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Group Heiko Heerklotz

Modes of action of membrane-active compounds

Biological membranes control integrity of cells along with export, import, signaling, division, and many more processes. Hence, they are the target of a wide variety of biomolecules, drugs and excipients, as well as detergents and other membrane interacting compounds used in laboratory, manufacturing, and agriculture. Our aim is a detailed, mechanistic, and quantitative understanding of the intermolecular interactions and mechanisms the govern membrane insertion, leakage, demixing, or shape changes. Current studies involve, for example, lipopeptides produced by Bacillus and Pseudomonas strains and are pursued in collaboration with EOS Rhizosphere and Bayer. Key methods of this project area are time correlated single photon counting (TCSPC) methods to characterize membrane binding, leakage, order, dynamics, and the positioning of molecules within the membrane. Isothermal titration calorimetry provides a comprehensive picture of binding processes and subsequent changes in membrane stability and structure. 

Membrane asymmetry: Models, functions and applications

Virtually all biological membranes show an asymmetric distribution of their lipid species between the outer and inner lipid leaflet. The considerable effort to establish, maintain and regulate this asymmetry implies important functions of this asymmetry. However, owing to the fact that the vast majority of membrane model studies so far has been carried out with symmetric models (including liposomes), very little insight has been obtained into these functions. To overcome this problem, we have been establishing new protocols and procedures to prepare asymmetric liposomes based on enzymatic and lipid-exchange strategies. A key analysis to monitor asymmetry of ionic lipids is the zeta potential measurement, since zeta is unaffected by the molecules in the inner leaflet. We are interested in the impact of asymmetry on membrane protein function and possible bioactive agents acting by lipid scrambling. Primary collaborator in this field is Prof. Hunte, Uni Freiburg. Asymmetric insertion or extraction of lipids or other molecules into or from one lipid leaflet creates a pressure asymmetry that promotes membrane bending and, ultimately, budding. This process is explored primarily by Asymmetric field flow field fractionation (AF4). 

Stabilization of biologics in solution

Active pharmaceutical ingredients of many modern drugs are antibodies and other large proteins. They offer amazing activity and selectivity but are enormously challenging to be formulated into a stable drug product. Related to our interest in the thermodynamic parameters governing protein folding in general, we seek to understand and optimize the action of protein stabilizing excipients. This project is a collaboration with Boehringer Ingelheim. Important methods are DSC/PPC to desribe unfolding, ITC to quantify binding, and DLS to monitor colloidal stability. 

Lipid-based drug delivery systems

Lecithin and other lipids are used in a wide variety of drug delivery systems, including vesicluar lipid gels (VLGs), mixed micelles, emulsions, and liposomes. Our aim is a fundamental understanding and rational optimization of such systems that often have been developed on a largely empirical basis. The main project pursued at this time is the characterization of thermoresponsive or thermosensitive liposomes (TSLs) that are loaded with drugs (typically cytostatics) and triggered to induce a sudden drug release in a body region of mild hyperthermia (the tumor). Key questions are the detailed, molecular role of lysolipids in such TSL formulations and their interactions with plasma proteins. This work is supported by the Phospholipid Research Center (Heidelberg) and Lipoid GmbH and collaborations exist with the lab of Prof. Christine Allen (University of Toronto), Prof. Lindner (LMU Munich) and ThermoSome GmbH (Martinsried). Another topic in this field has been the sustained release of exenatide, a peptide used for the treatment of diabetes, from VPGs. This work is in collaboration with Prof. Winter, LMU Munich.

 

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