Early stages of Golgi vesicle and tubule formation require diacylglycerol

Early signs of neurodegenerative diseases: Possible mechanisms and targets for Golgi stress

The Golgi apparatus plays a crucial role in mediating the modification, transport, and sorting of intracellular proteins and lipids. The morphological changes occurring in the Golgi apparatus are exceptionally important for maintaining its function. When exposed to external pressure or environmental stimulation, the Golgi apparatus undergoes adaptive changes in both structure and function, which are known as Golgi stress. Although certain signal pathway responses or post-translational modifications have been observed following Golgi stress, further research is needed to comprehensively summarize and understand the related mechanisms. Currently, there is evidence linking Golgi stress to neurodegenerative diseases; however, the role of Golgi stress in the progression of neurodegenerative diseases such as Alzheimer's disease remains largely unexplored. This review focuses on the structural and functional alterations of the Golgi apparatus during stress, elucidating potential mechanisms underlying the involvement of Golgi stress in regulating immunity, autophagy, and metabolic processes. Additionally, it highlights the pivotal role of Golgi stress as an early signaling event implicated in the pathogenesis and progression of neurodegenerative diseases. Furthermore, this study summarizes prospective targets that can be therapeutically exploited to mitigate neurodegenerative diseases by targeting Golgi stress. These findings provide a theoretical foundation for identifying novel breakthroughs in preventing and treating neurodegenerative diseases.

The function of sphingolipids in membrane trafficking and cell signaling in plants, in comparison with yeast and animal cells

Sphingolipids are essential membrane components involved in a wide range of cellular, developmental and signaling processes. Sphingolipids are so essential that knock-out mutation often leads to lethality. In recent years, conditional or weak allele mutants as well as the broadening of the pharmacological catalog allowed to decipher sphingolipid function more precisely in a less invasive way. This review intends to provide a discussion and point of view on the function of sphingolipids with a main focus on endomembrane trafficking, Golgi-mediated protein sorting, cell polarity, cell-to-cell communication and cell signaling at the plasma membrane. While our main angle is the plant field research, we will constantly refer to and compare with the advances made in the yeast and animal field. In this review, we will emphasize the role of sphingolipids not only as a membrane component, but also as a key player at a center of homeostatic regulatory networks involving direct or indirect interaction with other lipids, proteins and ion fluxes.

C9orf72 Toxic Species Affect ArfGAP-1 Function

Compelling evidence indicates that defects in nucleocytoplasmic transport contribute to the pathogenesis of amyotrophic lateral sclerosis (ALS). In particular, hexanucleotide (G4C2) repeat expansions in C9orf72, the most common cause of genetic ALS, have a widespread impact on the transport machinery that regulates the nucleocytoplasmic distribution of proteins and RNAs. We previously reported that the expression of G4C2 hexanucleotide repeats in cultured human and mouse cells caused a marked accumulation of poly(A) mRNAs in the cell nuclei. To further characterize the process, we set out to systematically identify the specific mRNAs that are altered in their nucleocytoplasmic distribution in the presence of C9orf72-ALS RNA repeats. Interestingly, pathway analysis showed that the mRNAs involved in membrane trafficking are particularly enriched among the identified mRNAs. Most importantly, functional studies in cultured cells and Drosophila indicated that C9orf72 toxic species affect the membrane trafficking route regulated by ADP-Ribosylation Factor 1 GTPase Activating Protein (ArfGAP-1), which exerts its GTPase-activating function on the small GTPase ADP-ribosylation factor 1 to dissociate coat proteins from Golgi-derived vesicles. We demonstrate that the function of ArfGAP-1 is specifically affected by expanded C9orf72 RNA repeats, as well as by C9orf72-related dipeptide repeat proteins (C9-DPRs), indicating the retrograde Golgi-to-ER vesicle-mediated transport as a target of C9orf72 toxicity.

Changes in the content of pollen total lipid and TAG in Arabidopsis thaliana DGAT1 mutant as11

In mature pollen grains, lipids are primarily stored in the form of lipid droplets that provide energy and act as a carbon source for normal pollen development and germination. Triacylglycerol (TAG) is the major form of stored plant lipids. Diacylglycerol transferase, which is encoded by DGAT1 in Arabidopsis thaliana, is an important enzyme regulating triacylglycerol synthesis. Within the seeds of the DGAT1 mutant as11, the content of TAG is significantly decreased and the fatty acid composition also differs from the wild type. Transcriptome data of mature anthers showed that the genes involved in the TAG synthesis pathway were downregulated in as11. Analysis of gene expression patterns via transcriptome data also revealed that the expression of PDAT1, which functions in a manner complementary to the DGAT1 gene, was significantly decreased in as11, whereas the amylopectin synthase genes SS1 and SS2 were upregulated in mutant as11. We also detected lower total lipid, TAG and fatty acid contents in mature as11 pollen, with palmitic acid (C16:0) and linolenic acid (C18:3) being the major fatty acids in mature pollen. The cytological results showed that the lipid droplet content was reduced in mature as11 pollen. In the binuclear pollen grain II stage, WT pollen contained lipid droplets that were primarily accumulated around the generative nucleus, whereas the pollen in the mutant as11 was rich in starch grains that were primarily distributed around the vegetative nucleus. Ultrastructural analysis indicated that during pollen development in as11, the amount of endoplasmic reticulum in tapetal cells and pollen grains decreased, whereas the Golgi body content increased, which directly or indirectly led to a decrease in the levels of lipidosomes and an increase in the starch content in as11. Changes in the lipid content and fatty acid composition of the mutant as11 differ from those in the wild type during pollen development.

Phosphatidic acid-PKA signaling regulates p38 and ERK1/2 functions in ligand-independent EGFR endocytosis

Ligand-independent epidermal growth factor receptor (EGFR) endocytosis is inducible by a variety of stress conditions converging upon p38 kinase. A less known pathway involves phosphatidic acid (PA) signaling toward the activation of type 4 phosphodiesterases (PDE4) that decrease cAMP levels and protein kinase A (PKA) activity. This PA/PDE4/PKA pathway is triggered with propranolol used to inhibit PA hydrolysis and induces clathrin-dependent and clathrin-independent endocytosis, followed by reversible accumulation of EGFR in recycling endosomes. Here we give further evidence of this signaling pathway using biosensors of PA, cAMP, and PKA in live cells and then show that it activates p38 and ERK1/2 downstream the PKA inhibition. Clathrin-silencing and IN/SUR experiments involved the activity of p38 in the clathrin-dependent route, while ERK1/2 mediates clathrin-independent EGFR endocytosis. The PA/PDE4/PKA pathway selectively increases the EGFR endocytic rate without affecting LDLR and TfR constitute endocytosis. This selectiveness is probably because of EGFR phosphorylation, as detected in Th1046/1047 and Ser669 residues. The EGFR accumulates at perinuclear recycling endosomes colocalizing with TfR, fluorescent transferrin, and Rab11, while a small proportion distributes to Alix-endosomes. A non-selective recycling arrest includes LDLR and TfR in a reversible manner. The PA/PDE4/PKA pathway involving both p38 and ERK1/2 expands the possibilities of EGFR transmodulation and interference in cancer.

