Hemoglobin transition in relation to metamorphosis in normal and isogenicXenopus

Changes in the immune system during metamorphosis of Xenopus

Profound immunological changes occur as tadpoles metamorphose into adult amphibians. These include the expression of a different antibody repertoire, a lessening of skin graft tolerance, the appearance on leukocytes of class I MHC antigens. Here Martin Flajnik and his colleagues review what is known of these changes in Xenopus and speculate on how they may occur.

Metamorphosis and the amphibian immune system

Studies of the ontogeny of immunity in a limited number of representative amphibians have shown that while the immune systems of the larval forms are competent to defend against potential pathogens in the temporary ponds they inhabit, they are not equivalent to the mature immune systems that develop after metamorphosis. Metamorphosis is a critical time of transition when increased concentrations of metamorphic hormones, principally thyroid hormones (TH) and corticosteroid hormones (CH), orchestrate the loss or reorganization of many tissues and organ systems, including the immune system. Immune system reorganization may serve to eliminate unnecessary lymphocytes that could be destructive if they recognized newly emerging adult-specific antigens on the adult tissues. Increased corticosteroids during metamorphosis appear to induce apoptosis of susceptible lymphocytes. This cell death can be inhibited in vitro or in vivo by the corticosteroid receptor antagonist, RU486. A coordinate increase in both TH and CH at metamorphosis may be common to all amphibians that undergo metamorphosis. Current evidence suggests that the central hypothalamic mediator that induces pituitary production of both thyroid-stimulating hormone and adrenocorticotropic hormone in larval amphibians is corticotropin-releasing hormone. Most amphibians probably survive the temporary immunosuppression associated with metamorphosis with no deleterious effects. However, it is hypothesized that if environmental stressors result in the induction of metamorphosis at a less than optimal body size and state of immune maturation, the immune system destruction would be more significant, and the amphibians could be at greater risk of infection and death.

Effects of thyroid hormone deprivation on immunity in postmetamorphic frogs

Previously, we showed that sodium perchlorate treatment of larval frogs (Xenopus laevis) interferes with the normal expansion of T and B lymphocytes and development of an adult-type T-cell population. It was unclear whether these effects resulted from preventing metamorphosis or from long-term thyroid hormone (TH) deprivation. To try to distinguish between these possibilities, we have now studied the effects of perchlorate treatment beginning immediately after metamorphosis. After 5 months, treated animals, but not untreated controls, had large thyroid goiters, were significantly smaller, and had significantly fewer erythrocytes, thymocytes, and splenocytes. Although the number of IgM- splenocytes and thymocytes was reduced, the estimated percent of major histocompatibility complex (MHC) class II+ cells within this subset was not significantly different from that of controls. Furthermore, splenocytes from perchlorate-treated frogs could respond normally by [3H]TdR incorporation to the T-cell mitogens, phytohemagglutinin-P (PHA) and concanavalin A (Con A). Thus, unlike perchlorate-treated larvae, perchlorate-treated juveniles appear to be able to develop T cells with an adult phenotype competent to respond to activation and proliferation signals.

Contribution of ventral blood island mesoderm to hematopoiesis in postmetamorphic and metamorphosis-inhibited Xenopus laevis

In an effort to label very early erythrocyte and lymphocyte populations and to follow their fate in normally developing postmetamorphic frogs and goitrogen-treated permanent larvae, diploid (2N) and triploid (3N) ventral blood island (VBI) mesoderm was exchanged between neurula stage embryos (about 16-22 hr old). Beginning at 15 days of age, half of the 2N or 3N hosts were treated with sodium perchlorate to prevent thyroxine-induced developmental changes. At larval stages 55-59 (41-48 days) and at 1-2 months postmetamorphosis (110-120 days), the untreated control chimeras and age-matched perchlorate-treated chimeras were killed for analysis of the VBI contribution to blood, spleen, and thymus populations by flow cytometry. The data suggest that grafting of ventral blood island mesoderm is an effective way to label an early larval erythrocyte population that declines after metamorphosis. In perchlorate-blocked permanent larvae this early VBI-derived erythrocyte population persists. In contrast, grafting of VBI mesoderm was less useful as a method to label a larvally distinct lymphocyte population in the thymus and spleen. At the late larval stages that we examined, the proportion of VBI-derived cells in thymus and spleen was not different from that observed after metamorphosis. Inhibition of metamorphosis interfered with the thymocyte expansion that normally occurs after metamorphosis, but the proportion of VBI-derived cells in thymus and spleen was not affected. This suggests that lymphopoiesis occurring in late larval life and after metamorphosis uses a stable persisting population of VBI-derived stem cells as well as dorsally derived stem cells.

