Using Xenopus oocyte extracts to study signal transduction.

Xenopus oocytes are naturally arrested at G2/M in prophase I of meiosis. Stimulation with progesterone initiates a nontranscriptional signaling pathway that culminates in the activation of Cdc2/cyclin B and reentry into meiosis. This pathway presents a paradigm for nongenomic signaling by steroid hormones and for the G2/M cell cycle transition. It has been extensively studied using intact oocytes, which are amenable to microinjection and biochemical analyses described elsewhere in this book. However, there are several experimental advantages in using in vitro systems consisting of cytosolic fractions of prophase-arrested oocytes. Because of their homogeneous nature, extracts avoid the difficulties of signaling asynchrony between individual oocytes. They are also amenable to biochemical manipulations such as protein immunodepletions, and proteins and pharmacological agents can be added easily. Despite these features, oocyte extracts have yet to achieve the widespread utility of Xenopus egg extracts, which can proceed through rounds of deoxyribonucleic acid (DNA) replication and mitosis in vitro. Here, we review the historical development of oocyte extracts and discuss the factors most crucial to success in reproducing the signaling pathway and the G2/M transition in vitro.