Tadpole Metamorphosis Research Articles

Comparative Pathology and Ecological Implications of Two Myxosporean Parasites in Native Australian Frogs and the Invasive Cane Toad

Myxosporean parasites Cystodiscus axonis and C. australis are pathogens of native and exotic Australian frog species. The pathology and ecological outcomes of infection with these parasites were investigated in this study. Gliosis was correlated to Cystodiscus axonis plasmodia in the brains of (9/60) tadpoles and (3/9) adult endangered Green and golden bell frogs using ordinal regression. Severe host reactions to C. axonis (haemorrhage, necrosis, and vasulitis) were observed in the brains of threatened Southern bell frogs (8/8), critically endangered Booroolong frogs (15/44) and Yellow spotted bell frogs (3/3). Severe brain lesions were associated with behavioural changes, neurological dysfunction, and spontaneous death. Both C. axonis and C. australis develop in the bile ducts of tadpoles, the plasmodia were significantly associated with biliary hyperplasia, inflammation and the loss of hepatocytes in (34/72) Green and golden bell frog tadpoles using ordinal regression. These lesions were so severe that in some cases 70% of the total liver was diseased. Normal liver function in tadpoles is necessary for metamorphosis, metabolism, and immune function. We postulate that this extensive liver damage would have significant host health impacts. Severe hepatic myxosporidiosis was more prevalent in tadpoles examined in autumn and winter (overwintered), suggestive of delayed metamorphosis in infected tadpoles, which would have serious flow-on effects in small populations. We compared the sensitivity of histopathology and species-specific PCR in the detection of C. australis and C. axonis. PCR was determined to be the most sensitive method (detection limit 1 myxospore equivalent of ribosomal DNA). Histology, however, had the advantage of assessing the impact of the parasite on the host. It was concluded that these parasites have the potential for significant ecological impacts, because of their high prevalence of infection and their ability to cause disease in some frogs.

Cartilage on the move: Cartilage lineage tracing during tadpole metamorphosis

The reorganization of cranial cartilages during tadpole metamorphosis is a set of complex processes. The fates of larval cartilage-forming cells (chondrocytes) and sources of adult chondrocytes are largely unknown. Individual larval cranial cartilages may either degenerate or remodel, while many adult cartilages appear to form de novo during metamorphosis. Determining the extent to which adult chondrocytes/cartilages are derived from larval chondrocytes during metamorphosis requires new techniques in chondrocyte lineage tracing. We have developed two transgenic systems to label cartilage cells throughout the body with fluorescent proteins. One system strongly labels early tadpole cartilages only. The other system inducibly labels forming cartilages at any developmental stage. We examined cartilages of the skull (viscero- and neurocranium), and identified larval cartilages that either resorb or remodel into adult cartilages. Our data show that the adult otic capsules, tecti anterius and posterius, hyale, and portions of Meckel's cartilage are derived from larval chondrocytes. Our data also suggest that most adult cartilages form de novo, though we cannot rule out the potential for extreme larval chondrocyte proliferation or de- and re-differentiation, which could dilute our fluorescent protein signal. The transgenic lineage tracing strategies developed here are the first examples of inducible, skeleton-specific, lineage tracing in Xenopus.

Expression profiles of metamorphosis-related genes during natural transformations in tadpoles of wild Wood Frogs (Lithobates sylvaticus)

Numerous studies using laboratory-reared tadpoles have shown the importance of thyroid hormones (TH), thyroid receptors (TR), and deiodinase (Dio) enzymes during anuran metamorphosis. Our study focuses on the analysis of thyroid-related genes in tadpoles of wild Wood Frogs ( Lithobates sylvaticus (LeConte, 1825); also known as Rana sylvatica (Cope, 1889)) during metamorphosis. Results showed that, in concordance with laboratory-reared studies, thyroid receptor beta (trb) gene expression profiles presented the most marked changes. At climax and compared with premetamorphic stages, brains, tails, and gonad–mesonephros complex (GMC) tissues increased trb expression levels 5-, 21-, and 41-fold, respectively (p < 0.05). In addition, gene expression levels of brain deiodinase type II and III showed opposite trends, where 3-fold decrease and 10-fold increase were, respectively, found. This finding supports the idea that thyroid hormone, as it has been demonstrated in laboratory-reared tadpoles, is also involved in natural metamorphosis in wild tadpoles. Interestingly, and contrary to our predictions, we observed that whole brain corticotropin-releasing factor (crf) and crf receptor 1 (crfr1) gene expression levels significantly decrease through metamorphosis in wild L. sylvaticus tadpoles. Further analyses are required to determine if a role of TH in the timing of anuran gonadal development exists, as well as the importance of cell-specific and tissue-specific expression of crf and crfr1 to metamorphosis.

