A model of transcriptional and morphological changes during thyroid hormone-induced metamorphosis of the axolotl

Histological changes of the skin during postembryonic development of the crested newt Triturus ivanbureschi (Urodela, Salamandridae)

BACKGROUND

Amphibian skin has been studied for many decades, especially the metamorphic changes in the skin of frogs. Less attention has been paid to salamander skin. Here, we describe changes in the skin structure during postembryonic development in a salamandrid species, the Balkan crested newt Triturus ivanbureschi.

METHOD

Using traditional histological techniques we examined the skin in the trunk region of three premetamorphic larval stages (hatchling, mid larval and late larval) and two postmetamorphic stages (juvenile, just after metamorphosis, and adult).

RESULTS

In larval stages, skin consists only of the epidermis, which gradually develops from the single epithelial cell layer in hatchlings, to a stratified epidermis with gland nests and characteristic Leydig cells at the late larval stage. During metamorphosis, Leydig cells disappear, and the dermal layer develops. In postmetamorphic stages, skin is differentiated on stratified epidermis and the dermis with well-developed glands. Three types of glands were observed in the skin of the postmetamorphic stages: mucous, granular and mixed. Gland composition appears to be stage- and sex-specific, with juveniles and adult female being more similar to each other. In juveniles and adult female, there are a similar proportion of glands in both dorsal and ventral skin, whereas in adult male granular glands dominated the dorsal skin, while mixed glands dominated the ventral skin.

CONCLUSION

Our results provide a baseline for future comparative research of skin anatomy in salamanders.

W, Chen H, Yu C, Yang L, Fu Y, Li J, Fu S, Sun Z, Fei L, Guo Q, Wang J, Xiao Y, Wang X, Zhang P, Ma L, Ge D, Xu S, Caballero-Pérez J, Cruz-Ramírez A, Zhou Y, Chen M, Fei JF, Han X, Guo G. Construction of the axolotl cell landscape using combinatorial hybridization sequencing at single-cell resolution

The Mexican axolotl (Ambystoma mexicanum) is a well-established tetrapod model for regeneration and developmental studies. Remarkably, neotenic axolotls may undergo metamorphosis, a process that triggers many dramatic changes in diverse organs, accompanied by gradually decline of their regeneration capacity and lifespan. However, the molecular regulation and cellular changes in neotenic and metamorphosed axolotls are still poorly investigated. Here, we develop a single-cell sequencing method based on combinatorial hybridization to generate a tissue-based transcriptomic landscape of the neotenic and metamorphosed axolotls. We perform gene expression profiling of over 1 million single cells across 19 tissues to construct the first adult axolotl cell landscape. Comparison of single-cell transcriptomes between the tissues of neotenic and metamorphosed axolotls reveal the heterogeneity of non-immune parenchymal cells in different tissues and established their regulatory network. Furthermore, we describe dynamic gene expression patterns during limb development in neotenic axolotls. This system-level single-cell analysis of molecular characteristics in neotenic and metamorphosed axolotls, serves as a resource to explore the molecular identity of the axolotl and facilitates better understanding of metamorphosis.

The Mexican axolotl is a well-established tetrapod model for regeneration and development. Here the authors report a scRNA-seq method to profile neotenic, metamorphic and limb development stages, highlighting unique perturbation patterns of cell type-related gene expression throughout metamorphosis.

The Tale-Tell Heart: Evolutionary tetrapod shift from aquatic to terrestrial life-style reflected in heart changes in axolotl (Ambystoma mexicanum)

BACKGROUND

During amphibian metamorphosis, the crucial moment lies in the rearrangement of the heart, reflecting the changes in circulatory demands. However, little is known about the exact shifts linked with this rearrangement. Here, we demonstrate such myocardial changes in axolotl (Ambystoma mexicanum) from the morphological and physiological point of view.

RESULTS

Micro-CT and histological analysis showed changes in ventricular trabeculae organization, completion of the atrial septum and its connection to the atrioventricular valve. Based on Myosin Heavy Chain and Smooth Muscle Actin expression we distinguished metamorphosis-induced changes in myocardial differentiation at the ventricular trabeculae and atrioventricular canal. Using optical mapping, faster speed of conduction through the atrioventricular canal was demonstrated in metamorphic animals. No differences between the groups were observed in the heart rates, ventricular activation times, and activation patterns.

