Interactive Fly, Drosophila

Thyroid receptor: a model for Ecdysone receptor in vertebrates

Thyroid receptor and Retinoic acid receptor are neither structural nor a functional homologs of the Ecdysone receptor. Nevertheless, the involvement of thyroid receptor in molting, and the heterodimerization of both thyroid receptor and Ecdysone receptor with RXR serve as a vertebrate model for the function of Ecdysone receptor. Information about Thyroid receptor and Retinoic acid receptor are included here in the hopes that this information will be of use to the reader.

The expression patterns of c-erbAalpha and c-erbAbeta, which encode the thyroid hormone receptors (T3Ralpha and T3Rbeta) have been examined during early chicken embryogenesis. Only c-erbAalpha expression is detected by RT-PCR and whole-mount in situ hybridization. c-erbAalpha transcripts are already present at low levels in embryos, before egg incubation. During neurulation a marked increase in transcript level is observed in neurectoderm. A reporter cell line was constructed and used to demonstrate the release of significant amounts of thyroid hormone (T3) from the egg yolk before gastrulation by area opaca cells. During gastrulation, T3 is enriched in the primitive streak and Hensen's node. Introduction of excess T3 frequently results in abnormal development of anterior structures, mainly neural tube defects and anencephalia. These observations suggest that T3Ralpha, like the closely related retinoic acid receptors, fulfills functions that are important for embryonic development well before the onset of thyroid gland function (Flamant, 1998).

Although thyroid hormone (TH) plays a significant role in vertebrate neural development, the molecular basis of TH action on the brain is poorly understood. Thirty four cDNAs were isolated for TH-regulated genes in the diencephalon of Xenopus tadpoles. The mRNAs are regulated by TH and are expressed during metamorphosis. Kinetic analyses shows that most of the genes are up-regulated by TH within 4-8 h and 13 are regulated by TH only in the brain. The identities of seven cDNAs were determined through homology with known genes; an additional five TH-regulated genes were identified by hybridization with known cDNA clones. These include five transcription factors (including two members of the steroid receptor superfamily), a TH-converting deiodinase, two metabolic enzymes, a protein disulfide isomerase-like protein that may bind TH, a neural-specific cytoskeletal protein, and two hypophysiotropic neuropeptides. This is the first successful attempt to isolate a large number of TH-target genes in the developing vertebrate brain. The gene identities allow predictions about the gene regulatory networks underlying TH action on the brain, and the cloned cDNAs provide tools for understanding the basic molecular mechanisms underlying neural cell differentiation (Denver, 1997).

Thyroid hormone (T3) plays a causative role in amphibian metamorphosis. This regulation is thought to be mediated by heterodimers of T3 receptors (TRs) and retinoid X receptors (RXRs). Xenopus TRs can form strong heterodimers with Xenopus RXRs on the T3 response element (TRE) present in Xenopus TR beta genes. Using a T3-responsive in vivo transcription system established by introducing TRs and RXRs into Xenopus oocytes, it has been demonstrated that TR-RXR heterodimers repress the TR beta gene promoter in the absence of T3 and activated the promoter in the presence of the hormone. Furthermore, by analyzing the expression of TR and RXR genes, it has been shown that TR and RXR genes are coordinately regulated in different tissues during metamorphosis. Thus high levels of their mRNAs are present in the limb during early stages of limb development when morphogenesis occurs and in the tail toward the end of metamorphosis, when the tail is being resorbed. Such correlations coupled with the TRE-binding and in vivo transcriptional activation experiments provide strong evidence that TRs and RXRs function together to mediate the effects of T3 during metamorphosis. These results further suggest a possible molecular basis for the temporal regulation of tissue-specific metamorphosis (Wong, 1995a).

The Xenopus thyroid hormone receptor betaA (TRbetaA) gene contains an important thyroid hormone response element (TRE) that is assembled into a positioned nucleosome. The translational position of the nucleosome containing the TRE was determined as well as 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 against the 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. These results indicate that the specific architecture of the nucleosome containing the TRE may have regulatory significance for expression of the TRbetaA gene (Wong, 1997b).

