Diacylglycerol induces fusion of nuclear envelope membrane precursor vesicles

Triglycerides Stabilize Water/Organic Interfaces of Changing Area via Conformational Flexibility

The role of triglycerides (TGs) in both natural and synthetic biological membranes has long been the subject of study, involving metabolism, disease, and colloidal synthesis. TGs have been found to be critical components for successful liposomal encapsulation via a water/oil/water double emulsion, which this work endeavors to explain. TGs can occupy multiple positions in biological membranes. The glycerol backbone can reside at the water/organic interface, adjacent to phospholipid headgroups ("m" conformation), typically with relatively low (<3%) solubility. The glycerol backbone can also occupy hydrophobic regions, where it is isolated from water ("h" or "oil" conformation). This can occur in either midmembrane positions or phospholipid-coated lipid droplets (LDs). These conformations can be distinguished using 13C-nuclear magnetic resonance spectroscopy (NMR), which determines the degree of hydration of the TG backbone. Using this method, it was revealed that TGs transition from "m" to "h" conformation as the organic solvent is removed via evaporation. A new transitional TG backbone position has been identified with a level of hydration between "m" and "h". These results suggest that TGs can temporarily coat and stabilize the large water/organic interfaces present after emulsification. As the organic solvent is removed and interfaces shrink, the TGs recede into midmembrane spaces or bud off into LDs, which are confirmed via transmission electron microscopy (TEM) and can be removed via centrifugation. Encapsulation efficiency is found to be inversely related to both the saturation and length of the TG acyl chains, indicating that membrane fluidization is a key property arising from the presence of TGs. Beyond clarification of a mechanism for high-efficiency liposomal encapsulation, these results implicate TGs as components that are able to stabilize biological membrane transitions involving a changing interfacial area and curvature. This role for TGs may be of use in the formulation of drug delivery systems as well as in the investigation of membrane transitions in life sciences.

Penalized Variable Selection for Lipid-Environment Interactions in a Longitudinal Lipidomics Study

Lipid species are critical components of eukaryotic membranes. They play key roles in many biological processes such as signal transduction, cell homeostasis, and energy storage. Investigations of lipid-environment interactions, in addition to the lipid and environment main effects, have important implications in understanding the lipid metabolism and related changes in phenotype. In this study, we developed a novel penalized variable selection method to identify important lipid-environment interactions in a longitudinal lipidomics study. An efficient Newton-Raphson based algorithm was proposed within the generalized estimating equation (GEE) framework. We conducted extensive simulation studies to demonstrate the superior performance of our method over alternatives, in terms of both identification accuracy and prediction performance. As weight control via dietary calorie restriction and exercise has been demonstrated to prevent cancer in a variety of studies, analysis of the high-dimensional lipid datasets collected using 60 mice from the skin cancer prevention study identified meaningful markers that provide fresh insight into the underlying mechanism of cancer preventive effects.

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.

The Use of Two-Photon FRET-FLIM to Study Protein Interactions During Nuclear Envelope Fusion In Vivo and In Vitro

FRET-FLIM techniques have wide application in the study of protein and protein-lipid interactions in cells. We have pioneered an imaging platform for accurate detection of functional states of proteins and their interactions in fixed cells. This platform, two-site-amplified Förster resonance energy transfer (a-FRET), allows greater signal generation while retaining minimal noise thus enabling application of fluorescence lifetime imaging microscopy (FLIM) to be routinely deployed in different types of cells and tissue. We have used the method described here, time-resolved FRET monitored by two-photon FLIM, to demonstrate the direct interaction of Phospholipase Cγ (PLCγ) by Src Family Kinase 1 (SFK1) during nuclear envelope formation and during male and female pronuclear membrane fusion in fertilized sea urchin eggs. We describe here a generic method that can be applied to monitor any proteins of interest.

