The erythroblastic island

Porphyrin overdrive rewires cancer cell metabolism

All cancer cells reprogram metabolism to support aberrant growth. Here, we report that cancer cells employ and depend on imbalanced and dynamic heme metabolic pathways, to accumulate heme intermediates, that is, porphyrins. We coined this essential metabolic rewiring "porphyrin overdrive" and determined that it is cancer-essential and cancer-specific. Among the major drivers are genes encoding mid-step enzymes governing the production of heme intermediates. CRISPR/Cas9 editing to engineer leukemia cell lines with impaired heme biosynthetic steps confirmed our whole-genome data analyses that porphyrin overdrive is linked to oncogenic states and cellular differentiation. Although porphyrin overdrive is absent in differentiated cells or somatic stem cells, it is present in patient-derived tumor progenitor cells, demonstrated by single-cell RNAseq, and in early embryogenesis. In conclusion, we identified a dependence of cancer cells on non-homeostatic heme metabolism, and we targeted this cancer metabolic vulnerability with a novel "bait-and-kill" strategy to eradicate malignant cells.

TFPI from erythroblasts drives heme production in central macrophages promoting erythropoiesis in polycythemia

Bleeding and thrombosis are known as common complications of polycythemia for a long time. However, the role of coagulation system in erythropoiesis is unclear. Here, we discover that an anticoagulant protein tissue factor pathway inhibitor (TFPI) plays an essential role in erythropoiesis via the control of heme biosynthesis in central macrophages. TFPI levels are elevated in erythroblasts of human erythroblastic islands with JAK2V617F mutation and hypoxia condition. Erythroid lineage-specific knockout TFPI results in impaired erythropoiesis through decreasing ferrochelatase expression and heme biosynthesis in central macrophages. Mechanistically, the TFPI interacts with thrombomodulin to promote the downstream ERK1/2-GATA1 signaling pathway to induce heme biosynthesis in central macrophages. Furthermore, TFPI blockade impairs human erythropoiesis in vitro, and normalizes the erythroid compartment in mice with polycythemia. These results show that erythroblast-derived TFPI plays an important role in the regulation of erythropoiesis and reveal an interplay between erythroblasts and central macrophages.

Posttraumatic pneumonia exacerbates bone marrow erythropoietic dysfunction

INTRODUCTION

Pneumonia is a common complication after severe trauma that is associated with worse outcomes with increased mortality. Critically ill trauma patients also have persistent inflammation and bone marrow dysfunction that manifests as persistent anemia. Terminal erythropoiesis, which occurs in bone marrow structures called erythroblastic islands (EBIs), has been shown to be impacted by trauma. Using a preclinical model of polytrauma (PT) and pneumonia, we sought to determine the effect of infection on bone marrow dysfunction and terminal erythropoiesis.

METHODS

Male and female Sprague-Dawley rats aged 9 to 11 weeks were subjected to either PT (lung contusion, hemorrhagic shock, cecectomy, and bifemoral pseudofracture) or PT with postinjury day 1 Pseudomonas pneumonia (PT-PNA) and compared with a naive cohort. Erythroblastic islands were isolated from bone marrow samples and imaged via confocal microscopy. Hemoglobin, early bone marrow erythroid progenitors, erythroid cells/EBI, and % reticulocytes/EBI were measured on day 7. Significance was defined as p < 0.05.

RESULTS

Day 7 hemoglobin was significantly lower in both PT and PT-PNA groups compared with naive (10.8 ± 0.6 and 10.9 ± 0.7 vs. 12.1 ± 0.7 g/dL [ p < 0.05]). Growth of bone marrow early erythroid progenitors (colony-forming units-granulocyte, erythrocyte, monocyte, megakaryocyte; erythroid burst-forming unit; and erythroid colony-forming unit) on day 7 was significantly reduced in PT-PNA compared with both PT and naive. Despite a peripheral reticulocytosis following PT and PT-PNA, the percentage of reticulocytes/EBI was not different between naive, PT, and PT-PNA. However, the number of erythroblasts/EBI was significantly lower in PT-PNA compared with naive (2.9 ± 1.5 [ p < 0.05] vs. 8.9 ± 1.1 cells/EBI macrophage). In addition to changes in EBI composition, EBIs were also found to have significant structural changes following PT and PT-PNA.

CONCLUSION

Multicompartmental PT altered late-stage erythropoiesis, and these changes were augmented with the addition of pneumonia. To improve outcomes following trauma and pneumonia, we need to better understand how alterations in EBI structure and function impact persistent bone marrow dysfunction and anemia.

A combined computational and experimental investigation of the filtration function of splenic macrophages in sickle cell disease

Being the largest lymphatic organ in the body, the spleen also constantly controls the quality of red blood cells (RBCs) in circulation through its two major filtration components, namely interendothelial slits (IES) and red pulp macrophages. In contrast to the extensive studies in understanding the filtration function of IES, fewer works investigate how the splenic macrophages retain the aged and diseased RBCs, i.e., RBCs in sickle cell disease (SCD). Herein, we perform a computational study informed by companion experiments to quantify the dynamics of RBCs captured and retained by the macrophages. We first calibrate the parameters in the computational model based on microfluidic experimental measurements for sickle RBCs under normoxia and hypoxia, as those parameters are not available in the literature. Next, we quantify the impact of key factors expected to dictate the RBC retention by the macrophages in the spleen, namely, blood flow conditions, RBC aggregation, hematocrit, RBC morphology, and oxygen levels. Our simulation results show that hypoxic conditions could enhance the adhesion between the sickle RBCs and macrophages. This, in turn, increases the retention of RBCs by as much as four-fold, which could be a possible cause of RBC congestion in the spleen of patients with SCD. Our study on the impact of RBC aggregation illustrates a 'clustering effect', where multiple RBCs in one aggregate can make contact and adhere to the macrophages, leading to a higher retention rate than that resulting from RBC-macrophage pair interactions. Our simulations of sickle RBCs flowing past macrophages for a range of blood flow velocities indicate that the increased blood velocity could quickly attenuate the function of the red pulp macrophages on detaining aged or diseased RBCs, thereby providing a possible rationale for the slow blood flow in the open circulation of the spleen. Furthermore, we quantify the impact of RBC morphology on their tendency to be retained by the macrophages. We find that the sickle and granular-shaped RBCs are more likely to be filtered by macrophages in the spleen. This finding is consistent with the observation of low percentages of these two forms of sickle RBCs in the blood smear of SCD patients. Taken together, our experimental and simulation results aid in our quantitative understanding of the function of splenic macrophages in retaining the diseased RBCs and provide an opportunity to combine such knowledge with the current knowledge of the interaction between IES and traversing RBCs to apprehend the complete filtration function of the spleen in SCD.

