Biochemical and Immunological Characterization of Collagenase in Tissues of Metamorphosing Bullfrog Tadpoles. (collagenase/bullfrog tadpole/thyroid hormone/metamorphosis/TIMP)

Tissue-dependent induction of apoptosis by matrix metalloproteinase stromelysin-3 during amphibian metamorphosis

Matrix metalloproteinases (MMPs) are a superfamily of Zn(2+)-dependent proteases that are capable of cleaving the proteinaceous component of the extracellular matrix (ECM). The ECM is a critical medium for cell-cell interactions and can also directly signal cells through cell surface ECM receptors, such as integrins. In addition, many growth factors and signaling molecules are stored in the ECM. Thus, ECM remodeling and/or degradation by MMPs are expected to affect cell fate and behavior during many developmental and pathological processes. Numerous studies have shown that the expression of MMP mRNAs and proteins associates tightly with diverse developmental and pathological processes, such as tumor metastasis and mammary gland involution. In vivo evidence to support the roles of MMPs in these processes has been much harder to get. Here, we will review some of our studies on MMP11, or stromelysin-3, during the thyroid hormone-dependent amphibian metamorphosis, a process that resembles the so-called postembryonic development in mammals (from a few months before to several months after birth in humans when organ growth and maturation take place). Our investigations demonstrate that stromelysin-3 controls apoptosis in different tissues via at least two distinct mechanisms.

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

Anuran (frog) metamorphosis has long-served as a model of how thyroid hormones regulate post-embryonic development in vertebrates. However, comparatively little is known about urodele (salamander) metamorphosis. We conducted a detailed time-course study of induced metamorphosis in the Mexican axolotl (Ambystoma mexicanum) that probed metamorphic changes in morphology and gene expression in the skin. Using morphometrics, quantitative PCR, histology, and in situ hybridization we demonstrate that the development of transcriptional markers is fundamental to the resolution of early metamorphic events in axolotls. We then use linear and piecewise linear models to identify a sequence of morphological and transcriptional changes that define larval to adult remodeling events throughout metamorphosis. In addition, we show that transcriptional biomarkers are expressed in specific larval and adult cell populations of the skin and that temporal changes in these biomarkers correlate with tissue remodeling. We compare our results with other studies of natural and induced metamorphosis in urodeles and highlight what appear to be conserved features between urodele and anuran metamorphosis.

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 and developmental analyses of thyroid hormone receptor function in Xenopus laevis, the African clawed frog

The current review focuses on the molecular mechanisms and developmental roles of thyroid hormone receptors (TRs) in gene regulation and metamorphosis in Xenopus laevis and discusses implications for TR function in vertebrate development and diversity. Questions addressed are: (1) what are the molecular mechanisms of gene regulation by TR, (2) what are the developmental roles of TR in mediating the thyroid hormone (TH) signal, (3) what are the roles of the different TR isoforms, and (4) how do changes in these molecular and developmental mechanisms affect evolution? Even though detailed knowledge of molecular mechanisms of TR-mediated gene regulation is available from in vitro studies, relatively little is known about how TR functions in development in vivo. Studies on TR function during frog metamorphosis are leading the way toward bridging the gap between in vitro and in vivo studies. In particular, a dual function model for the role of TR in metamorphosis has been proposed and investigated. In this model, TRs repress genes allowing tadpole growth in the absence of TH during premetamorphosis and activate genes important for metamorphosis when TH is present. Despite the lack of metamorphosis in most other vertebrates, TR has important functions in development across vertebrates. The underlying molecular mechanisms of TR in gene regulation are conserved through evolution, so other mechanisms involving TH-target genes and TH tissue-sensitivity and dependence underlie differences in role of TR across vertebrates. Continued analysis of molecular and developmental roles of TR in X. laevis will provide the basis for understanding how TR functions in gene regulation in vivo across vertebrates and how TR is involved in the generation of evolutionary diversity.

