Non-genomic actions of thyroid hormone in brain development

Thyroid hormone membrane receptor binding and transcriptional regulation in the sea urchin Strongylocentrotus purpuratus

Thyroid hormones (THs) are small amino acid derived signaling molecules with broad physiological and developmental functions in animals. Specifically, their function in metamorphic development, ion regulation, angiogenesis and many others have been studied in detail in mammals and some other vertebrates. Despite extensive reports showing pharmacological responses of invertebrate species to THs, little is known about TH signaling mechanisms outside of vertebrates. Previous work in sea urchins suggests that non-genomic mechanisms are activated by TH ligands. Here we show that several THs bind to sea urchin (Strongylocentrotus purpuratus) cell membrane extracts and are displaced by ligands of RGD-binding integrins. A transcriptional analysis across sea urchin developmental stages shows activation of genomic and non-genomic pathways in response to TH exposure, suggesting that both pathways are activated by THs in sea urchin embryos and larvae. We also provide evidence associating TH regulation of gene expression with TH response elements in the genome. In ontogeny, we found more differentially expressed genes in older larvae compared to gastrula stages. In contrast to gastrula stages, the acceleration of skeletogenesis by thyroxine in older larvae is not fully inhibited by competitive ligands or inhibitors of the integrin membrane receptor pathway, suggesting that THs likely activate multiple pathways. Our data confirms a signaling function of THs in sea urchin development and suggests that both genomic and non-genomic mechanisms play a role, with genomic signaling being more prominent during later stages of larval development.

Şizofreni hastalarında TSH, fT3 ve fT4 düzeylerinin nörobilişsel belirtiler üzerine etkisi

Amaç: Bu çalışmamızda ötiroid psikoz hastalarında tiroid hormon düzeyleri ile pozitif, negatif, genel ve bilişsel belirtiler arasındaki ilişkiyi incelemeyi amaçladık. Gereç ve Yöntem: Çalışmaya 33 şizofreni hastası dahil edildi. Katılımcıların sosyodemografik ve klinik verileri, Pozitif ve Negatif Belirtileri Değerlendirme Ölçeği (PANSS), İşlevselliğin Genel Değerlendirilmesi Ölçeği (IGD), Klinik Global İzlenim Ölçeği (KGI) skorları kaydedildi. İz sürme, Sözel Akıcılık Testi, İleri ve Geri Sayı Menzili ile Wisconsin Kart Sayma Testinden (WKST) oluşan bir nörobilişsel batarya uygulanan hastaların bilişsel performanslarının TSH, sT3 ve sT4 düzeyleri ile ilişkisi araştırıldı. Bulgular: sT4 düzeyleri ile İz Sürme B (r=-,376; p=0,031) ve Sözel Akıcılık test (p=,355; p=0,043) performansları arasında anlamlı derecede ilişki saptandı. Linear regresyon modelinde sT3 düzeylerinin WKST- tamamlanan kategori sayısını (UB=1.680; p=0.029) anlamlı derecede yordadığı saptandı.. Sonuç: Çalışmamızın sonuçları sT3 ün özellikle yürütücü işlevler üzerinde sT4’e kıyasla daha belirleyici etkileri olduğunu düşündürmüştür. İleride yapılacak daha geniş katılımlı araştırmalar, bilişsel belirtilerin tedavisinde tiroid hormon replasmanının etkinliğini belirlemek açısından önemli görünmektedir.

The Role of Thyroid Hormone in Neuronal Protection

Thyroid hormone is essential for brain development and brain function in the adult. During development, thyroid hormone acts in a spatial and temporal-specific manner to regulate the expression of genes essential for normal neural cell differentiation, migration, and myelination. In the adult brain, thyroid hormone is important for maintaining normal brain function. Thyroid hormone excess, hyperthyroidism, and thyroid hormone deficiency, hypothyroidism, are associated with disordered brain function, including depression, memory loss, impaired cognitive function, irritability, and anxiety. Adequate thyroid hormone levels are required for normal brain function. Thyroid hormone acts through a cascade of signaling components: activation and inactivation by deiodinase enzymes, thyroid hormone membrane transporters, and nuclear thyroid hormone receptors. Additionally, the hypothalamic-pituitary-thyroid axis, with negative feedback of thyroid hormone on thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion, regulates serum thyroid hormone levels in a narrow range. Animal and human studies have shown both systemic and local reduction in thyroid hormone availability in neurologic disease and after brain trauma. Treatment with thyroid hormone and selective thyroid hormone analogs has resulted in a reduction in injury and improved recovery. This article will describe the thyroid hormone signal transduction pathway in the brain and the role of thyroid hormone in the aging brain, neurologic diseases, and the protective role when administered after traumatic brain injury. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Involvement of Thyroid Hormones in Brain Development and Cancer

The development and maturation of the mammalian brain are regulated by thyroid hormones (THs). Both hypothyroidism and hyperthyroidism cause serious anomalies in the organization and function of the nervous system. Most importantly, brain development is sensitive to TH supply well before the onset of the fetal thyroid function, and thus depends on the trans-placental transfer of maternal THs during pregnancy. Although the mechanism of action of THs mainly involves direct regulation of gene expression (genomic effects), mediated by nuclear receptors (THRs), it is now clear that THs can elicit cell responses also by binding to plasma membrane sites (non-genomic effects). Genomic and non-genomic effects of THs cooperate in modeling chromatin organization and function, thus controlling proliferation, maturation, and metabolism of the nervous system. However, the complex interplay of THs with their targets has also been suggested to impact cancer proliferation as well as metastatic processes. Herein, after discussing the general mechanisms of action of THs and their physiological effects on the nervous system, we will summarize a collection of data showing that thyroid hormone levels might influence cancer proliferation and invasion.

Role of thyroid hormones in normal and abnormal central nervous system myelination in humans and rodents

Thyroid hormones (THs) are instrumental in promoting the molecular mechanisms which underlie the complex nature of neural development and function within the central nervous system (CNS) in vertebrates. The key neurodevelopmental process of myelination is conserved between humans and rodents, of which both experience peak fetal TH concentrations concomitant with onset of myelination. The importance of supplying adequate levels of THs to the myelin producing cells, the oligodendrocytes, for promoting their maturation is crucial for proper neural function. In this review we examine the key TH distributor and transport proteins, including transthyretin (TTR) and monocarboxylate transporter 8 (MCT8), essential for supporting proper oligodendrocyte and myelin health; and discuss disorders with impaired TH signalling in relation to abnormal CNS myelination in humans and rodents. Furthermore, we explore the importance of using novel TH analogues in the treatment of myelination disorders associated with abnormal TH signalling.

Thyroid hormone: sex-dependent role in nervous system regulation and disease

Thyroid hormone (TH) regulates many functions including metabolism, cell differentiation, and nervous system development. Alteration of thyroid hormone level in the body can lead to nervous system-related problems linked to cognition, visual attention, visual processing, motor skills, language, and memory skills. TH has also been associated with neuropsychiatric disorders including schizophrenia, bipolar disorder, anxiety, and depression. Males and females display sex-specific differences in neuronal signaling. Steroid hormones including testosterone and estrogen are considered to be the prime regulators for programing the neuronal signaling in a male- and female-specific manner. However, other than steroid hormones, TH could also be one of the key signaling molecules to regulate different brain signaling in a male- and female-specific manner. Thyroid-related diseases and neurological diseases show sex-specific incidence; however, the molecular mechanisms behind this are not clear. Hence, it will be very beneficial to understand how TH acts in male and female brains and what are the critical genes and signaling networks. In this review, we have highlighted the role of TH in nervous system regulation and disease outcome and given special emphasis on its sex-specific role in male and female brains. A network model is also presented that provides critical information on TH-regulated genes, signaling, and disease.

