Formation of mammalian erythrocytes: chromatin condensation and enucleation

Selective colorimetric urine glucose detection by paper sensor functionalized with polyaniline nanoparticles and cell membrane

For the diabetes diagnosis, noninvasive methods are preferred to invasive methods; urine glucose measurement is an example of a noninvasive method. However, conventional noninvasive methods for urine glucose measurement are not intuitive. Furthermore, such methods exhibit low selectivity because they can detect interfering molecules in addition to glucose. Herein, we fabricate a noninvasive, intuitive, and highly selective paper sensor consisting of polyaniline nanoparticles (PAni-NPs) and red blood cell membranes (RBCMs). The PAni-NPs (adsorbed on the paper) are highly sensitive to hydrogen ions and change color from emeraldine blue to emeraldine green within a few seconds. The RBCM (coated on the PAni-NP-adsorbed paper) having the glucose transporter-1 protein plays the role of a smart filter that transports glucose but rejects other interfering molecules. In particular, the selectivity of the RBCM-coated PAni-NP-based paper sensor was approximately improved ∼85%, compared to the uncoated paper sensors. The paper sensor could detect urine glucose over the range of 0-10 mg/mL (0-56 mM), with a limit of detection of 0.54 mM. The proposed paper sensor will facilitate the development of a highly selective and colorimetric urine glucose monitoring system.

Understanding terminal erythropoiesis: An update on chromatin condensation, enucleation, and reticulocyte maturation

A characteristic feature of terminal erythropoiesis in mammals is extrusion of the highly condensed nucleus out of the cytoplasm. Other vertebrates, including fish, reptiles, amphibians, and birds, undergo nuclear condensation but do not enucleate. Enucleation provides mammals evolutionary advantages by gaining extra space for hemoglobin and being more flexible to migrate through capillaries. Nascent reticulocytes further mature into red blood cells through membrane and proteome remodeling and organelle clearance. Over the past decade, novel molecular mechanisms and signaling pathways have been uncovered that play important roles in chromatin condensation, enucleation, and reticulocyte maturation. These advances not only increase understanding of the physiology of erythropoiesis, but also facilitate efforts in generating in vitro red blood cells for various translational application. In the present review, recent studies in epigenetic modification and release of histones during chromatin condensation are highlighted. New insights in enucleation, including protein sorting, vesicle trafficking, transcriptional regulation, noncoding RNA, cytoskeleton remodeling, erythroblastic islands, and cytokinesis, are summarized. Moreover, organelle clearance and proteolysis mediated by ubiquitin-proteasome degradation during reticulocytes maturation is also examined. Perspectives for future directions in this rapidly evolving research area are also provided.

Erythroid enucleation: a gateway into a "bloody" world

Erythropoiesis is an intricate process starting in hematopoietic stem cells and leading to the daily production of 200 billion red blood cells (RBCs). Enucleation is a greatly complex and rate-limiting step during terminal maturation of mammalian RBC production involving expulsion of the nucleus from the orthochromatic erythroblasts, resulting in the formation of reticulocytes. The dynamic enucleation process involves many factors ranging from cytoskeletal proteins to transcription factors to microRNAs. Lack of optimum terminal erythroid maturation and enucleation has been an impediment to optimum RBC production ex vivo. Major efforts in the past two decades have exposed some of the mechanisms that govern the enucleation process. This review focuses in detail on mechanisms implicated in enucleation and discusses the future perspectives of this fascinating process.

Erythrocytes 3D genome organization in vertebrates

Generation of mature red blood cells, consisting mainly of hemoglobin, is a remarkable example of coordinated action of various signaling networks. Chromatin condensation is an essential step for terminal erythroid differentiation and subsequent nuclear expulsion in mammals. Here, we profiled 3D genome organization in the blood cells from ten species belonging to different vertebrate classes. Our analysis of contact maps revealed a striking absence of such 3D interaction patterns as loops or TADs in blood cells of all analyzed representatives. We also detect large-scale chromatin rearrangements in blood cells from mammals, birds, reptiles and amphibians: their contact maps display strong second diagonal pattern, representing an increased frequency of long-range contacts, unrelated to TADs or compartments. This pattern is completely atypical for interphase chromosome structure. We confirm that these principles of genome organization are conservative in vertebrate erythroid cells.

Genome-wide analysis of pseudogenes reveals HBBP1's human-specific essentiality in erythropoiesis and implication in β-thalassemia

The human genome harbors 14,000 duplicated or retroposed pseudogenes. Given their functionality as regulatory RNAs and low conservation, we hypothesized that pseudogenes could shape human-specific phenotypes. To test this, we performed co-expression analyses and found that pseudogene exhibited tissue-specific expression, especially in the bone marrow. By incorporating genetic data, we identified a bone-marrow-specific duplicated pseudogene, HBBP1 (η-globin), which has been implicated in β-thalassemia. Extensive functional assays demonstrated that HBBP1 is essential for erythropoiesis by binding the RNA-binding protein (RBP), HNRNPA1, to upregulate TAL1, a key regulator of erythropoiesis. The HBBP1/TAL1 interaction contributes to a milder symptom in β-thalassemia patients. Comparative studies further indicated that the HBBP1/TAL1 interaction is human-specific. Genome-wide analyses showed that duplicated pseudogenes are often bound by RBPs and less commonly bound by microRNAs compared with retropseudogenes. Taken together, we not only demonstrate that pseudogenes can drive human evolution but also provide insights on their functional landscapes.

Membrane skeleton modulates erythroid proteome remodeling and organelle clearance

The final stages of mammalian erythropoiesis involve enucleation, membrane and proteome remodeling, and organelle clearance. Concomitantly, the erythroid membrane skeleton establishes a unique pseudohexagonal spectrin meshwork that is connected to the membrane through junctional complexes. The mechanism and signaling pathways involved in the coordination of these processes are unclear. The results of our study revealed an unexpected role of the membrane skeleton in the modulation of proteome remodeling and organelle clearance during the final stages of erythropoiesis. We found that diaphanous-related formin mDia2 is a master regulator of the integrity of the membrane skeleton through polymerization of actin protofilament in the junctional complex. The mDia2-deficient terminal erythroid cell contained a disorganized and rigid membrane skeleton that was ineffective in detaching the extruded nucleus. In addition, the disrupted skeleton failed to activate the endosomal sorting complex required for transport-III (ESCRT-III) complex, which led to a global defect in proteome remodeling, endolysosomal trafficking, and autophagic organelle clearance. Chmp5, a component of the ESCRT-III complex, is regulated by mDia2-dependent activation of the serum response factor and is essential for membrane remodeling and autophagosome-lysosome fusion. Mice with loss of Chmp5 in hematopoietic cells in vivo resembled the phenotypes in mDia2-knockout mice. Furthermore, overexpression of Chmp5 in mDia2-deficient hematopoietic stem and progenitor cells significantly restored terminal erythropoiesis in vivo. These findings reveal a formin-regulated signaling pathway that connects the membrane skeleton to proteome remodeling, enucleation, and organelle clearance during terminal erythropoiesis.

