Type 3 deiodinase, a thyroid-hormone-inactivating enzyme, controls survival and maturation of cone photoreceptors

The Circadian Clock Gene Bmal1 Controls Thyroid Hormone-Mediated Spectral Identity and Cone Photoreceptor Function

Circadian clocks regulate various aspects of photoreceptor physiology, but their contribution to photoreceptor development and function is unclear. Cone photoreceptors are critical for color vision. Here, we define the molecular function of circadian activity within cone photoreceptors and reveal a role for the clock genes Bmal1 and Per2 in regulating cone spectral identity. ChIP analysis revealed that BMAL1 binds to the promoter region of the thyroid hormone (TH)-activating enzyme type 2 iodothyronine deiodinase (Dio2) and thus regulates the expression of Dio2. TH treatment resulted in a partial rescue of the phenotype caused by the loss of Bmal1, thus revealing a functional relationship between Bmal1 and Dio2 in establishing cone photoreceptor identity. Furthermore, Bmal1 and Dio2 are required to maintain cone photoreceptor functional integrity. Overall, our results suggest a mechanism by which circadian proteins can locally regulate the availability of TH and influence tissue development and function.

Histological Evaluation of Diabetic Neurodegeneration in the Retina of Zucker Diabetic Fatty (ZDF) Rats

In diabetes, retinal dysfunctions exist prior to clinically detectable vasculopathy, however the pathology behind these functional deficits is still not fully established. Previously, our group published a detailed study on the retinal histopathology of type 1 diabetic (T1D) rat model, where specific alterations were detected. Although the majority of human diabetic patients have type 2 diabetes (T2D), similar studies on T2D models are practically absent. To fill this gap, we examined Zucker Diabetic Fatty (ZDF) rats - a model for T2D - by immunohistochemistry at the age of 32 weeks. Glial reactivity was observed in all diabetic specimens, accompanied by an increase in the number of microglia cells. Prominent outer segment degeneration was detectable with changes in cone opsin expression pattern, without a decrease in the number of labelled elements. The immunoreactivity of AII amacrine cells was markedly decreased and changes were detectable in the number and staining of some other amacrine cell subtypes, while most other cells examined did not show any major alterations. Overall, the retinal histology of ZDF rats shows a surprising similarity to T1D rats indicating that despite the different evolution of the disease, the neuroretinal cells affected are the same in both subtypes of diabetes.

Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration

Thyroid hormone (TH) signaling regulates cell proliferation, differentiation, and metabolism. Recent studies have implicated TH signaling in cone photoreceptor viability. Using mouse models of retinal degeneration, we demonstrated that antithyroid drug treatment and targeting iodothyronine deiodinases (DIOs) to suppress cellular tri-iodothyronine (T3) production or increase T3 degradation preserves cones. In this work, we investigated the effectiveness of inhibition of the TH receptor (TR). Two genes, THRA and THRB, encode TRs; THRB2 has been associated with cone viability. Using TR antagonists and Thrb2 deletion, we examined the effects of TR inhibition. Systemic and ocular treatment with the TR antagonists NH-3 and 1-850 increased cone density by 30-40% in the Rpe65-/- mouse model of Leber congenital amaurosis and reduced the number of TUNEL+ cells. Cone survival was significantly improved in Rpe65-/- and Cpfl1 (a model of achromatopsia with Pde6c defect) mice with Thrb2 deletion. Ventral cone density in Cpfl1/Thrb2-/- and Rpe65-/- /Thrb2-/- mice was increased by 1- to 4-fold, compared with age-matched controls. Moreover, the expression levels of TR were significantly higher in the cone-degeneration retinas, suggesting locally elevated TR signaling. This work shows that the effects of antithyroid treatment or targeting DIOs were likely mediated by TRs and that suppressing TR protects cones. Our findings support the view that inhibition of TR locally in the retina is a therapeutic strategy for retinal degeneration management.-Ma, H., Yang, F., Butler, M. R., Belcher, J., Redmond, T. M., Placzek, A. T., Scanlan, T. S., Ding, X.-Q. Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration.

Retinal Photoreceptor Functions Are Compromised in Patients With Resistance to Thyroid Hormone Syndrome (RTHβ)

CONTEXT

In animal models, disruption of thyroid hormone (TH) receptor-β (TRβ) reduces the long/medium wavelength (L/M) and increases the short-wavelength (S) cones. Retinal photoreceptor (RP) functions are unknown in patients with resistance to TH syndrome (RTHβ) with dominant-negative TRβ mutations.

OBJECTIVE

To investigate RP functions in RTHβ.

DESIGN, SETTING, AND PARTICIPANTS

Case-control study involving 27 RTHβ patients and 31 age/sex-matched controls, conducted in two tertiary referral centers in Italy.

MAIN OUTCOME MEASURES

Color vision sensitivity assessed by Farnsworth; central macular thickness (CMT) of the outer retinal layer measured by spectral-domain optical coherence tomography; and retinal function tested by full-field electroretinogram (ERG) and S-cone ERG.

RESULTS

Color sensitivity was worse in RTHβ patients than controls (P = 0.002). CMT was overlapping between the study groups but directly correlated with sex hormone-binding globuline levels in RTHβ. We found a significant reduction in amplitude of the cone (P = 0.024) and of the rod response (P = 0.006) in the ERG of RTHβ patients compared with controls. The response of the L/M cones measured by a specialized ERG test was lower in RTHβ than controls (P = 0.027), whereas no differences were found in the S-cone response. No correlations were found between TH levels, total error score, or electrophysiological results. Furthermore, no differences were found between patients with maternal or de novo/paternal inheritance.

CONCLUSIONS

We report, to our knowledge, the first in vivo evidence of functional defects of RP in RTHβ. These changes occur independently of endogenous TH levels or the prenatal exposure to high or normal maternal TH.

Germain DL, Hernandez A, Forrest D. Deletion of the Thyroid Hormone-Activating Type 2 Deiodinase Rescues Cone Photoreceptor Degeneration but Not Deafness in Mice Lacking Type 3 Deiodinase

Type 2 deiodinase amplifies and type 3 deiodinase depletes levels of the active form of thyroid hormone, triiodothyronine. Given the opposing activities of these enzymes, we tested the hypothesis that they counteract each other's developmental functions by investigating whether deletion of type 2 deiodinase (encoded by Dio2) modifies sensory phenotypes in type 3 deiodinase-deficient (Dio3-/-) mice. Dio3-/- mice display degeneration of retinal cones, the photoreceptors that mediate daylight and color vision. In Dio2-/- mice, cone function was largely normal but deletion of Dio2 in Dio3-/- mice markedly recovered cone numbers and electroretinogram responses, suggesting counterbalancing roles for both enzymes in cone survival. Both Dio3-/- and Dio2-/- strains exhibit deafness with cochlear abnormalities. In Dio3-/-;Dio2-/- mice, deafness was exacerbated rather than alleviated, suggesting unevenly balanced actions by these enzymes during auditory development. Dio3-/- mice also exhibit an atrophic thyroid gland, low thyroxine, and high triiodothyronine levels, but this phenotype was ameliorated in Dio3-/-;Dio2-/- mice, indicating counterbalancing roles for the enzymes in determining the thyroid hormone status. The results suggest that the composite action of these two enzymes is a critical determinant in visual and auditory development and in setting the systemic thyroid hormone status.

