Basic echocardiography for undergraduate students: a comparison of different peer-teaching approaches

BACKGROUND

The aim of this study was to assess the impact of different teaching interventions in a peer-teaching environment on basic echocardiography skills and to examine the influence of gender on learning outcomes.

METHODS

We randomly assigned 79 s year medical students (55 women, 24 men) to one of four groups: peer teaching (PT), peer teaching using Peyton's four-step approach (PPT), team based learning (TBL) and video-based learning (VBL). All groups received theoretical and practical hands-on training according to the different approaches. Using a pre-post-design we assessed differences in theoretical knowledge [multiple choice (MC) exam], practical skills (Objective Structured Practical Examination, OSPE) and evaluation results with respect to gender.

RESULTS

There was a significant gain in theoretical knowledge for all students. There were no relevant differences between the four groups regarding the MC exam and OSPE results. The majority of students achieved good or very good results. Acceptance of the peer-teaching concept was moderate and all students preferred medical experts to peer tutors even though the overall rating of the instructors was fairly good. Students in the Video group would have preferred a different training method. There was no significant effect of gender on evaluation results.

CONCLUSIONS

Using different peer-teaching concepts proved to be effective in teaching basic echocardiography. Gender does not seem to have an impact on effectiveness of the instructional approach. Qualitative analysis revealed limited acceptance of peer teaching and especially of video-based instruction.

Massive respiratory dysfunction as sign of fulminant peripartum cardiomyopathy (PPCM)

OBJECTIVE

Case report of a 35-year-old gravida 3, para 2, at 40 + 6 weeks with massive respiratory dysfunction with need of oxygenation, requiring cesarean section.

CASE REPORT

Postpartum investigations revealed pathological cardiomegaly with left ventricular failure (NYHAIV). Cardiac biopsy diagnosed postpartum dilatative cardiomyopathy. Despite medication with bromocriptine and levosimendan, cardiac function continued to decrease, requiring surgical intervention and implantation of an intracorporal, left ventricular assist device. Following surgery, cardiac function progressively improved and stabilized.

OBJECTIVE

Peripartum cardiomyopathy (PPCM) is a rare, pregnancy-induced disease and requires an interdisciplinary approach for diagnostics and therapeutical treatment.

Lipid flippases and their biological functions

The typically distinct phospholipid composition of the two leaflets of a membrane bilayer is generated and maintained by bi-directional transport (flip-flop) of lipids between the leaflets. Specific membrane proteins, termed lipid flippases, play an essential role in this transport process. Energy-independent flippases allow common phospholipids to equilibrate rapidly between the two monolayers and also play a role in the biosynthesis of a variety of glycoconjugates such as glycosphingolipids, N-glycoproteins, and glycosylphosphatidylinositol (GPI)-anchored proteins. ATP-dependent flippases, including members of a conserved subfamily of P-type ATPases and ATP-binding cassette transporters, mediate the net transfer of specific phospholipids to one leaflet of a membrane and are involved in the creation and maintenance of transbilayer lipid asymmetry of membranes such as the plasma membrane of eukaryotes. Energy-dependent flippases also play a role in the biosynthesis of glycoconjugates such as bacterial lipopolysaccharide. This review summarizes recent progress on the identification and characterization of the various flippases and the demonstration of their biological functions.

Sterol trafficking between the endoplasmic reticulum and plasma membrane in yeast

We recently showed that transport of ergosterol from the ER (endoplasmic reticulum) to the sterol-enriched PM (plasma membrane) in yeast occurs by a non-vesicular (Sec18p-independent) mechanism that results in the equilibration of sterol pools in the two organelles [Baumann, Sullivan, Ohvo-Rekilä, Simonot, Pottekat, Klaassen, Beh and Menon (2005) Biochemistry 44, 5816–5826]. To explore how this occurs, we tested the role of proteins that might act as sterol transporters. We chose to study oxysterol-binding protein homologues (Osh proteins), a family of seven proteins in yeast, all of which contain a putative sterol-binding pocket. Recent structural analyses of one of the Osh proteins [Im, Raychaudhuri, Prinz and Hurley (2005) Nature (London) 437, 154–158] suggested a possible transport cycle in which Osh proteins could act to equilibrate ER and PM pools of sterol. Our results indicate that the transport of newly synthesized ergosterol from the ER to the PM in an OSH deletion mutant lacking all seven Osh proteins is slowed only 5-fold relative to the isogenic wild-type strain. Our results suggest that the Osh proteins are not sterol transporters themselves, but affect sterol transport in vivo indirectly by affecting the ability of the PM to sequester sterols.

Endoplasmic reticulum proteins involved in glycosylphosphatidylinositol-anchor attachment: photocrosslinking studies in a cell-free system

Assembly of glycosylphosphatidylinositol (GPtdIns)-anchored proteins requires translocation of the nascent polypeptide chain across the endoplasmic reticulum (ER) membrane and replacement of the C-terminal signal sequence with a GPtdIns moiety. The anchoring reaction is carried out by an ER enzyme, GPtdIns transamidase. Genetic studies with yeast indicate that the transamidase consists of a dynamic complex of at least two subunits, Gaa1p and Gpi8p. To study the GPtdIns-anchoring reaction, we used a small reporter protein that becomes GPtdIns-anchored when the corresponding mRNA is translated in the presence of microsomes, in conjunction with site-specific photocrosslinking to identify ER membrane components that are proximal to the reporter during its conversion to a GPtdIns-anchored protein. We generated variants of the reporter protein such that upon in vitro translation in the presence of Nepsilon-(5-azido-2-nitrobenzoyl)-lysyl-tRNA, photoreactive lysine residues would be incorporated in the protein specifically near the GPtdIns-attachment site. We analyzed photoadducts resulting from UV irradiation of the samples. We show that proproteins can be crosslinked to the transamidase subunit Gpi8p, as well as to ER proteins of molecular mass approximately 60 kDa, approximately 70 kDa, and approximately 120 kDa. The identification of a photoadduct between a proprotein and Gpi8p provides the first direct evidence of an interaction between a proprotein substrate and one of the genetically identified transamidase subunits. The approximately 70-kDa protein that we identified may correspond to the other subunit Gaa1p, while the other proteins possibly represent additional, hitherto unidentified subunits of the mammalian GPtdIns transamidase complex.