Soft erythrocyte-based bacterial microswimmers for cargo delivery

Bacteria-propelled biohybrid microswimmers have recently shown to be able to actively transport and deliver cargos encapsulated into their synthetic constructs to specific regions locally. However, usage of synthetic materials as cargo carriers can result in inferior performance in load-carrying efficiency, biocompatibility, and biodegradability, impeding clinical translation of biohybrid microswimmers. Here, we report construction and external guidance of bacteria-driven microswimmers using red blood cells (RBCs; erythrocytes) as autologous cargo carriers for active and guided drug delivery. Multifunctional biohybrid microswimmers were fabricated by attachment of RBCs [loaded with anticancer doxorubicin drug molecules and superparamagnetic iron oxide nanoparticles (SPIONs)] to bioengineered motile bacteria, Escherichia coli MG1655, via biotin-avidin-biotin binding complex. Autonomous and on-board propulsion of biohybrid microswimmers was provided by bacteria, and their external magnetic guidance was enabled by SPIONs loaded into the RBCs. Furthermore, bacteria-driven RBC microswimmers displayed preserved deformability and attachment stability even after squeezing in microchannels smaller than their sizes, as in the case of bare RBCs. In addition, an on-demand light-activated hyperthermia termination switch was engineered for RBC microswimmers to control bacteria population after operations. RBCs, as biological and autologous cargo carriers in the biohybrid microswimmers, offer notable advantages in stability, deformability, biocompatibility, and biodegradability over synthetic cargo-carrier materials. The biohybrid microswimmer design presented here transforms RBCs from passive cargo carriers into active and guidable cargo carriers toward targeted drug and other cargo delivery applications in medicine.

GATA factor-regulated solute carrier ensemble reveals a nucleoside transporter-dependent differentiation mechanism

Developmental-regulatory networks often include large gene families encoding mechanistically-related proteins like G-protein-coupled receptors, zinc finger transcription factors and solute carrier (SLC) transporters. In principle, a common mechanism may confer expression of multiple members integral to a developmental process, or diverse mechanisms may be deployed. Using genetic complementation and enhancer-mutant systems, we analyzed the 456 member SLC family that establishes the small molecule constitution of cells. This analysis identified SLC gene cohorts regulated by GATA1 and/or GATA2 during erythroid differentiation. As >50 SLC genes shared GATA factor regulation, a common mechanism established multiple members of this family. These genes included Slc29a1 encoding an equilibrative nucleoside transporter (Slc29a1/ENT1) that utilizes adenosine as a preferred substrate. Slc29a1 promoted erythroblast survival and differentiation ex vivo. Targeted ablation of murine Slc29a1 in erythroblasts attenuated erythropoiesis and erythrocyte regeneration in response to acute anemia. Our results reveal a GATA factor-regulated SLC ensemble, with a nucleoside transporter component that promotes erythropoiesis and prevents anemia, and establish a mechanistic link between GATA factor and adenosine mechanisms. We propose that integration of the GATA factor-adenosine circuit with other components of the GATA factor-regulated SLC ensemble establishes the small molecule repertoire required for progenitor cells to efficiently generate erythrocytes.

GATA transcription factors endow blood stem and progenitor cells with activities to produce progeny that transport oxygen to protect cells and tissues, evade pathogens and control physiological processes. GATA factors regulate hundreds of genes, and the actions of these genes mediate important biological functions. While the genes have been documented, many questions remain regarding how the “network” components mediate biological functions. The networks include members of large gene families, and the relationships between the regulation and function of individual family members is not well understood. Analyzing datasets from genetic complementation and enhancer mutant systems revealed that GATA factors regulate an ensemble of membrane transporters termed solute carrier proteins (SLCs), which dictate the small molecule composition of cells. Genetic analyses with Slc29a1, which transports adenosine, revealed its function to promote erythrocyte development, and Slc29a1 attenuated anemia in a mouse model. This study revealed the importance of SLC transporters in GATA factor networks. We propose that the GATA factor-adenosine circuit integrates with other SLCs to establish/maintain the small molecule constitution of progenitor cells as a new mechanism to control blood cell development.

Putative Origins of Cell-Free DNA in Humans: A Review of Active and Passive Nucleic Acid Release Mechanisms

Through various pathways of cell death, degradation, and regulated extrusion, partial or complete genomes of various origins (e.g., host cells, fetal cells, and infiltrating viruses and microbes) are continuously shed into human body fluids in the form of segmented cell-free DNA (cfDNA) molecules. While the genetic complexity of total cfDNA is vast, the development of progressively efficient extraction, high-throughput sequencing, characterization via bioinformatics procedures, and detection have resulted in increasingly accurate partitioning and profiling of cfDNA subtypes. Not surprisingly, cfDNA analysis is emerging as a powerful clinical tool in many branches of medicine. In addition, the low invasiveness of longitudinal cfDNA sampling provides unprecedented access to study temporal genomic changes in a variety of contexts. However, the genetic diversity of cfDNA is also a great source of ambiguity and poses significant experimental and analytical challenges. For example, the cfDNA population in the bloodstream is heterogeneous and also fluctuates dynamically, differs between individuals, and exhibits numerous overlapping features despite often originating from different sources and processes. Therefore, a deeper understanding of the determining variables that impact the properties of cfDNA is crucial, however, thus far, is largely lacking. In this work we review recent and historical research on active vs. passive release mechanisms and estimate the significance and extent of their contribution to the composition of cfDNA.

Codanin-1 mutations engineered in human erythroid cells demonstrate role of CDAN1 in terminal erythroid maturation

The generation of a functional erythrocyte from a committed progenitor requires significant changes in gene expression during hemoglobin accumulation, rapid cell division, and nuclear condensation. Congenital dyserythropoietic anemia type I (CDA-I) is an autosomal recessive disease that presents with erythroid hyperplasia in the bone marrow. Erythroblasts in patients with CDA-I are frequently binucleate and have chromatin bridging and defective chromatin condensation. CDA-1 is most commonly caused by mutations in Codanin-1 (CDAN1). The function of CDAN1 is poorly understood but it is thought to regulate histone incorporation into nascent DNA during cellular replication. The study of CDA-1 has been limited by the lack of in vitro models that recapitulate key features of the disease, and most studies on CDAN1 function have been done in nonerythroid cells. To model CDA-I we generated HUDEP2 mutant lines with deletion or mutation of R1042 of CDAN1, mirroring mutations found in CDA-1 patients. CDAN1 mutant cell lines had decreased viability and increased intercellular bridges and binucleate cells. Further, they had alterations in histone acetylation associated with prematurely elevated erythroid gene expression, including gamma globin. Together, these data imply a specific functional role for CDAN1, specifically R1042 on exon 24, in the regulation of DNA replication and organization during erythroid maturation. Most importantly, generation of models with specific patient mutations, such as R1042, will provide further mechanistic insights into CDA-I pathology.

The role of ORC4 in enucleation of Murine Erythroleukemia (MEL) cells is similar to that in oocyte polar body extrusion

The Origin Replication Complex subunit 4 (ORC4) is one in six subunits of the Origin Replication Complexes (ORCs) which is essential for initiating licensing at DNA replication origins and recruiting adaptor molecules necessary for various cellular processes. Previously, we reported that ORC4 also plays a vital role in polar body extrusion (PBE) during oogenesis in which half the chromosomes are extruded from the oocyte. We hypothesized that ORC4 might play a broader role in chromatin elimination. We tested its role in enucleation during the development of erythrocytes. Murine erythroleukemia (MEL) cells can be propagated in culture indefinitely and can be induced to enucleate their DNA by treatment with Vacuolin-1, thereby mimicking normal erythrocyte enucleation. We found that ORC4 appeared around the nuclei of the MEL cells with Vacuolin-1 treatment, gradually increasing in thickness before enucleation. We then tested whether ORC4 was required for MEL enucleation by down regulating ORC4 with siRNA-ORC4 during Vacuolin-1 treatment and found that this prevented MEL enucleation. These data are consistent with the model that ORC4 is required for erythroblast enucleation just as it is for oocyte PBE. They suggest a new model in which ORC4 expression is a marker for the initiation to the enucleation pathway.