Inhibiting Matrix Metalloproteinases, an Old Story with New Potentials for Cancer Treatment

Five decades of extensive research have passed since the description for the first time of the existence of an enzyme, which had the ability to degrade collagen during the metamorphosis of tadpoles. In fact, during these years, a large family of enzymes possessing the unique ability of degrading the extra cellular matrix (ECM) has been discovered. These enzymes are widely known as Matrix Metalloproteinases (MMPs) and it is noteworthy that many members of this family are directly linked to several human diseases such as arthritis and cancer. Moreover, due to the critical role of certain members of MMPs in cancer invasion and metastasis, great efforts have been made in order to find new inhibitory compounds against these MMPs. In this work we attempt to summarize the current status of the intervention strategies against MMPs, using inhibitory compounds that could block the activation of MMPs directly or indirectly. Furthermore we will try to shed light towards new potential strategies of MMP inhibitors using monoclonal antibodies.

Salinity Acclimation Affects Survival and Metamorphosis of Crab-eating Frog Tadpoles

Abstract Previous studies showed that Fejervarya cancrivora tadpoles were salt-tolerant (up to 24 parts per thousand [ppt] in the field and 35 ppt in the laboratory) but metamorphosed only at or below 6.6 ppt. We hypothesized that the discrepancy in salt tolerance of tadpoles is due to a mixture of results drawn from short- and long-term experiments and different acclimation procedures, which needed to be re-examined. We compared the salinity tolerance of acclimated and nonacclimated tadpoles and assessed salinity effects on growth, development, and metamorphosis among tadpoles of F. cancrivora throughout larval period. In Experiment 1, we determined 48-h tadpole survivorship by assigning 40 tadpoles to one of the 24 treatments ranging from 3-ppt to 30-ppt salinity. In Experiment 2, we assigned tadpoles to 3–15-ppt salinity at intervals of 3 ppt and monitored tadpole growth and development through metamorphosis. In Experiment 3, we exposed 15 tadpoles to a stepwise increase of salinity (i.e., 2 ppt every ...

Effects of combinations of malathion and cypermethrin on survivability and time of metamorphosis of tadpoles of Indian cricket frog (Fejervarya limnocharis)

This study was undertaken to determine the effect of environmentally realistic concentrations of two commonly used pesticides viz., malathion and cypermethrin, using a fully 3 × 3 factorial experiments on the survivability and time of metamorphosis in a common rice paddy field frog (cricket frog) Fejervarya limnocharis under laboratory conditions. The results suggest that cypermethrin is more toxic than malathion and combinations of higher concentrations of cypermethrin (50 μg/L) with malathion (250 and 500 μg/L) are more deleterious to the survivability of tadpoles. With increasing cypermethrin concentration, the survivability of tadpole decreased (r = −0.986, P = 0.108). But cypermethrin alone induced early metamorphosis among the surviving tadpoles. However, there was a delay in the time required for metamorphosis induced by malathion and its combination with cypermethrin. The delay in metamorphosis may indicate the altered physiological fitness of the individual. The emergent froglets will be subjected to environmental stressors like high temperature and less humidity of post-monsoon tropical climate that could enhance negative influence triggered by pesticides.

Influence of Nitrate and Nitrite on Thyroid Hormone Responsive and Stress-Associated Gene Expression in Cultured Rana catesbeiana Tadpole Tail Fin Tissue

