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

Integrative proteome and metabolome analyses reveal molecular basis of the tail resorption during the metamorphic climax of Nanorana pleskei

During the metamorphosis of anuran amphibians, the tail resorption process is a necessary and crucial change. One subject that has received relatively little or no attention is the expression patterns of proteins and metabolites in the different tail portions during metamorphosis, especially in highland amphibians. The mechanisms of tail resorption in three portions (the tip, middle and root) of the tail were investigated in N. pleskei G43 tadpole based on two omics (proteomic and metabolomic). Integrin αVβ3 was found to be high expressed in the distal portion of the tail, which could improve the sensitiveness to thyroid hormones in the distal portion of the tail. Muscle regression displayed a spatial pattern with stronger regression in distal and weaker one in proximal portion. Probably, this stronger regression was mainly performed by the proteases of proteasome from the active translation by ribosomes. The suicide model and murder model coexisted in the tail resorption. Meanwhile, fatty acids, amino acids, pyrimidine, and purine which derived from the breakdown of tissues can be used as building blocks or energy source for successful metamorphosis. Our data improved a better comprehension of the tail resorption mechanisms underlying the metamorphism of N. pleskei tadpole through identifying important participating proteins and metabolites.

Revealing mitf functions and visualizing allografted tumor metastasis in colorless and immunodeficient Xenopus tropicalis

Transparent immunodeficient animal models not only enhance in vivo imaging investigations of visceral organ development but also facilitate in vivo tracking of transplanted tumor cells. However, at present, transparent and immunodeficient animal models are confined to zebrafish, presenting substantial challenges for real-time, in vivo imaging studies addressing specific biological inquiries. Here, we employed a mitf-/-/prkdc-/-/il2rg-/- triple-knockout strategy to establish a colorless and immunodeficient amphibian model of Xenopus tropicalis. By disrupting the mitf gene, we observed the loss of melanophores, xanthophores, and granular glands in Xenopus tropicalis. Through the endogenous mitf promoter to drive BRAFV600E expression, we confirmed mitf expression in melanophores, xanthophores and granular glands. Moreover, the reconstruction of the disrupted site effectively reinstated melanophores, xanthophores, and granular glands, further highlighting the crucial role of mitf as a regulator in their development. By crossing mitf-/- frogs with prkdc-/-/il2rg-/- frogs, we generated a mitf-/-/prkdc-/-/il2rg-/- Xenopus tropicalis line, providing a colorless and immunodeficient amphibian model. Utilizing this model, we successfully observed intravital metastases of allotransplanted xanthophoromas and migrations of allotransplanted melanomas. Overall, colorless and immunodeficient Xenopus tropicalis holds great promise as a valuable platform for tumorous and developmental biology research.

Effects of nitrite exposure on metamorphosis and skeletal development of Bufo gargarizans

Nitrite, as a part of nitrogen cycle, is one of the most common toxic compounds in aquatic ecosystems. Since skeletal development is an essential process during amphibian metamorphosis, exposure of larval amphibians to nitrite might disrupt skeletal development. To evaluate whether nitrite affects skeletal development of amphibian larvae, Bufo gargarizans larvae at Gs26 were exposed to 10, 100, 500 and 1000 μg/L nitrite-nitrogen (NO₂-N) in the present study. The metamorphosis rate, body weight, body length, forelimb length and hindlimb length of B. gargarizans exposed to NO₂-N were decreased. The microscopic structures of thyroid gland were altered under NO₂-N exposure at Gs42. The skeletal lengths of the humerus, femur and fibulare of tadpole at Gs42 were significantly reduced under 100, 500 and 1000 μg/L NO₂-N treatment groups, and the lengths of humerus, tibia-fibula and tibiale of tadpole at Gs46 were significantly reduced under 1000 μg/L NO₂-N treatment groups. In addition, the expression levels of thyroid hormone (TH) and endochondral ossification-related genes of tadpoles at Gs42 and Gs46 were tested by qRT-PCR. Overall, NO₂-N exposure could affect the expressions of these genes and then may influence the activity and function of thyroid gland, further disturbing the amphibian metamorphosis and skeletal development of amphibian larvae.

