes severe impairment of LTP and long-term memory. Collectively, our results identify an essential role for PI4KIIIα-dependent PI4P synthesis in synaptic plasticity of central nervous system neurons.

Acyl-chain saturation regulates the order of phosphatidylinositol 4,5-bisphosphate nanodomains

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) plays a critical role in the regulation of various plasma membrane processes and signaling pathways in eukaryotes. A significant amount of cellular resources are spent on maintaining the dominant 1-stearoyl-2-arachidonyl PI(4,5)P₂ acyl-chain composition, while less abundant and more saturated species become more prevalent in response to specific stimuli, stress or aging. Here, we report the impact of acyl-chain structure on the biophysical properties of cation-induced PI(4,5)P₂ nanodomains. PI(4,5)P₂ species with increasing levels of acyl-chain saturation cluster in progressively more ordered nanodomains, culminating in the formation of gel-like nanodomains for fully saturated species. The formation of these gel-like domains was largely abrogated in the presence of 1-stearoyl-2-arachidonyl PI(4,5)P_2. This is, to the best of our knowledge, the first report of the impact of PI(4,5)P₂ acyl-chain composition on cation-dependent nanodomain ordering, and provides important clues to the motives behind the enrichment of PI(4,5)P₂ with polyunsaturated acyl-chains. We also show how Ca²⁺-induced PI(4,5)P₂ nanodomains are able to generate local negative curvature, a phenomenon likely to play a role in membrane remodeling events.

An Emerging Role for Phosphoinositides in the Pathophysiology of Parkinson’s Disease

Recent data support an involvement of defects in homeostasis of phosphoinositides (PIPs) in the pathophysiology of Parkinson’s disease (PD). Genetic mutations have been identified in genes encoding for PIP-regulating and PIP-interacting proteins, that are associated with familial and sporadic PD. Many of these proteins are implicated in vesicular membrane trafficking, mechanisms that were recently highlighted for their close associations with PD. PIPs are phosphorylated forms of the membrane phospholipid, phosphatidylinositol. Their composition in the vesicle’s membrane of origin, as well as membrane of destination, controls vesicular membrane trafficking. We review the converging evidence that points to the involvement of PIPs in PD. The review describes PD- and PIP-associated proteins implicated in clathrin-mediated endocytosis and autophagy, and highlights the involvement of α-synuclein in these mechanisms.

Integration of population and functional genomics to understand mechanisms of artemisinin resistance in Plasmodium falciparum

Resistance to antimalarial drugs, and in particular to the artemisinin derivatives and their partner drugs, threatens recent progress toward regional malaria elimination and eventual global malaria eradication. Population-level studies utilizing whole-genome sequencing approaches have facilitated the identification of regions of the parasite genome associated with both clinical and in vitro drug-resistance phenotypes. However, the biological relevance of genes identified in these analyses and the establishment of a causal relationship between genotype and phenotype requires functional characterization. Here we examined data from population genomic and transcriptomic studies in the context of data generated from recent functional studies, using a new population genetic approach designed to identify potential favored mutations within the region of a selective sweep (iSAFE). We identified several genes functioning in pathways now known to be associated with artemisinin resistance that were supported in early population genomic studies, as well as potential new drug targets/pathways for further validation and consideration for treatment of artemisinin-resistant Plasmodium falciparum. In addition, we establish the utility of iSAFE in identifying positively-selected mutations in population genomic studies, potentially accelerating the time to functional validation of candidate genes.

An overview of the synaptic vesicle lipid composition

Chemical neurotransmission is the major mechanism of neuronal communication. Neurotransmitters are released from secretory organelles, the synaptic vesicles (SVs) via exocytosis into the synaptic cleft. Fusion of SVs with the presynaptic plasma membrane is balanced by endocytosis, thus maintaining the presynaptic membrane at steady-state levels. The protein machineries responsible for exo- and endocytosis have been extensively investigated. In contrast, less is known about the role of lipids in synaptic transmission and how the lipid composition of SVs is affected by dynamic exo-endocytotic cycling. Here we summarize the current knowledge about the composition, organization, and function of SV membrane lipids. We also cover lipid biogenesis and maintenance during the synaptic vesicle cycle.

Two StAR-related lipid transfer proteins play specific roles in endocytosis, exocytosis, and motility in the parasitic protist Entamoeba histolytica

Lipid transfer proteins (LTPs) are the key contributor of organelle-specific lipid distribution and cellular lipid homeostasis. Here, we report a novel implication of LTPs in phagocytosis, trogocytosis, pinocytosis, biosynthetic secretion, recycling of pinosomes, and motility of the parasitic protist E. histolytica, the etiological agent of human amoebiasis. We show that two StAR-related lipid transfer (START) domain-containing LTPs (named as EhLTP1 and 3) are involved in these biological pathways in an LTP-specific manner. Our findings provide novel implications of LTPs, which are relevant to the elucidation of pathophysiology of the diseases caused by parasitic protists.