Morphology of Tissue Disruption at Sites of High-Grade Tumors

Background

Invasive solid cancers originating from diverse organs like breast, ovary and lung metastasize to distant sites. The structural changes at the primary sites of these high-grade tumors have not been well characterized. The main aim of the current study was to examine if there is any morphological overlap of metastasizing tissues of different invasive tumors.

Methods

Whole slide hematoxylin and eosin (H&E) stained images from web repository of multiple tumor specimens were used for this study. ImageJ was used for image processing and analyses.

Results

The metastatic tissue(s) at the primary sites of different examined high-grade tumors appeared similar, irrespective of organ of origin of the primary tumor. Numerous excrescences with the repetitive appearance of a bulb-like projection with a narrowed-off trailing end were seen emanating from the tumor cell membrane. Many of them contained nuclei, while others were empty vesicles. Although these structures were not exactly equal in their dimensions, the rubrics of architectural distortion in different high-grade tumors were conserved.

Conclusions

The preliminary observations suggest for the first time that there is structural similarity of the epithelial dysmorphia in many high-grade invasive tumors, irrespective of their parental tissue of origin. This commonality of morphological prints of metastases suggests that similar pathways of cytosolic force generation are activated during temporal progression of cancer, resulting in the conserved mushroom-shaped appearance of the dismantling individual cell or cell clusters from the parental epithelium. The conserved genomic mechanisms underlying these fascinating observations merit testing and validation in future studies.

miR-144/451 inhibits c-Myc to promote erythroid differentiation

Ablation of miR-144/451 disrupts homeostasis of erythropoiesis. Myc, a protooncogenic protein, is essential for erythroblast proliferation but commits rapid downregulation during erythroid maturation. How erythroblasts orchestrate maturation processes through coding and non-coding genes is largely unknown. In this study, we use miR-144/451 knockout mice as in vivo model, G1E, MEL erythroblast lines and erythroblasts from fresh mouse fetal livers as in vitro systems to demonstrate that targeted depletion of miR-144/451 blocks erythroid nuclear condensation and enucleation. This is due, at least in part, to the continued high expression of Myc in erythroblasts when miR-144/451 is absent. Specifically, miR-144/451 directly inhibits Myc in erythroblasts. Loss of miR-144/451 locus derepresses, and thus, increases the expression of Myc. Sustained high levels of Myc in miR-144/451-depleted erythroblasts blocks erythroid differentiation. Moreover, Myc reversely regulates the expression of miR-144/451, forming a positive miR-144/451-Myc feedback to ensure the complete shutoff of Myc during erythropoiesis. Given that erythroid-specific transcription factor GATA1 activates miR-144/451 and inactivates Myc, our findings indicate that GATA1-miR-144/451-Myc network safeguards normal erythroid differentiation. Our findings also demonstrate that disruption of the miR-144/451-Myc crosstalk causes anemia, suggesting that miR-144/451 might be a potential therapeutic target in red cell diseases.

Wdr26 regulates nuclear condensation in developing erythroblasts

Mammalian red blood cells lack nuclei. The molecular mechanisms underlying erythroblast nuclear condensation and enucleation, however, remain poorly understood. Here we show that Wdr26, a gene upregulated during terminal erythropoiesis, plays an essential role in regulating nuclear condensation in differentiating erythroblasts. Loss of Wdr26 induces anemia in zebrafish and enucleation defects in mouse erythroblasts because of impaired erythroblast nuclear condensation. As part of the glucose-induced degradation-deficient ubiquitin ligase complex, Wdr26 regulates the ubiquitination and degradation of nuclear proteins, including lamin B. Failure of lamin B degradation blocks nuclear opening formation leading to impaired clearance of nuclear proteins and delayed nuclear condensation. Collectively, our study reveals an unprecedented role of an E3 ubiquitin ligase in regulating nuclear condensation and enucleation during terminal erythropoiesis. Our results provide mechanistic insights into nuclear protein homeostasis and vertebrate red blood cell development.

GATA Factor-Mediated Gene Regulation in Human Erythropoiesis

Erythroid commitment and differentiation are regulated by the coordinated action of a host of transcription factors, including GATA2 and GATA1. Here, we explored GATA-mediated transcriptional regulation through the integrative analysis of gene expression, chromatin modifications, and GATA factors' binding in human multipotent hematopoietic stem/progenitor cells, early erythroid progenitors, and late precursors. A progressive loss of H3K27 acetylation and a diminished usage of active enhancers and super-enhancers were observed during erythroid commitment and differentiation. GATA factors mediate transcriptional changes through a stage-specific interplay with regulatory elements: GATA1 binds different sets of regulatory elements in erythroid progenitors and precursors and controls the transcription of distinct genes during commitment and differentiation. Importantly, our results highlight a pivotal role of promoters in determining the transcriptional program activated upon erythroid differentiation. Finally, we demonstrated that GATA1 binding to a stage-specific super-enhancer sustains the expression of the KIT receptor in human erythroid progenitors.

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GATA2/1 binding to regulatory regions and transcriptional changes during erythropoiesis

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GATA1 sustains KIT expression in human erythroid progenitors

Computer simulations of protein-membrane systems

The interactions between proteins and membranes play critical roles in signal transduction, cell motility, and transport, and they are involved in many types of diseases. Molecular dynamics (MD) simulations have greatly contributed to our understanding of protein-membrane interactions, promoted by a dramatic development of MD-related software, increasingly accurate force fields, and available computer power. In this chapter, we present available methods for studying protein-membrane systems with MD simulations, including an overview about the various all-atom and coarse-grained force fields for lipids, and useful software for membrane simulation setup and analysis. A large set of case studies is discussed.

Circle-Seq: Isolation and Sequencing of Chromosome-Derived Circular DNA Elements in Cells

Chromosome-derived extrachromosomal circular DNA elements (eccDNAs) are detected in all eukaryotes examined so far. Here I describe the Circle-Seq protocol, applicable for physical enrichment of eccDNAs of a broad size range, combined with sequence confirmation of circular structures.Briefly, by concise alkaline treatment and gentle gravity flow-through an ion-exchange column, eccDNAs are enriched in the eluate fraction. EccDNAs are enzymatically isolated by extensive Plasmid-Safe DNase digestion of linear chromosomes and further enriched by φ29 rolling circle amplification. By means of high throughput sequencing of amplified eccDNA and custom eccDNA mapping software, around ten-thousand unique eccDNA types could be detected at nucleotide resolution in a million human muscle nuclei by this method.

