Scanning Electron Microscopy Reveals Two Distinct Classes of Erythroblastic Island Isolated from Adult Mammalian Bone Marrow

Erythroblastic island: the niche for erythroid terminal differentiation and beyond

The erythroblastic island (EBI) is a multicellular structure defined by the presence of 1 or 2 central macrophages surrounded by at least 3 erythroblasts. EBIs were initially proposed as a specialized microenvironment exclusively for erythroid terminal differentiation. Recent advancements in techniques such as lineage tracing mouse models, imaging flow cytometry, and single-cell RNA sequencing, accumulating evidence has provided novel insights that challenge this conventional view. Notably, the erythropoietin receptor has been identified as a novel marker for EBI macrophages. Additionally, neutrophils have been identified as novel cellular components of EBIs, raising the intriguing hypothesis that EBIs may support other hematopoietic lineage cells as well. Beyond the diverse cellular components of various hematopoietic lineages, even within the erythroid lineage, an immune-prone erythroblast subpopulation has been reported, although it remains unclear whether and how these immune-prone erythroblasts mature in EBIs. These observations indicate that EBIs are a heterogeneous population. In this review, we summarize the most recent findings on EBIs, discuss their potential immune functions, and provide a perspective for future investigations.

Role of MiRNA in the Regulation of Blood Group Expression

Background

MicroRNAs (miRNAs) are small, endogenous non-coding RNA molecules that inhibit gene expression through either destabilization of the target mRNA or translational repression. MiRNAs recognize target sites, most commonly found in the 3'-untranslated regions of cognate mRNAs. This review aims to provide a state-of-the-art overview of the role of miRNAs in the regulation of major blood group antigens such as ABH as well as cancer-specific glycans.

Summary

Besides their known roles in the control of developmental processes, proliferation, apoptosis, and carcinogenesis, miRNAs have recently been identified to play a regulatory role during erythropoiesis and blood group antigen expression. Since only little is known about the function of the red cell membrane proteins carrying blood group antigens, it is of great interest to shed light on the regulatory mechanisms of blood group gene expression. Some carrier proteins of blood group antigens are not restricted to red blood cells and are widely expressed in other bodily fluids and tissues and quite a few play a crucial role in tumor cells, as either tumor suppressors or promoters.

Key Message

All available data point at a tremendous physiological as well as pathophysiological relevance of miRNAs in context of blood group regulation. Furthermore, miRNAs are involved in the regulation of pleiotropic genetic pathways such as hematopoiesis and tumorigenesis and thus have to be studied in future research on this subject.

Role of Macrophages in Sickle Cell Disease Erythrophagocytosis and Erythropoiesis

Sickle cell disease (SCD) is an inherited blood disorder caused by a β-globin gene point mutation that results in the production of sickle hemoglobin that polymerizes upon deoxygenation, causing the sickling of red blood cells (RBCs). RBC deformation initiates a sequence of events leading to multiple complications, such as hemolytic anemia, vaso-occlusion, chronic inflammation, and tissue damage. Macrophages participate in extravascular hemolysis by removing damaged RBCs, hence preventing the release of free hemoglobin and heme, and triggering inflammation. Upon erythrophagocytosis, macrophages metabolize RBC-derived hemoglobin, activating mechanisms responsible for recycling iron, which is then used for the generation of new RBCs to try to compensate for anemia. In the bone marrow, macrophages can create specialized niches, known as erythroblastic islands (EBIs), which regulate erythropoiesis. Anemia and inflammation present in SCD may trigger mechanisms of stress erythropoiesis, intensifying RBC generation by expanding the number of EBIs in the bone marrow and creating new ones in extramedullary sites. In the current review, we discuss the distinct mechanisms that could induce stress erythropoiesis in SCD, potentially shifting the macrophage phenotype to an inflammatory profile, and changing their supporting role necessary for the proliferation and differentiation of erythroid cells in the disease. The knowledge of the soluble factors, cell surface and intracellular molecules expressed by EBI macrophages that contribute to begin and end the RBC’s lifespan, as well as the understanding of their signaling pathways in SCD, may reveal potential targets to control the pathophysiology of the disease.

Digital image analysis of erythroblastic islands in myelodysplastic syndromes

INTRODUCTION

Myelodysplastic syndromes (MDS) encompass a diverse group of myeloid neoplasms for which the diagnosis of low-grade subtypes remains challenging. Erythroblastic islands (EBIs) are highly organized units of erythroid proliferation, differentiation, and enucleation. EBI disruption is frequently observed and is believed to be one of the early changes in MDS.

METHODS

In this study, we digitally analyzed bone marrow biopsies dual stained with alpha-hemoglobin stabilizing protein (AHSP) and CD163 to quantitatively study features of EBIs in MDS, among MDS subtypes, as well as those in normal marrows and marrows with other causes of anemia.

RESULTS

EBIs in MDS specimens were smaller in size and higher in density compared to both normal and non-MDS anemia specimens. Increased CD163 expression within the EBIs is observed in both MDS and other causes of anemia. A combination of increased EBI density and CD163 expression is seen in association with MDS with high-risk cytogenetics and multiple adverse mutations.

