Xenopus tropicalis: an ideal experimental animal in amphibia

A minimally invasive procedure for blood extraction from Xenopus laevis allows follow up studies without euthanasia

Blood extraction is extremely important for the development of scientific research; however, the existing methods for amphibian´s blood sampling are invasive, mainly leading to the euthanasia of the animal. Therefore, less intrusive methods that allow the obtention of multiple samples from the same individual, are needed as an alternative to the common methods available. Hence, the aim of this study was to propose a minimally invasive method for obtaining blood from the hind leg of Xenopus laevis, that allows continuous sampling without compromising the wellbeing of the organisms. With this method, it was possible to extract blood and plasma from adults and juveniles, and the amount of sample was enough to perform biochemical and molecular assays to assess the viability of the blood. The results also revealed that this method is a convenient alternative to obtain blood without affecting the welfare of the experimental organisms, avoiding the cull of the animals, and the samples are viable for their use in follow up studies.

Developmental expression of peroxiredoxin gene family in early embryonic development of Xenopus tropicalis

Peroxidase genes (Prdx) encode a family of antioxidant proteins, which can protect cells from oxidative damage by reducing various cellular peroxides. This study investigated the spatiotemporal expression patterns of gene members in this family during the early development of Xenopus tropicalis. Real-time quantitative PCR showed that all members of this gene family have a distinct temporal expression pattern during the early development of X. tropicalis embryos. Additionally, whole mount in situ hybridization revealed that individual prdx genes display differential expression patterns, with overlapping expression in lymphatic vessels, pronephros, proximal tubule, and branchial arches. This study provides a basis for further study of the function of the prdx gene family.

Generation of translucent Xenopus tropicalis through triple knockout of pigmentation genes

Amphibians generally have three types of pigment cells, namely, melanophores (black and brown), xanthophores (yellow and red), and iridophores (iridescent). Single knockout of the tyr, slc2a7, and hps6 genes in Xenopus tropicalis results in the absence of melanophores, xanthophores, and iridophores, respectively. The generation of triple- knockout (3KO) X. tropicalis for these three genes could allow for observation of internal organs without sacrificing the animals, which would be transparent due to the absence of pigments. In this study, we generated 3KO X. tropicalis, which is one of the most widely used model amphibians, through crossing of a slc2a7 single-knockout frog with a tyr and hps6 double-knockout frog, followed by intercrossing of their offspring. The 3KO tadpoles had transparent bodies like the nop mutant and the frogs had translucent bodies. This translucency allowed us to observe the heart, lungs, stomach, liver, and digestive tract through the ventral body skin without surgery. After intravital staining, 3KO X. tropicalis showed much clearer fluorescent signals of mineralized tissues compared with the wild type. These 3KO X. tropicalis provide a useful mutant line for continuous observation of internal organs and fluorescent signals in the body. In particular, such 3KO frogs would revolutionize fluorescence monitoring in transgenic tadpoles and frogs expressing fluorescent proteins.

Isolation and evaluation of erythroid progenitors in the livers of larval, froglet, and adult Xenopus tropicalis

Xenopus liver maintains erythropoietic activity from the larval to the adult stage. During metamorphosis, thyroid hormone mediates apoptosis of larval-type erythroid progenitors and proliferation of adult-type erythroid progenitors, and a globin switch occurs during this time. In addition, the whole-body mass and the liver also change; however, whether there is a change in the absolute number of erythroid progenitors is unclear. To isolate and evaluate erythroid progenitors in the Xenopus liver, we developed monoclonal ER9 antibodies against the erythropoietin receptor (EPOR) of Xenopus. ER9 recognized erythrocytes, but not white blood cells or thrombocytes. The specificity of ER9 for EPOR manifested as its inhibitory effect on the proliferation of a Xenopus EPOR-expressing cell line. Furthermore, ER9 recognition was consistent with epor gene expression. ER9 staining with Acridine orange (AO) allowed erythrocyte fractionation through fluorescence-activated cell sorting. The ER9+ and AO-red (AOr)high fractions were highly enriched in erythroid progenitors and primarily localized to the liver. The method developed using ER9 and AO was also applied to larvae and froglets with different progenitor populations from adult frogs. The liver to body weight and the number of ER9+ AOrhigh cells per unit body weight were significantly higher in adults than in larvae and froglets, and the number of ER9+ AOrhigh cells per unit liver weight was the highest in froglets. Collectively, our results show increased erythropoiesis in the froglet liver and demonstrate growth-dependent changes in erythropoiesis patterns in specific organs of Xenopus.

Long-term Xenopus laevis tadpole -heart-organ-culture: Physiological changes in cholinergic and adrenergic sensitivities of tadpole heart with thyroxine-treatment

The present study clarified changes in physiological sensitivities of cultured Nieuwkoop and Faber stage 57 Xenopus laevis tadpole-organ-heart exposed to thyroxine (T4) using acetylcholine (ACh), norepinephrine (NE) and atropine. For preliminary life span and the chemical tests, 60% minimum essential medium (MEM), two types of modified Hank's balanced salt-solution-culture-media (MHBSS-CM) I and II containing relatively lower concentrations of amino acids and collagen were prepared. In preliminary lifespan-test of cultured tadpole hearts, the hearts maintained in 60% MEM was 50 days on average, whereas that of the tadpole-hearts in MHBSS-CMs was extended by 109 days on average, showing superior effectiveness of MHBSS-CMs. 4 min-stimulation by 5 × 10-9 M T4 tended to increase the tadpole heartbeat. 10-9 M ACh decreased the tadpole heartbeat. Frog-heart at 2-4 weeks after metamorphosis completion and tadpole heart treated with 5 × 10-10 M T4 for 45 h also responded to 10-9 M ACh, and low-resting hearts were restored to the control level with the competitive muscarinic antagonist 10-8 M atropine, whereas excessive exposure of 10-5 M atropine to T4-treated tadpole heart did not increase heartbeat in spite of the increased frog heartbeat over the control. 10-14 -10-12 M NE increase the tadpole heartbeat in a concentration-dependent manner, however, 10-12 M NE did not act to stimulate adrenergic receptors on both T4-treated tadpole- and the frog-hearts. These results suggest that T4 induces the desensitization of atropine-sensitive muscarinic and adrenergic receptors in organ-cultured tadpole-heart.

Echocardiographic assessment of Xenopus tropicalis heart regeneration

BACKGROUND

Recently, it was reported that the adult X. tropicalis heart can regenerate in a nearly scar-free manner after injury via apical resection. Thus, a cardiac regeneration model in adult X. tropicalis provides a powerful tool for recapitulating a perfect regeneration phenomenon, elucidating the underlying molecular mechanisms of cardiac regeneration in an adult heart, and developing an interventional strategy for the improvement in the regeneration of an adult heart, which may be more applicable in mammals than in species with a lower degree of evolution. However, a noninvasive and rapid real-time method that can observe and measure the long-term dynamic change in the regenerated heart in living organisms to monitor and assess the regeneration and repair status in this model has not yet been established.

RESULTS

In the present study, the methodology of echocardiographic assessment to characterize the morphology, anatomic structure and cardiac function of injured X. tropicalis hearts established by apex resection was established. The findings of this study demonstrated for the first time that small animal echocardiographic analysis can be used to assess the regeneration of X. tropicalis damaged heart in a scar-free perfect regeneration or nonperfect regeneration with adhesion manner via recovery of morphology and cardiac function.

CONCLUSIONS

Small animal echocardiography is a reliable, noninvasive and rapid real-time method for observing and assessing the long-term dynamic changes in the regeneration of injured X. tropicalis hearts.

