Histone deacetylases and repression of the gonadotropin genes

Epigenetic repression of gonadotropin gene expression via a GnRH-mediated DNA delivery system

The reproductive axis is activated by gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gonadotropes to secrete hormones that drive gonadal function and steroidogenesis. Thus repression of this axis, which is conserved across mammals and sexes, can reduce steroid levels and/or prevent reproduction. Steroid-dependent pathologies, including various cancers, are commonly treated with GnRH super-analogs which have long-term side-effects, while humane solutions for controlling reproduction in domestic and wild animal populations are lacking. GnRH-conjugated toxins are undergoing clinical trials for GnRHR-expressing cancer cells, and have been examined for gonadotrope ablation in animals, but showed low and/or transient effects and administration of toxins has many potential complications. Here we exploit GnRH targeting to gonadotropes to deliver DNA encoding an effector that induces gonadotropin gene repressive epigenetic modifications which are perpetuated over time. Several layers of specificity are endowed through targeting to GnRHR-expressing cells and due to local cleavage of the peptide packaging the DNA; the DNA-encoded effector is expressed and directed to the target genes by the DNA binding domain of a highly specific transcription factor. This design has multiple advantages over existing methods of shutting down the reproductive axis, and its modular design should allow adaptation for broad applications.

Epigenetic regulation of 5α reductase-1 underlies adaptive plasticity of reproductive function and pubertal timing

Background

Women facing increased energetic demands in childhood commonly have altered adult ovarian activity and shorter reproductive lifespan, possibly comprising a strategy to optimize reproductive success. Here, we sought to understand the mechanisms of early-life programming of reproductive function, by integrating analysis of reproductive tissues in an appropriate mouse model with methylation analysis of proxy tissue DNA in a well-characterized population of Bangladeshi migrants in the UK. Bangladeshi women whose childhood was in Bangladesh were found to have later pubertal onset and lower age-matched ovarian reserve than Bangladeshi women who grew-up in England. Subsequently, we aimed to explore the potential relevance to the altered reproductive phenotype of one of the genes that emerged from the screens.

Results

Of the genes associated with differential methylation in the Bangladeshi women whose childhood was in Bangladesh as compared to Bangladeshi women who grew up in the UK, 13 correlated with altered expression of the orthologous gene in the mouse model ovaries. These mice had delayed pubertal onset and a smaller ovarian reserve compared to controls. The most relevant of these genes for reproductive function appeared to be SRD5A1, which encodes the steroidogenic enzyme 5α reductase-1. SRD5A1 was more methylated at the same transcriptional enhancer in mice ovaries as in the women’s buccal DNA, and its expression was lower in the hypothalamus of the mice as well, suggesting a possible role in the central control of reproduction. The expression of Kiss1 and Gnrh was also lower in these mice compared to controls, and inhibition of 5α reductase-1 reduced Kiss1 and Gnrh mRNA levels and blocked GnRH release in GnRH neuronal cell cultures. Crucially, we show that inhibition of this enzyme in female mice in vivo delayed pubertal onset.

Conclusions

SRD5A1/5α reductase-1 responds epigenetically to the environment and its downregulation appears to alter the reproductive phenotype. These findings help to explain diversity in reproductive characteristics and how they are shaped by early-life environment and reveal novel pathways that might be targeted to mitigate health issues caused by life-history trade-offs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01219-6.

The role of the hypothalamus and pituitary epigenomes in central activation of the reproductive axis at puberty

Puberty is programmed through a multifactorial gene network which works to activate the pulsatile secretion of the gonadotropin releasing hormone (GnRH), and subsequently elevate circulating levels of the pituitary gonadotropins that stimulate gonadal activity. Although this developmental transition normally occurs at a limited age-range in individuals of the same genetic background and environment, pubertal onset can occur prematurely or be delayed following changes in ambient conditions, or due to genetic variations or mutations, many of which have remained elusive due to their location in distal regulatory elements. Growing evidence is pointing to a pivotal role for the epigenome in regulating key genes in the reproductive hypothalamus and pituitary at this time, which might mediate some of the plasticity of pubertal timing. This review will address epigenetic mechanisms which have been demonstrated in the KNDy neurons that increase the output of pulsatile GnRH, and those involved in activation of the GnRH gene and its receptor, and describes how GnRH utilizes epigenetic mechanisms to stimulate transcription of the pituitary gonadotropin genes in the context of the chromatin landscape.

