Interaction of α-tocopherol with a polyunsaturated lipid studied by MD simulations

poster abstractPolyunsaturated phospholipids are essential components of neural membranes and their effect on membrane architecture is proposed to be the molecular origin of a myriad of health benefits. A downside of polyunsaturated phospholipids is that they are highly susceptible to oxidation due to the presence of multiple double bonds. α-Tocopherol is the most biologically active component in a family of phenolic compounds that comprise vitamin E, which is the major lipid soluble antioxidant in cell membranes. To investigate whether α-tocopherol preferentially interacts with polyunsaturated phospholipids to optimize protection against oxidation, we performed MD simulations on 1-stearoyl-2-docosahexaenoylphosphatiylcholine (SDPC, 18:0-22:6PC) and 1-stearoyl-2-oleoylphosphatidylcholine (SOPC, 18:0-18:1PC) bilayers containing α-tocopherol. SDPC with a docosahexaenoyl sn-2 chain is polyunsaturated, while SOPC with an oleoyl sn-2 chain serves as a monounsaturated control. The simulations were run under constant pressure for 200 ns on a system that comprised 80 phospholipid molecules, 20 α-tocopherol molecules and 2165 water molecules. We discovered significant differences between the two systems. Analysis of the simulations indicates that the α-tocopherol has a strong interaction with the polyunsaturated fatty acid. The flip-flop of α-tocopherol across the bilayer is also much faster in SDPC than in SOPC. Solid state NMR, neutron scattering and complementary experiments are now underway to test the predictions from the MD simulations

α-Tocopherol is well designed to protect polyunsaturated fatty acids

poster abstractPolyunsaturated fatty acids (PUFA) are an influential constituent in cell membranes, but are extremely vulnerable to oxidation. The presumptive role for α-tocopherol (α-toc), the molecular form of vitamin E retained by the human body, is to protect PUFA-containing lipids from oxidation. To investigate whether α-toc preferentially interacts with PUFA in support of this function, we performed MD simulations on lipid bilayers composed of 1-stearoyl-2-docosahexaenoylphosphatidylcholine (SDPC, 18:0-22-6PC) and 1-stearoyl-2-oleoylphosphatidylcholine (SOPC, 18:0-18:1PC) in the presence of α-toc. SDPC with docosahexaenoic acid (DHA) for the sn-2 chain is polyunsaturated, while SOPC with oleic acid (OA) for the sn-2 chain serves as a monounsaturated control. The simulations were run at 37 °C under constant pressure for 200 ns on a system that comprised 80 phospholipid molecules, 20 α-toc molecules and 2165 water molecules. In qualitative agreement with our results from solid state 2H NMR and neutron scattering experiments, the simulations show that α-toc increases order inside the bilayer and that the chromanol headgroup sits near the surface in both SDPC and SOPC. Analyses of the density distribution of the lipid chains relative to α-toc show that the α-toc’s chromanol headgroup, the part of the molecule that protects against oxidation, would have more chance to interact with PUFA chains than saturated chains. A major prediction from our simulations is that α-toc undergoes flip-flop across the bilayer and that the rate is an order of magnitude greater in SDPC than SOPC. This is a remarkable finding that reveals a possible mechanism by which the chromanol group would not only wait at the membrane surface but would also patrol the membrane interior to meet lipid radicals and terminate the chain reaction by which lipid peroxidation proceeds

Influence of ceramide on lipid domain stability studied with small-angle neutron scattering: The role of acyl chain length and unsaturation

Ceramides and diacylglycerols are groups of lipids capable of nucleating and stabilizing ordered lipid domains, structures that have been implicated in a range of biological processes. Previous studies have used fluorescence reporter molecules to explore the influence of ceramide acyl chain structure on sphingolipid-rich ordered phases. Here, we use small-angle neutron scattering (SANS) to examine the ability of ceramides and diacylglycerols to promote lipid domain formation in the well-characterized domain- forming mixture DPPC/DOPC/cholesterol. SANS is a powerful, probe-free technique for interrogating membrane heterogeneity, as it is differentially sensitive to hydrogen\u27s stable isotopes protium and deuterium. Specifcally, neutron contrast is generated through selective deuteration of lipid species, thus enabling the detection of nanoscopic domains enriched in deuterated saturated lipids dispersed in a matrix of protiated un- saturated lipids. Using large unilamellar vesicles, we found that upon replacing 10 mol % DPPC with either C16:0 or C18:0 ceramide, or 16:0 diacylglycerol (dag), lipid domains persisted to higher temperatures. However, when DPPC was replaced with short chain (C6:0 or C12:0) or very long chain (C24:0) ceramides, or ceramides with unsaturated acyl chains of any length (C6:1(3), C6:1(5), C18:1, and C24:1), as well as C18:1-dag, lipid domains were destabilized, melting at lower temperatures than those in the DPPC/DOPC/cholesterol system. These results show how ceramide acyl chain length and unsaturation influence lipid domains, and have implications for how cell membranes might modify their function through the generation of different ceramide species