Thyroid Hormone Signaling in the Mouse Retina

Thyroid hormone is a crucial regulator of gene expression in the developing and adult retina. Here we sought to map sites of thyroid hormone signaling at the cellular level using the transgenic FINDT3 reporter mouse model in which neurons express β-galactosidase (β-gal) under the control of a hybrid Gal4-TRα receptor when triiodothyronine (T3) and cofactors of thyroid receptor signaling are present. In the adult retina, nearly all neurons of the ganglion cell layer (GCL, ganglion cells and displaced amacrine cells) showed strong β-gal labeling. In the inner nuclear layer (INL), a minority of glycineric and GABAergic amacrine cells showed β-gal labeling, whereas the majority of amacrine cells were unlabeled. At the level of amacrine types, β-gal labeling was found in a large proportion of the glycinergic AII amacrines, but only in a small proportion of the cholinergic/GABAergic 'starburst' amacrines. At postnatal day 10, there also was a high density of strongly β-gal-labeled neurons in the GCL, but only few amacrine cells were labeled in the INL. There was no labeling of bipolar cells, horizontal cells and Müller glia cells at both stages. Most surprisingly, the photoreceptor somata in the outer nuclear layer also showed no β-gal label, although thyroid hormone is known to control cone opsin expression. This is the first record of thyroid hormone signaling in the inner retina of an adult mammal. We hypothesize that T3 levels in photoreceptors are below the detection threshold of the reporter system. The topographical distribution of β-gal-positive cells in the GCL follows the overall neuron distribution in that layer, with more T3-signaling cells in the ventral than the dorsal half-retina.

Targeting iodothyronine deiodinases locally in the retina is a therapeutic strategy for retinal degeneration

Recent studies have implicated thyroid hormone (TH) signaling in cone photoreceptor viability. Using mouse models of retinal degeneration, we found that antithyroid treatment preserves cones. This work investigates the significance of targeting intracellular TH components locally in the retina. The cellular TH level is mainly regulated by deiodinase iodothyronine (DIO)-2 and -3. DIO2 converts thyroxine (T4) to triiodothyronine (T3), which binds to the TH receptor, whereas DIO3 degrades T3 and T4. We examined cone survival after overexpression of DIO3 and inhibition of DIO2 and demonstrated the benefits of these manipulations. Subretinal delivery of AAV5-IRBP/GNAT2-DIO3, which directs expression of human DIO3 specifically in cones, increased cone density by 30-40% in a Rpe65^-/- mouse model of Lebers congenital amaurosis (LCA) and in a Cpfl1 mouse with Pde6c defect model of achromatopsia, compared with their respective untreated controls. Intravitreal and topical delivery of the DIO2 inhibitor iopanoic acid also significantly improved cone survival in the LCA model mice. Moreover, the expression levels of DIO2 and Slc16a2 were significantly higher in the diseased retinas, suggesting locally elevated TH signaling. We show that targeting DIOs protects cones, and intracellular inhibition of TH components locally in the retina may represent a novel strategy for retinal degeneration management.-Yang, F., Ma, H., Belcher, J., Butler, M. R., Redmond, T. M., Boye, S. L., Hauswirth, W. W., Ding, X.-Q. Targeting iodothyronine deiodinases locally in the retina is a therapeutic strategy for retinal degeneration.

Age-Dependent Changes of Monocarboxylate Transporter 8 Availability in the Postnatal Murine Retina

The thyroid hormones (TH) triiodothyronine (T3) and its prohormone thyroxine (T4) are crucial for retinal development and function, and increasing evidence points at TH dysregulation as a cause for retinal degenerative diseases. Thus, precise regulation of retinal TH supply is required for proper retinal function, but knowledge on these mechanisms is still fragmentary. Several transmembrane transporters have been described as key regulators of TH availability in target tissues of which the monocarboxylate transporter 8 (MCT8), a high affinity transporter for T4 and T3, plays an essential role in the central nervous system. Moreover, in the embryonic chicken retina, MCT8 is highly expressed, but the postnatal availability of MCT8 in the mammalian retina was not reported to date. In the present study, spatiotemporal retinal MCT8 availability was examined in mice of different age. For this purpose, we quantified expression levels of Mct8 via Real-Time Reverse-Transcriptase PCR in mouse eyecups (C57BL/6) of juvenile and adult age groups. Additionally, age-dependent MCT8 protein levels were quantified via Western blotting and localized via immunofluorescence confocal microscopy. While no difference in Mct8 expression levels could be detected between age groups, MCT8 protein levels in juvenile animals were about two times higher than in adult animals based on Western blot analyses. Immunohistochemical analyses showed that MCT8 immunoreactivity in the eyecup was restricted to the retina and the retinal pigment epithelium. In juvenile mice, MCT8 was broadly observed along the apical membrane of the retinal pigment epithelium, tightly surrounding photoreceptor outer segments. Distinct immunopositive staining was also detected in the inner nuclear layer and the ganglion cell layer. However, in adult specimens, immunoreactivity visibly declined in all layers, which was in line with Western blot analyses. Since MCT8 was abundantly present in juvenile and about twofold lower in adult retinae, our findings suggest a pivotal role of MCT8 especially during postnatal maturation. The present study provides novel insights into age-dependent retinal TH supply, which might help to understand different aspects regarding retinal development, function, and disorders.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Deiodinase knockdown affects zebrafish eye development at the level of gene expression, morphology and function

Retinal development in vertebrates relies extensively on thyroid hormones. Their local availability is tightly controlled by several regulators, including deiodinases (Ds). Here we used morpholino technology to explore the roles of Ds during eye development in zebrafish. Transcriptome analysis at 3 days post fertilization (dpf) revealed a pronounced effect of knockdown of both T4-activating Ds (D1D2MO) or knockdown of T3-inactivating D3 (D3bMO) on phototransduction and retinoid recycling. This was accompanied by morphological defects (studied from 1 to 7 dpf) including reduced eye size, disturbed retinal lamination and strong reduction in rods and all four cone types. Defects were more prominent and persistent in D3-deficient fish. Finally, D3-deficient zebrafish larvae had disrupted visual function at 4 dpf and were less sensitive to a light stimulus at 5 dpf. These data demonstrate the importance of TH-activating and -inactivating Ds for correct zebrafish eye development, and point to D3b as a central player.

The Type 3 Deiodinase Is a Critical Determinant of Appropriate Thyroid Hormone Action in the Developing Testis

Timely and appropriate levels of thyroid hormone (TH) signaling are necessary to ensure normal developmental outcomes in many tissues. Studies using pharmacological models of altered TH status have revealed an influence of these hormones on testis development and size, but little is known about the role of endogenous determinants of TH action in the developing male gonads. Using a genetic approach, we demonstrate that the type 3 deiodinase (D3), which inactivates TH and protects developing tissues from undue TH action, is a key factor. D3 is highly expressed in the developing testis, and D3-deficient (D3KO) mice exhibit thyrotoxicosis and cell proliferation arrest in the neonatal testis, resulting in an approximately 75% reduction in testis size. This is accompanied by larger seminiferous tubules, impaired spermatogenesis, and a hormonal profile indicative of primary hypogonadism. A deficiency in the TH receptor-α fully normalizes testis size and adult testis gene expression in D3KO mice, indicating that the effects of D3 deficiency are mediated through this type of receptor. Similarly, genetic deficiencies in the D2 or in the monocarboxylate transporter 8 partially rescue the abnormalities in testis size and gonadal axis gene expression featured in the D3KO mice. Our study highlights the testis as an important tissue in which determinants of TH action coordinately converge to ensure normal development and identifies D3 as a critical factor in testis development and in testicular protection from thyrotoxicosis.

Thyroid Hormone Signaling and Cone Photoreceptor Viability

Thyroid hormone (TH) signaling regulates cell proliferation, differentiation, and apoptosis. In the retina, TH signaling plays a central role in cone opsin expression. TH signaling inhibits S opsin expression, stimulates M opsin expression, and promotes dorsal-ventral opsin patterning. TH signaling has also been associated with cone photoreceptor viability. Treatment with thyroid hormone triiodothyronine (T3) or induction of high T3 by deleting the hormone-inactivating enzyme type 3 iodothyronine deiodinase (DIO3) causes cone death in mice. This effect is reversed by deletion of the TH receptor (TR) gene. Consistent with the T3 treatment effect, suppressing TH signaling preserves cones in mouse models of retinal degeneration. The regulation of cone survival by TH signaling appears to be independent of its regulatory role in cone opsin expression. The mechanism by which TH signaling regulates cone viability remains to be identified. The current understanding of TH signaling regulation in photoreceptor viability suggests that suppressing TH signaling locally in the retina may represent a novel strategy for retinal degeneration management.