The precocious induction of amphibian metamorphosis is an ideal system for analyzing the developmental action of TH, while the hormonal activation of tadpole tail regression offers the further advantage of studying programmed cell death. One of the striking features of thyroid hormone (TH)-induced tail regression in Xenopus (as with the morphogenetic responses of all tadpole tissues) is the rapid autoinduction of TRbeta gene, but it is not known how TH affects the expression of the genes encoding TR's heterodimeric partner, retinoid X receptor (RXR). The synthetic glucocorticoid dexamethasone (Dex) potentiates and prolactin (PRL) suppresses the 3,3',5-triiodothyronine (T3)-induced regression of pre-metamorphic Xenopus tadpole tails in organ culture. T3 strongly upregulates (11-35-fold) the concentration of Xenopus TRbeta (xTRbeta) mRNA in these cultures while downregulating by 50% that of Xenopus RXRgamma (xRXRgamma) mRNA in the same samples of tail RNA. DEX and PRL either enhance or diminish (respectively) the T3-regulated expression of these two transcripts, which parallels their other effects in whole tadpoles or cultured tails. The contrasting effects of the three hormones on the steady-state levels of xTRbeta and XRXRgamma mRNAs are time- and dose-dependent. T3 and DEX also strongly upregulate the transcription of xTRbeta gene transfected into Xenopus XTC-2 cells but PRL fails to prevent this autoinduction. The actions of these three hormones involved in amphibian metamorphosis, as judged by the expression of xTRbeta and xRXRgamma genes, reveal a new facet of hormonal interplay underlying their developmental actions (Iwamuro, 1995).

Expression of genes up-regulated by thyroid hormone (TH) during amphibian tail resorption was localized by in situ hybridization. The constitutive thyroid hormone receptor (TRalpha) and its heterodimeric partners (RXRalpha and RXRbeta) are expressed ubiquitously in the resorbing tail. A group of early response genes, including those encoding transcription factors, are expressed at greatest levels within tissues whose cells attempt to grow and differentiate in the tail, but eventually succumb to the resorption program. The TH-inducible TR isoform, TRbeta, is expressed ubiquitously in the tail, but especially high in fibroblasts. Similarly, a group of delayed response genes including two proteolytic enzymes that appear to execute the tail resorption program, is expressed specifically in fibroblasts that line and surround the notochord and lie beneath the epidermal lamella (subepidermal fibroblasts). During active tail resorption these fibroblasts invade their neighboring epidermal and notochord lamellae as part of the resorption process. Expression analysis implicates the single layer of invasive subepidermal fibroblasts as crucial in tail resorption. Stromelysin-3 is up-regulated by TH with early kinetics, and is expressed most actively in fibroblasts within the tail fins. None of the proteases are expressed in the tadpole epidermis, which is replaced entirely during metamorphosis. While very few TH response genes are expressed in tadpole muscle, many are activated in fibroblasts that surround muscle and could induce muscle cell death by proteolysis of the extracellular matrix. These distinct localization patterns suggest that the common fate of all cell types within the tail is the result of multiple genetic programs (Berry, 1998a).

Thyroid hormone (TH) induces dramatic skeletal and tissue remodeling of the anuran head and body at metamorphosis. The expression pattern of TH up-regulated genes has been correlated with tissues that either grow or resorb at metamorphosis. Whereas the expression of the thyroid hormone receptors in Xenopus laevis tadpoles is ubiquitous, the locations where many of the TH up-regulated genes are activated fall into distinct classes. Genes in the early response class are expressed predominantly in cartilage and brain regions undergoing cell proliferation and at a higher level in the remodeling and growing body than in the resorbing tail. In contrast, expression of genes in the delayed response class is highest in resorbing tissues and higher in the tail than in the body within the subepidermal fibroblast layer, further indicating that this single cell layer is involved in tissue resorption. The expression boundary of delayed response class genes in the subepidermal fibroblasts in the body correlates with epidermal lamella invasion and subsequent adult skin differentiation. Differences in the expression patterns of stromelysin-3 and the delayed response proteinases in the head delineate separate programs of tissue resorption: one for the loss of epithelial structures, and one for the loss of cartilages. Expression of the type III deiodinase is up-regulated in growing tissues nearing completion of their metamorphic changes, suggesting a role for the deiodinase in modulating the influence of TH on these tissues (Berry, 1998b).

