Transcriptional regulatory patterns of the myelin basic protein and malic enzyme genes by the thyroid hormone receptors alpha1 and beta1

Thyroid hormone action in adult neurogliogenic niches: the known and unknown

Over the last decades, thyroid hormones (THs) signaling has been established as a key signaling cue for the proper maintenance of brain functions in adult mammals, including humans. One of the most fascinating roles of THs in the mature mammalian brain is their ability to regulate adult neurogliogenic processes. In this respect, THs control the generation of new neuronal and glial progenitors from neural stem cells (NSCs) as well as their final differentiation and maturation programs. In this review, we summarize current knowledge on the cellular organization of adult rodent neurogliogenic niches encompassing well-established niches in the subventricular zone (SVZ) lining the lateral ventricles, the hippocampal subgranular zone (SGZ), and the hypothalamus, but also less characterized niches in the striatum and the cerebral cortex. We then discuss critical questions regarding how THs availability is regulated in the respective niches in rodents and larger mammals as well as how modulating THs availability in those niches interferes with lineage decision and progression at the molecular, cellular, and functional levels. Based on those alterations, we explore the novel therapeutic avenues aiming at harnessing THs regulatory influences on neurogliogenic output to stimulate repair processes by influencing the generation of either new neurons (i.e. Alzheimer's, Parkinson's diseases), oligodendrocytes (multiple sclerosis) or both (stroke). Finally, we point out future challenges, which will shape research in this exciting field in the upcoming years.

Development of cerebral mitochondrial respiratory function is impaired by thyroid hormone deficiency before birth in a region-specific manner

Thyroid hormones regulate adult metabolism partly through actions on mitochondrial oxidative phosphorylation (OXPHOS). They also affect neurological development of the brain, but their role in cerebral OXPHOS before birth remains largely unknown, despite the increase in cerebral energy demand during the neonatal period. Thus, this study examined prepartum development of cerebral OXPHOS in hypothyroid fetal sheep. Using respirometry, Complex I (CI), Complex II (CII), and combined CI&CII OXPHOS capacity were measured in the fetal cerebellum and cortex at 128 and 142 days of gestational age (dGA) after surgical thyroidectomy or sham operation at 105 dGA (term ~145 dGA). Mitochondrial electron transfer system (ETS) complexes, mRNA transcripts related to mitochondrial biogenesis and ATP production, and mitochondrial density were quantified using molecular techniques. Cerebral morphology was assessed by immunohistochemistry and stereology. In the cortex, hypothyroidism reduced CI-linked respiration and CI abundance at 128 dGA and 142 dGA, respectively, and caused upregulation of PGC1α (regulator of mitochondrial biogenesis) and thyroid hormone receptor β at 128 dGA and 142 dGA, respectively. In contrast, in the cerebellum, hypothyroidism reduced CI&II- and CII-linked respiration at 128 dGA, with no significant effect on the ETS complexes. In addition, cerebellar glucocorticoid hormone receptor and adenine nucleotide translocase (ANT1) were downregulated at 128 dGA and 142 dGA, respectively. These alterations in mitochondrial function were accompanied by reduced myelination. The findings demonstrate the importance of thyroid hormones in the prepartum maturation of cerebral mitochondria and have implications for the etiology and treatment of the neurodevelopmental abnormalities associated with human prematurity and congenital hypothyroidism.

Thyroid Hormone and Neural Stem Cells: Repair Potential Following Brain and Spinal Cord Injury

Neurodegenerative diseases are characterized by chronic neuronal and/or glial cell loss, while traumatic injury is often accompanied by the acute loss of both. Multipotent neural stem cells (NSCs) in the adult mammalian brain spontaneously proliferate, forming neuronal and glial progenitors that migrate toward lesion sites upon injury. However, they fail to replace neurons and glial cells due to molecular inhibition and the lack of pro-regenerative cues. A major challenge in regenerative biology therefore is to unveil signaling pathways that could override molecular brakes and boost endogenous repair. In physiological conditions, thyroid hormone (TH) acts on NSC commitment in the subventricular zone, and the subgranular zone, the two largest NSC niches in mammals, including humans. Here, we discuss whether TH could have beneficial actions in various pathological contexts too, by evaluating recent data obtained in mammalian models of multiple sclerosis (MS; loss of oligodendroglial cells), Alzheimer’s disease (loss of neuronal cells), stroke and spinal cord injury (neuroglial cell loss). So far, TH has shown promising effects as a stimulator of remyelination in MS models, while its role in NSC-mediated repair in other diseases remains elusive. Disentangling the spatiotemporal aspects of the injury-driven repair response as well as the molecular and cellular mechanisms by which TH acts, could unveil new ways to further exploit its pro-regenerative potential, while TH (ant)agonists with cell type-specific action could provide safer and more target-directed approaches that translate easier to clinical settings.

Identifying and Validating Genes with DNA Methylation Data in the Context of Biological Network for Chinese Patients with Graves' Orbitopathy

AIM

This study investigated the association of DNA methylation with Graves' orbitopathy (GO) incidence through a combined analysis in the context of biological network to identify and validate potential genes for Chinese patients with GO.

METHODS

A genome-scale screening of DNA methylation was performed on the peripheral blood sample of six patients with GO and six controls. After extracting differentially methylated regions (DMRs), the study focused on two classes of genes with obviously different methylation levels: low methylated genes (LMGs) and high methylated genes (HMGs). Mutual information was applied to construct LMG- and HMG-regulated networks, and the top 10 LMGs and HMGs were extracted based on the topological properties. Then, 9 candidate genes were extracted to validate their association with GO in an expanded population (48 patients with GO vs. 24 normal controls) using single-cell methylation sequencing.

RESULTS

In the LMG-regulated network, some LMGs displayed a higher degree, such as HIST1H2AL, EFCAB1, and BOLL. Similarly, in the HMG-regulated network, some HMGs, such as MBP, ANGEL1, and LYAR, also showed a higher degree. For validation using an enlarged population, BOLL still displayed the lower methylation level whereas CDK5 and MBP still displayed the higher methylation level in patients with GO in the multivariable logistic regression analysis adjusted by age and gender (P < 0.01).

CONCLUSIONS

BOLL, CDK5, and MBP are potential genes associated with GO. This study was novel in clinically investigating the relation of these genomic loci with GO. The findings might provide new insights into understanding this disease.