We showed that EhLTP1, but not EhLTP3, is involved in secretion of cysteine protease, the well-established degrading factor of host cells and the extracellular matrix, and in pseudopod formation and migration. In contrast, EhLTP3, but not EhLTP1, is exclusively involved in pinocytosis of the fluid-phase marker. Both EhLTP1 and EhLTP3 are also involved in trogocytosis (ingestion by nibbling) of live mammalian cells and phagocytosis of dead cells. In trogocytosis and phagcytosis, these two LTPs displayed distinct patterns of recruitment: e.g., EhLTP1 was associated at the ligand attachment site at the initiation of trogocytosis, followed by the recruitment of EhLTP3 onto the “trogocytic tunnel” at the intermediate stage of trogocytosis before the closure of the trogosome. Such tempo-spatially coordinated involvement of LTPs in the course of trogo- and phagocytosis has never been demonstrated in unicellular eukaryotes. Neither has LTP been shown to be involved in both endocytosis and exocytosis.

SNARE complex alters the interactions of the Ca2+ sensor synaptotagmin 1 with lipid bilayers

Release of neuronal transmitters from nerve terminals is triggered by the molecular Ca²⁺ sensor synaptotagmin 1 (Syt1). Syt1 is a transmembrane protein attached to the synaptic vesicle (SV), and its cytosolic region comprises two domains, C2A and C2B, which are thought to penetrate into lipid bilayers upon Ca²⁺ binding. Before fusion, SVs become attached to the presynaptic membrane (PM) by the four-helical SNARE complex, which is thought to bind the C2B domain in vivo. To understand how the interactions of Syt1 with lipid bilayers and the SNARE complex trigger fusion, we performed molecular dynamics (MD) simulations at a microsecond scale. We investigated how the isolated C2 modules and the C2AB tandem of Syt1 interact with membranes mimicking either SV or PM. The simulations showed that the C2AB tandem can either bridge SV and PM or insert into PM with its Ca²⁺-bound tips and that the latter configuration is more favorable. Surprisingly, C2 domains did not cooperate in penetrating into PM but instead mutually hindered their insertion into the bilayer. To test whether the interaction of Syt1 with lipid bilayers could be affected by the C2B-SNARE attachment, we performed systematic conformational analysis of the C2AB-SNARE complex. Notably, we found that the C2B-SNARE interface precludes the coupling of C2 domains and promotes their insertion into PM. We performed the MD simulations of the prefusion protein complex positioned between the lipid bilayers mimicking PM and SV, and our results demonstrated in silico that the presence of the Ca²⁺ bound C2AB tandem promotes lipid merging. Altogether, our MD simulations elucidated the role of the Syt1-SNARE interactions in the fusion process and produced the dynamic all-atom model of the prefusion protein-lipid complex.

PI(4,5)P2 Clustering and Its Impact on Biological Functions

Located at the inner leaflet of the plasma membrane (PM), phosphatidyl-inositol 4,5-bisphosphate [PI(4,5)P₂] composes only 1-2 mol% of total PM lipids. With its synthesis and turnover both spatially and temporally regulated, PI(4,5)P₂ recruits and interacts with hundreds of cellular proteins to support a broad spectrum of cellular functions. Several factors contribute to the versatile and dynamic distribution of PI(4,5)P₂ in membranes. Physiological multivalent cations such as Ca²⁺ and Mg²⁺ can bridge between PI(4,5)P₂ headgroups, forming nanoscopic PI(4,5)P₂-cation clusters. The distinct lipid environment surrounding PI(4,5)P₂ affects the degree of PI(4,5)P₂ clustering. In addition, diverse cellular proteins interacting with PI(4,5)P₂ can further regulate PI(4,5)P₂ lateral distribution and accessibility. This review summarizes the current understanding of PI(4,5)P₂ behavior in both cells and model membranes, with emphasis on both multivalent cation- and protein-induced PI(4,5)P₂ clustering. Understanding the nature of spatially separated pools of PI(4,5)P₂ is fundamental to cell biology.

Calcium-Prolactin Secretion Coupling in Rat Pituitary Lactotrophs Is Controlled by PI4-Kinase Alpha

The role of calcium, but not of other intracellular signaling molecules, in the release of pituitary hormones by exocytosis is well established. Here, we analyzed the contribution of phosphatidylinositol kinases (PIKs) to calcium-driven prolactin (PRL) release in pituitary lactotrophs: PI4Ks - which control PI4P production, PIP5Ks - which synthesize PI(4, 5)P2 by phosphorylating the D-5 position of the inositol ring of PI4P, and PI3KCs - which phosphorylate PI(4, 5)P₂ to generate PI(3, 4, 5)P₃. We used common and PIK-specific inhibitors to evaluate the strength of calcium-secretion coupling in rat lactotrophs. Gene expression was analyzed by single-cell RNA sequencing and qRT-PCR analysis; intracellular and released hormones were assessed by radioimmunoassay and ELISA; and single-cell calcium signaling was recorded by Fura 2 imaging. Single-cell RNA sequencing revealed the expression of Pi4ka, Pi4kb, Pi4k2a, Pi4k2b, Pip5k1a, Pip5k1c, and Pik3ca, as well as Pikfyve and Pip4k2c, in lactotrophs. Wortmannin, a PI3K and PI4K inhibitor, but not LY294002, a PI3K inhibitor, blocked spontaneous action potential driven PRL release with a half-time of ~20 min when applied in 10 µM concentration, leading to accumulation of intracellular PRL content. Wortmannin also inhibited increase in PRL release by high potassium, the calcium channel agonist Bay K8644, and calcium mobilizing thyrotropin-releasing hormone without affecting accompanying calcium signaling. GSK-A1, a specific inhibitor of PI4KA, also inhibited calcium-driven PRL secretion without affecting calcium signaling and Prl expression. In contrast, PIK93, a specific inhibitor of PI4KB, and ISA2011B and UNC3230, specific inhibitors of PIP5K1A and PIP5K1C, respectively, did not affect PRL release. These experiments revealed a key role of PI4KA in calcium-secretion coupling in pituitary lactotrophs downstream of voltage-gated and PI(4, 5)P2-dependent calcium signaling.