The DXD motif is required for GM2 synthase activity but is not critical for nucleotide binding

We tested the importance of the aspartate-any residue-aspartate (DXD) motif for the enzymatic activity and nucleotide binding capacity of the Golgi glycosyltransferase GM2 synthase. We prepared point mutations of the motif, which is found in the sequence 352-VLWVDDDFV, and analyzed cells that stably expressed the mutated proteins. Whereas the folding of the mutated proteins was not seriously disrupted as judged by assembly into homodimers, Golgi localization, and secretion of a soluble form of the enzyme, exchange of the highly conserved aspartic acid residues at position 356 or 358 with alanine or asparagine reduced enzyme activity to background levels. In contrast, the D356E and D357N mutations retained weak activity, while the activity of V352A and W354A mutants was 167% and 24% that of wild-type enzyme, respectively. Despite the major effect of the DXD motif on enzymatic activity, nucleotide binding was not altered in the triple mutant D356N/D357N/D358N as revealed by binding to UDP-beads and labeling with the photoaffinity reagent, P(3)-(4-azidoanilido)uridine 5'-triphosphate (AAUTP). In summary, rather than being critical for nucleotide binding, this motif may function during catalysis in GM2 synthase, as has been proposed elsewhere for the SpsA glycosyltransferase based on its crystal structure.

Aortico-pulmonary paraganglioma associated with bilateral carotid body tumors. Diagnostic presentation and clinical implications

A case of mediastinal paraganglioma in association with bilateral carotid body tumors is presented. Characteristic radiological findings included a hypointense signal in T1-weighted, a hyperintense signal in T2-weighted magnetic resonance (MR) images and a vascular enhancement pattern in dynamic contrast enhanced MR imaging. Thus, feeding vessels could be depicted noninvasively. The importance of family screening in affected individuals is stressed, as a hereditary form of the disease exists in which multiple paragangliomas are common.

Soluble GPI8 restores glycosylphosphatidylinositol anchoring in a trypanosome cell-free system depleted of lumenal endoplasmic reticulum proteins

We previously established an in vitro assay for glycosylphosphatidylinositol (GPI) anchoring of proteins using trypanosome membranes. We now show that GPI anchoring is lost when the membranes are washed at high pH and restored to physiological pH prior to assay. We show that soluble component(s) of the endoplasmic reticulum that are lost in the high-pH wash are required for GPI anchoring. We reconstituted the high-pH extract with high-pH-treated membranes and demonstrated restoration of activity. Size fractionation of the high-pH extract indicated that the active component(s) was 30-50 kDa in size and was inactivated by iodoacetamide. Activity could also be restored by reconstituting the inactivated membranes with Escherichia coli-expressed, polyhistidine-tagged Leishmania mexicana GPI8 (GPI8-His; L. mexicana GPI8 is a soluble homologue of yeast and mammalian Gpi8p). No activity was seen when iodoacetamide-treated GPI8-His was used; however, GPI8-His could restore activity to iodoacetamide-treated membranes. Antibodies raised against L. mexicana GPI8 detected a protein of approx. 38 kDa in an immunoblot of the high-pH extract of trypanosome membranes. Our data indicate (1) that trypanosome GPI8 is a soluble lumenal protein, (2) that the interaction between GPI8 and other putative components of the transamidase may be dynamic, and (3) that GPI anchoring can be biochemically reconstituted using an isolated transamidase component.

Photoaffinity labelling with P3-(4-azidoanilido)uridine 5'-triphosphate identifies gpi3p as the UDP-GlcNAc-binding subunit of the enzyme that catalyses formation of GlcNAc-phosphatidylinositol, the first glycolipid intermediate in glycosylphosphatidylinositol synthesis

Glycosylphosphatidylinositols (GPIs) are made by all eukaryotes. The first step in their synthesis is the transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol (PI). Four proteins in mammals and at least three in yeast make up a complex that carries out this reaction. Three of the proteins are highly conserved between yeast and mammals: the Gpi1 protein, the Pig-C/Gpi2 protein and the Pig-A/Gpi3 protein. The function of the individual subunits is not known, but of the three, the Pig-A/Gpi3 proteins resemble members of a large family of nucleotide-sugar-utilizing glycosyltransferases. To establish whether Gpi3p is the UDP-GlcNAc-binding subunit of the yeast GlcNAc-PI synthetic complex, we tested its ability to become cross-linked to the photoactivatable substrate analogue P(3)-(4-azidoanilido)-uridine 5'-triphosphate (AAUTP). We report that Gpi3p bearing the FLAG epitope at its C-terminus becomes cross-linked to AAUTP[alpha-(32)P], but that Gpi2p-FLAG does not. Furthermore, Gpi3p-FLAG expressed in Escherichia coli is also cross-linked. These results indicate that Gpi3p is the UDP-GlcNAc-binding and probable catalytic subunit of the GlcNAc-PI synthetic complex.

Reconstitution and partial characterization of phospholipid flippase activity from detergent extracts of the Bacillus subtilis cell membrane

In bacteria, phospholipids are synthesized on the inner leaflet of the cytoplasmic membrane and must translocate to the outer leaflet to propagate a bilayer. Transbilayer movement of phospholipids has been shown to be fast and independent of metabolic energy, and it is predicted to be facilitated by membrane proteins (flippases) since transport across protein-free membranes is negligible. However, it remains unclear as to whether proteins are required at all and, if so, whether specific proteins are needed. To determine whether bacteria contain specific proteins capable of translocating phospholipids across the cytoplasmic membrane, we reconstituted a detergent extract of Bacillus subtilis into proteoliposomes and measured import of a water-soluble phospholipid analog. We found that the proteoliposomes were capable of transporting the analog and that transport was inhibited by protease treatment. Active proteoliposome populations were also able to translocate a long-chain phospholipid, as judged by a phospholipase A(2)-based assay. Protein-free liposomes were inactive. We show that manipulation of the reconstitution mixture by prior chromatographic fractionation of the detergent extract, or by varying the protein/phospholipid ratio, results in populations of vesicles with different specific activities. Glycerol gradient analysis showed that the majority of the transport activity sedimented at approximately 4S, correlating with the presence of specific proteins. Recovery of activity in other gradient fractions was low despite the presence of a complex mixture of proteins. We conclude that bacteria contain specific proteins capable of facilitating transbilayer translocation of phospholipids. The reconstitution methodology that we describe provides the basis for purifying a facilitator of transbilayer phospholipid translocation in bacteria.

