Embryonic Anemia In Pop2ca-deleted Mice

Protein phosphatase2A (PP2A) is the most abundant phosphatase specific for serine and threonine phosphorylation. It is composed of a common heteromeric core enzyme, that is composed of a catalytic subunit (C) and a scaffolding subunit (A), which associates with a variety of regulatory subunits (B). The catalytic subunit (PP2Ac) has two isoforms:PP2Acα (encoded by the Ppp2ca gene) and PP2Acβ(encoded by the Ppp2cb gene). PP2Acα is considered as the main functional PP2Ac isoform, because of its10-fold abundance than PP2Acβ in most mouse tissues. Dephosphorylation of substrates targeted by PP2A mediates a large variety of biological processes, such as proliferation, survival, differentiation, migration, and adhesion. The vast archive of chemical inhibitors, animal models and related studies demonstrate that PP2A phosphatase activity is important in tumorigenesis, nerve degeneration, hematopoiesis, myocardium contractile maintenance, and energy balance. However, precise knowledge about its function in developmental biology, especially the establishment of the vascular and blood system, needs to be analyzed in detail.In this study, we deleted the floxed Ppp2co locus in endothelial and hematopoietic progenitor cells with a Tie2Cre transgene. Briefly,40%Tie2Cre+/Ppp2ca/fl/fl embryos died between E10.5and E14.5, whereas11.6%mutants were born normally. Unexpectedly, PP2Aca-deficient embryos did not manifest any defects in vascular development. However, deletion of Ppp2ca in Tie2+cells perturbed fetal liver erythropoiesis with partial loss of late progenitor cells, blockage of erythroid differentiation, and diminished globin expression. Meanwhile, the erythropoietic cytokines, including erythropoietin and stem cell factor, were not affected by loss of the Ppp2ca allele. These results indicated us that PP2Acα controls erythroid cells in a cell-autonomous manner.We further analyzed whether the initial seeding of fetal liver with definitive hematopoietic progenitor cells was affected by Ppp2ca deletion. We found that absolute Lin-c-Kit+Sca-1+(LSK) cell numbers remained unchanged (at E12.5) or even upregulated at later stage (at E14.5) in mutant fetal livers. Meanwhile, we noticed that loss of PP2Acoα resulted in an impaired expansion of the fetal erythroid compartment that is associated with decreased proliferation and increased apoptosis. The increased programmed cell death of committed erythroid cells was accompanied by reduction in the tyrosine-phosphorylated form of STAT5(Tyr694) and total protein level of anti-apoptotic factor Bcl-xL. Consistent with these observations, inhibition of PP2A activity by okadaic acid (OA) administration on primary fetal liver cells also impaired the STAT5-Bcl-xL pathway.Taken together, our data indicate a crucial role for Ppp2ca in fetal liver erythropoiesis during the development of the blood system. The decreased expression of Bcl-xL in mutant fetal liver cells is likely to be a principal cause of their decreased survival. However, we are still on the way to set up the links from deletion of Ppp2ca, to decreased Tyr694phosphorylation of STAT5, and finally to fetal anemia.