Biomedical subjects

Z Mirghani

Publications and source records attributed to Z Mirghani.

4 recordsLinked to original sources

PubMed1996

Effect of physical constraints on the mechanisms of membrane fusion: bolaform lipid vesicles as model systems.

Bolaform lipid vesicles were used to study the effect of physical constraints on membrane fusion. In these vesicles the membrane is organized in a single monolayer, because of the presence of covalent bonds in its middle plane. Therefore, the formation of fusion intermediates is subject to higher energy barriers and greater geometrical constraints than is usual in bilayer membranes. Bolaform lipids were extracted from the thermophilic archaeon Sulfolobus solfataricus. These lipids can be divided into two classes, the monosubstituted molecules, in which one of the polar heads is glycerol, and the bisubstituted molecules, endowed with two complex polar heads. The fusion process in vesicles composed of different mixtures of monosubstituted/bisubstituted molecules was studied by means of fluorescence techniques. Ca2+ or poly(ethylene glycol) was employed as a fusogenic agent. We found that fusion of such constrained membranes is still possible, provided that molecules able to mediate a structural rearrangement of the membrane are present. This condition is fulfilled by monosubstituted molecules, which are able to partition the glycerol headgroup in the apolar moiety. In addition, the presence of traces (approximately 5%) of the monopolar compound diphytanylglycerol is an important factor for fusion to occur. On the contrary, vesicles formed by bisubstituted molecules are unable to fuse, irrespective of the fusogen employed.

Cell Membrane↗

PubMed1994

Calcium-induced interaction and fusion of archaeobacterial lipid vesicles: a fluorescence study.

The lipids extracted from the membrane of the thermophilic archaeobacterium Sulfolobus solfataricus have an unusual bipolar structure. Each molecule is formed by two isoprenoid chains (with up to four cyclopentane groups per chain) ether-linked at both ends to glycerol or nonitol groups. These groups can be variably substituted, mainly with complex sugars. Fluorescence resonance energy transfer, aqueous contents mixing and calcein release assays were employed to assess whether bipolar lipid vesicles were able to undergo a calcium-induced fusion process. The possibility of getting fusion depends strongly on the phase behaviour of the lipids. With vesicles formed by the natural polar lipid extract (PLE), a mixture showing a complex polymorphic behaviour, the fusion process was observed above the temperature T congruent to 60 degrees C at 15 mM Ca2+. By contrast, no fusion was observed in vesicles of P2, a fraction displaying only the lamellar phase. A dramatic change of the fusion process was observed when egg PC or P2 was added to PLE. In this case only lipid mixing, but not a real fusion process occurred at T > or = 60 degrees C. The dependence of such a process on ionic conditions has also been studied. Additional experiments involving surface tension measurements on monolayers have been performed to assess the importance of a surface tension increase to get fusion. In contrast to other monopolar lipid systems, no detectable change in surface tension has been observed in our bipolar lipids even in cases in which the fusion process is present.

Calcium↗

PubMed1992

Molecular packing parameters of bipolar lipids.

The bipolar lipid fractions extracted from the thermophilic archaeobacterium Sulfolobus solfataricus have different chemical structures and geometrical shapes. The conditions which lead to the formation of vesicles were investigated in order to study the self-assembly of these molecules. Such conditions are fulfilled when an appropriate mixture of two different molecular species (both bipolar or bipolar and monopolar) is used. According to the theory introduced by Israelachvili and co-workers, lipid self-assembly results from the balance of interaction free energy, entropy and molecular geometry. We have shown that this theory can be extended to bipolar lipids, in spite of their more complex nature, and the experimental results obtained combining 1H-NMR, light scattering and entrapped volume techniques closely match theoretical expectations. To carry out calculations, it was necessary to introduce hypotheses about the disposition of bipolar molecules in the vesicle membrane. These hypotheses have been tested indirectly by measuring the transport properties mediated by carriers or channels, whose transport mechanism can be considered to be a probe of the membrane structure.

Hot Temperature↗

PubMed1990

Monopolar-bipolar lipid interactions in model membrane systems.

1H-NMR, dynamic light scattering and negative staining electron microscopy have been used to study the formation and physico-chemical properties of aqueous dispersions of mixtures of monopolar lipids extracted from Sulfolobus solfataricus. This microorganism is a thermophilic archaeobacterium growing optimally at about 85 degrees C and pH 3. The two hydrolytic fractions of the membrane complex lipids that have been studied are: the symmetric lipid glycerol dialkyl glycerol tetraether (GDGT) and the asymmetric lipid glycerol dialkyl nonitol tetraether (GDNT). Electron micrographs of pure and mixed GDNT and GDGT dispersions show the formation of complex structures. Only above a critical monopolar/bipolar lipid ratio, typical of the bipolar lipid, could closed structures be formed and good agreement was obtained in sizing with NMR, electron microscopy and dynamic light scattering. NMR spectra have been carried out at several temperatures from 25 degrees to 85 degrees C, to obtain information on the temperature-dependent structural, dynamic and permeability properties of the co-dispersed vesicles. The results are discussed in terms of the steric constraints and the chemico-physical interactions occurring among the different parts of the molecules and compared with previous studies performed with different physical techniques.

Glyceryl Ethers↗