Xenopus thyroid hormone (xTR) and retinoid X (xRXR) were overexpressed in cells and the response to various ligands was studied. 3,3'5-triiodothyronine (T3) strongly upregulates xTR beta mRNA in XTC-2 cells, but not xTR alpha or xRXR alpha mRNAs, while xRXR gamma transcripts cannot be detected. 9-cis-retinoic acid (9-cis-RA) does not substantially influence the expression of any of these four receptor genes. Measurements of transcription activity were taken from three different thyroid response elements (TREs): a palindromic TREpal, an inverted repeat +6 [F2] and a direct repeat +4[DR+4], as present in the promoter of xTR beta gene. Only T3 upregulates transcription, while 9-cis-RA, either alone or together with T3, is ineffective. 9-cis-RA however enhances transcription from an RXR responsive element (RXR-RE). A second approach involved overexpression of xTR beta and xRXR alpha in premetamorphic Xenopus tadpole tail muscle followed by measuring the response of the tails to T3 in organ culture. T3 enhances transcription from the xTR beta DR +4 TRE in tails in which xTR beta is overexpressed, but the overexpression of xRXR alpha fails to modify this response. It is concluded that in both XTC cells and tadpole tails exogenous 9-cis-RA is ineffective, and that overexpressed xRXR fails to modify the enhanced transcriptional response of endogenous and overexpressed xTR beta to T3 (Ulisse, 1997).

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. There is 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, it has been demonstrated 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; 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 (Puzianowska-Kuznicka, 1997).

The biological activities of thyroid hormones are thought to be mediated by receptors generated by the TRalpha and TRbeta loci. The existence of several receptor isoforms suggests that different functions are mediated by specific isoforms and raises the possibility of functional redundancies. Both TRalpha and TRbeta genes have been inactivated by homologous recombination in the mouse and the phenotypes of wild-type, and single and double mutant mice were compared. The TRbeta receptors are the most potent regulators of the production of thyroid stimulating hormone (TSH). However, in the absence of TRbeta, the products of the TRalpha gene can fulfill this function as, in the absence of any receptors, TSH and thyroid hormone concentrations reach very high levels. TRbeta, in contrast to TRalpha, is dispensable for the normal development of bone and intestine. In bone, the disruption of both TRalpha and TRbeta genes does not modify the maturation delay observed in TRalpha -/- mice. In the ileum, the absence of any receptor results in a much more severe impairment than that observed in TRalpha -/- animals. It is concluded that each of the two families of proteins mediate specific functions of triiodothyronin (T3), and that redundancy is only partial and concerns a limited number of functions (Gauthier, 1999).

Transactivation-defective retinoid X and thyroid hormone receptors have been used to examine mechanisms for hormonal activation. Activation and repression of transcription by retinoid X and thyroid hormone receptors are shown to be mediated by physically distinct and functionally independent regions of the hormone binding domain. Nevertheless, the ability of receptors to respond to hormone requires communication between both functional domains. Deletion of the hormone-dependent transactivation function of the retinoid X receptor, the common subunit of heterodimeric nuclear receptors, significantly impairs hormone-dependent transcription by retinoic acid, thyroid hormone, and vitamin D receptors. The results indicate that receptors do not exist in static off and on conformations but that hormone alters an equilibrium between inactive and active states (Schulman, 1996).