Nitrate and nitrite are common aqueous pollutants that are known to disrupt the thyroid axis. In amphibians, thyroid hormone (TH)-dependent metamorphosis is affected, although whether the effect is acceleration or deceleration of this developmental process varies from study to study. One mechanism of action of these nitrogenous compounds is through alteration of TH synthesis. However, direct target tissue effects on TH signaling are hypothesized. The present study uses the recently developed cultured tail fin biopsy (C-fin) assay to study possible direct tissue effects of nitrate and nitrite. Tail biopsies obtained from premetamorphic Rana catesbeiana tadpoles were exposed to 5 and 50 mg/L nitrate (NO3–N) and 0.5 and 5 mg/L nitrite (NO2–N) in the absence and presence of 10 nM T3. Thyroid hormone receptor β (TRβ) and Rana larval keratin type I (RLKI), both of which are TH-responsive gene transcripts, were measured using quantitative real time polymerase chain reaction. To assess cellular stress which could affect TH signaling and metamorphosis, heat shock protein 30, and catalase (CAT) transcript levels were also measured. We found that nitrate and nitrite did not significantly change the level of any of the four transcripts tested. However, nitrate exposure significantly increased the heteroscedasticity in response of TRβ and RLKI transcripts to T3. Alteration in population variation in such a way could contribute to the previously observed alterations of metamorphosis in frog tadpoles, but may not represent a major mechanism of action.

Effects of Hydroperiod Duration on Survival, Developmental Rate, and Size at Metamorphosis in Boreal Chorus Frog Tadpoles (Pseudacris maculata)

Understanding the relationship between climate-driven habitat conditions and survival is key to preserving biodiversity in the face of rapid climate change. Hydroperiod—the length of time water is in a wetland—is a critical limiting habitat variable for amphibians as larvae must metamorphose before ponds dry. Changes in precipitation and temperature patterns are affecting hydroperiod globally, but the impact of these changes on amphibian persistence is poorly understood. We studied the responses of Boreal Chorus Frog (Pseudacris maculata) tadpoles to simulated hydroperiods (i.e., water level reductions) in the laboratory using individuals collected from ponds spanning a range of natural hydroperiods (Colorado Front Range, USA). To assess the effects of experimental hydroperiod reduction, we measured mortality, time to metamorphosis, and size at metamorphosis. We found that tadpoles grew at rates reflecting the hydroperiods of their native ponds, regardless of experimental treatment. Tadpoles from permanent ponds metamorphosed faster than those from ephemeral ponds across all experimental treatments, a pattern which may represent a predation selection gradient or countergradient variation in developmental rates. Size at metamorphosis did not vary across experimental treatments. Mortality was low overall but varied with pond of origin. Our results suggest that adaptation to local hydroperiod and/or predation and temperature conditions is important in P. maculata. Moreover, the lack of a plastic response to reduced hydroperiods suggests that P. maculata may not be able to metamorphose quickly enough to escape drying ponds. These results have important implications for amphibian persistence in ponds predicted to dry more quickly due to rapid climate change.

Impacts of artificial drainage on amphibian breeding sites in hemiboreal forests

Artificial drainage is widely used to increase timber production and improve its quality in northern boreal and temperate forests, but its impact on biodiversity remains largely unexplored. Amphibians are particularly dependent on forest wetland qualities to complete their complex life cycles. We explored the supply of water bodies, their occupancy by amphibians, and persistence during the larval period comparatively in drained and natural forest plots in two years in Estonia. The water bodies in drained plots were more abundant but less diverse and of generally lower quality than in natural plots; however, the drainage ditches were favoured by newts. In the dry spring of 2009, water bodies were used more frequently in natural than in drained plots, while in the typical spring of 2010, several natural depressions appeared in drained plots and the frequency of use of water bodies did not differ between natural and drained plots. However, most natural depressions used for breeding in drained plots dried out before tadpole metamorphosis; such drying was not observed in natural plots. Thus, by profoundly changing natural hydrology, intensive large-scale forest drainage reduces the variety of water bodies and the quantity and quality of amphibian breeding sites. Moreover, we suspect that, at least in some years, temporary water bodies in drained areas function as ecological traps for amphibians. We suggest that, where possible, intact wet-forest areas with remaining natural hydrology should stay undrained. In areas already drained, breeding sites for amphibians can be provided by creating depression-like enlargements to some ditches.