The Intrinsic Activity of Thyroxine Is Critical for Survival and Growth and Regulates Gene Expression in Neonatal Liver

Background: Thyroxine (T4) is generally considered to be a prohormone that requires conversion to triiodothyronine (T3) to exert biological activity. Although evidence suggests that T4 has intrinsic activity, it is questionable if this activity has any physiological relevance. Methods: To answer this question, triple knockout (KO) mice (Triples) that cannot express the types 1 (D1) and 2 (D2) deiodinase and the Pax8 genes were generated. Thus, they lack a thyroid and cannot convert T4 to T3. Triples were injected on alternate days with either vehicle or physiological doses of T4, T3, or T3+T4 from postnatal days 2-14. They were euthanized at P15, and RNA-seq was employed to profile gene expression in the liver. In another experiment, Pax8KO mice were injected with T3, T4, or T4+T3, and growth rate and survival to P84 were determined. Results: The growth retardation of Triples was not improved by either T3 or T4 alone but was significantly improved by T4+T3. In the liver, T4 significantly regulated the expression of genes that were also regulated by T3, but the proportion of genes that were negatively regulated was higher in mice treated with T4 than in mice treated with T3. Treatment with T4+T3 identified genes that were regulated synergistically by T3 and T4, and genes that were regulated only by T4+T3. Analysis of these genes revealed enrichment in mechanisms related to cell proliferation and cholesterol physiology, suggesting a unique contribution of T4 to these biological functions. Pax8KO mice all survived to P84 when injected with T4 or T4+T3. However, survival rate with T3 was only 50% and 10% at 3.5 and 12 weeks of life, respectively. Conclusions: T4 has intrinsic activity in vivo and is critical for survival and growth. At a physiological level, T4 per se can upregulate or downregulate many T3 target genes in the neonatal liver. While most of these genes are also regulated by T3, subsets respond exclusively to T4 or demonstrate enhanced or normalized expression only in the presence of both hormones. These studies demonstrate for the first time a complex dependency on both T4 and T3 for normal mammalian growth and development.

Nongenomic Actions of Thyroid Hormone: The Integrin Component

The extracellular domain of plasma membrane integrin αvβ3 contains a cell surface receptor for thyroid hormone analogues. The receptor is largely expressed and activated in tumor cells and rapidly dividing endothelial cells. The principal ligand for this receptor is l-thyroxine (T₄), usually regarded only as a prohormone for 3,5,3'-triiodo-l-thyronine (T₃), the hormone analogue that expresses thyroid hormone in the cell nucleus via nuclear receptors that are unrelated structurally to integrin αvβ3. At the integrin receptor for thyroid hormone, T₄ regulates cancer and endothelial cell division, tumor cell defense pathways (such as anti-apoptosis), and angiogenesis and supports metastasis, radioresistance, and chemoresistance. The molecular mechanisms involve signal transduction via mitogen-activated protein kinase and phosphatidylinositol 3-kinase, differential expression of multiple genes related to the listed cell processes, and regulation of activities of other cell surface proteins, such as vascular growth factor receptors. Tetraiodothyroacetic acid (tetrac) is derived from T₄ and competes with binding of T₄ to the integrin. In the absence of T₄, tetrac and chemically modified tetrac also have anticancer effects that culminate in altered gene transcription. Tumor xenografts are arrested by unmodified and chemically modified tetrac. The receptor requires further characterization in terms of contributions to nonmalignant cells, such as platelets and phagocytes. The integrin αvβ3 receptor for thyroid hormone offers a large panel of cellular actions that are relevant to cancer biology and that may be regulated by tetrac derivatives.

Maternal and fetal thyroid dysfunction following porcine reproductive and respiratory syndrome virus2 infection

To better understand the host response to porcine reproductive and respiratory virus-2 (PRRSV2) we evaluated circulating thyroid hormone and associated gene expression in a late gestation challenge model. Pregnant gilts were inoculated at gestation day 85 and fetal samples collected at either 12 or 21 days post-infection (dpi). A subset of fetuses was selected for analysis based on viability and viral load categorized as either uninfected-viable (UNIF), high viral load viable (HV-VIA) or high viral load meconium stained (HV-MEC) and were compared with gestational age matched controls (CON). In dams, circulating levels of total T3 and T4 decreased in the acute period following infection and rebounded by 21 dpi. A similar effect was observed in fetuses, but was largely restricted to HV-VIA and HV-MEC, with minimal decrease noted in UNIF relative to CON at 21 dpi. Gene expression in fetal heart at 12 dpi showed significant decompensatory transcription of thyroid hormone transporters (SLC16A2) and deiodinases (DIO2, DIO3), which was not observed in brain. Correspondingly, genes associated with cell cycle progression (CDK1,2,4) were downregulated in only the heart of highly infected fetuses, while expression of their inhibitor (CDKN1A) was upregulated in both tissues. Finally, expression of genes associated with cardiac stress including CAMKD and AGT were upregulated in the hearts of highly infected fetuses, and a shift in expression of MYH6 to MYH7 was observed in HV-MEC fetuses specifically. Collectively, the results suggest PRRSV2 infection causes a hypothyroid state that disproportionally impacts the fetal heart over the brain.

Free Thyroxine Concentrations Moderate the Response to a Cognitive Remediation Therapy in People With Early Psychosis: A Pilot Randomized Clinical Trial

INTRODUCTION

Cognitive deficits are a cause of functional disability in psychotic disorders. Cognitive remediation therapy (CRT) might be applied to improve these deficits. We conducted a pilot study to explore whether thyroid hormones might predict the response to CRT in patients with recent-onset psychosis (ROP).

METHODS

Twenty-eight stable ROP outpatients (9 women) were randomized to receive computerized CRT (N=14) or treatment as usual (TAU) (N=14), over three months. Both cognitive and thyroid functions were assessed at the baseline and after those three months to all patients. A full cognitive battery (CANTAB) was administered before and after the treatment. Serum levels of thyroid-stimulating hormone (TSH) and free thyroxine (FT4) were measured. FT4 concentrations were recoded into a dichotomic variable (FT4 group) based on the median of the sample (1.2 ng/dL). Data were analyzed on an intention-to-treat basis with linear mixed models. Afterwards, we offered CRT to all participants from the TAU group and seven enrolled CRT, reassessing them when finished. Secondary analyses were repeated in a sample of 14 participants who completed the CRT (either from the beginning or after the TAU period) and attended at least one third of the sessions.

RESULTS

The linear mixed models showed a significant time x CRT x FT4 group effect in two cognitive tasks dealing with executive functions and sustained attention (participants with higher FT4 concentrations worsened executive functions but improved sustained attention after CRT). In the secondary analysis including all patients assigned to CRT, higher FT4 concentrations were associated with a poorer response in verbal memory but a better response in spatial working memory.

