Sequential up-regulation of thyroid hormone beta receptor, ornithine transcarbamylase, and carbamyl phosphate synthetase mRNAs in the liver of Rana catesbeiana tadpoles during spontaneous and thyroid hormone-induced metamorphosis

Utilization of temperature-mediated activation of thyroid hormone-induced molecular memory to evaluate early signaling events in the olfactory epithelium of Rana [Lithobates] catesbeiana tadpoles

The amphibian olfactory system is highly distinct between aquatic tadpole and terrestrial frog life stages and therefore must remodel extensively during thyroid hormone (TH)-dependent metamorphosis. Developmentally appropriate functioning of the olfactory epithelium is critical for survival. Previous studies in other Rana [Lithobates] catesbeiana premetamorphic tadpole tissues showed that initiation of TH-induced metamorphosis can be uncoupled from execution of TH-dependent programs by holding tadpoles in the cold rather than at warmer permissive temperatures. TH-exposed tadpoles at the nonpermissive (5 °C) temperature do not undergo metamorphosis but retain a "molecular memory" of TH exposure that is activated upon shift to a permissive warm temperature. Herein, premetamorphic tadpoles were held at permissive (24 °C) or nonpermissive (5 °C) temperatures and injected with 10 pmoles/g body weight 3,5,3'-triiodothyronine (T3) or solvent control. Olfactory epithelium was collected at 48 h post-injection. RNA-sequencing (RNA-Seq) and reverse transcriptase quantitative real-time polymerase chain reaction (RT-qPCR) analyses generated differentially expressed transcript profiles of 4328 and 54 contigs for permissive and nonpermissive temperatures, respectively. Translation, rRNA, spliceosome, and proteolytic processes gene ontologies were enriched by T3 treatment at 24 °C while negative regulation of cell proliferation was enriched by T3 at 5 °C. Of note, as found in other tissues, TH-induced basic leucine zipper-containing protein-encoding transcript, thibz, was significantly induced by T3 at both temperatures, suggesting a role in the establishment of molecular memory in the olfactory epithelium. The current study provides critical insights by deconstructing early TH-induced induction of postembryonic processes that may be targets for disruption by environmental contaminants.

Crosstalk between Thyroid Hormone and Corticosteroid Signaling Targets Cell Proliferation in Xenopus tropicalis Tadpole Liver

Thyroid hormones (TH) and glucocorticoids (GC) are involved in numerous developmental and physiological processes. The effects of individual hormones are well documented, but little is known about the joint actions of the two hormones. To decipher the crosstalk between these two hormonal pathways, we conducted a transcriptional analysis of genes regulated by TH, GC, or both hormones together in liver of Xenopus tropicalis tadpoles using RNA-Seq. Among the differentially expressed genes (DE), 70.5% were regulated by TH only, 0.87% by GC only, and 15% by crosstalk between the two hormones. Gene ontology analysis of the crosstalk-regulated genes identified terms referring to DNA replication, DNA repair, and cell-cycle regulation. Biological network analysis identified groups of genes targeted by the hormonal crosstalk and corroborated the gene ontology analysis. Specifically, we found two groups of functionally linked genes (chains) mainly composed of crosstalk-regulated hubs (highly interactive genes), and a large subnetwork centred around the crosstalk-regulated genes psmb6 and cdc7. Most of the genes in the chains are involved in cell-cycle regulation, as are psmb6 and cdc7, which regulate the G2/M transition. Thus, the biological action of these two hormonal pathways acting together in the liver targets cell-cycle regulation.

Developmental gene expression patterns in the brain and liver of Xenopus tropicalis during metamorphosis climax

Thyroid hormones (THs) induce metamorphosis in amphibians, causing dynamic changes, whereas mammalian newborns undergo environmental transition from placenta to open air at birth. The similarity between amphibian metamorphosis and the mammalian perinatal periods has been repeatedly discussed. However, a corresponding developmental gene expression analysis has not yet been reported. In this study, we examined the developmental gene expression profiles in the brain and liver of Xenopus tropicalis during metamorphosis climax and compared them to the respective gene expression profiles of newborn rodents. Many upregulated genes identified in the tadpole brain during metamorphosis are also upregulated in the rodent brain during the first three postnatal weeks when the TH surge occurs. The upregulation of some genes in the brain was inhibited in thyroid hormone receptor α (TRα) knockout tadpoles but not in TRβ-knockout tadpoles, implying that brain metamorphosis is mainly mediated by TRα. The expression of some genes was also increased in the liver during metamorphosis climax. Our data suggest that the rodent brain undergoes TH-dependent remodeling during the first three postnatal weeks as observed in X. tropicalis during the larva-to-adult metamorphosis.

Behavioral and molecular analyses of olfaction-mediated avoidance responses of Rana (Lithobates) catesbeiana tadpoles: Sensitivity to thyroid hormones, estrogen, and treated municipal wastewater effluent

Olfaction is critical for survival, facilitating predator avoidance and food location. The nature of the olfactory system changes during amphibian metamorphosis as the aquatic herbivorous tadpole transitions to a terrestrial, carnivorous frog. Metamorphosis is principally dependent on the action of thyroid hormones (THs), l-thyroxine (T₄) and 3,5,3'-triiodothyronine (T₃), yet little is known about their influence on olfaction during this phase of postembryonic development. We exposed Taylor Kollros stage I-XIII Rana (Lithobates) catesbeiana tadpoles to physiological concentrations of T₄, T₃, or 17-beta-estradiol (E₂) for 48h and evaluated a predator cue avoidance response. The avoidance response in T₃-exposed tadpoles was abolished while T₄- or E₂-exposed tadpoles were unaffected compared to control tadpoles. qPCR analyses on classic TH-response gene transcripts (thra, thrb, and thibz) in the olfactory epithelium demonstrated that, while both THs produced molecular responses, T₃ elicited greater responses than T₄. Municipal wastewater feed stock was spiked with a defined pharmaceutical and personal care product (PPCP) cocktail and treated with an anaerobic membrane bioreactor (AnMBR). Despite substantially reduced PPCP levels, exposure to this effluent abolished avoidance behavior relative to AnMBR effluent whose feed stock was spiked with vehicle. Thibz transcript levels increased upon exposure to either effluent indicating TH mimic activity. The present work is the first to demonstrate differential TH responsiveness of the frog tadpole olfactory system with both behavioral and molecular alterations. A systems-based analysis is warranted to further elucidate the mechanism of action on the olfactory epithelium and identify further molecular bioindicators linked to behavioral response disruption.

Xenopus metamorphosis as a model to study thyroid hormone receptor function during vertebrate developmental transitions

A hormone-dependent developmental transition from aquatic to terrestrial existence occurs in all tetrapod vertebrates, such as birth, hatching, and metamorphosis. Thyroid hormones (TH) and their receptors (TRs) are key players in the tissue transformations comprising vertebrate developmental transitions. The African clawed frog, Xenopus, is a premier model for the role of TRs in developmental transitions because of the numerous and dramatic TH-dependent tissue transformations during metamorphosis and because of the endocrine, molecular, and genomic resources available. TRs are nuclear receptors that repress TH-response genes when plasma TH is minimal and that activate those same genes to induce tissue-specific gene regulation cascades when TH plasma levels increase. Tissue-specific TR expression levels help determine tissue sensitivity and responsivity to TH thereby regulating the initiation and rate of developmental change in TH-sensitive tissues which govern the tissue developmental asynchrony observed during metamorphosis. This review highlighting Xenopus presents the key experimental findings underpinning the roles TRs play in control of vertebrate developmental transitions.

