Transforming growth factor alpha gene expression and peptide localization in porcine ovarian follicles

Epidermal growth factor (EGF) receptor ligands in the chicken ovary: I. Evidence for heparin-binding EGF-like growth factor (HB-EGF) as a potential oocyte-derived signal to control granulosa cell proliferation and HB-EGF and kit ligand expression

There is increasing evidence that epidermal growth factor (EGF) receptor (EGFR) ligand and Kit ligand (KL) play critical roles in controlling follicular development in mammals. Because little is known about their expressions in the ovary of nonmammalian vertebrate, our study aimed to examine the expression, hormonal regulation, and interaction of HB-EGF and KL in the chicken ovary. Using semiquantitative RT-PCR, we demonstrated that ovarian HB-EGF expression increased dramatically with the posthatching ovarian growth. In line with this finding, HB-EGF was shown to be produced primarily by the growing oocytes and capable of stimulating the proliferation of granulosa cells in prehierarchal (3 mm) and preovulatory follicles (F5 and F1). Although HB-EGF expression is mainly restricted to the oocytes, its expression in cultured granulosa cells could be transiently yet strongly induced by HB-EGF and other EGFR ligands including EGF and TGF-alpha. And the inducing effect of HB-EGF was completely abolished by AG1478 (10 microM) or PD98059 (100 microM), indicating that the action of HB-EGF is mediated by EGFR and intracellular MAPK/ERK signaling pathway. Unlike mammals, only KL-1, not the other three isoforms identified (KL-2, -3, and -4), was detected to be predominantly expressed in the chicken ovary. Interestingly, KL expression in undifferentiated and differentiated granulosa cells could be transiently down-regulated by HB-EGF, implying an intrafollicular communication between growing oocyte and surrounding granulosa cells through the interplay of EGFR ligand and KL. Collectively, our data suggest that HB-EGF is likely a paracrine signal from the oocyte to regulate granulosa cell proliferation and HB-EGF and KL expression during ovarian follicular development.

Oocyte-somatic cell-endocrine interactions in pigs

Oocyte-somatic cell communication is bi-directional and essential for both oocyte and follicular granulosa and theca cell function and development. We have shown that the oocyte secretes factors that stimulate porcine granulosa cell proliferation in serum-free culture, and suppress progesterone production, thereby preventing premature luteinisation. Possible candidates for mediating some of these effects are the bone morphogenetic proteins (BMPs) that belong to the transforming growth factor beta family. They are emerging as a family of proteins critical for fertility and ovulation rate in several mammals, and they are expressed in various cell types in the ovary. We have evidence for a functional BMP system in the porcine ovary and BMP receptors are present in the egg nests in the fetal ovary and in the granulosa cells, oocytes and occasional theca cells throughout subsequent development. In addition to paracrine interactions in the ovary, the porcine oocyte and its developmental potential can also be influenced by nutritional manipulation in vivo. We have demonstrated that feeding a high plane of nutrition to gilts for 19 days prior to ovulation increased oocyte quality compared to control animals fed a maintenance diet, as determined by oocyte maturation in vitro. This was associated with a number of changes in circulating reproductive and metabolic hormones and also in the follicular fluid in which the oocyte is nurtured. Further studies showed a similar increase in prenatal survival on Day 30 of gestation, demonstrating a direct link between oocyte quality/maturation and embryo survival. Collectively, these studies emphasise the importance of the interactions that occur between the oocyte and somatic cells and also with endocrine hormones for ovarian development, and ultimately for the production of oocytes with optimal developmental potential.

Regulation of primordial follicle assembly and development

The assembly of the primordial follicles early in ovarian development and the subsequent development and transition of the primordial follicle to the primary follicle are critical processes in ovarian biology. These processes directly affect the number of oocytes available to a female throughout her reproductive life. Once the pool of primordial follicles is depleted a series of physiological changes known as menopause occur. The inappropriate coordination of these processes contributes to ovarian pathologies such as premature ovarian failure (POF) and infertility. Primordial follicle assembly and development are coordinated by locally produced paracrine and autocrine growth factors. Endocrine factors such as progesterone have also been identified that influence follicular assembly. Locally produced factors that promote the primordial to primary follicle transition include growth factors such as kit ligand (KL), leukaemia inhibitory factor (LIF), bone morphogenic proteins (BMP's), keratinocyte growth factor (KGF) and basic fibroblast growth factor (bFGF). Factors mediating both precursor theca-granulosa cell interactions and granulosa-oocyte interactions have been identified. A factor produced by preantral and antral follicles, Müllerian inhibitory substance, can act to inhibit the primordial to primary follicle transition. Observations suggest that a complex network of cell-cell interactions is required to control the primordial to primary follicle transition. Elucidation of the molecular and cellular control of primordial follicle assembly and the primordial to primary follicle transition provides therapeutic targets to regulate ovarian function and treat ovarian disease.

Evidence for autocrine or paracrine roles of alpha2-macroglobulin in regulation of estradiol production by granulosa cells and development of dominant follicles

alpha(2)-Macroglobulin (alpha(2)-M) inhibits proteinases and modulates the actions of growth factors and cytokines. Despite the key roles proteinases, growth factors, and cytokines have in folliculogenesis, the role of alpha(2)-M in follicular development is unknown. Our objectives were to: 1) determine whether granulosa cells produce alpha(2)-M and have alpha(2)-M receptors, 2) examine the effect of alpha(2)-M on estradiol production by granulosa cells, 3) establish whether amounts of alpha(2)-M and alpha(2)-M receptors were altered during dominant nonovulatory follicle development, and 4) examine alpha(2)-M's mechanism of action. The results demonstrated that bovine granulosa cells contain 5.2- and 15-kb mRNAs and 720- and 500-kDa proteins that correspond, respectively, to sizes of mRNAs and proteins for alpha(2)-M and the alpha(2)-M receptor. Treatment of granulosa cells with alpha(2)-M resulted in a specific dose-responsive increase in estradiol production. Cell viability, cell number, and the amount of aromatase in granulosa cells were not altered by alpha(2)-M. Treatment of granulosa cells with factors that bind alpha(2)-M or its receptor did not mimic alpha(2)-M action. Although intrafollicular amounts of alpha(2)-M remained unchanged, amounts of alpha(2)-M receptor in granulosa cells were strongly inversely associated with concentrations of estradiol in dominant and subordinate follicles. Based on these results, we concluded that alpha(2)-M may have autocrine or paracrine roles in granulosa cells potentially important for regulation of estradiol production and development of dominant follicles.

Gene expression and peptide localization for epidermal growth factor receptor and its ligands in porcine luteal cells

The present study was undertaken to determine the expression of the genes for epidermal growth factor receptor (EGF-R), and two of its ligands, epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) in isolated large (LLC) and small (SLC) porcine luteal cells by reverse transcription-polymerase chain reaction (RT-PCR), and to localize their peptides in whole corpora lutea (CL) as well in isolated LLC and SLC by immunocytochemistry. RT-PCR revealed strong cDNA signals for EGF-R and EGF in both LLC and SLC, and for TGF-alpha in LLC. The signal for TGF-alpha message in SLC was relatively weaker but detectable. Immunocytochemistry revealed intense EGF-R staining in LLC and SLC in both isolated and intact CL preparations. On the other hand, immunoreactive EGF and TGF-alpha appeared to be present only in LLC in intact CL sections, and isolated luteal cell preparations confirmed their presence in LLC and absence in SLC. These results suggest an autocrine/paracrine role for EGF and TGF-alpha in luteal functions.