CONCLUSIONS

Free thyroxine concentrations moderate the response to a CRT in patients with early psychosis.

Systemic endocrinopathies (thyroid conditions and diabetes): impact on postnatal life of the offspring

Fetal programming may influence childhood and adult life, determining the risk of specific diseases. During earlier stages of pregnancy, the transfer of maternal thyroid hormones to the fetus is vital for adequate neurologic development. The presence of severe maternal thyroid dysfunction, particularly severe iodine deficiency, is devastating, leading to irreversible neurologic sequelae. Moreover, mild maternal thyroid conditions, such as a mild-to-moderate iodine deficiency, may also lead to milder neurologic and behavioral conditions later during the life of the offspring. Maternal dysglycemia due to pregestational or gestational diabetes mellitus is another common situation in which fetal development encounters a hostile environment. Hyperglycemia in utero may trigger metabolic conditions in the offspring, including abnormalities of glucose tolerance and weight excess. Physicians assisting pregnant women have to be aware about these conditions, because they may go unnoticed if not properly screened. Because an early diagnosis and appropriate management may prevent most of the possible negative consequences for the progeny, the prevention, early diagnosis, and proper management of these endocrine conditions should be offered to all women undergoing pregnancy. Here, we comprehensively review the current evidence about the effects of maternal thyroid dysfunction and maternal dysglycemia on the cognitive function and carbohydrate metabolism in the offspring, two prevalent conditions of utmost importance for the child's health and development.

Thyroid hormone metabolites and analogues

Several metabolic products that derive from L-thyroxine (T4) and 3,3'5-L-triiodothyronine (T3), the main thyroid hormones secreted by the thyroid gland, possess biologic activities. Among these metabolites or derivatives showing physiological actions some have received greater attention: diiodothyronines, iodothyronamines, acetic acid analogues. It is known that increased thyroid hormone (T3 and T4) levels can improve serum lipid profiles and reduce body fat. These positive effects are, however, counterbalanced by adverse effects on the heart, muscle and bone, limiting their use. In addition to the naturally occurring metabolites, thyroid hormone analogues have been developed that either have selective effects on specific tissues or bind selectively to thyroid hormone receptor (TR) isoform. Among these GC-1, KB141, KB2115, and DITPA were deeply investigated and displayed promising therapeutic results in the potential treatment of conditions such as dyslipidemias and obesity. In this review, we summarize the current knowledge of metabolites and analogues of T4 and T3 with reference to their possible clinical application in the treatment of human diseases.

Molecular Actions of Thyroid Hormone on Breast Cancer Cell Migration and Invasion via Cortactin/N-WASP

The thyroid hormone triiodothyronine (T3) plays a fundamental role in growth regulation, differentiation, metabolism and cellular movement. These processes are particularly important considering that deregulation of T3 levels could promote abnormal responsiveness of mammary epithelial cells, which may lead to the development and progression of breast cancer (BC). Once cells migrate and invade different tissues, BC metastasis is the main cause of cancer-related death because it is particularly difficult to revert this multistep process. Cell migration integrates several steps that induce changes in cell structure and morphology to promote BC cell invasion. These sequential steps include actin cytoskeleton remodeling, focal adhesion complex formation and, finally, the turnover of branched actin filament networks. In this article, we demonstrate that T3 has the ability to modify the Epithelial-Mesenchymal Transition process. In addition, we show that T3 induces actin cytoskeleton reorganization, triggers focal adhesion formation and, as a consequence, promotes actin nucleation via non-genomic pathway. These events are specifically modulated by T3 via integrin αvβ3 to FAK/paxillin/cortactin/N-WASP/Arp2/3 complex signaling pathway, increasing cell adhesion, migration and invasion of T-47D BC cells. We suggest that T3 influences the progression of tumor metastasis by controlling signaling pathways that converge in cell motility. This knowledge is crucial for the development of novel therapeutic strategies for BC treatment.

Reverse T3 interacts with αvβ3 integrin receptor and restores enzyme activities in the hippocampus of hypothyroid developing rats: Insight on signaling mechanisms

In the present study we provide evidence that 3,3',5'-triiodothyronine (reverse T₃, rT₃) restores neurochemical parameters induced by congenital hypothyroidism in rat hippocampus. Congenital hypothyroidism was induced by adding 0.05% propylthiouracil in the drinking water from gestation day 8 and continually up to lactation day 15. In the in vivo rT₃ exposure, hypothyroid 12-day old pups were daily injected with rT₃ (50 ng/kg body weight) or saline until day 14. In the ex vivo rT₃ treatment, hippocampal slices from 15-day-old hypothyroid pups were incubated for 30 min with or without rT₃ (1 nM). We found that ex vivo and/or in vivo exposure to rT₃ failed in restoring the decreased ¹⁴C-glutamate uptake; however, restored the phosphorylation of glial fibrillary acidic protein (GFAP), ⁴⁵Ca²⁺ influx, aspartate transaminase (AST), glutamine synthetase (GS) and gamma-glutamate transferase (GGT) activities, as well as glutathione (GSH) levels in hypothyroid hippocampus. In addition, rT₃ improved ¹⁴C-2-deoxy-D-glucose uptake and lactate dehydrogenase (LDH) activity. Receptor agonists/antagonists (RGD peptide and AP-5), kinase inhibitors of p38MAPK, ERK1/2, CaMKII, PKA (SB239063, PD98059, KN93 and H89, respectively), L-type voltage-dependent calcium channel blocker (nifedipine) and intracellular calcium chelator (BAPTA-AM) were used to determine the mechanisms of the nongenomic rT₃ action on GGT activity. Using molecular docking analysis, we found rT₃ interaction with αvβ3 integrin receptors, nongenomically activating signaling pathways (PKA, CaMKII, p38MAPK) that restored GGT activity. We provide evidence that rT₃ is an active TH metabolite and our results represent an important contribution to elucidate the nonclassical mechanism of action of this metabolite in hypothyroidism.

Thyroid Hormone Enhances Neurite Outgrowth in Neuroscreen 1 Cells

Objectives

Alzheimer's disease (AD) is a neurodegenerative disorder that affects millions of individuals. Moreover, hypothyroidism has been identified as one of the risk factors that may contribute to the development of AD. Here, we investigated whether there was a correlation among expression levels of proteins involved in the formation of AD lesions, neurite outgrowth, and thyroid hormone levels.

Methods

Cells were grown in media supplemented with different levels of 3,5,3'-triiodothyronine (T3) and then processed for neurite outgrowth and to prepare RNA samples. RNA samples were analysed using quantitative real-time PCR. Protein levels were measured using in cell-Western blotting analysis.

Results

By using neurite outgrowth studies, it was demonstrated that T3 treatment enhanced neurite outgrowth in NS-1 cells in a time- and dose-dependent manner. Quantitative real-time PCR studies further confirmed that NS-1 cells expressed substantial levels of TRα and significantly less TRβ, either of which could be responsible for the T3-dependent effects on neurite outgrowth. Although the overall tau protein expression was not affected in response to T3 treatment, the splicing of tau exon 10 was impacted in the direction of producing more tau molecules that excluded the exon (tau 3R).

Conclusion

The results of this study are critical not only to understand the probable link between hypothyroidism and AD but also in providing the basis for future prevention and treatment of AD in hypothyroid patients.

