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Erythropoiesis is one of the most efficient cellular processes in the human body producing approximately 2.5 million red blood cells every second. This process occurs in a bone marrow niche comprised of a central resident macrophage surrounded by differentiating erythroblasts, termed an erythroblastic island. It is not known what initially attracts the macrophage to erythroblasts to form these islands. The ephrin/Eph receptor family are known to regulate heterophilic cell-cell adhesion. We find that human VCAM1 + and VCAM1 - bone marrow macrophages and in vitro cultured macrophages are ephrin-B2 positive, whereas differentiating human erythroblasts express EPHB4, EPHB6 and EPHA4. Furthermore, we detect a rise in integrin activation on erythroblasts at the stage at which the cells bind which is independent of EPH receptor presence. Using a live cell imaging assay, we show that specific inhibitory peptides or shRNA depletion of EPHB4 cause a significant reduction in the ability of macrophages to interact with erythroblasts but do not affect integrin activation. This study demonstrates for the first time that EPHB4 expression is required on erythroblasts to facilitate the initial recognition and subsequent interaction with macrophages, alongside the presence of active integrins., (Copyright© 2020 Ferrata Storti Foundation.)

The classical central macrophage found in erythroblastic islands plays an important role in erythroblast differentiation, proliferation and enucleation in the bone marrow. Convenient human in vitro models to facilitate the study of erythroid-macrophage interactions are desired. Recently, we demonstrated that cultured monocytes/macrophages enhance in vitro erythropoiesis by supporting hematopoietic stem and progenitor cell survival. Herein, we describe that these specific macrophages also support erythropoiesis. Human monocytes cultured in serum-free media supplemented with stem cell factor, erythropoietin, lipids and dexamethasone differentiate towards macrophages expressing CD16, CD163, CD169, CD206, CXCR4 and the phagocytic TAM-receptor family. Phenotypically, they resemble both human bone marrow and fetal liver resident macrophages. This differentiation is dependent on glucocorticoid receptor activation. Proteomic studies confirm that glucocorticoid receptor activation differentiates monocytes to anti-inflammatory tissue macrophages with a M2 phenotype, termed GC-macrophages. Proteins involved in migration, tissue residence and signal transduction/receptor activity are upregulated whilst lysosome and hydrolase activity GO-categories are downregulated. Functionally, we demonstrate that GC-macrophages are highly mobile and can interact to form clusters with erythroid cells of all differentiation stages and phagocytose the expelled nuclei, recapitulating aspects of erythroblastic islands. In conclusion, glucocorticoid-directed monocyte differentiation to macrophages represents a convenient model system to study erythroid-macrophage interactions., (Copyright© 2018 Ferrata Storti Foundation.)

Ankyrin-R provides a key link between band 3 and the spectrin cytoskeleton that helps to maintain the highly specialized erythrocyte biconcave shape. Ankyrin deficiency results in fragile spherocytic erythrocytes with reduced band 3 and protein 4.2 expression. We use in vitro differentiation of erythroblasts transduced with shRNAs targeting ANK1 to generate erythroblasts and reticulocytes with a novel ankyrin-R 'near null' human phenotype with less than 5% of normal ankyrin expression. Using this model, we demonstrate that absence of ankyrin negatively impacts the reticulocyte expression of a variety of proteins, including band 3, glycophorin A, spectrin, adducin and, more strikingly, protein 4.2, CD44, CD47 and Rh/RhAG. Loss of band 3, which fails to form tetrameric complexes in the absence of ankyrin, alongside GPA, occurs due to reduced retention within the reticulocyte membrane during erythroblast enucleation. However, loss of RhAG is temporally and mechanistically distinct, occurring predominantly as a result of instability at the plasma membrane and lysosomal degradation prior to enucleation. Loss of Rh/RhAG was identified as common to erythrocytes with naturally occurring ankyrin deficiency and demonstrated to occur prior to enucleation in cultures of erythroblasts from a hereditary spherocytosis patient with severe ankyrin deficiency but not in those exhibiting milder reductions in expression. The identification of prominently reduced surface expression of Rh/RhAG in combination with direct evaluation of ankyrin expression using flow cytometry provides an efficient and rapid approach for the categorization of hereditary spherocytosis arising from ankyrin deficiency., (Copyright© Ferrata Storti Foundation.)

