N-stearoyl-phosphatidylserine: synthesis and role in divalent-cation-induced aggregation and fusion

Formation of tethered bilayer lipid membranes on gold surfaces: QCM-Z and AFM study

Recently, tethered bilayer lipid membranes (tBLMs) have shown high potential as biomimetic systems due to their high stability and electrical properties, and have been used in applications ranging from membrane protein incorporation to biosensors. However, the kinetics of their formation remains largely uninvestigated. By using quartz crystal microbalance with impedance analysis (QCM-Z), we were able to monitor both the kinetics and viscoelastic properties of tether adsorption and vesicle fusion. Formation of the tether monolayer was shown to follow pseudo-first-order Langmuir kinetics with association and dissociation rate constants of 21.7 M-1 s(-1) and 7.43 x 10-6 s(-1), respectively. Moreover, the QCM-Z results indicate a rigid layer at the height of deposition, which then undergoes swelling as indicated by AFM. The deposition of vesicles to the tether layer also followed pseudo-first-order Langmuir kinetics with observed rate constants of 5.58 x 10(-2) and 2.41 x 10-2 s(-1) in water and buffer, respectively. Differential analysis of the QCM-Z data indicated deposition to be the fast kinetic step, with the rate-limiting steps being water release and fusion. Atomic force microscopy pictures taken complement the QCM-Z data, showing the major stages of tether adsorption and vesicle fusion, while providing a road map to successful tBLM formation.

Interactions of Ca(2+) with sphingomyelin and dihydrosphingomyelin

The changes induced by Ca(2+) on human lens sphingolipids, sphingomyelin (SM), and dihydrosphingomyelin were investigated by infrared spectroscopy. Ca(2+)-concentration-dependent studies of the head group region revealed that, for both sphingolipids, Ca(2+) partially dehydrates some of the phosphate groups and binds to others. Ca(2+) affects the interface of each sphingolipid differently. In SM, Ca(2+) shifts the amide I' band to frequencies lower than those in dehydrated samples of SM alone. This could be attributed to the direct binding of Ca(2+) to carbonyl groups and/or strong tightening of interlipid H-bonds to levels beyond those in dehydrated samples of SM only. In contrast, Ca(2+) induces relatively minor dehydration around the amide groups of dihydrosphingomyelin and a slight enhancement of direct lipid-lipid interactions. Temperature-dependent studies reveal that 0.2 M Ca(2+) increases the transition temperature T(m) from 31.6 +/- 1.0 degrees C to 35.7 +/- 1.1 degrees C for SM and from 45.5 +/- 1.1 degrees C to 48.2 +/- 1.0 degrees C for dihydrosphingomyelin. Binding of Ca(2+) to some phosphate groups remains above T(m). The strength of the interaction is, however, weaker. This allows for the partial rehydration of these moieties. Similarly, above T(m), Ca(2+)-lipid and/or direct inter-lipid interactions are weakened and lead to the rehydration of amide groups.

Incorporating lipids into boar sperm decreases chilling sensitivity but not capacitation potential

Fresh boar sperm were incubated with small unilamellar liposomes composed of either the total lipids extracted from head plasma membranes (HPM) of fresh boar sperm or selected lipids (SL) of five defined phospholipids with specific acyl chains. To optimize fusion, liposomes with 2 mol% octadecyl rhodamine fluorophore in Beltsville Thawing Solution +/- 1 mM CaCl(2) were incubated at 35 degrees C with 1;ts 10(7) or 10(8) spermatozoa/ml and monitored over 60 min, using flow cytometry and fluorescence microscopy. The HPM fused to both sperm concentrations faster than SL but was equivalent by 30 min (10(8) sperm/ml) or 60 min (10(7) sperm/ml; 57.5 +/- 3% and 67.1 +/- 8% sperm fused to HPM and SL, respectively) +/- Ca(2+). Neither HPM nor SL affected onset of capacitation or spontaneous or ionophore-induced acrosome reactions at 0 or 3 h (chlortetracycline and fluorescein isothiocyanate-Pisum sativum agglutinin; n = 3). During cooling and after cryopreservation (n = 4 ejaculates), SL but not HPM significantly improved sperm motility and viability (Sybr14/propidium iodide staining) +/- 20% egg yolk, but egg yolk alone was more effective than SL alone. Liposomes of complex composition can fuse to boar sperm without harming in vitro capacitation or acrosome reaction and reduce sperm chilling sensitivity.

Aggregation and fusion of vesicles composed of N-palmitoyl derivatives of membrane phospholipids

N-Acylphosphatidylethanolamines and N-acylphosphatidylserines have been isolated from mammalian cells and have been associated with some tissue degenerative changes, although the relationship between their synthesis and the uncontrolled sequence of events that ends in irreversible tissue damage is not completely established. Our results show that monovalent and divalent cations induce aggregation and fusion of liposomes constituted by N-palmitoylphosphatidylethanolamine (NPPE) and N-palmitoylphosphatidylserine (NPPS). The effectiveness of cations to induce the aggregation of NPPE and NPPS liposomes is Ca2+ > Mg2+ >> Na+. NPPS liposomes aggregate at lower concentrations of divalent cations than NPPE liposomes, but with sodium NPPE liposomes aggregate to a higher extent than NPPS liposomes. The reaction order for the aggregation processes depends on the lipid and the cation nature and range from 1.04 to 1.64. Dynamic light scattering shows an irreversible increase of the size of the aggregates in the presence of all cations tested. The irreversibility of the aggregation process and the intermixing of bilayer lipids, as studied by resonance energy transfer assay, suggest that fusion, rather than aggregation, occurs. The existence of a real fusion was demonstrated by the coalescence of the aqueous contents of both NPPS and NPPE liposomes in the presence of either monovalent or divalent cations. The different binding sensitivity of Ca2+ to NPPS and NPPE liposomes, determined by zeta potential measurements, agrees with the results obtained in the aggregation and fusion assays. Our results suggest that the synthesis in vivo of N-acylated phospholipids can introduce important changes in membrane-mediated processes.