The Role of Bilayer Tilt Difference in Equilibrium Membrane Shapes

Lipid bilayer membranes below their main transition have two tilt order parameters, corresponding to the two monolayers. These two tilts may be strongly coupled to membrane shape but only weakly coupled to each other. We discuss some implications of this observation for rippled and saddle phases, bilayer tubules, and bicontinuous phases. Tilt difference introduces a length scale into the elastic theory of tilted fluid membranes. It can drive an instability of the flat phase; it also provides a simple mechanism for the spontaneous breaking of inversion symmetry seen in some recent experiments.Comment: Latex file; .ps available at http://dept.physics.upenn.edu/~nelson/saddle.p

Lipid bilayer thickness determines cholesterol's location in model membranes

Cholesterol is an essential biomolecule of animal cell membranes, and an important precursor for the biosynthesis of certain hormones and vitamins. It is also thought to play a key role in cell signaling processes associated with functional plasma membrane microdomains (domains enriched in cholesterol), commonly referred to as rafts. In all of these diverse biological phenomena, the transverse location of cholesterol in the membrane is almost certainly an important structural feature. Using a combination of neutron scattering and solid-state 2H NMR, we have determined the location and orientation of cholesterol in phosphatidylcholine (PC) model membranes having fatty acids of different lengths and degrees of unsaturation. The data establish that cholesterol reorients rapidly about the bilayer normal in all the membranes studied, but is tilted and forced to span the bilayer midplane in the very thin bilayers. The possibility that cholesterol lies flat in the middle of bilayers, including those made from PC lipids containing polyunsaturated fatty acids (PUFAs), is ruled out. These results support the notion that hydrophobic thickness is the primary determinant of cholesterol's location in membranes

Bovine Spongiform Encephalopathy and Public Health

Η σπογγιόμορφη εγκεφαλοπάθεια των βοοειδών είναι μεταδοτική εκφυλιστική νόσος του κεντρικού νευρικού συστήματος και ανήκει σε ομάδα  ασθενειών, οι οποίες προσβάλλουν τον άνθρωπο και διάφορα είδη ζώων και έχουν παρόμοια ιστοπαθολογική εικόνα. Ο βλαπτικός παράγοντας της BSE, αλλά και όλων των άλλων σπογγιόμορφων εγκεφαλοπαθειών, δεν έχει πλήρως διευκρινιστεί. Η επικρατέστερη σήμερα άποψη είναι ότι αυτός αποτελείται κυρίως ή και μόνον από μη φυσιολογική πρωτεΐνη, που ονομάστηκε prion. Στις διάφορες παρατηρήσεις ο βλαπτικός παράγοντας γίνεται αντιληπτός με τη μορφή πρωτεϊνικών κυλίνδρων, οι οποίοι αποτελούνται από συγκεντρώσεις ή πολυμερισμένη μορφή του βλαπτικού παράγοντα και ονομάστηκε πρωτεΐνη prion (prion protein - PrP). Αποδείχθηκε ότι υπάρχουν δυο ισόμορφες της PrP. Η μία που σημειώνεται ως PrF παράγεται από αρκετά κύτταρα του ανθρώπου και των ζώων και αποτελεί κυτταρικό δομικό στοιχείο. Η δεύτερη που σημειώνεται ως PrPst παρουσιάζει ιδιαίτερες ιδιότητες, που την καθιστούν παθολογική και υπεύθυνη για τη δημιουργία των σπογγιόμορφων εγκεφαλοπαθειών. Ο αναδιπλασιασμός της PrPM φαίνεται ότι διενεργείται στα λυσοσώματα κυττάρων του νευρικού συστήματος και των δενδριτικών, καθώς και λοιπών κυττάρων του δικτυωτού των λεμφοκυτογόνων οργάνων με μετατροπή της PrPc σε PrPsc. Όπως φαίνεται η BSE προκλήθηκε στα βοοειδή εξαιτίας της κατανάλωσης από αυτά κρεαταλεύρων και οστεαλεύρων, που προέρχονταν από πρόβατα μολυσμένα από τη scrapie. Αναφορικά με την παθογένεια φαίνεται από πειραματικά δεδομένα ότι αρχικά η PrPsr εισέρχεται με την τροφή και εγκαθίσταται στα διάφορα λεμφοκυτογόνα όργανα όπου και γίνεται ο πρώτος αναδιπλασιασμός της. Πιστεύεται ότι, από τα λεμφοκυτογόνα όργανα, μεταφέρεται με τα νεύρα στο κεντρικό νευρικό σύστημα, όπου και δημιουργεί τις χαρακτηριστικές αλλοιώσεις της κενοτοπιώδους εκφύλισης των νευρικών κυττάρων και τη σπογγίωση, οπότε και εμφανίζεται κλινικά η νόσος. Τα νευρικά συμπτώματα χαρακτηρίζονται κυρίως από αλλαγή στη συμπεριφορά των ζώων και από κινητικές ανωμαλίες. Η διάγνωση της νόσου γίνεται με την παρατήρηση των ιστοπαθολογικών αλλοιώσεων, την ανίχνευση ινιδίων συνδεμένων με τη scrapie-SAF και την ανοσοϊστοχημική ανίχνευση των μορίων της PrPsc σε ιστολογικές τομές ή με ηλεκτροφόρηση (Western blotting test). Η BSE αποδείχθηκε ότι μπορεί να μεταδοθεί σε άλλα ζώα και υπάρχει η πιθανότητα να μεταδίδεται και στον άνθρωπο με την τροφική αλυσίδα. Ύστερα από αυτά και σ' όλο το διάστημα που διέρρευσε, απότην εμφάνιση της μέχρι σήμερα, λήφθηκαν μέτρα, τόσο από τη Μ. Βρεττανία, όσο και από την ΕΕ για την εκρίζωση της νόσου και την προστασία της δημόσιας υγείας. Τα μέτρα αυτά θα πρέπει να τηρούνται από τις επίσημες αρχές και επιπρόσθετα θα πρέπει να ενημερωθεί ο καταναλωτής για την πιθανή επικινδυνότητα των διαφόρων ζωικών προϊόντων.Bovine Spongiform Encephalopathy (BSE) is a transmissible degenerative disease of the central nervous system. It belongs to a group of diseases which affect man and various kinds of animals and they have a similar histopathological appearance. The harmful agent of BSE and all the others spongiform encephalopathies have not been totally clarified. Today according to the predominant opinion this agent is consisted mainly or/and only of an abnormal protein, which is called prion. In various observations the harmful agent appears like proteinaceous cylinders which are consisted of aggregations or polymerized forms of the agent and it is called prion-protein (PrP). It has been proved that there are two isoforms of PrP. The first of them, called PrPc, is produced from many cells of man and animals and consists a cellular structural element. The second, called PrPs t, due to its specific properties, it is considered to be pathological and responsible for the spongiform encephalopathies. The replication of PrPsc seems to take place in the lysosomes of central nervous system cells, dendritic, and other reticular cells of the lymphatic organs through transformation of PrPc into PrPsc. It appears BSE caused by feeding meat and bone meals to cattle which were originated from scrapie infected sheep. Refering to the pathogenesis originating from experimental data it seems that initially the PrF* enters the body by food and afterwards is settled in various lymphoid organs where the first replication takes place. It is believed that BSE is transmitted through the nerves to the CNS, where it creates the characteristic lesions of vacuolar degeneration of the neurons and finally the spongiosis. Then the clinical signs are expressed. The nervous signs characterised by behavioural alterations of the animals and kinetic abnormalities. The diagnosis of the disease is made by the observation of the histopathological lesions, the detection of Scrapie Associated Fibrils-SAF by EM, the immunohistochemical detection of prpsc i n histological samples or by electrophoresis (Western blotting test). BSE was proved to be transmissible to other animals and there is a possibility that it could be done to man through the food chain. According to the above in these years, from the appearance of the disease until now, have been taken bans from Great Britain as well as from E.U. for the eradication of the disease and the protection of the public health. These instructions should be followed by the authorities and additionally the consumers ought to be informed for the possible danger of various animal products

