Regulatory effects of 5 beta-reduced steroids

Extraembryonic metabolism of corticosterone protects against effects of exposure

When females experience stress during reproduction, developing embryos can be exposed to elevated levels of glucocorticoids, which can permanently affect offspring development, physiology, and behavior. However, the embryo can regulate exposure to glucocorticoids. In placental species, the placenta regulates embryonic exposure to maternal steroids via metabolism. In a comparable way, recent evidence has shown the extraembryonic membranes of avian species also regulate embryonic exposure to a number of maternal steroids deposited in the yolk via metabolism early in development. However, despite the known effects of embryonic exposure to glucocorticoids, it is not yet understood how glucocorticoids are metabolized early in development. To address this knowledge gap, we injected corticosterone into freshly laid chicken (Gallus gallus) eggs and identified corticosterone metabolites, located metabolomic enzyme transcript expression, tracked metabolomic enzyme transcript expression during the first six days of development, and determined the effect of corticosterone and metabolites on embryonic survival. We found that yolk corticosterone was metabolized before day four of development into two metabolites: 5β-corticosterone and 20β-corticosterone. The enzymes, AKR1D1 and CBR1 respectively, were expressed in the extraembryonic membranes. Expression was dynamic during early development, peaking on day two of development. Finally, we found that corticosterone exposure is lethal to the embryos, yet exposure to the metabolites is not, suggesting that metabolism protects the embryo. Ultimately, we show that the extraembryonic membranes of avian species actively regulate their endocrine environment very early in development.

Characterizing the timing of yolk testosterone metabolism and the effects of etiocholanolone on development in avian eggs

Maternal transfer of steroids to eggs can elicit permanent effects on offspring phenotype. Although testosterone was thought to be a key mediator of maternal effects in birds, we now know that vertebrate embryos actively regulate their exposure to maternal testosterone through steroid metabolism, suggesting testosterone metabolites, not testosterone, may elicit the observed phenotypic effects. To address the role steroid metabolism plays in mediating yolk testosterone effects, we used European starling (Sturnus vulgaris) eggs to characterize the timing of testosterone metabolism and determine whether etiocholanolone, a prominent metabolite of testosterone in avian embryos, is capable of affecting early embryonic development. Tritiated testosterone was injected into freshly laid eggs to characterize steroid movement and metabolism during early development. Varying levels of etiocholanolone were also injected into eggs, with incubation for either 3 or 5 days, to test whether etiocholanolone influences the early growth of embryonic tissues. The conversion of testosterone to etiocholanolone was initiated within 12 h of injection, but the increase in etiocholanolone was transient, indicating that etiocholanolone is also subject to metabolism, and that exposure to maternal etiocholanolone is limited to a short period during early development. Exogenous etiocholanolone manipulation had no significant effect on the growth rate of the embryos or extra-embryonic membranes early in development. Thus, the conversion of testosterone to etiocholanolone may be an inactivation pathway that buffers the embryo from maternal steroids, with any effects of yolk testosterone resulting from testosterone that escapes metabolism; alternatively, etiocholanolone may influence processes other than growth or take additional time to manifest.

In ovo metabolism of progesterone to 5β-pregnanedione in chicken eggs: Implications for how yolk progesterone influences embryonic development

Progesterone has received substantial attention for the essential role it plays in establishing and maintaining pregnancy in placental vertebrates. Despite the prevalence of progesterone during development, relatively little is known about how embryos respond to progesterone. This is true of placental vertebrates as well as egg-laying vertebrates where levels of progesterone in the yolk tend to be higher than most other steroids in the yolk. Bird eggs provide an opportunity to investigate the effects of progesterone on embryonic development because progesterone can be easily manipulated without any confounding effects on maternal physiology. To understand how progesterone might influence embryonic development, it is important to characterize the metabolic fate of progesterone given its potential to be converted to a wide range of steroids. We investigated the metabolic fate of tritiated progesterone over the first four days of development using chicken eggs (Gallus gallus) and identified 5β-pregnanedione as the primary metabolite during this period. After only one day of development, 5β-pregnanedione could be detected within the yolk. Levels of 5β-pregnanedione in both the yolk and albumen tended to rise early in development but conjugated metabolites began to accumulate towards the end of our sampling period. Additionally, in vitro assays using embryo homogenates collected after 72 h of development demonstrated that embryos were capable of carrying out the conversion of progesterone to 5β-pregnanedione. Overall these results have important implications for deciphering the mechanisms through which yolk progesterone might influence embryonic development. Effects could arise via progesterone receptors or receptors capable of binding 5β-pregnanedione but we found no evidence that progesterone is serving as a precursor for androgen or estrogen production.

Steroids in chicken egg yolk: metabolism and uptake during early embryonic development

Effects of maternal hormones may adaptively adjust offspring development to prevailing conditions. However, Darwinian fitness of parents is maximized by investing in more than one offspring while each individual offspring benefits from receiving maximal investment. The control of mother and offspring over hormone-mediated maternal effects is thought to play a key role in the outcome of parent-offspring conflict, but these control mechanisms have hardly been studied. We investigated the potential embryonic control by analysing the changes in distribution and metabolism of steroid hormones in the egg during the first 6 days of incubation using injections of radiolabelled testosterone and corticosterone in freshly laid eggs. After 1 day of incubation the highest amount of radioactivity was concentrated in a small area at the top of the yolk. This challenges the use of hormones in oil as mimicking natural exposure. During incubation radioactivity spread within the egg with highest concentrations in yolk and yolk sac and lower concentrations in albumen, embryo, allantois, and amnion. Steroids were metabolised to other unconjugated and conjugated steroids, perhaps facilitating embryonic steroid uptake. Our study shows that the injected radiolabel is metabolised in the egg and taken up by the embryo, giving the embryo potential control over the effects of maternal hormones and thereby limiting maternal control over the outcome of hormone-mediated maternal effects.