Human TLR8 induces inflammatory bone marrow erythromyeloblastic islands and anemia in SLE-prone mice

Anemia commonly occurs in systemic lupus erythematosus, a disease characterized by innate immune activation by nucleic acids. Overactivation of cytoplasmic sensors by self-DNA or RNA can cause erythroid cell death, while sparing other hematopoietic cell lineages. Whereas chronic inflammation is involved in this mechanism, less is known about the impact of systemic lupus erythematosus on the BM erythropoietic niche. We discovered that expression of the endosomal ssRNA sensor human TLR8 induces fatal anemia in Sle1.Yaa lupus mice. We observed that anemia was associated with a decrease in erythromyeloblastic islands and a block in differentiation at the CFU-E to proerythroblast transition in the BM. Single-cell RNAseq analyses of isolated BM erythromyeloblastic islands from human TLR8-expressing mice revealed that genes associated with essential central macrophage functions including adhesion and provision of nutrients were down-regulated. Although compensatory stress erythropoiesis occurred in the spleen, red blood cell half-life decreased because of hemophagocytosis. These data implicate the endosomal RNA sensor TLR8 as an additional innate receptor whose overactivation causes acquired failure of erythropoiesis via myeloid cell dysregulation.

Crosstalk between terminal erythropoiesis and granulopoiesis within their common niche: the erythromyeloblastic island

PURPOSE OF REVIEW

The identity of the erythroblastic island (EBI) macrophage (Mϕ) has been under investigation for decades since it was recognized as the first hematopoietic niche 'nursing' terminal erythropoiesis. This review will focus on the current insights to the characteristics and the role of the EBI Mϕ balancing terminal erythropoiesis and granulopoiesis.

RECENT FINDINGS

While the EBI has long been known as the niche for erythroid precursors, significant advancements in biology research technologies, including optimization of EBI enrichment protocols, single-cell ribonucleic acid sequencing, and imaging flow cytometry, have recently revealed that granulocytic precursors co-exist in this niche, termed erythromyeloblastic island (EMBI). More importantly, the balance noted at baseline between terminal granulopoiesis and erythropoiesis within EBIs/EMBIs is altered with diseases affecting hematopoiesis, such as stress erythropoiesis and inflammatory conditions causing anemia of inflammation. The role of the EMBI niche has yet to be fully investigated mechanistically, however, a notable degree of transcriptional and cell surface marker heterogeneity has been identified for the EMBI Mϕ, implicating its plasticity and diverse function.

SUMMARY

Terminal erythropoiesis and granulopoiesis are regulated within the EMBI. Investigations of their balance within this niche in health and disease may reveal new targets for treatment of diseases of terminal hematopoiesis.

A combined computational and experimental investigation of the filtration function of splenic macrophages in sickle cell disease

Being the largest lymphatic organ in the body, the spleen also constantly controls the quality of red blood cells (RBCs) in circulation through its two major filtration components, namely interendothelial slits (IES) and red pulp macrophages. In contrast to the extensive studies in understanding the filtration function of IES, there are relatively fewer works on investigating how the splenic macrophages retain the aged and diseased RBCs, i.e., RBCs in sickle cell disease (SCD). Herein, we perform a computational study informed by companion experiments to quantify the dynamics of RBCs captured and retained by the macrophages. We first calibrate the parameters in the computational model based on microfluidic experimental measurements for sickle RBCs under normoxia and hypoxia, as those parameters are not available in the literature. Next, we quantify the impact of a set of key factors that are expected to dictate the RBC retention by the macrophages in the spleen, namely, blood flow conditions, RBC aggregation, hematocrit, RBC morphology, and oxygen levels. Our simulation results show that hypoxic conditions could enhance the adhesion between the sickle RBCs and macrophages. This, in turn, increases the retention of RBCs by as much as five-fold, which could be a possible cause of RBC congestion in the spleen of patients with SCD. Our study on the impact of RBC aggregation illustrates a 'clustering effect', where multiple RBCs in one aggregate can make contact and adhere to the macrophages, leading to a higher retention rate than that resulting from RBC-macrophage pair interactions. Our simulations of sickle RBCs flowing past macrophages for a range of blood flow velocities indicate that the increased blood velocity could quickly attenuate the function of the red pulp macrophages on detaining aged or diseased RBCs, thereby providing a possible rationale for the slow blood flow in the open circulation of the spleen. Furthermore, we quantify the impact of RBC morphology on their tendency to be retained by the macrophages. We find that the sickle and granular-shaped RBCs are more likely to be filtered by macrophages in the spleen. This finding is consistent with the observation of low percentages of these two forms of sickle RBCs in the blood smear of SCD patients. Taken together, our experimental and simulation results aid in our quantitative understanding of the function of splenic macrophages in retaining the diseased RBCs and provide an opportunity to combine such knowledge with the current knowledge of the interaction between IES and traversing RBCs to apprehend the complete filtration function of the spleen in SCD.

Modelling the erythroblastic island niche of dyserythropoietic anaemia type IV patients using induced pluripotent stem cells

Introduction: Congenital dyserythropoietic anaemia (CDA) type IV has been associated with an amino acid substitution, Glu325Lys (E325K), in the transcription factor KLF1. These patients present with a range of symptoms, including the persistence of nucleated red blood cells (RBCs) in the peripheral blood which reflects the known role for KLF1 within the erythroid cell lineage. The final stages of RBCs maturation and enucleation take place within the erythroblastic island (EBI) niche in close association with EBI macrophages. It is not known whether the detrimental effects of the E325K mutation in KLF1 are restricted to the erythroid lineage or whether deficiencies in macrophages associated with their niche also contribute to the disease pathology. Methods: To address this question, we generated an in vitro model of the human EBI niche using induced pluripotent stem cells (iPSCs) derived from one CDA type IV patient as well as two iPSC lines genetically modified to express an KLF1-E325K-ERT2 protein that could be activated with 4OH-tamoxifen. The one patient iPSC line was compared to control lines from two healthy donors and the KLF1-E325K-ERT2 iPSC line to one inducible KLF1-ERT2 line generated from the same parental iPSCS. Results: The CDA patient-derived iPSCs and iPSCs expressing the activated KLF1-E325K-ERT2 protein showed significant deficiencies in the production of erythroid cells with associated disruption of some known KLF1 target genes. Macrophages could be generated from all iPSC lines but when the E325K-ERT2 fusion protein was activated, we noted the generation of a slightly less mature macrophage population marked by CD93. A subtle trend in their reduced ability to support RBC enucleation was also associated with macrophages carrying the E325K-ERT2 transgene. Discussion: Taken together these data support the notion that the clinically significant effects of the KLF1-E325K mutation are primarily associated with deficiencies in the erythroid lineage but it is possible that deficiencies in the niche might have the potential to exacerbate the condition. The strategy we describe provides a powerful approach to assess the effects of other mutations in KLF1 as well as other factors associated with the EBI niche.

Recent updates of stem cell-based erythropoiesis

Blood transfusions are now an essential part of modern medicine. Transfusable red blood cells (RBCs) are employed in various therapeutic strategies; however, the processes of blood donation, collection, and administration still involve many limitations. Notably, a lack of donors, the risk of transfusion-transmitted disease, and recent pandemics such as COVID-19 have prompted us to search for alternative therapeutics to replace this resource. Originally, RBC production was attempted via the ex vivo differentiation of stem cells. However, a more approachable and effective cell source is now required for broader applications. As a viable alternative, pluripotent stem cells have been actively used in recent research. In this review, we discuss the basic concepts related to erythropoiesis, as well as early research using hematopoietic stem cells ex vivo, and discuss the current trend of in vitro erythropoiesis using human-induced pluripotent stem cells.