CONCLUSION

As a proof-of-concept study, we show that EBI features can be relatively easily quantified with AHSP/CD163 dual immunohistochemistry and open-source imaging analysis software, highlighting those that are unique to MDS, and which may be prognostically relevant. Further studies of the measurable EBI features may provide valuable and novel tools to aid MDS diagnosis and prognostication in the era of digital pathology.

Macrophages: key players in erythrocyte turnover

The development of red blood cells (RBCs), or erythropoiesis, occurs in specialized niches in the bone marrow, called erythroblastic islands, composed of a central macrophage surrounded by erythroblasts at different stages of differentiation. Upon anemia or hypoxemia, erythropoiesis extends to extramedullary sites, mainly spleen and liver, a process known as stress erythropoiesis, leading to the expansion of erythroid progenitors, iron recruitment and increased production of reticulocytes and mature RBCs. Macrophages are key cells in both homeostatic and stress erythropoiesis, providing conditions for erythroid cells to survive, proliferate and differentiate. During RBCs aging and injury, macrophages play a fundamental role again, performing the clearance of these cells and recycling iron for new erythroblasts in development. Thus, macrophages are crucial components of the RBCs turnover and in this review, we aimed to cover the main known mechanisms involved in the process of birth and death of RBCs, highlighting the importance of macrophage functions in the whole RBC lifecycle.

Transcriptional Control of Gene Expression and the Heterogeneous Cellular Identity of Erythroblastic Island Macrophages

During definitive erythropoiesis, maturation of erythroid progenitors into enucleated reticulocytes requires the erythroblastic island (EBI) niche comprising a central macrophage attached to differentiating erythroid progenitors. Normally, the macrophage provides a nurturing environment for maturation of erythroid cells. Its critical physiologic importance entails aiding in recovery from anemic insults, such as systemic stress or acquired disease. Considerable interest in characterizing the central macrophage of the island niche led to the identification of putative cell surface markers enriched in island macrophages, enabling isolation and characterization. Recent studies focus on bulk and single cell transcriptomics of the island macrophage during adult steady-state erythropoiesis and embryonic erythropoiesis. They reveal that the island macrophage is a distinct cell type but with widespread cellular heterogeneity, likely suggesting distinct developmental origins and biological function. These studies have also uncovered transcriptional programs that drive gene expression in the island macrophage. Strikingly, the master erythroid regulator EKLF/Klf1 seems to also play a major role in specifying gene expression in island macrophages, including a putative EKLF/Klf1-dependent transcription circuit. Our present review and analysis of mouse single cell genetic patterns suggest novel expression characteristics that will enable a clear enrichment of EBI subtypes and resolution of island macrophage heterogeneity. Specifically, the discovery of markers such as Epor, and specific features for EKLF/Klf1-expressing island macrophages such as Sptb and Add2, or for SpiC-expressing island macrophage such as Timd4, or for Maf/Nr1h3-expressing island macrophage such as Vcam1, opens exciting possibilities for further characterization of these unique macrophage cell types in the context of their critical developmental function.

Fragmentation of tissue-resident macrophages during isolation confounds analysis of single-cell preparations from mouse hematopoietic tissues

Mouse hematopoietic tissues contain abundant tissue-resident macrophages that support immunity, hematopoiesis, and bone homeostasis. A systematic strategy to characterize macrophage subsets in mouse bone marrow (BM), spleen, and lymph node unexpectedly reveals that macrophage surface marker staining emanates from membrane-bound subcellular remnants associated with unrelated cells. Intact macrophages are not present within these cell preparations. The macrophage remnant binding profile reflects interactions between macrophages and other cell types in vivo. Depletion of CD169⁺ macrophages in vivo eliminates F4/80⁺ remnant attachment. Remnant-restricted macrophage-specific membrane markers, cytoplasmic fluorescent reporters, and mRNA are all detected in non-macrophage cells including isolated stem and progenitor cells. Analysis of RNA sequencing (RNA-seq) data, including publicly available datasets, indicates that macrophage fragmentation is a general phenomenon that confounds bulk and single-cell analysis of disaggregated hematopoietic tissues. Hematopoietic tissue macrophage fragmentation undermines the accuracy of macrophage ex vivo molecular profiling and creates opportunity for misattribution of macrophage-expressed genes to non-macrophage cells.

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.

K562 erythroleukemia line as a possible reticulocyte source to culture Plasmodium vivax and its surrogates

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miR-26a and miR-30a knockdowns promote differentiation in Fy-transduced K562 cell lines.

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miR-26a and miR-30a knockdowns promote enucleation in Fy-transduced K562 cell lines.

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Data denote an interplay in the mode of action of miR-26a and miR-30a in erythropoiesis.

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Plasmodium cynomolgi and P. knowlesi invade, albeit inefficiently, Fy-transduced K562 cells.

