The metamorphic switch in hemoglobin phenotype ofXenopus laevis involves erythroid cell replacement

Contribution of ventral and dorsal mesoderm to primitive and definitive erythropoiesis in the salamander Hynobius retardatus

Previously, we found that the conversion of hemoglobins (Hbs) from the larval to the adult type occurred within a single erythroid cell population in a salamander, Hynobius retardatus ("Hb switching" model), whereas the transition involves replacement of red-blood-cell (RBC) populations ("RBC replacement" model) in many amphibians (M. Yamaguchi, H. Takahashi, and M. Wakahara, 2000, Dev. Gene Evol. 210, 180-189). To further characterize the Hb transition, developmental changes in the erythropoietic sites have been intensively analyzed using larval- and adult-specific globin antibodies and globin and GATA-3 RNA probes. Cells of the ventral blood island (VBI) and the dorsolateral plate (DLP) in embryos differentiate in situ to erythroid cells that contain larval globin mRNA, suggesting that both the VBI and the DLP contribute to "primitive" erythropoiesis. In contrast, the expression pattern of the GATA-3 gene suggests that cells of the DLP may contribute to "definitive" hematopoiesis. In order to determine whether it is possible to define a definitive erythropoiesis in H. retardatus or not, further experiments were done: (1) when metamorphosing larvae were treated with phenylhydrazine to induce anemia and then bled at the postmetamorphic stage after recovery from the anemia, a precocious Hb transition was observed in these animals; (2) an RBC population expressing only adult Hb was confirmed by subtracting the number of RBCs expressing larval Hb from the total number of RBCs during metamorphosis. All these results support the existence of a definitive erythroid cell population that contributes only adult RBCs in this species.

Humoral regulation of hemoglobin transition from larval to adult types in a salamander, Hynobius retardatus

In amphibians hemoglobin (Hb) transition from larval to adult types occurs during metamorphosis. Hemoglobin transition in the salamander Hynobius retardatus also occurs during metamorphosis, but depends on activity of the pituitary gland, rather than that of the thyroid gland. These findings were supported by the fact that the transplanted pituitary gland to hypophysectomized (Hx) larvae exerted accelerating effects on Hb transition. Contrary to this, a homogenate of the pituitary gland from Xenopus laevis had no accelerating effect on Hb transition when injected into Hx larvae of H. retardatus. Whereas exogeneously applied triiodothyronine (T3) or a combination of T3 and hydrocortisone had an accelerating effect on Hb transition in the Hx larvae, a single treatment with hydrocortisone in the Hx larvae had no effect. When an inhibitor of synthesis of adrenocortico-steroid hormones, metyrapone, was applied in combination with goitrogens to intact larvae, Hb transition from larval to adult types was suppressed compared with normal controls, suggesting that the thyroid-corticoid system is involved in Hb transition. These results suggest that Hb transition in H. retardatus is controlled by two independent accelerating factors, from both the pituitary gland and the thyroid-corticoid system. These control mechanisms are different from the general regulation of metamorphosis that occurs in many amphibians. A tentative model of hormonal controls in Hb transition from larval to adult types during the metamorphosis in H. retardatus is presented.

Occurrence of DNase gamma-like apoptotic endonucleases in hematopoietic cells in Xenopus laevis and their relation to metamorphosis

DNase gamma has been suggested to be an endonuclease responsible for thymic apoptosis in mammals. Using the frog, Xenopus laevis, we examined whether any hematopoietic cells other than thymocytes have DNase gamma activity. The activity gel assays and HeLa cell nuclear assays for DNase revealed that nuclei from red blood cells and cells in liver (an erythropoietic organ) contain DNase gamma-like activities (molecular mass of 38 and 36 kDa) indicated by their biochemical features (internucleosomal DNA cleavage, requirement of Ca2+ and Mg2+, and sensitivity to Zn2+). Distribution of these two (38 and 36 kDa) forms of enzyme was determined by cell fractionation analysis of liver cells: the 38 kDa form of enzyme was found in erythrocytes/erythroblasts and hepatocytes, whereas the 36 kDa form was in lymphocytes/macrophages. Interestingly, these two enzyme activities increased during metamorphic climax, at which time larval-type cells are converted to adult ones. It thus appears that these gamma-type DNases are involved in programmed cell death during metamorphosis.