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How do cells work? Glycoconjugate cell coat models provide new answers

At the Max Planck Institute for Metal Research in Stuttgart, researchers are developing model systems that imitate the sugar coats of living cells. They hope to gain new insights into the regulation of biological functions and develop the system into a platform for biosensoric applications.

Many living cells are coated with a gel-like coat up to several micrometres thick. Is this layer really only filling material? Scientists have thought so for a long time. Hyaluronan, a linear, non-sulfated glycosaminoglycan up to several micrometres long, is strongly charged and attracts water molecules.
“Already at low concentrations hyaluronan is able to form a viscous and pressure-resistant matrix that fills hollow areas for example such as those in the eye,” said Dr. Ralf Richter of the Max Planck Institute for Metal Research in Stuttgart.

Indeed, it was previously assumed that the main function of hyaluronan in the organism was to structurally maintain water-filled space. The medics are using this effect by using hyaluronan together with water in order to create a hollow space inside the body, in which they find it easier to operate. The ‘removal’ of skin wrinkles is also based on the bolstering effect of hyaluronan.

Sugar structures are more than support

Dr. Ralf Richter is the head of the research group at the Stuttgart-based Max Planck Institute for Metal Research that is developing model systems of glycoconjugate cell coats in a project funded by the BMBF © private
However, it is now known that this molecule can do a lot more. It forms part of the cell coat where it plays an important role in cellular metabolism. “Approximately 20 years ago, a paradigm shift took place and it gradually became clear that the sugar structures have important biological functions, for example in cellular communication processes,” said Richter who is currently setting up his own research group in Spain that will focus on the investigation of lipid membranes and sugar-containing cell envelopes. In Germany, Richter won the Glycobiotechnology Workgroup Contest of the BMBF. Located in San Sebastian in Spain, Richter is also leading a team at the MPI for Metal Research in Stuttgart that is developing model systems of glycoconjugate cell coats. The project officially got underway on the 1st February 2008 and will be financed by the BMBF for three years.

It is assumed that the model system will facilitate research into the structure and function of the hyaluronan-containing cell coat since it is difficult to investigate these sugars in their natural cell environment. Even the best imaging methods are unable to resolve the structures due to low contrast caused by the high water content. In addition, dryness makes the coats collapse easily. The researchers plan to develop a controllable system that enables them to accurately investigate what happens when individual parameters are changed.

Clarification of structure and function is the main objective

Richter is well aware of the huge challenges they face: “The hyaluronans form a kind of polymer network, the composition and function of which is still hardly known. It is also highly complex; up to thousand proteins can bind to a single hyaluronan molecule. We are planning to imitate parts of the biological system and hope that this will provide us with fundamental insights into the functional mechanism.”

The pericellular coat around an adherent cartilage cell (chondrocyte), visualized by an exclusion assay. A halo of several micrometers in thickness around the cell cannot be penetrated by the dense population of surrounding probe objects (red blood cells)
The pericellular coat around an adherent cartilage cell (chondrocyte), visualized by an exclusion assay. A halo of several micrometers in thickness around the cell cannot be penetrated by the dense population of surrounding probe objects (red blood cells). The coat is invisible in common light microscopy, since its extreme hydration renders the optical contrast very low. The grafting of hyaluronan to the cell membrane and its interaction with hyaluronan-binding molecules can give rise to different supramolecular structures (indicated schematically). © J. E. Curtis (The School of Physics, Georgia Institute of Technology) and H. Boehm (Biophysical Chemistry, University of Heidelberg and New Materials and Biosystems, Max Planck Institute in Stuttgart)
The researchers intend to imitate the spatial restriction on the cell surface by binding the hyaluronans to carrier materials, for example cover glasses or glass or silicon microscope slides, which can also be covered with gold. “The type of carrier used depends on the subsequent analysis,” said Richter explaining that a standard method is the coating of the carriers with lipid membranes to which the sugar molecules can be attached. The researchers plan to use hyaluronan molecules of different sizes because “there is evidence that short-chain hyaluronans behave completely differently from long-chain ones. We will then add the proteins that are known to interact with hyaluronans and investigate how networks develop,” said Richter explaining the first steps of the experiment.

Modelling interactions

The molecular arrangement of the hyaluronans and their binding partners is important for functions such as the regulation and control of cellular properties. Richter and his team are hoping that the model system will enable them to specifically investigate the interactions and eventually also control them. “We have plans to use the model as cell sensor in order to examine different processes such as adhesion and differentiation in cell cultures,” said Richter.

How to construct a hyaluronan coat? The solid support (silica) was first functionalized with a lipid membrane and a protein linker, by exposure to a solution of lipid vesicles and proteins, respectively. Hyaluronan molecules were then immobilized with one of their ends.The surface functionalization and the immobilization of hyaluronan can be followed in real time by QCM-D. Pronounced changes in frequency, accompanied by small changes in dissipation, are characteristic for the lipid membrane and the protein layer. The strong increase in dissipation upon hyaluronan binding reflects the highly hydrated and viscoelastic state of the hyaluronan-brush that reaches a thickness of up to 500 nm. Important properties of the hyaluronan film, such as its permeability, can be probed directly. The film shown here is permeable to lipid vesicles of around 30 nm in size © Richter / Max Planck Institute for Metal Research
For Richter, the hyaluronan model is a kind of a start-up system. “There are many other molecules that can be investigated in a similar way,” said Richter who already has plans to investigate gel-like and strongly hydratised glycan layers. “For example, we are interested in mucous layers that are important for the substance transport in the intestine, where – just like in other parts of the body - they are important for the immune defense,” said the scientist.

Platform will be used for other systems such as mucous membranes

Since glycobiology in general, and even more so, the investigation of the cellular glycan coats is still a young field of research, Richter’s team faces many open questions but is in the comfortable position that the competition is not that stiff. “A lot of work is being done on the molecular structure of the hyaluronans and some groups are investigating specific phenomena on the cell and tissue level, but hardly anybody is combining the complex cell-biological level and the molecular approach,” said Richter who is combining these two approaches.

Richter does not have to plough a lonely furrow, but has an excellent international network. He enjoys working with other groups who have complementary skills: “For example, we receive hyaluronan-binding molecules and valuable feedback from teams of biologists in Oxford and Manchester. Jennifer Curtis’ group in Atlanta supports us in the work on the comparison between model and cell systems.”

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/how-do-cells-work-glycoconjugate-cell-coat-models-provide-new-answers