Abstract
Myo-inositol (Ins) is a major compatible osmolyte in many cells, including those of Mozambique tilapia (Oreochromis mossambicus). Ins biosynthesis is highly up-regulated in tilapia and other euryhaline fish exposed to hyperosmotic stress. In this study, enzymatic regulation of two enzymes of Ins biosynthesis, Ins phosphate synthase (MIPS) and inositol monophosphatase (IMPase), by direct ionic effects is analyzed. Specific MIPS and IMPase isoforms from Mozambique tilapia (MIPS-160 and IMPase 1) were selected based on experimental, phylogenetic, and structural evidence supporting their role for Ins biosynthesis during hyperosmotic stress. Recombinant tilapia IMPase 1 and MIPS-160 activity was assayed in vitro at ionic conditions that mimic changes in the intracellular milieu during hyperosmotic stress. The in vitro activities of MIPS-160 and IMPase 1 are highest at alkaline pH of 8.8. IMPase 1 catalytic efficiency is strongly increased during hyperosmolality (particularly for the substrate D-Ins-3-phosphate, Ins-3P), mainly as a result of [Na+] elevation. Furthermore, the substrate-specificity of IMPase 1 towards D-Ins-1-phosphate (Ins-1P) is lower than towards Ins-3P. Because MIPS catalysis results in Ins-3P this results represents additional evidence for IMPase 1 being the isoform that mediates Ins biosynthesis in tilapia. Our data collectively demonstrate that the Ins biosynthesis enzymes are activated under ionic conditions that cells are exposed to during hypertonicity, resulting in Ins accumulation, which, in turn, results in restoration of intracellular ion homeostasis. We propose that the unique and direct ionic regulation of the activities of Ins biosynthesis enzymes represents an efficient biochemical feedback loop for regulation of intracellular physiological ion homeostasis during hyperosmotic stress.
Highlights
IntroductionWith very few exceptions (some halophilic archaea), all cells maintain intracellular inorganic ion homeostasis within narrow limits and studies directed at the mechanisms by which such homeostasis is maintained during extracellular osmotic stress are of ubiquitous interest [1, 2].PLOS ONE | DOI:10.1371/journal.pone.0123212 June 11, 2015Inositol Biosynthetic Pathway Modulated by Osmolality in FishPlant and bacterial cells subjected to droughts or altered soil composition, renal inner medullary cells of mammals, and epithelial cells of aquatic organisms that inhabit variable salinity environments (estuaries, desert lakes) are all equipped with a high physiological capacity for maintaining intracellular inorganic ion homeostasis [3,4,5,6,7]
We cloned, expressed, purified and characterized the two enzymes that are required for Ins biosynthesis in Mozambique tilapia
Several inositol monophosphatase (IMPase) loci and at least two splice variants of MIPS have been identified in tilapia
Summary
With very few exceptions (some halophilic archaea), all cells maintain intracellular inorganic ion homeostasis within narrow limits and studies directed at the mechanisms by which such homeostasis is maintained during extracellular osmotic stress are of ubiquitous interest [1, 2].PLOS ONE | DOI:10.1371/journal.pone.0123212 June 11, 2015Inositol Biosynthetic Pathway Modulated by Osmolality in FishPlant and bacterial cells subjected to droughts or altered soil composition, renal inner medullary cells of mammals, and epithelial cells of aquatic organisms that inhabit variable salinity environments (estuaries, desert lakes) are all equipped with a high physiological capacity for maintaining intracellular inorganic ion homeostasis [3,4,5,6,7]. A high physiological capacity for responding to hypertonic stress depends on the ability for compensating passive loss of water across the semi-permeable cell membrane by 1) regulatory volume increase to restore cell volume homeostasis followed by 2) replacement of excessive intracellular inorganic ions by compatible organic osmolytes to restore intracellular electrolyte homeostasis [3, 6, 8, 9]. To avoid and alleviate macromolecular crowding during hypertonic stress, cell volume is rapidly restored when disturbed by hypertonic stress (within seconds to minutes) This restoration of cell volume is a result of activation of inorganic ion uptake, which is mediated largely by sodium-coupled secondarily active transporters, including Na+/K+/2Cl- (NKCC) cotransporters, and Na+/H+ exchangers (NHE) [10, 11]. Transport of extracellular Ins is mediated through sodium/Ins (SMIT) [18] and hydrogen/Ins (HMIT) [19] cotransporters
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
