Erythroid enucleation: a gateway into a "bloody" world

RNA Editors Sculpt the Transcriptome During Terminal Erythropoiesis

Selective RNA degradation during terminal erythropoiesis results in a globin-rich transcriptome in mature erythrocytes, but the specific RNA decay pathways remain unknown. We found that deficiency of the terminal uridylyl transferase enzyme Zcchc6 and the 3'-5' exoribonuclease Dis3l2 in mouse models led to fetal and perinatal reticulocytosis, an accumulation of RNA-rich precursors of terminal erythroid cells, suggesting their crucial roles in terminal red cell differentiation. Notably, knockout embryos exhibited persistent high-level expression of Hbb-bh1 globin, the ortholog of human fetal γ- globin. Perturbation of the Zcchc6-Dis3l2 pathway in mice engineered to express the human β-globin locus likewise increased γ -globin levels in fetal erythroid cells, suggesting that globin switching entails post-transcriptional mechanisms of mRNA destabilization in addition to transcriptional down-regulation. We cultured human hematopoietic stem and progenitor cells (HSPCs), performed CRISPR/Cas9-mediated knockout of ZCCHC6 and DIS3L2, and observed accumulation of RNA and elevated γ-globin levels in terminal erythroid cells. Our findings reveal a conserved role for the ZCCHC6/DIS3L2 RNA editors in terminal erythropoiesis and demonstrate a post-transcriptional mechanism for γ- globin gene switching, advancing research into in vitro erythrocyte generation and γ- globin stabilization to ameliorate hemoglobinopathies.

Cell death signaling in human erythron: erythrocytes lose the complexity of cell death machinery upon maturation

Over the recent years, our understanding of the cell death machinery of mature erythrocytes has been greatly expanded. It resulted in the discovery of several regulated cell death (RCD) pathways in red blood cells. Apoptosis (eryptosis) and necroptosis of erythrocytes share certain features with their counterparts in nucleated cells, but they are also critically different in particular details. In this review article, we summarize the cell death subroutines in the erythroid precursors (apoptosis, necroptosis, and ferroptosis) in comparison to mature erythrocytes (eryptosis and erythronecroptosis) to highlight the consequences of organelle clearance and associated loss of multiple components of the cell death machinery upon erythrocyte maturation. Recent advances in understanding the role of erythrocyte RCDs in health and disease have expanded potential clinical applications of these lethal subroutines, emphasizing their contribution to the development of anemia, microthrombosis, and endothelial dysfunction, as well as their role as diagnostic biomarkers and markers of erythrocyte storage-induced lesions. Fas signaling and the functional caspase-8/caspase-3 system are not indispensable for eryptosis, but might be retained in mature erythrocytes to mediate the crosstalk between both erythrocyte-associated RCDs. The ability of erythrocytes to switch between eryptosis and necroptosis suggests that their cell death is not a simple unregulated mechanical disintegration, but a tightly controlled process. This allows investigation of eventual pharmacological interventions aimed at individual cell death subroutines of erythrocytes.

Progress in Development of Functional Biological and Synthetic Blood Products to Augment Transfusable Blood Supply in Operational Medicine

A reliable, accessible, and high-quality blood supply is critical for the sustainment of any healthcare system. World events such as the COVID-19 pandemic have proven that maintaining the supply of blood presents a logistical challenge. The current blood supply is overseen by extensive donor programs around the world. In the United States, as in other countries, the need for blood has increased, with a decline in blood donations and increasing exclusions for blood donor qualification. While there is a need to improve blood donation participation, there is also need for new alternatives to traditional donation to ensure readiness to treat hemorrhagic shock common in the setting of trauma, as often occurs during a natural disaster or conflict. These operational medicine scenarios require significant blood availability which may tax the current blood supply chain. Aside from a walking blood bank (WBB) model for blood collection in suboptimal conditions, researchers have proposed alternatives for blood that include the manufacturing of blood from stem cell sources. Other alternatives include synthetic liquids that can carry oxygen such as Perfluoro-Chemicals (PFCs) and hemoglobin-based oxygen-carrying systems (HBCOs). Here, we review some of these alternatives to the traditional donor blood model. Researchers now have the technology that makes it feasible to develop blood alternatives that one day may supplement and help alleviate the limitations in blood supply.

Human erythrocytes' perplexing behaviour: erythrocytic microRNAs

Erythrocytes have the potential role in erythropoiesis and disease diagnosis. Thought to have lacked nucleic acid content, mammalian erythrocytes are nevertheless able to function for 120-140 days, metabolize heme, maintain oxidative stress, and so on. Mysteriously, erythrocytes proved as largest repositories of microRNAs (miRNAs) some of which are selectively retained and function in mature erythrocytes. They have unique expression patterns and have been found to be linked to specific conditions such as sickle cell anaemia, high-altitude hypoxia, chronic mountain sickness, cardiovascular and metabolic conditions as well as host-parasite interactions. They also have been implicated in cell storage-related damage and the regulation of its survival. However, the mechanism by which miRNAs function in the cell remains unclear. Investigations into the molecular mechanism of miRNAs in erythrocytes via extracellular vesicles have provided important clues in research studies on Plasmodium infection. Erythrocytes are also the primary source of circulating miRNAs but, how they affect the plasma/serum miRNAs profiles are still poorly understood. Erythrocyte-derived exosomal miRNAs, can interact with various body cell types, and have easy access to all regions, making them potentially crucial in various pathophysiological conditions. Which can also improve our understanding to identify potential treatment options and discovery related to non-invasive diagnostic markers. This article emphasizes the importance of erythrocytic miRNAs while focusing on the enigmatic behaviour of erythrocytes. It also sheds light on how this knowledge may be applied in the future to enhance the state of erythrocyte translational research from the standpoint of erythrocytic miRNAs.