TRPM8 thermosensation in poikilotherms mediates both skin colour and locomotor performance responses to cold temperature

Thermoregulation is a homeostatic process to maintain an organism's internal temperature within a physiological range compatible with life. In poikilotherms, body temperature fluctuates with that of the environment, with both physiological and behavioral responses employed to modify body temperature. Changing skin colour/reflectance and locomotor activity are both well-recognized temperature regulatory mechanisms, but little is known of the participating thermosensor/s. We find that Xenopus laevis tadpoles put in the cold exhibit a temperature-dependent, systemic, and rapid melanosome aggregation in melanophores, which lightens the skin. Cooling also induces a reduction in the locomotor performance. To identify the cold-sensor, we focus on transient receptor potential (trp) channel genes from a Trpm family. mRNAs for several Trpms are present in Xenopus tails, and Trpm8 protein is present in skin melanophores. Temperature-induced melanosome aggregation is mimicked by the Trpm8 agonist menthol (WS12) and blocked by a Trpm8 antagonist. The degree of skin lightening induced by cooling is correlated with locomotor performance, and both responses are rapidly regulated in a dose-dependent and correlated manner by the WS12 Trpm8 agonist. We propose that TRPM8 serves as a cool thermosensor in poikilotherms that helps coordinate skin lightening and behavioural locomotor performance as adaptive thermoregulatory responses to cold.

Acute and chronic toxicity tests of systemic insecticides, four neonicotinoids and fipronil, using the tadpoles of the western clawed frog Silurana tropicalis

Extensive use of neonicotinoids and fipronil, which are popular systemic insecticides used in Japanese rice paddies, has raised concerns about their impacts on nontarget aquatic organisms such as amphibians. This study employed premetamorphic tadpoles of Silurana tropicalis and addressed the toxicity of four neonicotinoids (acetamiprid, clothianidin, dinotefuran, and imidacloprid) and fipronil. Acute toxicity tests were conducted under a 96-h semistatic exposure regime and median lethal concentration (LC50) values were calculated at 24-h intervals. All LC50 values of the four neonicotinoids exceeded 100 mg/L, suggesting their low acute toxicity to amphibians. Fipronil yielded much lower LC50 values (3.00-1.34 mg/L) and was highly toxic compared to the four neonicotinoids. Additionally, exposure to fipronil at >1 mg/L induced axial malformations, suggesting its teratogenicity. However, the LC50 values of fipronil were three orders of magnitude higher than the realistic concentrations in paddy water. Chronic toxicity tests were conducted with morphometric, gravimetric, and thyroid-histological endpoints. Premetamorphic tadpoles were exposed to each insecticide at two test concentrations: 0.1 and 1.0 mg/L for the four neonicotinoids; and 1/100 and 1/10 of the 96-h LC50 value for fipronil. Exposure to each insecticide continued until all tadpoles in the control reached late prometamorphic stages or the initial stage of metamorphic climax. At test termination, all insecticides showed no significant differences in any of the endpoints between the respective controls and chemical exposure groups. Overall, our results suggest that these insecticides alone do not directly affect amphibians through their larval stages at concentrations that likely occur in paddy water.

Generation of no-yellow-pigment Xenopus tropicalis by slc2a7 gene knockout

BACKGROUND

Amphibians possess three kinds of dermal chromatophore: melanophores, iridophores, and xanthophores. Knockout Xenopus tropicalis that lack the pigmentation of melanophores and iridophores have been reported. The identification of the causal genes for xanthophore pigmentation or differentiation could lead to the creation of a see-through frog without three chromatophores. The genes causing xanthophore differentiation mutants are slc2a11b and slc2a15b in Japanese medaka (Oryzias latipes).

RESULTS

To obtain a heritable line of X tropicalis mutants without yellow pigment, we generated slc2a7 and slc2a15a knockout animals because they have the greatest similarity to the O latipes slc2a11b and slc2a15b genes. The slc2a7 knockout frog had a bluish skin and there were no visible yellow pigments in stereo microscope and skin section observations. Furthermore, no pterinosomes, which are characteristic of xanthophores, were observed via transmission electron microscopy in the skin of knockout animals.

CONCLUSIONS

We report the successful generation of a heritable no-yellow-pigment X tropicalis mutant after knock out of the slc2a7 gene. This finding will enable the creation of a see-through frog with no chromatophores.

Xenopus: Experimental Access to Cardiovascular Development, Regeneration Discovery, and Cardiovascular Heart-Defect Modeling

Xenopus has been used to study a wide array of developmental processes, benefiting from vast quantities of relatively large, externally developing eggs. Xenopus is particularly amenable to examining the cardiac system because many of the developmental processes and genes involved in cardiac specification, differentiation, and growth are conserved between Xenopus and human and have been characterized in detail. Furthermore, compared with other higher vertebrate models, Xenopus embryos can survive longer without a properly functioning heart or circulatory system, enabling investigation of later consequences of early embryological manipulations. This biology is complemented by experimental technology, such as embryonic explants to study the heart, microinjection of overexpression constructs, and, most recently, the generation of genetic mutations through gene-editing technologies. Recent investigations highlight Xenopus as a powerful experimental system for studying injury/repair and regeneration and for congenital heart disease (CHD) modeling, which reinforces why this model system remains ideal for studying heart development.

Ambient temperature alters body size and gut microbiota of Xenopus tropicalis

Temperature is important to determine physiological status of ectotherms. However, it is still not fully understood how amphibians and their symbiotic microbiota acclimate to ambient temperature. In this study, we investigated the changes of gut microbiota of Xenopus tropicalis at different temperatures under controlled laboratory conditions. The results showed that microbial communities were distinct and shared only a small overlap among froglet guts, culture water and food samples. Furthermore, the dominant taxa harbored in the gut exhibited low relative abundance in water and food. It indicates that bacterial taxa selected by amphibian gut were generally of low abundance in the external environment. Temperature could affect beta-diversity of gut microbiota in terms of phylogenetic distance, but it did not affect alpha diversity. The composition of gut microbiota was similar in warm and cool treatments. However, signature taxa in different temperature environments were identified. The relationships between temperature, gut microbiota and morphology traits of X. tropicalis revealed in this study help us to predict the consequences of environmental changes on ectothermic animals.

Xenopus Resources: Transgenic, Inbred and Mutant Animals, Training Opportunities, and Web-Based Support

Two species of the clawed frog family, Xenopus laevis and X. tropicalis, are widely used as tools to investigate both normal and disease-state biochemistry, genetics, cell biology, and developmental biology. To support both frog specialist and non-specialist scientists needing access to these models for their research, a number of centralized resources exist around the world. These include centers that hold live and frozen stocks of transgenic, inbred and mutant animals and centers that hold molecular resources. This infrastructure is supported by a model organism database. Here, we describe much of this infrastructure and encourage the community to make the best use of it and to guide the resource centers in developing new lines and libraries.

Functional properties of axolotl transient receptor potential ankyrin 1 revealed by the heterologous expression system

Transient receptor potential ankyrin 1 (TRPA1) from tetrapod vertebrates except rodents are activated by high temperature with a relatively clear threshold. Our recent investigation suggested that a gradual heat activation without clear threshold might be a common feature for TRPA1 of fish. To approach which animal first acquires TRPA1 as a threshold detector instead of a gradual heat sensor, here, we focused on TRPA1 from axolotls (Ambystoma mexicanum). We isolated a full-length cDNA of axolotl transient receptor potential ankyrin 1 (axTRPA1) and studied the functional properties by two-electrode voltage clamp method using Xenopus oocytes. Allyl isothiocyanate, caffeine, methyl anthranilate and carvacrol activated axTRPA1 channels. The results indicated that axTRPA1 is heat activated with the average threshold of 39.7°C, suggesting that axTRPA1 already has acquired the functional property of land animals.

Flow cytometric analysis of Xenopus laevis and X. tropicalis blood cells using acridine orange

Automated blood cell counters can distinguish cells based on their size and the presence or absence of a nucleus. However, most vertebrates have nucleated blood cells that cannot be counted automatically. We established an alternative automatic method for counting peripheral blood cells by staining cells with the fluorescent dye acridine orange (AO) and analysing cell populations using flow cytometry (FCM). As promising new animal models, we chose Xenopus laevis and three inbred strains of X. tropicalis. We compared the haematological phenotypes, including blood cell types, cell sizes, cellular structure, and erythrocyte lifespans/turnover rate among X. laevis and the three inbred strains of X. tropicalis. Each cell type from X. laevis was sorted according to six parameters: forward- and side-scattered light emission, AO red and green fluorescence intensity, and cellular red and green fluorescence. Remarkably, the erythrocyte count was the highest in the Golden line, suggesting that genetic factors were associated with the blood cells. Furthermore, immature erythrocytes in anaemic X. laevis could be separated from normal blood cells based on red fluorescence intensity. These results show that FCM with AO staining allows for an accurate analysis of peripheral blood cells from various species.