Metamorphosis of the amphibian eye

For many metamorphosing amphibians, the visual system must remain functional as the animal changes from an aquatic to a terrestrial habitat. Thyroid hormone, the trigger for metamorphosis, brings about changes at all levels of the animal, and profoundly alters the visual system, from cellular changes within the eye to new central connections subserving the binocular vision that develops during metamorphosis in some species. I will survey the alterations in the visual system in the metamorphosis of several Amphibian groups, and consider the role of thyroid hormone in bringing about these transformations through action at the molecular level.

MHC class I antigens as surface markers of adult erythrocytes during the metamorphosis of Xenopus

An alloantiserum produced against Xenopus MHC class I antigens has been used to distinguish different erythrocyte populations at metamorphosis. By analysis using a fluorescence-activated cell sorter (FACS) analyzer, tadpole (stage 55) and adult erythrocytes have distinct volume differences and tadpole cells have no MHC antigens on the cell surface. Both tadpole and adult erythrocytes express a "mature erythrocyte" antigen marker, recognized by its monoclonal antibody (F1F6). During metamorphosis, immature erythrocytes, at various stages of differentiation, which express adult levels of cell-surface MHC antigens by 12 days after tail resorption, are found in the bloodstream. These immature cells are biosynthetically active, produce adult hemoglobin, and mature by 60 days after the completion of metamorphosis. Percoll gradient-density fractionation has shown that all of the cells in the new erythrocyte series express adult levels of MHC antigens but there is only a gradual increase in the amount of "mature erythrocyte" antigen. Tadpole erythrocytes, which are biosynthetically active during larval stages, produce small amounts of surface MHC antigens before the metamorphic climax and then become metabolically inactive. They are completely cleared from the circulation by 60 days after metamorphosis. Erythrocytes from tadpoles arrested in their development for long periods of time express intermediate levels of MHC antigens, suggesting a "leaky" expression of these molecules in the tadpole cells. The most abundant erythrocyte cell-surface proteins from tadpoles and adults, as judged by two-dimensional gel electrophoresis, are very different.

In vivo regulation of hemoglobin phenotypes of developing Rana catesbeiana

We have examined the effects of phenylhydrazine-induced anemia on the in vivo synthesis of specific hemoglobins at larval, metamorphic, and post-metamorphic stages of the bullfrog Rana catesbeiana, and have found that at all stages the animals qualitatively and quantitatively regenerate their pre-anemia hemoglobin profiles, with one exception: Animals approaching or undergoing the metamorphic hemoglobin switch synthesize only adult hemoglobin during recovery from anemia. We conclude that the ontogenetic progression of hemoglobins in R. catesbeiana is regulated at the level of differentiation of distinct erythroid cell lines, each committed to expressing a particular hemoglobin phenotype; this regulation is unperturbed by anemia.

Conserved sequences and cell-specific DNase I hypersensitive sites upstream from the co-ordinately expressed alpha I- and alpha II-globin genes of Xenopus laevis

The globin gene family of Xenopus laevis comprises pairs of closely related genes that are arranged in two clusters, each pair of genes being co-ordinately and stage-specifically expressed. To get information on putative regulatory elements, we compared the DNA sequences and the chromatin conformation 5' to the co-ordinately expressed adult alpha-globin genes. Sequence analysis revealed a relatively conserved region from the cap site up to position -289, and further upstream seven distinct boxes of homology, separated by more diverged sequences or deletions/insertions. The homology boxes comprise 22 to 194 base-pairs showing 78 to 95% homology. Analysis of chromatin conformation showed that DNase I preferentially cuts the upstream region of both genes at similar positions, 5' to the T-A-T-A and the C-C-A-A-T boxes, only in chromatin of adult erythroblasts and erythrocytes, where adult globin genes are expressed, but not in chromatin of adult liver cells or larval erythrocytes, where these genes are silent. This suggests that cell- and stage-specific activation of these genes coincides with specific changes in chromatin conformation within the proximal upstream region. No difference was found in the nucleotide sequence within the DNase I hypersensitive region proximal to the adult alpha 1-globin gene in DNA from embryonic cells, in which this gene is inactive, and adult erythrocytes, expressing this gene.

Sequence analysis of the larval beta II-globin gene of Xenopus laevis

The 1822 bp sequence of the larval Xenopus laevis beta II-globin gene is reported together with 240 bp upstream of the gene and 190 bp beyond the site of polyadenylation. The mRNA start point was determined by primer extension as well as nuclease S1 mapping and the polyadenylation site by comparison of the gene sequence to the mRNA sequence derived from a corresponding cDNA clone. Like other vertebrate globin genes, this gene comprises three exons interrupted by two intervening sequences (IVS). IVS I spans over 582 nucleotides and interrupts the exon sequences within codon 30. IVS II is located between the codons 104/105 and spans over 617 nucleotides. The 5' region of the gene contains the canonical TATAA homology at position -31. Comparison of the upstream sequence to that of Xenopus laevis larval beta I-globin gene revealed a conserved sequence, located between nucleotide positions -60 and -87, which might function as regulatory element of transcription. Whereas the upstream region of the larval beta II-globin gene does not contain a CAAT box, we notice a reiterated AAATGA motif and discuss its possible significance.

