Identification of critical amino-acid residues on the erythroid intercellular adhesion molecule-4 (ICAM-4) mediating adhesion to alpha V integrins

Glycosylation of blood cells during the onset and progression of atherosclerosis and myocardial infarction

Protein glycosylation controls cell-cell and cell-extracellular matrix (ECM) communication in immune, vascular, and inflammatory processes, underlining the critical role of this process in the identification of disease biomarkers and the design of novel therapies. Emerging evidence highlights the critical role of blood cell glycosylation in the pathophysiology of atherosclerosis (ATH) and myocardial infarction (MI). Here, we review the role of glycosylation in the interplay between blood cells, particularly erythrocytes, and endothelial cells (ECs), highlighting the involvement of this critical post/cotranslational modification in settings of cardiovascular disease (CVD). Importantly, we focus on emerging preclinical studies and clinical trials based on glycan-targeted drugs to validate their therapeutic potential. These findings may help establish new trends in preventive medicine and delineate novel targeted therapies in CVD.

Normal and dysregulated crosstalk between iron metabolism and erythropoiesis

Erythroblasts possess unique characteristics as they undergo differentiation from hematopoietic stem cells. During terminal erythropoiesis, these cells incorporate large amounts of iron in order to generate hemoglobin and ultimately undergo enucleation to become mature red blood cells, ultimately delivering oxygen in the circulation. Thus, erythropoiesis is a finely tuned, multifaceted process requiring numerous properly timed physiological events to maintain efficient production of 2 million red blood cells per second in steady state. Iron is required for normal functioning in all human cells, the erythropoietic compartment consuming the majority in light of the high iron requirements for hemoglobin synthesis. Recent evidence regarding the crosstalk between erythropoiesis and iron metabolism sheds light on the regulation of iron availability by erythroblasts and the consequences of insufficient as well as excess iron on erythroid lineage proliferation and differentiation. In addition, significant progress has been made in our understanding of dysregulated iron metabolism in various congenital and acquired malignant and non-malignant diseases. Finally, we report several actual as well as theoretical opportunities for translating the recently acquired robust mechanistic understanding of iron metabolism regulation to improve management of patients with disordered erythropoiesis, such as anemia of chronic inflammation, β-thalassemia, polycythemia vera, and myelodysplastic syndromes.

Precision Medicine and Sickle Cell Disease

Sickle cell disease (SCD) is characterized by variable clinical outcomes, with some patients suffering life-threatening complications during childhood, and others living relatively symptom-free into old age. Because of this variability, there is an important potential role for precision medicine, in which particular different treatments are selected for different groups of patients. However, the application of precision medicine in SCD is limited by difficulties in identifying different prognostic groups and the small number of available treatments. The main genetic determinant of outcomes in SCD is the underlying β-globin genotype, with sickle cell anemia (HbSS) and hemoglobin SC disease (HbSC) forming the 2 major forms of the disease in most populations of African origin. Although there are clear differences in clinical outcomes between these conditions, treatments approaches are very similar, with little evidence on how to treat HbSC in particular. Other genomic information, such as the co-inheritance of α-thalassemia, or high fetal hemoglobin (HbF) levels, is of some prognostic value but insufficient to determine treatments. Precision medicine is further limited by the fact that the 2 main drugs used in SCD, penicillin and hydroxyurea, are currently recommended for all patients. Newer treatments, such as crizanlizumab and voxelotor, raise the possibility that groups will emerge who respond best to particular drugs or combinations. Perhaps the best current example of precision medicine in SCD is the selective use of blood transfusions as primary stroke prevention in children with evidence of cerebral vasculopathy. More precise treatments may emerge as we understand more about the pathology of SCD, including problems with erythropoiesis.

Erythroblastic Island Macrophages Shape Normal Erythropoiesis and Drive Associated Disorders in Erythroid Hematopoietic Diseases

Erythroblastic islands (EBIs), discovered more than 60 years ago, are specialized microenvironments for erythropoiesis. This island consists of a central macrophage with surrounding developing erythroid cells. EBI macrophages have received intense interest in the verifications of the supporting erythropoiesis hypothesis. Most of these investigations have focused on the identification and functional analyses of EBI macrophages, yielding significant progresses in identifying and isolating EBI macrophages, as well as verifying the potential roles of EBI macrophages in erythropoiesis. EBI macrophages express erythropoietin receptor (Epor) both in mouse and human, and Epo acts on both erythroid cells and EBI macrophages simultaneously in the niche, thereby promoting erythropoiesis. Impaired Epor signaling in splenic niche macrophages significantly inhibit the differentiation of stress erythroid progenitors. Moreover, accumulating evidence suggests that EBI macrophage dysfunction may lead to certain erythroid hematological disorders. In this review, the heterogeneity, identification, and functions of EBI macrophages during erythropoiesis under both steady-state and stress conditions are outlined. By reviewing the historical data, we discuss the influence of EBI macrophages on erythroid hematopoietic disorders and propose a new hypothesis that erythroid hematopoietic disorders are driven by EBI macrophages.

