Differential regulation of three thyroid hormone-responsive matrix metalloproteinase genes implicates distinct functions during frog embryogenesis

Matrix metalloproteinases (MMPs) are a family of Zn(2+)-dependent extracellular proteases capable of degrading various proteinaceous components of the extracellular matrix (ECM). They are expressed in developmental and pathological processes such as postlactation mammary gland involution and tumor metastasis. Relatively few studies have been carried out to investigate the function of MMPs during embryogenesis and postembryonic organ development. Using Xenopus development as a model system, we and others have previously isolated three MMP genes as thyroid hormone response genes. They have distinct temporal and organ-specific regulations during thyroid hormone-dependent metamorphosis. We demonstrate here that three MMPs-stromelysin-3 (ST3), collagenases-3 (Col3), and collagenases-4 (Col4)-also have distinct spatial and temporal expression profiles during embryogenesis. Consistent with earlier suggestions that ST3 is a direct thyroid hormone response gene whereas Col3 and Col4 are not, we show that precocious overexpression of thyroid hormone receptors in the presence of thyroid hormone lead to increased expression of ST3, but not Col3. Furthermore, our whole-mount in situ hybridizations reveal a tight but distinct association of individual MMPs with tissue remodeling in different regions of the animal during embryogenesis. These results suggest that ST3 is likely to play a role in ECM remodeling that facilitate apoptotic tissue remodeling or resorption, whereas Col3 and Col4 appear to participate in connective tissue degradation during development.

Thyroid hormone transport by 4F2hc-IU12 heterodimers expressed in Xenopus oocytes

Thyroid hormone (TH) action and metabolism require hormone transport across cell membranes. We have investigated the possibility that TH are substrates of amino acid transport (System L) mediated by heterodimers of 4F2 heavy-chain (hc) and the light-chain (lc) permease IU12. Co-expression of 4F2hc and IU12 cDNAs injected into Xenopus oocytes induces saturable, Na(+) -independent transport of tri-iodothyronine (T(3)), thyroxine (T(4)) (K(m) of 1.8 and 6.3 microM respectively), tryptophan and phenylalanine. Induced TH and tryptophan uptakes are inhibited by excess BCH (synthetic System L substrate). Induced TH uptake is also inhibited by excess reverse tri-iodothyronine (rT(3)), but not by triodothyroacetic acid (TRIAC) (TH analogue lacking anamino acid moiety). T(3) and tryptophan exhibit reciprocal inhibition of their 4F2hc-IU12 induced uptake. Transport pathways produced by 4F2hc-lc permease complexes may therefore be important routes for movement and exchange of TH (as well as amino acids) across vertebrate cell membranes, with a potential role in modulating TH action.

Functional analysis of the SIN3-histone deacetylase RPD3-RbAp48-histone H4 connection in the Xenopus oocyte

We investigated the protein associations and enzymatic requirements for the Xenopus histone deacetylase catalytic subunit RPD3 to direct transcriptional repression in Xenopus oocytes. Endogenous Xenopus RPD3 is present in nuclear and cytoplasmic pools, whereas RbAp48 and SIN3 are predominantly nuclear. We cloned Xenopus RbAp48 and SIN3 and show that expression of RPD3, but not RbAp48 or SIN3, leads to an increase in nuclear and cytoplasmic histone deacetylase activity and transcriptional repression of the TRbetaA promoter. This repression requires deacetylase activity and nuclear import of RPD3 mediated by a carboxy-terminal nuclear localization signal. Exogenous RPD3 is not incorporated into previously described oocyte deacetylase and ATPase complexes but cofractionates with a component of the endogenous RbAp48 in the oocyte nucleus. We show that RPD3 associates with RbAp48 through N- and C-terminal contacts and that RbAp48 also interacts with SIN3. Xenopus RbAp48 selectively binds to the segment of the N-terminal tail immediately proximal to the histone fold domain of histone H4 in vivo. Exogenous RPD3 may be targeted to histones through interaction with endogenous RbAp48 to direct transcriptional repression of the Xenopus TRbetaA promoter in the oocyte nucleus. However, the exogenous RPD3 deacetylase functions to repress transcription in the absence of a requirement for association with SIN3 or other targeted corepressors.

