CAREER: Lipid Transport in Membranes

University Of Illinois At Chicago, Chicago IL

Investigators

Abstract

Non Technical Abstract: In animal cells, a specialized layer of compounds combine to form what is called a lipid membrane or lipid bilayer that not only separates the interior of the cell from the outside environment but also surrounds inner compartments of the cell such as the cell's nucleus. Communication through these lipid membranes was thought to be primarily due to protein molecules with the lipid barrier playing a passive role. However, recent discoveries show that cells tightly control both the protein and lipid composition of its membranes to perform a broad range of functions. As a result this creates very unique and uneven distributions of lipids and proteins across different membranes. These uneven distributions or asymmetries can only occur if the cell is putting in additional energy because normal temperature driven effects will tend to mix lipids uniformly. Even more interesting is that if the cell fails to maintain this asymmetry, the result can lead to disease or even cell death. In this proposal the researchers want to answer a key question: What is the energetic cost of maintaining lipid asymmetry? This question has proven difficult to answer since multiple measurements have given varied results. Preliminary findings using a unique advanced technique called time resolved small angle neutron scattering (TR-SANS) was that cholesterol and other lipids move very slowly across a single membrane, taking tens to hundreds of minutes to move, or "flip-flop", across the lipid membrane. Our result on cholesterol is particularly interesting, and controversial, since the consensus is that the cholesterol moves rapidly through a single membrane - taking less than a second. Interestingly, it was recently shown that cholesterol itself is very unevenly distributed across the membrane and, if cholesterol moves fast across the membrane, this would mean that proteins are working very hard and using more energy to keep this necessary difference across the membrane. This project will investigate the movement of lipids and cholesterol across the membrane?s lipid bilayer. The results of this study will provide direct information of the factors influencing lipid and cholesterol distribution across biological membranes and the energetic cost of maintaining this distribution. Understanding of membrane regulation and dynamics will therefore be greatly enhanced. The long-term goal of the project is to address the grand challenge of finding key principles that govern the movement of lipids between and within membranes. Ultimately this project seeks to connect how membrane structure is maintained, how the membrane lipids actively engage during signaling processes, and how this affects their dynamics. The findings will be communicated to the scientific community through publications and conference presentations, and to the public through general science lecture events in local science museums and libraries. The principal investigator will train undergraduate and graduate students in STEM fields. In addition, summer outreach research projects engaging undergraduate students from colleges based in the Chicago area and whose backgrounds are under-represented in the physical sciences will be pursued. Technical Abstract: This CAREER award by the Biomaterials program in the Division of Materials Research to The University of Illinois at Chicago is to investigate the transport of lipids in membranes. The great variety of lipid molecules in the cell membrane suggests they play a complex role in cell function. Distinct lipid composition amongst different membranes within the cell suggests directed functionality. Within the plasma membrane itself, there is an asymmetric distribution of lipids between the outer or exocellular facing leaflet and the inner or cytoplasmic facing leaflet, and the physiological fate of cells depends on the strict maintenance of this asymmetry. This uneven distribution is mediated by concrete lipid transport mechanisms whose function maintains the identities of the different cellular membranes. Identifying and understanding these mechanisms is currently a subject of intense study. Reliable values of passive lipid translocation rates are a necessary starting point for a detailed mechanistic understanding of the lipid distribution landscape in cellular membranes, and the purpose of this study is to provide them. Using an in-situ non invasive technique: time-resolved small angle scattering, as well as nuclear magnetic resonance as a complementary technique, the researchers will directly obtain the flip-flop rates of lipids and cholesterol in membranes and study changes to the rates in different biological contexts. Time resolved small angle neutron scattering (TR-SANS) is a technique with nano spatial resolution capable of measuring the time dependent population of lipids, distinguished by deuteration, in vesicles. Deuteration has the advantages of not altering the chemical identity of the lipids under study, which have been found to have a profound effect on how they move across membranes. Further, deuteration of lipids - particularly of cholesterol - will allow probing membranes as complex as those found naturally in cells. The findings will be communicated to the scientific community through publications and conference presentations, and to the public through general science lecture events in local science museums and libraries. The principal investigator will train undergraduate and graduate students in STEM fields. In addition, summer outreach research projects engaging undergraduate students from colleges based in the Chicago area and whose backgrounds are under-represented in the physical sciences will be pursued. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.