Conservation of proteo-lipid nuclear membrane fusion machinery during early embryogenesis

The fusogenic lipid diacylglycerol is essential for remodeling gamete and zygote nuclear envelopes (NE) during early embryogenesis. It is unclear whether upstream signaling molecules are likewise conserved. Here we demonstrate PLCγ and its activator SFK1, which co-operate during male pronuclear envelope formation, also promote the subsequent male and female pronuclear fusion. PLCγ and SFK1 interact directly at the fusion site leading to PLCγ activation. This is accompanied by a spatially restricted reduction of PtdIns(4,5)P2. Consequently, pronuclear fusion is blocked by PLCγ or SFK1 inhibition. These findings identify new regulators of events in the early embryo and suggest a conserved "toolkit" of fusion machinery drives successive NE fusion events during embryogenesis.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

Lipidomic profiling of di- and tri-acylglycerol species in weight-controlled mice

Weight control by dietary calorie restriction (DCR) or exercise has been shown to prevent cancer in various models. However, the mechanisms as to how weight control is beneficial are not well understood. While previous reports have investigated the effects of weight control on total lipid levels or lipid composition within cellular membranes, there has been little work surrounding changes to individual lipids following weight control interventions. In this study, using a model of skin carcinogenesis centered on the tumor promotion stage, CD-1 mice were randomly assigned into 4 groups: ad libitum and sedentary (control), ad libitum with exercise (AL+Exe), exercise with pair feeding of a diet isocaloric with control (PF+Exe), and sedentary with 20% DCR compared to control. After ten weeks, body weight and body fat percentages significantly decreased in the PF+Exe and DCR groups but not AL+Exe when compared with sedentary controls. Murine skin and plasma samples were obtained for analysis. Lipidomics using electrospray ionization MS/MS was employed to profile triacylglycerol (TG) and diacylglycerol (DG) species. Both plasma and tissue TG species containing fatty acid chains with length 18:1 were significantly decreased following DCR when compared to sedentary control animals. In regards to DG, the most significant changes occurred in the plasma. DG species containing fatty acids with lengths 16:1 or 18:1 were significantly decreased in PF+Exe and DCR groups when compared to sedentary controls. Due to the significant role of TG in energy storage and DG in cellular signaling, our findings of the effects of weight control on individual TG and DG species in plasma and skin tissue following exposure to a tumor promoter, may provide insight into the mechanism of weight control on cancer prevention.

Characterization and quantification of diacylglycerol species in biological extracts after one-step derivatization: a shotgun lipidomics approach

Diacylglycerols (DAGs) are important intermediates of lipid metabolism and cellular signaling. It is well-known that the mass levels of DAG are altered under disease states. Therefore, quantitative analysis of DAGs in biological samples can provide critical information to uncover underlying mechanisms of various cellular functional disorders. Although great efforts on the analysis of individual DAG species have recently been made by utilizing mass spectrometry with or without derivatization, cost-effective and high throughput methodologies for identification and quantification of all DAG species including regioisomers, particularly in an approach of shotgun lipidomics, are still missing. Herein, we described a novel method for directly identifying and quantifying DAG species including regioisomers present in lipid extracts of biological samples after facile one-step derivatization with dimethylglycine based on the principles of multidimensional mass spectrometry-based shotgun lipidomics. The established method provided substantial sensitivity (low limit of quantification at amol/μL), high specificity, and broad linear dynamics range (2500-fold) without matrix effects. By exploiting this novel method, we revealed a 16-fold increase of total DAG mass in the livers of ob/ob mice compared to their wild type controls at 4 months of age (an insulin-resistant state) versus a 5-fold difference between 3 month old mice (with normal insulin). These results demonstrated the importance and power of the method for studying biochemical mechanisms underpinning disease states.

A structural role for lipids in organelle shaping

The importance of proteins in shaping the membranes that define the perimeters of organelles is well documented. By forming cross-links, motors, or scaffolds or by inserting into membranes, proteins can harness energy to deform membranes, particularly when high degrees of curvature are necessitated-as in small membrane vesicles, tubules of the endoplasmic reticulum, the edges of endoplasmic reticulum sheets or Golgi apparatus cisternae, and membrane fusion intermediates (stalks). Here we propose that membrane lipids displaying negative curvature act in concert with membrane proteins to contribute to the alteration and maintenance of bending in biological membranes. We emphasize recent data from studies of sea urchin eggs and embryos and suggest how novel approaches can lead to future directions for investigating the roles of such lipids in vivo.

