Heterogeneity of eel thyrotropin beta mRNAs is due to a minisatellite in the 3' untranslated region of the gene

Characterization of a novel disease resistance gene rtp3 and its association with VNN disease resistance in Asian seabass

Asian seabass, an important food fish in Southeast Asia, has suffered from nervous necrosis virus (NNV) infection, resulting in massive mortality of Asian seabass larvae and enormous economic losses. Identification and characterization of disease resistance genes is important. Previous transcriptome analysis of Asians seabass epithelial cells after NNV infection revealed a highly inducible gene, receptor-transporting protein 3 (rtp3), indicating it could play an important role in Asian seabass - NNV interaction. To characterize this gene, we determined its expression pattern and subcellular localization. The rtp3 was highly induced in most examined tissues and organs of Asian seabass after NNV infection, and protein Rtp3 was localized in cytoplasm. Further association study in multiple families revealed that a microsatellite marker, (GT)ntt(GT)n, in the 3' UTR of rtp3 was significantly associated with VNN disease resistance in Asian seabass. Our results imply that rtp3 may be a novel disease resistance gene in Asian seabass. This data could improve our understanding of molecular interaction between Asian seabass and NNV, and has the potential to be applied in marker-assisted selection for disease resistance breeding in Asian seabass.

Validation and comparison of microsatellite markers derived from Senegalese sole (Solea senegalensis, Kaup) genomic and expressed sequence tags libraries

In this work, we tested 100 potential new microsatellites (SSRs) equally derived from expressed sequence tag (EST) and enriched genomic-DNA libraries from Senegalese sole (Solea senegalensis, Kaup), a valuable cultured flatfish species. A final set of 69 new polymorphic microsatellites were validated after a population analysis, 37 of which corresponded to the first EST library constructed for Senegalese sole (EST-SSR). Although differences were not significant, EST sequences provided a higher proportion of quality markers (74%) than anonymous ones (64%). Most of the rejected anonymous SSRs (17 loci) were discarded because they did not generate PCR products; only one was monomorphic. On the contrary, all EST-SSRs gave PCR products, although monomorphism was more frequent (26%). Altogether, the number of alleles per locus was fairly similar in both SSR types, ranging from 2 to 19. The observed and expected heterozygosities varied from 0.105 to 1 and from 0.108 to 0.937, respectively. The main difference between the two sets was the percentage of annotated loci, being higher in EST-SSRs, as expected. Within the EST-SSRs, 46% of them showed flanking regions that significantly matched with EST sequences from other three flatfish species; however, the microsatellite itself was present only on half of these cases. These two new SSR sets constitute a suitable tool for fingerprinting, gene flow, genetic diversity, genome mapping studies and molecular-assisted breeding in this species.

First evidence for a direct inhibitory effect of kisspeptins on LH expression in the eel, Anguilla anguilla

The kisspeptin system has emerged as one of the main puberty gatekeepers among vertebrates. The European eel (Anguilla anguilla) is a remarkable model due to its phylogenetical position at the basis of teleosts, and its unique life cycle with a blockade of puberty before reproductive migration. We cloned the full-length coding sequence of a kisspeptin receptor (Kissr) in the eel. Comparison of Kissr sequences assigned the eel Kissr to a basal position in a clade including most of the known teleost Kissr, in agreement with the eel phylogenetical position. Eel Kissr tissue distribution was analyzed by quantitative real-time PCR. Eel Kissr was highly expressed in the brain, especially in the telencephalon and di-/mes-encephalon, while a very low or undetectable expression was observed in various peripheral organs. A high expression of Kissr was also found in the pituitary indicating a possible direct pituitary role of kisspeptin. Primary cultures of eel pituitary cells were performed to investigate the direct effects of kisspeptin on pituitary hormone expression. Human/lamprey kisspeptin exerted a time- and dose-dependent inhibitory effect on LHβ expression. All other tested kisspeptins had a similar inhibitory effect on LHβ expression. The inhibitory effect of kisspeptins was exerted specifically on LHβ as no change was induced on the expression of other glycoprotein hormone subunits (GPα, FSHβ and TSHβ) nor of growth hormone. These data provide the first evidence for the existence, in the European eel, of a kisspeptin system, which may play a direct inhibitory role on pituitary LHβ expression.

