Membrane lipid domains: Techniques for visualization and characterization

Early studies in to lipids that comprised cellular membranes noted an asymmetrical distribution of lipids between different membranes which contrasted with the contemporary concept of the plasma membrane as a homogeneous mixture of lipid in which both lipids and membrane-associated proteins exhibit unfettered mobility. The ability of lipids to form inhomogeneities capable of restricting lateral mobility has since been demonstrated in both model and cell membranes. It has further been shown that lipids possess the ability to preferentially associate and co-exist in domains rich in particular classes of lipid, most notably cholesterol, sphingolipids, and glycolipids. In cells, 'rafts' further appear to be enriched in raft-targeted proteins. These observations have led to the hypothesis that phase-separated domains may act as signaling platforms capable of recruiting proteins thus facilitating their interaction for transduction of cellular signals. Experimental evidence largely based upon detergent isolation of raft domains has since been provided implicating laterally organized lipid domains in a litany of both physiological and pathophysiological processes including T-cell activation, B-cell antigen signaling, thromboregulation, Alzheimers disease and atherosclerosis. However, the field is often criticized for interpretations which may lack physiological relevance due to artifact arising from isolation procedures. Thus, investigators are increasingly looking to spectroscopic techniques to obtain information about phase-separated domains. This review seeks to summarize the current knowledge and understanding of raft structure, and provide a first foray in to the fundamentals and application of spectroscopic and microscopic techniques to characterize attributes and dynamics of cellular lipid microdomains.