Insulin granule biogenesis and exocytosis

Insulin is produced by pancreatic β-cells, and once released to the blood, the hormone stimulates glucose uptake and suppresses glucose production. Defects in both the availability and action of insulin lead to elevated plasma glucose levels and are major hallmarks of type-2 diabetes. Insulin is stored in secretory granules that form at the trans-Golgi network. The granules undergo extensive modifications en route to their release sites at the plasma membrane, including changes in both protein and lipid composition of the granule membrane and lumen. In parallel, the insulin molecules also undergo extensive modifications that render the hormone biologically active. In this review, we summarize current understanding of insulin secretory granule biogenesis, maturation, transport, docking, priming and eventual fusion with the plasma membrane. We discuss how different pools of granules form and how these pools contribute to insulin secretion under different conditions. We also highlight the role of the β-cell in the development of type-2 diabetes and discuss how dysregulation of one or several steps in the insulin granule life cycle may contribute to disease development or progression.

Defining the subcellular distribution and metabolic channeling of phosphatidylinositol

Phosphatidylinositol (PI) is an essential structural component of eukaryotic membranes that also serves as the common precursor for polyphosphoinositide (PPIn) lipids. Despite the recognized importance of PPIn species for signal transduction and membrane homeostasis, there is still a limited understanding of the relationship between PI availability and the turnover of subcellular PPIn pools. To address these shortcomings, we established a molecular toolbox for investigations of PI distribution within intact cells by exploiting the properties of a bacterial enzyme, PI-specific PLC (PI-PLC). Using these tools, we find a minor presence of PI in membranes of the ER, as well as a general enrichment within the cytosolic leaflets of the Golgi complex, peroxisomes, and outer mitochondrial membrane, but only detect very low steady-state levels of PI within the plasma membrane (PM) and endosomes. Kinetic studies also demonstrate the requirement for sustained PI supply from the ER for the maintenance of monophosphorylated PPIn species within the PM, Golgi complex, and endosomal compartments.

Excess diacylglycerol at the endoplasmic reticulum disrupts endomembrane homeostasis and autophagy

Background

When stressed, eukaryotic cells produce triacylglycerol (TAG) to store nutrients and mobilize autophagy to combat internal damage. We and others previously reported that in yeast, elimination of TAG synthesizing enzymes inhibits autophagy under nitrogen starvation, yet the underlying mechanism has remained elusive.

Results

Here, we show that disruption of TAG synthesis led to diacylglycerol (DAG) accumulation and its relocation from the vacuolar membrane to the endoplasmic reticulum (ER). We further show that, beyond autophagy, ER-accumulated DAG caused severe defects in the endomembrane system, including disturbing the balance of ER-Golgi protein trafficking, manifesting in bulging of ER and loss of the Golgi apparatus. Genetic or chemical manipulations that increase consumption or decrease supply of DAG reversed these defects. In contrast, increased amounts of precursors of glycerolipid synthesis, including phosphatidic acid and free fatty acids, did not replicate the effects of excess DAG. We also provide evidence that the observed endomembrane defects do not rely on Golgi-produced DAG, Pkc1 signaling, or the unfolded protein response.

Conclusions

This work identifies DAG as the critical lipid molecule responsible for autophagy inhibition under condition of defective TAG synthesis and demonstrates the disruption of ER and Golgi function by excess DAG as the potential cause of the autophagy defect.

Biophysical parameters of the Sec14 phospholipid exchange cycle - Effect of lipid packing in membranes

Sec14, a yeast phosphatidylinositol/phosphatidylcholine transfer protein, functions at the trans-Golgi membranes. It lacks domains involved in protein-protein or protein-lipid interactions and consists solely of the Sec14 domain; hence, the mechanism underlying Sec14 function at proper sites remains unclear. In this study, we focused on the lipid packing of membranes and evaluated its association with in vitro Sec14 lipid transfer activity. Phospholipid transfer assays using pyrene-labelled phosphatidylcholine suggested that increased membrane curvature as well as the incorporation of phosphatidylethanolamine accelerated the lipid transfer. The quantity of membrane-bound Sec14 significantly increased in these membranes, indicating that "packing defects" of the membranes promote the membrane binding and phospholipid transfer of Sec14. Increased levels of phospholipid unsaturation promoted Sec14-mediated PC transfer, but had little effect on the membrane binding of the protein. Our results demonstrate the possibility that the location and function of Sec14 are regulated by the lipid packing states produced by a translocase activity at the trans-Golgi network.

Mutant p53 induces Golgi tubulo-vesiculation driving a prometastatic secretome

TP53 missense mutations leading to the expression of mutant p53 oncoproteins are frequent driver events during tumorigenesis. p53 mutants promote tumor growth, metastasis and chemoresistance by affecting fundamental cellular pathways and functions. Here, we demonstrate that p53 mutants modify structure and function of the Golgi apparatus, culminating in the increased release of a pro-malignant secretome by tumor cells and primary fibroblasts from patients with Li-Fraumeni cancer predisposition syndrome. Mechanistically, interacting with the hypoxia responsive factor HIF1α, mutant p53 induces the expression of miR-30d, which in turn causes tubulo-vesiculation of the Golgi apparatus, leading to enhanced vesicular trafficking and secretion. The mut-p53/HIF1α/miR-30d axis potentiates the release of soluble factors and the deposition and remodeling of the ECM, affecting mechano-signaling and stromal cells activation within the tumor microenvironment, thereby enhancing tumor growth and metastatic colonization.

Chromogranin A in the early steps of the neurosecretory pathway

Chromogranin A (CgA) is a soluble glycoprotein stored with hormones and neuropeptides in secretory granules (SG) of most (neuro)endocrine cells and neurons. Since its discovery in 1967, many studies have reported its structural characteristics, biological roles, and mechanisms of action. Indeed, CgA is both a precursor of various biologically active peptides and a granulogenic protein regulating the storage and secretion of hormones and neuropeptides. This review emphasizes the findings and theoretical concepts around the CgA-linked molecular machinery controlling hormone/neuropeptide aggregation and the interaction of CgA-hormone/neuropeptide aggregates with the trans-Golgi membrane to allow hormone/neuropeptide targeting and SG biogenesis. We will also discuss the intriguing alteration of CgA expression and secretion in various neurological disorders, which could provide insights to elucidate the molecular mechanisms underlying these pathophysiological conditions.

