Research Topics

The main topic of our research is the regulation of ion transport across epithelia. Epithelia are barriers in organisms, separate compartments, and guarantee a regulated vectorial exchange of substances (absorption and secretion). In so called tight epithelia this transport is transcellular, and the ions enter the cells via specific transporters in the apical (luminal) membrane. Other transporters in the basolateral membrane extrude the substances. Both transporters are precisely coordinated and regulated. This is achieved by steroid hormones (aldosterone, corticosterone), by peptide hormones (antidiuretic hormone), and by thyroid hormones (triiodeothyronine, thyroxine). Most of the underlying cellular mechanisms of these regulatory processes are still unknown.

We concentrate on investigations of the electrogenic sodium-transport and its hormonal regulation, and use several model systems like epithelial cells of human lung, amphibian lung epithelia, leech integument and colonic epithelia of annelids. We focus on the investigation of apical sodium channels which are the main regulated sites for cellular sodium entry. Although several groups have cloned the so called classical epithelial sodium channel (ENaC) from epithelia of human, rat and amphibians, it is still uncertain if these molecules, which each consist of 3 subunits, are part of a more complex structure, which contains also additional regulatory proteins. It seems that this channel-type, which is found early in evolution in nematodes, is arranged in different versions according to the needs and hormones of the particular species. Our comparative-molecular-physiological approach aims to answer such questions. We use molecular biological methods (heterologous expression), electrophysiological techniques (patch clamp, voltage clamp, intracellular microelectrodes, current-fluctuation-analysis), as well as optical fluorescence techniques (laser-confocal-microscopy).

The following projects are currently pursued:

Molecular regulation of the epithelial sodium channel (ENaC) - Berk, Fronius, Clauss

Our group has isolated m-RNA from guinea-pig colonic epithelial cells, and expressed the epithelial sodium channel of this species (gpENaC) functionally in oocytes from Xenopus laevis. This enabled us to characterize the electrophysiological properties of this ion channel with voltage-clamp and patch-clamp techniques. In these experiments we could stimulate the expressed gpENaC with cAMP (Pflügers Arch. - Eur. J. Physiol 431 (1996) 913-922). This has not been possible to date with all others of the so far cloned ENaCs. We suppose therefore that either the molecular structure of the gpENaC is different, or we have expressed additional regulatory proteins. Therefore, we have started to clone the gpENaC. So far we have cloned the alpha-subunit of the channel and have expressed it together with the beta- and gamma-subunits of the rat ENaC in oocytes. Currently we are investigating the electrophysiological properties of this chimera. By domain swaps of N- and C-termini between rat-alpha- and guinea-pig-alpha-subunits we were able to localize the action of cAMP at the extracellular loop (J. Membr. Biol. 183 (2001) 15-23). Further combinations of these swaps with subunits of the Xenopus-ENaC make it possible to compare these channels with regard to their function and regulation on a molecular level.

Cooperation with Prof. Dr. Jean-Daniel Horisberger, Pharmacological Institute, University of Lausanne

Supported by DFG (German Reseach Council)

Regulation of ion transport in leech integument - Clauss

Our previous studies showed, that leech integument possesses transport mechanisms for electrogenic sodium absorption (J. Comp. Physiol. B 163 (1993) 153-159, Am. J. Physiol. (Regulatory Integrative and Comparative Physiology) 37 (1995) R605 - R613). This transport is regulated by neuropeptides (J. Biol. Chem. 271 (1996) 7237-7243), and can be modulated by toxic substances of environmental pollution (Exp. Biol. Online 1:8 (1996)). This preparation is ideal because it responds already after exposure to very low toxic concentrations. Therefore it can be used as bioindicator. Currently we are investigating the intracellular regulation (second messenger system via cAMP and cGMP) of this mechanism.

