IntroductionCell Signaling

Use of Nanoprobes to Monitor Cell Signaling

Under certain conditions, extracellular chemical traces guide the movement of a cell in a particular direction. Extracellular signals are transmitted across the cell membrane by receptors for G-protein-mediated signaling pathways. The G-protein signal transduction system regulates the cytoskeleton through the generation of second messengers, such as phospholipids, which activate or inactivate a number of actin binding proteins or cytoskeletal protein kinases. Thus signaling in the cell membrane, a composite of lipids and proteins, affects the cytoskeleton. Since temperature, viscosity, and molecule size cannot control diffusive speed in the membrane, there has been much excitement about recent experiments indicating anomalous diffusion -particularly subdiffusion- in addition to normal diffusion. However, the limited observation area due to small cell size (~10 µm) leads to statistically inconclusive data. Diffusive transport differs significantly between Brownian and anomalous diffusion due to changes in molecular motion. Whereas normal diffusion is well understood, fundamental questions remain in the physics of anomalous diffusion. For a general study of diffusion in 2-D liquid crystals such as cell membranes, we have developed a lipid monolayer based system with an observation area 100 x larger than that of a cell. By labeling single lipids on a Langmuir monolayer with a gold nano-particle, we can track them individually using darkfield microscopy. In the fluid-crystalline coexistence phase of the monolayer, our initial results show a transition between normal and subdiffusion depending on the crystalline fraction. Our final aim is a comprehensive study of lipid diffusion with a special focus on the possibility of inducing transitions between normal diffusion and subdiffusion. It has been recently shown that amyloid peptide linked to Alzheimer's disease changes the size and structure of lipid rafts. Thus, we will investigate with our new technique how lipid diffusion changes in lipid mixtures generating lipid rafts when amyloid peptide is added to the subphase.