Spatial and temporal regulation of collagenases-3, -4, and stromelysin -3 implicates distinct functions in apoptosis and tissue remodeling during frog metamorphosis

Matrix metalloproteinases (MMPs) are a family of extracellular proteases capable of degrading various proteinaceous components of the extracellular matrix (ECM). They have been implicated to play important roles in a number of developmental and pathological processes, such as tumor metastasis and inflammation. Relatively few studies have been carried out to investigate the function of MMPs during postembryonic organ-development. Using Xenopus laevis development as a model system, we demonstrate here that three MMPs, stromelysin-3 (ST3), collagenases-3 (Col3), and Col4, have distinct spatial and temporal expression profiles during metamorphosis as the tadpole transforms into a frog. In situ hybridizations reveal a tight, but distinct, association of individual MMPs with tissue remodeling in the tail and intestine during metamorphosis. In particular, ST3 expression is strongly correlated with apoptosis in both organs as demonstrated by analyses of serial sections with in situ hybridization for ST3 mRNA and TUNEL (terminal deoxyribonucleotidyl transferase-mediated dUTP-biotin nick end labeling) for apoptosis, respectively. On the other hand, Col3 and Col4 are present in regions where extensive connective tissue remodeling take place. These results indicate that ST3 is likely to play a role in ECM-remodeling that facilitate apoptotic tissue remodeling or resorption while Col3 and Col4 appear to participate in connective tissue degradation during development.

Modification of Chromatin Structure by the Thyroid Hormone Receptor

Pioneering experiments and recent observations have established the thyroid hormone receptor as a master manipulator of the chromosomal environment in targeting the activation and repression of transcription. Here we review how the thyroid hormone receptor is assembled into chromatin, where in the absence of thyroid hormone the receptor recruits histone deacetylase to silence transcription. On addition of hormone, the receptor undergoes a conformational change that leads to the release of deacetylase, while facilitating the recruitment of transcriptional coactivators that act as histone acetyltransferases. We discuss the biological importance of these observations for gene control by the thyroid hormone receptor and for oncogenic transformation by the mutated thyroid hormone receptor, v-ErbA.

Molecular and cellular basis of tissue remodeling during amphibian metamorphosis

Amphibian metamorphosis involves systematic transformations of various tadpole organs/tissues. Three major types of changes take place during this process. These are remodeling, resorption, and de novo development, all of which appear to involve both cell proliferation and apoptosis (programmed cell death). All metamorphic changes are controlled by thyroid hormone (T3) and are organ-autonomous. Recent studies using primary cell cultures and a stably transformed cell line from tadpole tissues have implicated that T3 induces apoptosis cell-autonomously. This T3-induced, metamorphosis-associated apoptosis is similar to cell death in other animal species and involves similar cell death executioners. Both the activation of these executioners and the pathways leading to cell proliferation and differentiation are believed to be through transcriptional regulation by T3 receptors (TRs). TRs can activate or repress target gene transcription depending upon the presence or absence of T3, respectively. Many direct T3-response genes have been isolated and found to encode a variety of proteins that can affect both intra- and extra-cellular events. The determinations of the identities of these response genes through sequence analyses and studies on their expression profiles during development have provided strong clues toward their roles in metamorphosis. However, future studies using organ and cell culture systems and/or transient or stable transgenic technologies are required to understand how these genes transduce the T3 signal to activate the downstream cell death and proliferation/differentiation pathways.

Identification and characterization of a membrane protein (y+L amino acid transporter-1) that associates with 4F2hc to encode the amino acid transport activity y+L. A candidate gene for lysinuric protein intolerance

