SUMMARYDiminished erythrocyte count and erythropoiesis have been reported during hypothermia in some ectothermic animals. In this study, the African clawed frog, Xenopus laevis, was used to investigate the cause of hypothermia-induced anemia. We developed a new model of hypothermia at 5°C and monitored blood cell count and erythropoiesis on several days. Erythrocyte count declined by 30% on the first day following cold exposure (5°C) and mRNA expression of hemeoxygenase-1 was enhanced 10-fold; accumulation of iron as a result of heme degradation was observed in the liver. One day after low-temperature exposure, erythropoietin mRNA expression was elevated in the liver and lung compared with that at normal temperature (22°C) by qRT-PCR analysis. Examination of liver sections (i.e. the erythropoietic organ) showed an increase in o-dianisidine-positive erythrocytes in the hepatic sinusoid 5days after the onset of low-temperature exposure compared with normal liver. Peripheral erythrocyte count remained low, indicating that newly produced erythrocytes did not migrate from the liver to the circulation during hypothermia. In conclusion, this study reveals hypothermic anemia as being associated with hepatic erythrocyte destruction; prolonged anemia during low-temperature exposure is concomitant with newly produced erythrocytes being confined to the liver and may lead to new insights into vertebrate hematopoiesis.
While hematopoiesis in individuals is strictly regulated for maintenance of homeostasis, it has been reported that the numbers of peripheral blood cells are modulated in response to environmental temperature in vertebrates including hamsters, rats, squirrels, dogs, and bullfrogs. To date, the physiological and molecular mechanisms have not been elucidated yet. Amphibians are poikilothermic vertebrates exposed to various fluctuations of environmental conditions; therefore they need to exert their capacities to acclimate to such changes. Additionally numerous studies have demonstrated a measurable metabolic reduction in the metabolic rate of cold-acclimated frogs. We examined hematological changes in response to environmental temperature in an aquatic amphibian, Xenopus laevis. Frogs initially maintained at 25°C were acclimated to 10°C, and hematological changes were observed. The numbers of erythrocytes, leukocytes and thrombocytes gradually reduced as the transient phase by 4 weeks, and subsequently reached the steady-state that sustained for more than 4 months. Whereas the reduction in the numbers of white blood cells and thrombocytes were moderate, the number of erythrocytes and the level of hemoglobin at nadir were remarkably low (approximately 40% of the initial values). It is known that oxygen levels may reduce in ice-cold water, and cold-acclimated animals therefore tolerate prolonged severe hypoxia; nevertheless cold-acclimated Xenopus exhibited severe erythrocytopenia. In addition, morphological change of peripheral blood cells and hematopoietic tissues (liver, spleen, kidney, and bone marrow) were examined. There were no remarkable cellular changes in cellular size and shape. However, increased numbers of mature erythrocytes were observed in the bone marrow of the steady state cold-acclimated Xenopus, while mature erythrocytes were not found in the bone marrow in Xenopus at 25 °C. This cold-temperature-induced pancytopenia was reversible when the temperature was put back to 25°C, as all of blood cell counts returned to the normal levels within 4 weeks in a reverse fashion as the transient phase of cold-acclimation. During the recovery phase, immature erythrocytes that were scarcely existed in the normal peripheral blood appeared in the circulation, suggesting that erythrocytes were newly produced at 25 °C after prolonged exposure to cold temperature. The possible explanations for the reduction in the numbers of circulating peripheral blood cells might be due to a number of various reasons such as reduced productions of hamatopoietic progenitors and/or related cytokines, alternation in the storage capacities and/or the life span of blood cells, and systemic suspension of normal activities. To compare the lifespan of erythrocytes between normal and cold-acclimated Xenopus, erythrocytes were covalently labeled with biotin. The surviving biotinylated erythrocytes in the circulation were quantitatively detected as avidin-biotin complex by microscopy and flowcytometry. Furthermore expression levels of several genes responsible for the hematopoietic regulation were comparatively examined. The cold-acclimated Xenopus model developed here may allow for a valuable approach aiming at exploring undiscovered systems in hematopoietic regulation.
To survive, organisms must adapt to changes in the ambient environment. Here, we describe a new model of anemia based on exposure of African clawed frog, Xenopus laevis to low-temperature. Frogs exposed at low-temperature (5ºC) for five days had decreased numbers of peripheral blood erythrocytes, leukocytes, and thrombocytes as well as low hemoglobin levels. By contrast, spleen erythrocytes increased in number. Cell counts returned to normal in frogs re-warmed at ambient temperature (22ºC) for two days. To confirm these observations in vivo, we labeled peripheral blood cells with fluorescent reagent CFSE. During five days at 5ºC, labeled erythrocytes in peripheral blood decreased in number while those in spleen increased. When the temperature was raised to 22ºC, however, their numbers increased in peripheral blood. The findings suggested that exposure to low-temperature resulted in splenic pooling of peripheral erythrocytes. Accordingly, we looked at recovery from anemia induced by phenylhydrazine (PHZ) in this model. PHZ-treated frogs maintained at 22ºC decreased numbers of peripheral erythrocytes that were minimal on day 8, and increased gradually thereafter. In the liver, we found erythrocyte progenitors expressing erythropoietin receptor and GATA1-A detected by reverse transcription polymerase chain reactions and immunocytochemical staining but no mature forms. In PHZ-treated frogs exposed to 5ºC, peripheral erythrocyte counts remained minimal from day 8, and reversibly recovered when temperature returned to 22ºC. Erythrocyte progenitors were present in liver on day 8 but absent on day 12. Conversely, mature erythrocytes were absent in liver on day 8 but present on day 12. Finally, to learn whether the progenitors proliferate and differentiate without migrating from liver to peripheral blood, we treated frogs with thymidine analog bromodeoxyuridine (BrdU). In frogs kept at 22 ºC, BrdU-labeled erythrocytes were abundant in both liver and peripheral blood. However, frogs cooled at 5ºC had labeled cells in liver but few in peripheral blood. The findings suggest low-temperature exposure cause this anemia by impairing migration of mature/immature erythrocytes from the liver. In summary, this amphibian model offers a new perspective for investigating physiological effects of environmental temperature on vertebrate erythropoiesis.
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