Decoding cell signalling and regulation of oligodendrocyte differentiation

Oligodendrocytes are fundamental for the functioning of the nervous system; they participate in several cellular processes, including axonal myelination and metabolic maintenance for astrocytes and neurons. In the mammalian nervous system, they are produced through waves of proliferation and differentiation, which occur during embryogenesis. However, oligodendrocytes and their precursors continue to be generated during adulthood from specific niches of stem cells that were not recruited during development. Deficiencies in the formation and maturation of these cells can generate pathologies mainly related to myelination. Understanding the mechanisms involved in oligodendrocyte development, from the precursor to mature cell level, will allow inferring therapies and treatments for associated pathologies and disorders. Such mechanisms include cell signalling pathways that involve many growth factors, small metabolic molecules, non-coding RNAs, and transcription factors, as well as specific elements of the extracellular matrix, which act in a coordinated temporal and spatial manner according to a given stimulus. Deciphering those aspects will allow researchers to replicate them in vitro in a controlled environment and thus mimic oligodendrocyte maturation to understand the role of oligodendrocytes in myelination in pathologies and normal conditions. In this study, we review these aspects, based on the most recent in vivo and in vitro data on oligodendrocyte generation and differentiation.

Blocked, delayed, or obstructed: What causes poor white matter development in intrauterine growth restricted infants?

Poor white matter development in intrauterine growth restricted (IUGR) babies remains a major, untreated problem in neonatology. New therapies, guided by an understanding of the mechanisms that underlie normal and abnormal oligodendrocyte development and myelin formation, are required. Much of our knowledge of the mechanisms that underlie impaired myelination come from studies in adult demyelinating disease, preterm brain injury, or experimental models of hypoxia-ischemia. However, relatively less is known for IUGR which is surprising because IUGR is a leading cause of perinatal mortality and morbidity, second only to premature birth. IUGR is also a significant risk factor for the later development of cerebral palsy, and is a greater risk compared to some of the more traditionally researched antecedents - asphyxia and inflammation. Recent evidence suggests that the white matter injury and reduced myelination in the brains of some preterm babies is due to impaired maturation of oligodendrocytes thereby resulting in the reduced capacity to synthesize myelin. Therefore, it is not surprising that the hypomyelination observable in the central nervous system of IUGR infants has similarly lead to investigations identifying a delay or blockade in the progress of maturation of oligodendrocytes in these infants. This review will discuss current ideas thought to account for the poor myelination often present in the neonate's brain following IUGR, and discuss novel interventions that are promising as treatments that promote oligodendrocyte maturation, and thereby repair the myelination deficits that otherwise persist into infancy and childhood and lead to neurodevelopmental abnormalities.

Pharmacological treatment and BBB-targeted genetic therapy for MCT8-dependent hypomyelination in zebrafish

Hypomyelination is a key symptom of Allan-Herndon-Dudley syndrome (AHDS), a psychomotor retardation associated with mutations in the thyroid-hormone (TH) transporter MCT8 (monocarboxylate transporter 8). AHDS is characterized by severe intellectual deficiency, neuromuscular impairment and brain hypothyroidism. In order to understand the mechanism for TH-dependent hypomyelination, we developed an mct8 mutant (mct8^-/-) zebrafish model. The quantification of genetic markers for oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes revealed reduced differentiation of OPCs into oligodendrocytes in mct8^-/- larvae and adults. Live imaging of single glial cells showed that the number of oligodendrocytes and the length of their extensions are reduced, and the number of peripheral Schwann cells is increased, in mct8^-/- larvae compared with wild type. Pharmacological analysis showed that TH analogs and clemastine partially rescued the hypomyelination in the CNS of mct8^-/- larvae. Intriguingly, triiodothyronine (T3) treatment rescued hypomyelination in mct8^-/- embryos before the maturation of the blood-brain barrier (BBB), but did not affect hypomyelination in older larvae. Thus, we expressed Mct8-tagRFP in the endothelial cells of the vascular system and showed that even relatively weak mosaic expression completely rescued hypomyelination in mct8^-/- larvae. These results suggest potential pharmacological treatments and BBB-targeted gene therapy that can enhance myelination in AHDS and possibly in other TH-dependent brain disorders.

Overexpression of malic enzyme in the larval stage extends Drosophila lifespan

Metabolic modifications during the developmental period can extend longevity. We found that malic enzyme (Men) overexpression during the larval period lengthened the lifespan of Drosophila. Men overexpression by S106-GeneSwitch-Gal4 driver increased pyruvate content and NADPH/NADP(+) ratio but reduced triglyceride, glycogen, and ATP levels in the larvae. ROS levels increased unexpectedly in Men-overexpressing larvae. Interestingly, adults exposed to larval Men-overexpression maintained ROS tolerance with enhanced expression levels of glutathione-S-transferase D2 and thioredoxin-2. Our results suggest that metabolic changes mediated by Men during development might be related to the control of ROS tolerance and the longevity of Drosophila.

Altered behavioral performance and live imaging of circuit-specific neural deficiencies in a zebrafish model for psychomotor retardation

The mechanisms and treatment of psychomotor retardation, which includes motor and cognitive impairment, are indefinite. The Allan-Herndon-Dudley syndrome (AHDS) is an X-linked psychomotor retardation characterized by delayed development, severe intellectual disability, muscle hypotonia, and spastic paraplegia, in combination with disturbed thyroid hormone (TH) parameters. AHDS has been associated with mutations in the monocarboxylate transporter 8 (mct8/slc16a2) gene, which is a TH transporter. In order to determine the pathophysiological mechanisms of AHDS, MCT8 knockout mice were intensively studied. Although these mice faithfully replicated the abnormal serum TH levels, they failed to exhibit the neurological and behavioral symptoms of AHDS patients. Here, we generated an mct8 mutant (mct8−/−) zebrafish using zinc-finger nuclease (ZFN)-mediated targeted gene editing system. The elimination of MCT8 decreased the expression levels of TH receptors; however, it did not affect the expression of other TH-related genes. Similar to human patients, mct8−/− larvae exhibited neurological and behavioral deficiencies. High-throughput behavioral assays demonstrated that mct8−/− larvae exhibited reduced locomotor activity, altered response to external light and dark transitions and an increase in sleep time. These deficiencies in behavioral performance were associated with altered expression of myelin-related genes and neuron-specific deficiencies in circuit formation. Time-lapse imaging of single-axon arbors and synapses in live mct8−/− larvae revealed a reduction in filopodia dynamics and axon branching in sensory neurons and decreased synaptic density in motor neurons. These phenotypes enable assessment of the therapeutic potential of three TH analogs that can enter the cells in the absence of MCT8. The TH analogs restored the myelin and axon outgrowth deficiencies in mct8−/− larvae. These findings suggest a mechanism by which MCT8 regulates neural circuit assembly, ultimately mediating sensory and motor control of behavioral performance. We also propose that the administration of TH analogs early during embryo development can specifically reduce neurological damage in AHDS patients.