Collaboration with Prof. M. Salzet, CNRS, University Lille, France

Supported by a center grant of the state of Hessen

Molecular regulation mechanisms of sodium transport in gut cells of annelids - Goebel, Krumm, Clauss

This is a new project which is realized at the hind gut of Lumbricus terrestris. The osmoregulation of land living annelids is so far poorly understood. Although some neuropeptids were isolated from Oligochaeta, there are almost no investigations into their effect on epithelial transport processes. That is why we also investigate the colon epithelium in the Ussing-chamber to proof the existence of the epithelial sodium channel (ENaC) and to analyse its regulation by neuropeptids. By isolation of m-RNA from epithelial cells of the gut and by functional expression in oocytes, we also want to investigate how the ENaC is regulated in these invertebrate epithelia. As annelids contain no mineral corticoids but mainly neuropeptids, we also aim to investigate the role of these hormones for osmoregulation in these species (Can. J. Zool. 79 (2001) 192-203).

Persistance of coccidia in epithelial cells of the gut - Behrendt, Schröpfer, Hipke, Clauss

This project is part of a priority program of the DFG (molecular mechanisms of persistance of parasites in hosts). We investigate infected epithelial cells from human and mammalian gut to understand how the parasite organises its intracellular environment in the host cell to survive and reproduce. The ion concentrations (Na, K, H) in the parasite, the vacuole, and the host cell are measured with confocal-laser-scanning microscopy. We use also electrophysiological methods (patch clamp) to study the functional alterations of ion channels in host cells, which lead to diarrhea and ultimately to cell death (Parasitology Res. 86 (2000) 294-300).

Collaboration with Prof. Zahner, FB Veterinary Medicine, University of Giessen

Initial support by DFG (German Reseach Council) in the priority grant program Molecular Parasitology

Effects of silver ions on the epithelial sodium channel (ENaC) - Bury, Clauss

Silver residues from technical processes (photographic developing) constitute a considerable factor in pollution of the environment. It has been shown that mainly water living animals - particularly fish - are threatened. Silver ions penetrate the gill epithelia of fish and cause toxic damage. As these molecular processes presumably are mediated via epithelial sodium channels, we investigate these issues using heterologously expressed ENaCs and native epithelia. For this research programm we regularly join in our laboratory with the English eco-physiologist Dr. Nic Bury.

Cooperation with Dr. Nic Bury, Kings College, London

Supported by DAAD

Influence of reactive oxidants on the sodium reabsorption of the lung - Berk, Clauss

Reactive oxidants (O2-radicals and nitrogen containing compounds) are in our breathing air and effect the alveolar epithelium of the lung. It is well known that these oxidants inhibit the sodium reabsorption of the lung which, however, is very important for keeping the lung free of fluid accumulation (lung oedema). Cell physiological investigations are planned to elucidate the mechanisms of molecular regulation of these processes. Both the heterologously expressed sodium channel of the human lung and the intact alveolar epithelium will be used in these experiments. This research is supposed to make a contribution to understanding the patho-physiological changes of fluid clearance in the alveoli.

Cooperation with the working group of Prof. W. Seeger and Prof. F. Grimminger, Center for Internal Medicine, University Medical Center Giessen;
Prof. M. Lazdunski and Dr. E. Linguelia, CNRS, University of Nice, France

Supported by VERUM foundation

The regulation of the epithelial sodium transport in the epithelia of lung alveoli - Fronius, Berk, Clauss

The regulation of the epithelial sodium transport in the lung effects fluid clearance and so helps to avoid oedema. Sodium enters the cells through an amilorid-sensitive epithelial sodium channel (ENaC). The regulation of these processes in epithelial cells of alveoli differs from that in epithelia of gut and kidney. E.g. nitrogen monoxide (NO), inflammation mediators (prostaglandins) and corticosteroids are very important. These mechanisms of regulation are investigated using heterologously expressed ENaCs from human lung and also native alveolar epithelium. This is done in close cooperation with working groups from the Center for Internal Medicine, in order to also characterize patho-physiological evidences as well as pharmacological effects.

Cooperation with the working group of Prof. W. Seeger and Prof. F. Grimminger, Center for Internal Medicine, University Medical Center Giessen;