Cell surface display and intracellular trafficking of free glycosylphosphatidylinositols in mammalian cells

In addition to serving as membrane anchors for cell surface proteins, glycosylphosphatidylinositols (GPIs) can be found abundantly as free glycolipids in mammalian cells. In this study we analyze the subcellular distribution and intracellular transport of metabolically radiolabeled GPIs in three different cell lines. We use a variety of membrane isolation techniques (subcellular fractionation, plasma membrane vesiculation to isolate pure plasma membrane fractions, and enveloped viruses to sample cellular membranes) to provide direct evidence that free GPIs are not confined to their site of synthesis, the endoplasmic reticulum, but can redistribute to populate other subcellular organelles. Over short labeling periods (2.5 h), radiolabeled GPIs were found at similar concentration in all subcellular fractions with the exception of a mitochondria-enriched fraction where GPI concentration was low. Pulse-chase experiments over extended chase periods showed that although the total amount of cellular radiolabeled GPIs decreased, the plasma membrane complement of labeled GPIs increased. GPIs at the plasma membrane were found to populate primarily the exoplasmic leaflet as detected using periodate oxidation of the cell surface. Transport of GPIs to the cell surface was inhibited by Brefeldin A and blocked at 15 degrees C, suggesting that GPIs are transported to the plasma membrane via a vesicular mechanism. The rate of transport of radiolabeled GPIs to the cell surface was found to be comparable with the rate of secretion of newly synthesized soluble proteins destined for the extracellular space.

Specific proteins are required to translocate phosphatidylcholine bidirectionally across the endoplasmic reticulum

BACKGROUND

A long-standing problem in understanding the mechanism by which the phospholipid bilayer of biological membranes is assembled concerns how phospholipids flip back and forth between the two leaflets of the bilayer. This question is important because phospholipid biosynthetic enzymes typically face the cytosol and deposit newly synthesized phospholipids in the cytosolic leaflet of biogenic membranes such as the endoplasmic reticulum (ER). These lipids must be transported across the bilayer to populate the exoplasmic leaflet for membrane growth. Transport does not occur spontaneously and it is presumed that specific membrane proteins, flippases, are responsible for phospholipid flip-flop. No biogenic membrane flippases have been identified and there is controversy as to whether proteins are involved at all, whether any membrane protein is sufficient, or whether non-bilayer arrangements of lipids support flip-flop.

RESULTS

To test the hypothesis that specific proteins facilitate phospholipid flip-flop in the ER, we reconstituted transport-active proteoliposomes from detergent-solubilized ER vesicles under conditions in which protein-free liposomes containing ER lipids were inactive. Transport was measured using a synthetic, water-soluble phosphatidylcholine and was found to be sensitive to proteolysis and associated with proteins or protein-containing complexes that sedimented operationally at 3.8S. Chromatographic analyses indicated the feasibility of identifying the transporter(s) by protein purification approaches, and raised the possibility that at least two different proteins are able to facilitate transport. Calculations based on a simple reconstitution scenario suggested that the transporters represent approximately 0.2% of ER membrane proteins.

CONCLUSIONS

Our results clearly show that specific proteins are required to translocate a phosphatidylcholine analogue across the ER membrane. These proteins are likely to be the flippases, which are required to translocate natural phosphatidylcholine and other phospholipids across the ER membrane. The methodology that we describe paves the way for identification of a flippase.

Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review)

Glycosylphosphatidylinositols (GPIs) represent an abundant and ubiquitous class of eukaryotic glycolipids. Although these structures were originally discovered in the form of GPI-anchored cell surface glycoproteins, it is becoming increasingly clear that a significant proportion of the GPI synthetic output of a cell is not directed to protein anchoring. Indeed, pools of non-protein-linked GPIs can approach 10(7) molecules per cell in some cell types, especially the protozoa, with a large proportion of these molecules being displayed at the cell surface. Recent studies which form the subject of this review indicate that there is (a) considerable diversity in the range of structural modifications found on GPI glycolipids within and between species and cell types, (b) complexity in the topological arrangement of the GPI biosynthetic pathway in the endoplasmic reticulum, and (c) spatial restriction of the biosynthetic pathway within the endoplasmic reticulum. Furthermore, consistent with additional functional roles for these lipids beyond serving as protein anchor precursors, products of the GPI biosynthetic pathway appear to be widely distributed in the cellular endomembrane system. These studies indicate that there is still much to learn about the organization of glycolipid biosynthetic pathways in eukaryotic cells, the nature and subcellular distribution of the lipid products of these pathways, and the function of these lipids within cells.

A cell-free assay for glycosylphosphatidylinositol anchoring in African trypanosomes. Demonstration of a transamidation reaction mechanism

We established an in vitro assay for the addition of glycosyl-phosphatidylinositol (GPI) anchors to proteins using procyclic trypanosomes engineered to express GPI-anchored variant surface glycoprotein (VSG). The assay is based on the premise that small nucleophiles, such as hydrazine, can substitute for the GPI moiety and effect displacement of the membrane anchor of a GPI-anchored protein or pro-protein causing release of the protein into the aqueous medium. Cell membranes containing pulse-radiolabeled VSG were incubated with hydrazine, and the VSG released from the membranes was measured by carbonate extraction, immunoprecipitation, and SDS-polyacrylamide gel electrophoresis/fluorography. Release of VSG was time- and temperature-dependent, was stimulated by hydrazine, and occurred only for VSG molecules situated in early compartments of the secretory pathway. No nucleophile-induced VSG release was seen in membranes prepared from cells expressing a VSG variant with a conventional transmembrane anchor (i.e. a nonfunctional GPI signal sequence). Pro-VSG was shown to be a substrate in the reaction by assaying membranes prepared from cells treated with mannosamine, a GPI biosynthesis inhibitor. When a biotinylated derivative of hydrazine was used instead of hydrazine, the released VSG could be precipitated with streptavidin-agarose, indicating that the biotin moiety was covalently incorporated into the protein. Hydrazine was shown to block the C terminus of the released VSG hydrazide because the released material, unlike a truncated form of VSG lacking a GPI signal sequence, was not susceptible to proteolysis by carboxypeptidases. These results firmly establish that the released material in our assay is VSG hydrazide and strengthen the proof that GPI anchoring proceeds via a transamidation reaction mechanism. The reaction could be inhibited with sulfhydryl alkylating reagents, suggesting that the transamidase enzyme contains a functionally important sulfhydryl residue.