MicroRNA 214 Is a Potential Regulator of Thyroid Hormone Levels in the Mouse Heart Following Myocardial Infarction, by Targeting the Thyroid-Hormone-Inactivating Enzyme Deiodinase Type III

Cardiac thyroid-hormone signaling is a critical determinant of cellular metabolism and function in health and disease. A local hypothyroid condition within the failing heart in rodents has been associated with the re-expression of the fetally expressed thyroid-hormone-inactivating enzyme deiodinase type III (Dio3). While this enzyme emerges as a common denominator in the development of heart failure, the mechanism underlying its regulation remains largely unclear. In the present study, we investigated the involvement of microRNAs (miRNAs) in the regulation of Dio3 mRNA expression in the remodeling left ventricle (LV) of the mouse heart following myocardial infarction (MI). In silico analysis indicated that of the miRNAs that are differentially expressed in the post-MI heart, miR-214 has the highest potential to target Dio3 mRNA. In accordance, a luciferase reporter assay, including the full-length 3'UTR of mouse Dio3 mRNA, showed a 30% suppression of luciferase activity by miR-214. In the post-MI mouse heart, miR-214 and Dio3 protein were shown to be co-expressed in cardiomyocytes, while time-course analysis revealed that Dio3 mRNA expression precedes miR-214 expression in the post-MI LV. This suggests that a Dio3-induced decrease of T3 levels is involved in the induction of miR-214, which was supported by the finding that cardiac miR-214 expression is down regulated by T3 in mice. In vitro analysis of human DIO3 mRNA furthermore showed that miR-214 is able to suppress both mRNA and protein expression. Dio3 mRNA is a target of miR-214 and the Dio3-dependent stimulation of miR-214 expression in post-MI cardiomyocytes supports the involvement of a negative feedback mechanism regulating Dio3 expression.

Scope and limitations of iodothyronine deiodinases in hypothyroidism

The coordinated expression and activity of the iodothyronine deiodinases regulate thyroid hormone levels in hypothyroidism. Once heralded as the pathway underpinning adequate thyroid-hormone replacement therapy with levothyroxine, the role of these enzymes has come into question as they have been implicated in both an inability to normalize serum levels of tri-iodothyronine (T3) and the incomplete resolution of hypothyroid symptoms. These observations, some of which were validated in animal models of levothyroxine monotherapy, challenge the paradigm that tissue levels of T3 and thyroid-hormone signalling can be fully restored by administration of levothyroxine alone. The low serum levels of T3 observed among patients receiving levothyroxine monotherapy occur as a consequence of type 2 iodothyronine deiodinase (DIO2) in the hypothalamus being fairly insensitive to ubiquitination. In addition, residual symptoms of hypothyroidism have been linked to a prevalent polymorphism in the DIO2 gene that might be a risk factor for neurodegenerative disease. Here, we discuss how these novel findings underscore the clinical importance of iodothyronine deiodinases in hypothyroidism and how an improved understanding of these enzymes might translate to therapeutic advances in the care of millions of patients with this condition.

Strong induction of iodothyronine deiodinases by chemotherapeutic selenocompounds

The biological activity of thyroid hormones (TH) is regulated by selenoenzymes of the iodothyronine deiodinase (DIO) family catalysing TH activating and inactivating reactions. Besides TH metabolism, several studies indicate an important role of DIO isoenzymes in tumorigenesis and cancer growth. It is therefore of therapeutic importance to identify modulators of DIO expression. We have synthesized and studied a series of selenocompounds containing a methyl- or benzyl-imidoselenocarbamate backbone. One of these novel compounds had chemotherapeutic activities in a murine xenograft tumour model by an unknown mechanism. Therefore, we tested their effects on DIO expression in vitro. In HepG2 hepatocarcinoma cells, DIO1 activity was strongly (up to 10-fold) increased by the methyl- but not by the corresponding benzyl-imidoselenocarbamates. Steady-state mRNA levels remained unaltered under these conditions indicating a post-transcriptional mode of action. The effects were further characterized in HEK293 cells stably expressing DIO1, DIO2 or DIO3. Even within the artificial genetic context of the expression vectors, all three DIO isoenzymes were up-regulated by the methyl- and to a lesser extent by the benzyl-imidoselenocarbamates. Consistent stimulating effects were observed with methyl-N,N'-di(quinolin-3-ylcarbonyl)-imidoselenocarbamate (EI201), a selenocompound known for its anti-tumour activity. DIO inducing effects were unrelated to the intracellular accumulation of selenium, yet the precise mode of action remains elusive. Collectively, our data highlight that these selenocompounds may constitute interesting pharmacological compounds for modifying DIO expression potentially affecting the balance between cell differentiation and proliferation.

Differentiation of human embryonic stem cells into cone photoreceptors through simultaneous inhibition of BMP, TGFβ and Wnt signaling

Cone photoreceptors are required for color discrimination and high-resolution central vision and are lost in macular degenerations, cone and cone/rod dystrophies. Cone transplantation could represent a therapeutic solution. However, an abundant source of human cones remains difficult to obtain. Work performed in model organisms suggests that anterior neural cell fate is induced ‘by default' if BMP, TGFβ and Wnt activities are blocked, and that photoreceptor genesis operates through an S-cone default pathway. We report here that Coco (Dand5), a member of the Cerberus gene family, is expressed in the developing and adult mouse retina. Upon exposure to recombinant COCO, human embryonic stem cells (hESCs) differentiated into S-cone photoreceptors, developed an inner segment-like protrusion, and could degrade cGMP when exposed to light. Addition of thyroid hormone resulted in a transition from a unique S-cone population toward a mixed M/S-cone population. When cultured at confluence for a prolonged period of time, COCO-exposed hESCs spontaneously developed into a cellular sheet composed of polarized cone photoreceptors. COCO showed dose-dependent and synergistic activity with IGF1 at blocking BMP/TGFβ/Wnt signaling, while its cone-inducing activity was blocked in a dose-dependent manner by exposure to BMP, TGFβ or Wnt-related proteins. Our work thus provides a unique platform to produce human cones for developmental, biochemical and therapeutic studies and supports the hypothesis that photoreceptor differentiation operates through an S-cone default pathway during human retinal development.

Bird eyes distinguish summer from winter: Retinal response to acute photoperiod change in the night-migratory redheaded bunting

Eyes are the part of the circadian timekeeping system but not involved in the photoperiod regulated seasonal physiology in songbirds. Here, two experiments tested whether eyes detect and respond to seasonal change in the photoperiod environment, by examining gene and protein expressions in the retinas of redheaded buntings exposed to a single long day (LD, 16L:8D), with controls on short days (SD, 8L:16D). In the first experiment, mRNA expression of genes implicated in the light perception (opsins, rhodopsin, neuropsin, melanopsin, peropsin) and photoperiod induction (eya3, tsh-β, dio2, dio3) was measured at hours 15 and 19 (hour 0 = light on) on the first long day. There was a significant increase in the eya3, tsh-β and dio2 mRNA expression, albeit with a temporal difference, and decrease in the neuropsin mRNA expression in buntings on the first long day. There was no change in the dio3, rhodopsin, melanopsin and peropsin mRNA expressions on exposure to long days. The second experiment immunohistochemically examined the eya3, tsh-β and rhodopsin peptide expressions. eya3 was expressed in both light conditions, but with a significant higher levels in the retinal photoreceptor layer (PRL) under LD, as compared to SD. Similarly, tsh-β was expressed in the PRL of LD retinas only. Rhodopsin levels were not significantly different between SD and LD conditions, however. These results for the first time show photoperiod-dependent molecular switches in the bunting retina, similar to the well documented thyroid hormone response genes based molecular cascades in the avian hypothalamus.