Molecular cloning and regulation of mRNA expression of the thyrotropin β and glycoprotein hormone α subunits in red drum, Sciaenops ocellatus

Full-length cDNAs for thyrotropin β (TSHβ) and glycoprotein hormone α (GSUα) subunits were cloned and sequenced from the red drum (Sciaenops ocellatus). The cDNAs for TSHβ (877 bp) and GSUα (661 bp) yielded predicted coding regions of 126 and 94 amino acid proteins, respectively. Both sequences contain all invariant cysteine and putative glycosylated asparagines characteristic of each as deduced by comparison with other GSUα and TSHβ sequences from representative vertebrate species. Multiple protein sequence alignments show that each subunit shares highest identity (79% for the TSHβ and 86% for the GSUα) with perciform fish. Furthermore, in a single joint phylogenetic analysis, each subunit segregates most closely with corresponding GSUα and TSHβ subunit sequences from closely related fish. Tissue-specific expression assays using RT-PCR showed expression of the TSHβ subunit limited to the pituitary. GSUα mRNA was predominantly expressed in the pituitary but was also detected in the testis and ovary of adult animals. Northern hybridization revealed the presence of a single transcript for both TSHβ and GSUα, each close in size to mRNA transcripts from other species. Dot blot assays from total RNA isolated from S. ocellatus pituitaries showed that in vivo T3 administration significantly diminished mRNA expression of both the TSHβ and GSUα subunits and that goitrogen treatment caused a significant induction of TSHβ mRNA only. Both TSHβ and GSUα mRNA expression in the pituitary varied significantly in vivo over a 24-h period. Maximal expression for both TSHβ and GSUα occurred during the early scotophase in relation to a peak in T4 blood levels previously documented. These results suggest the production of TSH in this species which may serve to drive daily cycles of thyroid activity. Readily quantifiable, variable, and thyroid hormone-responsive pituitary TSH expression, coupled with previously described dynamic daily cycles of circulating T4 and extensive background on the growth, nutrition, and laboratory culture of red drum, suggests that this species will serve as a useful model for experimental studies of the physiological regulation of TSH production.

Thyrotropin in teleost fish

Thyrotropin (TSH), a pituitary glycoprotein hormone that stimulates the thyroid gland, has been cloned and sequenced from over a dozen teleost fish species. Although TSH is established as a primary driver of systemic thyroid status in mammals, its importance in the regulation of fish thyroid function is still uncertain. We review recent studies indicating that TSH structure is highly conserved across species representing six teleost families. These studies have found TSH messenger RNA consistently expressed in teleost pituitary tissue, although ectopic expression, particularly in gonads, has also been observed. They have also provided evidence for negative feedback inhibition of TSH expression by thyroid hormones, as well as stimulation by hypothalamic peptides. Descriptive studies have found increased TSHbeta expression associated with life history events thought to be promoted by thyroid hormones. These results, coupled with the discovery of a G-protein coupled TSH receptor in several teleost species, supports an active and conserved role for TSH in the regulation of teleost thyroid function. The relative importance of central pathways in regulating thyroid hormone provision to targets and the identity of a proposed thyrotropin-inhibiting factor in teleost fish are still unanswered questions whose resolution will be facilitated by development of methods to measure circulating TSH and its secretion from the pituitary gland.

Molecular cloning and sequence analysis of multiple cDNA variants for thyroid-stimulating hormone beta subunit (TSHbeta) in the fathead minnow (Pimephales promelas)

We cloned and sequenced full-length cDNAs encoding the beta subunit of thyroid-stimulating hormone (TSHbeta) from the pituitary of fathead minnow (Pimephales promelas) using 5'- and 3'-rapid amplification of cDNA ends (RACE). Three cDNA variants for TSHbeta with lengths of 1184-, 1093-, and 818-bp were identified. The cDNA variant of 1184-bp included 453-bp of open-reading frame and 610-bp of 3' UTR followed by a poly(A)site. This cDNA encodes 150 amino acids including a 19 residue signal peptide and a mature TSHbeta protein of 131 residues with sequence identities of 97-53% to other fishes and 42-39% to mammals. The 1093-bp cDNA variant was identical to the 1184-bp variant in the open-reading frame, but contained a deletion of 40-bp in the 3' UTR. The 818-bp cDNA variant, however, contained 498-bp of open-reading frame followed by 227-bp of 3' UTR and a poly(A)site. The deduced amino acid sequence for this cDNA variant showed 99.2% homology with the 1184- and 1093-bp variants of TSHbeta, but a single deletion of 332-bp nucleotides spanning the predicted stop codon and 3' UTR resulted in a deduced amino acid sequence with 15 additional residues on the C terminus. The presence of this 818-bp cDNA variant in the pituitary was further confirmed by PCR using primers developed to the 5' and 3' UTR. PCR and Southern blot analyses of genomic DNA suggested only one gene for TSHbeta. Sequencing of this gene revealed a hairpin loop structure of approximately 300-bp located in the 3' UTR and corresponding to the region of the 332-bp deletion in the 818-bp transcript.

