Background
Ovarian folliculogenesis includes growth of follicles gradually and development of competent oocytes. Development of primordial follicles to preantral follicles takes place in a gonadotropin independent manner, whereas development of antral follicles to ovulatory follicles comprises the gonadotropin dependent phase of the folliculogenesis. Direct interactions between granulosa cells and oocytes regulate pre-antral follicle development under the control of two oocyte specific members of the transforming growth factor-β (TGF-β) super family, growth differentiation factor 9 (GDF-9) and bone morphogenetic factor 15 (BMP-15) [
1]. Further development of selected ovarian follicles proceeds, whereas most follicles undergo atresia by follicle cell apoptosis [
2]. Follicles selected for further development are thought to receive precise signals from gonadotropins and locally produced growth factors for survival, whereas follicular atresia or granulosa cell apoptosis minimally evident [
3,
4]. Kit ligand is one of the growth factors that has important roles in maintaining the primordial follicle reserve. In primordial follicles, Kit ligand is expressed by developing granulosa cells and its corresponding receptor is localized to the oocyte membrane [
5]. Follicular survival and activation is promoted by stimulation of intracellular phosphoinositide 3-kinase (PI3K)/Akt signaling pathway by Kit ligand in primordial oocytes [
6,
7]. The PI3K/Akt signaling pathway is reported to have roles in primordial follicle activation as well as stem cell maintenance and organogenesis [
8].
Akt, also known as protein kinase B (PKB), is a serine/threonine protein kinase and its activity is modulated downstream of PI3K in response to many growth factors and cytokines. Akt is recruited to plasma membrane after binding to the 3-phosphoinositide produced by PI3K [
9]. Akt isoform Akt1 has been reported to be critical for female fertility. Akt1 is localized in granulosa cells and oocytes of both human [
10] and rodent [
11] ovaries. In the porcine ovary, Akt1 is localized in granulosa cells of primordial follicles and in the basal layers of the granulosa cells of preantral and antral follicles.
Mitogen activated protein kinases (MAPK) are present in a number of cell types and their roles in cell cycle, proliferation and differentiation are controlled by a series of extracellular signals [
12]. MAPK3 (p44ERK1) and MAPK1 (p42ERK2) isoforms of MAPK are found in mammalian oocytes and these kinases are shown to be activated during meiotic maturation [
13]. MAPK has been shown to be upregulated by epidermal growth factor/epidermal growth factor receptor (EGF/EGFR) to induce growth of primordial to secondary follicles [
14].
Activin is one of the factors responsible for follicle activation and preantral development [
15,
16]. Activin was first discovered as a heterodimeric protein composed of the two -β subunits of inhibins A and B linked by interchain disulphide bond(s) and defined as a “FSH-releasing substance”. Since structural organization of Activin was found to be homologous to that of TGF-β, it was included in this superfamily [
17]. Three different isoforms of activin referred to as activin A (βA-βA), activin AB (βA-βB) and activin B (βB-βB) are formed by dimerization of –β subunits [
18]. Activin has putative biological actions in a wide variety of tissues such as the pituitary, bone, gonad, liver and kidney as well as hematopoietic cells [
19,
20].
Though in the former studies, activin was found to be expressed only in the granulosa cells rather than oocytes [
21], presence of activin in both oocytes and granulosa cells was proved in rodent [
22], porcine [
23] and bovine follicles [
24]. Activin A was localized in the outer ooplasm and zona pellucida of immature bovine oocytes, whereas it was localized in the zona pellucida, perivitellin space and oolemma after maturation and fertilization [
25]. Both types of activin receptors (type 1 and type 2), are expressed in mammalian ovaries [
26]. Activin receptors are expressed both in cumulus cells and oocytes [
27,
28]. Activin promotes the release of FSH from the anterior pituitary [
29]. Additionally, activin has a role in promoting aromatase activity, antral cavity formation, and granulosa cell proliferation [
15,
22,
30]. Activin is found to promote FSH receptor expression on undifferentiated rat granulosa cells [
31]. Activin A (ActA) is thought to be involved in the early follicular phase FSH rise [
32]. Activin function is antagonized by follistatin and inhibin binding to its receptors [
33].
