Thyroid Hormone Deiodinases: Dynamic Switches in Developmental Transitions

Transgenerational epigenetic self-memory of Dio3 dosage is associated with Meg3 methylation and altered growth trajectories and neonatal hormones

Intergenerational and transgenerational epigenetic effects resulting from conditions in previous generations can contribute to environmental adaptation as well as disease susceptibility. Previous studies in rodent and human models have shown that abnormal developmental exposure to thyroid hormone affects endocrine function and thyroid hormone sensitivity in later generations. Since the imprinted type 3 deiodinase gene (Dio3) regulates sensitivity to thyroid hormones, we hypothesize its epigenetic regulation is altered in descendants of thyroid hormone overexposed individuals. Using DIO3-deficient mice as a model of developmental thyrotoxicosis, we investigated Dio3 total and allelic expression and growth and endocrine phenotypes in descendants. We observed that male and female developmental overexposure to thyroid hormone altered total and allelic Dio3 expression in genetically intact descendants in a tissue-specific manner. This was associated with abnormal growth and neonatal levels of thyroid hormone and leptin. Descendant mice also exhibited molecular abnormalities in the Dlk1-Dio3 imprinted domain, including increased methylation in Meg3 and altered foetal brain expression of other genes of the Dlk1-Dio3 imprinted domain. These molecular abnormalities were also observed in the tissues and germ line of DIO3-deficient ancestors originally overexposed to thyroid hormone in utero. Our results provide a novel paradigm of epigenetic self-memory by which Dio3 gene dosage in a given individual, and its dependent developmental exposure to thyroid hormone, influences its own expression in future generations. This mechanism of epigenetic self-correction of Dio3 expression in each generation may be instrumental in descendants for their adaptive programming of developmental growth and adult endocrine function.

Thyroid Hormone Signaling in Retinal Development and Function: Implications for Diabetic Retinopathy and Age-Related Macular Degeneration

Thyroid Hormones (THs) play a central role in the development, cell growth, differentiation, and metabolic homeostasis of neurosensory systems, including the retina. The coordinated activity of various components of TH signaling, such as TH receptors (THRs) and the TH processing enzymes deiodinases 2 and 3 (DIO2, DIO3), is required for proper retinal maturation and function of the adult photoreceptors, Müller glial cells, and pigmented epithelial cells. Alterations of TH homeostasis, as observed both in frank or subclinical thyroid disorders, have been associated with sight-threatening diseases leading to irreversible vision loss i.e., diabetic retinopathy (DR), and age-related macular degeneration (AMD). Although observational studies do not allow causal inference, emerging data from preclinical models suggest a possible correlation between TH signaling imbalance and the development of retina disease. In this review, we analyze the most important features of TH signaling relevant to retinal development and function and its possible implication in DR and AMD etiology. A better understanding of TH pathways in these pathological settings might help identify novel targets and therapeutic strategies for the prevention and management of retinal disease.

Mice lacking DIO3 exhibit sex-specific alterations in circadian patterns of corticosterone and gene expression in metabolic tissues

Disruption of circadian rhythms is associated with neurological, endocrine and metabolic pathologies. We have recently shown that mice lacking functional type 3 deiodinase (DIO3), the enzyme that clears thyroid hormones, exhibit a phase shift in locomotor activity, suggesting altered circadian rhythm. To better understand the physiological and molecular basis of this phenotype, we used Dio3+/+ and Dio3-/- mice of both sexes at different zeitgeber times (ZTs) and analyzed corticosterone and thyroxine (T4) levels, hypothalamic, hepatic, and adipose tissue expression of clock genes, as well as genes involved in the thyroid hormone action or physiology of liver and adipose tissues. Wild type mice exhibited sexually dimorphic circadian patterns of genes controlling thyroid hormone action, including Dio3. Dio3-/- mice exhibited altered hypothalamic expression of several clock genes at ZT12, but did not disrupt the overall circadian profile. Expression of clock genes in peripheral tissues was not disrupted by Dio3 deficiency. However, Dio3 loss in liver and adipose tissues disrupted circadian profiles of genes that determine tissue thyroid hormone action and physiology. We also observed circadian-specific changes in serum T4 and corticosterone as a result of DIO3 deficiency. The circadian alterations manifested sexual dimorphism. Most notable, the time curve of serum corticosterone was flattened in Dio3-/- females. We conclude that Dio3 exhibits circadian variations, influencing the circadian rhythmicity of thyroid hormone action and physiology in liver and adipose tissues in a sex-specific manner. Circadian disruptions in tissue physiology may then contribute to the metabolic phenotypes of DIO3-deficient mice.