Band 3 is the most abundant protein in the erythrocyte membrane and forms the core of a major multiprotein complex. The absence of band 3 in human erythrocytes has only been reported once, in the homozygous band 3 Coimbra patient. We used in vitro culture of erythroblasts derived from this patient, and separately short hairpin RNA-mediated depletion of band 3, to investigate the development of a band 3-deficient erythrocyte membrane and to specifically assess the stability and retention of band 3 dependent proteins in the absence of this core protein during terminal erythroid differentiation. Further, using lentiviral transduction of N-terminally green fluorescent protein-tagged band 3, we demonstrated the ability to restore expression of band 3 to normal levels and to rescue secondary deficiencies of key proteins including glycophorin A, protein 4.2, CD47 and Rh proteins arising from the absence of band 3 in this patient. By transducing band 3-deficient erythroblasts from this patient with band 3 mutants with absent or impaired ability to associate with the cytoskeleton we also demonstrated the importance of cytoskeletal connectivity for retention both of band 3 and of its associated dependent proteins within the reticulocyte membrane during the process of erythroblast enucleation., (Copyright© Ferrata Storti Foundation.)

Congenital dyserythropoietic anemia type II is an autosomally recessive form of hereditary anemia caused by SEC23B gene mutations. Patients exhibit characteristic phenotypes including multinucleate erythroblasts, erythrocytes with hypoglycosylated membrane proteins and an apparent double plasma membrane. Despite ubiquitous expression of SEC23B, the effects of mutations in this gene are confined to the erythroid lineage and the basis of this erythroid specificity remains to be defined. In addition, little is known regarding the stage at which the disparate phenotypes of this disease manifest during erythropoiesis. We employ an in vitro culture system to monitor the appearance of the defining phenotypes associated with congenital dyserythropoietic anemia type II during terminal differentiation of erythroblasts derived from small volumes of patient peripheral blood. Membrane protein hypoglycosylation was detected by the basophilic stage, preceding the onset of multinuclearity in orthochromatic erythroblasts that occurs coincident with the loss of secretory pathway proteins including SEC23A during erythropoiesis. Endoplasmic reticulum remnants were observed in nascent reticulocytes of both diseased and healthy donor cultures but were lost upon further maturation of normal reticulocytes, implicating a defect of ER clearance during reticulocyte maturation in congenital dyserythropoietic anemia type II. We also demonstrate distinct isoform and species-specific expression profiles of SEC23 during terminal erythroid differentiation and identify a prolonged expression of SEC23A in murine erythropoiesis compared to humans. We propose that SEC23A is able to compensate for the absence of SEC23B in mouse erythroblasts, providing a basis for the absence of phenotype within the erythroid lineage of a recently described SEC23B knockout mouse.

The study of human erythropoiesis in health and disease requires a robust culture system that consistently and reliably generates large numbers of immature erythroblasts that can be induced to differentiate synchronously. We describe a culture method modified from Leberbauer et al. (2005) and obtain a homogenous population of erythroblasts from peripheral blood mononuclear cells (PBMC) without prior purification of CD34(+) cells. This pure population of immature erythroblasts can be expanded to obtain 4x10(8) erythroblasts from 1x10(8) PBMC after 13-14 days in culture. Upon synchronized differentiation, high levels of enucleation (80-90%) and low levels of cell death (<10%) are achieved. We compared the yield of erythroblasts obtained from PBMC, CD34(+) cells or PBMC depleted of CD34(+) cells and show that CD34(-) cells represent the most significant early erythroid progenitor population. This culture system may be particularly useful for investigating the pathophysiology of anemic patients where only small blood volumes are available.

Background: Protein 4.2 deficiency caused by mutations in the EPB42 gene results in hereditary spherocytosis with characteristic alterations of CD47, CD44 and RhAG. We decided to investigate at which stage of erythropoiesis these hallmarks of protein 4.2 deficiency arise in a novel protein 4.2 patient and whether they cause disruption to the band 3 macrocomplex., Design and Methods: We used immunoprecipitations and detergent extractability to assess the strength of protein associations within the band 3 macrocomplex and with the cytoskeleton in erythrocytes. Patient erythroblasts were cultured from peripheral blood mononuclear cells to study the effects of protein 4.2 deficiency during erythropoiesis., Results: We report a patient with two novel mutations in EPB42 resulting in complete protein 4.2 deficiency. Immunoprecipitations revealed a weakened ankyrin-1-band 3 interaction in erythrocytes resulting in increased band 3 detergent extractability. CD44 abundance and its association with the cytoskeleton were increased. Erythroblast differentiation revealed that protein 4.2 and band 3 appear simultaneously and associate early in differentiation. Protein 4.2 deficiency results in lower CD47, higher CD44 expression and increased RhAG glycosylation starting from the basophilic stage. The normal downregulation of CD44 expression was not seen during protein 4.2(-) erythroblast differentiation. Knockdown of CD47 did not increase CD44 expression, arguing against a direct reciprocal relationship., Conclusions: We have established that the characteristic changes caused by protein 4.2 deficiency occur early during erythropoiesis. We postulate that weakening of the ankyrin-1-band 3 association during protein 4.2 deficiency is compensated, in part, by increased CD44-cytoskeleton binding.