Modeling the Distribution of Diprotic Basic Drugs in Liposomal Systems: Perspectives on Malaria Nanotherapy

Understanding how polyprotic compounds distribute within liposome (LP) suspensions is of major importance to design effective drug delivery strategies. Advances in this research field led to the definition of LP-based active drug encapsulation methods driven by transmembrane pH gradients with evidenced efficacy in the management of cancer and infectious diseases. An accurate modeling of membrane-solution drug partitioning is also fundamental when designing drug delivery systems for poorly endocytic cells, such as red blood cells (RBCs), in which the delivered payloads rely mostly on the passive diffusion of drug molecules across the cell membrane. Several experimental models have been proposed so far to predict the partitioning of polyprotic basic/acid drugs in artificial membranes. Nevertheless, the definition of a model in which the membrane-solution partitioning of each individual drug microspecies is studied relative to each other is still a topic of ongoing research. We present here a novel experimental approach based on mathematical modeling of drug encapsulation efficiency (EE) data in liposomal systems by which microspecies-specific partition coefficients are reported as a function of pH and phospholipid compositions replicating the RBC membrane in a simple and highly translatable manner. This approach has been applied to the study of several diprotic basic antimalarials of major clinical importance (quinine, primaquine, tafenoquine, quinacrine, and chloroquine) describing their respective microspecies distribution in phosphatidylcholine-LP suspensions. Estimated EE data according to the model described here closely fitted experimental values with no significant differences obtained in 75% of all pH/lipid composition-dependent conditions assayed. Additional applications studied include modeling drug EE in LPs in response to transmembrane pH gradients and lipid bilayer asymmetric charge, conditions of potential interest reflected in our previously reported RBC-targeted antimalarial nanotherapeutics.

Altering microtubule stability affects microtubule clearance and nuclear extrusion during erythropoiesis

Mammalian erythrocytes are highly specialized cells that have adapted to lose their nuclei and cellular components during maturation to ensure oxygen delivery. Nuclear extrusion, the most critical event during erythropoiesis, represents an extreme case of asymmetric partitioning that requires a dramatic reorganization of the cytoskeleton. However, the precise role of the microtubule cytoskeleton in the enucleation process remains controversial. In this study, we show that microtubule reorganization is critical for microtubule clearance and nuclear extrusion during erythropoiesis. Using a rodent anemia model, we found that microtubules were present in erythroblasts and reticulocytes but were undetectable in erythrocytes. Further analysis demonstrated that microtubules became disordered in reticulocytes and revealed that microtubule stabilization was critical for tubulin degradation. Disruption of microtubule dynamics using the microtubule-stabilizing agent paclitaxel or the microtubule-destabilizing agent nocodazole did not affect the efficiency of erythroblast enucleation. However, paclitaxel treatment resulted in the retention of tubulin in mature erythrocytes, and nocodazole treatment led to a defect in pyrenocyte morphology. Taken together, our data reveals a critical role for microtubules in erythrocyte development. Our findings also implicate the disruption of microtubule dynamics in the pathogenesis of anemia-associated diseases, providing new insight into the pathogenesis of the microtubule-targeted agent-associated anemia frequently observed during cancer chemotherapy.

The Evolving Erythrocyte: Red Blood Cells as Modulators of Innate Immunity

The field of red cell biology is undergoing a quiet revolution. Long assumed to be inert oxygen carriers, RBCs are emerging as important modulators of the innate immune response. Erythrocytes bind and scavenge chemokines, nucleic acids, and pathogens in circulation. Depending on the conditions of the microenvironment, erythrocytes may either promote immune activation or maintain immune quiescence. We examine erythrocyte immune function through a comparative and evolutionary lens, as this framework may offer perspective into newly recognized roles of human RBCs. Next, we review the known immune roles of human RBCs and discuss their activity in the context of sepsis where erythrocyte function may prove important to disease pathogenesis. Given the limited success of immunomodulatory therapies in treating inflammatory diseases, we propose that the immunologic function of RBCs provides an understudied and potentially rich area of research that may yield novel insights into mechanisms of immune regulation.

More than just the numbers-contrasting response of snake erythrocytes to thermal acclimation

Acclimation to lower temperatures decreases energy expenditure in ectotherms but increases oxygen consumption in most endotherms, when dropped below thermoneutrality. Such differences should be met by adjustments in oxygen transport through blood. Changes in hematological variables in correspondence to that in metabolic rates are, however, not fully understood, particularly in non-avian reptiles. We investigated the effect of thermal acclimation on a snake model, the grass snakes (Natrix natrix). After 6 months of acclimation to either 18 °C or 32 °C hematocrit, hemoglobin concentration, erythrocyte number, and size were assessed. All variables revealed significantly lower values under warm compared to cold ambient temperature. Our data suggest that non-avian reptiles, similarly as birds, reduce erythrocyte fraction under energy-demanding temperatures. Due to low deformability of nucleated erythrocytes in sauropsids, such reduced fraction may be important in decreasing blood viscosity to optimize blood flow. Novel findings on flexible erythrocyte size provide an important contribution to this optimization process.

Single-cell transcriptomic landscape of nucleated cells in umbilical cord blood

BACKGROUND

For both pediatric and adult patients, umbilical cord blood (UCB) transplant is a therapeutic option for a variety of hematologic diseases, such as blood cancers, myeloproliferative disorders, genetic diseases, and metabolic disorders. However, the level of cellular heterogeneity and diversity of nucleated cells in UCB has not yet been assessed in an unbiased and systemic fashion. In the present study, nucleated cells from UCB were subjected to single-cell RNA sequencing to simultaneously profile the gene expression signatures of thousands of cells, generating a rich resource for further functional studies. Here, we report the transcriptomes of 17,637 UCB cells, covering 12 major cell types, many of which can be further divided into distinct subpopulations.

RESULTS

Pseudotemporal ordering of nucleated red blood cells identifies wave-like activation and suppression of transcription regulators, leading to a polarized cellular state, which may reflect nucleated red blood cell maturation. Progenitor cells in UCB also comprise 2 subpopulations with activation of divergent transcription programs, leading to specific cell fate commitment. Detailed profiling of cytotoxic cell populations unveiled granzymes B and K signatures in natural killer and natural killer T-cell types in UCB.

CONCLUSIONS

Taken together, our data form a comprehensive single-cell transcriptomic landscape that reveals previously unrecognized cell types, pathways, and mechanisms of gene expression regulation. These data may contribute to the efficacy and outcome of UCB transplant, broadening the scope of research and clinical innovations.

Entering the post-epigenomic age: back to epigenetics

It is undeniably one of the greatest findings in biology that (with some very minor exceptions) every cell in the body possesses the whole genetic information needed to generate a complete individual. Today, this concept has been so thoroughly assimilated that we struggle to still see how surprising this finding actually was: all cellular phenotypes naturally occurring in one person are generated from genetic uniformity, and thus are per definition epigenetic. Transcriptional mechanisms are clearly critical for developing and protecting cell identities, because a mis-expression of few or even single genes can efficiently induce inappropriate cellular programmes. However, how transcriptional activities are molecularly controlled and which of the many known epigenomic features have causal roles remains unclear. Today, clarification of this issue is more pressing than ever because profiling efforts and epigenome-wide association studies (EWAS) continuously provide comprehensive datasets depicting epigenomic differences between tissues and disease states. In this commentary, we propagate the idea of a widespread follow-up use of epigenome editing technology in EWAS studies. This would enable them to address the questions of which features, where in the genome, and which circumstances are essential to shape development and trigger disease states.