Xenopus Tadpole Tissue Harvest

The procedures described here apply to Xenopus tadpoles from the beginning of feeding through the major changes of metamorphosis and are appropriate for downstream postoperative snap freezing for molecular analysis, fixation for histological analysis, and sterile organ culture. To the uninitiated, the most difficult aspects of tadpole tissue dissections are likely knowing the appearance and location of organs, and the difficulty manipulating and holding tadpoles in place to carry out the oftentimes fine and precise dissections. Therefore, images and stepwise instructions are given for the harvest of external organs (tail, head, eyes, tail skin, back skin, gills, thymus, hind limbs, forelimbs) and peritoneal organs (intestine, pancreas, liver, spleen, lungs, fat bodies, kidney/gonad complex), as well as brain, heart, and blood. Dissections are typically done under a dissection stereomicroscope, and two pairs of fine straight forceps, one pair of fine curved forceps, and one pair of microdissection scissors are sufficient for most tissue harvests.

Frogs model man: In vivo thyroid hormone signaling during development

Thyroid hormone (TH) signaling comprises TH transport across cell membranes, metabolism by deiodinases, and molecular mechanisms of gene regulation. Proper TH signaling is essential for normal perinatal development, most notably for neurogenesis and fetal growth. Knowledge of perinatal TH endocrinology needs improvement to provide better treatments for premature infants and endocrine diseases during gestation and to counteract effects of endocrine disrupting chemicals. Studies in amphibians have provided major insights to understand in vivo mechanisms of TH signaling. The frog model boasts dramatic TH-dependent changes directly observable in free-living tadpoles with precise and easy experimental control of the TH response at developmental stages comparable to fetal stages in mammals. The hormones, their receptors, molecular mechanisms, and developmental roles of TH signaling are conserved to a high degree in humans and amphibians, such that with respect to developmental TH signaling "frogs are just little people that hop." The frog model is exceptionally illustrative of fundamental molecular mechanisms of in vivo TH action involving TH receptors, transcriptional cofactors, and chromatin remodeling. This review highlights the current need, recent successes, and future prospects using amphibians as a model to elucidate molecular mechanisms and functional roles of TH signaling during post-embryonic development.

Regulation of thyroid hormone-induced development in vivo by thyroid hormone transporters and cytosolic binding proteins

Differential tissue sensitivity/responsivity to hormones can explain developmental asynchrony among hormone-dependent events despite equivalent exposure of each tissue to circulating hormone levels. A dramatic vertebrate example is during frog metamorphosis, where transformation of the hind limb, brain, intestine, liver, and tail are completely dependent on thyroid hormone (TH) but occurs asynchronously during development. TH transporters (THTs) and cytosolic TH binding proteins (CTHBPs) have been proposed to affect the timing of tissue transformation based on expression profiles and in vitro studies, but they have not been previously tested in vivo. We used a combination of expression pattern, relative expression level, and in vivo functional analysis to evaluate the potential for THTs (LAT1, OATP1c1, and MCT8) and CTHBPs (PKM2, CRYM, and ALDH1) to control the timing of TH-dependent development. Quantitative PCR analysis revealed complex expression profiles of THTs and CTHBPs with respect to developmental stage, tissue, and TH receptor β (TRβ) expression. For some tissues, the timing of tissue transformation was associated with a peak in the expression of some THTs or CTHBPs. An in vivo overexpression assay by tail muscle injection showed LAT1, PKM2, and CRYM increased TH-dependent tail muscle cell disappearance. Co-overexpression of MCT8 and CRYM had a synergistic effect on cell disappearance. Our data show that each tissue examined has a unique developmental expression profile of THTs and CTHBPs and provide direct in vivo evidence that the ones tested are capable of affecting the timing of developmental responses to TH.

Influence of temperature on thyroid hormone signaling and endocrine disruptor action in Rana (Lithobates) catesbeiana tadpoles

Thyroid hormones (THs) are essential for normal growth, development, and metabolic control in vertebrates. Their absolute requirement during amphibian metamorphosis provides a powerful means to detect and assess the impact of environmental contaminants on TH signaling in the field and laboratory. As poikilotherms, frogs can experience considerable temperature fluctuations. Previous work demonstrated that low temperature prevents precocious TH-dependent induction of metamorphosis. However, a shift to a permissive higher temperature allows resumption of the induced metamorphic program regardless of whether or not TH remains. We investigated the impact of temperature on the TH-induced gene expression programs of premetamorphic Rana (Lithobates) catesbeiana tadpoles following a single injection of 10pmol/g body wet weight 3,3',5-triiodothyronine (T3). Abundance profiles of several T3-responsive mRNAs in liver, brain, lung, back skin, and tail fin were characterized under permissive (24°C), nonpermissive (5°C), or temperature shift (5-24°C) conditions. While responsiveness to T3 was retained to varying degrees at nonpermissive temperature, T3 modulation of thibz occurred in all tissues at 5°C suggesting an important role for this transcription factor in initiation of T3-dependent gene expression programs. Low temperature immersion of tadpoles in water containing 10nM T3 and the nonsteroidal anti-inflammatory drug, ibuprofen, or the antimicrobial agent, triclosan, perturbed some aspects of the gene expression programs of tail fin and back skin that was only evident upon temperature shift. Such temporal uncoupling of chemical exposure and resultant biological effects in developing frogs necessitates a careful evaluation of environmental temperature influence in environmental monitoring programs.

Chronic sublethal exposure to silver nanoparticles disrupts thyroid hormone signaling during Xenopus laevis metamorphosis

Nanoparticles (NPs) are engineered in the nanoscale (<100 nm) to have unique physico-chemical properties from their bulk counterparts. Nanosilver particles (AgNPs) are the most prevalent NPs in consumer products due to their strong antimicrobial action. While AgNP toxicity at high concentrations has been thoroughly investigated, the sublethal effects at or below regulatory guidelines are relatively unknown. Amphibian metamorphosis is mediated by thyroid hormone (TH), and initial studies with bullfrogs (Rana catesbeiana) indicate that low concentrations of AgNPs disrupt TH-dependent responses in premetamorphic tadpole tailfin tissue. The present study examined the effects of low, non-lethal, environmentally-relevant AgNP concentrations (0.018, 0.18 or 1.8 μg/L Ag; ∼10 nm particle size) on naturally metamorphosing Xenopus laevis tadpoles in two-28 day chronic exposures beginning with either pre- or prometamorphic developmental stages. Asymmetric flow field flow fractionation with online inductively coupled plasma mass spectrometry and nanoparticle tracking analysis indicated a mixture of single AgNPs with homo-agglomerates in the exposure water with a significant portion (∼30-40%) found as dissolved Ag. Tadpoles bioaccumulated AgNPs and displayed transient alterations in snout/vent and hindlimb length with AgNP exposure. Using MAGEX microarray and quantitative real time polymerase chain reaction transcript analyses, AgNP-induced disruption of five TH-responsive targets was observed. The increased mRNA abundance of two peroxidase genes by AgNP exposure suggests the presence of reactive oxygen species even at low, environmentally-relevant concentrations. Furthermore, differential responsiveness to AgNPs was observed at each developmental stage. Therefore, low concentrations of AgNPs had developmental stage-specific endocrine disrupting effects during TH-dependent metamorphosis.