In a wide range of brain disorders, mutations in specific genes cause alterations in the development and function of neural circuits that ultimately affect behavior. A major challenge is to uncover the mechanism and provide treatment which is capable of preventing brain damage. Allan-Herndon-Dudley syndrome (AHDS) is a severe psychomotor retardation characterized by intellectual disabilities, neurological impairment and abnormal thyroid hormone (TH) levels. Mutations in the TH transporter MCT8 are associated with AHDS. Mice that lack the MCT8 protein exhibited impaired TH levels, as is the case in human patients; however, they lack neurological defects. Here, we generated an mct8 mutant (mct8−/−) zebrafish, which exhibited neurological and behavioral deficiencies and mimics pathological conditions of AHDS patients. The zebrafish is a simple transparent vertebrate and its nervous system is conserved with mammals. Time-lapse live imaging of single axons and synapses, and video-tracking of behavior revealed deficiencies in neural circuit assembly, which are associated with disturbed sleep and altered locomotor activity. In addition, since the mct8−/− larvae provides a highthroughput platform for testing therapeutic drugs, we showed that TH analogs can recover neurological deficiencies in an animal model for psychomotor retardation.

A selective thyroid hormone β receptor agonist enhances human and rodent oligodendrocyte differentiation

Nerve conduction within the mammalian central nervous system is made efficient by oligodendrocyte-derived myelin. Historically, thyroid hormones have a well described role in regulating oligodendrocyte differentiation and myelination during development; however, it remains unclear which thyroid hormone receptors are required to drive these effects. This is a question with clinical relevance since nonspecific thyroid receptor stimulation can produce deleterious side-effects. Here we report that GC-1, a thyromimetic with selective thyroid receptor β action and a potentially limited side-effect profile, promotes in vitro oligodendrogenesis from both rodent and human oligodendrocyte progenitor cells. In addition, we used in vivo genetic fate tracing of oligodendrocyte progenitor cells via PDGFαR-CreER;Rosa26-eYFP double-transgenic mice to examine the effect of GC-1 on cellular fate and find that treatment with GC-1 during developmental myelination promotes oligodendrogenesis within the corpus callosum, occipital cortex and optic nerve. GC-1 was also observed to enhance the expression of the myelin proteins MBP, CNP and MAG within the same regions. These results indicate that a β receptor selective thyromimetic can enhance oligodendrocyte differentiation in vitro and during developmental myelination in vivo and warrants further study as a therapeutic agent for demyelinating models.

Oligodendrogenesis from neural stem cells: perspectives for remyelinating strategies

Mobilization of remyelinating cells spontaneously occurs in the adult brain. These cellular resources are specially active after demyelinating episodes in early phases of multiple sclerosis (MS). Indeed, oligodendrocyte precursor cells (OPCs) actively proliferate, migrate to and repopulate the lesioned areas. Ultimately, efficient remyelination is accomplished when new oligodendrocytes reinvest nude neuronal axons, restoring the normal properties of impulse conduction. As the disease progresses this fundamental process fails. Multiple causes seem to contribute to such transient decline, including the failure of OPCs to differentiate and enwrap the vulnerable neuronal axons. Regenerative medicine for MS has been mainly centered on the recruitment of endogenous self-repair mechanisms, or on transplantation approaches. The latter commonly involves grafting of neural precursor cells (NPCs) or neural stem cells (NSCs), with myelinogenic potential, in the injured areas. Both strategies require further understanding of the biology of oligodendrocyte differentiation and remyelination. Indeed, the success of transplantation largely depends on the pre-commitment of transplanted NPCs or NSCs into oligodendroglial cell type, while the endogenous differentiation of OPCs needs to be boosted in chronic stages of the disease. Thus, much effort has been focused on finding molecular targets that drive oligodendrocytes commitment and development. The present review explores several aspects of remyelination that must be considered in the design of a cell-based therapy for MS, and explores more deeply the challenge of fostering oligodendrogenesis. In this regard, we discuss herein a tool developed in our research group useful to search novel oligodendrogenic factors and to study oligodendrocyte differentiation in a time- and cost-saving manner.

Nuclear receptor Rev-erbalpha: a heme receptor that coordinates circadian rhythm and metabolism

Nuclear receptor Rev-erbα (NR1D1), previously considered to be an orphan nuclear receptor, is a receptor for heme, which promotes transcriptional repression via recruitment of the NCoR-HDAC3 corepressor complex. Rev-erbα gene regulation is circadian, and Rev-erbα comprises a critical negative limb of the core circadian clock by directly repressing the expression of the positive clock component, Bmal1. Rev-erbα also regulates the metabolic gene pathway, thus serving as a heme sensor for coordination of circadian and metabolic pathways.

Thyroid hormone induces glial lineage of primary neurospheres derived from non-pathological and pathological rat brain: implications for remyelination-enhancing therapies

Thyroid hormone exerts a critical role in developmental myelination, acting on the production and maturation of oligodendrocyte, and on the expression of genes encoding for myelin protein. Since remyelination is considered a recapitulation of cellular and molecular events occurring during development, we tested the possibility of stimulating the oligodendroglial lineage and maturation in neurospheres derived from the subventricular zone of adult rats using 3,5,3'-L-triiodothyronine (T3). Both non-pathological and pathological brains derived from rats affected by the inflammatory-demyelinating disease experimental allergic encephalomyelitis (EAE) were included in the study. We investigated the effect of in vitro T3 exposure on: (i) the expression of nuclear thyroid hormone receptors; (ii) proliferation rate; (iii) differentiation into neurons, astrocytes and oligodendrocytes, focusing our attention on oligodendrocyte maturation. T3 reduced the proliferation rate of neurospheres when cultured in the presence of mitogens, shifting towards oligodendroglial lineage as indicated by increased expression of olig-1, and also favoring oligodendrocyte maturation, as indicated by the expression of antigens associated with different maturation stages. Neurospheres derived from EAE rats show a strong limitation in oligodendrocyte generation, which is completely restored by T3 treatment. These results indicate that T3 is a key factor in regulating neurosphere biology, when derived either from non-pathological or pathological adult brains, suggesting that T3 might be an important factor in favoring remyelination in demyelinating disorders.

