subject: OOCYTE GROWTH AND EARLY FOLLICULAR activation new concepts, [print this page] OOCYTE GROWTH AND EARLY FOLLICULAR activation new concepts,
OOCYTE GROWTH AND EARLY FOLLICULAR DEVELOPMENT
by G.M.Wani, PhD,DVM(Germany)
Ovarian dormancy
The suppression of Foxo3a and activation of P13K pathway has been reported to trigger activation of the oocyte(Liu,2006,Liu et al,2006,Reddy et al,2005)The FOXO3a is expressed mainly in the nuclei of the oocytes encaped as primodial or early primary follicles(.Liu et al,20The current view is that Foxo3a is responsible for the dormant follicle (Acciii and Aeden,2004,Arden,2004,Arden and Biggs iii 2002)This Foxo3a is the factor responsible for the dormancy of the primodial follicle.It prevents its activation.
Fox03a factor
Thus the first step towards restoring activstion of primodial follicles is to suppress the action of Fox03a factor.The costant expression of this factor i retards the growth and development of the early oocytes.(Epifano et al,1995)This also leads to anovulation and luetinization of the unruptured follicles.These are the cause of infertility.It is now regarded as important intra-oocyte signaling molecule which negatively regulates the oocyte growth and development during early stages.Thus one can conviently deduce that suppressing this Foxo3a factor may initiate expression of other factors responsible for the activation of the oocyte like Bmp 15,connexin 37,connexin 43.
This fact has been proved by the invstigations of Castrillion et al,2003.He demonstrated that the Foxo3a knock out mice exhibt excessive activation of primodial follicles.The signals for the follicular activation and development are thus carried by oocyte and not sperms as was the common notion till yester years.This signls a possibility of initiating oocyte activation through an energy wave like one which suppress the Foxo3a factors.(Albertini and Barrett,2003, Eppig,2001,Matzuk et al,2002)
Role of Gap junctions in folliculogenesis The connexin mutigene family ,connexin 37 is important for the establishment of oocyte-granulosa gap junctions (Kidder and Mhawi,2002)The importance of the connexin 37 can be assessed from the fact that in connexin 37 knock out mice oocyte gorws uninterrupted upto a diameter of 52 m(Carabatos et al,2000,Gittens and Kidder,205)Connexin 43 deficiency arrests the follicular growth at at early preantral stages.(Gittens et al 2003)It can be thus concluded that Foxo3a negatively regulate connexin 37 and connexin 43,thereby retarding the oocyte growth.
The low levels of connexin 37 may effect oocyte-granulosa communication system as it prevents the formation of oocyte-granulosa gap junctions.It is these gap junctions which facilitate transport of nutrients and ions from surrounding granulose cells to the encapsulated oocyte within them.The granulose cell proliferation is hindered by the reduced connexin 43.The mode of action of these pathways is yet to be elloborated and needs futher research. connexin 37 and connexin 43 expression in follicles are not currently known. Paracrine signaling pathways. Oocyts are found to produce Bmp15 and Gdf9( TGF- family members),which activate the paracrine signaling pathways.They are known to activate Smad pathways in granulose cells and thus paly a role in oocyte-granulosa cell connections and thus in follicular development.(Vitt et al,2000,Shimasaki,2002,Hashimoto et al,2005,Pangas and Matzuk,2004Moore and Shimasaki,2005 )It is supposed that Foxo3a suppresses the activation of oocyte by suppressing the Smad pathways through negatively regulating the production of Bmp15.
Thus the proliferation of surrounding granulose cells as well as their proliferation is effected.This down regulated the oocyte-Granuosa gap junction formation and the communication system between the Granulosa cells and the oocyte is disturbed and activation suppressed.This is one of the future research fields of research in follicular biology .There is an postulation that since Foxo3a binding sites have been located in the Bmp15,the direct regulation of the Bmp15 transcription may be controlled by it too (Shimasaki et al ,2004). .