Unravelling the role of epigenetics in reproductive adaptations to early-life environment

Reproductive function adjusts in response to environmental conditions in order to optimize success. In humans, this plasticity includes age of pubertal onset, hormone levels and age at menopause. These reproductive characteristics vary across populations with distinct lifestyles and following specific childhood events, and point to a role for the early-life environment in shaping adult reproductive trajectories. Epigenetic mechanisms respond to external signals, exert long-term effects on gene expression and have been shown in animal and cellular studies to regulate normal reproductive function, strongly implicating their role in these adaptations. Moreover, human cohort data have revealed differential DNA methylation signatures in proxy tissues that are associated with reproductive phenotypic variation, although the cause-effect relationships are difficult to discern, calling for additional complementary approaches to establish functionality. In this Review, we summarize how adult reproductive function can be shaped by childhood events. We discuss why the influence of the childhood environment on adult reproductive function is an important consideration in understanding how reproduction is regulated and necessitates consideration by clinicians treating women with diverse life histories. The resolution of the molecular mechanisms responsible for human reproductive plasticity could also lead to new approaches for intervention by targeting these epigenetic modifications.

Middle-aged female rats lack changes in histone H3 acetylation in the anterior hypothalamus observed in young females on the day of a luteinizing hormone surge

Histone acetylation has recently been implicated in gene transcription and estradiol (E2) actions in the hypothalamus. This study aims to determine the involvement of histone acetylation in mediating E2-induced luteinizing hormone (LH) surge to understand the mechanism underlying LH surge dysfunction in female reproductive aging. Young and middle-aged female rats were ovariectomized (OVX) and treated with hormone or oil once per day for two days. At the time of the expected LH surge, blood samples were taken for LH assay. The anterior and posterior hypothalami were dissected, histone H3/H4 acetylation and histone deacetylases (HDACs) 4, -5, -10 and -11 protein expressions were measured using Western blotting. Our results show that in the young females, E2 markedly increased histone H3 acetylation while significantly reducing HDAC10 protein expression in the anterior hypothalamus. Notably, E2-induced alterations of histone H3 acetylation and HDAC10 in the anterior hypothalamus were absent in middle-aged females, associated with a reduced LH release. However, age alters histone H4 acetylation in both the anterior and posterior hypothalamus and significantly increased HDAC 4 and -5 protein expression in the anterior hypothalamus. Taken together, these data suggest that histone H3 acetylation in the anterior hypothalamus may mediate E2 regulation of LH surge and the process possibly through decreasing HDAC10. The missed responsiveness of histone H3 acetylation and HDAC10 expression to E2 in the anterior hypothalamus may contribute to LH surge failure that occurs in female reproductive aging.

Multifaceted Targeting of the Chromatin Mediates Gonadotropin-Releasing Hormone Effects on Gene Expression in the Gonadotrope

Gonadotropin-releasing hormone (GnRH) stimulates the expression of multiple genes in the pituitary gonadotropes, most notably to induce synthesis of the gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH), but also to ensure the appropriate functioning of these cells at the center of the mammalian reproductive endocrine axis. Aside from the activation of gene-specific transcription factors, GnRH stimulates through its membrane-bound receptor, alterations in the chromatin that facilitate transcription of its target genes. These include changes in the histone and DNA modifications, nucleosome positioning, and chromatin packaging at the regulatory regions of each gene. The requirements for each of these events vary according to the DNA sequence which determines the basal chromatin packaging at the regulatory regions. Despite considerable progress in this field in recent years, we are only beginning to understand some of the complexities involved in the role and regulation of this chromatin structure, including new modifications, extensive cross talk, histone variants, and the actions of distal enhancers and non-coding RNAs. This short review aims to integrate the latest findings on GnRH-induced alterations in the chromatin of its target genes, which indicate multiple and diverse actions. Understanding these processes is illuminating not only in the context of the activation of these hormones during the reproductive life span but may also reveal how aberrant epigenetic regulation of these genes leads to sub-fertility.

