Scientific Mission: We aim to understand the mechanisms of assembly and function of biological hydrogels and cellular membranes. To directly assess the supramolecular level of interactions, we tailor make and study well-defined model systems.
The self-organization of molecules into dynamic and hierarchical supramolecular assemblies is a key feature of biological structures. The resulting architectures exhibit new qualities that are distinct from those that characterize its individual components. Our group is particularly interested in two types of assemblies: hydrogel-like structures that are made of flexible biopolymers and cellular membranes.
For a thorough investigation of the physical principles underlying the organization, dynamics and function of these supramolecular architectures, it is desirable to move from living cells to well-controlled models with tunable complexity. We create such model systems on solid supports. Modern techniques of surface biofunctionalization and patterning are employed to guide the assembly down to the nanometer-scale. For the characterization of the model systems, we develop and use a toolbox of biophysical in situ characterization techniques, including quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM), reflection interference contrast microscopy (RICM), spectroscopic ellipsometry (SE) and fluorescence methods.
Under inflammatory conditions and ovulation, hyaluronan-rich coats undergo significant remodeling. The inflammation-associated protein TSG-6 was hypothesized to be implicated in the coat remodeling by cross-linking HA chains. With the aid of our model systems, we could provide evidence that TSG-6 can indeed act as an effective HA cross-linker, and shed insight into the cross-linking mechanism. The cross-linking units are TSG-6 oligomers which form upon binding of TSG-6 to HA (Baranova et al. 2011). TSG-6 collapses and rigidifies hyaluronan films. Cross-linking might hence influence the mechanical environment of cells and the local organization of the pericellular coat (glycocalyx) might also serve as a signal for leukocyte recruitment to sites of inflammation.
CD44 is a major cell surface receptor for hyaluronan (HA). It is found on many cell types, and of particular importance in inflammation-like processes and tumor metastasis. The interaction of hyaluronan with the cell membrane is thought to be stabilized by multivalent interactions between polymeric HA and several cell surface receptors. The molecular (and supra molecular) mechanisms behind the regulation of binding, and in particular the different biological functions that are elicited by HA of different molecular weight, are currently not well understood.
We have designed tunable in-vitro model systems that mimic the cell surface in the sense that the HA binding domain of CD44 is immobilized in its native orientation and at controlled density to a supported lipid bilayer (SLB). Employing techniques such as QCM-D, SE and RICM and concepts from polymer theory, we analyze qualitatively and quantitatively, how the multivalent presentation of CD44 on the membrane surface regulates the binding and self-organization of hyaluronan and its complexes (Wolny et al. 2010).
Quartz crystal microbalance with dissipation monitoring (QCM-D) has become a popular tool to investigate biomolecular interaction phenomena at surfaces (Reviakine et al. 2011). In contrast to optical mass-sensitive techniques, which commonly detect the adsorbed nonhydrated mass, the mechanically coupled mass measured by QCM-D includes a significant amount of water. A mechanistic and quantitative picture of how the surrounding liquid couples to the deposited molecules has long been elusive for apparently simple phenomena like the random adsorption of proteins or other nanometre-sized particles on a planar surface.
We employ in situ combinations of QCM-D with other sensing techniques (in particular ellipsometry) and theoretical modelling to elucidate this question (Bingen et al. 2008). The insights are used for the development of novel sensing applications. With this methodology, it is for example possible to detect the clustering of proteins on supported lipid bilayers without any labels.