EP45 accumulates in growing Xenopus laevis oocytes and has oocyte-maturation-enhancing activity involved in oocyte quality.
The capacity of oocytes to fully support meiotic maturation develops gradually during oocyte growth. Growing oocytes accumulate proteins and mRNAs required for this process. However, little is known about the identity of these factors. We performed a differential proteomic screen comparing the proteomes of growing stage-IV oocytes, which do not undergo meiotic maturation in response to progesterone, with fully grown stage-VI ones, which do. In 2D gels of stage-VI oocytes, we identified a group of four protein spots as EP45 (estrogen-regulated protein 45 kDa), which belongs to the family of serine protease inhibitors and is also known as Seryp or pNiXa. Western blot analysis after mono- and bi-dimensional electrophoreses confirmed the accumulation of certain forms of this protein in oocytes between stages IV and VI. EP45 mRNA was not detectable in oocytes or ovaries, but was expressed in the liver. A low-mobility isoform of EP45 was detected in liver and blood, whereas two (occasionally three or four) higher-mobility isoforms were found exclusively in oocytes, suggesting that liver-synthesized protein is taken up by oocytes from the blood and rapidly modified. Alone, overexpression of RNA encoding either full-length or N-terminally truncated protein had no effect on meiotic resumption in stage-IV or -VI oocytes. However, in oocytes moderately reacting to low doses of progesterone, it significantly enhanced germinal-vesicle breakdown, showing a novel and unsuspected activity of this protein. Thus, EP45 accumulates in growing oocytes through uptake from the blood and has the capacity to act as an 'oocyte-maturation enhancer' ('Omen').
Marteil
Gaëlle
G
CNRS UMR 6061, Institute of Genetics and Development of Rennes, Mitosis and Meiosis Group, University of Rennes 1, IFR 140 GFAS, Faculty of Medicine, 35 043, Rennes, France.
D'Inca
Romain
R
Pascal
Aude
A
Guitton
Nathalie
N
Midtun
Torbjørn
T
Goksøyr
Anders
A
Richard-Parpaillon
Laurent
L
Kubiak
Jacek Z
JZ
D000818:Animals; D016203:CDC2 Protein Kinase / Q000378:metabolism; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D004625:Embryo, Nonmammalian; D005260:Female; D020869:Gene Expression Profiling; D008099:Liver / Q000196:embryology / Q000378:metabolism*; D008540:Meiosis / Q000235:genetics; D009865:Oocytes / Q000254:growth & development / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011374:Progesterone / Q000378:metabolism; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D040901:Proteomics; D015843:Serpins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D029867:Xenopus Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis / Q000196:embryology / Q000235:genetics
D010446:Peptide Fragments; D020033:Protein Isoforms; D015843:Serpins; D029867:Xenopus Proteins; C077804:SERPINA6 protein, Xenopus; D011374:Progesterone; D016203:CDC2 Protein Kinase