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

Anuran larval skin undergoes a process of metamorphosis into pre-adult and adult skin. Basal skein, larval basal and adult basal cells are basement membrane-attaching cells in the larval, pre-adult and adult epidermis, respectively, and are identified as cells expressing genes of RLK (Rana larval keratin), both RLK and RAK (Rana adult keratin), and RAK. Larval to pre-adult skin conversion takes place in the histological entity called the skin transformation center (STC). The present study performed a cDNA subtractive gene screening on cDNA of the larval and the pre-adult skin, and cloned the secreted protein acidic and rich in cysteine (SPARC) gene as an upregulated gene in the larva to pre-adult skin conversion. RAK gene-positive basal skein cells and fibroblasts in and around the STC were weakly and strongly sparc-positive, respectively. Using sparc and rak, we redefined the STC and visualized it on a histological section as an approximately 150 microm-long region that contained about 20 rak-negative and weakly sparc-positive basal cells. Intense sparc expression was observed in basal skein cells, but not in larval basal cells, suggesting that SPARC acts as a suppressor of rak during epidermal differentiation. This suggestion was tested by investigating the effect of SPARC on cultured larval basal cells. We observed that SPARC suppressed the expression of rak, but not rlk.

Matrix metalloproteinase activity correlates with blastema formation in the regenerating MRL mouse ear hole model

The MRL mouse was proposed as a model of mammalian regeneration because it can close ear holes completely with the restoration of normal tissue. This regeneration process involves the formation of a blastema during healing, the re-appearance of cartilage and hair follicles, and healing without scarring. Such a process requires extensive tissue remodeling. To characterize differences in ear wounding responses between regenerating and nonregenerating mice, we examined and compared the extracellular matrix remodeling and the matrix metalloproteinase (MMP) and tissue inhibitor of metalloproteinase (TIMP) response in the MRL and C57BL/6 mouse strains after injury. We found a correlation between the MRL's ability to break down the basement membrane, form a blastema, and close ear hole wounds and an inflammatory response with neutrophils and macrophages seen in the ear after injury. These cells were positive for MMP-2 and MMP-9 as well as TIMP-2 and TIMP-3. Clear differences between the MRL and B6 response to injury were seen that could explain the differences in healing and blastema formation in the MRL and lack of it in the B6 mice. This finding was further supported by enzyme activity as determined by gelatin zymography.

Unique expression patterns of matrix metalloproteinases in regenerating newt limbs

The process of regeneration of urodele limbs includes a drastic remodeling of extracellular matrices (ECMs) that is induced by matrix metalloproteinases (MMPs) and is thought to be one of the triggers of the regeneration. We studied this remodeling in limbs of Japanese newt, Cynops pyrrhogaster, by using five genes of newt MMPs (nMMPs) as probes: nMMP9, nMMP3/10-a, nMMP3/10-b, and nMMP13 that had been characterized previously, and nMMPe that was newly cloned in the present study. nMMPe was 502 amino acid residues long and showed a low homology to other known vertebrate MMPs. Reverse transcriptase-polymerase chain reactions analysis localized the transcript of nMMPe in the apical epidermal cap (AEC) and the non-blastemal wound epidermis but not in the blastemal mesenchyme or the normal epidermis. Northern blot analysis localized the transcripts of nMMP9, nMMP3/10-a, and nMMP13 in the bone of regenerating limbs, whereas those of nMMP3/10-b in AEC. mRNA in situ hybridization experiments identified the nMMP-expressing cells. nMMP9 gene was strongly expressed in chondrocytes of the cartilage of epiphysis. Of interest, basal cells of AEC, but not those of the normal skin, expressed nMMP3/10-b intensely. Immunohistochemical analysis showed that the nMMP9 proteins synthesized by chondrocytes were secreted and distributed widely in the basement membrane of bone and ECMs of the amputation plane. These nMMPs characterized in the present study might cooperatively work to remodel ECMs of regenerating limbs.