Effects of acute exposure to the non-steroidal anti-inflammatory drug ibuprofen on the developing North American Bullfrog (Rana catesbeiana) tadpole

A variety of pharmaceutical chemicals can represent constituents of municipal effluent outflows that are dispersed into aquatic receiving environments worldwide. Increasingly, there is concern as to the potential of such bioactive substances to interact with wildlife species at sensitive life stages and affect their biology. Using a combination of DNA microarray, quantitative real-time polymerase chain reaction, and quantitative nuclease protection assays, we assessed the ability of sub-lethal and environmentally relevant concentrations of ibuprofen (IBF), a non-steroidal anti-inflammatory agent and prevalent environmental contaminant, to function as a disruptor of endocrine-mediated post-embryonic development of the frog. While the LC50 of IBF for pre-metamorphic Rana catesbeiana tadpoles is 41.5 mg/L (95% confidence interval: 32.3-53.5 mg/L), exposure to concentrations in the ppb range elicited molecular responses both in vivo and in organ culture. A nominal concentration of 15 μg/L IBF (actual = 13.7 μg/L) altered the abundance of 26 mRNA transcripts within the liver of exposed pre-metamorphic R. catesbeiana tadpoles within 6 d. IBF-treated animals demonstrated subsequent disruption of thyroid hormone-mediated reprogramming in the liver transcriptome affecting constituents of several metabolic, developmental, and signaling pathways. Cultured tadpole tail fin treated with IBF for 48 h also demonstrated altered mRNA levels at drug concentrations as low as 1.5 μg/L. These observations raise the possibility that IBF may alter the post-embryonic development of anuran species in freshwater environs, where IBF is a persistent or seasonal pollutant.

Enabling comparative gene expression studies of thyroid hormone action through the development of a flexible real-time quantitative PCR assay for use across multiple anuran indicator and sentinel species

Studies performed across diverse frog species have made substantial contributions to our understanding of basic vertebrate development and the natural or anthropogenic environmental factors impacting sensitive life stages. Because, anurans are developmental models, provide ecosystems services, and act as sentinels for the identification of environmental chemical contaminants that interfere with thyroid hormone (TH) action during postembryonic development, there is demand for flexible assessment techniques that can be applied to multiple species. As part of the "thyroid assays across indicator and sentinel species" (TAXISS) initiative, we have designed and validated a series of cross-species real time quantitative PCR (qPCR) primer sets that provide information on transcriptome components in evolutionarily distant anurans. Validation for fifteen gene transcripts involved a rigorous three-tiered quality control within tissue/development-specific contexts. Assay performance was confirmed on multiple tissues (tail fin, liver, brain, and intestine) of Rana catesbeiana and Xenopus laevis tadpoles enabling comparisons between tissues and generation of response profiles to exogenous TH. This revealed notable differences in TH-responsive gene transcripts including thra, thrb, thibz, klf9, col1a2, fn1, plp1, mmp2, timm50, otc, and dio2, suggesting differential regulation and susceptibility to contaminant effects. Evidence for the applicability of the TAXISS anuran qPCR assay across seven other species is also provided with five frog families represented and its utility in defining genome structure was demonstrated. This novel validated approach will enable meaningful comparative studies between frog species and aid in extending knowledge of developmental regulatory pathways and the impact of environmental factors on TH signaling in frog species for which little or no genetic information is currently available.

Unliganded thyroid hormone receptor regulates metamorphic timing via the recruitment of histone deacetylase complexes

Anuran metamorphosis involves a complex series of tissue transformations that change an aquatic tadpole to a terrestrial frog and resembles the postembryonic perinatal period in mammals. Thyroid hormone (TH) plays a causative role in amphibian metamorphosis and its effect is mediated by TH receptors (TRs). Molecular analyses during Xenopus development have shown that unliganded TR recruits histone deacetylase (HDAC)-containing N-CoR/SMRT complexes and causes histone deacetylation at target genes while liganded TR leads to increased histone acetylations and altered histone methylations at target genes. Transgenic studies involving mutant TR-cofactors have shown that corepressor recruitment by unliganded TR is required to ensure proper timing of the onset of metamorphosis while coactivator levels influence the rate of metamorphic progression. In addition, a number of factors that can influence cellular free TH levels appear to contribute the timing of metamorphic transformations of different organs by regulating the levels of unliganded vs. liganded TR in an organ-specific manner. Thus, the recruitment of HDAC-containing corepressor complexes by unliganded TR likely controls both the timing of the initiation of metamorphosis and the temporal regulation of organ-specific transformations. Similar mechanisms likely mediate TR function in mammals as the maturation of many organs during postembryonic development is dependent upon TH and resembles organ metamorphosis in amphibians.

Mechanisms of thyroid hormone receptor action during development: lessons from amphibian studies

BACKGROUND

Thyroid hormone (TH) receptor (TR) plays critical roles in vertebrate development. However, the in vivo mechanism of TR action remains poorly explored.

SCOPE OF REVIEW

Frog metamorphosis is controlled by TH and mimics the postembryonic period in mammals when high levels of TH are also required. We review here some of the findings on the developmental functions of TH and TR and the associated mechanisms obtained from this model system.

MAJOR CONCLUSION

A dual function model for TR in Anuran development was proposed over a decade ago. That is, unliganded TR recruits corepressors to TH response genes in premetamorphic tadpoles to repress these genes and prevent premature metamorphic changes. Subsequently, when TH becomes available, liganded TR recruits coactivators to activate these same genes, leading to metamorphic changes. Over the years, molecular and genetic approaches have provided strong support for this model. Specifically, it has been shown that unliganded TR recruits histone deacetylase containing corepressor complexes during larval stages to control metamorphic timing, while liganded TR recruits multiple histone modifying and chromatin remodeling coactivator complexes during metamorphosis. These complexes can alter chromatin structure via nucleosome position alterations or eviction and histone modifications to contribute to the recruitment of transcriptional machinery and gene activation.

GENERAL SIGNIFICANCE

The molecular mechanisms of TR action in vivo as revealed from studies on amphibian metamorphosis are very likely applicable to mammalian development as well. These findings provide a new perspective for understanding the diverse effects of TH in normal physiology and diseases caused by TH dysfunction. This article is part of a Special Issue entitled Thyroid hormone signalling.

