Requirement for erythroblast-macrophage protein (Emp) in definitive erythropoiesis

RanBP9 controls the oligomeric state of CTLH complex assemblies

The CTLH (C-terminal to lissencephaly-1 homology motif) complex is a multisubunit RING E3 ligase with poorly defined substrate specificity and flexible subunit composition. Two key subunits, muskelin and Wdr26, specify two alternative CTLH complexes that differ in quaternary structure, thereby allowing the E3 ligase to presumably target different substrates. With the aid of different biophysical and biochemical techniques, we characterized CTLH complex assembly pathways, focusing not only on Wdr26 and muskelin but also on RanBP9, Twa1, and Armc8β subunits, which are critical to establish the scaffold of this E3 ligase. We demonstrate that the ability of muskelin to tetramerize and the assembly of Wdr26 into dimers define mutually exclusive oligomerization modules that compete with nanomolar affinity for RanBP9 binding. The remaining scaffolding subunits, Armc8β and Twa1, strongly interact with each other and with RanBP9, again with nanomolar affinity. Our data demonstrate that RanBP9 organizes subunit assembly and prevents higher order oligomerization of dimeric Wdr26 and the Armc8β-Twa1 heterodimer through its tight binding. Combined, our studies define alternative assembly pathways of the CTLH complex and elucidate the role of RanBP9 in governing differential oligomeric assemblies, thereby advancing our mechanistic understanding of CTLH complex architectures.

VPS39-deficiency observed in type 2 diabetes impairs muscle stem cell differentiation via altered autophagy and epigenetics

Insulin resistance and lower muscle quality (strength divided by mass) are hallmarks of type 2 diabetes (T2D). Here, we explore whether alterations in muscle stem cells (myoblasts) from individuals with T2D contribute to these phenotypes. We identify VPS39 as an important regulator of myoblast differentiation and muscle glucose uptake, and VPS39 is downregulated in myoblasts and myotubes from individuals with T2D. We discover a pathway connecting VPS39-deficiency in human myoblasts to impaired autophagy, abnormal epigenetic reprogramming, dysregulation of myogenic regulators, and perturbed differentiation. VPS39 knockdown in human myoblasts has profound effects on autophagic flux, insulin signaling, epigenetic enzymes, DNA methylation and expression of myogenic regulators, and gene sets related to the cell cycle, muscle structure and apoptosis. These data mimic what is observed in myoblasts from individuals with T2D. Furthermore, the muscle of Vps39+/- mice display reduced glucose uptake and altered expression of genes regulating autophagy, epigenetic programming, and myogenesis. Overall, VPS39-deficiency contributes to impaired muscle differentiation and reduced glucose uptake. VPS39 thereby offers a therapeutic target for T2D.

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.

The Redox Balance and Membrane Shedding in RBC Production, Maturation, and Senescence

Membrane shedding in the form of extracellular vesicles plays a key role in normal physiology and pathology. Partial disturbance of the membrane-cytoskeleton linkage and increased in the intracellular Ca content are considered to be mechanisms underlying the process, but it is questionable whether they constitute the primary initiating steps. Homeostasis of the redox system, which depends on the equilibrium between oxidants and antioxidants, is crucial for many cellular processes. Excess oxidative power results in oxidative stress, which affects many cellular components, including the membrane. Accumulating evidence suggests that oxidative stress indirectly affects membrane shedding most probably by affecting the membrane-cytoskeleton and the Ca content. In red blood cells (RBCs), changes in both the redox system and membrane shedding occur throughout their life-from birth-their production in the bone marrow, to death-aging in the peripheral blood and removal by macrophages in sites of the reticuloendothelial system. Both oxidative stress and membrane shedding are disturbed in diseases affecting the RBC, such as the hereditary and acquired hemolytic anemias (i.e., thalassemia, sickle cell anemia, and autoimmune hemolytic anemia). Herein, I review some data-based and hypothetical possibilities that await experimental confirmation regarding some aspects of the interaction between the redox system and membrane shedding and its role in the normal physiology and pathology of RBCs.

