Designing biologically-relevant cell membrane models with natural lipid mixtures

Cell membranes provide vital biological functions by separating the cell components from the surrounding environment and providing a bioactive interface for several biological processes. The direct extraction of intact cell membranes and their investigation with biophysical methods is heavily limited by their compositional complexity and intrinsic fragility. Therefore, over the years, membrane models including vesicles, lipid monolayers, supported lipid bilayers and lipid multilayers, were suggested as alternatives to study the physico-chemical properties of cell membranes. These membrane models are typically produced with 1-3 synthetic lipid species and their application is therefore restricted by their composition, being too simple as compared to native cell membranes. In this review, we discuss the latest efforts towards producing more biologically relevant membrane models by utilizing natural lipid extracts. These are produced by extracting and purifying lipids expressed by different types of microbial cells. In part I, we present a detailed discussion of the methods currently available for obtaining the extracts, and in part II, we discuss how to use them for preparing cell membrane models and characterizing their structure. The majority of the discussed studies refer to Escherichia coli and Pichia pastoris glycerophospholipid extracts. For these extracts, optimized extraction and purification protocols were recently reported, which enable the efficient production of both hydrogenous and deuterated natural lipid mixtures. Deuterated lipids are of particular relevance for membrane characterization with techniques such as NMR, vibrational spectroscopies, and neutron scattering. In part III, we provide some future perspectives on the application of the currently available natural lipid extracts as well as on the development of protocols for the production of extracts from other cell types, e.g. mammalian cells and for isolating individual lipid molecules from such extracts. We also discuss methods to design genetically engineer microbial strains for enhancing the biosynthesis of target lipid molecules.