Thyroid hormone receptor/c-erbA: control of commitment and differentiation in the neuronal/chromaffin progenitor line PC12

Vitamin D opposes multilineage cell differentiation induced by Notch inhibition and BMP4 pathway activation in human colon organoids

Understanding the mechanisms involved in colonic epithelial differentiation is key to unraveling the alterations causing inflammatory conditions and cancer. Organoid cultures provide an unique tool to address these questions but studies are scarce. We report a differentiation system toward enterocytes and goblet cells, the two major colonic epithelial cell lineages, using colon organoids generated from healthy tissue of colorectal cancer patients. Culture of these organoids in medium lacking stemness agents resulted in a modest ultrastructural differentiation phenotype with low-level expression of enterocyte (KLF4, KRT20, CA1, FABP2) and goblet cell (TFF2, TFF3, AGR2) lineage markers. BMP pathway activation through depletion of Noggin and addition of BMP4 resulted in enterocyte-biased differentiation. Contrarily, blockade of the Notch pathway using the γ-secretase inhibitor dibenzazepine (DBZ) favored goblet cell differentiation. Combination treatment with BMP4 and DBZ caused a balanced strong induction of both lineages. In contrast, colon tumor organoids responded poorly to BMP4 showing only weak signals of cell differentiation, and were unresponsive to DBZ. We also investigated the effects of 1α,25-dihydroxyvitamin D3 (calcitriol) on differentiation. Calcitriol attenuated the effects of BMP4 and DBZ on colon normal organoids, with reduced expression of differentiation genes and phenotype. Consistently, in normal organoids, calcitriol inhibited early signaling by BMP4 as assessed by reduction of the level of phospho-SMAD1/5/8. Our results show that BMP and Notch signaling play key roles in human colon stem cell differentiation to the enterocytic and goblet cell lineages and that calcitriol modulates these processes favoring stemness features.

Factors and Mechanisms of Thyroid Hormone Activity in the Brain: Possible Role in Recovery and Protection

Thyroid hormones (THs) are essential in normal brain development, and cognitive and emotional functions. THs act through a cascade of events including uptake by the target cells by specific cell membrane transporters, activation or inactivation by deiodinase enzymes, and interaction with nuclear thyroid hormone receptors. Several thyroid responsive genes have been described in the developing and in the adult brain and many studies have demonstrated a systemic or local reduction in TH availability in neurologic disease and after brain injury. In this review, the main factors and mechanisms associated with the THs in the normal and damaged brain will be evaluated in different regions and cellular contexts. Furthermore, the most common animal models used to study the role of THs in brain damage and cognitive impairment will be described and the use of THs as a potential recovery strategy from neuropathological conditions will be evaluated. Finally, particular attention will be given to the link observed between TH alterations and increased risk of Alzheimer's Disease (AD), the most prevalent neurodegenerative and dementing condition worldwide.

Nuclear hormone receptors in demyelinating diseases

Demyelination results from the pathological loss of myelin and is a hallmark of many neurodegenerative diseases. Despite the prevalence of demyelinating diseases, there are no disease modifying therapies that prevent the loss of myelin or promote remyelination. This review aims to summarize studies in the field that highlight the importance of nuclear hormone receptors in the promotion and maintenance of myelination and the relevance of nuclear hormone receptors as potential therapeutic targets for demyelinating diseases. These nuclear hormone receptors include the estrogen receptor, progesterone receptor, androgen receptor, vitamin D receptor, thyroid hormone receptor, peroxisome proliferator-activated receptor, liver X receptor, and retinoid X receptor. Pre-clinical studies in well-established animal models of demyelination have shown a prominent role of these nuclear hormone receptors in myelination through their promotion of oligodendrocyte maturation and development. The activation of the nuclear hormone receptors by their ligands also promotes the synthesis of myelin proteins and lipids in mouse models of demyelination. There are limited clinical studies that focus on how the activation of these nuclear hormone receptors could alleviate demyelination in patients with diseases such as multiple sclerosis (MS). However, the completed clinical trials have reported improved clinical outcome in MS patients treated with the ligands of some of these nuclear hormone receptors. Together, the positive results from both clinical and pre-clinical studies point to nuclear hormone receptors as promising therapeutic targets to counter demyelination.

Strategic design of peptide-decorated aligned nanofibers impregnated with triiodothyronine for neural regeneration

Nerve injuries are often debilitating as its regeneration occurs in a slow and laborious manner. Remediation of nerve injury is a colossal task as functional restoration in larger gaps seldom occurs due to the complex nerve regeneration mechanism. A nanofiber-based graft material has been fabricated to provide topographical and biochemical cues to encourage neural differentiation. Laminin plays a crucial role in supporting peripheral nerve regeneration and hence aligned polyvinyl cinnamate nanofibers surface-conjugated with laminin-derived cell-adhesion peptides have been fabricated to improve selective neural adhesion and regeneration. Further, triiodothyronine has been encapsulated within the nanofibers enabling its sustained release so as to bolster regeneration and reinstate the lost functionality to the damaged nerve. The fabricated nanofibers were characterized for its physicochemical, morphological, and topographical properties. Nanofibers were biocompatible, improved cell adhesion rate, and illustrated favourable interaction with cells. Gene expression (showed 9.5 and 4.1 fold increase in β-tubulin and MAP 2 expression, respectively) and protein expression (immunofluorescence, flow cytometry, and western blot) studies confirmed the positive influence of the scaffold over cell differentiation. The studies were extrapolated to adult zebrafish model with a surgical incision in posterior lateral line. The biocomposite treated group showed earlier functional restoration of the nerve compared with control groups detected by touch-evoked response. Thus, the combination of aligned nanofibers providing topographical cue, along with the peptides and triiodothyronine serving as biochemical cues, has a robust potential to restore functionality to the injured nerve, thereby opening avenues for fabrication of regenerative nerve grafts.

Hypothyroidism and Cognitive Disorders during Development and Adulthood: Implications in the Central Nervous System

Thyroid hormones (THs) play a critical function in fundamental signaling of the body regulating process such as metabolism of glucose and lipids, cell maturation and proliferation, and neurogenesis, to name just a few. THs trigger biological effects both by directly affecting gene expression through the interaction with nuclear receptors (genomic effects) and by activating protein kinases and/or ion channels (short-term effects). For years, a close relationship between the THs hormones and the central nervous system (CNS) has been described, not only for neuronal cells but also for glial development and differentiation. A deficit in thyroid hormones triiodothyronine (T₃) and thyroxine (T₄) is observed in the hypothyroid condition, generated by a iodine deficiency or an autoimmune response of the body. In the hypothyroid condition, several cellular deregulation and alterations have been described in dendrite spine morphology, cell migration and proliferation, and impaired synaptic transmission in the hippocampus, among others. The aim of this review is to describe the role of the thyroid hormones with focus in brain function and neurodegenerative disorders.