Expression of novel ING variants is regulated by thyroid hormone in the Xenopus laevis tadpole

The candidate tumor suppressor gene, ING1, encodes several protein isoforms as a result of alternative splicing that may possess agonistic and antagonistic roles in the control of cell proliferation and apoptosis. Recently a related gene, ING2, was isolated in human whose expression is increased in adenocarcinomas. Little is known about the cellular function and regulation of these ING family members, but the fact that ING proteins contain a plant homeodomain finger suggests that these proteins may modulate transcription factor-mediated pathways. To elucidate how ING may interact in different tissues to modulate function, we used amphibian metamorphosis as a model system in which a single stimulus, thyroid hormone (TH), initiates tissue-specific proliferation, differentiation, and apoptosis. We have isolated the first Xenopus laevis ING2 and demonstrate that transcript levels increase in response to TH treatment. We provide evidence for the existence of splice variants that are differentially expressed in tissues with different TH-induced fates. Western blots using an antibody directed against the highly conserved C-terminal end of ING proteins reveal a tissue-specific pattern of ING isoform expression in adult Xenopus tissues. Analyses of premetamorphic tadpole tissues show a TH-induced accumulation of ING proteins in tail, whereas the levels in the leg are not affected. This TH-induced accumulation is also observed in serum-free tail organ cultures and is prevented by inhibitors of tail apoptosis. Therefore, this work presents the first link between ING expression and a hormonally regulated nuclear transcription factor-mediated apoptotic response opening the possibility that ING family members may be involved in transducing the signal initiated by TH that determines cell fate.

Novel Rana keratin genes and their expression during larval to adult epidermal conversion in bullfrog tadpoles

The conversion of the larval to adult epidermis during metamorphosis of tadpoles of bullfrog, Rana catesbeiana, was investigated utilizing newly cloned Rana keratin cDNAs as probes. Rana larval keratin (RLK) cDNA (rlk) was cloned using highly specific antisera against Xenopus larval keratin (XLK). Tail skin proteins of bullfrog tadpoles were separated by 2-dimensional gel electrophoresis and subjected to Western blot analysis with anti-XLK antisera. The Rana antigen detected by this method was sequenced and identified as a type II keratin. We cloned rlk from tadpole skin by PCR utilizing primers designed from these peptide sequences of RLK. RLK predicted by nucleotide sequences of rlk was a 549 amino acid -long type II keratin. Subtractive cloning between the body and the tail skin of bullfrog tadpole yielded a cDNA (rak) of Rana adult keratin (RAK). RAK was a 433 amino acid-long type I keratin. We also cloned a Rana keratin 8 (RK8) cDNA (rk8) from bullfrog tadpole epidermis. RK8 was 502 amino acid-long and homologous to cytokeratin 8. Northern blot analyses and in situ hybridization experiments showed that rlk was actively expressed through prometamorphosis in larva-specific epidermal cells called skein cells and became completely inactive at the climax stage of metamorphosis and in the adult skin. RAK mRNA was expressed in basal cells of the tadpole epidermis and germinative cells in the adult epidermis. The expression of rlk and rak was down- and up-regulated by thyroid hormone (TH), respectively. In contrast, there was no change in the expression of RK8 during spontaneous and TH-induced metamorphosis. RK8 mRNA was exclusively expressed in apical cells of the larval epidermis. These patterns of keratin gene expression indicated that the expression of keratin genes is differently regulated by TH depending on the type of larval epidermal cells. The present study demonstrated the usefulness of these genes for the study of molecular mechanism of postembryonic epidermal development and differentiation.