Acute manipulation of diacylglycerol reveals roles in nuclear envelope assembly & endoplasmic reticulum morphology

The functions and morphology of cellular membranes are intimately related and depend not only on their protein content but also on the repertoire of lipids that comprise them. In the absence of in vivo data on lipid asymmetry in endomembranes, it has been argued that motors, scaffolding proteins or integral membrane proteins rather than non-lamellar bilayer lipids such as diacylglycerol (DAG), are responsible for shaping of organelles, local membrane curvature and fusion. The effects of direct alteration of levels of such lipids remain predominantly uninvestigated. Diacylglycerol (DAG) is a well documented second messenger. Here we demonstrate two additional conserved functions of DAG: a structural role in organelle morphology, and a role in localised extreme membrane curvature required for fusion for which proteins alone are insufficient. Acute and inducible DAG depletion results in failure of the nuclear envelope (NE) to reform at mitosis and reorganisation of the ER into multi-lamellar sheets as revealed by correlative light and electron microscopy and 3D reconstructions. Remarkably, depleted cells divide without a complete NE, and unless rescued by 1,2 or 1,3 DAG soon die. Attenuation of DAG levels by enzyme microinjection into echinoderm eggs and embryos also results in alterations of ER morphology and nuclear membrane fusion. Our findings demonstrate that DAG is an in vivo modulator of organelle morphology in mammalian and echinoderm cells, indicating a fundamental role conserved across the deuterostome superphylum.

Dynamics of PLCγ and Src family kinase 1 interactions during nuclear envelope formation revealed by FRET-FLIM

The nuclear envelope (NE) breaks down and reforms during each mitotic cycle. A similar process happens to the sperm NE following fertilisation. The formation of the NE in both these circumstances involves endoplasmic reticulum membranes enveloping the chromatin, but PLCγ-dependent membrane fusion events are also essential. Here we demonstrate the activation of PLCγ by a Src family kinase (SFK1) during NE assembly. We show by time-resolved FRET for the first time the direct in vivo interaction and temporal regulation of PLCγ and SFK1 in sea urchins. As a prerequisite for protein activation, there is a rapid phosphorylation of PLCγ on its Y783 residue in response to GTP in vitro. This phosphorylation is dependent upon SFK activity; thus Y783 phosphorylation and NE assembly are susceptible to SFK inhibition. Y783 phosphorylation is also observed on the surface of the male pronucleus (MPN) in vivo during NE formation. Together the corroborative in vivo and in vitro data demonstrate the phosphorylation and activation of PLCγ by SFK1 during NE assembly. We discuss the potential generality of such a mechanism.

Diacylglycerol stimulates acrosomal exocytosis by feeding into a PKC- and PLD1-dependent positive loop that continuously supplies phosphatidylinositol 4,5-bisphosphate

Acrosomal exocytosis involves a massive fusion between the outer acrosomal and the plasma membranes of the spermatozoon triggered by stimuli that open calcium channels at the plasma membrane. Diacylglycerol has been implicated in the activation of these calcium channels. Here we report that this lipid promotes the efflux of intraacrosomal calcium and triggers exocytosis in permeabilized human sperm, implying that diacylglycerol activates events downstream of the opening of plasma membrane channels. Furthermore, we show that calcium and diacylglycerol converge in a signaling pathway leading to the production of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Addition of diacylglycerol promotes the PKC-dependent activation of PLD1. Rescue experiments adding phosphatidic acid or PIP(2) and direct measurement of lipid production suggest that both PKC and PLD1 promote PIP(2) synthesis. Inhibition of different steps of the pathway was reverted by adenophostin, an agonist of IP(3)-sensitive calcium channels, indicating that PIP(2) is necessary to keep these channels opened. However, phosphatidic acid, PIP(2), or adenophostin could not trigger exocytosis by themselves, indicating that diacylglycerol must also activate another factor. We found that diacylglycerol and phorbol ester stimulate the accumulation of the GTP-bound form of Rab3A. Together our results indicate that diacylglycerol promotes acrosomal exocytosis by i) maintaining high levels of IP(3) - an effect that depends on a positive feedback loop leading to the production of PIP(2) - and ii) stimulating the activation of Rab3A, which in turn initiates a cascade of protein interactions leading to the assembly of SNARE complexes and membrane fusion.