Multiple massive domestication and recent amplification of Kolobok superfamily transposons in the clawed frog Xenopus

BACKGROUND

DNA transposons are generally destroyed by mutations and have short lifespans in hosts, as they are neutral or harmful to the host and therefore not conserved by natural selection. The clawed frog Xenopus harbors many DNA transposons and certain families, such as T2-MITE, have extremely long lives. These have ancient origins, but have shown recent transposition activity. In addition, certain transposase genes may have been "domesticated" by Xenopus and conserved over long time periods by natural selection. The aim of this study was to elucidate the evolutionary interactions between the host and the long-lived DNA transposon family it contains. Here, we investigated the molecular evolution of the Kolobok DNA transposon superfamily. Kolobok is thought to contribute to T2-MITE transposition.

RESULTS

In the diploid western clawed frog Xenopus tropicalis and the allotetraploid African clawed frog Xenopus laevis, we searched for transposase genes homologous to those in the Kolobok superfamily. To determine the amplification and domestication of these genes, we used molecular phylogenetics and analyses of copy numbers, conserved motifs, orthologous gene synteny, and coding sequence divergence between the orthologs of X. laevis and X. tropicalis, or between those of two distant X. tropicalis lineages. Among 38 X. tropicalis and 24 X. laevis prospective transposase genes, 10 or more in X. tropicalis and 14 or more in X. laevis were apparently domesticated. These genes may have undergone multiple independent domestications from before the divergence of X. laevis and X. tropicalis. In contrast, certain other transposases may have retained catalytic activity required for transposition and could therefore have been recently amplified.

CONCLUSION

Multiple domestication of certain transposases and prolonged conservation of the catalytic activity in others suggest that Kolobok superfamily transposons were involved in complex, mutually beneficial relationships with their Xenopus hosts. Some transposases may serve to activate long-lived T2-MITE subfamilies.

Xenopus embryos to study fetal alcohol syndrome, a model for environmental teratogenesis

Vertebrate model systems are central to characterize the outcomes of ethanol exposure and the etiology of fetal alcohol spectrum disorder (FASD), taking advantage of their genetic and morphological closeness and similarity to humans. We discuss the contribution of amphibian embryos to FASD research, focusing on Xenopus embryos. The Xenopus experimental system is characterized by external development and accessibility throughout embryogenesis, large clutch sizes, gene and protein activity manipulation, transgenesis and genome editing, convenient chemical treatment, explants and conjugates, and many other experimental approaches. Taking advantage of these methods, many insights regarding FASD have been obtained. These studies characterized the malformations induced by ethanol including quantitative analysis of craniofacial malformations, induction of fetal growth restriction, delay in gut maturation, and defects in the differentiation of the neural crest. Mechanistic, biochemical, and molecular studies in Xenopus embryos identified early gastrula as the high alcohol sensitivity window, targeting the embryonic organizer and inducing a delay in gastrulation movements. Frog embryos have also served to demonstrate the involvement of reduced retinoic acid production and an increase in reactive oxygen species in FASD. Amphibian embryos have helped pave the way for our mechanistic, molecular, and biochemical understanding of the etiology and pathophysiology of FASD.

Chronic toxicity of 1,3,5-triazine herbicides in the postembryonic development of the western clawed frog Silurana tropicalis

Seven 1,3,5- triazine (s-triazine) herbicides (ametryn, prometryn, dimethametryn, simazine, atrazine, propazine, and cyanazine) were tested using an amphibian (Silurana tropicalis) metamorphosis assay focusing on morphometric, gravimetric, and thyroid-histological endpoints. Premetamorphic tadpoles were exposed to each s-triazine at 2 concentrations between 1/1000 and 1/10 of the 96-h acute toxicity values, until all tadpoles in the control group reached either the late prometamorphosic stages or the initial stage of metamorphic climax. All s-triazines tested induced significant retardation in growth and development at the higher concentrations (0.2-1.0mg/L), and some of them induced similar effects even at the lower concentrations (0.02-0.1mg/L) while each showing a linear dose-response. Total size of the thyroid glands tended to be reduced corresponding to the delayed development, but without showing histomorphological lesions typical of anti-thyroid chemicals. These consistent results suggest that the s-triazines can act as a chemical stressor inhibiting tadpole growth and development, possibly without disrupting the thyroid axis. In addition, tadpoles exhibiting spinal curvatures appeared in either one or both of the lower and higher concentration groups for each s-triazine tested. The incidence rate in the s-triazine exposure groups where tadpoles with scoliosis were observed ranged from 3.3% to 63.3%, some of which were significantly higher than that in the respective control groups (0-6.7%). It is speculated that the s-triazines may promote to occur axial malformations in developing tadpoles.

Heart regeneration in adult Xenopus tropicalis after apical resection

Background

Myocardium regeneration in adult mammals is very limited, but has enormous therapeutic potentials. However, we are far from complete understanding the cellular and molecular mechanisms by which heart tissue can regenerate. The full functional ability of amphibians to regenerate makes them powerful animal models for elucidating how damaged mature organs are naturally reconstituted in an adult organism. Like other amphibians, such as newts and axolotls, adult Xenopus displays high regenerative capacity such as retina. So far, whether the adult frog heart processes regenerative capacity after injury has not been well delineated.

Results

We examined the regeneration of adult cardiac tissues of Xenopus tropicalis after resection of heart apex. We showed, for the first time, that the adult X. tropicalis heart can regenerate perfectly in a nearly scar-free manner approximately 30 days after injury via apical resection. We observed that the injured heart was sealed through coagulation immediately after resection, which was followed by transient fibrous tissue production. Finally, the amputated area was regenerated by cardiomyocytes. During the regeneration process, the cardiomyocytes in the border area of the myocardium adjacent to the wound exhibited high proliferation after injury, thus contribute the newly formed heart tissue.

Conclusions

Establishing a cardiac regeneration model in adult X. tropicalis provides a powerful tool for recapitulating a perfect regeneration phenomenon and elucidating the underlying molecular mechanisms of cardiac regeneration in an adult heart, and findings from this model may be applicable in mammals.

Electronic supplementary material

The online version of this article (10.1186/s13578-017-0199-6) contains supplementary material, which is available to authorized users.

EVI and MDS/EVI are required for adult intestinal stem cell formation during postembryonic vertebrate development

The gene ectopic viral integration site 1 (EVI) and its variant myelodysplastic syndrome 1 (MDS)/EVI encode zinc-finger proteins that have been recognized as important oncogenes in various types of cancer. In contrast to the established role of EVI and MDS/EVI in cancer development, their potential function during vertebrate postembryonic development, especially in organ-specific adult stem cells, is unclear. Amphibian metamorphosis is strikingly similar to postembryonic development around birth in mammals, with both processes taking place when plasma thyroid hormone (T3) levels are high. Using the T3-dependent metamorphosis in Xenopus tropicalis as a model, we show here that high levels of EVI and MDS/EVI are expressed in the intestine at the climax of metamorphosis and are induced by T3. By using the transcription activator-like effector nuclease gene editing technology, we have knocked out both EVI and MDS/EVI and have shown that EVI and MDS/EVI are not essential for embryogenesis and premetamorphosis in X. tropicalis On the other hand, knocking out EVI and MDS/EVI causes severe retardation in the growth and development of the tadpoles during metamorphosis and leads to tadpole lethality at the climax of metamorphosis. Furthermore, the homozygous-knockout animals have reduced adult intestinal epithelial stem cell proliferation at the end of metamorphosis (for the few that survive through metamorphosis) or during T3-induced metamorphosis. These findings reveal a novel role of EVI and/or MDS/EVI in regulating the formation and/or proliferation of adult intestinal adult stem cells during postembryonic development in vertebrates.-Okada, M., Shi, Y.-B. EVI and MDS/EVI are required for adult intestinal stem cell formation during postembryonic vertebrate development.

