The production of a vertebrate egg is a complex process controlled by systemic and ovarian hormones. Critical gaps exist in knowledge of the regulation of the early stages of development, including humans, when a variety of maternal materials that support early embryonic development are laid down in a coordinated fashion in the developing egg (oocyte). Thus, many aspects of "egg quality", the term used to describe an egg that can be successively fertilized and lead to a viable offspring, are determined during this time. Using a novel tissue culture system and molecular and cell biology approaches, this project will characterize the effects of sex steroid hormones on early development of oocytes of coho salmon, determine their modes of action, and determine the complex interplay between systemic and locally acting ovarian hormones in regulating development. It is anticipated that androgens and estrogens act in a steroid-specific and stage-dependent fashion, and that a number of novel, steroid-regulated genes will be identified. Because of the conserved nature of egg development in vertebrates, the research will improve fundamental knowledge on the systems regulating egg development in vertebrates, including humans and agriculturally important animals, permit the development of an experimental platform that will allow critical assessment of the potential impact of environmental endocrine disrupting contaminants that interfere with signaling by endogenous steroids, and may lead to the development of improved methods to overcome reproductive dysfunction of captive-reared fish in conservation and commercial aquaculture. This project will also have a significant impact on the education of a postdoctoral trainee, a MS student and undergraduate researchers.
This project investigated how early ovarian development of a teleost fish, the coho salmon is regulated by sex steroids, androgens and estrogens. Using a combination of in vivo and in vitro tissue culture approaches coupled with measurement of hormone levels and gene expression within the ovary, we have identified the physiological roles of steroids during two critical periods of early oogenesis: the progression of the primary oocyte from the early to late perinucleolar stage and the transition of primary oocytes into secondary (cortical alveoli) growth. Our major findings from in vitro studies are that androgens, once considered to merely act as substrate for estrogen production, have direct actions on the ovarian follicle, promoting growth and development of primary follicles. By contrast, estrogens have only minor actions on the primary follicle, but promote development of early secondary follicles. We have shown that in vivo treatment with either androgens or estrogens promote ovariandevelopment in a similar fashion, and we have assessed changes in expression in a number of ovarian genes during natural ovarian development. Both types of steroids appear to be necessary for ovarian follicles to survive. We have sequenced the transcriptomes of ovarian tissue from fish treated with androgens and shown that expression of hundreds of genes is altered by androgens. Some of the androgen-regulated genes include those encoding proteins implicated in hormonal signaling pathways, and genes implicated in ovarian development. Several highly regulated genes are not identifiable in databases. These results, along with analysis of which biological pathways and networks have been altered in response to androgen, give insights into regulation of ovarian development as well as identifying new androgen-sensitive targets for further study. In addition to improving fundamental knowledge of regulation of ovarian function, and contributing to a major shift in thinking on the role of androgens, these findings are now being used to develop hormonal therapies to overcome the block to puberty that occurs in many species cultured in captivity. The results also imply that any environmental contaminants that affect androgen and estrogen signaling could have profound effects on reproductive fitness. The award has provided training opportunities for one MS student, one PhD student, one postdoctoral researcher and one undergraduate research.
