Seasonal environmental changes modulate the prolactin receptor expression in an eurythermal fish

Effects of salinity and endocrine-disrupting chemicals on expression of prolactin and prolactin receptor genes in the euryhaline hermaphroditic fish, Kryptolebias marmoratus

Prolactin plays an essential role in ion uptake as well as reduction in ion and water permeability of osmoregulatory surfaces in euryhaline fish. Kryptolebias marmoratus is a euryhaline fish with unique internal self-fertilization. In order to understand the effect of different salinities and environmental endocrine-disrupting chemicals (EDCs) on the regulation of prolactin (PRL) and prolactin receptor (PRLR) genes, the full-length sequences of PRL and two PRLR genes were cloned from K. marmoratus. The expression pattern of K. marmoratus PRL (Km-PRL) and PRLR (Km-PRLR1, Km-PRLR2) mRNAs was analyzed in different developmental stages (2dpf to 5h post-hatching) and tissues of hermaphrodite fish. To investigate the effects of salinity changes and EDC exposure, the mRNA expression pattern of PRL, PRLR1 and PRLR2 was analyzed in exposed fish. The Km-PRL mRNA in the hermaphrodite was predominantly expressed in the brain/pituitary, the Km-PRLR1 mRNA was highly expressed in the intestine, while the Km-PRLR2 mRNA was intensively expressed in the gills. The expression of the Km-PRL mRNA generally increased from stage 1 (2 dpf) to stage 3 (12 dpf) in a developmental, stage-dependent manner. It decreased in stage 4 (12 dpf) and the hatching stage (stage 5). Km-PRLR1 and Km-PRLR2 mRNAs showed a gradual increase in expression from stage 1 (2 dpf) to stage 4 (12 dpf) and decreased by stage 5 (5 h post-hatching). Also, both mRNAs of PRLR showed a different expression pattern after exposure to different salinity concentrations (0, 33, and 50 ppt) in juvenile fish. The expression of PRL mRNA was upregulated at 0 ppt, but was downregulated at a moderately higher salinity concentration (33 to 50 ppt). The Km-PRLR1 mRNA showed upregulation at freshwater stress (0 ppt) compared to other concentrations of salinity (33 ppt to 50 ppt). The Km-PRLR2 mRNA was marginally upregulated at freshwater stress (0 ppt), but was downregulated at a higher salinity concentration (50 ppt) and showed no significant change in expression at 33 ppt salinity. Interestingly, both mRNAs showed upregulation in the brain (e.g. Km-PRL) and intestine (e.g. Km-PRLR1) after EDC exposure. These findings suggested that Km-PRL and two Km-PRLR mRNAs would be useful in analyzing the effect of different salinities as well as the modulatory effect of EDC exposure on these gene expressions in K. marmoratus.

Seasonal changes in peptide, receptor and ion channel mRNA expression in the caudal neurosecretory system of the European flounder (Platichthys flesus)

The caudal neurosecretory system (CNSS) of the euryhaline flounder Platichthys flesus has suggested roles in osmoregulatory, reproductive and nutritional adaptation, as fish migrate between seawater (winter) and brackish/freshwater (summer) environments. This study examined seasonal changes in mRNA expression profile of functionally important genes in the CNSS. cDNAs encoding neuropeptides, receptors and ion channels were cloned by reverse transcriptase polymerase chain reaction (RT-PCR) and screening of a flounder CNSS cDNA library. The expression profile of cloned genes was determined by real-time RT-PCR at 2-month intervals throughout the year in CNSS from seawater-adapted fish. Plasma cortisol (measured by radioimmunoassay) showed a peak in April, the time of spawning. Expression levels of mRNA for peptides urotensins I and II (UI, UII) and corticotropin releasing factor (CRF) all showed a seasonal cycle, with lowest expression in April and highest in August-October. The expression of CRF2(UI), UT(UII) and CRF1 receptors was not correlated with corresponding peptide expression. Receptors for potential neuromodulators of CNSS activity also displayed a seasonal mRNA expression profile. Glucocorticoid, 5-hydroxytryptamine, kappa-opioid and glutamate receptor expression peaked around April, suggesting that modulation of electrical activity of the neurosecretory Dahlgren cells is of particular importance at this time. Expression of mRNA for L-type Ca(2+) and Ca-activated K(+) channels was lower during the summer months. These channels underlie electrical bursting activity in Dahlgren cells. Ion channel mRNA expression was also lower in CNSS from flounder fully adapted to freshwater as opposed to seawater, consistent with previously reported observations of reduced bursting activity in Dahlgren cells from freshwater-adapted CNSS. These findings support the hypothesis that the CNSS is functionally reprogrammed to cope with changes in physiological challenge as fish migrate between sea and estuaries in winter and spring.

