Four IMPA genes have been identified in both the eel ( A.
Although only two IMPA genes have been reported in most vertebrate species, teleost-specific tandem and whole genome duplication events have enabled fish the potential to harbor up to six different IMPA transcripts ( 24). Adaptation of euryhaline fish to SW or hyper-saline environments has been reported to result in an increase in expression of IMPA in a number of tissues from the eel ( 24, 25), killifish ( 45), and tilapia ( 15, 24, 25, 39, 44). Alternatively, in animal cells, the cyclic alcohol can be sequestered as the result of the actions of two types of membrane transporter, the sodium- and the proton-dependent myo-inositol transporters (abbreviated as SMIT and HMIT, respectively) ( 41), which use the transmembrane sodium or proton gradients to accumulate the metabolite to concentrations of up to 1,000 times that found in the extracellular environment. IMPA can dephosphorylate both the inositol 3-phosphate that is generated from MIPS activity and also any inositol 1-phosphate that is produced from cell signaling and the turnover of membrane inositol phospholipids ( 36).
Using 4peaks free#
Inositol can be synthesized de novo from glucose 6-phosphate (G 6-P) by the sequential actions of two enzymes, myo- d-inositol 3-phosphate synthase (MIPS), which converts G 6-P to inositol 3-phosphate (IP) and inositol monophosphatase (IMPA), which dephosphorylates IP to free inositol. The osmolyte is particularly important in epithelial cells that are directly exposed to the external SW environment, such as in the skin, fin, and gill. Intracellular inositol levels have been shown to increase in a variety of tissues to help compensate for the increases in extracellular osmolality associated with the movement of fish from freshwater (FW) to seawater (SW) or to hyper-SW environments ( 10, 24, 25). Although inositol's role as a compatible intracellular osmolyte has been well documented in salinity adaptation in plants and in volume regulation in mammalian renal epithelia, it is only recently that its importance in osmoregulation in euryhaline teleosts, such as the eel ( Anguilla anguilla) and tilapia ( Oreochromis niloticus and Oreochromis mossambicus), has been investigated ( 15, 24, 25, 38, 39, 44). Inositol and its chemically modified congeners (e.g., phosphorylated derivatives such as inositol mono-, bis-, and tris-phosphate) are involved in many intracellular processes, such as hormonal signaling, regulation of gene expression, cell growth, membrane biogenesis and trafficking, protein stabilization, and cellular osmoregulation ( 8, 9, 13, 14, 16, 18).
The cyclic polyol, inositol, is a cellular metabolite with universal existence in all organisms from bacteria to humans.
Using 4peaks skin#
Immunofluorescence indicated that MIPS was expressed within gill chondrocytes and epithelial cells of the primary filaments, basal epithelial cell layers of the skin and fin, the cytosol of columnar intestinal epithelial and mucous cells, as well as unknown entero-endocrine-like cells. Immunohistochemical investigations revealed specific immunoreactivity in the gill, fin, skin, and intestine taken from only SW-acclimated tilapia. However, a major 67-kDa immunoreactive protein (presumed to be MIPS) found in tilapia tissues exhibited 11- and 54-fold increases in expression in gill and fin samples from SW-acclimated fish. SW acclimation failed to affect the abundance of any immunoreactive protein in any tissue tested from the eel. Western blots identified a number of species- and tissue-specific immunoreactive MIPS proteins ranging from 40 to 67 kDa molecular weight. SW acclimation resulted in large (6- to 32-fold) increases in mRNA expression for both MIPS (s) and MIPS (l) in all tilapia tissues tested, whereas in the eel, changes in expression were limited to a more modest 2.5-fold increase and only in the kidney. In most tilapia tissues, the MIPS (s) transcript exhibits much higher abundance (generally >10-fold) with the exception of white skeletal muscle and oocytes, in which the MIPS (l) transcript predominates. The larger tilapia transcript results from the inclusion of the 87-nucleotide intron between exons 5 and 6 in the genomic sequence. A single MIPS gene ( Isyna1/ Ino1) exists in eel and tilapia genomes with a single myo- d-inositol 3-phosphate synthase (MIPS) transcript identified in all eel tissues, although two MIPS spliced variants are found in all tilapia tissues.
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