CONCLUSIONS

Transition from aquatic to terrestrial life-style is reflected in the heart morphology and function. Rebuilding of the axolotl heart during metamorphosis was connected with reorganization of ventricular trabeculae, completion of the atrial septum and its connection to the atrioventricular valve, and acceleration of AV conduction.

Investigating Acute and Subchronic Effects of Neonicotinoids on Northwestern Salamander Larvae

This research investigated the adverse effects of neonicotinoids on the Northwestern salamander (Ambystoma gracile; NWS) after acute and subchronic exposures during early aquatic life stages via whole organism (i.e., growth, development) and molecular (i.e., gene expression) level endpoints. In a 96-h exposure, NWS larvae were exposed to four imidacloprid concentrations (250, 750, 2250, 6750 µg/L) and a water control treatment, and no effects on survival, body weight, snout-vent length (SVL), and total body length were observed. However, a significant 1.70- and 2.33-fold decrease in thyroid receptor β (TRβ) mRNA expression levels were detected in the larvae exposed to 750 and 2250 µg/L imidacloprid, respectively, compared with the larvae in the water control. In subsequent subchronic experiments, NWS larvae were exposed for 35 days to imidacloprid alone and an equal part mixture of neonicotinoids (imidacloprid, clothianidin, and thiamethoxam (ICT)) at three concentrations (10, 100 and 1000 µg total neonicotinoids/L) and a water control. In these experiments, there were no effects on larval survival, body weight, SVL, and total body length. However, advanced development of larvae in the 100 µg/L imidacloprid treatment was observed compared with the control after 35-day imidacloprid exposure, providing some evidence of disruption of the thyroid endocrine axis at an environmentally relevant concentration. Ultimately, there is a paucity of studies conducted examining the sensitivity of salamanders to pollutants; thus, this study reports novel findings that will contribute to understanding the sensitivity of a Caudate amphibian model to a common environmental pollutant.

Appendage regeneration in anamniotes utilizes genes active during larval-metamorphic stages that have been lost or altered in amniotes: The case for studying lizard tail regeneration

This review elaborates the idea that organ regeneration derives from specific evolutionary histories of vertebrates. Regenerative ability depends on genomic regulation of genes specific to the life-cycles that have differentially evolved in anamniotes and amniotes. In aquatic environments, where fish and amphibians live, one or multiple metamorphic transitions occur before the adult stage is reached. Each transition involves the destruction and remodeling of larval organs that are replaced with adult organs. After organ injury or loss in adult anamniotes, regeneration uses similar genes and developmental process than those operating during larval growth and metamorphosis. Therefore, the broad presence of regenerative capability across anamniotes is possible because generating new organs is included in their life history at metamorphic stages. Soft hyaluronate-rich regenerative blastemas grow in submersed or in hydrated environments, that is, essential conditions for regeneration, like during development. In adult anamniotes, the ability to regenerate different organs decreases in comparison to larval stages and becomes limited during aging. Comparisons of genes activated during metamorphosis and regeneration in anamniotes identify key genes unique to these processes, and include thyroid, wnt and non-coding RNAs developmental pathways. In the terrestrial environment, some genes or developmental pathways for metamorphic transitions were lost during amniote evolution, determining loss of regeneration. Among amniotes, the formation of soft and hydrated blastemas only occurs in lizards, a morphogenetic process that evolved favoring their survival through tail autotomy, leading to a massive although imperfect regeneration of the tail. Deciphering genes activity during lizard tail regeneration would address future attempts to recreate in other amniotes regenerative blastemas that grow into variably completed organs.

