Initiation of Steroidogenesis Precedes Expression of Cholesterologenic Enzymes in the Fetal Mouse Testes

Placental structure, function, and mitochondrial phenotype relate to fetal size in each fetal sex in mice†

Fetal growth depends on placental function, which requires energy from mitochondria. Here we investigated whether mitochondrial function in the placenta relates to the growth of the lightest and heaviest fetuses of each sex within the litter of mice. Placentas from the lightest and heaviest fetuses were taken to evaluate placenta morphology (stereology), mitochondrial energetics (high-resolution respirometry), mitochondrial regulators, nutrient transporters, hormone handling, and signaling pathways (qPCR and Western blotting). We found that mitochondrial complex I and II oxygen consumption rate was greater for placentas supporting the lightest female fetuses, although placental complex I abundance of the lightest females and complexes III and V of the lightest males were decreased compared to their heaviest counterparts. Expression of mitochondrial biogenesis (Nrf1) and fission (Drp1 and Fis1) genes was lower in the placenta from the lightest females, whilst biogenesis-related gene Tfam was greater in the placenta of the lightest male fetuses. In addition, placental morphology and steroidogenic gene (Cyp17a1 and Cyp11a1) expression were aberrant for the lightest females, but glucose transporter (Slc2a1) expression was lower in only the lightest males versus their heaviest counterparts. Differences in intra-litter placental phenotype were related to changes in the expression of hormone-responsive (androgen receptor) and metabolic signaling (AMPK, AKT, and PPARγ) pathways. Thus, in normal mouse pregnancy, placental structure, function, and mitochondrial phenotype are differentially responsive to the growth of the female and male fetus. This study may inform the design of sex-specific therapies for placental insufficiency and fetal growth abnormalities with life-long benefits for the offspring.

Contributions of Sex Chromosomes and Gonadal Hormones to the Male Bias in a Maternal Antibody-Induced Model of Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a group of neurodevelopmental conditions that is four times more commonly diagnosed in males than females. While susceptibility genes located in the sex chromosomes have been identified in ASD, it is unclear whether they are sufficient to explain the male bias or whether gonadal hormones also play a key role. We evaluated the sex chromosomal and hormonal influences on the male bias in a murine model of ASD, in which mice are exposed in utero to a maternal antibody reactive to contactin-associated protein-like 2 (Caspr2), which was originally cloned from a mother of a child with ASD (termed C6 mice henceforth). In this model, only male mice are affected. We used the four-core-genotypes (FCG) model in which the Sry gene is deleted from the Y chromosome (Y⁻) and inserted into autosome 3 (TgSry). Thus, by combining the C6 and FCG models, we were able to differentiate the contributions of sex chromosomes and gonadal hormones to the development of fetal brain and adult behavioral phenotypes. We show that the presence of the Y chromosome, or lack of two X chromosomes, irrespective of gonadal sex, increased the susceptibility to C6-induced phenotypes including the abnormal growth of the developing fetal cerebral cortex, as well as a behavioral pattern of decreased open-field exploration in adult mice. Our results indicate that sex chromosomes are the main determinant of the male bias in the maternal C6-induced model of ASD. The less dominant hormonal effect may be due to modulation by sex chromosome genes of factors involved in gonadal hormone pathways in the brain.

Cholesterol Contributes to Male Sex Differentiation Through Its Developmental Role in Androgen Synthesis and Hedgehog Signaling

Two specialized functions of cholesterol during fetal development include serving as a precursor to androgen synthesis and supporting hedgehog (HH) signaling activity. Androgens are produced by the testes to facilitate masculinization of the fetus. Recent evidence shows that intricate interactions between the HH and androgen signaling pathways are required for optimal male sex differentiation and defects of either can cause birth anomalies indicative of 46,XY male variations of sex development (VSD). Further, perturbations in cholesterol synthesis can cause developmental defects, including VSD, that phenocopy those caused by disrupted androgen or HH signaling, highlighting the functional role of cholesterol in promoting male sex differentiation. In this review, we focus on the role of cholesterol in systemic androgen and local HH signaling events during fetal masculinization and their collective contributions to pediatric VSD.