Perturbation of calcium homeostasis invokes eryptosis-like cell death in enucleated bone marrow stem cells

Enucleated cells, also known as cytoplasts, are valuable tools with a wide range of applications. However, their potential for bio-engineering is greatly restricted by the short lifespan. We postulated that the enucleation process damages the integrity of the plasma membrane and thus activates a cell death program(s). The results showed that a tiny hole was generated transiently on the plasma membrane when the nucleus was spun off, while force-gated ion channels were activated in response to the pulling by the nucleus. Influx of extracellular calcium stimulated the opening of calcium channels and the release of calcium from endoplasmic reticulum and mitochondria. Long lasting calcium transient increased protein phosphorylation and activated caspase 9 and calpain proteinase activities. Subsequently, mitochondria membrane permeability and Reactive Oxygen Species (ROS) levels were significantly elevated, which eventually led to eryptosis-like cell death. When extracellular calcium was maintained at optimal concentration, the lifespan of enucleated cells was extended; however, huge amounts of vacuoles appeared in the cytoplasm, possibly derived from enlarged autophagosomes. Inhibition of vacuolation by inhibitors of autophagy or in co-culture with primary muscle cells did not rescue cells dying from the paraptosis-like pathway. These results offer valuable insights for further investigation into the intricate mechanisms underlying enucleated cell death.

New insights into the mechanisms of red blood cell enucleation: From basics to clinical applications

Background

Red blood cell (RBC) enucleation is a crucial step in the process of erythropoiesis. By removing the nucleus, RBCs gain greater flexibility, enabling them to traverse narrow capillaries with ease, thereby enhancing the efficiency of oxygen and carbon dioxide transport. This transformation underscores the intricate balance between cellular structure and function essential for maintaining homeostasis.

Topic

This review delves into the multifaceted enucleation process, outlining its complex steps that encompass protein sorting, vesicle trafficking, cytoskeletal remodeling, and apoptosis regulation, while also exploring the potential of enhancing the enucleation rate of RBCs in vitro. We emphasize the intricate regulation of this process, which is orchestrated by multiple factors. This includes transcription factors that meticulously guide protein synthesis and sorting through the modulation of gene expression, as well as non-coding RNAs that play a pivotal role in post-transcriptional regulation during various stages of RBC enucleation. Additionally, macrophages participate in the enucleation process by engulfing and clearing the extruded nuclei, further ensuring the proper development of RBCs. Although many studies have deeply explored the molecular mechanisms of enucleation, the roles of apoptosis and anti-apoptotic processes in RBC enucleation remain incompletely understood.

Implication

In this review, we aim to comprehensively summarize the RBC enucleation process and explore the progress made in ex vivo RBC generation. In the future, a deeper understanding of the enucleation process could provide significant benefits to patients suffering from anemia and other related conditions.

Characterization of immortalized bone marrow erythroid progenitor adult (imBMEP-A)-The first inducible immortalized red blood cell progenitor cell line derived from bone marrow CD71-positive cells

BACKGROUND AIMS

Ex vivo production of red blood cells (RBCs) represents a promising alternative for transfusion medicine. Several strategies have been described to generate erythroid cell lines from different sources, including embryonic, induced pluripotent, and hematopoietic stem cells. All these approaches have in common that they require elaborate differentiation cultures whereas the yield of enucleated RBCs is inefficient.

METHODS

We generated a human immortalized adult erythroid progenitor cell line derived from bone marrow CD71-positive erythroid progenitor cells (immortalized bone marrow erythroid progenitor adult, or imBMEP-A) by an inducible expression system, to shorten differentiation culture necessary for terminal erythroid differentiation. It is the first erythroid cell line that is generated from direct reticulocyte progenitors and demonstrates robust hemoglobin production in the immortalized state.

RESULTS

Morphologic analysis of the immortalized cells showed that the preferred cell type of the imBMEP-A line corresponds to hemoglobin-producing basophilic erythroblasts. In addition, we were able to generate a stable cell line from a single cell clone with the triple knockout of RhAG, RhDCE and KELL. After removal of doxycycline, part of the cells differentiated into normoblasts and reticulocytes within 5-7 days.

CONCLUSIONS

Our results demonstrate that the imBMEP-A cell line can serve as a stable and straightforward modifiable platform for RBC engineering in the future.

Erythroblast enucleation at a glance

Erythroid enucleation, the penultimate step in mammalian erythroid terminal differentiation, is a unique cellular process by which red blood cells (erythrocytes) remove their nucleus and accompanying nuclear material. This complex, multi-stage event begins with chromatin compaction and cell cycle arrest and ends with generation of two daughter cells: a pyrenocyte, which contains the expelled nucleus, and an anucleate reticulocyte, which matures into an erythrocyte. Although enucleation has been compared to asymmetric cell division (ACD), many mechanistic hallmarks of ACD appear to be absent. Instead, enucleation appears to rely on mechanisms borrowed from cell migration, endosomal trafficking and apoptosis, as well as unique cellular interactions within the microenvironment. In this Cell Science at a Glance article and the accompanying poster, we summarise current insights into the morphological features and genetic drivers regulating the key intracellular events that culminate in erythroid enucleation and engulfment of pyrenocytes by macrophages within the bone marrow microenvironment.

PROTAC-mediated vimentin degradation promotes terminal erythroid differentiation of pluripotent stem cells

BACKGROUND

Human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs), can undergo erythroid differentiation, offering a potentially invaluable resource for generating large quantities of erythroid cells. However, the majority of erythrocytes derived from hPSCs fail to enucleate compared with those derived from cord blood progenitors, with an unknown molecular basis for this difference. The expression of vimentin (VIM) is retained in erythroid cells differentiated from hPSCs but is absent in mature erythrocytes. Further exploration is required to ascertain whether VIM plays a critical role in enucleation and to elucidate the underlying mechanisms.