Mitogen- and stress-activated protein kinase 1 is required for gonadotropin-releasing hormone-mediated activation of gonadotropin α-subunit expression

Pituitary gonadotropin hormones are regulated by gonadotropin-releasing hormone (GnRH) via MAPK signaling pathways that stimulate gene transcription of the common α-subunit (Cga) and the hormone-specific β-subunits of gonadotropin. We have reported previously that GnRH-induced activities at these genes include various histone modifications, but we did not examine histone phosphorylation. This modification adds a negative charge to residues of the histone tails that interact with the negatively charged DNA, is associated with closed chromatin during mitosis, but is increased at certain genes for transcriptional activation. Thus, the functions of this modification are unclear. We initially hypothesized that GnRH might induce phosphorylation of Ser-10 in histone 3 (H3S10p) as part of its regulation of gonadotropin gene expression, possibly involving cross-talk with H3K9 acetylation. We found that GnRH increases the levels of both modifications around the Cga gene transcriptional start site and that JNK inhibition dramatically reduces H3S10p levels. However, this modification had only a minor effect on Cga expression and no effect on H3K9ac. GnRH also increased H3S28p and H3K27ac levels and also those of activated mitogen- and stress-activated protein kinase 1 (MSK1). MSK1 inhibition dramatically reduced H3S28p levels in untreated and GnRH-treated cells and also affected H3K27ac levels. Although not affecting basal Cga expression, MSK1/2 inhibition repressed GnRH activation of Cga expression. Moreover, ChIP analysis revealed that GnRH-activated MSK1 targets the first nucleosome just downstream from the TSS. Given that the elongating RNA polymerase II (RNAPII) stalls at this well positioned nucleosome, GnRH-induced H3S28p, possibly in association with H3K27ac, would facilitate the progression of RNAPII.

An epigenetic switch repressing Tet1 in gonadotropes activates the reproductive axis

The TET enzymes catalyze conversion of 5-methyl cytosine (5mC) to 5-hydroxymethyl cytosine (5hmC) and play important roles during development. TET1 has been particularly well-studied in pluripotent stem cells, but Tet1-KO mice are viable, and the most marked defect is abnormal ovarian follicle development, resulting in impaired fertility. We hypothesized that TET1 might play a role in the central control of reproduction by regulating expression of the gonadotropin hormones, which are responsible for follicle development and maturation and ovarian function. We find that all three TET enzymes are expressed in gonadotrope-precursor cells, but Tet1 mRNA levels decrease markedly with completion of cell differentiation, corresponding with an increase in expression of the luteinizing hormone gene, Lhb We demonstrate that poorly differentiated gonadotropes express a TET1 isoform lacking the N-terminal CXXC-domain, which represses Lhb gene expression directly and does not catalyze 5hmC at the gene promoter. We show that this isoform is also expressed in other differentiated tissues, and that it is regulated by an alternative promoter whose activity is repressed by the liganded estrogen and androgen receptors, and by the hypothalamic gonadotropin-releasing hormone through activation of PKA. Its expression is also regulated by DNA methylation, including at an upstream enhancer that is protected by TET2, to allow Tet1 expression. The down-regulation of TET1 relieves its repression of the methylated Lhb gene promoter, which is then hydroxymethylated and activated by TET2 for full reproductive competence.

H2A.Z controls the stability and mobility of nucleosomes to regulate expression of the LH genes

The structure and dynamics of promoter chromatin have a profound effect on the expression levels of genes. Yet, the contribution of DNA sequence, histone post-translational modifications, histone variant usage and other factors in shaping the architecture of chromatin, and the mechanisms by which this architecture modulates expression of specific genes are not yet completely understood. Here we use optical tweezers to study the roles that DNA sequence and the histone variant H2A.Z have in shaping the chromatin landscape at the promoters of two model genes, Cga and Lhb. Guided by MNase mapping of the promoters of these genes, we reconstitute nucleosomes that mimic those located near the transcriptional start site and immediately downstream (+1), and measure the forces required to disrupt these nucleosomes, and their mobility along the DNA sequence. Our results indicate that these genes are basally regulated by two distinct strategies, making use of H2A.Z to modulate separate phases of transcription, and highlight how DNA sequence, alternative histone variants and remodelling machinery act synergistically to modulate gene expression.