Differential Roles of Lipin1 and Lipin2 in the Hepatitis C Virus Replication Cycle

Although their origin, nature and structure are not identical, a common feature of positive-strand RNA viruses is their ability to subvert host lipids and intracellular membranes to generate replication and assembly complexes. Recently, lipin1, a cellular enzyme that converts phosphatidic acid into diacylglycerol, has been implicated in the formation of the membranous web that hosts hepatitis C virus (HCV) replicase. In the liver, lipin1 cooperates with lipin2 to maintain glycerolipid homeostasis. We extended our previous study of the lipin family on HCV infection, by determining the impact of the lipin2 silencing on viral replication. Our data reveal that lipin2 silencing interferes with HCV virion secretion at late stages of the infection, without significantly affecting viral replication or assembly. Moreover, uninfected lipin2-, but not lipin1-deficient cells display alterations in mitochondrial and Golgi apparatus morphology, suggesting that lipin2 contributes to the maintenance of the overall organelle architecture. Finally, our data suggest a broader function of lipin2 for replication of HCV and other RNA viruses, in contrast with the specific impact of lipin1 silencing on HCV replication. Overall, this study reveals distinctive functions of lipin1 and lipin2 in cells of hepatic origin, a context in which they are often considered functionally redundant.

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.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Biophysical Parameters of the Sec14 Phospholipid Exchange Cycle

Sec14, the major yeast phosphatidylcholine (PC)/phosphatidylinositol (PI) transfer protein (PITP), coordinates PC and PI metabolism to facilitate an appropriate and essential lipid signaling environment for membrane trafficking from trans-Golgi membranes. The Sec14 PI/PC exchange cycle is essential for its essential biological activity, but fundamental aspects of how this PITP executes its lipid transfer cycle remain unknown. To address some of these outstanding issues, we applied time-resolved small-angle neutron scattering for the determination of protein-mediated intervesicular movement of deuterated and hydrogenated phospholipids in vitro. Quantitative analysis by small-angle neutron scattering revealed that Sec14 PI- and PC-exchange activities were sensitive to both the lipid composition and curvature of membranes. Moreover, we report that these two parameters regulate lipid exchange activity via distinct mechanisms. Increased membrane curvature promoted both membrane binding and lipid exchange properties of Sec14, indicating that this PITP preferentially acts on the membrane site with a convexly curved face. This biophysical property likely constitutes part of a mechanism by which spatial specificity of Sec14 function is determined in cells. Finally, wild-type Sec14, but not a mixture of Sec14 proteins specifically deficient in either PC- or PI-binding activity, was able to effect a net transfer of PI or PC down opposing concentration gradients in vitro.

PLCζ Induced Ca2+ Oscillations in Mouse Eggs Involve a Positive Feedback Cycle of Ca2+ Induced InsP3 Formation From Cytoplasmic PIP2

Egg activation at fertilization in mammalian eggs is caused by a series of transient increases in the cytosolic free Ca²⁺ concentration, referred to as Ca²⁺ oscillations. It is widely accepted that these Ca²⁺ oscillations are initiated by a sperm derived phospholipase C isoform, PLCζ that hydrolyses its substrate PIP₂ to produce the Ca²⁺ releasing messenger InsP₃. However, it is not clear whether PLCζ induced InsP₃ formation is periodic or monotonic, and whether the PIP₂ source for generating InsP₃ from PLCζ is in the plasma membrane or the cytoplasm. In this study we have uncaged InsP₃ at different points of the Ca²⁺ oscillation cycle to show that PLCζ causes Ca²⁺ oscillations by a mechanism which requires Ca²⁺ induced InsP₃ formation. In contrast, incubation in Sr²⁺ media, which also induces Ca²⁺ oscillations in mouse eggs, sensitizes InsP₃-induced Ca²⁺ release. We also show that the cytosolic level Ca²⁺ is a key factor in setting the frequency of Ca²⁺ oscillations since low concentrations of the Ca²⁺ pump inhibitor, thapsigargin, accelerates the frequency of PLCζ induced Ca²⁺ oscillations in eggs, even in Ca²⁺ free media. Given that Ca²⁺ induced InsP₃ formation causes a rapid wave during each Ca²⁺ rise, we use a mathematical model to show that InsP₃ generation, and hence PLCζ's substate PIP₂, has to be finely distributed throughout the egg cytoplasm. Evidence for PIP₂ distribution in vesicles throughout the egg cytoplasm is provided with a rhodamine-peptide probe, PBP10. The apparent level of PIP₂ in such vesicles could be reduced by incubating eggs in the drug propranolol which also reversibly inhibited PLCζ induced, but not Sr²⁺ induced, Ca²⁺ oscillations. These data suggest that the cytosolic Ca²⁺ level, rather than Ca²⁺ store content, is a key variable in setting the pace of PLCζ induced Ca²⁺ oscillations in eggs, and they imply that InsP₃ oscillates in synchrony with Ca²⁺ oscillations. Furthermore, they support the hypothesis that PLCζ and sperm induced Ca²⁺ oscillations in eggs requires the hydrolysis of PIP₂ from finely spaced cytoplasmic vesicles.

The Arabidopsis thaliana non-specific phospholipase C2 is involved in the response to Pseudomonas syringae attack

Background and Aims

The non-specific phospholipase C (NPC) is a new member of the plant phospholipase family that reacts to abiotic environmental stresses, such as phosphate deficiency, high salinity, heat and aluminium toxicity, and is involved in root development, silicon distribution and brassinolide signalling. Six NPC genes (NPC1-NPC6) are found in the Arabidopsis genome. The NPC2 isoform has not been experimentally characterized so far.

Methods

The Arabidopsis NPC2 isoform was cloned and heterologously expressed in Escherichia coli. NPC2 enzyme activity was determined using fluorescent phosphatidylcholine as a substrate. Tissue expression and subcellular localization were analysed using GUS- and GFP-tagged NPC2. The expression patterns of NPC2 were analysed via quantitative real-time PCR. Independent homozygous transgenic plant lines overexpressing NPC2 under the control of a 35S promoter were generated, and reactive oxygen species were measured using a luminol-based assay.

Key Results

The heterologously expressed protein possessed phospholipase C activity, being able to hydrolyse phosphatidylcholine to diacylglycerol. NPC2 tagged with GFP was predominantly localized to the Golgi apparatus in Arabidopsis roots. The level of NPC2 transcript is rapidly altered during plant immune responses and correlates with the activation of multiple layers of the plant defence system. Transcription of NPC2 decreased substantially after plant infiltration with Pseudomonas syringae, flagellin peptide flg22 and salicylic acid treatments and expression of the effector molecule AvrRpm1. The decrease in NPC2 transcript levels correlated with a decrease in NPC2 enzyme activity. NPC2-overexpressing mutants showed higher reactive oxygen species production triggered by flg22.