Thyroid hormone- and estrogen receptor interactions with natural ligands and endocrine disruptors in the cerebellum

Although the effects of phytoestrogens on brain function is widely unknown, they are often regarded as "natural" and thus as harmless. However, the effects of phytoestrogens or environmental pollutants on brain function is underestimated. Estrogen (17beta-estradiol, E2) and thyroid hormones (THs) play pivotal roles in brain development. In the mature brain, these hormones regulate metabolism on cellular and organismal levels. Thus, E2 and THs do not only regulate the energy metabolism of the entire organism, but simultaneously also regulate important homeostatic parameters of neurons and glia in the CNS. It is, therefore, obvious that the mechanisms through which these hormones exert their effects are pleiotropic and include both intra- and intercellular actions. These hormonal mechanisms are versatile, and the experimental investigation of simultaneous hormone-induced mechanisms is technically challenging. In addition, the normal physiological settings of metabolic parameters depend on a plethora of interactions of the steroid hormones. In this review, we discuss conceptual and experimental aspects of the gonadal and thyroid hormones as they relate to in vitro models of the cerebellum.

Triiodothyronine differentially modulates the LH and FSH synthesis and secretion in male rats

Hypothyroidism and thyrotoxicosis produce adverse effects in male reproduction by unknown mechanisms. We investigated whether triiodothyronine (T3) modulates luteinizing hormone (LH) and follicle stimulating hormone (FSH) synthesis/secretion, by inducing different thyroid states. In hypothyroidism, the content of Lhb and Fshb mRNAs was increased, while their association to ribosomes and the protein content were reduced and the serum LH and FSH concentrations were augmented and decreased, respectively. Thyrotoxicosis reduced Lhb mRNA and LH serum concentration, and increased Lhb mRNA translational rate. The Fshb mRNA content and its association to ribosomes were also increased, whereas FSH serum concentrations were comparable to euthyroid levels. Acute T3 treatment decreased the total content of Lhb and Fshb mRNAs, and increased their association to ribosomes, as well as the LHB and FSHB contents in secretory granules. This study shows that T3 acts on gonadotrophs, resulting in direct effects on LH and FSH synthesis/secretion of male rats, suggesting that some reproductive disorders observed in men may be associated with thyroid hormone imbalances.

Novel aspects of T3 actions on GH and TSH synthesis and secretion: physiological implications

Thyroid hormones (THs) classically regulate the gene expression by transcriptional mechanisms. In pituitary, the encoding genes for growth hormone (GH) and thyroid-stimulating hormone (TSH) are examples of genes regulated by triiodothyronine (T₃) in a positive and negative way, respectively. Recent studies have shown a rapid adjustment of GH and TSH synthesis/secretion induced by T₃ posttranscriptional actions. In somatotrophs, T₃ promotes an increase in Gh mRNA content, poly(A) tail length and binding to the ribosome, associated with a rearrangement of actin cytoskeleton. In thyrotrophs, T₃ reduces Tshb mRNA content, poly(A) tail length and its association with the ribosome. In parallel, it promotes a redistribution of TSH secretory granules to more distal regions of the cell periphery, indicating a rapid effect of T₃ inhibition of TSH secretion. T₃ was shown to affect the content of tubulin and the polymerization of actin and tubulin cytoskeletons in the whole anterior pituitary gland, and to increase intracellular alpha (CGA) content. This review summarizes genomic and non-genomic/posttranscriptional actions of TH on the regulation of several steps of GH and TSH synthesis and secretion. These distinct mechanisms induced by T₃ can occur simultaneously, even though non-genomic effects are promptly elicited and precede the genomic actions, coexisting in a functional network within the cells.

Mosaic Expression of Thyroid Hormone Regulatory Genes Defines Cell Type-Specific Dependency in the Developing Chicken Cerebellum

The cerebellum is a morphologically unique brain structure that requires thyroid hormones (THs) for the correct coordination of key cellular events driving its development. Unravelling the interplay between the multiple factors that can regulate intracellular TH levels is a key step to understanding their role in the regulation of these cellular processes. We therefore investigated the regional/cell-specific expression pattern of TH transporters and deiodinases in the cerebellum using the chicken embryo as a model. In situ hybridisation revealed expression of the TH transporters monocarboxylate transporter 8 (MCT8) and 10 (MCT10), L-type amino acid transporter 1 (LAT1) and organic anion transporting polypeptide 1C1 (OATP1C1) as well as the inactivating type 3 deiodinase (D3) in the fourth ventricle choroid plexus, suggesting a possible contribution of the resulting proteins to TH exchange and subsequent inactivation of excess hormone at the blood-cerebrospinal fluid barrier. Exclusive expression of LAT1 and the activating type 2 deiodinase (D2) mRNA was found at the level of the blood-brain barrier, suggesting a concerted function for LAT1 and D2 in the direct access of active T₃ to the developing cerebellum via the capillary endothelial cells. The presence of MCT8 mRNA in Purkinje cells and cerebellar nuclei during the first 2 weeks of embryonic development points to a potential role of this transporter in the uptake of T₃ in central neurons. At later stages, together with MCT10, detection of MCT8 signal in close association with the Purkinje cell dendritic tree suggests a role of both transporters in TH signalling during Purkinje cell synaptogenesis. MCT10 was also expressed in late-born cells in the rhombic lip lineage with a clear hybridisation signal in the outer external granular layer, indicating a potential role for MCT10 in the proliferation of granule cell precursors. By contrast, expression of D3 in the first-born rhombic lip-derived population may serve as a buffering mechanism against high T₃ levels during early embryonic development, a hypothesis supported by the pattern of expression of a fluorescent TH reporter in this lineage. Overall, this study builds a picture of the TH dependency in multiple cerebellar cell types starting from early embryonic development.

Thyroid hormones and learning-associated neuroplasticity

Thyroid hormones (THs) are crucial for brain development and maturation in all vertebrates. Especially during pre- and perinatal development, disruption of TH signaling leads to a multitude of neurological deficits. Many animal models provided insight in the role of THs in brain development, but specific data on how they affect the brain's ability to learn and adapt depending on environmental stimuli are rather limited. In this review, we focus on a number of learning processes like spatial learning, fear conditioning, vocal learning and imprinting behavior and on how abnormal TH signaling during development shapes subsequent performance. It is clear from multiple studies that TH deprivation leads to defects in learning on all fronts, and interestingly, changes in local expression of the TH activator deiodinase type 2 seem to have an important role. Taking into account that THs are regulated in a very space-specific manner, there is thus increasing pressure to investigate more local TH regulators as potential factors involved in neuroplasticity. As these learning processes are also important for proper adult human functioning, further elucidating the role of THs in developmental neuroplasticity in various animal models is an important field for advancing both fundamental and applied knowledge on human brain function.