Cholesterol signaling in single cells: lessons from STAR and sm-FISH

Cholesterol is an important regulator of cell signaling, both through direct impacts on cell membranes and through oxy-metabolites that activate specific receptors (steroids, hydroxy-cholesterols, bile acids). Cholesterol moves slowly through and between cell membranes with the assistance of specific binding proteins and transfer processes. The prototype cholesterol regulator is the Steroidogenesis Acute Regulatory (STAR), which moves cholesterol into mitochondria, where steroid synthesis is initiated by cytochrome P450 11A1 in multiple endocrine cell types. CYP27A1 generates hydroxyl cholesterol metabolites that activate LXR nuclear receptors to control cholesterol homeostatic and transport mechanisms. LXR regulation of cholesterol transport and storage as cholesterol ester droplets is shared by both steroid-producing cells and macrophage. This cholesterol signaling is crucial to brain neuron regulation by astrocytes and microglial macrophage, mediated by ApoE and sensitive to disruption by β-amyloid plaques. sm-FISH delivers appreciable insights into signaling in single cells, by resolving single RNA molecules as mRNA and by quantifying pre-mRNA at gene loci. sm-FISH has been applied to problems in physiology, embryo development and cancer biology, where single cell features have critical impacts. sm-FISH identifies novel features of STAR transcription in adrenal and testis cells, including asymmetric expression at individual gene loci, delayed splicing and 1:1 association of mRNA with mitochondria. This may represent a functional unit for the translation-dependent cholesterol transfer directed by STAR, which integrates into mitochondrial fusion dynamics. Similar cholesterol dynamics repeat with different players in the cycling of cholesterol between astrocytes and neurons in the brain, which may be abnormal in neurodegenerative diseases.

SOX4 regulates gonad morphogenesis and promotes male germ cell differentiation in mice

The group C SOX transcription factors SOX4, -11 and -12 play important and mutually overlapping roles in development of a number of organs. Here, we examined the role of SoxC genes during gonadal development in mice. All three genes were expressed in developing gonads of both sexes, predominantly in somatic cells, with Sox4 being most strongly expressed. Sox4 deficiency resulted in elongation of both ovaries and testes, and an increased number of testis cords. While female germ cells entered meiosis normally, male germ cells showed reduced levels of differentiation markers Nanos2 and Dnmt3l and increased levels of pluripotency genes Cripto and Nanog, suggesting that SOX4 may normally act to restrict the pluripotency period of male germ cells and ensure their proper differentiation. Finally, our data reveal that SOX4 (and, to a lesser extent, SOX11 and -12) repressed transcription of the sex-determining gene Sox9 via an upstream testis-specific enhancer core (TESCO) element in fetal gonads, raising the possibility that SOXC proteins may function as transcriptional repressors in a context-dependent manner.

Origin and Differentiation of Androgen-Producing Cells in the Gonads

Sexual reproduction is dependent on the activity of androgenic steroid hormones to promote gonadal development and gametogenesis. Leydig cells of the testis and theca cells of the ovary are critical cell types in the gonadal interstitium that carry out steroidogenesis and provide key androgens for reproductive organ function. In this chapter, we will discuss important aspects of interstitial androgenic cell development in the gonad, including: the potential cellular origins of interstitial steroidogenic cells and their progenitors; the molecular mechanisms involved in Leydig cell specification and differentiation (including Sertoli-cell-derived signaling pathways and Leydig-cell-related transcription factors and nuclear receptors); the interactions of Leydig cells with other cell types in the adult testis, such as Sertoli cells, germ cells, peritubular myoid cells, macrophages, and vascular endothelial cells; the process of steroidogenesis and its systemic regulation; and a brief discussion of the development of theca cells in the ovary relative to Leydig cells in the testis. Finally, we will describe the dynamics of steroidogenic cells in seasonal breeders and highlight unique aspects of steroidogenesis in diverse vertebrate species. Understanding the cellular origins of interstitial steroidogenic cells and the pathways directing their specification and differentiation has implications for the study of multiple aspects of development and will help us gain insights into the etiology of reproductive system birth defects and infertility.