Erythroblast island macrophages: recent discovery and future perspectives

Erythroblastic island (EBI), composed of a central macrophage surrounded by developing erythroid cells, is a structure found in hematopoietic tissues such as fetal liver and bone marrow. It is the first described hematopoietic niche that predominantly supports erythropoiesis. Although it is well accepted that EBIs and EBI macrophage play important roles during erythropoiesis, the mechanisms by which they support erythropoiesis remain largely unclear due to our inability to identify and isolate EBI macrophages. Earlier efforts to identify surface markers for EBI macrophages have focused on the adhesion molecules which are involved in macrophage's interaction with erythroblasts. These include EMP, Vcam1, CD169, CD163, and αV integrin. Findings from these earlier studies suggested that combination of Vcam1, CD169, and mouse macrophage surface marker F4/80 can be used to define mouse EBI macrophage. We found that not all F4/80⁺Vcam1⁺CD169⁺ macrophages are EBI macrophages. Instead, we discovered that EBI macrophages are characterized by the expression of Epor in both mouse and man. RNA-seq analyses of the newly identified EBI macrophages revealed that EBI macrophages have involved specialized function in supporting erythropoiesis. Our findings provide foundation for future studies. Here we will review current knowledge of EBI macrophages and discuss future perspectives.

Identification and transcriptome analysis of erythroblastic island macrophages

The erythroblastic island (EBI), composed of a central macrophage and surrounding erythroid cells, was the first hematopoietic niche discovered. The identity of EBI macrophages has thus far remained elusive. Given that Epo is essential for erythropoiesis and that Epor is expressed in numerous nonerythroid cells, we hypothesized that EBI macrophages express Epor so that Epo can act on both erythroid cells and EBI macrophages simultaneously to ensure efficient erythropoiesis. To test this notion, we used Epor-eGFPcre knockin mouse model. We show that in bone marrow (BM) and fetal liver, a subset of macrophages express Epor-eGFP. Imaging flow cytometry analyses revealed that >90% of native EBIs comprised F4/80⁺Epor-eGFP⁺ macrophages. Human fetal liver EBIs also comprised EPOR⁺ macrophages. Gene expression profiles of BM F4/80⁺Epor-eGFP⁺ macrophages suggest a specialized function in supporting erythropoiesis. Molecules known to be important for EBI macrophage function such as Vcam1, CD169, Mertk, and Dnase2α were highly expressed in F4/80⁺Epor-eGFP⁺ macrophages compared with F4/80⁺Epor-eGFP^- macrophages. Key molecules involved in iron recycling were also highly expressed in BM F4/80⁺Epor-eGFP⁺ macrophages, suggesting that EBI macrophages may provide an iron source for erythropoiesis within this niche. Thus, we have characterized EBI macrophages in mouse and man. Our findings provide important resources for future studies of EBI macrophage function during normal as well as disordered erythropoiesis in hematologic diseases such as thalassemia, polycythemia vera, and myelodysplastic syndromes.

The macrophage contribution to stress erythropoiesis: when less is enough

Although the importance of native bone marrow and spleen macrophages in enhancing baseline and stress erythropoiesis has been emphasized over several decades, their kinetic and phenotypic changes during a variety of stress responses have been unclear. Furthermore, whether monocyte-derived recruited macrophages can functionally substitute for inadequate or functionally impaired native macrophages has been controversial and seem to be not only tissue- but also stress-type dependent. To provide further insight into these issues, we made detailed observations at baseline and post-erythroid stress (E-stress) in 2 mouse models with genetically depressed macrophage numbers and compared them to their controls. We documented that, irrespective of the stress-induced (hemolytic or post-erythropoietin [Epo]) treatment, only native CD11b(lo) splenic macrophages expand dramatically post-stress in normal mice without significant changes in the monocyte-derived CD11b(hi) subset. The latter remained a minority and did not change post-stress in 2 genetic models lacking either Spi-C or VCAM-1 with impaired native macrophage proliferative expansion. Although CD11b(lo) macrophages in these mice were one-fifth of normal at their peak response, surprisingly, their erythroid response was not compromised and was similar to controls. Thus, despite the prior emphasis on numerical macrophage reliance to provide functional rescue from E-stress, our data highlight the importance of previously described non-macrophage-dependent pathways activated under certain stress conditions to compensate for low macrophage numbers.