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

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

Thyroid function and age-related macular degeneration: a prospective population-based cohort study--the Rotterdam Study

BACKGROUND

In animal models, lack of thyroid hormone is associated with cone photoreceptor preservation, while administration of high doses of active thyroid hormone leads to deterioration. The association between thyroid function and age-related macular degeneration (AMD) has not been investigated in the general population.

METHODS

Participants of age ≥ 55 years from the Rotterdam Study with thyroid-stimulating hormone (TSH) and/or free thyroxine (FT4) measurements and AMD assessment were included. We conducted age- and sex-adjusted Cox proportional hazards models to explore the association of TSH or FT4 with AMD, in the full range and in those with TSH (0.4-4.0 mIU/L) and/or FT4 in normal range (11-25 pmol/L). Cox proportional hazards models were performed for the association of TSH or FT4 with retinal pigment alterations (RPA), as an early marker of retinal changes. Multivariable models additionally included cardiovascular risk factors and thyroid peroxidase antibodies positivity. We also performed stratification by age and sex. A bidirectional look-up in genome-wide association study (GWAS) data for thyroid parameters and AMD was performed. Single nucleotide polymorphisms (SNPs) that are significantly associated with both phenotypes were identified.

RESULTS

We included 5,573 participants with a median follow-up of 6.9 years (interquartile range 4.4-10.8 years). During follow-up 805 people developed AMD. TSH levels were not associated with increased risk of AMD. Within normal range of FT4, participants in the highest FT4 quintile had a 1.34-fold increased risk of developing AMD, compared to individuals in the middle group (95% confidence interval [CI] 1.07-1.66). Higher FT4 values in the full range were associated with a higher risk of AMD (hazard ratio 1.04, CI, 1.01-1.06 per 1 pmol/L increase). Higher FT4 levels were similarly associated with a higher risk of RPA. Restricting analyses to euthyroid individuals, additional multivariable models, and stratification did not change estimates. We found a SNP (rs943080) in the VEGF-A gene, associated with AMD, to be significant in the TSH GWAS (P = 1.2 x 10(-4)). Adding this SNP to multivariable models did not change estimates.

CONCLUSIONS

Higher FT4 values are associated with increased risk of AMD - even in euthyroid individuals - and increased risk of RPA. Our data suggest an important role of thyroid hormone in pathways leading to AMD.

Hypothyroidism and brain developmental players

Most of our knowledge on the mechanisms of thyroid hormone (TH) dependent brain development is based on clinical observations and animal studies of maternal/fetal hypothyroidism. THs play an essential role in brain development and hormone deficiency during critical phases in fetal life may lead to severe and permanent brain damage. Maternal hypothyroidism is considered the most common cause of fetal TH deficiency, but the problem may also arise in the fetus. In the case of congenital hypothyroidism due to defects in fetal thyroid gland development or hormone synthesis, clinical symptoms at birth are often mild as a result of compensatory maternal TH supply. TH transporters (THTs) and deiodinases (Ds) are important regulators of intracellular triiodothyronine (T3) availability and therefore contribute to the control of thyroid receptors (TRs)-dependent CNS development and early embryonic life. Defects in fetal THTs or Ds may have more impact on fetal brain since they can result in intracellular T3 deficiency despite sufficient maternal TH supply. One clear example is the recent discovery of mutations in the TH transporter (monocarboxylate transporter 8; MCT8) that could be linked to a syndrome of severe and non reversible psychomotor retardation. Even mild and transient changes in maternal TH levels can directly affect and alter the gene expression profile, and thus disturb fetal brain development. Animal studies are needed to increase our understanding of the exact role of THTs and Ds in prenatal brain development.

Differential function of Gγ13 in rod bipolar and ON cone bipolar cells

Heterotrimeric G-proteins (comprising Gα and Gβγ subunits) are critical for coupling of metabotropic receptors to their downstream effectors. In the retina, glutamate released from photoreceptors in the dark activates metabotropic glutamate receptor 6 (mGluR6) receptors in ON bipolar cells; this leads to activation of Go , closure of transient receptor potential melastatin 1 channels and hyperpolarization of these cells. Go comprises Gαo , Gβ3 and a Gγ. The best Gγ candidate is Gγ13, although functional data to support this are lacking. Thus, we tested Gγ13 function by generating Gng13(-/-) knockout (KO) mice, recording electroretinograms (ERG) and performing immunocytochemical staining. The amplitude of scotopic ERG b-waves in KO mice was lower than in wild-type (WT) mice. Furthermore, in both KO and WT mice, the ERG b-wave decreased with age; this decrease was much more pronounced in KO mice. By contrast, the photopic ERG b-waves in KO mice were hardly affected at any age. In KO mice retinas, immunostaining for Gβ3 and for the GTPase activating proteins RGS7, RGS11, R9AP and Gβ5 decreased significantly in rod bipolar cells but not in ON cone bipolar cells. Staining for Gαo and certain other cascade elements decreased only slightly. Analysis of our ON bipolar cDNA library showed that these cells express mRNAs for Gγ5, Gγ10 and Gγ11. Quantitative RT-PCR of retinal cDNA showed greater values for these transcripts in retinas of KO mice, although the difference was not significant. Our results suggest that Gγ13 contributes to mGluR6 signalling in rod bipolar cells more than in ON cone bipolar cells, and that this contribution includes both coupling the receptor and maintaining a stable localization of the mGluR6-related cascade elements.

Selenoproteins in nervous system development and function

Selenoproteins are a distinct class of proteins that are characterized by the co-translational incorporation of selenium (Se) in the form of the 21st amino acid selenocysteine. Selenoproteins provide a key defense against oxidative stress, as many of these proteins participate in oxidation-reduction reactions neutralizing reactive oxygen species, where selenocysteine residues act as catalytic sites. Many selenoproteins are highly expressed in the brain, and mouse knockout studies have determined that several are required for normal brain development. In parallel with these laboratory studies, recent reports of rare human cases with mutations in genes involved in selenoprotein biosynthesis have described individuals with an assortment of neurological problems that mirror those detailed in knockout mice. These deficits include impairments in cognition and motor function, seizures, hearing loss, and altered thyroid metabolism. Additionally, due to the fact that oxidative stress is a key feature of neurodegenerative disease, there is considerable interest in the therapeutic potential of selenium supplementation for human neurological disorders. Studies performed in cell culture and rodent models have demonstrated that selenium administration attenuates oxidative stress, prevents neurodegeneration, and counters cell signaling mechanisms known to be dysregulated in certain disease states. However, there is currently no definitive evidence in support of selenium supplementation to prevent and/or treat common neurological conditions in the general population. It appears likely that, in humans, supplementation with selenium may only benefit certain subpopulations, such as those that are either selenium-deficient or possess genetic variants that affect selenium metabolism.