Segregation of glycosylphosphatidylinositol biosynthetic reactions in a subcompartment of the endoplasmic reticulum

Glycosylphosphatidylinositols (GPIs) are synthesized in the endoplasmic reticulum (ER) via the sequential addition of monosaccharides, fatty acid, and phosphoethanolamine(s) to phosphatidylinositol (PI). While attempting to establish a mammalian cell-free system for GPI biosynthesis, we found that the assembly of mannosylated GPI species was impaired when purified ER preparations were substituted for unfractionated cell lysates as the enzyme source. To explore this problem we analyzed the distribution of the various GPI biosynthetic reactions in subcellular fractions prepared from homogenates of mammalian cells. The results indicate the following: (i) the initial reaction of GPI assembly, i.e. the transfer of GlcNAc to PI to form GlcNAc-PI, is uniformly distributed in the ER; (ii) the second step of the pathway, i.e. de-N-acetylation of GlcNAc-PI to yield GlcN-PI, is largely confined to a subcompartment of the ER that appears to be associated with mitochondria; (iii) the mitochondria-associated ER subcompartment is enriched in enzymatic activities involved in the conversion of GlcN-PI to H5 (a singly mannosylated GPI structure containing one phosphoethanolamine side chain; and (iv) the mitochondria-associated ER subcompartment, unlike bulk ER, is capable of the de novo synthesis of H5 from UDP-GlcNAc and PI. The confinement of these GPI biosynthetic reactions to a domain of the ER provides another example of the compositional and functional heterogeneity of the ER. The implications of these findings for GPI assembly are discussed.

Identification and purification of the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter

Glycosylation of glycoproteins, proteoglycans, and glycosphingolipids occurs mainly in the lumen of the endoplasmic reticulum and the Golgi apparatus. Nucleotide sugars, donors of all the sugars involved in Golgi glycosylation reactions, are synthesized in the cytoplasm and require specialized transporters to be translocated into the lumen of the Golgi apparatus. By controlling the supply of sugar nucleotides in the lumen of the Golgi apparatus, these transporters directly regulate the glycosylation of macromolecules transiting the Golgi. We have identified and purified the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter. The transporter was purified to apparent homogeneity by a combination of conventional and dye color chromatography. An approximately 63,000-fold purification (6% yield) was achieved starting from crude rat liver Golgi membranes and resulting in a protein with an apparent molecular mass of 43 kDa. The transporter was active when reconstituted into phosphatidylcholine vesicles and could be specifically photolabeled with P3-(4-azidoanilido)-uridine-5'-[P1-32P]triphosphate, an analog of UDP-N-acetylgalactosamine. Native functional size determination on a glycerol gradient suggested that the transporter exists as a homodimer within the Golgi membrane.

Left ventricular geometry and cardiac function during minimally invasive coronary artery bypass grafting

BACKGROUND

This investigation was designed to study the changes in function and geometry of the left ventricle during two critical steps of minimally invasive direct coronary artery bypass procedures: placement of an epicardial stabilizer and occlusion of the left anterior descending coronary artery.

METHODS

Between February 1997 and January 1998, 28 patients underwent bypass grafting with the left internal thoracic artery to the left anterior descending coronary artery (minimally invasive direct coronary artery bypass technique). Transesophageal echocardiography was used for determination of fractional area change and to assess left ventricular (LV) diameters in two dimensions and at the apex.

RESULTS

Placement of the epicardial stabilizer resulted in a small decrease in LV end-systolic and end-diastolic dimensions; cardiac function remained unchanged. Subsequent occlusion of the left anterior descending coronary artery caused a moderate decline in cardiac index and fractional area change, an increase in LV diameters, and the development of hypokinetic segments within the LV myocardium.

CONCLUSIONS

The use of an epicardial stabilizer provides a safe and effective means to stabilize the operative field during minimally invasive direct coronary artery bypass procedures. Monitoring of LV function by transesophageal echocardiography enhances the safety of such procedures and is highly recommended.

Identification of endoplasmic reticulum proteins involved in glycan assembly: synthesis and characterization of P3-(4-azidoanilido)uridine 5'-triphosphate, a membrane-topological photoaffinity probe for uridine diphosphate-sugar binding proteins

Much of the enzymic machinery required for the assembly of cell surface carbohydrates is located in the endoplasmic reticulum (ER) of eukaryotic cells. Structural information on these proteins is limited and the identity of the active polypeptide(s) is generally unknown. This paper describes the synthesis and characteristics of a photoaffinity reagent that can be used to identify and analyse members of the ER glycan assembly apparatus, specifically those glycosyltransferases, nucleotide phosphatases and nucleotide-sugar transporters that recognize uridine nucleotides or UDP-sugars. The photoaffinity reagent, P3-(4-azidoanilido)uridine 5'-triphosphate (AAUTP), was synthesized easily from commercially available precursors. AAUTP inhibited the activity of ER glycosyltransferases that utilize UDP-GlcNAc and UDP-Glc, indicating that it is recognized by UDP-sugar-binding proteins. In preliminary tests AAUTP[alpha-32P] labelled bovine milk galactosyltransferase, a model UDP-sugar-utilizing enzyme, in a UV-light-dependent, competitive and saturable manner. When incubated with rat liver ER vesicles, AAUTP[alpha-32P] labelled a discrete subset of ER proteins; labelling was light-dependent and metal ion-specific. Photolabelling of intact ER vesicles with AAUTP[alpha-32P] caused selective incorporation of radioactivity into proteins with cytoplasmically disposed binding sites; UDP-Glc:glycoprotein glucosyltransferase, a lumenal protein, was labelled only when the vesicle membrane was disrupted. These data indicate that AAUTP is a membrane topological probe of catalytic sites in target proteins. Strategies for using AAUTP to identify and study novel ER proteins involved in glycan assembly are discussed.