Role for Cystathionine γ Lyase (CSE) in an Ethanol (E)-Induced Lesion in Fetal Brain GSH Homeostasis

Earlier, we reported that gestational ethanol (E) can dysregulate neuron glutathione (GSH) homeostasis partially via impairing the EAAC1-mediated inward transport of Cysteine (Cys) and this can affect fetal brain development. In this study, we investigated if there is a role for the transulfuration pathway (TSP), a critical bio-synthetic point to supply Cys in E-induced dysregulation of GSH homeostasis. These studies utilized an in utero E binge model where the pregnant Sprague⁻Dawley (SD) rat dams received five doses of E at 3.5 g/kg by gastric intubation beginning embryonic day (ED) 17 until ED19 separated by 12 h. The postnatal day 7 (PN7) alcohol model employed an oral dosing of 4 g/kg body weight split into 2 feedings at 2 h interval and an iso-caloric and iso-volumic equivalent maltose-dextrin milk solution served as controls. The in vitro model consisted of cerebral cortical neuron cultures from embryonic day (ED) 16⁻17 fetus from SD rats and differentiated neurons from ED18 rat cerebral cortical neuroblasts. E concentrations were 4 mg/mL. E induced an accumulation of cystathionine in primary cortical neurons (PCNs), 2nd trimester equivalent in utero binge, and 3rd trimester equivalent PN7 model suggesting that breakdown of cystathionine, a required process for Cys supply is impaired. This was associated with a significant reduction in cystathionine γ-lyase (CSE) protein expression in PCN (p < 0.05) and in fetal cerebral cortex in utero (53%, p < 0.05) without a change in the expression of cystathionine β-synthase (CBS). Concomitantly, E decreased Cse mRNA expression in PCNs (by 32% within 6 h of exposure, p < 0.05) and in fetal brain (33%, p < 0.05). In parallel, knock down of CSE in differentiated rat cortical neuroblasts exaggerated the E-induced ROS, GSH loss with a pronounced caspase-3 activation and cell death. These studies illustrate the importance of TSP in CSE-related maintenance of GSH and the downstream events via Cys synthesis in neurons and fetal brain.

Tailored release of triiodothyronine and retinoic acid from a spatio-temporally fabricated nanofiber composite instigating neuronal differentiation

Regeneration of the central and peripheral nervous system is challenging since the functional restoration of injured nerves is an incredible task. The fabrication of an ideal nerve guide that fulfills the requirement to regenerate nerve tissue is a herculean challenge requiring a combination of both biochemical and topographical cues. The present study explores the combinatorial effect of aligned nanofibers and the regulated delivery of triiodothyronine and retinoic acid on nerve regeneration. A sequential release mechanism is adopted in fabricating the nanofiber scaffold, with triiodothyronine incorporated into the nanofiber shell ensuring its prior release, followed by retinoic acid (entrapped within zein nanoparticles) from the core. The composite nanofibers thus fabricated possess excellent mechanical, physical and thermal properties and good topographical morphology and were highly biocompatible. The nanofibers were scrutinized for their efficacy in stimulating differentiation to a neuronal phenotype. The elongation factor (E-factor) of the neural cells had doubled in the bioactive incorporated composite compared to other scaffolds, as observed on phalloidin staining of their cytoskeleton, which endorsed enhanced neural differentiation on the fabricated nanofiber scaffold. There was a significant increase in the expression of neural-lineage specific markers on investigation of mRNA by real time PCR, showing a 10 fold increase in the gene expression of β-III-tubulin, a 5.5 fold increase for microtubule associated protein 2 gene and 3.5 fold for neurofilament M gene in the cells cultured over bioactive incorporated aligned nanofiber composites. Similarly protein expression was analyzed by immunofluorescence and flow cytometry studies, which showed an increase in the expression of β-III-tubulin in the composite nanofiber. This corroborates that neuronal differentiation is enhanced by the aligned nanotopography and spatio-temporal delivery of triiodothyronine and retinoic acid, opening avenues for nerve regenerative graft fabrication.

A Novel Population of Inner Cortical Cells in the Adrenal Gland That Displays Sexually Dimorphic Expression of Thyroid Hormone Receptor-β1

The development of the adrenal cortex involves the formation and then subsequent regression of immature or fetal inner cell layers as the mature steroidogenic outer layers expand. However, controls over this remodeling, especially in the immature inner layer, are incompletely understood. Here we identify an inner cortical cell population that expresses thyroid hormone receptor-β1 (TRβ1), one of two receptor isoforms encoded by the Thrb gene. Using mice with a Thrb(b1) reporter allele that expresses lacZ instead of TRβ1, β-galactosidase was detected in the inner cortex from early stages. Expression peaked at juvenile ages in an inner zone that included cells expressing 20-α-hydroxysteroid dehydrogenase, a marker of the transient, so-called X-zone in mice. The β-galactosidase-positive zone displayed sexually dimorphic regression in males after approximately 4 weeks of age but persisted in females into adulthood in either nulliparous or parous states. T3 treatment promoted hypertrophy of inner cortical cells, induced some markers of mature cortical cells, and, in males, delayed the regression of the TRβ1-positive zone, suggesting that TRβ1 could partly divert the differentiation fate and counteract male-specific regression of inner zone cells. TRβ1-deficient mice were resistant to these actions of T3, supporting a functional role for TRβ1 in the inner cortex.

The timecourse of apoptotic cell death during postnatal remodeling of the mouse cochlea and its premature onset by triiodothyronine (T3)

Apoptosis underlies various forms of tissue remodeling during development. Prior to the onset of hearing, thyroid hormone (T3) promotes cochlear remodeling, which involves regression of the greater epithelial ridge (GER), a transient structure of columnar cells adjacent to the mechanosensory hair cells. We investigated the timecourse of apoptosis in the GER and the influence of ectopic T3 on apoptosis. In saline-treated mice, activated caspase 3-positive cells were detected in the GER between postnatal days 7 and 13 and appeared progressively along the cochlear duct from base to apex over developmental time. T3 given on P0 and P1 advanced the overall program of apoptosis and remodeling by ~4 days. Thyroid hormone receptor β was required for these actions, suggesting a receptor-mediated process of initiation of apoptosis. Finally, T3 given only at P0 or P1 resulted in deafness in adult mice, thus revealing a transient period of susceptibility to long-term damage in the neonatal auditory system.

Ethanol-induced transcriptional activation of programmed cell death 4 (Pdcd4) is mediated by GSK-3β signaling in rat cortical neuroblasts

Ingestion of ethanol (ETOH) during pregnancy induces grave abnormalities in developing fetal brain. We have previously reported that ETOH induces programmed cell death 4 (PDCD4), a critical regulator of cell growth, in cultured fetal cerebral cortical neurons (PCNs) and in the cerebral cortex in vivo and affect protein synthesis as observed in Fetal Alcohol Spectrum Disorder (FASD). However, the mechanism which activates PDCD4 in neuronal systems is unclear and understanding this regulation may provide a counteractive strategy to correct the protein synthesis associated developmental changes seen in FASD. The present study investigates the molecular mechanism by which ethanol regulates PDCD4 in cortical neuroblasts, the immediate precursor of neurons. ETOH treatment significantly increased PDCD4 protein and transcript expression in spontaneously immortalized rat brain neuroblasts. Since PDCD4 is regulated at both the post-translational and post-transcriptional level, we assessed ETOH's effect on PDCD4 protein and mRNA stability. Chase experiments demonstrated that ETOH does not significantly impact either PDCD4 protein or mRNA stabilization. PDCD4 promoter-reporter assays confirmed that PDCD4 is transcriptionally regulated by ETOH in neuroblasts. Given a critical role of glycogen synthase kinase 3β (GSK-3β) signaling in regulating protein synthesis and neurotoxic mechanisms, we investigated the involvement of GSK-3β and showed that multifunctional GSK-3β was significantly activated in response to ETOH in neuroblasts. In addition, we found that ETOH-induced activation of PDCD4 was inhibited by pharmacologic blockade of GSK-3β using inhibitors, lithium chloride (LiCl) and SB-216763 or siRNA mediated silencing of GSK-3β. These results suggest that ethanol transcriptionally upregulates PDCD4 by enhancing GSK-3β signaling in cortical neuroblasts. Further, we demonstrate that canonical Wnt-3a/GSK-3β signaling is involved in regulating PDCD4 protein expression. Altogether, we provide evidence that GSK-3β/PDCD4 network may represent a critical modulatory point to manage the protein synthetic anomalies and growth aberrations of neural cells seen in FASD.