A short C-terminal sequence that is deleted in the v-ErbA oncoprotein (a C-terminally truncated form of the thyroid hormone receptor alpha [T3R alpha]) and conserved in members of the nuclear receptor superfamily, is required for normal biological function of its normal cellular counterpart, T3R alpha. An extensive mutational analysis was carried out in this region based on the crystal structure of the hormone-bound ligand binding domain of T3R alpha. Mutagenesis of either Leu398 or Glu401, which are surface exposed according to the crystal structure, completely blocks or significantly impairs T3-dependent transcriptional activation but does not affect or only partially diminishes interference with AP-1 activity. These are the first mutations that clearly dissociate these activities for T3R alpha. Substitution of Leu400, which is also surface exposed, does not affect interference with AP-1 activity and only partially diminishes T3-dependent transactivation. None of the mutations affect ligand-independent transactivation, consistent with previous findings that this activity is mediated by the N-terminal domain of T3R alpha. The loss of ligand-dependent transactivation for some mutants can largely be reversed in the presence of GRIP1 (a coactivator of T3Ralpha), which acts as a strong ligand-dependent coactivator for wild-type T3R alpha. There is excellent correlation between T3-dependent in vitro association of GRIP1 with T3R alpha mutants and their ability to support T3-dependent transcriptional activation. Therefore, GRIP1, previously found to interact with the glucocorticoid, estrogen, and androgen receptors, may also have a role in T3R alpha-mediated ligand-dependent transcriptional activation. When fused to a heterologous DNA binding domain, that of the yeast transactivator GAL4, the conserved C terminus of T3R alpha functions as a strong ligand-independent activator in both mammalian and yeast cells. However, point mutations within this region have drastically different effects on these activities compared to their effect on the full-length T3R alpha. It is concluded that the C-terminal conserved region contains a recognition surface for GRIP1 or a similar coactivator that facilitates its interaction with the basal transcriptional apparatus. While important for ligand-dependent transactivation, this interaction surface is not directly involved in transrepression of AP-1 activity. It is thought that transcripitonal interference between liganded nuclear receptors and AP-1 is due to competition for a cofactor that is required for efficient transactivation by either protein. Thus the C-terminal portion of T3R alpha has two functions: intertaction with Grip and interaction with a cofactor shared with AP-1 (Saatcioglu, 1997).

Combinatorial regulation of transcription implies flexible yet precise assembly of multiprotein regulatory complexes in response to signals. Biochemical and crystallographic analyses reveal that hormone binding leads to the formation of a hydrophobic groove within the ligand binding domain (LBD) of the thyroid hormone receptor that interacts with an LxxLL motif-containing alpha-helix from GRIP1, a coactivator. Residues immediately adjacent to the motif modulate the affinity of the interaction; the motif and the adjacent sequences are employed to different extents in binding to different receptors. Such interactions of amphipathic alpha-helices with hydrophobic grooves define protein interfaces in other regulatory complexes as well. It is suggested that these common structural elements impart flexibility to combinatorial regulation, whereas side chains at the interface impart specificity (Darimont, 1998).

Ligand-dependent activation of gene transcription by nuclear receptors is dependent on the recruitment of coactivators, including a family of related NCoA/SRC factors, via a region containing three helical domains sharing an LXXLL core consensus sequence, referred to as LXDs. Here is reported the receptor-specific differential utilization of LXXLL-containing motifs of the NCoA-1/SRC-1 coactivator. Whereas a single LXD is sufficient for activation by the estrogen receptor, different combinations of two LXDs, appropriately spaced, are required for actions of the thyroid hormone, retinoic acid, peroxisome proliferator-activated, or progesterone receptors. The specificity of LXD usage in the cell appears to be dictated, at least in part, by specific amino acids carboxy-terminal to the core LXXLL motif that may make differential contacts with helices 1 and 3 (or 3') in receptor ligand-binding domains. Intriguingly, distinct carboxy-terminal amino acids are required for PPARgamma activation in response to different ligands. Related LXXLL-containing motifs in NCoA-1/SRC-1 are also required for a functional interaction with CBP, potentially interacting with a hydrophobic binding pocket. Together, these data suggest that the LXXLL-containing motifs have evolved to serve overlapping roles that are likely to permit both receptor-specific and ligand-specific assembly of a coactivator complex, and that these recognition motifs underlie the recruitment of coactivator complexes required for nuclear receptor function (McInerney, 1998).

To decipher the mechanism of Rb function at the molecular level, a number of Rb-interacting proteins have been systematically characterized, among these, a clone termed C5 that encodes a protein of 1,978 amino acids with an estimated molecular mass of 230 kDa. The corresponding gene was assigned to chromosome 14q31, the same region where genetic alterations have been associated with several abnormalities of thyroid hormone response. The protein uses two distinct regions to bind Rb and thyroid hormone receptor (TR), respectively, and was named Trip230. Trip230 binds to Rb independently of thyroid hormone while it forms a complex with TR in a thyroid hormone-dependent manner. Ectopic expression of the protein Trip230 in cells, but not a mutant form that does not bind to TR, specifically enhances TR-dependent transcriptional activity. Coexpression of wild-type Rb, but not mutant Rb that fails to bind to Trip230, inhibits such activity. These results not only identify a coactivator molecule that modulates TR activity, but also uncover a role for Rb in a pathway that responds to thyroid hormone (Chang, 1997).

