Julia A. Elvin

Oocyte-expressed TGF-beta superfamily members in female fertility.

Folliculogenesis is regulated by the interplay of extraovarian and intraovarian factors, and the importance of each type of regulation varies depending on the developmental stage of the follicle. Preantral follicle development is regulated predominantly by factors produced locally within the ovary and within the follicle itself. The oocyte has been shown to produce soluble factor(s), which regulate a number of processes in follicular development, including cumulus expansion in the periovulatory period. Members of the TGFbeta superfamily are potent regulators of cell proliferation and differentiation in a number of organ systems, and three members, growth differentiation factor 9 (GDF-9), bone morphogenetic protein 15 (BMP-15) and BMP-6 are expressed by the oocyte and may mediate effects attributed to the oocyte. Based on knockout mouse models BMP-6 does not play an essential role in ovarian function, but GDF-9 is absolutely required for preantral follicle development. GDF-9 also alters the periovulatory expression of granulosa cell genes and stimulates cumulus expansion. Although BMP-15 is expressed identically to GDF-9, its role in regulating ovarian function is still unknown. This review examines the similarities and differences in sequence, expression, and function of the oocyte-expressed TGFbeta family members with respect to regulating folliculogenesis.

Regulation of Growth Differentiation Factor 9 Expression in Oocytes In Vivo: A Key Role of the E-Box

Abstract Growth differentiation factor 9 (GDF9) is preferentially expressed in oocytes and is essential for female fertility. To identify regulatory elements that confer high-level expression of GDF9 in the ovary but repression in other tissues, we generated transgenic mice in which regions of the Gdf9 locus were fused to reporter genes. Two transgenes (−10.7/+5.6mGdf9-GFP) and (−3.3/+5.6mGdf9-GFP) that contained sequences either 10.7 or 3.3 kb upstream and 5.6 kb downstream of the Gdf9 initiation codon demonstrated expression specifically in oocytes, thereby mimicking endogenous Gdf9 expression. In contrast, transgenes −10.7mGdf9-Luc and −3.3mGdf9-Luc, which lacked the downstream 5.6-kb region, demonstrated reporter expression not only in oocytes but also high expression in male germ cells. This suggests that the downstream 5.6-kb sequence contains a testis-specific repressor element and that 3.3 kb of 5′-flanking sequence contains all the cis-acting elements for directing high expression of Gdf9 to female (and male) germ cells. To define sequences responsible for oocyte expression of Gdf9, we analyzed sequences of Gdf9 genes from 16 mammalian species. The approximately 400 proximal base pairs upstream of these Gdf9 genes are highly conserved and contain a perfectly conserved E-box (CAGCTG) sequence. When this 400-bp region was placed upstream of a luciferase reporter (−0.4mGdf9-Luc), oocyte-specific expression was observed. However, a similar transgene construct (−0.4MUT-mGdf9-Luc) with a mutation in the E-box abolished oocyte expression. Likewise, the presence of an E-box mutation in a longer construct (−3.3MUT-mGdf9-Luc) abolished expression in the ovary but not in the testis. These observations indicate that the E-box is a key regulatory sequence for Gdf9 expression in the ovary.

Control of Ovarian Function

Female fertility depends on prenatal development of the fetal gonad into an ovary and the complex interactions between the intraovarian and extraovarian factors that regulate the postnatal process of folliculogenesis. At birth, the ovary has a finite oocyte population. Folliculogenesis initiates when some oocytes within primordial follicles begin to grow in response to undiscovered intragonadal factors; other oocytes will remain quiescent until later in life, resulting in a prolonged period of fertility. Follicular development is controlled locally by paracrine factors and at a distance by endocrine hormones, such as the pituitary hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Intragonadal factors initiate growth of the follicle and coordinate development of the oocyte, granulosa cells, and thecal cells (1). Extragonadal factors, particularly hormones from the pituitary, synchronize granulosa cell and theca cell function later in folliculogenesis to initiate puberty and integrate the reproductive system with overall female physiology (2).

Transgenic Models to Study Reproduction, Oncogenesis, and Development

In mammals, there are approximately 100,000 genes that govern the development of an organism. For development to proceed normally, there must be coordinate interaction of thousands of these gene products in any given cell of the organism. Beginning with fertilization, precise expression of these gene products is required during embryonic, fetal, postnatal, and adult development. Aberrant synthesis of even one of these gene products can be disastrous: Birth defects, cancer, infertility, and even death are all possible consequences when this developmental program is altered. To understand these processes in humans fully, it is necessary to have physiological models that closely mimic developmental events that occur during the creation of a human being.