Tissue-specific regulation of type III iodothyronine 5-deiodinase gene expression mediates the effects of prolactin and growth hormone in Xenopus metamorphosis

Characterization of the mechanistic linkages between iodothyronine deiodinase inhibition and impaired thyroid-mediated growth and development in Xenopus laevis using iopanoic acid

Iodothyronine deiodinases (DIO) are key enzymes that influence tissue-specific thyroid hormone levels during thyroid-mediated amphibian metamorphosis. Within the larger context of evaluating chemicals for thyroid system disrupting potential, chemical activity toward DIOs is being evaluated using high-throughput in vitro screening assays as part of U.S. EPA's ToxCast program. However, existing data gaps preclude any inferences between in vitro chemical inhibition of DIOs and in vivo outcomes relevant to ecological risk assessment. This study aimed to generate targeted data in a laboratory model species (Xenopus laevis) using a model DIO inhibitor, iopanoic acid (IOP), to characterize linkages between in vitro potency, in vivo biochemical responses, and adverse organismal outcomes. In vitro potency of IOP toward DIOs was evaluated using previously developed in vitro screening assays, which showed concentration-dependent inhibition of human DIO1 (IC50: 97 µM) and DIO2 (IC50: 231 µM) but did not inhibit human or X. laevis DIO3 under the assay conditions. In vivo exposure of larval X. laevis to 0, 2.6, 5.3 and 10.5 µM IOP caused thyroid-related biochemical profiles in the thyroid gland and plasma consistent with hyperthyroxinemia but resulted in delayed metamorphosis and significantly reduced growth in the highest two exposure concentrations. Independent evaluations of dio gene expression ontogeny, together with existing literature, supported interpretation of IOP-mediated effects resulting in a proposed adverse outcome pathway for DIO2 inhibition leading to altered amphibian metamorphosis. This study highlights the types of mechanistic data needed to move toward predicting in vivo outcomes of regulatory concern from in vitro bioactivity data.

Development of the hypothalamo-hypophyseal system in amphibians with special reference to metamorphosis

In this review article, topics of the embryonic origin of the adenohypophysis and hypothalamus and the development of the hypothalamo-hypophyseal system for the completion of metamorphosis in amphibians are included. The primordium of the adenohypophysis as well as the primordium of the hypothalamus in amphibians is of neural origin as shown in other vertebrates, and both are closely associated with each other at the earliest stage of development. Metamorphosis progresses via the interaction of thyroid hormone and adrenal corticosteroids, of which secretion is enhanced by thyrotropin and corticotropin, respectively. However, unlike in mammals, the hypothalamic releasing factor for thyrotropin is not thyrotropin-releasing hormone (TRH), but corticotropin-releasing factor (CRF) and the major releasing factor for corticotropin is arginine vasotocin (AVT). Prolactin, the release of which is profoundly enhanced by TRH at the metamorphic climax, is another pituitary hormone involved in metamorphosis. Prolactin has a dual role: modulation of the metamorphic speed and the development of organs for adult life. The secretory activities of the pituitary cells containing the three above-mentioned pituitary hormones are elevated toward the metamorphic climax in parallel with the activities of the CRF, AVT, and TRH neurons.

An Inhibitor of Thyroid Hormone Synthesis Protects Tail Skin Grafts Transplanted to Syngenic Adult Frogs

Tail regression in amphibian tadpoles during metamorphosis is one of the most dynamic morphological changes in animal development and is induced by thyroid hormone (TH). It has been proposed that tail resorption is driven by immunological rejection in Xenopus laevis, based on experimental evidence showing that larval skin grafts become atrophic on syngenic recipient adult frogs. This led to the hypothesis that tail regression is induced by an immunological rejection against larval skin-specific antigens called Ouro proteins. However, our group has demonstrated that ouro-knockout tadpoles undergo normal metamorphosis, including tail resorption in Xenopus tropicalis, which indicates that the expression of ouro genes is not necessary for tail regression. In the present study, we showed that an inhibitor of TH synthesis promotes the survival of larval tail skin grafts on syngenic adult Xenopus tropicalis frogs. The levels of endogenous THs in adult frogs were also comparable to those in metamorphosing tadpoles of Xenopus laevis with a regressing tail, and TH induced the regression of tadpole tail tips of Xenopus tropicalis in organ culture. Taken together, these results strongly suggest that endogenous THs in the recipient adult frog induce the degeneration of syngenic tail skin grafts.