A role for phosphoinositides in regulating plant nuclear functions

Nuclear localized inositol phospholipids and inositol phosphates are important for regulating many essential processes in animal and yeast cells such as DNA replication, recombination, RNA processing, mRNA export and cell cycle progression. An overview of the current literature indicates the presence of a plant nuclear phosphoinositide (PI) pathway. Inositol phospholipids, inositol phosphates, and enzymes of the PI pathway have been identified in plant nuclei and are implicated in DNA replication, chromatin remodeling, stress responses and hormone signaling. In this review, the potential functions of the nuclear PI pathway in plants are discussed within the context of the animal and yeast literature. It is anticipated that future research will help shed light on the functional significance of the nuclear PI pathway in plants.

Key role of polyphosphoinositides in dynamics of fusogenic nuclear membrane vesicles

The role of phosphoinositides has been thoroughly described in many signalling and membrane trafficking events but their function as modulators of membrane structure and dynamics in membrane fusion has not been investigated. We have reconstructed models that mimic the composition of nuclear envelope precursor membranes with naturally elevated amounts of phosphoinositides. These fusogenic membranes (membrane vesicle 1(MV1) and nuclear envelope remnants (NER) are critical for the assembly of the nuclear envelope. Phospholipids, cholesterol, and polyphosphoinositides, with polyunsaturated fatty acid chains that were identified in the natural nuclear membranes by lipid mass spectrometry, have been used to reconstruct complex model membranes mimicking nuclear envelope precursor membranes. Structural and dynamic events occurring in the membrane core and at the membrane surface were monitored by solid-state deuterium and phosphorus NMR. "MV1-like" (PC∶PI∶PIP∶PIP(2), 30∶20∶18∶12, mol%) membranes that exhibited high levels of PtdIns, PtdInsP and PtdInsP(2) had an unusually fluid membrane core (up to 20% increase, compared to membranes with low amounts of phosphoinositides to mimic the endoplasmic reticulum). "NER-like" (PC∶CH∶PI∶PIP∶PIP(2), 28∶42∶16∶7∶7, mol%) membranes containing high amounts of both cholesterol and phosphoinositides exhibited liquid-ordered phase properties, but with markedly lower rigidity (10-15% decrease). Phosphoinositides are the first lipids reported to counterbalance the ordering effect of cholesterol. At the membrane surface, phosphoinositides control the orientation dynamics of other lipids in the model membranes, while remaining unchanged themselves. This is an important finding as it provides unprecedented mechanistic insight into the role of phosphoinositides in membrane dynamics. Biological implications of our findings and a model describing the roles of fusogenic membrane vesicles are proposed.

Spatial regulation of membrane fusion controlled by modification of phosphoinositides

Membrane fusion plays a central role in many cell processes from vesicular transport to nuclear envelope reconstitution at mitosis but the mechanisms that underlie fusion of natural membranes are not well understood. Studies with synthetic membranes and theoretical considerations indicate that accumulation of lipids characterised by negative curvature such as diacylglycerol (DAG) facilitate fusion. However, the specific role of lipids in membrane fusion of natural membranes is not well established. Nuclear envelope (NE) assembly was used as a model for membrane fusion. A natural membrane population highly enriched in the enzyme and substrate needed to produce DAG has been isolated and is required for fusions leading to nuclear envelope formation, although it contributes only a small amount of the membrane eventually incorporated into the NE. It was postulated to initiate and regulate membrane fusion. Here we use a multidisciplinary approach including subcellular membrane purification, fluorescence spectroscopy and Förster resonance energy transfer (FRET)/two-photon fluorescence lifetime imaging microscopy (FLIM) to demonstrate that initiation of vesicle fusion arises from two unique sites where these vesicles bind to chromatin. Fusion is subsequently propagated to the endoplasmic reticulum-derived membranes that make up the bulk of the NE to ultimately enclose the chromatin. We show how initiation of multiple vesicle fusions can be controlled by localised production of DAG and propagated bidirectionally. Phospholipase C (PLCγ), GTP hydrolysis and (phosphatidylinsositol-(4,5)-bisphosphate (PtdIns(4,5)P₂) are required for the latter process. We discuss the general implications of membrane fusion regulation and spatial control utilising such a mechanism.

