Androgen receptors in a cichlid fish, Astatotilapia burtoni: structure, localization, and expression levels

Neural and hormonal mechanisms of reproductive-related arousal in fishes

The major classes of chemicals and brain pathways involved in sexual arousal in mammals are well studied and are thought to be of an ancient, evolutionarily conserved origin. Here we discuss what is known of these neurochemicals and brain circuits in fishes, the oldest and most species-rich group of vertebrates from which tetrapods arose over 350 million years ago. Highlighted are case studies in vocal species where well-delineated sensory and motor pathways underlying reproductive-related behaviors illustrate the diversity and evolution of brain mechanisms driving sexual motivation between (and within) sexes. Also discussed are evolutionary insights from the neurobiology and reproductive behavior of elasmobranch fishes, the most ancient lineage of jawed vertebrates, which are remarkably similar in their reproductive biology to terrestrial mammals.

Distribution of sex steroid hormone receptors in the brain of an African cichlid fish, Astatotilapia burtoni

Sex steroid hormones released from the gonads play an important role in mediating social behavior across all vertebrates. Many effects of these gonadal hormones are mediated by nuclear steroid hormone receptors, which are crucial for integration in the brain of external (e.g., social) signals with internal physiological cues to produce an appropriate behavioral output. The African cichlid fish Astatotilapia burtoni presents an attractive model system for the study of how internal cues and external social signals are integrated in the brain as males display robust plasticity in the form of two distinct, yet reversible, behavioral and physiological phenotypes depending on the social environment. In order to better understand where sex steroid hormones act to regulate social behavior in this species, we have determined the distribution of the androgen receptor, estrogen receptor alpha, estrogen receptor beta, and progesterone receptor mRNA and protein throughout the telencephalon and diencephalon and some mesencephalic structures of A. burtoni. All steroid hormone receptors were found in key brain regions known to modulate social behavior in other vertebrates including the proposed teleost homologs of the mammalian amygdalar complex, hippocampus, striatum, preoptic area, anterior hypothalamus, ventromedial hypothalamus, and ventral tegmental area. Overall, there is high concordance of mRNA and protein labeling. Our results significantly extend our understanding of sex steroid pathways in the cichlid brain and support the important role of nuclear sex steroid hormone receptors in modulating social behaviors in teleosts and across vertebrates.

Mammalian glucocorticoid metabolites act as androgenic endocrine disruptors in the medaka (Oryzias latipes)

Glucocorticoid metabolites enter the aquatic environment via mammalian excrements. Molecular structures of their C19O3 metabolites strongly resemble the major fish androgen 11-ketotestosterone. Therefore, we tested the hypothesis that the cortisol metabolite 5alpha-androstan-3,11,17-trione acts similarly to 11-ketotestosterone by employing a fish screening assay for endocrine-active substances. After 21 d, both 11-oxygenated compounds had masculinized sex characteristics of the anal fin in female medaka in a dose-dependent manner.

Specific in vitro toxicity of crude and refined petroleum products: II. Estrogen (alpha and beta) and androgen receptor-mediated responses in yeast assays

The present study is the second in a series aiming at a systematic inventory of specific toxic effects of oils. By employing a recombinant yeast stably transfected with human estrogen receptor-alpha (ERalpha) or -beta (ERbeta) or androgen receptor (AR) and expressing yeast enhanced green fluorescent protein, the (anti-)estrogenicity and (anti-)androgenicity of 11 crude oils and refined products were studied. None of the oils tested had significant estrogenic effects in the ERalpha assay or androgenic effects in the AR assay. However, all oils were capable of inducing estrogenic responses in the ERbeta assay, with several responses being above even the maximal response of the standard 17beta-estradiol (E2). Based on the lowest effect concentrations, the potencies of oils in all the assays were between four and seven orders of magnitude lower than those of the standards E2 or testosterone (T). The potencies of the actual individual petrochemical agonists may, however, be relatively high, considering the complex composition of oils. Additive effects, antagonistic effects, and a synergistic effect were measured in the assays upon coexposure to a fixed concentration of standard (E2 or T) and increasing concentrations of oils. To investigate whether the observed effects were receptor-mediated, coexposures to the synthetic inhibitors ICI 182,780 (ERbeta assay) or flutamide (AR assay), a fixed concentration of standard, and various concentrations of oils were performed. The results suggested that the androgenic effects were receptor mediated, whereas the estrogenic effects may be only partially mediated via the receptor. The present study indicates that oils contain compounds with possible endocrine-disrupting potential, some of them acting via the hormone receptors.

