Cellular Distribution of Aquaporin 3 Within the Gill, Kidney and Gastrointestinal Tract of the European Eel (Anguilla anguilla)

In teleost fish such an the European eel, water transport plays a crucial role in osmoregulation in both freshwater (FW) and seawater (SW) environments and a number of tissues work together to maintain body fluid homeostasis. A key water channel protein Aquaporin-3 (AQP3) has been identified in the 'silver' life stage of the European eel, Anguilla anguilla. Using a specific polyclonal antibody directed against the C-terminal of the eel protein, AQP3 was found to be expressed in the renal, intestinal, branchial and oesophageal tissues of the SW- and FW-acclimated eels and also in the intestinal rectal segment of SW-fish. Western blotting identified a 24 kDa protein in the gills from both FW and SW eels and immunohistochemistry localised the protein to membrances within the chloride cells. SW acclimation induced a three-fold decrease in the branchial protein level. In both FW and SW fish, AQP3 was detected in the brush border membrane of a subset of renal tubules, on the plasma membranes of basal epithelial cells and in epithelial cells within the anterior part of the oesphagus-staining in the posterior segments was restricted to mucous cells. AQP3 also stained macrophages within the intestinal epithelium of FW and SW eels and mucous cells in the rectal epithelium of SW-fish. These results suggest that AQP3 may play an important functional role in water and/or small solute transport in the major osmoregulatory organs; the role of this protein in the physiological processes associated with the acclimation of eels to different environmental salinities will be discussed.