Disruption of erythroid nuclear opening and histone release in myelodysplastic syndromes

Mammalian terminal erythropoiesis involves several characteristic phenomena including chromatin condensation and enucleation. One of the newly identified features of terminal erythropoiesis in mouse is a dynamic nuclear opening and histone release process, which is required for chromatin condensation. However, it is unclear whether the same feature is present in human. Here, we use an in vitro human CD34‐positive hematopoietic stem and progenitor cell culture system and reveal that nuclear openings and histone release are also identified during human terminal erythropoiesis. In contrast to mouse in which each erythroblast contains a single opening, multiple nuclear openings are present in human erythroblast, particularly during the late‐stage differentiation. The nuclear opening and histone release process is mediated by caspase‐3. Inhibition of caspase‐3 blocks nuclear opening, histone release, chromatin condensation, and terminal differentiation. We confirm the finding of histone cytosolic release in paraffin‐embedded human bone marrow in vivo. Importantly, we find that patients with myelodysplastic syndrome (MDS) exhibit significant defects in histone release in the dysplastic erythroblasts. Our results reveal developmentally conserved nuclear envelop and histone dynamic changes in human terminal erythropoiesis and indicate that disruption of the histone release process plays a critical role in the pathogenesis of dyserythropoiesis in MDS.

Multifunctional roles of tropomodulin-3 in regulating actin dynamics

Tropomodulins (Tmods) are proteins that cap the slow-growing (pointed) ends of actin filaments (F-actin). The basis for our current understanding of Tmod function comes from studies in cells with relatively stable and highly organized F-actin networks, leading to the view that Tmod capping functions principally to preserve F-actin stability. However, not only is Tmod capping dynamic, but it also can play major roles in regulating diverse cellular processes involving F-actin remodeling. Here, we highlight the multifunctional roles of Tmod with a focus on Tmod3. Like other Tmods, Tmod3 binds tropomyosin (Tpm) and actin, capping pure F-actin at submicromolar and Tpm-coated F-actin at nanomolar concentrations. Unlike other Tmods, Tmod3 can also bind actin monomers and its ability to bind actin is inhibited by phosphorylation of Tmod3 by Akt2. Tmod3 is ubiquitously expressed and is present in a diverse array of cytoskeletal structures, including contractile structures such as sarcomere-like units of actomyosin stress fibers and in the F-actin network encompassing adherens junctions. Tmod3 participates in F-actin network remodeling in lamellipodia during cell migration and in the assembly of specialized F-actin networks during exocytosis. Furthermore, Tmod3 is required for development, regulating F-actin mesh formation during meiosis I of mouse oocytes, erythroblast enucleation in definitive erythropoiesis, and megakaryocyte morphogenesis in the mouse fetal liver. Thus, Tmod3 plays vital roles in dynamic and stable F-actin networks in cell physiology and development, with further research required to delineate the mechanistic details of Tmod3 regulation in the aforementioned processes, or in other yet to be discovered processes.

Splitting of circulating red blood cells as an in vivo mechanism of erythrocyte maturation in developing zebrafish, chick and mouse embryos

Nucleated circulating red blood cells (RBCs) of developing zebrafish, chick and mouse embryos can actively proliferate. While marrow- or organ-mediated erythropoiesis has been widely studied, transforming in vivo processes of circulating RBCs are under little scrutiny. We employed confocal, stereo- and electron microscopy to document the maturation of intravascular RBCs. In zebrafish embryos (32-72 h post-fertilization), RBC splitting in the caudal vein plexus follows a four-step program: (i) nuclear division with continued cytoplasmic connection between somata; (ii) dumbbell-shaped RBCs tangle at transluminal vascular pillars; (iii) elongation; and (iv) disruption of soma-to-soma connection. Dividing RBCs of chick embryos, however, retain the nucleus in one of their somata. Here, RBC splitting acts to pinch off portions of cytoplasm, organelles and ribosomes. Dumbbell-shaped primitive RBCs re-appeared as circulation constituents in mouse embryos. The splitting of circulating RBCs thus represents a biologically relevant mechanism of RBC division and maturation during early vertebrate ontogeny.

Enhanced phosphocholine metabolism is essential for terminal erythropoiesis

Red cells contain a unique constellation of membrane lipids. Although much is known about regulated protein expression, the regulation of lipid metabolism during erythropoiesis is poorly studied. Here, we show that transcription of PHOSPHO1, a phosphoethanolamine and phosphocholine phosphatase that mediates the hydrolysis of phosphocholine to choline, is strongly upregulated during the terminal stages of erythropoiesis of both human and mouse erythropoiesis, concomitant with increased catabolism of phosphatidylcholine (PC) and phosphocholine as shown by global lipidomic analyses of mouse and human terminal erythropoiesis. Depletion of PHOSPHO1 impaired differentiation of fetal mouse and human erythroblasts, and, in adult mice, depletion impaired phenylhydrazine-induced stress erythropoiesis. Loss of PHOSPHO1 also impaired phosphocholine catabolism in mouse fetal liver progenitors and resulted in accumulation of several lipids; adenosine triphosphate (ATP) production was reduced as a result of decreased oxidative phosphorylation. Glycolysis replaced oxidative phosphorylation in PHOSPHO1-knockout erythroblasts and the increased glycolysis was used for the production of serine or glycine. Our study elucidates the dynamic changes in lipid metabolism during terminal erythropoiesis and reveals the key roles of PC and phosphocholine metabolism in energy balance and amino acid supply.

Flow Cytometric Analysis of Erythroblast Enucleation

Enucleation is the final step in mammalian erythropoiesis. In this process, the nucleus is extruded by budding off from the erythroblast, forming the reticulocyte. Herein, we describe the flow cytometry-based assays for enucleation assessment. The separation of nucleated erythroblasts, reticulocytes, and extruded nuclei by flow cytometry is based on DNA staining, surface expression of erythrocyte specific markers, or forward scatter (FSC). The enucleation of murine erythroblasts is assessed by the surface expression of murine erythrocyte marker Ter119 and DNA staining. Three discrete populations that represent nucleated erythroblasts, reticulocytes, and extruded nuclei are defined as Hoechst^medTER119^high, Hoechst^lowTER119^high, and Hoechst^highTER119^med, respectively. Another nuclei acid staining dye, SYTO16, is used for the assessment of human enucleation in combination with FSC. For human cells, the three populations that represent nucleated erythroblasts, reticulocyte, and extruded nuclei are identified as FSC^high SYTO16⁺, FSC^high SYTO16^-, FSC^lowSYTO16⁺, respectively.

The diverse origins of circulating cell-free DNA in the human body: a critical re-evaluation of the literature

Since the detection of cell-free DNA (cfDNA) in human plasma in 1948, it has been investigated as a non-invasive screening tool for many diseases, especially solid tumours and foetal genetic abnormalities. However, to date our lack of knowledge regarding the origin and purpose of cfDNA in a physiological environment has limited its use to more obvious diagnostics, neglecting, for example, its potential utility in the identification of predisposition to disease, earlier detection of cancers, and lifestyle-induced epigenetic changes. Moreover, the concept or mechanism of cfDNA could also have potential therapeutic uses such as in immuno- or gene therapy. This review presents an extensive compilation of the putative origins of cfDNA and then contrasts the contributions of cellular breakdown processes with active mechanisms for the release of cfDNA into the extracellular environment. The involvement of cfDNA derived from both cellular breakdown and active release in lateral information transfer is also discussed. We hope to encourage researchers to adopt a more holistic view of cfDNA research, taking into account all the biological pathways in which cfDNA is involved, and to give serious consideration to the integration of in vitro and in vivo research. We also wish to encourage researchers not to limit their focus to the apoptotic or necrotic fraction of cfDNA, but to investigate the intercellular messaging capabilities of the actively released fraction of cfDNA and to study the role of cfDNA in pathogenesis.