Nuclear envelope formation: mind the gaps

During mitosis in metazoans, the nuclear envelope (NE) breaks down at prophase and reassembles at telophase. The regulation of NE assembly is essential to correct cell functioning. The complex issue of the regulation of NE formation remains to be solved. It is still uncertain that a single mechanism depicts NE formation during mitosis. The aim of this review is to address some of the cytological, biophysical, and molecular aspects of models of NE formation. Our emphasis is on the role of lipids and their modifying enzymes in envelope assembly. We consider how the NE can be used as a model in characterizing membrane dynamics during membrane fusion. Fusion mechanisms that give insight into the formation of the double membrane of the envelope are summarized. We speculate on the possible roles of phosphoinositides in membrane fusion and NE formation.

Lipid signaling on the mitochondrial surface

Regulated production and elimination of the signaling lipids phosphatidic acid (PA), diacylglycerol (DAG), and phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) creates a complex and interconnected signaling network that modulates a wide variety of eukaryotic cell biological events. PA production at the plasma membrane and on trafficking membrane organelles by classical Phospholipase D (PLD) through the hydrolysis of phosphatidylcholine (PC) has been studied widely. In this chapter, we review a newly identified, non-canonical member of the PLD superfamily, MitoPLD, which localizes to the mitochondrial surface and plays a role in mitochondrial fusion via the hydrolysis of cardiolipin (CL) to generate PA. The role of PA in facilitating the mitochondrial fusion event carried out by proteins known as Mitofusins is intriguing in light of the role classic PLD-generated PA plays in facilitating SNARE-mediated fusion of secretory membrane vesicles into the plasma membrane. In addition, however, PA on the mitochondrial surface may also trigger a signaling cascade that elevates DAG, leading to downstream events that affect mitochondrial fission and energy production. PA production on the mitochondrial surface may also stimulate local production of PI4,5P(2) to facilitate mitochondrial fission and subcellular trafficking or facilitate Ca(2+) influx.

Phosphatidylinositol metabolism and membrane fusion

Membrane fusion underlies many cellular events, including secretion, exocytosis, endocytosis, organelle reconstitution, transport from endoplasmic reticulum to Golgi and nuclear envelope formation. A large number of investigations into membrane fusion indicate various roles for individual members of the phosphoinositide class of membrane lipids. We first review the phosphoinositides as membrane recognition sites and their regulatory functions in membrane fusion. We then consider how modulation of phosphoinositides and their products may affect the structure and dynamics of natural membranes facilitating fusion. These diverse roles underscore the importance of these phospholipids in the fusion of biological membranes.

Nuclear envelope remnants: fluid membranes enriched in sterols and polyphosphoinositides

Background

The cytoplasm of eukaryotic cells is a highly dynamic compartment where membranes readily undergo fission and fusion to reorganize the cytoplasmic architecture, and to import, export and transport various cargos within the cell. The double membrane of the nuclear envelope that surrounds the nucleus, segregates the chromosomes from cytoplasm and regulates nucleocytoplasmic transport through pores. Many details of its formation are still unclear. At fertilization the sperm devoid of nuclear envelope pores enters the egg. Although most of the sperm nuclear envelope disassembles, remnants of the envelope at the acrosomal and centriolar fossae do not and are subsequently incorporated into the newly forming male pronuclear envelope. Remnants are conserved from annelid to mammalian sperm.

Methodology/Principal Findings

Using lipid mass spectrometry and a new application of deuterium solid-state NMR spectroscopy we have characterized the lipid composition and membrane dynamics of the sperm nuclear envelope remnants in isolated sperm nuclei.

Conclusions/Significance

We report nuclear envelope remnants are relatively fluid membranes rich in sterols, devoid of sphingomyelin, and highly enriched in polyphosphoinositides and polyunsaturated phospholipids. The localization of the polybasic effector domain of MARCKS illustrates the non-nuclear aspect of the polyphosphoinositides. Based on their atypical biophysical characteristics and phospholipid composition, we suggest a possible role for nuclear envelope remnants in membrane fusion leading to nuclear envelope assembly.