In vitro thyroid disrupting effects of organic extracts from WWTPs in Beijing

It is generally known that there are many endocrine disrupting compounds (EDCs) in the effluents from wastewater treatment plants (WWTPs). Most research has focused on the occurrence of estrogenic or androgenic activities, while ignoring that there are environmental chemicals disrupting thyroid system, which is essential for growth and development in both humans and animals. In the present work, a two-hybrid yeast assay was conducted to evaluate the removal efficiencies of agonistic or antagonistic thyroid receptor (TR) mediated effects in different treatment processes of three WWTPs located in Beijing. We found no TR agonistic, but TR antagonistic activities in all processes from the WWTPs. The TR antagonistic activities in organic extracts of water samples were then calibrated regarding to a known TR-inhibitor, amiodarone hydrochloride (AH). The observed concentration of TR disrupting substances ranged from 2.35 x 10(-8) to 6.19 x 10(-7) mol/L AH in Gaobeidian WWTP, 3.76 x 10(-8) to 8.75 x 10(-8) mol/L AH in Lugouqiao WWTP, and 4.80 x 10(-9) to 2.55 x 10(-8) mol/L AH in Beixiaohe WWTP. Of the three WWTPs, the removal rates were 92.7%, 42.2%, and 23.1% respectively. Industrial sewage may contain more TR disrupting substances compared with domestic sewage. The recipient waters were found to contain considerable concentrations of TR disrupting substances that may cause adverse effects on the exposed organisms.

Dibutyl phthalate contributes to the thyroid receptor antagonistic activity in drinking water processes

It has long been recognized that thyroid hormone (TH) is essential for normal brain development in both humans and animals, and there is growing evidence that environmental chemicals can disrupt the thyroid system. In the present work, we used a two-hybrid yeast assay to screen for agonistic or antagonistic thyroid receptor (TR) mediated effects in drinking waters. We found no TR agonistic, but TR antagonistic activities in all samples from the drinking water processes. The TR antagonistic activities in organic extracts of water samples were then calibrated regarding to a known TR-inhibitor, NH3, and were expressed as the NH3 equivalents (TEQbio). The observed TEQbio in waters ranged from 180.8+/-24.8 to 280.2+/-48.2 microg/L NH3. To identify the specific compounds responsible for TR disrupting activities, the concentrations of potentially thyroid-disrupting chemicals including organochlorine pesticides (OCPs), phenols, and phthalates in organic extracts were quantitatively determined and their toxic equivalents with respect to NH3 (TEQcal) were estimated from their concentration-dependent relationships, respectively, using the same set of bioassays. Based on the TEQ approach, it was revealed that dibutyl phthalate (DBP) accounted for 53.7+/-8.2% to 105.5+/-16.7% of TEQbio. There was no effective removal of these potential thyroid disrupting substances throughout drinking water treatment processes.

Peering into molecular mechanisms of action with frogSCOPE

Exposure of critical life stages to harmful chemicals at low, environmentally-relevant concentrations can alter how hormones function, and change metabolic pathways or developmental processes that impact reproduction, behavior, or susceptibility to disease later in life. These alterations can be captured through evaluation of changes to transcriptomes, proteomes, and metabolomes occurring at those critical life stages thereby enabling more effective and earlier identification of mechanism of action, individual susceptibilities and adaptation, and prediction of detrimental sublethal effects. Amphibians are "wet canaries in the coalmine" as indicators for environmental health. There are more than 6000 species living in a variety of ecological niches worldwide yet limited 'omics resources and approaches exist. To provide for a means of addressing this challenge, frogSCOPE (frog Sentinel species Comparative "Omics" for the Environment) combines transcriptomics, proteomics, and metabolomics together to form the foundation for the identification of biological response indicators of harmful effects on a species of wild frog (Rana catesbeiana) at a sensitive tadpole stage. Various exposure and sampling methodologies are possible including standard in vivo exposures, tail fin biopsies, and the C-fin assay. frogSCOPE establishes methodological and analytical approaches applicable to wildlife by using a uniquely-designed frog cDNA array developed to accommodate cross-species hybridization and quantitative real-time polymerase chain reaction (QPCR) assays on poorly genetically-characterized wildlife species. Combination with proteomics (isobaric tags for relative and absolute protein quantitation; iTRAQ) and metabolomics (mass spectrometry) enable the generation of molecular fingerprints to identify mechanisms of action in a more comprehensive fashion to better define suitable indicators of deleterious biological outcomes to wildlife.

Amphibians and agricultural chemicals: review of the risks in a complex environment

Agricultural landscapes, although often highly altered in nature, provide habitat for many species of amphibian. However, the persistence and health of amphibian populations are likely to be compromised by the escalating use of pesticides and other agricultural chemicals. This review examines some of the issues relating to exposure of amphibian populations to these chemicals and places emphasis on mechanisms of toxicity. Several mechanisms are highlighted, including those that may disrupt thyroid activity, retinoid pathways, and sexual differentiation. Special emphasis is also placed on the various interactions that may occur between different agro-chemicals and between chemicals and other environmental factors. We also examine the indirect effects on amphibian populations that occur when their surrounding pond communities are altered by chemicals.

Characterization of vtg-1 mRNA expression during ontogeny in Oreochromis mossambicus (PETERS)

The yolk-precursor lipoprotein, vitellogenin (VTG) has been widely recognized as a biomarker for the detection of estrogenic activity in water-borne chemical pollutants. We characterized the expression status of this important constituent of reproduction in the Mozambique tilapia (Oreochromis mossambicus), a tilapiine freshwater fish species indigenous to Southern Africa, and investigated its utility in detection of exposure to estrogen using a quantitative real-time polymerase chain reaction (QPCR) assay. We initially isolated a 3kb upstream promoter region of the vtg gene and identified putative binding sites for several regulatory factors including estrogen receptor (ESR). Evidence for the expression of several splice-site vtg mRNA variants was found in a number of tissue types. A quantitative real-time polymerase chain reaction (QPCR) assay was subsequently developed based upon a specific primer pair (OMV6/9) that selectively amplified the liver-enriched transcript. The level of this transcript in liver tissue was high in females and lower, but detectable, in males and was significantly increased in male fish following laboratory exposure to 17beta-estradiol (E(2)). This study further established that juvenile whole body homogenates (WBHs) contained extremely low levels of liver-specific vtg mRNA between 5 and 110 days post-fertilization (dpf) compared to adult male liver. Subsequent exposure of 20 dpf juveniles to E(2) showed a substantial increase in this transcript within hours, and when compared to classic male model under same conditions, the juveniles were remarkably more sensitive. We therefore conclude that the quantification, using QPCR methodology, of vtg mRNA expression in 20 dpf O. mossambicus juveniles has promise for assessing estrogenic EDC activity in aquatic sources.

Developmental regulation and function of thyroid hormone receptors and 9-cis retinoic acid receptors during Xenopus tropicalis metamorphosis

Amphibian metamorphosis serves as an excellent model to study T3 function during postembryonic development in vertebrate due to its total dependence on T3. Earlier molecular studies in the model species Xenopus laevis have led to a number of important in vivo findings on the function and mechanisms of T3 receptor (TR) action during vertebrate development. However, the lack of genomic sequence information, its tetraploid genome, and lengthy developmental cycle hinder further analyses on TR functions. In this regard, the highly related species, Xenopus tropicalis, is much more advantageous. Toward developing X. tropicalis for genome-wide and genetic studies of TR function, we analyzed the expression profiles of TRs and their heterodimerization partners, retinoid X receptors (RXRs) or 9-cis retinoic acid receptors. We show that their expression correlates with transformations in different organs and that TR/RXR heterodimers are capable of repressing and activating gene expression in vivo in the absence and presence of T3, respectively. We further demonstrate that TRs are bound to endogenous target genes in X. tropicalis tadpoles. Our results thus support a role of TRs in mediating the metamorphic effects of T3 in X. tropicalis. More importantly, the similarities in the expression and function between X. tropicalis and X. laevis TRs and RXRs as demonstrated by our study also pave the way to take advantages of existing morphological, molecular, and cellular knowledge of X. laevis development and the genetic and sequence superiority of X. tropicalis to dissect the molecular pathways governing tissue/organ-specific transformations during vertebrate postembryonic development.

