Domains in Biologically Relevant Lipid Membranes

细胞膜是脂质和蛋白质的复杂集合，脂质自然倾向于形成双层。膜脂具有很大的结构可变性，最近的证据表明，许多生物膜含有结构域，可通过组成和物理特性加以区分。这些结构域通常富含鞘脂，它们往往具有相对较高的熔融温度，以及固醇，如胆固醇，但由于它们的小尺寸和动态性，在生命系统中极难表征。基于对“模型”膜的研究，即具有确定成分的膜，通常不含蛋白质，假设这些结构域具有不同的物理状态（“液体有序相”），其中脂质具有比周围膜更构象有序的液晶结构。膜内长期相共存强烈影响膜特性：局部膜厚度和曲率、膜内脂质和蛋白质的扩散、脂质和蛋白质之间的相互作用以及膜蛋白的功能/灵活性都可能在含有这些结构域的细胞膜中受到影响。

Cell membranes are complex assemblies of lipids and proteins held together by lipids' natural tendency to form bilayers. Membrane lipids have large structural variability and recent evidence suggests that many biological membranes contain domains, distinguishable by both composition and physical characteristics. These domains are often enriched in sphingolipids, which tend in isolation to have relatively high melting temperatures, and sterols such as cholesterol, but are extremely difficult to characterize in living systems due to their small size and dynamic nature. Based on work with ‘model’ membranes, i.e. membranes with defined composition, often protein-free, such domains are postulated to have a different physical state ("the liquid ordered phase"), where the lipids have a more conformationally ordered liquid crystalline structure than the surrounding membrane. Long-lived phase coexistence within a membrane strongly influences membrane character: local membrane thickness and curvature, diffusion of lipids and proteins within the membrane, interactions between lipids and proteins, and membrane protein function/flexibility could all be affected in cell membranes containing such domains. We will map out the fundamental interactions between phospholipids and sterols in model membranes designed to mimic the outer surface of cells. Many of these interactions are poorly understood even though they form the backdrop for the interplay among other cell membrane components. Deuterium nuclear magnetic resonance (NMR) is a powerful experimental approach to the study of phase or domain coexistence in membranes; different phases result in clearly distinguishable spectra, and the quantity of labeled lipid in a particular phase can be measured unambiguously. Importantly, deuterium NMR does not rely on large unnatural probes to monitor membrane domains, as even small amounts of such probes can modify domain properties. A new direction for us is monitoring the tendency of lipids used to manufacture specialized “lipid nanoparticles” (LNPs) to form non-bilayer phases. LNPs are used in gene therapy: they are taken up by the liver and then release short strands of RNA inside the cell. The release mechanism is thought to involve a transition from a typical bilayer to a different membrane morphology, consisting of tiny water-filled lipid cylinders, that has a distinct NMR signature. In collaboration with experts in the computer simulation of membrane structure, we will better understand the lipid qualities that favour the phase transition and in turn more accurately predict the mechanism of drug release. Knowledge gained from the proposed research will aid cell biologists investigating phenomena related to membrane domains, e.g. programmed cell death, protein localization and signaling, neuronal maturation and bacterial or viral infection. Cell physiologists will be able to use our work to better understand fast, non-genomic responses of tissues to active molecules, such as steroid hormones, in circulation. Finally, the membrane simulation community relies on reliable data about the structure and dynamics of lipids. Thus, our work will benefit a broad segment of the cell biological and biophysical research communities. In addition it will provide Canada with highly trained scientists whose knowledge will provide leadership to future industrial, medical and academic research.