Steroid metabolism in granulosa and theca interna cells from preovulatory follicles of domestic hen (Gallus domesticus)

The capability of granulosa and theca interna cells, from preovulatory follicles of the domestic hen, to metabolize steroid precursors was evaluated. Granulosa and theca interna cells were isolated from ovarian preovulatory follicles at three different developmental stages: F1, F3 and F5. Tritiated pregnenolone (P5), progesterone (P4), dehydroepiandrosterone (DHEA), androstenedione (A4) and testosterone (T) were employed as precursors and their metabolic products were evaluated. The major metabolite of P5 by granulosa cells was P4, but we also observed low amounts of 5beta-pregnandione. DHEA metabolism by granulosa cells yielded mainly A4, and minute quantities of 5beta-androstan-3,17-dione (5beta-dione) were detected. The only significant metabolite obtained in granulosa cells from A4 was 5beta-dione, whereas T was only transformed into A4. On the other hand, P5 metabolism by theca interna cells yielded A4 as the main product, also P4, 17alpha-OHP4, 17alpha-OHP5, 5beta-pregnandione, and DHEA, were found. When DHEA was the precursor A4 was produced in higher amounts than 5beta-dione. A4 was mainly transformed into 5beta-dione. In similar conditions, T was transformed into A4. These results show that granulosa cells have enzymatic activities of 3beta-hydroxysteroid dehydrogenase/5-4 isomerase (3beta-HSD from P5 and DHEA), 17beta-hydroxysteroid dehydrogenase (17beta-HSD from T) and 5beta-reductase (from P5, DHEA and A4). Whereas theca interna cells have enzymatic activities of cytochrome P450c17 (from P5 and P4), 3beta-HSD (from P5 and DHEA), 17beta-HSD (from T) and 5beta-reductase (from P4, DHEA and A4). These data support the concept that theca interna cells have the ability to synthesize androgens from progestins produced in granulosa cells. In addition, since theca interna cells did not show the capacity to aromatize androgens suggests that interaction between theca interna and theca externa cells occurs in vivo, thus confirming the three cell model for estrogen production. Furthermore, the fact that other metabolites were produced both in granulosa and theca interna cells, but in a different extent, suggests that complex mechanisms are participating in the regulation of steroid synthesis in avian ovary follicles.

Comparative modulation by 3 alpha,5 alpha and 3 beta,5 beta neurosteroids of GABA binding sites during avian central nervous system development

Neurosteroids are endogenous Central Nervous System (CNS) compounds which act mainly by allosteric modulation of the GABAA receptor complex. The presence of a 3 alpha-hydroxyl group and a 5 alpha-hydrogen atom have been found to be essential structural requirements for biological activity in mammals. In the present work we report the enhancing activity on [3H]GABA binding to its receptor sites in chick optic lobe produced by progesterone metabolites 3 alpha-hydroxy,5 alpha-pregnan-20-one (3 alpha,5 alpha-P) and 3 beta-hydroxy,5 beta-pregnan-20-one (3 beta,5 beta-P). Both steroids were found able to enhance [3H]GABA binding along ontogeny, displaying a similar profile at early developmental stages, while in adulthood 3 alpha,5 alpha-P had greater potency (EC50 0.22 microM) and enhancing effect (Emax: 122%). In adult synaptic membranes, the two compounds displayed a complex interaction with the GABAA receptor, disclosed by a Schild plot with slope below one and an incomplete displacement of 3 alpha,5 alpha-P by its 3 beta,5 beta isomer. Such complexity could be related to the steroidogenic profile in avian CNS, with 5 alpha-reduced progesterone metabolites present since early development, while 3 alpha,5 alpha-P is found only in adulthood. Bearing in mind differences between avian and mammalian steroidogenic profiles and the relevance of 5 beta-steroids in early avian development, we propose that 3 beta,5 beta-P, instead of the classical potent 3 alpha,5 alpha-steroids, may be the endogenous modulator of GABAergic activity in developing avian brain.

3 beta-OH-5 beta-pregnan-20-one enhances [3H]GABA binding in developing chick optic lobe

The neurosteroids are synthesized in the CNS and act mainly through allosteric modulation of the GABAA receptor. Structure-activity relationship studies in mammalian CNS have shown that a 3 alpha-hydroxyl group and a 5 alpha-reduced A-ring are striking features for their biological activity, while the 3 beta,5 beta structures as in 3 beta,5 beta-P are completely inactive. In this work we report the enhancing activity of epipregnanolone on [3H]GABA binding to its receptor sites in the chick optic lobe. Concentration-effect curves for this neurosteroid showed a concentration-dependent activity with different potencies at the three developmental stages studied, the hatching stage being the most sensitive to the steroid stimulatory effect. The displacement of a potent 3 alpha,5 alpha steroid by epipregnanolone indicated that this steroid behaved as a partial agonist of the steroid recognition site. Considering the developmental profile for steroidogenesis in avian tissues and the biological relevance of 5 beta-reduced steroids in early development, we propose that 3 or its 3 alpha-epimer, pregnanolone, instead of the potent 3 alpha,5 alpha neurosteroids, modulates GABAA receptors in the chick optic lobe during development.