Transcription factor pit-1 expression is modulated upon seasonal acclimatization of eurythermal ectotherms: identification of two pit-1 genes in the carp

Differential enrichment of TTF-I and Tip5 in the T-like promoter structures of the rDNA contribute to the epigenetic response of Cyprinus carpio during environmental adaptation

To ensure homeostasis, ectothermic organisms adapt to environmental variations through molecular mechanisms. We previously reported that during the seasonal acclimatization of the common carp Cyprinus carpio, molecular and cellular functions are reprogrammed, resulting in distinctive traits. Importantly, the carp undergoes a drastic rearrangement of nucleolar components during adaptation. This ultrastructural feature reflects a fine modulation of rRNA gene transcription. Specifically, we identified the involvement of the transcription termination factor I (TTF-I) and Tip-5 (member of nucleolar remodeling complex, NoRC) in the control of rRNA transcription. Our results suggest that differential Tip5 enrichment is essential for silencing carp ribosomal genes and that the T0 element is key for regulating the ribosomal gene during the acclimatization process. Interestingly, the expression and content of Tip5 were significantly higher in winter than in summer. Since carp ribosomal gene expression is lower in the winter than in summer, and considering that expression concomitantly occurs with nucleolar ultrastructural changes of the acclimatization process, these results indicate that Tip5 importantly contributes to silencing the ribosomal genes. In conclusion, the current study provides novel evidence on the contributions of TTF-I and NoRC in the environmental reprogramming of ribosomal genes during the seasonal adaptation process in carp.

Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions

Relative to mammals, the neuroendocrine control of pituitary growth hormone (GH) secretion and synthesis in teleost fish involves numerous stimulatory and inhibitory regulators, many of which are delivered to the somatotrophs via direct innervation. Among teleosts, how multifactorial regulation of somatotroph functions are mediated at the level of post-receptor signalling is best characterized in goldfish. Supplemented with recent findings, this review focuses on the known intracellular signal transduction mechanisms mediating the ligand- and function-specific actions in multifactorial control of GH release and synthesis, as well as basal GH secretion, in goldfish somatotrophs. These include membrane voltage-sensitive ion channels, Na(+)/H(+) antiport, Ca(2+) signalling, multiple pharmacologically distinct intracellular Ca(2+) stores, cAMP/PKA, PKC, nitric oxide, cGMP, MEK/ERK and PI3K. Signalling pathways mediating the major neuroendocrine regulators of mammalian somatotrophs, as well as those in other major teleost study model systems are also briefly highlighted. Interestingly, unlike mammals, spontaneous action potential firings are not observed in goldfish somatotrophs in culture. Furthermore, three goldfish brain somatostatin forms directly affect pituitary GH secretion via ligand-specific actions on membrane ion channels and intracellular Ca(2+) levels, as well as exert isoform-specific action on basal and stimulated GH mRNA expression, suggesting the importance of somatostatins other than somatostatin-14.

At least two expressed genes for transcription factors Pitx2 and Rpx are present in common carp and are upregulated during winter acclimatization

The mechanisms of seasonal acclimatization in eurythermal fish such as common carp are not fully understood. Here, we concentrate on the regulation of pituitary factors, as this organ was shown to be highly affected by seasonal changes. We cloned and sequenced two different cDNAs for each of the transcription factors Pitx2 and Rpx, known to play a role in pituitary development. We show that these genes are conserved throughout evolution, to different degrees depending on the specific domain considered. Finally, we show that the cDNAs for both factors are clearly up-regulated during the winter season, in sharp contrast to other regulators such as Pit1 or pituitary hormone genes such as prolactin (prl) and growth hormone (gh). Our results suggest that increased expression of Pitx2 and Rpx contributes to seasonal adaptation of common carp to winter conditions.

Activation of the black seabream (Acanthopagrus schlegeli) somatolactin-alpha gene promoter by Pit-1c in the Hepa-T1 cell-line

Somatolactin (SL) is a pituitary hormone of the growth hormone (GH) gene family found only in fish. To understand the regulation of this hormone at the level of gene transcription, we obtained a SLalpha gene from black seabream (bsb), with its 5' flanking promoter region carrying several putative transcription factors including seven binding sites for pituitary-specific transcription factor 1 (Pit-1). To study the actions of Pit-1 on this gene promoter, we cloned three variants of bsbPit-1 (Pit-1a, Pit-1b and Pit-1c) derived from alternative splicing of mRNA or differential transcription start sites from black seabream pituitary. The deduced amino acid sequences of these Pit-1s contained 371 amino acids (aa), 333 and 311aa for the three Pit-1 variants, Pit-1a, Pit-1b and Pit-1c, respectively, with diverse regions of Pit-1 located at the transactivation domain. The actions of bsbPit-1 variants on the bsbSL gene promoter were investigated using a co-transfection assay, with a reporter gene using a transient expression assay in Hepa-T1 cells. The N-terminus truncated isoform bsbPit-1c showed the highest level of activity on SLalpha gene promoter activation in Hepa-T1 cells; however, neither Pit-1a nor Pit-1b activated the bsbSL gene promoter in the same study.