Loss of pleckstrin-2 reverts lethality and vascular occlusions in JAK2V617F-positive myeloproliferative neoplasms

V617F driver mutation of JAK2 is the leading cause of the Philadelphia-chromosome-negative myeloproliferative neoplasms (MPNs). Although thrombosis is a leading cause of mortality and morbidity in MPNs, the mechanisms underlying their pathogenesis are unclear. Here, we identified pleckstrin-2 (Plek2) as a downstream target of the JAK2/STAT5 pathway in erythroid and myeloid cells, and showed that it is upregulated in a JAK2V617F-positive MPN mouse model and in patients with MPNs. Loss of Plek2 ameliorated JAK2V617F-induced myeloproliferative phenotypes including erythrocytosis, neutrophilia, thrombocytosis, and splenomegaly, thereby reverting the widespread vascular occlusions and lethality in JAK2V617F-knockin mice. Additionally, we demonstrated that a reduction in red blood cell mass was the main contributing factor in the reversion of vascular occlusions. Thus, our study identifies Plek2 as an effector of the JAK2/STAT5 pathway and a key factor in the pathogenesis of JAK2V617F-induced MPNs, pointing to Plek2 as a viable target for the treatment of MPNs.

Dynamic Regulation of a Ribosome Rescue Pathway in Erythroid Cells and Platelets

Protein synthesis continues in platelets and maturing reticulocytes, although these blood cells lack nuclei and do not make new mRNA or ribosomes. Here, we analyze translation in primary human cells from anucleate lineages by ribosome profiling and uncover a dramatic accumulation of post-termination unrecycled ribosomes in the 3' UTRs of mRNAs. We demonstrate that these ribosomes accumulate as a result of the natural loss of the ribosome recycling factor ABCE1 during terminal differentiation. Induction of the ribosome rescue factors PELO and HBS1L is required to support protein synthesis when ABCE1 levels fall and for hemoglobin production during blood cell development. Our observations suggest that this distinctive loss of ABCE1 in anucleate blood lineages could sensitize them to defects in ribosome homeostasis, perhaps explaining in part why genetic defects in the fundamental process of ribosome production ("ribosomopathies") often affect hematopoiesis specifically.

Nucleated Teleost Erythrocytes Play an Nk-Lysin- and Autophagy-Dependent Role in Antiviral Immunity

With the exception of mammals, vertebrate erythrocytes are nucleated. Nevertheless, these cells are usually considered as mere carriers of hemoglobin. In this work, however, we describe for the first time an unrecognized role of teleost red blood cells (RBCs). We found that Nk-lysin (Nkl), an antimicrobial peptide produced by NK-cells and cytotoxic T-lymphocytes, was also expressed in flatfish turbot (Scophthalmus maximus) erythrocytes. Although the antiviral role of Nkl remains to be elucidated, we found a positive correlation between the transcription of nkl and the resistance to an infection with Rhabdovirus in a teleost fish. Surprisingly, Nkl was found to be present in the autophagolysosomes of erythrocytes, and therefore this higher resistance provided by Nkl could be related to autophagy. The organelles of RBCs are degraded through autophagy during the maturation process of these cells. In this work, we observed that the blockage of autophagy increased the replication of viral hemorrhagic septicemia virus in nucleated teleost erythrocytes, which suggests that this mechanism may also be a key process in the defense against viruses in these cells. Nkl, which possesses membrane-perturbing ability and was affected by this modulation of RBC autophagy, could also participate in this process. For the first time, autophagy has been described not only as a life cycle event during the maturation of erythrocytes but also as a pivotal antiviral mechanism in nucleated erythrocytes. These results suggest a role of erythrocytes and Nkl in the antiviral immunity of fish and other vertebrates with nucleated RBCs.

High-Resolution Fluorescence Microscope Imaging of Erythroblast Structure

During erythropoiesis, erythroblasts undergo dramatic morphological changes to produce mature erythrocytes. Many unanswered questions regarding the molecular mechanisms behind these changes can be addressed with high-resolution fluorescence imaging. Immunofluoresence staining enables localization of specific molecules, organelles, and membrane components in intact cells at different phases of erythropoiesis. Confocal laser scanning microscopy can provide high-resolution, three-dimensional images of stained structures, which can be used to dissect the molecular mechanisms driving erythropoiesis. The sample preparation, staining procedure, imaging parameters, and image analysis methods used directly affect the quality of the confocal images and the amount and accuracy of information that they can provide. Here, we describe methods to dissect erythropoietic tissues from mice, to perform immunofluorescence staining and confocal imaging of various molecules, organelles and structures of interest in erythroblasts, and to present and quantitatively analyze the data obtained in these fluorescence images.

Unravelling pathways downstream Sox6 induction in K562 erythroid cells by proteomic analysis

The Sox6 transcription factor is crucial for terminal maturation of definitive red blood cells. Sox6-null mouse fetuses present misshapen and nucleated erythrocytes, due to impaired actin assembly and cytoskeleton stability. These defects are accompanied with a reduced survival of Sox6^−/− red blood cells, resulting in a compensated anemia. Sox6-overexpression in K562 cells and in human primary ex vivo erythroid cultures enhances erythroid differentiation and leads to hemoglobinization, the hallmark of erythroid maturation. To obtain an overview on processes downstream to Sox6 expression, we performed a differential proteomic analysis on human erythroid K562 cells overexpressing Sox6. Sox6-overexpression induces dysregulation of 64 proteins, involved in cytoskeleton remodeling and in protein synthesis, folding and trafficking, key processes for erythroid maturation. Moreover, 43 out of 64 genes encoding for differentially expressed proteins contain within their proximal regulatory regions sites that are bound by SOX6 according to ENCODE ChIP-seq datasets and are possible direct SOX6 targets. SAR1B, one of the most induced proteins upon Sox6 overexpression, shares a conserved regulatory module, composed by a double SOX6 binding site and a GATA1 consensus, with the adjacent SEC24 A gene. Since both genes encode for COPII components, this element could concur to the coordinated expression of these proteins during erythropoiesis.

Orchestration of late events in erythropoiesis by KLF1/EKLF

PURPOSE OF REVIEW

Transcriptional regulators provide the molecular and biochemical basis for the cell specific properties and characteristics that follow from their central role in establishing tissue-restricted expression. Precise and sequential control of terminal cell divisions, nuclear condensation, and enucleation are defining characteristics within erythropoietic differentiation. This review is focused on KLF1, a central global regulator of this process.

RECENT FINDINGS

Studies in the past year have brought a number of proteins that are targets of KLF1 regulation into focus with respect to their roles in terminal erythroid differentiation. Many of these are involved in fine control of the cell cycle at both early (E2F2, Cyclin A2) and later (p18, p27, p19) stages of differentiation, or are directly involved in enucleation (p18, p27). Dramatic biophysical changes controlled at the nuclear lamin by caspase 3 enable histone release and nuclear condensation, whereas dematin association with structural proteins alters the timing of enucleation. Conditional ablation of mDia2 has established its role in late stage cell cycle and enucleation.

SUMMARY

Transcription factors such as KLF1, along with epigenetic modifiers, play crucial roles in establishing the proper onset and progression of terminal differentiation events. Studies from the past year show a remarkable multifaceted convergence on cell cycle control, and establish that the orthochromatic erythroblast stage is a critical nodal point for many of the effects on enucleation. These studies are relevant to understanding the underlying causes of anemia and hematologic disease where defective enucleation predicts a poor clinical outcome.