α-Synuclein facilitates endocytosis by elevating the steady-state levels of phosphatidylinositol 4,5-bisphosphate

α-Synuclein (α-Syn) is a protein implicated in the pathogenesis of Parkinson's disease (PD). It is an intrinsically disordered protein that binds acidic phospholipids. Growing evidence supports a role for α-Syn in membrane trafficking, including, mechanisms of endocytosis and exocytosis, although the exact role of α-Syn in these mechanisms is currently unclear. Here we investigate the associations of α-Syn with the acidic phosphoinositides (PIPs), phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2). Our results show that α-Syn colocalizes with PIP2 and the phosphorylated active form of the clathrin adaptor protein 2 (AP2) at clathrin-coated pits. Using endocytosis of transferrin as an indicator for clathrin-mediated endocytosis (CME), we find that α-Syn involvement in endocytosis is specifically mediated through PI(4,5)P2 levels on the plasma membrane. In accord with their effects on PI(4,5)P2 levels, the PD associated A30P, E46K, and A53T mutations in α-Syn further enhance CME in neuronal and nonneuronal cells. However, lysine to glutamic acid substitutions at the KTKEGV repeat domain of α-Syn, which interfere with phospholipid binding, are ineffective in enhancing CME. We further show that the rate of synaptic vesicle (SV) endocytosis is differentially affected by the α-Syn mutations and associates with their effects on PI(4,5)P2 levels, however, with the exception of the A30P mutation. This study provides evidence for a critical involvement of PIPs in α-Syn-mediated membrane trafficking.

Phosphatidylinositol 4,5-bisphosphate in the Control of Membrane Trafficking

Phosphoinositides are membrane lipids generated by phosphorylation on the inositol head group of phosphatidylinositol. By specifically distributed to distinct subcellular membrane locations, different phosphoinositide species play diverse roles in modulating membrane trafficking. Among the seven known phosphoinositide species, phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) is the one species most abundant at the plasma membrane. Thus, the PI4,5P₂ function in membrane trafficking is first identified in controlling plasma membrane dynamic-related events including endocytosis and exocytosis. However, recent studies indicate that PI4,5P₂ is also critical in many other membrane trafficking events such as endosomal trafficking, hydrolases sorting to lysosomes, autophagy initiation, and autophagic lysosome reformation. These findings suggest that the role of PI4,5P₂ in membrane trafficking is far beyond just plasma membrane. This review will provide a concise synopsis of how PI4,5P₂ functions in multiple membrane trafficking events. PI4,5P₂, the enzymes responsible for PI4,5P₂ production at specific subcellular locations, and distinct PI4,5P₂ effector proteins compose a regulation network to control the specific membrane trafficking events.

Signalling Pinpointed to the Tip: The Complex Regulatory Network That Allows Pollen Tube Growth

Plants display a complex life cycle, alternating between haploid and diploid generations. During fertilisation, the haploid sperm cells are delivered to the female gametophyte by pollen tubes, specialised structures elongating by tip growth, which is based on an equilibrium between cell wall-reinforcing processes and turgor-driven expansion. One important factor of this equilibrium is the rate of pectin secretion mediated and regulated by factors including the exocyst complex and small G proteins. Critically important are also non-proteinaceous molecules comprising protons, calcium ions, reactive oxygen species (ROS), and signalling lipids. Among the latter, phosphatidylinositol 4,5-bisphosphate and the kinases involved in its formation have been assigned important functions. The negatively charged headgroup of this lipid serves as an interaction point at the apical plasma membrane for partners such as the exocyst complex, thereby polarising the cell and its secretion processes. Another important signalling lipid is phosphatidic acid (PA), that can either be formed by the combination of phospholipases C and diacylglycerol kinases or by phospholipases D. It further fine-tunes pollen tube growth, for example by regulating ROS formation. How the individual signalling cues are intertwined or how external guidance cues are integrated to facilitate directional growth remain open questions.

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.