Multicompartmental trauma alters bone marrow erythroblastic islands

BACKGROUND

Trauma is associated with widespread inflammation, neuroendocrine activation, and an inadequate bone marrow response to anemia. During late-stage erythropoiesis, erythroid progenitors/erythroblasts form clusters on the surface of specialized bone marrow macrophages where they are supported through terminal differentiation and enucleation. We hypothesized that these erythroblastic islands (EBIs) are adversely impacted by severe trauma.

METHODS

Male Sprague-Dawley rats (n = 8/group) were subjected to either multiple injuries (PT) (lung contusion, hemorrhagic shock, cecectomy, and bifemoral pseudofractures), PT plus 2 hours of daily chronic restraint stress (PT/CS), or naive controls. Bone marrow was harvested on days 2 and 7. Nuclear-stained, enriched bone marrow EBIs were fixed and stained for CD71, VCAM-1, and CD163, and confocal images were obtained at 20 times magnification. Numbers of erythroid cells/EBI and ratio of reticulocytes/EBI were counted by a blinded observer. Differences were compared using analysis of variance, with significance defined as p < 0.05.

RESULTS

PT and PT/CS had significantly reduced numbers of erythroid cells per EBI on day 2 when compared with naive (PT: 5.9 ± 1.0 cells [ p < 0.05], PT/CS: 6.8 ± 0.8 cells [ p < 0.05] vs. naive: 8.5 ± 0.8 cells). On day 7, the number of erythroid cells/EBI increased following PT (8.3 ± 0.4 cells) but remained reduced following PT/CS (5.9 ± 0.5 cells [ p < 0.05]). This correlated with an increased proportion of reticulocytes/EBI on day 7 following PT, which was not present following PT/CS (PT: 54% [ p < 0.05] vs. PT/CS: 28%).

CONCLUSION

Late-stage erythropoiesis was altered following multicompartmental PT early after injury, and these alterations persisted with the addition of daily chronic stress. Alterations in EBI structure and function after severe trauma and critical illness may serve as a promising new area of study to improve mechanistic understanding of persistent anemia after trauma.

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.

Origin, production and molecular determinants of macrophages for their therapeutic targeting

Macrophages, the most heterogeneous cells of the hematopoietic system and the giant eaters of the immune system that present either as tissue-resident cells or infiltrated immune cells, eliminate foreign pathogens and microbes and also play different physiological roles to maintain the body's immune response. In this review, we basically provide a broad overview of macrophages from their origin, functional diversity to M1-M2 polarization, specialized markers, and their role as important therapeutic targets in different diseases based on the current research and evidence. Apart from this, we have precisely discussed about tumor-associated macrophages (TAMs) and their role in tumor progression and newly discovered lesser-known markers of TAMs that could be used as potential therapeutic targets to treat life-threatening diseases. It is really very important to understand the diversity of macrophages to develop TAM-modulating strategies to activate our own immune system against diseases and to overcome immune resistance.

Krüppel-Like Factor 1: A Pivotal Gene Regulator in Erythropoiesis

Krüppel-like factor 1 (KLF1) plays a crucial role in erythropoiesis. In-depth studies conducted on mice and humans have highlighted its importance in erythroid lineage commitment, terminal erythropoiesis progression and the switching of globin genes from γ to β. The role of KLF1 in haemoglobin switching is exerted by the direct activation of β-globin gene and by the silencing of γ-globin through activation of BCL11A, an important γ-globin gene repressor. The link between KLF1 and γ-globin silencing identifies this transcription factor as a possible therapeutic target for β-hemoglobinopathies. Moreover, several mutations have been identified in the human genes that are responsible for various benign phenotypes and erythroid disorders. The study of the phenotype associated with each mutation has greatly contributed to the current understanding of the complex role of KLF1 in erythropoiesis. This review will focus on some of the principal functions of KLF1 on erythroid cell commitment and differentiation, spanning from primitive to definitive erythropoiesis. The fundamental role of KLF1 in haemoglobin switching will be also highlighted. Finally, an overview of the principal human mutations and relative phenotypes and disorders will be described.

New Avenues of Heme Synthesis Regulation

During erythropoiesis, there is an enormous demand for the synthesis of the essential cofactor of hemoglobin, heme. Heme is synthesized de novo via an eight enzyme-catalyzed pathway within each developing erythroid cell. A large body of data exists to explain the transcriptional regulation of the heme biosynthesis enzymes, but until recently much less was known about alternate forms of regulation that would allow the massive production of heme without depleting cellular metabolites. Herein, we review new studies focused on the regulation of heme synthesis via carbon flux for porphyrin synthesis to post-translations modifications (PTMs) that regulate individual enzymes. These PTMs include cofactor regulation, phosphorylation, succinylation, and glutathionylation. Additionally discussed is the role of the immunometabolite itaconate and its connection to heme synthesis and the anemia of chronic disease. These recent studies provide new avenues to regulate heme synthesis for the treatment of diseases including anemias and porphyrias.

An Erythropoietin-Independent Mechanism of Erythrocytic Precursor Proliferation Underlies Hypoxia Tolerance in Sea Nomads

The Bajau Sea Nomads were recently demonstrated to have evolved larger spleens as an adaptation to millennia of a marine foraging lifestyle. The large-spleen phenotype appears to derive from increases in thyroid hormone (TH) production as a result of reduced expression of phosphodiesterase 10A (PDE10A), though the exact mechanism remains unknown. Through pharmacological inhibition of PDE10A using the selective inhibitor MP-10 in mice, we were able to mimic the Bajau adaptation and show that treated mice had significantly larger spleens than control animals. This difference appears connected to an excess of early stage erythrocytes and an apparent increase in red blood cell (RBC) precursor proliferation in response to increased TH. However, we determined that the stimulation of RBC production in the mouse model via TH is Erythropoietin (EPO)-independent, unlike in the altitude (chronic hypoxemia) response. We confirmed this using human GWAS data; although the Bajau PDE10A variants are significantly associated with increased TH levels and RBC count, they are not associated with EPO levels, nor are other strongly thyroid-associated SNPs. We therefore suggest that an EPO-independent mechanism of stimulating RBC precursor proliferation via TH upregulation underlies the increase in spleen size observed in Sea Nomad populations.

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.