Biosynthesis, Engineering, and Delivery of Selenoproteins

Selenocysteine (Sec) was discovered as the 21st genetically encoded amino acid. In nature, site-directed incorporation of Sec into proteins requires specialized biosynthesis and recoding machinery that evolved distinctly in bacteria compared to archaea and eukaryotes. Many organisms, including higher plants and most fungi, lack the Sec-decoding trait. We review the discovery of Sec and its role in redox enzymes that are essential to human health and important targets in disease. We highlight recent genetic code expansion efforts to engineer site-directed incorporation of Sec in bacteria and yeast. We also review methods to produce selenoproteins with 21 or more amino acids and approaches to delivering recombinant selenoproteins to mammalian cells as new applications for selenoproteins in synthetic biology.

Localized T3 production modifies the transcriptome and promotes the hepatocyte-like lineage in iPSC-derived hepatic organoids

Thyroid hormone (TH) levels are low during development, and the deiodinases control TH signaling through tissue-specific activation or inactivation of TH. Here, we studied human induced pluripotent stem cell-derived (iPSC-derived) hepatic organoids and identified a robust induction of DIO2 expression (the deiodinase that activates T4 to T3) that occurs in hepatoblasts. The surge in DIO2-T3 (the deiodinase that activates thyroxine [T4] to triiodothyronine [T3]) persists until the hepatoblasts differentiate into hepatocyte- or cholangiocyte-like cells, neither of which expresses DIO2. Preventing the induction of the DIO2-T3 signaling modified the expression of key transcription factors, decreased the number of hepatocyte-like cells by ~60%, and increased the number of cholangiocyte-like cells by ~55% without affecting the growth or the size of the mature liver organoid. Physiological levels of T3 could not fully restore the transition from hepatoblasts to mature cells. This indicates that the timed surge in DIO2-T3 signaling critically determines the fate of developing human hepatoblasts and the transcriptome of the maturing hepatocytes, with physiological and clinical implications for how the liver handles energy substrates.

Neonatal estrogen induces male-like expression of astroglial markers of maturation and plasticity in the neocortex of female mice

Astroglia play a crucial role in various aspects of neurodevelopment including building, maintaining, and modulating neuronal circuits that underly complex behaviours in the neocortex. Telencephalic regions exhibit sex differences in neuronal networks that arise early in development. Astroglia express receptors for gonadal hormones responsible for the organization of sex differences, such as estrogen, placing them in a key position to modulate sex differences in the development of neuronal networks. Astroglial cells express specific proteins related to their morphology, function, and maturation. We have previously shown that P7-P14 is a key transition period for neocortical astroglial maturation and that males reach a mature phenotype earlier than females, at P7. In this study, we investigated whether administration of perinatal estradiol to female mice is sufficient to masculinize astroglial protein and gene expression related to maturation that we previously observed at P7. We found that canonical astroglial markers like glial fibrillary acidic protein and glutamine synthetase are not affected by perinatal estrogen, but markers of astroglial maturation, Vimentin, Aldh1a1, Dio2, and the number of actively dividing astroglia are masculinized by perinatal estradiol administration. These findings suggest that sex differences in neocortical astroglial maturation are at least in-part due to the role of perinatal estrogen. Given the higher prevalence of neurodevelopmental disorders in males compared to females and the involvement of astroglia in virtually all neurodevelopmental disorders, further research is needed to determine other contributions to sex differences in neocortical astroglial cells.

Paternal developmental thyrotoxicosis disrupts neonatal leptin leading to increased adiposity and altered physiology of the melanocortin system

Background

The genetic code does not fully explain individual variability and inheritance of susceptibility to endocrine conditions, suggesting the contribution of epigenetic factors acting across generations.