Conclusions

This first experimental characterization of NPC2 provides new insights into the role of the non-specific phospholipase C protein family. The results suggest that NPC2 is involved in the response of Arabidopsis to P. syringae attack.

Bidirectional traffic between the Golgi and the endosomes - machineries and regulation

The bidirectional transport between the Golgi complex and the endocytic pathway has to be finely regulated in order to ensure the proper delivery of newly synthetized lysosomal enzymes and the return of sorting receptors from degradative compartments. The high complexity of these routes has led to experimental difficulties in properly dissecting and separating the different pathways. As a consequence, several models have been proposed during the past decades. However, recent advances in our understanding of endosomal dynamics have helped to unify these different views. We provide here an overview of the current insights into the transport routes between Golgi and endosomes in mammalian cells. The focus of the Commentary is on the key molecules involved in the trafficking pathways between these intracellular compartments, such as Rab proteins and sorting receptors, and their regulation. A proper understanding of the bidirectional traffic between the Golgi complex and the endolysosomal system is of uttermost importance, as several studies have demonstrated that mutations in the factors involved in these transport pathways result in various pathologies, in particular lysosome-associated diseases and diverse neurological disorders, such as Alzheimer's and Parkinson's disease.

Cholesterol-Independent SREBP-1 Maturation Is Linked to ARF1 Inactivation

Lipogenesis requires coordinated expression of genes for fatty acid, phospholipid, and triglyceride synthesis. Transcription factors, such as SREBP-1 (Sterol regulatory element binding protein), may be activated in response to feedback mechanisms linking gene activation to levels of metabolites in the pathways. SREBPs can be regulated in response to membrane cholesterol and we also found that low levels of phosphatidylcholine (a methylated phospholipid) led to SBP-1/SREBP-1 maturation in C. elegans or mammalian models. To identify additional regulatory components, we performed a targeted RNAi screen in C. elegans, finding that both lpin-1/Lipin 1 (which converts phosphatidic acid to diacylglycerol) and arf-1.2/ARF1 (a GTPase regulating Golgi function) were important for low-PC activation of SBP-1/SREBP-1. Mechanistically linking the major hits of our screen, we find that limiting PC synthesis or LPIN1 knockdown in mammalian cells reduces the levels of active GTP-bound ARF1. Thus, changes in distinct lipid ratios may converge on ARF1 to increase SBP-1/SREBP-1 activity.

Lipids implicated in the journey of a secretory granule: from biogenesis to fusion

The regulated secretory pathway begins with the formation of secretory granules by budding from the Golgi apparatus and ends by their fusion with the plasma membrane leading to the release of their content into the extracellular space, generally following a rise in cytosolic calcium. Generation of these membrane-bound transport carriers can be classified into three steps: (i) cargo sorting that segregates the cargo from resident proteins of the Golgi apparatus, (ii) membrane budding that encloses the cargo and depends on the creation of appropriate membrane curvature, and (iii) membrane fission events allowing the nascent carrier to separate from the donor membrane. These secretory vesicles then mature as they are actively transported along microtubules toward the cortical actin network at the cell periphery. The final stage known as regulated exocytosis involves the docking and the priming of the mature granules, necessary for merging of vesicular and plasma membranes, and the subsequent partial or total release of the secretory vesicle content. Here, we review the latest evidence detailing the functional roles played by lipids during secretory granule biogenesis, recruitment, and exocytosis steps. In this review, we highlight evidence supporting the notion that lipids play important functions in secretory vesicle biogenesis, maturation, recruitment, and membrane fusion steps. These effects include regulating various protein distribution and activity, but also directly modulating membrane topology. The challenges ahead to understand the pleiotropic functions of lipids in a secretory granule's journey are also discussed. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015).

Diacylglycerol kinases in membrane trafficking

Diacylglycerol kinases (DGKs) belong to a family of cytosolic kinases that regulate the phosphorylation of diacylglycerol (DAG), converting it into phosphatidic acid (PA). There are 10 known mammalian DGK isoforms, each with a different tissue distribution and substrate specificity. These differences allow regulation of cellular responses by fine-tuning the delicate balance of cellular DAG and PA. DGK isoforms are best characterized as mediators of signal transduction and immune function. However, since recent studies reveal that DAG and PA are also involved in the regulation of endocytic trafficking, it is therefore anticipated that DGKs also plays an important role in membrane trafficking. In this review, we summarize the literature discussing the role of DGK isoforms at different stages of endocytic trafficking, including endocytosis, exocytosis, endocytic recycling, and transport from/to the Golgi apparatus. Overall, these studies contribute to our understanding of the involvement of PA and DAG in endocytic trafficking, an area of research that is drawing increasing attention in recent years.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

Differential dependencies on [Ca2+] and temperature of the monolayer spontaneous curvatures of DOPE, DOPA and cardiolipin: effects of modulating the strength of the inter-headgroup repulsion

Biomembranes assume nonlamellar structures in many cellular events, with the tendency of forming a nonlamellar structure quantified by the monolayer spontaneous curvature, C(0), and with many of these events involving the acts of Ca(2+). Despite this biologically important intimacy, how C(0) is affected by [Ca(2+)] is unknown. In this study, we use the X-ray diffraction technique and the reconstruction of electron density profiles to measure the C(0)s of a zwitterionic phospholipid, DOPE, and two anionic phospholipids, DOPA and 18 : 1 (9Z) cardiolipin, at temperatures from 20 °C to 40 °C and [Ca(2+)]s from 0 mM to 100 mM; these phospholipids are chosen to examine the contributions of the electric charge density per molecule. While showing a strong dependence on temperature, C(0,DOPE) is nearly independent of [Ca(2+)]. In contrast, C(0,DOPA) and C(0),cardiolipin are almost unresponsive to the temperature change but affected by the [Ca(2+)] variation; and C(0,DOPA) varies with [Ca(2+)] ∼1.5 times more strongly than C(0,cardiolipin), with the phase preferences of DOPA and cardiolipin shifting to the H(II) phase and remaining on the Lα phase, respectively, at [Ca(2+)] = 100 mM. From these observations, we reveal the effects of modulating the strength of the inter-headgroup repulsion and discuss the mechanisms underlying the phase behaviour and cellular functions of the investigated phospholipids. Most importantly, this study recognizes that the headgroup charge density is dominant in dictating the phase behaviour of the anionic phospholipids, and that the unique molecular characteristics of cardiolipin are critically needed both for maintaining the structural integrity of cardiolipin-rich biomembranes and for fulfilling the biological roles of the phospholipid.