Modular changes of cis-regulatory elements from two functional Pit1 genes in the duplicated genome of Cyprinus carpio

The pituitary-specific transcription factor Pit1 is involved in its own regulation and in a network of transcriptional regulation of hypothalamo-hypophyseal factors including prolactin (PRL) and growth hormone (GH). In the ectotherm teleost Cyprinus carpio, Pit1 plays an important role in regulation of the adaptive response to seasonal environmental changes. Two Pit1 genes exist in carp, a tetraploid vertebrate and transcripts of both genes were detected by RT-PCR analysis. Powerful comparative analyses of the 5'-flanking regions revealed copy specific changes comprising modular functional units in the naturally evolved promoters. These include the precise replacement of four nucleotides around the transcription start site embedded in completely conserved regions extending upstream of the TATA-box, an additional transcription factor binding site in the 5'-UTR of gene-I and, instead, duplication of a 9 bp element in gene-II. Binding of nuclear factors was assessed by electro mobility shift assays using extracts from rat pituitary cells and carp pituitary. Binding was confirmed at one conserved Pit1, one conserved CREB and one consensus MTF1. Interestingly, two functional Pit1 sites and one putative MTF1 binding site are unique to the Pit1 gene-I. In situ hybridization experiments revealed that the expression of gene-I in winter carp was significantly stronger than that of gene-II. Our data suggest that the specific control elements identified in the proximal regulatory region are physiologically relevant for the function of the duplicated Pit1 genes in carp and highlight modular changes in the architecture of two Pit1 genes that evolved for at least 12 MYA in the same organism.

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.

Differential expression of duplicated LDH-A genes during temperature acclimation of weatherfish Misgurnus fossilis

Temperature acclimation in poikilotherms entails metabolic rearrangements provided by variations in enzyme properties. However, in most cases the underlying molecular mechanisms that result in structural changes in the enzymes are obscure. This study reports that acclimation to low (5 degrees C) and high (18 degrees C) temperatures leads to differential expression of alternative forms of the LDH-A gene in white skeletal muscle of weatherfish, Misgurnus fossilis. Two isoforms of LDH-A mRNA were isolated and characterized: a short isoform (= 1332 bp) and a long isoform ( = 1550 bp), which both have 5'-UTRs and ORFs of the same length (333 amino acid residues), but differ in the length of the 3'-UTR. In addition, these two mRNAs have 44 nucleotide point mismatches of an irregular pattern along the complete sequence, resulting in three amino acid mismatches (Gly214Val; Val304Ile and Asp312Glu) between protein products from the short and long mRNA forms, correspondingly LDH-A(alpha) and LDH-A(beta) subunits. It is expected that the beta-subunit is more aliphatic due to the properties of the mismatched amino acids and therefore sterically more restricted. According to molecular modelling of M. fossilis LDH-A, the Val304Ile mismatch is located in the subunit contact area of the tetramer, whereas the remaining two mismatches surround the contact area; this is expected to manifest in the kinetic and thermodynamic properties of the assembled tetramer. In warm-acclimated fish the relative expression between alpha and beta isoforms of the LDH-A mRNA is around 5 : 1, whereas in cold-acclimated fish expression of is reduced almost to zero. This indicates that at low temperature the pool of total tetrameric LDH-A is more homogeneous in terms of alpha/beta-subunit composition. The temperature acclimation pattern of proportional pooling of subunits with different kinetic and thermodynamic properties of the tetrameric enzyme may result in fine-tuning of the properties of skeletal LDH-A, which is in line with previously observed kinetic and thermodynamic differences between 'cold' and 'warm' LDH-A purified from weatherfish. Also, an irregular pattern of nucleotide mismatches indicates that these mRNAs are the products of two independently evolving genes, i.e. paralogues. Karyotype analysis has confirmed that the experimental population of M. fossilis is tetraploid (2n = 100), therefore gene duplication, possibly through tetraploidy, may contribute to the adaptability towards temperature variation.

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.

