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Our group is interested in the process of electrolyte transport that takes place in epithelial tissues from kidney, airways and intestine. Ion transport in these tissues controls blood pressure (kidney), mucociliary clearance (airways) and intestinal transport (colon). Similar transport proteins are in charge of ion transport in all of these organs, such as Cl- channels (CFTR, CaCC), epithelial Na+ channels (ENaC) and potassium channels (BK, SK4, KCNQ).
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Electrolyte secretion occurs mainly via luminal Cl- channels such as cAMP regulated CFTR (cystic fibrosis transmembrane conductance regulator) and Ca2+ activated Cl- channels (CaCC). Large amounts of fluid can be produced in glands or by the intestinal epithelium. |
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In contrast, Na+ absorption in the kidney or in excretory ducts of glands are in charge of fluid absorption. Thus the renal tubular system is able to reabsorb large quantities of fluid, which has been primarily filtrated through the glomerular sieve. |
Bestrophins are a family of recently identified proteins. Bestrophin 1 is expressed in the retinal pigment epithelium of the eye and has been claimed to be a Ca2+ activated Cl- channel. The protein is non-functional in the so called Best disease, a form of retina macular dystrophy. We try to identify the role of bestrophins for Ca2+ activated Cl- transport in kidney, airways, colon and glands. Ca2+ activated Cl- secretion is induced by ATP binding to purinergic P2Y receptors and acetylcholine (or carbachol, CCH) binding to muscarinic M3 receptors.
Mucociliary clearance of the airways is a protective mechanism and requires both secretion and absorption of electrolytes in order to maintain an airway surface fluid (ASL) layer (left). ENaC, CFTR, and CaCC (maybe formed by bestrophin proteins) control the ASL (right). Ca2+ activated Cl- channels are activated through stimulation of purinergic P2Y2 receptors. In kidney tubules and airways ATP is released by shear stress. ATP binds to P2Y2 receptors or is further hydrolyzed by the Ecto-5'-Nucleotidase to generate adenosine (ADO), which stimulates adenosine (A2B) receptors. Adenosine appears to be a major player in Asthma.
CFTR is a cAMP regulated Cl- channel that is essential for Cl- transport in most epithelia. Its function is impaired in Cystic Fibrosis. CFTR does not operate as a single isolated protein, but is forming a macromolecular complex in the cell membrane, together with many interacting proteins. It actually translocates an entire signaling machinery to the luminal membrane of epithelial cells. Identification of the individual components will allow for the development of novel therapeutic targets.
The epithelial Na+ channel is a tetramer of three different subunits. The channel has a central role in a variety of diseases such as Hypertension, the Acute respiratory distress syndrome (ARDS) and Cystic fibrosis. Similar to CFTR, ENaC is extensively regulated by associated receptors (P2Y2), kinases (e.g. Grk2 & CK2) and additional proteins such as Nedd4-2. Phosphorylation of the ß-subunit of ENaC reduces binding of Nedd4-2 to ENaC and enhances both membrane expression and channel activity. Some of these regulatory pathways are studied by our team.
Epithelial Na+ absorption through ENaC appears to be impaired in Infectious diseases caused by virus, fungus, and bacteria. The host-pathogen interaction that takes place on the epithelial surface triggers intracellular second messenger pathways that lead to malabsorption and/or hypersecretion. For example, NSP4 toxin is produced by the rotavirus, which is the most common cause worldwide for infectious diarrhea in infants. NSP4, apart from stimulating a Cl- secretion, may also inhibit Na+ and glucose absorption by ENaC and the Na+/glucose- cotransporter SGLT1, respectively. All three effects contribute to this severe form of diarrhea. We try to unravel the underlying signaling pathways.
Potassium channels are essential in nearly all living cells. We found recently that voltage and Ca2+ gated K+ channels are upregulated in Cancer of prostate and colon. We are particularly interested in the abnormal expression of ion channels in colonic crypts during carcinogenesis. These channels support cell proliferation and could therefore reflect new potential diagnostic and proliferative targets.
In our studies we use a mix of in vitro, ex vivo and in vivo techniques. Molecular, biochemical and functional aspects of ion channels, activatory or inhibitory signaling pathways and disease mechanisms are studied.
Epithelial transport are measured in Ussing chambers, ion channels are studied by patch clamping. Primary cell cultures and permanent cell lines as well as freshly isolated tissues are used for analysis. Overexpressed proteins are examined in double electrode voltage clamp experiments in Xenopus oocytes.