Matrix metalloproteinase production in regenerating axolotl spinal cord

In urodele amphibian spinal cord regeneration, the ependymal cells lining the central canal remodel the lesion site to favor axonal regrowth. We profiled the production of matrix metalloproteinases by injury-reactive mesenchymal ependymal cells in vivo and in vitro and found that matrix metalloproteinases are involved in this remodeling process in the axolotl (Ambystoma mexicanum). The production of cell-associated matrix metalloproteinases in vivo was shown to be identical to that in our cultured ependymal cell model system. Activated and zymogen forms of matrix metalloproteinases were identified using zymography, chemical inhibitors of matrix metalloproteinases, and cleavage of propeptides by organomercurials. The principal cellular proteinases consisted of matrix metalloproteinase-2 (gelatinase A) and matrix metalloproteinase-1 (type I collagenase), which display characteristic shifts in molecular weight following proenzyme processing by organomercurials. In addition, ependymal cell conditioned medium contained secreted forms of the enzyme undetectable in situ. Matrix metalloproteinase-9 (gelatinase B) as well as matrix metalloproteinase-2 and matrix metalloproteinase-1 were secreted and casein substrate zymography showed the presence of a small amount of a very high molecular weight matrix metalloproteinase-3 (prostromelysin) secreted into the culture medium. Matrix metalloproteinases were still present at 4 weeks post-lesioning when the ependymal cells have just re-epithelialized, but decreased near the completion of regeneration (8 weeks post-lesioning). Zymography showed no detectable matrix metalloproteinases in unlesioned cord but the presence of tissue inhibitor of metalloproteinase-1 in intact cord was seen by Western blotting. This study shows that matrix metalloproteinases are associated with urodele spinal cord regeneration and validates the use of our ependymal cell tissue culture model system to evaluate ependymal cell behavior during spinal cord regeneration.

Dual functions of thyroid hormone receptors during Xenopus development

Thyroid hormone (TH) plays a causative role in anuran metamorphosis. This effect is presumed to be manifested through the regulation of gene expression by TH receptors (TRs). TRs can act as both activators and repressors of a TH-inducible gene depending upon the presence and absence of TH, respectively. We have been investigating the roles of TRs during Xenopus laevis development, including premetamorphic and metamorphosing stages. In this review, we summarize some of the studies on the TRs by others and us. These studies reveal that TRs have dual functions in frog development as reflected in the following two aspects. First, TRs function initially as repressors of TH-inducible genes in premetamorphic tadpoles to prevent precocious metamorphosis, thus ensuring a proper period of tadpole growth, and later as activators of these genes to activate the metamorphic process. Second, TRs can promote both cell proliferation and apoptosis during metamorphosis, depending upon the cell type in which they are expressed.

Characterization of a collagenolytic enzyme released from wounded planarians Dugesia japonica

Planarians (Dugesia japonica) were cultured on gels of type I bovine collagen under various conditions that damaged the worms to test the possibility that the animals secrete a collagenolytic enzyme when they are wounded. The planarians were shown to release a potent collagenolytic enzyme around their body when the animals received damage-inducing treatments such as heating at 37 degrees C, freezing and thawing, and amputation. This release of the enzyme did not require the synthesis of RNA or protein. Isolated planarian cells did not release the enzyme even when cultured at 37 degrees C. The collagenolytic activity was found in the supernatants, but not in the insoluble fraction prepared from disintegrated tissues of the wounded animals. These results indicated that the enzyme was stored extracellularly in the normal planarian body. Partial purification of the enzyme and the action spectra of various protease inhibitors on the enzyme showed that the enzyme was a neutral type I collagen-degrading 40 kDa metalloproteinase. We named this enzyme the planarian collagenase.

Cell-type specific and thyroid hormone-dependent expression of genes of alpha1(I) and alpha2(I) collagen in intestine during amphibian metamorphosis

Both the epithelium and the mesenchyme of the larval small intestine of anurans undergoes metamorphic conversion into the adult counterparts. The conversion of the mesenchyme has been poorly understood especially at the molecular level, whereas the changes of the epithelium have been extensively studied. The present study investigated the metamorphic changes of the mesenchyme of tadpoles of bullfrog, Rana catesbeiana, focusing on the expression of genes of type I collagen. By using the cDNA clones coding for a 1(I) and a 2(I) collagen as probes, expression of each collagen gene was examined. These genes were drastically up-regulated at the climax period of spontaneous metamorphosis, which was precociously mimicked by treating tadpoles with thyroid hormone. The increased expression of these genes at the climax stage was well correlated with the conversion of the thin larval mesenchyme to more thick and dense adult connective tissues of the intestine. In situ hybridization identified the fibroblasts that were actively expressing the collagen genes and, therefore, were thought to be responsible for the remodeling. These results strongly suggest that the expression of type I collagen genes is regulated during the intestinal remodeling in a cell-type specific and thyroid hormone-dependent manner.