METHODS

In this study, we established a hESC line with reversible vimentin degradation (dTAG-VIM-H9) using the proteolysis-targeting chimera (PROTAC) platform. Various time-course studies, including erythropoiesis from CD34+ human umbilical cord blood and three-dimensional (3D) organoid culture from hESCs, morphological analysis, quantitative real-time PCR (qRT-PCR), western blotting, flow cytometry, karyotyping, cytospin, Benzidine-Giemsa staining, immunofluorescence assay, and high-speed cell imaging analysis, were conducted to examine and compare the characteristics of hESCs and those with vimentin degradation, as well as their differentiated erythroid cells.

RESULTS

Vimentin expression diminished during normal erythropoiesis in CD34+ cord blood cells, whereas it persisted in erythroid cells differentiated from hESC. Depletion of vimentin using the degradation tag (dTAG) system promotes erythroid enucleation in dTAG-VIM-H9 cells. Nuclear polarization of erythroblasts is elevated by elimination of vimentin.

CONCLUSIONS

VIM disappear during the normal maturation of erythroid cells, whereas they are retained in erythroid cells differentiated from hPSCs. We found that retention of vimentin during erythropoiesis impairs erythroid enucleation from hPSCs. Using the PROTAC platform, we validated that vimentin degradation by dTAG accelerates the enucleation rate in dTAG-VIM-H9 cells by enhancing nuclear polarization.

Mononuclear Phagocytes, Cellular Immunity, and Nobel Prizes: A Historic Perspective

The mononuclear phagocyte system includes monocytes, macrophages, some dendritic cells, and multinuclear giant cells. These cell populations display marked heterogeneity depending on their differentiation from embryonic and bone marrow hematopoietic progenitors, tissue location, and activation. They contribute to tissue homeostasis by interacting with local and systemic immune and non-immune cells through trophic, clearance, and cytocidal functions. During evolution, they contributed to the innate host defense before effector mechanisms of specific adaptive immunity emerged. Mouse macrophages appear at mid-gestation and are distributed throughout the embryo to facilitate organogenesis and clear cells undergoing programmed cell death. Yolk sac, AGM, and fetal liver-derived tissue-resident macrophages persist throughout postnatal and adult life, supplemented by bone marrow-derived blood monocytes, as required after injury and infection. Nobel awards to Elie Metchnikoff and Paul Ehrlich in 1908 drew attention to cellular phagocytic and humoral immunity, respectively. In 2011, prizes were awarded to Jules Hoffmann and Bruce Beutler for contributions to innate immunity and to Ralph Steinman for the discovery of dendritic cells and their role in antigen presentation to T lymphocytes. We trace milestones in the history of mononuclear phagocyte research from the perspective of Nobel awards bearing directly and indirectly on their role in cellular immunity.

Mitochondrial regulation of erythropoiesis in homeostasis and disease

Erythroid cells undergo a highly complex maturation process, resulting in dynamic changes that generate red blood cells (RBCs) highly rich in haemoglobin. The end stages of the erythroid cell maturation process primarily include chromatin condensation and nuclear polarization, followed by nuclear expulsion called enucleation and clearance of mitochondria and other organelles to finally generate mature RBCs. While healthy RBCs are devoid of mitochondria, recent evidence suggests that mitochondria are actively implicated in the processes of erythroid cell maturation, erythroblast enucleation and RBC production. However, the extent of mitochondrial participation that occurs during these ultimate steps is not completely understood. This is specifically important since abnormal RBC retention of mitochondria or mitochondrial DNA contributes to the pathophysiology of sickle cell and other disorders. Here we review some of the key findings so far that elucidate the importance of this process in various aspects of erythroid maturation and RBC production under homeostasis and disease conditions.

Decoding human in vitro terminal erythropoiesis originating from umbilical cord blood mononuclear cells and pluripotent stem cells

Ex vivo red blood cell (RBC) production generates unsatisfactory erythroid cells. A deep exploration into terminally differentiated cells is required to understand the impairments for RBC generation and the underlying mechanisms. Here, we mapped an atlas of terminally differentiated cells from umbilical cord blood mononuclear cells (UCBMN) and pluripotent stem cells (PSC) and observed their dynamic regulation of erythropoiesis at single-cell resolution. Interestingly, we detected a few progenitor cells and non-erythroid cells from both origins. In PSC-derived erythropoiesis (PSCE), the expression of haemoglobin switch regulators (BCL11A and ZBTB7A) were significantly absent, which could be the restraint for its adult globin expression. We also found that PSCE were less active in stress erythropoiesis than in UCBMN-derived erythropoiesis (UCBE), and explored an agonist of stress erythropoiesis gene, TRIB3, could enhance the expression of adult globin in PSCE. Compared with UCBE, there was a lower expression of epigenetic-related proteins (e.g., CASPASE 3 and UBE2O) and transcription factors (e.g., FOXO3 and TAL1) in PSCE, which might restrict PSCE's enucleation. Moreover, we characterized a subpopulation with high proliferation capacity marked by CD99high in colony-forming unit-erythroid cells. Inhibition of CD99 reduced the proliferation of PSC-derived cells and facilitated erythroid maturation. Furthermore, CD99-CD99 mediated the interaction between macrophages and erythroid cells, illustrating a mechanism by which macrophages participate in erythropoiesis. This study provided a reference for improving ex vivo RBC generation.

Progresses in overcoming the limitations of in vitro erythropoiesis using human induced pluripotent stem cells

Researchers have attempted to generate transfusable oxygen carriers to mitigate RBC supply shortages. In vitro generation of RBCs using stem cells such as hematopoietic stem and progenitor cells (HSPCs), embryonic stem cells (ESCs), and induced pluripotent stem cells (iPSCs) has shown promise. Specifically, the limited supplies of HSPCs and ethical issues with ESCs make iPSCs the most promising candidate for in vitro RBC generation. However, researchers have encountered some major challenges when using iPSCs to produce transfusable RBC products, such as enucleation and RBC maturation. In addition, it has proven difficult to manufacture these products on a large scale. In this review, we provide a brief overview of erythropoiesis and examine endeavors to recapitulate erythropoiesis in vitro using various cell sources. Furthermore, we explore the current obstacles and potential solutions aimed at enabling the large-scale production of transfusable RBCs in vitro.