In vitro assessment of thyroidal and estrogenic activities in poultry and broiler manure

Among the many chemicals found in avian manure, endocrine disruptors (EDs), of natural or anthropogenic origin, are of special environmental concern. Nowadays, an increasing amount of estrogens is being released into the environment via the use of manure to fertilize agricultural land. While most research in this field has focused on estrogenic phenomena, little is known about alterations related to other endocrine systems, such as the thyroidal one. Here we simultaneously assessed the potential estrogenic and thyroidal activity of poultry and broiler litter manure using in vitro approaches based on estrogen receptor (Er) and thyroid receptor (Tr) transactivation assays. In addition, leaching experiments were performed to assess whether the EDs present in the manure pass through a soil column and potentially reach the groundwater. Manure from four broiler and four poultry farms was collected in two sampling campaigns carried out in two seasons (fall and spring). Extracts from broiler and poultry manure exhibited strong thyroidal activity. Only poultry manure showed estrogenic activity, which is consistent with the low levels of estrogens expected in hatchlings. Leakage experiments were performed in columns with two kinds of arable soils: sandy and loamy. No estrogenicity or thyroidal activity was detectable in soils treated with the manure or in the corresponding leachates. These results indicate that substances with estrogenic or thyroidal activity were degraded in the soil under our experimental conditions. However, the long-term effects associated with the constant and intensive application of manure to agricultural land in some regions require further research.

Thyroid hormones differentially regulate phosphorylation of ERK and Akt via integrin αvβ3 receptor in undifferentiated and differentiated PC-12 cells

The effects of 3,5,3'-triiodo-l-thyronine (T3) and l-thyroxine (T4) on the integrin αvβ3 receptor of thyroid hormones (TH) were investigated in pheochromocytoma PC-12 cells. Differentiation was induced by treatment of PC-12 cells with fisetin and the levels of phosphorylated extracellular signal-regulated kinase (ERK) and Akt in cytoplasm, as well as the content of FoxO6 transcription factor in nuclei was analysed in undifferentiated and differentiated conditions. We have found that in undifferentiated PC-12 cells, tetraiodothyroacetic acid (TETRAC), a known inhibitor of binding of T4 and T3 to plasma membrane integrin αvβ3 receptor inhibits T4-dependent phosphorylation of ERK, whereas in differentiated PC-12 cells, TETRAC abolishes the effect of T3. In undifferentiated PC-12 cells, both TH increase the level of p-Akt, and this enhancement is not sensitive to TETRAC. In differentiated PC-12 cells, both TH increase the level of p-Akt; however, only T3-dependent activation of Akt is sensitive to the TETRAC. Furthermore, our results have shown that in differentiated PC-12 cells, the expression of FoxO6 was higher than in undifferentiated PC-12 cells, and this elevation has not changed under the action of TH. Only in undifferentiated PC-12 cells the T3-dependent expression of FoxO6 was sensitive to the TETRAC. We propose that PC-12 cells contain integrin αvβ3 receptor, which T3 and T3/T4 sites are differentially regulated by TH in undifferentiated and differentiated conditions.

Unliganded thyroid hormone receptor alpha1 impairs adult hippocampal neurogenesis

Thyroid hormone regulates adult hippocampal neurogenesis, a process involved in key functions, such as learning, memory, and mood regulation. We addressed the role of thyroid hormone receptor TRα1 in adult hippocampal neurogenesis, using mice harboring a TRα1 null allele (TRα1(-/-)), overexpressing TRα1 6-fold (TRα2(-/-)), and a mutant TRα1 (TRα1(+/m)) with a 10-fold lower affinity to the ligand. While hippocampal progenitor proliferation was unaltered, TRα1(-/-) mice exhibited a significant increase in doublecortin-positive immature neurons and increased survival of bromodeoxyuridine-positive (BrdU(+)) progenitors as compared to wild-type controls. In contrast, the TRα1(+/m) and the TRα2(-/-) mice, where the overexpressed TRα1 acts as an aporeceptor, showed a significant decline in surviving BrdU(+) progenitors. TRα1(-/-) and TRα2(-/-) mice showed opposing effects on neurogenic markers like polysialylated neural cell adhesion molecule and stathmin. The decreased progenitor survival in the TRα2(-/-) and TRα1(+/m) mice could be rescued by thyroid hormone treatment, as was the decline in neuronal differentiation seen in the TRα1(+/m) mice. These mice also exhibited a decrease in NeuroD(+) cell numbers in the dentate gyrus, suggesting an effect on early postmitotic progenitors. Our results provide the first evidence of a role for unliganded TRα1 in modulating the deleterious effects of hypothyroidism on adult hippocampal neurogenesis.

The thyroid hormone receptor alpha1 protein is expressed in embryonic postmitotic neurons and persists in most adult neurons

Thyroid hormone is essential for brain development where it acts mainly through the thyroid hormone receptor α1 (TRα1) isoform. However, the potential for the hormone to act in adult neurons has remained undefined due to difficulties in reliably determining the expression pattern of TR proteins in vivo. We therefore created a mouse strain that expresses TRα1 and green fluorescent protein as a chimeric protein from the Thra locus, allowing examination of TRα1 expression during fetal and postnatal development and in the adult. Furthermore, the use of antibodies against other markers enabled identification of TRα1 expression in subtypes of neurons and during specific stages of their maturation. TRα1 expression was first detected in postmitotic cells of the cortical plate in the embryonic telencephalon and preceded the expression of the mature neuronal protein NeuN. In the cerebellum, TRα1 expression was absent in proliferating cells of the external granular layer, but switched on as the cells migrated towards the internal granular layer. In addition, TRα1 was expressed transiently in developing Purkinje cells, but not in mature cells. Glial expression was found in tanycytes in the hypothalamus and in the cerebellum. In the adult brain, TRα1 expression was detected in essentially all neurons. Our data demonstrate that thyroid hormone, unexpectedly, has the capacity to play an important role in virtually all developing and adult neurons. Because the role of TRα1 in most neuronal cell types in vivo is largely unknown, our findings suggest that novel functions for thyroid hormone remain to be identified in the brain.

T3 differentially regulates TRH expression in developing hypothalamic neurons in vitro

Triiodothyronine (T3) plays an important role during development of the central nervous system. T3 effects on gene expression are determined in part by the type of thyroid hormone receptors (TRs) expressed in a given cell type. Previous studies have demonstrated that thyrotropin releasing hormone (TRH) transcription in the adult hypothalamus is subjected to negative regulation by thyroid hormones. However, the role of T3 on the development of TRH expression is unknown. In this study we used primary cultures derived from 17-day-old fetal rat hypothalamus to analyze the effects of T3 on TRH gene expression during development. T3 increased TRH mRNA expression in immature cultures, but decreased it in mature cultures. In addition, T3 up-regulated TRalpha1 and TRbeta2 mRNA expression. TRalpha1 expression coincided chronologically with that of TRH in the rat hypothalamus in vivo. Maturation of TRH expression in the hypothalamus may involve T3 acting through TRalpha1.