Decreased cyclin-dependent kinase activity promotes thyroid hormone-dependent tail regression in Rana catesbeiana

The thyroid hormone (TH), 3,5,3'-triiodothyronine (T(3)), is an important regulator of diverse cellular processes including cell proliferation, differentiation, and apoptosis, with increasing evidence that the modulation of the phosphoproteome is an important factor in the TH-mediated response. However, little is understood regarding the mechanisms whereby phosphorylation may contribute to T(3)-mediated cellular outcomes during development. The cyclin-dependent kinases (Cdks) and mitogen-activated protein kinases (MAPK/ERK) have been implicated in TH signaling in mammalian cells. In this study, we have investigated, in frogs, the possible role that these kinases may have in the promotion of tail regression during tadpole metamorphosis, an important postembryonic process that is completely TH-dependent. Cdk2 steady state levels and activity increase in the tail concurrent with progression through the growth phase of metamorphosis, followed by a precipitous decrease coinciding with tail regression. Cyclin-A-associated kinase activity also follows a similar trend except that its associated kinase activity is maintained longer before a decrease in activity. Protein steady state levels of ERK1 and ERK2 remain relatively constant, and their kinase activities do not decrease until much later during tail regression. Tail tips cultured in serum-free medium in the presence of T(3) undergo regression, which is accelerated by coincubation with a specific Cdk2 inhibitor. Coincubation with PD098059, a MAPK inhibitor, has no effect. Thus, T(3)-dependent tail regression does not require MAPKs, but a decrease in Cdk2 activity promotes tail regression.

Genistein prevents thyroid hormone-dependent tail regression ofRana catesbeiana tadpoles by targetting protein kinase C and thyroid hormone receptor α

Thyroid hormone (TH)-regulated gene expression is mainly mediated by TH binding to nuclear thyroid hormone receptors (TRs). Despite extensive studies in mammalian cell lines that show that phosphorylation signaling pathways are important in TH action, little is known about their roles on TH signaling in vivo during development. Anuran metamorphosis is a postembryonic process that is absolutely dependent upon TH and tadpole tail resorption can be precociously induced by exogenous administration of 3,5,3'-triiodothyronine (T(3)). We demonstrate that genistein (a major isoflavone in soy products and tyrosine kinase inhibitor) and the PKC inhibitor (H7) prevent T(3)-induced regression of the Rana catesbeiana tadpole tail. T(3)-induced protein kinase C tyrosine phosphorylation and kinase activity are inhibited by genistein while T(3)-induced up-regulation of TRbeta mRNA, but not TRalpha mRNA, is significantly attenuated, most likely through inhibition of T(3)-dependent phosphorylation of the TRalpha protein. This phosphorylation may be modulated through PKC. These data demonstrate that T(3) signaling in the context of normal cells in vivo includes phosphorylation as an important factor in establishing T(3)-dependent tail regression during development.

Isolation of the alligator (Alligator mississippiensis) thyroid hormone receptor alpha and beta transcripts and their responsiveness to thyroid stimulating hormone

Thyroid hormones (THs) play key regulatory roles in growth, development and metabolism in vertebrates. Modulation of the cellular hormonal response is largely through the activity of two nuclear TH receptors, TRalpha and TRbeta, which act as transcription factors and alter gene expression programs. Little information is available regarding their structure and regulation in reptiles. We have cloned the expressed sequences encoding these two receptors in the American alligator, Alligator mississippiensis. The encoded putative proteins share a high degree of amino acid sequence conservation with other vertebrates, however, both alligator TRs contain putative N-terminal truncations. This phenomenon is shared with the chicken for TRbeta, but not for TRalpha, making this the first demonstration of this type of TRalpha isoform. We measured the steady-state levels of TR transcripts in heart, lung, liver, thyroid, cliterophallus/phallus, and gonad of juvenile alligators 24 and 48 h after injection with thyroid stimulating hormone (TSH). TRalpha transcript levels were increased in the heart, decreased in the lung and cliterophallus/phallus, and unaffected in the liver, thyroid, and gonad. TRbeta transcript levels were increased in the heart, lung, and gonad whereas estrogen receptor alpha transcript levels were elevated by TSH treatment only in the gonad. Modulation of these transcripts in the gonad is consistent with TH playing an important role in this tissue's function since seasonal TH fluctuations coincide with reproductive events. These data demonstrate that alligator tissues are differentially responsive to TSH by regulation of TR expression and provide an important comparative framework among vertebrates.

Evaluation of the effect of acetochlor on thyroid hormone receptor gene expression in the brain and behavior of Rana catesbeiana tadpoles

The thyroid hormones (THs) including 3,5,3'-triiodothyronine (T3), are important regulators of growth and development of the brain in vertebrates. Previous studies showed that acetochlor, a widely used herbicide, accelerates T3 -induced frog tadpole metamorphosis and elevates the T3 -dependent accumulation of the mRNAs encoding the TH receptors, TRalpha and beta, in the tail. Here we show that acetochlor affects the expression of these TR isoforms in the brain of Rana catesbeiana tadpoles. Premetamorphic tadpoles exposed to 10 nM acetochlor with and without 100 nM T(3) for 4 days showed substantial increases in TRalpha and TRbeta transcript levels and significant decreases in the TRalpha/TRbeta ratios in their brains. This change in TR ratios is recapitulated with 10 nM acetochlor in R. catesbeiana tadpole brains during prometamorphosis, a period in which THs are endogenously produced. Tail fin biopsies revealed an elevation in TRalpha and beta mRNA levels compared to control animals when exposed to 1 and 10 nM acetochlor for 6 days. When subsequently reared in clean water for 59 days, no alterations in metamorphic hallmarks (forelimb emergence, mouth development, tail regression) were noted compared to the controls. Since alterations in TR ratios/levels may impact brain development, we tested the escape behavior in premetamorphic tadpoles exposed to 10 nM acetochlor for 4 days. We did not detect any statistically significant differences that would indicate that acetochlor affects escape behavior. However, since the gene expression data suggest that brain function may be affected, additional studies examining different behaviors upon acetochlor exposure at environmentally-relevant concentrations are warranted.

Molecular and developmental analyses of thyroid hormone receptor function in Xenopus laevis, the African clawed frog

The current review focuses on the molecular mechanisms and developmental roles of thyroid hormone receptors (TRs) in gene regulation and metamorphosis in Xenopus laevis and discusses implications for TR function in vertebrate development and diversity. Questions addressed are: (1) what are the molecular mechanisms of gene regulation by TR, (2) what are the developmental roles of TR in mediating the thyroid hormone (TH) signal, (3) what are the roles of the different TR isoforms, and (4) how do changes in these molecular and developmental mechanisms affect evolution? Even though detailed knowledge of molecular mechanisms of TR-mediated gene regulation is available from in vitro studies, relatively little is known about how TR functions in development in vivo. Studies on TR function during frog metamorphosis are leading the way toward bridging the gap between in vitro and in vivo studies. In particular, a dual function model for the role of TR in metamorphosis has been proposed and investigated. In this model, TRs repress genes allowing tadpole growth in the absence of TH during premetamorphosis and activate genes important for metamorphosis when TH is present. Despite the lack of metamorphosis in most other vertebrates, TR has important functions in development across vertebrates. The underlying molecular mechanisms of TR in gene regulation are conserved through evolution, so other mechanisms involving TH-target genes and TH tissue-sensitivity and dependence underlie differences in role of TR across vertebrates. Continued analysis of molecular and developmental roles of TR in X. laevis will provide the basis for understanding how TR functions in gene regulation in vivo across vertebrates and how TR is involved in the generation of evolutionary diversity.

