Reconstitution of ATP-dependent aminophospholipid translocation in proteoliposomes

Reconstitution of ATP-dependent lipid transporters: gaining insight into molecular characteristics, regulation, and mechanisms

Lipid transporters play a crucial role in supporting essential cellular processes such as organelle assembly, vesicular trafficking, and lipid homeostasis by driving lipid transport across membranes. Cryo-electron microscopy has recently resolved the structures of several ATP-dependent lipid transporters, but functional characterization remains a major challenge. Although studies of detergent-purified proteins have advanced our understanding of these transporters, in vitro evidence for lipid transport is still limited to a few ATP-dependent lipid transporters. Reconstitution into model membranes, such as liposomes, is a suitable approach to study lipid transporters in vitro and to investigate their key molecular features. In this review, we discuss the current approaches for reconstituting ATP-driven lipid transporters into large liposomes and common techniques used to study lipid transport in proteoliposomes. We also highlight the existing knowledge on the regulatory mechanisms that modulate the activity of lipid transporters, and finally, we address the limitations of the current approaches and future perspectives in this field.

Asymmetric Distribution of Plasmalogens and Their Roles-A Mini Review

Plasmalogens are a unique family of cellular glycerophospholipids that contain a vinyl-ether bond. The synthesis of plasmalogens is initiated in peroxisomes and completed in the endoplasmic reticulum. Plasmalogens are transported to the post-Golgi compartment, including endosomes and plasma membranes, in a manner dependent on ATP, but not vesicular transport. Plasmalogens are preferentially localized in the inner leaflet of the plasma membrane in a manner dependent on P4-type ATPase ATP8B2, that associates with the CDC50 subunit. Plasmalogen biosynthesis is spatiotemporally regulated by a feedback mechanism that senses the amount of plasmalogens in the inner leaflet of the plasma membrane and controls the stability of fatty acyl-CoA reductase 1 (FAR1), the rate-limiting enzyme for plasmalogen biosynthesis. The physiological consequences of such asymmetric localization and homeostasis of plasmalogens are discussed in this review.

Regulation of phospholipid distribution in the lipid bilayer by flippases and scramblases

Cellular membranes function as permeability barriers that separate cells from the external environment or partition cells into distinct compartments. These membranes are lipid bilayers composed of glycerophospholipids, sphingolipids and cholesterol, in which proteins are embedded. Glycerophospholipids and sphingolipids freely move laterally, whereas transverse movement between lipid bilayers is limited. Phospholipids are asymmetrically distributed between membrane leaflets but change their location in biological processes, serving as signalling molecules or enzyme activators. Designated proteins - flippases and scramblases - mediate this lipid movement between the bilayers. Flippases mediate the confined localization of specific phospholipids (phosphatidylserine (PtdSer) and phosphatidylethanolamine) to the cytoplasmic leaflet. Scramblases randomly scramble phospholipids between leaflets and facilitate the exposure of PtdSer on the cell surface, which serves as an important signalling molecule and as an 'eat me' signal for phagocytes. Defects in flippases and scramblases cause various human diseases. We herein review the recent research on the structure of flippases and scramblases and their physiological roles. Although still poorly understood, we address the mechanisms by which they translocate phospholipids between lipid bilayers and how defects cause human diseases.

The transferred translocases: An old wine in a new bottle

The role of translocases was underappreciated and was not included as a separate class in the enzyme commission until August 2018. The recent research interests in proteomics of orphan enzymes, ionomics, and metallomics along with high-throughput sequencing technologies generated overwhelming data and revamped this enzyme into a separate class. This offers a great opportunity to understand the role of new or orphan enzymes in general and specifically translocases. The enzymes belonging to translocases regulate/permeate the transfer of ions or molecules across the membranes. These enzyme entries were previously associated with other enzyme classes, which are now transferred to a new enzyme class 7 (EC 7). The entries that are reclassified are important to extend the enzyme list, and it is the need of the hour. Accordingly, there is an upgradation of entries of this class of enzymes in several databases. This review is a concise compilation of translocases with reference to the number of entries currently available in the databases. This review also focuses on function as well as dysfunction of translocases during normal and disordered states, respectively.

Extracellular Vesicles in the Fungi Kingdom

Extracellular vesicles (EVs) are membranous, rounded vesicles released by prokaryotic and eukaryotic cells in their normal and pathophysiological states. These vesicles form a network of intercellular communication as they can transfer cell- and function-specific information (lipids, proteins and nucleic acids) to different cells and thus alter their function. Fungi are not an exception; they also release EVs to the extracellular space. The vesicles can also be retained in the periplasm as periplasmic vesicles (PVs) and the cell wall. Such fungal vesicles play various specific roles in the lives of these organisms. They are involved in creating wall architecture and maintaining its integrity, supporting cell isolation and defence against the environment. In the case of pathogenic strains, they might take part in the interactions with the host and affect the infection outcomes. The economic importance of fungi in manufacturing high-quality nutritional and pharmaceutical products and in remediation is considerable. The analysis of fungal EVs opens new horizons for diagnosing fungal infections and developing vaccines against mycoses and novel applications of nanotherapy and sensors in industrial processes.

The transport mechanism of P4 ATPase lipid flippases

P4 ATPase lipid flippases are ATP-driven transporters that translocate specific lipids from the exoplasmic to the cytosolic leaflet of biological membranes, thus establishing a lipid gradient between the two leaflets that is essential for many cellular processes. While substrate specificity, subcellular and tissue-specific expression, and physiological functions have been assigned to a number of these transporters in several organisms, the mechanism of lipid transport has been a topic of intense debate in the field. The recent publication of a series of structural models based on X-ray crystallography and cryo-EM studies has provided the first glimpse into how P4 ATPases have adapted the transport mechanism used by the cation-pumping family members to accommodate a substrate that is at least an order of magnitude larger than cations.

Cadmium-induced oxidative stress, histopathology, and transcriptome changes in the hepatopancreas of freshwater crayfish (Procambarus clarkii)

Cadmium (Cd) is a common contaminant in environment. Crayfish are considered suitable for indicating the impact of heavy metals on the environment. However, there is limited information on the mechanisms causing damage to the hepatopancreas of Procambarus clarkii exposed to Cd. We exposed adult male P. clarkii to 2.0, 5.0, and 10.0 mg/L Cd for 24, 48, and 72 h to explore Cd toxicity. Afterwards, we measured bioaccumulations in the hepatopancreas and determined malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), and glutathione S-transferase (GST). Additionally, the hepatopancreas histopathology was analyzed and the transcriptome analysis of the P. clarkii hepatopancreas under Cd stress was conducted. The results revealed that hepatopancreas could accumulate Cd in a time- and dose-dependent manner. Cd induced significant changes in MDA content and antioxidant enzyme activity. Severe histological alterations were observed in crayfish hepatopancreas. After 72 h exposure to 2.0, 5.0, and 10.0 mg/L Cd, transcriptome analysis identified 1061, 747, and 1086 differentially expressed genes (DEGs), respectively. Exposure to 5.0 mg/L Cd inhibited heme binding, tetrapyrrole binding, iron ion binding and activity of oxidoreductase and sulfotransferase, while exposure to 10.0 mg/L Cd enhanced the export of matters from nucleus. In the hepatopancreas treated with 10.0 mg/L Cd, pathways related to diseases and immune system were significantly enriched. Meanwhile, 31, 31, 24, 7, and 12 identified DEGs were associated with the oxidation-reduction process, immune system, ion homeostasis, digestion and absorption, and ATPases, respectively. Our study provides comprehensive information for exploring the toxic mechanisms of Cd and candidate biomarkers for aquatic Cd risk evaluation.

