The decrease in maturation-promoting factor (MPF) activity precedes that in
mitogen-activated protein kinase (MAPK) activity after egg activation, but
the cellular functions of this delayed inactivation of MAPK are still
unclear. The present study was conducted to examine the essential role of
MAPK activity for supporting the transition from metaphase to interphase
in porcine oocytes matured in vitro. The increases in the phosphorylated
forms of MAPK and the activities of MAPK and histone H1 kinase (H1K) were
shown in oocytes arrested at the metaphase II (MII) stage. After additional
incubation of MII-arrested oocytes in medium with added U0126, a specific
inhibitor of MAPK kinase, 24% of oocytes completed the second meiotic
division and underwent entry into interphase with pronucleus (PN) formation,
but not second polar body (PB-2) emission. The intensities of the
phosphorylated forms of MAPK and the activities of MAPK and H1K in
matured oocytes treated with U0126 were significantly decreased by the
treatment with U0126. Electrostimulation to induce artificial activation
caused both H1K and MAPK inactivation; the inactivation of H1K preceded
the inactivation of MAPK and sustained high levels of MAPK activity were
detected during the period of PB-2 emission. However, the time sequence
required for MAPK inactivation was significantly reduced by the addition
of U0126 to the culture medium following electrostimulation, resulting in
the dramatic inactivation of MAPK distinct from that of H1K. In these
oocytes, PB-2 emission was markedly inhibited but little difference was
found in the time course of PN formation compared with oocytes not treated
with U0126. These findings suggest that the decrease in MAPK activity is
partly involved in driving matured oocytes out of metaphase to induce PN
development, and that the delayed MAPK inactivation after the onset of MPF
inactivation in activated oocytes has a crucial role for PB-2 emission to
accomplish the transition from meiosis to mitosis.