Erythroblastic islands foster granulopoiesis in parallel to terminal erythropoiesis.

The erythroblastic island (EBI), composed of a central macrophage surrounded by maturing erythroblasts, is the erythroid precursor niche. Despite numerous studies, its precise composition is still unclear. Using multispectral imaging flow cytometry, in vitro island reconstitution, and single-cell RNA sequencing of adult mouse bone marrow (BM) EBI-component cells enriched by gradient sedimentation, we present evidence that the CD11b+ cells present in the EBIs are neutrophil precursors specifically associated with BM EBI macrophages, indicating that erythro-(myelo)-blastic islands are a site for terminal granulopoiesis and erythropoiesis. We further demonstrate that the balance between these dominant and terminal differentiation programs is dynamically regulated within this BM niche by pathophysiological states that favor granulopoiesis during anemia of inflammation and favor erythropoiesis after erythropoietin stimulation. Finally, by molecular profiling, we reveal the heterogeneity of EBI macrophages by cellular indexing of transcriptome and epitope sequencing of mouse BM EBIs at baseline and after erythropoietin stimulation in vivo and provide a searchable online viewer of these data characterizing the macrophage subsets serving as hematopoietic niches. Taken together, our findings demonstrate that EBIs serve a dual role as niches for terminal erythropoiesis and granulopoiesis and the central macrophages adapt to optimize production of red blood cells or neutrophils.

Quantitative analysis of murine terminal erythroid differentiation in vivo: novel method to study normal and disordered erythropoiesis.

Terminal erythroid differentiation is the process during which proerythroblasts differentiate to produce enucleated reticulocytes. Although it is well established that during murine erythropoiesis in vivo, 1 proerythroblast undergoes 3 mitosis to generate sequentially 2 basophilic, 4 polychromatic, and 8 orthochromatic erythroblasts, currently there is no method to quantitatively monitor this highly regulated process. Here we outline a method that distinguishes each distinct stage of erythroid differentiation in cells from mouse bone marrow and spleen based on expression levels of TER119, CD44, and cell size. Quantitative analysis revealed that the ratio of proerythroblasts:basophilic:polychromatic:orthromatic erythroblasts follows the expected 1:2:4:8 ratio, reflecting the physiologic progression of terminal erythroid differentiation in normal mice. Moreover, in 2 stress erythropoiesis mouse models, phlebotomy-induced acute anemia and chronic hemolytic anemia because of 4.1R deficiency, the ratio of these erythroblast populations remains the same as that of wild-type bone marrow. In contrast, in anemic ß-thalassemia intermedia mice, there is altered progression which is restored to normal by transferrin treatment which was previously shown to ameliorate the anemic phenotype. The means to quantitate in vivo murine erythropoiesis using our approach will probably have broad application in the study of altered erythropoiesis in various red cell disorders.

Isolation and functional characterization of human erythroblasts at distinct stages: implications for understanding of normal and disordered erythropoiesis in vivo.

Terminal erythroid differentiation starts from morphologically recognizable proerythroblasts that proliferate and differentiate to generate red cells. Although this process has been extensively studied in mice, its characterization in humans is limited. By examining the dynamic changes of expression of membrane proteins during in vitro human terminal erythroid differentiation, we identified band 3 and α4 integrin as optimal surface markers for isolating 5 morphologically distinct populations at successive developmental stages. Functional analysis revealed that these purified cell populations have distinct mitotic capacity. Use of band 3 and α4 integrin enabled us to isolate erythroblasts at specific developmental stages from primary human bone marrow. The ratio of erythroblasts at successive stages followed the predicted 1:2:4:8:16 pattern. In contrast, bone marrows from myelodysplastic syndrome patients exhibited altered terminal erythroid differentiation profiles. Thus, our findings not only provide new insights into the genesis of the red cell membrane during human terminal erythroid differentiation but also offer a means of isolating and quantifying each developmental stage during terminal erythropoiesis in vivo. Our findings should facilitate a comprehensive cellular and molecular characterization of each specific developmental stage of human erythroblasts and should provide a powerful means of identifying stage-specific defects in diseases associated with pathological erythropoiesis.