Transbilayer movement of fluorescent phospholipids in Bacillus megaterium membrane vesicles

We investigated the transbilayer movement or flip-flop of phospholipids in vesicles derived from the cytoplasmic membrane of Bacillus megaterium. Since common assay techniques were found to be inapplicable to the Bacillus system, we exploited and elaborated a newly described method in which fluorescent phospholipids (1-myristoyl-2-C6-NBD phospholipids) are used as tracers to monitor flip-flop. These lipids were introduced into Bacillus vesicles from synthetic donor vesicles containing a fluorescence quencher. Transport was measured by monitoring the increase in fluorescence as the tracers departed the quenched environment of the donor vesicle and entered first the outer membrane leaflet and subsequently the inner leaflet of Bacillus vesicles. Independent experiments involving cobalt quenching of NBD fluorescence provided results consistent with the existence of pools of fluorescent phospholipid in the outer and inner leaflets of Bacillus vesicles at the completion of transport. Using the assay we show that phospholipid flip-flop in Bacillus vesicles occurs rapidly (half-time approximately 30 s at 37 degrees C) with no preference for a particular phospholipid headgroup and that it is sensitive to proteolysis. We also establish that flip-flop does not occur in synthetic phospholipid vesicles or vesicles made from Bacillus phospholipids. We conclude that Bacillus vesicles possess the ability to promote rapid transbilayer movement of phospholipids, and that the transport is probably protein (flippase)-mediated.

Phosphatidylinositol hydrolysis by Trypanosoma brucei glycosylphosphatidylinositol phospholipase C

Detergent-solubilized glycosylphosphatidylinositol (GPI)-anchored structures can be cleaved by C-type phospholipases isolated from peanuts and bloodstream cells of the African trypanosome, Trypanosoma brucei. The two enzymes differ in their reported ability to hydrolyze phosphatidylinositol (PI); while the peanut enzyme readily hydrolyzes PI in vitro, the T. brucei enzyme was reported to be virtually inactive against PI and consequently named GPI-specific phospholipase C (GPI-PLC). In this paper, we describe experiments in which we reinvestigated the substrate specificity of T. brucei GPI-PLC by incubating the purified enzyme with Triton X-100/PI-mixed micelles and by studying PI hydrolysis. We found that PI hydrolysis occurred in a detergent-dependent fashion over the range of concentrations tested (5 microM to 1 mM PI). At 5 microM PI, hydrolysis was maximal at 0.005% Triton X-100, whereas at 1 mM PI, maximal hydrolysis required 0.05% Triton X-100. Hydrolysis of both PI and GPI was strongly affected by the presence of phospholipids. Endogenous PI was hydrolyzed during osmotic and detergent lysis of trypanosomes under conditions used to obtain quantitative hydrolysis of the GPI-anchored trypanosome variant surface glycoprotein. PI hydrolysis in the lysates was inhibited by sodium p-chloromercuriphenylsulfonate but unaffected by EGTA, consistent with the proposal that hydrolysis is due to GPI-PLC. These results suggest that the function of T. brucei GPI-PLC may be to regulate PI as well as (or instead of) GPI levels.

-Congenital hypoplasia of the anterior tricuspid leaflet as a cause of high-grade tricuspid valve insufficiency in a 64-year-old patient-

In a 64-year-old man a heart murmur was present since childhood and could be identified 8 years ago as isolated tricuspid regurgitation. Because of progressive dyspnea there was an indication for surgical treatment. Intraoperatively, we found a hypoplastic anterior leaflet, though the other two leaflets were preserved in totally good condition, without any evidence of endocarditis. Tricuspid valve replacement was performed as there was no chance of reconstruction. To our knowledge, this is the first description of a congenital hypoplastic anterior leaflet of the tricuspid valve.

Nonpolarized distribution of glycosylphosphatidylinositols in the plasma membrane of polarized Madin-Darby canine kidney cells

Glycosylphosphatidylinositols (GPIs) are ubiquitous in eukaryotes and serve to anchor a variety of proteins to the exoplasmic leaflet of cellular membranes. GPIs are synthesized in the endoplasmic reticulum (ER), in excess of the amount needed for protein modification. The fate of the excess GPIs is unknown, but they may be retained in the ER, transported to other membranes, and/or metabolized. In relation to this problem, we were interested in determining whether GPIs were transported to the plasma membrane and whether, like GPI-anchored proteins, their presence was confined to the apical plasma membrane domain in polarized epithelial cells. Polarized Madin-Darby canine kidney epithelial cell monolayers were incubated with [3H]mannose or [3H]ethanolamine to label GPIs and then infected with enveloped viruses. We used influenza virus (flu) and vesicular stomatitis virus (VSV) for these experiments as these viruses are assembled at the cell surface and acquire their envelope lipids from the plasma membrane. Furthermore, flu and VSV bud specifically from the apical and basolateral plasma membrane domains, respectively. Flu and VSV were isolated from the apical and basolateral media, respectively, and subjected to lipid analysis. Radiolabeled GPIs were found in both viruses. Moreover, the membrane concentration of GPIs (i.e. GPI radioactivity normalized to membrane mass) in the two viruses was essentially the same. These observations suggest that (i) non-protein-linked GPIs are located at the plasma membrane; (ii) since GPIs are synthesized in the ER, they must be transported from the ER to the plasma membrane; and (iii) transport of nonprotein-linked GPIs is not influenced by the sorting processes that target GPI-anchored proteins exclusively to the apical plasma membrane.

Soluble constituents of the ER lumen are required for GPI anchoring of a model protein

Transfer of a glycosylphosphatidylinositol (GPI) anchor to proteins carrying a C-terminal GPI-directing signal sequence occurs after protein translocation across the endoplasmic reticulum (ER). We describe the translocation and GPI modification of a model protein, preprominiPLAP, in ER microsomes depleted of lumenal content by high pH washing. In untreated microsomes preprominiPLAP was processed to prominiPLAP and GPI-anchored miniPLAP. Both products were fully translocated, since they resisted proteinase K treatment of the microsomes, and both behaved as membrane proteins by the carbonate extraction criterion. Microsomes depleted of lumenal content were able to translocate and process preprominiPLAP to give protease-protected prominiPLAP, but were unable to convert prominiPLAP to miniPLAP. Loss of GPI anchoring capacity occurred with a wash of pH > 9.5. If the alkaline wash was performed after formation of prominiPLAP conversion to miniPLAP was relatively unimpaired. The results indicate that constituents of the ER lumen, possibly chaperones interacting with the proprotein and/or the GPI anchor precursor, are required in the initial steps of GPI anchoring.

