identification-murine-erythroblast-subpopulation-enriched-enucleating

Identification of a Murine Erythroblast Subpopulation  Enriched in Enucleating Events by Multi-spectral Imaging Flow Cytometry

Erythropoiesis in mammals concludes with the dramatic process of enucleation that results in reticulocyte formation. The mechanism of enucleation has not ...

University of Cincinnati College of Medicine

Actin oligomers are a significant structural component of the erythrocyte cytoskeleton. Rac1 and Rac2 GTPases regulate actin structures and have multiple overlapping as well as distinct roles in hematopoietic cells; therefore, we studied their role in red blood cells (RBCs). Conditional gene targeting with a loxP-flanked Rac1 gene allowed Crerecombinase-induced deletion of Rac1 on a Rac2 null genetic background. The Rac1(-/-);Rac2(-/-) mice developed microcytic anemia with a hemoglobin drop of about 20% and significant anisocytosis and poikilocytosis. Reticulocytes increased more than 2-fold. Rac1(-/-);Rac2(-/-) RBCs stained with rhodamine-phalloidin demonstrated F-actin meshwork gaps and aggregates under confocal microscopy. Transmission electron microscopy of the cytoskeleton demonstrated junctional aggregates and pronounced irregularity of the hexagonal spectrin scaffold. Ektacytometry confirmed that these cytoskeletal changes in Rac1(-/-);Rac2(-/-) erythrocytes were associated with significantly decreased cellular deformability. The composition of the cytoskeletal proteins was altered with an increased actin-to-spectrin ratio and increased phosphorylation (Ser724) of adducin, an F-actin capping protein. Actin and phosphorylated adducin of Rac1(-/-);Rac2(-/-) erythrocytes were more easily extractable by Triton X-100, indicating weaker association to the cytoskeleton. Thus, deficiency of Rac1 and Rac2 GTPases in mice alters actin assembly in RBCs and causes microcytic anemia with reticulocytosis, implicating Rac GTPases as dynamic regulators of the erythrocyte cytoskeleton organization.

Rac GTPases regulate the morphology and deformability of the erythrocyte cytoskeleton.

Normal production of RBCs requires that the antiapoptotic protein Bcl-xl be induced at end stages of differentiation in response to erythropoietin (Epo) signaling. The critical proapoptotic pathways inhibited by Bcl-xl in erythroblasts are unknown. We used gene targeting in the mouse to evaluate the BH3-only factor Nix, which is transcriptionally up-regulated during Epo-stimulated in vitro erythrocyte differentiation. Nix null mice are viable and fertile. Peripheral blood counts revealed a profound reticulocytosis and thrombocytosis despite normal serum Epo levels and blood oxygen tension. Nix null mice exhibited massive splenomegaly, with splenic and bone marrow erythroblastosis and reduced apoptosis in vivo during erythrocyte maturation. Hematopoietic progenitor populations were unaffected. Cultured Nix null erythroid cells were hypersensitive to Epo and resistant to apoptosis stimulated by cytokine deprivation and calcium ionophore. Transcriptional profiling of Nix null spleens revealed increased expression of cell cycle and erythroid genes, including Bcl-xl, and diminished expression of cell death and B cell-related genes. Thus, cell-autonomous Nix-mediated apoptosis in opposition to the Epo-induced erythroblast survival pathway appears indispensable for regulation of erythrocyte production and maintenance of hematological homeostasis. These results suggest that physiological codependence and coordinated regulation of pro- and antiapoptotic Bcl2 family members may represent a general regulatory paradigm in hematopoiesis.

Unrestrained erythroblast development in Nix-/- mice reveals a mechanism for apoptotic modulation of erythropoiesis.

The Rho GTPase Cdc42 regulates adhesion, migration, and homing, as well as cell cycle progression, of hematopoietic stem cells, but its role in multilineage blood development remains unclear. We report here that inducible deletion of cdc42 in cdc42-floxed mouse bone marrow by the interferon-responsive, Mx1-Cre-mediated excision led to myeloid and erythroid developmental defects. Cdc42 deletion affected the number of early myeloid progenitors while suppressing erythroid differentiation. Cdc42-deficient mice developed a fatal myeloproliferative disorder manifested by significant leukocytosis with neutrophilia, myeloid hyperproliferation, and myeloid cell infiltration into distal organs. Concurrently, Cdc42 deficiency caused anemia and splenomegaly accompanied with decreased bone marrow erythroid burst-forming units (BFU-Es) and colony-forming units-erythroid (CFU-Es) activities and reduced immature erythroid progenitors, suggesting that Cdc42 deficiency causes a block in the early stage of erythropoiesis. Cdc42 activity is responsive to stimulation by SCF, IL3, SDF-1alpha, and fibronectin. The increased myelopoiesis and decreased erythropoiesis of the knockout mice are associated with an altered gene transcription program in hematopoietic progenitors, including up-regulation of promyeloid genes such as PU.1, C/EBP1alpha, and Gfi-1 in the common myeloid progenitors and granulocyte-macrophage progenitors and down-regulation of proerythroid gene such as GATA-2 in the megakaryocyte-erythroid progenitors. Thus, Cdc42 is an essential regulator of the balance between myelopoiesis and erythropoiesis.

Cdc42 critically regulates the balance between myelopoiesis and erythropoiesis.

The small Rho GTPases Rac1 and Rac2 have both overlapping and distinct roles in actin organization, cell survival, and proliferation in various hematopoietic cell lineages. The role of these Rac GTPases in erythropoiesis has not yet been fully elucidated.

Rac1 and Rac2 GTPases are necessary for early erythropoietic expansion in the bone marrow but not in the spleen.

