Abstract
The nucleolus undergoes significant functional changes and plays important roles during mammalian oocyte meiotic maturation. Fibrillarin (FBL) is the component of nucleolar small nuclear ribonucleoprotein (snRNP) particle and localizes to the dense fibrillar component (DFC) of the nucleolus. We found that FBL–GFP displays an uneven and cloudy localization in the nucleolus of non-surrounded nucleolus (NSN) oocytes, while it distributes evenly and to a few bright dots in the surrounded nucleolus (SN) oocytes. Accordingly, NSN oocytes showed active nascent RNA transcription, while the SN group was transcriptionally quiescent. NSN geminal vesicles also contained more DNA damage marker γH2AX foci. Based on different FBL–GFP patterns in live oocytes, the ones with superior meiotic maturation potential can be identified. Global transcriptome profiling revealed a significant difference in single SN and NSN oocytes. Thus, FBL–GFP can serve as a marker for nucleolus activity, which also correlates with transcription activity and the quality of oocytes.
Highlights
The nucleolus is important for mammalian oocyte meiotic maturation
DAPI staining indicates that germinal vesicle (GV) oocytes with such FBL–GFP localization are non-surrounded nucleolus (NSN) GV oocytes (Figure 1A), which is in agreement with earlier reports (Zatsepina et al, 2000)
The fluorescence quantification showed that the amount of FBL–GFP protein is significantly less in surrounded nucleolus (SN) oocytes than in NSN oocytes (Figure 1C)
Summary
Oocytes are arrested in the prophase with a large nucleus called the germinal vesicle (GV). During oocyte meiosis I (M I), the germinal vesicle breakdown (GVBD), chromosomes condense and align at the metaphase I spindle. After completing the first meiotic division, oocytes are arrested at metaphase II, waiting to be fertilized (Fulka et al, 1998). The nucleolus of mammalian oocytes is termed nucleolus-like body (NLB) due to their unusual ultrastructure and different functions from the active somatic nucleolus. Somatic fibrillo-granular nucleoli are assembled around nucleolar organizer regions (NORs) (Boisvert et al, 2007), which are divided into subregions named fibrillar centers (FC), dense fibrillar components (DFC), and granular components (GC) (Carmo-Fonseca et al, 2000). Nucleolar rRNA transcription and processing status correlated with nuclear chromatin configuration
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