Identification and functional analyses of disease-associated P4-ATPase phospholipid flippase variants in red blood cells

ATP-dependent phospholipid flippase activity crucial for generating lipid asymmetry was first detected in red blood cell (RBC) membranes, but the P4-ATPases responsible have not been directly determined. Using affinity-based MS, we show that ATP11C is the only abundant P4-ATPase phospholipid flippase in human RBCs, whereas ATP11C and ATP8A1 are the major P4-ATPases in mouse RBCs. We also found that ATP11A and ATP11B are present at low levels. Mutations in the gene encoding ATP11C are responsible for blood and liver disorders, but the disease mechanisms are not known. Using heterologous expression, we show that the T415N substitution in the phosphorylation motif of ATP11C, responsible for congenital hemolytic anemia, reduces ATP11C expression, increases retention in the endoplasmic reticulum, and decreases ATPase activity by 61% relative to WT ATP11C. The I355K substitution in the transmembrane domain associated with cholestasis and anemia in mice was expressed at WT levels and trafficked to the plasma membrane but was devoid of activity. We conclude that the T415N variant causes significant protein misfolding, resulting in low protein expression, cellular mislocalization, and reduced functional activity. In contrast, the I355K variant folds and traffics normally but lacks key contacts required for activity. We propose that the loss in ATP11C phospholipid flippase activity coupled with phospholipid scramblase activity results in the exposure of phosphatidylserine on the surface of RBCs, decreasing RBC survival and resulting in anemia.

Structure and function analysis of the C. elegans aminophospholipid translocase TAT-1

The Caenorhabditis elegans aminophospholipid translocase TAT-1 maintains phosphatidylserine (PS) asymmetry in the plasma membrane and regulates endocytic transport. Despite these important functions, the structure-function relationship of this protein is poorly understood. Taking advantage of the tat-1 mutations identified by the C. elegans million mutation project, we investigated the effects of 16 single amino acid substitutions on the two functions of the TAT-1 protein. Two substitutions that alter a highly conserved PISL motif in the fourth transmembrane domain and a highly conserved DKTGT phosphorylation motif, respectively, disrupt both functions of TAT-1, leading to a vesicular gut defect and ectopic PS exposure on the cell surface, whereas most other substitutions across the TAT-1 protein, often predicted to be deleterious by bioinformatics programs, do not affect the functions of TAT-1. These results provide in vivo evidence for the importance of the PISL and DKTGT motifs in P4-type ATPases and improve our understanding of the structure-function relationship of TAT-1. Our study also provides an example of how the C. elegans million mutation project helps decipher the structure, functions, and mechanisms of action of important genes.

Oxidative Stress, Amyloid-β Peptide, and Altered Key Molecular Pathways in the Pathogenesis and Progression of Alzheimer's Disease

Oxidative stress is implicated in the pathogenesis and progression of Alzheimer's disease (AD) and its earlier stage, amnestic mild cognitive impairment (aMCI). One source of oxidative stress in AD and aMCI brains is that associated with amyloid-β peptide, Aβ1-42 oligomers. Our laboratory first showed in AD elevated oxidative stress occurred in brain regions rich in Aβ1-42, but not in Aβ1-42-poor regions, and was among the first to demonstrate Aβ peptides led to lipid peroxidation (indexed by HNE) in AD and aMCI brains. Oxidatively modified proteins have decreased function and contribute to damaged key biochemical and metabolic pathways in which these proteins normally play a role. Identification of oxidatively modified brain proteins by the methods of redox proteomics was pioneered in the Butterfield laboratory. Four recurring altered pathways secondary to oxidative damage in brain from persons with AD, aMCI, or Down syndrome with AD are interrelated and contribute to neuronal death. This "Quadrilateral of Neuronal Death" includes altered: glucose metabolism, mTOR activation, proteostasis network, and protein phosphorylation. Some of these pathways are altered even in brains of persons with preclinical AD. We opine that targeting these pathways pharmacologically and with lifestyle changes potentially may provide strategies to slow or perhaps one day, prevent, progression or development of this devastating dementing disorder. This invited review outlines both in vitro and in vivo studies from the Butterfield laboratory related to Aβ1-42 and AD and discusses the importance and implications of some of the major achievements of the Butterfield laboratory in AD research.

Assay of Flippase Activity in Proteoliposomes Using Fluorescent Lipid Derivatives

Specific membrane proteins, termed lipid flippases, play a central role in facilitating the movement of lipids across cellular membranes. In this protocol, we describe the reconstitution of ATP-driven lipid flippases in liposomes and the analysis of their in vitro flippase activity based on the use of fluorescent lipid derivatives. Working with purified and reconstituted systems provides a well-defined experimental setup and allows to directly characterize these membrane proteins at the molecular level.

P4-ATPases as Phospholipid Flippases-Structure, Function, and Enigmas

P4-ATPases comprise a family of P-type ATPases that actively transport or flip phospholipids across cell membranes. This generates and maintains membrane lipid asymmetry, a property essential for a wide variety of cellular processes such as vesicle budding and trafficking, cell signaling, blood coagulation, apoptosis, bile and cholesterol homeostasis, and neuronal cell survival. Some P4-ATPases transport phosphatidylserine and phosphatidylethanolamine across the plasma membrane or intracellular membranes whereas other P4-ATPases are specific for phosphatidylcholine. The importance of P4-ATPases is highlighted by the finding that genetic defects in two P4-ATPases ATP8A2 and ATP8B1 are associated with severe human disorders. Recent studies have provided insight into how P4-ATPases translocate phospholipids across membranes. P4-ATPases form a phosphorylated intermediate at the aspartate of the P-type ATPase signature sequence, and dephosphorylation is activated by the lipid substrate being flipped from the exoplasmic to the cytoplasmic leaflet similar to the activation of dephosphorylation of Na(+)/K(+)-ATPase by exoplasmic K(+). How the phospholipid is translocated can be understood in terms of a peripheral hydrophobic gate pathway between transmembrane helices M1, M3, M4, and M6. This pathway, which partially overlaps with the suggested pathway for migration of Ca(2+) in the opposite direction in the Ca(2+)-ATPase, is wider than the latter, thereby accommodating the phospholipid head group. The head group is propelled along against its concentration gradient with the hydrocarbon chains projecting out into the lipid phase by movement of an isoleucine located at the position corresponding to an ion binding glutamate in the Ca(2+)- and Na(+)/K(+)-ATPases. Hence, the P4-ATPase mechanism is quite similar to the mechanism of these ion pumps, where the glutamate translocates the ions by moving like a pump rod. The accessory subunit CDC50 may be located in close association with the exoplasmic entrance of the suggested pathway, and possibly promotes the binding of the lipid substrate. This review focuses on properties of mammalian and yeast P4-ATPases for which most mechanistic insight is available. However, the structure, function and enigmas associated with mammalian and yeast P4-ATPases most likely extend to P4-ATPases of plants and other organisms.

On the molecular mechanism of flippase- and scramblase-mediated phospholipid transport

Phospholipid flippases are key regulators of transbilayer lipid asymmetry in eukaryotic cell membranes, critical to many trafficking and signaling pathways. P4-ATPases, in particular, are responsible for the uphill transport of phospholipids from the exoplasmic to the cytosolic leaflet of the plasma membrane, as well as membranes of the late secretory/endocytic pathways, thereby establishing transbilayer asymmetry. Recent studies combining cell biology and biochemical approaches have improved our understanding of the path taken by lipids through P4-ATPases. Additionally, identification of several protein families catalyzing phospholipid 'scrambling', i.e. disruption of phospholipid asymmetry through energy-independent bi-directional phospholipid transport, as well as the recent report of the structure of such a scramblase, opens the way to a deeper characterization of their mechanism of action. Here, we discuss the molecular nature of the mechanism by which lipids may 'flip' across membranes, with an emphasis on active lipid transport catalyzed by P4-ATPases. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Specific mutations in mammalian P4-ATPase ATP8A2 catalytic subunit entail differential glycosylation of the accessory CDC50A subunit

P4-ATPases, or flippases, translocate phospholipids between the two leaflets of eukaryotic biological membranes. They are essential to the physiologically crucial phospholipid asymmetry and involved in severe diseases, but their molecular structure and mechanism are still unresolved. Here, we show that in an extensive mutational alanine screening of the mammalian flippase ATP8A2 catalytic subunit, five mutations stand out by leading to reduced glycosylation of the accessory subunit CDC50A. These mutations may disturb the interaction between the subunits.