Disruption of tumour-host communication by downregulation of LFA-1 reduces COX-2 and e-NOS expression and inhibits brain metastasis growth

Over 20% of cancer patients will suffer metastatic spread to the brain, and prognosis remains poor. Communication between tumour cells and host tissue is essential during metastasis, yet little is known of the processes underlying such interactions in the brain.Here we test the hypothesis that cross-talk between tumour cells and host brain cells, through tumour cell leukocyte function associated protein-1 (LFA-1), is critical in metastasis development. Temporal expression of LFA-1 and its major ligand intercellular adhesion molecule-1 (ICAM-1) was determined in two different mouse models of brain metastasis. Marked upregulation of both proteins was found, co-localising with astrocytes, microglia and tumour cells themselves. Silencing of LFA-1 expression in MDA231Br-GFP cells prior to intracerebral injection resulted in > 70% reduction in tumour burden compared to control MDA231Br-GFP cells (p < 0.005, n = 5). Subsequent qRT-PCR analysis of brain tissue revealed significant reductions in COX-2, VEGF and eNOS from host brain tissue, but not tumour cells, in mice injected with LFA-1 knockdown cells (p < 0.0001, n = 5). Finally, expression of both LFA-1 and ICAM-1 was demonstrated in human brain metastasis samples.The results of this study suggest LFA-1 as a new target in brain metastasis therapy and highlight the potential synergy with current anti-COX-2 and anti-NOS therapies.

Extrinsic and intrinsic control by EKLF (KLF1) within a specialized erythroid niche

The erythroblastic island provides an important nutritional and survival support niche for efficient erythropoietic differentiation. Island integrity is reliant on adhesive interactions between erythroid and macrophage cells. We show that erythroblastic islands can be formed from single progenitor cells present in differentiating embryoid bodies, and that these correspond to erythro-myeloid progenitors (EMPs) that first appear in the yolk sac of the early developing embryo. Erythroid Krüppel-like factor (EKLF; KLF1), a crucial zinc finger transcription factor, is expressed in the EMPs, and plays an extrinsic role in erythroid maturation by being expressed in the supportive macrophage of the erythroblastic island and regulating relevant genes important for island integrity within these cells. Together with its well-established intrinsic contributions to erythropoiesis, EKLF thus plays a coordinating role between two different cell types whose interaction provides the optimal environment to generate a mature red blood cell.

Genetic variation of the whole ICAM4 gene in Caucasians and African Americans

BACKGROUND

Landsteiner-Wiener (LW) is the human blood group system Number 16, which comprises two antithetical antigens, LW(a) and LW(b) and the high-prevalence antigen LW(ab) . LW is encoded by the intracellular adhesion molecule 4 (ICAM4) gene. The ICAM4 protein is part of the Rhesus complex in the red cell membrane and is involved in cell-cell adhesion.

STUDY DESIGN AND METHODS

We developed a method to sequence the whole 1.9-kb ICAM4 gene from genomic DNA in one amplicon. We determined the nucleotide sequence of Exons 1 to 3, the two introns, and 402-bp 5'-untranslated region (UTR) and 347-bp 3'-UTR in 97 Caucasian and 91 African American individuals.

RESULTS

Seven variant ICAM4 alleles were found, distinct from the wild-type ICAM4 allele (GenBank KF712272), known as LW*05 and encoding LW(a) . An effect of the LW(a) /LW(b) amino acid substitution on the protein structure was predicted by two of the three computational modeling programs used.

CONCLUSIONS

We describe a practical approach for sequencing and determining the ICAM4 alleles using genomic DNA. LW*05 is the ancestral allele, which had also been observed in a Neanderthal sample. All seven variant alleles are immediate derivatives of the prevalent LW*05 and caused by one single-nucleotide polymorphism (SNP) in each allele. Our data were consistent with the NHLBI GO Exome Sequencing Project (ESP) and the dbSNP databases, as all SNPs had been observed previously. Our study has the advantage over the other databases in that it adds haplotype (allele) information for the ICAM4 gene, clinically relevant in the field of transfusion medicine.

Hydroxycarbamide decreases sickle reticulocyte adhesion to resting endothelium by inhibiting endothelial lutheran/basal cell adhesion molecule (Lu/BCAM) through phosphodiesterase 4A activation

Vaso-occlusive crises are the main acute complication in sickle cell disease. They are initiated by abnormal adhesion of circulating blood cells to vascular endothelium of the microcirculation. Several interactions involving an intricate network of adhesion molecules have been described between sickle red blood cells and the endothelial vascular wall. We have shown previously that young sickle reticulocytes adhere to resting endothelial cells through the interaction of α4β1 integrin with endothelial Lutheran/basal cell adhesion molecule (Lu/BCAM). In the present work, we investigated the functional impact of endothelial exposure to hydroxycarbamide (HC) on this interaction using transformed human bone marrow endothelial cells and primary human pulmonary microvascular endothelial cells. Adhesion of sickle reticulocytes to HC-treated endothelial cells was decreased despite the HC-derived increase of Lu/BCAM expression. This was associated with decreased phosphorylation of Lu/BCAM and up-regulation of the cAMP-specific phosphodiesterase 4A expression. Our study reveals a novel mechanism for HC in endothelial cells where it could modulate the function of membrane proteins through the regulation of phosphodiesterase expression and cAMP-dependent signaling pathways.