Isolation of Healthy F4/80+ Macrophages from Embryonic day E13.5 Mouse Fetal Liver Using Magnetic Nanoparticles for Single Cell Sequencing

In vivo erythropoiesis occurs in the erythroblast island niche (EBI), comprising of a central macrophage that attaches to and aids the maturation of erythroid progenitors into mature reticulocytes. Macrophages in hematopoietic tissue such as embryonic fetal liver are heterogeneous and express the cell surface protein F4/80. Earlier methods of isolating F4/80+ macrophages from hematopoietic tissue relied on FACS sorting, but the relatively low numbers of F4/80+ cells obtained after FACS sometimes led to poor RNA quality. Additionally, since EBI macrophages are attached to erythroblasts, care must be taken to avoid contamination with bound erythroblasts. We have developed a novel method for isolating F4/80+ cells from E13.5 mouse fetal liver using magnetic nanoparticles, which can be performed on the lab bench. During cell suspension and homogenization, we also add a peptide that disrupts erythroid macrophage interactions and generates F4/80+ single cells free of erythroid contamination. Thus, our protocol generates a population enriched in F4/80+ cells that are healthy and ready for sensitive techniques such as single cell sequencing.

The roles of free iron, heme, haemoglobin, and the scavenger proteins haemopexin and alpha-1-microglobulin in preeclampsia and fetal growth restriction

BACKGROUND

Preeclampsia (PE) is a complex pregnancy syndrome characterised by maternal hypertension and organ damage after 20 weeks of gestation and is associated with an increased risk of cardiovascular disease later in life. Extracellular haemoglobin (Hb) and its metabolites heme and iron are highly toxic molecules and several defence mechanisms have evolved to protect the tissue.

OBJECTIVES

We will discuss the roles of free iron, heme, Hb, and the scavenger proteins haemopexin and alpha-1-microglobulin in pregnancies complicated by PE and fetal growth restriction (FGR).

CONCLUSION

In PE, oxidative stress causes syncytiotrophoblast (STB) stress and increased shedding of placental STB-derived extracellular vesicles (STBEV). The level in maternal circulation correlates with the severity of hypertension and supports the involvement of STBEVs in causing maternal symptoms in PE. In PE and FGR, iron homeostasis is changed, and iron levels significantly correlate with the severity of the disease. The normal increase in plasma volume taking place during pregnancy is less for PE and FGR and therefore have a different impact on, for example, iron concentration, compared to normal pregnancy. Excess iron promotes ferroptosis is suggested to play a role in trophoblast stress and lipotoxicity. Non-erythroid α-globin regulates vasodilation through the endothelial nitric oxide synthase pathway, and hypoxia-induced α-globin expression in STBs in PE placentas is suggested to contribute to hypertension in PE. Underlying placental pathology in PE with and without FGR might be amplified by iron and heme overload causing oxidative stress and ferroptosis. As the placenta becomes stressed, the release of STBEVs increases and affects the maternal vasculature.

Improving cancer treatments via dynamical biophysical models

Despite significant advances in oncological research, cancer nowadays remains one of the main causes of mortality and morbidity worldwide. New treatment techniques, as a rule, have limited efficacy, target only a narrow range of oncological diseases, and have limited availability to the general public due their high cost. An important goal in oncology is thus the modification of the types of antitumor therapy and their combinations, that are already introduced into clinical practice, with the goal of increasing the overall treatment efficacy. One option to achieve this goal is optimization of the schedules of drugs administration or performing other medical actions. Several factors complicate such tasks: the adverse effects of treatments on healthy cell populations, which must be kept tolerable; the emergence of drug resistance due to the intrinsic plasticity of heterogeneous cancer cell populations; the interplay between different types of therapies administered simultaneously. Mathematical modeling, in which a tumor and its microenvironment are considered as a single complex system, can address this complexity and can indicate potentially effective protocols, that would require experimental verification. In this review, we consider classical methods, current trends and future prospects in the field of mathematical modeling of tumor growth and treatment. In particular, methods of treatment optimization are discussed with several examples of specific problems related to different types of treatment.

Use of dual-electron probes reveals the role of ferritin as an iron depot in ex vivo erythropoiesis

In the finely regulated process of mammalian erythropoiesis, the path of the labile iron pool into mitochondria for heme production is not well understood. Existing models for erythropoiesis do not include a central role for the ubiquitous iron storage protein ferritin; one model proposes that incoming endosomal Fe³⁺ bound to transferrin enters the cytoplasm through an ion transporter after reduction to Fe²⁺ and is taken up into mitochondria through mitoferrin-1 transporter. Here, we apply a dual three-dimensional imaging and spectroscopic technique, based on scanned electron probes, to measure Fe³⁺ in ex vivo human hematopoietic stem cells. After seven days in culture, we observe cells displaying a highly specialized architecture with anchored clustering of mitochondria and massive accumulation of nanoparticles containing high iron concentrations localized to lysosomal storage depots, identified as ferritin. We hypothesize that lysosomal ferritin iron depots enable continued heme production after expulsion of most of the cellular machinery.

Sex Matters: Physiological Abundance of Immuno-Regulatory CD71+ Erythroid Cells Impair Immunity in Females

Mature erythrocytes are the major metabolic regulators by transporting oxygen throughout the body. However, their precursors and progenitors defined as CD71+ Erythroid Cells (CECs) exhibit a wide range of immunomodulatory properties. Here, we uncover pronounced sexual dimorphism in CECs. We found female but not male mice, both BALB/c and C57BL/6, and human females were enriched with CECs. CECs, mainly their progenitors defined as CD45+CECs expressed higher levels of reactive oxygen species (ROS), PDL-1, VISTA, Arginase II and Arginase I compared to their CD45- counterparts. Consequently, CECs by the depletion of L-arginine suppress T cell activation and proliferation. Expansion of CECs in anemic mice and also post-menstrual cycle in women can result in L-arginine depletion in different microenvironments in vivo (e.g. spleen) resulting in T cell suppression. As proof of concept, we found that anemic female mice and mice adoptively transferred with CECs from anemic mice became more susceptible to Bordetella pertussis infection. These observations highlight the role of sex and anemia-mediated immune suppression in females. Notably, enriched CD45+CECs may explain their higher immunosuppressive properties in female BALB/c mice. Finally, we observed significantly more splenic central macrophages in female mice, which can explain greater extramedullary erythropoiesis and subsequently abundance of CECs in the periphery. Thus, sex-specific differences frequency in the frequency of CECs might be imprinted by differential erythropoiesis niches and hormone-dependent manner.