Effects of Separate and Combined Exposure of Cadmium and Lead on the Endochondral Ossification in Bufo gargarizans

Cadmium (Cd) and lead (Pb) are ubiquitous in aquatic environments and most studies have examined the potential effects of Cd or Pb alone on aquatic organisms. In the present study, chronic effects of Cd and Pb, alone and in combination, on Bufo gargarizans were investigated by exposing embryos to these contaminants throughout metamorphosis. Significant reductions in body mass and snout-to-vent length were observed in B. gargarizans at Gosner stage 42 (Gs 42) and Gs 46 exposed to a Cd/Pb mixture. Single and combined exposure with Cd and Pb induced histological alterations of the thyroid gland characterized by reduced colloid area and thickness of epithelial cells. There was a significant decrease in the maximum jump distance of froglets exposed to Cd alone and the Cd/Pb mixture, and the jumping capacity showed a positive correlation with hind limb length and tibia/fibula. Moreover, single metals and their mixture induced reduction of endochondral bone formation in B. gargarizans. Transcriptomic and real-time quantitative polymerase chain reaction results showed that genes involved in skeletal ossification (TRα, TRβ, Dio2, Dio3, MMP9, MMP13, Runx1, Runx2, and Runx3) were transcriptionally dysregulated by Cd and Pb exposure alone or in combination. Our results suggested that despite the low concentration tested, the Cd/Pb mixture induced more severe impacts on B. gargarizans. In addition, the Cd/Pb mixture might reduce chances of survival for B. gargarizans froglets by decreasing size at metamorphosis, impaired skeletal ossification, and reduction in jumping ability, which might result from dysregulation of genes involved in thyroid hormone action and endochondral ossification. The findings obtained could add a new dimension to understanding of the mechanisms underpinning skeletal ossification response to heavy metals in amphibians. Environ Toxicol Chem 2022;41:1228-1245. © 2022 SETAC.

Altered temperature affect body condition and endochondral ossification in Bufo gargarizans tadpoles

Bufo gargarizans is one kind of economic animals with higher medicinal value in China. In this study, B. gargarizans (Bufo gargarizans) tadpoles were reared at three different water temperature (15, 22 and 29 °C) from Gosner stages 28-46. We investigated the effects of temperature on growth, development, survival, metamorphic duration, size and skeletal ossification at Gosner stage 40, 42, and 46, as well as thyroid tissue reached metamorphic climax (Gs42). Besides, we examined the transcription levels of endochondral ossification-related genes in hind limb at metamorphic climax (Gs42). Our results showed that the growth and development of tadpoles conform to the temperature-size rule (TSR). While warm temperature resulted in the decrease in body size and hind limb length, and shorten larval period, cold temperature led to increase in body size and hind limb length but prolonged larval period. Histological examinations revealed that warm and cold temperatures caused damage to thyroid tissue. Also, warm and cold temperatures inhibited the degree of ossification with the double staining methodology. Additionally, the real-time PCR results suggested that warm and cold temperatures significantly up-regulated Runx2, VEGF and VEGFR mRNA levels, and down-regulated TRβ, MMP9, MMP13 and Runx3 mRNA levels. The up-regulation of Dio2 level and down-regulation of Dio3 level were observed in warm temperature. TRα mRNA level was significantly increased in warm temperature, but decreased in cold temperature. Collectively, these observations demonstrated that warm and cold temperatures affected endochondral ossification in B. gargarizans tadpoles, which might influence their capacity to terrestrial locomotion.

Modeling by disruption and a selected-for partner for the nude locus

A long‐standing problem in biology is how to dissect traits for which no tractable model exists. Here, we screen for genes like the nude locus (Foxn1)—genes central to mammalian hair and thymus development—using animals that never evolved hair, thymi, or Foxn1. Fruit flies are morphologically disrupted by the FOXN1 transcription factor and rescued by weak reductions in fly gene function, revealing molecules that potently synergize with FOXN1 to effect dramatic, chaotic change. Strong synergy/effectivity in flies is expected to reflect strong selection/functionality (purpose) in mammals; the more disruptive a molecular interaction is in alien contexts (flies), the more beneficial it will be in its natural, formative contexts (mammals). The approach identifies Aff4 as the first nude‐like locus, as murine AFF4 and FOXN1 cooperatively induce similar cutaneous/thymic phenotypes, similar gene expression programs, and the same step of transcription, pre‐initiation complex formation. These AFF4 functions are unexpected, as AFF4 also serves as a scaffold in common transcriptional‐elongation complexes. Most likely, the approach works because an interaction's power to disrupt is the inevitable consequence of its selected‐for power to benefit.

