Comparative transcriptomic analysis of follicle-enclosed oocyte maturational and developmental competence acquisition in two non-mammalian vertebrates

<p>Abstract</p> <p>Background</p> <p>In vertebrates, late oogenesis is a key period during which the oocyte acquires its ability to resume meiosis (<it>i.e</it>. maturational competence) and to develop, once fertilized, into a normal embryo (<it>i.e</it>. developmental competence). However, the molecular mechanisms involved in these key biological processes are far from being fully understood. In order to identify key mechanisms conserved among teleosts and amphibians, we performed a comparative analysis using ovarian tissue sampled at successive steps of the maturational competence acquisition process in the rainbow trout (<it>Oncorhynchus mykiss</it>) and in the clawed toad (<it>Xenopus laevis</it>). Our study aimed at identifying common differentially expressed genes during late oogenesis in both species. Using an existing transcriptomic analysis that had previously been carried out in rainbow trout, candidate genes were selected for subsequent quantitative PCR-based comparative analysis.</p> <p>Results</p> <p>Among the 1200 differentially expressed clones in rainbow trout, twenty-six candidate genes were selected for further analysis by real-time PCR in both species during late oogenesis. Among these genes, eight had similar expression profiles in trout and <it>Xenopus</it>. Six genes were down-regulated during oocyte maturation (<it>cyp19a1, cyp17a1, tescalcin, tfr1, cmah, hsd11b3</it>) while two genes exhibited an opposite pattern (<it>apoc1, star</it>). In order to document possibly conserved molecular mechanisms, four genes (<it>star, cyp19a1, cyp17a1 </it>and <it>hsd11b3</it>) were further studied due to their known or suspected role in steroidogenesis after characterization of the orthology relationships between rainbow trout and <it>Xenopus </it>genes. <it>Apoc1 </it>was also selected for further analysis because of its reported function in cholesterol transport, which may modulate steroidogenesis by regulating cholesterol bioavailability in the steroidogenic cells.</p> <p>Conclusions</p> <p>We have successfully identified orthologous genes exhibiting conserved expression profiles in the ovarian follicle during late oogenesis in both trout and <it>Xenopus</it>. While some identified genes were previously uncharacterized during <it>Xenopus </it>late oogenesis, the nature of these genes has pointed out molecular mechanisms possibly conserved in amphibians and teleosts. It should also be stressed that in addition to the already suspected importance of steroidogenesis in maturational competence acquisition, our approach has shed light on other regulatory pathways which may be involved in maturational and developmental competence acquisitions that will require further studies.</p

Caractérisation d'acteurs moléculaires impliqués dans l'acquisition de la compétence ovocytaire à la maturation et au développement chez deux vertébrés non mammaliens

In vertebrates, the follicle-enclosed growing oocyte progressively acquires both maturational and developmental competence (ability to resume meiosis and to develop once fertilized, respectively). The aim of this work was to characterize molecular actors involved in acquisition of oocyte competence in two non-mammalian vertebrates: the rainbow trout (Oncorhynchus mykiss) and the clawed toad Xenopus (Xenopus laevis). Eight genes have been studied: star, cyp19a1, cyp17a1, hsd11b3, apoC1, cmah, tfr1 and tesc. We evidenced in both species the expression by the oocyte of a functional aromatase (cyp19a1) that had been under-estimated so far. Moreover, the identification of the oocyte expression of a transcriptional cofactor of the estradiol receptor, Rbm39, contributes to stress the significance of estradiol action in regulating the acquisition of vertebrate oocyte maturational and/or developmental competence.Chez les vertébrés, l'ovocyte acquiert progressivement lors de sa croissance une double compétence à la maturation et au développement (capacités à reprendre sa méiose et à se développer une fois fécondé). L’objectif de ce travail était de caractériser de nouveaux acteurs moléculaires impliqués dans cette acquisition de compétences chez 2 vertébrés non mammaliens : la truite arc-en-ciel (Oncorhynchus mykiss) et le xénope (Xenopus laevis). Huit gènes ont été identifiés comme différentiellement exprimés au cours de cette acquisition de compétence : star, cyp19a1, cyp17a1, hsd11b3, apoC1, cmah, tfr1 et tesc. La mise en évidence de l’expression d’une aromatase (cyp19a1) fonctionnelle par l’ovocyte des deux espèces suggère une activité œstrogénique ovocytaire. De plus, l'identification d'une expression ovocytaire régulée de Rbm39, un cofacteur transcriptionnel du récepteur aux œstrogènes, contribue à renforcer l’importance de l’œstradiol lors de l’acquisition de ces compétences par l’ovocyte chez les vertébrés

Acquisition of developmental competence by the follicle-enclosed oocyte: a comparative genomic approach in lower vertebrates

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Aromatase is expressed and active in the rainbow trout oocyte during final oocyte maturation.

International audienceWhile it is generally well accepted that the ovarian follicular sites of estradiol-17β (E2) synthesis are restricted to somatic cells, the possible contribution of the germinal compartment has received little or no attention in teleosts. In order to demonstrate the expression of ovarian aromatase in the oocyte, cyp19a1a mRNA was studied in ovarian follicles by in situ hybridization. In addition, the expression of cyp19a1a was studied in both somatic and germinal compartments of the ovarian follicle in rainbow trout (Oncorhynchus mykiss) during final oocyte maturation (i.e., maturational competence acquisition and subsequent meiosis resumption) by real-time PCR. The enzymatic activity of ovarian aromatase was also studied in both somatic and germinal compartments of the ovarian follicle. Finally, E2 levels were monitored in follicle-enclosed oocytes throughout the pre-ovulatory period. We were able to demonstrate a significant ovarian aromatase expression and activity in the late vitellogenic oocyte. Furthermore, a dramatic decrease in aromatase expression and activity occurs in the oocyte during late oogenesis, concomitantly with the trend observed in surrounding follicular layers. We also report an unexpected increase of E2 levels in the oocyte during the pre-ovulatory period. To our knowledge, these observations are reported for the first time in any teleost species. Together, our data support the hypothesis of the participation of the germinal compartment in follicular estrogen synthesis and a biological role of E2 during oocyte and/or early embryo development