Thyroid hormone receptor alpha plays an essential role in the normalisation of adult-onset hypothyroidism-related hypoexpression of synaptic plasticity target genes in striatum

Thyroid hormone (TH) deficiency leads to molecular changes resulting in behavioural deficits. TH action is mediated by two types of nuclear receptors (TRs), TRalpha and TRbeta, which control target gene transcription. The relative contributions of the two TR products in mediating adult TH responses are poorly understood. As TRalpha1 transcripts are widely distributed in the brain, they presumably mediate most of the TH effects. This report examines the role and specific functions of T3 receptor isoforms on regulation of striatal synaptic plasticity indicators using adult hypothyroid mutant mice that fail to express single or multiple TR gene products. We then evaluated the effect of this hypothyroidism, with or without subsequent administration of T3, on T3 nuclear receptor (TRalpha1, TRbeta) and synaptic plasticity gene expression in TRalpha(0/0), TRbeta(-/-) and wild-type 129/SV mice. Hypothyroid wild-type mice exhibited reduced TRbeta, RC3, CaMKII and Rhes expression. The mRNA levels of Rhes and CaMKII were the same in all three hypothyroid substrains. By contrast, hypothyroid TRbeta(-/-) mice had higher RC3 mRNA levels than wild-type. T3 administration restored TRbeta, RC3 and CaMKII levels in hypothyroid wild-type mice, without significant Rhes upregulation. T3 administration normalised expression of all genes studied in hypothyroid TRbeta(-/-) but not TRalpha(0/0) mice. Thus, TRalpha apparently plays an essential role in restoring the expression of the TH-regulated genes potentially involved in striatal synaptic plasticity.

T3 administration in adult hypothyroid mice modulates expression of proteins involved in striatal synaptic plasticity and improves motor behavior

Adult-onset hypothyroidism is associated with neurological changes such as cognitive dysfunction and impaired learning, which may be related to alterations of synaptic plasticity. We investigate the consequence of adult-onset hypothyroidism on thyroid-mediated transcription events in striatal synaptic plasticity, and the effect of triiodothyronine (T3) replacement. We used hypothyroid mice, treated with propylthiouracil (PTU) and methimazole (MMI), with or without subsequent administration of T3. We evaluated the amount of T3 nuclear receptors (TRalpha1, TRbeta) and striatal plasticity indicators: neurogranin (RC3), Ras homolog enriched in striatum (Rhes), Ca2+/calmodulin-dependent protein kinase (CaMKII), and dopamine- and cAMP-regulated phosphoprotein (DARPP-32). In addition, we assessed hypothyroid mice motor behavior as related to striatum synaptic functions. Hypothyroid mice exhibited significantly reduced TRbeta, RC3 and Rhes expression. T3 administration reversed the expression of TRbeta, RC3, and up-regulated CaMKII levels as well as motor behavior, and decreased DARPP-32 protein phosphorylation. We suggest that thyroid hormone modulation had a major impact on striatal synaptic plasticity of adult mice which produced in turn motor behavior modifications.

Thyroid hormones promote differentiation of oligodendrocyte progenitor cells and improve remyelination after cuprizone-induced demyelination

In the present work we analyzed the capacity of thyroid hormones (THs) to improve remyelination using a rat model of cuprizone-induced demyelination previously described in our laboratories. Twenty one days old Wistar rats were fed a diet containing 0.6% cuprizone for two weeks to induce demyelination. After cuprizone withdrawal, rats were injected with triiodothyronine (T3). Histological studies carried out in these animals revealed that remyelination in the corpus callosum (CC) of T3-treated rats improved markedly when compared to saline treated animals. The cellular events occurring in the CC and in the subventricular zone (SVZ) during the first week of remyelination were analyzed using specific oligodendroglial cell (OLGc) markers. In the CC of saline treated demyelinated animals, mature OLGcs decreased and oligodendroglial precursor cells (OPCs) increased after one week of spontaneous remyelination. Furthermore, the SVZ of these animals showed an increase in early progenitor cell numbers, dispersion of OPCs and inhibition of Olig and Shh expression compared to non-demyelinated animals. The changes triggered by demyelination were reverted after T3 administration, suggesting that THs could be regulating the emergence of remyelinating oligodendrocytes from the pool of proliferating cells residing in the SVZ. Our results also suggest that THs receptor beta mediates T3 effects on remyelination. These results support a potential role for THs in the remyelination process that could be used to develop new therapeutic approaches for demyelinating diseases.

The endocrine disruptor monoethyl-hexyl-phthalate is a selective peroxisome proliferator-activated receptor gamma modulator that promotes adipogenesis

The ability of pollutants to affect human health is a major concern, justified by the wide demonstration that reproductive functions are altered by endocrine disrupting chemicals. The definition of endocrine disruption is today extended to broader endocrine regulations, and includes activation of metabolic sensors, such as the peroxisome proliferator-activated receptors (PPARs). Toxicology approaches have demonstrated that phthalate plasticizers can directly influence PPAR activity. What is now missing is a detailed molecular understanding of the fundamental basis of endocrine disrupting chemical interference with PPAR signaling. We thus performed structural and functional analyses that demonstrate how monoethyl-hexyl-phthalate (MEHP) directly activates PPARgamma and promotes adipogenesis, albeit to a lower extent than the full agonist rosiglitazone. Importantly, we demonstrate that MEHP induces a selective activation of different PPARgamma target genes. Chromatin immunoprecipitation and fluorescence microscopy in living cells reveal that this selective activity correlates with the recruitment of a specific subset of PPARgamma coregulators that includes Med1 and PGC-1alpha, but not p300 and SRC-1. These results highlight some key mechanisms in metabolic disruption but are also instrumental in the context of selective PPAR modulation, a promising field for new therapeutic development based on PPAR modulation.

