Deletion of Dicer in late erythroid cells results in impaired stress erythropoiesis in mice.

MicroRNAs (miRNAs) have been shown to influence erythroid lineage commitment and differentiation; however, our knowledge of miRNA function in terminal erythropoiesis remains limited. To address this issue, we generated a novel animal model, where the miRNA-processing enzyme, Dicer, is selectively inactivated in erythropoietin receptor positive erythroid cells beginning with CFU-e/proerythroblast cells. This results in significant depletion of all miRNAs from the proerythroblast stage onwards, with one exception, miR-451, which is processed by Ago2 in a Dicer-independent manner. We observed that mature Dicer-dependent miRNAs, like miR-451, are dispensable under steady-state conditions, but these mutants have an impaired response to stress erythropoiesis, as demonstrated by a delay in recovery from anemia. This defect was specific to later maturing erythroid cells, as progenitor numbers were unaffected. In addition to generating a novel mouse model to study miRNA function in late erythroid cells, we conclude that miRNAs (both Dicer-dependent and independent) act primarily to regulate the optimal response to stress among late erythroid cells.

FLVCR is necessary for erythroid maturation, may contribute to platelet maturation, but is dispensable for normal hematopoietic stem cell function.

Heme is a pleiotropic molecule that is important for oxygen and oxidative metabolism, most notably as the prosthetic group of hemoglobin and cytochromes. Because excess free intracellular heme is toxic, organisms have developed mechanisms to tightly regulate its concentration. One mechanism is through active heme export by the group C feline leukemia virus receptor (FLVCR). Previously, we have shown that FLVCR is necessary for embryonic and postnatal erythropoiesis. However, FLVCR is also expressed in numerous other tissues, including hematopoietic stem cells (HSCs). To explore a possible role for FLVCR in HSC function, we performed serial, competitive repopulation transplant experiments using FLVCR-deleted and control bone marrow cells, along with wild-type competitor cells. Loss of FLVCR did not impact HSC function under steady-state or myelotoxic stress conditions (such as arsenic or radiation exposure), nor did FLVCR deletion result in alterations in the various progenitor compartments. However, even when 95% of the donor bone marrow cells lacked FLVCR, all red cells in recipient mice were wild type. This is due to the increased apoptosis of FLVCR-deleted proerythroblasts. Also, remarkably, loss of FLVCR increased megakaryocyte ploidy. Together, these findings show FLVCR is redundant in stem cells but has critical and contrasting stage-specific roles in discrete hematopoietic lineages.

MicroRNAs: Allies or foes in erythropoiesis?

MicroRNAs are small non-coding RNAs that negatively regulate gene expression through mRNA degradation or translational repression. It is becoming increasingly recognized that miRNAs play central roles in almost all cellular processes, and especially during development. The function of miRNAs in hematopoiesis, including erythropoiesis, is beginning to be elucidated. In this review, we will focus on what is known about miRNA function in various aspects of erythropoiesis and red cell physiology.