We have identified a new human cDNA (y+L amino acid transporter-1 (y+LAT-1)) that induces system y+L transport activity with 4F2hc (the surface antigen 4F2 heavy chain) in oocytes. Human y+LAT-1 is a new member of a family of polytopic transmembrane proteins that are homologous to the yeast high affinity methionine permease MUP1. Other members of this family, the Xenopus laevis IU12 and the human KIAA0245 cDNAs, also co-express amino acid transport activity with 4F2hc in oocytes, with characteristics that are compatible with those of systems L and y+L, respectively. y+LAT-1 protein forms a approximately 135-kDa, disulfide bond-dependent heterodimer with 4F2hc in oocytes, which upon reduction results in two protein bands of approximately 85 kDa (i.e. 4F2hc) and approximately 40 kDa (y+LAT-1). Mutation of the human 4F2hc residue cysteine 109 (Cys-109) to serine abolishes the formation of this heterodimer and drastically reduces the co-expressed transport activity. These data suggest that y+LAT-1 and other members of this family are different 4F2 light chain subunits, which associated with 4F2hc, constitute different amino acid transporters. Human y+LAT-1 mRNA is expressed in kidney >> peripheral blood leukocytes >> lung > placenta = spleen > small intestine. The human y+LAT-1 gene localizes at chromosome 14q11.2 (17cR approximately 374 kb from D14S1350), within the lysinuric protein intolerance (LPI) locus (Lauteala, T., Sistonen, P. , Savontaus, M. L., Mykkanen, J., Simell, J., Lukkarinen, M., Simmell, O., and Aula, P. (1997) Am. J. Hum. Genet. 60, 1479-1486). LPI is an inherited autosomal disease characterized by a defective dibasic amino acid transport in kidney, intestine, and other tissues. The pattern of expression of human y+LAT-1, its co-expressed transport activity with 4F2hc, and its chromosomal location within the LPI locus, suggest y+LAT-1 as a candidate gene for LPI.

Regulation of apoptosis during development: input from the extracellular matrix (review)

Programmed cell death or apoptosis is an important aspect in organogenesis and tissue remodeling during development. Extensive investigations have led to the identification of many genes that participate in the regulation of cell death execution. These include the caspases and nucleases, which are involved in the degradation of cellular proteins and nuclear DNA to initiate the irreversible death process. In addition, several families of proteins like Bcl-2 superfamily can either prevent or promote cell death. The function of these proteins are getting to be understood. On the other hand, how these proteins are regulated remains to be investigated. This is in part due to the presence of diverse upstream signals that can influence cell fate. One such signal is the remodeling of the extracellular matrix (ECM), which is largely due to the action of matrix metalloproteinases (MMPs). The regulation of MMPs and ECM remodeling has been shown to affect apoptosis in different systems, including the apoptotic remodeling of the intestine during Xenopus laevis metamorphosis and post-lactation involution of the mouse mammary gland. Current evidence suggests that ECM regulates cell fate at least in part through its membrane receptors, the integrins, which in turn send the signal through yet poorly understood pathways to the nucleus to regulate gene expression.

Thyroid hormone regulation of Xenopus laevis metamorphosis: functions of thyroid hormone receptors and roles of extracellular matrix remodeling

The regulatory effects of the thyroid hormone on amphibian metamorphosis is mediated by thyroid hormone receptors. Using Xenopus laevis as a model system, we and others have shown that the mRNA levels of thyroid hormone receptors and 9-cis retinoic acid receptors, which form the functional heterodimers with thyroid hormone receptors, are regulated temporally in a tissue-dependent manner so that high levels of their mRNAs are present in an organ when metamorphosis is occurring. By overexpressing thyroid hormone receptors, 9-cis retinoic acid receptors, or both into developing Xenopus embryos, we have shown that both thyroid hormone receptors and 9-cis retinoic acid receptors are required for mediating the effects of thyroid hormone on embryo development and precocious but specific regulation of the genes, which are normally regulated by thyroid hormone during metamorphosis. Analyses of the developmental expression of one class of thyroid hormone response genes, which encode extracellular matrix-degrading metalloproteinases, suggest that extra cellular remodeling plays an important role during tissue remodeling, including cell death (apoptosis) and cell proliferation and differentiation. This effect of extracellular matrix on cell behavior has been supported directly by in vitro primary cell culture experiments, in which intestinal epithelial cells undergo thyroid hormone-induced apoptosis, just like that during natural metamorphosis.

Unique organization and involvement of GAGA factors in transcriptional regulation of the Xenopus stromelysin-3 gene

Expression of the matrix metalloproteinase (MMP) gene stromelysin-3 ( ST3 ) has been shown to be tightly associated with cell migration and apoptosis inmammals and amphibians. This contrasts with most other MMP genes. We demonstrate here that the Xenopus ST3 gene also has a structure distinct from other MMP genes, with its C-terminal half (the hemopexin domain) encoded by 4 instead of 6 exons, as in other MMP genes. Our primer extension analysis reveals the existence of two transcription start sites and at least one is needed for transcription of the promoter in transient transfection assays. Furthermore, our deletion analysis has demonstrated a requirement for at least one GAGA factor binding site for promoter function. In vitro DNA binding and mutational studies have provided strong evidence for the participation of GAGA or GAGA-like factors in transcriptional regulation of the frog ST3 gene. This contrasts with regulation of the human ST3 promoter. These results suggest that the ST3 gene evolved prior to most other metalloproteinase genes and uses distinct regulation pathways to achieve similar expression profiles and serve similar functions in mammals and amphibians.