Promoters and Antagonists of Phagocytosis: A Plastic and Tunable Response

Recent observations indicate that, rather than being an all-or-none response, phagocytosis is finely tuned by a host of developmental and environmental factors. The expression of key phagocytic determinants is regulated via transcriptional and epigenetic means that confer memory on the process. Membrane traffic, the cytoskeleton, and inside-out signaling control the activation of phagocytic receptors and their ability to access their targets. An exquisite extra layer of complexity is introduced by the coexistence of distinct "eat-me" and "don't-eat-me" signals on targets and of corresponding "eat" and "don't-eat" receptors on the phagocyte surface. Moreover, assorted physical barriers constitute "don't-come-close-to-me" hurdles that obstruct the engagement of ligands by receptors. The expression, mobility, and accessibility of all these determinants can be modulated, conferring extreme plasticity on phagocytosis and providing attractive targets for therapeutic intervention in cancer, atherosclerosis, and dementia. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A quantitative systems pharmacology model of hyporesponsiveness to erythropoietin in rats

Recombinant human erythropoietin (rHuEPO) is effective in managing chronic kidney disease and chemotherapy-induced anemia. However, hyporesponsiveness to rHuEPO treatment was reported in about 10% of the patients. A decreased response in rats receiving a single or multiple doses of rHuEPO was also observed. In this study, we aimed to develop a quantitative systems pharmacology (QSP) model to examine hyporesponsiveness to rHuEPO in rats. Pharmacokinetic (PK) and pharmacodynamic (PD) data after a single intravenous dose of rHuEPO (100 IU/kg) was obtained from a previous study (Yan et al. in Pharm Res, 30:1026-1036, 2013) including rHuEPO plasma concentrations, erythroid precursors counts in femur bone marrow and spleen, reticulocytes (RETs), red blood cells (RBCs), and hemoglobin (HGB) in circulation. Parameter values were obtained from literature or calibrated with experimental data. Global sensitivity analysis and model-based simulations were performed to assess parameter sensitivity and hyporesponsiveness. The final QSP model adequately characterizes time courses of rHuEPO PK and nine PD endpoints in both control and treatment groups simultaneously. The model indicates that negative feedback regulation, neocytolysis, and depletion of erythroid precursors are major factors leading to hyporesponsiveness to rHuEPO treatment in rats.

Differences in Steady-State Erythropoiesis in Different Mouse Bones and Postnatal Spleen

Adult erythropoiesis is a highly controlled sequential differentiation of hematopoietic stem cells (HSCs) to mature red blood cells in the bone marrow (BM). The bones which contain BM are diverse in their structure, embryonic origin, and mode of ossification. This has created substantial heterogeneity in HSCs function in BM of different bones, however, it is not known if this heterogeneity influences erythropoiesis in different bones and different regions of the same bone. In this study, we examined steady state BM erythroid progenitors and precursors from different bones - the femur, tibia, pelvis, sternum, vertebrae, radius, humerus, frontal, parietal bone, and compared all to the femur. Trabecular and cortical regions of the femur were also compared for differences in erythropoiesis. In addition, mouse spleen was studied to determine at which age erythropoietic support by the spleen was lost postnatally. We report that total erythroid cells, and erythroid precursors in the femur are comparable to tibia, pelvis, humerus and sternum, but are significantly reduced in the vertebrae, radius, frontal, and parietal bones. Erythroid progenitors and multipotential progenitor numbers are comparable in all the bones except for reduced number in the parietal bone. In the femur, the epiphysis and metaphysis have significantly reduced number of erythroid precursors and progenitors, multipotential progenitors and myeloid progenitors compared to the diaphysis region. These results show that analysis of erythroid precursors from diaphysis region of the femur is representative of tibia, pelvis, humerus and sternum and have significant implications on the interpretation of the steady-state erythropoiesis finding from adult BM. Postnatal spleen supports erythroid precursors until 6 weeks of age which coincides with reduced number of red pulp macrophages. The residual erythroid progenitor support reaches the adult level by 3 months of age. In conclusion, our findings provide insights to the differences in erythropoiesis between different bones, between trabecular and cortical regions of the femur, and developmental changes in postnatal spleen erythropoiesis.

Epo receptor signaling in macrophages alters the splenic niche to promote erythroid differentiation

Anemic stress induces stress erythropoiesis, which rapidly generates new erythrocytes to restore tissue oxygenation. Stress erythropoiesis is best understood in mice where it is extramedullary and occurs primarily in the spleen. However, both human and mouse stress erythropoiesis use signals and progenitor cells that are distinct from steady-state erythropoiesis. Immature stress erythroid progenitors (SEPs) are derived from short-term hematopoietic stem cells. Although the SEPs are capable of self-renewal, they are erythroid restricted. Inflammation and anemic stress induce the rapid proliferation of SEPs, but they do not differentiate until serum erythropoietin (Epo) levels increase. Here we show that rather than directly regulating SEPs, Epo promotes this transition from proliferation to differentiation by acting on macrophages in the splenic niche. During the proliferative stage, macrophages produce canonical Wnt ligands that promote proliferation and inhibit differentiation. Epo/Stat5-dependent signaling induces the production of bioactive lipid mediators in macrophages. Increased production of prostaglandin J2 (PGJ2) activates peroxisome proliferator-activated receptor γ (PPARγ)-dependent repression of Wnt expression, whereas increased production of prostaglandin E2 (PGE2) promotes the differentiation of SEPs.

EKLF/KLF1 expression defines a unique macrophage subset during mouse erythropoiesis

Erythroblastic islands are a specialized niche that contain a central macrophage surrounded by erythroid cells at various stages of maturation. However, identifying the precise genetic and transcriptional control mechanisms in the island macrophage remains difficult due to macrophage heterogeneity. Using unbiased global sequencing and directed genetic approaches focused on early mammalian development, we find that fetal liver macrophages exhibit a unique expression signature that differentiates them from erythroid and adult macrophage cells. The importance of erythroid Krüppel-like factor (EKLF)/KLF1 in this identity is shown by expression analyses in EKLF-/- and in EKLF-marked macrophage cells. Single-cell sequence analysis simplifies heterogeneity and identifies clusters of genes important for EKLF-dependent macrophage function and novel cell surface biomarkers. Remarkably, this singular set of macrophage island cells appears transiently during embryogenesis. Together, these studies provide a detailed perspective on the importance of EKLF in the establishment of the dynamic gene expression network within erythroblastic islands in the developing embryo and provide the means for their efficient isolation.

Red cell manufacturing using parallel stirred-tank bioreactors at the final stages of differentiation enhances reticulocyte maturation

The aim of this study was to develop a robust, quality controlled, and reproducible erythroid culture system to obtain high numbers of mature erythroblasts and red blood cells (RBCs). This was achieved using a fully controlled stirred-tank bioreactor by the design of experiments (DOE) methods in the serum-free medium by defining the appropriate culture parameters. Human cord blood CD34+ cells were first cultured in static flasks and then inoculated to stirred-tank bioreactors. Cell diameter was gradually decreased and final RBC yields were significantly higher when cells were inoculated at sizes smaller than 12 μm. The larger immature cells in the basophilic stage did not survive, while smaller mature erythroid cells were successfully expanded at high agitation speeds, demonstrating that appropriate seeding timing is critical. A high inoculation cell density of 5 × 10⁶  cells/ml was achieved reaching 1.5 × 10⁷  cells/ml. By using DOE analysis fitted to precise stages of erythropoiesis, we were able to acquire the optimal culture parameters for pH (7.5), temperature (37°C), dissolved oxygen, agitation speed (500 rpm), inoculation timing (cell diameter 12-13 μm), media feeding regimen, and cell seeding density (5 × 10⁶  cells/ml). The final pure RBCs showed appropriate functions compared with fresh donor RBCs, confirming that manufacturing mature RBCs with reproducibility is possible.