Tetraspanins CD81 and CD82 facilitate α4β1-mediated adhesion of human erythroblasts to vascular cell adhesion molecule-1

The proliferation and terminal differentiation of erythroid progenitors occurs in human bone marrow within erythroblastic islands, specialised structures consisting of a central macrophage surrounded by developing erythroid cells. Many cell-cell and cell-matrix adhesive interactions maintain and regulate the co-ordinated daily production of reticulocytes. Erythroid cells express only one integrin, α4β1, throughout differentiation, and its interactions with both macrophage Vascular Cell Adhesion Molecule-1 and with extracellular matrix fibronectin are critical for erythropoiesis. We observed that proerythroblasts expressed a broad tetraspanin phenotype, and investigated whether any tetraspanin could modulate integrin function. A specific association between α4β1 and CD81, CD82 and CD151 was demonstrated by confocal microscopy and co-immune precipitation. We observed that antibodies to CD81 and CD82 augmented adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 but not to the fibronectin spliceoforms FnIII_12-IIICS-15 and FnIII_12–15. In contrast, different anti-CD151 antibodies augmented or inhibited adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 and the fibronectin spliceoform FnIII_12-IIICS-15 but not to FnIII_12–15. These results strongly suggest that tetraspanins have a functional role in terminal erythropoiesis by modulating interactions of erythroblast α4β1 with both macrophages and extracellular matrix.

Evaluation of erythroblast macrophage protein related to erythroblastic islands in patients with hematopoietic stem cell transplantation

Background

Hematopoietic evaluation of the patients after Hematopoietic stem cell transplantation (HSCT) is very important. Erythroblast macrophage protein (Emp) is a key protein with function in normal differentiation of erythroid cells and macrophages. Emp expression correlates with erythroblastic island formation, a process widely believed to be associated with hematopoiesis in bone marrow. We aimed to investigate the hematopoietic function of bone marrow from 46 HSCT patients and 16 inpatients with severe anemia applied to the treatment of EPO by measuring Emp expression level.

Methods

Emp mRNA and protein expression levels in mononuclear cells of bone marrow and peripheral blood samples were detected by RT-PCR and Western blotting method respectively.

Results

While hematopoiesis occurs in bone marrow, Emp expression level was elevated and more erythroblastic islands were found , and Emp is upregulated in bone marrow in response to erythropoietin (EPO) treatment.

Conclusions

Emp expression correlates with erythroblastic island formation and has an important function for bone marrow hematopoiesis. Emp could be a potential biomarker for hematopoietic evaluation of HSCT patients.

Decreased sickle red blood cell adhesion to laminin by hydroxyurea is associated with inhibition of Lu/BCAM protein phosphorylation

Sickle cell disease is characterized by painful vaso-occlusive crises during which abnormal interactions between erythroid adhesion molecules and vessel-wall proteins are thought to play a critical role. Hydroxyurea, the only drug with proven benefit in sickle cell disease, diminishes these interactions, but its mechanism of action is not fully understood. We report that, under hydroxyurea, expression of the unique erythroid laminin receptor Lu/BCAM was increased, but red blood cell adhesion to laminin decreased. Because Lu/BCAM phosphorylation is known to activate cell adhesion to laminin, it was evaluated and found to be dramatically lower in hydroxyurea-treated patients. Analysis of the protein kinase A pathway showed decreased intracellular levels of the upstream effector cyclic adenosine monophosphate during hydroxyurea treatment. Using a cellular model expressing recombinant Lu/BCAM, we showed that hydroxyurea led to decreased intracellular cyclic adenosine monophosphate levels and diminished Lu/BCAM phosphorylation and cell adhesion. We provide evidence that hydroxyurea could reduce abnormal sickle red blood cell adhesion to the vascular wall by regulating the activation state of adhesion molecules independently of their expression level.

