Molecular identification of the skin transformation center of anuran larval skin using genes of Rana adult keratin (RAK) and SPARC as probes

Evaluation of the potential teratogenic and toxic effect of the herbicide 2,4-D (DMA® 806) in bullfrog embryos and tadpoles (Lithobates catesbeianus)

The objective of this work was to evaluate the potential teratogenic and toxic effect of the herbicide 2,4-D (DMA® 806) on bullfrog embryos and tadpoles (Lithobates catesbeianus). We used the FETAX (Frog Embryo Teratogenesis Assay Xenopus) assay for embryos, and for tadpoles, we used acute (96 h) and chronic (49 days) toxicity tests and evaluated aspects of healthiness, hematology, and histopathology. The LC_50-144h (Median Lethal Concentration), EC_50-144h (Median Effective Concentration), MCIG (Minimum Concentration to Inhibit Growth) and TI (Teratogenic Index) for embryos were 792 mg/L, 593 mg/L, 150 mg/L of 2,4-D (DMA) and 1.34, respectively. For tadpoles, the LC_50-96h was 700 mg/L of 2,4-D (DMA) and chronic test indicated an inflammatory process and erythrocytosis (with possible polycythemia), with consequent reduction of the spleen. This demonstrates physiological stress probably due to dehydration, which can be proven by the gill tufts widening intercellular space and gill tuft fusions. We also found injuries to the kidneys and skin of the animals even in the lowest concentration tested. Our results indicated that this pesticide is minimally teratogenic and has a low toxicity on L. catesbeianus embryos and tadpoles, but it can inhibit embryo growth in concentrations lower than those tested in this study. We hypothesized that the herbicide 2,4-D (DMA® 806) may be a respiratory allergen for L. catesbeianus tadpoles and recommend precautionary measures for prolonged exposure of aquatic organisms to this pesticide.

Teleost Metamorphosis: The Role of Thyroid Hormone

In most teleosts, metamorphosis encompasses a dramatic post-natal developmental process where the free-swimming larvae undergo a series of morphological, cellular and physiological changes that enable the larvae to become a fully formed, albeit sexually immature, juvenile fish. In all teleosts studied to date thyroid hormones (TH) drive metamorphosis, being the necessary and sufficient factors behind this developmental transition. During metamorphosis, negative regulation of thyrotropin by thyroxine (T4) is relaxed allowing higher whole-body levels of T4 that enable specific responses at the tissue/cellular level. Higher local thyroid cellular signaling leads to cell-specific responses that bring about localized developmental events. TH orchestrate in a spatial-temporal manner all local developmental changes so that in the end a fully functional organism arises. In bilateral teleost species, the most evident metamorphic morphological change underlies a transition to a more streamlined body. In the pleuronectiform lineage (flatfishes), these metamorphic morphological changes are more dramatic. The most evident is the migration of one eye to the opposite side of the head and the symmetric pelagic larva development into an asymmetric benthic juvenile. This transition encompasses a dramatic loss of the embryonic derived dorsal-ventral and left-right axis. The embryonic dorsal-ventral axis becomes the left-right axis, whereas the embryonic left-right axis becomes, irrespectively, the dorsal-ventral axis of the juvenile animal. This event is an unparalleled morphological change in vertebrate development and a remarkable display of the capacity of TH-signaling in shaping adaptation and evolution in teleosts. Notwithstanding all this knowledge, there are still fundamental questions in teleost metamorphosis left unanswered: how the central regulation of metamorphosis is achieved and the neuroendocrine network involved is unclear; the detailed cellular and molecular events that give rise to the developmental processes occurring during teleost metamorphosis are still mostly unknown. Also in flatfish, comparatively little is still known about the developmental processes behind asymmetric development. This review summarizes the current knowledge on teleost metamorphosis and explores the gaps that still need to be challenged.

Matricellular Proteins: Functional Insights From Non-mammalian Animal Models

The extracellular matrix (ECM) has central roles in tissue integrity and remodeling throughout the life span of animals. While collagens are the most abundant structural components of ECM in most tissues, tissue-specific molecular complexity is contributed by ECM glycoproteins. The matricellular glycoproteins are categorized primarily according to functional criteria and represented predominantly by the thrombospondin, tenascin, SPARC/osteonectin, and CCN families. These proteins do not self-assemble into ECM fibrils; nevertheless, they shape ECM properties through interactions with structural ECM proteins, growth factors, and cells. Matricellular proteins also promote cell migration or morphological changes through adhesion-modulating or counter-adhesive actions on cell-ECM adhesions, intracellular signaling, and the actin cytoskeleton. Typically, matricellular proteins are most highly expressed during embryonic development. In adult tissues, expression is more limited unless activated by cues for dynamic tissue remodeling and cell motility, such as occur during inflammatory response and wound repair. Many insights in the complex roles of matricellular proteins have been obtained from studies of gene knockout mice. However, with the exception of chordate-specific tenascins, these are highly conserved proteins that are encoded in many animal phyla. This review will consider the increasing body of research on matricellular proteins in nonmammalian animal models. These models provide better access to the very earliest stages of embryonic development and opportunities to study biological processes such as limb and organ regeneration. In aggregate, this research is expanding concepts of the functions and mechanisms of action of matricellular proteins.