Crystal structure of an FIV/HIV chimeric protease complexed with the broad-based inhibitor, TL-3

We have obtained the 1.7 Å crystal structure of FIV protease (PR) in which 12 critical residues around the active site have been substituted with the structurally equivalent residues of HIV PR (12X FIV PR). The chimeric PR was crystallized in complex with the broad-based inhibitor TL-3, which inhibits wild type FIV and HIV PRs, as well as 12X FIV PR and several drug-resistant HIV mutants [1-4]. Biochemical analyses have demonstrated that TL-3 inhibits these PRs in the order HIV PR > 12X FIV PR > FIV PR, with K_i values of 1.5 nM, 10 nM, and 41 nM, respectively [2-4]. Comparison of the crystal structures of the TL-3 complexes of 12X FIV and wild-typeFIV PR revealed theformation of additinal van der Waals interactions between the enzyme inhibitor in the mutant PR. The 12X FIV PR retained the hydrogen bonding interactions between residues in the flap regions and active site involving the enzyme and the TL-3 inhibitor in comparison to both FIV PR and HIV PR. However, the flap regions of the 12X FIV PR more closely resemble those of HIV PR, having gained several stabilizing intra-flap interactions not present in wild type FIV PR. These findings offer a structural explanation for the observed inhibitor/substrate binding properties of the chimeric PR.

Thyroid hormone deficiency changes the distribution of oligodendrocyte/myelin markers during oligodendroglial differentiation in vitro

Myelination depends on the proper differentiation of oligodendrocytes and several factors may influence this event. For instance, thyroid hormone (T3) affects the timing of differentiation and regulates the expression of several enzymes involved in the synthesis of complex lipids and in the expression of some myelin structural proteins. We investigated the effect of T3 deficiency on oligodendroglial differentiation and in the distribution of oligodendrocyte/myelin proteins 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and myelin basic protein (MBP). Oligodendroglial-enriched cultures were obtained from cerebra of neonate rats grown in a modified medium. The T3-deficient status was induced by using medium devoid of T3. We observed a delay, in T3-deficient cultures, in oligodendroglial maturation characterized by less extensive processes and membrane vellum than in controls. In control cultures, CNPase immunoreactivity was punctated, showing cell bodies and processes at earlier stages and redistribution to cytoskeleton vein-like structures in later stages. In T3-deficient cultures, CNPase remained in a punctated pattern and only at 10 days in vitro we observed CNPase redistribution to the presumptive cytoskeleton vein-like structures. MBP in control cultures was distributed through the whole cell body and processes whereas in T3-deficient cultures, MBP immunoreactivity was concentrated in the perinuclear region. These results reinforce the hypothesis that T3 is an important factor in oligodendrocyte differentiation, particularly regarding the distribution of myelin proteins.

One of the duplicated matrix metalloproteinase-9 genes is expressed in regressing tail during anuran metamorphosis

The drastic morphological changes of the tadpole are induced during the climax of anuran metamorphosis, when the concentration of endogenous thyroid hormone is maximal. The tadpole tail, which is twice as long as the body, shortens rapidly and disappears completely in several days. We isolated a cDNA clone, designated as Xl MMP-9TH, similar to the previously reported Xenopus laevis MMP-9 gene, and showed that their Xenopus tropicalis counterparts are located tandemly about 9 kb apart from each other in the genome. The Xenopus MMP-9TH gene was expressed in the regressing tail and gills and the remodeling intestine and central nervous system, and induced in thyroid hormone-treated tail-derived myoblastic cultured cells, while MMP-9 mRNA was detected in embryos. Three thyroid hormone response elements in the distal promoter and the first intron were involved in the upregulation of the Xl MMP-9TH gene by thyroid hormone in transient expression assays, and their relative positions are conserved between X. laevis and X. tropicalis promoters. These data strongly suggest that the MMP-9 gene was duplicated, and differentiated into two genes, one of which was specialized in a common ancestor of X. laevis and X. tropicalis to be expressed in degenerating and remodeling organs as a response to thyroid hormone during metamorphosis.

Expression of myelin basic protein in two oligodendroglial cell lines is modulated by apotransferrin through different transcription factors

We have shown that apotransferrin (aTf) promotes the differentiation of two oligodendroglial cell (OLGc) lines, N19 and N20.1, representing different stages of OLGc maturation. Although in both cell lines aTf promoted myelin basic protein (MBP) expression, an increase in cAMP levels and CREB phosphorylation was observed only in the less mature cells (N19), suggesting that the maturation induced by aTf is achieved probably through different signaling pathways. We transfected both cell lines with the proximal region of the human MBP promoter fused to the lacZ reporter gene. In both transfected cell lines, addition of aTf produced an activation of the promoter. To elucidate the mechanisms involved in this action, Western blot analysis, EMSAs, and RT-PCR were performed for different transcription factors involved in mbp regulation. In the N20.1 line, treatment with aTf increased the expression and the DNA-binding capacity of thyroid hormone (TH) receptors, Sp1, and nuclear factor-kappaB (NFkappaB). For these cells we found that an inductor of NFkappaB (tumor necrosis factor-alpha) promoted MBP messenger synthesis, whereas mithramycin, a specific inibitor of Sp1, and a cAMP analog (db-cAMP) inhibited its transcription. In the N19 cell line, aTf stimulated NF-I and NFkappaB activation, but, aside from aTf, only db-cAMP induced mbp transcription. These data suggest that, depending on the OLGc maturational stage, aTf modulates MBP expression and OLGc differentiation through different signaling pathways and different transcription factors.

Thyroid hormone regulates oligodendrocyte accumulation in developing rat brain white matter tracts

Thyroid hormone (TH) is necessary for normal axonal myelination. Myelin basic protein (MBP) is a structural protein essential for myelin function. In this study, we demonstrate that perinatal hypothyroidism regulates MBP mRNA levels via indirect mechanisms. We observed decreased MBP mRNA accumulation in the hypothyroid rat brain at postnatal (PN) d 10 and 50. Acute TH replacement did not rescue hypothyroid MBP mRNA levels at PN5, 10, or 50. TH is necessary for normal intrahemispheric commissure development including the anterior commissure (AC) and the corpus callosum (CC). We determined that perinatal hypothyroidism decreases AC area and cellularity in the developing rat brain by PN10 and 50. In the developing CC, hypothyroidism initially increases area and cellularity by PN5, but then ultimately decreases area and cellularity by PN50. MBP-expressing oligodendrocytes are a recognized target of TH and are responsible for myelination within intrahemispheric commissures. We found that hypothyroidism reduces the number of mature oligodendrocytes within both the AC and CC. This reduction is noted at PN5, 10, and 50 in the AC and by PN10 and 50 in the CC. Together, these data suggest that TH regulates MBP mRNA levels through indirect mechanisms. These data demonstrate the complex mechanisms whereby TH regulates myelination in the developing brain.

