Kazumichi Nagasawa

Immortalization of Erythroblasts by c-MYC and BCL-XL Enables Large-Scale Erythrocyte Production from Human Pluripotent Stem Cells

Summary The lack of knowledge about the mechanism of erythrocyte biogenesis through self-replication makes the in vitro generation of large quantities of cells difficult. We show that transduction of c-MYC and BCL-XL into multipotent hematopoietic progenitor cells derived from pluripotent stem cells and gene overexpression enable sustained exponential self-replication of glycophorin A+ erythroblasts, which we term immortalized erythrocyte progenitor cells (imERYPCs). In an inducible expression system, turning off the overexpression of c-MYC and BCL-XL enabled imERYPCs to mature with chromatin condensation and reduced cell size, hemoglobin synthesis, downregulation of GCN5, upregulation of GATA1, and endogenous BCL-XL and RAF1, all of which appeared to recapitulate normal erythropoiesis. imERYPCs mostly displayed fetal-type hemoglobin and normal oxygen dissociation in vitro and circulation in immunodeficient mice following transfusion. Using critical factors to induce imERYPCs provides a model of erythrocyte biogenesis that could potentially contribute to a stable supply of erythrocytes for donor-independent transfusion.

Structural and biological properties of erythropoietin in Xenopus laevis

OBJECTIVE Erythropoietin (EPO) and its receptor (EPOR) are key regulators of red blood cell production in mammals and fish. We aimed to investigate the structural and functional conservation of the EPO-EPOR system in amphibian erythropoiesis, using Xenopus laevis as a model. MATERIALS AND METHODS X. laevis epo (xlepo) complementary DNA was identified by referring to the Xenopus tropicalis genome database. Biological activity of recombinant xlEPO expressed in COS-1 cells was evaluated using xlEPOR-expressing murine FDC/P2 cells and human EPO-dependent UT-7/EPO cells. Expression of xlepo messenger RNA in adult X. laevis tissues in the normal state and under the condition of phenylhydrazine-induced anemia was evaluated by real-time reverse transcription polymerase chain reaction. RESULTS In the encoded protein, the positions of four cysteine residues were conserved; however, xlEPO had only 38% identity with human EPO. N-glycosylation sites were absent. Recombinant xlEPO induced proliferation of cell lines expressing xlEPOR and UT-7/EPO, confirming biological activity and cross-species reactivity. Despite little primary amino acid sequence similarity, the evolutionary highly conserved sequence NFLRGK was identified in the EPOR-binding site 1 region as in the human EPO protein. Strong expression of xlepo messenger RNA was detected in the lung and liver, especially in fractionated hepatocytes. No marked increase in xlepo expression was seen in the lung and liver of phenylhydrazine-induced anemic X. laevis. CONCLUSION We confirmed that xlEPO is the ligand to the previously reported xlEPOR in X. laevis. xlEPO shares structural and functional similarities and differences with mammalian counterparts, and regulation of xlepo expression and its influence on the erythropoietic system appears to be unique.

Identification of erythroid progenitors induced by erythropoietic activity in Xenopus laevis

Oxygen is essential for the survival of animals. Red blood cells in the circulation, i.e. peripheral erythrocytes, are responsible for transporting oxygen to tissues. The regulation of erythropoiesis in vertebrates other than mammals is yet to be elucidated. Recently we identified erythropoietin, a primary regulator of erythropoiesis, in Xenopus laevis, which should enable us to identify target cells, including erythroid progenitors, and to investigate the production and development of erythroid cells in amphibians. Here, we established a semi-solid colony-forming assay in Xenopus laevis to clarify the existence of colony-forming unit-erythroid cells, the functional erythroid progenitors identified in vitro. Using this assay, we showed that recombinant xlEPO induces erythroid colony formation in vitro and detected an increased level of erythropoietin activity in blood serum during acute anemic stress. In addition, our study demonstrated the possible presence of multiple, non-xlEPO, factors in anemic serum supportive of erythroid colony formation. These results indicate that erythropoiesis mediated by erythropoietin is present in amphibian species and, furthermore, that the regulatory mechanisms controlling peripheral erythrocyte number may vary among vertebrates.