Transcription from the thyroid hormone-dependent promoter of the Xenopus laevis thyroid hormone receptor betaA gene requires a novel upstream element and the initiator, but not a TATA Box

The thyroid hormone receptor (TR) beta genes in Xenopus laevis are regulated by thyroid hormone in all organs of an animal during metamorphosis. This autoregulation appears to be critical for systematic transformations of different organs as a tadpole is transformed into a frog. To understand this autoregulation, we have previously identified a thyroid hormone response element in the hormone-dependent promoter of the X. laevis TRbetaA gene. We report here the detailed characterization of the promoter. We have now mapped the transcription start site and demonstrated the existence of an initiator element at the start site critical for promoter function. More important, our deletion and mutational experiments revealed a novel upstream DNA element that is located 125 base pairs upstream of the start site and that is essential for active transcription from the promoter. Promoter reconstitution experiments showed that this novel element does not function as an enhancer, but acts as a core promoter element, which, together with the initiator, directs accurate transcription from the promoter. Finally, we provide evidence for the existence of a protein(s) that specifically recognizes this element. Our studies thus demonstrate that the TRbetaA promoter has a unique organization consisting of an initiator and a novel upstream promoter element. Such an organization may be important for the ubiquitous but tissue-dependent temporal regulation of the gene by thyroid hormone during amphibian metamorphosis.

Distinct functions are implicated for the GATA-4, -5, and -6 transcription factors in the regulation of intestine epithelial cell differentiation

Based on conserved expression patterns, three members of the GATA family of transcriptional regulatory proteins, GATA-4, -5, and -6, are thought to be involved in the regulation of cardiogenesis and gut development. Functions for these factors are known in the heart, but relatively little is understood regarding their possible roles in the regulation of gut-specific gene expression. In this study, we analyze the expression and function of GATA-4, -5, and -6 using three separate but complementary vertebrate systems, and the results support a function for these proteins in regulating the terminal-differentiation program of intestinal epithelial cells. We show that xGATA-4, -5, and -6 can stimulate directly activity of the promoter for the intestinal fatty acid-binding protein (xIFABP) gene, which is a marker for differentiated enterocytes. This is the first direct demonstration of a target for GATA factors in the vertebrate intestinal epithelium. Transactivation by xGATA-4, -5, and -6 is mediated at least in part by a defined proximal IFABP promoter element. The expression patterns for cGATA-4, -5, and -6 are markedly distinct along the proximal-distal villus axis. Transcript levels for cGATA-4 increase along the axis toward the villus tip; likewise, cGATA-5 transcripts are largely restricted to the distal tip containing differentiated cells. In contrast, the pattern of cGATA-6 transcripts is complementary to cGATA-5, with highest levels detected in the region of proliferating progenitor cells. Undifferentiated and proliferating human HT-29 cells express hGATA-6 but not hGATA-4 or hGATA-5. Upon stimulation to differentiate, the transcript levels for hGATA-5 increase, and this occurs prior to increased transcription of the terminal differentiation marker intestinal alkaline phosphatase. At the same time, hGATA-6 steady-state transcript levels decline appreciably. All of the data are consistent with evolutionarily conserved but distinct roles for these factors in regulating the differentiation program of intestinal epithelium. Based on this data, we suggest that GATA-6 might function primarily within the proliferating progenitor population, while GATA-4 and GATA-5 function during differentiation to activate terminal-differentiation genes including IFABP.