α-Synuclein facilitates endocytosis by elevating the steady-state levels of phosphatidylinositol 4,5-bisphosphate.. [DOI: 10.1101/2020.06.18.158709] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

α-Synuclein (α-Syn) is a protein implicated in the pathogenesis of Parkinson’s disease (PD). It is an intrinsically disordered protein that binds acidic phospholipids. Growing evidence supports a role for α-Syn in membrane trafficking, including, mechanisms of endocytosis and exocytosis, although the exact role of α-Syn in these mechanisms is currently unclear. Here we have investigated the role of α-Syn in membrane trafficking through its association with acidic phosphoinositides (PIPs), such as phosphatidylinositol 4,5-bisphosphate (PI4,5P2) and phosphatidylinositol 3,4-bisphosphate (PI3,4P2). Our results show that α-Syn colocalizes with PIP2 and the phosphorylated active form of the clathrin adaptor AP2 at clathrin-coated pits. Using endocytosis of transferrin, an indicator of clathrin mediated endocytosis (CME), we find that α-Syn involvement in endocytosis is specifically mediated through PI4,5P2 levels. We further show that the rate of synaptic vesicle (SV) endocytosis is differentially affected by α-Syn mutations. In accord with their effects on PI4,5P2 levels at the plasma membrane, the PD associated E46K and A53T mutations further enhance SV endocytosis. However, neither A30P mutation, nor Lysine to Glutamic acid substitutions at the KTKEGV repeat domain of α-Syn, that interfere with phospholipid binding, affect SV endocytosis. This study provides evidence for a critical involvement of PIPs in α-Syn-mediated membrane trafficking.

Significance Statement

α-Synuclein (α-Syn) protein is known for its causative role in Parkinson’s disease. α-Syn is normally involved in mechanisms of membrane trafficking, including endocytosis, exocytosis and synaptic vesicles cycling. However, a certain degree of controversy regarding the exact role of α-Syn in these mechanisms persists. Here we show that α-Syn acts to increase plasma membrane levels PI4,5P2 and PI3,4P2 to facilitate clathrin mediated and synaptic vesicles endocytosis. Based on the results, we suggest that α-Syn interactions with the acidic phosphoinositides facilitate a shift in their homeostasis to support endocytosis.

CAPS1 Suppresses Tumorigenesis in Cholangiocarcinoma

BACKGROUND

CAPS1 (calcium-dependent activator protein for secretion) is a multi-domain protein involved in regulating exocytosis of synaptic vesicles and dense-core vesicles. However, the expression and function of CAPS1 in cholangiocarcinoma (CCA) remains unclear. In the present study, we explored the role of CAPS1 in CCA carcinogenesis.

METHODS

CAPS1 expression was explored using western blotting and immunohistochemistry in four CCA cell lines and clinical samples from 90 cases of CCA. The clinical significance of CAPS1 was analyzed. The biological function of CAPS1 in CCA cells was detected in vitro and in vivo. The underlying mechanism of CAPS1 function was explored by detecting the expression of critical molecules in its associated signaling pathways. The mechanism of CAPS1 downregulation in tumor tissues was explored using in silico prediction and luciferase reporter assays.

RESULTS

CAPS1 expression was reduced in CCA cell lines and human tumor tissues. Loss of CAPS1 in tumor tissues was closely associated with poor prognosis of patients with CCA. Moreover, CAPS1 expression correlated significantly with tumor-node-metastasis stage, lymph node metastasis, and vascular invasion. Lentivirus-mediated CAPS1 overexpression substantially prevented clone formation, cell proliferation, and cell cycle progression. CAPS1 overexpression also suppressed carcinogenesis in nude mice. Mechanistically, CAPS1 overexpression greatly accelerated the ERK and p38 MAPK signal pathways. In addition, microRNA miR-30e-5p negatively regulated CAPS1 expression.

CONCLUSION

These data showed that CAPS1 functions as a tumor suppressor in CCA. Reduced CAPS1 expression could indicate poor prognosis of patients with CCA.

Phospholipases in neuronal function: A role in learning and memory?

Despite the human brain being made of nearly 60% fat, the vast majority of studies on the mechanisms of neuronal communication which underpin cognition, memory and learning, primarily focus on proteins and/or (epi)genetic mechanisms. Phospholipids are the main component of all cellular membranes and function as substrates for numerous phospholipid-modifying enzymes, including phospholipases, which release free fatty acids (FFAs) and other lipid metabolites that can alter the intrinsic properties of the membranes, recruit and activate critical proteins, and act as lipid signalling molecules. Here, we will review brain specific phospholipases, their roles in membrane remodelling, neuronal function, learning and memory, as well as their disease implications. In particular, we will highlight key roles of unsaturated FFAs, particularly arachidonic acid, in neurotransmitter release, neuroinflammation and memory. In light of recent findings, we will also discuss the emerging role of phospholipase A₁ and the creation of saturated FFAs in the brain.

