Circulating primitive erythroblasts establish a functional, protein 4.1R-dependent cytoskeletal network prior to enucleating

Developmental regulation of primitive erythropoiesis

PURPOSE OF REVIEW

In this review, we present an overview of recent studies of primitive erythropoiesis, focusing on advances in deciphering its embryonic origin, defining species-specific differences in its developmental regulation, and better understanding the molecular and metabolic pathways involved in terminal differentiation.

RECENT FINDINGS

Single-cell transcriptomics combined with state-of-the-art lineage tracing approaches in unperturbed murine embryos have yielded new insights concerning the origin of the first (primitive) erythroid cells that arise from mesoderm-derived progenitors. Moreover, studies examining primitive erythropoiesis in rare early human embryo samples reveal an overall conservation of primitive erythroid ontogeny in mammals, albeit with some interesting differences such as localization of erythropoietin (EPO) production in the early embryo. Mechanistically, the repertoire of transcription factors that critically regulate primitive erythropoiesis has been expanded to include regulators of transcription elongation, as well as epigenetic modifiers such as the histone methyltransferase DOT1L. For the latter, noncanonical roles aside from enzymatic activity are being uncovered. Lastly, detailed surveys of the metabolic and proteomic landscape of primitive erythroid precursors reveal the activation of key metabolic pathways such as pentose phosphate pathway that are paralleled by a striking loss of mRNA translation machinery.

SUMMARY

The ability to interrogate single cells in vivo continues to yield new insights into the birth of the first essential organ system of the developing embryo. A comparison of the regulation of primitive and definitive erythropoiesis, as well as the interplay of the different layers of regulation - transcriptional, epigenetic, and metabolic - will be critical in achieving the goal of faithfully generating erythroid cells in vitro for therapeutic purposes.

High-throughput total RNA sequencing in single cells using VASA-seq

Most methods for single-cell transcriptome sequencing amplify the termini of polyadenylated transcripts, capturing only a small fraction of the total cellular transcriptome. This precludes the detection of many long non-coding, short non-coding and non-polyadenylated protein-coding transcripts and hinders alternative splicing analysis. We, therefore, developed VASA-seq to detect the total transcriptome in single cells, which is enabled by fragmenting and tailing all RNA molecules subsequent to cell lysis. The method is compatible with both plate-based formats and droplet microfluidics. We applied VASA-seq to more than 30,000 single cells in the developing mouse embryo during gastrulation and early organogenesis. Analyzing the dynamics of the total single-cell transcriptome, we discovered cell type markers, many based on non-coding RNA, and performed in vivo cell cycle analysis via detection of non-polyadenylated histone genes. RNA velocity characterization was improved, accurately retracing blood maturation trajectories. Moreover, our VASA-seq data provide a comprehensive analysis of alternative splicing during mammalian development, which highlighted substantial rearrangements during blood development and heart morphogenesis.

Facile discovery of red blood cell deformation and compromised membrane/skeleton assembly in Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a rare congenital disease with genetic alterations in chromosome 15. Although genetic disorders and DNA methylation abnormalities involved in PWS have been investigated to a significant degree, other anomalies such as those in erythrocytes may occur and these have not been clearly elucidated. In the present study, we uncovered slight anemia in children with PWS that was associated with increased red blood cell (RBC) distribution width (RDW) and contrarily reduced hematocrit (HCT) values. Intriguingly, the increased ratio in RDW to HCT allowed sufficient differentiation between the PWS patients from the healthy controls and, importantly, with individuals exhibiting conventional obesity. Further morphologic examinations revealed a significant deformity in erythrocytes and mild hemolysis in PWS patients. Comprehensive mechanistic investigations unveiled compromised membrane skeletal assembly and membrane lipid composition, and revealed a reduced F-actin/G-actin ratio in PWS patients. We ascribed these phenotypic changes in erythrocytes to the observed genetic defects, including DNA methylation abnormalities. Our collective data allowed us to uncover RBC deformation in children with PWS, and this may constitute an auxiliary indicator of PWS in early childhood.