Human Type IV P-type ATPases That Work as Plasma Membrane Phospholipid Flippases and Their Regulation by Caspase and Calcium

In plasma membranes, flippases translocate aminophospholipids such as phosphatidylserine and phosphatidylethanolamine from the extracellular to the cytoplasmic leaflet. Mammalian ATP11C, a type IV P-type ATPase, acts as a flippase at the plasma membrane. Here, by expressing 12 human type IV P-type ATPases in ATP11C-deficient cells, we determined that ATP8A2 and ATP11A can also act as plasma membrane flippases. As with ATP11C, ATP8A2 and ATP11A localized to the plasma membrane in a CDC50A-dependent manner. ATP11A was cleaved by caspases during apoptosis, and a caspase-resistant ATP11A blocked apoptotic PtdSer exposure. In contrast, ATP8A2 was not cleaved by caspase, and cells expressing ATP8A2 did not expose PtdSer during apoptosis. Similarly, high Ca(2+) concentrations inhibited the ATP11A and ATP11C PtdSer flippase activity, but ATP8A2 flippase activity was relatively resistant to Ca(2+). ATP11A and ATP11C were ubiquitously expressed in human and mouse adult tissues. In contrast, ATP8A2 was expressed in specific tissues, such as the brain and testis. Thus, ATP8A2 may play a specific role in translocating PtdSer in these tissues.

Mice deficient in the putative phospholipid flippase ATP11C exhibit altered erythrocyte shape, anemia, and reduced erythrocyte life span

Transmembrane lipid transporters are believed to establish and maintain phospholipid asymmetry in biological membranes; however, little is known about the in vivo function of the specific transporters involved. Here, we report that developing erythrocytes from mice lacking the putative phosphatidylserine flippase ATP11C showed a lower rate of PS translocation in vitro compared with erythrocytes from wild-type littermates. Furthermore, the mutant mice had an elevated percentage of phosphatidylserine-exposing mature erythrocytes in the periphery. Although erythrocyte development in ATP11C-deficient mice was normal, the mature erythrocytes had an abnormal shape (stomatocytosis), and the life span of mature erythrocytes was shortened relative to that in control littermates, resulting in anemia in the mutant mice. Thus, our findings uncover an essential role for ATP11C in erythrocyte morphology and survival and provide a new candidate for the rare inherited blood disorder stomatocytosis with uncompensated anemia.

Critical roles of isoleucine-364 and adjacent residues in a hydrophobic gate control of phospholipid transport by the mammalian P4-ATPase ATP8A2

P4-ATPases (flippases) translocate specific phospholipids such as phosphatidylserine from the exoplasmic leaflet of the cell membrane to the cytosolic leaflet, upholding an essential membrane asymmetry. The mechanism of flipping this giant substrate has remained an enigma. We have investigated the importance of amino acid residues in transmembrane segment M4 of mammalian P4-ATPase ATP8A2 by mutagenesis. In the related ion pumps Na(+),K(+)-ATPase and Ca(2+)-ATPase, M4 moves during the enzyme cycle, carrying along the ion bound to a glutamate. In ATP8A2, the corresponding residue is an isoleucine, which recently was found mutated in patients with cerebellar ataxia, mental retardation, and dysequilibrium syndrome. Our analyses of the lipid substrate concentration dependence of the overall and partial reactions of the enzyme cycle in mutants indicate that, during the transport across the membrane, the phosphatidylserine head group passes near isoleucine-364 (I364) and that I364 is critical to the release of the transported lipid into the cytosolic leaflet. Another M4 residue, N359, is involved in recognition of the lipid substrate on the exoplasmic side. Our functional studies are supported by structural homology modeling and molecular dynamics simulations, suggesting that I364 and adjacent hydrophobic residues function as a hydrophobic gate that separates the entry and exit sites of the lipid and directs sequential formation and annihilation of water-filled cavities, thereby enabling transport of the hydrophilic phospholipid head group in a groove outlined by the transmembrane segments M1, M2, M4, and M6, with the hydrocarbon chains following passively, still in the membrane lipid phase.

Where do they come from and where do they go: candidates for regulating extracellular vesicle formation in fungi

In the past few years, extracellular vesicles (EVs) from at least eight fungal species were characterized. EV proteome in four fungal species indicated putative biogenesis pathways and suggested interesting similarities with mammalian exosomes. Moreover, as observed for mammalian exosomes, fungal EVs were demonstrated to be immunologically active. Here we review the seminal and most recent findings related to the production of EVs by fungi. Based on the current literature about secretion of fungal molecules and biogenesis of EVs in eukaryotes, we focus our discussion on a list of cellular proteins with the potential to regulate vesicle biogenesis in the fungi.

ATP11B mediates platinum resistance in ovarian cancer

Platinum compounds display clinical activity against a wide variety of solid tumors; however, resistance to these agents is a major limitation in cancer therapy. Reduced platinum uptake and increased platinum export are examples of resistance mechanisms that limit the extent of DNA damage. Here, we report the discovery and characterization of the role of ATP11B, a P-type ATPase membrane protein, in cisplatin resistance. We found that ATP11B expression was correlated with higher tumor grade in human ovarian cancer samples and with cisplatin resistance in human ovarian cancer cell lines. ATP11B gene silencing restored the sensitivity of ovarian cancer cell lines to cisplatin in vitro. Combined therapy of cisplatin and ATP11B-targeted siRNA significantly decreased cancer growth in mice bearing ovarian tumors derived from cisplatin-sensitive and -resistant cells. In vitro mechanistic studies on cellular platinum content and cisplatin efflux kinetics indicated that ATP11B enhances the export of cisplatin from cells. The colocalization of ATP11B with fluorescent cisplatin and with vesicular trafficking proteins, such as syntaxin-6 (STX6) and vesicular-associated membrane protein 4 (VAMP4), strongly suggests that ATP11B contributes to secretory vesicular transport of cisplatin from Golgi to plasma membrane. In conclusion, inhibition of ATP11B expression could serve as a therapeutic strategy to overcome cisplatin resistance.

Mammalian P4-ATPases and ABC transporters and their role in phospholipid transport

Transport of phospholipids across cell membranes plays a key role in a wide variety of biological processes. These include membrane biosynthesis, generation and maintenance of membrane asymmetry, cell and organelle shape determination, phagocytosis, vesicle trafficking, blood coagulation, lipid homeostasis, regulation of membrane protein function, apoptosis, etc. P(4)-ATPases and ATP binding cassette (ABC) transporters are the two principal classes of membrane proteins that actively transport phospholipids across cellular membranes. P(4)-ATPases utilize the energy from ATP hydrolysis to flip aminophospholipids from the exocytoplasmic (extracellular/lumen) to the cytoplasmic leaflet of cell membranes generating membrane lipid asymmetry and lipid imbalance which can induce membrane curvature. Many ABC transporters play crucial roles in lipid homeostasis by actively transporting phospholipids from the cytoplasmic to the exocytoplasmic leaflet of cell membranes or exporting phospholipids to protein acceptors or micelles. Recent studies indicate that some ABC proteins can also transport phospholipids in the opposite direction. The importance of P(4)-ATPases and ABC transporters is evident from the findings that mutations in many of these transporters are responsible for severe human genetic diseases linked to defective phospholipid transport. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Calcium and Magnesium ATPase Activities in Women with Varying BMIs

OBJECTIVE

Intracellular calcium (Ca) is increased in obese humans, and magnesium (Mg)-ATPase activity is increased in monosodium glutamate-induced obese rats. The aims of this study were to test the hypotheses that Ca-ATPase activity is negatively correlated with BMI, and that Mg-ATPase activity is positively correlated with BMI and Ca-ATPase activity in obese women.