Free radical theory of apoptosis and metamorphosis

Reactive oxygen species (ROS) are the major factors that induce oxidative modification of DNA and gene mutation. ROS can elicit oxidative stress and affect a wide variety of physiological and pathological processes including embryonal development, maturation and aging.

Low-temperature arrest of the triiodothyronine-dependent transcription in Rana catesbeiana red blood cells

We examined possible molecular mechanisms for the low-temperature arrest of T3-induced Rana catesbeiana metamorphosis. Scatchard plots revealed that the ratios of maximum binding capacity/dissociation constant for high-affinity sites of tadpole serum proteins for T3 at 20 and 28 C was 3.3-4.6 times less than that at 4 C, due to the decrease in maximum binding capacity values. Kinetic studies of T3 uptake into tadpole red blood cells demonstrated that the ratio of maximum uptake rate/Michaelis constant at 23 C was approximately 13 times greater than that at 4 C. The process of intracellular transport of T3 into the nucleus was not arrested at 4 C. The ratio of T3 incorporated into nuclei to that taken up into red blood cells was not significantly different at 4, 20, and 28 C, indicating the absence of temperature-sensitive sites in this process. T3 binding to the T3 receptors alpha and beta were not temperature sensitive at least at 4 and 20 C. Transcription of the tr genes, early primary T3 response genes, was activated by 10 nM T3 at 20 and 28 C but was barely detected at 4 C. These results indicate that the major molecular event causing the low-temperature arrest of amphibian metamorphosis occurs after T3 entry into the nucleus but before or during the transcriptional activation of the tr genes. Plasma proteins binding T3 and the cellular thyroid hormone uptake system on the plasma membrane may contribute to the slowing of the incorporation of T3 into nucleus at 4 C by decreasing the uptake velocity of T3.

Amphibian cardiac troponin I gene's organization, developmental expression, and regulatory properties are different from its mammalian homologue

In mammals, the expression of the troponin I-slow (TnIs) isoform is predominant in the heart during embryogenesis and, shortly after birth, is replaced by the cardiac-specific isoform, TnIc; a developmental switch thought to be mediated by thyroid hormone. Whereas, in Xenopus, TnIc is expressed at the onset of heart formation and is the only TnI isoform expressed in the heart. Herein, we demonstrate that the expression patterns of these genes appear to be common within the anuran lineage and, unlike their mammalian counterparts, are not affected by thyroid hormone. To elucidate the regulatory mechanism(s) governing the expression of the amphibian TnIc gene, we characterized the TnIc gene from Rana catesbeiana and used its 5'-flanking region to drive expression of green fluorescent protein in the Xenopus transgenic system. Our results demonstrate that a 300-bp minimal promoter containing intact GATA and CArG-box elements is sufficient to drive expression of this reporter gene in a pattern that mimics, both spatially and temporally, the expression of the endogenous Xenopus TnIc gene.

Distinctive gene profiles occur at key points during natural metamorphosis in the Xenopus laevis tadpole tail

Thyroid hormones (THs) are essential for tadpole metamorphosis into a juvenile frog; however, a complex interplay between additional hormones and signaling events also contributes to this dramatic developmental phase. A major mechanism of TH action is the nuclear receptor-mediated regulation of gene transcription of responsive genes. By using the precocious metamorphic model, several genes have been identified as TH responsive in the regressing tail. Many of these genes also exhibit altered expression during natural metamorphosis. Although identification of these genes provides insight into the mechanism whereby TH acts, complex interplay between TH and other hormones and the developmental stage-dependency of tissue responses contribute to the timing and coordination of metamorphic events. We investigated the temporal gene expression profile in Xenopus laevis tadpole tails from premetamorphosis through metamorphic climax by using a combination of a novel frog cDNA array containing 420 genes and quantitative real-time PCR. Seventy-nine genes were identified whose steady-state mRNA expression levels were altered in the tadpole tail during natural metamorphosis, of which 34 have previously been identified to be TH responsive in frogs or mammals. Of these genes, 75 clustered into 13 groups that displayed distinct developmental expression profiles. The levels of 28 transcripts were altered during premetamorphosis, 31 during prometamorphosis, and 43 with the onset of tail regression. This work establishes an important baseline for determining the mechanisms whereby tissues undergo differing metamorphic fates.

Excretory nitrogen metabolism in the juvenile axolotl Ambystoma mexicanum: differences in aquatic and terrestrial environments

The fully grown but nonmetamorphosed (juvenile) axolotl Ambystoma mexicanum was ureogenic and primarily ureotelic in water. A complete ornithine-urea cycle (OUC) was present in the liver. Aerial exposure impeded urea (but not ammonia) excretion, leading to a decrease in the percentage of nitrogen excreted as urea in the first 24 h. However, urea and not ammonia accumulated in the muscle, liver, and plasma during aerial exposure. By 48 h, the rate of urea excretion recovered fully, probably due to the greater urea concentration gradient in the kidney. It is generally accepted that an increase in carbamoyl phosphate synthetase activity is especially critical in the developmental transition from ammonotelism to ureotelism in the amphibian. Results from this study indicate that such a transition in A. mexicanum would have occurred before migration to land. Aerial exposure for 72 h exhibited no significant effect on carbamoyl phosphate synthetase-I activity or that of other OUC enzymes (with the exception of ornithine transcarbamoylase) from the liver of the juvenile A. mexicanum. This supports our hypothesis that the capacities of OUC enzymes present in the liver of the aquatic juvenile axolotl were adequate to prepare it for its invasion of the terrestrial environment. The high OUC capacity was further supported by the capability of the juvenile A. mexicanum to survive in 10 mM NH(4)Cl without accumulating amino acids in its body. The majority of the accumulating endogenous and exogenous ammonia was detoxified to urea, which led to a greater than twofold increase in urea levels in the muscle, liver, and plasma and a significant increase in urea excretion by hour 96. Hence, it can be concluded that the juvenile axolotl acquired ureotelism while submerged in water, and its hepatic capacity of urea synthesis was more than adequate to handle the toxicity of endogenous ammonia during migration to land.