Methods

We used a mouse model of developmental thyrotoxicosis (Dio3-/- mouse) to analyze endocrine outcomes in the adult offspring of Dio3-/- males using standard methods for body composition, and baseline and fasting hormonal and gene expression determinations in serum and tissues of relevance to the control of energy balance.

Results

Compared to controls, adult females with an exposed father (EF females) exhibited higher body weight and fat mass, but not lean mass, a phenotype that was much milder in EF males. After fasting, both EF females and males exhibited a more pronounced decrease in body weight than controls. EF females also showed markedly elevated serum leptin, increased white adipose tissue mRNA expression of leptin and mesoderm-specific transcript but decreased expression of type 2 deiodinase. EF females exhibited decreased serum ghrelin, which showed more pronounced post-fasting changes in EF females than in control females. EF female hypothalami also revealed significant decreases in the expression of pro-opiomelanocortin, agouti-related peptide, neuropeptide Y and melanocortin receptor 4. These markers also showed larger changes in response to fasting in EF females than in control females. Adult EF females showed no abnormalities in serum thyroid hormones, but pituitary expression of thyrotropin-releasing hormone receptor 1 and thyroid gland expression of thyroid-stimulating hormone receptor, thyroid peroxidase and iodotyrosine deiodinase were increased at baseline and showed differential regulation after fasting, with no increase in Trhr1 expression and more pronounced reductions in Tshr, Tpo and Iyd. In EF males, these abnormalities were generally milder. In addition, postnatal day 14 (P14) serum leptin was markedly reduced in EF pups.

Discussion

A paternal excess of thyroid hormone during development modifies the endocrine programming and energy balance in the offspring in a sexually dimorphic manner, with baseline and dynamic range alterations in the leptin-melanocortin system and thyroid gland, and consequences for adiposity phenotypes. We conclude that thyroid hormone overexposure may have important implications for the non-genetic, inherited etiology of endocrine and metabolic pathologies.

The importance of thyroid hormone signaling during early development: Lessons from the zebrafish model

The zebrafish is an optimal experimental model to study thyroid hormone (TH) involvement in vertebrate development. The use of state-of-the-art zebrafish genetic tools available for the study of the effect of gene silencing, cell fate decisions and cell lineage differentiation have contributed to a more insightful comprehension of molecular, cellular, and tissue-specific TH actions. In contrast to intrauterine development, extrauterine embryogenesis observed in zebrafish has facilitated a more detailed study of the development of the hypothalamic-pituitary-thyroid axis. This model has also enabled a more insightful analysis of TH molecular actions upon the organization and function of the brain, the retina, the heart, and the immune system. Consequently, zebrafish has become a trendy model to address paradigms of TH-related functional and biomedical importance. We here compilate the available knowledge regarding zebrafish developmental events for which specific components of TH signaling are essential.

Epigenetic developmental programming and intergenerational effects of thyroid hormones

Mounting evidence is showing that altered signaling through the nuclear hormone receptor superfamily can cause abnormal, long-term epigenetic changes which translate into pathological modifications and susceptibility to disease. These effects seem to be more prominent if the exposure occurs early in life, when transcriptomic profiles are rapidly changing. At this time, the coordination of the complex coordinated processes of cell proliferation and differentiation that characterize mammalian development. Such exposures may also alter the epigenetic information of the germ line, potentially leading to developmental changes and abnormal outcomes in subsequent generations. Thyroid hormone (TH) signaling is mediated by specific nuclear receptors, which have the ability to markedly change chromatin structure and gene transcription, and can also regulate other determinants of epigenetic marks. TH exhibits pleiotropic effects in mammals, and during development, its action is regulated in a highly dynamic manner to suit the rapidly evolving needs of multiple tissues. Their molecular mechanisms of action, timely developmental regulation and broad biological effects place THs in a central position to play a role in the developmental epigenetic programming of adult pathophysiology and, through effects on the germ line, in inter- and trans-generational epigenetic phenomena. These areas of epigenetic research are in their infancy, and studies regarding THs are limited. In the context of their characteristics as epigenetic modifiers and their finely tuned developmental action, here we review some of the observations underscoring the role that altered TH action may play in the developmental programming of adult traits and in the phenotypes of subsequent generations via germ line transmission of altered epigenetic information. Considering the relatively high prevalence of thyroid disease and the ability of some environmental chemicals to disrupt TH action, the epigenetic effects of abnormal levels of TH action may be important contributors to the non-genetic etiology of human disease.

