野田 洋一 京都大学, 医学部, 助教授 (50115911)
MANFRED J Lo カルガリー大学, 生物学教室, 助教授
JAMES L Mall コロラド大学, ハワードヒューズ医学研究所, 教授
田中 実 岡崎国立共同研究機構, 基礎生物学研究所, 助手 (80202175)
山下 正兼 北海道大学, 理学部, 助教授 (30202378)
吉国 通庸 岡崎国立共同研究機構, 基礎生物学研究所, 助手 (50210662)
LOHKA Manfre カルガリー大学, 助教授
MALLER James コロラド大学, ハワードヒューズ医学研究所, 教授
IZUMI Tetsuro University of Colorado, Postdoctoral Fellow
NODA Youichi University of Kyoto, Associate Professor
MALLER James L. University of Colorado School of Medicine, Professor
LOHKA Manfred J. University of Calgary, Professor
TANAKA Minoru National Institute for Basic Biology, Assistant Prof.
YAMASHITA Masakane Hokkaido University, Associate Professor
YOSHIKUNI Michiyasu National Institute for Basic Biology, Assistant Prof.
|Budget Amount *help
¥7,600,000 (Direct Cost : ¥7,600,000)
Fiscal Year 1993 : ¥3,500,000 (Direct Cost : ¥3,500,000)
Fiscal Year 1992 : ¥4,100,000 (Direct Cost : ¥4,100,000)
The purpose of this cooperative project was that two research groups directed by Drs.Yoshitaka Nagahama (Okazaki, Japan) and J.L.Maller (Denver, USA) collaborate to clarify the general principle involved in the regulatory mechanism of oocyte maturation in vertebrates, in particular amphibians (Maller, USA) and fishes (Nagahama, Japan).
In both amphibians (Xenopus) and fishes (salmonids, goldfish, etc.), gonadotropins (probably LH-like gonadotropin) are responsible for triggering oocyte maturation. However, in both cases, gonadotropins do not act directly on the oocyte, but act on the ovarian follicle cells to produce maturation-inducing hormones (progesterone in amphibians and 17alpha, 20beta-dihydroxy-4-pregnen-3-one, 17alpha, 20beta-DP, in fishes). The site of the action of both maturation-inducing hormones is the surface of the oocytes ; specific surface receptors responsible for the progesterone-and 17alpha, 20beta-DP-induced oocyte maturation were found on the plasma membrane of Xe
nopus, rainbow trout and flounder oocytes. Two research groups will continuously collaborate to purify and characterize these steroid hormone membrane receptors.
Microinjection of progesterone and 17alpha, 20beta-DP failed to induce oocyte maturation, suggesting that they do not act directly on the germinal vesicle but seems to act through a third mediator in the oocyte cytoplasm. This third mediator of cytoplasmic nature has been called maturation-promoting factor (MPF). MPF was purified and characterized from oocytes of Xenopus and goldfish (carp), consisting of two common components, cdc2 kinase and cyclin B. Although the components of MPF in Xenopus and goldfish oocytes, are the same, the mechanisms of MPF activation are different between these two species. In goldfish, 17alpha, 20beta-DP induces immature oocytes to synthesize cyclin B, which in turn activates preexisting 35-kDa cdc2 kinase through its threonine (Thr161) phosphorylation, producing the 34-kDa active cdc2 kinase. These mechanisms of MPF activation in fish oocytes apparently differ from those in Xenopus, in which cyclin B is present in immature oocytes and forms a complex with cdc2 kinase (pre-MPF). Furthermore, in Xenopus oocytes, dephosphorylation of threonine (Thr14) and tyrosine (Tyr15) is required for cdc2 kinase activation. However, it is doubtful that tyrosine (Tyr15) dephosphorylation of cdc2 kinase is not a prerequisite for its activation during goldfish oocyte maturation, since the activation is not inhibited by vanadade, a protein phosphatase inhibitor commonly used for inhibiting cdc25 activity that dephosphorylates tyrosine (Tyr15) of cdc2 kinase, thereby inhibits its activation. Less