Evidence that two tilapia (Oreochromis niloticus) prolactins have different osmoregulatory functions during adaptation to a hyperosmotic environment

Abstract

Two forms of prolactin (tiPRLI and tiPRLII), with only 69% sequence identity, have been previously described in the cichlid fish tilapia (Oreochromis species). In the present study we have attempted to investigate the biological activity of these two prolactin forms during adaptation to a hyperosmotic environment. For this purpose, we have developed two highly sensitive (sensitivity: 0.05 ng/ml) and specific (cross-reactivity < 0.04%) radioimmunoassays for tiPRLI and tiPRLII, using recombinant hormones. When fish were directly transferred from fresh to brackish water, the measured levels of plasma tiPRLI and tiPRLII dropped abruptly until 12 h after transfer. Thereafter, plasma tiPRLII remained stable (around 0.5 ng/ml) until the end of the experiment, whereas plasma tiPRLI continued to decrease to undetectable levels. These different patterns of change are reflected in the calculated ratio of plasma tiPRLII to tiPRLI, which increased from 2-3 in fresh water-adapted fish to over 10 in fish which had spent 3 days or more in brackish water. The pituitary contents of tiPRLI and tiPRLII varied in a qualitatively similar fashion after transfer to brackish water. The tiPRLI content dropped continuously after 12 h, reaching one-twelfth of its initial level after 2 weeks. The pituitary tiPRLII content, on the other hand, did not decrease significantly until day 7, and after a 2-week exposure to brackish water it had only decreased by 50%. When injected into tilapia adapted to brackish water, both ovine prolactin and recombinant tiPRLI induced a clear dose-dependent ion-retaining effect. In contrast, the effect induced by tiPRLII treatment was markedly smaller and not dose-dependent. Northern blot analysis of tiPRL mRNAs using either a tiPRLI or a tiPRLII cDNA probe indicated the presence of two mRNAs differing in size: a 1.7 kb mRNA coding for tiPRLI and a 1.3 kb mRNA coding for tiPRLII. After transfer to brackish water, levels of the two mRNAs decreased similarly. The present study indicates that, in O. niloticus, the two forms of prolactin have different osmoregulatory roles during adaptation to brackish water. Accordingly, their synthesis are differentially regulated after transfer to a hyperosmotic environment, presumably at a post-transcriptional level.

Immunohistochemically detected ontogeny of prolactin and growth hormone cells in the African catfish Clarias gariepinus

“…Recombinant tPRL I is more sensitive to the transfer from fresh to salt water in comparison to recombinant tPRL II [3,13] while it may possess somatotropic actions similar to GH [39]. The tilapia PRL II hormone comprises 177 amino acids and is known to be present in the same granules as PRL I [40].…”

Expression cloning of a cDNA encoding a fish prolactin receptor.

“…Polyadenylylated RNA was purified from tissues removed from pools of adult or juvenile tilapias reared in freshwater or brackish water. Northern blot analysis and membrane hybridizations were conducted as described (10,28), with a 0.9-kb probe corresponding to the extracellular domain of tiPRL receptor and a rainbow trout ,B-actin cDNA (32) (33) and found to contain two identical positive clones with an insert of -3 kb. point of 5.53.…”

“…These two tilapia PRLs (tiPRLs) were shown to be differentially regulated during adaptation to a hyperosmotic environment (9)(10)(11) and to exhibit both common and distinct biological effects and potencies (6,10,12). These effects are mediated by a specific cell membrane receptor.…”

“…We report in this paper the isolation of a tiPRL receptor cDNAI by an expression cloning approach (25) (10). The specific activity was 80-140 gCi/p,g for 125I-labeled hGH (125I-hGH) and [30][31][32][33][34][35][36][37][38][39][40] ,tCi/,g for 125I[ labeled tiPRL188 (1251-tiPRL188; 1 Ci = 37 GBq).…”

Is the primitive regulation of pituitary prolactin (tPRL177 and tPRL188) secretion and gene expression in the euryhaline tilapia (Oreochromis mossambicus) hypothalamic or environmental?

“…The amino acid sequence identity of the two prolactins is only 69%, with each form being encoded by separate genes (Yamaguchi et al 1988, Rentier-Delrue et al 1989. Given these differences, studies have aimed at determining their unique physiological functions in ion and water balance (Specker et al 1985, Young et al 1988, Specker et al 1989, Auperin et al 1994, 1995, Flik et al 1994, growth (Shepherd et al 1997b), reproduction (Rubin & Specker 1992, Oshima et al 1996 and pigmentation (Kitta et al 1993, Oshima et al 1996. Both forms of PRL are colocalized within the same cells of the rostral pars distalis (RPD) of the pituitary , Specker et al 1993, but when the ratio of the two prolactins (tPRL 188 :tPRL 177 ) is examined, it becomes evident that they are differentially regulated during development (Ayson et al 1994), and by changes in environmental salinity (Borski et al 1992, Auperin et al 1994, Yada et al 1994, Yoshikawa-Ebesu et al 1995 and nutrition (Vijayan et al 1996, Shepherd et al 1997a.…”