Forever young: Endocrinology of paedomorphosis in the Mexican axolotl (Ambystoma mexicanum)

The Mexican axolotl (Ambystoma mexicanum) is a salamander species that does not undergo metamorphosis, resulting in the retention of juvenile characteristics in the mature breeding stage (paedomorphosis). Here we review the endocrinological studies investigating the proximate cause of axolotl paedomorphosis with a focus on the hypothalamo-pituitary-thyroid (HPT) axis. It is well established that axolotl paedomorphosis is a consequence of low activity of the HPT axis. The pituitary hormone thyrotropin (TSH) is capable of inducing metamorphosis in the axolotl, which indicates that all processes and interactions in the HPT axis below the pituitary level are functional, but that TSH release is impaired. In metamorphosing species, TSH secretion is largely controlled by the hypothalamic neuropeptide corticotropin-releasing hormone (CRH), which seems to have lost its thyrotropic activity in the axolotl. However, preliminary experiments have not yet confirmed a role for faulty CRH signalling in axolotl paedomorphosis. Other hypothalamic factors and potential pituitary inhibitors need to be investigated to identify their roles in amphibian metamorphosis and axolotl paedomorphosis.

Coordinated expression and regulation of deiodinases and thyroid hormone receptors during metamorphosis in the Japanese flounder (Paralichthys olivaceus)

In vertebrates, thyroid hormone receptors (TRs) and deiodinases are essential for developmental events driven by the thyroid hormones (THs). However, the significance of deiodinases during the metamorphosis of the Japanese flounder (Paralichthys olivaceus) remains unclear. Moreover, regulation and response of the TRs and deiodinases to THs in this fish are poorly understood. Therefore, we detected the expression patterns of THs, deiodinases, and TRs in drug-treated larvae and untreated larvae of P. olivaceus by using enzyme-linked immunosorbent assay and quantitative real-time PCR during P. olivaceus metamorphosis. To further understand the roles of these elements, a rescue assay was performed. Our results show the importance of THs, TRs, and deiodinases in flatfish metamorphosis. Our results also confirm that D1 and D2 activate THs and D3 plays the opposite and complementary role. Moreover, we demonstrated that both TRα and TRβ have important but different roles during P. olivaceus metamorphosis.

Prolactin and blood vessels: a comparative endocrinology perspective

The hormone prolactin (PRL), fundamental for lactation in mammals, is known to exert a wide diversity of actions in the various vertebrate groups. Blood vessels are surfacing as important PRL targets, contributing to these hormonal functions. PRL promotes the growth of new blood vessels (angiogenesis) and is proteolytically cleaved to vasoinhibins, a family of peptides (including 16-kDa PRL) with potent antiangiogenic and blood vessel regression effects. These opposing actions point to the regulation of the proteases responsible for PRL cleavage as an efficient way to balance blood vessel growth and involution. This review briefly summarizes the effects of PRL and vasoinhibins on blood vessels in mammals and discusses whether similar vascular actions could contribute to the effects of PRL on the development, growth, and reproduction of lower vertebrates. A comparative study in diverse species may lead to a better understanding of blood vessels as a driving force for the biological actions of PRL.