Molecular Dynamics Simulations of Lipid Bilayers: Major Artifacts due to Truncating Electrostatic Interactions

We study the influence of truncating the electrostatic interactions in a fully hydrated pure dipalmitoylphosphatidylcholine (DPPC) bilayer through 20 ns molecular dynamics simulations. The computations in which the electrostatic interactions were truncated are compared to similar simulations using the Particle-Mesh Ewald (PME) technique. All examined truncation distances (1.8 to 2.5 nm) lead to major effects on the bilayer properties, such as enhanced order of acyl chains together with decreased areas per lipid. The results obtained using PME, on the other hand, are consistent with experiments. These artifacts are interpreted in terms of radial distribution functions $g(r)$ of molecules and molecular groups in the bilayer plane. Pronounced maxima or minima in g(r) appear exactly at the cutoff distance indicating that the truncation gives rise to artificial ordering between the polar phosphatidyl and choline groups of the DPPC molecules. In systems described using PME, such artificial ordering is not present

Performance-Based Seismic Design and Assessment of Bridges

Current trends in the seismic design and assessment of bridges are discussed, with emphasis on two procedures that merit some particular attention, displacement-based procedures and deformation-based procedures. The available performance-based methods for bridges are critically reviewed and a number of critical issues are identified, which arise in all procedures. Then two recently proposed methods are presented in some detail, one based on the direct displacement-based design approach, using equivalent elastic analysis and properly reduced displacement spectra, and one based on the deformation-based approach, which involves a type of partially inelastic response-history analysis for a set of ground motions and wherein pier ductility is included as a design parameter, along with displacement criteria. The current trends in seismic assessment of bridges are then summarised and the more rigorous assessment procedure, i.e. nonlinear dynamic response-history analysis, is used to assess the performance of bridges designed to the previously described procedures. Finally some comments are offered on the feasibility of including such methods in the new generation of bridge codes