PLCγ1 participates in protein transport and diacylglycerol production triggered by cargo arrival at the Golgi

Diacylglycerol (DAG) is required for membrane traffic and structural organization at the Golgi. DAG is a lipid metabolite of several enzymatic reactions present at this organelle, but the mechanisms by which they are regulated are still unknown. Here, we show that cargo arrival at the Golgi increases the recruitment of the DAG-sensing constructs C1-PKCθ-GFP and the PKD-wt-GFP. The recruitment of both constructs was reduced by PLCγ1 silencing. Post-Golgi trafficking of transmembrane and soluble proteins was impaired in PLCγ1-silenced cells. Under basal conditions, PLCγ1 contributed to the maintenance of the pool of DAG associated with the Golgi and to the structural organization of the organelle. Finally, we show that cytosolic phospholipase C (PLC) can hydrolyse phosphatidylinositol 4-phosphate in isolated Golgi membranes. Our results indicate that PLCγ1 is part of the molecular mechanism that couples cargo arrival at the Golgi with DAG production to co-ordinate the formation of transport carriers for post-Golgi traffic.

ARFGAP1 is dynamically associated with lipid droplets in hepatocytes

The ARF GTPase Activating Protein 1 (ARFGAP1) associates mainly with the cytosolic side of Golgi cisternal membranes where it participates in the formation of both COPI and clathrin-coated vesicles. In this study, we show that ARFGAP1 associates transiently with lipid droplets upon addition of oleate in cultured cells. Also, that addition of cyclic AMP shifts ARFGAP1 from lipid droplets to the Golgi apparatus and that overexpression and knockdown of ARFGAP1 affect lipid droplet formation. Examination of human liver tissue reveals that ARFGAP1 is found associated with lipid droplets at steady state in some but not all hepatocytes.

Phospholipase D2 is involved in the formation of Golgi tubules and ArfGAP1 recruitment

Lipids and lipid-modifying enzymes play a key role in the biogenesis, maintenance and fission of transport carriers in the secretory and endocytic pathways. In the present study we demonstrate that phosphatidic acid generated by phospholipase D2 (PLD2) is involved in the formation of Golgi tubules. The main evidence to support this is: 1) inhibitors of phosphatidic acid formation and PLD2 depletion inhibit the formation of tubules containing resident enzymes and regulators of intra-Golgi transport in a low temperature (15°C) model of Golgi tubulation but do not affect brefeldin A-induced tubules, 2) inhibition of PLD2 enzymatic activity and PLD2 depletion in cells cultured under physiological conditions (37°C) induce the formation of tubules specifically containing Golgi matrix proteins, and, 3) over-expression of PLD2 induces the formation of a tubular network. In addition, it was found that the generation of this lipid by the isoenzyme is necessary for ArfGAP1 recruitment to Golgi membranes. These results suggest that both proteins are involved in the molecular mechanisms which drive the formation of different types of Golgi tubules.

Lipin-1 regulates autophagy clearance and intersects with statin drug effects in skeletal muscle

LPIN1 encodes lipin-1, a phosphatidic acid phosphatase (PAP) enzyme that catalyzes the dephosphorylation of phosphatidic acid to form diacylglycerol. Homozygous LPIN1 gene mutations cause severe rhabdomyolysis, and heterozygous LPIN1 missense mutations may promote statin-induced myopathy. We demonstrate that lipin-1-related myopathy in the mouse is associated with a blockade in autophagic flux and accumulation of aberrant mitochondria. Lipin-1 PAP activity is required for maturation of autolysosomes, through its activation of the protein kinase D (PKD)-Vps34 phosphatidylinositol 3-kinase signaling cascade. Statin treatment also reduces PKD activation and autophagic flux, which are compounded by diminished mammalian target of rapamycin (mTOR) abundance in lipin-1-haploinsufficent and -deficient muscle. Lipin-1 restoration in skeletal muscle prevents myonecrosis and statin toxicity in vivo, and activated PKD rescues autophagic flux in lipin-1-deficient cells. Our findings identify lipin-1 PAP activity as a component of the macroautophagy pathway and define the basis for lipin-1-related myopathies.

Acyl-CoA synthetase 3 promotes lipid droplet biogenesis in ER microdomains

Acyl-CoA synthetase 3 is recruited early to lipid droplet assembly sites on the ER, where it is required for efficient lipid droplet nucleation and lipid storage.

Control of lipid droplet (LD) nucleation and copy number are critical, yet poorly understood, processes. We use model peptides that shift from the endoplasmic reticulum (ER) to LDs in response to fatty acids to characterize the initial steps of LD formation occurring in lipid-starved cells. Initially, arriving lipids are rapidly packed in LDs that are resistant to starvation (pre-LDs). Pre-LDs are restricted ER microdomains with a stable core of neutral lipids. Subsequently, a first round of “emerging” LDs is nucleated, providing additional lipid storage capacity. Finally, in proportion to lipid concentration, new rounds of LDs progressively assemble. Confocal microscopy and electron tomography suggest that emerging LDs are nucleated in a limited number of ER microdomains after a synchronized stepwise process of protein gathering, lipid packaging, and recognition by Plin3 and Plin2. A comparative analysis demonstrates that the acyl-CoA synthetase 3 is recruited early to the assembly sites, where it is required for efficient LD nucleation and lipid storage.

Epidermal growth factor receptor endocytic traffic perturbation by phosphatidate phosphohydrolase inhibition: new strategy against cancer

Epidermal growth factor receptor (EGFR) exaggerated (oncogenic) function is currently targeted in cancer treatment with drugs that block receptor ligand binding or tyrosine kinase activity. Because endocytic trafficking is a crucial regulator of EGFR function, its pharmacological perturbation might provide a new anti-tumoral strategy. Inhibition of phosphatidic acid (PA) phosphohydrolase (PAP) activity has been shown to trigger PA signaling towards type 4 phosphodiesterase (PDE4) activation and protein kinase A inhibition, leading to internalization of empty/inactive EGFR. Here, we used propranolol, its l- and d- isomers and desipramine as PAP inhibitors to further explore the effects of PAP inhibition on EGFR endocytic trafficking and its consequences on EGFR-dependent cancer cell line models. PAP inhibition not only made EGFR inaccessible to stimuli but also prolonged the signaling lifetime of ligand-activated EGFR in recycling endosomes. Strikingly, such endocytic perturbations applied in acute/intermittent PAP inhibitor treatments selectively impaired cell proliferation/viability sustained by an exaggerated EGFR function. Phospholipase D inhibition with FIPI (5-fluoro-2-indolyl des-chlorohalopemide) and PDE4 inhibition with rolipram abrogated both the anti-tumoral and endocytic effects of PAP inhibition. Prolonged treatments with a low concentration of PAP inhibitors, although without detectable endocytic effects, still counteracted cell proliferation, induced apoptosis and decreased anchorage-independent growth of cells bearing EGFR oncogenic influences. Overall, our results show that PAP inhibitors can counteract EGFR oncogenic traits, including receptor overexpression or activating mutations resistant to current tyrosine kinase inhibitors, perturbing EGFR endocytic trafficking and perhaps other as yet unknown processes, depending on treatment conditions. This puts PAP activity forward as a new suitable target against EGFR-driven malignancy.