Morphological and immunohistochemical reactions of the larval epidermis in the Italian newt (Lissotriton italicus) after exposure to low pH

Mounting evidence suggests that amphibians are globally and currently the most threatened group of vertebrates and different causes might be responsible for this phenomenon. Acidification of water bodies is a global environmental issue that has been proposed as a possible cause for amphibian populations decline. Indeed, it has been widely demonstrated that low pH may exert harmful effects on amphibians, either directly or by increasing the adverse effects of other stressors. Surprisingly only few studies documented the response of amphibian integument to acidic pH conditions and no data are available on the effects of a non-lethal level of pH onto the amphibian larval epidermis. The present study showed that acidic pH (4.5) condition has severe effects on the epidermis of the Italian newt (Lissotriton italicus, formerly Triturus italicus) inducing both morphological and functional alterations. The increase of mucus is the first evident effect of acid injury, followed by the flattening of the epithelium and the appearance of a keratinized shedding layer. The immunolabeling of cytokeratins substantially changes acquiring an adult-like pattern. Also aquaporin 3 and iNOS expression modify their distribution according to a change of the histological features of the epidermis. These results clearly indicate that a short-term exposure to a sub-lethal pH disrupts the epidermis morphology and function in L. italicus larvae. Since the skin exerts a prominent role in both respiration and osmoregulation, the described alterations may adversely affect the overall ionic balance, with a long chain of cascading effects significantly decreasing newts survival probabilities when environmental pH lowering occurs.

Larval epidermis of the red eye tree frog Agalychnis callidryas (Anura, Hylidae): ultrastructural investigation on the Kugelzellen, specialized forms of the constitutive skein cell line

An ultrastructural study was carried out on the epidermis of Agalychnis callidryas tadpoles during limb development. Larval epidermis consisted of four cell layers: basal, lower intermediate, upper intermediate, and surface or apical layers. Basal cells represented the stem compartment of intermediate cells: both belong to the skein cell (SC) lineage, described in several anuran species, on account of the conspicuous intracytoplasmic tonofilament bundles. Apical cells were secretory in nature and released mucus on the body surface. Intermediate SCs exhibited a hydrated central cytoplasm and peripheral tonofilament bundles. They closely resembled the epidermal ball-like cells, Kugelzellen (KZn) of Xenopus laevis tadpoles, and possibly shared their turgor-stiffness properties. In A. callidryas, the stratification of intermediated SCs on their stem cell layer provided the chance to study their cytodifferentiation in a suitable sequence, until basal cell differentiation shifted toward the keratinocyte lineage in premetamorphic stages. Present data assign A. callidryas to the anuran species with a constitutive SC population in larval epidermis, and demonstrate that KZn express the ultimate specialization of such cell line. SCs were arranged in the fashion of a random-rubble stone groundwork, and possessed long processes. These cytoplasmic outgrowths contained a tonofilament axial rod and held together contiguous cells. Ultrastructural findings suggest that this complex structure may impart compressive as well as sliding strengths to the larval epidermis, representing a possible adaption to the fresh water environment.

Molecular characterization, gene expression and dependence on thyroid hormones of two type I keratin genes (sseKer1 and sseKer2) in the flatfish Senegalese sole (Solea senegalensis Kaup)

BACKGROUND

Keratins make up the largest subgroup of intermediate filaments, and, in chordates, represent the most abundant proteins in epithelial cells. They have been associated with a wide range of functions in the cell, but little information is still available about their expression profile and regulation during flatfish metamorphosis. Senegalese sole (Solea senegalensis) is a commercially important flatfish in which no keratin gene has been described yet.

