Bilayer nanotubes and helical ribbons formed by hydrated galactosylceramides: acyl chain and headgroup effects

Abstract

The molecular basis of bilayer tubule formation in hydrated galactosylceramide (GalCer) dispersions has been investigated by synthesizing different chain-pure GalCers and examining their aqueous mesomorphic phase structure by freeze fracture and negative-stain electron microscopy. Thermotropic characterization of the GalCer species by differential scanning calorimetry provided supplementary information that verified the phase state under which morphological observations were carried out. Under aqueous conditions and at room temperature, N-24:1 delta 15(cis) GalSph, the predominant monounsaturated, nonhydroxy acyl species of bovine brain GalCer (NFA-GalCer), formed cylindrical mesomorphic self-assemblies consisting almost exclusively of "nanotubes," i.e., lipid bilayer tubules of relatively uniform length and diameter (length, 250-400 nm; diameter, 25-30 nm). In contrast, N-24:0 GalSph, the major saturated, nonhydroxy acyl species of bovine brain GalCer, displayed no tendency to form these relatively small "nanotubes." Rather, N-24:0 GalSph formed larger, variable-length ribbon-like structures (length, 5,000-10,000 nm) that often appeared to undulate and, occasionally, appeared to be helically twisted. Interestingly, bovine brain GalCer, which contains high levels of the N-24:1 delta 15(cis) and N-24:0 species as well as 2-hydroxy acyl chains, formed multilamellar liposomes of variable size and showed little tendency to form cylindrical structures. This result suggested that changes to the polar interface/headgroup region imparted by the 2-hydroxy acyl species strongly influenced bilayer tubule and cylinder formation in GalCer. To define this influence more clearly, other sphingoid-based and glycerol-based lipids were investigated. Morphological characterization of N-24:1 delta 15(cis) sphingosylphosphorylcholine (24:1 SM) revealed no evidence of bilayer cylinder or tubule formation. Similar results were obtained with aqueous dispersions of 1-palmitoyl-2-nervonoyl phosphatidylcholine (16:0, 24:1 PC). Hence, the bulkier, more hydrated, zwitterionic phosphocholine headgroup inhibited the formation of bilayer nanotubes and cylinders under physiological saline conditions.

Intermediate aggregates resulting in the interaction of bile salt with liposomes studied by transmission electron microscopy and light scattering techniques

“…The ring observed around the larger vesicles is generally detected when observing liposomes using this TEM technique (Wertz et al, 1986;Lasic, 1993a;Yarosh et al, 1994;Zeidel et al, 1994;Memoli et al, 1995;Kulkarni et al, 1995;Son & Alkan, 1989;Polozova et al, 1995). The fact that this ring was much larger in the steps near to vesicle disintegration has also been reported by our research group for solubilization of PC liposomes with different surfactants (de la Maza & Parra, 1994a,b, 1995.…”

“…It has been previously demonstrated that the TEM technique applied to negatively stained liposomes constituted an appropriate technique to study the formation and morphology of different liposome structures and their interaction with biological tissues (Wertz et al, 1986;Lasic, 1993a;Yarosh et al, 1994;Zeidel et al, 1994;Kulkarni et al, 1995;Memoli et al, 1995). This technique has been demonstrated to be very convenient in the characterization of the aggregates (vesicles and mixed micelles) formed during the interaction of different amphiphilic compounds with liposomes (Beigel et al, 1988;Son & Alkan, 1989;Lasic, 1993b;Polozova et al, 1995;de la Maza & Parra, 1994a,b, 1995.…”

Cholesterol-Induced Interfacial Area Condensations of Galactosylceramides and Sphingomyelins with Identical Acyl Chains

“…Egg and bovine brain SM were purchased from Avanti Polar Lipids (Alabaster, AL), and psychosine was obtained from Matreya, Inc. (Pleasant Gap, PA). GalCers and SMs containing homogeneous acyl chains were synthesized by reacylating psychosine and sphingosylphosphocholine, respectively, with the N-hydroxysuccinimide ester of the desired fatty acid (Kulkarni et al, 1995;Smaby et al, 1996). No dihydropsychosine was evident by thin layer chromatography TLC).…”

Lipid nanotubes as substrates for helical crystallization of macromolecules

“…In the work described here, we have combined the principles of the lipid layer crystallization methodology with the propensity of certain lipids to form nanotubes (26) to develop a general approach for helical crystallization of macromolecules. By incorporating charged lipids or lipids that have been chemically modified with high affinity ligands into the nanotubes, molecules from solution can be adsorbed specifically to the nanotube surface where crystallization is favored.…”