Protein Amphipathic Helix Insertion: A Mechanism to Induce Membrane Fission

One of the fundamental features of biomembranes is the ability to fuse or to separate. These processes called respectively membrane fusion and fission are central in the homeostasis of events such as those related to intracellular membrane traffic. Proteins that contain amphipathic helices (AHs) were suggested to mediate membrane fission via shallow insertion of these helices into the lipid bilayer. Here we analyze the AH-containing proteins that have been identified as essential for membrane fission and categorize them in few subfamilies, including small GTPases, Atg proteins, and proteins containing either the ENTH/ANTH- or the BAR-domain. AH-containing fission-inducing proteins may require cofactors such as additional proteins (e.g., lipid-modifying enzymes), or lipids (e.g., phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], phosphatidic acid [PA], or cardiolipin). Both PA and cardiolipin possess a cone shape and a negative charge (-2) that favor the recruitment of the AHs of fission-inducing proteins. Instead, PtdIns(4,5)P2 is characterized by an high negative charge able to recruit basic residues of the AHs of fission-inducing proteins. Here we propose that the AHs of fission-inducing proteins contain sequence motifs that bind lipid cofactors; accordingly (K/R/H)(K/R/H)xx(K/R/H) is a PtdIns(4,5)P2-binding motif, (K/R)x6(F/Y) is a cardiolipin-binding motif, whereas KxK is a PA-binding motif. Following our analysis, we show that the AHs of many fission-inducing proteins possess five properties: (a) at least three basic residues on the hydrophilic side, (b) ability to oligomerize, (c) optimal (shallow) depth of insertion into the membrane, (d) positive cooperativity in membrane curvature generation, and (e) specific interaction with one of the lipids mentioned above. These lipid cofactors favor correct conformation, oligomeric state and optimal insertion depth. The most abundant lipid in a given organelle possessing high negative charge (more negative than -1) is usually the lipid cofactor in the fission event. Interestingly, naturally occurring mutations have been reported in AH-containing fission-inducing proteins and related to diseases such as centronuclear myopathy (amphiphysin 2), Charcot-Marie-Tooth disease (GDAP1), Parkinson's disease (α-synuclein). These findings add to the interest of the membrane fission process whose complete understanding will be instrumental for the elucidation of the pathogenesis of diseases involving mutations in the protein AHs.

Phosphatidic acid in membrane rearrangements

Phosphatidic acid (PA) is the simplest cellular glycerophospholipid characterized by unique biophysical properties: a small headgroup; negative charge; and a phosphomonoester group. Upon interaction with lysine or arginine, PA charge increases from -1 to -2 and this change stabilizes protein-lipid interactions. The biochemical properties of PA also allow interactions with lipids in several subcellular compartments. Based on this feature, PA is involved in the regulation and amplification of many cellular signalling pathways and functions, as well as in membrane rearrangements. Thereby, PA can influence membrane fusion and fission through four main mechanisms: it is a substrate for enzymes producing lipids (lysophosphatidic acid and diacylglycerol) that are involved in fission or fusion; it contributes to membrane rearrangements by generating negative membrane curvature; it interacts with proteins required for membrane fusion and fission; and it activates enzymes whose products are involved in membrane rearrangements. Here, we discuss the biophysical properties of PA in the context of the above four roles of PA in membrane fusion and fission.

The Caenorhabditis elegans Tubby homolog dynamically modulates olfactory cilia membrane morphogenesis and phospholipid composition

Plasticity in sensory signaling is partly mediated via regulated trafficking of signaling molecules to and from primary cilia. Tubby-related proteins regulate ciliary protein transport; however, their roles in remodeling cilia properties are not fully understood. We find that the C. elegans TUB-1 Tubby homolog regulates membrane morphogenesis and signaling protein transport in specialized sensory cilia. In particular, TUB-1 is essential for sensory signaling-dependent reshaping of olfactory cilia morphology. We show that compromised sensory signaling alters cilia membrane phosphoinositide composition via TUB-1-dependent trafficking of a PIP5 kinase. TUB-1 regulates localization of this lipid kinase at the cilia base in part via localization of the AP-2 adaptor complex subunit DPY-23. Our results describe new functions for Tubby proteins in the dynamic regulation of cilia membrane lipid composition, morphology, and signaling protein content, and suggest that this conserved family of proteins plays a critical role in mediating cilia structural and functional plasticity.

Synaptotagmin oligomers are necessary and can be sufficient to form a Ca2+ -sensitive fusion clamp

The buttressed‐ring hypothesis, supported by recent cryo‐electron tomography analysis of docked synaptic‐like vesicles in neuroendocrine cells, postulates that prefusion SNAREpins are stabilized and organized by Synaptotagmin (Syt) ring‐like oligomers. Here, we use a reconstituted single‐vesicle fusion analysis to test the prediction that destabilizing the Syt1 oligomers destabilizes the clamp and results in spontaneous fusion in the absence of Ca²⁺. Vesicles in which Syt oligomerization is compromised by a ring‐destabilizing mutation dock and diffuse freely on the bilayer until they fuse spontaneously, similar to vesicles containing only v‐SNAREs. In contrast, vesicles containing wild‐type Syt are immobile as soon as they attach to the bilayer and remain frozen in place, up to at least 1 h until fusion is triggered by Ca²⁺.

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.

Non-vesicular Lipid Transport Machinery in Entamoeba histolytica

Eukaryotic cells are organized into separate membrane-bound compartments that have specialized biochemical signature and function. Maintenance and regulation of distinct identity of each compartment is governed by the uneven distribution and intra-cellular movement of two essential biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid transfer proteins plays a pivotal role in intra-cellular lipid trafficking and homeostasis whereas vesicular transport plays a central role in protein trafficking. Comparative study of lipid transport machinery in protist helps to better understand the pathogenesis and parasitism, and provides insight into eukaryotic evolution. Amebiasis, which is caused by Entamoeba histolytica, is one of the major enteric infections in humans, resulting in 40–100 thousand deaths annually. This protist has undergone remarkable alterations in the content and function of its sub-cellular compartments as well represented by its unique diversification of mitochondrion-related organelle, mitosome. We conducted domain-based search on AmoebaDB coupled with bioinformatics analyses and identified 22 potential lipid transfer protein homologs in E. histolytica, which are grouped into several sub-classes. Such in silico analyses have demonstrated the existence of well-organized lipid transport machinery in this parasite. We summarized and discussed the conservation and unique features of the whole repertoire of lipid transport proteins in E. histolytica.