Histidine decarboxylase deficiency inhibits NBP-induced extramedullary hematopoiesis by modifying bone marrow and spleen microenvironments

Histidine decarboxylase (HDC), a histamine synthase, is expressed in various hematopoietic cells and is induced by hematopoietic cytokines such as granulocyte colony-stimulating factor (G-CSF). We previously showed that nitrogen-containing bisphosphonate (NBP)-treatment induces extramedullary hematopoiesis via G-CSF stimulation. However, the function of HDC in NBP-induced medullary and extramedullary hematopoiesis remains unclear. Here, we investigated changes in hematopoiesis in wild-type and HDC-deficient (HDC-KO) mice. NBP treatment did not induce anemia in wild-type or HDC-KO mice, but did produce a gradual increase in serum G-CSF levels in wild-type mice. NBP treatment also enhanced Hdc mRNA expression and erythropoiesis in the spleen and reduced erythropoiesis in bone marrow and the number of vascular adhesion molecule 1 (VCAM-1)-positive macrophages in wild-type mice, as well as increased the levels of hematopoietic progenitor cells and proliferating cells in the spleen and enhanced expression of bone morphogenetic protein 4 (Bmp4), CXC chemokine ligand 12 (Cxcl12), and hypoxia inducible factor 1 (Hif1) in the spleen. However, such changes were not observed in HDC-KO mice. These results suggest that histamine may affect hematopoietic microenvironments of the bone marrow and spleen by changing hematopoiesis-related factors in NBP-induced extramedullary hematopoiesis.

Splenic erythroid progenitors decrease TNF-α production by macrophages and reduce systemic inflammation in a mouse model of T cell-induced colitis

In inflammatory bowel disease (IBD), inflammation can occur beyond the intestine and spread systemically causing complications such as arthritis, cachexia, and anemia. Here, we determine the impact of CD45, a pan-leukocyte marker and tyrosine phosphatase, on IBD. Using a mouse model of T cell transfer colitis, CD25^- CD45RB^high CD4⁺ T cells were transferred into Rag1-deficient mice (RAGKO) and CD45-deficient RAGKO mice (CD45RAGKO). Weight loss and systemic wasting syndrome were delayed in CD45RAGKO mice compared to RAGKO mice, despite equivalent inflammation in the colon. CD45RAGKO mice had reduced serum levels of TNF-α, and reduced TNF-α production by splenic myeloid cells. CD45RAGKO mice also had increased numbers of erythroid progenitors in the spleen, which had previously been shown to be immunosuppressive. Adoptive transfer of these erythroid progenitors into RAGKO mice reduced their weight loss and TNF-α expression by splenic red pulp macrophages. In vitro, erythroid cells suppressed TNF-α expression in red pulp macrophages in a phagocytosis-dependent manner. These findings show a novel role for erythroid progenitors in suppressing the pro-inflammatory function of splenic macrophages and cachexia associated with IBD.

Stress erythropoiesis: definitions and models for its study

Steady-state erythropoiesis generates new erythrocytes at a constant rate, and it has enormous productive capacity. This production is balanced by the removal of senescent erythrocytes by macrophages in the spleen and liver. Erythroid homeostasis is highly regulated to maintain sufficient erythrocytes for efficient oxygen delivery to the tissues, while avoiding viscosity problems associated with overproduction. However, there are times when this constant production of erythrocytes is inhibited or is inadequate; at these times, erythroid output is increased to compensate for the loss of production. In some cases, increased steady-state erythropoiesis can offset the loss of erythrocytes but, in response to inflammation caused by infection or tissue damage, steady-state erythropoiesis is inhibited. To maintain homeostasis under these conditions, an alternative stress erythropoiesis pathway is activated. Emerging data suggest that the bone morphogenetic protein 4 (BMP4)-dependent stress erythropoiesis pathway is integrated into the inflammatory response and generates a bolus of new erythrocytes that maintain homeostasis until steady-state erythropoiesis can resume. In this perspective, we define the mechanisms that generate new erythrocytes when steady-state erythropoiesis is impaired and discuss experimental models to study human stress erythropoiesis.

Inside Out Integrin Activation Mediated by PIEZO1 Signaling in Erythroblasts

The non-selective mechanosensitive ion channel PIEZO1 controls erythrocyte volume homeostasis. Different missense gain-of-function mutations in PIEZO1 gene have been identified that cause Hereditary Xerocytosis (HX), a rare autosomal dominant haemolytic anemia. PIEZO1 expression is not limited to erythrocytes and expression levels are significantly higher in erythroid precursors, hinting to a role in erythropoiesis. During erythropoiesis, interactions between erythroblasts, central macrophages, and extracellular matrix within erythroblastic islands are important. Integrin α4β1 and α5β1 present on erythroblasts facilitate such interactions in erythroblastic islands. Here we found that chemical activation of PIEZO1 using Yoda1 leads to increased adhesion to VCAM1 and fibronectin in flowing conditions. Integrin α4, α5, and β1 blocking antibodies prevented this PIEZO1-induced adhesion suggesting inside-out activation of integrin on erythroblasts. Blocking the Ca²⁺ dependent Calpain and PKC pathways by using specific inhibitors also blocked increased erythroid adhesion to VCAM1 and fibronectins. Cleavage of Talin was observed as a result of Calpain and PKC activity. In conclusion, PIEZO1 activation results in inside-out integrin activation, facilitated by calcium-dependent activation of PKC and Calpain. The data introduces novel concepts in Ca²⁺ signaling during erythropoiesis with ramification on erythroblastic island homeostasis in health and disease like Hereditary Xerocytosis.

An Overview of Different Strategies to Recreate the Physiological Environment in Experimental Erythropoiesis

Human erythropoiesis is a complex process leading to the production of mature, enucleated erythrocytes (RBCs). It occurs mainly at bone marrow (BM), where hematopoietic stem cells (HSCs) are engaged in the early erythroid differentiation to commit into erythroid progenitor cells (burst-forming unit erythroid (BFU-E) and colony-forming unit erythroid (CFU-E)). Then, during the terminal differentiation, several erythropoietin-induced signaling pathways trigger the differentiation of CFU-E on successive stages from pro-erythroblast to reticulocytes. The latter are released into the circulation, finalizing their maturation into functional RBCs. This process is finely regulated by the physiological environment including the erythroblast-macrophage interaction in the erythroblastic island (EBI). Several human diseases have been associated with ineffective erythropoiesis, either by a defective or an excessive production of RBCs, as well as an increase or a hemoglobinization defect. Fully understanding the production of mature red blood cells is crucial for the comprehension of erythroid pathologies as well as to the field of transfusion. Many experimental approaches have been carried out to achieve a complete differentiation in vitro to produce functional biconcave mature RBCs. However, the various protocols usually fail to achieve enough quantities of completely mature RBCs. In this review, we focus on the evolution of erythropoiesis studies over the years, taking special interest in efforts that were made to include the microenvironment and erythroblastic islands paradigm. These more physiological approaches will contribute to a deeper comprehension of erythropoiesis, improve the treatment of dyserythropoietic disorders, and break through the barriers in massive RBCs production for transfusion.

