For a long time reactive oxygen species (ROS) were primarily known for their toxic effects. However, the last decade has seen a change in perspective: It has become clear that in many physiological situations hydrogen peroxide (H2O2) is produced in a controlled manner and acts as a messenger in signal transduction. Endogenous changes of oxidant levels are now recognized to fulfill adaptive roles in that they contribute to signal processing and cell fate decisions, such as proliferation and differentiation.

A central theme has emerged over the last years, which is a common denominator in organisms as diverse as fungi and humans: oxidants are typically perceived by protein thiol groups which act as reversible molecular switches. H2O2 leads to transient posttranslational thiol modifications that can either be inhibitory or activating. For example, oxidants generated during growth factor signaling lead to the selective and transient inactivation of particular protein tyrosine phosphatases, thus promoting phosphorylation cascades.

The duality of oxidant action (adaptive vs. damaging) likely represents a challenge for biological systems, to allow for oxidant sensing and signaling on the one hand but to restrict uncontrolled oxidation on the other. Perhaps unsurprisingly, prolonged pro-oxidative changes and altered redox homeostasis are frequently found at the boundary between physiological and pathophysiological processes.

Our research interests can be broadly subdivided into five interconnected thematic areas, as briefly summarized below.

The role of H2O2 sensing and signaling in inflammation

H2O2 is released following tissue injury and may act as one of the early signals for the recruitment and activation of immune cells. The elevation of H2O2 in the cytosol of immune cells appears to be a key signal that links a broad variety of biotic and abiotic stresses to the triggering of the inflammatory response. Recently, we have identified oxidant-sensitive proteins involved in inflammatory signaling and immune regulation. Our goal is to understand the specific links between inflammatory activators, H2O2 and immune effector mechanisms. In particular, we are interested in the balancing of migration vs. adhesion in lymphoid cells and in the regulation of inflammatory pathways, TLR signaling and inflammasome activation, in myeloid cells. This work is part of the collaborative research center (SFB) "Milieu-dependent regulation of immunological reactivity".

Redox homeostasis in tumor cells

Many, if not most, tumor cells differ in redox homeostasis as compared to their normal counterparts. For example, the endogenous production of H2O2 is typically increased, but so are the stress defense mechanisms that protect against excessive oxidants. It may be possible to exploit such homeostatic differences to interfere with tumor growth in a selective manner. One of the redox-sensitive enzymes that is frequently overexpressed in tumor cells is glutathione S-transferase pi (GST-pi). We are investigating signaling pathways by which GST-pi contributes to increased stress resistance in tumor cells. Along similar lines, some natural products from plants may be of value in cancer prevention because they modulate redox-regulated signaling pathways. In collaboration with C. Gerhäuser (DKFZ) and the EC-funded research training network RedCat we are addressing the question of how the chemoprophylactic flavonoid Xanthohumol influences redox homeostasis in human cells.

Molecular mechanisms of oxidative signaling

How are H2O2-sensitive signaling proteins oxidized in an efficient and specific manner? It is increasingly recognized that protein thiol oxidation in signaling is unlikely to be a spontaneous process, but rather depends on dedicated catalysts and specific protein-protein interactions. Growing evidence suggests that certain thiol-based peroxidases, and maybe other redox enzymes, can act as primary oxidant receptors and then pass on the oxidizing equivalents to target protein thiols through specific protein-protein interactions. To shed light on such fundamental principles of redox signaling, we are following two complementary approaches: Firstly, starting with proteins well known to be subject to oxidative modification in vivo, we ask how they are oxidized. Secondly, starting with particular peroxidase enzymes, we ask if and how they oxidize client proteins in a physiologically relevant setting.

Monitoring redox changes in cells, tissues and organisms

One of the major limitations in the field of redox biology is the lack of techniques to measure clearly defined redox processes in intact cells and within the physiological context of the living organism. Based on the idea that oxidants can be relayed efficiently and specifically from a primary oxidant receptor to a target protein, we recently developed a novel approach in redox imaging, namely the coupling of redox-sensitive GFP (roGFP) to oxidant-transmitting redox enzymes (reviewed by Meyer and Dick, 2010). On the one hand, we introduced Grx1-roGFP2, a glutathione-specific redox biosensor in which the oxidoreductase Grx transmits oxidative equivalents from GSSG to GFP (Gutscher et al., 2008). On the other hand, we created roGFP2-Orp1, a probe for H2O2 in which a peroxidase transmits oxidative equivalents from H2O2 to GFP (Gutscher et al., 2009). A major goal is to enable the study of redox processes as they occur in the physiological context of the whole organism. Ongoing projects involve various collaborators and address the feasibility of creating and analyzing redox biosensor-transgenic model organisms, including flies and mice.

Fundamental principles of redox homeostasis

The glutathione system is central to eukaryotic redox homeostasis but remains incompletely understood. We are combining redox imaging and reverse genetics in the model eukaryote S. cerevisiae to address basic questions about the glutathione system. Glutathione homeostasis depends upon a large number of factors, the most poorly understood being regulation on a cellular sub-compartment specific level and redox communication between such sub-compartments. One of our goals is to understand the contribution of glutathione transport between cellular compartments to the maintenance of cellular and compartmental glutathione homeostasis. Further we are interested in how such glutathione transport processes may be influenced by coupling to proton and other ion gradients. In collaboration with B. Schwappach (University of Göttingen) we recently identified a role for the CLC anion transporter Gef1 in coupled proton and glutathione redox homeostasis (Braun et al., 2010). Another research focus is to understand exactly how perturbation of glutathione redox potential on both a whole-cell and a sub-compartment level impacts upon cell viability. Associated with this topic, we are interested in how H2O2 crosses cellular membranes and what factors mediate its interaction with the glutathione system.