Pivotal functions and impact of long con-coding RNAs on cellular processes and genome integrity

Recent advances in uncovering the mysteries of the human genome suggest that long non-coding RNAs (lncRNAs) are important regulatory components. Although lncRNAs are known to affect gene transcription, their mechanisms and biological implications are still unclear. Experimental research has shown that lncRNA synthesis, subcellular localization, and interactions with macromolecules like DNA, other RNAs, or proteins can all have an impact on gene expression in various biological processes. In this review, we highlight and discuss the major mechanisms through which lncRNAs function as master regulators of the human genome. Specifically, the objective of our review is to examine how lncRNAs regulate different processes like cell division, cell cycle, and immune responses, and unravel their roles in maintaining genomic architecture and integrity.

The miR-144/Hmgn2 regulatory axis orchestrates chromatin organization during erythropoiesis

Differentiation of stem and progenitor cells is a highly regulated process that involves the coordinated action of multiple layers of regulation. Here we show how the post-transcriptional regulatory layer instructs the level of chromatin regulation via miR-144 and its targets to orchestrate chromatin condensation during erythropoiesis. The loss of miR-144 leads to impaired chromatin condensation during erythrocyte maturation. Among the several targets of miR-144 that influence chromatin organization, the miR-144-dependent regulation of Hmgn2 is conserved from fish to humans. Our genetic probing of the miR-144/Hmgn2 regulatory axis establish that intact miR-144 target sites in the Hmgn2 3'UTR are necessary for the proper maturation of erythrocytes in both zebrafish and human iPSC-derived erythroid cells while loss of Hmgn2 rescues in part the miR-144 null phenotype. Altogether, our results uncover miR-144 and its target Hmgn2 as the backbone of the genetic regulatory circuit that controls the terminal differentiation of erythrocytes in vertebrates.

The miR-144/Hmgn2 regulatory axis orchestrates chromatin organization during erythropoiesis

Differentiation of stem and progenitor cells is a highly regulated process that involves the coordinated action of multiple layers of regulation. Here we show how the post-transcriptional regulatory layer instructs the level of chromatin regulation via miR-144 and its targets to orchestrate chromatin condensation during erythropoiesis. The loss of miR-144 leads to impaired chromatin condensation during erythrocyte maturation. Among the several targets of miR-144 that influence chromatin organization, the miR-144-dependent regulation of Hmgn2 is conserved from fish to humans. Our genetic probing of the miR-144/Hmgn2 regulatory axis established that intact miR-144 target sites in the Hmgn2 3'UTR are necessary for the proper maturation of erythrocytes in both zebrafish and human iPSC-derived erythroid cells while loss of Hmgn2 rescues in part the miR-144 null phenotype. Altogether, our results uncover miR-144 and its target Hmgn2 as the backbone of the genetic regulatory circuit that controls the terminal differentiation of erythrocytes in vertebrates.

Comprehensive Characterization and Global Transcriptome Analysis of Human Fetal Liver Terminal Erythropoiesis

The fetal liver (FL) is the key erythropoietic organ during fetal development, but knowledge on human FL erythropoiesis is very limited. In this study, we sorted primary erythroblasts from FL cells and performed RNA sequencing (RNA-seq) analyses. We found that temporal gene expression patterns reflected changes in function during primary human FL terminal erythropoiesis. Notably, the expression of genes enriched in proteolysis and autophagy was up-regulated in orthochromatic erythroblasts (OrthoEs), suggesting the involvement of these pathways in enucleation. We also performed RNA-seq of in vitro cultured erythroblasts derived from FL CD34+ cells. Comparison of transcriptomes between the primary and cultured erythroblasts revealed significant differences, indicating impacts of the culture system on gene expression. Notably, the expression of lipid metabolism-related genes was increased in cultured erythroblasts. We further immortalized erythroid cell lines from FL and cord blood (CB) CD34+ cells (FL-iEry and CB-iEry, respectively). FL-iEry and CB-iEry were immortalized at the proerythroblast stage and can be induced to differentiate into OrthoEs, but their enucleation ability was very low. Comparison of the transcriptomes between OrthoEs with and without enucleation capability revealed the down-regulation of pathways involved in chromatin organization and mitophagy in OrthoEs without enucleation capacity, indicating that defects in chromatin organization and mitophagy contribute to the inability of OrthoEs to enucleate. Additionally, the expression of HBE1, HBZ, and HBG2 was up-regulated in FL-iEry compared with CB-iEry, and such up-regulation was accompanied by down-regulated expression of BCL11A and up-regulated expression of LIN28B and IGF2BP1. Our study provides new insights into human FL erythropoiesis and rich resources for future studies.

Peroxiredoxins in erythrocytes: far beyond the antioxidant role

The red blood cells (RBCs) are essential to transport oxygen (O2) and nutrients throughout the human body. Changes in the structure or functioning of the erythrocytes can lead to several deficiencies, such as hemolytic anemias, in which an increase in reactive oxidative species generation is involved in the pathophysiological process, playing a significant role in the severity of several clinical manifestations. There are important lines of defense against the damage caused by oxidizing molecules. Among the antioxidant molecules, the enzyme peroxiredoxin (Prx) has the higher decomposition power of hydrogen peroxide, especially in RBCs, standing out because of its abundance. This review aimed to present the recent findings that broke some paradigms regarding the three isoforms of Prxs found in RBC (Prx1, Prx2, and Prx6), showing that in addition to their antioxidant activity, these enzymes may have supplementary roles in transducing peroxide signals, as molecular chaperones, protecting from membrane damage, and maintenance of iron homeostasis, thus contributing to the overall survival of human RBCs, roles that seen to be disrupted in hemolytic anemia conditions.