Pathophisiology of sickle cell disease and new drugs for the treatment

A homozygous mutation in the gene for β globin, a subunit of adult hemoglobin A (HbA), is the proximate cause of sickle cell disease (SCD). Sickle hemoglobin (HbS) shows peculiar biochemical properties, which lead to polymerizing when deoxygenated. HbS polymerization is associated with a reduction in cell ion and water content (cell dehydration), increased red cell density which further accelerate HbS polymerization. Dense, dehydrated erythrocytes are likely to undergo instant polymerization in conditions of mild hypoxia due to their high HbS concentration, and HbS polymers may be formed under normal oxygen pressure. Pathophysiological studies have shown that the dense, dehydrated red cells may play a central role in acute and chronic clinical manifestations of sickle cell disease, in which intravascular sickling in capillaries and small vessels leads to vaso-occlusion and impaired blood flow in a variety of organs and tissue. The persistent membrane damage associated with HbS polymerization also favors the generation of distorted rigid cells and further contributes to vaso-occlusive crisis (VOCs) and cell destruction in the peripheral circulation. These damaged, dense sickle red cells also show a loss of phospholipid asymmetry with externalization of phosphatidylserine (PS), which is believed to play a significant role in promoting macrophage recognition with removal of erythrocytes (erythrophagocytosis). Vaso-occlusive events in the microcirculation result from a complex scenario involving the interactions between different cell types, including dense, dehydrated sickle cells, reticulocytes, abnormally activated endothelial cells, leukocytes, platelets and plasma factors such as cytokine and oxidized pro-inflammatory lipids. Hydroxycarbamide (hydroxyurea) is currently the only drug approved for chronic administration in adult patients with sickle cell disease to prevent acute painful crises and reduce the incidence of transfusion and acute chest crises. Here, we will focus on consolidated and experimental therapeutic strategies for the treatment of sickle cell disease, including:

agents which reduce or prevent sickle cell dehydration

agents which reduce sickle cell-endothelial adhesive events

nitric oxide (NO) or NO-related compounds

anti-oxidant agents

Correction of the abnormalities ranging from membrane cation transport pathways to red cell-endothelial adhesive events, might constitute new pharmacological targets for treating sickle cell disease.

Generation and characterisation of Rhd and Rhag null mice

Mouse Rhd* and Rhag* genes were targeted using insertional vectors; the resulting knockout mice, and double-knockout descendants, were analysed. Rhag glycoprotein deficiency entailed defective assembly of the erythroid Rh complex with complete loss of Rh and intercellular adhesion molecule 4 (ICAM-4), but not CD47, expression. Absence of the Rh protein induced a loss of ICAM-4, and only a moderate reduction of Rhag expression. Double knockout phenotype was similar to that of Rhag targeted mice. Rhd and Rhag deficient mice exhibited neither the equivalent of human Rh(null) haemolytic anaemia nor any clinical or cellular abnormalities. Rhd-/- and Rhag-/- erythrocytes showed decreased basal adhesion to an endothelial cell line resulting from defective ICAM-4 membrane expression. There was no difference in recovery from phenylhydrazine-induced haematopoietic stress for double knockout mice as compared to controls, suggesting that ICAM-4 might be dispensable during stress erythropoiesis. Ammonia and methylammonia transport in erythrocytes was severely impaired in Rhag-/- but only slightly in Rhd-/- animals that significantly expressed Rhag, supporting the view that RhAG and Rhag, but not Rh, may act as ammonium transporters in human and mouse erythrocytes. These knockout mice should prove useful for further dissecting the physiological roles of Rh and Rhag proteins in the red cell membrane.

Sickle red cell-endothelium interactions

Periodic recurrence of painful vaso-occlusive crisis is the defining feature of sickle cell disease. Among multiple pathologies associated with this disease, sickle red cell-endothelium interaction has been implicated as a potential initiating mechanism in vaso-occlusive events. This review focuses on various interrelated mechanisms involved in human sickle red cell adhesion. We discuss in vitro and microcirculatory findings on sickle red cell adhesion, its potential role in vaso-occlusion, and the current understanding of receptor-ligand interactions involved in this pathological phenomenon. In addition, we discuss the contribution of other cellular interactions (leukocytes recruitment and leukocyte-red cell interaction) to vaso-occlusion, as observed in transgenic sickle mouse models. Emphasis is given to recently discovered adhesion molecules that play a predominant role in mediating human sickle red cell adhesion. Finally, we analyze various therapeutic approaches for inhibiting sickle red cell adhesion by targeting adhesion molecules and also consider therapeutic strategies that target stimuli involved in endothelial activation and initiation of adhesion.