Flippases

A critical feature in the biogenesis of cellular membranes is the translocation (flipping) of phospholipids and glycolipids from one leaflet of a membrane bilayer to the opposing leaflet. In some cases, flipping results in a pronounced transbilayer lipid asymmetry which has important functional consequences. In general, flipping occurs only very slowly in artificial membranes but is accelerated to a biologically relevant rate in some biomembranes. Current data suggest that this acceleration is most likely brought about by protein catalysts (termed flippases). This article reviews available information on flippases, including the recent isolation of two flippases operating at the plasma membrane of animal cells.

Production of a nested set of glycosylphosphatidylinositol structures from a glycosylphosphatidylinositol-anchored protein

Glycosylphosphatidylinositol (GPI) membrane anchors are synthesized in the endoplasmic reticulum of eukaryotic cells. Synthesis of the core GPI structure is achieved by the sequential transfer of monosaccharides and phosphoethanolamine to phosphatidylinositol. The assembly process can be reproduced in vitro using membrane preparations supplemented with sugar nucleotides. With one exception, however, none of the biosynthetic enzymes involved have been isolated. One impediment to progress in the isolation of these enzymes is the nonavailability of adequate amounts of partially assembled GPI structures for use as assay substrates. In this paper we present procedures to prepare these structures from a GPI-anchored protein. The methods described include selective dephosphorylation of the GPI-anchored variant surface glycoprotein from Trypanosoma brucei variant 118 to generate Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN alpha 1-6-myo-inositol-P-dimyristoylglycerol (Man3GlcN-PI), followed by exoglycosidase treatments and N-acetylation to produce Man2GlcN-PI, Man1GlcN-PI, GlcN-PI, and GlcNAc-PI. Procedures are also described for the stabilization and purification of these structures. It is anticipated that the convenient preparation of this range of partially assembled GPIs will be useful not only in developing assays for the eventual purification of the GPI biosynthetic enzymes but will also contribute to evaluating the specificity of the phospholipases that hydrolyze GPI anchors.

The GPI anchor of cell-surface proteins is synthesized on the cytoplasmic face of the endoplasmic reticulum

Glycosylphosphatidylinositol (GPI) membrane protein anchors are synthesized from sugar nucleotides and phospholipids in the ER and transferred to newly synthesized proteins destined for the cell surface. The topology of GPI synthesis in the ER was investigated using sealed trypanosome microsomes and the membrane-impermeant probes phosphatidylinositol-specific phospholipase C, Con A, and proteinase K. All the GPI biosynthetic intermediates examined were found to be located on the external face of the microsomal vesicles suggesting that the principal steps of GPI assembly occur in the cytoplasmic leaflet of the ER. Protease protection experiments showed that newly GPI-modified trypanosome variant surface glycoprotein was primarily oriented towards the ER lumen, consistent with eventual expression at the cell surface. The unusual topographical arrangement of the GPI assembly pathway suggests that a biosynthetic intermediate, possibly the phosphoethanolamine-containing anchor precursor, must be translocated across the ER membrane bilayer in the process of constructing a GPI anchor.

Topology of GPI biosynthesis in the endoplasmic reticulum

Glycosylphosphatidylinositol (GPI) anchors are constructed in the endoplasmic reticulum (ER) through the action of at least seven unique enzymes. Using cell-free systems, mainly derived from African trypanosomes, it has been experimentally possible to re-create many aspects of the GPI biosynthetic pathway in vitro and to obtain a series of glycosylated phosphatidylinositol structures that correspond to biosynthetic intermediates. This approach led to the identification of the biosynthetic donors of individual components of the GPI glycan, and the discovery of unusual fatty acid re-modelling reactions in the GPI pathway in trypanosomes. Despite this progress, questions remain concerning the enzymology of the pathway, particularly the topological distribution of the different assembly steps in the ER membrane. In the work described here we have attempted to define the transbilayer orientation of different GPI biosynthetic intermediates in the ER membrane bilayer. The experiments were performed with a microsomal fraction derived from bloodstream-form Trypanosoma brucei, and standard radiolabeling procedures. The orientation of GPIs was probed with bacterial phosphatidylinositol-specific phospholipase C (PI-PLC) and the jackbean lectin Concanavalin A. Contrary to expectations based on other ER glycosylation reactions, most notably the reactions involved in the dolichol pathway of N-glycosylation, our results suggest that non-inositol-acylated (PI-PLC-sensitive) GPIs are synthesized in the cytoplasmic leaflet of the ER membrane bilayer and that the final reaction product, a phosphoethanolamine-containing GPI, flips into the luminal leaflet for transfer to protein.

Early lipid intermediates in glycosyl-phosphatidylinositol anchor assembly are synthesized in the ER and located in the cytoplasmic leaflet of the ER membrane bilayer

Glycosylated phosphoinositides serve as membrane anchors for numerous eukaryotic cell surface glycoproteins. Recent biochemical and genetic studies indicate that the glycolipids are assembled by sequential addition of components (monosaccharides and phosphoethanolamine) to phosphatidylinositol. The biosynthetic steps are presumed to occur in the ER, but formal proof of this is lacking. We describe experiments designed to establish the subcellular location of the initial steps in glycosyl-phosphatidylinositol (GPI) anchor biosynthesis and to define the transmembrane distribution of early biosynthetic lipid intermediates. The experiments were performed with the thymoma cell line BW5147.3. A subcellular fractionation protocol was used to show that early biosynthetic steps in GPI assembly, i.e., synthesis and deacetylation of N-acetylglucosaminyl phosphatidylinositol, occur in the ER. GPI biosynthetic intermediates were synthesized by incubating the microsomes with UDP-[3H]GlcNAc, and the transmembrane distribution of the labeled lipids was probed with phosphatidylinositol-specific phospholipase C (PI-PLC). Treatment of the radiolabeled microsomes with PI-PLC showed that > 70% of the N-acetylglucosaminyl phosphatidylinositol and glucosaminyl phosphatidylinositol could be hydrolyzed, indicating that the two lipids were primarily distributed in the cytoplasmic (outer) leaflet of the microsomes. Similar cleavage results were obtained using Streptolysin O-permeabilized thymoma cells. When permeabilized cells were incubated with UDP-[3H]GlcNAc and treated with PI-PLC, approximately 85% of the radiolabeled N-acetylglucosaminyl phosphatidylinositol and glucosaminyl phosphatidylinositol could be cleaved, indicating that they were accessible to the enzyme. The cumulative data indicate that early GPI intermediates are primarily located in the cytoplasmic leaflet of the ER, and are probably synthesized from PI located in the cytoplasmic leaflet and UDP-GlcNAc synthesized in the cytosol.

