Characterization of tilapia (Oreochromis niloticus) gonadotropins by modeling and immunoneutralization Joseph Aizen a , Noga Kowalsman b , Masha Y. Niv b,c , Berta Levavi-Sivan a,⇑ a The Robert H. Smith Faculty of Agriculture, Food and Environment, Department of Animal Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel b The Robert H. Smith Faculty of Agriculture, Food and Environment, The Institute of Biochemistry, Food Science and Nutrition, The Hebrew University of Jerusalem, Rehovot 76100, Israel c The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel article info Article history: Available online 2 June 2014 Keywords: 11-ketotestosterone LH FSH GnRH Gonadotropin receptor abstract In fish, both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) play important roles in reproduction. Here we explored the structure and differential specificity of tilapia (t) gonadotropins (GTHs) to delineate their physiological relevance and the nature of their regulation. We generated struc- tural models of tGTHs and GTH receptors (R) that enabled us to better understand the hormone–receptor interacting region. In tilapia, FSH release is under the control of the hypothalamic decapeptide GnRH, an effect that was abolished by specific bioneutralizing antisera [anti-recombinant (r) tFSHb]. These antisera also reduced the basal secretion and delayed GnRH-stimulated production of 11-ketotestosterone (11KT), and dramatically reduced LH levels. Immunoneutralization of tLH using anti-rtLHb significantly reduced its GnRH-stimulated levels. Basal 11KT and FSH levels were also reduced. Taken together, these results suggest a feedback mechanism between FSH and LH release in tilapia. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The general model of vertebrate reproduction involves the con- trol of gonadal functions by two heterodimeric plasma glycopro- teins (gonadotropins; GTHs) secreted from the gonadotrophs in the anterior pituitary. In fish, two distinct GTHs—GTH-I and GTH- II—have been purified, and the cDNAs of the corresponding specific subunits have been cloned from more than 56 fish species repre- senting at least 14 teleost orders. Based on their molecular structures and physiological effects, a new nomenclature has emerged, with fish GTH-I and GTH-II being referred to as follicle- stimulating hormone (FSH) and luteinizing hormone (LH), respec- tively. FSH and LH are members of the glycoprotein hormone family, which also includes thyroid stimulating hormone (TSH) and chorionic gonadotropin (CG) (reviewed by (Levavi-Sivan et al., 2010). In ovarian follicles, FSH regulates granulosa cell proliferation, the synthesis of cell-cycle-regulatory proteins and induction of differentiation-specific genes’ expression. LH, on the other hand, promotes follicular maturation, ovulation and the synthesis of ovarian steroid hormones. In the testes, FSH determines Sertoli cell proliferation and is involved in germ cell maturation, while LH sup- ports Leydig cell functions and stimulates the synthesis of andro- gens, regulating the final stages of spermatogenesis (reviewed by (Yaron and Sivan, 2006). The presence of two distinct GTH receptors (GTHRs) in a single fish species was confirmed by the molecular cloning of two differ- ent cDNAs in several fish species from different taxa (Levavi-Sivan et al., 2010). The FSH and LH receptors (FSHR and LHR, respec- tively) are G-protein-coupled receptors (GPCRs) from family A of the rhodopsin-like receptors (Gether, 2000). Despite their overall similarity to mammalian GTHRs, in vitro binding and functional studies have revealed promiscuous ligand recognition of fish GTHRs, in contrast to the strict ligand selectivity described in mammals. Studies on GTHRs from representatives of the piscine orders Siluriformes (Kumar et al., 2001a,b; Vischer et al., 2003) and Cypriniformes (So et al., 2005) have indicated that FSHRs show a preference for FSH but also respond to LH, whereas LHRs respond specifically to LH. Studies on receptors from a salmonid fish, the amago salmon (Oba et al., 1999a,b), concluded that LHR, but not FSHR, responds to both GTHs. However, in the sea bass, each GTHR is exclusively activated by its corresponding hormone (Rocha et al., http://dx.doi.org/10.1016/j.ygcen.2014.05.028 0016-6480/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Address: Department of Animal Sciences, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University, P.O. Box 12, Rehovot 76100, Israel. Fax: +972 8 9465763. E-mail address: berta.sivan@mail.huji.ac.il (B. Levavi-Sivan). General and Comparative Endocrinology 207 (2014) 28–33 Contents lists available at ScienceDirect General and Comparative Endocrinology journal homepage: www.elsevier.com/locate/ygcen