Hemoglobin transitions in the bullfrog, Rana catesbeiana, during spontaneous and induced metamorphosis

Insufficiency of Thyroid Hormone in Frog Metamorphosis and the Role of Glucocorticoids

Thyroid hormone (TH) is the most important hormone in frog metamorphosis, a developmental process which will not occur in the absence of TH but can be induced precociously by exogenous TH. However, such treatments including in-vitro TH treatments often do not replicate the events of natural metamorphosis in many organs, including lung, brain, blood, intestine, pancreas, tail, and skin. A potential explanation for the discrepancy between natural and TH-induced metamorphosis is the involvement of glucocorticoids (GCs). GCs are not able to advance development by themselves but can modulate the rate of developmental progress induced by TH via increased tissue sensitivity to TH. Global gene expression analyses and endocrine experiments suggest that GCs may also have direct actions required for completion of metamorphosis independent of their effects on TH signaling. Here, we provide a new review and analysis of the requirement and necessity of TH signaling in light of recent insights from gene knockout frogs. We also examine the independent and interactive roles GCs play in regulating morphological and molecular metamorphic events dependent upon TH.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Analysis of the Rana catesbeiana tadpole tail fin proteome and phosphoproteome during T3-induced apoptosis: identification of a novel type I keratin

BACKGROUND

Thyroid hormones (THs) are vital in the maintenance of homeostasis and in the control of development. One postembryonic developmental process that is principally regulated by THs is amphibian metamorphosis. This process has been intensively studied at the genomic level yet very little information at the proteomic level exists. In addition, there is increasing evidence that changes in the phosphoproteome influence TH action.

RESULTS

Here we identify components of the proteome and phosphoproteome in the tail fin that changed within 48 h of exposure of premetamorphic Rana catesbeiana tadpoles to 10 nM 3,5,3'-triiodothyronine (T3). To this end, we developed a cell and protein fractionation method combined with two-dimensional gel electrophoresis and phosphoprotein-specific staining. Altered proteins were identified using mass spectrometry (MS). We identified and cloned a novel Rana larval type I keratin, RLK I, which may be a target for caspase-mediated proteolysis upon exposure to T3. In addition, the RLK I transcript is reduced during T3-induced and natural metamorphosis which is consistent with a larval keratin. Furthermore, GILT, a protein involved in the immune system, is changed in phosphorylation state which is linked to its activation. Using a complementary MS technique for the analysis of differentially-expressed proteins, isobaric tags for relative and absolute quantitation (iTRAQ) revealed 15 additional proteins whose levels were altered upon T3 treatment. The success of identifying proteins whose levels changed upon T3 treatment with iTRAQ was enhanced through de novo sequencing of MS data and homology database searching. These proteins are involved in apoptosis, extracellular matrix structure, immune system, metabolism, mechanical function, and oxygen transport.

CONCLUSION

We have demonstrated the ability to derive proteomics-based information from a model species for postembryonic development for which no genome information is currently available. The present study identifies proteins whose levels and/or phosphorylation states are altered within 48 h of the induction of tadpole tail regression prior to overt remodeling of the tail. In particular, we have identified a novel keratin that is a target for T3-mediated changes in the tail that can serve as an indicator of early response to this hormone.

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

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.

Hemoglobin transition in the anuran Pelodytes punctatus

In the anuran Pelodytes punctatus the larval hemoglobin produces five electrophoretic bands. In the premetamorphic period, two other bands appear which are typical of the adult. They gradually substitute the larval bands completely as in other anurans. The pathway of the hemoglobin shift indicates that the synthesis of the adult fractions does not depend on the thyroid hormones for its activation, whereas the disappearance of the larval fractions depends on the destruction of the "larval" red blood cell line. As in other species of anurans which are considered primitive, the hemoglobinic change of Pelodytes punctatus starts earlier and develops slower than in other more evolved anurans. In normal developing specimens of Pelodytes punctatus the change in the hemoglobin fractions occurs very slowly and finishes 4 weeks after the metamorphosis, whereas in "hibernated" specimens the rate of change is higher and it ends earlier, probably as a consequence in the retarded larvae, of the increased sensibility of the hemopoietic tissues to the metamorphosis factors.

Tadpole erythrocytes: luminescent properties with dark field microscopy

To characterize and classify erythrocytes of ranid tadpoles, alcohol-fixed blood smears were studied with dark field illumination. All preclimax stage (limb bud-foot) Rana castesbeiana tadpoles from ponds in Massachusetts had red blood cells that were polymorphic. The majority of cells (88%) showed a bright, granular luminescence varying from white to blue-grey, whereas, cytoplasm of the other cells was smooth, black, and nonluminescent. On the other hand, tadpoles in similar stages from other species (Rana clamitans, Mass. and Rana pipens, Vermont) and from R. catesbeiana tadpoles from other locations (Wisconsin and North Carolina) had no observable cytoplasmic luminescence in any of their red blood cells. Moreover, as Mass. R. catesbeiana underwent metamorphic climax their luminescent cells disappeared and were replaced by small, round, dark, nonluminescent cells, precursors of the oval, nonluminescent erythrocytes characteristic of adult frogs. Cells with black nonluminescent cytoplasm generally contained nuclei which were luminescent. In conclusion, two main types of red blood cells-those with and those without cytoplasmic luminescence-are distinguishable with dark field microscopy. Luminescence of the cells varies with species, geographic location, and developmental stage of the tadpoles.