EHBP1L1, an apicobasal polarity regulator, is critical for nuclear polarization during enucleation of erythroblasts

Cell polarity, the asymmetric distribution of proteins and organelles, is permanently or transiently established in various cell types and plays an important role in many physiological events. epidermal growth factor receptor substrate 15 homology domain-binding protein 1-like 1 (EHBP1L1) is an adapter protein that is localized on recycling endosomes and regulates apical-directed transport in polarized epithelial cells. However, the role of EHBP1L1 in nonepithelial cells, remains unknown. Here, Ehbp1l1-/- mice showed impaired erythroblast enucleation. Further analyses showed that nuclear polarization before enucleation was impaired in Ehbp1l1-/- erythroblasts. It was also revealed that EHBP1L1 interactors Rab10, Bin1, and dynamin were involved in erythroblast enucleation. In addition, Ehbp1l1-/- erythrocytes exhibited stomatocytic morphology and dehydration. These defects in erythroid cells culminated in early postnatal anemic lethality in Ehbp1l1-/- mice. Moreover, we found the mislocalization of nuclei and mitochondria in the skeletal muscle cells of Ehbp1l1-/- mice, as observed in patients with centronuclear myopathy with genetic mutations in Bin1 or dynamin 2. Taken together, our findings indicate that the Rab8/10-EHBP1L1-Bin1-dynamin axis plays an important role in multiple cell polarity systems in epithelial and nonepithelial cells.

Dynamics of human hematopoietic stem and progenitor cell differentiation to the erythroid lineage

Erythropoiesis, the development of erythrocytes from hematopoietic stem cells, occurs through four phases: erythroid progenitor (EP) development, early erythropoiesis, terminal erythroid differentiation (TED), and maturation. According to the classical model that is based on immunophenotypic profiles of cell populations, each of these phases comprises multiple differentiation states that arise in a hierarchical manner. After segregation of lymphoid potential, erythroid priming begins during progenitor development and progresses through progenitor cell types that have multilineage potential. Complete separation of the erythroid lineage is achieved during early erythropoiesis with the formation of unipotent EPs: burst-forming unit-erythroid and colony-forming unit-erythroid. These erythroid-committed progenitors undergo TED and maturation, which involves expulsion of the nucleus and remodeling to form functional biconcave, hemoglobin-filled erythrocytes. In the last decade or so, many studies employing advanced techniques such as single-cell RNA-sequencing (scRNA-seq) as well as the conventional methods, including colony-forming cell assays and immunophenotyping, have revealed heterogeneity within the stem, progenitor, and erythroblast stages, and uncovered alternate paths for segregation of erythroid lineage potential. In this review, we provide an in-depth account of immunophenotypic profiles of all cell types within erythropoiesis, highlight studies that demonstrate heterogeneous erythroid stages, and describe deviations to the classical model of erythropoiesis. Overall, although scRNA-seq approaches have provided new insights, flow cytometry remains relevant and is the primary method for validation of novel immunophenotypes.

Post-transcriptional regulation of erythropoiesis

Erythropoiesis is a complex, precise, and lifelong process that is essential for maintaining normal body functions. Its strict regulation is necessary to prevent a variety of blood diseases. Normal erythropoiesis is precisely regulated by an intricate network that involves transcription levels, signal transduction, and various epigenetic modifications. In recent years, research on post-transcriptional levels in erythropoiesis has expanded significantly. The dynamic regulation of splicing transitions is responsible for changes in protein isoform expression that add new functions beneficial for erythropoiesis. RNA-binding proteins adapt the translation of transcripts to the protein requirements of the cell, yielding mRNA with dynamic translation efficiency. Noncoding RNAs, such as microRNAs and lncRNAs, are indispensable for changing the translational efficiency and/or stability of targeted mRNAs to maintain the normal expression of genes related to erythropoiesis. N6-methyladenosine-dependent regulation of mRNA translation plays an important role in maintaining the expression programs of erythroid-related genes and promoting erythroid lineage determination. This review aims to describe our current understanding of the role of post-transcriptional regulation in erythropoiesis and erythroid-associated diseases, and to shed light on the physiological and pathological implications of the post-transcriptional regulation machinery in erythropoiesis. These may help to further enrich our understanding of the regulatory network of erythropoiesis and provide new strategies for the diagnosis and treatment of erythroid-related diseases.

Immortalized erythroid cells as a novel frontier for in vitro blood production: current approaches and potential clinical application

BACKGROUND

Blood transfusions represent common medical procedures, which provide essential supportive therapy. However, these procedures are notoriously expensive for healthcare services and not without risk. The potential threat of transfusion-related complications, such as the development of pathogenic infections and the occurring of alloimmunization events, alongside the donor's dependence, strongly limits the availability of transfusion units and represents significant concerns in transfusion medicine. Moreover, a further increase in the demand for donated blood and blood transfusion, combined with a reduction in blood donors, is expected as a consequence of the decrease in birth rates and increase in life expectancy in industrialized countries.

MAIN BODY

An emerging and alternative strategy preferred over blood transfusion is the in vitro production of blood cells from immortalized erythroid cells. The high survival capacity alongside the stable and longest proliferation time of immortalized erythroid cells could allow the generation of a large number of cells over time, which are able to differentiate into blood cells. However, a large-scale, cost-effective production of blood cells is not yet a routine clinical procedure, as being dependent on the optimization of culture conditions of immortalized erythroid cells.

CONCLUSION

In our review, we provide an overview of the most recent erythroid cell immortalization approaches, while also describing and discussing related advancements of establishing immortalized erythroid cell lines.