It has been reported that female Bmp15-knockout mice are subfertile, with reduced ovulation rate but with minimal histopathological defects in the ovary (Yan et al., 2001). In Bmp15-/- Gdf9+/- double-mutant (DM) mice, defects in COC expansion were observed and the DM oocytes did not support expansion of the oocytectomized WT cumulus cells, indicating that secretion of both Bmp15 and Gdf9 by oocytes is important for supporting the process of cumulus expansion (Su et al., 2004). In the current study, the cumulus expansion defect observed in our Zp3-Foxo3a Tg mice is comparable to what was observed in Bmp15-/- Gdf9+/- DM mice (Su et al., 2004). On the other hand, in our Tg mice, cumulus cells surrounding the oocyte always showed a tight structure, which is the opposite situation to that with the Bmp15-/- Gdf9+/- DM cumulus cells, which are loosely attached and readily fall off the oocyte (Yan et al., 2001). Thus, our Tg mouse model cannot be considered to be identical to Bmp15-/- mice or Bmp15-/- Gdf9+/- DM mice, as a low level of Bmp15 mRNA was still found to be expressed in oocytes, and the levels of Gdf9 mRNA were similar in primary/secondary follicles of WT and Tg mice at PD8. It is not clear yet whether or not Gdf9 levels are lower in the few `preovulatory' oocytes in the Tg mice, and further studies will be required to investigate how Foxo3a in oocytes may regulate Gdf9 transcription in preovulatory follicles, which may in turn regulate cumulus expansion prior to ovulation. We believe that the defect in ovulation in our Tg mice is caused by multiple molecular mechanisms that are regulated by Foxo3a in oocytes.
The p27 protein is a Cdk inhibitor that suppresses cell growth. After being phosphorylated, p27 shuttles from the nucleus to the cytoplasm, whereby its inhibitory effects can be abolished (Cunningham et al., 2004; Shin et al., 2005; Viglietto et al., 2002). Our data from the current study demonstrate that in Zp3-Foxo3a Tg mice, p27 expression is maintained in the nuclei of oocytes, indicating that Foxo3a in oocytes prevents the shuttling of p27 from the nucleus to the cytoplasm, or prevents the downregulation of p27 expression, thereby maintaining the growth-inhibitory function of p27 in oocytes. This notion is supported by previous studies with other cell types showing that Foxo3a can enhance the expression of p27 (Chandramohan et al., 2004; Dijkers et al., 2000). Moreover, our unpublished data also suggest that p27 in primary oocytes may participate in the suppression of primordial follicle activation, because in p27-knockout mice primordial follicles were prematurely activated. The underlying mechanism of how Foxo3a may regulate p27 levels and regulate the localization of p27 in mouse oocytes is being investigated in our laboratory. Another interesting finding from the current study is that in the Tg mice, expression of the Foxo3a transgene in oocytes resulted in luteinization of unruptured `preovulatory' follicles, with oocytes being trapped in the CL. In spite of the possible regulation through altered production of Bmp15 and Gdf9, further analyses have shown that the anovulation phenotype in our Tg mice may be caused by the dramatically reduced expression of PR in large follicles. PR is a nuclear receptor transcription factor that is induced in the granulosa cells of preovulatory follicles in response to the surge of luteinizing hormone. PR has been shown to be essential for ovulation, as mice lacking this molecule fail to ovulate and are infertile (Robker et al., 2000). The assumption that the considerably reduced PR levels may cause anovulation in the Tg mice is supported by our finding that cathepsin L, a protease that facilitates ovulation, was also downregulated in granulosa cells of large follicles in the Tg mice (not shown), a phenomenon similar to that observed in PR-knockout mice (Robker et al., 2000). At this stage of the study, however, it is not clear whether Foxo3a produced by oocytes can directly regulate the expression of PR in preovulatory granulosa cells. Based on the results of the current study and from our previous report that Kitl can induce the phosphorylation/suppression of Foxo3a via the activation of PI3K/Akt in cultured mouse and rat oocytes (Reddy et al., 2005),
we suggest that a well-balanced activation of the intra-oocyte PI3K pathway is of importance in controlling the rates of oocyte growth and follicular development (Liu, 2006; Liu et al., 2006). In other words, a deregulated PI3K pathway in the oocyte may lead to disturbed follicular development and impaired fertility, as is the case in the Zp3-Foxo3a Tg mice. In addition, as Foxo3a is a transcription factor, other downstream genes and related regulation mechanisms in oocytes remain to be elucidated. Additional Foxo3a-independent pathways in oocytes, such as the mTOR-p70S6 kinase-mediated cascades and the MAPK pathway, are still just as active in the Tg oocytes, indicating that there exist overlapping signaling mechanisms that account for the rapid oocyte growth during follicular activation and early development. In summary, our study has revealed the functional roles of intraoocyte Foxo3a in the regulation of oocyte growth, follicular development and ovulation.
The findings from the current study may provide some useful information in the search for oocyte-borne genetic aberrations that lead to defects in follicular development and ovulation in human diseases, such as premature ovarian failure
