SLC20A1 at the Interface between Macrophages and Red Blood Cells

Red blood cells (RBCs) comprise 84% of our body's cells and have the essential function of transporting oxygen from the lungs to all organs. The daily production rate of RBCs is enormous and the pathways mediating this process are quite complex. One important aspect hereby is the synthesis of hemoglobin and heme, the iron-containing prosthetic group that enables hemoglobin to bind oxygen.RBCs differentiate within the erythropoietic niche, which consists of erythroid precursors and specialized macrophages. These so-called nursing macrophages provide nutrients and iron, a fundamental component of heme.

The precise mechanisms of heme transport within a cell and the potential of heme transfer between cells is not completely understood. To get insights into heme transport we took advantage of the cytotoxic capacity of heme and performed a CRISPR-Cas9 loss of function screen by focusing on SLC transporters. We identified SLC20A1 as an essential protein mediating heme toxicity and verified that intracellular heme levels as well as heme-induced downstream gene inductions were reduced in the absence of Slc20a1. Based on these evidences we hypothesized that SLC20A1 might be involved in the trafficking of heme, possibly important during erythropoiesis, since mice lacking Slc20a1 exhibit embryonic lethality due to liver apoptosis and anemia (Beck L. et al., PLoS One 2013).

To test the biological role of Slc20a1 in adult mice, we conditionally deleted Slc20a1 using tamoxifen in ERT2-Cre Slc20a1^fl animals and observed the spontaneous development of severe anemia and splenomegaly within 2 weeks after tamoxifen administration. We further discovered that the anemia-related expansion of red pulp macrophages (RPM) consisted predominantly of non-recombined wild type cells, whereas successfully recombined (Slc20a1 deficient) cells seemed stuck in the monocytic stage. These data suggest that Slc20a1 might be involved in the differentiation and maturation of monocytes to RPMs. When we performed a specific Slc20a1 deletion in nursing macrophages (CD169 Cre), mice neither showed signs of anemia, nor experienced impaired recovery upon anemia induction following phlebotomy or induction of hemolysis. However, deleting Sc20a1 specifically in the erythroid compartment using Slc20a1^fl mice crossed to erythropoietin receptor-cre (EpoR-Cre) animals, resulted in embryonic lethality. Mice died around E12.5 due to severe anemia. Analysis of E11.5 animals disclosed erythroid precursors to be arrested in the pro-erythroblast stage.These data suggest that SLC20A1 is a protein involved in heme-mediated toxicity and possibly in the trafficking of heme with strong impact on fetal and adult erythropoiesis.