Developmental profiles of progesterone receptor transcripts and molecular responses to gestagen exposure during Silurana tropicalis early development

Highlights

• P4-receptors are expressed in early larval development in S. tropicalis.

• P4 upregulated gene expression of ipgr, mpgrβ, pgrmc1, ar, and srd5α1, 2, and 3.

• MGA exposure induced a dissimilar molecular response to P4.

• MGA exposure inhibited transcription of srd5α3 in a concentration-dependent manner.

Abstract

Environmental gestagens are an emerging class of contaminants that have been recently measured in surface water and can interfere with reproduction in aquatic vertebrates. Gestagens include endogenous progestogens, such as progesterone (P4), which bind P4-receptors and have critically important roles in vertebrate physiology and reproduction. Gestagens also include synthetic progestins, which are components of human and veterinary drugs, such as melengestrol acetate (MGA). Endogenous progestogens are essential in the regulation of reproduction in mammalian species, but the role of P4 in amphibian larval development remains unclear. This project aims to understand the roles and the regulatory mechanisms of P4 in amphibians and to assess the consequences of exposures to environmental gestagens on the P4-receptor signaling pathways in frogs. Here, we established the developmental profiles of the P4 receptors: the intracellular progesterone receptor (ipgr), the membrane progesterone receptor β (mpgrβ), and the progesterone receptor membrane component 1 (pgrmc1) in Western clawed frog (Silurana tropicalis) embryos using real-time qPCR. P4-receptor mRNAs were detected throughout embryogenesis. Transcripts for ipgr and pgrmc1 were detected in embryos at Nieuwkoop and Faber (NF) stage 2 and 7, indicative of maternal transfer of mRNA. We also assessed the effects of P4 and MGA exposure in embryonic and early larval development. Endocrine responses were evaluated through transcript analysis of a suite of gene targets of interest, including: ipgr, mpgrβ, pgrmc1, androgen receptor (ar), estrogen receptor α (erα), follicle stimulating hormone β (fshβ), prolactin (prl), and the steroid 5-alpha reductase family (srd5α1, 2, and 3). Acute exposure (NF 12–46) to P4 caused a 2- to 5-fold change increase of ipgr, mpgrβ, pgrmc1, and ar mRNA levels at the environmentally relevant concentration of 195 ng/L P4. Acute exposure to MGA induced a 56% decrease of srd5α3 at 1140 ng/L MGA. We conclude that environmental exposure to P4 induced multiple endocrine-related transcript responses in amphibians; however, the differential responses of MGA suggest that the effects of MGA are not mediated through the classical P4 signaling pathway in S. tropicalis.

Introduction

Gestagens are a class of chemicals that exhibit progestogenic activity, including the endogenous progestogens (e.g., progesterone; P4), and synthetic progestins used in human and veterinary drugs (e.g., melengestrol acetate; MGA) (Fig. 1). Progestogens have critical function in vertebrate physiology and reproduction and P4 is a key metabolic intermediate for the production of other endogenous steroids. In mammals, P4 is crucial to pregnancy with roles in regulating the reproductive cycle, ovulation, preparation of the uterus for implantation, maintaining gestation, and differentiation of the mammary gland (Graham and Clarke, 1997, Rozenbaum, 2001). Progestogens also play an essential role in reproduction and regulation of gamete maturation in lower vertebrates, such as amphibians and fish (Nagahama and Yamashita, 2008). In seasonally breeding amphibians, plasma P4 levels peak during the preovulatory period and gradually decline during breeding (Paolucci and DiFiore, 1994). Progestogens have been shown to induce gamete maturation in frogs (Schuetz and Lessman, 1982, Wasserman et al., 1982) and in several species of teleost fishes (Tubbs et al., 2010, Boni et al., 2007, Kazeto et al., 2005, Zhu et al., 2003a, Nagahama, 1997, Thomas and Trant, 1989).

Physiological functions of gestagens are mediated through binding and activating progesterone receptors (PRs), including intracellular and membrane PRs (iPGR and mPGR, respectively), and P4 membrane receptor (PGRMC) proteins (Thomas, 2008). The intracellular iPGR is a ligand-activated transcription factor that induces transcription of specific P4 response genes (reviewed in Jacobsen and Horwitz, 2012). The biological roles of iPGR in mammalian species are well documented and include ovulation, sexual behaviour, uterine function, and mammary gland morphology (Conneely et al., 2002, Chappell et al., 1999, Chappell et al., 1997, Lydon et al., 1995). In non-mammalian vertebrates, evidence suggests that iPGR serves critical roles in ovulation (Tang et al., 2016).