Though FSH receptors are present on the granulosa cells of early preantral follicles [
34], activation of follicle growth and initial development is accepted to be independent of FSH [
35]. Follicle growth was found to become critically dependent on FSH at the emerging antral stage [
36]. Besides its roles in antral follicle development, FSH is considered to be related with the actions of activin regarding early follicular development [
37], since FSH is found to improve the actions of activin [
15,
38]. It has been found that FSH is required for mouse, human and rhesus in vitro pre-antral follicle development [
39‐
41]. In vitro studies on preantral follicle development revealed that ActA decreases the proportion of atretic follicles, whereas combined treatment of ActA with FSH increases the proportion of atretic preantral oocytes [
42]. Although, there is evidence that both FSH and ActA are critical for preantral follicle development and survival, their effects on the downstream signaling molecules have not been fully identified in the developing follicles. Hence, in our study we aim to find out the effects of FSH and ActA on p-MAPK and p-Akt protein levels of in vitro cultured bovine ovarian cortical strips including follicles at earlier stages of development.
Discussion
Findings of the current study indicate that involvement of rhFSH and rhActA in cortical strip culture of bovine ovaries significantly affects pAkt levels that have remarkable roles during early folliculogenesis. When rhActA was applied in combination with rhFSH, pAkt protein displayed significantly the lowest level. Akt is known to be phosphorylated at two sites: the Thr308 residue phosphorylated by the phosphoinositide-dependent kinase 1 (PDK1) [
45]; the Ser473 residue phosphorylated by the mammalian target of rapamycin (mTOR) [
46] as well as integrin-linked kinase (ILK) [
47] and mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK2) [
48]. Since we investigated the levels of pAkt phosphorylated at Ser473, the decrease in the pAkt levels in the presence of rhActA and rhFSH might be associated with impairment of mTOR, ILK or MAPKAPK2.
In previous studies, the effects of rhFSH and rhActA on ovarian follicles cultured in vitro were assessed in different species in a morphological manner. rhFSH and rhActA, both alone and in the combined form was included in human cortical strip culture [
49] or bovine [
37,
50], human [
43] and primate [
51] pre-antral follicle culture followed by strip culture as well as rodent ovary organ cultures [
42,
52] and their effects on in vitro follicle development was evaluated. Proportion of human pre-antral follicles cultured in vitro reaching antral stage was reported to be higher in the presence of rhActA [
43], whereas another study showed that rhActA had a better impact on bovine ovarian follicle diameter when used in the combined form with rhFSH, though oocyte diameter alone was not affected by these combination [
50]. It was also reported that rhActA alone had a better impact on oocyte morphology of cultured bovine preantral follicles when compared to rhFSH or combination of rhActA and rhFSH groups of culture [
37]. On the other hand, rhActA was reported to have an inhibitory effect on human primordial follicle activation [
49]. Reversely, in our study conducting bovine ovarian strip culture, we achieved an enhancement in secondary follicle development when we applied rhActA alone. rhFSH was reported to promote nest breakdown and primordial follicle formation at low levels of estradiol (E2) in rodents [
52], whereas pre-antral follicle culture of primates revealed that rhFSH disrupted the integrity of oocyte and cumulus cells resulting in impaired follicle health [
51]. In our study, addition of rhFSH alone did not cause a remarkable disruption in the follicles developed until the secondary stages.
For an effective evaluation of the ovarian follicles developed in vitro, the signaling components affecting the developmental potential of the follicles must also be considered through molecular techniques. There are particular studies suggesting an enhancement in FSH receptor, activin βA, βB subunit m-RNA levels in neonatal [
42] and fetal rodent ovary culture model [
52] in the presence of rhActA [
42] and rhFSH [
52]. In the current study, rhActA alone had a better impact on follicular development when compared to rhFSH. However, this impact was not reflected to the enhancement of p-Akt and p-MAPK levels that have critical roles in follicle development and survival [
53,
54]. Interestingly, control group of culture which had the highest level of these proteins among the culture groups, also had the highest percentage of healthy primordial follicles. Thus, it can be suggested that, especially p-Akt levels that were significantly affected by culture conditions, might be more critical for primordial follicles rather than further stages.