Effects of low-dose embryonic thyroid disruption and rearing temperature on the development of the eye and retina in zebrafish

Thyroid hormones are required for vertebrate development, and disruption of the thyroid system in developing embryos can result in a large range of morphologic and physiologic changes, including in the eye and retina. In this study, our anatomic analyses following low-dose, chronic thyroid inhibition reveal that both methimazole (MMI) exposure and rearing temperature affect eye development in a time- and temperature-dependent fashion. Maximal sensitivity to MMI for external eye development occurred at 65 hr postfertilization (hpf) for zebrafish reared at 28°C, and at 69 hpf for those reared at 31°C. Changes in eye diameter corresponded to changes in thickness of two inner retinal layers: the ganglion cell layer and the inner plexiform layer, with irreversible MMI-induced decreases in layer thickness observed in larvae treated with MMI until 66 hpf at 28°C. We infer that maximal sensitivity to MMI between 65 and 66 hpf at 28°C indicates a critical period of thyroid-dependent eye and retinal development. Furthermore, our results support previous work that shows spontaneous escape from MMI-induced effects potentially due to embryonic compensatory actions, as our data show that embryos treated beyond the critical period generally resemble controls.

Genomic imprinting variations in the mouse type 3 deiodinase gene between tissues and brain regions

The Dio3 gene, which encodes for the type 3 deiodinase (D3), controls thyroid hormone (TH) availability. The lack of D3 in mice results in tissue overexposure to TH and a broad neuroendocrine phenotype. Dio3 is an imprinted gene, preferentially expressed from the paternally inherited allele in the mouse fetus. However, heterozygous mice with paternal inheritance of the inactivating Dio3 mutation exhibit an attenuated phenotype when compared with that of Dio3 null mice. To investigate this milder phenotype, the allelic expression of Dio3 was evaluated in different mouse tissues. Preferential allelic expression of Dio3 from the paternal allele was observed in fetal tissues and neonatal brain regions, whereas the biallelic Dio3 expression occurred in the developing eye, testes, and cerebellum and in the postnatal brain neocortex, which expresses a larger Dio3 mRNA transcript. The newborn hypothalamus manifests the highest degree of Dio3 expression from the paternal allele, compared with other brain regions, and preferential allelic expression of Dio3 in the brain relaxed in late neonatal life. A methylation analysis of two regulatory regions of the Dio3 imprinted domain revealed modest but significant differences between tissues, but these did not consistently correlate with the observed patterns of Dio3 allelic expression. Deletion of the Dio3 gene and promoter did not result in significant changes in the tissue-specific patterns of Dio3 allelic expression. These results suggest the existence of unidentified epigenetic determinants of tissue-specific Dio3 imprinting. The resulting variation in the Dio3 allelic expression between tissues likely explains the phenotypic variation that results from paternal Dio3 haploinsufficiency.

PDE9A is expressed in the inner retina and contributes to the normal shape of the photopic ERG waveform

The ubiquitous second messenger cGMP is synthesized by guanylyl cyclase and hydrolyzed by phosphodiesterase (PDE). cGMP mediates numerous signaling pathways in multiple tissues. In the retina, cGMP regulates signaling in nearly every cell class including photoreceptors, bipolar cells, amacrine cells, and ganglion cells. In order to understand the specific role of cGMP and its regulating enzymes in different cell types, it is first necessary to localize these components and dissect their influence on the circuits. Here we tested the contribution of PDE9A to retinal processing by recording the electroretinograms (ERG) of PDE9A^™/™ (KO) mice and by localizing the enzyme. We found that while the scotopic ERG of KO was the same as that of wild type (WT) in both amplitude and kinetics, the photopic ERG was greatly affected. The greatest effect was on the recovery of the b-wave; the falling phase and the b-wave duration were significantly longer in the KO mice for all photopic stimuli (UV, green, or saturating white flashes). The rising phase was slower in KO than in WT for UV and green stimuli. For certain stimuli, amplitudes of both the a- and b-waves were smaller than in WT. Using Lac-Z expression in KO retinas as a reporter for PDE9A expression pattern, we found that PDE9A is localized to GABA-positive and GABA-negative amacrine cells, and likely also to certain types of ganglion cells. Our results indicate that PDE9A, by controlling the level of cGMP, modulates inhibitory processes within the cone pathway. We speculate that these circuits involve NO/cGMP signaling pathways.

Thyroid hormone signaling during early neurogenesis and its significance as a vulnerable window for endocrine disruption

The essential roles of thyroid hormone (TH) in perinatal brain development have been known for decades. More recently, many of the molecular mechanisms underlying the multiple effects of TH on proliferation, differentiation, migration, synaptogenesis and myelination in the developing nervous system have been elucidated. At the same time data from both epidemiological studies and animal models have revealed that the influence of thyroid signaling on development of the nervous system, extends to all periods of life, from early embryogenesis to neurogenesis in the adult brain. This review focuses on recent insights into the actions of TH during early neurogenesis. A key concept is that, in contrast to the previous ideas that only the unliganded receptor was implicated in these early phases, a critical role of the ligand, T3, is increasingly recognized. These findings are considered in the light of increasing knowledge of cell specific control of T3 availability as a function of deiodinase activity and transporter expression. These requirements for TH in the early stages of neurogenesis take on new relevance given the increasing epidemiological data on adverse effects of TH lack in early pregnancy on children's neurodevelopmental outcome. These ideas lead logically into a discussion on how the actions of TH during the first phases of neurogenesis can be potentially disrupted by gestational iodine lack and/or chemical pollution. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

The sonic hedgehog-induced type 3 deiodinase facilitates tumorigenesis of basal cell carcinoma by reducing Gli2 inactivation

Thyroid hormone (TH) is an important regulator of growth, development, and metabolism. Most of the active TH T3 is generated by peripheral TH metabolism mediated by the iodothyronine deiodinases. Type 3 deiodinase (D3) inactivates T3 via specific deiodination reactions. It is an oncofetal protein frequently expressed in neoplastic tissues and is a direct target of the sonic hedgehog (Shh) pathway in basal cell carcinomas (BCCs). However, the molecular mechanisms triggered by T3 in BCC are still mostly unrevealed. Here, we demonstrate that D3 action is critical in the proliferation and survival of BCC cells. D3 depletion or T3 treatment induce apoptosis of BCC cells and attenuate Shh signaling. This is achieved through a direct impairment of Gli2 protein stability by T3. T3 induces protein kinase A, which in turn destabilizes Gli2 protein via its C-terminal degron. Finally, in a mouse model of BCC, T3-topical treatment significantly reduces tumor growth. These results demonstrate the existence of a previously unrecognized cross talk between TH and Gli2 oncogene, providing functional and mechanistic evidence of the involvement of TH metabolism in Shh-induced cancer. TH-mediated Gli2 inactivation would be beneficial for therapeutically purposes, because the inhibition of Shh-Gli2 signaling is an attractive target for several anticancer drugs, currently in clinical trials.

Thyroid hormones in fetal growth and prepartum maturation

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are essential for normal growth and development of the fetus. Their bioavailability in utero depends on development of the fetal hypothalamic-pituitary-thyroid gland axis and the abundance of thyroid hormone transporters and deiodinases that influence tissue levels of bioactive hormone. Fetal T4 and T3 concentrations are also affected by gestational age, nutritional and endocrine conditions in utero, and placental permeability to maternal thyroid hormones, which varies among species with placental morphology. Thyroid hormones are required for the general accretion of fetal mass and to trigger discrete developmental events in the fetal brain and somatic tissues from early in gestation. They also promote terminal differentiation of fetal tissues closer to term and are important in mediating the prepartum maturational effects of the glucocorticoids that ensure neonatal viability. Thyroid hormones act directly through anabolic effects on fetal metabolism and the stimulation of fetal oxygen consumption. They also act indirectly by controlling the bioavailability and effectiveness of other hormones and growth factors that influence fetal development such as the catecholamines and insulin-like growth factors (IGFs). By regulating tissue accretion and differentiation near term, fetal thyroid hormones ensure activation of physiological processes essential for survival at birth such as pulmonary gas exchange, thermogenesis, hepatic glucogenesis, and cardiac adaptations. This review examines the developmental control of fetal T4 and T3 bioavailability and discusses the role of these hormones in fetal growth and development with particular emphasis on maturation of somatic tissues critical for survival immediately at birth.

