An analysis of the control of the Xenopus oocyte meiotic cell cycles

Andrew David Johnson, Purdue University

Abstract

Xenopus oocytes are arrested in the first meiotic cell cycle during which time they grow, and accumulate the maternal components that regulate early embryonic development. Oocytes are triggered to resume the cell cycle by secretion of progesterone by the follicle cells that encase the oocytes. Only the largest oocytes are capable of responding to the progesterone signal, and they do so by activating a cytoplasmic component, MPF, that regulates entry into M phase in all eukaryotic cells. Although smaller oocytes contain the precursor to MPF, pre-MPF, they are unable to activate it in response to hormonal signals. Small oocytes can only activate MPF after experimental manipulations. I have injected cyclin mRNA into small oocytes and shown that it can activate MPF. This is similar to the situation seen when cyclin is injected into large oocytes, and suggests that small oocytes contain all of the components necessary for MPF activation other than cyclin. MPF has proven very difficult to purify due its extremely labile nature. When extracts are made from cyclin injected oocytes, however, the MPF appears very stable. This is likely due to protection of the major component of MPF, p34cdc2, which can be inactivated by phosphorylation. The stability of MPF in these crude extracts directly reflected the stability of cyclin. In additional experiments the activities of cyclin A and cyclin B were compared. In these experiments cyclin A was much more stable than cyclin B. This is likely to have biological relevance as only cyclin B mRNA has been seen in the oocyte to date. RNA encoding an active ras oncogene was also injected into oocytes. Although ras could induce MPF activation in stage 6 oocytes it could not in stage 4 oocytes. Instead ras caused these oocytes to activate. Activation is the response normally seen at fertilization. The activation response is involved in the destruction of MPF and exit from M phase of the second meiotic cell cycle. These data suggest that ras might be involved in the normal response of eggs to fertilization. Furthermore, these results may demonstrate a role for ras in controlling exit from M phase in cleaving embryonic cells, or eukaryotic cells in general.

Degree

Ph.D.

Advisors

Smith, Purdue University.

Subject Area

Biology

COinS