Stella’s role in women’s fertility
It has been established that since 1950 the female fertility rate has halved. What could be the reasons? Is it a woman’s choice? Is it perhaps due to the fact that over the last years women have acquired an important social and working role? Is the cause to be found in environmental factors? Or is it within women’s body? n interesting discovery has made it possible to understand that infertility can be due to genetic factors. In fact, in 2007 a study brought to light the importance of Stella in regulating the genome of the oocyte. Stella is a protein that has a key function in protecting the maternal genome from hypermetilation and it is therefore essential in the processes of genetic reprogramming after fertilization. Stella can only perform this task when it is in the nucleus [1]. In 2013, it was also discovered that the gain of DNA methylation in the germ cell vesicle of mice affected more non-GC sites [2]. After fertilization between the egg cell and the sperm cell the zygote is formed, and it will undergo various divisions. The zygote inherits numerous proteins essential for the development from the oocyte, including Stella. In the subsequent stages of development, particularly in the division phases, the inherited protein pattern is destroyed and the zygotic genes are activated [3].
In the study by Yingfeng Li and collaborators [4] are provided interesting new results about the role of Stella. Analysis of the oocyte showed an ectopic accumulation of UHRF1 in the nucleus in the absence of Stella. UHRF1 is a DNMT1 methyltransferase regulator, responsible for methylation in GC sites [5]. Starting from this cue, the authors wanted to understand how Stella regulated UHRF1 in mouse oocytes. Two immuno-blotting tests were used to analyse the sublocation of Stella and UHRF1 in both Stella+/- (maternal deleted Stella) and Stella KO oocytes. In the first case, Stella was found to be localized in both the nucleus and the cytoplasm, while UHRF1 is enriched in the cytoplasm. Instead, in the absence of Stella, UHRF1 is localized in the nucleus. The results obtained suggest that the presence of Stella is essential for the correct distribution of UHRF1.
They also identify by mutants the Stella domain interacting with UHRF1 by the analysis of Stella mutants. It has been observed that the presence of UHRF1 in the nucleus in Stella KO oocytes causes an ectopic nuclear accumulation of DNMT1. In particular, the presence of DNMT1 in the pericentric chromatin has been detected, as opposed to its usual localization (diffused chromatin). As a result of this, an increase in methylation was observed in transcriptionally inert genomic regions through RRBS sequencing. All sites that have gained aberrant methylation have been defined as extra ooAMRs. The importance of UHRF1 was intelligently re-tested in an experiment in which the latter was removed with Zp3-cre transgene and a decrease in methylation was obtained. The crucial role of UHRF1 in the formation of an erroneous methylome was confirmed by doing the experiment in reverse. Hypermetilation in important regions in the genome, as well as in promoters, intergenic and intragenic regions of genes essential for development causes infertility. It has been observed that in the absence of Stella, the embryo stops at the four-cell stage and does not progress to the blastocyst stage. In the genome generally the balance between the levels of 5-methylcytosine and 5-hydroxy-methylcytosine is important for cell differentiation. Through UHPLC-MS/MS, the authors observed that the concentration of 5-hmC in StellaΔm/+ oocytes was increased compared to WT. This increase was due to the absence of Stella, which has the task of regulating the enzyme TET, responsible for hydroxylation. Therefore, the loss of maternal Stella is responsible for a zygotic hypermethylated genome.
Figure 1
A question arises spontaneously: can the absence of Stella influence the fertility of a male mouse? By analysing the percentage of methylation on the genome of the sperm cell Stella KO and comparing it with that of a sperm cell WT, it was clear that the levels of methylation were almost identical, supporting the fact that in males there is no loss of fertility. However, if paternal pronuclei from a fecundation between female Stella KO and male WT are taken, a genomic misregulation with gain of methylation can be observed. Thanks to such in-depth analyses, the authors conclude that the presence of the maternal Stella guarantees a correct genomic arrangement. Looking at the effects that a misregulated genome in Stella KO oocytes may have on the transcriptome, numerous downregulated and upregulated zygotic genes have been detected. Genes related to hypermethylated promoters, transmitted exclusively from the maternal genome, have been the most affected by this phenomenon. Finally, to confirm that DNMT1 was responsible for the hypermethylation and to exclude the contribution of DNMT3A they analysed the double mutant Stella KO either with DNMT3A or with DNMT1 knockout. The levels of DNA methylation were reduced only in DNMT1, deleted oocytes.
In conclusion, this work convincingly demonstrates that Stella is an essential factor for the proper embryonic development as it regulates the localization of UHRF1 and consequently the function of the DNMT1 methyltransferase. Zygotes without Stella will never be able to progress to the blastocyst stage, confirming that the absence of Stella causes infertility in women. It is interesting to note that in this work, in order to ascertain the validity of the results obtained, the authors carry out a number of different experiments in replication. Moreover, it is interesting that the authors confirmed experiences from previous studies and have also given a complete idea of the pattern of factors involved in the formation of aberrant methylome. In our opinion, this work can represent a good starting point to allow new technologies to progress in resolving genetic causes, such as mutations in the Stella locus, related to infertility. The next step could be to understand whether genomic interventions can be applied to cure such infertility probles and whether they can be considered ethical.
References
- Nakamura, T. et al. PGC7/Stella protects against DNA demethylation in early embryogenesis. Nat. Cell Biol. 9, 64–71 (2007).
- Shirane, K. et al. Mouse oocyte methylomes at base resolution reveal genome-wide accumulation of non-CpG methylation and role of DNA methyltransferases. PLoS Genet. 9, e1003439 (2013).
- Huang, Y. et al. Stella modulates transcriptional and endogenous retrovirus programs during maternal-to-zygotic transition. eLife 6, e22345 (2017).
- Yingfeng Li, Zhuqiang Zhang, Jiayu Chen, Wenqiang Liu, Weiyi Lai, Baodong Liu, Xiang Li, Liping Liu, Shaohua Xu, Qiang Dong, Mingzhu Wang, Xiaoya Duan, Jiajun Tan, Yong Zheng, Pumin Zhang, Guoping Fan, Jiemin Wong, Guo-Liang Xu, Zhigao Wang, Hailin Wang, Shaorong Gao & Bing Zhu. Stella safeguards the oocyte methylome by preventing de novo methylation mediated by DNMT1. Nature 564 (2018).
- Bostick, M. et al. UHRF1 plays a role in maintaining DNA methylation in mammalian cells. Science 317, 1760–1764 (2007).