Hemoglobin switching in Rana/Xenopus erythroid heterokaryons: Factors mediating the metamorphic hemoglobin switch are conserved

Abstract

Hemoglobin switching, which occurs in all classes of vertebrates as well as in certain invertebrates, is due to developmental regulation of different globin genes which are typically arranged in clustered families. By fusing erythroid cells of different developmental programs, trans-acting factors that regulate this switch in gene expression have been detected [Ramseyer et al. (1989): Dev Biol 133:262-271]. Adult erythroid cells of one anuran species, Xenopus laevis, were fused with tadpole erythroid cells of another frog, Rana catesbeiana, creating developmental erythroid heterokaryons that synthesize adult Rana globin mRNA and hemoglobins. The results show that factors from adult Xenopus erythroid cells are capable of inducing adult Rana globin gene expression in the Rana tadpole erythroid cell nucleus. We have used the cross-induction of adult Rana hemoglobin synthesis in these adult Xenopus/Rana tadpole erythroid heterokaryons to address two practical questions, answers to which may be helpful in isolating developmental stage-specific globin gene regulatory proteins: 1) Are erythroblasts which are actively expressing globin mRNAs and hemoglobins richer in specific globin-inducing activities than other stages of erythroid cellular differentiation? 2) Do mature, circulating erythrocytes still have the activities necessary to mediate the cross-induction of Hb synthesis? The results reported here show that the answers to both questions are affirmative and show that quiescent, fully differentiated adult erythroid cells are still capable of expressing the trans-activator(s). These findings show that factors which mediate the metamorphic hemoglobin switch are conserved between these two genera of frogs.

Rapid and selective removal of larval erythrocytes from systemic circulation during metamorphosis of the bullfrog, Rana catesbeiana

“…The present results strongly support the hypothesis that the metamorphic transition of the RBC type is brought about by replacement of the entire population of erythrocytes rather than by altered globin gene expression within a single cell population. The previous report that L‐RBC synthesize adult‐type globin molecules when exposed to transacting factor(s) transferred from A‐RBC ( Broyles et al 1994 ) does not necessarily mean that similar events occur in vivo . Furthermore, the demonstration of RBC with intermediate parametric features ( Yamaguchi & Wakahara 1997) by no means proves that such cells are actually transitional intermediates.…”

“…By contrast to this circumstantial evidence for the replacement hypothesis, two reports have provided evidence against this hypothesis. According to Broyles et al (1994) , L‐RBC can be induced to synthesize adult‐type globin molecules by exposure to transacting factor(s) transferred from the A‐RBC. Other authors have supported the single cell population hypothesis based on the observation that RBC with intermediate parametric features, in terms of electrophoretic mobility and buoyant density, appear transiently in the salamander Hynobius retardatus ( Yamaguchi & Wakahara 1997).…”

The analysis of complex developmental programmes: amphibian metamorphosis

“…The tadpole immune system is lost and replaced by an adult one that can no longer recognize tadpole skin as self in a graft (Izutsu & Yoshizato 1993). Thyroid hormone induces the transition from larval to adult globin synthesis (see Broyles et al 1994).…”