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

Eurythermal fish have evolved compensatory responses to the cyclical seasonal changes of the environment. The complex adaptive mechanisms include the transduction of the physical parameters variations into molecular signals. Studies in carp have indicated that prolactin and growth hormone expression is associated with acclimatization, suggesting that the pituitary gland is a relevant physiological node in the generation of the homeostatic rearrangement that occurs in this adaptive process. Here, we report the cloning and characterization of a full-length carp prolactin receptor cDNA, which codes for the long form of the protein resembling that found in mammalian prolactin receptors. We identified up to three receptor transcript isoforms in different tissues of the teleost and assessed cell- and temporal-specific transcription and protein expression in carp undergoing seasonal acclimatization. The distinctive pattern of expression that carp prolactin receptor (cPRLr) depicts upon seasonal acclimatization supports the hypothesis that prolactin and its receptor are clearly involved in the new homeostatic stage that the eurythermal fish needs to survive during the cyclical changes of its habitat.

An increased expression of nucleolin is associated with a physiological nucleolar segregation

Nucleolar segregation is the most striking cellular phenotypic feature of cold-acclimatized carp and depicts the cyclical reprogramming that the physiology of the fish undergoes between summer and winter, where a clear differential expression of some nucleolar related genes occurs. We characterized carp nucleolin, a nucleolar protein involved in multiple steps of ribosome biogenesis, and evaluated its expression upon fish acclimatization. We show that the carp cDNA deduced amino acid sequence exhibits the same tripartite structural organization found in other species. Nevertheless, we observed that nucleolin mRNA expression was strongly induced in the cold-adapted carp as was the nuclear protein content, assessed by immunocytochemistry in liver sections. The physiological up-regulation of nucleolin in the cold-acclimatized carp, where rRNA transcription and processing are depressed concomitantly with the nucleolus segregation, is consistent with the notion that nucleolin plays a fundamental role in repressing rRNA synthesis.

Expression of ayu (Plecoglossus altivelis) Pit-1 in Escherichia coli: its purification and immunohistochemical detection using monoclonal antibody

The pituitary-specific transcription factor Pit-1 belongs to the family of POU-domain proteins and is known to play an important role in the differentiation of pituitary cells. Here we report the complete nucleotide sequence of cDNA encoding Pit-1 from the brackish water fish, ayu (Plecoglossus altivelis). Nucleotide sequence analysis of 1910 bp of ayu Pit-1 cDNA revealed an open reading frame of 1074 bp that encodes a protein of 358 amino acids containing a POU-specific domain, POU homeodomain, and an STA (Ser/Thr-rich activation) transactivation domain. We inserted the coding region of Pit-1 cDNA, obtained by PCR, into a pET-20b(+) plasmid to produce recombinant Pit-1 in Escherichia coli BL21 (DE3) pLysS cells. Upon induction with isopropyl beta-D-thiogalactopyranoside, Pit-1 was expressed and accumulated as inclusion bodies in E. coli. The protein was then purified in one step by affinity chromatography on a nickel-nitrilotriacetic acid agarose column under denaturing conditions. This method yielded 0.7 mg of highly pure and stable protein per 200 ml of bacterial culture. A band of 40 kDa, resolved as recombinant ayu Pit-1 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, agrees well with the molecular mass calculated from the translated cDNA sequence. The purified recombinant Pit-1 was confirmed in vitro through Western blot analysis, using its monoclonal antibody. This monoclonal antibody detected Pit-1 in the nuclei of ayu developing pituitary by immunohistochemical reaction. It serves as a good reagent for the detection of ayu Pit-1 in situ.

Environmental acclimatization of the carp modulates the transcription of beta-actin

A cascade of mechanisms involving changes in gene expression are substantial to shape the adaptive responses that a eurythermal fish requires upon environmental changes in its habitat. We have previously shown that the cyclical reprogramming of rRNA transcription is a remarkable feature in carp under seasonal acclimatization. Using in situ hybridization and competitive RT-PCR we found significant differences in beta-actin transcripts, generally accepted to be coded by a typical housekeeping gene, in tissues from summer- and winter-acclimatized carp. The physiological differential beta-actin transcription herein reported places us on the alert for the reference genes estimated to be constitutive to quantitatively assess gene transcripts.

Seasonal adaptation modulates the expression of the protein kinase CK2 beta subunit gene in the carp

Carp fish seasonal acclimatization induces a cyclical transcriptional modulation of several genes. Its most dramatic expression results in a concomitant structural rearrangement of the nucleolar components that phenotypically represents profound shifts in the level of ribosomal RNA synthesis. In connection with the recent studies that implicate CK2 in the control of rRNA synthesis in vertebrates, we characterized the cDNA of carp protein kinase CK2beta subunit and assessed its transcriptional behavior in winter- and summer-acclimatized fish. We found a remarkable differential gene expression of CK2beta subunit between summer- and winter-acclimatized carp which correlates with the modulatory pattern observed in rRNA transcription.