CD14+ monocytes repress gamma globin expression at early stages of erythropoiesis

In β-hemoglobinopathies, reactivation of gamma- at the expense of beta-globin is a prominent therapeutic option. Expression of the globin genes is not strictly intrinsically regulated during erythropoiesis, supported by the observation that fetal erythroid cells switch to adult hemoglobin expression when injected in mice. We show cultured erythroblasts are a mix of HbA restrictive and HbA/HbF expressing cells and that the proportion of cells in the latter population depends on the starting material. Cultures started from CD34+ cells contain more HbA/HbF expressing cells compared to erythroblasts cultured from total peripheral blood mononuclear cells (PBMC). Depletion of CD14+ cells from PBMC resulted in higher HbF/HbA percentages. Conversely, CD34+ co-culture with CD14+ cells reduced the HbF/HbA population through cell–cell proximity, indicating that CD14+ actively repressed HbF expression in adult erythroid cultures. RNA-sequencing showed that HbA and HbA/HbF populations contain a limited number of differentially expressed genes, aside from HBG1/2. Co-culture of CD14+ cells with sorted uncommitted hematopoietic progenitors and CD34-CD36+ erythroblasts showed that hematopoietic progenitors prior to the hemoglobinized erythroid stages are more readily influenced by CD14+ cells to downregulate expression of HBG1/2, suggesting temporal regulation of these genes. This possibly provides a novel therapeutic avenue to develop β-hemoglobinopathies treatments.

DIA-based systems biology approach unveils E3 ubiquitin ligase-dependent responses to a metabolic shift

The yeast Saccharomyces cerevisiae is a powerful model system for systems-wide biology screens and large-scale proteomics methods. Nearly complete proteomics coverage has been achieved owing to advances in mass spectrometry. However, it remains challenging to scale this technology for rapid and high-throughput analysis of the yeast proteome to investigate biological pathways on a global scale. Here we describe a systems biology workflow employing plate-based sample preparation and rapid, single-run, data-independent mass spectrometry analysis (DIA). Our approach is straightforward, easy to implement, and enables quantitative profiling and comparisons of hundreds of nearly complete yeast proteomes in only a few days. We evaluate its capability by characterizing changes in the yeast proteome in response to environmental perturbations, identifying distinct responses to each of them and providing a comprehensive resource of these responses. Apart from rapidly recapitulating previously observed responses, we characterized carbon source-dependent regulation of the GID E3 ligase, an important regulator of cellular metabolism during the switch between gluconeogenic and glycolytic growth conditions. This unveiled regulatory targets of the GID ligase during a metabolic switch. Our comprehensive yeast system readout pinpointed effects of a single deletion or point mutation in the GID complex on the global proteome, allowing the identification and validation of targets of the GID E3 ligase. Moreover, this approach allowed the identification of targets from multiple cellular pathways that display distinct patterns of regulation. Although developed in yeast, rapid whole-proteome-based readouts can serve as comprehensive systems-level assays in all cellular systems.

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

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

Ephrin/Eph receptor interaction facilitates macrophage recognition of differentiating human erythroblasts

Erythropoiesis is one of the most efficient cellular processes in the human body producing approximately 2.5 million red blood cells every second. This process occurs in a bone marrow niche comprised of a central resident macrophage surrounded by differentiating erythroblasts, termed an erythroblastic island. It is not known what initially attracts the macrophage to erythroblasts to form these islands. The ephrin/Eph receptor family are known to regulate heterophilic cell-cell adhesion. We find that human VCAM1⁺ and VCAM1^- bone marrow macrophages and in vitro cultured macrophages are ephrin-B2 positive, whereas differentiating human erythroblasts express EPHB4, EPHB6 and EPHA4. Furthermore, we detect a rise in integrin activation on erythroblasts at the stage at which the cells bind which is independent of EPH receptor presence. Using a live cell imaging assay, we show that specific inhibitory peptides or shRNA depletion of EPHB4 cause a significant reduction in the ability of macrophages to interact with erythroblasts but do not affect integrin activation. This study demonstrates for the first time that EPHB4 expression is required on erythroblasts to facilitate the initial recognition and subsequent interaction with macrophages, alongside the presence of active integrins.

Differential tissue expression of erythroblast macrophage protein in a MRL/lpr mouse model of lupus

OBJECTIVE

The objective of this study was to observe the expression features of erythroblast macrophage protein (EMP) between the tissues of MRL/lpr mice, a mouse model of systemic lupus erythematosus (SLE), and control mice.

METHODS

We examined the serum ANA in both mice groups through indirect immunofluorescence (IIF). Expression features of EMP in bone marrow, liver, renal, spleen, brain, and lung tissues of the MRL/lpr mice and control mice groups were followed using quantitative real-time polymerase chain reaction (Q-PCR). Meanwhile, the expression of EMP was located through immunohistochemical (IHC) studies and the expressive cell identified through double immunofluorescent labeling.