Actions of Thyroid Hormone Analogues on Chemokines

The extracellular domain of plasma membrane integrin αvβ3 contains a receptor for thyroid hormone (L-thyroxine, T₄; 3,5,3′-triiodo-L-thyronine, T₃); this receptor also binds tetraiodothyroacetic acid (tetrac), a derivative of T₄. Tetrac inhibits the binding of T₄ and T₃ to the integrin. Fractalkine (CX3CL1) is a chemokine relevant to inflammatory processes in the CNS that are microglia-dependent but also important to normal brain development. Expression of the CX3CL1 gene is downregulated by tetrac, suggesting that T₄ and T₃ may stimulate fractalkine expression. Independently of its specific receptor (CX3CR1), fractalkine binds to αvβ3 at a site proximal to the thyroid hormone-tetrac receptor and changes the physical state of the integrin. Tetrac also affects expression of the genes for other CNS-relevant chemokines, including CCL20, CCL26, CXCL2, CXCL3, and CXCL10. The chemokine products of these genes are important to vascularity of the brain, particularly of the choroid plexus, to inflammatory processes in the CNS and, in certain cases, to neuroprotection. Thyroid hormones are known to contribute to regulation of each of these CNS functions. We propose that actions of thyroid hormone and hormone analogues on chemokine gene expression contribute to regulation of inflammatory processes in brain and of brain blood vessel formation and maintenance.

Emerging role of thyroid hormone metabolites

Thyroid hormones (THs) are essential for the regulation of development and metabolism in key organs. THs produce 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). Such activations can be either direct, in the case of ion channels, or mediated by membrane or cytoplasmic receptors. Short-term-activated signalling pathways often play a role in the regulation of genomic effects. Several TH intermediate metabolites, which were previously considered without biological activity, have now been associated with a broad range of actions, mostly attributable to short-term effects. Here, we give an overview of the physiological roles and mechanisms of action of THs, focusing on the emerging position that TH metabolites are acquiring as important regulators of physiology and metabolism.

Free thyroxine levels are associated with cognitive changes in individuals with a first episode of psychosis: A prospective 1-year follow-up study

The results of previous cross-sectional studies suggest that free thyroxine (FT4) levels are associated with cognitive abilities (particularly attention/vigilance) during the early stages of psychosis. We aimed to explore whether hypothalamic-pituitary-thyroid hormones predict cognitive changes in a 1-year longitudinal study following first episodes of psychosis (FEP). We studied 36 FEP patients and a control group of 50 healthy subjects (HS). Plasma levels of thyroid-stimulating hormone (TSH) and FT4 were measured. Cognitive assessment was performed with the MATRICS Cognitive Consensus Cognitive Battery (MCCB). FEP patients were assessed twice (baseline and after 1year), whereas HS were assessed only once. We compared cognitive changes at 1year between three groups based on baseline FT4 levels: 1) lowest quartile (Q1, FT4<1.16ng/dL); 2) medium quartiles (Q2-Q3, FT4 1.16-1.54ng/dL); and 3) highest quartile (Q4, FT4>1.54ng/dL). No differences in TSH or FT4 levels were found between HS and FEP patients. All participants had FT4 levels within the normal range. HS outperformed FEP patients in all cognitive tasks. In relation to the relationship between FT4 levels and cognitive changes, a U-shaped pattern was observed: FEP patients from the middle quartiles (Q2-Q3) improved in attention/vigilance, whereas both extreme quartiles (Q1 and Q4) showed a worsening in this cognitive domain over time. Patients with lower FT4 (Q1) showed poorer baseline attention; therefore, lower baseline FT4 levels predicted a poorer prognosis in terms of attention performance. Our study suggests that baseline FT4 levels are associated with changes in attention and vigilance performance over one year in FEP patients.

Gestational Hypothyroxinemia Affects Glutamatergic Synaptic Protein Distribution and Neuronal Plasticity Through Neuron-Astrocyte Interplay

Gestational hypothyroxinemia, characterized by low levels of maternal thyroxine (T₄) during gestation, is closely associated with cognitive impairment in offspring. Studies in animal models have shown that this condition alters neuronal glutamatergic synapses in the hippocampus. Given that astrocytes critically contribute to the establishment and functioning of synapses, the aim of this study was to determine the effects of gestational hypothyroxinemia on the capacity of astrocytes to regulate glutamatergic synapses. In an in vitro co-culture model of astrocytes and hippocampal neurons, gestational hypothyroxinemia profoundly affected the synaptic patterns of GluN1 and CD3ζ in an astrocyte-dependent manner. These effects were associated with impaired plasticity that was dependent on both neuronal and astrocyte contributions. These results highlight the importance of neuron-astrocyte interplay in the deleterious effects of gestational hypothyroxinemia and the timely diagnosis and treatment of this condition during gestation to ensure proper central nervous system development in offspring.

Thyroid Hormone, Hormone Analogs, and Angiogenesis

Modulation by thyroid hormone and hormone analogs of angiogenesis in the heart after experimental infarction, and in other organs, has been appreciated for decades. Description of a plasma membrane receptor for thyroid hormone on the extracellular domain of integrin αvβ3 on endothelial cells has revealed the complexity of the nongenomic regulation of angiogenesis by the hormone. From αvβ3, the hormone directs transcription of specific vascular growth factor genes, regulates growth factor receptor/growth factor interactions and stimulates endothelial cell migration to a vitronectin cue; these actions are implicated experimentally in tumor-relevant angiogenesis and angioproliferative pulmonary hypertension. Derived from L-thyroxine (T4), tetraiodothyroacetic acid (tetrac) can be covalently bound to a polymer and as Nanotetrac acts exclusively at the hormone receptor on αvβ3 to block actions of T4 and 3,5,3'-triiodo-L-thyronine (T3) on angiogenesis. Other antiangiogenic actions of Nanotetrac include disruption of crosstalk between integrin αvβ3 and adjacent cell surface vascular growth factor receptors, resulting in disordered vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF; FGF2) actions at their respective plasma membrane receptors. From αvβ3, Nanotetrac also downregulates expression of VEGFA and epidermal growth factor receptor (EGFR) genes, upregulates transcription of the angiogenesis suppressor gene, thrombospondin 1 (THBS1; TSP1) and decreases cellular abundance of Ang-2 protein and matrix metalloproteinase-9. Existence of this receptor provides new insights into the multiple mechanisms by which thyroid hormone and hormone analogs may regulate angiogenesis at the molecular level. The receptor also offers pharmacological opportunities for interruption of pathological angiogenesis via integrin αvβ3.

Free thyroxine levels are associated with cognitive abilities in subjects with early psychosis

INTRODUCTION

Subjects with a psychotic disorder show mild to moderate cognitive impairment, which is an important determinant of functional outcome. The underlying biological process of cognitive impairment in psychosis is unclear. We aimed to explore whether hypothalamic-pituitary-thyroid axis hormones or thyroid autoimmunity modulate cognitive functioning in subjects with early psychosis.

METHODS

We studied 70 patients with a psychotic disorder (<3years of illness) and a control group of 37 healthy subjects (HS). Plasma levels of thyroid-stimulating hormone (TSH), free thyroxine (FT4) and thyroid-peroxidase (TPO-Abs) and thyroglobulin antibodies (TG-Abs) were determined. Cognitive assessment was performed with the MATRICS Cognitive Consensus Cognitive Battery. We also explored the relationship between thyroid variables and cognition in three subgroups of psychotic patients: psychosis not otherwise specified, affective psychosis (bipolar disorder or schizoaffective disorder) and non-affective psychosis (schizophrenia or schizophreniphorm disorder).