New avenues for systematically inferring cell-cell communication: through single-cell transcriptomics data

For multicellular organisms, cell-cell communication is essential to numerous biological processes. Drawing upon the latest development of single-cell RNA-sequencing (scRNA-seq), high-resolution transcriptomic data have deepened our understanding of cellular phenotype heterogeneity and composition of complex tissues, which enables systematic cell-cell communication studies at a single-cell level. We first summarize a common workflow of cell-cell communication study using scRNA-seq data, which often includes data preparation, construction of communication networks, and result validation. Two common strategies taken to uncover cell-cell communications are reviewed, e.g., physically vicinal structure-based and ligand-receptor interaction-based one. To conclude, challenges and current applications of cell-cell communication studies at a single-cell resolution are discussed in details and future perspectives are proposed.

Nrf2 deficiency in mice attenuates erythropoietic stress-related macrophage hypercellularity

Erythropoiesis in the bone marrow and spleen depends on intricate interactions between the resident macrophages and erythroblasts. Our study focuses on identifying the role of nuclear factor erythroid 2-related factor 2 (Nrf2) during recovery from stress erythropoiesis. To that end, we induced stress erythropoiesis in Nrf2^+/+ and Nrf2-null mice and evaluated macrophage subsets known to support erythropoiesis and erythroid cell populations. Our results confirm macrophage and erythroid hypercellularity after acute blood loss. Importantly, Nrf2 depletion results in a marked numerical reduction of F4/80⁺/CD169⁺/CD11b⁺ macrophages, which is more prominent under the induction of stress erythropoiesis. The observed macrophage deficiency is concomitant to a significantly impaired erythroid response to acute stress erythropoiesis in both murine bone marrow and murine spleen. Additionally, peripheral blood reticulocyte count as a response to acute blood loss is delayed in Nrf2-deficient mice compared with age-matched controls (11.0 ± 0.6% vs. 14.8 ± 0.6%, p ≤ 0.001). Interestingly, we observe macrophage hypercellularity in conjunction with erythroid hyperplasia in the bone marrow during stress erythropoiesis in Nrf2^+/+ controls, with both impaired in Nrf2^-/- mice. We further confirm the finding of macrophage hypercellularity in another model of erythroid hyperplasia, the transgenic sickle cell mouse, characterized by hemolytic anemia and chronic stress erythropoiesis. Our results revealed the role of Nrf2 in stress erythropoiesis in the bone marrow and that macrophage hypercellularity occurs concurrently with erythroid expansion during stress erythropoiesis. Macrophage hypercellularity is a previously underappreciated feature of stress erythropoiesis in sickle cell disease and recovery from blood loss.

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.

Reappraising the role of α5 integrin and the microenvironmental support in stress erythropoiesis

We previously studied the role of β1 integrin and some of its different α partners relevant to erythropoiesis. Although clear and consistent answers regarding the role of α4β1 (VLA-4) were evident, the role of its companion integrin α5β1 (VLA-5) was clouded by inconsistent outcomes in all prior publications. Furthermore, the functional consequences of integrin deficiencies only in microenvironmental (ME) cells supporting erythroid cell expansion and maturation post stress have never been explored. In the study described here, we created several additional mouse models in the aim of addressing unanswered questions regarding functional consequences of single or combined integrin deficiencies in erythroid cells or only in ME supporting cells. Our novel and expansive data solidified the intrinsic requirement of both α4 and α5 integrins in erythroid cells for their proliferative expansion and maturation in response to stress; α5 integrin alone, deleted either early in all hematopoietic cells or only in erythroid cell, has only a redundant role in proliferative expansion and is dispensable for erythroid maturation. By contrast, α4 integrin, on its own, exerts a dominant effect on timely and optimal erythroid maturation. Deficiency of both α4 and α5 integrins in ME cells, including macrophages, does not negatively influence stress response by normal erythroid cells, in great contrast to the effect of ME cells deficient in all β1 integrins. Collectively the present data offer deeper insight into the coordination of different β1 integrin functional activities in erythroid cells or in ME cells for optimal erythroid stress response.

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.

Enhanced Ex Vivo Generation of Erythroid Cells from Human Induced Pluripotent Stem Cells in a Simplified Cell Culture System with Low Cytokine Support

Red blood cell (RBC) differentiation from human induced pluripotent stem cells (hiPSCs) offers great potential for developmental studies and innovative therapies. However, ex vivo erythropoiesis from hiPSCs is currently limited by low efficiency and unphysiological conditions of common culture systems. Especially, the absence of a physiological niche may impair cell growth and lineage-specific differentiation. We here describe a simplified, xeno- and feeder-free culture system for prolonged RBC generation that uses low numbers of supporting cytokines [stem cell factor (SCF), erythropoietin (EPO), and interleukin 3 (IL-3)] and is based on the intermediate development of a “hematopoietic cell forming complex (HCFC).” From this HCFC, CD43⁺ hematopoietic cells (purity >95%) were continuously released into the supernatant and could be collected repeatedly over a period of 6 weeks for further erythroid differentiation. The released cells were mainly CD34⁺/CD45⁺ progenitors with high erythroid colony-forming potential and CD36⁺ erythroid precursors. A total of 1.5 × 10⁷ cells could be harvested from the supernatant of one six-well plate, showing 100- to 1000-fold amplification during subsequent homogeneous differentiation into GPA⁺ erythroid cells. Mean enucleation rates near 40% (up to 60%) further confirmed the potency of the system. These benefits may be explained by the generation of a niche within the HCFC that mimics the spatiotemporal signaling of the physiological microenvironment in which erythropoiesis occurs. Compared to other protocols, this method provides lower complexity, less cytokine and medium consumption, higher cellular output, and better enucleation. In addition, slight modifications in cytokine addition shift the system toward continuous generation of granulocytes and macrophages.

Infused wild-type macrophages reside and self-renew in the liver to rescue the hemolysis and anemia of Hmox1-deficient mice

Heme oxygenase 1 (HMOX1), the inducible enzyme that catabolizes the degradation of heme into biliverdin, iron, and carbon monoxide, plays an essential role in the clearance of senescent and damaged red blood cells, systemic iron homeostasis, erythropoiesis, vascular hemostasis, and oxidative and inflammatory stress responses. In humans, HMOX1 deficiency causes a rare and lethal disease, characterized by severe anemia, intravascular hemolysis, as well as vascular and tissue damage. Hmox1 knockout (KO) mice recapitulated the phenotypes of HMOX1-deficiency patients and could be rescued by bone marrow (BM) transplantation that engrafted donor's hematopoietic stem cells into the recipient animals after myeloablation. To find better therapy and elucidate the contribution of macrophages to the pathogenesis of HMOX1-deficiency disease, we infused wild-type (WT) macrophages into Hmox1 KO mice. Results showed that WT macrophages engrafted and proliferated in the livers of Hmox1 KO mice, which corrected the microcytic anemia, rescued the intravascular hemolysis, restored iron homeostasis, eliminated kidney iron overload and tissue damage, and provided long-term protection. These results showed that a single macrophage infusion delivered a long-term curative effect in Hmox1 KO mice, obviating the need for BM transplantation, and suggested that the HMOX1 disease stems mainly from the loss of viable reticuloendothelial macrophages. Our work provides new insights into the etiology of HMOX1 deficiency and demonstrates the potential of infusion of WT macrophages to prevent disease in patients with HMOX1 deficiency and potentially other macrophage-related diseases.