Endocytosis of the thrombopoietin receptor Mpl regulates megakaryocyte and erythroid maturation in mice

Dnm2fl/fl Pf4-Cre (Dnm2Plt-/- ) mice lacking the endocytic GTPase dynamin 2 (DNM2) in platelets and megakaryocytes (MKs) develop hallmarks of myelofibrosis. At the cellular level, the tyrosine kinase JAK2 is constitutively active but decreased in expression in Dnm2Plt-/- platelets. Additionally, Dnm2Plt-/- platelets cannot endocytose the thrombopoietin (TPO) receptor Mpl, leading to elevated circulating TPO levels. Here, we assessed whether the hyperproliferative phenotype of Dnm2Plt-/- mice was due to JAK2 constitutive activation or to elevated circulating TPO levels. In unstimulated Dnm2Plt-/- platelets, STAT3 and, to a lower extent, STAT5 were phosphorylated, but their phosphorylation was slowed and diminished upon TPO stimulation. We further crossed Dnm2Plt-/- mice in the Mpl-/- background to generate Mpl-/-Dnm2Plt-/- mice lacking Mpl ubiquitously and DNM2 in platelets and MKs. Mpl-/- Dnm2Plt-/- platelets had severely reduced JAK2 and STAT3 but normal STAT5 expression. Mpl-/- Dnm2Plt-/- mice had severely reduced bone marrow MK and hematopoietic stem and progenitor cell numbers. Additionally, Mpl-/- Dnm2Plt-/- mice had severe erythroblast (EB) maturation defects, decreased expression of hemoglobin and heme homeostasis genes and increased expression of ribosome biogenesis and protein translation genes in spleen EBs, and developed anemia with grossly elevated plasma erythropoietin (EPO) levels, leading to early fatality by postnatal day 25. Mpl-/- Dnm2Plt+/+ mice had impaired EB development at three weeks of age, which normalized with adulthood. Together, the data shows that DNM2-dependent Mpl-mediated endocytosis in platelets and MKs is required for steady-state hematopoiesis and provides novel insights into a developmentally controlled role for Mpl in normal erythropoiesis, regulating hemoglobin and heme production.

Membrane Properties of Human Induced Pluripotent Stem Cell-Derived Cultured Red Blood Cells

Cultured red blood cells from human induced pluripotent stem cells (cRBC_iPSCs) are a promising source for future concepts in transfusion medicine. Before cRBC_iPSCs will have entrance into clinical or laboratory use, their functional properties and safety have to be carefully validated. Due to the limitations of established culture systems, such studies are still missing. Improved erythropoiesis in a recently established culture system, closer simulating the physiological niche, enabled us to conduct functional characterization of enucleated cRBC_iPSCs with a focus on membrane properties. Morphology and maturation stage of cRBC_iPSCs were closer to native reticulocytes (nRETs) than to native red blood cells (nRBCs). Whereas osmotic resistance of cRBC_iPSCs was similar to nRETs, their deformability was slightly impaired. Since no obvious alterations in membrane morphology, lipid composition, and major membrane associated protein patterns were observed, reduced deformability might be caused by a more primitive nature of cRBC_iPSCs comparable to human embryonic- or fetal liver erythropoiesis. Blood group phenotyping of cRBC_iPSCs further confirmed the potency of cRBC_iPSCs as a prospective device in pre-transfusional routine diagnostics. Therefore, RBC membrane analyses obtained in this study underscore the overall prospects of cRBC_iPSCs for their future application in the field of transfusion medicine.

Circulating primitive murine erythroblasts undergo complex proteomic and metabolomic changes during terminal maturation

Terminal maturation of primary murine primitive erythroid precursors is characterized by loss of organelles and anabolic components.

Metabolic reprogramming includes depression of mitochondrial metabolism and upregulation of the pentose phosphate pathway and redox metabolism.