Retinoid receptor-induced alteration of the chromatin assembled on a ligand-responsive promoter in Xenopus oocytes

Retinoic acid (RA) stimulates transcription from the retinoic acid receptor beta2 (RARbeta2) promoter in mammalian embryonal cells. Evidence by in vivo deoxyribonuclease I (DNase I) hypersensitivity assay indicates that RA treatment of these cells results in an alteration of chromatin structure in and near the promoter. To study the role of chromatin in RA-activated transcription, we assembled the RARbeta2 promoter into chromatin in Xenopus oocytes. Ectopic expression of RAR and retinoid X receptor (RXR) enhanced transcription without ligand, irrespective of whether chromatin was assembled in a replication-dependent or -independent manner, although ligand addition led to a further, marked increase in transcription. Moreover, expression of RAR and RXR, without ligand addition, induced DNase I-hypersensitive sites in the chromatin-assembled promoter. Furthermore, expression of RAR and RXR in oocytes led to local disruption of chromatin assembled over the promoter without ligand. Similar ligand-independent, but RXR/RAR-dependent nucleosomal disruption was observed in an in vitro chromatin reconstitution system using Drosophila embryonic extracts. Thus, unliganded receptors expressed in oocytes are capable of accessing to the chromatin-assembled promoter and activating transcription without ligand, indicating that chromatin assembly per se is not sufficient to reproduce ligand-dependent chromatin changes and promoter activation seen in mammalian cells. The oocyte system may serve as a model to study mechanisms of RA-dependent alterations of chromatin structure.

Auto-regulation of thyroid hormone receptor genes during metamorphosis: roles in apoptosis and cell proliferation

Amphibian metamorphosis is an excellent model system for studying postembryonic development in vertebrates. It involves specific degeneration of larval cells through programmed cell death with apoptotic morphology and selective proliferation and differentiation of adult cell types. Thyroid hormone (T3) plays a causative role in this process and the effects of T3 is presumed to be mediated by T3 receptors (TRs). Studies in other systems have suggested that TRs function as heterodimers formed with RXRs (9-cis retinoic acid receptors) and require the presence of various cofactor in transcriptional activation and repression in the presence and absence of T3, respectively. The T3-induced transcriptional activation leads to chromatin remodeling which may involve some of the cofactors. Recent investigation on receptor expression has implicated a role of TRs in T3-induced apoptosis in larval tissues and proliferation of adult cell types. Functional studies in tadpoles and developing embryos have provide strong support for such a role and further demonstrate the importance of RXR in mediating the effect of T3.

Distinct requirements for chromatin assembly in transcriptional repression by thyroid hormone receptor and histone deacetylase

Histone deacetylase and chromatin assembly contribute to the control of transcription of the Xenopus TRbetaA gene promoter by the heterodimer of Xenopus thyroid hormone receptor and 9-cis retinoic acid receptor (TR-RXR). Addition of the histone deacetylase inhibitor Trichostatin A (TSA) relieves repression of transcription due to chromatin assembly following microinjection of templates into Xenopus oocyte nuclei, and eliminates regulation of transcription by TR-RXR. Expression of Xenopus RPD3p, the catalytic subunit of histone deacetylase, represses the TRbetaA promoter, but only after efficient assembly of the template into nucleosomes. In contrast, the unliganded TR-RXR represses templates only partially assembled into nucleosomes; addition of TSA also relieves this transcriptional repression. This result indicates the distinct requirements for chromatin assembly in mediating transcriptional repression by the deacetylase alone, compared with those needed in the presence of unliganded TR-RXR. In addition, whereas hormone-bound TR-RXR targets chromatin disruption as assayed through changes in minichromosome topology and loss of a regular nucleosomal ladder on micrococcal nuclease digestion, addition of TSA relieves transcriptional repression but does not disrupt chromatin. Thus, TR-RXR can facilitate transcriptional repression in the absence of hormone through mechanisms in addition to recruitment of deacetylase, and disrupts chromatin structure through mechanisms in addition to the inhibition or release of deacetylase.

Thyroid hormone induces apoptosis in primary cell cultures of tadpole intestine: cell type specificity and effects of extracellular matrix

Thyroid hormone (T3 or 3,5,3'-triiodothyronine) plays a causative role during amphibian metamorphosis. To investigate how T3 induces some cells to die and others to proliferate and differentiate during this process, we have chosen the model system of intestinal remodeling, which involves apoptotic degeneration of larval epithelial cells and proliferation and differentiation of other cells, such as the fibroblasts and adult epithelial cells, to form the adult intestine. We have established in vitro culture conditions for intestinal epithelial cells and fibroblasts. With this system, we show that T3 can enhance the proliferation of both cell types. However, T3 also concurrently induces larval epithelial apoptosis, which can be inhibited by the extracellular matrix (ECM). Our studies with known inhibitors of mammalian cell death reveal both similarities and differences between amphibian and mammalian cell death. These, together with gene expression analysis, reveal that T3 appears to simultaneously induce different pathways that lead to specific gene regulation, proliferation, and apoptotic degeneration of the epithelial cells. Thus, our data provide an important molecular and cellular basis for the differential responses of different cell types to the endogenous T3 during metamorphosis and support a role of ECM during frog metamorphosis.