Current and Potential Rodent Screens and Tests for Thyroid Toxicants

This article reviews current rodent screens and tests to detect thyroid toxicants. Many points of disruption for thyroid toxicants are outlined and include: (a) changes in serum hormone level; (b) thyroperoxidase inhibitors; (c) the perchlorate discharge test; (d) inhibitors of iodide uptake; (e) effects on iodothyronine deiodinases; (f) effects on thyroid hormone action; and (g) role of binding proteins (e.g., rodent transthyretin). The major thyroid endpoints currently utilized in existing in vivo assay protocols of the Organization for Economic Cooperation and Development (OECD), Japanese researchers, and U.S. Environmental Protection Agency (EPA) include thyroid gland weight, histopathology, circulating thyroid hormone measurements, and circulating thyroid-stimulating hormone (TSH). These endpoints can be added into the existing in vivo assays for reproduction, development, and neurodevelopment that are outlined in this chapter. Strategic endpoints for possible addition to existing protocols to detect effects on developmental and adult thyroid endpoints are discussed. Many of these endpoints for detecting thyroid system disruption require development and additional research before they can be considered in existing assays. Examples of these endpoints under development include computer-assisted morphometry of the brain and evaluation of treatment-related changes in gene expression, thyrotropin-releasing hormone (TRH) and TSH challenge tests, and tests to evaluate thyroid hormone (TH)-dependent developmental events, especially in the rodent brain (e.g., measures of cerebellar and cortical proliferation, differentiation, migration, apoptosis, planimetric measures and gene expression, and oligodendrocyte differentiation). Finally, TH-responsive genes and proteins as well as enzyme activities are being explored. Existing in vitro tests are also reviewed, for example, thyroid hormone (TH) metabolism, receptor binding, and receptor activation assays, and their restrictions are described. The in vivo assays are currently the most appropriate for understanding the potential effects of a thyroid toxicant on the thyroid system. The benefits and potential limitations of the current in vivo assays are listed, and a discussion of the rodent thyroid system in the context of human health is touched upon. Finally, the importance of understanding the relationship between timing of exposure, duration of dose, and time of acquisition of the endpoints in interpreting the results of the in vivo assays is emphasized.

Maternal thyroid hormone: a strong repressor of neuronal nitric oxide synthase in rat embryonic neocortex

Understanding of how maternal thyroid inadequacy during early gestation poses a risk for developmental outcomes is still a challenge for the neuroendocrine community. Early neocortical neurogenesis is accompanied by maternal thyroid hormone (TH) transfer to fetal brain, appearance of TH receptors, and absence of antineurogenesis signals, followed by optimization of neuronal numbers through apoptosis. However, the effects of TH deprivation on neurogenesis and neuronal cell death before the onset of fetal thyroid are still not clear. We show that maternal TH deficiency during early gestational period causes massive premature elevation in the expression of neuronal nitric oxide synthase (nNOS) with an associated neuronal death in embryonic rat neocortex. Maternal hypothyroidism was induced by feeding methimazole (0.025% wt/vol) in the drinking water to pregnant Sprague Dawley rats from embryonic d 6. Cerebral cortices from fetuses were harvested at different embryonic stages (embryonic d 14, 16, and 18) of hypothyroid and euthyroid groups. Immunoblotting and real-time PCR results showed that both protein and RNA levels of nNOS were prematurely increased under maternal hypothyroidism, and showed reversibility upon T4 administration. Immunohistochemistry revealed an increased nNOS immunoreactivity in both the cortical plate and proliferative zone of neocortex along with a corroborative decrease in the microtubule associated protein-2 positive neurons under maternal TH insufficiency. Results combined, put forth nNOS as a novel target of maternal TH action in embryonic neocortex, and underscore the importance of prenatal screening and timely rectification of maternal TH insufficiency, even of a moderate degree.

Multigenic control of thyroid hormone functions in the nervous system

Thyroid hormone (TH) has a remarkable range of actions in the development and function of the nervous system. A multigenic picture is emerging of the mechanisms that specify these diverse functions in target tissues. Distinct responses are mediated by alpha and beta isoforms of TH receptor which act as ligand-regulated transcription factors. Receptor activity can be regulated at several levels including that of uptake of TH ligand and the activation or inactivation of ligand by deiodinase enzymes in target tissues. Processes under the control of TH range from learning and anxiety-like behaviour to sensory function. At the cellular level, TH controls events as diverse as axonal outgrowth, hippocampal synaptic activity and the patterning of opsin photopigments necessary for colour vision. Overall, TH coordinates this variety of events in both central and sensory systems to promote the function of the nervous system as a complete entity.

A new bioluminescent cellular assay to measure the transcriptional effects of chemicals that modulate the alpha-1 thyroid hormone receptor

Interactions of environmental pollutants with the thyroid endocrine axis have received much attention especially because thyroid hormones (THs) play a major role in mammalian brain development. In order to screen for compounds that act on the triiodothyronine (T3) signaling pathway, we developed a new reporter gene assay expressing luciferase under the control of the TH receptor (TR). PC12 cells expressing the alpha1-isoform of TR of avian origin were stably transfected with a luciferase gene controlled by the SV40 promoter, and enhanced by a four-spaced direct repeat (DR4) thyroid response element (TRE). The resulting PC-DR-LUC cells were used to optimize a T3 assay in multiwell microplates. This assay was highly sensitive (30 pM T3) and reproducible, and responded as expected to TH analogues. Several halogenated phenolic (3,3',5,5'-tetrabromobisphenol A, 3,3',5,5'-tetrachlorobisphenol A, 4-hydroxy-2',3,4',5,6'-pentachlorobiphenyl) and phenol (pentachlorophenol, 2,4,6-triiodophenol) compounds suspected of being thyroid-disrupting environmental chemicals induced partial agonistic and/or complex competitive/uncompetitive antagonistic responses in PC-DR-LUC cells at micromolar concentrations. A cell viability test indicated that these effects were not related to cytotoxicity of the chemicals. These results suggest that the PC-DR-LUC assay could be a valuable tool for the large-scale screening for thyroid receptor agonists and antagonists in vitro, and for detecting thyroid disruptors in the environment.

Thyroid hormone receptors in brain development and function

Thyroid hormones are important during development of the mammalian brain, acting on migration and differentiation of neural cells, synaptogenesis, and myelination. The actions of thyroid hormones are mediated through nuclear thyroid hormone receptors (TRs) and regulation of gene expression. The purpose of this article is to review the role of TRs in brain maturation. In developing humans maternal and fetal thyroid glands provide thyroid hormones to the fetal brain, but the timing of receptor ontogeny agrees with clinical data on the importance of the maternal thyroid gland before midgestation. Several TR isoforms, which are encoded by the THRA and THRB genes, are expressed in the brain, with the most common being TRalpha1. Deletion of TRalpha1 in rodents is not, however, equivalent to hormone deprivation and, paradoxically, even prevents the effects of hypothyroidism. Unliganded receptor activity is, therefore, probably an important factor in causing the harmful effects of hypothyroidism. Accordingly, expression of a mutant receptor with impaired triiodothyronine (T(3)) binding and dominant negative activity affected cerebellar development and motor performance. TRs are also involved in adult brain function. TRalpha1 deletion, or expression of a dominant negative mutant receptor, induces consistent behavioral changes in adult mice, leading to severe anxiety and morphological changes in the hippocampus.

c-Jun N-terminal protein kinase signalling pathway mediates lovastatin-induced rat brain neuroblast apoptosis

We have previously shown that lovastatin, an HMG-CoA reductase inhibitor, induces apoptosis in rat brain neuroblasts. c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) are implicated in regulation of neuronal apoptosis. In this work, we investigated the role of JNK and p38 MAPK in neuroblast apoptosis induced by lovastatin. We found that lovastatin induced the activation of JNK, but not p38 MAPK. It also induced c-Jun phosphorylation with a subsequent increase in activator protein-1 (AP-1) binding, AP-1-mediated gene expression and BimEL protein levels. The effects of lovastatin were prevented by mevalonate. Pre-treatment with iJNK-I (a selective JNK inhibitor) prevented the effect of lovastatin on both neuroblast apoptosis and the activation of the JNK cascade. Furthermore, we found that the activation of the JNK signalling pathway triggered by lovastatin is accompanied by caspase-3 activation which is also inhibited by iJNK-I pre-treatment. Finally, a specific inhibitor of p38 MAPK, SB203580, had no effect on lovastatin-induced neuroblast apoptosis. Taken together, our data suggest that the activation of the JNK/c-Jun/BimEL signalling pathway plays a crucial role in lovastatin-induced neuroblast apoptosis. Our findings may also contribute to elucidate the intracellular mechanisms involved in the central nervous system side effects associated with statin therapy.

