Title: Compositional And Environmental Effects On Transport And Lipid Ordering In Membranes
Program: Doctor of Philosophy in Biomolecular Sciences
Advisor: Dr. Daniel Fologea, Physics
Committee Members: Dr. Julia Oxford, Biological Sciences and Dr. Denise Wingett, Biological Sciences
Lipid membranes exploit self-assembly to create barriers, which partition cells from extracellular environments to regulate the transport of ions and substances critical to cell survival. The permeability and fluidity of membranes also impact the function of associated proteins and our ability to use artificial lipid structures for potential therapeutics. As such, investigating factors modulating the biophysical and transportation properties of lipid bilayers has potential for far-reaching impact on our understanding of key cellular processes. This work focuses on filling gaps in knowledge on the effects of lipid bilayer composition, lipid ordering, and transport in both cellular and planar lipid membranes. Our electrophysiology experiments on planar bilayer lipid membranes under hydrostatic pressure indicate the occurrence of unassisted inorganic ion transport. The same experiments, carried out with varied cholesterol ratios in the membrane, confirm the importance of cholesterol in the membrane’s mechanical and transport properties. We show that a membrane’s permeability to inorganic ions increases with the applied hydrostatic pressure as a result of the induced curvature, while cholesterol plays a stabilizing role. Using a similar electrophysiological model system, we show that organic ion permeability varies with both temperature and cholesterol concentration of the bilayer. Temperature-induced phase transitions of lipids, associated with changes in lipid ordering, are mitigated by the addition of cholesterol, leading to a decreased permeability. While the temperatures needed to attain such transitions are often outside the physiological range for many cells, we asked whether changes in lipid ordering can also be observed in cellular membranes at constant, physiological temperature in response to stress effectors such as hypo-osmosis or protein insertion. For this assessment, we employed membrane sensitive probes embedded in red blood cell membranes and membrane-status indicator dyes. We show that hypo-osmotic pressure leads to significant lipid disordering, which is a consequence of diminished short-range interactions in tensioned membranes. We also observe a significant lipid disordering upon insertion of exogenous toxins in the membranes, which we ascribe as a consequence of the hydrophobic mismatch between the inserted protein and the hydrophobic core of the membrane. Both stressors indicate sigmoidal variations of the analyzed parameters (i.e., generalized polarization and anisotropy), suggesting the occurrence of isothermal phase transitions. This collection of work provides new methodologies and knowledge, supplementing the growing field of membrane biophysics and its applicability in medicine, biotechnology, and cellular physiology.