In the Spotlight

MicroRNAs (miRNAs) are a class of noncoding RNA molecules that are about 22 nucleotides long and that have in the last decades been recognized as pivotal regulators of gene expression through gene silencing at the posttranscriptional level. It is believed that miRNAs fine-tune the expression of approximately 30% of the mammalian protein-encoded genes, playing an important role in many different biological processes, from cell proliferation to apoptosis. Not surprisingly, abnormal expression of miRNAs has also been reported to be associated with multiple human reproductive disorders, including endometriosis, preeclampsia, ectopic pregnancy, and recurrent miscarriages. In the current issue of Reproductive Sciences, the importance of miRNAs in the reproductive field is reinforced by the report by Gu and colleagues, who explore the role of specific miRNAs in human implantation. Gu et al had previously observed that the expression level of miR-3074-5p in placental villi of patients with recurrent miscarriage was significantly higher as compared to the controls. In the mouse, the expression of miR-3074-5p was also observed to be markedly decreased at implantation sites as compared to nonimplantation sites, suggesting that an increased expression of miR-3074-5p at the maternal-fetal interface might result in adverse effects on the establishment and maintenance of pregnancy. Therefore, Gu and colleagues aimed to observe the effects of miR-30745p overexpression in human trophoblast cells in vitro. For that, the authors transfected HTR8/SVneo cells with miR-3074-5p and determined cell apoptosis and invasion using flow cytometry and transwell assay. Additionally, they determined the expression levels of different genes by complementary DNA microarray assay and Western blot analysis, assessing their biofunctions through gene ontology analysis. Gu and colleagues’ results showed that the overexpression of miR-30745p in the transfected cells promoted cell apoptosis and inhibited cell invasion. At the molecular level, P27, BCL-2, and BCL-G were significantly upregulated with miR-3074-5p overexpression, in accordance with the previously observed elevated expression of P27 in a small sample of the placental villi of patients with recurrent miscarriages. The authors observed that 411 genes were downregulated while 397 were upregulated by at least 2-fold in the transfected cells and that the genes involved play important roles in pregnancy. As such, the data suggest that mi-3074-5p is crucial for the normal physiology of trophoblast cells, while its deregulation may be on the basis of recurrent miscarriages. Also, in the present issue of this journal, the importance of another class of noncoding RNAs in reproduction, the PIWIinteracting RNAs (piRNAs) is highlighted by the report by Cui et al. PIWI-interacting RNAs is the largest class of small noncoding RNA molecules expressed in animal cells, with 26 to 31 nucleotides. Their name comes from their interaction with the PIWI family proteins, forming complexes that have been associated with epigenetic and posttranscriptional gene silencing particularly during spermatogenesis. Thus, Cui and colleagues aimed to investigate whether there is a relationship between specific piRNAs levels and semen parameters and to determine the role of these piRNAs on fertilization and embryo development. For this study, the authors recruited 186 patients with idiopathic infertility undergoing a first cycle of infertility treatment involving intracytoplasmic sperm injection (ICSI) to determine the levels of piRNAs in spermatozoa as measured by real-time RT-PCR. Routine spermiogram analysis’ parameters, rates of fertilization after ICSI, and parameters of good embryo development, such as early embryo cleavage and day 3 morphology, were assessed. The results showed that the levels of expression of piR-31704 and piR-39888 in the spermatozoa of the male factor infertility group were lower than in the control group. In addition, the expression levels were lower in the subnormal sperm concentration samples as compared to the normal sperm concentration ones, while there was no correlation between the piRNA expression levels and the rates of sperm progressive motility or morphology parameters. Regarding the fertilization rates, the authors observed a significant increase in the levels of the 3 piRNAs tested for the spermatozoa leading to the higher 2 pronuclei formation rate, whereas they did not identify a correlation between piRNAs expression and rates of embryo early cleavage, good morphology on day 3, or achievement of pregnancy. Altogether, the results above lead Cui and colleagues to conclude that piRNA levels correlate with sperm concentration and embryo fertilization rate, suggesting that they may play an important role in the fertilization process.