The synthetic gestagen levonorgestrel impairs metamorphosis in Xenopus laevis by disruption of the thyroid system

Synthetic gestagens, including levonorgestrel (LNG), are active compounds in contraceptives, and several studies report their occurrence in surface waters. However, information about endocrine-disrupting effects in nontarget organisms is scarce. The present study investigated effects of LNG exposure on thyroid hormone-dependent metamorphosis of Xenopus laevis. Premetamorphic X. laevis tadpoles at Nieuwkoop and Faber (NF) stage 48 were exposed in a flow-through culture system to four LNG concentrations (10(-11), 10(-10), 10(-9), and 10(-8)M) over the period of metamorphosis. At NF 58 and 66, tadpoles were examined sex specifically. Developmental time and organismal responses were recorded and correlated with molecular and histopathological endpoints. Exposure to 10(-8)M LNG caused an inhibition of metamorphosis resulting in developmental arrest at early climax stages as giant tadpoles or tailed frogs. In brain-pituitary tissue of NF 58 tadpoles, gene expression of thyroid-stimulating hormone (β-subunit; TSHβ), TH receptor β (TRβ), and deiodinase type 3 (D3) was not changed. Instead, prolactin (PRL) messenger RNA (mRNA) was significantly increased by 10(-9)M LNG in females and by 10(-8)M LNG in both sexes. In NF 66 tadpoles, mRNA levels of TSHβ mRNA were significantly increased in the 10(-9) and 10(-8)M LNG treatment groups indicating a hypothyroid state. No changes of TRβ, D3, and PRL gene expression were detected. Histopathological evaluation of thyroid gland sections revealed no typical sign of hypothyroidism but rather an inactivated appearance of the thyroid. In conclusion, our data demonstrate for the first time a completely new aspect of thyroid system disruption caused by synthetic gestagens in developing amphibians.

Serum IGF-1 and IGFBP-3 levels in subclinical hypothyroid women

Thyroid status is known to influence growth in mammals. The aim of this study is to investigate the possible relationship between autoimmune subclinical hypothyroidism and growth hormone (GH), insulin-like growth factor-1(IGF-1) and insulin-like growth factor binding protein-3(IGFBP-3) levels. Thirty-five women with autoimmune subclinical hypothyroidism, 33 years of age, were used as controls and enrolled in the study. Free triiodothyronin (FT3), free thyroxin(FT4), thyrotropin(TSH), anti-thyroid peroxidase(Anti-TPO), anti-thyroglobuline(Anti-Tg), GH, IGF-1 and IGFBP-3 levels were measured in blood samples and correlations among these parameters were evaluated. We found no significant differences in GH, IGF-1 or IGFBP-3 between patients and controls. In patients and controls, there were no correlations among thyroid hormones and IGF-1 or IGFBP-3 levels, but GH levels were correlated with FT3, FT4 and TSH only in patients’ group. In controls, only IGF-1 and IGFBP-3 levels were correlated. The present study suggests that subclinical hypothyroidism with high TSH and antibody status does not affect IGF-1 and IGFBP-3 levels in adult women. To our knowledge, this is the first study concerning the relationship between autoimmune subclinical hypothyroidism and IGF-1 and IGFBP-3 levels.

Developmental regulation of gene expression in the thyroid gland of Xenopus laevis tadpoles