Knockdown of type 3 iodothyronine deiodinase severely perturbs both embryonic and early larval development in zebrafish

Exposure to appropriate levels of thyroid hormones (THs) at the right time is of key importance for normal development in all vertebrates. Type 3 iodothyronine deiodinase (D3) is the prime TH-inactivating enzyme, and its expression is highest in the early stages of vertebrate development, implying that it may be necessary to shield developing tissues from overexposure to THs. We used antisense morpholino knockdown to examine the role of D3 during early development in zebrafish. Zebrafish possess 2 D3 genes, dio3a and dio3b. Here, we show that both genes are expressed during development and both contribute to in vivo D3 activity. However, dio3b mRNA levels in embryos are higher, and the effects of dio3b knockdown on D3 activity and on the resulting phenotype are more severe. D3 knockdown induced an overall delay in development, as determined by measurements of otic vesicle length, eye and ear size, and body length. The time of hatching was also severely delayed in D3-knockdown embryos. Importantly, we also observed a severe disturbance of several aspects of development. Swim bladder development and inflation was aberrant as was the development of liver and intestine. Furthermore, D3-knockdown larvae spent significantly less time moving, and both embryos and larvae exhibited perturbed escape responses, suggesting that D3 knockdown affects muscle development and/or functioning. These data indicate that D3 is essential for normal zebrafish embryonic and early larval development and show the value of morpholino knockdown in this model to further elucidate the specific role of D3 in some aspects of vertebrate development.

Suppressing thyroid hormone signaling preserves cone photoreceptors in mouse models of retinal degeneration

Cone phototransduction and survival of cones in the human macula is essential for color vision and for visual acuity. Progressive cone degeneration in age-related macular degeneration, Stargardt disease, and recessive cone dystrophies is a major cause of blindness. Thyroid hormone (TH) signaling, which regulates cell proliferation, differentiation, and apoptosis, plays a central role in cone opsin expression and patterning in the retina. Here, we investigated whether TH signaling affects cone viability in inherited retinal degeneration mouse models. Retinol isomerase RPE65-deficient mice [a model of Leber congenital amaurosis (LCA) with rapid cone loss] and cone photoreceptor function loss type 1 mice (severe recessive achromatopsia) were used to determine whether suppressing TH signaling with antithyroid treatment reduces cone death. Further, cone cyclic nucleotide-gated channel B subunit-deficient mice (moderate achromatopsia) and guanylate cyclase 2e-deficient mice (LCA with slower cone loss) were used to determine whether triiodothyronine (T3) treatment (stimulating TH signaling) causes deterioration of cones. We found that cone density in retinol isomerase RPE65-deficient and cone photoreceptor function loss type 1 mice increased about sixfold following antithyroid treatment. Cone density in cone cyclic nucleotide-gated channel B subunit-deficient and guanylate cyclase 2e-deficient mice decreased about 40% following T3 treatment. The effect of TH signaling on cone viability appears to be independent of its regulation on cone opsin expression. This work demonstrates that suppressing TH signaling in retina dystrophy mouse models is protective of cones, providing insights into cone preservation and therapeutic interventions.

Immunocytochemical analysis of misplaced rhodopsin-positive cells in the developing rodent retina

During the first postnatal weeks of the developing rodent retina, rhodopsin can be detected in a number of neuron-like cells in the inner retina. In the present study, we aim to characterize the morphology, number and staining characteristics of this peculiar population. Misplaced rhodopsin-positive cells (MRCs) were analyzed on retinas of four rodent species, labeled with various rhodopsin-specific antibodies. To investigate their possible relation with non-photoreceptor cells, sections were double-stained against distinct retinal cell types and proteins of the phototransduction cascade. The possibility of synapse formation and apoptosis were also investigated. In all species studied, misplaced cells comprised a few percent of all rhodopsin-positive elements. This ratio declined from the end of the second week and MRCs disappeared nearly completely from the retina by P24. MRCs resembled resident neurons of the inner retina, while outer segment-like processes were seen only rarely. MRCs expressed no other photopigment types and showed no colocalization with any of the bipolar, horizontal, amacrine and ganglion cell markers used. While all MRCs colabeled for arrestin and recoverin, other proteins of the phototransduction cascade were only detectable in a minority of the population. Only a few MRCs were shown to form synaptic-like endings. Our results showed that, during development, some rhodopsin-expressing cells are displaced to the inner retinal layers. Although most MRCs lack morphological features of photoreceptors, they contain some but not all, elements of the phototransduction cascade, indicating that they are most probably misplaced rods that failed to complete differentiation and integrate into the photoreceptor mosaic.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Decreased translation of Dio3 mRNA is associated with drug-induced hepatotoxicity

Recent work has demonstrated the importance of post-transcriptional gene regulation in toxic responses. In the present study, we used two rat models to investigate mRNA translation in the liver following xenobiotic-induced toxicity. By combining polysome profiling with genomic methodologies, we were able to assess global changes in hepatic mRNA translation. Dio3 (iodothyronine deiodinase type III) was identified as a gene that exhibited specific translational repression and had a functional role in a number of relevant canonical pathways. Western blot analysis indicated that this repression led to reduced D3 (the protein expressed by Dio3) levels, enhanced over time and with increased dose. Using Northern blotting techniques and qRT-PCR (quantitative reverse transcription–PCR), we confirmed further that there was no reduction in Dio3 mRNA, suggesting that translational repression of Dio3 is an important determinant of the reduced D3 protein expression following liver damage. Finally, we show that drug-induced hepatotoxicity appears to cause localized disruptions in thyroid hormone levels in the liver and plasma. We suggest that this leads to reduced translation of Dio3 mRNA, which results in decreased D3 production. It may therefore be possible that this is an important mechanism by which the liver can, upon early signs of damage, act rapidly to maintain its own energy equilibrium, thereby avoiding global disruption of the hypothalamic–pituitary–thyroid axis.

Cones respond to light in the absence of transducin β subunit

Mammalian cones respond to light by closing a cGMP-gated channel via a cascade that includes a heterotrimeric G-protein, cone transducin, comprising Gαt2, Gβ3 and Gγt2 subunits. The function of Gβγ in this cascade has not been examined. Here, we investigate the role of Gβ3 by assessing cone structure and function in Gβ3-null mouse (Gnb3(-/-)). We found that Gβ3 is required for the normal expression of its partners, because in the Gnb3(-/-) cone outer segments, the levels of Gαt2 and Gγt2 are reduced by fourfold to sixfold, whereas other components of the cascade remain unaltered. Surprisingly, Gnb3(-/-) cones produce stable responses with normal kinetics and saturating response amplitudes similar to that of the wild-type, suggesting that cone phototransduction can function efficiently without a Gβ subunit. However, light sensitivity was reduced by approximately fourfold in the knock-out cones. Because the reduction in sensitivity was similar in magnitude to the reduction in Gαt2 level in the cone outer segment, we conclude that activation of Gαt2 in Gnb3(-/-) cones proceeds at a rate approximately proportional to its outer segment concentration, and that activation of phosphodiesterase and downstream cascade components is normal. These results suggest that the main role of Gβ3 in cones is to establish optimal levels of transducin heteromer in the outer segment, thereby indirectly contributing to robust response properties.

Making sense with thyroid hormone--the role of T(3) in auditory development

The senses are our window to the world, our interface with the habitat in which we live in and the basis for our communication with each other. Although sensory systems are not generally viewed as major targets of endocrine regulation, sensory development is profoundly influenced by thyroid hormone (T(3)) signalling. In this article, we discuss this developmental role of T(3) and highlight the auditory system as the best-studied example of the interplay between systemic and local tissue mechanisms by which T(3) stimulates the onset of sensory function. Several genes that mediate the action of T(3) are known to promote sensory development in mice, including genes that encode T(3) receptors and deiodinase enzymes that amplify or deplete levels of T(3). We also discuss the current knowledge of sensory defects in human genetic disorders in which T(3) signalling is impaired. As sensory input provides the only means of acquiring information from the environment, the stimulation of sensory development is one of the most fundamental functions of T(3) signalling.