Lineage of anuran epidermal basal cells and their differentiation potential in relation to metamorphic skin remodeling

The anuran remodels the larval epidermis into the adult one during metamorphosis. Larval and adult epidermal cells of the bullfrog were characterized by determining the presence of huge cytoplasmic keratin bundles and the expression profiles of specific marker genes, namely colalpha1 (collagen alpha1 (I)), rlk (larval keratin) and rak (adult keratin). We identified four types of epidermal basal cells: (i) basal skein cells that have keratin bundles and express colalpha1 and rlk; (ii) rak+-basal skein cells that have keratin bundles and express colalpha1, rlk, and rak; (iii) larval basal cells that express rlk and rak; and (iv) adult basal cells that express rak. These traits suggested that these basal cells are on the same lineage in which basal skein cells are the original progenitor cells that consecutively differentiate into rak+-basal skein cells into larval basal cells, and finally into adult basal cells. To directly verify the differentiation potential of larval basal cells into adult ones, the mono-layered epidermis composed of larval basal cells was cultured in the presence of aldosterone and thyroid hormone. In this culture, larval basal cells differentiated into adult basal cells that reconstituted the adult epidermis. Thus, it was concluded that larval basal cells are the direct progenitor cells of the adult epidermal stem cells.

Expression of novel ING variants is regulated by thyroid hormone in the Xenopus laevis tadpole

The candidate tumor suppressor gene, ING1, encodes several protein isoforms as a result of alternative splicing that may possess agonistic and antagonistic roles in the control of cell proliferation and apoptosis. Recently a related gene, ING2, was isolated in human whose expression is increased in adenocarcinomas. Little is known about the cellular function and regulation of these ING family members, but the fact that ING proteins contain a plant homeodomain finger suggests that these proteins may modulate transcription factor-mediated pathways. To elucidate how ING may interact in different tissues to modulate function, we used amphibian metamorphosis as a model system in which a single stimulus, thyroid hormone (TH), initiates tissue-specific proliferation, differentiation, and apoptosis. We have isolated the first Xenopus laevis ING2 and demonstrate that transcript levels increase in response to TH treatment. We provide evidence for the existence of splice variants that are differentially expressed in tissues with different TH-induced fates. Western blots using an antibody directed against the highly conserved C-terminal end of ING proteins reveal a tissue-specific pattern of ING isoform expression in adult Xenopus tissues. Analyses of premetamorphic tadpole tissues show a TH-induced accumulation of ING proteins in tail, whereas the levels in the leg are not affected. This TH-induced accumulation is also observed in serum-free tail organ cultures and is prevented by inhibitors of tail apoptosis. Therefore, this work presents the first link between ING expression and a hormonally regulated nuclear transcription factor-mediated apoptotic response opening the possibility that ING family members may be involved in transducing the signal initiated by TH that determines cell fate.

Role of ECM remodeling in thyroid hormone-dependent apoptosis during anuran metamorphosis

Programmed cell death or apoptosis is an important aspect in organogenesis and tissue remodeling. It is precisely controlled both temporally and spatially during development. Amphibian metamorphosis is an excellent model to study developmental control of apoptosis in vertebrates. This process involves the transformation of essentially every organ/tissue as tadpoles change to frogs, yet is controlled by a single hormone, thyroid hormone (TH). Although different organs and tissues undergo vastly different developmental changes, including de novo development and total resorption, most require apoptotic elimination of at least some cell types. Such properties and the dependence on TH make frog metamorphosis a unique model to isolate and functionally characterize genes participating in the regulation of tissue specific cell death during organ development in vertebrates. Indeed, molecular studies of the TH-dependent gene regulation cascade have led to the discovery of a group of genes encoding matrix metalloproteinases (MMPs) participating in metamorphosis. In vivo and in vitro studies have provided strong evidence to support a role of MMP-mediated remodeling of the extracellular matrix in regulating apoptotic tissue remodeling during metamorphosis.

Thyroid hormone regulation of apoptotic tissue remodeling: implications from molecular analysis of amphibian metamorphosis

Organogenesis and tissue remodeling are critical processes during postembryonic animal development. Anuran metamorphosis has for nearly a century served as an excellent model to study these processes in vertebrates. Frogs not only have essentially the same organs with the same functions as higher vertebrates such as humans, but also employ similar organogenic processes involving highly conserved genes. Development of frog organs takes place during metamorphosis, which is free of any maternal influences but absolutely dependent on the presence of thyroid hormone. Furthermore, this process can be easily manipulated both in intact tadpoles and in organ cultures by controlling the availability of thyroid hormone. These interesting properties have led to extensive morphological, cellular, and biochemical studies on amphibian metamorphosis. More recently, the cloning of thyroid hormone receptors and the demonstration that they are transcription factors have encouraged enormous interest in the molecular pathways controlling tissue remodeling induced by thyroid hormone during metamorphosis. This article summarizes some of the recent studies on the mechanisms of gene regulation by thyroid hormone receptors and isolation and functional characterization of genes induced by thyroid hormone during Xenopus metamorphosis. Particular focus is placed on the remodeling of the animal intestine, which involves both apoptosis (programmed cell death) of larval cells and de novo development of adult tissues, and the roles of thyroid hormone-induced genes that encode matrix metalloproteinases during this process.

Postembryonic expression of the myosin heavy chain genes in the limb, tail, and heart muscles of metamorphosing amphibian tadpoles

Thyroid hormone is presumed to play a role in initiating and/or orchestrating the postembryonic expression of the genes encoding isoforms of the myosin heavy chains (MHCs) that characterize the muscle fibres in an adult organism. The fact that the postembryonic development of a free-living amphibian tadpole takes place during its thyroid hormone-dependent metamorphosis has made the metamorphosing tadpole an ideal system for elucidating the molecular mechanism(s) by which this hormone affects these postembryonic changes. In this review, we summarize the results from recent studies focused on the postembryonic expression of the MHC genes in the skeletal muscles and hearts of metamorphosing anuran (Rana catesbeiana) tadpoles. The demonstration that mRNAs encoding at least five of the MHC isoforms present in the tadpole tail muscles are also present in the adult hind-limb muscles and that an mRNA encoding a cardiac-specific MHC isoform is present in the heart of both the tadpole and adult organism, rules out the possibility that thyroid hormone initiates the expression of these MHC genes. Instead, it seems more likely that this hormone acts by modulating the expression of one or more of the genes encoding these particular MHC isoforms. Whatever the case, the fact that sequence homology suggests that the five distinct skeletal muscle-specific MHCs are all "fast" isoforms raises the question of how these MHCs are distributed among the three different fibre types described for Rana. On the other hand, the possibility exists that the mRNAs for one or more of these fast MHC isoforms encode developmental isoforms that are present but not translated in the muscles of the tadpole and/or adult frog. Finally, an evaluation of the evolutionary relatedness of the R. catesbeiana MHCs to the MHCs in another species of Rana and to the MHCs in other vertebrates discloses, among other things, that the nucleotide sequence in the R. catesbeiana cardiac MHC isoform is more closely related to the chicken ventricular MHC isoform than it is to any of the other MHC isoforms examined.

