Molecular mechanisms of estrogen action in female genital tract development
Introduction
In eutherian mammals the female reproductive tract consists of the gonads (ovaries), oviducts, uterus/i, cervix/ices, and vagina. The female reproductive tract of non-eutherian mammals is divergent in development and function, and thus lies out of the scope of this review. While the eutherian female reproductive tract consists of conserved organs, the developmental timeline of the reproductive tract varies depending on the organism in question. With respect to human and murine female reproductive tract development, superficial differentiation of stromal and uterine glandular tissue occurs gestationally by 21 weeks in humans, where in mice it occurs postnatally at day 9 (Gray et al., 2001). Additionally, murine Müllerian (paramesonephric) duct development and elongation occurs until E13.5, and differentiation of these tissues into distinct gland containing structures isn't histologically detected several weeks after birth (Gray et al., 2001; Orvis and Behringer, 2007; Taylor et al., 1997). In humans, the Müllerian structures are morphologically formed by 22 weeks of gestation, roughly half way through the prenatal period (Cunha et al., 2018). These differences in development over the gestational period require analysis of female reproductive tract development based on morphology and gene expression at multiple developmental time points, rather than linear gestational staging, or post-natal development staging, for comparative endocrinology and comparative developmental analysis. Therefore, the impact of estrogens in the context of female reproductive tract can be broken into three separate time points: Müllerian duct organogenesis, pre-sexual maturation development, and post-sexual maturation hormonal regulation.
Estrogens are steroid hormones that are necessary for the development of female sex characteristics and regulation of the adult female reproductive system's function. In humans and non-human primates, there are four main molecules that make up the family of estrogens: estrone (E1), estradiol (E2), estriol (E3) and estetrol (E4), which are different from one another by the number of hydroxyl groups attached to the core steroidal structure (Fig. 1A). The primary site of estrogen steroidogenesis in the adult human/non-human primate female is the ovaries, where estrone and estradiol biosynthesis occurs via the uptake of cholesterol precursors and subsequent enzymatic processing or steroidogenesis (Miller and Auchus, 2011). While estradiol biosynthesis decreases after menopause in humans, estrone biosynthesis continues in peripheral tissues and becomes the primary circulating estrogen post-menopause (Judd et al., 1976). The remaining human/non-human primate estrogens, estriol and estetrol are primarily produced during pregnancy by placental enzymatic processing of fetal-adrenal synthesized 16α-DHEAS and maternal estradiol, respectively (Fig. 2) (Falah et al., 2015; Holinka et al., 2008). The production of estrogens in the maternal-fetal unit are not affected by the sex of the fetus, and thus while estrogens can be thought of as hormones associated with female secondary sexual development, levels in fetal circulation do not differ between male and female fetuses (Enninga et al., 2015).
At a genetic level, organogenesis is controlled by a series of transcription factors including homeobox genes (HOX), which are responsible for axial patterning of invertebrates and vertebrates (Du and Taylor, 2015; Lewis, 1978; McGinnis and Krumlauf, 1992). In mammals, there are four paralogous clusters of homeobox genes (HOX A-D), containing 39 hox genes, organized in an anterior 3′ to posterior 5’ organization across several chromosomes (Parker, 2020). In the nascent female reproductive tract, the Müllerian ducts express, in an anterior to posterior order, Hoxa9, Hoxa10, Hoxa11, and Hoxa13 (Du and Taylor, 2015; Taylor et al., 1997). While the expression of these Hox genes is not limited to the female reproductive tract, disruption of Hoxa10 and Hoxa11 has been shown to lead to infertility in mice by preventing proper Müllerian duct formation, and normal uterine organogenesis (Benson et al., 1996; Hsieh-Li et al., 1995; Warot et al., 1997). The transcription factors and extra-cellular interactions that lead to the patterned activation of Hox cluster genes along the anterior to posterior axis of the Müllerian duct have not been identified at this time, however the activation of specific Hox clusters appear to be regulated using an interplay of classical epigenetic chromatin regulation (histone/DNA modification), 3D chromatin dynamics, and lncRNA interactions with the HOX loci (Mallo and Alonso, 2013).
While Hox genes play a functional role in the early stages of organogenesis, these functions in the early female reproductive tract (Müllerian ducts) are estrogen independent. Several studies have utilized knockouts of the estrogen receptors Esr1 (ERα) and Esr2 (ERβ) (Curtis et al., 1999; Krege et al., 1998), and found no gross morphological changes to early uterine development, or female reproductive tract development. These studies are elegant in that while steroidogenic pathways are left unperturbed, the receptors that recognize and potentiate the effects of the estrogens are knocked out from the organism. While estrogen signaling has no impact on early Mullerian duct differentiation and gross patterning for the oviducts, uterus, cervix, and upper vagina, these tissues do becomes sensitive to estrogen signaling in the context of the ongoing differentiation of functional epithelial tissues such as the uterine endometrium (Lubahn et al., 1993). This is in contrast to the development of the external genitalia, including the lower segment of the vagina, where sex steroids play a role in early organogenesis and cellular differentiation (Blaschko et al., 2012; Kurita 2010; Suk Kim et al., 2004; Suzuki et al., 2002).
Section snippets
Estrogen signaling pathways in female reproductive tract development pre-sexual maturation
While data from estrogen receptor knockout studies indicates that estrogen signaling has little impact on early Hox patterning of the developing Müllerian tract, clinical experiences in the 1980's from patients exposed to the synthetic estrogen Diethylstilbestrol (DES) showed an impact of estrogens in reproductive tract development. DES is a synthetic estrogen-like compound (xenoestrogen) that binds to ERα/ERβ with a four-fold increased relative binding affinity compared to Estradiol (Blair et
Canonical estrogen receptor signaling
Canonical estrogen receptor signaling occurs when either ERα or ERβ proteins bind to a ligand (such as E2), and form homodimers. These homodimers then bind to exposed segments of DNA with a specific sequence identified as the Estrogen Response Element (ERE) (Klein-Hitpass et al., 1988) and act to recruit transcription factors leading to the upregulation of the gene downstream from this promotor site. Given the degeneracy of the ERE sequence, in silica analysis of the murine genome has found
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