Thyroid hormones (TH) are the primary morphogen regulating amphibian metamorphosis. However, knowledge about molecular mechanisms regulating thyroid gland activity in anuran tadpoles is very scarce. In this study, we characterized gene expression profiles in thyroids of Xenopus laevis tadpoles during spontaneous metamorphosis. Using real-time PCR, elevated expression of slc5a5, tpo, tshr, and sar1a mRNAs was detected at late prometamorphic and climax stages. For dio2 and dio3 but not dio1, developmental regulation of thyroidal expression was evident from a strong up-regulation at late stages. Conversely, expression of the DNA replication markers mcm2 and pcna declined at climax stages. The presence of functional feedback mechanisms at premetamorphic stages was examined in two experiments. Stage 52 tadpoles were exposed for 72 h to 1.0 microg/l thyroxine (T4). This treatment caused reduced mRNA expression of slc5a5, tpo, and dio2, whereas no significant changes were detectable for tshr expression in thyroids and tshb expression in the pituitary. In another experiment, stage 46 tadpoles were treated with 20 mg/l sodium perchlorate (PER) for 5 and 10 days. Within this period of time, control tadpoles developed to stages 50 and 52, respectively. PER treatment resulted in up-regulation of slc5a5, tpo, and tshr mRNAs at both time points and increased dio2 mRNA expression at day 10. Effects of PER on thyroid histology were only apparent on day 10. Together, our analyses of thyroidal gene expression demonstrate a marked developmental regulation for functional markers of thyroid activity, two deiodinases as well as for DNA replication markers. Expression patterns detected in PER- and T4-treated tadpoles indicate that functional feedback signaling controlling thyroid activity is already active during premetamorphosis.

Molecular mechanisms of corticosteroid synergy with thyroid hormone during tadpole metamorphosis

Corticosteroids (CS) act synergistically with thyroid hormone (TH) to accelerate amphibian metamorphosis. Earlier studies showed that CS increase nuclear 3,5,3'-triiodothyronine (T(3)) binding capacity in tadpole tail, and 5' deiodinase activity in tadpole tissues, increasing the generation of T(3) from thyroxine (T(4)). In the present study we investigated CS synergy with TH by analyzing expression of key genes involved in TH and CS signaling using tadpole tail explant cultures, prometamorphic tadpoles, and frog tissue culture cells (XTC-2 and XLT-15). Treatment of tail explants with T(3) at 100 nM, but not at 10 nM caused tail regression. Corticosterone (CORT) at three doses (100, 500 and 3400 nM) had no effect or increased tail size. T(3) at 10 nM plus CORT caused tails to regress similar to 100 nM T(3). Thyroid hormone receptor beta (TRbeta) mRNA was synergistically upregulated by T(3) plus CORT in tail explants, tail and brain in vivo, and tissue culture cells. The activating 5' deiodinase type 2 (D2) mRNA was induced by T(3) and CORT in tail explants and tail in vivo. Thyroid hormone increased expression of glucocorticoid (GR) and mineralocorticoid receptor (MR) mRNAs. Our findings support that the synergistic actions of TH and CS in metamorphosis occur at the level of expression of genes for TRbeta and D2, enhancing tissue sensitivity to TH. Concurrently, TH enhances tissue sensitivity to CS by upregulating GR and MR. Environmental stressors can modulate the timing of tadpole metamorphosis in part by CS enhancing the response of tadpole tissues to the actions of TH.

Coordination of deiodinase and thyroid hormone receptor expression during the larval to juvenile transition in sea bream (Sparus aurata, Linnaeus)

To test the hypothesis that THs play an important role in the larval to juvenile transition in the marine teleost model, sea bream (Sparus auratus), key elements of the thyroid axis were analysed during development. Specific RT-PCR and Taqman quantitative RT-PCR were established and used to measure sea bream iodothyronine deiodinases and thyroid hormone receptor (TR) genes, respectively. Expression of deiodinases genes (D1 and D2) which encode enzymes producing T3, TRs and T4 levels start to increase at 20-30 days post-hatch (dph; beginning of metamorphosis), peak at about 45 dph (climax) and decline to early larval levels after 90-100 dph (end of metamorphosis) when fish are fully formed juveniles. The profile of these different TH elements during sea bream development is strikingly similar to that observed during the TH driven metamorphosis of flatfish and suggests that THs play an analogous role in the larval to juvenile transition in this species and probably also in other pelagic teleosts. However, the effect of T3 treatment on deiodinases and TR transcript abundance in sea bream is not as clear cut as in larval flatfish and tadpoles indicating divergence in the responsiveness of TH axis elements and highlighting the need for further studies of this axis during development of fish.