Erythroid adhesion molecules in sickle cell disease: effect of hydroxyurea

In sickle cell disease, the complex scenario of vaso-occlusive crisis (VOC) typical of this disease is clearly multifactorial and not fully understood. Cell-cell and cell-cell matrix interactions mediated by adhesive molecules present on blood cells and endothelial cells (ECs) are thought to play an important role. Early studies have shown that sickle red blood cells (RBCs) are abnormally adherent to ECs and some of the molecules involved in these interactions have been identified, such as the alpha4beta1 integrin and CD36, exclusively present on stress reticulocytes, and CD47 on mature RBCs. More recently, attention focused on Lu/BCAM, the unique RBC receptor for laminin, and on ICAM-4, a red cell-specific adhesion receptor, which is a ligand for a large repertoire of integrins (alphaLbeta2, alphaMbeta2, alphaxbeta2, alphaVbeta3). The counter-receptors on ECs and the role of plasma proteins forming bridges between blood cells and ECs have been clarified in part. It has also been shown that reticulocytes from SCD patients express higher levels of alpha4beta1 integrin and CD36, and that under hydroxyurea (HU) therapy, both cell adhesion to ECs or extracellular matrix proteins and the levels of these adhesion molecules are reduced. These findings are consistent with the view that enhanced adhesion of blood cells to ECs is largely determined by the membrane expression level of adhesion molecules and could be a crucial factor for triggering or aggravating vaso-occlusion. In SCD patients, membrane expression of Lu/BCAM (and perhaps ICAM-4) is enhanced on RBCs whose adherence to laminin or ECs is also increased. Interestingly, Lu/BCAM- and ICAM-4-mediated adhesion are enhanced by the stress mediator epinephrine through a PKA-dependent pathway initiated by a rise in intracellular cAMP and leading to receptor activation by phosphorylation according to the same signaling pathway. More recently, studies based on quantitative expression analysis of adhesion molecules on RBCs and during erythroid differentiation in patients undergoing HU therapy, surprisingly revealed that Lu/BCAM level was enhanced, although alpha4beta1, CD36 and ICAM-4 (to a lower extent) levels were indeed reduced. CD47 and CD147 expression were also enhanced in HU-treated patients. Based on these findings we suggest that the signalization cascade leading to receptor activation rather than the expression level only of adhesion molecules may be the critical factor regulating cell adhesion, although both mechanisms are not mutually exclusive.

Requirement for erythroblast-macrophage protein (Emp) in definitive erythropoiesis

Emp, erythroblast-macrophage protein was initially identified as a mediator of erythroblast-macrophage interactions during erythroid differentiation. More recent studies have shown that targeted disruption of Emp leads to abnormal erythropoiesis in the fetal liver, and fetal demise. To further address the activity of Emp in the hematopoietic lineage in adult bone marrow, we conducted fetal liver HSC reconstitution assay. Emp null fetal liver cells were transplanted into lethally irradiated wild-type sibling mice, and assessed the erythropoietic activity. We found that Emp null cells rescued lethally irradiated mice with efficiency comparable to that of wild-type cells. However, the recipients of Emp null cells showed abnormal erythropoiesis as indicated by the presence of persistent anemia, extensive extramedullary erythropoiesis, and increased apoptosis of erythroid precursors. Extramedullary erythropoiesis suggests perturbed interactions between the Emp-deficient hematopoietic cells and the wild-type niche. Furthermore, in spleen colony-forming unit assays, proliferation rates of the Emp null cells were greater than those of the wild-type cells. Similarly, in vitro burst-forming unit-erythroid and colony-forming unit-erythroid assays showed increased erythroid colony numbers from Emp null livers. Morphologic examination showed that Emp null CFU-E-derived erythroblasts were immature compared to those derived from wild-type CFU-Es, suggesting that loss of Emp function in erythroid cells results in impaired proliferation and terminal differentiation. These results demonstrate that Emp plays a cell intrinsic role in the erythroid lineage.

Importance of molecular studies on major blood groups--intercellular adhesion molecule-4, a blood group antigen involved in multiple cellular interactions

Several blood groups, including the LW-blood group were discovered in the first part of last century, but their biochemical characteristics and cellular functions have only more recently been elucidated. The LW-blood group, renamed ICAM-4 (CD242), is red cell specific and belongs to the intercellular adhesion molecule family. ICAM-4 binds to several integrin receptors on blood and endothelial cells and is thus able to form large cellular complexes containing red cells. Its physiological function(s) has remained incompletely understood, but recent work shows that macrophage integrins can bind red cells through this ligand. In this article we discuss molecular properties of major blood group antigens, describe ICAM-4 in more detail, and show that phagocytosis of senescent red cells is in part ICAM-4/beta(2)-integrin dependent.

Small-molecule cyclic αVβ3 antagonists inhibit sickle red cell adhesion to vascular endothelium and vasoocclusion

Abnormal adhesion of sickle red blood cells (SS RBCs) to vascular endothelium may play an important role in vasoocclusion in sickle cell disease. Accruing evidence shows that endothelial αVβ3-integrin has an important role in SS RBC adhesion because of its ability to bind several adhesive proteins implicated in this interaction. In the present studies, we tested therapeutic efficacy of small-molecule cyclic pentapeptides for their ability to block αVβ3-mediated SS RBC adhesion by using two well-established assay systems, i.e., cultured human umbilical vein endothelial cells (HUVEC) and artificially perfused mesocecum vasculature of the rat under flow conditions. We tested the efficacy of two RGD-containing cyclic pentapeptides, i.e., cRGDFV (EMD 66203) and cRGDF-ACHA (α-amino cyclohexyl carboxylic acid) (EMD 270179), based on their known ability to bind αVβ3. An inactive peptide, EMD 135981 (cRβ-ADFV) was used as control. Cyclization and the introduction of d-Phe (F) results in a marked increase in the ability of cyclic peptides to selectively bind αVβ3 receptors. In the mesocecum vasculature, both EMD 66203 and EMD 270179 ameliorated platelet-activating factor-induced enhanced SS RBC adhesion, postcapillary blockage, and significantly improved hemodynamic behavior. Infusion of a fluorescent derivative of EMD 66203 resulted in colocalization of the antagonist with vascular endothelium. Also, pretreatment of HUVEC with either αVβ3 antagonist resulted in a significant decrease in SS RBC adhesion. Because of their metabolic stability, the use of these cyclic αVβ3 antagonists may constitute a novel therapeutic strategy to block SS RBC adhesion and associated vasoocclusion under flow conditions.