Thyroid hormone receptor (T3R) is a member of the steroid hormone receptor gene family of nuclear hormone receptors. In most cells T3R activates gene expression only in the presence of its ligand, L-triiodothyronine (T3). However, in certain cell types (e.g., GH4C1 cells) expression of T3R leads to hormone-independent constitutive activation. This activation by unliganded T3R occurs with a variety of gene promoters and appears to be independent of the binding of T3R to specific thyroid hormone response elements (TREs). Previous studies indicate that this constitutive activation results from the titration of an inhibitor of transcription. Since the tumor suppresser p53 is capable of repressing a wide variety of gene promoters, the possibility was considered that the inhibitor is p53. Evidence to support this comes from studies indicating that expression of p53 blocks T3R-mediated constitutive activation in GH4C1 cells. In contrast with hormone-independent activation by T3R, p53 has little or no effect on T3-dependent stimulation which requires TREs. In addition, p53 mutants which oligomerize with wild-type p53 and interfere with its function also increase promoter activity. This enhancement is of similar magnitude to the stimulation mediated by unliganded T3R, but not additive with it, suggesting that liganded and unliganded T3R target the same factor. Since p53 mutants are known to target wild-type p53 in the cell, this suggests that T3R also interacts with p53 in vivo and that endogenous levels of p53 act to suppress promoter activity. Evidence supporting both functional and physical interactions for T3R and p53 in the cell is presented. The DNA binding domain (DBD) of T3R is important in mediating constitutive activation, and the receptor DBD appears to functionally interact with the N terminus of p53 in the cell. In vitro binding studies indicate that the T3R DBD is important for interaction of T3R with p53 and that this interaction is reduced by T3. These findings are consistent with the in vivo studies indicating that p53 blocks constitutive activation but not ligand-dependent stimulation. These studies provide insight into mechanisms by which unliganded nuclear hormone receptors can modulate gene expression and may provide an explanation for the mechanism of action of the v-erbA oncoprotein, a retroviral homolog of chicken T3R alpha (Qi, 1997).

To elucidate the role of thyroid hormone receptors (TRs) alpha1 and beta in the development of hearing, cochlear functions have been investigated in mice lacking TRalpha1 or TRbeta. TRs are ligand-dependent transcription factors expressed in the developing organ of Corti, and loss of TRbeta is known to impair hearing in mice and in humans. Here, TRalpha1-deficient [TRalpha1(-/-)] mice are shown to display a normal auditory-evoked brainstem response, indicating that only TRbeta, and not TRalpha1, is essential for hearing. Because cochlear morphology was normal in TRbeta-/- mice, it has been postulated that TRbeta regulates functional rather than morphological development of the cochlea. At the onset of hearing, inner hair cells (IHCs) in wild-type mice express a fast-activating potassium conductance, IK,f, that transforms the immature IHC from a regenerative, spiking pacemaker to a high-frequency signal transmitter. Expression of IK,f is significantly retarded in TRbeta-/- mice, whereas the development of the endocochlear potential and other cochlear functions, including mechanoelectrical transduction in hair cells, progresses normally. TRalpha1(-/-) mice expressed IK,f normally, in accord with their normal auditory-evoked brainstem response. These results establish that the physiological differentiation of IHCs depends on a TRbeta-mediated pathway. When defective, this may contribute to deafness in congenital thyroid diseases (Rusch, 1997).

Thyroid hormone (T3) has widespread functions in development and homeostasis, although the receptor pathways by which this diversity arises are unclear. Deletion of the T3 receptors TRalpha1 or TRbeta individually reveals only a small proportion of the phenotypes that arise in hypothyroidism, implying that additional pathways must exist. Mice lacking both TRalpha1 and TRbeta [TRalpha1(-/-)beta-/-] display a novel array of phenotypes not found in single receptor-deficient mice, including an extremely hyperactive pituitary-thyroid axis, poor female fertility and retarded growth and bone maturation. These results establish that major T3 actions are mediated by common pathways in which TRalpha1 and TRbeta cooperate with or substitute for each other. Thus, varying the balance of use of TRalpha1 and TRbeta individually or in combination facilitates control of an extended spectrum of T3 actions. There was no evidence for any previously unidentified T3 receptors in TRalpha1(-/-)beta-/- mouse tissues. Compared to the debilitating symptoms of severe hypothyroidism, the milder overall phenotype of TRalpha1(-/-)beta-/- mice, lacking all known T3 receptors, indicates divergent consequences for hormone versus receptor deficiency. These distinctions suggest that the T3-independent actions of T3 receptors, demonstrated previously in vitro, may be a significant function in vivo (Gothe, 1999).