Experimental Platform Using the Amphibian Xenopus laevis for Research in Fundamental and Medical Immunology

The amphibian Xenopus constitutes a powerful, versatile, and cost-effective nonmammalian model with which to investigate important contemporary issues of immunity relevant to human health such as ontogeny of immunity, self-tolerance, wound healing, autoimmunity, cancer immunity, immunotoxicology, and adaptation of host immune defenses to emerging pathogens. This model system presents several attractive features: an external developmental environment free of maternal influence that allows for easy experimental access from early life stages; an immune system that is remarkably similar to that of mammals; the availability of large-scale genetic and genomic resources; invaluable major histocompatibility complex (MHC)-defined inbred strains of frogs; and useful tools such as lymphoid tumor cell lines, monoclonal antibodies, and MHC tetramers. Modern reverse genetic loss-of-function and genome-editing technologies applied to immune function further empower this model. Finally, the evolutionary distance between Xenopus and mammals permits distinguishing species-specific adaptation from more conserved features of the immune system. In this introduction, the advantages and features of Xenopus for immunological research are outlined, as are existing tools, resources, and methods for using this model system.

Expression of TCR genes in adult and larval Xenopus laevis

In order to better understand the development and function of γδ T cells in Xenopus frogs, it is necessary to determine where and when γδ T cells are found in Xenopus tissues. This study examined the expression of TCR genes, focused primarily on TCR γ, in tissues of adult and larval Xenopus laevis and provide new data about the expression pattern of these different TCR genes in this anuran amphibian. TCR gene expression was detected by RT-PCR in adult frog tissues including the thymus, spleen, skin, intestine, lung, and liver, but not the testes. TCR γ and β genes were detected in the larval (tadpole) tail and intestine. The absence of RAG-1 expression in these larval tissues is consistent with differentiation of the T cells in the thymus. Together, these data provide evidence that migration of these cells from the thymus likely occurs relatively early in larval development. These studies provide a necessary foundation for future studies of the functions of γδ T cells in amphibians, which are placed at an intermediate position flanked by fishes on one end and mammals and chickens on the other.

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

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

Tail Resorption During Metamorphosis in Xenopus Tadpoles

Tail resorption in anuran tadpoles is one of the most physically and physiologically notable phenomena in developmental biology. A tail that is over twice as long as the tadpole trunk is absorbed within several days, while concurrently the tadpole's locomotive function is continuously managed during the transition of the driving force from the tail to hindlimbs. Elaborate regulation is necessary to accomplish this locomotive switch. Tadpole's hindlimbs must develop from the limb-bud size to the mature size and the nervous system must be arranged to control movement before the tail is degenerated. The order of the development and growth of hindlimbs and the regression of the tail are regulated by the increasing levels of thyroid hormones (THs), the intracellular metabolism of THs, the expression levels of TH receptors, the expression of several effector genes, and other factors that can modulate TH signaling. The tail degeneration that is induced by the TH surge occurs through two mechanisms, direct TH-responsive cell death (suicide) and cell death caused by the degradation of the extracellular matrix and a loss of cellular anchorage (murder). These pathways lead to the collapse of the notochord, the contraction of surviving slow muscles, and, ultimately, the loss of the tail. In this review, I focus on the differential TH sensitivity of the tail and hindlimbs and the mechanism of tail resorption during Xenopus metamorphosis.

Transcriptome analyses reveal molecular mechanisms that regulate endochondral ossification in amphibian Bufo gargarizans during metamorphosis

BACKGROUND

A developmental transition from aquatic to terrestrial existence is one of the most important events in the evolution of terrestrial vertebrates. Amphibian metamorphosis is a classic model to study this transition. The development of the vertebrate skeleton can reflect its evolutionary history. Endochondral ossification serves a vital role in skeletal development. Thus, we sought to unravel molecular mechanisms that regulate endochondral ossification during Bufo gargarizans metamorphosis.

METHODS

The alizarin red-alcian blue double staining method was used to visualize the skeletal development of B. gargarizans during metamorphosis. RNA sequencing (RNA-seq) was used to explore the transcriptome of B. gargarizans in four key developmental stages during metamorphosis. Real-time quantitative PCR (RT-qPCR) was used to validate the expression patterns of endochondral ossification related genes.

