A Heme Export Protein Is Required for Red Cell Differentiation and May Facilitate Iron Homeostasis.

Heme is a component of cytochromes, catalases, glutathione peroxidase, hydroxylases, and nitric oxide synthase, as well as myoglobin and hemoglobin, and thus is critical to all aerobic cells. It is also a transcriptional and translational initiator of globin synthesis. However, free heme is toxic so that cells must balance synthesis with use. We recently determined that the cell surface receptor for Feline Leukemia Virus, subgroup C, (FLVCR), which causes pure red cell aplasia in cats, is a heme export protein (Cell; 118:6, 2004). To understand its physiologic significance, we generated constitutive (Flvcr^+/−) and inducible (Flvcr^flox/flox;Mx-cre) FLVCR null mice by targeted deletion of exon 3, and confirmed this produced a nonfunctional protein by zinc mesoporphyrin (ZnMP) and ⁵⁵Fe-heme export studies. Intercrossing Flvcr^+/− mice produced 0 null animals among 109 progeny genotyped. FLVCR null embryos die either before E8.5 or at E14.5, suggesting two separate mechanisms for their demise. In situ hybridization studies in normal mouse embryos show highest Flvcr transcript levels in the ectoplacental cone which forms placenta (E8.5), the placenta (E9.5), and high levels in the fetal liver (E12.5). These data plus studies with Flvcr^flox/flox x Meox2-Cre mice (Cre inactivates Flvcr in all embryonic tissue, but not extraembryonic visceral endoderm and placenta) suggest that the early lethality of null mice is due to impaired maternal-embryo heme-iron transport. At E14.5, residual null mice were small with deformed limb buds and craniofacial abnormalities resembling the congenital anomalies of Diamond-Blackfan anemia. Flow cytometric analyses of fetal liver cells from deleted animals with Ter119 and CD71 demonstrated a block in erythroid differentiation at the CFU-E/proerythroblast stage, implying that their death results from a stage-specific failure in definitive erythropoiesis. Heterozygous null animals (Flvcr^+/−) were viable with normal hematologic parameters. These mice had 30.9% of normal Flvcr mRNA by q RT-PCR, however, they demonstrated normal protein by western blot analysis and normal heme export function by ZnMP studies. Thus, heterozygous null mice compensate at the protein level, confirming FLVCR is critical in vivo. We next utilized our inducible mutant line (Flvcr^flox/flox;Mx-cre) to generate a viable null mouse for study. Flvcr deleted mice developed pure red cell aplasia characterized by macrocytic anemia (deleted: HCT 13.8±1.2, MCV 68.35± 1.6; control: HCT 50.24 ± 2.0, MCV 51.3±1.5), reticulocytopenia, and reactive thrombocytosis. Marrow and spleen had excess cells with a proerythroblast morphology and lacked maturing red cells. Flow cytometry confirmed maturation arrest at the CFU-E/proerythroblast stage. CFU-E from FLVCR null mice were undetectable in culture. Deleted mice also demonstrated iron loading of hepatocytes. FLVCR is highly expressed in multiple human tissues with high heme-flux by western blot analyses, including liver, placenta, duodenum, and uterus, in addition to the bone marrow. Our studies prove that FLVCR is required for the survival or differentiation of proerythroblasts (that stage where heme synthesis intensifies). These results, plus additional data not shown, suggest that FLVCR may also protect nonerythroid tissues with high free heme exposure (uterus, duodenum), may mediate maternal-embryo heme-iron transport, and may mediate the transport of heme from hepatocyte to bile which would imply a novel mechanism for iron export from the body.