The role of an orphan nuclear hormone receptor, ROR alpha (Drosophila homolog: Hormone receptor-like in 46), on thyroid hormone (TH) receptor (TR)-mediated transcription on a TH-response element (TRE) was investigated. A transient transfection study using various TREs [i.e., F2 (chick lysozyme TRE), DR4 (direct repeat), and palindrome TRE] and TR and ROR alpha1 was performed. When ROR alpha1 and TR were cotransfected into CV1 cells, ROR alpha1 enhanced the transactivation by liganded-TR on all TREs tested without an effect on basal repression by unliganded TR. However, by electrophoretic mobility shift assay, although ROR alpha bound to all TREs tested as a monomer, no (or weak) TR and ROR alpha1 heterodimer formation was observed on various TREs except when a putative ROR-response element was present. The transactivation by ROR alpha1 on a ROR-response element, which does not contain a TRE, was not enhanced by TR. The effect of ROR alpha1 on the TREs is unique, because, whereas other nuclear hormone receptors (such as vitamin D receptor) may competitively bind to TRE to exert dominant negative function, ROR alpha1 augments TR action. These results indicate that ROR alpha1 may modify the effect of liganded TR on TH-responsive genes. Because TR and ROR alpha are coexpressed in cerebellar Purkinje cells, and perinatal hypothyroid animals and ROR alpha-disrupted animals show similar abnormalities of this cell type, cross-talk between these two receptors may play a critical role in Purkinje cell differentiation (Koibuchi, 1999).

The timing of oligodendrocyte development is regulated by thyroid hormone (TH) in vitro and in vivo, but it is still uncertain which TH receptors mediate this regulation. TH acts through nuclear receptors that are encoded by two genes, TRa and TRß. Direct evidence is provided for the involvement of the TRa1 receptor isoform in vivo, by showing that the number of oligodendrocytes in the postnatal day 7 (P7) and P14 optic nerve of TRa1-/- mice is decreased compared with normal. TRa1 mediates the normal differentiation-promoting effect of TH on oligodendrocyte precursor cells (OPCs): unlike wild-type OPCs, postnatal TRa1-/- OPCs fail to stop dividing and differentiate in response to TH in culture. Overexpression of TRa1 accelerates oligodendrocyte differentiation in culture, suggesting that the level of TRa1 expression is normally limiting for TH-dependent OPC differentiation. Finally, evidence is provided that the inhibitory isoforms of TRa are unlikely to play a part in the timing of OPC differentiation (Billon, 2002).

Alien has been described as a corepressor for the thyroid hormone receptor (TR). Corepressors are coregulators that mediate gene silencing of DNA-bound transcriptional repressors. Alien gene expression in vivo is regulated by thyroid hormone both in the rat brain and in cultured cells. In situ hybridization revealed that Alien is widely expressed in the mouse embryo and also throughout the rat brain. Hypothyroid animals exhibit lower expression of both Alien mRNAs and protein levels as compared with normal animals. Accordingly, Alien gene is found to be inducible after thyroid hormone treatment both in vivo and in cell culture. In cultured cells, the hormonal induction is mediated by either TRalpha or TRß, while cells lacking detectable amounts of functional TR lack hormonal induction of Alien. Two Alien-specific mRNAs are detectable by Northern experiments and two Alien-specific proteins are detected in vivo and in cell lines by Western analysis; one of the two Alien forms represent the CSN2 subunit of the COP9 signalosome. Interestingly, both Alien mRNAs and both detected proteins are regulated by thyroid hormone in vivo and in cell lines. Furthermore, evidence is provided for the existence of at least two Alien genes in rodents. Taken together, it is concluded that Alien gene expression is under control of TR and thyroid hormone. This suggests a negative feedback mechanism between TR and its own corepressor. Thus, the reduction of corepressor levels may represent a control mechanism of TR-mediated gene silencing (Tenbaum, 2003).

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