Red or green? Enucleation traffic light.

In this issue of Blood, Keerthivasan and colleagues provide compelling support for the novel concept that the formation, movement, and fusion of endocytic vesicles in the region between the extruding nucleus and nascent reticulocyte are critical steps in erythroblast enucleation.

Hereditary spherocytosis and hereditary elliptocytosis: aberrant protein sorting during erythroblast enucleation.

During erythroblast enucleation, membrane proteins distribute between extruded nuclei and reticulocytes. In hereditary spherocytosis (HS) and hereditary elliptocytosis (HE), deficiencies of membrane proteins, in addition to those encoded by the mutant gene, occur. Elliptocytes, resulting from protein 4.1R gene mutations, lack not only 4.1R but also glycophorin C, which links the cytoskeleton and bilayer. In HS resulting from ankyrin-1 mutations, band 3, Rh-associated antigen, and glycophorin A are deficient. The current study was undertaken to explore whether aberrant protein sorting, during enucleation, creates these membrane-spanning protein deficiencies. We found that although glycophorin C sorts to reticulocytes normally, it distributes to nuclei in 4.1R-deficient HE cells. Further, glycophorin A and Rh-associated antigen, which normally partition predominantly to reticulocytes, distribute to both nuclei and reticulocytes in an ankyrin-1-deficient murine model of HS. We conclude that aberrant protein sorting is one mechanistic basis for protein deficiencies in HE and HS.

The water channel aquaporin-1 partitions into exosomes during reticulocyte maturation: implication for the regulation of cell volume.

Aquaporin-1 (AQP-1), the universal water channel, is responsible for rapid response of cell volume to changes in plasma tonicity. In the membrane of the red cell the concentration of the protein is tightly controlled. Here, we show that AQP-1 is partially lost during in vitro maturation of mouse reticulocytes and that it is associated with exosomes, released throughout this process. AQP-1 in young reticulocytes localizes to the plasma membrane and also in endosomal compartments and exosomes, formed both in vitro and in vivo. During maturation a part of the total pool of AQP-1 is differentially sorted and released via the exosomal pathway. A proteasome inhibitor, MG132, suppresses secretion of AQP-1, implying that ubiquitination is a sorting signal for its release. We further show that modulation of medium tonicity in vitro regulates the secretion of AQP-1, thus showing that extracellular osmotic conditions can drive sorting of selected proteins by the exosomal pathway. These results lead us to suggest that AQP-1 sorting into exosomes may be the mechanism by which the reticulocyte adapts to environmental changes during its maturation.

Protein 4.1R-dependent multiprotein complex: new insights into the structural organization of the red blood cell membrane.

Protein 4.1R (4.1R) is a multifunctional component of the red cell membrane. It forms a ternary complex with actin and spectrin, which defines the nodal junctions of the membrane-skeletal network, and its attachment to the transmembrane protein glycophorin C creates a bridge between the protein network and the membrane bilayer. We now show that deletion of 4.1R in mouse red cells leads to a large diminution of actin accompanied by extensive loss of cytoskeletal lattice structure, with formation of bare areas of membrane. Whereas band 3, the preponderant transmembrane constituent, and proteins known to be associated with it are present in normal or increased amounts, glycophorin C is missing and XK, Duffy, and Rh are much reduced in the 4.1R-deficient cells. The inference that these are associated with 4.1R was borne out by the results of in vitro pull-down assays. Furthermore, whereas Western blot analysis showed normal levels of band 3 and Kell, flow cytometric analysis using an antibody against the extracellular region of band 3 or Kell revealed reduction of these two proteins, suggesting a conformational change of band 3 and Kell epitopes. Taken together, we suggest that 4.1R organizes a macromolecular complex of skeletal and transmembrane proteins at the junctional node and that perturbation of this macromolecular complex not only is responsible for the well characterized membrane instability but may also remodel the red cell surface.

Erythroblastic islands: niches for erythropoiesis.