Metabolomic approach for identifying and visualizing molecular tissue markers in tadpoles of Xenopus tropicalis by mass spectrometry imaging

In developmental and cell biology it is crucial to evaluate the dynamic profiles of metabolites. An emerging frog model system using Xenopus tropicalis, whose genome sequence and inbred strains are available, is now ready for metabolomics investigation in amphibians. In this study we applied matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI) analysis to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We detected tissue-specific peaks and visualized their distribution in tissues, and distinguished 19 tissues and their specific peaks. We identified, for the first time, some of their molecular localizations via tandem mass spectrometric analysis: hydrocortisone in artery, L-DOPA in rhombencephalon, taurine in eye, corticosterone in gill, heme in heart, inosine monophosphate and carnosine in muscle, dopamine in nerves, and phosphatidylethanolamine (16:0/20:4) in pharynx. This is the first MALDI-MSI study of X. tropicalis tadpoles, as in small tadpoles it is hard to distinguish and dissect the various organs. Furthermore, until now there has been no data about the metabolomic profile of each organ. Our results suggest that MALDI-MSI is potentially a powerful tool for examining the dynamics of metabolomics in metamorphosis as well as conformational changes due to metabolic changes.

Summary: We applied matrix-assisted laser desorption/ionization−mass spectrometry imaging analyses to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We found new molecular markers in various tissues and cells.

Evolution of Heat Sensors Drove Shifts in Thermosensation between Xenopus Species Adapted to Different Thermal Niches

Temperature is one of the most critical environmental factors affecting survival, and thus species that inhabit different thermal niches have evolved thermal sensitivities suitable for their respective habitats. During the process of shifting thermal niches, various types of genes expressed in diverse tissues, including those of the peripheral to central nervous systems, are potentially involved in the evolutionary changes in thermosensation. To elucidate the molecular mechanisms behind the evolution of thermosensation, thermal responses were compared between two species of clawed frogs (Xenopus laevis and Xenopus tropicalis) adapted to different thermal environments. X. laevis was much more sensitive to heat stimulation than X. tropicalis at the behavioral and neural levels. The activity and sensitivity of the heat-sensing TRPA1 channel were higher in X. laevis compared with those of X. tropicalis The thermal responses of another heat-sensing channel, TRPV1, also differed between the two Xenopus species. The species differences in Xenopus TRPV1 heat responses were largely determined by three amino acid substitutions located in the first three ankyrin repeat domains, known to be involved in the regulation of rat TRPV1 activity. In addition, Xenopus TRPV1 exhibited drastic species differences in sensitivity to capsaicin, contained in chili peppers, between the two Xenopus species. Another single amino acid substitution within Xenopus TRPV1 is responsible for this species difference, which likely alters the neural and behavioral responses to capsaicin. These combined subtle amino acid substitutions in peripheral thermal sensors potentially serve as a driving force for the evolution of thermal and chemical sensation.

Towards the bridging of molecular genetics data across Xenopus species

BACKGROUND

The clawed African frog Xenopus laevis has been one of the main vertebrate models for studies in developmental biology. However, for genetic studies, Xenopus tropicalis has been the experimental model of choice because it shorter life cycle and due to a more tractable genome that does not result from genome duplication as in the case of X. laevis. Today, although still organized in a large number of scaffolds, nearly 85% of X. tropicalis and 89% of X. laevis genomes have been sequenced. There is expectation for a comparative physical map that can be used as a Rosetta Stone between X. laevis genetic studies and X. tropicalis genomic research.

RESULTS

In this work, we have mapped using coarse-grained alignment the 18 chromosomes of X. laevis, release 9.1, on the 10 reference scaffolds representing the haploid genome of X. tropicalis, release 9.0. After validating the mapping with theoretical data, and estimating reference averages of genome sequence identity, 37 to 44% between the two species, we have carried out a synteny analysis for 2,112 orthologous genes. We found that 99.6% of genes are in the same organization.

CONCLUSIONS

Taken together, our results make possible to establish the correspondence between 62 and 65.5% of both genomes, percentage of identity, synteny and automatic annotation of transcripts of both species, providing a new and more comprehensive tool for comparative analysis of these two species, by allowing to bridge molecular genetics data among them.

Ouro proteins are not essential to tail regression during Xenopus tropicalis metamorphosis

Tail regression is one of the most prominent transformations observed during anuran metamorphosis. A tadpole tail that is twice as long as the tadpole trunk nearly disappears within 3 days in Xenopus tropicalis. Several years ago, it was proposed that this phenomenon is driven by an immunological rejection of larval-skin-specific antigens, Ouro proteins. We generated ouro-knockout tadpoles using the TALEN method to reexamine this immunological rejection model. Both the ouro1- and ouro2-knockout tadpoles expressed a very low level of mRNA transcribed from a targeted ouro gene, an undetectable level of Ouro protein encoded by a target gene and a scarcely detectable level of the other Ouro protein from the untargeted ouro gene in tail skin. Furthermore, congenital athymic frogs were produced by Foxn1 gene modification. Flow cytometry analysis showed that mutant frogs lacked splenic CD8(+) T cells, which play a major role in cytotoxic reaction. Furthermore, T-cell-dependent skin allograft rejection was dramatically impaired in mutant frogs. None of the knockout tadpoles showed any significant delay in the process of tail shortening during the climax of metamorphosis, which shows that Ouro proteins are not essential to tail regression at least in Xenopus tropicalis and argues against the immunological rejection model.

Inbreeding Ratio and Genetic Relationships among Strains of the Western Clawed Frog, Xenopus tropicalis

The Western clawed frog, Xenopus tropicalis, is a highly promising model amphibian, especially in developmental and physiological research, and as a tool for understanding disease. It was originally found in the West African rainforest belt, and was introduced to the research community in the 1990s. The major strains thus far known include the Nigerian and Ivory Coast strains. However, due to its short history as an experimental animal, the genetic relationship among the various strains has not yet been clarified, and establishment of inbred strains has not yet been achieved. Since 2003 the Institute for Amphibian Biology (IAB), Hiroshima University has maintained stocks of multiple X. tropicalis strains and conducted consecutive breeding as part of the National BioResource Project. In the present study we investigated the inbreeding ratio and genetic relationship of four inbred strains at IAB, as well as stocks from other institutions, using highly polymorphic microsatellite markers and mitochondrial haplotypes. Our results show successive reduction of heterozygosity in the genome of the IAB inbred strains. The Ivory Coast strains clearly differed from the Nigerian strains genetically, and three subgroups were identified within both the Nigerian and Ivory Coast strains. It is noteworthy that the Ivory Coast strains have an evolutionary divergent genetic background. Our results serve as a guide for the most effective use of X. tropicalis strains, and the long-term maintenance of multiple strains will contribute to further research efforts.

Evidence for an amphibian sixth digit

INTRODUCTION

Despite the great diversity in digit morphology reflecting the adaptation of tetrapods to their lifestyle, the number of digits in extant tetrapod species is conservatively stabilized at five or less, which is known as the pentadactyl constraint.

RESULTS

We found that an anuran amphibian species, Xenopus tropicalis (western clawed frog), has a clawed protrusion anteroventral to digit I on the foot. To identify the nature of the anterior-most clawed protrusion, we examined its morphology, tissue composition, development, and gene expression. We demonstrated that the protrusion in the X. tropicalis hindlimb is the sixth digit, as is evident from anatomical features, development, and molecular marker expression.

CONCLUSION

Identification of the sixth digit in the X. tropicalis hindlimb strongly suggests that the prehallux in other Xenopus species with similar morphology and at the same position as the sixth digit is also a vestigial digit. We propose here that the prehallux seen in various species of amphibians generally represents a rudimentary sixth digit.

