A porcine homolog of human integrin alpha 6 is a differentiation antigen of granulosa cells

Dedifferentiated follicular granulosa cells derived from pig ovary can transdifferentiate into osteoblasts

Transdifferentiation is the conversion of cells from one differentiated cell type into another. How functionally differentiated cells already committed to a specific cell lineage can transdifferentiate into other cell types is a key question in cell biology and regenerative medicine. In the present study we show that porcine ovarian follicular GCs (granulosa cells) can transdifferentiate into osteoblasts in vitro and in vivo. Pure GCs isolated and cultured in Dulbecco's modified Eagle's medium supplemented with 20% FBS (fetal bovine serum) proliferated and dedifferentiated into fibroblast-like cells. We referred to these cells as DFOG (dedifferentiated follicular granulosa) cells. Microarray analysis showed that DFOG cells lost expression of GC-specific marker genes, but gained the expression of osteogenic marker genes during dedifferentiation. After osteogenic induction, DFOG cells underwent terminal osteoblast differentiation and matrix mineralization in vitro. Furthermore, when DFOG cells were transplanted subcutaneously into SCID mice, these cells formed ectopic osteoid tissue. These results indicate that DFOG cells derived from GCs can differentiate into osteoblasts in vitro and in vivo. We suggest that GCs provide a useful model for studying the mechanisms of transdifferentiation into other cell lineages in functionally differentiated cells.

Actions of anti-Mullerian hormone on the ovarian transcriptome to inhibit primordial to primary follicle transition

The oocytes found within the primordial follicles of mammalian ovaries remain quiescent for months to years until they receive the appropriate signals to undergo the primordial to primary follicle transition and initiate folliculogenesis. The molecular mechanisms and extracellular signaling factors that regulate this process remain to be fully elucidated. The current study investigates the mechanisms utilized by anti-Müllerian hormone (AMH; i.e. Müllerian inhibitory substance) to inhibit the primordial to primary follicle transition. Ovaries from 4-day-old rats were placed into organ culture and incubated in the absence or presence of AMH, either alone or in combination with known stimulators of follicle transition, including basic fibroblast growth factor (bFGF), kit ligand (KITL), or keratinocyte growth factor (KGF). Following 10 days of culture, the ovaries were sectioned, stained, and morphologically evaluated to determine the percentage of primordial versus developing follicles. As previously demonstrated, AMH treatment decreased primordial to primary follicle transition. Interestingly, AMH inhibited the stimulatory actions of KITL, bFGF, and KGF. Therefore, AMH can inhibit the basal and stimulated development of primordial follicles. To investigate the mechanism of AMH actions, the influence AMH has on the ovarian transcriptome was analyzed. AMH treatment when compared with controls was found to alter the expression of 707 genes. The overall effect of AMH exposure is to decrease the expression of stimulatory factors, increase the expression of inhibitory factors, and regulate cellular pathways (e.g. transforming growth factor beta signaling pathway) that result in the inhibition of primordial follicle development. Analysis of the regulatory factors and cellular pathways altered by AMH provides a better understanding of the molecular control of primordial follicle development.

Cytoskeleton reorganization mediates alpha6beta1 integrin-associated actions of laminin on proliferation and survival, but not on steroidogenesis of ovine granulosa cells

BACKGROUND

Laminin (LN) is one of the most abundant extracellular matrix components of the basal lamina and granulosa cell layers of ovarian follicles. Culture of ovine granulosa cells (GC) on LN substratum induces cell spreading, enhances cell survival and proliferation, and promotes luteinization. Previous investigations have shown that these effects are mostly mediated by the alpha6beta1 integrin, but its signalization pathways have not been investigated. This study aimed to assess the importance of the cytoskeleton in the alpha6beta1 integrin-mediated actions of laminin on survival, proliferation and steroidogenesis of ovine GC.

METHODS

The relationships between morphology and functions of ovine GC cultured on substrata containing LN or/and RGD peptides were investigated. The effects of (1) cytochalasin D, an actin cytoskeleton-disrupting drug, (2) a specific function-blocking antibody raised against alpha6 integrin subunit (anti-alpha6 IgG), and (3) an inhibitor of the ERK1/2 signalization pathway (PD98059) were assessed for GC shape, pyknosis and proliferation rates, oestradiol and progesterone secretions.

RESULTS

Cytoskeleton disruption by cytochalasin D induced cell rounding, inhibited proliferation, promoted pyknosis, inhibited progesterone secretion and enhanced oestradiol secretion by GC cultured on LN. When GC were cultured on various substrata containing LN and/or RGD peptides in the presence or absence of anti-alpha6 IgG, both the existence of close correlations between the percentage of round cells, and the GC proliferation rate (r = -0.87) and pyknotic rate (r = 0.76) were established, but no relationship was found between cell shape and steroidogenesis. Inhibition of the ERK1/2 signalization pathway by PD98059 had no effect on GC shape, proliferation or pyknotic rates. However, it dramatically reduced progesterone secretion, expression of cytochrome P450 cholesterol side-chain cleavage and 3beta-hydroxysteroid deshydrogenase enzymes, and enhanced oestradiol secretion, thereby reproducing all the effects of the anti-alpha6 IgG on steroidogenesis of GC cultured on LN.