Multi-organ transcriptomic landscape of Ambystoma velasci metamorphosis

Metamorphosis is a postembryonic developmental process that involves morphophysiological and behavioral changes, allowing organisms to adapt into a novel environment. In some amphibians, aquatic organisms undergo metamorphosis to adapt in a terrestrial environment. In this process, these organisms experience major changes in their circulatory, respiratory, digestive, excretory and reproductive systems. We performed a transcriptional global analysis of heart, lung and gills during diverse stages of Ambystoma velasci to investigate its metamorphosis. In our analyses, we identified eight gene clusters for each organ, according to the expression patterns of differentially expressed genes. We found 4064 differentially expressed genes in the heart, 4107 in the lung and 8265 in the gills. Among the differentially expressed genes in the heart, we observed genes involved in the differentiation of cardiomyocytes in the interatrial zone, vasculogenesis and in the maturation of coronary vessels. In the lung, we found genes differentially expressed related to angiogenesis, alveolarization and synthesis of the surfactant protein. In the case of the gills, the most prominent biological processes identified are degradation of extracellular matrix, apoptosis and keratin production. Our study sheds light on the transcriptional responses and the pathways modulation involved in the transformation of the facultative metamorphic salamander A. velasci in an organ-specific manner.

Multiple stressors produce differential transcriptomic patterns in a stream-dwelling salamander

BACKGROUND

Global biodiversity is decreasing at an alarming rate and amphibians are at the forefront of this crisis. Understanding the factors that negatively impact amphibian populations and effectively monitoring their health are fundamental to addressing this epidemic. Plasma glucocorticoids are often used to assess stress in amphibians and other vertebrates, but these hormones can be extremely dynamic and impractical to quantify in small organisms. Transcriptomic responses to stress hormones in amphibians have been largely limited to laboratory models, and there have been few studies on vertebrates that have evaluated the impact of multiple stressors on patterns of gene expression. Here we examined the gene expression patterns in tail tissues of stream-dwelling salamanders (Eurycea tynerensis) chronically exposed to the stress hormone corticosterone under different temperature regimes.

RESULTS

We found unique transcriptional signatures for chronic corticosterone exposure that were independent of temperature variation. Several of the corticosterone responsive genes are known to be involved in immune system response (LY-6E), oxidative stress (GSTM2 and TRX), and tissue repair (A2M and FX). We also found many genes to be influenced by temperature (CIRBP, HSC71, HSP40, HSP90, HSP70, ZNF593). Furthermore, the expression patterns of some genes (GSTM2, LY-6E, UMOD, ZNF593, CIRBP, HSP90) show interactive effects of temperature and corticosterone exposure, compared to each treatment alone. Through a series of experiments we also showed that stressor induced patterns of expression were largely consistent across ages, life cycle modes, and tissue regeneration.

CONCLUSIONS

Outside of thermal stressors, the application of transcriptomes to monitor the health of non-human vertebrate systems has been vastly underinvestigated. Our study suggests that transcriptomic patterns harbor stressor specific signatures that can be highly informative for monitoring the diverse stressors of amphibian populations.

Joints in the appendicular skeleton: Developmental mechanisms and evolutionary influences

The joints are a diverse group of skeletal structures, and their genesis, morphogenesis, and acquisition of specialized tissues have intrigued biologists for decades. Here we review past and recent studies on important aspects of joint development, including the roles of the interzone and morphogenesis of articular cartilage. Studies have documented the requirement of interzone cells in limb joint initiation and formation of most, if not all, joint tissues. We highlight these studies and also report more detailed interzone dissection experiments in chick embryos. Articular cartilage has always received special attention owing to its complex architecture and phenotype and its importance in long-term joint function. We pay particular attention to mechanisms by which neonatal articular cartilage grows and thickens over time and eventually acquires its multi-zone structure and becomes mechanically fit in adults. These and other studies are placed in the context of evolutionary biology, specifically regarding the dramatic changes in limb joint organization during transition from aquatic to land life. We describe previous studies, and include new data, on the knee joints of aquatic axolotls that unlike those in higher vertebrates, are not cavitated, are filled with rigid fibrous tissues and resemble amphiarthroses. We show that when axolotls metamorph to life on land, their intra-knee fibrous tissue becomes sparse and seemingly more flexible and the articular cartilage becomes distinct and acquires a tidemark. In sum, there have been considerable advances toward a better understanding of limb joint development, biological responsiveness, and evolutionary influences, though much remains unclear. Future progress in these fields should also lead to creation of new developmental biology-based tools to repair and regenerate joint tissues in acute and chronic conditions.

