Lipids and Lipid-Intermediate Structures in the Fusion of Biological Membranes

Processes involved in assisted reproduction technologies significantly increase sperm DNA fragmentation and phosphatidylserine translocation

Sperm preparation techniques in assisted reproduction technologies (ART) are potential generators of exogenous stresses that cause additional DNA damage. DNA fragmentation tests, such as the sperm chromatin structure assay, involve freezing sperm samples in the absence of cryoprotectant. Thermal, oxidative stress (OS) and freezing are detrimental to sperm DNA fragmentation and phosphatidylserine (PS) translocation. The primary aim of this study was to subject mature sperm to environmental insults that normally occur during ART. We tested the hypotheses that OS, thermal stress and freeze-thawing caused sperm nuclear and membrane damage and that a positive correlation exists between PS translocation and DNA fragmentation. Sperm DNA integrity deteriorates in semen samples from men with advancing age and a sperm concentration of <15 m ml(-1) . The significant increase in sperm DNA fragmentation at 37 °C after merely 1 h is important clinically as semen liquefaction and short-term sperm storage in an ART cycle involve incubating samples at this temperature. Freezing without a cryoprotectant significantly increases the level of sperm nuclear damage, so it is important not to freeze neat semen prior to DNA fragmentation testing. This study highlights the importance of minimising the production of exogenous stresses during sperm preparation in ART.

Incorporation of semi-fluorinated alkanes in the bilayer of small unilamellar vesicles of phosphatidylserine: impact on fusion kinetics

Semi-fluorinated alkanes C(n)F(2n+1)C(m)H(2m+1) (FnHm) can be co-dispersed with standard phospholipids to form 'fluorinated' vesicles, i.e. vesicles with an internal fluorinated film within their bilayer membrane. This paper reports the effect of the presence of such FnHm diblocks in phosphatidylserine (PS)-based small unilamellar vesicles (SUVs) on their kinetics of fusion. Fusion was induced by calcium ions and monitored by the terbium/dipicolinic acid assay. The diblocks were composed of a 10-carbon long linear hydrocarbon segment and of a linear fluorocarbon segment of four, six or eight carbon atoms. We found that the incorporation of FnHm in the PS membrane considerably modifies the kinetics of the process of fusion, with Ca(2+) concentration having a much more limited effect on the fluorinated vesicles. Both the rates of fusion and the rates of release of the internal content, as evaluated by the release of 5,6-carboxyfluorescein, were much lower for the fluorinated SUVs than for those based on phosphatidylserine alone, the highest effect being obtained for F6H10 with a 10 times slower rate of fusion and a 40-fold reduction in the release of content. FnHm molecules are proposed to have a dual action: by hindering fusion and release by creating an inert, hydrophobic and lipophobic fluorinated film in the core of the membrane, and by stabilizing the membrane by increasing van der Waals interactions in the hydrocarbon region.

Modulation of lipid polymorphism by the feline leukemia virus fusion peptide: implications for the fusion mechanism

The structural effects of the fusion peptide of feline leukemia virus (FeLV) on lipid polymorphism were studied, using differential scanning calorimetry (DSC), 31P nuclear magnetic resonance (NMR), and time-resolved X-ray diffraction. This peptide lowers the bilayer to inverted hexagonal phase transition temperature, TH, of dipalmitoleoylphosphatidylethanolamine (DiPoPE) at peptide mole fractions of up to 1.5 x 10(-3) at pH 5.0 and at pH 7.4. The temperature at which isotropic 31P NMR signals for monomethyldioleoylphosphatidylethanolamine (MeDOPE) first occurred is lowered by the FeLV peptide. The amount of isotropic signal seen at 40 degrees C is directly correlated to the peptide:lipid molar ratio. In the peptide-containing samples, more lipid remains in the isotropic state over the whole recorded temperature range. Isotropic 31P NMR signals were observed for DiPoPE in the presence of the FeLV peptide for the entire recorded temperature range of 35-50 degrees C, while pure DiPoPE showed no significant amount of isotropic signal. X-ray studies of DiPoPE show the formation of a new lipid phase with peptide, which is not seen in the pure lipid samples. Disordering of the Lalpha phase is evidenced by broadening of the diffraction peaks, and the hexagonal cell parameter is decreased with peptide present. Our results suggest that the FeLV peptide is increasing the negative curvature of the lipid system, which is thought to be crucial to the formation of highly bent, high-energy structural fusion intermediates, such as the "stalk" model. Fusion activity for this putative fusogenic peptide was also demonstrated, using a resonance energy transfer (RET) lipid mixing assay. To our knowledge, this work provides the first published experimental evidence of both fusogenic activity and effects on lipid polymorphism for the FeLV fusion peptide.