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.

The macrophage CD163 surface glycoprotein is an erythroblast adhesion receptor

Erythropoiesis occurs in erythroblastic islands, where developing erythroblasts closely interact with macrophages. The adhesion molecules that govern macrophage-erythroblast contact have only been partially defined. Our previous work has implicated the rat ED2 antigen, which is highly expressed on the surface of macrophages in erythroblastic islands, in erythroblast binding. In particular, the monoclonal antibody ED2 was found to inhibit erythroblast binding to bone marrow macrophages. Here, we identify the ED2 antigen as the rat CD163 surface glycoprotein, a member of the group B scavenger receptor cysteine-rich (SRCR) family that has previously been shown to function as a receptor for hemoglobin-haptoglobin (Hb-Hp) complexes and is believed to contribute to the clearance of free hemoglobin. CD163 transfectants and recombinant protein containing the extracellular domain of CD163 supported the adhesion of erythroblastic cells. Furthermore, we identified a 13-amino acid motif (CD163p2) corresponding to a putative interaction site within the second scavenger receptor domain of CD163 that could mediate erythroblast binding. Finally, CD163p2 promoted erythroid expansion in vitro, suggesting that it enhanced erythroid proliferation and/or survival, but did not affect differentiation. These findings identify CD163 on macrophages as an adhesion receptor for erythroblasts in erythroblastic islands, and suggest a regulatory role for CD163 during erythropoiesis.

Changing pattern of the subcellular distribution of erythroblast macrophage protein (Emp) during macrophage differentiation

Erythroblast macrophage protein (Emp) mediates the attachment of erythroid cells to macrophages and is required for normal differentiation of both cell lineages. In erythroid cells, Emp is believed to be involved in nuclear extrusion, however, its role in macrophage differentiation is unknown. Information on the changes in the expression level and subcellular distribution of Emp in differentiating macrophages is essential for understanding the function of Emp. Macrophages of varying maturity were examined by immunofluorescence microscopy and biochemical methods. Our data show that Emp is expressed in all stages of maturation, but its localization pattern changes dramatically during maturation: in immature macrophages, a substantial fraction of Emp is associated with the nuclear matrix, whereas in more mature cells, Emp is expressed largely at cell surface. Pulse-chase experiments show that nascent Emp migrates intracellularly from the cytoplasm to the plasma membrane more efficiently in mature macrophages than in immature cells. Incubation of erythroid cells with macrophages in culture shows that erythroid cells attach to mature macrophages but not to immature macrophage precursors. Together, our data show that the temporal and spatial expression of Emp correlates with its role in erythroblastic island formation and suggest that Emp may be involved in multiple cellular functions.

Red-cell ICAM-4 is a ligand for the monocyte/macrophage integrin CD11c/CD18: characterization of the binding sites on ICAM-4

Intercellular adhesion molecule 4 (ICAM-4) is a unique member of the ICAM family because of its specific expression on erythroid cells and ability to interact with several types of integrins expressed on blood and endothelial cells. The first reported receptors for ICAM-4 were CD11a/CD18 and CD11b/CD18. In contrast to these 2, the cellular ligands and the functional role of the third beta2 integrin, CD11c/CD18, have not been well defined. Here, we show that ICAM-4 functions as a ligand for the monocyte/macrophage-specific CD11c/CD18. Deletion of the individual immunoglobulin domains of ICAM-4 demonstrated that both its domains contain binding sites for CD11c/CD18. Analysis of a panel of ICAM-4 point mutants identified residues that affected binding to the integrin. By molecular modeling the important residues were predicted to cluster in 2 distinct but spatially close regions of the first domain with an extension to the second domain spatially distant from the other residues. We also identified 2 peptides derived from sequences of ICAM-4 that are capable of modulating the binding to CD11c/CD18. CD11c/CD18 is expressed on macrophages in spleen and bone marrow. Inhibition of erythrophagocytosis by anti-ICAM-4 and anti-integrin antibodies suggests a role for these interactions in removal of senescent red cells.

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.

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.