Topological regulation of lipid balance in cells

Lipids are unevenly distributed within and between cell membranes, thus defining organelle identity. Such distribution relies on local metabolic branches and mechanisms that move lipids. These processes are regulated by feedback mechanisms that decipher topographical information in organelle membranes and then regulate lipid levels or flows. In the endoplasmic reticulum, the major lipid source, transcriptional regulators and enzymes sense changes in membrane features to modulate lipid production. At the Golgi apparatus, lipid-synthesizing, lipid-flippase, and lipid-transport proteins (LTPs) collaborate to control lipid balance and distribution within the membrane to guarantee remodeling processes crucial for vesicular trafficking. Open questions exist regarding LTPs, which are thought to be lipid sensors that regulate lipid synthesis or carriers that transfer lipids between organelles across long distances or in contact sites. A novel model is that LTPs, by exchanging two different lipids, exploit one lipid gradient between two distinct membranes to build a second lipid gradient.

Propranolol restricts the mobility of single EGF-receptors on the cell surface before their internalization

The epidermal growth factor receptor is involved in morphogenesis, proliferation and cell migration. Its up-regulation during tumorigenesis makes this receptor an interesting therapeutic target. In the absence of the ligand, the inhibition of phosphatidic acid phosphohydrolase activity by propranolol treatment leads to internalization of empty/inactive receptors. The molecular events involved in this endocytosis remain unknown. Here, we quantified the effects of propranolol on the mobility of single quantum-dot labelled receptors before the actual internalization took place. The single receptors showed a clear stop-and-go motion; their diffusive tracks were continuously interrupted by sub-second stalling events, presumably caused by transient clustering. In the presence of propranolol we found that: i) the diffusion rate reduced by 22 %, which indicates an increase in drag of the receptor. Atomic force microscopy measurements did not show an increase of the effective membrane tension, such that clustering of the receptor remains the likely mechanism for its reduced mobility. ii) The receptor got frequently stalled for longer periods of multiple seconds, which may signal the first step of the internalization process.

Oxysterol-binding protein (OSBP) is required for the perinuclear localization of intra-Golgi v-SNAREs

OSBP regulates the Golgi cholesterol level. This study demonstrates that OSBP and cholesterol are essential for localization of Golgi v-SNAREs. Knockdown of ArfGAP1 restores v-SNARE localization in OSBP-depleted cells, suggesting that OSBP-regulated cholesterol ensures proper COP-I vesicle transport.

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) have been implicated in the distribution of sterols among intracellular organelles. OSBP regulates the Golgi cholesterol level, but how it relates to Golgi function is elusive. Here we report that OSBP is essential for the localization of intra-Golgi soluble vesicle N-ethylmaleimide-sensitive fusion attachment protein receptors (v-SNAREs). Depletion of OSBP by small interfering RNA causes mislocalization of intra-Golgi v-SNAREs GS28 and GS15 throughout the cytoplasm without affecting the perinuclear localization of Golgi target-SNARE syntaxin5 and reduces the abundance of a Golgi enzyme, mannosidase II (Man II). GS28 mislocalization and Man II reduction are also induced by cellular cholesterol depletion. Three domains of OSBP—an endoplasmic reticulum–targeting domain, a Golgi-targeting domain, and a sterol-binding domain—are all required for Golgi localization of GS28. Finally, GS28 mislocalization and Man II reduction in OSBP-depleted cells are largely restored by depletion of ArfGAP1, a regulator of the budding of coat protein complex (COP)-I vesicles. From these results, we postulate that Golgi cholesterol level, which is controlled by OSBP, is essential for Golgi localization of intra-Golgi v-SNAREs by ensuring proper COP-I vesicle transport.

Golgi tubules: their structure, formation and role in intra-Golgi transport

Tubules are common Golgi elements that can form extensive networks associated with the cis-, lateral and trans-Golgi sides, but despite this, they have almost been forgotten for decades. The molecular mechanisms involved in their formation, elongation and fission are only just beginning to be understood. However, the role of these membranes is not well understood. In the present review, we analyze the mechanisms that induce Golgi tubulation or, conversely, disrupt tubules in order to throw some lights on the nature of these elements. The putative role of these elements in the framework of current models for intra-Golgi transport is also discussed.

Extracting curvature preferences of lipids assembled in flat bilayers shows possible kinetic windows for genesis of bilayer asymmetry and domain formation in biological membranes

Studies on the assembly of pure lipid components allow mechanistic insights toward understanding the structural and functional aspects of biological membranes. Molecular dynamic (MD) simulations on membrane systems provide molecular details on membrane dynamics that are difficult to obtain experimentally. A large number of MD studies have remained somewhat disconnected from a key concept of amphipathic assembly resulting in membrane structures--shape parameters of lipid molecules in those structures in aqueous environments. This is because most of the MD studies have been done on flat lipid membranes. With the above in view, we analyzed MD simulations of 26 pure lipid patches as a function of (1) lipid type(s) and (2) time of MD simulations along with 35-40 ns trajectories of five pure lipids. We report, for the first time, extraction of curvature preferences of lipids from MD simulations done on flat bilayers. Our results may lead to mechanistic insights into the possible origins of bilayer asymmetries and domain formation in biological membranes.

Lipid phosphate phosphatase 3 participates in transport carrier formation and protein trafficking in the early secretory pathway

The inhibition of phosphatidic acid phosphatase (PAP) activity by propanolol indicates that diacylglycerol (DAG) is required for the formation of transport carriers at the Golgi and for retrograde trafficking to the ER. Here we report that the PAP2 family member lipid phosphate phosphatase 3 (LPP3, also known as PAP2b) localizes in compartments of the secretory pathway from ER export sites to the Golgi complex. The depletion of human LPP3: (i) reduces the number of tubules generated from the ER-Golgi intermediate compartment and the Golgi, with those formed from the Golgi being longer in LPP3-silenced cells than in control cells; (ii) impairs the Rab6-dependent retrograde transport of Shiga toxin subunit B from the Golgi to the ER, but not the anterograde transport of VSV-G or ssDsRed; and (iii) induces a high accumulation of Golgi-associated membrane buds. LPP3 depletion also reduces levels of de novo synthesized DAG and the Golgi-associated DAG contents. Remarkably, overexpression of a catalytically inactive form of LPP3 mimics the effects of LPP3 knockdown on Rab6-dependent retrograde transport. We conclude that LPP3 participates in the formation of retrograde transport carriers at the ER-Golgi interface, where it transitorily cycles, and during its route to the plasma membrane.