Nucleated red blood cells explain most of the association between DNA methylation and gestational age

Determining if specific cell type(s) are responsible for an association between DNA methylation (DNAm) and a given phenotype is important for understanding the biological mechanisms underlying the association. Our EWAS of gestational age (GA) in 953 newborns from the Norwegian MoBa study identified 13,660 CpGs significantly associated with GA (p_Bonferroni<0.05) after adjustment for cell type composition. When the CellDMC algorithm was applied to explore cell-type specific effects, 2,330 CpGs were significantly associated with GA, mostly in nucleated red blood cells [nRBCs; n = 2,030 (87%)]. Similar patterns were found in another dataset based on a different array and when applying an alternative algorithm to CellDMC called Tensor Composition Analysis (TCA). Our findings point to nRBCs as the main cell type driving the DNAm-GA association, implicating an epigenetic signature of erythropoiesis as a likely mechanism. They also explain the poor correlation observed between epigenetic age clocks for newborns and those for adults.

The accumulation of miR-125b-5p is indispensable for efficient erythroblast enucleation

Erythroblast enucleation is a precisely regulated but not clearly understood process. Polycythemia shows pathological erythroblast enucleation, and we discovered a low miR-125b-5p level in terminal erythroblasts of patients with polycythemia vera (PV) compared to those of healthy controls. Exogenous upregulation of miR-125b-5p levels restored the enucleation rate to normal levels. Direct downregulation of miR-125b-5p in mouse erythroblasts simulated the enucleation issue found in patients with PV, and miR-125b-5p accumulation was found in enucleating erythroblasts, collectively suggesting the importance of miR-125b-5p accumulation for erythroblast enucleation. To elucidate the role of miR-125b-5p in enucleation, gain- and loss-of-function studies were performed. Overexpression of miR-125b-5p improved the enucleation of erythroleukemia cells and primary erythroblasts. Infused erythroblasts with higher levels of miR-125b-5p also exhibited accelerated enucleation. In contrast, miR-125b-5p inhibitors significantly suppressed erythrocyte enucleation. Intracellular imaging revealed that in addition to cytoskeletal assembly and nuclear condensation, miR-125b-5p overexpression resulted in mitochondrial reduction and depolarization. Real-time PCR, western blot analysis, luciferase reporter assays, small molecule inhibitor supplementation and gene rescue assays revealed that Bcl-2, as a direct target of miR-125b-5p, was one of the key mediators of miR-125b-5p during enucleation. Following suppression of Bcl-2, the activation of caspase-3 and subsequent activation of ROCK-1 resulted in cytoskeletal rearrangement and enucleation. In conclusion, this study is the first to reveal the pivotal role of miR-125b-5p in erythroblast enucleation.

The inner nuclear membrane protein NEMP1 supports nuclear envelope openings and enucleation of erythroblasts

Nuclear envelope membrane proteins (NEMPs) are a conserved family of nuclear envelope (NE) proteins that reside within the inner nuclear membrane (INM). Even though Nemp1 knockout (KO) mice are overtly normal, they display a pronounced splenomegaly. This phenotype and recent reports describing a requirement for NE openings during erythroblasts terminal maturation led us to examine a potential role for Nemp1 in erythropoiesis. Here, we report that Nemp1 KO mice show peripheral blood defects, anemia in neonates, ineffective erythropoiesis, splenomegaly, and stress erythropoiesis. The erythroid lineage of Nemp1 KO mice is overrepresented until the pronounced apoptosis of polychromatophilic erythroblasts. We show that NEMP1 localizes to the NE of erythroblasts and their progenitors. Mechanistically, we discovered that NEMP1 accumulates into aggregates that localize near or at the edge of NE openings and Nemp1 deficiency leads to a marked decrease of both NE openings and ensuing enucleation. Together, our results for the first time demonstrate that NEMP1 is essential for NE openings and erythropoietic maturation in vivo and provide the first mouse model of defective erythropoiesis directly linked to the loss of an INM protein.

Impact of Staining Methods and Human Factors on Accuracy of Manual Reticulocyte Enumeration

Although peripheral blood reticulocyte enumeration reflects bone marrow functional integrity, which is important for differential diagnosis of hematological diseases, the factors affecting its accuracy have not been adequately addressed. Using 100 consecutive venous blood samples being processed with four supravital staining techniques [i.e., brilliant cresyl blue (BCB), new methylene blue (NMB), and BCB/NMB with Liu’s stain] for reticulocyte enumeration, two technologists (senior vs. junior) conducted microscopic counting. The results were compared with those obtained with an automated system (Sysmex XE-5000) that served as the standard. The aims of this study were to identify (1) the technique that gave the most reliable outcome, and (2) possible human factors (i.e., seniority, repeated counting) that may affect the counting results. Analysis showed least bias (i.e., deviation from automated counting) associated with BCB staining, followed by NMB. In addition, the senior observer exhibited a higher bias in counting compared with their junior counterpart. Repeated counting also correlated with a higher rate of bias. Nevertheless, inter-observer consistency was high (intraclass correlation coefficient >0.95) and inter-/intra-observer variations were non-significant (both p > 0.05). Our results supported the use of BCB stain for reticulocyte enumeration and the reliability of manual counting despite the involvement of human factors, which had negligible impacts on the final outcomes.

Discovery of N-arylcinnamamides as novel erythroblast enucleation inducers

Derivation of mature red blood cells (RBCs) from stem cells in vitro is a promising solution to the current shortage of blood supply, in which terminal enucleation is the rate-limiting step. Here we discovered two cinnamamides B8 and B16 showed potential activities of enhancing the enucleation of erythroblasts through the screening of "in-house" compound library. Subsequently, twenty-four N-arylcinnamamides were rationally designed and synthesized on the basis of the structure of B8 and B16, in which N-(9H-carbazol-2-yl)cinnamamide (KS-2) significantly elevated the percentage of reticulocytes in the cultured mouse fetal liver cells in vitro (relative enucleation = 2.43). The underlying mechanism of KS-2 in promoting mouse erythroid enucleation is accelerating the process of cell cycle exit via p53 activation in late stage erythrocytes. These results strongly suggest that compound KS-2 is worthy of further study as a potential erythrocyte enucleation inducer.