Assessment of thyroid system disruption in Rana pipiens tadpoles chronically exposed to UVB radiation and 4-tert-octylphenol

Many studies have considered recent increases in ultraviolet B radiation (UVBR) and endocrine disrupting chemicals polluting the environment as possible contributing factors to the reduction in amphibian populations. It has been demonstrated that exposure of amphibians to estrogenic chemicals or UVBR can affect the timing of larval development and metamorphosis. However, amphibians in the wild are exposed to multiple environmental stressors simultaneously. Therefore, our study examines the effects of UVBR and the estrogenic chemical 4-tert-octylphenol (OP), alone and in combination, on the thyroid system of Rana pipiens tadpoles, which is the main regulator of amphibian metamorphosis. Results demonstrate that thyroid gland histomorphology measurements in Gosner stage 31 tadpoles continuously exposed to UVBR (0.21W/m(2)) were not different than those measured in animals from the control group. In a separate experiment, tadpoles exposed to environmentally relevant levels of UVBR (0.22W/m(2)) and/or OP (0.01nM or 10nM) exhibited significantly delayed development starting from Gosner stage 29, given that fewer tadpoles developed past stage 29 in these groups. In addition, significantly fewer UVBR-treated tadpoles developed past stage 34 and metamorphosed. Samples were collected from stages 29 and 34 tadpoles for gene expression analysis in tail tissue and measurements of T3 (triiodothyronine) whole body levels (minus tail). UVBR and/or OP exposure did not affect T3 levels in stages 29 and 34 tadpoles. However, a decrease in deiodinase type 2 (D2) or increase in deiodinase type 3 (D3) mRNA levels was observed in groups of tadpoles with slowed developmental rates at those developmental stages. Given that D2 activates and D3 inactivates thyroid hormones (TH), UVBR/OP mediated disruptions in development are likely caused by dysfunctions in the localized metabolism of THs through alterations in the expression of these enzymes in peripheral tissues. This is the first study to our knowledge reporting a potential thyroid-based mechanism of action for the developmental delays in amphibians exposed to UVBR and/or OP.

Corticosteroids disrupt amphibian metamorphosis by complex modes of action including increased prolactin expression

Although thyroid hormones (TH) are the primary morphogens regulating amphibian metamorphosis, other hormones including corticosteroids are known to participate in this regulation. The present study investigated effects of corticosteroids on larval development of the amphibian Xenopus laevis. Premetamorphic tadpoles (stage 51) were treated with aldosterone (ALDO; 100 nM), corticosterone (B; 10, 100, 500 nM) and dexamethasone (DEX; 10, 100, 500 nM) for 21 days and organismal responses were assessed by gross morphology determining stage development, whole body length (WBL), and hind limb length (HLL). B and DEX reduced WBL and HLL and caused abnormal development including the lack of fore limb emergence while ALDO treatment showed no significant effect. Gene expression analyses using RT-PCR revealed up-regulation of prolactin (PRL) in brain, but down-regulation of type III deiodinase in tail tissue induced by the glucocorticoids B and DEX. Additionally, stromelysin-3 transcript in tail tissue was decreased by B. ALDO at 100 nM had no effect on mRNA expression, neither in brain nor in tail tissue. These findings indicate that corticosteroids modulate TH-dependent metamorphosis by complex mechanisms that even include indirect effects triggered by increased PRL mRNA expression.

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.