LW protein: a promiscuous integrin receptor activated by adrenergic signaling

The LW blood group antigen glycoprotein, although part of the Rh macromolecular complex, is nonetheless a member of the intercellular adhesion molecule (ICAM) family. Thus, while it is only rarely clinically important in the setting of transfusion and pregnancy, LW is likely to contribute to red cell adhesion in a variety of settings, including during hematopoiesis, as well as in vascular disorders. The best documentation of a pathophysiological role for LW in human disease is in sickle cell disease, where it contributes to red cell adhesion to endothelial cells and the development of vaso-occlusion, the hallmark of that disease. LW may also contribute to other intravascular processes, such as both venous and arterial thrombosis, due to its ability to interact with both activated platelets as well as leukocytes. The evidence that LW itself can undergo activation on red cells holds promise that pharmacotherapeutic maneuvers may be found to prevent such pathophysiologic interactions.

Rh proteins: Key structural and functional components of the red cell membrane

Rh (Rhesus) proteins (D, CcEe) are expressed in red cells (RBC) in association with other membrane proteins (RhAG, LW, CD47 and GPB). By interacting with the spectrin-based skeleton through protein 4.2 and ankyrin, the Rh complex contributes to the maintenance of the mechanical properties of the erythrocyte membrane. The RH system is one of the most immunogenic and polymorphic human blood group system. Molecular basis of most Rh phenotypes, including the Rh(null) phenotype associated with hemolytic anemia, have been determined. The demonstration that the RHD-positive locus is composed of the RHD and RHCE genes, whereas the RHD gene is deleted in most RhD-negative individuals, allowed fetal RhD genotyping by non-invasive PCR assays for antenatal diagnosis of pregnancy at risk for Rh hemolytic disease of the newborn. In mammals, the Rh protein family includes two non-erythroid members, RhBG and RhCG, mainly expressed in liver and kidney, two organs specialized in ammonia genesis and excretion. Functional analyses in heterologous systems revealed that RhAG, RhBG and RhCG can mediate ammonium (NH(3) and/or NH(4)(+)) transport across the cell membrane and might represent mammalian specific ammonium transporters. Furthermore, recent studies performed in human and murine red blood cells (RBC) indicate that RhAG facilitates CH(3)NH(2)/NH(3) movement across the membrane and represents a potential example of gas channel. The crystallographic structure of the bacterial ammonia channel AmtB and functional studies showing that AmtB conducts NH(3) into reconstituted vesicles is fully consistent with these latter studies. In RBCs, RhAG may transport NH(3) to detoxifying organs like kidney and liver and with non-erythroid tissues orthologs may contribute to regulation of the acid-base balance.

Erythrocyte adhesion receptors: blood group antigens and related molecules

During the second half of the 20th century, blood bankers quickly expanded our knowledge of human erythrocyte blood group antigens. By the dawn of the 21st century, several hundred blood group antigen polymorphisms had been identified. Hot on the heels of the serologists, membrane biochemists and molecular geneticists defined both the biochemical and genetic bases of most of these antigens. Perhaps to their surprise, this work has led to the discovery of functionally diverse and important membrane proteins expressed on the surface of red cells, including numerous adhesion molecules. Red cells express an unexpected number of such adhesion receptors, some of which contribute to human disease, as well as to normal red cell development. And perhaps most interestingly, study of these molecules has elucidated ways in which even mature red cells respond to external stimuli, such as adrenergic hormones.

Integrin receptor specificity for human red cell ICAM-4 ligand. Critical residues for alphaIIbeta3 binding

The red cell intercellular adhesion molecule-4 (ICAM-4) binds to different members of the integrin receptor families. To better define the ICAM-4 integrin receptor specificity, cell transfectants individually expressing various integrins were used to demonstrate that alphaLbeta2, alphaMbeta2, and alphaIIbbeta3 (activated) bind specifically and dose dependently to the recombinant ICAM-4-Fc protein. We also show that cell surface ICAM-4 interacts with the cell surface alphaVbeta3 integrin. In addition, using a alpha4beta1 cell transfectant and beta2 integrin-deficient LAD cells, we show here that ICAM-4 failed to interact with alpha4beta1 even after alpha4beta1 activation by phorbol ester or with the monoclonal antibody TS2/16 (+ Mn2+). ICAM-4 amino acids that are critical for alphaIIbbeta3 and alphaVbeta3 interaction were identified by domain deletion analysis, site-directed mutagenesis and synthetic peptide inhibition. Our results provide evidence that the beta3 integrin binding sites encompass the first and second Ig-like domains of ICAM-4. However, while the alphaIIbbeta3 contact site comprises the ABED face of domain D1 with an extension in the C'-E loop of domain D2, the alphaVbeta3 contact site comprises residues on both faces of D1 and in the C'-E loop of D2. These data, together with our previous results, demonstrate that different integrins bind to different but partly overlapping sites on ICAM-4, and that ICAM-4 may accommodate multiple integrin receptors present on leukocytes, platelets and endothelial cells.