The influence of different membrane components on the electrical stability of bilayer lipid membranes

A cell membrane forms an impermeable barrier to foreign entities, among which genes, drugs, particles, and certain dyes. For many applications, such as DNA transfection or drug research, it is necessary to transport these substances into a cell, and this requires the transient permeabilization of the cell membrane. A commonly used technique for this purpose is electroporation. Thereby, pores are temporarily created in the membrane upon application of a short but high external electric field. These last few decades, the popularity of the technique of electroporation has been increasing, notably for cell transfection. However, the whole process is difficult to control at the level of a cell population as the outcome of the electroporation process depends on a number of uncontrollable parameters, such as the cell size, shape and “vulnerability”.

In this article, we get a better insight into the mechanism of electropore formation. For that purpose, we demonstrate the influence of the membrane composition and of its individual constituents on its (electrical) stability. Our approach consists of using artificial membrane models, bilayer lipid membranes (BLMs) and assessing the membrane stability by measuring the electroporation threshold (V_th). This threshold is defined as the potential at which pores are observed in the membrane. The basic “building block” we use to prepare the membranes is a synthetic phospholipid, 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC). This phospholipid is composed of two identical saturated hydrocarbon chains functionalized with methyl groups and a relatively large head group. It yields densely packed and very stable membranes (V_th ~200 mV). In a first stage, we show to which extent the electrical stability is affected by the properties of the individual phospholipids present in the membrane. For that purpose, BLMs are prepared using a mixture of two phospholipids; DPhPC mixed with other phospholipids found in natural cell membranes that are unsaturated and have a non-zero intrinsic monolayer curvature. Following this, we demonstrate how cholesterol affects pore formation in DPhPC membranes, and finally the contribution of one type of channel proteins, α-hemolysin, on the same process.

While the phospholipid composition has a slight effect (100 mV ≤ V_th ≤ 290 mV), cholesterol gives a concentration-dependent effect: a slight stabilization until 5% weight (V_th ~250 mV) followed by a noticeable destabilization (V_th ~100 mV at 20%). Interestingly, the presence of a model protein, α-hemolysin, dramatically disfavours membrane poration and V_th shows a 4-fold increase (~800 mV) from a protein density in the membrane of 24 × 10^-3 proteins/μm². In general, we establish that pore formation is affected by the molecular organization (packing and ordering) in the membrane and by its thickness. We correlate the resulting changes in molecular interactions to theories on pore formation.

A detailed description can be found in the full paper published in BBA - Biomembranes: http://dx.doi.org/10.1016/j.bbamem.2009.10.003

For more information please contact Iris van Uitert: i.vanuitert@ewi.utwente.nl