Characterization of the yeast actin patch protein App1p phosphatidate phosphatase

Yeast App1p is a phosphatidate phosphatase (PAP) that associates with endocytic proteins at cortical actin patches. App1p, which catalyzes the conversion of phosphatidate (PA) to diacylglycerol, is unique among Mg(2+)-dependent PAP enzymes in that its reaction is not involved with de novo lipid synthesis. Instead, App1p PAP is thought to play a role in endocytosis because its substrate and product facilitate membrane fission/fusion events and regulate enzymes that govern vesicular movement. App1p PAP was purified from yeast and characterized with respect to its enzymological, kinetic, and regulatory properties. Maximum PAP activity was dependent on Triton X-100 (20 mm), PA (2 mm), Mg(2+) (0.5 mm), and 2-mercaptoethanol (10 mm) at pH 7.5 and 30 °C. Analysis of surface dilution kinetics with Triton X-100/PA-mixed micelles yielded constants for surface binding (Ks(A) = 11 mm), interfacial PA binding (Km(B) = 4.2 mol %), and catalytic efficiency (Vmax = 557 μmol/min/mg). The activation energy, turnover number, and equilibrium constant were 16.5 kcal/mol, 406 s(-1), and 16.2, respectively. PAP activity was stimulated by anionic lipids (cardiolipin, phosphatidylglycerol, phosphatidylserine, and CDP-diacylglycerol) and inhibited by zwitterionic (phosphatidylcholine and phosphatidylethanolamine) and cationic (sphinganine) lipids, nucleotides (ATP and CTP), N-ethylmaleimide, propranolol, phenylglyoxal, and divalent cations (Ca(2+), Mn(2+), and Zn(2+)). App1p also utilized diacylglycerol pyrophosphate and lyso-PA as substrates with specificity constants 4- and 7-fold lower, respectively, when compared with PA.

The Saccharomyces cerevisiae actin patch protein App1p is a phosphatidate phosphatase enzyme

BACKGROUND

Phosphatidate phosphatase (PAP) plays diverse roles in lipid metabolism and cell signaling.

RESULTS

A novel yeast PAP is identified as the actin patch protein encoded by APP1.

CONCLUSION

APP1 and other known genes (PAH1, DPP1, LPP1) are responsible for all detectable PAP activity in yeast.

SIGNIFICANCE

Identification of App1p as a PAP enzyme will facilitate the understanding of its cellular function. Phosphatidate phosphatase (PAP) catalyzes the dephosphorylation of phosphatidate to yield diacylglycerol. In the yeast Saccharomyces cerevisiae, PAP is encoded by PAH1, DPP1, and LPP1. The presence of PAP activity in the pah1Δ dpp1Δ lpp1Δ triple mutant indicated another gene(s) encoding the enzyme. We purified PAP from the pah1Δ dpp1Δ lpp1Δ triple mutant by salt extraction of mitochondria followed by chromatography with DE52, Affi-Gel Blue, phenyl-Sepharose, MonoQ, and Superdex 200. Liquid chromatography/tandem mass spectrometry analysis of a PAP-enriched sample revealed multiple putative phosphatases. By analysis of PAP activity in mutants lacking each of the proteins, we found that APP1, a gene whose molecular function has been unknown, confers ~30% PAP activity of wild type cells. The overexpression of APP1 in the pah1Δ dpp1Δ lpp1Δ mutant exhibited a 10-fold increase in PAP activity. The PAP activity shown by App1p heterologously expressed in Escherichia coli confirmed that APP1 is the structural gene for the enzyme. Introduction of the app1Δ mutation into the pah1Δ dpp1Δ lpp1Δ triple mutant resulted in a complete loss of PAP activity, indicating that distinct PAP enzymes in S. cerevisiae are encoded by APP1, PAH1, DPP1, and LPP1. Lipid analysis of cells lacking the PAP genes, singly or in combination, showed that Pah1p is the only PAP involved in the synthesis of triacylglycerol as well as in the regulation of phospholipid synthesis. App1p, which shows interactions with endocytic proteins, may play a role in vesicular trafficking through its PAP activity.

The plant non-specific phospholipase C gene family. Novel competitors in lipid signalling

Non-specific phospholipases C (NPCs) were discovered as a novel type of plant phospholipid-cleaving enzyme homologous to bacterial phosphatidylcholine-specific phospholipases C and responsible for lipid conversion during phosphate-limiting conditions. The six-gene family was established in Arabidopsis, and growing evidence suggests the involvement of two articles NPCs in biotic and abiotic stress responses as well as phytohormone actions. In addition, the diacylglycerol produced via NPCs is postulated to participate in membrane remodelling, general lipid metabolism and cross-talk with other phospholipid signalling systems in plants. This review summarises information concerning this new plant protein family and focusses on its sequence analysis, biochemical properties, cellular and tissue distribution and physiological functions. Possible modes of action are also discussed.

Retrograde vesicle transport in the Golgi

The Golgi apparatus is the central sorting and biosynthesis hub of the secretory pathway, and uses vesicle transport for the recycling of its resident enzymes. This system must operate with high fidelity and efficiency for the correct modification of secretory glycoconjugates. In this review, we discuss recent advances on how coats, tethers, Rabs and SNAREs cooperate at the Golgi to achieve vesicle transport. We cover the well understood vesicle formation process orchestrated by the COPI coat, and the comprehensively documented fusion process governed by a set of Golgi localised SNAREs. Much less clear are the steps in-between formation and fusion of vesicles, and we therefore provide a much needed update of the latest findings about vesicle tethering. The interplay between Rab GTPases, golgin family coiled-coil tethers and the conserved oligomeric Golgi (COG) complex at the Golgi are thoroughly evaluated.

Golgi membrane dynamics and lipid metabolism

The striking morphology of the Golgi complex has fascinated cell biologists since its discovery over 100 years ago. Yet, despite intense efforts to understand how membrane flow relates to Golgi form and function, this organelle continues to baffle cell biologists and biochemists alike. Fundamental questions regarding Golgi function, while hotly debated, remain unresolved. Historically, Golgi function has been described from a protein-centric point of view, but we now appreciate that conceptual frameworks for how lipid metabolism is integrated with Golgi biogenesis and function are essential for a mechanistic understanding of this fascinating organelle. It is from a lipid-centric perspective that we discuss the larger question of Golgi dynamics and membrane trafficking. We review the growing body of evidence for how lipid metabolism is integrally written into the engineering of the Golgi system and highlight questions for future study.