Insecticide has asymmetric effects on two tadpole species despite priority effects

Priority effects are defined as asymmetric responses of different species colonizing the same habitat in different sequences and are important in structuring communities, particularly for ephemeral systems that are colonized annually. Amphibians often use ephemeral ponds and have been shown to experience priority effects from interspecific competition. Pesticide exposure can impact amphibian species interactions like competition, and has been linked to declines. We investigated whether insecticide exposure interacted with colonization history to impact priority effects in Bufo americanus and Rana pipiens. We predicted that exposure to the insecticide carbaryl and colonization history would affect tadpole metamorphosis independently and interactively. Our results indicated that Bufo time to metamorphosis experienced priority effects: Bufo introduced early reached metamorphosis sooner when Rana were absent, but Bufo introduced late reached metamorphosis sooner when Rana were also late. Insecticide exposure eliminated differences in Bufo time to metamorphosis due to Rana colonization history. Rana survival and mass at metamorphosis were affected by intraspecific colonization history. Bufo had significantly fewer survivors and Rana had more survivors in insecticide-exposed treatments. Bufo did better when reared without Rana, but Rana was unaffected by Bufo presence. The effects on anuran metamorphosis occurred without any effects on periphyton abundance. We suggest that the effects of insecticide exposure on amphibian metamorphosis are not related to colonization history, although each factor alone can have substantial impacts. These results preliminarily suggest that managing pesticide applications with sensitivity to amphibians may be less complex than implied by traditional food web theory, although more research is needed.

KRÜPPEL-LIKE FACTOR 9 ENHANCES THYROID HORMONE RECEPTOR β AUTOINDUCTION IN TADPOLE BRAIN IN VIVO, INCREASING TISSUE SENSITIVITY TO THYROID HORMONE AND ACCELERATING METAMORPHOSIS

Event Abstract Back to Event KRÜPPEL-LIKE FACTOR 9 ENHANCES THYROID HORMONE RECEPTOR β AUTOINDUCTION IN TADPOLE BRAIN IN VIVO, INCREASING TISSUE SENSITIVITY TO THYROID HORMONE AND ACCELERATING METAMORPHOSIS Fang Hu1, Joseph Knoedler1 and Robert J. Denver1* 1 University of Michigan, Molecular, Cellular and Developmental Biology, United States Thyroid hormone (T3) receptor β (TRβ) is a direct T3 target gene that is strongly induced (autoinduced) during tadpole metamorphosis. TRβ autoinduction is thought to be critical for hormone action during metamorphosis. The transcription factor Krüppel-like factor 9 (KLF9) is the most rapidly responding T3 target gene thus far identified. Previously, we showed in the Xenopus cell line XTC-2 that KLF9 associates with the TRβ promoter to accelerate and enhance TRβ autoinduction. Here we show that KLF9 promotes TRβ autoinduction in tadpole brain in vivo. We used electroporation-mediated (EM) gene transfer to transfect plasmids into the brains of early prometamorphic tadpoles to express wild type or mutant forms of KLF9. Forced expression of KLF9 increased basal TRβ mRNA expression in euthyroid tadpoles, but had no effect in methimazole-treated, hypothyroid animals, suggesting that KLF9’s major action is to enhance liganded TR action. Similar to our findings in XTC-2 cells, forced expression of KLF9 accelerated TRβ autoinduction in tadpole brain in vivo and enhanced T3-dependent induction of the TR target gene TH/bZip. Consistent with our previous mutagenesis experiments conducted in XTC-2 cells, the actions of KLF9 in vivo did not depend on its DNA binding activity, but required the second transactivation domain ‘B’. Chronic, forced expression of KLF9 in tadpole brain accelerated metamorphosis, which we hypothesize is due to KLF9 increasing TRβ expression, and thereby enhancing tissue sensitivity to T3. In support of this hypothesis, we found that forced expression of TRβ in tadpole brain in vivo by EM gene transfer increased expression of TR target genes. We conclude that the immediate early transcription factor KLF9 participates in a transcriptional regulatory network to promote TRβ autoinduction in vivo, forming a positive feedback loop that sensitizes the cell to further hormonal stimulation owing to the production of more TRβ. Acknowledgements Supported by NIH grant R01 NS046690 and NSF grant IOS 0922583 to RJD Keywords: Autoinduction, Hypothalamus, metamorphosis, nuclear receptor, Thyroid hormone, Xenopus Conference: NASCE 2011: The inaugural meeting of the North American Society for Comparative Endocrinology, Ann Arbor, United States, 13 Jul - 16 Jul, 2011. Presentation Type: Poster Topic: Nuclear receptors Citation: Hu F, Knoedler J and Denver RJ (2011). KRÜPPEL-LIKE FACTOR 9 ENHANCES THYROID HORMONE RECEPTOR β AUTOINDUCTION IN TADPOLE BRAIN IN VIVO, INCREASING TISSUE SENSITIVITY TO THYROID HORMONE AND ACCELERATING METAMORPHOSIS. Front. Endocrinol. Conference Abstract: NASCE 2011: The inaugural meeting of the North American Society for Comparative Endocrinology. doi: 10.3389/conf.fendo.2011.04.00019 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Jul 2011; Published Online: 09 Aug 2011. * Correspondence: Prof. Robert J Denver, University of Michigan, Molecular, Cellular and Developmental Biology, Ann Arbor, United States, Rdenver@umich.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Fang Hu Joseph Knoedler Robert J Denver Google Fang Hu Joseph Knoedler Robert J Denver Google Scholar Fang Hu Joseph Knoedler Robert J Denver PubMed Fang Hu Joseph Knoedler Robert J Denver Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

