The midblastula transition in Xenopus embryos activates multiple pathways to prevent apoptosis in response to DNA damage

The midblastula transition in Xenopus embryos activates multiple pathways to prevent apoptosis in response to DNA damage

Howard Hughes Medical Institute and Department of Pharmacology, University of Colorado School of Medicine, Denver, CO 80262

Communicated by Joan V. Ruderman, Harvard Medical School, Boston, MA (received for review September 19, 2000)

Abstract

Apoptosis is controlled by a complex interplay between regulatory proteins. Previous work has shown that Xenopus embryos remove damaged cells by apoptosis when irradiated before, but not after, the midblastula transition (MBT). Here we demonstrate that Akt/protein kinase B is activated and mediates an antiapoptotic signal only in embryos irradiated after the MBT. In addition, an increase in xBcl-2/xBax oligomerization and a decrease in xBax homodimerization promote a protective effect against apoptosis only after the MBT. The post-MBT survival mechanism arrests cells in G₁ phase by increasing expression of the cyclin-dependent kinase inhibitor p27^Xic1. p27^Xic1 associates with cyclin D/Cdk4 and cyclin A/Cdk2 complexes to cause G₁/S arrest, perhaps allowing more time for DNA repair. Taken together, the results define the DNA damage response as an element of the MBT and indicate that multiple mechanisms prevent apoptosis after the MBT.

Footnotes

↵ * To whom reprint requests should be addressed at: Department of Pharmacology, University of Colorado School of Medicine, Box C-236, Denver, CO 80262. E-mail: Jim.Maller{at}uchsc.edu.
Data deposition: The sequence reported in this paper has been deposited in the GenBank database (accession no. AF288809).
Abbreviations:

MBT,

midblastula transition;

cdk,

cyclin-dependent kinase;

CKI,

cdk inhibitor;

γ-IR,

γ-irradiation, γ-irradiated;

GST,

glutathione S-transferase;

xBax,

Xenopus Bax homolog;

WT,

wild type;

DN,

dominant negative;

CA,

constitutively active