Lipid levels in sperm, eggs, and during fertilization in Xenopus laevis

Critical developmental periods, such as fertilization, involve metabolic activation, membrane fusion events such as sperm-egg or plasma membrane-cortical granule merger, and production and hydrolysis of phospholipids. However, there has been no large-scale quantification of phospholipid changes during fertilization. Using an enzymatic assay, traditional FA analysis by TLC and gas chromatography, along with a new method of phospholipid measurement involving HPLC separation and evaporative light-scattering detection, we report lipid levels in eggs, sperm, and during fertilization in Xenopus laevis. Sperm were found to contain different amounts of phospholipids as compared with eggs. During fertilization, total phosphatidylinositol, lysophosphatidylcholine, sphingomyelin, and phosphatidylserine decreased, and ceramide increased, whereas there was no change in phosphatidylcholine, cardiolipin, or phosphatidylethanolamine. FA analysis of phospholipids found numerous changes during fertilization. Because there is an increase in sn-1,2-diacylglycerol at fertilization, the FAs associated with this increase and the source of the increase in this neutral lipid were examined. Finally, activation of phospholipase C, phospholipase D, phospholipase A2, autotoxin, and sphingomyelinase at fertilization is discussed.

A role for diacylglycerol in annexin A7-mediated fusion of lung lamellar bodies

Lung surfactant secretion in alveolar type II cells occurs following lamellar body fusion with plasma membrane. Annexin A7 is a Ca2+-dependent membrane-binding protein that is postulated to promote membrane fusion during exocytosis in some cell types including type II cells. Since annexin A7 preferably binds to lamellar body membranes, we postulated that specific lipids could modify the mode of annexin A7 interaction with membranes and its membrane fusion activity. Initial studies with phospholipid vesicles containing phosphatidylserine and other lipids showed that certain lipids affected protein interaction with vesicle membranes as determined by change in protein tryptophan fluorescence, protein interaction with trans membranes, and by protein sensitivity to limited proteolysis. The presence of signaling lipids, diacylglycerol or phosphatidylinositol-4,5-bisphosphate, as minor components also modified the lipid vesicle effect on these characteristics and membrane fusion activity of annexin A7. In vitro incubation of lamellar bodies with diacylglycerol or phosphatidylinositol-4,5-bisphosphate caused their enrichment with either lipid, and increased the annexin A7 and Ca2+-mediated fusion of lamellar bodies. Treatment of isolated lung lamellar bodies with phosphatidylinositol- or phosphatidylcholine phospholipase C to increase diacylglycerol, without or with preincubation with phosphatidylinositol-4,5-bisphosphate, augmented the fusion activity of annexin A7. Thus, increased diacylglycerol in lamellar bodies following cell stimulation with secretagogues may enhance membrane fusion activity of annexin A7.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Quantification of diacylglycerol molecular species in biological samples by electrospray ionization mass spectrometry after one-step derivatization

Diacylglycerols (DAGs) are important lipid intermediates in cellular trafficking and signaling. Their concentrations are altered in diabetes, cancer, and other disease states. Quantification of DAGs in biological samples may provide critical information to uncover molecular mechanisms leading to various cellular functional disorders. Recent advances in lipidomics using mass spectrometry have greatly accelerated global lipid analysis and quantification. Quantification of DAGs by electrospray mass spectrometry (ESI/MS), however, is challenged by the absence of a permanent charge on the molecule, its low proton affinity and acidity, and its low abundance under normal biological conditions. We describe here the introduction of a quaternary ammonium cation to DAG molecules, using N-chlorobetainyl chloride, to afford a derivatized DAG that gives 2 orders of magnitude higher signal intensities than their underivatized sodium adducts. A linear calibration curve in which peak intensity ratios are plotted versus molar ratios can be achieved by using ESI/MS with dilauroyl glycerol as the internal standard. Employing this new approach to this analyte, we found a 9-fold increase of total DAGs in the livers of obese db/db mice as compared to their heterozygous lean controls. This proven strategy can be used to detect and quantify DAG molecular species from biological samples using ESI/MS after one-step derivatization.