Complete mitochondrial genome of "Xenopus tropicalis" Asashima line (Anura: Pipidae), a possible undescribed species

The diploid Xenopus tropicalis, with its small nuclear genomic size and short generation time compared to the traditional experimental amphibian X. laevis, is considered a next-generation model animal. Several experimental X. tropicalis lines have been used in research studies. Previous studies showed that the mtDNA sequence of the Asashima line is divergent from other lines and that this line may represent a distinct species. Here, we report the complete nucleotide sequence of this unique X. tropicalis experimental line. The genome is 17,700 bp in length and contains 37 genes commonly found in animal mtDNAs. The 16S rRNA gene sequence in Asashima line differed by over 6% from the standard Nigerian lines (a 3% difference is considered the species threshold in anurans), suggesting that this experimental line is a distinct species from the true X. tropicalis.

Molecular cloning, expression, and signaling pathway of four melanin-concentrating hormone receptors from Xenopus tropicalis

Melanin-concentrating hormone (MCH) mainly regulates feeding in mammals and pigmentation in teleosts. It acts via two G-protein-coupled receptors, MCH receptor 1 (MCHR1) and MCHR2. Although many studies exploring the MCH system in teleosts and mammals have been carried out, studies on other organisms are limited. In this study, we cloned and characterized four MCHR subtypes from the diploid species Xenopus tropicalis (X-MCHRs; X-MCHR1a, R1b, R2a, and R2b). According to a phylogenetic tree of the X-MCHRs, X-MCHR1a and R2a are close to mammalian MCHRs, while X-MCHR1b and R2b are close to teleostean MCHRs. We previously reported that the G-protein coupling capacity of the MCHR subtypes differed between mammals (R1: Gαi/o and Gαq; R2: Gαq) and teleosts (R1: Gαq; R2: Gαi/o and Gαq) in mammalian cell-based assays. By using Ca(2+) mobilization assays with pertussis toxin in CHO dhfr(-) cells, we found that X-MCHR1a promiscuously coupled to both Gαi/o and Gαq, while X-MCHR1b and R2a exclusively coupled to Gαq. However, no Ca(2+) influx was detected in cells transfected with X-MCHR2b. Reverse transcription-PCR showed that the X-MCHR mRNAs were expressed in various tissues. In particular, both X-MCHR1b and R2b were exclusively found in melanophores of the dorsal skin. In skin pigment migration assays, melanophores were weakly aggregated at low concentrations but dispersed at high concentrations of MCH, suggesting possible interactions between X-MCHR1b and R2b for the regulation of body color. These findings demonstrate that X. tropicalis has four characteristic MCHRs and will be useful for elucidating the nature of MCHR evolution among vertebrates.

Retinal stem/progenitor cells in the ciliary marginal zone complete retinal regeneration: a study of retinal regeneration in a novel animal model

Our research group has extensively studied retinal regeneration in adult Xenopus laevis. However, X. laevis does not represent a suitable model for multigenerational genetics and genomic approaches. Instead, Xenopus tropicalis is considered as the ideal model for these studies, although little is known about retinal regeneration in X. tropicalis. In the present study, we showed that a complete retina regenerates at approximately 30 days after whole retinal removal. The regenerating retina was derived from the stem/progenitor cells in the ciliary marginal zone (CMZ), indicating a novel mode of vertebrate retinal regeneration, which has not been previously reported. In a previous study, we showed that in X. laevis, retinal regeneration occurs primarily through the transdifferentiation of retinal pigmented epithelial (RPE) cells. RPE cells migrate to the retinal vascular membrane and reform a new epithelium, which then differentiates into the retina. In X. tropicalis, RPE cells also migrated to the vascular membrane, but transdifferentiation was not evident. Using two tissue culture models of RPE tissues, it was shown that in X. laevis RPE culture neuronal differentiation and reconstruction of the retinal three-dimensional (3-D) structure were clearly observed, while in X. tropicalis RPE culture neither ßIII tubulin-positive cells nor 3-D retinal structure were seen. These results indicate that the two Xenopus species are excellent models to clarify the cellular and molecular mechanisms of retinal regeneration, as these animals have contrasting modes of regeneration; one mode primarily involves RPE cells and the other mode involves stem/progenitor cells in the CMZ.

Targeted gene disruption in the Xenopus tropicalis genome using designed TALE nucleases

Transcription activator-like effector nucleases (TALENs) are attractive and powerful molecular tools for targeted gene disruption because of their simple design and quick assembly. To evaluate the utility of TALENs in genome editing in Xenopus tropicalis, we prepared nine pairs of TALENs for the tyrosinase, noggin and MMP-9TH genes. All of the TALENs had some activity in a single-strand annealing assay using a cultured frog cell line, suggesting double-stranded DNA cleavage activity by the TALENs at their target site. The injection of mRNAs encoding TALENs into fertilized X. tropicalis embryos resulted in Cel-1 cleavage of the PCR fragment containing the target site amplified from embryo genomic DNA, indicating that a mutation in the target gene had occurred during embryogenesis. These mutations were confirmed by the sequencing of clones derived from the PCR fragments of genomic DNA. Patches of vitiligo were observed in tadpoles raised from fertilized eggs that had been injected with mRNAs of TALENs for the tyrosinase gene. TALENs containing the repeat variable di-residue (RVD) NN appeared to show more activity than TALENs containing RVD NK, although both RVD NN and NK preferentially associate with a G nucleotide.

Expression of the amelogenin gene in the skin of Xenopus tropicalis

Anuran skin contains a calcified dermal layer, referred to as the Eberth-Kastschenko (EK) layer, which is found between the stratum spongiosum and the stratum compactum. Although it is established that some anuran species possess the EK layer, little is known about this layer from the standpoint of evolutionary and developmental biology. We conducted a morphological analysis by staining the dorsal skin from many species with alizarin red S to investigate the calcified layer. This layer was observed in all of the anurans tested, as well as in fishes and one species of caecilian with dermal scales, but not in urodeles, amniotes, or a scaleless caecilian. All of the investigated species with dermal scales exhibited a calcified layer in their dermis, while the anurans showed the EK layer, but no scales. We also analyzed the expression of genes related to scale formation (sparc, mmp9, and mmp2) in the dorsal skin of X. tropicalis. These genes were highly expressed at the metamorphic climax stage, which preceded the deposition of calcium. Furthermore, we examined the gene expression profile of amelogenin, the major protein found in the enamel matrix of the developing tooth. In X. tropicalis, amelogenin was upregulated in the skin at the climax stage and was expressed in the adult dermis at a high level. These data provide the first experimental evidence of the expression of amelogenin in the skin. These findings will lead to a better understanding of the developmental formation of the EK layer and the function of amelogenin.

Application of an amphibian (Silurana tropicalis) metamorphosis assay to the testing of the chronic toxicity of three rice paddy herbicides: simetryn, mefenacet, and thiobencarb

We examined the chronic toxicity of three rice paddy herbicides (simetryn, mefenacet, and thiobencarb) using an amphibian (Silurana tropicalis) metamorphosis assay (a 28-day semistatic test under an individual-separated exposure system). Each herbicide was tested at two concentrations (1/100 and 1/10 of the 96-h LC50 value reported previously) with morphometric, gravimetric, and thyroid-histological endpoints. Simetryn caused significant retardation in growth and development at both test concentrations (0.04 and 0.40mg/L), as indicated by significantly shorter total body lengths and hind limb lengths, smaller wet body masses, and delayed developmental stages compared to those observed in the control tadpoles. However, no clear histopathology was observed in the thyroid glands of the tadpoles exposed to simetryn. These results suggest that simetryn can act as a chemical stressor retarding tadpole growth and development without disrupting thyroid functions, even at 1/100 of the 96-h LC50 value. In addition, scoliosis near the tail base was observed in the tadpoles exposed to 0.40mg/L of simetryn at a significantly high incidence (7/30=23.3%). Therefore, simetryn can also act as a teratogen inducing axial malformations at 1/10 of the 96-h LC50 value. During the 28 days of exposure, neither mefenacet (0.03 and 0.30mg/L) nor thiobencarb (0.008 and 0.080mg/L) induced any abnormalities, although the test concentrations measured immediately before the solution renewals decreased to nearly 50 percent of the nominal concentrations since day 14. Because the concentrations tested for simetryn are likely to occur in paddy water, wild anuran tadpoles in paddy water may therefore be adversely impacted by simetryn.