Erythroblastic islands, the specialized niches in which erythroid precursors proliferate, differentiate, and enucleate, were first described 50 years ago by analysis of transmission electron micrographs of bone marrow. These hematopoietic subcompartments are composed of erythroblasts surrounding a central macrophage. A hiatus of several decades followed, during which the importance of erythroblastic islands remained unrecognized as erythroid progenitors were shown to possess an autonomous differentiation program with a capacity to complete terminal differentiation in vitro in the presence of erythropoietin but without macrophages. However, as the extent of proliferation, differentiation, and enucleation efficiency documented in vivo could not be recapitulated in vitro, a resurgence of interest in erythroid niches has emerged. We now have an increased molecular understanding of processes operating within erythroid niches, including cell-cell and cell-extracellular matrix adhesion, positive and negative regulatory feedback, and central macrophage function. These features of erythroblast islands represent important contributors to normal erythroid development, as well as altered erythropoiesis found in such diverse diseases as anemia of inflammation and chronic disease, myelodysplasia, thalassemia, and malarial anemia. Coupling of historical, current, and future insights will be essential to understand the tightly regulated production of red cells both in steady state and stress erythropoiesis.

Adhesive activity of Lu glycoproteins is regulated by interaction with spectrin.

The Lutheran (Lu) and Lu(v13) blood group glycoproteins function as receptors for extracellular matrix laminins. Lu and Lu(v13) are linked to the erythrocyte cytoskeleton through a direct interaction with spectrin. However, neither the molecular basis of the interaction nor its functional consequences have previously been delineated. In the present study, we defined the binding motifs of Lu and Lu(v13) on spectrin and identified a functional role for this interaction. We found that the cytoplasmic domains of both Lu and Lu(v13) bound to repeat 4 of the alpha spectrin chain. The interaction of full-length spectrin dimer to Lu and Lu(v13) was inhibited by repeat 4 of alpha-spectrin. Further, resealing of this repeat peptide into erythrocytes led to weakened Lu-cytoskeleton interaction as demonstrated by increased detergent extractability of Lu. Importantly, disruption of the Lu-spectrin linkage was accompanied by enhanced cell adhesion to laminin. We conclude that the interaction of the Lu cytoplasmic tail with the cytoskeleton regulates its adhesive receptor function.

Chromatin condensation in terminally differentiating mouse erythroblasts does not involve special architectural proteins but depends on histone deacetylation.

Terminal erythroid differentiation in vertebrates is characterized by progressive heterochromatin formation and chromatin condensation and, in mammals, culminates in nuclear extrusion. To date, although mechanisms regulating avian erythroid chromatin condensation have been identified, little is known regarding this process during mammalian erythropoiesis. To elucidate the molecular basis for mammalian erythroblast chromatin condensation, we used Friend virus-infected murine spleen erythroblasts that undergo terminal differentiation in vitro. Chromatin isolated from early and late-stage erythroblasts had similar levels of linker and core histones, only a slight difference in nucleosome repeats, and no significant accumulation of known developmentally regulated architectural chromatin proteins. However, histone H3(K9) dimethylation markedly increased while histone H4(K12) acetylation dramatically decreased and became segregated from the histone methylation as chromatin condensed. One histone deacetylase, HDAC5, was significantly upregulated during the terminal stages of Friend virus-infected erythroblast differentiation. Treatment with histone deacetylase inhibitor, trichostatin A, blocked both chromatin condensation and nuclear extrusion. Based on our data, we propose a model for a unique mechanism in which extensive histone deacetylation at pericentromeric heterochromatin mediates heterochromatin condensation in vertebrate erythroblasts that would otherwise be mediated by developmentally-regulated architectural proteins in nucleated blood cells.

Contractile forces sustain and polarize hematopoiesis from stem and progenitor cells.

Self-renewal and differentiation of stem cells depend on asymmetric division and polarized motility processes that in other cell types are modulated by nonmuscle myosin-II (MII) forces and matrix mechanics. Here, mass spectrometry-calibrated intracellular flow cytometry of human hematopoiesis reveals MIIB to be a major isoform that is strongly polarized in hematopoietic stem cells and progenitors (HSC/Ps) and thereby downregulated in differentiated cells via asymmetric division. MIIA is constitutive and activated by dephosphorylation during cytokine-triggered differentiation of cells grown on stiff, endosteum-like matrix, but not soft, marrow-like matrix. In vivo, MIIB is required for generation of blood, while MIIA is required for sustained HSC/P engraftment. Reversible inhibition of both isoforms in culture with blebbistatin enriches for long-term hematopoietic multilineage reconstituting cells by 5-fold or more as assessed in vivo. Megakaryocytes also become more polyploid, producing 4-fold more platelets. MII is thus a multifunctional node in polarized division and niche sensing.