Phosphoinositides and their functions in apicomplexan parasites

Phosphoinositides are the phosphorylated derivatives of the structural membrane phospholipid phosphatidylinositol. Single or combined phosphorylation at the 3, 4 and 5 positions of the inositol ring gives rise to the seven different species of phosphoinositides. All are quantitatively minor components of cellular membranes but have been shown to have important functions in multiple cellular processes. Here we describe our current knowledge of phosphoinositide metabolism and functions in apicomplexan parasites, mainly focusing on Toxoplasma gondii and Plasmodium spp. Even though our understanding is still rudimentary, phosphoinositides have already shown their importance in parasite biology and revealed some very particular and parasite-specific functions. Not surprisingly, there is a strong potential for phosphoinositide synthesis to be exploited for future anti-parasitic drug development.

The Dual Function of the Polybasic Juxtamembrane Region of Syntaxin 1A in Clamping Spontaneous Release and Stimulating Ca2+-Triggered Release in Neuroendocrine Cells

The exact function of the polybasic juxtamembrane region (5RK) of the plasma membrane neuronal SNARE, syntaxin 1A (Syx), in vesicle exocytosis, although widely studied, is currently not clear. Here, we addressed the role of 5RK in Ca²⁺-triggered release, using our Syx-based intramolecular fluorescence resonance energy transfer (FRET) probe, which previously allowed us to resolve a depolarization-induced Ca²⁺-dependent close-to-open transition (CDO) of Syx that occurs concomitant with evoked release, both in PC12 cells and hippocampal neurons and was abolished upon charge neutralization of 5RK. First, using dynamic FRET analysis in PC12 cells, we show that CDO occurs following assembly of SNARE complexes that include the vesicular SNARE, synaptobrevin 2, and that the participation of 5RK in CDO goes beyond its participation in the final zippering of the complex, because mutations of residues adjacent to 5RK, believed to be crucial for final zippering, do not abolish this transition. In addition, we show that CDO is contingent on membrane phosphatidylinositol 4,5-bisphosphate (PIP2), which is fundamental for maintaining regulated exocytosis, as depletion of membranal PIP2 abolishes CDO. Prompted by these results, which underscore a potentially significant role of 5RK in exocytosis, we next amperometrically analyzed catecholamine release from PC12 cells, revealing that charge neutralization of 5RK promotes spontaneous and inhibits Ca²⁺-triggered release events. Namely, 5RK acts as a fusion clamp, making release dependent on stimulation by Ca²⁺SIGNIFICANCE STATEMENT Syntaxin 1A (Syx) is a central protein component of the SNARE complex, which underlies neurotransmitter release. Although widely studied in relation to its participation in SNARE complex formation and its interaction with phosphoinositides, the function of Syx's polybasic juxtamembrane region (5RK) remains unclear. Previously, we showed that a conformational transition of Syx, related to calcium-triggered release, reported by a Syx-based FRET probe, is abolished upon charge neutralization of 5RK (5RK/A). Here we show that this conformational transition is dependent on phosphatidylinositol 4,5-bisphosphate (PIP2) and is related to SNARE complex formation. Subsequently, we show that the 5RK/A mutation enhances spontaneous release and inhibits calcium-triggered release in neuroendocrine cells, indicating a previously unrecognized role of 5RK in neurotransmitter release.

Fusion pore in exocytosis: More than an exit gate? A β-cell perspective

Secretory vesicle exocytosis is a fundamental biological event and the process by which hormones (like insulin) are released into the blood. Considerable progress has been made in understanding this precisely orchestrated sequence of events from secretory vesicle docked at the cell membrane, hemifusion, to the opening of a membrane fusion pore. The exact biophysical and physiological regulation of these events implies a close interaction between membrane proteins and lipids in a confined space and constrained geometry to ensure appropriate delivery of cargo. We consider some of the still open questions such as the nature of the initiation of the fusion pore, the structure and the role of the Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor (SNARE) transmembrane domains and their influence on the dynamics and regulation of exocytosis. We discuss how the membrane composition and protein-lipid interactions influence the likelihood of the nascent fusion pore forming. We relate these factors to the hypothesis that fusion pore expansion could be affected in type-2 diabetes via changes in disease-related gene transcription and alterations in the circulating lipid profile. Detailed characterisation of the dynamics of the fusion pore in vitro will contribute to understanding the larger issue of insulin secretory defects in diabetes.