Gene structure and seasonal expression of carp fish prolactin short receptor isoforms

The complex adaptive mechanisms that eurythermal fish have evolved in response to the seasonal changes of the environment include the transduction of the physical parameter variations into neuroendocrine signals. Studies in carp (Cyprinus carpio) have indicated that prolactin (PRL) and growth hormone (GH) expression is associated with acclimatization, suggesting that the pituitary gland is a relevant physiological node in this adaptive process. Also, the distinctive pattern of expression that carp prolactin receptor (PRLr) protein depicts upon seasonal acclimatization supports the hypothesis that PRL and its receptor clearly are involved in the new homeostatic stage that the eurythermal fish needs to survive during the cyclical changes of its habitat. Here, we characterize the first prolactin receptor gene in a teleost and show that its expression is not associated with alternative promoters, unlike in humans and rodents. Using the regulatory region to direct the transcription of green fluorescent protein (GFP) in zebrafish embryos, we mapped the appearance of this hormone receptor during fish development. This is the first report identifying a fish prolactin receptor gene expressing transcript isoforms encoding for short forms of the protein (45 kDa). These have been found in osmoregulatory tissues of the carp and are regulated in connection with the seasonal acclimatization of the fish.

Copeptin, derived from isotocin precursor, is a probable prolactin releasing factor in carp

Control of prolactin (PRL) release is of crucial importance for the multiple functions exerted by PRL in vertebrates. Recently identified hypothalamic PRL-releasing peptides displayed additional neuromodulatory activities and in fish only few could be detected close to lactotrophs. Here we describe the C-terminal peptide processed from the carp isotocin precursor as probable physiologically relevant regulator of PRL release in carp. The amino acid sequence derived from the complete isotocin precursor gene of Cyprinus carpio, predicted a C-terminal peptide uncleaved between the neurophysin (Np) and copeptin (Cp) domain. Accordingly, antibodies against synthetic Np- and Cp-specific oligopeptides both immunodetected a 13kDa protein (cNpCp) in total pituitary proteins and showed abundant immunoreaction in hypothalamic axons in direct contact with lactotrophs in the rostral pars distalis of carp pituitary gland sections. Finally, incubation of cultured carp pituitary explants with purified carp cNpCp resulted in a potent stimulation of PRL release.

Seasonal modulation of growth hormone mRNA and protein levels in carp pituitary: evidence for two expressed genes

Adaptation of eurythermal fish to naturally varying environmental conditions involves modulation of expressions of various factors in the hypothalamo-hypophyseal axis. Here we used three complementary approaches to assess the seasonal variation of growth hormone (GH) protein and mRNA levels in pituitary glands of acclimatized carp fish. First, a polyclonal antibody raised against an oligopeptide derived from the carp GH sequence was used for immunohistochemistry; second, oligonucleotides specific for GH transcripts were used for in situ hybridization. Specific immunodetection of GH coincides with visualization of GH mRNA in the proximal pars distalis, the specific location of somatotroph cells in carp pituitary gland. Finally, competitive RT-PCR analyses confirmed that GH expression exhibits seasonal cyclical reprogramming with higher levels in summer- than in winter-adapted fish. The expression pattern suggests an important role for GH in the molecular mechanisms underlying the acclimatization process. In parallel, amplification of sequences from the fourth intron and adjacent sites from exons IV and V demonstrates the existence of a new GH gene previously undescribed. The detection of transcripts corresponding to each gene suggests that both GH gene copies are active in the duplicated carp genome and that they are similarly affected by seasonal adaptation.