In vivo activation of PPAR target genes by RXR homodimers

The ability of a retinoid X receptor (RXR) to heterodimerize with many nuclear receptors, including LXR, PPAR, NGF1B and RAR, underscores its pivotal role within the nuclear receptor superfamily. Among these heterodimers, PPAR:RXR is considered an important signalling mediator of both PPAR ligands, such as fatty acids, and 9-cis retinoic acid (9-cis RA), an RXR ligand. In contrast, the existence of an RXR/9-cis RA signalling pathway independent of PPAR or any other dimerization partner remains disputed. Using in vivo chromatin immunoprecipitation, we now show that RXR homodimers can selectively bind to functional PPREs and induce transactivation. At the molecular level, this pathway requires stabilization of the homodimer-DNA complexes through ligand-dependent interaction with the coactivator SRC1 or TIF2. This pathway operates both in the absence and in the presence of PPAR, as assessed in cells carrying inactivating mutations in PPAR genes and in wild-type cells. In addition, this signalling pathway via PPREs is fully functional and can rescue the severe hypothermia phenotype observed in fasted PPARalpha-/- mice. These observations have important pharmacological implications for the development of new rexinoid-based treatments.

Triiodothyronine administration reverses vitamin A deficiency-related hypo-expression of retinoic acid and triiodothyronine nuclear receptors and of neurogranin in rat brain

Recent studies have revealed that retinoids play an important role in the adult central nervous system and cognitive functions. Previous investigations in mice have shown that vitamin A deficiency (VAD) generates a hypo-expression of retinoic acid (RA, the active metabolite of vitamin A) receptors and of neurogranin (RC3, a neuronal protein involved in synaptic plasticity) and a concomitant selective behavioural impairment. Knowing that RC3 is both a triiodothyronine (T3) and a RA target gene, and in consideration of the relationships between the signalling pathways of retinoids and thyroid hormones, the involvement of T3 on RA signalling functionality in VAD was investigated. Thus, the effects of vitamin A depletion and subsequent administration with RA and/or T3 on the expression of RA nuclear receptors (RAR, RXR), T3 nuclear receptor (TR) and on RC3 in the brain were examined. Rats fed a vitamin A-deficient diet for 10 weeks exhibited a decreased expression of RAR, RXR and TR mRNA and of RC3 mRNA and proteins. RA administration to these vitamin A-deficient rats reversed only the RA hypo-signalling in the brain. Interestingly, T3 is able to restore its own brain signalling simultaneously with that of vitamin A and the hypo-expression of RC3. These results obtained in vivo revealed that one of the consequences of VAD is a dysfunction in the thyroid signalling pathway in the brain. This seems of crucial importance since the down regulation of RC3 observed in the depleted rats was corrected only by T3.

Distinctive gene profiles occur at key points during natural metamorphosis in the Xenopus laevis tadpole tail

Thyroid hormones (THs) are essential for tadpole metamorphosis into a juvenile frog; however, a complex interplay between additional hormones and signaling events also contributes to this dramatic developmental phase. A major mechanism of TH action is the nuclear receptor-mediated regulation of gene transcription of responsive genes. By using the precocious metamorphic model, several genes have been identified as TH responsive in the regressing tail. Many of these genes also exhibit altered expression during natural metamorphosis. Although identification of these genes provides insight into the mechanism whereby TH acts, complex interplay between TH and other hormones and the developmental stage-dependency of tissue responses contribute to the timing and coordination of metamorphic events. We investigated the temporal gene expression profile in Xenopus laevis tadpole tails from premetamorphosis through metamorphic climax by using a combination of a novel frog cDNA array containing 420 genes and quantitative real-time PCR. Seventy-nine genes were identified whose steady-state mRNA expression levels were altered in the tadpole tail during natural metamorphosis, of which 34 have previously been identified to be TH responsive in frogs or mammals. Of these genes, 75 clustered into 13 groups that displayed distinct developmental expression profiles. The levels of 28 transcripts were altered during premetamorphosis, 31 during prometamorphosis, and 43 with the onset of tail regression. This work establishes an important baseline for determining the mechanisms whereby tissues undergo differing metamorphic fates.

Thyroid hormone actions on neural cells

1. In addition to its role in cellular metabolic activity, thyroid hormone (TH) is critically involved in growth, development, and function of the central nervous system. In the brain, as in other structures, TH is described to exert its major action by the binding of L-3,5,3'-triiodothyronine (T3), considered as the bioactive form of the hormone, to nuclear thyroid hormone receptors (TR) that function as ligand-dependent transcription factors. 2. The transcription of numerous brain genes was indeed shown to be positively or negatively regulated by TH, turning these TR-mediated effects one explanation for the physiological effects of TH. In this context, the knowledge from TR-knockout studies provides some surprising results, since neonatal hypothyroidism is associated to more significant abnormalities than is TR deficiency. Some (nonexclusive) hypotheses include a permissive effect of TH, allowing derepression of unliganded-TR effects and non-TR-mediated effects of the hormone, further emphasizing the importance of a controlled accessibility of neural cells to TH. 3. On the other hand, T3 was demonstrated to directly act not only on neuronal but also on glial cells proliferation and differentiation, contributing to the harmonious development of the brain. Interestingly, in addition to these direct actions on neuronal and glial cells, several lines of evidence, notably developped in our laboratory, point out the role of thyroid hormone in neuronal-glial interactions.