RESULTS

In patients with early psychosis, higher FT4 levels (but not TSH or thyroid antibodies) were associated with better cognitive performance in attention/vigilance and overall cognition. The relationship between FT4 levels and the attention/vigilance domain remained significant in a multivariate analysis after adjusting for education level, age, gender, substance use, and benzodiazepine and antipsychotic treatments. We did not find a significant association between FT4 and cognitive performance in HS. In the exploratory analysis by psychotic subtypes, subjects with affective psychosis had increased FT4 levels and better cognitive profile than those with non-affective psychosis.

CONCLUSIONS

Our study suggests that FT4 levels are associated with cognitive abilities (attention/vigilance and overall cognition) in individuals with early psychosis.

Overlapping nongenomic and genomic actions of thyroid hormone and steroids

The genomic actions of thyroid hormone and steroids depend upon primary interactions of the hormones with their specific nuclear receptor proteins. Formation of nuclear co-activator or co-repressor complexes involving the liganded receptors subsequently result in transcriptional events-either activation or suppression-at genes that are specific targets of thyroid hormone or steroids. Nongenomic actions of thyroid hormone and steroids are in contrast initiated at binding sites on the plasma membrane or in cytoplasm or organelles and do not primarily require formation of intranuclear receptor protein-hormone complexes. Importantly, hormonal actions that begin nongenomically outside the nucleus often culminate in changes in nuclear transcriptional events that are regulated by both traditional intranuclear receptors as well as other nuclear transcription factors. In the case of thyroid hormone, the extranuclear receptor can be the classical "nuclear" thyroid receptor (TR), a TR isoform, or integrin αvβ3. In the case of steroid hormones, the membrane receptor is usually, but not always, the classical "nuclear" steroid receptor. This concept defines the paradigm of overlapping nongenomic and genomic hormone mechanisms of action. Here we review some examples of how extranuclear signaling by thyroid hormone and by estrogens and androgens modulates intranuclear hormone signaling to regulate a number of vital biological processes both in normal physiology and in cancer progression. We also point out that nongenomic actions of thyroid hormone may mimic effects of estrogen in certain tumors.

Functional status of thyroid and cognitive functions after menopause

Background

Thyroid activity plays a role in cognition. However, the relation between the functional state of thyroid and neuropsychiatric changes proceeding with age among people without clinical symptoms of thyroid dysfunction is still unknown. The aim of this study was analysis of cognitive function levels in reference to thyroid examination: thyroid-stimulating hormone (TSH), total thyroxin (TT4), triiodothyronine (TT3), free thyroxin (FT4), free triiodothyronine (FT3), thyroperoxidase antibodies (TPO-AB), and thyroglobulin antibodies (Tg-AB), TSH receptor antibodies (AB-TSHR) in women after menopause.

Material/Methods

A group of 383 women was recruited for the study. The inclusion criteria were: minimum two years after the last menstruation and no dementia signs on Montreal Cognitive Assessment (MoCA). Computerized battery of Central Nervous System Vital Signs (CNS VS) test was used to diagnostic cognitive functions. The blood plasma values were determined: TSH, FT3, FT4, TT3, TT4, TPO-AB, Tg-AB, and AB-TSHR. Statistical analysis was performed using Pearson’s correlation coefficient and analysis of variance in STATISTICA software.

Results

In women after menopause, TSH was negatively correlated with NCI results, executive functions, complex attention, and cognitive flexibility. FT4 was positively correlated with results of psychomotor speed. TT3 and TT4 were negatively correlated with results of memory and verbal memory. Furthermore, TT4 was negatively correlated with NCI, executive functions, and cognitive flexibility. TPO-AB was negatively correlated with results of memory, verbal memory, and psychomotor speed. Tg-AB was positively correlated with results of reaction time. AB-TSHR was negatively correlated with NCI results, memory, executive functions, psychomotor speed, complex attention, and cognitive flexibility.

Conclusions

Our study supports the importance of thyroid functionality in cognitive functioning in a group of women after menopause. The values of TSH, TT3, TT4, TPO-AB, and AB-TSHR were higher and FT4 was lower in examined women. The results were poorer in examination of cognitive functions measured with a battery of CNS-VS tests.

Thyroid hormone and astroglia: endocrine control of the neural environment

Thyroid hormones (THs) play key roles in brain development and function. The lack of THs during childhood is associated with the impairment of several neuronal connections, cognitive deficits and mental disorders. Several lines of evidence point to astrocytes as TH targets and as mediators of TH action in the central nervous system; however, the mechanisms underlying these events are still not completely known. In this review, we focus on advances in our understanding of the effects of THs on astroglial cells and the impact of these effects on neurone-astrocyte interactions. First, we discuss the signalling pathways involved in TH metabolism and the molecular mechanisms underlying TH receptor function. Then, we discuss data related to the effects of THs on astroglial cells, as well as studies regarding the generation of mutant TH receptor transgenic mice that have contributed to our understanding of TH function in brain development. We argue that astrocytes are key mediators of hormone actions on development of the cerebral cortex and cerebellum and that the identification of the molecules and pathways involved in these events might be important for determining the molecular-level basis of the neural deficits associated with endocrine diseases.

Thyroid hormone inhibits the proliferation of piglet Sertoli cell via PI3K signaling pathway

Accumulating researches show that thyroid hormone (TH) inhibits Sertoli cells (SCs) proliferation and stimulates their functional maturation in prepubertal rat testis, confirming that TH plays a key role in testicular development. However, the mechanism under the T3 regulation of piglet SC proliferation remains unclear. In the present study, in order to investigate the possible mechanism of T3 on the suppression of SC proliferation, the expression pattern of TRα1 and cell cycle-related molecules, effect of T3 on SC proliferation, and the role of phosphoinositide 3-kinase (PI3K)/Akt signaling pathway on the T3-mediated SC proliferation in piglet testis were explored. Our results demonstrated that TRα1 was expressed in all tested stages of SCs and decreased along with the ages. T3 inhibited the proliferation of SCs in a time- and dose-dependent manner, and T3 treatment downregulated the expressions of cell cycling molecules, such as cyclinA2, cyclinD1, cyclinE1, PCNA, and Skp2, but upregulated the p27 expression in SCs. Most importantly, the suppressive effects of T3 on SC proliferation seemed dependent on the inhibition of PI3K/Akt signaling pathway, and pre-stimulation of PI3K could enhance such suppressive effects. Together, our findings demonstrate that TH inhibits the proliferation of piglet SCs via the suppression of PI3K/Akt signaling pathway.

Persistent organic pollutants and transthyretin-bound thyroxin in plasma of Inuit women of childbearing age

The Inuit population of Nunavik (Northern Quebec, Canada) is highly exposed to persistent organic pollutants (POPs) through their traditional diet. Some POPs, i.e., hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs), pentachlorophenol (PCP), and perfluorooctane sulfonate (PFOS), compete with thyroxin (T4) for binding sites on transthyretin (TTR), a T4 transport protein found in plasma and cerebrospinal fluid. We tested the hypothesis that these TTR-binding compounds decrease circulating concentrations of T4 bound to TTR (T4-TTR) in Inuit women of reproductive age. We measured the concentration of T4-TTR in plasma samples obtained from 120 Inuit women (18-39 years old) by combining native-polyacrylamide gel electrophoresis and liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques. Total T4, TTR, and thyroxin-binding globulin (TBG) concentrations were also determined, while POPs levels had been previously measured. The mean T4-TTR concentration was 8.4 nmol/L (SD = 2.4) with values ranging from 2.9 to 14.4 nmol/L. Linear regression analysis revealed that TTR, TBG, and total T4 concentrations were significant predictors (p < 0.002) of T4-TTR levels (total adjusted R-squared = 0.26, p < 0.0001) but not levels of OH-PCBs, chlorophenols, or PFOS. Our results suggest that circulating levels of these TTR-binding compounds in Inuit women of childbearing age are not high enough to affect TTR-mediated thyroid hormone transport. The possibility of increased delivery of these compounds to the developing brain requires further investigation.