Efficient high-throughput sequencing of a laser microdissected chromosome arm

BACKGROUND

Genomic sequence assemblies are key tools for a broad range of gene function and evolutionary studies. The diploid amphibian Xenopus tropicalis plays a pivotal role in these fields due to its combination of experimental flexibility, diploid genome, and early-branching tetrapod taxonomic position, having diverged from the amniote lineage ~360 million years ago. A genome assembly and a genetic linkage map have recently been made available. Unfortunately, large gaps in the linkage map attenuate long-range integrity of the genome assembly.

RESULTS

We laser dissected the short arm of X. tropicalis chromosome 7 for next generation sequencing and computational mapping to the reference genome. This arm is of particular interest as it encodes the sex determination locus, but its genetic map contains large gaps which undermine available genome assemblies. Whole genome amplification of 15 laser-microdissected 7p arms followed by next generation sequencing yielded ~35 million reads, over four million of which uniquely mapped to the X. tropicalis genome. Our analysis placed more than 200 previously unmapped scaffolds on the analyzed chromosome arm, providing valuable low-resolution physical map information for de novo genome assembly.

CONCLUSION

We present a new approach for improving and validating genetic maps and sequence assemblies. Whole genome amplification of 15 microdissected chromosome arms provided sufficient high-quality material for localizing previously unmapped scaffolds and genes as well as recognizing mislocalized scaffolds.

Generation of gene disruptions by transcription activator-like effector nucleases (TALENs) in Xenopus tropicalis embryos

Transcription activator-like effector nucleases (TALENs) are novel engineered DNA nucleases, and have been proven to be effective for gene specific targeting in various species. Recently we reported gene disruptions in Xenopus embryos by using TALENs. Here we summarize the protocol that is used in our studies for gene disruption. This protocol covers selection of TALEN targeting sites, TALEN assembly with a modified Golden Gate method, and injection of TALEN mRNAs into Xenopus tropicalis embryos. We also provide details for detection of somatic and germ line transmitted mutations. And finally, we briefly describe establishment of knockout Xenopus lines. This protocol will facilitate broader applications of TALENs in studies of Xenopus biology.

Characterization of an outward rectifying chloride current of Xenopus tropicalis oocytes

Here, we describe an outward rectifying current in Xenopus tropicalis oocytes that we have called xtClC-or. The current has two components; the major component is voltage activated and independent of intracellular or extracellular Ca(2+), whereas the second is a smaller component that is Ca(2+) dependent. The properties of the Ca(2+)-independent current, such as voltage dependence and outward rectification, resemble those of ClC anion channels/transporters. This current is sensitive to NPPB and NFA, insensitive to 9AC and DIDS, and showed a whole-cell conductance sequence of SCN(-)>I(-)>Br(-)>CI(-). RT-PCR revealed the expression in oocytes of ClC-2 to ClC-7, and major reductions of current amplitudes were observed when a ClC-5 antisense oligonucleotide was injected into oocytes. The Ca(2+)-dependent component was abated after injection of 10mM BAPTA or EGTA, whereas 10mMMg(2+) inhibited the current to 26±3.1%. This component was blocked by 9-AC, NFA, and NPPB, whereas DIDS did not elicit any evident effect. The ion sequence selectivity was SCN=I(-)>Br(-)>Cl(-). To try to determine the molecular identity that gives rise to this component we assessed by RT-PCR the expression of the Ca(2+)-dependent Cl(-) channel TMEM16A, which was found to be present in the oocytes. However, injection of antisense TMEM16A oligonucleotides did not inhibit the transient outward current. This result fits well with the electrophysiological data. Together, these results suggest that ClC-5 is a major, but not the sole channel responsible for this outwardly rectifying Cl(-) current.

Embryonic exposure to propylthiouracil disrupts left-right patterning in Xenopus embryos

Antithyroid medications are the preferred therapy for the treatment of Graves' disease during pregnancy. Propylthiouracil (PTU) is favored over methimazole (MMI) due to potential teratogenic concerns with MMI. This study was to determine the teratogenic potential of MMI and PTU using a validated Xenopus tropicalis embryo model. Embryos were exposed to 1 mM PTU (EC(50)=0.88 mM), 1 mM MMI, or vehicle control (water) from stages 2 to 45. Treated embryos were examined for gross morphological defects, ciliary function, and gene expression by in situ hybridization. Exposure to PTU, but not MMI, led to cardiac and gut looping defects and shortening along the anterior-posterior axis. PTU exposure during gastrulation (stage 8-12.5) was identified as the critical period of exposure leading to left-right (LR) patterning defects. Abnormal cilia polarization, abnormal cilia-driven leftward flow at the gastrocoel roof plate (GRP), and aberrant expression of both Coco and Pitx2c were associated with abnormal LR symmetry observed following PTU exposure. PTU is teratogenic during late blastula, gastrulation, and neurulation; whereas MMI is not. PTU alters ciliary-driven flow and disrupts the normal genetic program involved in LR axis determination. These studies have important implications for women taking PTU during early pregnancy.

Inositol-requiring enzyme 1α is required for gut development in Xenopus lavies embryos

AIM: To investigate the role of inositol-requiring enzyme 1α (IRE1α) in gut development of Xenopus lavies embryos.

METHODS: Xenopus embryos were obtained with in vitro fertilization and cultured in 0.1 × MBSH. One and half nanogram of IRE1α, 1 ng of IRE1α-GR mRNA, 1 ng of IRE1αΔC-GR mRNA, and 50 ng of IRE1α morpholino oligonucleotide (MO) or XBP1(C)MO were injected into four blastomeres at 4-cell stage for scoring the phenotype and marker gene analysis. To rescue the effect of IRE1α MO, 1 ng of IRE1α-GR mRNA was co-injected with 50 ng of MO. For the activation of the GR-fusion proteins, dexamethasone was prepared as 5 mmol/L stock solutions in 100% ethanol and applied to the mRNA injected embryos at desired stages in a concentration of 10 μmol/L in 0.1 × MBSH. Embryos were kept in dexamethasone up to stage 41. Whole-mount in situ hybridization was used to determine specific gene expression, such as IRE1α, IRE1β, Xbra and Xsox17α. IRE1α protein expression during Xenopus embryogenesis was detected by Western blotting.

RESULTS: In the whole-mount in situ hybridization analysis, xenopus IRE1α and IRE1β showed quite different expression pattern during tadpole stage. The relatively higher expression of IRE1α was observed in the pancreas, and significant transcription of IRE1β was found in the liver. IRE1α protein could be detected at all developmental stages analyzed, from stage 1 to stage 42. Gain-of-function assay showed that IRE1α mRNA injected embryos at tailbud stage were nearly normal and the expression of the pan-mesodermal marker gene Xbra and the endodermal gene Xsox17α at stage 10.5 was not significantly changed in embryos injected with IRE1α mRNA as compared to uninjected control embryos. And at tadpole stage, the embryos injected with IRE1α-GR mRNA did not display overt phenotype, such as gut-coiling defect. Loss-of-function assay demonstrated that the IRE1α MO injected embryos were morphologically normal before the tailbud stages. We did not observe a significant change of mesodermal and endodermal marker gene expression, while after stage 40, about 80% of the MO injected embryos exhibited dramatic gut defects in which the guts did not coil, but other structures outside the gastrointestinal tract were relatively normal. To test if the phenotypes were specifically caused by the knockdown of IRE1α, a rescue experiment was performed by co-injection of IRE1α-GR mRMA with IRE1α MO. The data obtained demonstrated that the gut coiling defect was rescued. The deletion mutant of IRE1α was constructed, consisting of the N-terminal part without the C-terminal kinase and RNase domains named IRE1αΔC, to investigate the functional domain of IRE1α. Injection of IRE1αΔC-GR mRNA caused similar morphological alterations with gut malformation by interfering with the function of endogenous xIRE1α. In order to investigate if IRE1α/XBP1 pathway was involved in gut development, 50 ng of XBP1 MO was injected and the results showed that knockdown of XBP1 resulted in similar morphological alterations with gut-coiling defect at tadpole stage.

CONCLUSION: IRE1α is not required for germ layer formation but for gut development in Xenopus lavies and it may function via XBP1-dependent pathway.

