Thermal Induced Modulation of Surface Charge of Sialoglycosphingolipid Micelles

Effect of protein-encapsulation on thermal structural stability of liposome composed of glycosphingolipid/cholesterol/phospholipid

We have studied the thermal structural stability of liposomes encapsulating proteins by using synchrotron radiation small- and wide-angle X-ray scattering (SR-SWAXS). Liposomes are known to be effective drug-delivery systems (DDSs) because they can reduce drug toxicity due to biodegradability and biocompatibility and can offer promising carriers of various types of drugs. However, in spite of numerous studies of liposomes, physicochemical characteristics of liposomes entrapping proteins are rarely known. The liposome studied is characterized by the lipid composition (mixture of acidic glycosphingolipid (ganglioside)/cholesterol/phospholipid). Gangliosides are one of the major constituents of so-called lipid rafts playing the role of a platform of cell-signaling. We have found that the encapsulation of proteins elevates the thermal transition temperature of the liposome membrane and suppresses the deformation of its shape. The present results suggest that not only membrane proteins, but also water-soluble proteins affect liposome stability through the revalence between osmotic pressure and membrane elasticity. In addition, we have found the presence of the size-effect depending on the molar content of gangliosides in the liposome, indicating the ability of ganglioside molecule controlling both the size and effective surface charge of the liposome. The present results would have significance from two different points of view. One is the confinement effect of proteins within a limited space like cell, and the other is a stability of a new type of DDS using gangliosides. Due to the intrinsic properties, gangliosides are expected to be promising agents for targeting and long-circulation properties of liposomal DDSs.

Structure of liposome encapsulating proteins characterized by X-ray scattering and shell-modeling

Lipid liposomes are promising drug delivery systems because they have superior curative effects owing to their high adaptability to a living body. Lipid liposomes encapsulating proteins were constructed and the structures examined using synchrotron radiation small- and wide-angle X-ray scattering (SR-SWAXS). The liposomes were prepared by a sequential combination of natural swelling, ultrasonic dispersion, freeze-throw, extrusion and spin-filtration. The liposomes were composed of acidic glycosphingolipid (ganglioside), cholesterol and phospholipids. By using shell-modeling methods, the asymmetric bilayer structure of the liposome and the encapsulation efficiency of proteins were determined. As well as other analytical techniques, SR-SWAXS and shell-modeling methods are shown to be a powerful tool for characterizing in situ structures of lipid liposomes as an important candidate of drug delivery systems.

Change of dynamics of raft-model membrane induced by amyloid-β protein binding

While the steady-state existence in the size and shape of liquid-ordered microdomains in cell membranes, the so-called "lipid rafts", still remain the subject of debate, glycosphingolipid-cholesterol rich regions in plasma membranes have been considered to have a function as platforms for signaling and sorting. In addition, recent spectroscopic studies show that the interaction between monosialoganglioside and amyloid beta (Aβ protein promotes the transition of Aβ from the native structure to the cross-beta fold in amyloid aggregates. However, there is few evidence on the dynamics of "lipid rafts" membranes. As the neutron spin-echo (NSE) technique is well known to detect directly slow dynamics of membrane systems in situ, by the combination of NSE and small-angle X-ray scattering we have studied the effect of the interaction between raft-model membrane and amyloid Aβ proteins on the structure and dynamics of a large uni-lamellar vesicle (LUV) consisting of monosialoganglioside-cholesterol-phospholipid ternary mixtures as a model of lipid-raft membrane. We have found that the interaction between the Aβ proteins and the model membrane at the liquid crystal phase significantly suppresses a bending-diffusion motion with a minor effect on the LUV structure. The present results would suggest a possibility of non-receptor-mediated disorder in signaling through a modulation of a membrane dynamics induced by the association of amyloidogenic peptides on a plasma membrane.

Determination of asymmetric structure of ganglioside-DPPC mixed vesicle using SANS, SAXS, and DLS

Functions of mammalian cell membrane microdomains being rich in glycosphingolipids, so-called rafts, are now one of the current hot topics in cell biology from the intimate relation to cell adhesion and signaling. However, little is known about the role of glycosphingolipids in the formation and stability of the domains. By the use of the inverse contrast variation method in small-angle neutron scattering (SANS), combined with small-angle x-ray scattering (SAXS) and dynamic light scattering (DLS), we have determined an asymmetric internal structure of the bilayer of the small unilamellar vesicle (SUV) of monosialoganglioside (G(M1))-dipalmitoylphosphatidylcholine (DPPC) mixture ([G(M1)]:[DPPC] = 0.1:1). A direct method using a shell-model fitting with a size distribution function describes consistently all experimental results of SANS, SAXS, and DLS. We have found that G(M1) molecules predominantly localize at SUV outer surface to form a highly hydrophilic layer which is dehydrated with the rise of temperature from 25 degrees C to 55 degrees C accompanied by the conformational change of the oligosaccharide chains. The average SUV size determined is approximately 200 A, which is comparable to the reported value 260 +/- 130 A of glycosphingolipids microdomains. The present results suggest that the preferential asymmetric distribution of gangliosides is essential to define the size and stability of the domains.