Developmental thyroid hormone action on pro-opiomelanocortin-expressing cells programs hypothalamic BMPR1A depletion and brown fat activation

Thyroid hormone excess secondary to global type 3 deiodinase (DIO3) deficiency leads to increased locomotor activity and reduced adiposity, but also to concurrent alterations in parameters of the leptin-melanocortin system that would predict obesity. To distinguish the underlying contributions to the energy balance phenotype of DIO3 deficiency, we generated mice with thyroid hormone excess targeted to pro-opiomelanocortin (POMC)-expressing cells via cell-specific DIO3 inactivation. These mice exhibit a male-specific phenotype of reduced hypothalamic Pomc expression, hyperphagia, and increased activity in brown adipose tissue, with adiposity and serum levels of leptin and thyroid hormones remained normal. These male mice also manifest a marked and widespread hypothalamic reduction in the expression of bone morphogenetic receptor 1a (BMPR1A), which has been shown to cause similar phenotypes when inactivated in POMC-expressing cells. Our results indicate that developmental overexposure to thyroid hormone in POMC-expressing cells programs energy balance mechanisms in a sexually dimorphic manner by suppressing adult hypothalamic BMPR1A expression.

Biphasic expression of thyroid hormone receptor TRβ1 in mammalian retina and anterior ocular tissues

The retina is increasingly recognized as a target of thyroid hormone. We previously reported critical functions for thyroid hormone receptor TRβ2, encoded by Thrb, in cones, the photoreceptors that mediate color vision. TRβ1, another Thrb receptor isoform, is widely expressed in other tissues but little studied in the retina. Here, we investigate these N-terminal isoforms by RNA-sequencing analysis and reveal a striking biphasic profile for TRβ1 in mouse and human retina. In contrast to the early TRβ2 peak, TRβ1 peaks later during retinal maturation or later differentiation of human retinal organoids. This switch in receptor expression profiles was confirmed using lacZ reporter mice. TRβ1 localized in cones, amacrine cells and ganglion cells in contrast to the restricted expression of TRβ2 in cones. Intriguingly, TRβ1 was also detected in the retinal pigmented epithelium and in anterior structures in the ciliary margin zone, ciliary body and iris, suggesting novel functions in non-retinal eye tissues. Although TRβ1 was detected in cones, TRβ1-knockout mice displayed only minor changes in opsin photopigment expression and normal electroretinogram responses. Our results suggest that strikingly different temporal and cell-specific controls over TRβ1 and TRβ2 expression may underlie thyroid hormone actions in a range of ocular cell types. The TRβ1 expression pattern suggests novel functions in retinal and non-neural ocular tissues.

Transcriptional control of cone photoreceptor diversity by a thyroid hormone receptor

Cone photoreceptor diversity allows detection of wavelength information in light, the first step in color (chromatic) vision. In most mammals, cones express opsin photopigments for sensitivity to medium/long (M, "green") or short (S, "blue") wavelengths and are differentially arrayed over the retina. Cones appear early in retinal neurogenesis but little is understood of the subsequent control of diversity of these postmitotic neurons, because cone populations are sparse and, apart from opsins, poorly defined. It is also a challenge to distinguish potentially subtle differences between cell subtypes within a lineage. Therefore, we derived a Cre driver to isolate individual M and S opsin-enriched cones, which are distributed in counter-gradients over the mouse retina. Fine resolution transcriptome analyses identified expression gradients for groups of genes. The postnatal emergence of gradients indicated divergent differentiation of cone precursors during maturation. Using genetic tagging, we demonstrated a role for thyroid hormone receptor β2 (TRβ2) in control of gradient genes, many of which are enriched for TRβ2 binding sites and TRβ2-regulated open chromatin. Deletion of TRβ2 resulted in poorly distinguished cones regardless of retinal location. We suggest that TRβ2 controls a bipotential transcriptional state to promote cone diversity and the chromatic potential of the species.

