Mitogen activated protein kinase plays a significant role in metaphase II arrest, spindle morphology, and maintenance of maturation promoting factor activity in bovine oocytes

Mammalian oocytes are arrested at the G2/M transition of the first meiotic division from which, after reaching full size and subsequent to an LH surge , they undergo final maturation. Oocyte maturation, which involves germinal vesicle breakdown, progression through metaphase I (MI), and arrest at Mil , is triggered and regulated by the coordinated action of two kinases, matu ration promoting factor (MPF) and mitogen activated protein kinase (MAPK). The importance of the role of MPF in mammalian oocyte maturation is well es tablished, while the role of MAPK, although well understood in mouse oocyte s, has not been fully elucidated in oocytes of targe domestic species, espe cially bovine oocytes. Here we show that injection of MKP-1 mRNA, which enc odes a dual specificity MAPK phosphatase, into germinal vesicle stage bovin e oocytes prevents the activation of MAPK during maturation. Despite the la ck of MAPK activity, MKP-1-injected oocytes resume and progress through mei osis, although they are unable to arrest at Mit stage and, by 22-26-hour po st-maturation, exhibit decondensed pronucleus-like chromatin, a clear sign of parthenogenetic activation. MKP-1-injected bovine oocytes exhibit normal activation of MPF activity; however, by 18-hour post-maturation, MPF activ ity starts to decline and by 22-26 hr MPF activity is absent. MKP-1-injecte d oocytes also show disorganized MII spindles with poorly aligned chromosom es. In summary, our results demonstrate that in bovine oocytes MAPK activit y is required for MII arrest, maintenance of MPF activity, and spindle orga nization. Mol. Reprod. Dev. 59:106-114, 2001. (C) 2001 Wiley-Liss, Inc.