Globin gene expression in erythroid cell lines during larval development of Pleurodeles waltlii

We have attempted to determine whether in Pleurodeles ontogenesis there exists a close relationship between the two following characteristics: change from primitive to definitive erythroid cell populations, which parallels the change of major erythropoietic site; change in the type of synthesized hemoglobin, larval or adult. The origin of red blood cells was investigated by embryonic grafts of hemopoietic anlage from 2n to 4n embryos. The larval or adult hemoglobin type was characterized by immunofluorescence by using specific antibodies. Our results show that in Pleurodeles, blood island-originating red blood cells and spleen-originating red blood cells are both able to synthesize either Hb L or Hb A at a given time, but in separate cells.

Heme oxygenase activity in liver microsomes of the bullfrog, Rana catesbeiana, and its change during the tadpole development

Heme oxygenase in the liver microsomes of tadpole and adult bullfrog, Rana catesbeiana, was studied in relation to hemoglobin metabolism during tadpole development. The results obtained are as follows. 1. The specific activity of the enzyme of premetamorphic tadpole liver was comparable to that of adult frog liver. The apparent Km for methemalbumin was about 25 microM for both tadpole and frog enzymes. 2. The enzyme activity was stimulated in vivo by the injection of methemalbumin, phenylhydrazine and triiodothyronine to the animals. 3. A marked increase in the enzyme activity was found in the liver of tadpole during prometamorphic stage and metamorphic climax.

Developmental time of the hemoglobin transition in the anuran Bombina orientalis

The electrophoretic pattern of the larval hemoglobin of the anuran Bombina orientalis presents two bands. In premetamorphic period gradually appear four other bands, corresponding to those of the adult hemoglobin; they substitute, within 15-16 days after metamorphosis, the larval pattern. The percentage of larval and adult fractions during the development is shown. In Bombina orientalis the change of the hemoglobinic fractions from larval to adult type is total as in most anurans. But it starts earlier than in other species studied and develops slower than most of the other species.

Enzyme clusters during the metamorphic period of Ambystoma mexicanum: role of thyroid hormone

Enzyme activities and DNA content have been measure in axolotl liver during the metamorphic period (4-8 months after spawning). Three different types of enzyme activity profiles were observed. In the type I profile (carbamoyl-phosphate synthase, arginase, ornithine transcarbamoylase, and glutamate dehydrogenase) enzyme activity is high in the youngest animals studied, and shows a minimum at 5 months followed by a maximum at 8 months of age. Thereafter activities do not change or slightly decrease. In the type II profile (tyrosine aminotransferase, glucose-6-phosphatase) enzyme activity shows a peak at 5 months of age and is low thereafter. Hexokinase, the enzyme with a type III profile, shows high activity throughout the metamorphic period. DNA content remains high throughout the metamorphic period but decreases 50% between 9 and 12 months of age, probably due to an increase in the size of the hepatocytes. No glucokinase activity was detected. High activities of cluster II enzymes represent early metamorphic events, while the rising part of cluster I is associated with late metamorphic events. The apparent molecular specific activity increases during natural development between 5 and 9 months of age, or precociously, upon thyroid hormone treatment. This change in apparent molecular specific activity is correlated to the advent of ureotelism.

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.

Changes in the density of circulating erythrocytes of the bullfrog tadpole, Rana catesbeiana, in relation to the transition of hemoglobin during metamorphosis

1. The circulating erythrocytes of Rana catesbeiana tadpoles at varying metamorphic stages were studied by the phthalate-microcapillary technique of Danon & Marikovsky (1964 J. Lab. Clin. Med. 64, 668-674) and by the method of Murphy (1973 J. Lab. Clin. Med. 82, 334-341), which are used for the separation of erythrocytes according to the density. 2. The erythrocytes of tadpoles were different in the mean density from those of adult frogs. 3. At the metamorphic climax, aged erythrocytes having the tadpole hemoglobin coexisted with young erythrocytes having the frog hemoglobin in the circulating blood. 4. The frog hemoglobin was preferentially synthesized during metamorphosis. 5. All the results suggested that the transition of hemoglobin during metamorphosis was associated with the replacement of erythrocyte population.

Metamorphic variations in the hemoglobins of Bombina variegata (L.)

1. The electrophoretic pattern of the anuran Bombina variegata presents one band only until the end of premetamorphosis. Afterwards three new bands appear which substitute the larval band within 15-18 days of metamorphosis. 2. The gradual change of larval and adult hemoglobin, correlated with the development, has been reported in a diagram. 3. In Bombina variegata the hemoglobin change is total, like in most amphibians, but starts earlier and develops slower than in most other species.

A quantitative analysis of plasma osmotic pressure during metamorphosis of the bullfrog, Rana catesbeiana

The plasma constituents contributing to osmotic pressure are, in decreasing order: Na+, Cl-, HCO3-, K+, glucose, amino acids, urea and protein. Plasma osmotic pressure increases from 180 mmoles/1 to 200 mmoles/1 throughout development.

Studies of the development of frog hemopoietic tissue in vitro. I. Spleen culture assay of an erythropoietic factor in anemic frog blood

A new in vitro technique has been described for demonstrating the presence of an erythropoietic factor in the circulating blood of frogs. The assay system consisted of MC33 medium, erythropoietically active spleen cells from Rana pipiens, and plasma or serum from frogs made anemic via phenylhydrazine or bleeding. The spleen cells, which remain erythropoietically active for up to nine days, were found to incorporate 59Fe, [3H]thymidine, [3H]uridine, and [3H]leucine at a greater rate in the presence of plasma or serum from anemic versus normal frogs. The hormones triiodothyronine, prolactin, and erythropoietin were not effective in eliciting an hemopoietic response. The data presented suggest that the spleen from that adult frog is a major site of erythroid differentiation and maturation.

Differentiation of red blood cells in vitro

Differentiation of red blood cells occurs in organ cultures of both livers and kidney tissue from tadpoles of the bullfrog Rana catesbeiana. Our evidence indicates that different red blood cell lines are produced by the two tissues and that these different cell lines contain different tadpole hemoglobins.