Mice lacking Period 1 and Period 2 circadian clock genes exhibit blue cone photoreceptor defects

Many aspects of retinal physiology are modulated by circadian clocks, but it is unclear whether clock malfunction impinges directly on photoreceptor survival, differentiation or function. Eyes from wild-type (WT) and Period1 (Per1) and Period2 (Per2) mutant mice (Per1(Brdm1) Per2(Brdm1) ) were examined for structural (histology, in vivo imaging), phenotypical (RNA expression, immunohistochemistry) and functional characteristics. Transcriptional levels of selected cone genes [red/green opsin (Opn1mw), blue cone opsin (Opn1sw) and cone arrestin (Arr3)] and one circadian clock gene (RORb) were quantified by real-time polymerase chain reaction. Although there were no changes in general retinal histology or visual responses (electroretinograms) between WT and Per1(Brdm1) Per2(Brdm1) mice, compared with age-matched controls, Per1(Brdm1) Per2(Brdm1) mice showed scattered retinal deformations by fundus inspection. Also, mRNA expression levels and immunostaining of blue cone opsin were significantly reduced in mutant mice. Especially, there was an alteration in the dorsal-ventral patterning of blue cones. Decreased blue cone opsin immunoreactivity was present by early postnatal stages, and remained throughout maturation. General photoreceptor differentiation was retarded in young mutant mice. In conclusion, deletion of both Per1 and Per2 clock genes leads to multiple discrete changes in retina, notably patchy tissue disorganization, reductions in cone opsin mRNA and protein levels, and altered distribution. These data represent the first direct link between Per1 and Per2 clock genes, and cone photoreceptor differentiation and function.

Cerebellar abnormalities in mice lacking type 3 deiodinase and partial reversal of phenotype by deletion of thyroid hormone receptor α1

Thyroid hormone serves many functions throughout brain development, but the mechanisms that control the timing of its actions in specific brain regions are poorly understood. In the cerebellum, thyroid hormone controls formation of the transient external germinal layer, which contains proliferative granule cell precursors, subsequent granule cell migration, and cerebellar foliation. We report that the thyroid hormone-inactivating type 3 deiodinase (encoded by Dio3) is expressed in the mouse cerebellum at embryonic and neonatal stages, suggesting a need to protect cerebellar tissues from premature stimulation by thyroid hormone. Dio3(-/-) mice displayed reduced foliation, accelerated disappearance of the external germinal layer, and premature expansion of the molecular layer at juvenile ages. Furthermore, Dio3(-/-) mice exhibited locomotor behavioral abnormalities and impaired ability in descending a vertical pole. To ascertain that these phenotypes resulted from inappropriate exposure to thyroid hormone, thyroid hormone receptor α1 (TRα1) was removed from Dio3(-/-) mice, which substantially corrected the cerebellar and behavioral phenotypes. Deletion of TRα1 did not correct the previously reported small thyroid gland or deafness in Dio3(-/-) mice, indicating that Dio3 controls the activation of specific receptor isoforms in different tissues. These findings suggest that type 3 deiodinase constrains the timing of thyroid hormone action during cerebellar development.

Gβ3 is required for normal light ON responses and synaptic maintenance

Heterotrimeric G-proteins, comprising Gα and Gβγ subunits, couple metabotropic receptors to various downstream effectors and contribute to assembling and trafficking receptor-based signaling complexes. A G-protein β subunit, Gβ(3), plays a critical role in several physiological processes, as a polymorphism in its gene is associated with a risk factor for several disorders. Retinal ON bipolar cells express Gβ(3), and they provide an excellent system to study its role. In the ON bipolar cells, mGluR6 inverts the photoreceptor's signal via a cascade in which glutamate released from photoreceptors closes the TRPM1 channel. This cascade is essential for vision since deficiencies in its proteins lead to complete congenital stationary night blindness. Here we report that Gβ(3) participates in the G-protein heterotrimer that couples mGluR6 to TRPM1. Gβ(3) deletion in mouse greatly reduces the light response under both scotopic and photopic conditions, but it does not eliminate it. In addition, Gβ(3) deletion causes mislocalization and downregulation of most cascade elements and modulators. Furthermore, Gβ(3) may play a role in synaptic maintenance since in its absence, the number of invaginating rod bipolar dendrites is greatly reduced, a deficit that was not observed at 3 weeks, the end of the developmental period.

Thyroid hormone signaling in the Xenopus laevis embryo is functional and susceptible to endocrine disruption

Thyroid hormone (TH) is essential for vertebrate brain development. Most research on TH and neuronal development focuses on late development, mainly the perinatal period in mammals. However, in human infants neuromotor development correlates best with maternal TH levels in the first trimester of pregnancy, suggesting that TH signaling could affect early brain development. Studying TH signaling in early embryogenesis in mammals is experimentally challenging. In contrast, free-living embryos, such as Xenopus laevis, permit physiological experimentation independent of maternal factors. We detailed key elements of TH signaling: ligands, receptors (TR), and deiodinases during early X. laevis development, before embryonic thyroid gland formation. Dynamic profiles for all components were found. Between developmental stages 37 and 41 (~48 h after hatching, coincident with a phase of continuing neurogenesis) significant increases in T(3) levels as well as in mRNA encoding deiodinases and TR occurred. Exposure of embryos at this developmental stage for 24 h to either a TH antagonist, NH-3, or to tetrabromobisphenol A, a flame retardant and known TH disruptor, differentially modulated the expression of a number of TH target genes implicated in neural stem cell function or neural differentiation. Moreover, 24-h exposure to either NH-3 or tetrabromobisphenol A diminished cell proliferation in the brain. Thus, these data show first, that TH signaling exerts regulatory roles in early X. laevis neurogenesis and second, that this period represents a potential window for endocrine disruption.

Mechanisms of thyroid hormone action

Our understanding of thyroid hormone action has been substantially altered by recent clinical observations of thyroid signaling defects in syndromes of hormone resistance and in a broad range of conditions, including profound mental retardation, obesity, metabolic disorders, and a number of cancers. The mechanism of thyroid hormone action has been informed by these clinical observations as well as by animal models and has influenced the way we view the role of local ligand availability; tissue and cell-specific thyroid hormone transporters, corepressors, and coactivators; thyroid hormone receptor (TR) isoform-specific action; and cross-talk in metabolic regulation and neural development. In some cases, our new understanding has already been translated into therapeutic strategies, especially for treating hyperlipidemia and obesity, and other drugs are in development to treat cardiac disease and cancer and to improve cognitive function.

Thyroid hormone's action on progenitor/stem cell biology: new challenge for a classic hormone?

BACKGROUND

Thyroid hormones are involved in developmental and homeostatic processes in several tissues. Their action results in different outcomes depending on the developmental stage, tissue and/or cellular context. Interestingly, their pleiotropic roles are conserved across vertebrates. It is largely documented that thyroid hormones act via nuclear receptors, the TRs, which are transcription factors and whose activity can be modulated by the local availability of the hormone T3. In the "classical view", the T3-induced physiological response depends on the expression of specific TR isoforms and the iodothyronine deiodinase selenoenzymes that control the local level of T3, thus TR activity.

SCOPE OF THE REVIEW

Recent data have clearly established that the functionality of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and propose a new and intriguing role for thyroid hormones in two selected examples.

MAJOR CONCLUSIONS

In the intestinal epithelium and the retina, TRα1 and TRβ2 are expressed at the level of the precursors where they induce cell proliferation and differentiation, respectively. Moreover, these different functions result from the integration of the hormone signal with other intrinsic pathways, which play a fundamental role in progenitor/stem cell physiology.