Primitive erythropoiesis is a critical component of the fetal cardiovascular network and is essential for the growth and survival of the mammalian embryo. The need to rapidly establish a functional cardiovascular system is met, in part, by the intravascular circulation of primitive erythroid precursors that mature as a single semisynchronous cohort. To better understand the processes that regulate erythroid precursor maturation, we analyzed the proteome, metabolome, and lipidome of primitive erythroblasts isolated from embryonic day (E) 10.5 and E12.5 of mouse gestation, representing their transition from basophilic erythroblast to orthochromatic erythroblast (OrthoE) stages of maturation. Previous transcriptional and biomechanical characterizations of these precursors have highlighted a transition toward the expression of protein elements characteristic of mature red blood cell structure and function. Our analysis confirmed a loss of organelle-specific protein components involved in messenger RNA processing, proteostasis, and metabolism. In parallel, we observed metabolic rewiring toward the pentose phosphate pathway, glycolysis, and the Rapoport-Luebering shunt. Activation of the pentose phosphate pathway in particular may have stemmed from increased expression of hemoglobin chains and band 3, which together control oxygen-dependent metabolic modulation. Increased expression of several antioxidant enzymes also indicated modification to redox homeostasis. In addition, accumulation of oxylipins and cholesteryl esters in primitive OrthoE cells was paralleled by increased transcript levels of the p53-regulated cholesterol transporter (ABCA1) and decreased transcript levels of cholesterol synthetic enzymes. The present study characterizes the extensive metabolic rewiring that occurs in primary embryonic erythroid precursors as they prepare to enucleate and continue circulating without internal organelles.

Development of Mechanical Stability in Late-Stage Embryonic Erythroid Cells: Insights From Fluorescence Imaged Micro-Deformation Studies

The combined use of fluorescence labeling and micro-manipulation of red blood cells has proven to be a powerful tool for understanding and characterizing fundamental mechanisms underlying the mechanical behavior of cells. Here we used this approach to study the development of the membrane-associated cytoskeleton (MAS) in primary embryonic erythroid cells. Erythropoiesis comes in two forms in the mammalian embryo, primitive and definitive, characterized by intra- and extra-vascular maturation, respectively. Primitive erythroid precursors in the murine embryo first begin to circulate at embryonic day (E) 8.25 and mature as a semi-synchronous cohort before enucleating between E12.5 and E16.5. Previously, we determined that the major components of the MAS become localized to the membrane between E10.5 and E12.5, and that this localization is associated with an increase in membrane mechanical stability over this same period. The change in mechanical stability was reflected in the creation of MAS-free regions of the membrane at the tips of the projections formed when cells were aspirated into micropipettes. The tendency to form MAS-free regions decreases as primitive erythroid cells continue to mature through E14.5, at least 2 days after all detectable cytoskeletal components are localized to the membrane, indicating continued strengthening of membrane cohesion after membrane localization of cytoskeletal components. Here we demonstrate that the formation of MAS-free regions is the result of a mechanical failure within the MAS, and not the detachment of membrane bilayer from the MAS. Once a "hole" is formed in the MAS, the skeletal network contracts laterally along the aspirated projection to form the MAS-free region. In protein 4.1-null primitive erythroid cells, the tendency to form MAS-free regions is markedly enhanced. Of note, similar MAS-free regions were observed in maturing erythroid cells from human marrow, indicating that similar processes occur in definitive erythroid cells. We conclude that localization of cytoskeletal components to the cell membrane of mammalian erythroid cells during maturation is insufficient by itself to produce a mature MAS, but that subsequent processes are additionally required to strengthen intraskeletal interactions.

Comparative genomics provides insights into the aquatic adaptations of mammals

Divergent lineages can respond to common environmental factors through convergent processes involving shared genomic components or pathways, but the molecular mechanisms are poorly understood. Here, we provide genomic resources and insights into the evolution of mammalian lineages adapting to aquatic life. Our data suggest convergent evolution, for example, in association with thermoregulation through genes associated with a surface heat barrier (NFIA) and internal heat exchange (SEMA3E). Combined with the support of previous reports showing that the UCP1 locus has been lost in many marine mammals independently, our results suggest that the thermostatic strategy of marine mammals shifted from enhancing heat production to limiting heat loss.