A structural model of the constitutive androstane receptor defines novel interactions that mediate ligand-independent activity

Unlike classical nuclear receptors that require ligand for transcriptional activity, the constitutive androstane receptor (CAR) is active in the absence of ligand. To determine the molecular contacts that underlie this constitutive activity, we created a three-dimensional model of CAR and verified critical structural features by mutational analysis. We found that the same motifs that facilitate ligand-dependent activity in classical receptors also mediated constitutive activity in CAR. This raises a critical question: how are these motifs maintained in an active conformation in unliganded CAR? The model identified several novel interactions that account for this activity. First, CAR possesses a short loop between helix 11 and the transactivation domain (helix 12), as well as a short carboxy-terminal helix. Together, these features favor ligand-independent docking of the transactivation domain in a position that is characteristic of ligand-activated receptors. Second, this active conformation is further stabilized by a charge-charge interaction that anchors the carboxy-terminal activation domain to helix 4. Mutational analysis of these interactions provides direct experimental support for this model. We also show that ligand-mediated repression of constitutive activity reflects both a displacement of coactivator and a recruitment of corepressor. Our data demonstrate that CAR utilizes the same conserved structural motifs and coregulator proteins as originally defined for classical nuclear receptors. Despite these remarkable similarities, our model demonstrates how a few critical changes in CAR can dramatically reverse the transcriptional activity of this protein.

Feedback on hypothalamic TRH transcription is dependent on thyroid hormone receptor N terminus

The beta thyroid hormone receptor (TRbeta), but not TRalpha1, plays a specific role in mediating T(3)-dependent repression of hypothalamic TRH transcription. To investigate the structural basis of isoform specificity, we compared the transcriptional regulation and DNA binding obtained with chimeric and N-terminally deleted TRs. Using in vivo transfection assays to follow hypothalamic TRH transcription in the mouse brain, we found that TRbeta1 and chimeras with the TRbeta1 N terminus did not affect either transcriptional activation or repression from the rat TRH promoter, whereas N-terminally deleted TRbeta1 impaired T(3)-dependent repression. TRalpha1 or chimeras with the TRalpha1 N terminus reduced T(3)-independent transcriptional activation and blocked T(3)-dependent repression of transcription. Full deletion of the TRalpha1 N terminus restored ligand-independent activation of transcription. No TR isoform specificity was seen after transcription from a positive thyroid hormone response element. Gel mobility assays showed that all TRs tested bound specifically to the main negative thyroid hormone response element in the TRH promoter (site 4). Addition of neither steroid receptor coactivator 1 nor nuclear extracts from the hypothalamic paraventricular nuclei revealed any TR isoform specificity in binding to site 4. Thus N-terminal sequences specify TR T(3)-dependent repression of TRH transcription but not DNA recognition, emphasizing as yet unknown neuron-specific contributions to protein-promoter interactions in vivo.

Tumor necrosis factor modulates transcription of myelin basic protein gene through nuclear factor kappa B in a human oligodendroglioma cell line

Tumor necrosis factor-alpha (TNF-alpha) is a major mediator of inflammation and it is involved in many neurological disorders such as multiple sclerosis. Levels of TNF-alpha and lymphotoxin-alpha have been found elevated in plaques, bloods, and cerebral spinal fluids from multiple sclerosis patients. The expression of myelin basic protein (MBP), a major protein of the myelin sheath, is affected by cytokines secreted by activated immune cells. To determine the signal transduction pathway involving tumor necrosis factor's action in myelination and demyelination, we have cloned and analyzed cis-elements on promoters of the human and mouse MBP genes. There are two putative nuclear factors kappa-B (NF-kappaB) cis-elements on the human and one on the mouse gene promoter. In an electrophoretic mobility shift assay, all three NF-kappaB cis-elements showed binding to a protein, which was recognized by an antibody against NF-kappaB P65 component. The specificity of the binding was demonstrated in a competitive assay using NF-kappaB consensus oligonucleotides. A two base pair site-directed mutation on the mouse NF-kappaB cis-element abolished its binding activity. We created a DNA construct by linking the mouse MBP gene promoter containing the NF-kappaB cis-element to luciferase gene. Transfection of this construct into a human oligodendroglioma cell line showed TNF-alpha increased the transgene expression. Furthermore the mutation of NF-kappaB site abolished TNF-alpha -induction of the transgene. The data demonstrate that NF-kappaB is the mediator between tumor necrosis factor's action and MBP gene expression. Elucidating the molecular mechanisms underlying TNF-alpha regulation of MBP gene expression provides new scientific bases for the development of therapy against oligodendrocyte-specific and myelin-related disorders such as multiple sclerosis.

Requirement of helix 1 and the AF-2 domain of the thyroid hormone receptor for coactivation by PGC-1

Although PGC-1 (peroxisome proliferator-activated receptor-gamma coactivator-1) has been previously shown to enhance thyroid hormone receptor (TR)/retinoid X receptor-mediated ucp-1 gene expression in a ligand-induced manner in rat fibroblast cells, the precise mechanism of PGC-1 modulation of TR function has yet to be determined. In this study, we show that PGC-1 can potentiate TR-mediated transactivation of reporter genes driven by natural thyroid hormone response elements both in a ligand-dependent and ligand-independent manner and that the extent of coactivation is a function of the thyroid hormone response element examined. Our data also show that PGC-1 stimulation of TR activity in terms of Gal4 DNA-binding domain fusion is strictly ligand-dependent. In addition, an E457A AF-2 mutation had no effect on the ligand-induced PGC-1 enhancement of TR activity, indicating that the conserved charged residue in AF-2 is not essential for this PGC-1 function. Furthermore, GST pull-down and mammalian two-hybrid assays demonstrated that the PGC-1 LXXLL motif is required for ligand-induced PGC-1/TR interaction. This agonist-dependent PGC-1/TR interaction also requires both helix 1 and the AF-2 region of the TR ligand-binding domain. Taken together, these results support the notion that PGC-1 is a bona fide TR coactivator and that PGC-1 modulates TR activity via a mechanism different from that utilized with peroxisome proliferator activator receptor-gamma.

Thyroid hormone is a critical determinant for the regulation of the cochlear motor protein prestin

The most impressive property of outer hair cells (OHCs) is their ability to change their length at high acoustic frequencies, thus providing the exquisite sensitivity and frequency-resolving capacity of the mammalian hearing organ. Prestin, a protein related to a sulfate/anion transport protein, recently has been identified and proposed as the OHC motor molecule. Homology searches of 1.5 kb of genomic DNA 5' of the coding region of the prestin gene allowed the identification of a thyroid hormone (TH) response element (TRE) in the first intron upstream of the prestin ATG codon. Prestin(TRE) bound TH receptors as a monomer or presumptive heterodimer and mediated a triiodothyronine-dependent transactivation of a heterologous promotor in response to triiodothyronine receptors alpha and beta. Retinoid X receptor-alpha had an additive effect. Expression of prestin mRNA and prestin protein was reduced strongly in the absence of TH. Although prestin protein typically was redistributed to the lateral membrane before the onset of hearing, an immature pattern of prestin protein distribution across the entire OHC membrane was noted in hypothyroid rats. The data suggest TH as a first transcriptional regulator of the motor protein prestin and as a direct or indirect modulator of subcellular prestin distribution.