Changes in the appendicular skeleton during metamorphosis in the axolotl salamander (Ambystoma mexicanum)

Axolotl salamanders (Ambystoma mexicanum) remain aquatic in their natural state, during which biomechanical forces on their diarthrodial limb joints are likely reduced relative to salamanders living on land. However, even as sexually mature adults, these amphibians can be induced to metamorphose into a weight-bearing terrestrial stage by environmental stress or the exogenous administration of thyroxine hormone. In some respects, this aquatic to terrestrial transition of axolotl salamanders through metamorphosis may model developmental and changing biomechanical skeletal forces in mammals during the prenatal to postnatal transition at birth and in the early postnatal period. To assess differences in the appendicular skeleton as a function of metamorphosis, anatomical and gene expression parameters were compared in skeletal tissues between aquatic and terrestrial axolotls that were the same age and genetically full siblings. The length of long bones and area of cuboidal bones in the appendicular skeleton, as well as the cellularity of cartilaginous and interzone tissues of femorotibial joints were generally higher in aquatic axolotls compared with their metamorphosed terrestrial siblings. A comparison of steady-state mRNA transcripts encoding aggrecan core protein (ACAN), type II collagen (COL2A1), and growth and differentiation factor 5 (GDF5) in femorotibial cartilaginous and interzone tissues did not reveal any significant differences between aquatic and terrestrial axolotls.

Transcriptional correlates of proximal-distal identify and regeneration timing in axolotl limbs

Cells within salamander limbs retain memories that inform the correct replacement of amputated tissues at different positions along the length of the arm, with proximal and distal amputations completing regeneration at similar times. We investigated the possibility that positional memory is associated with variation in transcript abundances along the proximal-distal limb axis. Transcripts were deeply sampled from Ambystoma mexicanum limbs at the time they were administered fore arm vs upper arm amputations, and at 19 post-amputation time points. After amputation and prior to regenerative outgrowth, genes typically expressed by differentiated muscle cells declined more rapidly in upper arms while cell cycle transcripts were expressed more highly. These and other expression patterns suggest upper arms undergo more robust tissue remodeling and cell proliferation responses after amputation, and thus provide an explanation for why the overall time to complete regeneration is similar for proximal and distal amputations. Additionally, we identified candidate positional memory genes that were expressed differently between fore and upper arms that encode a surprising number of epithelial proteins and a variety of cell surface, cell adhesion, and extracellular matrix molecules. Also, genes were discovered that exhibited different, bivariate patterns of gene expression between fore and upper arms, implicating dynamic transcriptional regulation for the first time in limb regeneration. Finally, 43 genes expressed differently between fore and upper arm samples showed similar transcriptional patterns during retinoic acid-induced reprogramming of fore arm blastema cells into upper arm cells. Our study provides new insights about the basis of positional information in regenerating axolotl limbs.

How Metamorphosis Is Different in Plethodontids: Larval Life History Perspectives on Life-Cycle Evolution

Plethodontid salamanders exhibit biphasic, larval form paedomorphic, and direct developing life cycles. This diversity of developmental strategies exceeds that of any other family of terrestrial vertebrate. Here we compare patterns of larval development among the three divergent lineages of biphasic plethodontids and other salamanders. We discuss how patterns of life-cycle evolution and larval ecology might have produced a wide array of larval life histories. Compared with many other salamanders, most larval plethodontids have relatively slow growth rates and sometimes exceptionally long larval periods (up to 60 mo). Recent phylogenetic analyses of life-cycle evolution indicate that ancestral plethodontids were likely direct developers. If true, then biphasic and paedomorphic lineages might have been independently derived through different developmental mechanisms. Furthermore, biphasic plethodontids largely colonized stream habitats, which tend to have lower productivity than seasonally ephemeral ponds. Consistent with this, plethodontid larvae grow very slowly, and metamorphic timing does not appear to be strongly affected by growth history. On the basis of this, we speculate that feeding schedules and stress hormones might play a comparatively reduced role in governing the timing of metamorphosis of stream-dwelling salamanders, particularly plethodontids.

