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

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.

Ultrastructural changes of the carp (Cyprinus carpio) hepatocyte nucleolus during seasonal acclimatization

BACKGROUND INFORMATION

The eurythermal fish carp (Cyprinus carpio) adjusts to the seasonal changes in the temperature and photoperiod of its habitat through diverse cellular and molecular mechanisms. We have observed that ribosomal biogenesis is modulated during the acclimatization process and correlates with profound phenotypic changes, reflecting a seasonal-dependent ultrastructural appearance of the nucleolar components. Previous studies using classical techniques showed that in winter-adapted carp the nucleolus appears to be segregated. In the present work, we have reassessed the nucleolar ultrastructural organization of the carp in summer- and winter-adapted fish by using more specific cytochemical and immunocytological techniques.

RESULTS

The acetylation method provided evidence that the nucleolar organization is different between winter- and summer-adapted carp. In winter-adapted fish the fibrillar component appears as a unique mass surrounded by several granular caps, whereas in summer-adapted carp the fibrillar component forms few cordons surrounded by granular masses. The nucleolar structure and distribution of the condensed chromatin observed varies upon seasonal acclimatization. In winter the nucleolar chromatin is densely packed in masses that surround the nucleolus, whereas during summer it displays a rather looser organization formed by filaments that not only surround the nucleolus, but also go through the nucleolar body. Using the TdT (terminal deoxynucleotidyl transferase)-immunogold labelling technique, we detected condensed and decondensed nucleolar chromatin, and found some labelling of fibrillar components in both seasons. When liver tissue from summer-adapted carp was treated with AMD (actinomycin D), we observed that the rearrangement of the nucleolar components and condensed chromatin were similar to that found in winter-adapted fish, with differences in the distribution of the perinucleolar chromatin.

CONCLUSIONS

The acetylation and TdT-immunogold labelling experiments indicated that the rearrangement of the nucleolar components of winter-adapted carp is very similar to the AMD-treated summer-adapted carp nucleolus, with the latter representing the repression of the ribosomal biogenesis that occurs during the cold season. Nevertheless, the distribution of the condensed perinucleolar chromatin in winter-adapted carp compared with AMD-treated cells suggests that the transcription of rRNA genes in winter-adapted fish is less strongly inhibited and does not lead to the classical segregation of the nucleolus of that described after AMD treatment. In addition, we have confirmed that carp hepatocyte nucleoli comprise only two main structural compartments: a fibrillar component and a granular component. Fibrillar centres were not observed.

Characterisation and expression of the paired box protein 7 (Pax7) gene in polymorphic Arctic charr (Salvelinus alpinus)

Arctic charr (Salvelinus alpinus L.) from Lake Thingvallavatn, Iceland occur as four distinct morphs: large benthivorous (LB), dwarf benthivorous (DB), piscivorous (PI) and planktonivorous (PL). The morphs differ with respect to body size, head morphology, growth rate, and life history. The aim of this study was to investigate the paired box protein 7 (Pax7) gene as a candidate for such polymorphisms due to its importance in cranio-facial, skeletal muscle, and central nervous system development. No variation in coding and intronic sequences was found between morphs. We identified 10 alternate Pax7 isoforms with insertions/deletions: a four-residue (GNRT) deletion, a GEASS insertion truncated by the first serine residue (GEAS), and a thirteen-residue insertion (GQYA/TGPEYVYCGT). The latter insertion with a threonine (T) contains a putative casein kinase II (CK-2) phosphorylation site. Pax7 spatial expression patterns were identical in embryos of DB-, LB-, and PL-morphs, and were similar to those described for zebrafish Pax7c, but a difference in temporal expression for segmentation was observed between DB and LB morphs. At the end of segmentation, novel expression was observed in the mandibular region as two bilateral domains. The potential role of multiple alternative splicing of the Pax7 gene for the generation of different Arctic charr morphs is briefly discussed.

Insight into the complex genetic network of tetraploid Atlantic salmon (Salmo salar L.): description of multiple novel Pax-7 splice variants

Paired box transcription factor 7 (Pax-7) cDNA was isolated from the skeletal muscle and brain of alevin and adult stages of Atlantic salmon, identifying 10 variants categorised as novel or established insertions (ins) or deletions (del). Two putative Pax-7 paralogs were identified (denoted Pax-7alpha and Pax-7beta) on the basis of the length and sequences of intron 3 (218 and 248 bp) and versions of ins1 and ins2. Pax-7beta contained a threonine variant of ins1 (GQY[T]GPEYVYCGT), and a shortened variant of ins2 (GEAS). Pattern identification revealed the threonine variant of ins1 includes a potential phosphorylation site (casein kinase II). Thus, the tetraploid Atlantic salmon genome appears to contain at least two putative copies and multiple splice variants of Pax-7. In situ hybridisation localised Pax-7 to mononuclear cells in the fast muscle of adult Atlantic salmon, while quantitative real-time PCR showed Pax-7alpha to be more highly expressed in brain than in skeletal muscle.

Seasonal environmental changes regulate the expression of the histone variant macroH2A in an eurythermal fish

Adaptation to cold and warm conditions requires dramatic change in gene expression. The acclimatization process of the common carp Cyprinus carpio L. in its natural habitat has been used to study how organisms respond to natural environmental changes. At the cellular level, adaptation to cold condition is accompanied by a dramatic alteration in nucleolar structure and a down regulation of the expression of ribosomal genes. We show that the enrichment of condensed chromatin in winter adapted cells is not correlated with an increase of the heterochromatin marker trimethyl and monomethyl K20H4. However, the expression of the tri methyl K4 H3 and of the variant histone macroH2A is significantly increased during the winter season together with a hypermethylation of CpG residues. Taking into account the properties of macroH2A toward chromatin structure and dynamics and its role in gene repression our data suggest that the increased expression of macroH2A and the hypermethylation of DNA which occurs upon winter-acclimatization plays a major role for the reorganization of chromatin structure and the regulation of gene expression during the physiological adaptation to a colder environment.

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.

Gene structure of the carp fish ribosomal protein L41: seasonally regulated expression

The seasonal acclimatization of the carp fish demands physiological compensatory responses. The process involves profound nucleolar adjustments and remarkable changes in rRNA synthesis, which affect ribosomal biogenesis. We have documented that protein kinase CK2, whose activity is related to ribosomal protein L41 and the regulation of rRNA synthesis, was expressed in notably higher amounts in summer-acclimatized carp compared to the cold-season adapted fish. Thus, we approached the study of the functional genomics of carp L41 protein. We report the first cloning of a fish L41 gene encoding the highly conserved 25 amino acids, including approximately 1700 bp regulatory upstream region and the 3(') polyadenylation signal, plus the isolation and characterization of two different L41 cDNAs. We found a clear differential expression of L41, which follows the same pattern as protein kinase CK2beta that transcribes at higher levels in the summer-acclimatized carp than it does in the winter-adapted fish.