Towards an understanding of protein kinase B (PKB/Akt) function in mouse development

Protein kinase B (PKB/Akt) belongs to a subfamily of serine/threonine protein kniases called AGC protein kinases. Homologues of PKB can be found in worms, flies and mammals. Three isoforms of PKB, termed PKBα/Akt1, PKBβ/Akt2 and PKBγ/Akt3 that are encoded by three distinct genes, have been identified in mammals like mice and humans. PKB can be activated by numerous growth factors, hormones, cytokines and other stimuli through a phosphatidylinositol 3-kinase (PI3K)-dependent manner. The signaling pathway of PI3K/PKB/Akt has been established and the significance of this pathway for numerous cellular and physiological processes has been recognized and widely accepted. The understanding of developmental principles in mouse is a big challenge. How PKB contributes to mouse development and why three isoforms exist in mice have been wondering researchers in this field since the identification of these proteins in this animal. Early mouse work using northern blotting and in situ hybridization showed expression of PKB/Akt in mouse embryos with isoform- and tissue-specific properties. Thus, PKB/Akt may play important roles in mouse development. In addition, the distinct tissue distribution patterns of the three isoforms suggest that these proteins have different functions. To address these questions, we generated mouse mutant for each isoform by homologous recombination. Characterization and analyses of these mice have provided new insights into the functions of PKB/Akt in mouse development. We found that PKBα/Akt1 was the predominant isoform in placenta. PKBα/Akt1 mutant mice were born small with increased neonatal mortality. The mutant placenta displayed reduced size and impaired development and glycogen-containing spongiotrophoblast cells are
rare. More significant is a decrease in vascularization of the mutant placenta. As the
size and structure of the placenta determines the growth of the fetus, we conclude that
PKBα/Akt1 modulates placental development and, thus, fetal growth.
In contrast to PKBα/Akt1 mutant mice, PKBγ/Akt3 mutant mice did not show
increased postnatal mortality and and grew normally. However, these mice displayed a
reduced brain size by 25% after birth. This indicates that PKBγ/Akt3 is an important
modulator of postnatal brain growth.
We crossed PKBα/Akt1 mutant mice with PKBγ/Akt3 mutant mice to produce
compound knockout mice and found that the two proteins have different roles in the
maintenance of animal survival. While Pkbα+/−Pkbγ −/− (Akt1+/-Akt3 -/-) mice survived
normally, almost all Pkbα -/-Pkbγ +/-(Akt1-/- Akt3+/-) mice died at an early age with
multiple pathologies. PKBα/γ (Akt1/3) double knockout mice were embryonic lethal at
around E12. The development of these mice was severely impaired, including the
branchial arch arteries, the brain and the placenta. We conclude that PKBα/Akt1 is
more important than PKBγ/Akt3 for animal survival but both are required for mouse
development.