Cloning of the cDNA for thyroid stimulating hormone beta subunit and changes in activity of the pituitary-thyroid axis during silvering of the Japanese eel, Anguilla japonica

Abstract

The purposes of this study were: (1) to clone the cDNA encoding pituitary thyroid-stimulating hormone beta subunit (TSH ) of the Japanese eel, Anguilla japonica, together with its genomic DNA sequence, for phylogenetic analysis, and to study the regulation of the TSH gene expression in cultured pituitaries; and (2) to investigate the transcript levels of pituitary TSH mRNA and the serum thyroxine profiles at different stages of ovarian development before and during silvering in the wild female eels. The maturity of female eels was divided into four stages, juvenile, sub-adult, pre-silver, and silver, based on skin color and oocyte diameter. The genomic DNA of the TSH subunit contains two introns and three exons, and the TSH protein possesses a putative signal peptide of 20 amino acids and a mature peptide of 127 amino acids. The amino acid sequence identities of TSH mature peptide of Japanese eel compared with those of teleosts and other vertebrates are: European eel (98·4%), salmonids (60·6 -61·3%), carps (52·0 -56·7%), sturgeon (48·4%), and tetrapods (42·9 -45·2%). In in vitro studies of the regulation of TSH mRNA it was found that thyrotropin-releasing hormone increased while thyroxine decreased its expression. RT-PCR and real-time quantitative PCR analysis showed that the transcript levels of TSH subunit increased during eel silvering. The serum thyroxine levels also increased in parallel with TSH mRNA expression during silvering, supporting the hypothesis that the hypothalamus-pituitary-thyroid axis is correlated to silvering in the wild female Japanese eels.

Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

“…Measurement of plasma levels of thyroid hormones during silvering in our study showed a moderate increase in thyroxine (T4) between intermediate and silver eels (×1.3) and no significant change in triidothyronine (T3). This is in agreement with the data obtained in Japanese eels in which serum T4 levels slightly increase (×1.6) between sub-adult and silver stages [53]. This lack of strong increase in TSH and thyroid hormones during the transition between yellow to silver eels argues against an involvement of the thyrotropic axis in eel silvering.…”

“…In the present study, in contrast to the strong variations in gonadotropin (LHβ and FSHβ) expression, a weak and nonsignificant increase (×1.2) was seen in TSHβ mRNA levels in silver eels. Similarly, in Japanese eel, Han et al [53] found a slight increase (×1.24) in TSHβ mRNA levels between juvenile and silver stages. Measurement of plasma levels of thyroid hormones during silvering in our study showed a moderate increase in thyroxine (T4) between intermediate and silver eels (×1.3) and no significant change in triidothyronine (T3).…”

Crossover of the Hypothalamic Pituitary–Adrenal/Interrenal, –Thyroid, and –Gonadal Axes in Testicular Development

“…(78, 79)]; however, in fish, mixed effects have been found. In bighead carp ( Aristichthys nobilis ) and Japanese eel ( Anguilla japonica ), TRH was shown to increase hypophyseal tsh- β expression (80, 81), while in coho salmon ( Oncorhynchus kisutch ), TRH-treatment did not stimulate TSH release (13). Furthermore, teleost fish have no portal systems that connect the CNS and the pituitary, in which hypothalamic neurons terminate very close to adenohypophysial cells (79).…”

Endocrine profiles during silvering of the European eel (Anguilla anguilla L.) living in saltwater

“…Studies on eel also showed increased thyroid and pituitary activity during metamorphosis (Callamand and Fontaine, 1942;Etienne, 1959;Knowles and Vollrath, 1966). Also in the study of Han et al (2004), serum thyroxine levels increased in parallel with TSH β mRNA expression during silvering, supporting the hypothesis that the hypothalamus-pituitary-thyroid axis is correlated to silvering in the wild Japanese eels. Also in salmons during parrsmolt transformation thyroxin is involved and a peak was found in salmon plasma in April (Dickhoff et al, 1978).…”