Cloning of Atlantic halibut growth hormone receptor genes and quantitative gene expression during metamorphosis

To gain insight into the possible regulatory role of the growth hormone (GH)-insulin-like growth factor I (IGF-I) system in flatfish metamorphosis, body GHR gene expression as well as IGF-I protein content was quantified in larval Atlantic halibut throughout metamorphosis (developmental stages 5-10). The cDNA of the full-length GH receptor (hhGHR) was cloned from adult liver and characterized. The hhGHR shows common features of a GHR, including a (Y/F)GEFS motif in the extracellular domain, a single transmembrane region, and an intracellular domain containing a Box 1 and Box 2. Additionally, a truncated GHR (hhGHRtr), similar to turbot and Japanese flounder GHRtr, was cloned and sequenced. These sequences are highly similar to the full-length and truncated GHRs in turbot (89%/86%) and Japanese flounder (93%/91%) with lower identity with other fish type I GHR (81%) and type II GHRs (58%). A quantitative real-time RT-PCR assay was used to measure hhGHR and hhGHRtr mRNA content in normally and abnormally metamorphosed individuals at six developmental stages, from early pre-metamorphosis to post-metamorphosis, when the fish is considered a juvenile. The level of hhGHR gene expression was highest at pre-metamorphic stage 6 and at stage 8 at the onset of metamorphosis, and then decreased during metamorphic climax and post-metamorphosis. Expression of hhGHRtr reached highest levels at stage 6 and then decreased to post-metamorphosis. The ratio of expression between the full-length and the truncated GHR (hhGHR:hhGHRtr) varied among stages and was highest at the onset of metamorphosis and at metamorphic climax. A radioimmunoassay was used to measure halibut IGF-I body content throughout metamorphosis. IGF-I increases from early metamorphosis to the onset of metamorphosis and then decreases towards post-metamorphosis. In comparison between normally and abnormally metamorphosing larvae, IGF-I content, hhGHR and hhGHRtr mRNA levels were reduced in the abnormal fish. These data indicate that the GH-IGF-I system either has a regulatory role in metamorphosis, or is being affected as a consequence of the abnormal metamorphosis.

Temperature and humidity alter prolactin receptor expression in the skin of toad (Bufo bankorensis and Bufo melanostictus)

Bufo bankorensis and Bufo melanostictus, the only two species of Bufonidae genus in Taiwan, live in habitats that differ in altitude and humidity. This study tested the hypothesis that prolactin receptor (PRLR) expression responds to environmental change. Western blot analysis showed that the PRLR protein was widely distributed in brain, lung, liver, kidney, dorsal skin and ventral skin of toads. The level PRLR protein was elevated in the dorsal skin of the two toad species treated with dry or wet conditions for 14 days. The increase in PRLR of dorsal skin in B. bankorensis was higher than that in B. melanostictus. This experimental result suggests that B. bankorensis secretes more mucus to reduce water evaporation from its thinner cuticle than B. melanostictus. The expression of PRLR protein was increased in the lung of B. bankorensis and decreased in the lung of B. melanostictus. Moreover, PRLR protein levels were increased in the kidneys in the two species toad, likely due to reduction in water lost through lung and urine. The two toad species were subjected to varying temperatures (25 degrees C, 15 degrees C and 10 degrees C) for 14 days. The lowest PRLR protein expression was observed at 10 degrees C. Comparison of the decreasing trend in PRLR protein levels demonstrated that the variation in B. bankorensis was significantly higher than that in B. melanostictus. Comparisons of variation in PRLR protein expression in the two species under different environments suggest that B. bankorensis is more adaptable to different environments than B. melanostictus.

The role of deiodinases in amphibian metamorphosis

Valerie Galton's prediction over 40 years ago that deiodinases would be important in amphibian metamorphosis has been confirmed. Expression of iodothyronine deiodinase type 2 (D2) and type 3 (D3) occurs in embryogenesis at relevant locations long before the thyroid gland has developed. The expression of D2, an enzyme that synthesizes the active hormone triiodothyronine (T3) from its circulating precursor thyroxine (T4), accurately marks cells at the time that they undergo thyroid hormone-dependent changes. D2 expression in the anterior pituitary controls the maturation of the negative feed back loop between the pituitary and the thyroid gland. The expression of D3, an enzyme that inactivates the hormone, protects cells from responding to the hormone. Its constitutive expression in the dorsal ciliary marginal zone of the tadpole retina results in asymmetric replication of the ventral retina. This in turn leads to ipsilateral projections at the climax of metamorphosis as the frog develops over lapping visual fields. The genes encoding D2 and D3 appear to function in a cell autonomous manner affecting just the cells in which they are expressed. Localized deiodinases represent one of the simplest and most elegant methods to control gene expression, by regulating local concentration of the active hormone in specific cells.