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.

Cytoskeleton Remodeling Induces Membrane Stiffness and Stability Changes of Maturing Reticulocytes

Reticulocytes, the precursors of erythrocytes, undergo drastic alterations in cell size, shape, and deformability during maturation. Experimental evidence suggests that young reticulocytes are stiffer and less stable than their mature counterparts; however, the underlying mechanism is yet to be fully understood. Here, we develop a coarse-grained molecular-dynamics reticulocyte membrane model to elucidate how the membrane structure of reticulocytes contributes to their particular biomechanical properties and pathogenesis in blood diseases. First, we show that the extended cytoskeleton in the reticulocyte membrane is responsible for its increased shear modulus. Subsequently, we quantify the effect of weakened cytoskeleton on the stiffness and stability of reticulocytes, via which we demonstrate that the extended cytoskeleton along with reduced cytoskeleton connectivity leads to the seeming paradox that reticulocytes are stiffer and less stable than the mature erythrocytes. Our simulation results also suggest that membrane budding and the consequent vesiculation of reticulocytes can occur independently of the endocytosis-exocytosis pathway, and thus, it may serve as an additional means of removing unwanted membrane proteins from reticulocytes. Finally, we find that membrane budding is exacerbated when the cohesion between the lipid bilayer and the cytoskeleton is compromised, which is in accord with the clinical observations that erythrocytes start shedding membrane surface at the reticulocyte stage in hereditary spherocytosis. Taken together, our results quantify the stiffness and stability change of reticulocytes during their maturation and provide, to our knowledge, new insights into the pathogenesis of hereditary spherocytosis and malaria.

Pro-inflammatory Aorta-Associated Macrophages Are Involved in Embryonic Development of Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) are generated from specialized endothelial cells of the embryonic aorta. Inflammatory factors are implicated in regulating mouse HSC development, but which cells in the aorta-gonad-mesonephros (AGM) microenvironment produce these factors is unknown. In the adult, macrophages play both pro- and anti-inflammatory roles. We sought to examine whether macrophages or other hematopoietic cells found in the embryo prior to HSC generation were involved in the AGM HSC-generative microenvironment. CyTOF analysis of CD45⁺ AGM cells revealed predominance of two hematopoietic cell types, mannose-receptor positive macrophages and mannose-receptor negative myeloid cells. We show here that macrophage appearance in the AGM was dependent on the chemokine receptor Cx3cr1. These macrophages expressed a pro-inflammatory signature, localized to the aorta, and dynamically interacted with nascent and emerging intra-aortic hematopoietic cells (IAHCs). Importantly, upon macrophage depletion, no adult-repopulating HSCs were detected, thus implicating a role for pro-inflammatory AGM-associated macrophages in regulating the development of HSCs.

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Yolk-sac-derived macrophages are the most abundant hematopoietic cells in the AGM

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Cx3cr1 mediates yolk-sac macrophage progenitor recruitment to the AGM niche

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AGM macrophages dynamically interact with emerging intra-aortic hematopoietic cells

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Pro-inflammatory AGM macrophages are positive regulators of HSC generation

Scrublet: Computational Identification of Cell Doublets in Single-Cell Transcriptomic Data

Single-cell RNA-sequencing has become a widely used, powerful approach for studying cell populations. However, these methods often generate multiplet artifacts, where two or more cells receive the same barcode, resulting in a hybrid transcriptome. In most experiments, multiplets account for several percent of transcriptomes and can confound downstream data analysis. Here, we present Single-Cell Remover of Doublets (Scrublet), a framework for predicting the impact of multiplets in a given analysis and identifying problematic multiplets. Scrublet avoids the need for expert knowledge or cell clustering by simulating multiplets from the data and building a nearest neighbor classifier. To demonstrate the utility of this approach, we test Scrublet on several datasets that include independent knowledge of cell multiplets. Scrublet is freely available for download at github.com/AllonKleinLab/scrublet.

Scrublet: Computational Identification of Cell Doublets in Single-Cell Transcriptomic Data

Single-cell RNA-sequencing has become a widely used, powerful approach for studying cell populations. However, these methods often generate multiplet artifacts, where two or more cells receive the same barcode, resulting in a hybrid transcriptome. In most experiments, multiplets account for several percent of transcriptomes and can confound downstream data analysis. Here, we present Single-Cell Remover of Doublets (Scrublet), a framework for predicting the impact of multiplets in a given analysis and identifying problematic multiplets. Scrublet avoids the need for expert knowledge or cell clustering by simulating multiplets from the data and building a nearest neighbor classifier. To demonstrate the utility of this approach, we test Scrublet on several datasets that include independent knowledge of cell multiplets. Scrublet is freely available for download at github.com/AllonKleinLab/scrublet.

Scrublet: Computational Identification of Cell Doublets in Single-Cell Transcriptomic Data

Single-cell RNA-sequencing has become a widely used, powerful approach for studying cell populations. However, these methods often generate multiplet artifacts, where two or more cells receive the same barcode, resulting in a hybrid transcriptome. In most experiments, multiplets account for several percent of transcriptomes and can confound downstream data analysis. Here, we present Single-Cell Remover of Doublets (Scrublet), a framework for predicting the impact of multiplets in a given analysis and identifying problematic multiplets. Scrublet avoids the need for expert knowledge or cell clustering by simulating multiplets from the data and building a nearest neighbor classifier. To demonstrate the utility of this approach, we test Scrublet on several datasets that include independent knowledge of cell multiplets. Scrublet is freely available for download at github.com/AllonKleinLab/scrublet.

Genetic programming of macrophages generates an in vitro model for the human erythroid island niche

Red blood cells mature within the erythroblastic island (EI) niche that consists of specialized macrophages surrounded by differentiating erythroblasts. Here we establish an in vitro system to model the human EI niche using macrophages that are derived from human induced pluripotent stem cells (iPSCs), and are also genetically programmed to an EI-like phenotype by inducible activation of the transcription factor, KLF1. These EI-like macrophages increase the production of mature, enucleated erythroid cells from umbilical cord blood derived CD34⁺ haematopoietic progenitor cells and iPSCs; this enhanced production is partially retained even when the contact between progenitor cells and macrophages is inhibited, suggesting that KLF1-induced secreted proteins may be involved in this enhancement. Lastly, we find that the addition of three secreted factors, ANGPTL7, IL-33 and SERPINB2, significantly enhances the production of mature enucleated red blood cells. Our study thus contributes to the ultimate goal of replacing blood transfusion with a manufactured product.

In vitro differentiation of red blood cells (RBCs) is a desirable therapy for various disorders. Here the authors develop a culture system using stem cell-derived macrophages to show that inducible expression of a transcription factor, KLF1, enhances RBC production, potentially through the induction of three soluble factors, ANGPTL7, IL33 and SERPINB2.