The SCL Transcription Factor Supports Erythroid Cell Survival and Differentiation Downstream of the Erythropoietin Receptor.

Erythroid cell development critically depends on the SCL/Tal1 transcription factor and on erythropoietin signalling. In the present study, we have taken several approaches to show that the two genes operate within the same pathway to consolidate the erythroid lineage. Signaling through the erythropoietin receptor (EpoR) upregulates SCL protein levels in a clonal cell line (TF-1) in vitro, and in murine fetal liver cells in vivo, when Epor^−/− cells were compared to those of wild type littermates at E12.5. In addition, we provide functional evidence for a linear pathway from EpoR to SCL that regulates erythropoiesis. Interfering with SCL induction or SCL function prevents the anti-apoptotic effect of Epo in TF-1 cells and conversely, ectopic SCL expression is sufficient to substitute for Epo to transiently maintain cell survival. In vivo, SCL gain of function complements the cellular defects in Epor^−/− embryos to support cell survival and maturation during primitive and definitive erythropoiesis, as assessed by cellular and histological analyses of Epor^−/− SCL^tg embryos. Moreover, several erythroid specific genes that are decreased in Epor^−/− embryos are rescued by the SCL transgene including glycophorinA, bH1 and b^maj globin, providing molecular confirmation of the functional and genetic interaction between Epor and SCL. Conversely, erythropoiesis becomes deficient in compound Epor^+/−SCL^+/− heterozygote mice, indicating that the genetic interaction between EpoR and SCL is synthetic. Finally, using EpoR mutants that harbour well defined signalling deficiencies, combined with gain and loss of function approaches for specific kinases, we identify MAPK as the major signal transduction pathway downstream of EpoR that upregulates SCL function, necessary for erythroid cell survival and differentiation. Taken together, our observations are consistent with the view that cytokines can influence cell fate by altering the dosage of lineage transcriptional regulators.