Differential Regulation of the Expression of the Two Thyrotropin Beta Subunit Paralogs by Salmon Pituitary Cells In Vitro

Abstract

We recently characterized two paralogs of the thyrotropin (TSH) beta subunit in Atlantic salmon,tshβaandtshβb, issued from teleost-specific whole genome duplication. The transcript expression oftshβb, but not oftshβa, peaks at the time of smoltification, which revealed a specific involvement oftshβbparalog in this metamorphic event.Tshβaandtshβbare expressed by distinct pituitary cells in salmon, likely related to TSH cells from thepars distalisandpars tuberalis, respectively, in mammals and birds. The present study aimed at investigating the neuroendocrine and endocrine factors potentially involved in the differential regulation oftshβaandtshβbparalogs, using primary cultures of Atlantic salmon pituitary cells. The effects of various neurohormones and endocrine factors potentially involved in the control of development, growth, and metabolism were tested. Transcript levels oftshβaandtshβbwere measured by qPCR, as well as those of growth hormone (gh), for comparison and validation. Corticotropin-releasing hormone (CRH) stimulatedtshβatranscript levels in agreement with its potential role in the thyrotropic axis in teleosts, but had no effect ontshβbparalog, while it also stimulatedghtranscript levels. Thyrotropin-releasing hormone (TRH) had no effect on neithertshβparalogs norgh. Somatostatin (SRIH) had no effects on bothtshβparalogs, while it exerted a canonical inhibitory effect onghtranscript levels. Thyroid hormones [triiodothyronine (T3) and thyroxine (T4)] inhibited transcript levels of bothtshβparalogs, as well asgh, but with a much stronger effect ontshβathan ontshβbandgh.Conversely, cortisol had a stronger inhibitory effect ontshβbthantshβa, while no effect ongh. Remarkably, insulin-like growth factor 1 (IGF1) dose-dependently stimulatedtshβbtranscript levels, while it had no effect ontshβa, and a classical inhibitory effect ongh. This study provides the first data on the neuroendocrine factors involved in the differential regulation of the expression of the twotshβparalogs. It suggests that IGF1 may be involved in triggering the expression peak of thetshβbparalog at smoltification, thus representing a potential internal signal in the link between body growth and smoltification metamorphosis.