Deconstructing mammalian reproduction: using knockouts to define fertility pathways

Identification of Neuregulin as a Factor Required for Formation of Aligned Spermatogonia

In the absence of somatic cells, medium conditioned by the SNL fibroblast line (SNL-CM) is able to stimulate primary cultures of rat type-A single spermatogonia to develop into chains of aligned spermatogonia at the 8-, 16-, and 32-cell stages. By comparison, medium conditioned by an MSC-1 Sertoli cell line is ineffective. Glial cell line-derived neurotrophic factor (GDNF)-like molecules were identified in SNL-CM and recombinant forms of GDNF, neurturin, and artemin were shown to stimulate formation of aligned spermatogonia, but principally to only the 4- and 8-cell stages. Because SNL-CM and GDNF-like molecules stimulated the formation of spermatogonial chain length differently, we purified components of SNL-CM to identify the additional contributing factor(s). A fraction was isolated that was dependent on GDNF, but required for effective formation of 16- and 32-cell chain lengths. Sequence analysis identified the factor as mouse neuregulin-1. At picomolar concentrations, recombinant neuregulin-1 in combination with GDNF effectively stimulated formation of aligned spermatogonia up to the 32-cell stage. Neuregulin in the absence of GDNF was relatively ineffective. Soluble receptors for neuregulins blocked the effects of GDNF and SNL-CM, suggesting that both neuregulin and GDNF are required for effective formation of long spermatogonial chains. Addition of neuregulin-1 to cultures on MSC-1 feeder layers resulted in spermatogonial behavior similar to that seen on feeder layers of SNL fibroblasts. In fact, SNL cells were found to express 100-fold higher levels of neuregulin-1 transcripts than MSC-1 cells. Thus, we identify neuregulin as a factor required for spermatogonial amplification and differentiation in culture.

CPEB controls oocyte growth and follicle development in the mouse

CPEB is a sequence-specific RNA-binding protein that regulates polyadenylation-induced translation. In Cpeb knockout mice, meiotic progression is disrupted at pachytene due to inhibited translation of synaptonemal complex protein mRNAs. To assess the function of CPEB after pachytene, we used the zona pellucida 3 (Zp3) promoter to generate transgenic mice expressing siRNA that induce the destruction of Cpeb mRNA. Oocytes from these animals do not develop normally; they undergo parthenogenetic cell division in the ovary, exhibit abnormal polar bodies, are detached from the cumulus granulosa cell layer, and display spindle and nuclear anomalies. In addition, many follicles contain apoptotic granulosa cells. CPEB binds several oocyte mRNAs, including Smad1, Smad5, spindlin, Bub1b, Mos, H1foo, Obox1, Dnmt1o, TiParp, Trim61 and Gdf9, a well described oocyte-expressed growth factor that is necessary for follicle development. In Cpeb knockdown oocytes, Gdf9 RNA has a shortened poly(A) tail and reduced expression. These data indicate that CPEB controls the expression of Gdf9 mRNA, which in turn is necessary for oocyte-follicle development. Finally, several phenotypes, i.e. progressive oocyte loss and infertility, elicited by the knockdown of CPEB in oocytes resemble those of the human premature ovarian failure syndrome.

[Animal models: Candidate genes for human male infertility]

More than 300 genes necessary for the normal completion of the spermatogenesis have been identified by means of the production of knockout mice. The data cover the whole male reproduction apparatus and thus allow defining candidate genes that could be related to various dysfunctions of human male fertility. Data obtained from mouse models have allowed identifying genetic mutations with loss of function for men with: (i) early meiotic arrest, (ii) maturation arrest of the round spermatid and (iii) morphological anomalies of the spermatozoa. Also numerous Drosophila mutants are models for the knowledge of genes involved in the spermatogenesis. Finally, there are other important models sharing cilia and flagella, and thus, having a structure in common with the sperm flagellum, the axoneme. First, these organisms have allowed the identification of genes involved in human respiratory diseases. But interestingly, these last two years, a great number of human syndromes have been discovered to be related to cilia pathologies, and among them, complex phenotypes including an abnormal spermatogenesis.

Control of mammalian oocyte growth and early follicular development by the oocyte PI3 kinase pathway: new roles for an old timer

A large amount of information has accumulated over the past decade on how gonadotropins, steroid hormones and growth factors regulate development of the mammalian ovarian follicle. Moreover, the bi-directional communication between mammalian oocytes and their surrounding somatic (granulosa) cells has also been shown to be crucial for this process. The intra-ovarian factors, or more specifically, the intra-oocyte signaling pathways that control oocyte growth and early follicular development are largely unknown, however. Based on both in vitro studies and in vivo functional studies using gene-modified mouse models, this review focuses on the key features of the phosphatidylinositol 3 kinase (PI3K) pathway in growing mouse oocytes and on the novel functions of the oocyte PI3K pathway in controlling mammalian oocyte growth and follicular development that have come to light only recently. We propose that the PI3K pathway in the oocyte, which is activated by granulosa cell-produced Kit ligand (KL) via the oocyte-surface receptor Kit, may serve as an intra-oocyte network that regulates both oocyte growth and the early development of ovarian follicles.