The antidepressant drug carbamazepine induces differential transcriptome expression in the brain of Atlantic salmon, Salmo salar

Concerns are being expressed recently over possible environmental effects of human pharmaceuticals. Although the likelihood of acute toxicity is low, the continuous discharge of pharmaceuticals into the aquatic environment means that sublethal effects on non-target organisms need to be seriously considered. One-year-old Atlantic salmon parr were exposed to 7.85±0.13μgL(-1) of the antidepressant drug Carbamazepine (CBZ) for five days to investigate changes of mRNA expression in the brain by means of a custom 17k Atlantic salmon cDNA microarray. The selected concentration is similar to upper levels that can be found in hospital and sewage treatment plant effluents. After treatment, 373 features were differently expressed with 26 showing up- or down-regulation of ≥2-fold (p≤0.05). Among the mRNAs showing the highest change were the pituitary hormones encoding features somatolactin, prolactin and somatotropin, or growth hormone. Functional enrichment and network analyses of up- and down-regulated genes showed that CBZ induced a highly different gene expression profile in comparison to untreated organisms. CBZ induced expression of essential genes of the focal adhesion and extracellular matrix - receptor interaction pathways most likely through integrin alpha-6 (itga6) activation. Negative regulation of apoptotic process, extracellular matrix organization and heme biosynthesis were the most enriched biological process related GO-terms, with the simultaneous enrichment of collagen and extracellular region related cellular component GO-terms, and extracellular matrix structural constituent, hormone activity and chromatin binding molecular function related GO-terms. These results show that relatively low doses of CBZ may affect brain physiology in exposed salmon parr, targeting similar processes as in human, indicating a high degree of conservation of targets of CBZ action. However, and since the mRNAs showing most changes in expression are critical for adaptation to different stressors and life history transitions in Atlantic salmon, more research should be undertaken to assess CBZ effects to avoid impairment of normal development and maintenance of natural populations.

Systematic mapping of occluded genes by cell fusion reveals prevalence and stability of cis-mediated silencing in somatic cells

Both diffusible factors acting in trans and chromatin components acting in cis are implicated in gene regulation, but the extent to which either process causally determines a cell's transcriptional identity is unclear. We recently used cell fusion to define a class of silent genes termed "cis-silenced" (or "occluded") genes, which remain silent even in the presence of trans-acting transcriptional activators. We further showed that occlusion of lineage-inappropriate genes plays a critical role in maintaining the transcriptional identities of somatic cells. Here, we present, for the first time, a comprehensive map of occluded genes in somatic cells. Specifically, we mapped occluded genes in mouse fibroblasts via fusion to a dozen different rat cell types followed by whole-transcriptome profiling. We found that occluded genes are highly prevalent and stable in somatic cells, representing a sizeable fraction of silent genes. Occluded genes are also highly enriched for important developmental regulators of alternative lineages, consistent with the role of occlusion in safeguarding cell identities. Alongside this map, we also present whole-genome maps of DNA methylation and eight other chromatin marks. These maps uncover a complex relationship between chromatin state and occlusion. Furthermore, we found that DNA methylation functions as the memory of occlusion in a subset of occluded genes, while histone deacetylation contributes to the implementation but not memory of occlusion. Our data suggest that the identities of individual cell types are defined largely by the occlusion status of their genomes. The comprehensive reference maps reported here provide the foundation for future studies aimed at understanding the role of occlusion in development and disease.

Gonadotrophin-releasing hormone signalling downstream of calmodulin

Gonadotrophin-releasing hormone (GnRH) regulates reproduction via binding a G-protein coupled receptor on the surface of the gonadotroph, through which it transmits signals, mostly via the mitogen-activated protein (MAPK) cascade, to increase synthesis of the gonadotrophin hormones: luteinising hormone (LH) and follicle-stimulating hormone (FSH). Activation of the MAPK cascade requires an elevation in cytosolic Ca(2+) levels, which is a result of both calcium influx and mobilisation from intracellular stores. However, Ca(2+) also transmits signals via an MAPK-independent pathway, through binding calmodulin (CaM), which is then able to bind a number of proteins to impart diverse downstream effects. Although the ability of GnRH to activate CaM was recognised over 20 years ago, only recently have some of the downstream effects been elucidated. GnRH was shown to activate the CaM-dependent phosphatase, calcineurin, which targets gonadotrophin gene expression both directly and indirectly via transcription factors such as nuclear factor of activated T-cells and Nur77, the Transducer of Regulated CREB (TORC) co-activators and also the prolyl isomerase, Pin1. Gonadotrophin gene expression is also regulated by GnRH-induced CaM-dependent kinases (CaMKs); CaMKI is able to derepress the histone deacetylase-inhibition of β-subunit gene expression, whereas CaMKII appears to be essential for the GnRH-activation of all three subunit genes. Asides from activating gonadotrophin gene expression, GnRH also exerts additional effects on gonadotroph function, some of which clearly occur via CaM, including the proliferation of immature gonadotrophs, which is dependent on calcineurin. In this review, we summarise these pathways, and discuss the additional functions that have been proposed for CaM with respect to modifying GnRH-induced signalling pathways via the regulation of the small GTP-binding protein, Gem, and/or the regulator of G-protein signalling protein 2.

