DDX3X-mediated rRNA condensate clearance during germinal vesicle breakdown facilitates chromosome segregation in oocytes.

During oocyte growth, rRNAs are transcribed in nucleolus and participate in ribosome formation, but dynamic changes of rRNAs during oocyte maturation haven't yet been studied. Using RNA-FISH, we found rRNAs in germinal vesicle breakdown (GVBD) stage oocytes and a small number of oocytes at the pro-metaphase of the first meiosis stage still associate with chromosomes, and can be labeled with 5-Ethynyl Uridine (5-EU). In oocytes, rDNA transcription factor UBTF are present within nucleoli at germinal vesicle (GV) stage or near chromosomes at GVBD stage. However, RNA polymerase I can only be found at GV but not GVBD oocytes. In contrast, RNA helicase DDX3X doesn't colocalize with UBTF in GV oocytes but does colocalize with UBTF in GVBD oocytes. During oocyte maturation, the proportion of oocyte with DDX3X focus gradually decrease after GVBD. At GVBD stage, both UBTF and DDX3X are associated with rRNA condensates. When treating oocytes with DDX3X inhibitor RK-33, the decrease of 5-EU signals in oocytes will be delayed. Both treatment of RK-33 or microinjection of mutated human DDX3X cRNAs significantly reduce polar body extrusion rate and increase rate of spindle abnormalities in oocytes. Based on these findings, we hypothesize that rRNA accumulated during GV stage may entangle condensed chromosomes during GVBD and impairs chromosome segregation, and RNA helicases such as DDX3X are recruited to rRNA condensate to facilitate its clearance, thereby promoting proper chromosome segregation in oocytes. These data reveal a novel mechanism regulating chromosome segregation in oocytes.