Characterization of the Xenopus homolog of an immediate early gene associated with cell activation: sequence analysis and regulation of its expression by thyroid hormone during amphibian metamorphosis

The complex transformation of a tadpole to a frog during amphibian development is under the control of thyroid hormone (T3). T3 is known to regulate gene transcription through its nuclear receptors. We have previously isolated many genes which are up-regulated by T3 in the intestine of Xenopus laevis tadpoles. We have now cloned a full-length cDNA for one such gene (IU12). Sequence analysis shows that the IU12 cDNA encodes a plasma membrane protein with 12 transmembrane domains and homologous to a mammalian gene associated with cell activation and organ development. Similarly, we have found that IU12 is activated during intestinal remodeling when both cell death and proliferation take place. Furthermore, IU12 is an early T3-response gene and its expression in the intestine during T3-induced metamorphosis mimics that during normal development. These results argue for a role of IU12 in the signal transduction pathways leading to intestinal metamorphosis.

Structural and functional features of a specific nucleosome containing a recognition element for the thyroid hormone receptor

The Xenopus thyroid hormone receptor betaA (TRbetaA) gene contains an important thyroid hormone response element (TRE) that is assembled into a positioned nucleosome. We determine the translational position of the nucleosome containing the TRE and the rotational positioning of the double helix with respect to the histone surface. Histone H1 is incorporated into the nucleosome leading to an asymmetric protection to micrococcal nuclease cleavage of linker DNA relative to the nucleosome core. Histone H1 association is without significant consequence for the binding of the heterodimer of thyroid hormone receptor and 9-cis retinoic acid receptor (TR/RXR) to nucleosomal DNA in vitro, or for the regulation of TRbetaA gene transcription following microinjection into the oocyte nucleus. Small alterations of 3 and 6 bp in the translational positioning of the TRE in chromatin are also without effect on the transcriptional activity of the TRbetaA gene, whereas a small change in the rotational position of the TRE (3 bp) relative to the histone surface significantly reduces the binding of TR/RXR to the nucleosome and decreases transcriptional activation directed by TR/RXR. Our results indicate that the specific architecture of the nucleosome containing the TRE may have regulatory significance for expression of the TRbetaA gene.

Anteroposterior gradient of epithelial transformation during amphibian intestinal remodeling: immunohistochemical detection of intestinal fatty acid-binding protein

To determine whether the remodeling of the well-organized intestinal epithelium during amphibian metamorphosis is regionally regulated along the anteroposterior axis of the intestine, we raised a polyclonal antibody against the Xenopus laevis intestinal fatty acid-binding protein (IFABP), which is known to be specifically expressed in intestinal absorptive cells, and examined immunohistochemically the differentiation, proliferation, and apoptosis of the epithelial cells throughout X. laevis small intestine. During pre- and prometamorphosis, IFABP-immunoreactive (ir) epithelial cells were localized only in the anterior half of the larval intestine. At the beginning of metamorphic climax, apoptotic cells detected by nick end-labeling (TUNEL) suddenly increased in number in the entire larval epithelium, concurrently with the appearance of adult epithelial primordia. Subsequently, the adult primordia in the anterior part of the intestine developed more rapidly by active cell proliferation than those in the posterior part, and replaced the larval epithelial cells earlier than those in the posterior part. IFABP-ir cells in the adult epithelium were first detectable at the tips of newly formed folds in the proximal part of the intestine. Thereafter, IFABP expression gradually progressed both in the anteroposterior direction and in the crest-trough direction of the folds. These results suggest that developmental processes of the adult epithelium in the X. laevis intestine are regionally regulated along the anteroposterior axis of the intestine, which is maintained throughout metamorphosis, and along the trough-crest axis of the epithelial folds, which is newly established during metamorphosis. Furthermore, the regional differences in IFABP expression along the anteroposterior axis of the intestine were reproduced in organ cultures in vitro. In addition, IFABP expression was first down-regulated and then reactivated in vitro when the anterior part, but not the posterior part, of the larval intestine was treated with thyroid hormone (TH) for extended periods. Therefore, it seems that, in addition to TH, an endogenous factor(s) localized in the intestine itself with an anteroposterior gradient participates in the development of the adult epithelium during amphibian metamorphosis.