Expression of the myosin heavy chain genes in the tail muscle of thyroid hormone-induced metamorphosing Rana catesbeiana tadpoles

In tadpoles of the North American bullfrog, Rana catesbeiana, spontaneous and thyroid hormone (T3)-induced metamorphosis is characterized by regression of the tail, which is preceded by a decrease in total protein synthesis in tail tissues. We have demonstrated that thyroid hormone treatment of a tadpole does not affect the synthesis of all proteins equally in the tadpole tail muscle. For example, the synthesis of myosin heavy chains (MHCs) is depressed within 1 day and decreases to 45% of control values after 5 days of T3 treatment, whereas the decreased synthesis of soluble muscle proteins is transient and returns to above control levels by day 5. To determine whether the hormone-induced decrease in MHC synthesis is the result of changes in the transcription of translation of MHC mRNAs, we isolated cDNAs complementary to five different MHC mRNAs from a tail muscle cDNA library and used them to examine the levels of each MHC mRNA in the tail muscle of T3-treated tadpoles. mRNAs that recognize the cDNAs for these five different MHCs are all expressed in the tadpole tail and limb muscles, as well as in the adult leg muscles. MHC mRNAs unique to tadpole tail were not detected. Interestingly, the relative amounts of mRNA for four of the five MHCs increase in tail muscle after T3 treatment of the tadpole, suggesting that repression of MHC gene expression at the protein level does not result from a decrease in the amount of MHC mRNAs. Rather, these results support the contention that the decreased synthesis of MHCs in the tail muscle of T3-treated tadpoles is caused by this hormone, either directly or indirectly, depressing the translation of the MHC mRNAs in this tissue. These results, coupled with the observation that the synthesis of soluble muscle proteins is depressed only in a transient fashion, suggest that T3 may be initiating the expression of a gene(s) that encodes a protein(s) responsible for inhibiting the translation of the MHCs and, perhaps, other structural proteins in the tadpole tail muscle. Whatever the case, the translational regulation of MHC synthesis occurs well before any degradation of the tail tissue is evident and appears to be one of the earliest events in the hormone-induced cell death program of the tadpole tail muscle.

Role for the Rana catesbeiana homologue of C/EBP alpha in the reprogramming of gene expression in the liver of metamorphosing tadpoles

During the spontaneous or thyroid hormone (TH)-induced metamorphosis of Rana catesbeiana, developmental changes occur in its liver that are necessary for the transition of this organism from an ammonotelic larva to a ureotelic adult. These changes include the coordinated expression of genes encoding the urea cycle enzymes carbamyl phosphate synthetase (CPS-I) and arnithine transcarbamylase (OTC). Although the expression of these genes is dependent on TH, the mechanisms(s) by which TH initiates this tissue-specific response is thought to be indirect and to involve early TH-induced upregulation of a gene(s), which, in turn, upregulates the coordinated expression of these urea-cycle enzyme genes. Herein, we demonstrate that mRNAs encoding the Rana homologue of the mammalian transcription factor C/EBP alpha (designated RcC/EBP-1) accumulate early in response to TH and that the product of these mRNAs can bind to and transactivate the promoters of both the Rana CPS-1 and OTC genes. These results support the contention that the reprogramming of gene expression in the liver of metamorphosing tadpoles involves a TH-induced cascade of gene activity in which RcC/EBP-1 and, perhaps, other transcription factors coordinate the expression of genes, such as those encoding CPS-I and OTC, whose products are characteristic of the adult liver phenotype.

Dual functions of thyroid hormone receptors during Xenopus development

Thyroid hormone (TH) plays a causative role in anuran metamorphosis. This effect is presumed to be manifested through the regulation of gene expression by TH receptors (TRs). TRs can act as both activators and repressors of a TH-inducible gene depending upon the presence and absence of TH, respectively. We have been investigating the roles of TRs during Xenopus laevis development, including premetamorphic and metamorphosing stages. In this review, we summarize some of the studies on the TRs by others and us. These studies reveal that TRs have dual functions in frog development as reflected in the following two aspects. First, TRs function initially as repressors of TH-inducible genes in premetamorphic tadpoles to prevent precocious metamorphosis, thus ensuring a proper period of tadpole growth, and later as activators of these genes to activate the metamorphic process. Second, TRs can promote both cell proliferation and apoptosis during metamorphosis, depending upon the cell type in which they are expressed.

Thyroid hormone-dependent metamorphosis in a direct developing frog

The direct developing anuran, Eleutherodactylus coqui, lacks a tadpole, hatching as a tiny frog. We investigated the role of the metamorphic trigger, thyroid hormone (TH), in this unusual ontogeny. Expression patterns of the thyroid hormone receptors, TRalpha and TRbeta, were similar to those of indirect developers. TRbeta mRNA levels increased dramatically around the time of thyroid maturation, when remodeling events reminiscent of metamorphosis occur. Treatment with the goitrogen methimazole inhibited this remodeling, which was reinitiated on cotreatment with TH. Despite their radically altered ontogeny, direct developers still undergo a TH-dependent metamorphosis, which occurs before hatching. We propose a new model for the evolution of anuran direct development.

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.

Transcription from the thyroid hormone-dependent promoter of the Xenopus laevis thyroid hormone receptor betaA gene requires a novel upstream element and the initiator, but not a TATA Box

The thyroid hormone receptor (TR) beta genes in Xenopus laevis are regulated by thyroid hormone in all organs of an animal during metamorphosis. This autoregulation appears to be critical for systematic transformations of different organs as a tadpole is transformed into a frog. To understand this autoregulation, we have previously identified a thyroid hormone response element in the hormone-dependent promoter of the X. laevis TRbetaA gene. We report here the detailed characterization of the promoter. We have now mapped the transcription start site and demonstrated the existence of an initiator element at the start site critical for promoter function. More important, our deletion and mutational experiments revealed a novel upstream DNA element that is located 125 base pairs upstream of the start site and that is essential for active transcription from the promoter. Promoter reconstitution experiments showed that this novel element does not function as an enhancer, but acts as a core promoter element, which, together with the initiator, directs accurate transcription from the promoter. Finally, we provide evidence for the existence of a protein(s) that specifically recognizes this element. Our studies thus demonstrate that the TRbetaA promoter has a unique organization consisting of an initiator and a novel upstream promoter element. Such an organization may be important for the ubiquitous but tissue-dependent temporal regulation of the gene by thyroid hormone during amphibian metamorphosis.

Both thyroid hormone and 9-cis retinoic acid receptors are required to efficiently mediate the effects of thyroid hormone on embryonic development and specific gene regulation in Xenopus laevis

Tissue culture transfection and in vitro biochemical studies have suggested that heterodimers of thyroid hormone receptors (TRs) and 9-cis retinoic acid receptors (RXRs) are the likely in vivo complexes that mediate the biological effects of thyroid hormone, 3,5,3'-triiodothyronine (T3). However, direct in vivo evidence for such a hypothesis has been lacking. We have previously reported a close correlation between the coordinated expression of TR and RXR genes and tissue-dependent temporal regulation of organ transformations during Xenopus laevis metamorphosis. By introducing TRs and RXRs either individually or together into developing Xenopus embryos, we demonstrate here that RXRs are critical for the developmental function of TRs. Precocious expression of TRs and RXRs together but not individually leads to drastic, distinct embryonic abnormalities, depending upon the presence or absence of T3, and these developmental effects require the same receptor domains as those required for transcriptional regulation by TR-RXR heterodimers. More importantly, the overexpressed TR-RXR heterodimers faithfully regulate endogenous T3 response genes that are normally regulated by T3 only during metamorphosis. That is, they repress the genes in the absence of T3 and activate them in the presence of the hormone. On the other hand, the receptors have no effect on a retinoic acid (RA) response gene. Thus, RA- and T3 receptor-mediated teratogenic effects in Xenopus embryos occur through distinct molecular pathways, even though the resulting phenotypes have similarities.