Search for the genes responsible for adaptation to marine environments: osmoregulatory hormone genes

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The sea, from which the life originates, is generally a comfortable environment for living organism. However, high osmotic pressure of seawater poses a heavy burden to marine teleosts that have plasma osmotic pressure only one third of seawater. Hormones play central roles in body fluid regulation in such hyperosmotic environmental. However, major hormones that play key roles in seawater adaptation have not been identified thus far, although vasopressin/vasotocin is identified as an essential hormone to adapt to the terrestrial environment. Therefore, we search for Na-extruding hormones in the genome database of pufferfish and medaka, and they are much diversified in teleost fish. For instance, natriuretic peptides (NPs) have 7 paralogs in teleost, much more than 3 in mammals, and one of the NPs, atrial NP (ANP), is found to be a major seawater-adapting hormone in teleost fish. ANP decreases plasma Na concentration in seawater eels by inhibiting drinking and subsequent absorption of Na … More Cl by the intestine, thereby promoting seawater adaptation. Adrenomedullin is a water and Na-extruding hormone in mammals, but it forms an independent subfamily of 5 paralogs (AM1, 2, 3, 4 and 5) in teleost fish. Further, we discovered AM2 and AM5 in mammals. AM2 has potent central actions for body fluid regulation such as inhibition of drinking and oxytocin secretion in mammals, while AM5 may be related to regulation of immune system. AM2 and AM5 have a potent dipsogenic and antidiuretic effects, so that these hormones may promoter seawater adaptation in teleost fish. Guanylin, an intestinal hormone, causes diarrhea in mammals by secreting Cl ions into the lumen of the intestine, with which water moves in parallel. In teleost fish, guanylin also stimulates Cl secretion in the intestine. However, the secreted Cl ions are absorbed with Na and K through Na-K-2Cl co-transporter, two more osmolytes moves from the lumen into the body, resulting in absorption of water. Therefore, guanylin may promote seawater adaptation by acquiring more water to cope with dehydration. In summary, we could identify major hormones that promote seawater adaptation through the three-year project supported by JSPS. Less