Both thyroid hormone and 9-cis retinoic acid receptors are required to efficiently mediate the effects of thyroid hormone on embryonic development and specific gene regulation in Xenopus laevis

Tissue culture transfection and in vitro biochemical studies have suggested that heterodimers of thyroid hormone receptors (TRs) and 9-cis retinoic acid receptors (RXRs) are the likely in vivo complexes that mediate the biological effects of thyroid hormone, 3,5,3'-triiodothyronine (T3). However, direct in vivo evidence for such a hypothesis has been lacking. We have previously reported a close correlation between the coordinated expression of TR and RXR genes and tissue-dependent temporal regulation of organ transformations during Xenopus laevis metamorphosis. By introducing TRs and RXRs either individually or together into developing Xenopus embryos, we demonstrate here that RXRs are critical for the developmental function of TRs. Precocious expression of TRs and RXRs together but not individually leads to drastic, distinct embryonic abnormalities, depending upon the presence or absence of T3, and these developmental effects require the same receptor domains as those required for transcriptional regulation by TR-RXR heterodimers. More importantly, the overexpressed TR-RXR heterodimers faithfully regulate endogenous T3 response genes that are normally regulated by T3 only during metamorphosis. That is, they repress the genes in the absence of T3 and activate them in the presence of the hormone. On the other hand, the receptors have no effect on a retinoic acid (RA) response gene. Thus, RA- and T3 receptor-mediated teratogenic effects in Xenopus embryos occur through distinct molecular pathways, even though the resulting phenotypes have similarities.

Determinants of chromatin disruption and transcriptional regulation instigated by the thyroid hormone receptor: hormone-regulated chromatin disruption is not sufficient for transcriptional activation

Chromatin disruption and transcriptional activation are both thyroid hormone-dependent processes regulated by the heterodimer of thyroid hormone receptor and 9-cis retinoic acid receptor (TR-RXR). In the absence of hormone, TR-RXR binds to nucleosomal DNA, locally disrupts histone-DNA contacts and generates a DNase I-hypersensitive site. Chromatin-bound unliganded TR-RXR silences transcription of the Xenopus TRbetaA gene within a canonical nucleosomal array. On addition of hormone, the receptor directs the extensive further disruption of chromatin structure over several hundred base pairs of DNA and activates transcription. We define a domain of the TR protein necessary for directing this extensive hormone-dependent chromatin disruption. Particular TR-RXR heterodimers containing mutations in this domain are able to bind both hormone and their thyroid hormone receptor recognition element (TRE) within chromatin, yet are unable to direct the extensive hormone-dependent disruption of chromatin or to activate transcription. We distinguish the hormone-dependent disruption of chromatin and transcriptional activation as independently regulated events through the mutagenesis of basal promoter elements and by altering the position and number of TREs within the TRbetaA promoter. Chromatin disruption alone on a minichromosome is shown to be insufficient for transcriptional activation of the TRbetaA gene.

Cyclosporin A but not FK506 inhibits thyroid hormone-induced apoptosis in tadpole intestinal epithelium

Amphibian metamorphosis and mammalian T cell development represent two of the best known systems where developmental programmed cell death through apoptosis takes place. Two immunosuppressants, cyclosporin A (CsA) and FK506, have been demonstrated to inhibit activation-induced cell death in immature T cells and T cell hybridomas. In this study, we have established an in vitro system in which isolated primary tadpole intestinal epithelial cells undergo typical apoptosis upon treatment with thyroid hormone (T3), the causative agent of metamorphosis. It is surprising that this T3-induced apoptosis was found to be inhibited only by CsA but not by FK506, whereas both immunosuppressants block activation-induced apoptosis in T cells. Since T3 exerts its effect primarily by regulating gene transcription through direct binding to nuclear thyroid hormone receptors, our results strongly suggest that except for their similarity in the T cell receptor-mediated signal transduction process, CsA, but not FK506, also blocks another yet-unidentified step during the induction of apoptosis. The identification of this novel function of CsA may provide an important clue toward the understanding of the mechanism of apoptosis and helps in designing better clinical applications of the immunosuppressants.