Molecular genetic system for regenerative studies using newts

Urodele newts have the remarkable capability of organ regeneration, and have been used as a unique experimental model for more than a century. However, the mechanisms underlying regulation of the regeneration are not well understood, and gene functions in particular remain largely unknown. To elucidate gene function in regeneration, molecular genetic analyses are very powerful. In particular, it is important to establish transgenic or knockout (mutant) lines, and systematically cross these lines to study the functions of the genes. In fact, such systems have been developed for other vertebrate models. However, there is currently no experimental model system using molecular genetics for newt regenerative research due to difficulties with respect to breeding newts in the laboratory. Here, we show that the Iberian ribbed newt (Pleurodeles waltl) has outstanding properties as a laboratory newt. We developed conditions under which we can obtain a sufficient number and quality of eggs throughout the year, and shortened the period required for sexual maturation from 18 months to 6 months. In addition, P. waltl newts are known for their ability, like other newts, to regenerate various tissues. We revealed that their ability to regenerate various organs is equivalent to that of Japanese common newts. We also developed a method for efficient transgenesis. These studies demonstrate that P. waltl newts are a suitable model animal for analysis of regeneration using molecular genetics. Establishment of this experimental model will enable us to perform comparable studies using these newts and other vertebrate models.

Examination of an amphibian metamorphosis assay under an individual-separated exposure system using Silurana tropicalis tadpoles

We examined the validity of an amphibian (Silurana tropicalis) metamorphosis assay (a 28-day semistatic test) under an individual-separated exposure system, where tadpoles were individually held in small glass beakers. We first conducted a comparative rearing experiment for 28 days between this exposure system and the traditional individual-grouped exposure system, both of which held 30 tadpoles (stages 49 and 50) in dechlorinated tap water (a control solution). The former system served to reduce interindividual variability in regard to three morphological measures (developmental stage, hind limb length, and total body length). Under this system, we tested thyroxine (T4, 1μg/L) and propylthiouracil (PTU, 75mg/L) for 28 days of exposure. The morphological data collected at 7-day intervals indicated that significant metamorphic acceleration and retardation were consistently induced in the tadpoles exposed to T4 and PTU, respectively. In addition, the thyroid glands of the tadpoles exposed to T4 and PTU clearly exhibited atrophy and hypertrophy accompanied with severe follicular cell hyperplasia, respectively. Our results are in agreement with the historical data generated from previous studies employing the traditional exposure system, thus indicating the validity of our alternative testing protocol.

Generation of albino Xenopus tropicalis using zinc-finger nucleases

To generate albino lines of Xenopus tropicalis, we injected fertilized eggs with mRNAs encoding zinc-finger nucleases (ZFNs) targeting the tyrosinase coding region. Surprisingly, vitiligo was observed on the skin of F0 frogs that had been injected with ZFN mRNAs, indicating that both tyrosinase genes in the genome were disrupted in all melanocytes within the vitiligo patches. Mutation analysis using genomic DNA from the skin revealed that two mosaic F0 frogs underwent spatially complex tyrosinase gene mutations. The data implies that the ZFN-induced tyrosinase gene ablations occurred randomly over space and time throughout the entire body, possibly until the young tadpole stage, and that melanocyte precursors lacking functional tyrosinase proliferated and formed vitiligo patches. Several albino X. tropicalis, which are compound heterozygotes for biallelic tyrosinase mutations, were obtained by mating the mosaic F0 frogs. To our knowledge, this is the first report of the albino vertebrates generated by the targeted gene knockout.

Analysis of transient receptor potential ankyrin 1 (TRPA1) in frogs and lizards illuminates both nociceptive heat and chemical sensitivities and coexpression with TRP vanilloid 1 (TRPV1) in ancestral vertebrates

Transient receptor potential ankyrin 1 (TRPA1) and TRP vanilloid 1 (V1) perceive noxious temperatures and chemical stimuli and are involved in pain sensation in mammals. Thus, these two channels provide a model for understanding how different genes with similar biological roles may influence the function of one another during the course of evolution. However, the temperature sensitivity of TRPA1 in ancestral vertebrates and its evolutionary path are unknown as its temperature sensitivities vary among different vertebrate species. To elucidate the functional evolution of TRPA1, TRPA1s of the western clawed (WC) frogs and green anole lizards were characterized. WC frog TRPA1 was activated by heat and noxious chemicals that activate mammalian TRPA1. These stimuli also activated native sensory neurons and elicited nocifensive behaviors in WC frogs. Similar to mammals, TRPA1 was functionally co-expressed with TRPV1, another heat- and chemical-sensitive nociceptive receptor, in native sensory neurons of the WC frog. Green anole TRPA1 was also activated by heat and noxious chemical stimulation. These results suggest that TRPA1 was likely a noxious heat and chemical receptor and co-expressed with TRPV1 in the nociceptive sensory neurons of ancestral vertebrates. Conservation of TRPV1 heat sensitivity throughout vertebrate evolution could have changed functional constraints on TRPA1 and influenced the functional evolution of TRPA1 regarding temperature sensitivity, whereas conserving its noxious chemical sensitivity. In addition, our results also demonstrated that two mammalian TRPA1 inhibitors elicited different effect on the TRPA1s of WC frogs and green anoles, which can be utilized to clarify the structural bases for inhibition of TRPA1.

Xenopus tropicalis as a model organism for genetics and genomics: past, present, and future

Xenopus tropicalis was introduced as a model system for genetic, and then genomic research, in the early 1990s, complementing work on the widely used model organism Xenopus laevis. Its shorter generation time and diploid genome has facilitated a number of experimental approaches. It has permitted multigenerational experiments (e.g., preparation of transgenic lines and generation of mutant lines) that have added powerful approaches for research by the Xenopus community. As a diploid animal, its simpler genome was sequenced before X. laevis, and has provided a highly valuable resource indispensable for all Xenopus researchers. As more sophisticated transgenic technologies for manipulating gene expression are developed, and mutations, particularly null mutations, are identified in widely studied genes involved in critical cellular and developmental processes, researchers will increasingly turn to X. tropicalis for definitive analysis of complex genetic pathways. This chapter describes the historical and conceptual development of X. tropicalis as a genetic and genomic model system for higher vertebrate development.

Molecular cloning and functional characterization of Xenopus tropicalis frog transient receptor potential vanilloid 1 reveal its functional evolution for heat, acid, and capsaicin sensitivities in terrestrial vertebrates

The functional difference of thermosensitive transient receptor potential (TRP) channels in the evolutionary context has attracted attention, but thus far little information is available on the TRP vanilloid 1 (TRPV1) function of amphibians, which diverged earliest from terrestrial vertebrate lineages. In this study we cloned Xenopus tropicalis frog TRPV1 (xtTRPV1), and functional characterization was performed using HeLa cells heterologously expressing xtTRPV1 (xtTRPV1-HeLa) and dorsal root ganglion neurons isolated from X. tropicalis (xtDRG neurons) by measuring changes in the intracellular calcium concentration ([Ca(2+)](i)). The channel activity was also observed in xtTRPV1-expressing Xenopus oocytes. Furthermore, we tested capsaicin- and heat-induced nocifensive behaviors of the frog X. tropicalis in vivo. At the amino acid level, xtTRPV1 displays ∼60% sequence identity to other terrestrial vertebrate TRPV1 orthologues. Capsaicin induced [Ca(2+)](i) increases in xtTRPV1-HeLa and xtDRG neurons and evoked nocifensive behavior in X. tropicalis. However, its sensitivity was extremely low compared with mammalian orthologues. Low extracellular pH and heat activated xtTRPV1-HeLa and xtDRG neurons. Heat also evoked nocifensive behavior. In oocytes expressing xtTRPV1, inward currents were elicited by heat and low extracellular pH. Mutagenesis analysis revealed that two amino acids (tyrosine 523 and alanine 561) were responsible for the low sensitivity to capsaicin. Taken together, our results indicate that xtTRPV1 functions as a polymodal receptor similar to its mammalian orthologues. The present study demonstrates that TRPV1 functions as a heat- and acid-sensitive channel in the ancestor of terrestrial vertebrates. Because it is possible to examine vanilloid and heat sensitivities in vitro and in vivo, X. tropicalis could be the ideal experimental lower vertebrate animal for the study of TRPV1 function.