The erythroid niche: molecular processes occurring within erythroblastic islands.

Erythroblasts terminally differentiate within specialized niches composed of erythroblast islands nesting in extracellular matrix proteins. A number of adhesion molecules active in erythroid island attachments have been identified. We have recently observed a receptor/counter receptor interaction that appears to maintain island integrity: erythroid ICAM-4 interacting with macrophage alphaV integrin. When ICAM-4/alphaV binding is blocked, a 70% decrease in islands is observed. Moreover, erythroblastic islands are markedly decreased in ICAM-4 null mice. Using erythropoietin to examine whether ICAM-4/alphaV binding plays a role in stress erythropoiesis, we found that the reticulocyte response is different in ICAM-4 null mice compared to control mice. We speculate that this may be a reflection of the baseline decrease in island number in the ICAM-4 null mice. Erythroblast enucleation also occurs within the erythroid niche. Earlier, we examined whether abnormal protein sorting during nuclear extrusion creates the deficiencies of membrane proteins that are well described in hereditary spherocytosis (HS) and hereditary elliptocytosis (HE). We observed that whereas glycophorin C partitions to reticulocytes in normal mouse cells, it sorts to extruding nuclei in murine hereditary elliptocytosis cells. Additionally, in a murine model of hereditary spherocytosis, band 3, glycophorin A and RhAG partition to both nuclei and reticulocytes, while in normal cells these three proteins distribute predominantly to reticulocytes. Hence, it appears that abnormal protein sorting generates specific protein deficiencies in hereditary elliptocytosis and hereditary spherocytosis.

The Laminin 511/521-binding site on the Lutheran blood group glycoprotein is located at the flexible junction of Ig domains 2 and 3.

The Lutheran blood group glycoprotein, first discovered on erythrocytes, is widely expressed in human tissues. It is a ligand for the alpha5 subunit of Laminin 511/521, an extracellular matrix protein. This interaction may contribute to vaso-occlusive events that are an important cause of morbidity in sickle cell disease. Using x-ray crystallography, small-angle x-ray scattering, and site-directed mutagenesis, we show that the extracellular region of Lutheran forms an extended structure with a distinctive bend between the second and third immunoglobulin-like domains. The linker between domains 2 and 3 appears to be flexible and is a critical determinant in maintaining an overall conformation for Lutheran that is capable of binding to Laminin. Mutagenesis studies indicate that Asp312 of Lutheran and the surrounding cluster of negatively charged residues in this linker region form the Laminin-binding site. Unusually, receptor binding is therefore not a function of the domains expected to be furthermost from the plasma membrane. These studies imply that structural flexibility of Lutheran may be essential for its interaction with Laminin and present a novel opportunity for the development of therapeutics for sickle cell disease.

Erythroblastic islands: specialized microenvironmental niches for erythropoiesis.

PURPOSE OF REVIEW: This review focuses on current understanding of molecular mechanisms operating within erythroblastic islands including cell-cell adhesion, regulatory feedback, and central macrophage function. RECENT FINDINGS: Erythroblasts express a variety of adhesion molecules and recently two interactions have been identified that appear to be critical for island integrity. Erythroblast macrophage protein, expressed on erythroblasts and macrophages, mediates cell-cell attachments via homophilic binding. Erythroblast intercellular adhesion molecule-4 links erythroblasts to macrophages through interaction with macrophage alphav integrin. In intercellular adhesion molecule-4 knockout mice, erythroblastic islands are markedly reduced, whereas the erythroblast macrophage protein null phenotype is severely anemic and embryonic lethal. Retinoblastoma tumor suppressor (Rb) protein stimulates macrophage differentiation by counteracting inhibition of Id2 on PU.1, a transcription factor that is a crucial regulator of macrophage differentiation. Rb-deficient macrophages do not bind Rb null erythroblasts and the Rb null phenotype is anemic and embryonic lethal. Lastly, extruded nuclei rapidly expose phosphatidylserine on their surface, providing a recognition signal similar to apoptotic cells. SUMMARY: Although understanding of molecular mechanisms operating within islands is at an early stage, tantalizing evidence suggests that erythroblastic islands are specialized niches where intercellular interactions in concert with cytokines play critical roles in regulating erythropoiesis.