Formation of the dermal chromatophore unit (DCU) in the tree frog Hyla arborea

In the tadpole of the tree frog Hyla arborea, the color of the dorsal skin was dark brown. Dermal melanophores, xanthophores, and iridophores were scattered randomly under the subepidermal collagen layer (SCL). After metamorphosis, the dorsal color of the animal changed to green and the animal acquired the ability of dramatic color change, demonstrating that the dermal chromatophore unit (DCU) was formed at metamorphosis. Fibroblasts invaded the SCL and divided it into two parts: the stratum spongiosum (SS) and the stratum compactum (SC). The activity of collagenase increased at metamorphosis. The fibroblasts appeared to dissolve the collagen matrix as they invaded the SCL. Then, three types of chromatophores migrated through the SCL and the DCU was formed in the SS. The mechanism how the three types of chromatophores were organized into a DCU is uncertain, but different migration rates of the three chromatophore types may be a factor that determines the position of the chromatophores in the DCU. Almost an equal number of each chromatophore type is necessary to form the DCUs. However, the number of dermal melanophores in the tadpoles was less than the number of xanthophores and iridophores. It was suggested that epidermal melanophores migrated to the dermis at metamorphosis and developed into dermal melanophores. This change may account for smaller number of dermal melanophores available to form the DCUs.

Visualization of the initiation and sequential expansion of the metamorphic conversion of anuran larval skin into the precursor of adult type

A tadpole of bullfrog, Rana catesbeiana, is originally covered with the larval skin over its entire body. Drastic changes arise in both the epidermis and the subcutaneous connective tissue at an early developmental stage, producing the precursor of adult type skin (pre-adult skin). It was found that calcium is a useful probe to detect the region where the precursor formation has occurred because its deposition in the upper part of subcutaneous collagen bundles coincides with the appearance of the pre-adult skin. Whole-mount in situ staining of tadpoles with alizarin red S revealed the initiation site of the premetamorphic transformation of the larval skin into the adult precursor and its ensuing region-dependent expansion. The pre-adult skin first emerged at TK II to III (TK, Taylor and Kollros staging) t lateral sides of the body, which led us to postulate that 'the center for premetamorphic skin transformation' is formed at the specific site in this region. This center moved dorsally and then ventrally, then reached to the most proximal region of the tail, yielding a unique sequential conversion pattern by around TK V when the conversion was completed in the trunk. The present study also visualized the process of the hindlimb skin transformation.

Regionally regulated conversion of protein expression in the skin during anuran metamorphosis

The back skin of anuran larvae undergoes drastic metamorphic changes which are quite different from the tail skin. To have an insight into the mechanism of the region-dependent changes, we made a thorough survey of metamorphic alterations in protein expression in both the body and tail skin of Xenopus laevis tadpoles utilizing high-resolution two-dimensional electrophoresis. Approximately 400 proteins were detected, 9 of which were identified as known proteins and were used as index proteins to standardize the readout of distribution of protein spots on gels. The overall pattern of polypeptide distribution (map) was classified into "larval type" and "adult type." The maps showed stage-specific changes. Up to stage 58 the back skin map remained the larval type, showed remarkable changes for the first time at stage 60 (whose pattern was a combination of both larval and adult types), and became the adult type at stage 62. The basic feature of the tail skin map remained the larval type through the climax stage of metamorphosis. Experiments using tadpoles treated with an antithyroid drug showed that most of back skin proteins detected on the map are thyroid hormone responsive in their expression. Expression of keratin II (adult-type keratin) was found to be uniquely regulated. As expected, silver staining detected it in the back but not in the tail. However, the protein-labeling experiment with [35S]methionine showed that the tail skin is actively synthesizing the adult keratin, suggesting that tail cells have a specific elimination mechanism of the protein.