Phosphatidylethanolamine is the donor of the terminal phosphoethanolamine group in trypanosome glycosylphosphatidylinositols

A variety of eukaryotic cell surface proteins, including the variant surface glycoproteins of African trypanosomes, rely on a covalently attached lipid, glycosylphosphatidylinositol (GPI), for membrane attachment. GPI anchors are synthesized in the endoplasmic reticulum by stepwise glycosylation of phosphatidylinositol (via UDP-GlcNAc and dolichol-P-mannose) followed by the addition of phosphoethanolamine. The experiments described in this paper are aimed at identifying the biosynthetic origin of the terminal phosphoethanolamine group. We show that trypanosome GPIs can be labelled via CDP-[3H]ethanolamine or [beta-32P]CDP-ethanolamine in a cell-free system, indicating that phosphoethanolamine is acquired en bloc. In pulse-chase experiments with CDP-[3H]ethanolamine we show that the GPI phosphoethanolamine is not derived directly from CDP-ethanolamine, but instead from a relatively stable metabolite, such as phosphatidylethanolamine (PE), generated from CDP-ethanolamine in the cell-free system. To test the possibility that PE is the immediate donor of the GPI phosphoethanolamine moiety, we describe metabolic labelling experiments with [3H]serine and show that GPIs can be labelled in the absence of detectable radiolabelled CDP-ethanolamine, presumably via [3H]PE generated from [3H]phosphatidylserine (PS). The data support the proposal that the terminal phosphoethanolamine group in trypanosome GPIs is derived from PE.

Molecular species analysis of phospholipids from Trypanosoma brucei bloodstream and procyclic forms

We present a quantitative description of the molecular species composition of the major phospholipid classes in bloodstream and procyclic forms of Trypanosoma brucei. Phospholipid classes were resolved by 2-dimensional thin-layer chromatography. Diradylglycerols were released from individual phospholipid classes by phospholipases C, converted into benzoate derivatives and separated into diacyl, alkylacyl and alk-1-enylacyl subclasses. Individual molecular species were quantitated and identified by HPLC and the assignments were confirmed by mass spectrometry. Comparison of the diacyl species of PC, PE and PI in bloodstream trypanosomes showed major differences in the relative amounts of individual molecular species between the different classes but not striking changes in the degree of saturation or overall chain length. In contrast, in procyclic trypanosomes the relative amounts of diacyl molecular species with polyunsaturated fatty acyl chains decreased in the order of PC > PE >> PI. Also, the alkylacyl and alk-1-enylacyl subclasses of PC and PE in bloodstream trypanosomes comprised a single molecular species, 18:0 18:2. Such exclusivity was not observed in procyclic trypanosomes among the same phospholipid subclasses, although 18:0 18:2 was the predominant species. Almost all the PI of bloodstream forms contained one 18:0 acyl species, which is consistent with the composition of the PI used for glycosylphosphatidylinositol synthesis.

Complexity of ethanolamine phosphate addition in the biosynthesis of glycosylphosphatidylinositol anchors in mammalian cells

Biosynthetic intermediates for the mammalian glycosylphosphatidylinositol (GPI) anchor have been described. The earliest GPI anchor precursor is N-acetylglucosaminylphosphatidylinositol, which is deacetylated to give glucosaminylphosphatidylinositol. This is followed by fatty acylation of the inositol ring, sequential addition of mannose residues donated by dolichyl mannosyl phosphate, and finally addition of ethanolamine phosphate. Here, we show that the final steps of GPI anchor biosynthesis are more complex than we have previously reported. Six distinct GPI anchor precursors were found to contain at least 1 ethanolamine phosphate residue. The headgroups of these glycolipids were purified and analyzed by a combination of Bio-Gel P4 chromatography and high resolution thin-layer chromatography. The sizes of neutral glycans were determined following HF dephosphorylation. The position of the ethanolamine phosphate residue was inferred from results of alpha-mannosidase treatment. Finally, the number of negative charges on the headgroups were determined by Mono Q chromatography. Our results show that the addition of ethanolamine phosphate is controlled by at least two different genes. Thus, the class F mutant, though unable to add ethanolamine phosphate to the third mannose residue, does incorporate ethanolamine phosphate into the first and second mannose residues. Only the wild type cells are capable of incorporating ethanolamine phosphate into the third mannose residue. Furthermore, the GPI core contains up to 3 ethanolamine phosphate residues. These results should facilitate the elucidation of the biochemical defects in paroxysmal nocturnal hemoglobinuria.

Phosphatidylethanolamine is the donor of the ethanolamine residue linking a glycosylphosphatidylinositol anchor to protein

Numerous cell surface glycoproteins from eukaryotic organisms including African trypanosomes and budding yeast (Saccharomyces cerevisiae), are anchored to the lipid bilayer by a glycophospholipid, glycosylphosphatidylinositol, covalently linked to the carboxyl terminus of the protein via a phosphoethanolamine bridge. In this paper we describe metabolic labeling experiments aimed at identifying the biosynthetic origin of the ethanolamine residue in the phosphoethanolamine bridge. Using yeast mutants generated by disruption of the ethanolaminephosphotransferase (EPT1) and cholinephosphotransferase (CPT1) genes, we report data consistent with the proposal that the ethanolamine residue is derived from phosphatidylethanolamine.

