Electroneutral NaCl Absorption is the major mechanism for intestinal salt and water absorption and is primarily mediated via the Na+/H+ exchanger isoform NHE3 and the Cl-/HCO3- exchanger DRA (SLC26A3). The agonist-induced inhibition of this transport process causes diarrhea and is operative in diarrheal diseases of most etiologies. Current dogma sees the formation of a multiprotein complex of transport-, PDZ-adapter, anchorproteins, the cytoskeleton, and the involved protein kinases as one crucial step in the inhibition of NaCl absorption. Drugs targeted to the PDZ-domain-interaction may be theoretically the most efficient antidiarrheal strategy. Surprisingly, cAMP-inhibited inhibition of salt absorption is lacking in the intestine of CFTR-deficient mice. At the same time, these mice display disturbances in intestinal PDZ-adapterprotein expression. In this project, we will investigate the physiological role of different PDZ-domain proteins in the regulation of intestinal salt/water absorption, as well as the molecular mechanisms and the interaction partners in the regulation of NHE3 transport activity. Besides classical molecular biological, biochemical and immunological techniques, we will use two-photon and conventional live-cell imaging in isolated epithelium and cultured cells of PDZ-adapter knockout mice. SiRNA-based approaches to modify expression levels of PDZ- and other interacting proteins will also be utilized. This will allow us to find out how the PDZ-adpter proteins regulate the interaction of different transport and regulatory proteins, and how the „switch“ from absorption to secretion in the enterocyte takes place on a molecular level.