Membrane Order Is a Key Regulator of Divalent Cation-Induced Clustering of PI(3,5)P2 and PI(4,5)P2

Although the evidence for the presence of functionally important nanosized phosphorylated phosphoinositide (PIP)-rich domains within cellular membranes has accumulated, very limited information is available regarding the structural determinants for compartmentalization of these phospholipids. Here, we used a combination of fluorescence spectroscopy and microscopy techniques to characterize differences in divalent cation-induced clustering of PI(4,5)P₂ and PI(3,5)P₂. Through these methodologies we were able to detect differences in divalent cation-induced clustering efficiency and cluster size. Ca²⁺-induced PI(4,5)P₂ clusters are shown to be significantly larger than the ones observed for PI(3,5)P₂. Clustering of PI(4,5)P₂ is also detected at physiological concentrations of Mg²⁺, suggesting that in cellular membranes, these molecules are constitutively driven to clustering by the high intracellular concentration of divalent cations. Importantly, it is shown that lipid membrane order is a key factor in the regulation of clustering for both PIP isoforms, with a major impact on cluster sizes. Clustered PI(4,5)P₂ and PI(3,5)P₂ are observed to present considerably higher affinity for more ordered lipid phases than the monomeric species or than PI(4)P, possibly reflecting a more general tendency of clustered lipids for insertion into ordered domains. These results support a model for the description of the lateral organization of PIPs in cellular membranes, where both divalent cation interaction and membrane order are key modulators defining the lateral organization of these lipids.

Hypothesis - buttressed rings assemble, clamp, and release SNAREpins for synaptic transmission

Neural networks are optimized to detect temporal coincidence on the millisecond timescale. Here, we offer a synthetic hypothesis based on recent structural insights into SNAREs and the C2 domain proteins to explain how synaptic transmission can keep this pace. We suggest that an outer ring of up to six curved Munc13 ‘MUN’ domains transiently anchored to the plasma membrane via its flanking domains surrounds a stable inner ring comprised of synaptotagmin C2 domains to serve as a work‐bench on which SNAREpins are templated. This ‘buttressed‐ring hypothesis’ affords straightforward answers to many principal and long‐standing questions concerning how SNAREpins can be assembled, clamped, and then released synchronously with an action potential.

Phosphatidylinositol 4,5-bisphosphate optical uncaging potentiates exocytosis

Phosphatidylinositol-4,5-bisphosphate [PI(4,5)P₂] is essential for exocytosis. Classical ways of manipulating PI(4,5)P₂ levels are slower than its metabolism, making it difficult to distinguish effects of PI(4,5)P₂ from those of its metabolites. We developed a membrane-permeant, photoactivatable PI(4,5)P₂, which is loaded into cells in an inactive form and activated by light, allowing sub-second increases in PI(4,5)P₂ levels. By combining this compound with electrophysiological measurements in mouse adrenal chromaffin cells, we show that PI(4,5)P₂ uncaging potentiates exocytosis and identify synaptotagmin-1 (the Ca²⁺ sensor for exocytosis) and Munc13-2 (a vesicle priming protein) as the relevant effector proteins. PI(4,5)P₂ activation of exocytosis did not depend on the PI(4,5)P₂-binding CAPS-proteins, suggesting that PI(4,5)P₂ uncaging may bypass CAPS-function. Finally, PI(4,5)P₂ uncaging triggered the rapid fusion of a subset of readily-releasable vesicles, revealing a rapid role of PI(4,5)P₂ in fusion triggering. Thus, optical uncaging of signaling lipids can uncover their rapid effects on cellular processes and identify lipid effectors.

Cells in our body communicate by releasing compounds called transmitters that carry signals from one cell to the next. Packages called vesicles store transmitters within the signaling cell. When the cell needs to send a signal, the vesicles fuse with the cell's membrane and release their cargo. For many signaling processes, such as those used by neurons, this fusion is regulated, fast, and coupled to the signal that the cell receives to activate release. Specialized molecular machines made up of proteins and fatty acid molecules called signaling lipids enable this to happen.

One signaling lipid called PI(4,5)P₂ (short for phosphatidylinositol 4,5-bisphosphate) is essential for vesicle fusion as well as for other processes in cells. It interacts with several proteins that help it control fusion and the release of transmitter. While it is possible to study the role of these proteins using genetic tools to inactivate them, the signaling lipids are more difficult to manipulate. Existing methods result in slow changes in PI(4,5)P₂ levels, making it hard to directly attribute later changes to PI(4,5)P₂.

Walter, Müller, Tawfik et al. developed a new method to measure how PI(4,5)P₂ affects transmitter release in living mammalian cells, which causes a rapid increase in PI(4,5)P₂ levels. The method uses a chemical compound called “caged PI(4,5)P₂” that can be loaded into cells but remains undetected until ultraviolet light is shone on it. The ultraviolet light uncages the compound, generating active PI(4,5)P₂ in less than one second. Walter et al. found that when they uncaged PI(4,5)P₂ in this way, the amount of transmitter released by cells increased. Combining this with genetic tools, it was possible to investigate which proteins of the release machinery were required for this effect.

The results suggest that two different types of proteins that interact with PI(4,5)P₂ are needed: one must bind PI(4,5)P₂ to carry out its role and the other helps PI(4,5)P₂ accumulate at the site of vesicle fusion. The new method also allowed Walter et al. to show that a fast increase in PI(4,5)P₂ triggers a subset of vesicles to fuse very rapidly. This shows that PI(4,5)P₂ rapidly regulates the release of transmitter. Caged PI(4,5)P₂ will be useful to study other processes in cells that need PI(4,5)P₂, helping scientists understand more about how signaling lipids control many different events at cellular membranes.

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.