Autism-associated chromatin remodeler CHD8 regulates erythroblast cytokinesis and fine-tunes the balance of Rho GTPase signaling

CHD8 is an ATP-dependent chromatin-remodeling factor whose monoallelic mutation defines a subtype of autism spectrum disorders (ASDs). Previous work found that CHD8 is required for the maintenance of hematopoiesis by integrating ATM-P53-mediated survival of hematopoietic stem/progenitor cells (HSPCs). Here, by using Chd8^F/FMx1-Cre combined with a Trp53^F/F mouse model that suppresses apoptosis of Chd8^−/− HSPCs, we identify CHD8 as an essential regulator of erythroid differentiation. Chd8^−/−P53^−/− mice exhibited severe anemia conforming to congenital dyserythropoietic anemia (CDA) phenotypes. Loss of CHD8 leads to drastically decreased numbers of orthochromatic erythroblasts and increased binucleated and multinucleated basophilic erythroblasts with a cytokinesis failure in erythroblasts. CHD8 binds directly to the gene bodies of multiple Rho GTPase signaling genes in erythroblasts, and loss of CHD8 results in their dysregulated expression, leading to decreased RhoA and increased Rac1 and Cdc42 activities. Our study shows that autism-associated CHD8 is essential for erythroblast cytokinesis.

Tu et al. report that CHD8, an autism-related chromatin remodeler, is essential for erythroid differentiation. Loss of CHD8 leads to unbalanced Rho GTPase signaling and defective erythroblast cytokinesis, mimicking that of congenital dyserythropoietic anemia.

Chicken cathepsin G-like - A highly specific serine protease with a peculiar tryptase specificity expressed by chicken thrombocytes

Serine proteases are major granule constituents of cells from several mammalian hematopoietic cell lineages. Despite the relatively extensive knowledge about these mammalian proteases, very little is known about their bird, reptile and amphibian homologs. In order to close this gap in our understanding of the evolution of these proteases, we have characterized the extended cleavage specificity and hematopoietic expression pattern of the chicken serine protease cathepsin G-like. This protease, which clusters in a separate subfamily of serine proteases among the vertebrate hematopoietic serine proteases, has been characterized using substrate phage display and further validated by using a panel of recombinant substrates. A preference for a lysine in the P1 position of a substrate, arginines in positions P2 and P3, and the aromatic amino acid tryptophane in the P4 position was observed. Based on the sequence alignment we could identify a consensus sequence for this protease as being PGGWRRK^↓ALSV. Mass spectrometry analysis of a peptide with the consensus sequence obtained by phage display showed that cleavage of this peptide occurred after the conserved Lys (K) residue. A screening of potential in vivo substrates based on the derived P5-P3' consensus sequence resulted in a relatively limited number of potential substrates, due to the high selectivity of this enzyme. The most interesting of these were PDGF-A, coagulation factor V and low-density lipoprotein receptor like-8. Immunohistochemical analysis of chicken white blood cells with antisera produced against chicken cathepsin G-like and chicken egg lysozyme, as a reference protein known to be expressed by hematopoietic cells, showed presence of chicken cathepsin G-like almost exclusively in thrombocytes whereas lysozyme was found at very high amounts in heterophils, and lower amounts in monocytes and thrombocytes.

Tpr Deficiency Disrupts Erythroid Maturation With Impaired Chromatin Condensation in Zebrafish Embryogenesis

Vertebrate erythropoiesis involves nuclear and chromatin condensation at the early stages of terminal differentiation, which is a unique process to distinguish mature erythrocytes from erythroblasts. However, the underlying mechanisms of chromatin condensation during erythrocyte maturation remain elusive. Here, we reported a novel zebrafish mutant cas7 with erythroid maturation deficiency. Positional cloning showed that a single base mutation in tprb gene, which encodes nucleoporin translocated promoter region (Tpr), is responsible for the disrupted erythroid maturation and upregulation of erythroid genes, including ae1-globin and be1-globin. Further investigation revealed that deficient erythropoiesis in tprb cas7 mutant was independent on HIF signaling pathway. The proportion of euchromatin was significantly increased, whereas the percentage of heterochromatin was markedly decreased in tprb cas7 mutant. In addition, TPR knockdown in human K562 cells also disrupted erythroid differentiation and dramatically elevated the expression of globin genes, which suggests that the functions of TPR in erythropoiesis are highly conserved in vertebrates. Taken together, this study revealed that Tpr played vital roles in chromatin condensation and gene regulation during erythroid maturation in vertebrates.

Macrophages form erythropoietic niches and regulate iron homeostasis to adapt erythropoiesis in response to infections and inflammation

It has recently emerged that tissue-resident macrophages are key regulators of several stem cell niches orchestrating tissue formation during development, as well as postnatally, when they also organize the repair and regeneration of many tissues including the hemopoietic tissue. The fact that macrophages are also master regulators and effectors of innate immunity and inflammation allows them to coordinate hematopoietic response to infections, injuries, and inflammation. After recently reviewing the roles of phagocytes and macrophages in regulating normal and pathologic hematopoietic stem cell niches, we now focus on the key roles of macrophages in regulating erythropoiesis and iron homeostasis. We review herein the recent advances in understanding how macrophages at the center of erythroblastic islands form an erythropoietic niche that controls the terminal differentiation and maturation of erythroblasts into reticulocytes; how red pulp macrophages in the spleen control iron recycling and homeostasis; how these macrophages coordinate emergency erythropoiesis in response to blood loss, infections, and inflammation; and how persistent infections or inflammation can lead to anemia of inflammation via macrophages. Finally, we discuss the technical challenges associated with the molecular characterization of erythroid island macrophages and red pulp macrophages.