A histological atlas of the tissues and organs of neotenic and metamorphosed axolotl

Axolotl (Ambystoma Mexicanum) has been emerging as a promising model in stem cell and regeneration researches due to its exceptional regenerative capacity. Although it represents lifelong lasting neoteny, induction to metamorphosis with thyroid hormones (THs) treatment advances the utilization of Axolotl in various studies. It has been reported that amphibians undergo anatomical and histological remodeling during metamorphosis and this transformation is crucial for adaptation to terrestrial conditions. However, there is no comprehensive histological investigation regarding the morphological alterations of Axolotl organs and tissues throughout the metamorphosis. Here, we reveal the histological differences or resemblances between the neotenic and metamorphic axolotl tissues. In order to examine structural features and cellular organization of Axolotl organs, we performed Hematoxylin & Eosin, Luxol-Fast blue, Masson's trichrome, Alcian blue, Orcein and Weigart's staining. Stained samples from brain, gallbladder, heart, intestine, liver, lung, muscle, skin, spleen, stomach, tail, tongue and vessel were analyzed under the light microscope. Our findings contribute to the validation of the link between newly acquired functions and structural changes of tissues and organs as observed in tail, skin, gallbladder and spleen. We believe that this descriptive work provides new insights for a better histological understanding of both neotenic and metamorphic Axolotl tissues.

The transcriptome of metamorphosing flatfish

BACKGROUND

Flatfish metamorphosis denotes the extraordinary transformation of a symmetric pelagic larva into an asymmetric benthic juvenile. Metamorphosis in vertebrates is driven by thyroid hormones (THs), but how they orchestrate the cellular, morphological and functional modifications associated with maturation to juvenile/adult states in flatfish is an enigma. Since THs act via thyroid receptors that are ligand activated transcription factors, we hypothesized that the maturation of tissues during metamorphosis should be preceded by significant modifications in the transcriptome. Targeting the unique metamorphosis of flatfish and taking advantage of the large size of Atlantic halibut (Hippoglossus hippoglossus) larvae, we determined the molecular basis of TH action using RNA sequencing.

RESULTS

De novo assembly of sequences for larval head, skin and gastrointestinal tract (GI-tract) yielded 90,676, 65,530 and 38,426 contigs, respectively. More than 57 % of the assembled sequences were successfully annotated using a multi-step Blast approach. A unique set of biological processes and candidate genes were identified specifically associated with changes in morphology and function of the head, skin and GI-tract. Transcriptome dynamics during metamorphosis were mapped with SOLiD sequencing of whole larvae and revealed greater than 8,000 differentially expressed (DE) genes significantly (p < 0.05) up- or down-regulated in comparison with the juvenile stage. Candidate transcripts quantified by SOLiD and qPCR analysis were significantly (r = 0.843; p < 0.05) correlated. The majority (98 %) of DE genes during metamorphosis were not TH-responsive. TH-responsive transcripts clustered into 6 groups based on their expression pattern during metamorphosis and the majority of the 145 DE TH-responsive genes were down-regulated.

CONCLUSIONS

A transcriptome resource has been generated for metamorphosing Atlantic halibut and over 8,000 DE transcripts per stage were identified. Unique sets of biological processes and candidate genes were associated with changes in the head, skin and GI-tract during metamorphosis. A small proportion of DE transcripts were TH-responsive, suggesting that they trigger gene networks, signalling cascades and transcription factors, leading to the overt changes in tissue occurring during metamorphosis.

Gene expression during the first 28 days of axolotl limb regeneration I: Experimental design and global analysis of gene expression

While it is appreciated that global gene expression analyses can provide novel insights about complex biological processes, experiments are generally insufficiently powered to achieve this goal. Here we report the results of a robust microarray experiment of axolotl forelimb regeneration. At each of 20 post‐amputation time points, we estimated gene expression for 10 replicate RNA samples that were isolated from 1 mm of heterogeneous tissue collected from the distal limb tip. We show that the limb transcription program diverges progressively with time from the non‐injured state, and divergence among time adjacent samples is mostly gradual. However, punctuated episodes of transcription were identified for five intervals of time, with four of these coinciding with well‐described stages of limb regeneration—amputation, early bud, late bud, and pallet. The results suggest that regeneration is highly temporally structured and regulated by mechanisms that function within narrow windows of time to coordinate transcription within and across cell types of the regenerating limb. Our results provide an integrative framework for hypothesis generation using this complex and highly informative data set.