Retardation of cochlear maturation and impaired hair cell function caused by deletion of all known thyroid hormone receptors

The deafness caused by early onset hypothyroidism indicates that thyroid hormone is essential for the development of hearing. We investigated the underlying roles of the TRalpha1 and TRbeta thyroid hormone receptors in the auditory system using receptor-deficient mice. TRalpha1 and TRbeta, which act as hormone-activated transcription factors, are encoded by the Thra and Thrb genes, respectively, and both are expressed in the developing cochlea. TRbeta is required for hearing because TRbeta-deficient (Thrb(tm1/tm1)) mice have a defective auditory-evoked brainstem response and retarded expression of a potassium current (I(K,f)) in the cochlear inner hair cells. Here, we show that although TRalpha1 is individually dispensable, TRalpha1 and TRbeta synergistically control an extended array of functions in postnatal cochlear development. Compared with Thrb(tm1/tm1) mice, the deletion of all TRs in Thra(tm1/tm1)Thrb(tm1/tm1) mice produces exacerbated and novel phenotypes, including delayed differentiation of the sensory epithelium, malformation of the tectorial membrane, impairment of electromechanical transduction in outer hair cells, and a low endocochlear potential. The induction of I(K,f) in inner hair cells was not markedly more retarded than in Thrb(tm1/tm1) mice, suggesting that this feature of hair cell maturation is primarily TRbeta-dependent. These results indicate that distinct pathways mediated by TRbeta alone or by TRbeta and TRalpha1 together facilitate control over an extended range of functions during the maturation of the cochlea.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Proteomic comparison of human and great ape blood plasma reveals conserved glycosylation and differences in thyroid hormone metabolism

Most blood plasma proteins are glycosylated. These glycoproteins typically carry sialic acid-bearing sugar chains, which can modify the observed molecular weights and isoelectric points of those proteins during electrophoretic analyses. To explore changes in protein expression and glycosylation that occurred during great ape and human evolution, we subjected multiple blood plasma samples from all these species to high-resolution proteomic analysis. We found very few species-specific differences, indicating a remarkable degree of conservation of plasma protein expression and glycosylation during approximately 12 million years of evolution. A few lineage-specific differences in protein migration were noted among the great apes. The only obvious differences between humans and all great apes were an apparent decrease in transthyretin (prealbumin) and a change in haptoglobin isoforms (the latter was predictable from prior genetic studies). Quantitative studies of transthyretin in samples of blood plasma (synthesized primarily by the liver) and of cerebrospinal fluid (synthesized locally by the choroid plexus of the brain) confirmed approximately 2-fold higher levels in chimpanzees compared to humans. Since transthyretin binds thyroid hormones, we next compared plasma thyroid hormone parameters between humans and chimpanzees. The results indicate significant differences in the status of thyroid hormone metabolism, which represent the first known endocrine difference between these species. Notably, thyroid hormones are known to play major roles in the development, differentiation, and metabolism of many organs and tissues, including the brain and the cranium. Also, transthyretin is known to be the major carrier of thyroid hormone in the cerebrospinal fluid, likely regulating delivery of this hormone to the brain. A potential secondary difference in retinoid (vitamin A) metabolism is also noted. The implications of these findings for explaining unique features of human evolution are discussed.

The mechanism of action of thyroid hormones

Thyroid hormone is essential for normal development, differentiation, and metabolic balance. Thyroid hormone action is mediated by multiple thyroid hormone receptor isoforms derived from two distinct genes. The thyroid hormone receptors belong to a nuclear receptor superfamily that also includes receptors for other small lipophilic hormones. Thyroid hormone receptors function by binding to specific thyroid hormone-responsive sequences in promoters of target genes and by regulating transcription. Thyroid hormone receptors often form heterodimers with retinoid X receptors. Heterodimerization is regulated through distinct mechanisms that together determine the specificity and flexibility of the sequence recognition. Amino-terminal regions appear to modulate thyroid hormone receptor function in an isoform-dependent manner. Unliganded thyroid hormone receptor represses transcription through recruitment of a corepressor complex, which also includes Sin3A and histone deacetylase. Ligand binding alters the conformation of the thyroid hormone receptor in such a way as to release the corepressor complex and recruit a coactivator complex that includes multiple histone acetyltransferases, including a steroid receptor family coactivator, p300/CREB-binding protein-associated factor (PCAF), and CREB binding protein (CBP). The existence of histone-modifying activities in the transcriptional regulatory complexes indicates an important role of chromatin structure. Stoichiometric, structural, and sequence-specific rules for coregulator interaction are beginning to be understood, as are aspects of the tissue specificity of hormone action. Moreover, knockout studies suggest that the products of two thyroid hormone receptor genes mediate distinct functions in vivo. The increased understanding of the structure and function of thyroid hormone receptors and their interacting proteins has markedly clarified the molecular mechanisms of thyroid hormone action.

A novel role for helix 12 of retinoid X receptor in regulating repression

Nutrients, drugs, and hormones influence transcription during differentiation and metabolism by binding to high-affinity nuclear receptors. In the absence of ligand, some but not all nuclear receptors repress transcription as a heterodimer with retinoid X receptor (RXR). Here we define a novel role for helix 12 (H12) in sterically masking the corepressor (CoR) binding site in apo-RXR. Removing H12 converts RXR to a potent transcriptional repressor. The length but not the specific sequence of H12 is critical for masking RXR's intrinsic repression function. This contrasts with the amphipathic character required for mediating ligand-dependent activation and coactivator recruitment. Physiologically, we show that heterodimerization of RXR with apo-thyroid hormone receptor (TR) unmasks the CoR binding site in RXR and allows the TR-RXR heterodimer to repress. A molecular mechanism that involves sequence-specific interaction between RXR H12 and the coactivator-binding surface of the nuclear receptor is proposed for this heterodimerization-mediated unmasking. Peroxisome proliferator-activated receptor gamma does not interact as well with RXR H12, thus explaining its inability to repress transcription as an RXR heterodimer. The requirement to unmask RXR's latent repression function explains why only certain RXR partners repress transcription.