Molecular cloning and sequence analysis of a cDNA encoding pituitary thyroid stimulating hormone beta-subunit of the Chinese soft-shell turtle Pelodiscus sinensis and regulation of its gene expression

A cDNA encoding thyroid stimulating hormone beta-subunit (TSHbeta) was cloned from pituitary of the Chinese soft-shell turtle, Pelodiscus sinensis, and its regulation of mRNA expression was investigated for the first time in reptile. The Chinese soft-shell turtle TSHbeta cDNA was cloned from pituitary RNA by reverse transcription and polymerase chain reaction (RT-PCR), and rapid amplification cDNA end (RACE) methods. The Chinese soft-shell turtle TSHbeta cDNA consists of 580-bp nucleotides, including 67-bp nucleotides of 5'-untranslated region (UTR), 402-bp of the open reading frame, and 97-bp of 3'-UTR followed by a 14 poly (A) trait. It encodes a precursor protein molecule of 133 amino acids with a putative signal peptide of 19 amino acids and a putative mature protein of 114 amino acids. The number and position of 12 cysteine residues, presumably forming six disulfide bonds, one putative asparagine-linked glycosylation site, and six proline residues that are found at positions for changing the backbone direction of the protein have been conserved in the turtle as in other vertebrate groups. The deduced amino acid sequence of the Chinese soft-shell turtle TSHbeta mature protein shares identities of 82-83% with birds, 71-72% with mammals, 49-57% with amphibians, and 44-61% with fish. The Chinese soft-shell turtle pituitaries were incubated in vitro with synthetic TRH (TSH-releasing hormone), thyroxine and triiodothyronine at doses of 10(-10) and 10(-8)M. TRH stimulated, while thyroid hormones suppressed, TSHbeta mRNA levels in dose-related manner. The sequences of cDNA and its deduced peptide of TSHbeta as well as the regulation of its mRNA level were reported for the first time in reptile.

Cloning of cDNAs encoding the three pituitary glycoprotein hormone beta subunit precursor molecules in the Japanese toad, Bufo japonicus

Complementary DNAs encoding precursor molecules of the beta subunits of three pituitary glycoprotein hormones (LH, FSH, and TSH) of the Japanese toad (Bufo japonicus) were isolated and sequenced. Unexpectedly large numbers of single nucleotide substitutions were found in all three beta subunit cDNAs. The eight isolated LH beta precursor cDNA clones were classified into six forms of nucleotide sequence, with four nucleotide substitutions each in the apoprotein coding region and in the 3' untranslated region (UTR). In the deduced amino acid sequence, the LH beta subunit showed two forms with a single amino acid substitution. The seven isolated FSH beta subunit cDNAs were classified into two forms, which differed from each other at 11 positions in the 3' UTR. The six isolated TSH beta subunit clones were classified into four forms with 2 and 5 nucleotide substitutions in the signal peptide and apoprotein coding regions, respectively. However, all the substitutions in the apoprotein coding region were silent. The substitution in the signal peptide coding region could produce three forms of signal peptide. Amino acid sequence comparison revealed that the toad LH beta subunit is more similar to the fish GTH II beta subunit than to mammalian and avian LH beta subunits. We found that the toad LH beta subunit molecule is a partial chimera of LH and FSH; amino acid residues located in 36th to 42nd and 96th to 99th are identical or similar to those of not LH- but FSH-beta subunit in mammalian, whereas it is more similar to LH- than FSH-beta subunit in total. We also found that the toad FSH beta subunit is more similar to the fish GTH II beta subunit than to the fish GTH I beta subunit and that the toad TSH beta subunit is more similar to tetrapod TSH beta subunits than to fish TSH beta subunits.

Isolation and characterization of the goldfish thyrotropin beta subunit gene including the 5'-flanking region

The complete gene encoding the beta subunit of thyrotropin (thyroid-stimulating hormone, TSH) was isolated from a goldfish genomic library. The goldfish TSHbeta subunit gene, which is approximately 2.0 kilobase pairs (kb) in length, consisted of three exons and two introns. The first intron was much longer (0.89 kb) than the second intron (0.3 kb) as are TSHbeta genes in mammalian species. On the basis of the location of the first intron, the goldfish TSHbeta gene belongs to the mammalian TSHbeta/FSHbeta gene group, which is distinct from the LHbeta group. Inspection of the 5'-flanking and exon 1 regions of the goldfish TSHbeta gene (1.2 kb) revealed the presence of several putative cis-acting elements, including the negative triiodothyronine (T(3))-responsive element (nTRE), Pit-1 element, and GATA-2 element. Comparison of the goldfish sequence with mammalian TSHbeta promoter sequences showed an identical region, nTRE, in the first exon-intron junction region. An in vitro study using dispersed goldfish pituitary cells showed that T(3) treatment (20 ng/ml) suppressed the TSHbeta mRNA level in the cells. These data indicate that (1) the basic structure of TSHbeta genes is highly conserved in vertebrates and that (2) T(3) acts directly on the pituitary and inhibits TSHbeta gene expression in goldfish, probably via the nTRE in the TSHbeta gene.