Genetically engineered red cells expressing single domain camelid antibodies confer long-term protection against botulinum neurotoxin

A short half-life in the circulation limits the application of therapeutics such as single-domain antibodies (VHHs). We utilize red blood cells to prolong the circulatory half-life of VHHs. Here we present VHHs against botulinum neurotoxin A (BoNT/A) on the surface of red blood cells by expressing chimeric proteins of VHHs with Glycophorin A or Kell. Mice whose red blood cells carry the chimeric proteins exhibit resistance to 10,000 times the lethal dose (LD₅₀) of BoNT/A, and transfusion of these red blood cells into naive mice affords protection for up to 28 days. We further utilize an improved CD34+ culture system to engineer human red blood cells that express these chimeric proteins. Mice transfused with these red blood cells are resistant to highly lethal doses of BoNT/A. We demonstrate that engineered red blood cells expressing VHHs can provide prolonged prophylactic protection against bacterial toxins without inducing inhibitory immune responses and illustrates the potentially broad translatability of our strategy for therapeutic applications.

The therapeutic use of single-chain antibodies (VHHs) is limited by their short half-life in the circulation. Here the authors engineer mouse and human red blood cells to express VHHs against botulinum neurotoxin A (BoNT/A) on their surface and show that an infusion of these cells into mice confers long lasting protection against a high dose of BoNT/A.

Tropomodulin 1 controls erythroblast enucleation via regulation of F-actin in the enucleosome

Biogenesis of mammalian red blood cells requires nuclear expulsion by orthochromatic erythoblasts late in terminal differentiation (enucleation), but the mechanism is largely unexplained. Here, we employed high-resolution confocal microscopy to analyze nuclear morphology and F-actin rearrangements during the initiation, progression, and completion of mouse and human erythroblast enucleation in vivo. Mouse erythroblast nuclei acquire a dumbbell-shaped morphology during enucleation, whereas human bone marrow erythroblast nuclei unexpectedly retain their spherical morphology. These morphological differences are linked to differential expression of Lamin isoforms, with primary mouse erythroblasts expressing only Lamin B and primary human erythroblasts only Lamin A/C. We did not consistently identify a continuous F-actin ring at the cell surface constriction in mouse erythroblasts, nor at the membrane protein-sorting boundary in human erythroblasts, which do not have a constriction, arguing against a contractile ring-based nuclear expulsion mechanism. However, both mouse and human erythroblasts contain an F-actin structure at the rear of the translocating nucleus, enriched in tropomodulin 1 (Tmod1) and nonmuscle myosin IIB. We investigated Tmod1 function in mouse and human erythroblasts both in vivo and in vitro and found that absence of Tmod1 leads to enucleation defects in mouse fetal liver erythroblasts, and in CD34⁺ hematopoietic stem and progenitor cells, with increased F-actin in the structure at the rear of the nucleus. This novel structure, the "enucleosome," may mediate common cytoskeletal mechanisms underlying erythroblast enucleation, notwithstanding the morphological heterogeneity of enucleation across species.

Strategy for erythroid differentiation in ex vivo cultures: Lentiviral genetic modification of human hematopoietic stem cells with erythropoietin gene

If cultured in appropriate conditions, such as supplementing culture media with costly cytokines and growth factors, hematopoietic stem/progenitor cells (HSPCs) from different origins have shown to be an adequate source of erythroid cells. This requirement turns erythroid cells production into a complicated process to be scaled-up for future applications. The aim of our work was to genetically modify HSPCs with human erythropoietin (hEPO) sequence by lentiviral transgenesis in order for cells to secrete the hormone into the culture medium. Initially, we evaluated erythroid differentiation in colony forming units (CFU) assays and further analyzed cell expansion and erythroid differentiation throughout time in suspension cultures by flow cytometry and May-Grünwald-Giemsa staining. Additionally, we studied hEPO production and its isoforms profile. The different assessment approaches demonstrated erythroid differentiation, which was attributed to the hEPO secreted by the HSPCs. Our data demonstrate that it is possible to develop culture systems in which recombinant HSPCs are self-suppliers of hEPO. This feature makes our strategy attractive to be applied in biotechnological production processes of erythroid cells that are currently under development.

The Epimmunity Theory: The Single Cell Defenses against Infectious and Genetic Diseases

Single cell defense against diseases defines “epimmunity.” Epimmunity is complementary to the immune system and can neither be substituted by innate nor by acquired immunity. Epimmunity, the proposed new branch of immunity, is further explored and analyzed for enucleated mature mammalian erythrocytes and nucleated erythrocytes of non-mammalian vertebrates leading to the development of “The Epimmunity Theory.” Enucleation of mammalian erythroblast and inactivation of nuclei in erythrocytes of non-mammalian vertebrates are major contributors to the collective immunity: epimmunity, innate, and acquired. The fact that diseases of mature erythrocytes (MEs) are rare supports the notion that a single cell can resist microbial and genetic diseases; MEs are refractory to malaria and cancer. Nucleated cells, such as B-cells, T-cells, hepatocytes, and cell developmental stages are susceptible to genetic and specific microbial diseases depending on their nuclear activities and the receptors they express; such cells show lower epimmunity relative to MEs. Epimmunity is important as a disease insulator that prevents the spread of diseases from an infected tissue to the majority of other tissues. Breakdown of epimmunity may lead to disease development.

Cellular function reinstitution of offspring red blood cells cloned from the sickle cell disease patient blood post CRISPR genome editing

Background

Sickle cell disease (SCD) is a disorder of red blood cells (RBCs) expressing abnormal hemoglobin-S (HbS) due to genetic inheritance of homologous HbS gene. However, people with the sickle cell trait (SCT) carry a single allele of HbS and do not usually suffer from SCD symptoms, thus providing a rationale to treat SCD.

Methods

To validate gene therapy potential, hematopoietic stem cells were isolated from the SCD patient blood and treated with CRISPR/Cas9 approach. To precisely dissect genome-editing effects, erythroid progenitor cells were cloned from single colonies of CRISPR-treated cells and then expanded for simultaneous gene, protein, and cellular function studies.

Results

Genotyping and sequencing analysis revealed that the genome-edited erythroid progenitor colonies were converted to SCT genotype from SCD genotype. HPLC protein assays confirmed reinstallation of normal hemoglobin at a similar level with HbS in the cloned genome-edited erythroid progenitor cells. For cell function evaluation, in vitro RBC differentiation of the cloned erythroid progenitor cells was induced. As expected, cell sickling assays indicated function reinstitution of the genome-edited offspring SCD RBCs, which became more resistant to sickling under hypoxia condition.

Conclusions

This study is an exploration of genome editing of SCD HSPCs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-017-0489-9) contains supplementary material, which is available to authorized users.

Activation of KLF1 Enhances the Differentiation and Maturation of Red Blood Cells from Human Pluripotent Stem Cells

Blood transfusion is widely used in the clinic but the source of red blood cells (RBCs) is dependent on donors, procedures are susceptible to transfusion-transmitted infections and complications can arise from immunological incompatibility. Clinically-compatible and scalable protocols that allow the production of RBCs from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have been described but progress to translation has been hampered by poor maturation and fragility of the resultant cells. Genetic programming using transcription factors has been used to drive lineage determination and differentiation so we used this approach to assess whether exogenous expression of the Erythroid Krüppel-like factor 1 (EKLF/KLF1) could augment the differentiation and stability of iPSC-derived RBCs. To activate KLF1 at defined time points during later stages of the differentiation process and to avoid transgene silencing that is commonly observed in differentiating pluripotent stem cells, we targeted a tamoxifen-inducible KLF1-ERT2 expression cassette into the AAVS1 locus. Activation of KLF1 at day 10 of the differentiation process when hematopoietic progenitor cells were present, enhanced erythroid commitment and differentiation. Continued culture resulted the appearance of more enucleated cells when KLF1 was activated which is possibly due to their more robust morphology. Globin profiling indicated that these conditions produced embryonic-like erythroid cells. This study demonstrates the successful use of an inducible genetic programing strategy that could be applied to the production of many other cell lineages from human induced pluripotent stem cells with the integration of programming factors into the AAVS1 locus providing a safer and more reproducible route to the clinic. Stem Cells 2017;35:886-897.