The Influence of Gonadal Steroid Hormones on Immunoreactive Kisspeptin in the Preoptic Area and Arcuate Nucleus of Developing Agonadal Mice with a Genetic Disruption of Steroidogenic Factor 1

Kisspeptin, a regulator of reproductive function and puberty in mammals, is expressed in the rostral (anteroventral) periventricular nucleus (AVPV) and arcuate nucleus (Arc), and its expression is at least partially regulated by estradiol in rodents. The aim of the present study was to determine contributions of genetic factors and gonadal steroid hormones to the sexual differentiation of kisspeptin-immunoreactive (kisspeptin-ir) cell populations in the AVPV and Arc during postnatal development using agonadal steroidogenic factor 1 (SF-1) knockout (KO) mice. To examine the effects of gonadal hormones on pubertal development of kisspeptin neurons, SF-1 KO mice were treated with estradiol benzoate (EB) from postnatal day (P)25 to P36, and their brains were examined at P36. No sex differences were observed in the SF-1 KO mice during postnatal development and after treatment with EB - which failed to increase the number of kisspeptin-ir cells at P36 to the levels found in wild-type (WT) control females. This suggests that specific time periods of estradiol actions or other factors are needed for sexual differentiation of the pattern of immunoreactive kisspeptin in the AVPV. Kisspeptin immunoreactivity in the Arc was significantly higher in gonadally intact WT and SF-1 KO females than in male mice at P36 during puberty. Further, in WT and SF-1 KO females, but not in males, adult levels were reached at P36. This suggests that maturation of the kisspeptin system in the Arc differs between sexes and is regulated by gonad-independent mechanisms.

Obif, a Transmembrane Protein, Is Required for Bone Mineralization and Spermatogenesis in Mice

BACKGROUND

Various kinds of transmembrane and secreted proteins play pivotal roles in development through cell-cell communication. We previously reported that Obif (Osteoblast induction factor, Tmem119), encoding a single transmembrane protein, is expressed in differentiating osteoblasts, and that Obif-/- mice exhibit significantly reduced bone volume in the femur. In the current study, we characterized the Obif protein and further investigated the biological phenotypes of a variety of tissues in Obif-/- mice.

RESULTS

First, we found that O-glycosylation of the Obif protein occurs at serine residue 36 in the Obif extracellular domain. Next, we observed that Obif-/- mice exhibit bone dysplasia in association with significantly increased osteoid volume per osteoid surface (OV/OS) and osteoid maturation time (Omt), and significantly decreased mineral apposition rate (MAR) and bone formation rate per bone surface (BFR/BS). In addition, we observed that Obif-/- mice show a significant decrease in testis weight as well as in sperm number. By histological analysis, we found that Obif is expressed in spermatocytes and spermatids in the developing testis and that spermatogenesis is halted at the round spermatid stage in the Obif-/- testis that lacks sperm. However, the number of litters fathered by male mice was slightly reduced in Obif-/- mice compared with wild-type mice, although this was not statistically significant.

CONCLUSIONS

Our results, taken together with previous observations, indicate that Obif is a type Ia transmembrane protein whose N-terminal region is O-glycosylated. In addition, we found that Obif is required for normal bone mineralization and late testicular differentiation in vivo. These findings suggest that Obif plays essential roles in the development of multiple tissues.