FSH significantly affects structure and function of follicular cells at later stages of follicular development in association with different signaling pathways. In granulosa and theca cell cultures, FSH actions on hormone production was reported to be Akt and MAPK dependent. Addition of FSH in the granulosa cell culture was shown to enhance phosphorylation levels of Akt and MAPK [
55]. FSH induces Akt phosphorylation in granulosa cells resulting in their differentiation [
56]. FSH is also known to act on granulosa cell proliferation through the activation of the MAPK pathway [
57] besides the Akt pathway [
58] and the inositol triphosphate and diacylglycerol pathways [
59]. In the current study, though rhFSH alone had a positive effect on follicle development until the secondary stage and secondary follicle survival, the percentage of healthy primordial follicles were low in rhFSH group when compared to the control group. The reason behind these observations might be that p-Akt and p-MAPK1/3 levels were not enhanced in rhFSH group. Though FSH was shown to enhance Akt phosphorylation in cultured granulosa cells [
55‐
58], in our study a significant effect of FSH on Akt phosphorylation was not observed in cortical strip cultures that specifically include ovarian follicles at primordial, primary and secondary stages. When FSH was evaluated in terms of its effects on molecular aspects via ovarian organ culture of immature rats that include only primordial and primary follicles, it was revealed that it had no significant effect on folistatin, activin receptor and FSH receptor mRNAs [
42]. Thus, the effect of FSH on different signaling molecules must be investigated through both in mRNA and protein levels.
In previous studies, EGF, as an upstream regulator of MAPK and PI3K signaling pathways, was shown to play a significant role in maintaining intraovarian primordial follicle viability and promoting ovarian cell proliferation in the prepubertal cat [
54]. There is evidence that during folliculogenesis granulosa and cumulus cells become responsive to TGF-β superfamily growth factors. EGF receptor– MAPK1/3 pathway was shown to be enabled by GDF9–SMAD3 signaling in granulosa and cumulus cells [
60]. Physiological concentrations of LH and FSH was shown to increase enzymatic activity of MAPK3 but not that of MAPK1 in the cytosol and of both MAPK1 and MAPK3 in the nucleus of porcine granulosa cells. Activation of MAPK3 by gonadotropins as well as cAMP was accompanied by increased tyrosine phosphorylation of the kinase. Following treatment with gonadotropins, translocation of MAPK to the nucleus was shown in porcine granulosa cells. EGF was reported to increase MAPK1 and MAPK3 associated kinase activity 7–8-fold in the cytoplasm of porcine granulosa cells, while kinase activity of cytoplasmic MAPK3 was enhanced 3–4-fold by LH, FSH, or cAMP [
61]. In the current study, phosphorylated MAPK1 and MAPK3 levels were not significantly affected by addition of rhFSH or rhActA in the culture. In order to obtain an enhancement in the phosphorylation and activation of these proteins to maintain the follicular development, addition of growth factors such as EGF that has a direct effect on these proteins might be essential.
Primordial follicle activation was closely associated with PI3K/Akt pathway in recent studies conducted in both human [
62] and mouse [
63]. It was reported that, when phosphatase and tensin homolog (PTEN), a suppressor of PI3K signaling, was inhibited, follicles were induced to progress to the secondary stage and increased activation of follicles was associated with increased Akt phosphorylation and nuclear export of FOXO3 (forkhead family transcription factor), though this application resulted in alterations in the survival of isolated secondary follicles [
62]. In a transgenic mice model, in which PI3K from oocyte was constitutively active, apoptosis rate was significantly reduced which resulted in an excess number of follicles per ovary. On the other hand, PTEN was introduced as a preventer of immature follicle activation, since neonatal Cre + mice was shown to remain dormant demonstrating a nuclear accumulation of PTEN. Akt phosphorylation was also increased in these follicles [
63]. Thus in our study, rhActA + rhFSH group having the lowest percentage of secondary follicles and the most impaired morphology at the end of the culture might be linked to the significantly low levels of p-Akt in this group of culture.
Though our study was limited to the treatment of single concentrations of follicle activators, according to our current results it can be suggested that use of these activators alone rather than in the combined form has a better impact on early follicular development. Different concentrations of these activators might also have various effects on the morphology of the follicles even eliminating the detrimental effect of rhFSH and rhActA in the combined form which was determined for preantral follicles in previous studies [
42]. In order to better understand the effects of these variable treatments, the follicles must be assessed in terms of the protein levels associated with follicular activation and survival besides their morphology.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
FT carried out isolation and culture of bovine ovarian cortical strips. Histological observations and follicle counting were performed by FT and GA. Western blot experiments were carried out by FT. GA conceived of the study, participated in its design and coordination and helped to draft the manuscript. Both of the authors read and approved the final manuscript.