Nuclear mRNA processing domain formed by PABPN1 - mRNA phase separation is essential for the growth of oocytes and female reproduction

The growing oocytes stored a large amount of maternal mRNA to support the subsequent "maternal-zygotic transition" process, which is a key regulatory events in female reproduction. The accumulation of maternal mRNA in the growing oocyte determines the developmental potential of the oocyte. At present, it is not clear how the growing oocytes store and process the newly transcribed mRNA under physiological conditions, including the distribution and localization of mRNA in the oocyte, as well as the molecular mechanism and physiological significance of this localization. In this study, we found that poly(A) tailed mRNA has the dynamic distributions of droplet fusion and gradual disappearance in the mouse nucleus as oocyte growth and meiotic maturation. Our research found that PABPN1 modulated the dynamic compartmentalization of mRNA with poly(A) tails to form NmPD (nuclear mRNA processing domain) during the growing oocytes through phase separation. The NmPD in Pabpn1-null oocytes could not form the compartmentalized distribution, and the Pabpn1 knockout females were sterile with defects of premature ovarian failure. Combined with multi-omics sequencing analysis, we investigate that NmPD was essential for long 3'-UTR isoform formation and the stability of mRNA in growing oocytes. Examination of WT and Pabpn1-null transcriptome in growing oocytes生长中的卵母细胞储存了大量母源mRNA（maternal mRNA），以支持后续的“母源-合子转换（maternal-zygotic transition）”过程——这是雌性生殖过程中的关键调控事件。生长中的卵母细胞内母源mRNA的积累水平，决定了卵母细胞的发育潜能。目前，生理条件下生长中的卵母细胞如何储存并加工新转录的mRNA，包括mRNA在卵母细胞内的分布与定位，以及该定位模式的分子机制与生理意义，尚未阐明。本研究发现，伴随小鼠卵母细胞的生长与减数分裂成熟，带有聚腺苷酸尾的mRNA（poly(A) tailed mRNA）在小鼠细胞核内呈现出液滴融合并逐渐消失的动态分布特征。进一步研究表明，在生长中的卵母细胞中，多聚腺苷酸结合蛋白核型1（PABPN1）通过相分离（phase separation）调控带有聚腺苷酸尾的mRNA的动态区室化过程，进而形成核mRNA加工结构域（NmPD, nuclear mRNA processing domain）。在Pabpn1完全敲除的卵母细胞中，无法形成该具区室化特征的NmPD；而Pabpn1基因敲除的雌性小鼠会出现卵巢早衰缺陷并导致不育。结合多组学测序联合分析，我们证实NmPD对于生长中的卵母细胞内长3'非翻译区（3'-UTR）异构体的形成以及mRNA的稳定性至关重要。对生长中的卵母细胞中野生型（WT）与Pabpn1基因敲除型的转录组进行分析