The ancestors of marine mammals once roamed the land and independently committed to an aquatic lifestyle. These macroevolutionary transitions have intrigued scientists for centuries. Here, we generated high-quality genome assemblies of 17 marine mammals (11 cetaceans and six pinnipeds), including eight assemblies at the chromosome level. Incorporating previously published data, we reconstructed the marine mammal phylogeny and population histories and identified numerous idiosyncratic and convergent genomic variations that possibly contributed to the transition from land to water in marine mammal lineages. Genes associated with the formation of blubber (NFIA), vascular development (SEMA3E), and heat production by brown adipose tissue (UCP1) had unique changes that may contribute to marine mammal thermoregulation. We also observed many lineage-specific changes in the marine mammals, including genes associated with deep diving and navigation. Our study advances understanding of the timing, pattern, and molecular changes associated with the evolution of mammalian lineages adapting to aquatic life.

Systematic review of hormonal and genetic factors involved in the nonsyndromic disorders of the lower jaw

Mandibular disorders are among the most common birth defects in humans, yet the etiological factors are largely unknown. Most of the neonates affected by mandibular abnormalities have a sequence of secondary anomalies, including airway obstruction and feeding problems, that reduce the quality of life. In the event of lacking corrective surgeries, patients with mandibular congenital disorders suffer from additional lifelong problems such as sleep apnea and temporomandibular disorders, among others. The goal of this systematic review is to gather evidence on hormonal and genetic factors that are involved in signaling pathways and interactions that are potentially associated with the nonsyndromic mandibular disorders. We found that members of FGF and BMP pathways, including FGF8/10, FGFR2/3, BMP2/4/7, BMPR1A, ACVR1, and ACVR2A/B, have a prominent number of gene-gene interactions among all identified genes in this review. Gene ontology of the 154 genes showed that the functional gene sets are involved in all aspects of cellular processes and organogenesis. Some of the genes identified by the genome-wide association studies of common mandibular disorders are involved in skeletal formation and growth retardation based on animal models, suggesting a potential direct role as genetic risk factors in the common complex jaw disorders. Developmental Dynamics 248:162-172, 2019. © 2018 Wiley Periodicals, Inc.

Continuous Change in Membrane and Membrane-Skeleton Organization During Development From Proerythroblast to Senescent Red Blood Cell

Within the context of erythropoiesis and the possibility of producing artificial red blood cells (RBCs) in vitro, a most critical step is the final differentiation of enucleated erythroblasts, or reticulocytes, to a fully mature biconcave discocyte, the RBC. Reviewed here is the current knowledge about this fundamental maturational process. By combining literature data with our own experimental evidence we propose that the early phase in the maturation of reticulocytes to RBCs is driven by a membrane raft-based mechanism for the sorting of disposable membrane proteins, mostly the no longer needed transferrin receptor (TfR), to the multivesicular endosome (MVE) as cargo of intraluminal vesicles that are subsequently exocytosed as exosomes, consistently with the seminal and original observation of Johnstone and collaborators of more than 30 years ago (Pan BT, Johnstone RM. Cell. 1983;33:967-978). According to a strikingly selective sorting process, the TfR becomes cargo destined to exocytosis while other molecules, including the most abundant RBC transmembrane protein, band 3, are completely retained in the cell membrane. It is also proposed that while this process could be operating in the early maturational steps in the bone marrow, additional mechanism(s) must be at play for the final removal of the excess reticulocyte membrane that is observed to occur in the circulation. This processing will most likely require the intervention of the spleen, whose function is also necessary for the continuous remodeling of the RBC membrane all along this cell's circulatory life.

Microfluidic assay of the deformability of primitive erythroblasts

Primitive erythroblasts (precursors of red blood cells) enter vascular circulation during the embryonic period and mature while circulating. As a result, primitive erythroblasts constantly experience significant hemodynamic shear stress. Shear-induced deformation of primitive erythroblasts however, is poorly studied. In this work, we examined the deformability of primitive erythroblasts at physiologically relevant flow conditions in microfluidic channels and identified the regulatory roles of the maturation stage of primitive erythroblasts and cytoskeletal protein 4.1 R in shear-induced cell deformation. The results showed that the maturation stage affected the deformability of primitive erythroblasts significantly and that primitive erythroblasts at later maturational stages exhibited a better deformability due to a matured cytoskeletal structure in the cell membrane.