Synthesis and bioassay of a boron-dipyrromethene derivative of estradiol for fluorescence imaging in vivo

C7α-substituted estradiols bind to estrogen receptors in cell nuclei, yet these derivatives remain little used in bioimaging. Here, we describe a fluorescent derivative of estradiol (E2) with a boron-dipyrromethene (BODIPY) moiety attached to C7α, synthesized by olefin metathesis reaction of 7α-allylestradiol and 9-decenyl-BODIPY. In ovariectomized rats and non-ovariectomized mice, E2-BODIPY promoted the growth of uterine tissue similar to the effect of estradiol. Twenty-four hours after subcutaneous injection of E2-BODIPY in non-ovariectomized mice, we observed fluorescence of E2-BODIPY in the nuclei of uterine epithelial cells. Our findings suggest that fluorescence microscopy can localize this derivative in E2-responsive cells during normal development and tumorigenesis in vivo.

Significant modulation of the hepatic proteome induced by exposure to low temperature in Xenopus laevis

Summary The African clawed frog, Xenopus laevis, is an ectothermic vertebrate that can survive at low environmental temperatures. To gain insight into the molecular events induced by low body temperature, liver proteins were evaluated at the standard laboratory rearing temperature (22°C, control) and a low environmental temperature (5°C, cold exposure). Using nano-flow liquid chromatography coupled with tandem mass spectrometry, we identified 58 proteins that differed in abundance. A subsequent Gene Ontology analysis revealed that the tyrosine and phenylalanine catabolic processes were modulated by cold exposure, which resulted in decreases in hepatic tyrosine and phenylalanine, respectively. Similarly, levels of pyruvate kinase and enolase, which are involved in glycolysis and glycogen synthesis, were also decreased, whereas levels of glycogen phosphorylase, which participates in glycogenolysis, were increased. Therefore, we measured metabolites in the respective pathways and found that levels of hepatic glycogen and glucose were decreased. Although the liver was under oxidative stress because of iron accumulation caused by hepatic erythrocyte destruction, the hepatic NADPH/NADP ratio was not changed. Thus, glycogen is probably utilized mainly for NADPH supply rather than for energy or glucose production. In conclusion, X. laevis responds to low body temperature by modulating its hepatic proteome, which results in altered carbohydrate metabolism.

The influence of artificially introduced N-glycosylation sites on the in vitro activity of Xenopus laevis erythropoietin.

Erythropoietin (EPO), the primary regulator of erythropoiesis, is a heavily glycosylated protein found in humans and several other mammals. Intriguingly, we have previously found that EPO in Xenopus laevis (xlEPO) has no N-glycosylation sites, and cross-reacts with the human EPO (huEPO) receptor despite low homology with huEPO. In this study, we introduced N-glycosylation sites into wild-type xlEPO at the positions homologous to those in huEPO, and tested whether the glycosylated mutein retained its biological activity. Seven xlEPO muteins, containing 1–3 additional N-linked carbohydrates at positions 24, 38, and/or 83, were expressed in COS-1 cells. The muteins exhibited lower secretion efficiency, higher hydrophilicity, and stronger acidic properties than the wild type. All muteins stimulated the proliferation of both cell lines, xlEPO receptor-expressing xlEPOR-FDC/P2 cells and huEPO receptor-expressing UT-7/EPO cells, in a dose-dependent manner. Thus, the muteins retained their in vitro biological activities. The maximum effect on xlEPOR-FDC/P2 proliferation was decreased by the addition of N-linked carbohydrates, but that on UT-7/EPO proliferation was not changed, indicating that the muteins act as partial agonists to the xlEPO receptor, and near-full agonists to the huEPO receptor. Hence, the EPO-EPOR binding site in X. laevis locates the distal region of artificially introduced three N-glycosylation sites, demonstrating that the vital conformation to exert biological activity is conserved between humans and X. laevis, despite the low similarity in primary structures of EPO and EPOR.

A comparative perspective on erythropoiesis

The acquisition of fundamental information by the use of recent technologies, including omics-based molecular analyses and total RNA sequencing, has opened the door to further advances in physiological studies on new animal models. Currently, we are endeavoring to develop a comparative hematology protocol in order to build a discovery platform. All vertebrates, with the exception of a few species, have universally peripheral erythrocytes and hemoglobin, suggesting erythropoiesis to be an evolutionary index.