Thyroid hormones activate TH/E2 receptor/regulator system and drive Na+/K+-ATPase in the ovarian wall of hypothyroid air-breathing fish (Anabas testudineus Bloch)

Abstract

In fish, as in other vertebrates, thyroid hormones (THs) act on many biological processes including growth and reproduction. Primary THs such as thyroxine (T₄) and triiodothyronine (T₃) are known for their direct action on osmoregulatory organs regulating ion osmotic homeostasis in many teleost fishes. However, it is unclear how these hormones interact with estradiol-17β (E₂), an ovarian hormone that regulates the development of oocytes. We thus examined the short-term in vivo action of varied THs such as T_4, T₃ and T₂, a potent TH metabolite diiodothyronine, on the expression pattern of receptors of THs and E₂ in the ovarian wall of the hypothyroid climbing perch to identify the interactive pattern of TH/E₂ receptor system and the molecular dynamics of Na⁺/K⁺−ATPase (NKA) subunits in the ovarian wall that provides structural and functional support to ovary. We found differential pattern of transcript abundance of NKA subunits isoforms such as nkaα1a, nkaα1b, nkaα1c atp1b1, atp1b2 and fxyd3, fxyd5, fxyd6, TH receptor isoforms (tr<, trβ, tr<B), deiodinases (deo1, deo2 and deo3) and TH transporter (mct8, mct10) and E₂ receptor (er<, erβ, gper), aromatases (cyp19a1a, cyp19a1b) and steroidogenic enzymes (17β-hsd, sf-1, star) in ovarian wall in response to T₄, T₃ and T₂ administration in MMI-treated fish. The transcript abundance pattern provides evidence for a direct role of THs in the ovarian wall of hypothyroid fish. It confirms a targeted interaction of THs with ER/cyp19a1 and NKA systems. Pattern analysis further revealed that T₄ could produce maximum activation of the TR/Deo/Mct and ER/cyp19a1 systems among three iodinated tyrosines, bringing synergistic interaction between the TH and E₂ systems in the ovarian wall. This novel evidence of direct interaction between the TH/E₂ system and the NKA system in the ovarian wall further testifies to an ion osmotic role for THs in this ovarian structure.