Thyroid hormone regulators in human cerebral cortex development

Brain development is critically dependent on the timely supply of thyroid hormones. The thyroid hormone transporters are central to the action of thyroid hormones in the brain, facilitating their passage through the blood-brain barrier. Mutations of the monocarboxylate transporter 8 (MCT8) cause the Allan-Herndon-Dudley syndrome, with altered thyroid hormone concentrations in the blood and profound neurological impairment and intellectual deficit. Mouse disease models have revealed interplay between transport, deiodination, and availability of T3 to receptors in specific cells. However, the mouse models are not satisfactory, given the fundamental differences between the mouse and human brains. The goal of the present work is to review human neocortex development in the context of thyroid pathophysiology. Recent developments in single-cell transcriptomic approaches aimed at the human brain make it possible to profile the expression of thyroid hormone regulators in single-cell RNA-Seq datasets of the developing human neocortex. The data provide novel insights into the specific cellular expression of thyroid hormone transporters, deiodinases, and receptors.

Selenoproteins and the senescence-associated epitranscriptome

Selenium is a naturally found trace element, which provides multiple benefits including antioxidant, anticancer, and antiaging, as well as boosting immunity. One unique feature of selenium is its incorporation as selenocysteine, a rare 21st amino acid, into selenoproteins. Twenty-five human selenoproteins have been discovered, and a majority of these serve as crucial antioxidant enzymes for redox homeostasis. Unlike other amino acids, incorporation of selenocysteine requires a distinctive UGA stop codon recoding mechanism. Although many studies correlating selenium, selenoproteins, aging, and senescence have been performed, it has not yet been explored if the upstream events regulating selenoprotein synthesis play a role in senescence-associated pathologies. The epitranscriptomic writer alkylation repair homolog 8 (ALKBH8) is critical for selenoprotein production, and its deficiency can significantly decrease levels of selenoproteins that are essential for reactive oxygen species (ROS) detoxification, and increase oxidative stress, one of the major drivers of cellular senescence. Here, we review the potential role of epitranscriptomic marks that govern selenocysteine utilization in regulating the senescence program.

Thyroid hormone elicits intergenerational epigenetic effects on adult social behavior and fetal brain expression of autism susceptibility genes

Genetic mutations identified in genome-wide association studies can only explain a small percentage of the cases of complex, highly heritable human conditions, including neurological and neurodevelopmental disorders. This suggests that intergenerational epigenetic effects, possibly triggered by environmental circumstances, may contribute to their etiology. We previously described altered DNA methylation signatures in the sperm of mice that experienced developmental overexposure to thyroid hormones as a result of a genetic defect in hormone clearance (DIO3 deficiency). Here we studied fetal brain gene expression and adult social behavior in genetically normal F2 generation descendants of overexposed mice. The brain of F2 generation E13.5 fetuses exhibited abnormal expression of genes associated with autism in humans, including Auts2, Disc1, Ldlr, Per2, Shank3, Oxtr, Igf1, Foxg1, Cd38, Grid2, Nrxn3, and Reln. These abnormal gene expression profiles differed depending on the sex of the exposed ancestor. In the three-chamber social box test, adult F2 generation males manifested significantly decreased interest in social interaction and social novelty, as revealed by decrease total time, distance traveled and time immobile in the area of interaction with novel strangers. F1 generation mice, compared to appropriate controls also exhibited altered profiles in fetal brain gene expression, although these profiles were substantially different to those in the F2 generation. Likewise adult F1 generation mice showed some abnormalities in social behavior that were sexually dimorphic and milder than those in F2 generation mice. Our results indicate that developmental overexposure to thyroid hormone causes intergenerational epigenetic effects impacting social behavior and the expression of autism-related genes during early brain development. Our results open the possibility that altered thyroid hormone states, by eliciting changes in the epigenetic information of the germ line, contribute to the susceptibility and the missing-but heriTables-etiology of complex neurodevelopmental conditions characterized by social deficits, including autism and schizophrenia.

