Bridging from the single cell to the cell population - Learning from mechanisms controlling erythropoietin (Epo)-induced cellular responses and erythroleukemia

In a healthy human individual, every second 2.5 million red blood cells (erythrocytes) are generated. Despite this high-proliferative capacity, erythroleukemia is rather rare. Key regulator of erythropoiesis is the hormone erythropoietin (Epo) that is secreted by the kidney and acts primarily on erythroid progenitor cells at the colony-forming unit-erythroid (CFU-E) stage. CFU-E cells express the Epo receptor (EpoR) and Epo is essential for their survival, proliferation and differentiation. Since CFU-E cells are readily accessible and can be cultivated in vitro, this system is highly suitable to establish strategies for high-quality data generation and to advance mathematical modeling approaches for resolving key regulatory mechanisms. For example, at the cell population level, a graded increase in survival of CFU-E cells in response to Epo stimulation is observed, whereas at the single cell level survival is a binary decision. By a mathematical modelling approach, we resolved that heterogeneity in Epo-induced STAT5 phosphorylation is due to variability in membrane associated processes such as the recruitment of the phosphatase SHP1 to the EpoR and the cytoplasmic volume, which defines a survival threshold. The variability in cytoplasmic volume explains how CFU-E cells convert a switch-like behavior in individual cells to graded population level responses (Adlung et al., Cell Rep 2021). In the absence of the EpoR the production of hemoglobin in CFU-E cells fails and a massive iron overload is observed in the fetal liver of EpoR knockout embryos, which is accompanied by a reduction in iron-sulfur cluster proteins involved in oxidative phosphorylation. Therefore, the link of erythropoiesis, regulation of iron homeostasis and metabolic reprogramming turns out to be crucial for the protection of the embryo from iron intoxication and for its survival (Chakraborty et al., Cell Rep 2022). Currently, the comprehensive dynamic pathway models established for Epo-induced signal transduction in CFU-E cells and cellular model systems (Schilling et al., Mol Syst Biol 2009; Becker et al., Science 2010, Bachmann et al., Mol Syst Biol 2011, Adlung et al., Mol Syst Biol 2017) are adapted to the situation in patient-derived erythroleukemia cells to unravel mechanisms that contribute to cancer progression and to propose promising intervention strategies.