Regulators of thyroid hormone availability and action in embryonic chicken brain development

Thyroid hormones (THs) are crucial elements in vertebrate brain development. They exert their action mainly through binding of 3,5,3'-triiodothyronine (T3) to nuclear receptors that directly influence the expression of TH-regulated genes. Intracellular TH action is therefore dependent on both the availability of T3 and its receptors. TH uptake in cells is regulated by specific TH transporters and local activation and inactivation is regulated by deiodinases. This review provides an overview of the general expression pattern of TH transporters, deiodinases and receptors during embryonic chicken brain development and compares it to the situation in mammals. It is clear that THs and their regulators are present in the embryonic brain from the early stages of development, long before the onset of embryonic thyroid gland functioning. The mechanism of TH uptake across the brain barriers during development is only partly understood. At the developing blood-brain-barrier expression of the TH-activating type 2 deiodinase is closely associated with the blood vessels, but contrary to the situation in (adult) mammals no expression of MCT8 or OATP1C1 TH transporters is found at that level in the developing chicken. At the blood-cerebrospinal fluid-barrier co-expression of the TH-inactivating type 3 deiodinase and MCT8 and OATP1C1 is found in birds and mammals. These comparative data show overlapping patterns, pointing to general mechanisms, but also indicate specific interspecies differences that may help to understand species-specific responses to regulator gene knockout/mutation.

The effect of low dose lipopolysaccharide on thyroid hormone-regulated actin cytoskeleton modulation and type 2 iodothyronine deiodinase activity in astrocytes

Systemic infection/inflammation can severely interfere with brain development. Lipopolysaccharide (LPS) is a major cell wall component of gram-negative bacteria and commonly used to model the response by infections. Since perinatal exposure to LPS shows neurodevelopmental defects partly similar to those seen in perinatal hypothyroidism, we examined the effect of LPS on thyroxin (T4)-mediated signalings in astrocytes. Initially, C6 rat glioma-derived clonal cells were used, whose biological nature is similar to that of astrocytes. To measure the effects of LPS and T4, actin polymerization and D2 activity assays were carried out. LPS treatment (10 ng/mL) markedly induced actin depolymerization, whereas 10 nM T4 promoted actin polymerization. Furthermore, T4 partly rescued LPS-induced actin depolymerization. LPS treatment (10 ng/mL) increased D2 activity, whereas T4 (10 nM) suppressed this activity. T4 restored LPS-increased D2 activity at 10 nM. LPS-induced actin depolymerization and D2 activity were blocked by p38 MAP kinase inhibitor. Such effects were not seen in T4-mediated changes. Furthermore, similar results were found in the cerebellar primary astrocyte. These results indicate that, although LPS affects T4-regulated cellular events such as actin polymerization and D2 activity, which may induce neurodevelopmental defects similar to those in perinatal hypothyroidism, LPS signaling pathways are independent of T4 signaling pathways.

Pain and thyroid hormones

The role of endocrine systems in chronic pain mechanisms is slowly getting increasing experimental and clinical consideration. Many painful conditions appear to be directly and/or indirectly induced, reduced or, in some cases, modulated by hormones. We have done much work in trying to understand the relationship between hormones and pain, with particular attention to the hypothalamus-pituitary-gonadal axis. To expand our knowledge of this field, we have directed our attention to another axis, the hypothalamus-pituitary-thyroid (HPT). The literature on thyroid functions is vast but very few studies have focused on the HPT axis and pain. The few available data are considered in the present review to stimulate interest in the possible interactions between the HPT axis and pain.

Recent insights into the cell biology of thyroid angiofollicular units

In thyrocytes, cell polarity is of crucial importance for proper thyroid function. Many intrinsic mechanisms of self-regulation control how the key players involved in thyroid hormone (TH) biosynthesis interact in apical microvilli, so that hazardous biochemical processes may occur without detriment to the cell. In some pathological conditions, this enzymatic complex is disrupted, with some components abnormally activated into the cytoplasm, which can lead to further morphological and functional breakdown. When iodine intake is altered, autoregulatory mechanisms outside the thyrocytes are activated. They involve adjacent capillaries that, together with thyrocytes, form the angiofollicular units (AFUs) that can be considered as the functional and morphological units of the thyroid. In response to iodine shortage, a rapid expansion of the microvasculature occurs, which, in addition to nutrients and oxygen, optimizes iodide supply. These changes are triggered by angiogenic signals released from thyrocytes via a reactive oxygen species/hypoxia-inducible factor/vascular endothelial growth factor pathway. When intra- and extrathyrocyte autoregulation fails, other forms of adaptation arise, such as euthyroid goiters. From onset, goiters are morphologically and functionally heterogeneous due to the polyclonal nature of the cells, with nodules distributed around areas of quiescent AFUs containing globules of compact thyroglobulin (Tg) and surrounded by a hypotrophic microvasculature. Upon TSH stimulation, quiescent AFUs are activated with Tg globules undergoing fragmentation into soluble Tg, proteins involved in TH biosynthesis being expressed and the local microvascular network extending. Over time and depending on physiological needs, AFUs may undergo repetitive phases of high, moderate, or low cell and tissue activity, which may ultimately culminate in multinodular goiters.

A segment of 97 amino acids within the translocation domain of Clostridium difficile toxin B is essential for toxicity

Clostridium difficile toxin B (TcdB) intoxicates target cells by glucosylating Rho GTPases. TcdB (269 kDa) consists of at least 4 functional domains including a glucosyltransferase domain (GTD), a cysteine protease domain (CPD), a translocation domain (TD), and a receptor binding domain (RBD). The function and molecular mode of action of the TD, which is the largest segment of TcdB and comprises nearly 50% of the protein, remain largely unknown. Here we show that a 97-amino-acid segment (AA1756 – 1852, designated as ?97 or D97), located in the C-terminus of the TD and adjacent to the RBD, is essential for the cellular activity of TcdB. Deletion of this segment in TcdB (designated as TxB-D97), did not adversely alter toxin enzymatic activities or its cellular binding and uptake capacity. TxB-D97 bound to and entered cells in a manner similar to TcdB holotoxin. Both wild type and mutant toxins released their GTDs similarly in the presence of inositol hexakisphosphate (InsP₆), and showed a similar glucosyltransferase activity in a cell-free glucosylating assay. Despite these similarities, the cytotoxic activity of TxB-D97 was reduced by more than 5 logs compared to wild type toxin, supported by the inability of TxB-D97 to glucosylate Rac1 of target cells. Moreover, the mutant toxin failed to elicit tumor necrosis factor alpha (TNF-α) in macrophages, a process dependent on the glucosyltransferase activity of the toxin. Cellular fractionation of toxin-exposed cells revealed that TxB-D97 was unable to efficiently release the GTD into cytosol. Thereby, we conclude the 97-amino-acid region of the TD C-terminus of TcdB adjacent to the RBD, is essential for the toxicity of TcdB.