Toward an unbiased evolutionary platform for unraveling Xenopus developmental gene networks

The availability of both the Xenopus tropicalis genome and the soon to be released Xenopus laevis genome provides a solid foundation for Xenopus developmental biologists. The Xenopus community has presently amassed expression data for ∼2,300 genes in the form of published images collected in the Xenbase, the principal Xenopus research database. A few of these genes have been examined in both X. tropicalis and X. laevis and the cross-species comparison has been proven invaluable for studying gene function. A recently published work has yielded developmental expression profiles for the majority of Xenopus genes across fourteen developmental stages spanning the blastula, gastrula, neurula, and the tail-bud. While this data was originally queried for global evolutionary and developmental principles, here we demonstrate its general use for gene-level analyses. In particular, we present the accessibility of this dataset through Xenbase and describe biases in the characterized genes in terms of sequence and expression conservation across the two species. We further indicate the advantage of examining coexpression for gene function discovery relating to developmental processes conserved across species. We suggest that the integration of additional large-scale datasets--comprising diverse functional data--into Xenbase promises to provide a strong foundation for researchers in elucidating biological processes including the gene regulatory programs encoding development.

Wormometry-on-a-chip: Innovative technologies for in situ analysis of small multicellular organisms

Small multicellular organisms such as nematodes, fruit flies, clawed frogs, and zebrafish are emerging models for an increasing number of biomedical and environmental studies. They offer substantial advantages over cell lines and isolated tissues, providing analysis under normal physiological milieu of the whole organism. Many bioassays performed on these alternative animal models mirror with a high level of accuracy those performed on inherently low-throughput, costly, and ethically controversial mammalian models of human disease. Analysis of small model organisms in a high-throughput and high-content manner is, however, still a challenging task not easily susceptible to laboratory automation. In this context, recent advances in photonics, electronics, as well as material sciences have facilitated the emergence of miniaturized bioanalytical systems collectively known as Lab-on-a-Chip (LOC). These technologies combine micro- and nanoscale sciences, allowing the application of laminar fluid flow at ultralow volumes in spatially confined chip-based circuitry. LOC technologies are particularly advantageous for the development of a wide array of automated functionalities. The present work outlines the development of innovative miniaturized chip-based devices for the in situ analysis of small model organisms. We also introduce a new term "wormometry" to collectively distinguish these up-and-coming chip-based technologies that go far beyond the conventional meaning of the term "cytometry."

Evolution of vertebrate transient receptor potential vanilloid 3 channels: opposite temperature sensitivity between mammals and western clawed frogs

Transient Receptor Potential (TRP) channels serve as temperature receptors in a wide variety of animals and must have played crucial roles in thermal adaptation. The TRP vanilloid (TRPV) subfamily contains several temperature receptors with different temperature sensitivities. The TRPV3 channel is known to be highly expressed in skin, where it is activated by warm temperatures and serves as a sensor to detect ambient temperatures near the body temperature of homeothermic animals such as mammals. Here we performed comprehensive comparative analyses of the TRPV subfamily in order to understand the evolutionary process; we identified novel TRPV genes and also characterized the evolutionary flexibility of TRPV3 during vertebrate evolution. We cloned the TRPV3 channel from the western clawed frog Xenopus tropicalis to understand the functional evolution of the TRPV3 channel. The amino acid sequences of the N- and C-terminal regions of the TRPV3 channel were highly diversified from those of other terrestrial vertebrate TRPV3 channels, although central portions were well conserved. In a heterologous expression system, several mammalian TRPV3 agonists did not activate the TRPV3 channel of the western clawed frog. Moreover, the frog TRPV3 channel did not respond to heat stimuli, instead it was activated by cold temperatures. Temperature thresholds for activation were about 16 °C, slightly below the lower temperature limit for the western clawed frog. Given that the TRPV3 channel is expressed in skin, its likely role is to detect noxious cold temperatures. Thus, the western clawed frog and mammals acquired opposite temperature sensitivity of the TRPV3 channel in order to detect environmental temperatures suitable for their respective species, indicating that temperature receptors can dynamically change properties to adapt to different thermal environments during evolution.

Evolution of temperature perception is crucial for adaptation to thermal environments; however, this process is poorly understood. Here we investigated the evolution of the vertebrate TRPV subfamily which contains several mammalian temperature receptors. We identified several novel TRPV genes that have not been found previously and discovered evolutionary flexibility of the TRPV3 gene during vertebrate evolution. TRPV3 channels perceive warm temperature and serve as sensors to detect ambient temperatures near the body temperature of homeothermic animals such as mammals. To examine the functional evolution of TRPV3 channels in vertebrate evolution, we cloned the gene from the western clawed frog and found that its N- and C-terminal regions were highly diversified from those of other terrestrial vertebrate TRPV3 channels. Characterization of the channel properties of western clawed frog TRPV3 revealed that it was not activated by heat stimuli, but instead was activated by cold stimuli. Temperature thresholds for activation were about 16 °C, slightly below the lower temperature limit for the western clawed frog. Thus, the western clawed frog and mammals acquired opposite temperature sensitivity of TRPV3 channels to detect environmental temperatures suitable for their respective species, indicating that temperature receptors can dynamically change properties to adapt to thermal environments during evolution.

Evolution of acetylcholinesterase and butyrylcholinesterase in the vertebrates: an atypical butyrylcholinesterase from the Medaka Oryzias latipes

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are thought to be the result of a gene duplication event early in vertebrate evolution. To learn more about the evolution of these enzymes, we expressed in vitro, characterized, and modeled a recombinant cholinesterase (ChE) from a teleost, the medaka Oryzias latipes. In addition to AChE, O. latipes has a ChE that is different from either vertebrate AChE or BChE, which we are classifying as an atypical BChE, and which may resemble a transitional form between the two. Of the fourteen aromatic amino acids in the catalytic gorge of vertebrate AChE, ten are conserved in the atypical BChE of O. latipes; by contrast, only eight are conserved in vertebrate BChE. Notably, the atypical BChE has one phenylalanine in its acyl pocket, while AChE has two and BChE none. These substitutions could account for the intermediate nature of this atypical BChE. Molecular modeling supports this proposal. The atypical BChE hydrolyzes acetylthiocholine (ATCh) and propionylthiocholine (PTCh) preferentially but butyrylthiocholine (BTCh) to a considerable extent, which is different from the substrate specificity of AChE or BChE. The enzyme shows substrate inhibition with the two smaller substrates but not with the larger substrate BTCh. In comparison, AChE exhibits substrate inhibition, while BChE does not, but may instead show substrate activation. The atypical BChE from O. latipes also shows a mixed pattern of inhibition. It is effectively inhibited by physostigmine, typical of all ChEs. However, although the atypical BChE is efficiently inhibited by the BChE-specific inhibitor ethopropazine, it is not by another BChE inhibitor, iso-OMPA, nor by the AChE-specific inhibitor BW284c51. The atypical BChE is found as a glycophosphatidylinositol-anchored (GPI-anchored) amphiphilic dimer (G(2) (a)), which is unusual for any BChE. We classify the enzyme as an atypical BChE and discuss its implications for the evolution of AChE and BChE and for ecotoxicology.

Recent transposition activity of Xenopus T2 family miniature inverted-repeat transposable elements

To investigate the recent transposition activity of T2 family miniature inverted-repeat transposable elements (MITEs) in Xenopus tropicalis (Western clawed frog), we analyzed the intraspecific polymorphisms associated with MITE insertion in X. tropicalis for three subfamilies of the T2 family (T2-A1, T2-C, and T2-E). A high frequency of MITE-insertion polymorphisms was observed at the T2-A1 (50%) and T2-C insertion loci (60%), but none were noted at the T2-E insertion locus (0%). Analyses of the collected data indicated that members of the T2-A1 and T2-C subfamilies may be currently active in the host species. Identification of these active transpositions will help us in understanding the mechanisms underlying the long-term survival (over several tens of millions of years) of the T2-A1 and T2-C subfamilies.