Dioxin silences gonadotropin expression in perinatal pups by inducing histone deacetylases: a new insight into the mechanism for the imprinting of sexual immaturity by dioxin

Maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes the impairment of reproduction and development in the pups. Our previous studies have revealed that maternal treatment with TCDD attenuates the fetal production of pituitary gonadotropins (luteinizing hormone (LH) and follicle-stimulating hormone) at gestational day (GD) 20, leading to the impairment of sexual behavior in adulthood. However, the mechanism underlying such a reduction has remained unknown until now. When pregnant rats at GD15 were given an oral dose of TCDD (1 μg/kg), the testicular expression of steroidogenic proteins was reduced between GD20 and postnatal days (PND) 2. In accordance with this, the pituitary expression of gonadotropin β-subunit and serum gonadotropin were also attenuated from GD20 to PND0 in a pup-specific fashion. To identify the target genes linked to a fetal reduction in gonadotropin β-subunit, we performed a DNA microarray analysis using the fetal pituitary and its regulatory organ, the hypothalamus. The results obtained showed that TCDD induced histone deacetylases (HDACs) in the fetal pituitary. In support with this, TCDD markedly deacetylated histones H3 and H4 twined around the promoter of the fetal LHβ gene. This effect was fetus- and LHβ-specific, and this was not observed in the maternal pituitary or for other pituitary hormone genes. Finally, an LHβ reduction caused by TCDD was completely restored by maternal co-treatment with valproic acid, an HDAC inhibitor. These results strongly suggest that the increased deacetylation of histone owing to HDAC induction plays a critical role in the TCDD-induced reduction in LHβ in the fetal pituitary.

GnRH regulation of Jun and Atf3 requires calcium, calcineurin, and NFAT

GnRH binds to its receptor on gonadotropes and activates multiple members of the MAPK signaling family that in turn regulates the expression of several immediate early genes (IEGs) including Jun, Fos, Atf3, and Egr1. These IEGs confer hormonal responsiveness to gonadotrope-specific genes including Gnrhr, Cga, Fshb, and Lhb. In this study we tested the hypothesis that GnRH specifically regulates the accumulation of Jun and Atf3 mRNA through a pathway that includes intracellular Ca²⁺, calcineurin, and nuclear factor of activated T cells (NFAT). Our results indicate that pretreatment of murine LβT2 cells with 1, 2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)-ester, a Ca²⁺ chelator, reduced the expression of all the IEGs to varying degrees, whereas treatment with thapsigargin, an intracellular Ca²⁺ protein pump inhibitor, increased the expression of the IEG. Furthermore, cyclosporin A, a calcineurin-specific inhibitor, reduced the ability of GnRH to regulate accumulation of Jun and Atf3 mRNA and to a lesser extent Fos. In contrast, Egr1 mRNA was unaffected. NFATs are transcription factors regulated by calcineurin and were detected in LβT2 cells. GnRH increased luciferase activity of an NFAT-dependent promoter reporter that was dependent on intracellular Ca²⁺ and calcineurin activity. Additionally, although small interfering RNA specific for Nfat4 only marginally reduced GnRH regulation of Jun, Fos, and Atf3 mRNA accumulation, activity of an activator protein-1-responsive reporter construct was reduced by 48%. Together these data suggest that calcineurin and NFAT are new members of the gonadotrope transcriptional network that confer hormonal responsiveness to several key genes required for gonadotropin synthesis and secretion.

Histone deacetylases regulate gonadotropin-releasing hormone I gene expression via modulating Otx2-driven transcriptional activity

BACKGROUND

Precise coordination of the hypothalamic-pituitary-gonadal axis orchestrates the normal reproductive function. As a central regulator, the appropriate synthesis and secretion of gonadotropin-releasing hormone I (GnRH-I) from the hypothalamus is essential for the coordination. Recently, emerging evidence indicates that histone deacetylases (HDACs) play an important role in maintaining normal reproductive function. In this study, we identify the potential effects of HDACs on Gnrh1 gene transcription.