RESULTS

The development of large-scale genomics of Senegalese sole has facilitated the identification of two different type I keratin genes referred to as sseKer1 and sseKer2. Main characteristics and sequence identities with other fish and mammal keratins are described. Phylogenetic analyses grouped sseKer1 and sseKer2 in a significant clade with other teleost epidermal type I keratins, and have allowed for the identification of sseKer2 as a novel keratin. The expression profile of both genes was studied during larval development and in tissues using a real-time approach. sseKer1 and sseKer2 mRNA levels were significantly higher in skin than in other tissues examined. During metamorphosis, sseKer1 transcripts increased significantly at first stages, and reduced thereafter. In contrast, sseKer2 mRNA levels did not change during early metamorphosis although a significant drop at metamorphosis climax and late metamorphosis was also detected. To study the possible regulation of sseKer gene expressions by thyroid hormones (THs), larvae were exposed to the goitrogen thiourea (TU). TU-treated larvae exhibited higher sseKer1 and sseKer2 mRNA levels than untreated control at both 11 and 15 days after treatment. Moreover, addition of exogenous T4 hormone to TU-treated larvae restored or even reduced the steady-state levels with respect to the untreated control, demonstrating that expression of both genes is negatively regulated by THs.

CONCLUSION

We have identified two keratin genes, referred to as sseKer1 and sseKer2, in Senegalese sole. Phylogenetic analyses revealed sseKer2 as a novel keratin. Although they exhibit different expression patterns during larval development, both of them are negatively regulated by THs. The co-regulation by THs could explain the reduction of both keratin transcripts after the metamorphosis climax, suggesting their role in the tissue remodelling processes that occur during metamorphosis.

Analysis of the Rana catesbeiana tadpole tail fin proteome and phosphoproteome during T3-induced apoptosis: identification of a novel type I keratin

BACKGROUND

Thyroid hormones (THs) are vital in the maintenance of homeostasis and in the control of development. One postembryonic developmental process that is principally regulated by THs is amphibian metamorphosis. This process has been intensively studied at the genomic level yet very little information at the proteomic level exists. In addition, there is increasing evidence that changes in the phosphoproteome influence TH action.

RESULTS

Here we identify components of the proteome and phosphoproteome in the tail fin that changed within 48 h of exposure of premetamorphic Rana catesbeiana tadpoles to 10 nM 3,5,3'-triiodothyronine (T3). To this end, we developed a cell and protein fractionation method combined with two-dimensional gel electrophoresis and phosphoprotein-specific staining. Altered proteins were identified using mass spectrometry (MS). We identified and cloned a novel Rana larval type I keratin, RLK I, which may be a target for caspase-mediated proteolysis upon exposure to T3. In addition, the RLK I transcript is reduced during T3-induced and natural metamorphosis which is consistent with a larval keratin. Furthermore, GILT, a protein involved in the immune system, is changed in phosphorylation state which is linked to its activation. Using a complementary MS technique for the analysis of differentially-expressed proteins, isobaric tags for relative and absolute quantitation (iTRAQ) revealed 15 additional proteins whose levels were altered upon T3 treatment. The success of identifying proteins whose levels changed upon T3 treatment with iTRAQ was enhanced through de novo sequencing of MS data and homology database searching. These proteins are involved in apoptosis, extracellular matrix structure, immune system, metabolism, mechanical function, and oxygen transport.

CONCLUSION

We have demonstrated the ability to derive proteomics-based information from a model species for postembryonic development for which no genome information is currently available. The present study identifies proteins whose levels and/or phosphorylation states are altered within 48 h of the induction of tadpole tail regression prior to overt remodeling of the tail. In particular, we have identified a novel keratin that is a target for T3-mediated changes in the tail that can serve as an indicator of early response to this hormone.

Microarray analysis identifies keratin loci as sensitive biomarkers for thyroid hormone disruption in the salamander Ambystoma mexicanum

Ambystomatid salamanders offer several advantages for endocrine disruption research, including genomic and bioinformatics resources, an accessible laboratory model (Ambystoma mexicanum), and natural lineages that are broadly distributed among North American habitats. We used microarray analysis to measure the relative abundance of transcripts isolated from A. mexicanum epidermis (skin) after exogenous application of thyroid hormone (TH). Only one gene had a >2-fold change in transcript abundance after 2 days of TH treatment. However, hundreds of genes showed significantly different transcript levels at days 12 and 28 in comparison to day 0. A list of 123 TH-responsive genes was identified using statistical, BLAST, and fold level criteria. Cluster analysis identified two groups of genes with similar transcription patterns: up-regulated versus down-regulated. Most notably, several keratins exhibited dramatic (1000 fold) increases or decreases in transcript abundance. Keratin gene expression changes coincided with morphological remodeling of epithelial tissues. This suggests that keratin loci can be developed as sensitive biomarkers to assay temporal disruptions of larval-to-adult gene expression programs. Our study has identified the first collection of loci that are regulated during TH-induced metamorphosis in a salamander, thus setting the stage for future investigations of TH disruption in the Mexican axolotl and other salamanders of the genus Ambystoma.

