Expression and functions of inositol monophosphatase in seawater adapted euryhaline teleosts such as the European eel (Anguilla anguilla) and tilapia (Oreochromis mossambicus)

Myo‐inositol is a major osmolyte, which plays an important role in cyto‐protection in many organisms exposed to a variety of environmental stresses. Myo‐inositol monophosphatase (IMPA) de‐phosphorylates any myo‐inositol 1‐phosphate recycled from the membrane phospholipid pool or synthesised de novo from glucose‐6 phosphate to generate myo‐inositol. Studies have identified changes in expression of a number of Impa isoforms when euryhaline fish move between freshwater (FW) and seawater (SW), however these changes in gene expression are isoform‐ and species‐specific. The Impa1 isoform is primarily expressed in epithelial tissues exposed to the aquatic environment. In eels and tilapia, Impa1 expression was located throughout the basal/germinal cell layers within the stratified epithelia of the oesophagus, skin, fin and epithelia of the branchial arch and primary filaments, the interstitial cells and/or glomerulae between renal tubules and, specifically in eels, also in branchial and ray fin chondrocytes. In all tissues examined from eels and tilapia, Impa1 expression and inositol contents were markedly up‐regulated after transfer of fish to SW. These results suggest that increased inositol concentrations promotes renal water retention and maintains the cell volume of epithelial and epidermal tissues when fish are exposed to the dehydrating effects of the SW environment.