Histone deacetylase 6 regulates cytokinesis and erythrocyte enucleation through deacetylation of formin protein mDia2

The formin protein mDia2 plays a critical role in a number of cellular processes through its ability to promote nucleation and elongation of actin filaments. In erythroblasts, this includes control of cytokinesis and enucleation by regulating contractile actin ring formation. Here we report a novel mechanism of how mDia2 is regulated: through acetylation and deacetylation at lysine 970 in the formin homology 2 domain. Ectopic expression of an acetyl-mimic mDia2 mutant in mouse erythroblasts is sufficient to abolish contractile actin ring formation at the cleavage furrow and subsequent erythrocyte cytokinesis and enucleation. We also identified that class II histone deacetylase 6 deacetylates and subsequently activates mDia2. Knockdown or inhibition of histone deacetylase 6 impairs contractile actin ring formation, and expression of a non-acetyl-mimic mDia2 mutant restores the contractile actin ring and rescues the impairment of enucleation. In addition to revealing a new step in mDia2 regulation, this study may unveil a novel regulatory mechanism of formin-mediated actin assembly, since the K970 acetylation site is conserved among Dia proteins

Antigen Density Dictates Immune Responsiveness following Red Blood Cell Transfusion

Although RBC transfusion can result in the development of anti-RBC alloantibodies that increase the probability of life-threatening hemolytic transfusion reactions, not all patients generate anti-RBC alloantibodies. However, the factors that regulate immune responsiveness to RBC transfusion remain incompletely understood. One variable that may influence alloantibody formation is RBC alloantigen density. RBC alloantigens exist at different densities on the RBC surface and likewise exhibit distinct propensities to induce RBC alloantibody formation. However, although distinct alloantigens reside on the RBC surface at different levels, most alloantigens also represent completely different structures, making it difficult to separate the potential impact of differences in Ag density from other alloantigen features that may also influence RBC alloimmunization. To address this, we generated RBCs that stably express the same Ag at different levels. Although exposure to RBCs with higher Ag levels induces a robust Ab response, RBCs bearing low Ag levels fail to induce RBC alloantibodies. However, exposure to low Ag-density RBCs is not without consequence, because recipients subsequently develop Ag-specific tolerance. Low Ag-density RBC-induced tolerance protects higher Ag-density RBCs from immune-mediated clearance, is Ag specific, and occurs through the induction of B cell unresponsiveness. These results demonstrate that Ag density can potently impact immune outcomes following RBC transfusion and suggest that RBCs with altered Ag levels may provide a unique tool to induce Ag-specific tolerance.

The Asymmetric Cell Division Regulators Par3, Scribble and Pins/Gpsm2 Are Not Essential for Erythroid Development or Enucleation

Erythroid enucleation is the process by which the future red blood cell disposes of its nucleus prior to entering the blood stream. This key event during red blood cell development has been likened to an asymmetric cell division (ACD), by which the enucleating erythroblast divides into two very different daughter cells of alternate molecular composition, a nucleated cell that will be removed by associated macrophages, and the reticulocyte that will mature to the definitive erythrocyte. Here we investigated gene expression of members of the Par, Scribble and Pins/Gpsm2 asymmetric cell division complexes in erythroid cells, and functionally tested their role in erythroid enucleation in vivo and ex vivo. Despite their roles in regulating ACD in other contexts, we found that these polarity regulators are not essential for erythroid enucleation, nor for erythroid development in vivo. Together our results put into question a role for cell polarity and asymmetric cell division in erythroid enucleation.

Characterizing the hypomethylated DNA methylation profile of nucleated red blood cells from cord blood

AIM

To provide insight into fetal nucleated red blood cell (nRBC) development using genome-wide DNA methylation (DNAm) profiling.

MATERIALS & METHODS

The DNAm profile (Illumina 450K array) of cord blood (n = 7) derived nRBCs was compared with B cells, CD4 and CD8 T cells, natural killer cells, granulocytes, monocytes and placenta (n = 5).

RESULTS

nRBCs and placenta had similarly low array-wide DNAm compared with white blood cells, but their patterns of hypomethylation differed at biologically relevant subsets of the array. High interindividual variability in nRBC DNAm was driven by a negative association between DNAm and nRBC count.

CONCLUSION

nRBC hypomethylation is likely an epigenetic signature of erythropoiesis rather than of early development. Variability in nRBC DNAm may stem from differences in the cell population's maturity or hematopoietic source.

Cyclin A2 regulates erythrocyte morphology and numbers

Cyclin A2 is an essential gene for development and in haematopoietic stem cells and therefore its functions in definitive erythropoiesis have not been investigated. We have ablated cyclin A2 in committed erythroid progenitors in vivo using erythropoietin receptor promoter-driven Cre, which revealed its critical role in regulating erythrocyte morphology and numbers. Erythroid-specific cyclin A2 knockout mice are viable but displayed increased mean erythrocyte volume and reduced erythrocyte counts, as well as increased frequency of erythrocytes containing Howell-Jolly bodies. Erythroblasts lacking cyclin A2 displayed defective enucleation, resulting in reduced production of enucleated erythrocytes and increased frequencies of erythrocytes containing nuclear remnants. Deletion of the Cdk inhibitor p27^Kip1 but not Cdk2, ameliorated the erythroid defects resulting from deficiency of cyclin A2, confirming the critical role of cyclin A2/Cdk activity in erythroid development. Loss of cyclin A2 in bone marrow cells in semisolid culture prevented the formation of BFU-E but not CFU-E colonies, uncovering its essential role in BFU-E function. Our data unveils the critical functions of cyclin A2 in regulating mammalian erythropoiesis.

Chromatin condensation during terminal erythropoiesis

Mammalian terminal erythropoiesis involves gradual but dramatic chromatin condensation steps that are essential for cell differentiation. Chromatin and nuclear condensation is followed by a unique enucleation process, which is believed to liberate more spaces for hemoglobin enrichment and enable the generation of a physically flexible mature red blood cell. Although these processes have been known for decades, the mechanisms are still unclear. Our recent study reveals an unexpected nuclear opening formation during mouse terminal erythropoiesis that requires caspase-3 activity. Major histones, except H2AZ, are partially released from the opening, which is important for chromatin condensation. Block of the nuclear opening through caspase inhibitor or knockdown of caspase-3 inhibits chromatin condensation and enucleation. We also demonstrate that nuclear opening and histone release are cell cycle regulated. These studies reveal a novel mechanism for chromatin condensation in mammalia terminal erythropoiesis.

Chromatin Condensation and Enucleation in Red Blood Cells: An Open Question

Differentiating erythroid cells undergo dramatic changes in morphology, with reduction in cell size, chromatin and nuclear condensation, and enucleation. In this issue of Developmental Cell, Zhao et al. (2016) show that these events are associated with the formation of transient, recurring nuclear openings and selective histone release mediated by caspase-3.