Regulation of the Golgi complex by phospholipid remodeling enzymes

The mammalian Golgi complex is a highly dynamic organelle consisting of stacks of flattened cisternae with associated coated vesicles and membrane tubules that contribute to cargo import and export, intra-cisternal trafficking, and overall Golgi architecture. At the morphological level, all of these structures are continuously remodeled to carry out these trafficking functions. Recent advances have shown that continual phospholipid remodeling by phospholipase A (PLA) and lysophospholipid acyltransferase (LPAT) enzymes, which deacylate and reacylate Golgi phospholipids, respectively, contributes to this morphological remodeling. Here we review the identification and characterization of four cytoplasmic PLA enzymes and one integral membrane LPAT that participate in the dynamic functional organization of the Golgi complex, and how some of these enzymes are integrated to determine the relative abundance of COPI vesicle and membrane tubule formation. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Sec14 like PITPs couple lipid metabolism with phosphoinositide synthesis to regulate Golgi functionality

An interface coordinating lipid metabolism with proteins that regulate membrane trafficking is necessary to regulate Golgi morphology and dynamics. Such an interface facilitates the membrane deformations required for vesicularization, forms platforms for protein recruitment and assembly on appropriate sites on a membrane surface and provides lipid co-factors for optimal protein activity in the proper spatio-temporally regulated manner. Importantly, Sec14 and Sec14-like proteins are a unique superfamily of proteins that sense specific aspects of lipid metabolism, employing this information to potentiate phosphoinositide production. Therefore, Sec14 and Sec14 like proteins form central conduits to integrate multiple aspects of lipid metabolism with productive phosphoinositide signaling.

Correlative light and electron microscopy of GFP

The correlation of light and electron microscopy (EM) is a powerful tool as it combines the investigation of dynamic processes in vivo with the resolution power of the electron microscope. The green fluorescent proteins (GFPs) and its derivatives revolutionized live-cell light microscopy. Hence, this review outlines correlative microscopy of GFP through photo-oxidation, a method that allows for the direct ultrastructural visualization of fluorophores upon illumination. Oxygen radicals generated during the GFP bleaching process photo-oxidize diaminobenzidine (DAB) into an electron dense precipitate that can be visualized both by routine EM of thin sections and by electron tomography for 3D analysis. There are different levels of correlative microscopy, i.e. the correlation of certain areas, cells, or organelles from light to EM, where photo-oxidation of DAB through GFP allows the highest possible degree--the correlation of specific molecules.

Bridging membrane and cytoskeleton dynamics in the secretory and endocytic pathways

Transport carriers regulate membrane flow between compartments of the secretory and endocytic pathways in eukaryotic cells. Carrier biogenesis is assisted by microtubules, actin filaments and their associated motors that link to membrane-associated coats, adaptors and accessory proteins. We summarize here how the biochemical properties of membranes inform their interactions with cytoskeletal regulators. We also discuss how the forces generated by the cytoskeleton and motor proteins alter the biophysical properties and the shape of membranes. The interplay between the cytoskeleton and membrane proteins ensures tight spatial and temporal control of carrier biogenesis, which is essential for cellular homeostasis.

Yeast lipin 1 orthologue pah1p regulates vacuole homeostasis and membrane fusion

Vacuole homotypic fusion requires a group of regulatory lipids that includes diacylglycerol, a fusogenic lipid that is produced through multiple metabolic pathways including the dephosphorylation of phosphatidic acid (PA). Here we examined the relationship between membrane fusion and PA phosphatase activity. Pah1p is the single yeast homologue of the Lipin family of PA phosphatases. Deletion of PAH1 was sufficient to cause marked vacuole fragmentation and abolish vacuole fusion. The function of Pah1p solely depended on its phosphatase activity as complementation studies showed that wild type Pah1p restored fusion, whereas the phosphatase dead mutant Pah1p(D398E) had no effect. We discovered that the lack of PA phosphatase activity blocked fusion by inhibiting the binding of SNAREs to Sec18p, an N-ethylmaleimide-sensitive factor homologue responsible for priming inactive cis-SNARE complexes. In addition, pah1Δ vacuoles were devoid of the late endosome/vacuolar Rab Ypt7p, the phosphatidylinositol 3-kinase Vps34p, and Vps39p, a subunit of the HOPS (homotypic fusion and vacuole protein sorting) tethering complex, all of which are required for vacuole fusion. The lack of Vps34p resulted in the absence of phosphatidylinositol 3-phosphate, a lipid required for SNARE activity and vacuole fusion. These findings demonstrate that Pah1p and PA phosphatase activity are critical for vacuole homeostasis and fusion.

COPI budding within the Golgi stack

The Golgi serves as a hub for intracellular membrane traffic in the eukaryotic cell. Transport within the early secretory pathway, that is within the Golgi and from the Golgi to the endoplasmic reticulum, is mediated by COPI-coated vesicles. The COPI coat shares structural features with the clathrin coat, but differs in the mechanisms of cargo sorting and vesicle formation. The small GTPase Arf1 initiates coating on activation and recruits en bloc the stable heptameric protein complex coatomer that resembles the inner and the outer shells of clathrin-coated vesicles. Different binding sites exist in coatomer for membrane machinery and for the sorting of various classes of cargo proteins. During the budding of a COPI vesicle, lipids are sorted to give a liquid-disordered phase composition. For the release of a COPI-coated vesicle, coatomer and Arf cooperate to mediate membrane separation.

COPI acts in both vesicular and tubular transport

Intracellular transport occurs through two general types of carrier, either vesicles or tubules. Coat proteins act as the core machinery that initiates vesicle formation, but the counterpart that initiates tubule formation has been unclear. Here, we find that the coat protein I (COPI) complex initially drives the formation of Golgi buds. Subsequently, a set of opposing lipid enzymatic activities determines whether these buds become vesicles or tubules. Lysophosphatidic acid acyltransferase-γ (LPAATγ) promotes COPI vesicle fission for retrograde vesicular transport. In contrast, cytosolic phospholipase A2-α (cPLA2α) inhibits this fission event to induce COPI tubules, which act in anterograde intra-Golgi transport and Golgi ribbon formation. These findings not only advance a molecular understanding of how COPI vesicle fission is achieved, but also provide insight into how COPI acts in intra-Golgi transport and reveal an unexpected mechanistic relationship between vesicular and tubular transport.