DISCOVERING THE REGULATORY LOGIC OF AN ANCIENT, EVOLUTIONARILY CONSERVED NUCLEAR RECEPTOR ENHANCER MODULE

Event Abstract Back to Event DISCOVERING THE REGULATORY LOGIC OF AN ANCIENT, EVOLUTIONARILY CONSERVED NUCLEAR RECEPTOR ENHANCER MODULE Pia Bagamasbad1, Ronald Bonett2 and Robert J. Denver1* 1 University of Michigan, Molecular, Cellular and Developmental Biology, United States 2 The University of Tulsa, Biological Science, United States Different hormones may cooperate to regulate physiology and development. A striking example is the acceleration of tadpole metamorphosis by the synergistic actions of thyroid hormone (T3) and glucocorticoid (GC). Krüppel-like factor 9 (KLF9) is a transcription factor that mediates hormone action during development; the Klf9 gene is directly regulated by T3 receptor (TR) and GC receptor (GR). We investigated whether the Klf9 gene is synergistically transactivated by T3 plus GC. Gene expression analysis in frog and mouse cell lines, and in tadpole brain showed that treatment with T3 plus GC synergistically transactivated Klf9. Kinetic analysis of mRNA and heteronuclear RNA accumulation supported that synergy occurred at the transcriptional level. Transfection assays with reporter constructs containing 1 kb fragments of the 5’ flanking region of the frog Klf9 gene identified a region between -5 and -6 kb that supported synergistic transactivation. Comparative sequence analysis showed an evolutionarily conserved region of ~180 bp within the -5 to -6 kb fragment. The frog and mouse ~180 bp sequences supported synergistic transactivation, supporting that this region comprises an enhancer module. In silico analysis identified several putative hormone response elements (HREs) for TR and GR. Using gel shift, DNAseI footprinting and chromatin immunoprecipitation (ChIP) assays we found that TR and GR associate with specific sequences within the enhancer module. Site-directed mutagenesis of any of the HRE half sites abolished the synergistic response, supporting that direct DNA binding by TR and GR are necessary for synergistic transactivation. ChIP assays spanning the Klf9 locus for acetylated histones 3 and 4 showed elevated histone acetylation at the NR synergy module, but no augmentation of acetylation by combined hormone treatment. We are now investigating differential recruitment of RNA polymerase II as a possible molecular basis for hormone synergy. Acknowledgements Supported by NIH grant R01 NS046690 and NSF grant IOS 0922583 to RJD Keywords: Chromatin, Glucorticoid, metamorphosis, nuclear receptor, Synergy, thyroid, transcription Conference: NASCE 2011: The inaugural meeting of the North American Society for Comparative Endocrinology, Ann Arbor, United States, 13 Jul - 16 Jul, 2011. Presentation Type: Oral Presentation Topic: Nuclear receptors Citation: Bagamasbad P, Bonett R and Denver RJ (2011). DISCOVERING THE REGULATORY LOGIC OF AN ANCIENT, EVOLUTIONARILY CONSERVED NUCLEAR RECEPTOR ENHANCER MODULE. Front. Endocrinol. Conference Abstract: NASCE 2011: The inaugural meeting of the North American Society for Comparative Endocrinology. doi: 10.3389/conf.fendo.2011.04.00002 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Jul 2011; Published Online: 09 Aug 2011. * Correspondence: Prof. Robert J Denver, University of Michigan, Molecular, Cellular and Developmental Biology, Ann Arbor, United States, Rdenver@umich.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Pia Bagamasbad Ronald Bonett Robert J Denver Google Pia Bagamasbad Ronald Bonett Robert J Denver Google Scholar Pia Bagamasbad Ronald Bonett Robert J Denver PubMed Pia Bagamasbad Ronald Bonett Robert J Denver Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Does detritus quality predict the effect of native and non‐native plants on the performance of larval amphibians?