Treatment strategies for glucose-6-phosphate dehydrogenase deficiency: past and future perspectives

Glucose-6-phosphate dehydrogenase (G6PD) maintains redox balance in a variety of cell types and is essential for erythrocyte resistance to oxidative stress. G6PD deficiency, caused by mutations in the G6PD gene, is present in ~400 million people worldwide, and can cause acute hemolytic anemia. Currently, there are no therapeutics for G6PD deficiency. We discuss the role of G6PD in hemolytic and nonhemolytic disorders, treatment strategies attempted over the years, and potential reasons for their failure. We also discuss potential pharmacological pathways, including glutathione (GSH) metabolism, compensatory NADPH production routes, transcriptional upregulation of the G6PD gene, highlighting potential drug targets. The needs and opportunities described here may motivate the development of a therapeutic for hematological and other chronic diseases associated with G6PD deficiency.

Blood Pharming – eine realistische Option?

Die Bluttransfusion ist ein wesentlicher und unersetzlicher Teil der modernen Medizin. Jedoch stellt vor allem bei Patienten mit sehr seltenen Blutgruppenkonstellationen der Mangel an Blutprodukten auch heute noch ein wichtiges Gesundheitsproblem weltweit dar. Um diesem Problem entgegenzutreten, versucht man seit einiger Zeit künstlich rote Blutzellen zu generieren. Diese haben potenzielle Vorteile gegenüber Spenderblut, wie z. B. ein verringertes Risiko für die Übertragung von Infektionskrankheiten. Diese Übersicht fasst die aktuellen Entwicklungen über den Prozess der Erythropoese, die Expansionsstrategien der erythrozytären Zellen, der verschiedenen Quellen für ex vivo expandierte Erythrozyten, die Hürden für die klinische Anwendung und die zukünftigen Möglichkeiten der Anwendung zusammen.

Mitochondrial localization and moderated activity are key to murine erythroid enucleation

Mammalian red blood cells (RBCs), which primarily contain hemoglobin, exemplify an elaborate maturation process, with the terminal steps of RBC generation involving extensive cellular remodeling. This encompasses alterations of cellular content through distinct stages of erythroblast maturation that result in the expulsion of the nucleus (enucleation) followed by the loss of mitochondria and all other organelles and a transition to anaerobic glycolysis. Whether there is any link between erythroid removal of the nucleus and the function of any other organelle, including mitochondria, remains unknown. Here we demonstrate that mitochondria are key to nuclear clearance. Using live and confocal microscopy and high-throughput single-cell imaging, we show that before nuclear polarization, mitochondria progressively move toward one side of maturing erythroblasts and aggregate near the nucleus as it extrudes from the cell, a prerequisite for enucleation to proceed. Although we found active mitochondrial respiration is required for nuclear expulsion, levels of mitochondrial activity identify distinct functional subpopulations, because terminally maturing erythroblasts with low relative to high mitochondrial membrane potential are at a later stage of maturation, contain greatly condensed nuclei with reduced open chromatin-associated acetylation histone marks, and exhibit higher enucleation rates. Lastly, to our surprise, we found that late-stage erythroblasts sustain mitochondrial metabolism and subsequent enucleation, primarily through pyruvate but independent of in situ glycolysis. These findings demonstrate the critical but unanticipated functions of mitochondria during the erythroblast enucleation process. They are also relevant to the in vitro production of RBCs as well as to disorders of the erythroid lineage.

Home Sweet Home: Plasmodium vivax-Infected Reticulocytes-The Younger the Better?

After a century of constant failure to produce an in vitro culture of the most widespread human malaria parasite Plasmodium vivax, recent advances have highlighted the difficulties to provide this parasite with a healthy host cell to invade, develop, and multiply under in vitro conditions. The actual level of understanding of the heterogeneous populations of cells—framed under the name ‘reticulocytes’—and, importantly, their adequate in vitro progression from very immature reticulocytes to normocytes (mature erythrocytes) is far from complete. The volatility of its individual stability may suggest the reticulocyte as a delusory cell, particularly to be used for stable culture purposes. Yet, the recent relevance gained by a specific subset of highly immature reticulocytes has brought some hope. Very immature reticulocytes are characterized by a peculiar membrane harboring a plethora of molecules potentially involved in P. vivax invasion and by an intracellular complexity dynamically changing upon its quick maturation into normocytes. We analyze the potentialities offered by this youngest reticulocyte subsets as an ideal in vitro host cell for P. vivax.

SETD2 is essential for terminal differentiation of erythroblasts during fetal erythropoiesis

SET domain-containing 2 (SETD2), the primary methyltransferase for histone 3 lysine-36 trimethylation (H3K36me3) in mammals, is associated with many hematopoietic diseases when mutated. Previous works have emphasized its role in maintaining adult hematopoietic stem cells or tumorigenesis, however, whether and how SETD2 regulates erythropoiesis during embryonic development is relatively unexplored. In this study, using a conditional SETD2 knockout (KO) mouse model, we reveal that SETD2 plays an essential role in fetal erythropoiesis. Loss of Setd2 in hematopoietic cells ablates H3K36me3, and leads to anemia with a significant decrease in erythroid cells in the peripheral blood at E18.5. This is due to impaired erythroblast differentiation in both spleen and liver. We also find increased proportions of nucleated erythrocytes in the blood of Setd2 KO embryos. Lastly, we ascribe embryonic erythropoiesis-related genes Vegfc, Vegfr3, and Prox1, as likely downstream targets of SETD2 regulation. Our study reveals a critical role of SETD2 in fetal erythropoiesis that precedes adult hematopoiesis, and provide unique insights into the defects in erythroid lineages, such as anemia.