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The role of the lipid matrix for membrane transport

Most drugs and many biomolecules cross cellular membranes by partitioning into and diffusion through the lipid matrix. Kinetics and thermodynamics of hydrophobic or amphiphilic ligands or substrates binding to membrane proteins depend on their interaction with the lipid bilayer [1]. Biologically active molecules may act by changing the properties of the lipid matrix, including permeability, order, dynamics, and lateral organization that modulate membrane protein function or membrane barrier integrity. We are interested in a better fundamental understanding of the mechanistic and thermodynamic processes governing these phenomena.

Fig.: Schematic representation of lipid matrix effects on membrane transport. This includes passive diffusion, membrane permeation and permeabilization by local defects or toroidal pores, lateral transport of hydrophobic and amphiphilic ligands to their receptor in the membrane, and indirect and direct effects of the membrane uptake of molecules onto membrane proteins.

The specific research topics of our two collegiates deal primarily with membrane permeabilization and the improvement of membrane models for transport studies.

Antimicrobial peptides are part of the innate immune system of a vast variety of organisms; they are believed to act by permeabilizing the membranes of target organisms. However, in spite of the urgent need for new classes of antibiotics, they have hardly found their way to medical applications because they are expensive to produce, unstable in the body, and not selective enough for systemic application. Recently, synthetic polymers are being developed that mimic key properties of AMPs [2] but do not suffer from their drawbacks. Our work aims at a deep, quantitative understanding of the activity and selectivity of these molecules to attack membranes of a given lipid composition. This approach is supported by efforts to implement new experimental assays to study membrane permeation, perturbation and permeabilization based on cutting edge microcalorimetry (ITC, DSC, PPC), subnanosecond time-resolved fluorescence spectroscopy and other techniques. Particular attention is being paid to effects accompanying the clustering of anionic lipids by polycationic, membrane-active polymers.

The second topic aims at improving models for membrane studies. Reconstitution of membrane proteins into proteoliposomes is considered to provide a more relevant environment for the protein than micelles and other systems. However, these liposomes still lack a key property that is adjusted and maintained in biological membranes with great effort: their compositional asymmetry between outer and inner lipid leaflets. We have introduced several new protocols to produce and monitor a lipid asymmetry, particularly of an anionic lipid [3]. This includes liposomes with phosphatidylglycerol in the outer or phosphatidylserine in the inner leaflet only, mimicking the charge asymmetry of bacterial and mammalian membranes, respectively. We seek to further develop such techniques and answer the question whether and how transport proteins are affected by an asymmetry of lipid species (and charges).


[1] How membrane partitioning modulates receptor activation: Parallel versus serial effects of hydrophobic ligands. H. Heerklotz and S. Keller (2013) Biophys. J. 105:2607-2610
[2] Correlating antimicrobial activity and model membrane leakage induced by nylon-3 polymers and detergents. Sara G. Hovakeemian, Runhui Liu, Samuel H. Gellman, and Heiko Heerklotz (2015) Soft Matter 11:6840-51
[3] Engineering asymmetric lipid vesicles: precise and convenient control of the outer leaflet lipid composition. M Markones, C. Zorzin, M. Kaiser, H. Heerklotz and S. Fiedler (submitted)



Prof. Dr. Heiko Heerklotz

Institute for Pharmaceutical Sciences - Pharmaceutics division
Hermann-Herder-Str. 9
79104 Freiburg

Phone: +49 (761) 203 6336
Fax: +49 (761) 203 6357