The small Rho GTPases Rac1 and Rac2 regulate actin structures and mediate reactive oxygen species (ROS) production via NADPH oxidase in a variety of cells. We have demonstrated that deficiency of Rac1 and Rac2 GTPases in mice disrupts the normal hexagonal organization of the RBC cytoskeleton and reduces erythrocyte deformability. This is associated with increased phosphorylation of adducin at Ser-724, (corresponding to Ser-726 in human erythrocytes), a domain target of protein kinase C (PKC). PKC phosphorylates adducin and leads to decreased F-actin capping and dissociation of spectrin from actin, implicating a significant role of such phosphorylation in cytoskeletal remodeling. We evaluated adducin phosphorylation in erythrocytes from patients with sickle cell disease and found it consistently increased at Ser-726. In addition, ROS concentration is elevated in sickle erythrocytes by 150-250% compared to erythrocytes from normal control individuals. Here, we review previous studies demonstrating that altered phosphorylation of erythrocyte cytoskeletal proteins and increased ROS production result in disruption of cytoskeleton stability in healthy and sickle cell erythrocytes. We discuss in particular the known and potential roles of protein kinase C and the Rac GTPases in these two processes.

Altered phosphorylation of cytoskeleton proteins in sickle red blood cells: the role of protein kinase C, Rac GTPases, and reactive oxygen species.

We report a novel glucose-6-phosphate dehydrogenase (G6PD) mutation, which we propose to name G6PD Cincinnati (c.1037A > T, p.N346I), found in combination with G6PD Gastonia (c.637G > T, p.V213L) in an infant who presented with neonatal cholestasis. The G6PD Cincinnati mutation results in a non-conservative amino acid substitution at the tetramer interface disturbing its formation, as seen by native gel electrophoresis and immunoblotting. G6PD Gastonia disrupts dimerization of the enzyme and by itself causes chronic non-spherocytic hemolytic anemia. The G6PD Cincinnati mutation may have aggravated the clinical picture of G6PD Gastonia with the result of severe perinatal hemolysis causing cholestasis and associated liver injury.

Cooperating G6PD mutations associated with severe neonatal hyperbilirubinemia and cholestasis.

To understand the role of cytoskeleton and membrane signaling molecules in erythroblast enucleation, we developed a novel analysis protocol of multiparameter high-speed cell imaging in flow. This protocol enabled us to observe F-actin and phosphorylated myosin regulatory light chain (pMRLC) assembled into a contractile actomyosin ring (CAR) between nascent reticulocyte and nucleus, in a population of enucleating erythroblasts. CAR formation and subsequent enucleation were not affected in murine erythroblasts with genetic deletion of Rac1 and Rac2 GTPases because of compensation by Rac3. Pharmacologic inhibition or genetic deletion of all Rac GTPases altered the distribution of F-actin and pMRLC and inhibited enucleation. Erythroblasts treated with NSC23766, cytochalasin-D, colchicine, ML7, or filipin that inhibited Rac activity, actin or tubulin polymerization, MRLC phosphorylation, or lipid raft assembly, respectively, exhibited decreased enucleation efficiency, as quantified by flow cytometry. As assessed by high-speed flow-imaging analysis, colchicine inhibited erythroblast polarization, implicating microtubules during the preparatory stage of enucleation, whereas NSC23766 led to absence of lipid raft assembly in the reticulocyte-pyrenocyte border. In conclusion, enucleation is a multistep process that resembles cytokinesis, requiring establishment of cell polarity through microtubule function, followed by formation of a contractile actomyosin ring, and coalescence of lipid rafts between reticulocyte and pyrenocyte.

Signaling and cytoskeletal requirements in erythroblast enucleation.

Chronic inflammation has emerged as an important pathogenic mechanism in sickle cell disease (SCD). One component of this inflammatory response is oxidant stress mediated by reactive oxygen species (ROS) generated by leukocytes, endothelial cells, plasma enzymes, and sickle red blood cells (RBC). Sickle RBC ROS generation has been attributed to sickle hemoglobin auto-oxidation and Fenton chemistry reactions catalyzed by denatured heme moieties bound to the RBC membrane. In this study, we demonstrate that a significant part of ROS production in sickle cells is mediated enzymatically by NADPH oxidase, which is regulated by protein kinase C, Rac GTPase, and intracellular Ca(2+) signaling within the sickle RBC. Moreover, plasma from patients with SCD and isolated cytokines, such as transforming growth factor β1 and endothelin-1, enhance RBC NADPH oxidase activity and increase ROS generation. ROS-mediated damage to RBC membrane components is known to contribute to erythrocyte rigidity and fragility in SCD. Erythrocyte ROS generation, hemolysis, vaso-occlusion, and the inflammatory response to tissue damage may therefore act in a positive-feedback loop to drive the pathophysiology of sickle cell disease. These findings suggest a novel pathogenic mechanism in SCD and may offer new therapeutic targets to counteract inflammation and RBC rigidity and fragility in SCD.

Suvarnamala Pushkaran

Cancer and Blood Diseases Institute,
Cincinnati Children's Hospital Medical Center,
Cancer and Blood Diseases Institute, Cincinnati Children's Hospital Medical Center

University of Cincinnati College of Medicine

Identification of a Murine Erythroblast Subpopulation Enriched in Enucleating Events by Multi-spectral Imaging Flow Cytometry

Suvarnamala Pushkaran

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University of Cincinnati College of Medicine

Identification of a Murine Erythroblast Subpopulation Enriched in Enucleating Events by Multi-spectral Imaging Flow Cytometry

Cancer and Blood Diseases Institute,
Cincinnati Children's Hospital Medical Center,
Cancer and Blood Diseases Institute, Cincinnati Children's Hospital Medical Center