Molecular Mechanism and Evolutional Significance of Epithelial–Mesenchymal Interactions in the Body‐ and Tail‐Dependent Metamorphic Transformation of Anuran Larval Skin

The epidermis of an anuran larva is composed of apical and skein cells that are both mitotically active and self-renewed through larval life. In contrast, the epidermis of an adult frog, with typical stratified squamous epithelium composed of germinative basal, spinous, granular, and cornified cells, is histologically identical to the mammalian epidermis. Two important issues have not yet been addressed in the study of the development of anuran skin. One is the origin of adult basal cells in the larval epidermis and the other is the mechanism by which larval basal cells are transformed into adult basal cells in a region- (body- and tail-) dependent manner. The cell lineage relationship between the larval and adult epidermal cells was determined by examining the expression profiles of several genes that are expressed specifically in larval and/or adult epidermal cells and differentiation profiles of larval basal cells cultured in the presence of thyroid hormone (TH). Histological analyses using several markers led to the identification of the skin transformation center (STC) where the conversion of larval skin to the adult counterpart is taking place. The STC emerges at a specific place in the body skin and at a specific stage of larval development. The STC progressively "moves" into and "invades" the adjacent larval region of the trunk skin as a larva develops, converting the larval skin into the preadult skin, but never into the tail region. The larva to preadult skin conversion requires an epidermal-mesenchymal interaction. The genesis of preadult basal cells is suppressed in the tail epidermis due to the influence of underlying mesenchyme in the tail region. PDGF signaling is one of the molecular cues of epidermal-mesenchymal interactions. In addition, a unique feature of anuran skin metamorphosis is presented referring to the skin of other vertebrates. Histological comparisons of the skin among vertebrate species strongly suggested a similarity between the anuran larval skin and the teleost fish adult skin and between the anuran adult skin and the adult skin of other tetrapod species. Based on these similarities, the evolutional significance of anuran skin metamorphosis is proposed. Finally, studies are reviewed that reveal the molecular mechanism of anuran metamorphosis in relation to TH-TR-TRE signaling. The results of these studies suggest new aspects of the biological significance of TH, and also enable us to envision concerted regulations of the expression of a gene in the frame of the gene network responsible for metamorphic remodeling of larval tissues. The present review will contribute to an understanding of the molecular mechanism of region-dependent skin development of anurans from not only a metamorphic but also from an evolutional point of view, and will provide a new way to understand the biological significance of TH in anurans.

Molecular, cellular and histological changes in skin from a larval to an adult phenotype during bony fish metamorphosis

Developmental models for skin exist in terrestrial and amphibious vertebrates but there is a lack of information in aquatic vertebrates. We have analysed skin epidermal development of a bony fish (teleost), the most successful group of extant vertebrates. A specific epidermal type I keratin cDNA (hhKer1), which may be a bony-fish-specific adaptation associated with the divergence of skin development (scale formation) compared with other vertebrates, has been cloned and characterized. The expression of hhKer1 and collagen 1alpha1 in skin taken together with the presence or absence of keratin bundle-like structures have made it possible to distinguish between larval and adult epidermal cells during skin development. The use of a flatfish with a well-defined larval to juvenile transition as a model of skin development has revealed that epidermal larval basal cells differentiate directly to epidermal adult basal cells at the climax of metamorphosis. Moreover, hhKer1 expression is downregulated at the climax of metamorphosis and is inversely correlated with increasing thyroxin levels. We suggest that, whereas early mechanisms of skin development between aquatic and terrestrial vertebrates are conserved, later mechanisms diverge.

A novel Xenopus laevis larval keratin gene, xlk2: its gene structure and expression during regeneration and metamorphosis of limb and tail

A novel cytokeratin (CK) gene, xlk2, was cloned from a cDNA library prepared from regenerating limbs of Xenopus larvae. The deduced amino acid sequence indicated that its product, XLK2, is a 48 kDa type I (acidic) CK and has a high similarity to CK13, 15, and 19 with the highest homology (58%) to mouse CK15. The gene of xlk2 exclusively expressed in basal cells of the bi-layered larval epidermis, but not in other cells in larvae and not in other periods of life. Its expression was down-regulated during spontaneous and thyroid hormone-induced metamorphosis. The basal cells of the apical epidermal cap (AEC) formed on the regenerate of larval limbs terminated the expression of xlk2, whereas those of the adjacent normal epidermis continued to express it. The AEC-basal cells did not re-express the gene in the regenerate. In contrast, the basal cells of the tail regenerate also once terminated the expression of xlk2, but was able to re-express xlk2 later, supporting a notion that the "de-differentiated" basal cells of the tail epidermal regenerate re-differentiate into larval normal epidermal cells.