Targeted gene deletion demonstrates that the cell adhesion molecule ICAM-4 is critical for erythroblastic island formation.

Erythroid progenitors differentiate in erythroblastic islands, bone marrow niches composed of erythroblasts surrounding a central macrophage. Evidence suggests that within islands adhesive interactions regulate erythropoiesis and apoptosis. We are exploring whether erythroid intercellular adhesion molecule 4 (ICAM-4), an immunoglobulin superfamily member, participates in island formation. Earlier, we identified alpha(V) integrins as ICAM-4 counterreceptors. Because macrophages express alpha(V), ICAM-4 potentially mediates island attachments. To test this, we generated ICAM-4 knock-out mice and developed quantitative, live cell techniques for harvesting intact islands and for re-forming islands in vitro. We observed a 47% decrease in islands reconstituted from ICAM-4 null marrow compared to wild-type marrow. We also found a striking decrease in islands formed in vivo in knock-out mice. Further, peptides that block ICAM-4/alpha(V) adhesion produced a 53% to 57% decrease in reconstituted islands, strongly suggesting that ICAM-4 binding to macrophage alpha(V) functions in island integrity. Importantly, we documented that alpha(V) integrin is expressed in macrophages isolated from erythroblastic islands. Collectively, these data provide convincing evidence that ICAM-4 is critical in erythroblastic island formation via ICAM-4/alpha(V) adhesion and also demonstrate that the novel experimental strategies we developed will be valuable in exploring molecular mechanisms of erythroblastic island formation and their functional role in regulating erythropoiesis.

Peptides based on alphaV-binding domains of erythrocyte ICAM-4 inhibit sickle red cell-endothelial interactions and vaso-occlusion in the microcirculation.

Growing evidence shows that adhesion molecules on sickle erythrocytes interact with vascular endothelium leading to vaso-occlusion. Erythrocyte intercellular adhesion molecule-4 (ICAM-4) binds alphaV-integrins, including alphaVbeta3 on endothelial cells. To explore the contribution of ICAM-4 to vascular pathology of sickle cell disease, we tested the effects of synthetic peptides, V(16)PFWVRMS (FWV) and T(91)RWATSRI (ATSR), based on alphaV-binding domains of ICAM-4 and capable of inhibiting ICAM-4 and alphaV-binding in vitro. For these studies, we utilized an established ex vivo microvascular model system that enables intravital microscopy and quantitation of adhesion under shear flow. In this model, the use of platelet-activating factor, which causes endothelial oxidant generation and endothelial activation, mimicked physiological states known to occur in sickle cell disease. Infusion of sickle erythrocytes into platelet-activating factor-treated ex vivo rat mesocecum vasculature produced pronounced adhesion of erythrocytes; small-diameter venules were sites of maximal adhesion and frequent blockage. Both FWV and ATSR peptides markedly decreased adhesion, and no vessel blockage was observed with either of the peptides, resulting in improved hemodynamics. ATSR also inhibited adhesion in unactivated microvasculature. Although infused fluoresceinated ATSR colocalized with vascular endothelium, pretreatment with function-blocking antibody to alphaVbeta3-integrin markedly inhibited this interaction. Our data strengthen the thesis that ICAM-4 on sickle erythrocytes binds endothelium via alphaVbeta3 and that this interaction contributes to vaso-occlusion. Thus peptides or small molecule mimetics of ICAM-4 may have therapeutic potential.

Nuclear substructure reorganization during late-stage erythropoiesis is selective and does not involve caspase cleavage of major nuclear substructural proteins.

