Morphology, mechanical properties and viability of encapsulated cells.

The morphological and mechanical properties of encapsulated yeast cells (Saccharomyces cerevisiae) have been investigated by atomic force microscopy (AFM). Single living cells have been coated through the alternate deposition of oppositely charged polyelectrolyte (PE) layers. The properties of cells coated by different numbers of PE layers and from PE solutions of different ionic strength have been investigated. AFM imaging indicates an increase in PE coating stability when decreasing the solution ionic strength. The Young's moduli of the different examined systems have been evaluated through a quantitative analysis of force-distance curves by using the Hertz-Sneddon model. The analysis indicates an increase in hybrid system stiffness when lowering the ionic strength of the PE solution. An evaluation of the viability of encapsulated cells was obtained by confocal laser scanning microscopy (CLSM) measurements. CLSM analysis indicates that cells preserve their subcellular structure and duplication capability after encapsulation. By coupling AFM and CLSM data, a correlation between local stiffness and duplication rate was obtained.

A thermodynamic and structural study of myelin basic protein in lipid membrane models.

Myelin basic protein (MBP) is a major protein of the myelin membrane in the central nervous system. It is believed to play a relevant role in the structure and function of the myelin sheath and is a candidate autoantigen in demyelinating processes such as multiple sclerosis. MBP has many features typical of soluble proteins but is capable of strongly interacting with lipids, probably via a conformation change. Its structure in the lipid membrane as well as the details of its interaction with the lipid membrane are still to be resolved. In this article we study the interaction of MBP with Langmuir films of anionic and neutral phospholipids, used as experimental models of the lipid membrane. By analyzing the equilibrium surface pressure/area isotherms of these films, we measured the protein partition coefficient between the aqueous solution and the lipid membrane, the mixing ratio between protein and lipid, and the area of the protein molecules inserted in the lipid film. The penetration depth of MBP in the lipid monolayer was evaluated by x-ray reflectivity measurements. The mixing ratio and the MBP molecular area decrease as the surface pressure increases, and at high surface pressure the protein is preferentially located at the lipid/water interface for both anionic and neutral lipids. The morphology of MBP adsorbed on lipid films was studied by atomic force microscopy. MBP forms bean-like structures and induces a lateral compaction of the lipid surface. Scattered MBP particles have also been observed. These particles, which are 2.35-nm high, 4.7-nm wide, and 13.3-nm long, could be formed by protein-lipid complexes. On the basis of their size, they could also be either single MBP molecules or pairs of c-shaped interpenetrating molecules.