RESULTS

IIF showed that lupus mice have strong positive fluorescence, but no significant fluorescence was observed in control mice. Q-PCR detection revealed that EMP was expressed in the marrow, liver, renal, spleen, lung, and brain tissues of lupus mice. The highest levels were observed in the bone marrow, but there was no statistical difference between these tissues. EMP mRNA expression in the liver (t = 2.747, p = 0.01) and bone marrow (t = 3.853, p = 0.008) of lupus mice was significantly higher than in the control mice. However, no differences in EMP mRNA expression were observed in the renal, spleen, lung, and brain tissues between the lupus and control mice (p > 0.05). In addition, the IHC results showed that EMP protein is ubiquitously expressed in all of the tissues of the lupus and control mice. The positive expression rate in the bone marrow and liver tissues of the lupus mice was higher than in the control mice, but without an obvious difference in the other tissues. The double IF staining method shows that EMP protein was expressed in macrophages in the tissues of the lupus mice and the control mice.

CONCLUSIONS

Our data showed that EMP is ubiquitously expressed in macrophages at all of the tissues of the lupus and control mice. However, the expression of EMP in bone marrow and liver tissues of lupus mice was higher than in the control mice, which indicates that EMP may be important in the development of SLE.

Maea expressed by macrophages, but not erythroblasts, maintains postnatal murine bone marrow erythroblastic islands

The erythroblastic island (EI), formed by a central macrophage and developing erythroblasts (EBs), was first described decades ago and was recently shown to play an in vivo role in homeostatic and pathological erythropoiesis. The exact molecular mechanisms, however, mediating the interactions between macrophages and EBs remain unclear. Macrophage-EB attacher (Maea) has previously been suggested to mediate homophilic adhesion bounds bridging macrophages and EBs. Maea-deficient mice die perinatally with anemia and defective erythrocyte enucleation, suggesting a critical role in fetal erythropoiesis. Here, we generated conditional knockout mouse models of Maea to assess its cellular and postnatal contributions. Deletion of Maea in macrophages using Csf1r-Cre or CD169-Cre caused severe reductions of bone marrow (BM) macrophages, EBs, and in vivo island formation, whereas its deletion in the erythroid lineage using Epor-Cre had no such phenotype, suggesting a dominant role of Maea in the macrophage for BM erythropoiesis. Interestingly, Maea deletion in spleen macrophages did not alter their numbers or functions. Postnatal Maea deletion using Mx1-Cre or function inhibition using a novel monoclonal antibody also impaired BM erythropoiesis. These results indicate that Maea contributes to adult BM erythropoiesis by regulating the maintenance of macrophages and their interaction with EBs via an as-yet-unidentified EB receptor.

Glucocorticoids induce differentiation of monocytes towards macrophages that share functional and phenotypical aspects with erythroblastic island macrophages

The classical central macrophage found in erythroblastic islands plays an important role in erythroblast differentiation, proliferation and enucleation in the bone marrow. Convenient human in vitro models to facilitate the study of erythroid-macrophage interactions are desired. Recently, we demonstrated that cultured monocytes/macrophages enhance in vitro erythropoiesis by supporting hematopoietic stem and progenitor cell survival. Herein, we describe that these specific macrophages also support erythropoiesis. Human monocytes cultured in serum-free media supplemented with stem cell factor, erythropoietin, lipids and dexamethasone differentiate towards macrophages expressing CD16, CD163, CD169, CD206, CXCR4 and the phagocytic TAM-receptor family. Phenotypically, they resemble both human bone marrow and fetal liver resident macrophages. This differentiation is dependent on glucocorticoid receptor activation. Proteomic studies confirm that glucocorticoid receptor activation differentiates monocytes to anti-inflammatory tissue macrophages with a M2 phenotype, termed GC-macrophages. Proteins involved in migration, tissue residence and signal transduction/receptor activity are upregulated whilst lysosome and hydrolase activity GO-categories are downregulated. Functionally, we demonstrate that GC-macrophages are highly mobile and can interact to form clusters with erythroid cells of all differentiation stages and phagocytose the expelled nuclei, recapitulating aspects of erythroblastic islands. In conclusion, glucocorticoid-directed monocyte differentiation to macrophages represents a convenient model system to study erythroid-macrophage interactions.