RESEARCH METHODS AND PROCEDURES

Thirty healthy adult women, with BMIs of 20 to 40, donated a single sample of whole blood and were interviewed as to medical history and family history of obesity. Erythrocyte membranes were isolated and assayed for Ca-ATPase and Mg-ATPase. Weight and height were self-reported. Regression analysis was used to determine relationship between BMI and enzyme activity. Family history of obesity served as a covariant.

RESULTS

Ca-ATPase was negatively correlated with increasing BMI (r = - 0.38, p = 0.02). The relationship between BMI and Ca-ATPase remained valid after controlling for family history of obesity (r = -0.36, p = 0.03). There was a positive correlation between Mg-ATPase activity and Ca-ATPase (r = 0.42, p = 0.024), and this relationship remained valid after controlling for BMI and family history of obesity (r = 0.41, p = 0.03).

DISCUSSION

Ca-ATPase activity decreases as BMI increases. Decreased ATPase activity may contribute to increased intracellular calcium, previously reported in obese persons. Further studies are needed to determine whether a drop in Ca-ATPase activity can serve as a marker for the development of obesity.

Phospholipid flippases: building asymmetric membranes and transport vesicles

Phospholipid flippases in the type IV P-type ATPase family (P4-ATPases) are essential components of the Golgi, plasma membrane and endosomal system that play critical roles in membrane biogenesis. These pumps flip phospholipid across the bilayer to create an asymmetric membrane structure with substrate phospholipids, such as phosphatidylserine and phosphatidylethanolamine, enriched within the cytosolic leaflet. The P4-ATPases also help form transport vesicles that bud from Golgi and endosomal membranes, thereby impacting the sorting and localization of many different proteins in the secretory and endocytic pathways. At the organismal level, P4-ATPase deficiencies are linked to liver disease, obesity, diabetes, hearing loss, neurological deficits, immune deficiency and reduced fertility. Here, we review the biochemical, cellular and physiological functions of P4-ATPases, with an emphasis on their roles in vesicle-mediated protein transport. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Caenorhabditis elegans numb inhibits endocytic recycling by binding TAT-1 aminophospholipid translocase

Numb regulates endocytosis in many metazoans, but the mechanism by which it functions is not completely understood. Here we report that the Caenorhabditis elegans Numb ortholog, NUM-1A, a regulator of endocytic recycling, binds the C isoform of transbilayer amphipath transporter-1 (TAT-1), a P4 family adenosine triphosphatase and putative aminophospholipid translocase that is required for proper endocytic trafficking. We demonstrate that TAT-1 is differentially spliced during development and that TAT-1C-specific splicing occurs in the intestine where NUM-1A is known to function. NUM-1A and TAT-1C colocalize in vivo. We have mapped the binding site to an NXXF motif in TAT-1C. This motif is not required for TAT-1C function but is required for NUM-1A's ability to inhibit recycling. We demonstrate that num-1A and tat-1 defects are both suppressed by the loss of the activity of PSSY-1, a phosphatidylserine (PS) synthase. PS is mislocalized in intestinal cells with defects in tat-1 or num-1A function. We propose that NUM-1A inhibits recycling by inhibiting TAT-1C's ability to translocate PS across the membranes of recycling endosomes.

Endocytic sorting and recycling require membrane phosphatidylserine asymmetry maintained by TAT-1/CHAT-1

Endocytic sorting is achieved through the formation of morphologically and functionally distinct sub-domains within early endosomes. Cargoes destined for recycling are sorted to and transported through newly-formed tubular membranes, but the processes that regulate membrane tubulation are poorly understood. Here, we identified a novel Caenorhabditis elegans Cdc50 family protein, CHAT-1, which acts as the chaperone of the TAT-1 P4-ATPase to regulate membrane phosphatidylserine (PS) asymmetry and endocytic transport. In chat-1 and tat-1 mutants, the endocytic sorting process is disrupted, leading to defects in both cargo recycling and degradation. TAT-1 and CHAT-1 colocalize to the tubular domain of the early endosome, the tubular endocytic recycling compartment (ERC), and the recycling endosome where PS is enriched on the cytosolic surface. Loss of tat-1 and chat-1 function disrupts membrane PS asymmetry and abrogates the tubular membrane structure. Our data suggest that CHAT-1 and TAT-1 maintain membrane phosphatidylserine asymmetry, thus promoting membrane tubulation and regulating endocytic sorting and recycling.

The process by which cells take up nutrients and other large molecules from the extracellular environment is known as endocytosis. At the cell surface, external molecules become enclosed in membrane spheres called endosomes. Early endosomes serve as a sorting station, directing the contents (cargo molecules) to the correct compartment within the cell. This is thought to be achieved by the formation of membrane structures with distinct shape and function. For example, cargoes destined for recycling and degradation are processed through tubular membrane structures and big vesicular compartments, respectively. However, it is poorly understood how early endosome membranes are shaped into different structures. Here we show that two proteins, CHAT-1 and TAT-1, regulate membrane structure and are important for normal endocytic transport in the nematode worm C. elegans. TAT-1 and CHAT-1 are found in tubular membrane structures along the sorting and recycling pathway, where they enrich the outer membrane layer with a lipid called phosphatidylserine (PS) and probably change the membrane curvature. Loss of tat-1 and chat-1 function disrupts the asymmetric distribution of PS, abolishes tubular membrane structures, and abrogates endocytic sorting/recycling. Our data support a role of TAT-1/CHAT-1–regulated membrane PS asymmetry in promoting membrane tubulation for endocytic cargo sorting and recycling.

In and out of the cation pumps: P-type ATPase structure revisited

Active transport across membranes is a crucial requirement for life. P-type ATPases build up electrochemical gradients at the expense of ATP by forming and splitting a covalent phosphoenzyme intermediate, coupled to conformational changes in the transmembrane section where the ions are translocated. The marked increment during the last three years in the number of crystal structures of P-type ATPases has greatly improved our understanding of the similarities and differences of pumps with different ion specificities, since the structures of the Ca2+-ATPase, the Na+,K+-ATPase and the H+-ATPase can now be compared directly. Mechanisms for ion gating, charge neutralization and backflow prevention are starting to emerge from comparative structural analysis; and in combination with functional studies of mutated pumps this provides a framework for speculating on how the ions are bound and released as well as on how specificity is achieved.

