Biochemical changes during progesterone-induced maturation in xenopus laevis oocytes

Summary

The main biochemical changes which occur, when maturation is induced in isolatedX. laevis oocytes by the addition of progesterone to the medium, are discussed. The following points are presented:

• 1. The peripheric region of the oocyte contains a specific receptor for progesterone; it is a protein with a sedimentation constant of 15–18S.

• 2. It is doubtful that cAMP plays a direct role in the initiation of maturation.

• 3. Experiments with inhibitors (KCN, dinitrophenol) show that maturation requires the production of energy.

• 4. There is no DNA synthesis — except mito-chondrial DNA synthesis — during maturation; the latter is possible in the presence of inhibitors of DNA synthesis (hydroxyurea, 2'-d-adenosine, cytosine arabinoside, bromodeoxyuridine, dimethylbenzyl-rifampicine, ethidium bromide, X-rays).

• 5. Actinomycin D and, to a lesser extent, α-amanitin speed up maturation. Cordycepin has no effect on this process.

• 6. Protein synthesis is required for maturation; but experiments with cycloheximide and puromycin have shown that the breakdown of the germinal vesicle (G.V.) is still possible when protein synthesis is inhibited by 50%. Progesterone induces a transient increase in protein synthesis, which is followed by a strong inhibition.

• 7. Progesterone induces the synthesis or release of 2 antagonistic factors: one of them (MPF) promotes maturation; the other (PIF) produces a special kind of abortive maturation, which has been calledpseudomaturation. PIF becomes detectable several hours before MPF; it produces, after injection into the oocytes, an 80 to 90% inhibition of protein synthesis. Preliminary characterization of PIF indicates that it is neither a protease nor a ribonuclease. It is localized in the outer part of the oocyte and is, in part, attached to membranes. It is thermolabile, non dialyzable.

It is concluded that progesterone controls maturation, at the post-transcriptional level, by the production of two antagonistic factors (MPF and PIF), which respectively exert a positive and a negative regulation.