Prof Sonia Antoranz Contera

Abstract:

Purple membranes (PM) are two-dimensional crystals formed by bacteriorhodopsin and a variety of lipids. The lipid composition and density in the cytoplasmic (CP) leaflet differ from those of the extracellular (EC) leaflet. A new way of differentiating the two sides of such asymmetric membranes using the phase signal in alternate contact atomic force microscopy is presented. This method does not require molecular resolution and is applied to study the stiffness and intertrimer lipid mobility in both leaflets of the PM independently over a broad range of pH and salt concentrations. PM stiffens with increasing salt concentration according to two different regimes. At low salt concentration, the membrane Young's normal modulus grows quickly but differentially for the EC and CP leaflets. At higher salt concentration, both leaflets behave similarly and their stiffness converges toward the native environment value. Changes in pH do not affect PM stiffness; however, the crystal assembly is less pronounced at pH > or = 10. Lipid mobility is high in the CP leaflet, especially at low salt concentration, but negligible in the EC leaflet regardless of pH or salt concentration. An independent lipid mobility study by solid-state NMR confirms and quantifies the atomic force microscopy qualitative observations.

Abstract:

Magnetic AC mode (MACmode) atomic force microscopy (AFM) was used to study murine (mouse) MC3T3-E1 preosteoblastic cells attached to biocompatible tantalum substrates. Cell volumes of attached cells derived from AFM images were compared to volumes of detached cells in suspension measured by the Coulter sizing technique. An increase of approximately 50% in cell volume was observed when the cells attached to planar tantalum substrates and developed a flattened structure including lamellipodia. We address thoroughly the issues general to the AFM determination of absolute cell volumes, and compare our magnetic AC mode AFM measurements to hitherto reported cell volume determinations by contact mode AFM.