Need for more focus on lipid species in studies of biological and model membranes

The lipid composition of cellular membranes can impact a number of physiological processes such as signaling, cell migration, endocytosis and intracellular transport. In this article we focus on some aspects concerning analysis of lipids and research on lipid structure and function in mammalian cells that in our opinion have not obtained sufficient attention. This includes interleaflet coupling between the two layers of the membrane, and the role of lipid species, i.e. the role of the complete structure of the lipids, including lipid chain length and the position of double bonds. We highlight the role of PS species for membrane function. We also discuss the large diversity of PS species in different biological samples and the possible functional consequences, and we provide an overview of PS species from 40 different samples. Furthermore, recent studies show that there seems to be a coregulation concerning the levels of sphingolipids and ether lipids. We review and discuss the published data indicating such a coregulation. Moreover, we point to some of the pitfalls in the field of lipidomics and present suggestions for improvement. Finally, we discuss the importance of using asymmetric membrane models with a composition of lipid species that are common in biological membranes.