Lovastatin inhibits the extracellular-signal-regulated kinase pathway in immortalized rat brain neuroblasts

We have shown previously that lovastatin, a 3-hydroxy-3-methyl- glutaryl coenzyme A reductase inhibitor, induces apoptosis in spontaneously immortalized rat brain neuroblasts. In the present study, we analysed the intracellular signal transduction pathways by which lovastatin induces neuroblast apoptosis. We showed that lovastatin efficiently inhibited Ras activation, which was associated with a significant decrease in ERK1/2 (extracellular-signal-regulated kinase 1/2) phosphorylation. Lovastatin also decreased CREB phosphorylation and CREB-mediated gene expression. The effects of lovastatin on the Ras/ERK1/2/CREB pathway were time- and concentration-dependent and fully prevented by mevalonate. In addition, we showed that two MEK [MAPK (mitogen-activated protein kinase)/ERK kinase] inhibitors, PD98059 and PD184352, were poor inducers of apoptosis in serum-treated neuroblasts. However, these inhibitors significantly increased apoptosis induced by lovastatin treatment. Furthermore, we showed that pharmacological inhibition of both MEK and phosphoinositide 3-kinase activities was able to induce neuroblast apoptosis with similar efficacy as lovastatin. Our results suggest that lovastatin triggers neuroblast apoptosis by regulating several signalling pathways, including the Ras/ERK1/2 pathway. These findings might also contribute to elucidate the intracellular mechanisms involved in the central nervous system side effects associated with statin therapy.

CCK1 and 2 receptors are expressed in immortalized rat brain neuroblasts: Intracellular signals after cholecystokinin stimulation

Cholecystokinin (CCK) is one of the most abundant neuropeptides in the central nervous system (CNS) where it promotes important functions by activation of receptors CCK1 and CCK2. Our aim was to investigate CCK receptors expression and their downstream intracellular signaling in immortalized rat brain neuroblasts. Results show that CCK1 and CCK2 receptor mRNAs and CCK2 receptor protein are expressed in neuroblasts. CCK incubation of neuroblasts leads to stimulation in a time-dependent manner of several signaling pathways, such as tyrosine phosphorylation of adaptor proteins paxillin and p130(Cas), phosphorylation of p44/p42 ERKs as well as PKB (Ser473). Moreover, CCK-8 stimulates the DNA-binding activity of the transcription factor AP-1. The CCK2 receptor agonist gastrin stimulates ERK1/2 phosphorylation in a comparable degree as CCK does. ERK1/2 phosphorylation activated by CCK-8 was markedly inhibited by the CCK2 receptor antagonist CR2945. Incubation for 48 h with CCK-8 increases neuroblasts viability in a similar degree as EGF. In summary, our data clearly identify CCK1 and CCK2 receptor mRNAs and CCK2 receptor protein in brain neuroblasts and show that incubation with CCK promotes cell proliferation and activates the phosphorylation of survival transduction pathways. Stimulation of ERK1/2 phosphorylation by CCK is mainly mediated by the CCK2 receptor. Moreover, this work might provide a novel model of proliferating neuronal cells to further study the biochemical mechanisms by which the neuropeptide CCK exerts its actions in the CNS.

Heat shock proteins protect both MPP+ and paraquat neurotoxicity

The exposure of immortalized rat neuroblast cells to MPP(+) and paraquat results in cell death. Heat shock pre-treatment prior to the addition of MPP(+) and paraquat significantly reduced cell death and led to an increase in the synthesis of Hsp 27 and Hsp70 proteins. Quercetin inhibits the synthesis of heat shock proteins (Hsp) and prevents their protective effect, which suggests that this protection was dependent on the Hsps synthesis. These data indicate that heat shock protects cells from the toxic effect of MPP(+) and paraquat. These results and the structural similarity between paraquat and MPP(+) support the role of paraquat as a putative risk factor in the etiology of Parkinson's disease.

Lovastatin inhibits the growth and survival pathway of phosphoinositide 3-kinase/protein kinase B in immortalized rat brain neuroblasts

We previously showed that lovastatin, an HMG-CoA reductase inhibitor, suppresses cell growth by inducing apoptosis in rat brain neuroblasts. Our aim was to study intracellular signalling induced by lovastatin in neuroblasts. Lovastatin significantly decreases the phosphoinositide 3-kinase (PI3-K) activity in a concentration-dependent manner. Expression of p85 subunit and its association with phosphotyrosine-containing proteins are unaffected by lovastatin. Lovastatin decreases protein kinase B (PKB)/Akt phosphorylation, and its downstream effectors, p70S6K and the eukaryotic initiation factor 4E (eIF4E) regulatory protein 1, 4E-BP1, in a concentration-dependent manner, and reduces p70S6K expression. Lovastatin effects are fully prevented with mevalonate. Only the highest dose of PI3-K inhibitors that significantly reduce PI3-K kinase activity induces apoptosis in neuroblasts but to a lower degree than lovastatin. In summary, this work shows that treatment of brain neuroblasts with lovastatin leads to an inhibition of the main pathway that controls cell growth and survival, PI3-K/PKB and the subsequent blockade of downstream proteins implicated in the regulation of protein synthesis. This work suggests that inactivation of the antiapoptotic PI3-K appears insufficient to induce the degree of neuroblasts apoptosis provoked by lovastatin, which must necessarily involve other intracellular pathways. These findings might contribute to elucidate the molecular mechanisms of some statins effects in the central nervous system.

Postnatal thyroid hormone supplementation rescues developmental abnormalities induced by congenital-neonatal hypothyroidism in the rat retina

Thyroid hormones (TH) play a key role in central nervous system development. We have studied the influence of congenital and neonatal hypothyroidism on retinal development and the effects of postnatal TH supplementation. An experimental model was set up using Wistar rats by inducing chemical thyroidectomy during gestation and suckling. Eyes from control (CG) and TH-depleted (THDG) groups of animals were obtained at postnatal days 10 and 25. In the THDG, there was a significant reduction in the retinal thickness and layering, retinal volume, cell number and nuclear volumes in all layers. A third group of rats, made hypothyroid during the gestational and neonatal period and then supplemented with TH (THSG), showed a recovery of both the retinal thickness [at P25: 188.5 +/- 9.2 microm (THSG) vs. 175.8 +/- 16.1 microm (THDG), p < 0.001, and 210.8 +/- 8.9 (CG)] and total retinal cell number [at P25: 6.9 x 10(6) (THSG) vs. 3.7 x 10(6) (THDG) cells, p < 0.001, and 5.3 x 10(6) cells (CG)]. Light and electron microscopy studies confirmed that TH deprivation altered the organization of the retina, which was mostly normalized by hormone administration. Our data show that TH regulates intrinsic mechanisms for controlling retinal cytoarchitecture and layering, and that alterations in retinal maturation induced by congenital-neonatal TH deficiency can be at least partially rescued by early hormonal treatment in vivo.