Enucleation, a rare feature of mammalian differentiation, occurs in 3 cell types: erythroblasts, lens epithelium, and keratinocytes. Previous investigations suggest that caspase activation functions in lens epithelial and keratinocyte enucleation, as well as in early erythropoiesis encompassing erythroid burst-forming unit (BFU-E) differentiation to proerythroblast. To determine whether caspase activation contributes to later erythropoiesis and whether nuclear substructures other than chromatin reorganize, we analyzed distributions of nuclear subcompartment proteins and assayed for caspase-induced cleavage of subcompartmental target proteins in mouse erythroblasts. We found that patterns of lamin B in the filamentous network interacting with both the nuclear envelope and DNA, nuclear matrix protein NuMA (Nuclear mitotic apparatus), and splicing factors Sm and SC35 persisted during nuclear condensation, consistent with effective transcription of genes expressed late in differentiation. Thus, nuclear reorganization prior to enucleation is selective, allowing maintenance of critical transcriptional processes independent of extensive chromosomal reorganization. Consistent with these data, we found no evidence for caspase-induced cleavage of major nuclear subcompartment proteins during late erythropoiesis, in contrast to what has been observed in early erythropoiesis and in lens epithelial and keratinocyte differentiation. These findings imply that nuclear condensation and extrusion during terminal erythroid differentiation involve novel mechanisms that do not entail major activation of apoptotic machinery.

Shear-response of the spectrin dimer-tetramer equilibrium in the red blood cell membrane.

The red cell membrane derives its elasticity and resistance to mechanical stresses from the membrane skeleton, a network composed of spectrin tetramers. These are formed by the head-to-head association of pairs of heterodimers attached at their ends to junctional complexes of several proteins. Here we examine the dynamics of the spectrin dimer-dimer association in the intact membrane. We show that univalent fragments of spectrin, containing the dimer self-association site, will bind to spectrin on the membrane and thereby disrupt the continuity of the protein network. This results in impairment of the mechanical stability of the membrane. When, moreover, the cells are subjected to a continuous low level of shear, even at room temperature, the incorporation of the fragments and the consequent destabilization of the membrane are greatly accentuated. It follows that a modest shearing force, well below that experienced by the red cell in the circulation, is sufficient to sever dimer-dimer links in the network. Our results imply 1) that the membrane accommodates the enormous distortions imposed on it during the passage of the cell through the microvasculature by means of local dissociation of spectrin tetramers to dimers, 2) that the network in situ is in a dynamic state and undergoes a "breathing" action of tetramer dissociation and re-formation.

Two protein 4.1 domains essential for mitotic spindle and aster microtubule dynamics and organization in vitro.

Multifunctional structural proteins belonging to the 4.1 family are components of nuclei, spindles, and centrosomes in vertebrate cells. Here we report that 4.1 is critical for spindle assembly and the formation of centrosome-nucleated and motor-dependent self-organized microtubule asters in metaphase-arrested Xenopus egg extracts. Immunodepletion of 4.1 disrupted microtubule arrays and mislocalized the spindle pole protein NuMA. Remarkably, assembly was completely rescued by supplementation with a recombinant 4.1R isoform. We identified two 4.1 domains critical for its function in microtubule polymerization and organization utilizing dominant negative peptides. The 4.1 spectrin-actin binding domain or NuMA binding C-terminal domain peptides caused morphologically disorganized structures. Control peptides with low homology or variant spectrin-actin binding domain peptides that were incapable of binding actin had no deleterious effects. Unexpectedly, the addition of C-terminal domain peptides with reduced NuMA binding caused severe microtubule destabilization in extracts, dramatically inhibiting aster and spindle assembly and also depolymerizing preformed structures. However, the mutant C-terminal peptides did not directly inhibit or destabilize microtubule polymerization from pure tubulin in a microtubule pelleting assay. Our data showing that 4.1 is a crucial factor for assembly and maintenance of mitotic spindles and self-organized and centrosome-nucleated microtubule asters indicates that 4.1 is involved in regulating both microtubule dynamics and organization. These investigations underscore an important functional context for protein 4.1 in microtubule morphogenesis and highlight a previously unappreciated role for 4.1 in cell division.