Research News

Researchers Reveal Molecular Mechanisms Underlying PIWI Translocation from the piRNA-producing Factory to piRNA-functioning Platform

Source: Time: 2024-03-21

In a study published in Nature Communications, researchers led by Drs. LIU Mofang from the Center for Excellence in Molecular Cell Science, Shanghai Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences, DING Deqiang from Tongji University, and WANG Xin from the Hangzhou Advanced Institute, UCAS, have discovered that piRNA loading facilitates the translocation of mouse PIWI (MIWI) protein from the intermitochondrial cement (IMC) to the chromatoid body (CB) during spermatogenesis in mice. 

PIWI proteins and its interacting small noncoding RNAs, i.e., piRNAs are specifically expressed in animal germ cells, which forms PIWI/piRNA complex to suppress transposable elements and regulate protein-coding genes, thereby contributing to germ cell development and fertility in animals. Germ granules, membrane-less organelles found in the cytoplasm of germ cells, known as the cellular sites for RNA metabolism, play critical roles during spermatogenesis in animals. Previous studies have shown that the PIWI/piRNA machinery is associated with specific germ granules in male germ cells. In particular, the IMC and CB, located between mitochondrial clusters and around the cell nucleus, respectively, are considered as the central sites for piRNA processing and functionality. The IMC is present in the early stages of spermatogenesis and disappears by the spermatid stage, whereas the CB becomes prominent as the IMC fades, eventually forming a singular punctate structure within spermatids.

Mouse PIWI proteins, including MIWI, MILI, and MIWI2, are specifically expressed in male germ cells, which are spatiotemporally regulated during spermatogenesis. MIWI protein is initially expressed in pachytene spermatocytes and recruited to the IMC through interaction with the mitochondrion-anchored Tudor domain-containing protein TDRKH for piRNA processing in mid-pachytene spermatocytes. Thereafter, MIWI is enriched in the CB in round spermatids, where it, in complex with piRNAs, is involved in post-transcriptionally regulating transposon and protein-coding mRNA transcripts. Despite these insights, the mechanism underlying the sequential translocation of MIWI from the piRNA-producing IMC to the piRNA-functioning CB during spermatogenesis has remained an open question.

Considering that MIWI is initially recruited to the IMC by TDRKH for piRNA processing and then localized to the CB in complex with piRNAs, the authors hypothesized that piRNA loading could be instrumental in the translocation of MIWI from the IMC to CB during spermatogenesis in mice. To test this hypothesis, the authors generated two piRNA loading-defective Miwi mutant mouse models (MiwiYY/YY and MiwiYK/YK). They found that both MIWI mutant proteins were correctly located to the IMC in spermatocytes, but failed to transport to the CB in round spermatids, underscoring the crucial role of piRNA binding to sequential translocation of MIWI from the IMC to CB during spermatogenesis. Their mechanistic studies revealed that piRNA loading weakens MIWI interaction with TDRKH, thereby allowing MIWI dissociation from the IMC. Without TDRKH binding, arginine residues at the N-terminal domain of MIWI are exposed and then methylated by PRMT5 (Protein arginine methyltransferase 5). This modification enhances MIWI binding to another Tudor family protein TDRD6, leading to its recruitment to the CB. The authors further showed that piRNA loading-deficient mutations in Miwi impair piRNA production and MIWI stability, leading to spermiogenic failure and male infertility in mice. Collectively, these findings establish the critical role of piRNA loading in MIWI translocation during spermatogenesis, offering new insights into piRNA biology in mammals.