Quantification and localization of erythropoietin-receptor-expressing cells in the liver of Xenopus laevis

Identification and characterization of myeloid cells localized in the tadpole liver cortex in Xenopus laevis

In the present study, using transgenic frogs that express GFP specifically in myeloid cells under the myeloperoxidase enhancer sequence, we found that myeloperoxidase-positive cells are localized in the liver cortex at the late tadpole stages. Immunohistochemical analysis revealed that myelopoiesis in the liver cortex became evident after st. 50 and reached its peak by st. 56. Transplantation experiments indicated that cells with a high density at the liver cortex were derived from the dorso-lateral plate tissue in the neurula embryo. Analysis of smear samples of the cells isolated from collagenase-treated liver tissues of the transgenic tadpoles indicated that myeloid cells were the major population of blood cells in the larval liver and that, in addition to myeloid colonies, erythroid colonies expanded in entire liver after metamorphosis. Cells that were purified from the livers of transgenic tadpoles according to the GFP expression exhibited the multi-lobed nuclei. The results of present study provide evidence that the liver cortex of the Xenopus tadpole is a major site of granulopoiesis.

Isolation and evaluation of erythroid progenitors in the livers of larval, froglet, and adult Xenopus tropicalis

Xenopus liver maintains erythropoietic activity from the larval to the adult stage. During metamorphosis, thyroid hormone mediates apoptosis of larval-type erythroid progenitors and proliferation of adult-type erythroid progenitors, and a globin switch occurs during this time. In addition, the whole-body mass and the liver also change; however, whether there is a change in the absolute number of erythroid progenitors is unclear. To isolate and evaluate erythroid progenitors in the Xenopus liver, we developed monoclonal ER9 antibodies against the erythropoietin receptor (EPOR) of Xenopus. ER9 recognized erythrocytes, but not white blood cells or thrombocytes. The specificity of ER9 for EPOR manifested as its inhibitory effect on the proliferation of a Xenopus EPOR-expressing cell line. Furthermore, ER9 recognition was consistent with epor gene expression. ER9 staining with Acridine orange (AO) allowed erythrocyte fractionation through fluorescence-activated cell sorting. The ER9+ and AO-red (AOr)high fractions were highly enriched in erythroid progenitors and primarily localized to the liver. The method developed using ER9 and AO was also applied to larvae and froglets with different progenitor populations from adult frogs. The liver to body weight and the number of ER9+ AOrhigh cells per unit body weight were significantly higher in adults than in larvae and froglets, and the number of ER9+ AOrhigh cells per unit liver weight was the highest in froglets. Collectively, our results show increased erythropoiesis in the froglet liver and demonstrate growth-dependent changes in erythropoiesis patterns in specific organs of Xenopus.

Flow cytometric analysis of Xenopus laevis and X. tropicalis blood cells using acridine orange

Automated blood cell counters can distinguish cells based on their size and the presence or absence of a nucleus. However, most vertebrates have nucleated blood cells that cannot be counted automatically. We established an alternative automatic method for counting peripheral blood cells by staining cells with the fluorescent dye acridine orange (AO) and analysing cell populations using flow cytometry (FCM). As promising new animal models, we chose Xenopus laevis and three inbred strains of X. tropicalis. We compared the haematological phenotypes, including blood cell types, cell sizes, cellular structure, and erythrocyte lifespans/turnover rate among X. laevis and the three inbred strains of X. tropicalis. Each cell type from X. laevis was sorted according to six parameters: forward- and side-scattered light emission, AO red and green fluorescence intensity, and cellular red and green fluorescence. Remarkably, the erythrocyte count was the highest in the Golden line, suggesting that genetic factors were associated with the blood cells. Furthermore, immature erythrocytes in anaemic X. laevis could be separated from normal blood cells based on red fluorescence intensity. These results show that FCM with AO staining allows for an accurate analysis of peripheral blood cells from various species.