Age-related loss of phospholipid asymmetry in APP(NLh)/APP(NLh) x PS-1(P264L)/PS-1(P264L) human double mutant knock-in mice: relevance to Alzheimer disease

Using APP(NLh)/APP(NLh) x PS-1(P246L)/PS-1(P246L) human double knock-in (APP/PS-1) mice, we examined whether phosphatidylserine (PtdSer) asymmetry is significantly altered in brain of this familial Alzheimer disease mouse model in an age-dependent manner as a result of oxidative stress, toxic Abeta(1-42) oligomer production, and/or apoptosis. Annexin V (AV) and NBD-PS fluorescence in synaptosomes of wild-type (WT) and APP/PS-1 mice were used to determine PtdSer exposure with age, while Mg(2+) ATPase activity was determined to correlate PtdSer asymmetry changes with PtdSer translocase, flippase, activity. AV and NBD-PS results demonstrated significant PtdSer exposure beginning at 9 months compared to 1-month-old WT controls for both assays, a trend that was exacerbated in synaptosomes of APP/PS-1 mice. Decreasing Mg(2+) ATPase activity confirms that the age-related loss of PtdSer asymmetry is likely due to loss of flippase activity, more prominent in APP/PS-1 brain. Two-site sandwich ELISA on SDS- and FA-soluble APP/PS-1 brain fractions were conducted to correlate Abeta(1-40) and Abeta(1-42) levels with age-related trends determined from the AV, NBD-PS, and Mg(2+) ATPase assays. ELISA revealed a significant increase in both SDS- and FA-soluble Abeta(1-40) and Abeta(1-42) with age, consistent with PtdSer and flippase assay trends. Lastly, because PtdSer exposure is affected by pro-apoptotic caspase-3, levels of both latent and active forms were measured. Western blotting results demonstrated an increase in both active fragments of caspase-3 with age, while levels of pro-caspase-3 decrease. These results are discussed with relevance to loss of lipid asymmetry and consequent neurotoxicity in brain of subjects with Alzheimer disease.

Screening for the drug-phospholipid interaction: correlation to phospholipidosis

Phospholipid bilayers represent a complex, anisotropic environment fundamentally different from bulk oil or octanol, for instance. Even "simple" drug association to phospholipid bilayers can only be fully understood if the slab-of-hydrocarbon approach is abandoned and the complex, anisotropic properties of lipid bilayers reflecting the chemical structures and organization of the constituent phospholipids are considered. The interactions of drugs with phospholipids are important in various processes, such as drug absorption, tissue distribution, and subcellular distribution. In addition, drug-lipid interactions may lead to changes in lipid-dependent protein activities, and further, to functional and morphological changes in cells, a prominent example being the phospholipidosis (PLD) induced by cationic amphiphilic drugs. Herein we briefly review drug-lipid interactions in general and the significance of these interactions in PLD in particular. We also focus on a potential causal connection between drug-induced PLD and steatohepatitis, which is induced by some cationic amphiphilic drugs.

Localization, purification, and functional reconstitution of the P4-ATPase Atp8a2, a phosphatidylserine flippase in photoreceptor disc membranes

P(4)-ATPases comprise a relatively new subfamily of P-type ATPases implicated in the energy-dependent translocation of aminophospholipids across cell membranes. In this study, we report on the localization and functional properties of Atp8a2, a member of the P(4)-ATPase subfamily that has not been studied previously. Reverse transcription-PCR revealed high expression of atp8a2 mRNA in the retina and testis. Within the retina, immunofluorescence microscopy and subcellular fractionation studies localized Atp8a2 to outer segment disc membranes of rod and cone photoreceptor cells. Atp8a2 purified from photoreceptor outer segments by immunoaffinity chromatography exhibited ATPase activity that was stimulated by phosphatidylserine and to a lesser degree phosphatidylethanolamine but not by phosphatidylcholine or other membrane lipids. Purified Atp8a2 was reconstituted into liposomes containing fluorescent-labeled phosphatidylserine to measure the ability of Atp8a2 to flip phosphatidylserine across the lipid bilayer. Fluorescence measurements showed that Atp8a2 flipped fluorescent-labeled phosphatidylserine from the inner leaflet of liposomes (equivalent to the exocytoplasmic leaflet of cell membranes) to the outer leaflet (equivalent to cytoplasmic leaflet) in an ATP-dependent manner. Our studies provide the first direct biochemical evidence that purified P(4)-ATPases can translocate aminophospholipids across membranes and further implicates Atp8a2 in the generation and maintenance of phosphatidylserine asymmetry in photoreceptor disc membranes.

Reconstitution of phospholipid translocase activity with purified Drs2p, a type-IV P-type ATPase from budding yeast

Type-IV P-type ATPases (P4-ATPases) are putative phospholipid translocases, or flippases, that translocate specific phospholipid substrates from the exofacial to the cytosolic leaflet of membranes to generate phospholipid asymmetry. In addition, the activity of Drs2p, a P4-ATPase from Saccharomyces cerevisiae, is required for vesicle-mediated protein transport from the Golgi and endosomes, suggesting a role for phospholipid translocation in vesicle budding. Drs2p is necessary for translocation of a fluorescent phosphatidylserine analogue across purified Golgi membranes. However, a flippase activity has not been reconstituted with purified Drs2p or any other P4-ATPase, so whether these ATPases directly pump phospholipid across the membrane bilayer is unknown. Here, we show that Drs2p can catalyze phospholipid translocation directly through purification and reconstitution of this P4-ATPase into proteoliposomes. The noncatalytic subunit, Cdc50p, also was reconstituted in the proteoliposome, although at a substoichiometric concentration relative to Drs2p. In proteoliposomes containing Drs2p, a phosphatidylserine analogue was actively flipped across the liposome bilayer to the outer leaflet in the presence of Mg(2+)-ATP, whereas no activity toward the phosphatidylcholine or sphingomyelin analogues was observed. This flippase activity was mediated by Drs2p, because protein-free liposomes or proteoliposomes reconstituted with a catalytically inactive form of Drs2p showed no translocation activity. These data demonstrate for the first time the reconstitution of a flippase activity with a purified P4-ATPase.

P-type ATPase TAT-2 negatively regulates monomethyl branched-chain fatty acid mediated function in post-embryonic growth and development in C. elegans

Monomethyl branched-chain fatty acids (mmBCFAs) are essential for Caenorhabditis elegans growth and development. To identify factors acting downstream of mmBCFAs for their function in growth regulation, we conducted a genetic screen for suppressors of the L1 arrest that occurs in animals depleted of the 17-carbon mmBCFA C17ISO. Three of the suppressor mutations defined an unexpected player, the P-type ATPase TAT-2, which belongs to the flippase family of proteins that are implicated in mediating phospholipid bilayer asymmetry. We provide evidence that TAT-2, but not other TAT genes, has a specific role in antagonizing the regulatory activity of mmBCFAs in intestinal cells. Interestingly, we found that mutations in tat-2 also suppress the lethality caused by inhibition of the first step in sphingolipid biosynthesis. We further showed that the fatty acid side-chains of glycosylceramides contain 20%–30% mmBCFAs and that this fraction is greatly diminished in the absence of mmBCFA biosynthesis. These results suggest a model in which a C17ISO-containing sphingolipid may mediate the regulatory functions of mmBCFAs and is negatively regulated by TAT-2 in intestinal cells. This work indicates a novel connection between a P-type ATPase and the critical regulatory function of a specific fatty acid.

Fatty acids serve diverse functions in organisms, including roles at the cell membrane to coordinate cell signaling processes. Monomethyl branched-chain fatty acids (mmBCFAs) are a special type of fatty acid that is commonly present in animals. Because mmBCFAs are a small component of the total fatty acid pool, their functions have not been a major research focus and are largely unclear. We tackled the problem using the nematode C. elegans. Our laboratory previously found that without mmBCFAs, worms cannot develop normally and die. To understand how these obscure fatty acids perform such important roles, we searched for other factors involved in the process by conducting a mutagenesis screen to uncover mutant worms that can recover the ability to grow without the presence of mmBCFAs. We found several such mutations in a single gene that codes for a protein called TAT-2. TAT-2 is one of several poorly understood P-type ATPases that likely help maintain the proper lipid structure in cell membranes. Our work indicates that TAT-2 antagonizes the growth regulatory function of mmBCFAs in intestinal cells. Studies on how mmBCFAs and this protein functionally interact explore a novel, interesting, and important problem that is only beginning to be understood.