Establishing an in vitro “red blood cell matrix” that would allow uninterrupted access to a stable, homogeneous reticulocyte population would facilitate the establishment of continuous, long-term in vitro Plasmodium vivax blood stage cultures. In this study, we have explored the suitability of the erythroleukemia K562 cell line as a continuous source of such reticulocytes and have investigated regulatory factors behind the terminal differentiation (and enucleation, in particular) of this cell line that can be used to drive the reticulocyte production process. The Duffy blood group antigen receptor (Fy), essential for P. vivax invasion, was stably introduced into K562 cells by lentiviral gene transfer. miRNA-26a-5p and miRNA-30a-5p were downregulated to promote erythroid differentiation and enucleation, resulting in a tenfold increase in the production of reticulocytes after stimulation with an induction cocktail compared with controls. Our results suggest an interplay in the mechanisms of action of miRNA-26a-5p and miRNA-30a-5p, which makes it necessary to downregulate both miRNAs to achieve a stable enucleation rate and Fy receptor expression. In the context of establishing P. vivax-permissive, stable, and reproducible reticulocytes, a higher enucleation rate may be desirable, which may be achieved by the targeting of further regulatory mechanisms in Fy-K562 cells; promoting the shift in hemoglobin production from fetal to adult may also be necessary. Despite the fact that K562 erythroleukemia cell lines are of neoplastic origin, this cell line offers a versatile model system to research the regulatory mechanisms underlying erythropoiesis.

Cellular dynamics of mammalian red blood cell production in the erythroblastic island niche

Red blood cells, or erythrocytes, make up approximately a quarter of all cells in the human body with over 2 billion new erythrocytes made each day in a healthy adult human. This massive cellular production system is coupled with a set of cell biological processes unique to mammals, in particular, the elimination of all organelles, and the expulsion and destruction of the condensed erythroid nucleus. Erythrocytes from birds, reptiles, amphibians and fish possess nuclei, mitochondria and other organelles: erythrocytes from mammals lack all of these intracellular components. This review will focus on the dynamic changes that take place in developing erythroid cells that are interacting with specialized macrophages in multicellular clusters termed erythroblastic islands. Proerythroblasts enter the erythroblastic niche as large cells with active nuclei, mitochondria producing heme and energy, and attach to the central macrophage via a range of adhesion molecules. Proerythroblasts then mature into erythroblasts and, following enucleation, in reticulocytes. When reticulocytes exit the erythroblastic island, they are smaller cells, without nuclei and with few mitochondria, possess some polyribosomes and have a profoundly different surface molecule phenotype. Here, we will review, step-by-step, the biophysical mechanisms that regulate the remarkable process of erythropoiesis with a particular focus on the events taking place in the erythroblastic island niche. This is presented from the biological perspective to offer insight into the elements of red blood cell development in the erythroblastic island niche which could be further explored with biophysical modelling systems.

The iron islands: Erythroblastic islands and iron metabolism

BACKGROUND

A healthy human can produce over 1 × 10¹⁵ blood cells throughout their life. This remarkable amount of biomass requires a concomitantly vast amount of iron to generate functional haemoglobin and functional erythrocytes.

SCOPE OF THE REVIEW

Erythroblasts form multicellular clusters with macrophages in the foetal liver, bone marrow and spleen termed erythroblastic islands. How the central erythroblastic island macrophage co-ordinates the supply of iron to the developing erythroblasts will be a central focus of this review.

MAJOR CONCLUSION

Despite being studied for over 60 years, the mechanisms by which the erythroblastic island niche serves to control erythroid cell iron metabolism are poorly resolved.

GENERAL SIGNIFICANCE

Over 2 billion people suffer from some form of anaemia. Iron deficiency anaemia is the most prevalent form of anaemia. Therefore, understanding the processes by which iron is trafficked to, and metabolised in developing erythrocytes, is crucially important.

Analyzing the Formation, Morphology, and Integrity of Erythroblastic Islands

The bone marrow is the primary site of erythropoiesis in healthy adult mammals. In the bone marrow, erythroid cells mature within specialized microenvironments termed erythroblastic islands (EBIs). EBIs are multi-cellular clusters comprised of a central macrophage surrounded by red blood cell (erythroid) progenitors. It has been proposed that the central macrophage functions as a "nurse-cell" providing iron, cytokines, and growth factors for the developing erythroid cells. The central macrophage also engulfs and destroys extruded erythroid nuclei. EBIs have recently been shown to play clinically important roles during human hematological disease. The molecular mechanisms regulating this hematopoietic niche are largely unknown. In this chapter, we detail protocols to study isolated EBIs using multiple microscopy platforms. Adhesion molecules regulate cell-cell interactions within the EBI and maintain the integrity of the niche. To improve our understanding of the molecular regulation of erythroid cells in EBIs, we have developed protocols for immuno-gold labeling of erythroid surface antigens to combine with scanning electron microscopy. These protocols have allowed imaging of EBIs at the nanometer scale, offering novel insights into the processes regulating red blood cell production.