Biological Significance
A growing body of evidence now suggests that cellular membranes manage their vast chemical diversity by sorting into specialized compartments or microdomains (Glaser 1993; Vaz & Almeida 1993). A particular class of membrane microdomains exhibiting significant enrichment of cholesterol and sphingolipids in their material composition has received considerable attention in recent years (Simons & Ikonen 1997; Brown 2002). These microdomains, commonly referred to as lipid rafts, are believed to play a role in a broad range of important biological processes including: a vast class of signaling pathways, cell adhesion and migration, synaptic transmission, cytoskeletal organization, membrane transport, protein sorting, and apoptosis. (Verkade & Simons 1997; Simons & Toomre 2000; Simons 2002). Biological systems use raft size and spacing distributions to control these processes. In addition, rafts have also been suggested to serve as adhesion targets for bacteria, viruses, and toxins, as well as provide microenvironments for prion and amyloid aggregation (Simons & Ehehalt 2002). It is believed that raft-initiated cellular events are the result of small regulated perturbations about a meta-stable equilibrium that the cell maintains through tuning the size and distribution of raft domains. This tuning is achieved physically and chemically by altering cellular compartmentalization and cholesterol concentration.
I am working on utilizing new techniques for capturing the nucleation, growth, and formation of lipid domains. Much lipid domain work has been done using fluorescence micrsoscopy as well as AFM. In a collaboration with Chris Orme from Lawrence Livermore National Lab, we are examining these systems using both AFM and Imaging Ellipsometry (IE). The combination of these techniques allows for real time, wide filed-of-view, label-free imaging with IE, while capturing nano-scale features with AFM. Both techniques additionally yield thickness or height measurements of the thin lipid film. By comparing the large statistical domain growth captured by IE and the detailed single domain nucleation and growth with AFM, we aim to better understand some of the kinetic and thermodynamic processes controlling these formations.
We are also interested in other complimentary methods of determining bilayer thicknesses. Through a collaboration with Sunil Sinha from UC San Diego, we have been exploring the limits of specular and off-specular X-ray and neutron scattering and how these techniques may help in exposing not only bilayer thicknesses, but domain sizes and spacings in a non-perturbative way.