Thyroid hormone-dependent gene expression in differentiated embryonic stem cells and embryonal carcinoma cells: identification of novel thyroid hormone target genes by deoxyribonucleic acid microarray analysis

T3 is required for normal early development, but relatively few T3-responsive target genes have been identified. In general, in vitro stem cell differentiation techniques stimulate a wide range of developmental programs, including thyroid hormone receptor (TR) pathways. We developed several in vitro stem cell models to more specifically identify TR-mediated gene expression in early development. We found that embryonic carcinoma (EC) cells have reduced T3 nuclear binding capacity and only modestly express the known T3 target genes, neurogranin (RC3) and Ca2+/calmodulin-dependent protein kinase IV (CaMKIV), in response to T3. Full T3 induction in transient transfection of EC cells was restored with cotransfection of a TR expression vector. We, therefore, performed gene expression profiles in wild-type embryonic stem (ES) cells compared with expression in cells with deficient (EC) or mutant TR (TRalpha P398H mutant ES cells), to identify T3 target genes. T3 stimulation of wild-type ES cells altered mRNA expression of 610 known genes (26% of those studied), although only approximately 60 genes (1%) met criteria for direct T3 stimulation based on the magnitude of induction and requirement for the presence of TR. We selected five candidate T3 target genes, neurexophilin 2, spermatid perinuclear RNA-binding protein (SPNR), kallikrein-binding protein (KBP), prostate-specific membrane antigen (PSMA), and synaptotagmin II, for more detailed study. T3 responsiveness of these genes was evaluated in both in vitro endogenous gene expression and in vivo mouse model systems. These genes identified in a novel stem cell system, including those induced and repressed in response to T3, may mediate thyroid hormone actions in early development.

Neuroserpin is post-transcriptionally regulated by thyroid hormone

Neuroserpin is a serine protease inhibitor expressed in the developing and the adult nervous system. Studies with genetically modified mice indicate a role of neuroserpin in the regulation of anxiety. Mutations in the neuroserpin gene cause protein polymerization and formation of inclusion bodies leading to progressive myoclonic epilepsy and neurodegeneration. Here we demonstrate that neuroserpin expression is regulated by thyroid hormone (T3). Neuroserpin RNA levels are down-regulated in cortical layers II/III and VIa, the hippocampus, the retrosplenial cortex and the medial habenular nucleus, but not in cortical layer V or other areas of the hypothyroid rat brain. Concordantly, neuroserpin RNA and protein expression was induced by T3 in rat PC12 cells containing appropriate thyroid hormone receptor levels. In run-on assays T3 did not affect the transcription rate of the neuroserpin gene, indicating that regulation was post-transcriptional. Moreover, T3 increased in vitro binding of cytoplasmic proteins to neuroserpin 3'-UTR RNA and caused biphasic regulation of the stability of this transcript in PC12 cells. Ectopic neuroserpin expression induced neurite extension in PC12 cells and enhanced neuritogenesis triggered by nerve growth factor. In summary, these results indicate that neuroserpin expression is post-transcriptionally regulated by T3 at the level of RNA stability.

The small GTP-binding protein, Rhes, regulates signal transduction from G protein-coupled receptors

The Ras homolog enriched in striatum, Rhes, is the product of a thyroid hormone-regulated gene during brain development. Rhes and the dexamethasone-induced Dexras1 define a novel distinct subfamily of proteins within the Ras family, characterized by an extended variable domain in the carboxyl terminal region. We have carried this study because there is a complete lack of knowledge on Rhes signaling. We show that in PC12 cells, Rhes is targeted to the plasma membrane by farnesylation. We demonstrate that about 30% of the native Rhes protein is bound to GTP and this proportion is unaltered by typical Ras family nucleotide exchange factors. However, Rhes is not transforming in murine fibroblasts. We have also examined the role of Rhes in cell signaling. Rhes does not stimulate the ERK pathway. By contrast, it binds to and activates PI3K. On the other hand, we demonstrate that Rhes impairs the activation of the cAMP/PKA pathway by thyroid-stimulating hormone, and by an activated beta2 adrenergic receptor by a mechanism that suggests uncoupling of the receptor to its cognate heterotrimeric complex. Overall, our results provide the initial insights into the role in signal transduction of this novel Ras family member.

The thyroid hormone receptor beta gene: structure and functions in the brain and sensory systems

Thyroid hormone profoundly influences the development of the vertebrate nervous system. The thyroid hormone receptor beta gene (Thrb) is a key mediator of many of these actions. The Thrb gene is complex, spanning up to 400 kb in mammals, and differentially expresses distinct receptor subtypes through independent tissue-specific promoters and alternative splicing. These receptors serve a range of functions in the brain as well as particularly sensitive functions in the auditory and visual sensory systems. The Thrb gene illustrates how versatility in neurodevelopmental control can be achieved at the receptor level.

Neuronal HuD gene encoding a mRNA stability regulator is transcriptionally repressed by thyroid hormone

Many genes governed by thyroid hormone (T3) lack binding sites for its receptor (TR) and are thought to be post-transcriptionally regulated by T3. Here we demonstrate that the HuD gene, which encodes a neurone-specific protein that binds to mRNA and modulates its stability, is regulated by T3. HuD RNA and protein expression were strongly up-regulated in specific areas of the hypothyroid rat brain, and reduced by T3 in rat PC12 and mouse N2a cells containing appropriate TR levels. Furthermore, T3 inhibited the transcription of HuD in run-on assays. Finally, HuD protein bound with high affinity to two sequences in acetylcholinesterase mRNA, and ectopic HuD expression increased its abundance in N2a cells. This is the first report of a gene encoding an mRNA stability regulator that is under T3 control. The results suggest that HuD may mediate some T3 effects by altering the half-life of mRNAs for acetylcholinesterase and other genes.

N-CAM expression and localization in PC12 cells modulated by extracellular peptides

The neural cell adhesion molecules (N-CAMs) play an important role in mediating cell-cell interactions in the nervous system. Different isoforms of these membrane proteins are involved in the formation of the neuronal network and in the dynamic phases of neuronal plasticity. We studied the early stages of the pseudo neuronal differentiation of PC12 cells induced by a class of small acidic peptides capable of modulating gene expression in these cells. The data presented here indicate that peptides with specific sequences induce an increase in N-CAM mRNA expression and protein translocation to the plasma membrane to a comparable degree as NGF.