Developmental variation of glycosylphosphatidylinositol membrane anchors in Trypanosoma brucei. In vitro biosynthesis of intermediates in the construction of the GPI anchor of the major procyclic surface glycoprotein

The African trypanosome, Trypanosoma brucei, expresses two abundant stage-specific glycosylphosphatidylinositol (GPI)-anchored glycoproteins, the procyclic acidic repetitive protein (PARP or procyclin) in the procyclic form, and the variant surface glycoprotein (VSG) in the mammalian bloodstream form. The GPI anchor of VSG can be readily cleaved by phosphatidylinositol (PI)-specific phospholipase C (PI-PLC), whereas that of PARP cannot, due to the presence of a fatty acid esterified to the inositol. In the bloodstream form trypanosome, a number of GPIs which are structurally related to the VSG GPI anchor have been identified. In addition, several structurally homologous GPIs have been described, both in vivo and in vitro, that contain acyl-inositol. In vivo the procyclic stage trypanosome synthesizes a GPI that is structurally homologous to the PARP GPI anchor, i.e. contains acyl-inositol. No PI-PLC-sensitive GPIs have been detected in the procyclic form. Using a membrane preparation from procyclic trypanosomes which is capable of synthesizing GPI lipids upon the addition of nucleotide sugars we find that intermediate glycolipids are predominantly of the acyl-inositol type, and the mature ethanolamine-phosphate-containing precursors are exclusively acylated. We suggest that the differences between the bloodstream and procyclic form GPI biosynthetic intermediates can be accounted for by the developmental regulation of an inositol acylhydrolase, which is active only in the bloodstream form, and a glyceride fatty acid remodeling system, which is only partially functional in the procyclic form.

Galactose-containing glycosylphosphatidylinositols in Trypanosoma brucei

Many eukaryotic surface glycoproteins, including the variant surface glycoproteins (VSGs) of Trypanosoma brucei, are synthesized with a carboxyl-terminal hydrophobic peptide extension that is cleaved and replaced by a complex glycosylphosphatidylinositol (GPI) membrane anchor within 1-5 min of the completion of polypeptide synthesis. We have reported the purification and partial characterization of candidate precursor glycolipids (P2 and P3) from T. brucei. P2 and P3 contain ethanolamine-phosphate-Man alpha 1-2Man alpha 1-6Man alpha 1-GlcN linked glycosidically to an inositol residue, as do all the GPI anchors that have been structurally characterized. The anchors on mature VSGs contain a heterogenously branched galactose structure attached alpha 1-3 to the mannose residue adjacent to the glucosamine. We report the identification of free GPIs that appear to be similarly galactosylated. These glycolipids contain diacylglycerol and alpha-galactosidase-sensitive glycan structures which are indistinguishable from the glycans derived from galactosylated VSG GPI anchors. We discuss the relevance of these galactosylated GPIs to the biosynthesis of VSG GPI anchors.

The mevalonate pathway in the bloodstream form of Trypanosoma brucei. Identification of dolichols containing 11 and 12 isoprene residues

The major surface antigen of the bloodstream form of Trypanosoma brucei, the variant surface glycoprotein, is attached to the plasma membrane via a glycosylphosphatidylinositol anchor. The biosynthesis of the glycosylphosphatidylinositol anchor, as well as the assembly of the asparagine-linked oligosaccharide chains found on the variant surface glycoproteins, involves polyisoprenoid lipids that act as sugar carriers. Preliminary observations (Menon, A.K., Schwarz, R.T., Mayor, and Cross, G.A.M. (1990) J. Biol. Chem. 265, 9033-9042) suggested that the sugar carriers in T. brucei were short-chain polyisoprenoids containing substantially fewer isoprene residues than polyisoprenols in mammalian cells. In this paper we describe metabolic labeling experiments with [3H]mevalonate, as well as chromatographic and mass spectrometric analyses of products of the mevalonate pathway in T. brucei. We report that cells of the bloodstream form of T. brucei contain a limited spectrum of short chain dolichols and dolichol phosphates (11 and 12 isoprene residues). The total dolichol content was estimated to be 0.28 nmol/10(9) cells; the dolichyl phosphate content was 0.07 nmol/10(9) cells. The same spectrum of dolichol chain lengths was also found in a polar lipid that could be labeled with [3H]mevalonate, [3H]glucosamine, and [3H]mannose, and which was characterized as Man5GlcNAc2-PP-dolichol. The most abundant product of the mevalonate pathway identified in T. brucei was cholesterol (140 nmol/10(9) cells). Ubiquinone (0.09 nmol/10(9) cells) with a solanesol side chain was also identified.

A glycosylphosphatidylinositol protein anchor from procyclic stage Trypanosoma brucei: lipid structure and biosynthesis

Cells of the insect (procyclic) stage of the life cycle of the African trypanosome, Trypanosoma brucei, express an abundant stage-specific glycosylated phosphatidylinositol (GPI) anchored glycoprotein, the procyclic acidic repetitive protein (PARP). The anchor is insensitive to the action of bacterial phosphatidylinositol-specific phospholipase C (PI-PLC), suggesting that it contains an acyl-inositol. We have recently described the structure of a PI-PLC resistant glycosylphosphatidylinositol, PP1, which is specific to the procyclic stage, and have presented preliminary evidence that the phosphatidylinositol portion of the protein-linked GPI on PARP has a similar structure. In this paper we show, by metabolic labelling with [3H]fatty acids, that the PARP anchor contains palmitate esterified to inositol, and stearate at sn-1, in a monoacylglycerol moiety, a structure identical to PP1. Using pulse-chase labelling, we show that both fatty acids are incorporated into the GPI anchor from a large pool of metabolic precursors, rather than directly from acyl-CoA. We also demonstrate that the addition of the GPI anchor moiety to PARP is dependent on de novo protein synthesis, excluding the possibility that incorporation of fatty acids into PARP can occur by a remodelling of pre-existing GPI anchors. Finally we show that the phosphatidylinositol (PI) species that are utilized for GPI biosynthesis are a subpopulation of the cellular PI molecular species. We propose that these observations may be of general validity since several other eukaryotic membrane proteins (e.g. human erythrocyte acetylcholine esterase and decay accelerating factor) have been reported to contain palmitoylated inositol residues.