Cdc50p plays a vital role in the ATPase reaction cycle of the putative aminophospholipid transporter Drs2p

Members of the P(4) subfamily of P-type ATPases are believed to catalyze transport of phospholipids across cellular bilayers. However, most P-type ATPases pump small cations or metal ions, and atomic structures revealed a transport mechanism that is conserved throughout the family. Hence, a challenging problem is to understand how this mechanism is adapted in P(4)-ATPases to flip phospholipids. P(4)-ATPases form heteromeric complexes with Cdc50 proteins. The primary role of these additional polypeptides is unknown. Here, we show that the affinity of yeast P(4)-ATPase Drs2p for its Cdc50-binding partner fluctuates during the transport cycle, with the strongest interaction occurring at a point where the enzyme is loaded with phospholipid ligand. We also find that specific interactions with Cdc50p are required to render the ATPase competent for phosphorylation at the catalytically important aspartate residue. Our data indicate that Cdc50 proteins are integral components of the P(4)-ATPase transport machinery. Thus, acquisition of these subunits may have been a crucial step in the evolution of flippases from a family of cation pumps.

P4 ATPases - lipid flippases and their role in disease

P4 ATPases (type 4 P-type ATPases) are multispan transmembrane proteins that have been implicated in phospholipid translocation from the exoplasmic to the cytoplasmic leaflet of biological membranes. Studies in Saccharomyces cerevisiae have indicated that P4 ATPases are important in vesicle biogenesis and are required for vesicular trafficking along several intracellular vesicular transport routes. Although little is known about mammalian P4 ATPases, some members of this subfamily appear to be associated with human disease or mouse pathophysiology. ATP8B1, a phosphatidylserine translocase, is the most extensively studied mammalian P4 ATPase. This protein is important for maintaining the detergent resistant properties of the apical membrane of the hepatocyte. Mutations in ATP8B1 give rise to severe liver disease. Furthermore, a role for Atp8b3 in mouse sperm cell capacitation has been suggested, whereas deficiency of Atp10a and Atp10d leads to insulin resistance and obesity in mice. Here we review the present status on the pathophysiological consequences of P4 ATPase deficiency.

The C. elegans P4-ATPase TAT-1 regulates lysosome biogenesis and endocytosis

P-type adenosine triphosphatases (ATPases) of the Drs2p family (P4-ATPases) are multipass transmembrane proteins required to generate and maintain phospholipid asymmetry in membrane bilayers. In Saccharomyces cerevisiae, several members of this family control distinct transport events within the endosomal and secretory pathways. Comparatively, little is known about the functions of P4-ATPases in multicellular organisms. In this study, we analyzed the role of the Caenorhabditis elegans Drs2p homologue transbilayer amphipath transporter (TAT)-1 in intracellular trafficking. tat-1 is expressed in many tissues including the intestine, the epidermis and the nervous system. In intestinal cells, tat-1 loss-of-function mutants accumulate large vacuoles of mixed endolysosomal identity positive for the lysosomal protein LMP-1. In addition, they lack the same class of storage granules as lmp-1 mutants, suggesting that part of the tat-1 phenotype might result from LMP-1 sequestration in an aberrant compartment. Epidermal cells mutant for tat-1 contain acidified giant hybrid multivesicular bodies probably corresponding to endolysosomal intermediate compartments or deficient lysosomes. Finally, TAT-1 is required for yolk uptake in oocytes and an early step of fluid-phase endocytosis in the intestine. Hence, TAT-1 is required at multiple steps of the endolysosomal pathway, at least in part by ensuring proper trafficking of cell-specific effector proteins.

Role of C. elegans TAT-1 protein in maintaining plasma membrane phosphatidylserine asymmetry

The asymmetrical distribution of phospholipids on the plasma membrane is critical for maintaining cell integrity and physiology and for regulating intracellular signaling and important cellular events such as clearance of apoptotic cells. How phospholipid asymmetry is established and maintained is not fully understood. We report that the Caenorhabditis elegans P-type adenosine triphosphatase homolog, TAT-1, is critical for maintaining cell surface asymmetry of phosphatidylserine (PS). In animals deficient in tat-1, PS is abnormally exposed on the cell surface, and normally living cells are randomly lost through a mechanism dependent on PSR-1, a PS-recognizing phagocyte receptor, and CED-1, which contributes to recognition and engulfment of apoptotic cells. Thus, tat-1 appears to function in preventing appearance of PS in the outer leaflet of plasma membrane, and ectopic exposure of PS on the cell surface may result in removal of living cells by neighboring phagocytes.

Loss of phospholipid asymmetry and elevated brain apoptotic protein levels in subjects with amnestic mild cognitive impairment and Alzheimer disease

Oxidative stress, a hallmark of Alzheimer disease (AD), has been shown to induce lipid peroxidation and apoptosis disrupting cellular homeostasis. Normally, the aminophospholipid phosphatidylserine (PtdSer) is asymmetrically distributed on the cytosolic leaflet of the lipid bilayer. Under oxidative stress conditions, asymmetry is altered, characterized by the appearance of PtdSer on the outer leaflet, to initiate the first stages of an apoptotic process. PtdSer asymmetry is actively maintained by the ATP-dependent translocase flippase, whose function is inhibited if covalently bound by lipid peroxidation products, 4-hydroxynonenal (HNE) and acrolein, within the membrane bilayer in which they are produced. Additionally, pro-apoptotic proteins Bax and caspase-3 have been implemented in the oxidative modification of PtdSer resulting in subsequent asymmetric collapse, while anti-apoptotic protein Bcl-2 has been found to prevent this process. The current investigation focused on detection of PtdSer on the outer leaflet of the bilayer in synaptosomes from brain of subjects with AD and amnestic mild cognitive impairment (MCI), as well as expression levels of apoptosis-related proteins Bcl-2, Bax, and caspase-3. Fluorescence and Western blot analysis suggest PtdSer exposure on the outer leaflet is significantly increased in brain from subjects with MCI and AD contributing to early apoptotic elevation of pro- and anti-apoptotic proteins and finally neuronal loss. MCI is considered a possible transition point between normal cognitive aging and probable AD. Brain from subjects with MCI is reported to have increased levels of tissue oxidation; therefore, the results of this study could mark the progression of patients with MCI into AD. This study contributes to a model of apoptosis-specific oxidation of phospholipids consistent with the notion that PtdSer exposure is required for apoptotic-cell death.

Diseases of intramembranous lipid transport

The maintenance of transbilayer distribution of phospholipids is crucial for proper cell function. Intramembrane transport of lipids is mediated by three activities termed floppases, flippases, and scramblases. Members of the ATP-binding cassette transporter family and P-type ATPase superfamily have been implicated in the translocation of lipids. The importance of these activities is exemplified by several severe human inherited disorders that are caused by defects in intramembranous transport of lipids. In order to elucidate the molecular mechanisms that underlie these disorders, the combination of in vivo, biochemical, and structural analyses on intramembrane transporters is crucial.