Summary1. The lack of consistent differences between the traits of native and non‐native plant species makes it difficult to make general predictions about the ecological impact of invasive plants; however, the increasing number of non‐native plants in many habitats makes the assessment of the impact of each individual species impracticable. General knowledge about how specific plant traits are linked to their effects on communities or ecosystems may be more useful for predicting the effect of plant invasions. Specifically, we hypothesised that higher carbon‐to‐nitrogen ratio (C:N) and percent lignin in plant detritus would reduce the rate of development and total mass at metamorphosis of tadpoles, resulting in lower metamorph production (total fresh biomass) and amphibian species richness.2. To test these hypotheses, we raised five species of tadpoles in mesocosms containing senescent leaves of three common native and three common non‐native wetland plants that varied in C:N ratio and % lignin.3. Leaf mass loss, total metamorph production and the number of species that metamorphosed declined as a function of increasing C:N ratio of plant leaves. Plant lignin content was not related to the production of metamorphs or the number of species that metamorphosed. The percentage of wood frog (Lithobates sylvaticus) and American toad (Anaxyrus americanus) tadpoles reaching metamorphosis declined as a function of increasing plant C:N ratio. Mean time to metamorphosis increased and mean mass at metamorphosis declined as a function of increasing plant C:N ratio. Tadpole performance and metamorph diversity and production (biomass) were similar between native and non‐native plant species with similar C:N ratio in leaves. Percent lignin was not a significant predictor of tadpole performance.4. Our results show that the impact of a plant invasion on tadpole performance could depend on differences between the quality of the detritus produced by the invading species and that of the native species it replaces. We suggest that plant community changes that lead to dominance by more recalcitrant plant species (those with higher leaf C:N ratio) may negatively affect amphibian populations.

PERSONAL CARE PRODUCTS: Triclosan Comes under Scrutiny

PERSONAL CARE PRODUCTS: Triclosan Comes under Scrutiny

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 (TRβ) 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 TRβ 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.

Effects of Salinity on the Survival, Growth, Development, and Metamorphosis ofFejervarya limnocharisTadpoles Living in Brackish Water

We studied salinity tolerance and the effects of salinity on growth, development, and metamorphosis in Fejervarya limnocharis tadpoles living in brackish water. Specifically, we examined whether tadpoles exhibit adaptive plasticity in development when exposed to different salinities. Tadpoles collected on Green and Orchid Islands off Southeastern Taiwan were assigned to salinities of 0, 3, 5, 7, 9, 11, and 13 parts per thousand (ppt). The daily survival, weekly growth, and development of tadpoles were recorded until metamorphosis. More than 50% of tadpoles survived in 9 ppt for over a month, and a few individuals survived in 11 ppt for 20 days, suggesting that F. limnocharis tadpoles tolerate salinity better than the tadpoles of most species studied to date. Tadpoles at 9 ppt had lower survivorship, and retarded growth and development (from Gosner stage 26 to 35) compared to the other treatments. Tadpoles metamorphosed early at a smaller size as salinity increased, suggesting the existence of adaptive developmental plasticity in F. limnocharis in response to osmotic stress. Phenotypic plasticity in the age and size at metamorphosis in response to salinity may provide a means for tadpoles to adapt to the unpredictable salinity variation in coastal rock pools.

Energetics of metamorphic climax in the pickerel frog ( Lithobates palustris)

Anuran metamorphosis, the transition from aquatic larvae to terrestrial juveniles, is accompanied by significant morphological, physiological, and behavioral changes. Timing of metamorphosis and final size, which can influence adult fitness, may depend on sufficient energy accumulated during the larval period to support metamorphosis. However, only two species of anurans have been examined for energetic costs of metamorphosis, Rana tigrina and Anaxyrus terrestris. Based on these species, it has been hypothesized that differences in energy expenditure are related to duration of metamorphosis. To compare energetic costs of metamorphosis among species and examine this hypothesis, we quantified the total energy required for metamorphosis of Lithobates palustris tadpoles by measuring oxygen consumption rates over the duration of metamorphic climax using closed-circuit respirometry. Total energy costs for L. palustris were positively related to tadpole mass and duration of metamorphic climax. However, larger tadpoles completed metamorphosis more efficiently because they used proportionally less total energy for metamorphic climax than smaller counterparts. Costs were intermediate to R. tigrina, a larger species with similar metamorphic duration, and A. terrestris, a smaller species with shorter metamorphic climax. The results supported the hypothesis that amphibian species with more slowly developing tadpoles, such as ranids, require more absolute energy for metamorphosis in comparison to more rapidly developing species like bufonids.