Thrombopoietin induces production of nucleated thrombocytes from liver cells in Xenopus laevis

The development of mammalian megakaryocytes (MKs) and platelets, which are thought to be absent in non-mammals, is primarily regulated by the thrombopoietin (TPO)/Mpl system. Although non-mammals possess nucleated thrombocytes instead of platelets, the features of nucleated thrombocyte progenitors remain to be clarified. Here, we provide the general features of TPO using Xenopus laevis TPO (xlTPO). Hepatic and splenic cells were cultured in liquid suspension with recombinant xlTPO. These cells differentiated into large, round, polyploid CD41-expressing cells and were classified as X. laevis MKs, comparable to mammalian MKs. The subsequent culture of MKs after removal of xlTPO produced mature, spindle-shaped thrombocytes that were activated by thrombin, thereby altering their morphology. XlTPO induced MKs in cultured hepatic cells for at least three weeks; however, this was not observed in splenic cells; this result demonstrates the origin of early haematopoietic progenitors in the liver rather than the spleen. Additionally, xlTPO enhanced viability of peripheral thrombocytes, indicating the xlTPO-Mpl pathway stimulates anti-apoptotic in peripheral thrombocytes. The development of thrombocytes from MKs via the TPO-Mpl system in X. laevis plays a crucial role in their development from MKs, comparable to mammalian thrombopoiesis. Thus, our results offer insight into the cellular evolution of platelets/MKs in vertebrates. (200/200).

Diverse of Erythropoiesis Responding to Hypoxia and Low Environmental Temperature in Vertebrates

Erythrocytes are responsible for transporting oxygen to tissue and are essential for the survival of almost all vertebrate animals. Circulating erythrocyte counts are tightly regulated and respond to erythrocyte mass and oxygen tension. Since the discovery of erythropoietin, the erythropoietic responses to environment and tissue oxygen tension have been investigated in mice and human. Moreover, it has recently become increasingly clear that various environmental stresses could induce the erythropoiesis via various modulating systems, while all vertebrates live in various environments and habitually adapt to environmental stress. Therefore, it is considered that investigations of erythropoiesis in vertebrates provide a lead to the various erythropoietic responses to environmental stress. This paper comparatively introduces the present understanding of erythropoiesis in vertebrates. Indeed, there is a wide range of variations in vertebrates' erythropoiesis. This paper also focused on erythropoietic responses to environmental stress, hypoxia, and lowered temperature in vertebrates.

Cellular characterization of thrombocytes in Xenopus laevis with specific monoclonal antibodies

Platelets are produced from megakaryocytes (MKs) in the bone marrow. In contrast, most nonmammalian vertebrates have nucleated and spindle-shaped thrombocytes instead of platelets in their circulatory systems, and the presence of MKs as thrombocyte progenitors has not been verified. In developing a new animal model in adult African clawed frog (Xenopus laevis), we needed to distinguish nucleated thrombocytes and their progenitors from other blood cells, because the cellular morphology of activated thrombocytes resembles lymphocytes and other cells. We initially generated two monoclonal antibodies, T5 and T12, to X. laevis thrombocytes. Whereas T5 recognized both thrombocytes and leukocytes, T12 specifically reacted to spindle-shaped thrombocytes. The T12(+) thrombocytes displayed much higher DNA ploidy than nucleated erythrocytes, and they expressed CD41 and Fli-1. In the presence of CaCl2, adenosine diphosphate, thrombin, or various collagens, T12(+) thrombocytes exhibited aggregation. These thrombocytes were located predominantly in the hepatic sinusoids and the splenic red pulp, suggesting that both organs are the sites of thrombopoiesis. Notably, circulating thrombocytes exhibited lower DNA ploidy than hepatic thrombocytes. Intraperitoneal administration of T12 produced immune thrombocytopenia in frogs, which reached a nadir 4 days postinjection, followed by recovery, suggesting that humoral regulation maintained the number of circulating thrombocytes. Although differences between MKs and thrombocytes in X. laevis remain to be defined, our results provide further insight into MK development and thrombopoiesis in vertebrates.

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

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.