Dr Ralf Richter

A fluorescence micrograph of a perineuronal net as an example of glycan-rich cell coats and extracellular matrices, and an illustration of the nuclear pore permeability barrier. Illustration shows hydrogels, multivalent binding and polymer brushes; the physical principles governing the behaviour of such systems are used to better understand the mechanisms of functions of soft biological interfaces.

Introduction

Our mission is to understand the mechanisms of assembly and function of soft biological interfaces, to advance fundamental knowledge and for applications in the life sciences. We focus on extracellular matrices that are rich in glycans: these microscopic hydrogel-like assemblies are important regulators of cell function and inter-cellular communication. Another main object of our research is the nuclear pore permeability barrier: this fuzzy meshwork of natively unfolded proteins controls macromolecular exchange between the cytosol and nucleus of cells and is crucial for orderly gene expression.

To understand how biological functions emerge from the assembly and dynamic reorganization of biomolecules, we adopt a multidisciplinary approach that combines living cells and tissues with well-controlled models of tuneable complexity. Exploiting surface science and chemistry tools, we tailor-make model systems by the directed self-assembly of purified components on solid supports. We develop new tools based on quartz crystal microbalance (QCM-D), atomic force microscopy, spectroscopic ellipsometry and optical microscopy for the quantitative analysis of these systems. We use concepts from soft matter physics to rationalize the properties of soft biological matter, and collaborate closely with biochemists and biologists to integrate our bottom-up biosynthetic approach with work at the levels of molecules, cells and living organisms.

Current major projects

  • Superselective targeting of cells and tissues
  • Glycocalyces: How do glycan-rich cell coats assemble, and how do they control cell adhesion, migration and inter-cellular communication?
  • Nucleo-cytoplasmic transport: How do fuzzy protein assemblies at the nanoscale control macromolecular transport?

Detailed research programme

Superselective targeting of cells and tissues

Illustration shows that a multivalent probe (made from a linear polymer scaffold with ligands attached) selectively binds to cell surfaces presenting a cognate receptor at high (but not at low) surface density. Graph shows the sharp (stronger than linear) dependence of probe binding on receptor surface density, termed superselectivity.

Targeting of cells and tissues, a basic requirement in biomedicine, relies on specific binding of a ‘ligand’ on a tailor-made probe to a ’receptor’ on the desired cell/tissue. Conventional probes efficiently distinguish a cell/tissue displaying a given receptor from others that do not, but perform poorly when the entities to be distinguished all display the receptor, albeit at different densities. Combining chemistry and soft matter physics tools, we have demonstrated how multivalent probes can be designed to sharply discriminate between different receptor densities. We are now developing such ‘superselective’ probes for diagnostic purposes, such as improved tumour cell targeting. We also aim to understand how our cells harness superselective targeting to communicate with each other, and how pathogens hijack such interactions to invade host cells.

Revealing how signalling molecules navigate through extracellular space

Illustration shows how signalling proteins (chemokines, morphogens, growth factors) are secreted by one cell, and need to travel through extracellular matrix to reach their target receptors on a distant cell. The extracellular matrix controls retention and transport of signalling molecules.

Directed migration of immune cells is a vital part of our immune and inflammatory responses. Chemokines (a family of soluble signalling proteins) guide cells, and glycosaminoglycans (GAGs; a family of linear polysaccharides) help control the distribution and presentation of chemokines in the extracellular space thus playing key roles in controlling cell behaviour. Probing the molecular mechanisms that drive these phenomena in vivo is challenging. We develop biomimetic surfaces to study GAG-chemokine interactions on the molecular and supramolecular levels, and to probe cellular responses to defined biochemical and biophysical cues to better understand GAG-mediated cell-cell and cell-extracellular matrix communication. This work is also relevant for tissue development, remodelling and repair where GAGs and other signalling proteins play similar cell guidance roles.

Hyaluronan-rich extracellular matrices

Micrograph on the left show perineuronal nets, net-like extracellular matrix structures rich in hyaluronan that surround neurons and control neuronal plasticity. Micrograph on the right shows that similar net-like morphologies can be reconstituted in vitro with surface-grafted hyaluronan and a hyaluronan cross-linking protein.

Many cells surround themselves with a hydrogel-like matrix that is rich in the polysaccharide hyaluronan. Such matrices are important in physiology and disease, and have unique properties. For example, we have found the hyaluronan matrix surrounding mammalian oocytes during ovulation and fertilization to be the softest elastic biological material reported to date. We reconstitute hyaluronan matrices in vitro to study the biochemical and physical mechanisms underpinning their self-organisation, and how their unique properties guide cell behaviour. This provides important new insight as to how the complexity of glycan-protein interactions contributes to physiological processes as diverse as inflammation and ovulation, immune cell recruitment, neuronal plasticity and cartilage function.

The permeability barrier of nuclear pore complexes

Illustrations shows a nuclear pore complex as a large (40 nm wide) channel filled by a meshwork of linear polymers (intrinsically disordered proteins called FG nups) that control the transport of macromolecules (mediated by nuclear transport receptors) between the cytosol and the nucleus of eukaryotic cells. An in vitro model of the nuclear pore permeability barrier, consisting of intrinsically disordered proteins grafted to a planar surface, is also shown.

Transport of proteins and nucleic acids between the cytosol and the nucleus of living cells is essential for the ordered course of gene expression. This transport occurs through nuclear pore complexes that perforate the nuclear envelope and is selective. A fuzzy meshwork of natively unfolded proteins (FG nups) fills the pore and effectively controls which macromolecules can pass and which cannot. We aim to understand the physical rules underpinning the function of this permeability barrier at the nanoscale. To this end, we combine biophysical experiments with reconstituted FG nup assemblies and polymer physics theory. A better mechanistic understanding of nuclear transport will also provide guidelines for the rational design of novel artificial separation devices for biotechnological applications.