Osmotic regulation and tissue localization of the myo‐inositol biosynthesis pathway in tilapia (Oreochromis mossambicus) larvae

Abstract

The myo-inositol biosynthesis (MIB) pathway converts glucose-6-phosphate to the compatible osmolyte myo-inositol, which protects cells from salinity stress. We exposed tilapia larvae just after yolk sac resorption to various hypersaline environments and recorded robust induction of the enzymes that constitute the MIB pathway, myo-inositol-phosphate synthase (MIPS), and inositol monophosphatase 1 (IMPA1). Strong up-regulation of these enzymes is evident at both mRNA (quantitative real-time PCR) and protein (densitometric analysis of Western blots) levels. The highest level of induction of these enzymes occurs at the highest salinity that larvae were exposed to (90 ppt). Less severe salinity stress causes a proportionately reduced induction of the MIB pathway. Two distinct MIPS mRNA variants are present in tilapia larvae and both are induced at comparable levels for all the salinity challenges tested (34, 70, and 90 ppt). Immunohistochemical localization of IMPA1 protein in sagittal sections of salinity stressed and control larvae identified tissues that are particularly potent in inducing the MIB pathway. These tissues include the skin (epidermis), gills, eye (ciliary epithelium) and heart. In particular, the epidermis directly facing the external milieu showed a very strong induction of IMPA1 immunoreactivity. IMPA1 induction in response to salinity stress was not observed in other tissues suggesting that tilapia larvae may also utilize compatible organic osmolytes other than solely myo-inositol for osmoprotection. We conclude that the MIB pathway plays an important role in protecting multiple (but not all) tissues of tilapia larvae from hyperosmotic salinity stress.

Myo-inositol phosphate synthase expression in the European eel (Anguilla anguilla) and Nile tilapia (Oreochromis niloticus): effect of seawater acclimation

“…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). 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). 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).…”

Osmolality/salinity-responsive enhancers (OSREs) control induction of osmoprotective genes in euryhaline fish

“…This pathway converts glucose-6-phosphate to the compatible osmolyte myo-inositol and is highly salinity-induced in multiple tissues of O. mossambicus, including gill epithelium, brain, heart, larval epidermis, and OmB cells (38,(41)(42)(43)51). Induction of these two genes is also evident in other euryhaline fish exposed to acute salinity stress.…”

“…Therefore, if the cellular and biochemical phenotypes observed in the tissues of whole organisms are reproducible in cell culture, their mechanistic basis can be revealed using cell lines as an alternative to animal models. For instance, the osmotic induction of the pathway for synthesis of the compatible organic osmolyte myo-inositol, which is evident in many O. mossambicus tissues, is fully reproducible in the OmB cell line derived from the brain of the tilapia O. mossambicus (38,(41)(42)(43).…”

“…This pathway plays a key physiological role in teleost osmoregulation, because it converts glucose-6-phosphate to the compatible organic osmolyte myoinositol, which protects cells from salinity-induced damage (44). IMPA1.1 and MIPS mRNA, protein, and activity are all highly induced by salinity stress, resulting in elevated levels of the metabolite myo-inositol (41,45). myo-Inositol is one of only a handful of compatible organic osmolytes that are universally used by all cells to protect macromolecular structure and function during osmotic stress (44,(46)(47)(48).…”

An osmolality/ salinity-responsive enhancer 1 (OSRE1) in intron 1 promotes salinity induction of tilapia glutamine synthetase

“…A remarkable adaptive trait of O. mossambicus is its ability to tolerate large and rapid salinity fluctuations 4 . This species has evolved molecular mechanisms for rapidly turning on and off genes that encode enzymes and transporters involved in hypo-and hyper-osmoregulation [5][6][7] . However, the regulatory and evolutionary mechanisms controlling environmental (e.g.…”