Conazole fungicides inhibit Leydig cell testosterone secretion and androgen receptor activation in vitro

Conazole fungicides are widely used in agriculture despite their suspected endocrine disrupting properties. In this study, the potential (anti-)androgenic effects of ten conazoles were assessed and mutually compared with existing data. Effects of cyproconazole (CYPRO), fluconazole (FLUC), flusilazole (FLUS), hexaconazole (HEXA), myconazole (MYC), penconazole (PEN), prochloraz (PRO), tebuconazole (TEBU), triadimefon (TRIA), and triticonazole (TRIT) were examined using murine Leydig (MA-10) cells and human T47D-ARE cells stably transfected with an androgen responsive element and a firefly luciferase reporter gene. Six conazoles caused a decrease in basal testosterone (T) secretion by MA-10 cells varying from 61% up to 12% compared to vehicle-treated control. T secretion was concentration-dependently inhibited after exposure of MA-10 cells to several concentrations of FLUS (IC₅₀ = 12.4 μM) or TEBU (IC₅₀ = 2.4 μM) in combination with LH. The expression of steroidogenic and cholesterol biosynthesis genes was not changed by conazole exposure. Also, there were no changes in reactive oxygen species (ROS) formation that could explain the altered T secretion after exposure to conazoles. Nine conazoles decreased T-induced AR activation (IC₅₀s ranging from 10.7 to 71.5 μM) and effect potencies (REPs) were calculated relative to the known AR antagonist flutamide (FLUT). FLUC had no effect on AR activation by T. FLUS was the most potent (REP = 3.61) and MYC the least potent (REP = 0.03) AR antagonist. All other conazoles had a comparable REP from 0.12 to 0.38. Our results show distinct in vitro anti-androgenic effects of several conazole fungicides arising from two mechanisms: inhibition of T secretion and AR antagonism, suggesting potential testicular toxic effects. These effects warrant further mechanistic investigation and clearly show the need for accurate exposure data in order to perform proper (human) risk assessment of this class of compounds.

Building the mammalian testis: origins, differentiation, and assembly of the component cell populations

Development of testes in the mammalian embryo requires the formation and assembly of several cell types that allow these organs to achieve their roles in male reproduction and endocrine regulation. Testis development is unusual in that several cell types such as Sertoli, Leydig, and spermatogonial cells arise from bipotential precursors present in the precursor tissue, the genital ridge. These cell types do not differentiate independently but depend on signals from Sertoli cells that differentiate under the influence of transcription factors SRY and SOX9. While these steps are becoming better understood, the origins and roles of many testicular cell types and structures-including peritubular myoid cells, the tunica albuginea, the arterial and venous blood vasculature, lymphatic vessels, macrophages, and nerve cells-have remained unclear. This review synthesizes current knowledge of how the architecture of the testis unfolds and highlights the questions that remain to be explored, thus providing a roadmap for future studies that may help illuminate the causes of XY disorders of sex development, infertility, and testicular cancers.

Immunohistochemical analysis of steroidogenic acute regulatory protein (StAR) and StAR-binding protein (SBP) expressions in the testes of mice during fetal development

The expression patterns of steroidogenic acute regulatory protein (StAR) and StAR-binding protein (SBP) in fetal Leydig cells were compared by using immunohistochemistry. While StAR immunoreactivity was detected during the first steps of testis differentiation, SBP expression was detected slightly later. The timing of SBP expression closely correlated with that of the testosterone surge, an event which is known to induce masculinization. Our results suggest that SBP plays an important role in male sexual development via interactions with StAR.

Expression of LH receptor in nonpregnant mouse endometrium: LH induction of 3β-HSD and de novo synthesis of progesterone

In mouse uterus, at the late diestrus stage LH binding sites have previously been described. The aim of our study was to confirm the existence of LH receptor (Lhr (Lhcgr)) mRNA and its protein in mouse endometrium. Endometrium at all stages of the estrous cycle contained Lhr mRNA, essentially identical to that found in mouse ovary. Endometrium also contained a 72  kDa immunoreactive receptor protein that bound to mouse anti-LHR antibody in western blot. Both receptor mRNA and protein were maximally expressed in the endometrium at metestrus and LH caused a significant increase in their expression levels. Endometrium also contained 3β-hydroxy steroid dehydrogenase (3β-hsd) mRNA and 3β-HSD protein. LH addition elevated their expression and activity as evident from increased conversion of labeled pregnenolone to progesterone (P(4)) and de novo P(4) synthesis. LH-induced endometrial P(4) synthesis is mediated through expression of steroidogenic acute regulatory (Star) gene. Results demonstrated that LH-induced P(4) synthesis in endometrium is possibly mediated through the cAMP pathway. Involvement of a MAPK pathway was also evident. Gonadotropin-stimulated endometrial P(4) synthesis was markedly attenuated by an antagonist of MEK1/2, PD98059. LH-stimulated MEK1/2-dependent phosphorylation of ERK1/2 in a concentration- and time-dependant manner in cultured endometrial tissues. Moreover, involvement of cAMP in LH-stimulated activation of ERK1/2 was also evident. It is therefore possible that the major signaling pathways regulating endometrial steroidogenesis in mouse, including the adenylate cyclase and MAP kinase pathways, converge at a point distal to activation of protein kinase A and ERK1/2.

