Investigating the asymmetry of the plasma membrane: model membranes mimicking healthy and tumor cells, and anti-tumor therapeutic strategies

Abstract

The plasma membrane is a very complex system that provides cellular protection and maintenance. This membrane is composed of a lipid bilayer, with leaflets of asymmetric lipid composition, where proteins are sorted in a lipid matrix of hundreds of lipids. The exoplasmic leaflet has a lipid composition that includes high melting (HTm) lipids, which enable coexistence of liquid phases ("lipid-rafts"), namely Ld (liquid disordered) and Lo (liquid ordered) phases. In contrast, the cytoplasmic leaflet lacks HTm lipids and thereby this cytoplasmic leaflet would form a single fluid phase by itself. The architecture of the membrane is important to cell signaling and function, and both leaflets must be somehow coupled together to transmit information across the membrane. The loss of asymmetry, including exposure of phosphatidylserine (PS) from the inner to the outer leaflet, could be a signal to apoptosis, a programmed death of the cell. In cancer cells, PS is exposed on the exoplasmic leaflet but not correctly eliminated because of a dysfunction in clearing away these cells. Antimicrobial peptides (AMPs) act in disrupting membranes by targeting the exposed PS. We plan to investigate the asymmetry of the plasma membrane using in vitro models of healthy cells and tumor cells. To build asymmetric membranes, we will use a new method developed by Enoki and Feigenson (2019), in which we control the hemifusion of giant unilamellar vesicles with a supported lipid bilayer, allowing the outer leaflets of these bilayers to exchange lipids by diffusion. We propose to characterize a realistic bilayer model of the exoplasmic and cytoplasmic leaflets, which includes a phase-separated leaflet creating an induced order domain in a leaflet that lacks HTm lipids. We observed that these induced order domains are enriched in chol, and we propose to further investigate the lipid distribution in the cytoplasmic model leaflet. We plan to investigate the fundamental interactions involved in the partial asymmetry loss of PS lipids in unhealthy cells. We will investigate how peptides with antitumor activities can perturb the physical-chemical and mechanical properties of asymmetric bilayers. We will correlate these perturbations with the activity of the peptide destroying the model membrane of a tumor cell. We seek to understand which perturbations can be more aggressive in disrupting the membrane. Asymmetric membranes provide a realistic model of the plasma membrane. We propose to find a different mechanism of interaction between AMPs and these mimetic membranes, which can yield new strategies to increase the antitumor activity of these peptides. Our study can provide access to information that is otherwise limited in the studies of symmetric membranes. (AU)