Human Cord Blood and Bone Marrow CD34+ Cells Generate Macrophages That Support Erythroid Islands

Recently, we developed a small molecule responsive hyperactive Mpl-based Cell Growth Switch (CGS) that drives erythropoiesis associated with macrophages in the absence of exogenous cytokines. Here, we compare the physical, cellular and molecular interaction between the macrophages and erythroid cells in CGS expanded CD34+ cells harvested from cord blood, marrow or G-CSF-mobilized peripheral blood. Results indicated that macrophage based erythroid islands could be generated from cord blood and marrow CD34+ cells but not from G-CSF-mobilized CD34+ cells. Additional studies suggest that the deficiency resides with the G-CSF-mobilized CD34+ derived monocytes. Gene expression and proteomics studies of the in vitro generated erythroid islands detected the expression of erythroblast macrophage protein (EMP), intercellular adhesion molecule 4 (ICAM-4), CD163 and DNASE2. 78% of the erythroblasts in contact with macrophages reached the pre reticulocyte orthochromatic stage of differentiation within 14 days of culture. The addition of conditioned medium from cultures of CD146+ marrow fibroblasts resulted in a 700-fold increase in total cell number and a 90-fold increase in erythroid cell number. This novel CD34+ cell derived erythroid island may serve as a platform to explore the molecular basis of red cell maturation and production under normal, stress and pathological conditions.

Intercellular network structure and regulatory motifs in the human hematopoietic system

The hematopoietic system is a distributed tissue that consists of functionally distinct cell types continuously produced through hematopoietic stem cell (HSC) differentiation. Combining genomic and phenotypic data with high-content experiments, we have built a directional cell-cell communication network between 12 cell types isolated from human umbilical cord blood. Network structure analysis revealed that ligand production is cell type dependent, whereas ligand binding is promiscuous. Consequently, additional control strategies such as cell frequency modulation and compartmentalization were needed to achieve specificity in HSC fate regulation. Incorporating the in vitro effects (quiescence, self-renewal, proliferation, or differentiation) of 27 HSC binding ligands into the topology of the cell-cell communication network allowed coding of cell type-dependent feedback regulation of HSC fate. Pathway enrichment analysis identified intracellular regulatory motifs enriched in these cell type- and ligand-coupled responses. This study uncovers cellular mechanisms of hematopoietic cell feedback in HSC fate regulation, provides insight into the design principles of the human hematopoietic system, and serves as a foundation for the analysis of intercellular regulation in multicellular systems.

Molecular phylogeny of a RING E3 ubiquitin ligase, conserved in eukaryotic cells and dominated by homologous components, the muskelin/RanBPM/CTLH complex

Ubiquitination is an essential post-translational modification that regulates signalling and protein turnover in eukaryotic cells. Specificity of ubiquitination is driven by ubiquitin E3 ligases, many of which remain poorly understood. One such is the mammalian muskelin/RanBP9/CTLH complex that includes eight proteins, five of which (RanBP9/RanBPM, TWA1, MAEA, Rmnd5 and muskelin), share striking similarities of domain architecture and have been implicated in regulation of cell organisation. In budding yeast, the homologous GID complex acts to down-regulate gluconeogenesis. In both complexes, Rmnd5/GID2 corresponds to a RING ubiquitin ligase. To better understand this E3 ligase system, we conducted molecular phylogenetic and sequence analyses of the related components. TWA1, Rmnd5, MAEA and WDR26 are conserved throughout all eukaryotic supergroups, albeit WDR26 was not identified in Rhizaria. RanBPM is absent from Excavates and from some sub-lineages. Armc8 and c17orf39 were represented across unikonts but in bikonts were identified only in Viridiplantae and in O. trifallax within alveolates. Muskelin is present only in Opisthokonts. Phylogenetic and sequence analyses of the shared LisH and CTLH domains of RanBPM, TWA1, MAEA and Rmnd5 revealed closer relationships and profiles of conserved residues between, respectively, Rmnd5 and MAEA, and RanBPM and TWA1. Rmnd5 and MAEA are also related by the presence of conserved, variant RING domains. Examination of how N- or C-terminal domain deletions alter the sub-cellular localisation of each protein in mammalian cells identified distinct contributions of the LisH domains to protein localisation or folding/stability. In conclusion, all components except muskelin are inferred to have been present in the last eukaryotic common ancestor. Diversification of this ligase complex in different eukaryotic lineages may result from the apparently fast evolution of RanBPM, differing requirements for WDR26, Armc8 or c17orf39, and the origin of muskelin in opisthokonts as a RanBPM-binding protein.