Polyunsaturated phosphatidylinositol and diacylglycerol substantially modify the fluidity and polymorphism of biomembranes: a solid-state deuterium NMR study

One of the main biological systems that can be used as a model for studying the molecular mechanisms involved in membrane fusion is the formation of the nuclear envelope (NE). NE assembly to form the male pronucleus at fertilization occurs by binding of NE membrane precursor vesicles to chromatin and their fusion. MV1 is an NE precursor vesicle population of low density, highly enriched in [18:0/20:4]PI. The modification of [18:0/20:4]PI to [18:0/20:4]DAG leads to NE formation, and the depletion of MV1 from the total membrane precursors results in the inhibition of NE assembly. Here we show by 2H NMR studies of various physiologically relevant model membranes made of [18:0/20:4]PI, [18:0/20:4]DAG, and saturated and unsaturated PC that membranes of composition similar to MV1 exhibit dramatically enhanced fluidity and non-lamellar structures, thus providing a possible explanation for the essential role of MV1 and the modification of PI to DAG in membrane precursor vesicles during NE assembly.

Transmembrane proteins are not required for early stages of nuclear envelope assembly

All identified membrane fusion proteins are transmembrane proteins. In the present study, we explored the post-mitotic reassembly of the NE (nuclear envelope). The proteins that drive membrane rearrangements in NE assembly remain unknown. To determine whether transmembrane proteins are prerequisite components of this fusion machinery, we have focused on nuclear reconstitution in a cell-free system. Mixing of soluble interphase cytosolic extract and MV (membrane vesicles) from amphibian eggs with chromatin results in the formation of functional nuclei. We replaced MV and cytosol with protein-free phosphatidylcholine LS (liposomes) that were pre-incubated with interphase cytosol. While later stages of NE assembly yielding functional nucleus did not proceed without integral proteins of MV, LS-associated cytosolic proteins were sufficient to reconstitute membrane targeting to the chromatin and GTP-dependent lipid mixing. Binding involved LS-associated A-type lamin, and fusion involved Ran GTPase. Thus in contrast with post-fusion stages, fusion initiation in NE assembly, like membrane remodelling in budding and fission, does not require transmembrane proteins.

PLCgamma is enriched on poly-phosphoinositide-rich vesicles to control nuclear envelope assembly

Nuclear envelope assembly is an essential event in each cell cycle but the proteins and lipids involved in its regulation remain mostly unknown. Assembly involves membrane fusions but neither specific SNAREs nor Rab GTPases have been identified in its control. We report that a precursor membrane population (MV1) required for NE assembly has a unique lipid composition consisting prominently of poly-phosphatidylinositides. The lipid composition was determined by adapting HPLC electrospray ionisation tandem mass spectrometry to phosphoinositide analysis, revealing the capacity of this technique to document dynamic lipid transitions of functional importance in natural membrane populations. MV1 is >100-fold enriched in endogenous PLCgamma and >25-fold enriched in the PLC substrate phosphatidylinositol bisphosphate (PtdInsP2) compared to the second membrane population, derived largely from endoplasmic reticulum (ER), that contributes most of the NE. During NE formation PLCgamma becomes transiently phosphorylated at the tyrosine 783 site indicative of its activation. In addition specific inhibition of PLCgamma blocks nuclear envelope formation. In vivo, PLCgamma is concentrated on vesicles of similar size to purified MV1. These associate with nuclei during the period of NE formation and are distinct from ER membranes. The unprecedented concentration of PLCgamma and its substrate PtdInsP2 in a subset of membranes that binds to only two regions of the nucleus, and activation of PLCgamma by GTP during initial stages of NE formation provide a mechanism for temporal control of NE assembly and offer an explanation for how such a process of membrane fusion can be spatially regulated.

Membranes of the world unite!

Despite diverse origins, cellular fusion mechanisms converge at a pathway of phospholipid bilayer fusion. In this mini-review, we discuss how proteins can mediate each of the three major stages in the fusion pathway: contact, hemifusion, and the opening of an expanding fusion pore.

The developing role of receptors and adaptors

The response of a cell to the myriad of signals that it receives is varied, and it is dependent on many different factors. The most-studied responses involve growth-factor signalling and these signalling cascades have become key targets for cancer therapy. Recent reports have indicated that growth-factor receptors and associated adaptors can accumulate in the nucleus. Are there novel functions for these proteins that might affect our understanding of their role in cancer and have implications for drug resistance?