A targeted thyroid hormone receptor alpha gene dominant-negative mutation (P398H) selectively impairs gene expression in differentiated embryonic stem cells

Thyroid hormone and retinoic acid (RA) are essential for normal neural development in vivo, yet all in vitro differentiation strategies of embryonic stem (ES) cells use only RA. We developed a novel differentiation strategy of mouse ES cells using T(3). A dominant-negative knock-in point mutation (P398H) was introduced into the thyroid hormone receptor alpha gene to determine the influence of T(3) on ES cell differentiation. Differentiation promoted by T(3) (1 nM), RA (1 microM), or combined T(3)/RA was assessed in wild-type (wt) and mutant (m) ES cells on the basis of neuronal-specific gene expression and cell cycle. T(3) alone stimulated neural differentiation in a similar fashion as that seen with RA in both wtES and mES cells. Expression of neurogranin and Ca(2+)/calmodulin-dependent kinase IV mRNA (identified in vivo as T(3)-regulated genes), however, was markedly reduced in mES, compared with wtES cells. RA treatment enhanced apoptosis, significantly greater than that seen with T(3) stimulation. T(3) treatment given with RA significantly reduced the apoptotic effects of RA, an effect not seen in mES cells. T(3)-induced ES cell neural differentiation of thyroid hormone alpha mutant and wtES cells provides an in vitro model to study T(3)-dependent gene regulation in neural development. This system could also be used to identify novel T(3)-regulated genes. The modulation of the apoptotic effects of RA by T(3) may have implications for stem cell therapy.

c-SRC mediates neurite outgrowth through recruitment of Crk to the scaffolding protein Sin/Efs without altering the kinetics of ERK activation

SRC family kinases have been consistently and recurrently implicated in neurite extension events, yet the mechanism underlying their neuritogenic role has remained elusive. We report that epidermal growth factor (EGF) can be converted from a non-neuritogenic into a neuritogenic factor through moderate activation of endogenous SRC by receptor-protein-tyrosine phosphatase alpha (a physiological SRC activator). We show that such a qualitative change in the response to EGF is not accompanied by changes in the extent or kinetics of ERK induction in response to this factor. Instead, the pathway involved relies on increased tyrosine phosphorylation of, and recruitment of Crk to, the SRC substrate Sin/Efs. The latter is a scaffolding protein structurally similar to the SRC substrate Cas, tyrosine phosphorylation of which is critical for migration in fibroblasts and epithelial cells. Expression of a dominant negative version of Sin interfered with receptor-protein-tyrosine phosphatase alpha/EGF- as well as fibroblast growth factor-induced neurite outgrowth. These observations uncouple neuritogenic signaling in PC12 cells from sustained activation of ERK kinases and for the first time identify an effector of SRC function in neurite extension.

Inhibition of proliferation and expression of T1 and cyclin D1 genes by thyroid hormone in mammary epithelial cells

The relationship between thyroid hormone (triiodothyronine, T(3)) and breast cancer is unclear. We studied the effect of the c-erbA/TR alpha proto-oncogene encoding a functional T(3) receptor (TR alpha 1), of its ligand T(3), and of its retroviral, mutated counterpart, the v-erbA oncogene, on the proliferation capacity of nontumorigenic mammary epithelial cells (EpH4). We found that EpH4 cells expressing ectopically TR (EpH4 + TR alpha 1) or v-erbA (EpH4 + v-erbA) proliferated faster than parental EpH4 cells that contained low levels of endogenous TR. T(3) inhibited DNA synthesis and proliferation in EpH4 + TR alpha 1 cells but not EpH4 or EpH4 + v-erbA cells. The study of cell-cycle genes showed that T(3) decreased cyclin D1 RNA and protein levels in EpH4 + TR alpha 1 cells. In addition, T(3) downregulated the expression of T1, a gene that is overexpressed in human breast adenocarcinomas and is induced by mitogens, serum, and several oncogenes and cytokines. Inhibition of the T1 gene by T(3) required both de novo mRNA and protein synthesis. Furthermore, T(3) abolished the induction of T1 by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate and inhibited the activity of an activation protein 1-dependent promoter (-73-Col-CAT) in EpH4 + TR alpha 1 cells, suggesting that interference with activation protein 1 transcription factor plays a part in the inhibition of the T1 gene. Our results showed that T(3) reduced the proliferation of mammary epithelial cells and inhibited the expression of cyclin D1 and T1 genes.

Thyroid hormone-dependent regulation of Talpha1 alpha-tubulin during brain development

Thyroid hormone (T3) is essential for brain development and most of its actions are exerted at the gene expression level after interaction with nuclear receptors. In particular, genes encoding cytoskeletal proteins are influenced by the thyroidal status. Thyroid hormone is involved in the normal downregulation of the Talpha1 alpha-tubulin gene during postnatal growth. The action of T3 on Talpha1 tubulin expression is complex and is exerted at least at two levels. In cultured cells, T3 induces a transient and fast decrease of Talpha1 mRNA concentration. This effect is enhanced when transcription is blocked by actinomycin D, suggesting that T3 increases mRNA degradation. In transgenic animals T3 affects the expression of beta-galactosidase under control of the Talpha1 promoter in the same way as the endogenous gene, supporting an effect mediated through the Talpha1 promoter. However, the Talpha1 promoter is not regulated by T3 in transfected cells and, therefore, the effects of the hormone in vivo are likely to be indirect. It is concluded that regulation of Talpha1 alpha-tubulin by thyroid hormone is the result of multiple influences including effects on mRNA half life and indirect effects at the promoter level.

Analysis of gene expression following sciatic nerve crush and spinal cord hemisection in the mouse by microarray expression profiling

1. The responses of periphery (PNS) and central nervous systems (CNS) towards nerve injury are different: while injured mammalian periphery nerons can successfully undergo regeneration, axons in the central nervous system are usually not able to regenerate. 2. In the present study, the genes which were differentially expressed in the PNS and CNS following nerve injury were identified and compared by microarray profiling techniques. 3. Sciatic nerve crush and hemisection of the spinal cord of adult mice were used as the models for nerve injury in PNS and CNS respectivey. 4. It was found that of all the genes examined, 14% (80/588) showed changes in expression following either PNS or CNS injury, and only 3% (18/588) showed changes in both types of injuries. 5. Among all the differentially expressed genes, only 8% (6/80) exhibited similar changes in gene expression (either up- or down-regulation) following injury in both PNS and CNS nerve injuries. 6. Our results indicated that microarray expression profiling is an efficient and useful method to identify genes that are involved in the regeneration process following nerve injuries, and several genes which are differentially expressed in the PNS and/or CNS following nerve injuries were identified in the present study.

Thyroid hormone regulates the obesity gene tub

Thyroid hormone T3/T4 is a major regulator of energy metabolism in vertebrates, and defects in thyroid status are frequently associated with changes in body weight. It is demonstrated here that thyroid hormone regulates in vivo and in vitro the tub gene, which when mutated in tubby mice causes obesity, insulin resistance and sensory deficits. Hypothyroidism in rats altered tub mRNA and protein in discrete brain areas. These changes could be attributed to thyroid hormone deficiency since T3/T4 treatment restored normal tub expression. T3 also upregulated tub mRNA within 4-6 h in neuronal cells in culture, suggesting that T3 is a positive regulator of tub gene expression. Thus, these results establish a novel pathway of T3 action and provide an important molecular link between thyroid status and the tubby-associated syndrome.

Lovastatin induces apoptosis of spontaneously immortalized rat brain neuroblasts: involvement of nonsterol isoprenoid biosynthesis inhibition

We have examined the effects of lovastatin and pravastatin (competitive HMG-CoA reductase inhibitors) on the growth and survival of rat brain neuroblasts. Lovastatin, but not pravastatin, suppressed cell growth by inducing apoptosis of neuroblasts in a dose- and time-dependent manner. Apoptosis was accompanied by a decrease in both Bcl-2 and Bcl-xL protein levels, suggesting that changes in the expression of these genes may contribute to apoptosis following lovastatin treatment. Lovastatin treatment was also associated with decreased prenylation of both Ras and Rho A proteins whereas Rac 1 geranylgeranylation was not affected. Lovastatin effects were fully prevented by mevalonate. The present data suggest that lovastatin induces apoptosis of rat brain neuroblasts by its capacity to decrease the prenylation of specific proteins involved in signal transduction pathways that control growth and survival of neuronal cells.