Emerging roles of ER-resident selenoproteins in brain physiology and physiopathology

The brain has a very high oxygen consumption rate and is particularly sensitive to oxidative stress. It is also the last organ to suffer from a loss of selenium (Se) in case of deficiency. Se is a crucial trace element present in the form of selenocysteine, the 21st proteinogenic amino acid present in selenoproteins, an essential protein family in the brain that participates in redox signaling. Among the most abundant selenoproteins in the brain are glutathione peroxidase 4 (GPX4), which reduces lipid peroxides and prevents ferroptosis, and selenoproteins W, I, F, K, M, O and T. Remarkably, more than half of them are proteins present in the ER and recent studies have shown their involvement in the maintenance of ER homeostasis, glycoprotein folding and quality control, redox balance, ER stress response signaling pathways and Ca2+ homeostasis. However, their molecular functions remain mostly undetermined. The ER is a highly specialized organelle in neurons that maintains the physical continuity of axons over long distances through its continuous distribution from the cell body to the nerve terminals. Alteration of this continuity can lead to degeneration of distal axons and subsequent neuronal death. Elucidation of the function of ER-resident selenoproteins in neuronal pathophysiology may therefore become a new perspective for understanding the pathophysiology of neurological diseases. Here we summarize what is currently known about each of their molecular functions and their impact on the nervous system during development and stress.

Effects of Thyroid Hormones on Lipid Metabolism Pathologies in Non-Alcoholic Fatty Liver Disease

The typical modern lifestyle contributes to the development of many metabolic-related disorders, as exemplified by metabolic syndrome. How to prevent, resolve, or avoid subsequent deterioration of metabolic disturbances and the development of more serious diseases has become an important and much-discussed health issue. Thus, the question of the physiological and pathological roles of thyroid hormones (THs) in metabolism has never gone out of fashion. Although THs influence almost all organs, the liver is one of the most important targets as well as the hub of metabolic homeostasis. When this homeostasis is out of balance, diseases may result. In the current review, we summarize the common features and actions of THs, first focusing on their effects on lipid metabolism in the liver. In the second half of the review, we turn to a consideration of non-alcoholic fatty liver disease (NAFLD), a disease characterized by excessive accumulation of fat in the liver that is independent of heavy alcohol consumption. NAFLD is a growing health problem that currently affects ~25% of the world’s population. Unfortunately, there are currently no approved therapies specific for NAFLD, which, if left uncontrolled, may progress to more serious diseases, such as cirrhosis or liver cancer. This absence of effective treatment can also result in the development of non-alcoholic steatohepatitis (NASH), an aggressive form of NAFLD that is the leading cause of liver transplantation in the United States. Because THs play a clear role in hepatic fat metabolism, their potential application in the prevention and treatment of NAFLD has attracted considerable research attention. Studies that have investigated the use of TH-related compounds in the management of NAFLD are also summarized in the latter part of this review. An important take-home point of this review is that a comprehensive understanding of the physiological and pathological roles of THs in liver fat metabolism is possible, despite the complexities of this regulatory axis—an understanding that has clinical value for the specific management of NAFLD.

Maternal LiCl exposure disrupts thyroid-cerebral axis in neonatal albino rats

This work aimed to elucidate whether maternal lithium chloride (LiCl) exposure disturbs the thyroid-cerebral axis in neonatal albino rats. 50 mg of LiCl/kg b.wt. is orally given for pregnant Wistar rats from gestational day (GD) 1 to lactation day (LD) 28. The maternal administration of LiCl induced follicular dilatation and degeneration, hyperplasia, lumen obliteration and colloid vacuolation in the maternal and neonatal thyroid gland at postnatal days (PNDs) 14, 21 and 28. Neuronal degeneration (spongiform), gliosis, nuclear pyknosis, perivascular oedema, and meningeal hyperaemia were observed in the neonatal cerebral cortex of the maternal LiCl-treated group at examined PNDs. This disturbance appears to depend on intensification in the neonatal cerebral malondialdehyde (MDA), nitric oxide (NO), and hydrogen peroxide (H₂ O₂ ) levels, and attenuation in the glutathione (GSH), total thiol (t-SH), catalase (CAT), and superoxide dismutase (SOD) levels. In the neonatal cerebrum, the fold change in the relative mRNA expression of deiodinases (DII and DIII) increased significantly at PNDs 21 and 14, respectively, in the maternal LiCl-treated group. These data suggest that maternal LiCl may perturb the thyroid-cerebrum axis generating neonatal neurodevelopmental disorder.