Proteins involved in lipid translocation in eukaryotic cells

Since the first discovery of ATP-dependent translocation of lipids in the human erythrocyte membrane in 1984, there has been much evidence of the existence of various ATPases translocating lipids in eukaryotic cell membranes. They include P-type ATPases involved in inwards lipid transport from the exoplasmic leaflet to the cytosolic leaflet and ABC proteins involved in outwards transport. There are also ATP-independent proteins that catalyze the passage of lipids in both directions. Five P-type ATPase involved in lipid transport have been genetically characterized in yeast cells, suggesting a pool of several proteins with partially redundant activities responsible for the regulation of lipid asymmetry. However, expression and purification of individual yeast proteins is still insufficient to allow reconstitution experiments in liposomes. In this review, we want to give an overview over current investigation efforts about the identification and purification of proteins that may be involved in lipid translocation.

mGluRI targets microglial activation and selectively prevents neuronal cell engulfment through Akt and caspase dependent pathways

Metabotropic glutamate receptors (mGluRs) are expressed throughout the mammalian central nervous system and integrate a host of signal transduction pathways that determine cellular function, plasticity and injury. Yet, one of the more unique regulatory functions of this family of GTP-binding proteins involves cytoprotection in the nervous system. Here, we demonstrate that activation of group I metabotropic glutamate receptors (mGluRIs) in primary hippocampal neurons not only provides intrinsic cellular protection for the maintenance of genomic DNA integrity, but also prevents inflammatory microglial activation and specific neuronal cell engulfment during free radical oxidative stress. Loss of cellular membrane asymmetry and exposure of membrane phosphatidylserine (PS) residues were necessary and sufficient to result in microglial activation and proliferation, since administration of an antibody to the PS receptor could block microglial activity. Through the continuous assessment of individual neurons in real time, activation of mGluRIs was documented to block neuronal PS exposure and prevented subsequent neuronal cell engulfment by microglia seeking "PS tagged" neurons. Furthermore, regulation of both cellular integrity and microglial activity by mGluRI activation was dependent upon the activation and phosphorylation of protein kinase B (Akt1), prevention of mitochondrial membrane depolarization with associated permeability transition pore complex formation, and the down regulation of caspase 9-like activity. Our work defines a significant role of mGluRIs for the modulation of cellular survival and inflammation in the nervous system during oxidative stress.

The type 4 subfamily of P-type ATPases, putative aminophospholipid translocases with a role in human disease

The maintenance of phospholipid asymmetry in membrane bilayers is a paradigm in cell biology. However, the mechanisms and proteins involved in phospholipid translocation are still poorly understood. Members of the type 4 subfamily of P-type ATPases have been implicated in the translocation of phospholipids from the outer to the inner leaflet of membrane bilayers. In humans, several inherited disorders have been identified which are associated with loci harboring type 4 P-type ATPase genes. Up to now, one inherited disorder, Byler disease or progressive familial intrahepatic cholestasis type 1 (PFIC1), has been directly linked to mutations in a type 4 P-type ATPase gene. How the absence of an aminophospholipid translocase activity relates to this severe disease is, however, still unclear. Studies in the yeast Saccharomyces cerevisiae have recently identified important roles for type 4 P-type ATPases in intracellular membrane- and protein-trafficking events. These processes require an (amino)phospholipid translocase activity to initiate budding or fusion of membrane vesicles from or with other membranes. The studies in yeast have greatly contributed to our cell biological insight in membrane dynamics and intracellular-trafficking events; if this knowledge can be translated to mammalian cells and organs, it will help to elucidate the molecular mechanisms which underlie severe inherited human diseases such as Byler disease.

Drs2p-coupled aminophospholipid translocase activity in yeast Golgi membranes and relationship to in vivo function

Aminophospholipid translocases (APLTs) are defined primarily by their ability to flip fluorescent or spin-labeled derivatives of phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the external leaflet of a membrane bilayer to the cytosolic leaflet and are thought to establish phospholipid asymmetry in biological membranes. The identities of APLTs remain unknown, although candidate proteins include the Drs2p/ATPase II subfamily of P-type ATPases. Drs2p from budding yeast localizes to the trans-Golgi network (TGN), and here we show that this membrane contains an ATP-dependent APLT that flips 7-nitro-2-1,3-benzoxadiazol-4-yl (NBD) PS and PE derivatives from the luminal to the cytosolic leaflet. To assess the contribution of Drs2p to this activity, TGN membranes were prepared from strains harboring WT or temperature-sensitive alleles of DRS2 and null alleles of three other potential APLT genes (DNF1, DNF2, and DNF3). Assay of these membranes indicated that Drs2p was required for the ATP-dependent translocation of NBD-PS, whereas no active translocation of NBD-PE or NBD-phosphatidylcholine was detected. The specificity of Drs2p for NBD-PS suggested that translocation of PS would be required for the function of Drs2p in protein transport from the TGN. However, cho1 yeast strains that are unable to synthesize PS do not phenocopy drs2 but instead transport proteins normally via the secretory pathway. In addition, a drs2 cho1 double mutant retains drs2 transport defects. Therefore, whereas NBD-PS is a preferred substrate for Drs2p in vitro, endogenous PS is not an obligatory substrate in vivo for the role Drs2p plays in protein transport.

Modulation of phospholipid asymmetry in synaptosomal membranes by the lipid peroxidation products, 4-hydroxynonenal and acrolein: implications for Alzheimer's disease

Membrane lipid bilayer asymmetry is maintained by the ATP-dependent enzyme flippase. An early signal of synaptosomal apoptosis is the loss of phospholipid asymmetry and the appearance of phosphatidylserine (PS) in the outer leaflet of the membrane. Two highly reactive products of lipid peroxidation, 4-hydroxynonenal (HNE) and acrolein, both elevated in Alzheimer's disease (AD) brain, have been shown to induce apoptosis and disrupt cellular ion homeostasis. These reactive aldehydes can structurally modify proteins by covalent interaction and inhibit enzyme function. Phospholipid asymmetry of PS is maintained by the ATP-requiring enzyme flippase. We have investigated the inactivation of the transmembrane enzyme aminophospholipid-translocase (or flippase) by HNE and acrolein. Flippase activity depends on a critical cysteine residue, a possible site of covalent modification by HNE or acrolein. The present study demonstrates that these alkenals induce the appearance of PS on the outer bilayer lamellae and suggests that increases in intracellular Ca(2+) might not be the sole cause for loss of flippase activity. Rather, other mechanisms that could modulate the function of flippase might be important in phospholipid asymmetry disruption. These results are discussed with potential relevance to neuronal loss in Alzheimer's disease brain.

Requirement for neo1p in retrograde transport from the Golgi complex to the endoplasmic reticulum

Neo1p from Saccharomyces cerevisiae is an essential P-type ATPase and potential aminophospholipid translocase (flippase) in the Drs2p family. We have previously implicated Drs2p in protein transport steps in the late secretory pathway requiring ADP-ribosylation factor (ARF) and clathrin. Here, we present evidence that epitope-tagged Neo1p localizes to the endoplasmic reticulum (ER) and Golgi complex and is required for a retrograde transport pathway between these organelles. Using conditional alleles of NEO1, we find that loss of Neo1p function causes cargo-specific defects in anterograde protein transport early in the secretory pathway and perturbs glycosylation in the Golgi complex. Rer1-GFP, a protein that cycles between the ER and Golgi complex in COPI and COPII vesicles, is mislocalized to the vacuole in neo1-ts at the nonpermissive temperature. These phenotypes suggest that the anterograde protein transport defect is a secondary consequence of a defect in a COPI-dependent retrograde pathway. We propose that loss of lipid asymmetry in the cis Golgi perturbs retrograde protein transport to the ER.

Functional cloning of the miltefosine transporter. A novel P-type phospholipid translocase from Leishmania involved in drug resistance

The antitumor drug miltefosine (hexadecylphosphocholine, MIL) has recently been approved as the first oral agent for the treatment of visceral leishmaniasis. Little is known about the mechanisms of action and uptake of MIL in either parasites or tumor cell lines. We have cloned a putative MIL transporter (LdMT) by functional rescue, using a Leishmania donovani-resistant line defective in the inward-directed translocation of both MIL and glycerophospholipids. LdMT is a novel P-type ATPase belonging to the partially characterized aminophospholipid translocase subfamily. Resistant parasites transfected with LdMT regain their sensitivity to MIL and edelfosine and the ability to normally take up [14C]MIL and fluorescent-labeled glycerophospholipids. Moreover, LdMT localizes to the plasma membrane, and its overexpression in Leishmania tarentolae, a species non-sensitive to MIL, significantly increases the uptake of [14C]MIL, strongly suggesting that this protein behaves as a true translocase. Finally, both LdMT-resistant alleles encompass single but distinct point mutations, each of which impairs transport function, explaining the resistant phenotype. These results demonstrate biochemically and genetically the direct involvement of LdMT in MIL and phospholipids translocation in Leishmania and describe for the first time a P-type ATPase involved in MIL uptake and potency in eukaryotic cells.