Ligand-induced changes in Oestrogen and thyroid hormone receptor expression in the developing rat cerebellum: A comparative quantitative PCR and Western blot study

Oestrogen (E2) and thyroid hormones (THs) are key regulators of cerebellar development. Recent reports implicate a complex mechanism through which E2 and THs influence the expression levels of each other's receptors (ERs and TRs) to precisely mediate developmental signals and modulate signal strength. We examined the modulating effects of E2 and THs on the expression levels of their receptor mRNAs and proteins in cultured cerebellar cells obtained from 7-day-old rat pups. Cerebellar granule cell cultures were treated with either E2, THs or a combination of these hormones, and resulting receptor expression levels were determined by quantitative PCR and Western blot techniques. The results were compared to non-treated controls and to samples obtained from 14-day-old in situ cerebella. Additionally, we determined the glial effects on the regulation of ER-TR expression levels. The results show that (i) ER and TR expression depends on the combined presence of E2 and THs; (ii) glial cells mediate the hormonal regulation of neuronal ER-TR expression and (iii) loss of tissue integrity results in characteristic changes in ER-TR expression levels. These observations suggest that both E2 and THs, in adequate amounts, are required for the precise orchestration of cerebellar development and that alterations in the ratio of E2/THs may influence signalling mechanisms involved in neurodevelopment. Comparison of data from in vitro and in situ samples revealed a shift in receptor expression levels after loss of tissue integrity, suggesting that such adjusting/regenerative mechanisms may function after cerebellar tissue injury as well.

Iodothyronine deiodinases and cancer

Thyroid hormones (TH) regulate key cellular processes, including proliferation, differentiation, and apoptosis in virtually all human cells. Disturbances in TH pathway and the resulting deregulation of these processes have been linked with neoplasia. The concentrations of TH in peripheral tissues are regulated via the activity of iodothyronine deiodinases. There are 3 types of these enzymes: type 1 and type 2 deiodinases are involved in TH activation while type 3 deiodinase inactivates TH. Expression and activity of iodothyronine deiodinases are disturbed in different types of neoplasia. According to the limited number of studies in cancer cell lines and mouse models changes in intratumoral and extratumoral T3 concentrations may influence proliferation rate and metastatic progression. Recent findings showing that increased expression of type 3 deiodinases may lead to enhanced tumoral proliferation support the idea that deiodinating enzymes have the potential to influence cancer progression. This review summarizes the observations of impaired expression and activity in different cancer types, published to date, and the mechanisms behind these alterations, including impaired regulation via TH receptors, transforming growth factor-β, and Sonic-hedgehog pathway. Possible roles of deiodinases as cancer markers and potential modulators of tumor progression are also discussed.

Thyroid hormone and the neuroglia: both source and target

Thyroid hormone plays a crucial role in the development and function of the nervous system. In order to bind to its nuclear receptor and regulate gene transcription thyroxine needs to be activated in the brain. This activation occurs via conversion of thyroxine to T3, which is catalyzed by the type 2 iodothyronine deiodinase (D2) in glial cells, in astrocytes, and tanycytes in the mediobasal hypothalamus. We discuss how thyroid hormone affects glial cell function followed by an overview on the fine-tuned regulation of T3 generation by D2 in different glial subtypes. Recent evidence on the direct paracrine impact of glial D2 on neuronal gene expression underlines the importance of glial-neuronal interaction in thyroid hormone regulation as a major regulatory pathway in the brain in health and disease.

Extranuclear signaling of mutated thyroid hormone receptors in promoting metastatic spread in thyroid carcinogenesis

Thyroid hormone receptors (TRs) mediate the critical activities of the thyroid hormone (T3) in growth, development, and differentiation. Decreased expression and/or somatic mutations of TRs have been shown to be associated with several types of human cancers including liver, breast, lung, and thyroid. A direct demonstration that TRβ mutants could function as oncogenes is evidenced by the spontaneous development of follicular thyroid carcinoma similar to human cancer in a knockin mouse model harboring a mutated TRβ (denoted as PV; Thrb(PV/PV) mice). PV is a dominant negative mutation identified in a patient with resistance to thyroid hormone. Analysis of altered gene expression and molecular studies of thyroid carcinogenesis in Thrb(PV/PV) mice show that the oncogenic activity of PV is mediated by both nucleus-initiated transcription and extranuclear actions to alter gene expression and signaling transduction activity. This article focuses on recent findings of novel extranuclear actions of PV that affect signaling cascades and thereby the invasiveness, migration, and motility of thyroid tumor cells. These findings have led to identification of potential molecular targets for treatment of metastatic thyroid cancer.

Effects of perinatal hypothyroidism on regulation of reelin and brain-derived neurotrophic factor gene expression in rat hippocampus: Role of DNA methylation and histone acetylation

Thyroid hormones have long been known to play important roles in the development and functions of the central nervous system, however, the precise molecular mechanisms that regulate thyroid hormone-responsive gene expression are not well understood. The present study investigated the role of DNA methylaion and histone acetylation in the effects of perinatal hypothyroidism on regulation of reelin and brain-derived neurotrophic factor (BDNF) gene expression in rat hippocampus. The findings indicated that the activities of DNA methyltransferase (DNMT), methylated reelin and BDNF genes were up-regulated, whereas, the activities of histone acetylases (HAT), the levels of global acetylated histone 3 (H3) and global acetylated histone 4 (H4), and acetylated H3, acetylated H4 at reelin promoter and at BDNF gene promoter for exon II were down-regulated in the hippocampus at the developmental stage of the hypothyroid animals. These results suggest that epigenetic modification of chromatin might underlie the mechanisms of hypothyroidism-induced down-regulation of reelin and BDNF gene expression in developmental rat hippocampus.

Minireview: thyroid hormone transporters: the knowns and the unknowns

The effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field.

Rapid stimulatory effect of thyroxine on plasma membrane transport systems: calcium uptake and neutral amino acid accumulation in immature rat testis

Although the biological effects of thyroid hormones are mediated by nuclear receptors (genomic mechanisms), interactions with receptors associated with the plasma membrane (non-genomic mechanisms) of target cells are not clear. In this study we investigated the rapid stimulatory effect of thyroxine (T(4)) on (45)Ca(2+) uptake as well as ionic currents and intracellular messengers involved in the stimulatory action of T(4) in amino acid accumulation in immature rat testes. Results indicated that 10(-9)M or 10(-6)M T(4) was able to increase immediately (45)Ca(2+) uptake after 60s of hormone exposure. These results indicate for the first time that voltage-dependent Ca(2+) channels and ATP-dependent K(+) channels can be seen as a set-point in the stimulatory effect of T(4) on amino acid accumulation. Apamin-sensitive small-conductance Ca(2+)-activated K(+) channels (SK(Ca)) and chloride channels were shown to be partially involved in this mechanism. The amino acid accumulation triggered by the PKC pathway suggests a functional link between different ion channel activities and the stimulatory effect of T(4) on amino acid accumulation. In conclusion, we show in this study a rapid and stimulatory effect of T(4) on calcium uptake and on amino acid accumulation, both events initiated at the plasma membrane, which strongly characterizes a non-genomic effect of T(4) in immature rat testes.