In a study published in Nature Cell Biology, the teams of Dr. WU Ligang from the Center for Excellence in Molecular Cell Science, Shanghai Institute of Biochemistry and Cell Biology of the Chinese Academy of Sciences, and Dr. LI Jianmin’s team from Nanjing Medical University, reported the indispensable roles of piRNA pathway in both female and male fertility in golden hamsters.
piRNAs are small non-coding RNAs (sncRNAs) that are associated with PIWI family proteins and function as an adaptive immune system to safeguard genome integrity from transposable elements (TEs) in the germline by silencing active transposons through transcriptional and post-transcriptional mechanisms. Besides, piRNA pathway could also regulate the stability and translation of many mRNAs in the germ cells. Despite its essential role for both female and male reproduction in flies and zebrafish, piRNA pathway appears to be dispensable for female fertility in the mouse. Given that piRNA pathway in mammals has been exclusively studied in mice, the idea that piRNA pathway is not required for female fertility has been accepted as a dogma by many researchers.
Although mouse is the most widely used animal model and has enormous contributions for understanding gene functions, physiology, and etiology of diseases, as well as drugs development and therapies, it may not always be the right choice. Knockout functional genes in mice shown no obvious phenotypes are not scarce and the phenotypical discrepancy might reflect the species-specific pattern of gene expression and functions. Surveying of the sncRNAs presented in the oocytes of various mammals, the researchers in Dr. Wu’s lab noticed that the endo-siRNAs exclusively expressed in the oocytes of mice are absent in other mammals. More importantly, the profile of both PIWIs and piRNAs expressed in mouse oocytes are not representative, indicating the previous conclusion of piRNA function based on the study in mouse oocytes warrant a revisiting using alternative animals.
The golden (Syrian) hamster (Mesocricetus auratus) belongs to the cricetidae family and has a long history employed for the studies on developmental and reproductive biology since 1963. It has several unique features, such as consistent estrous cycle (4 days), high responsiveness to conventional superovulation regimens, and the short gestation period (16 days), and notably, the piRNA composition in oocytes resembles that in human oocytes.
Having optimized the protocols for embryo injection and CRISPR–Cas9 genome-editing in the golden hamster, the researchers knockout Piwil1 and Mov10l, two genes that encode key components of the piRNA pathway. Surprisingly, they found that disruption of Piwil1 or Mov10l1 resulted in the sterility of both males and females in hamsters. In the mutants, oocytes were produced normally and could be fertilized in vivo. However, the zygotes resulting from the fertilization of Piwil1- or Mov10l1-deficient oocytes with wild-type sperm failed to develop beyond the two-cell stage.
Small RNA sequencing in single oocyte/embryo revealed a substantial decrease in the populations of 23- or 29-nt piRNAs in Piwil1-deficient oocytes and all three piRNA populations in Mov10l1-deficient oocytes. Transcriptome sequencing results showed an obvious accumulation of both LTR and LINE1 retrotransposons in Piwil1-deficient oocytes. Furthermore, reduced degradation of maternal mRNAs and a concurrent failure in zygotic genome activation (ZGA) were detected in maternal Piwil1-deficient embryos. These findings suggest that transposon upregulation and/or a perturbed ZGA could be responsible for the developmental arresting at the two-cell stage in maternal Piwil1-deficient embryos.
Collectively, these findings suggest piRNA pathway is essential for female fertility in golden hamsters and the results might be representative in most other mammalian species, including humans, therefore, open a new avenue for investigating the aetiology of human infertility.