CONCLUSION

LN may participate in the paracrine control of follicular development through different mechanisms. It could enhance proliferation and survival of GC through its alpha6beta1 integrin-mediated actions on cytoskeleton. In contrast, its stimulating action on GC luteinization could be partly mediated by the ERK1/2 pathway, irrespective of cell shape.

Efficacy of adhesive interactions in pig-to-human xenotransplantation

Successful xenotransplantation depends on many factors, one being the interactions of cross-species adhesion molecule-ligand pairs. Depending on the approach used to facilitate xenotransplantation, these interactions can play differing roles. Here, André Simon, Anthony Warrens and Megan Sykes review the existing information on pig-to-human adhesive interactions and its implication for different approaches to pig-to-human xenotransplantation.

Putative mechanism through which N-cadherin-mediated cell contact maintains calcium homeostasis and thereby prevents ovarian cells from undergoing apoptosis

To date most of the studies involving the maintenance of ovarian cell viability have focused on the endocrine, paracrine, and autocrine factors that inhibit these cells from undergoing programmed cell death or apoptosis. Recently, studies have demonstrated that cell contact also prevents ovarian cells from dying via an apoptotic mechanism. In this commentary, the role that homophilic binding of the cell adhesion molecule, N-cadherin, plays in maintaining ovarian cell viability is presented. These studies showed that N-cadherin homophilic binding (1) is part of the mechanism through which cell contact maintains cell viability, (2) results in the activation (i.e. tyrosine phosphorylation) of the fibroblast growth factor (FGF) receptor, and (3) prevents a sustained elevation in intracellular free calcium ([Ca2+]i) which triggers apoptosis. These studies also revealed that hepatocyte growth factor (HGF), also known as scatter factor (SF), disrupts cell contact, which leads to a sustained increase in [Ca2+]i levels and ultimately to cell death. Based on these studies, this commentary presents a putative mechanism that relates the cellular and molecular mechanism through which basic FGF, N-cadherin, and HGF/SF interact to regulate [Ca2+]i levels and ultimately ovarian cell survival.

Integrin alpha6 is involved in follicular growth in mice

We previously reported that integrin alpha6 is expressed on granulosa cells in the inner layers of the human and porcine ovarian follicles, where granulosa cells have no direct contact with basal lamina. In this study, we examined the physiological role of integrin alpha6 on follicular growth with an immature superovulated mice model using the anti-integrin alpha6 monoclonal antibody, GoH3. In the ovaries of 9- to 20-day-old mice, integrin alpha6 was detected on all the layers of granulosa cells in the primordial, primary, and secondary follicles by immunohistochemistry. The 13-day-old female mice were superovulated by pregnant mare serum gonadotropin and human chorionic gonadotropin with the treatment of intraperitoneal administration of GoH3, or a control antibody, or PBS. In the group of GoH3 treatment, successful ovulation was observed in 57+/-25.7% of the animals, whereas no ovulation was observed in the control groups (p<0.01). These findings indicate that integrin alpha6 is involved in gonadotropin-induced follicular growth.

Differential effect of components of the extracellular matrix on differentiation and apoptosis

BACKGROUND

Epithelial cells are closely associated with a basement membrane, but the intimate relationships that affect growth, differentiation and survival remain enigmatic. We have previously reported that granulosa cells adjacent to the basement membrane of the ovarian follicle have a higher degree of differentiation compared with cells located distal to the basement membrane. By contrast, granulosa cells distal to the basement membrane are the first to undergo apoptosis during follicular atresia. Moreover, growth of granulosa cells in vitro on a naturally produced basement-membrane-like extracellular matrix (ECM) enhances progesterone production and the cellular response to gonadotropic hormones by an undefined mechanism.

RESULTS

To investigate the effect of the ECM on granulosa cell differentiation and death, primary granulosa cells were cultured on ECMs that lacked or contained bFGF (basic fibroblast growth factor). These otherwise identical ECMs were deposited by HR9 mouse endodermal cells, which do not synthesize bFGF, or by HR9 cells transfected with the bFGF gene. Both ECMs provided protection against apoptosis in serum-free medium, but only the bFGF-containing ECM maintained expression of the steroidogenic P450scc enzyme system and the production of progesterone. Moreover, culturing the cells on this ECM enhanced the expression of the 30 kDa steroid acute regulatory protein which plays a key role in steroid hormone biosynthesis. Laminin, but not fibronectin, was able to replace the ECM in protecting the cells from apoptosis; but not in maintaining steroidogenesis, whereas bFGF was able to enhance steroidogenesis without protecting the cells against apoptosis. Cells cultured on both ECMs or laminin had a well-developed actin cytoskeleton compared with cells cultured on non-coated dishes, which underwent apoptosis.

CONCLUSIONS

Cellular responses to ECM are mediated by the combined action of macromolecular constituents and regulatory molecules, such as bFGF, that are sequestered and stored in the ECM. ECM or laminin protects against cell death by interacting with specific integrin receptors and maintaining a well-developed actin cytoskeleton. ECM-bound bFGF provides differentiation signals for granulosa cells, which are in intimate contact with the ECM. Thus, a clear distinction can be made between the survival activity and the differentiation stimulus exerted by the ECM on epithelial cells.