Regulation of the L1 cell adhesion molecule by thyroid hormone in the developing brain

Thyroid hormone is essential for brain maturation, regulating neuronal differentiation and migration, myelination, and synaptogenesis. Mutations in the cell adhesion molecule L1 cause severe neurological abnormalities in humans. We studied the effect of thyroid hormone deprivation and administration on L1 expression. Northern and in situ hybridization studies showed that hypothyroidism induces a marked increase in L1 mRNA levels in the caudate putamen, cerebral cortex, amygdala, and some thalamic nuclei. L1 protein was overexpressed in embryonic and newborn hypothyroid rats in the caudate putamen, internal capsule, habenula, and neocortex. Later in development, an abnormally high L1 expression was found in the cortical and cerebellar white matter, corpus callosum, anterior commissure, thalamocortical projections, and striatal fiber tracts of hypothyroid animals. Thyroid hormone administration reversed the upregulation of L1 expression in vivo and in cultured cells. Thus, alterations of L1 expression may contribute to the profound abnormalities caused by hypothyroidism in the developing brain.

Thyroidal dysfunction and environmental chemicals--potential impact on brain development

Certain polyhalogenated aromatic hydrocarbons such as polychlorinated biphenyls (PCBs) and dibenzo-p-dioxins (dioxins, 2,3,7, 8-tetrachlorodibenzo-p-dioxin) have been shown to have neurotoxic effects and to alter thyroid function during critical periods of thyroid hormone-dependent brain development. This has led to the suggestion that some of the neurotoxic effects of these compounds could be mediated through the thyroid system. Thyroid hormones are essential for normal brain development during a critical period beginning in utero and extending through the first 2 years postpartum. They regulate neuronal proliferation, migration, and differentiation in discrete regions of the brain during definitive time periods. Even transient disruption of this normal pattern can impair brain development. Thyroid hormones are necessary for normal cytoskeletal assembly and stability and the cytoskeletal system is essential for migration and neuronal outgrowth. In addition, they regulate development of cholinergic and dopaminergic systems serving the cerebral cortex and hippocampus. Animals perinatally exposed to certain environmental organohalogens such as many of the PCBs and dioxins have abnormal thyroid function and neurologic impairment. Although there are both species and congener variabilities, most reports show exposure results in thyroid enlargement and reduced serum T(4) levels with normal T(3) levels. Initial research concentrated on studying the direct actions of xenobiotics on the thyroid; however, some of these compounds bear a structural resemblance to the natural thyroid hormones and have high affinity with thyroid hormone-binding proteins such as transthyretin. These compounds could act as agonists or antagonists for receptors of the thyroid/steroid/retinoic acid superfamily. These structurally similar organohalogens could act at multiple points to alter thyroid hormone action. The similarity of the neurologic impairment seen in thyroid disorders to that seen following PCB or dioxin exposure suggests that one mechanism of neurotoxicity of these compounds could involve interaction with the thyroid system.

Thyroidal dysfunction and environmental chemicals--potential impact on brain development

Certain polyhalogenated aromatic hydrocarbons such as polychlorinated biphenyls (PCBs) and dibenzo-p-dioxins (dioxins, 2,3,7, 8-tetrachlorodibenzo-p-dioxin) have been shown to have neurotoxic effects and to alter thyroid function during critical periods of thyroid hormone-dependent brain development. This has led to the suggestion that some of the neurotoxic effects of these compounds could be mediated through the thyroid system. Thyroid hormones are essential for normal brain development during a critical period beginning in utero and extending through the first 2 years postpartum. They regulate neuronal proliferation, migration, and differentiation in discrete regions of the brain during definitive time periods. Even transient disruption of this normal pattern can impair brain development. Thyroid hormones are necessary for normal cytoskeletal assembly and stability and the cytoskeletal system is essential for migration and neuronal outgrowth. In addition, they regulate development of cholinergic and dopaminergic systems serving the cerebral cortex and hippocampus. Animals perinatally exposed to certain environmental organohalogens such as many of the PCBs and dioxins have abnormal thyroid function and neurologic impairment. Although there are both species and congener variabilities, most reports show exposure results in thyroid enlargement and reduced serum T(4) levels with normal T(3) levels. Initial research concentrated on studying the direct actions of xenobiotics on the thyroid; however, some of these compounds bear a structural resemblance to the natural thyroid hormones and have high affinity with thyroid hormone-binding proteins such as transthyretin. These compounds could act as agonists or antagonists for receptors of the thyroid/steroid/retinoic acid superfamily. These structurally similar organohalogens could act at multiple points to alter thyroid hormone action. The similarity of the neurologic impairment seen in thyroid disorders to that seen following PCB or dioxin exposure suggests that one mechanism of neurotoxicity of these compounds could involve interaction with the thyroid system.

Thyroid hormone regulates reelin and dab1 expression during brain development

The reelin and dab1 genes are necessary for appropriate neuronal migration and lamination during brain development. Since these processes are controlled by thyroid hormone, we studied the effect of thyroid hormone deprivation and administration on the expression of reelin and dab1. As shown by Northern analysis, in situ hybridization, and immunohistochemistry studies, hypothyroid rats expressed decreased levels of reelin RNA and protein during the perinatal period [embryonic day 18 (E18) and postnatal day 0 (P0)]. The effect was evident in Cajal-Retzius cells of cortex layer I, as well as in layers V/VI, hippocampus, and granular neurons of the cerebellum. At later ages, however, Reelin was more abundant in the cortex, hippocampus, cerebellum, and olfactory bulb of hypothyroid rats (P5), and no differences were detected at P15. Conversely, Dab1 levels were higher at P0, and lower at P5 in hypothyroid animals. In line with these results, reelin RNA and protein levels were higher in cultured hippocampal slices from P0 control rats compared to those from hypothyroid animals. Significantly, thyroid-dependent regulation of reelin and dab1 was confirmed in vivo and in vitro by hormone treatment of hypothyroid rats and organotypic cultures, respectively. In both cases, thyroid hormone led to an increase in reelin expression. Our data suggest that the effects of thyroid hormone on neuronal migration may be in part mediated through the control of reelin and dab1 expression during brain ontogenesis.

Identification of the mammalian homolog of the splicing regulator Suppressor-of-white-apricot as a thyroid hormone regulated gene

Mammalian brain development is controlled by thyroid hormone through the regulation of target genes. In this study, we describe for the first time that a splicing regulator gene is under thyroid hormone control in the rat brain during the critical period of neuronal differentiation. By differential display, we have identified the mammalian homolog of the Drosophila splicing regulator Suppressor-of-white-apricot (SWAP) as a thyroid hormone-regulated gene in an immortal line of rat neuroblasts, E18 cells. Using Northern blotting and in situ hybridization, we found that expression of SWAP is under thyroid control in the developing rat brain. SWAP gene expression is highest during the first 10 days of life (P0-P10), preferentially in cerebral cortex, cerebellum, subventricular epithelium, piriform cortex, hippocampus, amygdala, and caudate putamen. At later stages (P15-P30) SWAP expression decreases, being detectable only in the cerebellum, hippocampus, and layers II/III of cerebral and piriform cortexes. We found that hypothyroidism causes an abnormal high level of SWAP RNA expression at P5-P15 throughout the brain except the cerebellum. Significantly, thyroid hormone treatment in vivo of hypothyroid animals led to a normalization of SWAP RNA expression. Furthermore, similar hormone treatment caused a decrease in SWAP expression in control rats. By modulating the expression of SWAP and perhaps other splicing regulators thyroid hormone may exert wide regulatory effects on multiple genes. The regulation of SWAP gene defines a novel mechanism of action of thyroid hormone which can be important for its effects in the developing brain.