Aminophospholipid asymmetry: A matter of life and death

Maintenance of membrane lipid asymmetry is a dynamic process that influences many events over the lifespan of the cell. With few exceptions, most cells restrict the bulk of the aminophospholipids to the inner membrane leaflet by means of specific transporters. Working in concert with each other, these proteins correct for sporadic incursions of the aminophospholipids to the outer membrane leaflet as a result of bilayer imbalances created by various cellular events. A shift in the relative contribution in each of these activities can result in sustained exposure of the aminophospholipids at the cell surface, which allows capture of the cells by phagocytes before the integrity of the plasma membrane is compromised. The absence of an efficient recognition and elimination mechanism can result in uncontrolled and persistent presentation of self-antigens to the immune system, with development of autoimmune syndromes. To prevent this, phagocytes have developed a diverse array of distinct and redundant receptor systems that drive the postphagocytic events along pathways that facilitate cross-talk between the homeostatic and the immune systems. In this work, we review the basis for the proposed mechanism(s) by which apoptotic ligands appear on the target cell surface and the phagocyte receptors that recognize these moieties.

Autoimmune disease as a cause of reproductive failure

High-risk pregnancy is the most common clinical association with antiphospholipid antibodies; the principal manifestations are pregnancy loss and early preeclampsia. Membership in this family of antibodies is continually growing and includes antibodies against a variety of phospholipids, phospholipid-protein complexes, and phospholipid-binding proteins. The current information in the literature is inadequate to clearly implicate a subgroup of antiphospholipid antibodies or a particular pathophysiologic mechanism as being responsible for poor pregnancy outcomes. It is clear, however, that prevalent diagnostic tests for LA and aCL are extremely useful to identify many of these patients, but are inadequate for diagnosis of all patients with autoimmune pregnancy loss or to elucidate the pathophysiology. Many patients who present clinically with autoimmune-like pregnancy complications currently are negative in tests for LA or aCL, but have antibodies against annexin V, phosphatidylserine, or other relevant antigens. The greatest risk for a complicated pregnancy is conveyed by a subgroup of antibodies that affect the normal function of placental trophoblast. As clinical laboratory tests designed to detect more members of the antiphospholipid antibody family become available, understanding of this complicated disease (APS) will increase.

Regulation of transbilayer plasma membrane phospholipid asymmetry

Lipids in biological membranes are asymmetrically distributed across the bilayer; the amine-containing phospholipids are enriched on the cytoplasmic surface of the plasma membrane, while the choline-containing and sphingolipids are enriched on the outer surface. The maintenance of transbilayer lipid asymmetry is essential for normal membrane function, and disruption of this asymmetry is associated with cell activation or pathologic conditions. Lipid asymmetry is generated primarily by selective synthesis of lipids on one side of the membrane. Because passive lipid transbilayer diffusion is slow, a number of proteins have evolved to either dissipate or maintain this lipid gradient. These proteins fall into three classes: 1) cytofacially-directed, ATP-dependent transporters ("flippases"); 2) exofacially-directed, ATP-dependent transporters ("floppases"); and 3) bidirectional, ATP-independent transporters ("scramblases"). The flippase is highly selective for phosphatidylserine and functions to keep this lipid sequestered from the cell surface. Floppase activity has been associated with the ABC class of transmembrane transporters. Although they are primarily nonspecific, at least two members of this class display selectivity for their substrate lipid. Scramblases are inherently nonspecific and function to randomize the distribution of newly synthesized lipids in the endoplasmic reticulum or plasma membrane lipids in activated cells. It is the combined action of these proteins and the physical properties of the membrane bilayer that generate and maintain transbilayer lipid asymmetry.

Transbilayer phospholipid movement and the clearance of apoptotic cells

When lymphocytes (and other cells) die by apoptosis, they orchestrate their own orderly removal by macrophages, and thereby prevent the inflammation that would otherwise attend cell lysis. As part of their demise, apoptotic cells disrupt the normal asymmetric distribution of phospholipids across their plasma membranes, an asymmetry normally maintained by an aminophospholipid translocase. This disruption of asymmetry, mediated by an activity known as the scramblase, generates ligands on the cell surface that trigger phagocytosis of the dying cell before lysis can occur. This crucial alteration of the plasma membrane is not dependent on caspase-mediated proteolysis, but quite unexpectedly, it is required both on the apoptotic target cell and on the phagocyte that engulfs it. At least in the phagocyte, this rearrangement may depend on the activity of an ABC ATPase, termed ABC1 in mammals and ced-7 in C. elegans.

An essential subfamily of Drs2p-related P-type ATPases is required for protein trafficking between Golgi complex and endosomal/vacuolar system

The Saccharomyces cerevisiae genome contains five genes encoding P-type ATPases that are potential aminophospholipid translocases (APTs): DRS2, NEO1, and three uncharacterized open reading frames that we have named DNF1, DNF2, and DNF3 for DRS2/NEO1 family. NEO1 is the only essential gene in APT family and seems to be functionally distinct from the DRS2/DNF genes. The drs2Delta dnf1Delta dnf2Delta dnf3Delta quadruple mutant is inviable, although any one member of this group can maintain viability, indicating that there is a substantial functional overlap between the encoded proteins. We have previously implicated Drs2p in clathrin function at the trans-Golgi network. In this study, we constructed strains carrying all possible viable combinations of null alleles from this group and analyzed them for defects in protein transport. The drs2Delta dnf1Delta mutant grows slowly, massively accumulates intracellular membranes, and exhibits a substantial defect in the transport of alkaline phosphatase to the vacuole. Transport of carboxypeptidase Y to the vacuole is also perturbed, but to a lesser extent. In addition, the dnf1Delta dnf2Delta dnf3Delta mutant exhibits a defect in recycling of GFP-Snc1p in the early endocytic-late secretory pathways. Drs2p and Dnf3p colocalize with the trans-Golgi network marker Kex2p, whereas Dnf1p and Dnf2p seem to localize to the plasma membrane and late exocytic or early endocytic membranes. We propose that eukaryotes express multiple APT subfamily members to facilitate protein transport in multiple pathways.

An aminophospholipid translocase associated with body fat and type 2 diabetes phenotypes

OBJECTIVE

We have shown that a region on proximal mouse chromosome 7, near the pink-eyed (p) dilution locus, contains an ATPase (pfatp), a putative aminophospholipid translocase. Studies have suggested that this gene is a prime candidate for modulating body fat or involved in lipid metabolism in mouse and humans. Toward further analyses, our objective was to generate the complete genomic structures of mouse and human genes.

RESEARCH METHODS AND PROCEDURES

The genomic structure of mouse pfatp was deduced by comparing the full-length cDNA sequence with the genomic sequence derived from a mouse BAC. The human ortholog was identified from the National Center for Biotechnology Information database. Full-length cDNA was generated, and the corresponding genomic structure was deduced from the Human Genome Database.

RESULTS

Murine pfatp, and its human ortholog, PFATP, belong to class V of the third subfamily of P-type ATPases. The gene organization is strikingly similar in both organisms and all exon-intron junctions are conserved. A putative promoter region of PFATP contains a strong CpG island. The 5' untranslated regions of the two cDNAs have potential binding sites for multiple transcription factors, including Sp1, USF, AP1, and AP2, involved in adipogenesis and adipocyte metabolism.

DISCUSSION

We report the generation of the complete genomic structure of a novel aminophospholipid translocase in mice and humans. Because the exact biological role and the subsequent relevance of these ATPases to obesity and diabetes are unknown, these data help to delineate the role of these genes in lipid/adipocyte metabolism.