Ontogeny of mammalian metabolizing enzymes in humans and animals used in toxicological studies

It is well recognized that expression of enzymes varies during development and growth. However, an in-depth review of this acquired knowledge is needed to translate the understanding of enzyme expression and activity into the prediction of change in effects (e.g. kinetics and toxicity) of xenobiotics with age. Age-related changes in metabolic capacity are critical for understanding and predicting the potential differences resulting from exposure. Such information may be especially useful in the evaluation of the risk of exposure to very low (µg/kg/day or ng/kg/day) levels of environmental chemicals. This review is to better understand the ontogeny of metabolizing enzymes in converting chemicals to either less-toxic metabolite(s) or more toxic products (e.g. reactive intermediate[s]) during stages before birth and during early development (neonate/infant/child). In this review, we evaluated the ontogeny of major "phase I" and "phase II" metabolizing enzymes in humans and commonly used experimental animals (e.g. mouse, rat, and others) in order to fill the information gap.

Species-specific dibutyl phthalate fetal testis endocrine disruption correlates with inhibition of SREBP2-dependent gene expression pathways

Fetal rat phthalate exposure produces a spectrum of male reproductive tract malformations downstream of reduced Leydig cell testosterone production, but the molecular mechanism of phthalate perturbation of Leydig cell function is not well understood. By bioinformatically examining fetal testis expression microarray data sets from susceptible (rat) and resistant (mouse) species after dibutyl phthalate (DBP) exposure, we identified decreased expression of several metabolic pathways in both species. However, lipid metabolism pathways transcriptionally regulated by sterol regulatory element-binding protein (SREBP) were inhibited in the rat but induced in the mouse, and this differential species response corresponded with repression of the steroidogenic pathway. In rats exposed to 100 or 500 mg/kg DBP from gestational days (GD) 16 to 20, a correlation was observed between GD20 testis steroidogenic inhibition and reductions of testis cholesterol synthesis endpoints including testis total cholesterol levels, Srebf2 gene expression, and cholesterol synthesis pathway gene expression. SREBP2 expression was detected in all fetal rat testis cells but was highest in Leydig cells. Quantification of SREBP2 immunostaining showed that 500 mg/kg DBP exposure significantly reduced SREBP2 expression in rat fetal Leydig cells but not in seminiferous cords. By Western analysis, total rat testis SREBP2 levels were not altered by DBP exposure. Together, these data suggest that phthalate-induced inhibition of fetal testis steroidogenesis is closely associated with reduced activity of several lipid metabolism pathways and SREBP2-dependent cholesterologenesis in Leydig cells.

The game plan: cellular and molecular mechanisms of mammalian testis development

In mammals, biological differences between males and females, which influence many aspects of their physical, social, and psychological environments, are solely determined genetically. In the presence of a Y chromosome, the gonadal primordium will differentiate into a testis, whereas in the absence of the Y chromosome an ovary will develop. Testis and ovary subsequently direct the differentiation of all secondary sex characteristics down the male and female pathway, respectively. The male-determining factor on the Y chromosome, SRY, was identified some 20 years ago. Since then, significant progress has been made toward understanding the molecular and cellular pathways that result in the formation of a testis. Here, we review what is known about testis differentiation in mice and humans, with reference to other species where appropriate.