METHODOLOGY/PRINCIPAL FINDINGS

Inhibition of HDACs activities by trichostatin A (TSA) and valproic acid (VPA) promptly and dramatically repressed transcription of Gnrh1 gene in the mouse immortalized mature GnRH neuronal cells GT1-7. The suppression was connected with a specific region of Gnrh1 gene promoter, which contains two consensus Otx2 binding sites. Otx2 has been known to activate the basal and also enhancer-driven transcription of Gnrh1 gene. The transcriptional activity of Otx2 is negatively modulated by Grg4, a member of the Groucho-related-gene (Grg) family. In the present study, the expression of Otx2 was downregulated by TSA and VPA in GT1-7 cells, accompanied with the opposite changes of Grg4 expression. Chromatin immunoprecipitation and electrophoretic mobility shift assays demonstrated that the DNA-binding activity of Otx2 to Gnrh1 gene was suppressed by TSA and VPA. Overexpression of Otx2 partly abolished the TSA- and VPA-induced downregulation of Gnrh1 gene expression.

CONCLUSIONS/SIGNIFICANCE

Our data indicate that HDAC inhibitors downregulate Gnrh1 gene expression via repressing Otx2-driven transcriptional activity. This study should provide an insight for our understanding on the effects of HDACs in the reproductive system and suggests that HDACs could be potential novel targets for the therapy of GnRH-related diseases.

Hormonal signaling to follicle stimulating hormone beta-subunit gene expression

Expression of the hormone-specific beta-subunit of follicle stimulating hormone (FSHbeta) is regulated primarily by gonadotropin releasing hormone (GnRH) and activin, with additional feedback by various steroids. While the nature of this hormonal regulation appears conserved, the molecular mechanisms mediating these effects appear less so. This is apparent from the diverse cis-elements required for hormonal stimulation in different species, distinct transcription factors that seem to mediate the effects, as well as the lack of conservation of several reportedly functional cis-elements across species. Recent additional information on the molecular mechanisms through which these regulatory hormones exert their effects, supports the possibility of species-specific mechanisms of regulation, while some redundancy may exist in signaling by the activated transcription factors which allows preservation of the hormonal regulation in these different promoter contexts.

The biology of gonadotroph regulation

PURPOSE OF REVIEW

To discuss recent progress in our understanding of pituitary gonadotroph development and gonadotropin gene regulation, with an emphasis on differential luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion and subunit synthesis, and the implications this may have on female reproductive health.

RECENT FINDINGS

In the mature gonadotroph, there is an emerging concept that differential synthesis of gonadotropin beta-subunit genes, essential for cyclic reproductive function, is associated with modification of activation and/or stability of important regulatory proteins and transcription factors. Recent studies suggest that cellular events, which affect histone modification, play an essential role in both gonadotroph development and the ontogeny of gonadotropin subunit gene expression. Such dynamic events are under the orchestration of the hypothalamic neuropeptide gonadotropin-releasing hormone (GnRH), potentially through the ability of GnRH to activate several distinct signaling cascades within the gonadotroph.

SUMMARY

Greater insight into the cellular events that are key to gonadotroph physiology will contribute to our understanding of abnormal gonadotropin secretion in disorders such as hypothalamic amenorrhea and polycystic ovarian syndrome (PCOS), and provide a context for the design of novel therapeutic approaches.

The molecular regulation of Chinook salmon gonadotropin beta-subunit gene transcription

Expression of the gonadotropin beta-subunit genes is tightly regulated both cell-specifically and by the regulatory hormones to achieve the appropriate gonadotropic hormone levels required for reproductive development and function. Although the cDNA sequences of these genes are highly conserved across species, their promoter sequences are not and few functional studies have been carried out to understand the molecular mechanisms through which their expression is regulated. We and others have carried out several studies on the LHbeta gene promoter of Chinook salmon (Oncorhynchus tschawytscha), and also isolated the FSHbeta gene from the same species. We present here a review of these studies and also novel data pertaining to both genes, in an attempt to collate the current understanding of the molecular regulation of the gonadotropin beta-subunit genes in these fish.