Globin gene expression in Xenopus laevis: anemia induces precocious globin transition and appearance of adult erythroblasts during metamorphosis

The expression of Xenopus laevis larval and adult globin genes after phenylhydrazine-induced anemia has been investigated at the cellular and molecular levels by means of cloned cDNA probes specific for the four main larval and the four main adult globin mRNA species. In the circulating blood of anemic metamorphic larvae there are at least two distinct populations of erythroblasts containing either larval or adult globin mRNA sequences. The cells expressing adult sequences replace those expressing larval ones at the end of metamorphosis. Under the influence of anemia the qualitative pattern of transcribed RNA species is not changed, but the larval to adult transition takes place earlier during development. This would imply that this transition is not strongly correlated with the morphological changes of metamorphosis. The abundance of the different larval globin mRNA species is similar and, as compared to control animals, not affected by the phenylhydrazine treatment. In anemic adults no reactivation of larval gene expression has been observed. The different adult globin mRNA species are present in comparable abundance, but the phenylhydrazine treatment appears to enhance expression of the alpha A II and beta A II genes, as compared to nonanemic individuals.

Two transitions of haemoglobin expression in Xenopus: from embryonic to larval and from larval to adult

Electrophoretic analyses of haemoglobin and globin phenotypes in families of Xenopus borealis and Xenopus l. laevis revealed two developmental haemoglobin transitions during ontogeny. The first transition occurs at the developmental stage when tadpoles begin to feed. It is characterized by the decline of embryonic-specific globins in favour of novel, tadpole-specific globins (X. borealis) correlated to changes in the haemoglobin pattern. We suppose that this switch results from the replacement of a primitive, ventral blood island-dependent erythrocyte population by tadpole erythrocytes from other erythropoietic sites. Several other globin chains and haemoglobins are present in both young tadpoles and throughout larval life. The second, well-known transition occurs during metamorphosis, where all tadpole haemoglobins are replaced by adult haemoglobins composed of entirely different globin chains.

The Xenopus laevis globin gene family: chromosomal arrangement and gene structure

Clones containing nine different larval and adult globin genes have been isolated from two genomic libraries of Xenopus laevis. They encompass three distinct DNA regions: a 70 kb region containing five genes in the order 5'-alpha Lla-alpha Llb-alpha Al-beta Al-beta Lla-3', all with the same transcriptional polarity; a 40 kb region with three genes, 5'-alpha Llla-alpha Lllb-alpha All-3', again with the same polarity; and a 10 kb segment comprising the beta Llla gene only. The beta All gene has not been found in our libraries. Genetic analysis has revealed two more beta L genes (beta Lllb, beta Lllb). Globin-like sequences have also been isolated, but have not been further characterized. The X. laevis globin gene family thus consists of 12 genes arranged in two clusters, each containing larval and adult alpha and beta genes in a unique, "symmetrical" arrangement. Electron microscopic analysis has revealed that Xenopus globin genes also comprise three exons and two introns, but differ from the globin genes of higher vertebrates in that the introns are much larger.

Effect of thyroid hormones on the switch from larval to adult hemoglobin synthesis in the salamander Pleurodeles waltlii

The evolution of globin chain synthesis was studied in larvae treated either with thyroxine or with an anti-thyroid substance. Thyroxine treatment accelerated the rate of Hb switching; it induced a preferential synthesis of adult globins while the synthesis of larval globins decreased rapidly. Treatment with thiourea did not prevent the Hb transition, which occurred even at concentrations of thyroid hormones that did not permit induction of anatomical metamorphosis.

Comparison of hemoglobins from the genus Xenopus (Amphibia salienta)

A survey of the electrophoretic mobility of hemoglobins of 14 species and subspecies of the genus Xenopus was undertaken. It was found that the variation between the different taxons considered was quite important in the number and the degree of the hemoglobin bands on polyacrylamide slab gels. The profiles of the hemoglobin electrophoretic pattern in tube gels gave corresponding results, confirming the variation between the different Xenopus species. The degree of resolution obtained on polyacrylamide slab gels was insufficient to allow one to distinguish between the subspecies of Xenopus laevis. Specific staining of the gels showed that the protein bands were in fact hemoglobins. From the results obtained it appears that there are relative differences in the number and mobility of hemoglobin bands between the different species, these differences being important enough to stimulate interest in further investigation in this field, but, as far as this preliminary investigation indicates, not clear-cut enough to use as a method of identification for isolated unknown species.