Reproductive status regulates expression of sex steroid and GnRH receptors in the olfactory bulb

Neuromodulators including gonadotropin-releasing hormone (GnRH) and sex steroids help integrate an animal's internal physiological state with incoming external cues, and can have profound effects on the processing of behaviorally relevant information, particularly from the olfactory system. While GnRH and steroid receptors are present in olfactory processing regions across vertebrates, little is known about whether their expression levels change with internal physiological state or external social cues. We used qRT-PCR to measure mRNA levels of two GnRH receptors (GnRH-R1, GnRH-R2), five sex steroid receptors (estrogen receptors: ERalpha, ERbetaa, ERbetab; androgen receptors: ARalpha, ARbeta), and aromatase in the olfactory bulb of the highly social African cichlid fish Astatotilapia burtoni. We asked whether these receptor levels changed with reproductive condition in females, or with social status, which regulates reproductive capacity in males. Our results reveal that mRNA levels of multiple sex steroid, GnRH receptor subtypes, and aromatase in the olfactory bulb vary with sex, social status in males, and reproductive condition in females, which highlights the potential importance of changing receptor levels in fine-tuning the olfactory system during the reproductive cycle. Further, steroid receptor mRNA levels were positively correlated with circulating steroid levels in males, but negatively correlated in females, suggesting different regulatory control between sexes. These results provide support for the hypothesis that the first-order olfactory relay station is a substrate for both GnRH and sex steroid modulation, and suggest that changes in receptor levels could be an important mechanism for regulating reproductive, social, and seasonal plasticity in olfactory perception observed across vertebrates.

Steroid receptor expression in the fish inner ear varies with sex, social status, and reproductive state

Background

Gonadal and stress-related steroid hormones are known to influence auditory function across vertebrates but the cellular and molecular mechanisms responsible for steroid-mediated auditory plasticity at the level of the inner ear remain unknown. The presence of steroid receptors in the ear suggests a direct pathway for hormones to act on the peripheral auditory system, but little is known about which receptors are expressed in the ear or whether their expression levels change with internal physiological state or external social cues. We used qRT-PCR to measure mRNA expression levels of multiple steroid receptor subtypes (estrogen receptors: ERα, ERβa, ERβb; androgen receptors: ARα, ARβ; corticosteroid receptors: GR2, GR1a/b, MR) and aromatase in the main hearing organ of the inner ear (saccule) in the highly social African cichlid fish Astatotilapia burtoni, and tested whether these receptor levels were correlated with circulating steroid concentrations.

Results

We show that multiple steroid receptor subtypes are expressed within the main hearing organ of a single vertebrate species, and that expression levels differ between the sexes. We also show that steroid receptor subtype-specific changes in mRNA expression are associated with reproductive phase in females and social status in males. Sex-steroid receptor mRNA levels were negatively correlated with circulating estradiol and androgens in both males and females, suggesting possible ligand down-regulation of receptors in the inner ear. In contrast, saccular changes in corticosteroid receptor mRNA levels were not related to serum cortisol levels. Circulating steroid levels and receptor subtype mRNA levels were not as tightly correlated in males as compared to females, suggesting different regulatory mechanisms between sexes.

Conclusions

This is the most comprehensive study of sex-, social-, and reproductive-related steroid receptor mRNA expression in the peripheral auditory system of any single vertebrate. Our data suggest that changes in steroid receptor mRNA expression in the inner ear could be a regulatory mechanism for physiological state-dependent auditory plasticity across vertebrates.

Distribution of androgen receptor mRNA expression in vocal, auditory, and neuroendocrine circuits in a teleost fish

Across all major vertebrate groups, androgen receptors (ARs) have been identified in neural circuits that shape reproductive-related behaviors, including vocalization. The vocal control network of teleost fishes presents an archetypal example of how a vertebrate nervous system produces social, context-dependent sounds. We cloned a partial cDNA of AR that was used to generate specific probes to localize AR expression throughout the central nervous system of the vocal plainfin midshipman fish (Porichthys notatus). In the forebrain, AR mRNA is abundant in proposed homologs of the mammalian striatum and amygdala, and in anterior and posterior parvocellular and magnocellular nuclei of the preoptic area, nucleus preglomerulosus, and posterior, ventral and anterior tuberal nuclei of the hypothalamus. Many of these nuclei are part of the known vocal and auditory circuitry in midshipman. The midbrain periaqueductal gray, an essential link between forebrain and hindbrain vocal circuitry, and the lateral line recipient nucleus medialis in the rostral hindbrain also express abundant AR mRNA. In the caudal hindbrain-spinal vocal circuit, high AR mRNA is found in the vocal prepacemaker nucleus and along the dorsal periphery of the vocal motor nucleus congruent with the known pattern of expression of aromatase-containing glial cells. Additionally, abundant AR mRNA expression is shown for the first time in the inner ear of a vertebrate. The distribution of AR mRNA strongly supports the role of androgens as modulators of behaviorally defined vocal, auditory, and neuroendocrine circuits in teleost fish and vertebrates in general.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