Development of neuroendocrine components of the thyroid axis in the direct-developing frog Eleutherodactylus coqui: formation of the median eminence and onset of pituitary TSH production

Direct-developing frogs lack, wholly or in part, a wide range of larval features found in metamorphosing species and form adult-specific features precociously, during embryogenesis. Most information on thyroid regulation of direct development relies on hormone manipulations; the ontogeny of many thyroid axis components has not been fully described. This analysis examines differentiation of the median eminence of the hypothalamus and production of thyroid-stimulating hormone (TSH) by the pituitary of the direct-developing frog Eleutherodactylus coqui. The median eminence is established two-thirds of the way through embryogenesis. Cells immunoreactive to human TSHβ antibodies are first detected during embryogenesis and quantitative changes in TSHβ-IR cells resemble those in metamorphosing amphibians. Formation of the median eminence of the hypothalamus and TSHβ production by the pituitary precede or coincide with morphological changes during embryogenesis that occur during metamorphosis in biphasic anurans. Thus, while the onset of neuroendocrine regulation has changed during the evolution of direct development, it is likely that these thyroid axis components still mediate the formation of adult features.

Towards the identification of the loci of adaptive evolution

1. Establishing the genetic and molecular basis underlying adaptive traits is one of the major goals of evolutionary geneticists in order to understand the connection between genotype and phenotype and elucidate the mechanisms of evolutionary change. Despite considerable effort to address this question, there remain relatively few systems in which the genes shaping adaptations have been identified. 2. Here, we review the experimental tools that have been applied to document the molecular basis underlying evolution in several natural systems, in order to highlight their benefits, limitations and suitability. In most cases, a combination of DNA, RNA and functional methodologies with field experiments will be needed to uncover the genes and mechanisms shaping adaptation in nature.

Experimentally induced metamorphosis in axolotls reduces regenerative rate and fidelity

While most tetrapods are unable to regenerate severed body parts, amphibians display a remarkable ability to regenerate an array of structures. Frogs can regenerate appendages as larva, but they lose this ability around metamorphosis. In contrast, salamanders regenerate appendages as larva, juveniles, and adults. However, the extent to which fundamental traits (e.g., metamorphosis, body size, aging, etc.) restrict regenerative ability remains contentious. Here we utilize the ability of normally paedomorphic adult axolotls (Ambystoma mexicanum) to undergo induced metamorphosis by thyroxine exposure to test how metamorphosis and body size affects regeneration in age‐matched paedomorphic and metamorphic individuals. We show that body size does not affect regeneration in adult axolotls, but metamorphosis causes a twofold reduction in regeneration rate, and lead to carpal and digit malformations. Furthermore, we find evidence that metamorphic blastemal cells may take longer to traverse the cell cycle and display a lower proliferative rate. This study identifies the axolotl as a powerful system to study how metamorphosis restricts regeneration independently of developmental stage, body size, and age; and more broadly how metamorphosis affects tissue‐specific changes.

New insights into vertebrate skin regeneration

Regeneration biology has experienced a renaissance as clinicians, scientists, and engineers have combined forces to drive the field of regenerative medicine. Studies investigating the mechanisms that regulate wound healing in adult mammals have led to a good understanding of the stereotypical processes that lead to scarring. Despite comparative studies of fetal wound healing in which no scar is produced, the fact remains that insights from this work have failed to produce therapies that can regenerate adult human skin. In this review, we analyze past and contemporary accounts of wound healing in a variety of vertebrates, namely, fish, amphibians, and mammals, in order to demonstrate how examples of skin regeneration in adult organisms can impact traditional wound-healing research. When considered together, these studies suggest that inflammation and reepithelialization are necessary events preceding both scarring and regeneration. However, the extent to which these processes may direct one outcome over another is likely weaker than currently accepted. In contrast, the extent to which newly deposited extracellular matrix in the wound bed can be remodeled into new skin, and the intrinsic ability of new epidermis to regenerate appendages, appears to underlie the divergence between scar-free healing and the persistence of a scar. We discuss several ideas that may offer areas of overlap between researchers using these different model organisms and which may be of benefit to the ultimate goal of scar-free human wound healing.

