The ins and outs of erythroid heme transport.

Heme plays critical roles in erythropoiesis not only as a structural component of hemoglobin but also as a regulator of erythroid proliferation by affecting the expression of proteins involved in iron transport and globin synthesis. Within the cell, heme is synthesized in the mitochondria and is subsequently transported to the cytosol for incorporation into hemoglobin and other hemoproteins. Because excess free heme is cytotoxic, intracellular heme must be maintained at levels that support function while preventing cellular damage. There is growing evidence that the tight control of intracellular heme levels is accomplished by coordination of heme synthesis, degradation, and trafficking between intracellular compartments. In contrast to the well-characterized biochemistry of heme synthesis and degradation, the mechanisms of heme transport remain somewhat obscure. Feline leukemia virus subgroup C receptor (Flvcr1) is the cellsurface receptor for a retrovirus that causes pure red cell aplasia in cats. It was first cloned by Janis Abkowitz’s group, who later demonstrated that Flvcr1 exports heme and is essential for fetal erythropoiesis using Flvcr1-null mice. Recently, Emanuela Tolosano and colleagues showed that human Flvcr1 encodes two distinct transcripts: a cell surface isoform Flvcr1a and a shorter, mitochondrial isoform that lacks the first exon, termed Flvcr1b. This group previously generated mice with a germline deletion of Flvcr1a that retains Flvcr1b expression and found normal erythroid development in Flvcr1a-null embryos. This is in sharp contrast to the lethal erythroid differentiation block seen in mice lacking both Flvcr1 isoforms, suggesting that the Flvcr1b is able to support erythroid differentiation in the absence of Flvcr1a and the erythroid defect in Flvcr1a/1b-null mice could be a result of cytosolic heme deficiency caused by the loss of Flvcr1b, rather than heme accumulation due to Flvcr1a deletion. While the mouse Flvcr1 knockout models strongly suggest a dispensable role of Flvcr1a in fetal erythropoiesis, it cannot be excluded that Flvcr1a may be important in other stages of erythropoiesis. Indeed, the function of Flvcr1b inferred from the Flvcr1a mutant phenotype had not been verified by in vivo models of Flvcr1b deficiency. In this issue of Haematologica, Mercurio and colleagues build on their previous analysis of germline Flvcr1a knockout mice and continue to dissect roles of Flvcr1a and Flvcr1b isoforms in erythroid tissue by using mouse, zebrafish and cell culture models. In mice and zebrafish, deletion of Flvcr1a resulted in anemia with a block in erythroid expansion, suggesting that Flvcr1a is required for the proliferation of erythroid progenitor cells. Although there are no data on Flvcr1b-specific knockouts, a distinct role of Flvcr1b in erythroid maturation can be deduced from the terminal maturation arrest in mouse bone marrow lacking both Flvcr1a/1b and the failure of Flvcr1a supplementation to restore the erythroid differentiation defect in Flvcr1a/1b-null zebrafish. In addition, heme depletion and supplementation in zebrafish morphants rescued erythroid defects associated with the loss of Flvcr1a and Flvcr1a/1b, providing indirect evidence for a causal relationship between cytosolic heme accumulation/deficiency and the erythroid phenotype. Knockdown experiments in human K562 cells further established a direct link between Flvcr1a and Flvcr1b silencing with cytosolic and mitochondrial heme accumulation and erythroid development defects. Taken together, these data suggest that Flvcr1a and Flvcr1b are involved in distinct stages of erythropoiesis and likely function as safety valves to maintain optimal cytosolic heme levels during erythroid development. The discovery of cellular heme transporters such as Flvcr1a and Flvcr1b reveals the exquisite mechanisms in controlling heme balance. However, many questions remain regarding the function and cellular characteristics of Flvcr1 isoforms in erythroid tissue. First, the critical role of Flvcr1b in erythroid maturation is largely inferred from animal models of Flvcr1a deficiency. This speculation needs to be verified in animals with specific deletion of Flvcr1b with intact Flvcr1a expression. Second, information on the subcellular localization of Flvcr1 isoforms was obtained from non-erythroid cells overexpressing the proteins. It is unclear whether erythroid cells share this expression pattern. Tissue-specific Flvcr1b is suggested by differential Flvcr1b expression in erythroid cells and hepatocytes in response to ablation of the Flvcr1a-specific exon 1. Conditional deletion of exon 1 in mouse bone marrow reported in this paper results in loss of Flvcr1a but also of Flvcr1b, while liver-specific deletion with the same targeted allele has no impact on Flvcr1b expression. Little is known about the regulation of Flvcr1 and how intracellular heme transport coordinates with heme synthesis and degradation to achieve optimal intracellular heme balance throughout erythroid development. Finally, to fully understand the physiological significance of Flvcr1 at the systemic level, the role of Flvcr1 isoforms in non-erythroid tissues with high heme trafficking activity, such as duodenum and reticuloendothelial macrophages, as well as their contribution to local and systemic heme homeostasis are important issues to address in future studies.