Vertebrate sex steroid receptors: evolution, ligands, and neurodistribution

This review focuses on our current understanding of vertebrate sex steroid receptors, with an emphasis on their evolutionary relationships. These relationships are discussed based on nucleotide and amino acid sequence data, which provide clues to the process by which structure-function relations have originated, evolved, and been maintained over time. The importance of the distribution of sex steroid receptors in the vertebrate brain is discussed using the example of androgen receptor sites and their relatively conserved localizations in the vertebrate brain.

The fate of the duplicated androgen receptor in fishes: a late neofunctionalization event?

BACKGROUND

Based on the observation of an increased number of paralogous genes in teleost fishes compared with other vertebrates and on the conserved synteny between duplicated copies, it has been shown that a whole genome duplication (WGD) occurred during the evolution of Actinopterygian fish. Comparative phylogenetic dating of this duplication event suggests that it occurred early on, specifically in teleosts. It has been proposed that this event might have facilitated the evolutionary radiation and the phenotypic diversification of the teleost fish, notably by allowing the sub- or neo-functionalization of many duplicated genes.

RESULTS

In this paper, we studied in a wide range of Actinopterygians the duplication and fate of the androgen receptor (AR, NR3C4), a nuclear receptor known to play a key role in sex-determination in vertebrates. The pattern of AR gene duplication is consistent with an early WGD event: it has been duplicated into two genes AR-A and AR-B after the split of the Acipenseriformes from the lineage leading to teleost fish but before the divergence of Osteoglossiformes. Genomic and syntenic analyses in addition to lack of PCR amplification show that one of the duplicated copies, AR-B, was lost in several basal Clupeocephala such as Cypriniformes (including the model species zebrafish), Siluriformes, Characiformes and Salmoniformes. Interestingly, we also found that, in basal teleost fish (Osteoglossiformes and Anguilliformes), the two copies remain very similar, whereas, specifically in Percomorphs, one of the copies, AR-B, has accumulated substitutions in both the ligand binding domain (LBD) and the DNA binding domain (DBD).

CONCLUSION

The comparison of the mutations present in these divergent AR-B with those known in human to be implicated in complete, partial or mild androgen insensitivity syndrome suggests that the existence of two distinct AR duplicates may be correlated to specific functional differences that may be connected to the well-known plasticity of sex determination in fish. This suggests that three specific events have shaped the present diversity of ARs in Actinopterygians: (i) early WGD, (ii) parallel loss of one duplicate in several lineages and (iii) putative neofunctionalization of the same duplicate in percomorphs, which occurred a long time after the WGD.

Expression profiling of candidate genes during ovary-to-testis trans-differentiation in rainbow trout masculinized by androgens

Fish gonadal phenotype is very sensitive to sex steroid and functional masculinizations can be obtained in most species using androgen treatments. To gain insight into the molecular effects of androgen-induced masculinization we characterized, in the rainbow trout, the gonadal expression profiles of 103 candidate genes involved in sex differentiation and early gametogenesis. The androgen treatment (11beta-hydroxyandrostenedione, 10 mg/kg of food for 3 months) was administered in a genetic all-female population. Gonads were sampled at different time points in genetic all-male and all-female control populations and in the androgen-treated group. Gene expression profiles were recorded by real-time RT-PCR and biological samples and gene expressions were compared using a global clustering analysis. This analysis revealed that masculinization with androgens acts firstly by repressing granulosa cell related genes, including genes involved in ovarian differentiation (foxl2a, fst, cyp19a1a), and subsequently by repressing genes important for early oogenesis (gdf9, bcl2lb, fancl, gcl, fshb, lhb, sox23, sox24, nup62 and vtgr). However, this masculinizing treatment did not induce a testicular differentiation similar to what was observed in the control male population. This was especially noticeable for many Leydig cell genes encoding proteins involved in steroidogenesis or its control (hsd3b1, star, cyp17a1, cyp11b2.1 and nr5a1b) that were down-regulated in the androgen-treated group. Concomitantly some Sertoli cells marker genes were up-regulated by the androgen treatment (sox9a.1, nr0b1, cldn11, dmrt1) whereas others were down-regulated (amh, sox9a.2), suggesting a partial differentiation of the Sertoli cell lineage. Overall, this suggests that the crucial step of this masculinization process is the de-differentiation of the granulosa cells.