Effects of cadmium on growth, metamorphosis and gonadal sex differentiation in tadpoles of the African clawed frog, Xenopus laevis

Xenopus laevis larvae were exposed to cadmium (Cd) at 0, 1, 8, 85 or 860 μg L −1 in FETAX medium from 0 to 86 d postfertilization. Premetamorphic tadpoles were sampled on day 31; pre and prometamorphic tadpoles on day 49; and frogs (NF stage 66) between days 50 and 86. Survival, snout-vent length (SVL), tail length, total length, hindlimb length (HLL), initiation of metamorphic climax, size at and completion of metamorphosis, and gonadal condition and sex ratio (assessed histologically) were determined. Survival was unaffected by Cd until day 49, but increased mortality was observed after day 49 at 860 μg Cd L −1. On day 31, when tadpoles were in early premetamorphosis, inhibitory effects on tadpole growth were observed only at 860 μg Cd L −1. On day 49, when most tadpoles where in late premetamorphosis/early prometamorphosis, reductions in SVL, HLL and total length were observed at 8 and 860 but not 85 μg L −1, thus creating a U-shaped size distribution at 0–85 μg Cd L −1. However, this U-shaped size pattern was not evident in postmetamorphic individuals. In fact, frog size at completion of metamorphosis was slightly smaller at 85 μg Cd L −1relative to control animals. These observations confirmed a recent report of a Cd concentration-dependent bimodal growth pattern in late-premetamorphic Xenopus tadpoles, but also showed that growth responses to varying Cd concentrations change with development. The fraction of animals initiating or completing metamorphosis during days 50–86 was reduced in a Cd concentration-dependent manner. Testicular histology and population sex ratios were unaffected by Cd suggesting that, unlike mammals, Cd is not strongly estrogenic in Xenopus tadpoles.

Neural stem cells at the crossroads: MMPs may tell the way

Matrix metalloproteinases (MMP) constitute a family of more than 25 enzymes which process a large number of pericellular substrates. Even though initially reported to have an ability to degrade almost all of the extracellular components, MMP are now known to play roles which are not limited to the breakdown of extracellular barriers. In fact, MMPs regulate many biological processes, being involved not only in physiological events, but also in pathological processes. Strikingly, MMPs have been found to be involved in the physiology of the Central Nervous System (CNS), taking part and playing important roles in several processes such as repair and ontogeny, as well as in pathological conditions of the CNS. Initially considered to be a static structure, lacking regenerative capability, the CNS has been considered for a long time to be a system without renewal capabilities. Recently, the discovery of constant neural replacement has changed our way of considering the adult brain, and the finding of the existence of neural stem cells has opened the way to exciting and fascinating perspectives of the CNS. So, could MMPs, originally found during metamorphosis in tadpoles, and now amazingly identified in the CNS, have something to do in neuronal function? In this review we take into consideration the possible roles of two metalloproteinases, MMP-2 and MMP-9, also called gelatinases, in controlling several aspects of CNS organization, including the modulation of neural stem cell properties and the differentiation of their progeny, both under normal and pathophysiological conditions.

Disruption of Thyroid Hormone Function by Environmental Pollutants

Ag reat number of synthetic chemicals are released into the environment, many of which are known or thought to interfere with normal thyroid hormone (TH) function. THs play important roles in regulating growth and development and maintaining metabolic homeostasis. For example, amphibian tadpole metamorphosis is a TH-triggered and controlled developmental process, and has proven to be useful as a screening tool for environmental pollutants suspected of disrupting TH functions. TH disruption is thought to be caused through a variety of mechanisms, including increased thyroxine (T4 )m etabolism by uridine diphosphate glucuronyl transferases (UDPGTs), blocking TH signaling through TH receptors (TRs), and inductio no fm itochondrial membrane permeability transition (MPT). As our knowledge concerning the specific effects of these chemicals is very limited, further research is needed to obtain accurate informatio nt ob eu se di ne stablishing guidelines for the protection of health in humans and wildlife.