Evaluation of erythroblast macrophage protein related to erythroblastic islands in patients with hematopoietic stem cell transplantation

Background

Hematopoietic evaluation of the patients after Hematopoietic stem cell transplantation (HSCT) is very important. Erythroblast macrophage protein (Emp) is a key protein with function in normal differentiation of erythroid cells and macrophages. Emp expression correlates with erythroblastic island formation, a process widely believed to be associated with hematopoiesis in bone marrow. We aimed to investigate the hematopoietic function of bone marrow from 46 HSCT patients and 16 inpatients with severe anemia applied to the treatment of EPO by measuring Emp expression level.

Methods

Emp mRNA and protein expression levels in mononuclear cells of bone marrow and peripheral blood samples were detected by RT-PCR and Western blotting method respectively.

Results

While hematopoiesis occurs in bone marrow, Emp expression level was elevated and more erythroblastic islands were found , and Emp is upregulated in bone marrow in response to erythropoietin (EPO) treatment.

Conclusions

Emp expression correlates with erythroblastic island formation and has an important function for bone marrow hematopoiesis. Emp could be a potential biomarker for hematopoietic evaluation of HSCT patients.

Distribution and shedding of the membrane phosphatidylserine during maturation and aging of erythroid cells

Maturation and aging of erythroid cells are accompanied by extensive remodeling of the membrane and a marked decrease in cell size, processes that are mediated by externalization and shedding of phosphatidylserine (PS). In the present study, we investigated the redistribution of PS in the plasma membrane of erythroid precursors during their maturation and of mature RBCs during senescence, and the involvement of changes in calcium (Ca)-flux in these processes. Maturation was studied by analyzing normal human bone marrow cells as well as cultured human normal erythroid precursors induced by erythropoietin and murine erythroleukemia cells induced by hexamethylene-bisacetamide. Senescence was studied in normal human peripheral RBCs following density fractionation. PS and Ca were determined by flow cytometry using annexin-V and Flu-3, respectively. The outer, inner and shed PS were quantified by a novel two-step binding inhibitory assay. The results indicate a bi-phasic modulation of intracellular Ca and PS externalization/shedding; both of which decreased during maturation and increased during aging. The role of intracellular Ca in PS externalization/shedding was demonstrated by modulating intracellular Ca: Ca was decreased by incubating the cells with an ion chelator (EDTA) or with decreasing concentrations of Ca, whereas treatment with the ionophore A23187 elevated intracellular Ca. The results showed that low Ca resulted in decreased outer and shed PS, whereas high Ca had the opposite effect. The results suggest that PS externalization and shedding are mediated by increased cellular Ca-flux, and that they play an important role in erythroid maturation and RBC senescence.

During EPO or anemia challenge, erythroid progenitor cells transit through a selectively expandable proerythroblast pool

Investigations of bone marrow (BM) erythroblast development are important for clinical concerns but are hindered by progenitor cell and tissue availability. We therefore sought to more specifically define dynamics, and key regulators, of the formation of developing BM erythroid cell cohorts. A unique Kit(-)CD71(high)Ter119(-) "stage E2" proerythroblast pool first is described, which (unlike its Kit(+) "stage E1" progenitors, or maturing Ter119(+) "stage E3" progeny) proved to selectively expand ∼ 7-fold on erythropoietin challenge. During short-term BM transplantation, stage E2 proerythroblasts additionally proved to be a predominantly expanded progenitor pool within spleen. This E1→E2→E3 erythroid series reproducibly formed ex vivo, enabling further characterizations. Expansion, in part, involved E1 cell hyperproliferation together with rapid E2 conversion plus E2 stage restricted BCL2 expression. Possible erythropoietin/erythropoietin receptor proerythroblast stage specific events were further investigated in mice expressing minimal erythropoietin receptor alleles. For a hypomorphic erythropoietin receptor-HM allele, major defects in erythroblast development occurred selectively at stage E2. In addition, stage E2 cells proved to interact productively with primary BM stromal cells in ways that enhanced both survival and late-stage development. Overall, findings reveal a novel transitional proerythroblast compartment that deploys unique expansion devices.