Using molecular, cellular, and genetic approaches, recent studies examining the role of the bone morphogenetic protein (BMP) family of growth factors in the reproductive system have led to significant breakthroughs in our understanding of mammalian reproduction and fertility. Gene expression studies have revealed that key components of the BMP system (ligands, receptors, signaling molecules, and binding proteins) exhibit coordinated spatial and temporal expression patterns in fundamental cell types throughout the reproductive system. Availability of recombinant BMPs has enabled functional studies that have demonstrated important biological activities of BMPs in controlling cellular proliferation, differentiation, and apoptosis in reproductive tissues. The physiological importance of the BMP system for mammalian reproduction has been further highlighted by the elucidation of the aberrant reproductive phenotypes of animals with naturally occurring mutations or targeted deletions of certain BMP family genes. Collectively, these studies have established the concept that the BMP system plays a crucial role in fertility in female and male mammals. The purpose of this article is to review the evidence underpinning the importance of the BMP system in mammalian reproduction.

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We have recently reported that oocyte-derived bone morphogenetic protein-15 (BMP-15) can directly modulate follicle-stimulating hormone (FSH) action in rat granulosa cells. Here, we investigate underlying mechanisms of this BMP-15 effect. Treatment with BMP-15 alone exerted no significant effect on the basal expression of mRNAs encoding steroidogenic acute regulatory protein, P450 side chain cleavage enzyme, P450 aromatase, 3␤-hydroxysteroid dehydrogenase, luteinization hormone receptor, and inhibin/activin subunits. However, BMP-15 markedly inhibited the FSH-induced increases in these messages. In striking contrast, BMP-15 did not change the forskolin-induced levels of these transcripts. Thus, the inhibitory effect of BMP-15 on FSH action must be upstream of cAMP signaling. We next examined changes in FSH receptor mRNA expression. Interestingly, BMP-15 severely reduced the levels of FSH receptor mRNA in both basal and FSH-stimulated cells. To determine whether this effect was at the level of FSH function, we investigated the effect of BMP-15 on FSH bioactivity. Consistent with the mRNA data, BMP-15 inhibited the biological response of FSH, but not that of forskolin. Based on these results, we propose that BMP-15 is an important determinant of FSH action through its ability to inhibit FSH receptor expression. Because FSH plays an essential role in follicle growth and development, our findings could have new implications for understanding how oocyte growth factors contribute to folliculogenesis.

Recently, we proposed an oocyte-growth differentiation factor-9 hypothesis that predicts alterations in the initial stages of folliculogenesis in polycystic ovary syndrome (PCOS) ovaries. Here, we test this hypothesis by scoring the composition of follicles in normal and PCOS ovaries. Follicles were classified as primordial, transitional primary, classic primary, secondary, and Graafian. A total of 2274 follicles were scored. The total number of growing follicles was significantly greater in PCOS ovaries than normal, but the number of nongrowing primordial follicles did not differ. Consequently, the increase in growing follicles in PCOS cannot be explained by increased primordial follicle recruitment. Differential counts showed that the number of growing follicles at each stage of development was significantly greater: PCOS had 2.7-fold more primary, 1.8-fold more secondary, and 2-fold more Graafian follicles than normal. The greatest effect was on the classic primary follicles where the number was almost 5-fold greater in PCOS ovaries. The absence of apoptosis in normal and PCOS preantral follicles argues that the increase in growing follicles in PCOS cannot be explained by changes in atresia. We conclude, therefore, that primary follicle growth is abnormally slow in PCOS and the dynamics are reflected in a stockpiling of classic primary follicles.

The purpose of this research was to test the hypothesis that insulin-like growth factor-I (IGF-I) regulates estradiol (E2) synthesis in human granulosa and granulosa luteal cells. Cells from individual follicles from spontaneous and human menopausal gonadotropin/CG-stimulated cycles were cultured in serum-free medium containing androstenedione, IGF-I, FSH, and/or CG. At 2, 4, and 6 days, E2 in the medium was measured by RIA. In the granulosa experiments, control cells produced basal levels of E2 at 2 days, and the levels increased with increasing follicle size. Treatment with FSH stimulated E2 production (on the average, 5-fold), and the effect was dose dependent (ED50 = 5 ng/mL or 16 mIU/mL). Incubation with IGF-I alone caused increases in E2 production comparable to those caused by FSH, and the IGF-I effect was dose dependent (ED50 = 8 ng/mL). In most cases, coincubation with FSH and IGF-I augmented E2 levels more than either hormone alone, and at 4 and 6 days the interaction was synergistic. The data from dose-response experiments suggested that the basis of the synergy between FSH and IGF-I was a marked potentiation by either hormone (approximately 10-fold) in the potency of the complementary hormone to stimulate E2 production. In the experiments with granulosa luteal cells from spontaneous and in vitro fertilization preovulatory follicles, the controls synthesized very high levels of E2 spontaneously at 2 days; however, E2 production declined 700% at 4 days, and no E2 was produced by control cells at 6 days. Treatment with FSH, CG, or IGF-I did not cause a significant increase in the high basal levels of E2 at 2 days. During subsequent culture, however, all three hormones stimulated E2 production at 4 and 6 days, but the increases were modest and not sustained. In contrast, coincubation of granulosa luteal cells with FSH plus IGF-I or CG plus IGF-I dramatically enhanced E2 production at 4 and 6 days (on the average, 4-fold), and the effects were sustained throughout the culture period. (ABSTRACT TRUNCATED AT 400 WORDS)