Prolactin, an Activator of Epithelial Na+Channel, Inhibits Basolateral K+Channels in Adult Tree Frog Skin

Adult amphibian skin actively transports Na+ from its apical to basolateral side while in turn, K+ is recycled through Na+, K+-ATPase and K+ channels located in the basolateral membrane. We previously found that PRL stimulates Na+ transport in the skin of the adult tree frog (Hyla arborea japonica) via an increase in the open-channel density of the epithelial Na+ channel (ENaC). If PRL also activates basolateral K+ channels, this activation would help to stimulate Na+ transport, too. Whether PRL does indeed stimulate basolateral K+ channels in the adult tree frog was examined by measuring the short-circuit current across nystatin-treated skin. Both tolbutamide, a K(ATP) channel blocker, and tetrapentylammonium (TPA), a KCa channel blocker, blocked the current, the effect of TPA being more powerful than that of tolbutamide. Contrary to expectation, PRL inhibited the basolateral K+ channels in this skin. In the presence of basolateral amiloride, PRL still inhibited the basolateral K+ current, suggesting that the (Na+)-H+ exchanger located in the basolateral membrane does not mediate the inhibitory effect of PRL on the basolateral K+ channels in Hyla.

Molecular cloning of bullfrog prolactin receptor cDNA: changes in prolactin receptor mRNA level during metamorphosis

In amphibian larvae, prolactin (PRL) is known to possess growth-promoting and anti-metamorphic activities. For further understanding of the role of PRL in larvae, bullfrog PRL receptor (bfPRLR) cDNA was obtained from the tail fin of premetamorphic tadpoles by use of the reverse transcription-polymerase chain reaction (RT-PCR) coupled with 5' - and 3' -rapid amplification of cDNA ends (RACE). The predicted bfPRLR was composed of 617 amino acids, contained a single transmembrane domain, and showed 33-57% sequence homologies with known sequences of vertebrate PRLRs. When bfPRLR was transiently expressed, specific binding of 125I-labeled bullfrog PRL (bfPRL) was observed. By Northern blot analysis, a 3-kb transcript was detected in the tail fin. By RT-PCR bfPRLR mRNA expression was detected mainly in the brain, kidney, skin, and tail throughout prometamorphic and middle climactic periods. The results of an RNase protection assay revealed that the bfPRLR mRNA level in the tail fin increased around the onset of climax (stage XX) and was maintained at a relatively high value at least until mid-climax (stage XXII). It also revealed that bfPRLR mRNA level in the kidney of larvae gradually rose as metamorphosis progressed. The results support the view that PRL in larval period acts not only on the larval organs but also on the organs that are necessary for the adult life to maintain or develop their structures and functions.

Thyroid hormone deiodinases: physiology and clinical disorders

Thyroid hormone action is achieved through the binding of 3,5,3'-triiodothyronine to its nuclear receptor, which results in alterations in gene expression. An impairment in thyroid hormone action during vertebrate development results in severe, irreversible abnormalities in tissue growth, maturation, and function. The deiodinases are a family of selenoproteins expressed in a number of fetal and adult tissues that catalyze the activation and inactivation of thyroid hormones. Their unique biochemical characteristics and tissue and developmental expression patterns suggest that deiodinases may control the concentration of active thyroid hormone available to specific tissues or cell types at certain stages of development. The deiodinases thus appear to play an important role in regulating thyroid hormone action at a prereceptor level. Current research focusing on a better understanding of the biochemistry, regulation, and physiologic role of these enzymes is the focus of this review.