Aromatase Is Expressed and Active in Growing Oocytes of Two Non-Mammalian Vertebrates (Xenopus laevis and Oncorhynchus mykiss).

Abstract

Numerous reports have previously demonstrated that ovarian follicles synthesize the three major classes of sex steroids (androgens, estrogens, and progestins) in amphibians and fish. Precisely, during late oogenesis, a shift in steroidogenesis occurs from estradiol-17β (E2) to maturation-inducing steroid production by the ovarian follicles. While most studies have focused on the steroidogenic capacity of somatic follicular layers, the oocyte compartment has received far less attention. However, a previous study identified cyp17 activity exclusively in Xenopus laevis oocytes, suggesting that Xenopus oocytes may produce androgens during late oogenesis and particularly during oocyte maturation. Somatic cells within the ovarian follicle synthesize E2 mainly during oocyte growth, also called vitellogenesis. The aim of the present study was to determine if the enzyme responsible for E2 synthesis, the aromatase, was also expressed in Xenopus oocyte during late vitellogenesis. Moreover in order to further investigate whether oocyte contribution to E2 synthesis was a mechanism shared by evolutionary distant non-mammalian vertebrates, we performed similar experiments in a teleost fish, the rainbow trout (Oncorhynchus mykiss) in which follicular steroidogenesis has been thoroughly documented. Using In Situ Hybridization, quantitative RT-PCR and aromatase activity measurements, we demonstrate that ovarian aromatase is not only expressed and active in the follicular cells but also in vitellogenic oocytes of both species. Moreover oocyte aromatase expression and activity decreased during late oogenesis concomitantly with the trend observed in surrounding follicular layers. Nevertheless we identified species features: Aromatase expression and activity measured in Xenopus vitellogenic oocytes correspond to 25% of that in the corresponding whole follicle versus 3% only in rainbow trout oocytes. This lower contribution of the rainbow trout germinal compartment to follicular aromatase activity may reflect existing biological differences between Xenopus and trout which exhibit distinct ovarian development types (asynchronous vs group-synchronous, respectively). Thus, E2 synthesized by the Xenopus oocyte may be directly used or stored by the oocyte itself and would not influence the development of other oocytes. In rainbow trout, as all vitellogenic oocytes develop simultaneously, it is hypothesized that E2 produced by germ and somatic cells is secreted and will control the whole ovarian development. Discrepancies of oocyte contribution between both species may also result from the different methodologies used to collect oocyte content. Altogether our results suggest that oocyte synthesize E2 during its growth in both species. Nevertheless the exact function of E2 production by the oocyte remains to be clarified. Supported by ANR-08-GENM-033 OSCILE. (platform)