Nongenomic progestogenic effects are mediated by membrane bound P4 receptors. mPGRs are G protein-coupled receptors that belong to the progestin and adipoQ receptor (PAQR) family of 7-transmembrane domain proteins (Thomas, 2008, Thomas et al., 2007, Tang et al., 2005, Zhu et al., 2003b). In general, vertebrates express three isoforms of mPGRs (mPGRα, mPGRβ, and mPRγ) (Thomas, 2008, Brinton et al., 2008, Zhu et al., 2003b). The biological functions of mPGRs include final oocyte maturation in female amphibians and fish (Nagahama and Yamashita, 2008) and sperm hypermotility in humans and fish (Thomas, 2008). Similarly, PGRMC1, a membrane bound protein, is capable of mediating rapid progestogenic functions. A diverse range of physiological roles and functions have been demonstrated or suggested for PGRMC1, including: steroid synthesis and metabolism, cholesterol regulation, antiapoptotic action in ovarian follicle cells, and initiation of the acrosome reaction in mammalian sperm cells (Thomas et al., 2014; reviewed in Thomas, 2008, Peluso, 2006, Buddhikot et al., 1999).

Exogenous compounds have been shown to interfere with normal endocrine function in aquatic vertebrates. These endocrine disrupting chemicals (EDCs) pose a threat to wild populations. Recently, gestagens have been identified as an emergent class of EDC in the aquatic environment (reviewed in Orlando and Ellestad, 2014). Gestagens are continuously released the aquatic environment from at least three sources: waste water treatment plant effluent, paper mill plant effluent, and animal agricultural runoff; and are generally detected in the range of 0.1–30 ng/L (reviewed in Fent, 2015; reviewed in Orlando and Ellestad, 2014). Both P4 and MGA are administered to beef cattle in animal feeding operations to synchronize estrus and encourage rapid anabolic development (Schiffer et al., 2001). Cattle excrete relatively high concentrations of both endogenous and synthetic steroid hormones in urine and manure (Hanselman et al., 2003) and MGA has been measured at concentrations of 0.3–8.0 ng/g in manure (Schiffer et al., 2001). Elevated levels of both P4 and MGA have been measured in flush water from animal agriculture operations in concentrations up to 11,900 ng/L and 500 ng/L, respectively (reviewed by Fent, 2015, Bartelt-hunt et al., 2012, Liu et al., 2012).

Despite the presence of gestagens in the aquatic environment, the data set regarding the effects of gestagen exposure in aquatic organisms is limited. Ecotoxicological studies demonstrated that some progestins (e.g., levonorgestrel (LNG), norethindrone (NET), and gestodene (GES)) cause masculinization of females and disruption of testicular development in males in amphibians (reviewed by Ziková et al., 2017, Säfholm et al., 2014 Hoffmann and Kloas, 2012, Säfholm et al., 2012, Kvarnryd et al., 2011, Lorenz et al., 2011a) and fish (Kroupova et al., 2014, Svensson et al., 2014, Runnalls et al., 2013, Svensson et al., 2013, Zucchi et al., 2012, Murack et al., 2011, Paulos et al., 2010). These effects may be mediated through crosstalk of progestins with receptors other than PRs such as the AR or glucocorticoid receptor (Africander et al., 2011). For example, LNG, NET, and GES are potent androgens with relatively high binding affinity to the AR (45%, 15%, and 85% of metribolone, respectively) (reviewed by Schindler et al., 2003). In contrast, MGA exhibits weak androgenic activity (approximately 1% of T and 0.3% dihydrotestosterone (Bauer et al., 2000)) but glucocorticoid activity comparable with that of hydrocortisone (Elliott et al., 1973, Greig et al., 1970, Duncan et al., 1964). Relative to other progestins, there are few studies to date that examine the ecotoxicity of MGA in aquatic organisms. In African clawed frog (Xenopus laevis) tadpoles, MGA exposure induced a significant reduction in body mass and snout-vent length at 100 ng/L MGA (Finch et al., 2013). While this study suggested that amphibians exposed to MGA in the environment may experience adverse effects in growth and development, further investigation into the molecular mechanism of MGA in frogs remains a nascent topic.

The objectives of the present study were to first establish the ontogenetic expression of ipgr, mpgrβ, and pgrmc1 during amphibian embryogenesis, and second, to investigate the effects of P4 and MGA exposure on development, survival, and sex steroid-related gene expression in Western clawed frog (Silurana tropicalis) larvae. Here, we established for the first time the expression profiles of ipgr, mpgrβ, and pgrmc1 from the commencement of amphibian embryogenesis at Nieuwkoop and Faber (NF) stage 2 to the beginning of larval development (NF 46) in S. tropicalis. We then conducted a series of acute exposures to P4 and MGA from NF 12–46 and evaluated the morphological and molecular effects of gestagen treatment.