Developmental diversity of amphibians

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes.

Identification of differentially expressed thyroid hormone responsive genes from the brain of the Mexican Axolotl (Ambystoma mexicanum)

The Mexican axolotl (Ambystoma mexicanum) presents an excellent model to investigate mechanisms of brain development that are conserved among vertebrates. In particular, metamorphic changes of the brain can be induced in free-living aquatic juveniles and adults by simply adding thyroid hormone (T4) to rearing water. Whole brains were sampled from juvenile A. mexicanum that were exposed to 0, 8, and 18 days of 50 nM T4, and these were used to isolate RNA and make normalized cDNA libraries for 454 DNA sequencing. A total of 1,875,732 high quality cDNA reads were assembled with existing ESTs to obtain 5884 new contigs for human RefSeq protein models, and to develop a custom Affymetrix gene expression array (Amby_002) with approximately 20,000 probe sets. The Amby_002 array was used to identify 303 transcripts that differed statistically (p<0.05, fold change >1.5) as a function of days of T4 treatment. Further statistical analyses showed that Amby_002 performed concordantly in comparison to an existing, small format expression array. This study introduces a new A. mexicanum microarray resource for the community and the first lists of T4-responsive genes from the brain of a salamander amphibian.

Microarray analysis of a salamander hopeful monster reveals transcriptional signatures of paedomorphic brain development

Background

The Mexican axolotl (Ambystoma mexicanum) is considered a hopeful monster because it exhibits an adaptive and derived mode of development - paedomorphosis - that has evolved rapidly and independently among tiger salamanders. Unlike related tiger salamanders that undergo metamorphosis, axolotls retain larval morphological traits into adulthood and thus present an adult body plan that differs dramatically from the ancestral (metamorphic) form. The basis of paedomorphic development was investigated by comparing temporal patterns of gene transcription between axolotl and tiger salamander larvae (Ambystoma tigrinum tigrinum) that typically undergo a metamorphosis.

Results

Transcript abundances from whole brain and pituitary were estimated via microarray analysis on four different days post hatching (42, 56, 70, 84 dph) and regression modeling was used to independently identify genes that were differentially expressed as a function of time in both species. Collectively, more differentially expressed genes (DEGs) were identified as unique to the axolotl (n = 76) and tiger salamander (n = 292) than were identified as shared (n = 108). All but two of the shared DEGs exhibited the same temporal pattern of expression and the unique genes tended to show greater changes later in the larval period when tiger salamander larvae were undergoing anatomical metamorphosis. A second, complementary analysis that directly compared the expression of 1320 genes between the species identified 409 genes that differed as a function of species or the interaction between time and species. Of these 409 DEGs, 84% exhibited higher abundances in tiger salamander larvae at all sampling times.

Conclusions

Many of the unique tiger salamander transcriptional responses are probably associated with metamorphic biological processes. However, the axolotl also showed unique patterns of transcription early in development. In particular, the axolotl showed a genome-wide reduction in mRNA abundance across loci, including genes that regulate hypothalamic-pituitary activities. This suggests that an axolotls failure to undergo anatomical metamorphosis late in the larval period is indirectly associated with a mechanism(s) that acts earlier in development to broadly program transcription. The axolotl hopeful monster provides a model to identify mechanisms of early brain development that proximally and ultimately affect the expression of adult phenotypes.

Ambystoma mexicanum, the axolotl: a versatile amphibian model for regeneration, development, and evolution studies

Adult salamanders are best known for their capacity to regenerate an astounding range of body structures including the whole limb and tail, the central nervous system, and tissues of the eye and heart. The axolotl (Ambystoma mexicanum) represents the salamander species that is most easily bred in the laboratory, and for which the most comprehensive genetic, genomic, and transgenesis tools have been developed. As such, it serves as an important vertebrate model for studying regeneration and tissue repair. Beyond regeneration, axolotls have a deep and rich history as primary amphibian models, especially in research areas concerning embryonic development--most notably the inductive mode of germ cell formation. The easily obtained oocytes, high quantities of embryos produced by each spawning, large size of the embryo, and ability to graft tissues from individual to individual at any stage without rejection make the axolotl an advantageous model system for the study of development, electrophysiology, and regeneration.