Annexins A2 and A8 in endothelial cell exocytosis and the control of vascular homeostasis

Blood vessel homeostasis is controlled by a variety of regulatory circuits that involve both the vessel-lining endothelial cells as well as the circulating blood cells and products thereof. One important feature is the control exerted by endothelial cells through regulated exocytosis of factors affecting blood coagulation and local inflammatory processes. These factors include two important adhesion proteins: the leukocyte receptor P-selectin and the pro-coagulant von Willebrand factor (VWF) that binds platelets and is involved in the formation of a platelet plug at sites of blood vessel injury. Failure to correctly produce and secrete P-selectin and VWF leads to pathologies such as von Willebrand disease, the most common inherited bleeding disorder. P-selectin and VWF are stored in unique secretory granules, the Weibel-Palade bodies (WPB), that undergo a complex maturation process and are acutely secreted following endothelial stimulation, e.g. in the course of inflammation or following blood vessel injury. Two annexins have been shown to be involved in different aspects of WPB biology: annexin A8 is required for proper WPB maturation and annexin A2 participates in late steps of WPB exocytosis. Thus, by affecting the stimulated release of P-selectin and VWF from endothelial cells, annexins fulfil important functions in the control of vascular homeostasis and could be considered as targets for influencing P-selectin- and VWF-dependent processes/pathologies.

Fission yeast myosin I facilitates PI(4,5)P2-mediated anchoring of cytoplasmic dynein to the cortex

Several key processes in the cell, such as vesicle transport and spindle positioning, are mediated by the motor protein cytoplasmic dynein, which produces force on the microtubule. For the functions that require movement of the centrosome and the associated nuclear material, dynein needs to have a stable attachment at the cell cortex. In fission yeast, Mcp5 is the anchor protein of dynein and is required for the oscillations of the horsetail nucleus during meiotic prophase. Although the role of Mcp5 in anchoring dynein to the cortex has been identified, it is unknown how Mcp5 associates with the membrane as well as the importance of the underlying attachment to the nuclear oscillations. Here, we set out to quantify Mcp5 organization and identify the binding partner of Mcp5 at the membrane. We used confocal and total internal reflection fluorescence microscopy to count the number of Mcp5 foci and the number of Mcp5 molecules in an individual focus. Further, we quantified the localization pattern of Mcp5 in fission yeast zygotes and show by perturbation of phosphatidylinositol 4-phosphate 5-kinase that Mcp5 binds to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. Remarkably, we discovered that the myosin I protein in fission yeast, Myo1, which is required for organization of sterol-rich domains in the cell membrane, facilitates the localization of Mcp5 and that of cytoplasmic dynein on the membrane. Finally, we demonstrate that Myo1-facilitated association of Mcp5 and dynein to the membrane determines the dynamics of nuclear oscillations and, in essence, dynein activity.

Ring-like oligomers of Synaptotagmins and related C2 domain proteins

We recently reported that the C2AB portion of Synaptotagmin 1 (Syt1) could self-assemble into Ca(2+)-sensitive ring-like oligomers on membranes, which could potentially regulate neurotransmitter release. Here we report that analogous ring-like oligomers assemble from the C2AB domains of other Syt isoforms (Syt2, Syt7, Syt9) as well as related C2 domain containing protein, Doc2B and extended Synaptotagmins (E-Syts). Evidently, circular oligomerization is a general and conserved structural aspect of many C2 domain proteins, including Synaptotagmins. Further, using electron microscopy combined with targeted mutations, we show that under physiologically relevant conditions, both the Syt1 ring assembly and its rapid disruption by Ca(2+) involve the well-established functional surfaces on the C2B domain that are important for synaptic transmission. Our data suggests that ring formation may be triggered at an early step in synaptic vesicle docking and positions Syt1 to synchronize neurotransmitter release to Ca(2+) influx.

F-actin waves, actin cortex disassembly and focal exocytosis driven by actin-phosphoinositide positive feedback

Actin polymerization is controlled by the phosphoinositide composition of the plasma membrane. However, the molecular mechanisms underlying the spatiotemporal regulation of actin network organization over extended length scales are still unclear. To observe phosphoinositide-dependent cytoskeletal dynamics we combined the model system of frustrated phagocytosis, total internal reflection microscopy and manipulation of the buffer tonicity. We found that macrophages interacting with IgG-coated glass substrates formed circular F-actin waves on their ventral surface enclosing a region of plasma membrane devoid of cortical actin. Plasma membrane free of actin cortex was strongly depleted of PI(4,5)P2 , but enriched in PI(3,4)P2 and displayed a fivefold increase in exocytosis. Wave formation could be promoted by application of a hypotonic shock. The actin waves were characteristic of a bistable wavefront at the boundary between the regions of membrane containing and lacking cortical actin. Phosphoinositide modifiers and RhoGTPase activities dramatically redistributed with respect to the wavefronts, which often exhibited spatial oscillations. Perturbation of either lipid or actin cytoskeleton-related pathways led to rapid loss of both the polarized lipid distribution and the wavefront. As waves travelled over the plasma membrane, wavefront actin was seen to rapidly polymerize and depolymerize at pre-existing clusters of FcγRIIA, coincident with rapid changes in lipid composition. Thus the potential of receptors to support rapid F-actin polymerization appears to depend acutely on the local concentrations of multiple lipid species. We propose that interdependence through positive feedback from the cytoskeleton to lipid modifiers leads to coordinated local cortex remodeling, focal exocytosis, and organizes extended actin networks.