RESULTS

Endochondral ossification increased gradually in skeletal system of B. gargarizans during metamorphosis. A total of 137,264 unigenes were assembled and 44,035 unigenes were annotated. 10,352 differentially expressed genes (DEGs) were further extracted among four key developmental stages. In addition, 28 endochondral ossification related genes were found by searching for DEG libraries in B. gargarizans. Of the 28 genes, 10 genes were validated using RT-qPCR.

CONCLUSIONS

The exquisite coordination of the 28 genes is essential for regulation of endochondral ossification during B. gargarizans metamorphosis.

GENERAL SIGNIFICANCE

The present study will not only provide an invaluable genomic resource and background for further research of endochondral ossification in amphibians but will also aid in enhancing our understanding of the evolution of terrestrial vertebrates.

Identification and differential regulation of microRNAs during thyroid hormone-dependent metamorphosis in Microhyla fissipes

BACKGROUND

Anuran metamorphosis, which is obligatorily initiated and sustained by thyroid hormone (TH), is a dramatic example of extensive morphological, biochemical and cellular changes occurring during post-embryonic development. Thus, it provides an ideal model to understand the actions of the hormone and molecular mechanisms underlying these developmental and apoptotic processes. In addition to transcriptional factors, microRNAs (miRNAs) play key roles in diverse biological processes via post-transcriptional repression of mRNAs. However, the possible role of miRNAs in anuran metamorphosis is not well understood. Screening and identification of TH-responding miRNAs are required to reveal the integrated regulatory mechanisms of TH during metamorphosis. Given the specific role of TRs during M. fissipes metamorphosis and the characteristics of M. fissipes as an ideal model, Illumina sequencing technology was employed to get a full scope of miRNA in M. fissipes metamorphosis treated by T3.

RESULTS

Morphological and histological analysis revealed that 24 h T3 treatment M. fissipes tadpoles resembled that at the climax of natural metamorphosis. Thus, small RNA libraries were constructed from control and 24 h T3 treatment groups. A total of 164 conserved miRNAs and 36 predicted novel miRNAs were characterized. Furthermore, 5' first and ninth nucleotides of miRNAs were significantly enriched in U in our study. In all, 21 miRNAs were differentially expressed between the T3 and control groups (p < 0.01). A total of 10,206 unigenes were identified as target genes of these differentially expressed miRNAs. KEGG pathway analysis indicated that the most overrepresented miRNA target genes were enriched in the "PI3k-Akt signaling pathway". In addition, a network associated with the TH signaling pathway provides an opportunity to further understand the complex biological processes that occur in metamorphosis.

CONCLUSIONS

We identified a large number of miRNAs during M. fissipes metamorphosis, and 21 of them were differentially expressed in the two groups that represented two different metamorphic stages. These miRNAs may play important roles during metamorphosis. The study gives us clues for further studies of the mechanisms of anuran metamorphosis and provides a model to study the mechanism of TH-affected biological processes in humans.

Mechanisms of tail resorption during anuran metamorphosis

Amphibian metamorphosis has historically attracted a good deal of scientific attention owing to its dramatic nature and easy observability. However, the genetic mechanisms of amphibian metamorphosis have not been thoroughly examined using modern techniques such as gene cloning, DNA sequencing, polymerase chain reaction or genomic editing. Here, we review the current state of knowledge regarding molecular mechanisms underlying tadpole tail resorption.

Using Xenopus to understand human disease and developmental disorders

Model animals are crucial to biomedical research. Among the commonly used model animals, the amphibian, Xenopus, has had tremendous impact because of its unique experimental advantages, cost effectiveness, and close evolutionary relationship with mammals as a tetrapod. Over the past 50 years, the use of Xenopus has made possible many fundamental contributions to biomedicine, and it is a cornerstone of research in cell biology, developmental biology, evolutionary biology, immunology, molecular biology, neurobiology, and physiology. The prospects for Xenopus as an experimental system are excellent: Xenopus is uniquely well-suited for many contemporary approaches used to study fundamental biological and disease mechanisms. Moreover, recent advances in high throughput DNA sequencing, genome editing, proteomics, and pharmacological screening are easily applicable in Xenopus, enabling rapid functional genomics and human disease modeling at a systems level.