Macrophage Erythroblast Attacher (MAEA), but Not VCAM1, Is Required for the Bone Marrow Erythroblastic Niche

Definitive mammalian erythropoiesis occurs in a specialized niche called erythroblastic island (EI), which is composed of a central macrophage surrounded by maturing erythroblasts. The attachment of the developing erythroblasts to the central macrophages within the islands has been suggested to be critical for the survival, proliferation and regulated differentiation of the developing erythrocytes both in vitro and in vivo. Several adhesion molecules have been suggested to mediate the interaction between the macrophage and erythroblasts in the EI niche. However, cell type-specific requirement of these molecules for EI formation and function in vivo hasn’t been examined. We sought to identify the crucial adhesion molecule(s) responsible for the in vivo EI function using macrophage-specific conditional deletion mouse models. As Vascular Cell Adhesion Molecule-1 (VCAM1) has been suggested to play a key role in erythropoiesis, we deleted the gene selectively in macrophage by crossing Vcam1^fl/fl mice with Csf1r-Cre transgenic mice. Our results revealed that macrophage VCAM1 was not required for steady-state erythropoiesis in vivo since bone marrow and spleen erythroblasts and hematocrit levels were not altered. Stress erythropoiesis induced by phenylhydrazine (PHZ) led to mild deficit in hematocrit recovery but no significant anemia, suggesting the contribution of other adhesion receptors. We next generated a conditional floxed allele of the Macrophage Erythroblast Attacher (Maea), which has also been suggested to be an important component of the erythroblastic niche (Soni et al. J. Biol. Chem, 2006). Efficient ablation of MAEA (~2-fold protein level reduction on macrophages) using Csf1r-Cre resulted in a more than 3-fold reduction of leukocyte counts (p=0.003), but no significant anemia in peripheral blood. However, the cellularity in the bone marrow was significantly reduced in Maea-deficient mice, owing largely to > 2-fold reduction of erythroblasts (p=0.01). Analyses of erythroblast maturation by FACS revealed a significant increase in the proportion of the less mature erythroblasts at the expenses of terminal differentiation, indicating that MAEA regulates erythroblast maturation.  Interestingly, BM macrophage numbers were also severely affected in Maea-deficient mice (~4-fold reduction, p=0.01), suggesting a role for MAEA in macrophage development.  By contrast, the spleen of Maea^fl/-; Csf1r-Cre mice did not exhibit any reduction in erythroblast numbers, indicating that the erythroblast-macrophage interaction may be differentially regulated in BM and spleen. Unexpectedly, Maea ablation impaired the engraftment ability of BM hematopoietic stem and progenitors (HSPCs) after transplantation, suggesting broader functions for this protein in hematopoiesis.  These studies identify MAEA as a critical adhesion mediator between the erythroblasts and central macrophages in adult murine bone marrow. Ongoing studies will shed light on its involvement in stress and pathological erythropoiesis and HSPC regulation.