Functional specificity of the rainbow trout (Oncorhynchus mykiss) gonadotropin receptors as assayed in a mammalian cell line

Abstract

In vertebrates, gonadotropins (GTHs) (FSH and LH) are two circulating pituitary glycoprotein hormones that play a major role in the regulation of gonadal functions, including gonadal cell proliferation/differentiation and steroidogenesis. In mammals, it is well known that their biological effects are mediated by highly specific membrane-bound receptors expressed preferentially on the somatic cells of the gonads. However, in fish, binding and functional studies have shown that cross-reactivity may occur in GTH receptors depending on the species. To understand the molecular mechanisms involved in GTH actions, functional characterization of trout GTH receptors and their gonadal gene expression pattern has been carried out. The present study describes the presence of two distinct GTH receptors in trout showing similarities with those of higher vertebrates but also differences in their structural determinants. In vitro functional studies demonstrate that rtLH specifically activates its cognate receptor (EC 50 Z117 ng/ml), whereas purified rainbow trout FSH (rtFSH) activates FSHR but also LHR at supraphysiological doses (EC 50 Z38 vs 598 ng/ml for FSHR and LHR respectively). The high doses of rtFSH required to activate LHR put into question the physiological relevance of this interaction. The use of heterologous chinook GTHs confirms the strong preference of each hormone for its cognate receptor. The gonadal expression pattern of the GTH receptor genes suggests that FSH may play an important role in regulating gonadal functions, not only at the early stages but also at the final stages of the male and female reproductive cycles, in addition to the LH pathway.

Fsh and Lh have common and distinct effects on gene expression in rainbow trout testis

“…This observation is consistent with the fact that in trout, Fsh and Lh activate preferentially their cognate receptor at the concentration used here (Sambroni et al 2007). Among the genes preferentially regulated by Fsh, we found the transcripts that were previously shown to be expressed in the Sertoli cells, such as amh in fish (Skaar et al 2011) and Wisp1, Abca1, Smtn, Krt18, Slc40a1, Col1a1, Ccnd1, and Fstl3 in mice (Chalmel et al 2007).…”

“…Besides, in fish, one could hypothesize that both gonadotropins activate the same receptor type in the target cells. However, as mentioned above, such promiscuous action of Lh on Fsh receptor activity appears unlikely because we previously showed that trout gonadotropins used at 500 ng/ml did not crossactivate their reciprocal receptor (Sambroni et al 2007). Finally, the common effects of the gonadotropins could, at least in part, be mediated through indirect actions resulting from the stimulation of sexual steroids.…”

“…In trout, gonadotropins reach their maximum levels just before and during the spawning period at stages VII-VIII (Gomez et al 1999). The positive regulation of the gonadotropin receptor genes observed in vitro may explain the dramatic increase in fshr and lhcgr gene expression described in vivo at stage VIII (Sambroni et al 2007). The increased availability of the gonadotropins and their receptors may result in the amplification of the gonadotropin effects on gene expression.…”

“…In African catfish (Bogerd et al 2001, zebrafish (So et al 2005), and Atlantic salmon (Andersson et al 2009), a promiscuous activation of Fsh receptors has been reported using high Lh concentrations. However, in two salmonid species, amago salmon and rainbow trout, only Fsh was able to activate Fshr (Oba et al 1999, Sambroni et al 2007). The physiological relevance of the promiscuity of the piscine gonadotropin receptors therefore remains a matter of debate.…”

Seasonal changes in gonadal expression of gonadotropin receptors, steroidogenic acute regulatory protein and steroidogenic enzymes in the European sea bass

“…4, B expression steadily increases during testicular maturation, while in sea bass and rainbow trout, 388 according to Sambroni et al(2007), LHR mRNA levels were maintained almost constant until 389 the end of the recrudescence stage. Analysis of LH levels in sea bass plasma (Fig.…”