Submitted July 7, 2003
Returned for revision July 28, 2003
Accepted October 21, 2003
Gamete Biology
Regulation of Ribosomal RNA Synthesis During the Final
Phases of Porcine Oocyte Growth
Bolette Bjerregaard ,
Christine Wrenzycki ,
Vlada V. Philimonenko ,
Pavel Hozak ,
Jozef Laurincik ,
Heiner Niemann ,
Jan Motlik ,
and
Poul Maddox-Hyttel *
* To whom correspondence should be addressed. E-mail: poh{at}kvl.dk.
Abstract
In porcine oocytes acquisition of meiotic competence
coincides with a decrease of general tran-scriptional
activity at the end of the oocyte growth phase and,
specifically, of ribosomal RNA (rRNA) synthesis in the
nucleolus. The present study investigated the regulation
of rRNA synthesis during porcine oocyte growth.
Localization and expression of components involved in
regulation of the rRNA synthesis, (the RNA polymerase
I-associated factor PAF53, upstream binding factor (UBF),
and the pocket proteins p130 and pRb) were assessed by
immunocytochemistry and semi-quantitative RT-PCR, and
correlated with ultrastructural analysis and
autoradiography following 3H-uridine incubation
in growing and fully grown porcine oocytes. In addition,
meiotic resumption, ultrastructure, and expression of
p130, UBF and PAF53 were analyzed in growing and fully
grown porcine oocytes cultured with 100 µM
butyrolactone I (BL-I), a potent inhibitor of cyclin
dependent kinases (cdk), to gain insight into the
regulation of rRNA transcription during meiotic arrest.
Immunocytochemical analysis demonstrated that p130 became
co-localized with UBF and PAF53, and that the intensity of
the PAF53 labeling decreased towards the end of the oocyte
growth phase. These data suggest that the decrease in rRNA
synthesis is regulated by inhibition of UBF by p130 as
well as by decreased availability of PAF53. Moreover,
expression of mRNA encoding PAF53 was decreased at the end
of the oocyte growth phase. At the morphological level
these events coin-cided with inactivation of the nucleolus
as visualized by the transformation of the
fibrillo-granular nucleolus to an electron-dense fibrillar
sphere with remnants of the fibrillar centers at the
surface. Meiotic inhibition with 100 µM BL-I had a
detrimental effect on the ability of porcine oocytes to
resume meiosis, and on nucleolus morphology resulting in
lack of RNA synthetic capability as the fibrillar
components, where rRNA transcription and initial
processing occur, condensed or even disintegrated.
Key words:
Gamete Biology •
Follicular development •
Oocyte development