GENERAL SIGNIFICANCE

Taken together, the interaction of TRs with other signaling pathways, specifically in stem/progenitor cells, is a new concept that may have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Function of thyroid hormone transporters in the central nervous system

BACKGROUND

Iodothyronines are charged amino acid derivatives that cannot passively cross a phospholipid bilayer. Transport of thyroid hormones across plasma membranes is mediated by integral membrane proteins belonging to several gene families. These transporters therefore allow or limit access of thyroid hormones into brain. Since thyroid hormones are essential for brain development and cell differentiation, it is expected that genetic deficiency of such transporters would result in neurodevelopmental derangements.

SCOPE OF REVIEW

We introduce concepts of thyroid hormone transport into the brain and into brain cells. Important thyroid hormone transmembrane transporters are presented along with their expression patterns in different brain cell types. A focus is placed on monocarboxylate transporter 8 (MCT8) which has been identified as an essential thyroid hormone transporter in humans. Mutations in MCT8 underlie one of the first described X-linked mental retardation syndromes, the Allan-Herndon-Dudley syndrome.

MAJOR CONCLUSIONS

Thyroid hormone transporter molecules are expressed in a developmental and cell type-specific pattern. Any thyroid hormone molecule has to cross consecutively the luminal and abluminal membranes of the capillary endothelium, enter astrocytic foot processes, and leave the astrocyte through the plasma membrane to finally cross another plasma membrane on its way towards its target nucleus.

GENERAL SIGNIFICANCE

We can expect more transporters being involved in or contributing to in neurodevelopmental or neuropsychiatric disease. Due to their expression in cellular components regulating the hypothalamus-pituitary-thyroid axis, mutations and polymorphisms are expected to impact on negative feedback regulation and hormonal setpoints. This article is part of a Special Issue entitled Thyroid hormone signalling.

Dynamic mRNA distribution pattern of thyroid hormone transporters and deiodinases during early embryonic chicken brain development

Maternal thyroid hormones (THs) are important in early brain development long before the onset of embryonic TH secretion, but information about the regulation of TH availability in the brain at these early stages is still limited. We therefore investigated in detail the mRNA distribution pattern of the TH activating type 2 and inactivating type 3 deiodinases (D2 and D3) and the TH transporters, organic anion transporting polypeptide 1c1 (Oatp1c1) and monocarboxylate transporter 8 (Mct8), in chicken embryonic brain as well as in retina and inner ear from day 3 to day 10 of development. Oatp1c1, Mct8 and D3 are expressed in the choroid plexus and its precursors allowing selective uptake of THs at the blood-cerebrospinal fluid-barrier with subsequent inactivation of excess hormone. In contrast, the developing blood-brain-barrier does not express Oatp1c1 or Mct8 but appears to be a site for TH activation by D2. Expression of D3 in several sensory brain centers may serve as protection against premature TH action. Expression of D2 and Mct8 but not D3 in the developing pituitary gland allows accumulation of active THs even at early stages. Mct8 is widely expressed in gray matter throughout the brain. This is the first comprehensive study on the dynamic distribution pattern of TH-transporters and deiodinases at stages of embryonic brain development when only maternal THs are available. It provides the essential background for further research aimed at understanding early developmental processes depending on maternal THs.

Thyroid hormone replacement therapy: three 'simple' questions, complex answers

Current guidelines recommend that hypothyroid patients should be treated with levothyroxine, which in the vast majority of the cases leads to resolution of the symptoms and normalization of serum free T4 (FT4), T3 and TSH levels. However, a small group of hypothyroid patients remain symptomatic for neurocognitive dysfunction despite normal serum FT4 and TSH, which could be explained by localized brain hypothyroidism. More than half of the T3 in the brain is produced locally via the action of the type II deiodinase (D2) and variability/defects in this pathway could explain the residual symptoms. If this rationale is correct, adding liothyronine to the replacement therapy could prove beneficial. However, with a few exceptions, several clinical trials failed to identify any beneficial effects of combined therapy. More recently, the results of a large clinical trial revealed a better neurocognitive outcome with combined therapy only in hypothyroid patients carrying a polymorphism in the DIO2 gene. This obviously needs to be confirmed by other groups but it is tempting to speculate that combined levothyroxine and liothyronine has a place in the treatment of hypothyroidism, for some.

The deiodinases and the control of intracellular thyroid hormone signaling during cellular differentiation

Background

Thyroid hormone influences gene expression in virtually all vertebrates. Its action is initiated by the activation of T4 to T3, an outer ring deiodination reaction that is catalyzed by the type 1 or the type 2 iodothyronine selenodeiodinases (D1 or D2). Inactivation of T4 and T3 occurs via inner ring deiodination catalyzed by the type 3 iodothyronine selenodeiodinases (D3). The T4 concentration is generally quite stable in human plasma, with T3 levels also remaining constant. Deiodinase actions are tightly regulated in both pre- and post-natal life when they are required to make local adjustments of intracellular T3 concentrations in a precise spatio- and temporal manner. Although all the signals governing the dynamic expression of deiodinases in specific cell types are not known, many important regulatory factors have been deciphered.

Scope of review

This review provides striking examples from the recent literature illustrating how the expression of D2 and D3 is finely tuned during maturation of different organs, and how their action play a critical role in different settings to control intracellular T3 availability.

Major conclusions

Emerging evidence indicates that in various cell contexts, D2 and D3 are expressed in a dynamic balance, in which the expression of one enzyme is coordinately regulated with that of the other to tightly control intracellular T3 levels commensurate with cell requirements at that time.

General significance

Deiodinases control TH action in a precise spatio-temporal fashion thereby providing a novel mechanism for the local paracrine and autocrine regulation of TH action. This remarkable tissue-specific regulation of intracellular thyroid status remains hidden due to the maintenance of constant circulating TH concentrations by the hypothalamic–pituitary–thyroid axis. This article is part of a Special Issue entitled Thyroid hormone signalling.

Minireview: the role of nuclear receptors in photoreceptor differentiation and disease

Rod and cone photoreceptors are specialized sensory cells that mediate vision. Transcriptional controls are critical for the development and long-term survival of photoreceptors; when these controls become ineffective, retinal dysfunction or degenerative disease may result. This review discusses the role of nuclear receptors, a class of ligand-regulated transcription factors, at key stages of photoreceptor life in the mammalian retina. Nuclear receptors with known ligands, such as retinoids or thyroid hormone, together with several orphan receptors without identified physiological ligands, complement other classes of transcription factors in directing the differentiation and functional maintenance of photoreceptors. The potential of nuclear receptors to respond to ligands introduces versatility into the control of photoreceptor development and function and may suggest new opportunities for treatments of photoreceptor disease.

Genomic imprinting of the type 3 thyroid hormone deiodinase gene: regulation and developmental implications

BACKGROUND

In recent years, findings in a number of animal and human models have ignited renewed interest in the type 3 deiodinase (D3), the main enzyme responsible for the inactivation of thyroid hormones. The induction of D3 in models of illness and injury has raised critical questions about the physiological significance of reduced thyroid hormone availability in those states. Phenotypes in transgenic mice lacking this enzyme also point to important developmental roles for D3. A critical determinant of D3 expression is genomic imprinting, an epigenetic phenomenon that regulates a small number of dosage-critical genes in the mammalian genome. The D3 gene (Dio3) is imprinted and preferentially expressed from one of the alleles in most tissues.

SCOPE OF REVIEW

In the context of the physiological significance of D3 and the characteristics and purported origins of genomic imprinting, we review the current knowledge about the epigenetic mechanisms specifying gene dosage in the Dio3 locus.

MAJOR CONCLUSIONS

Altered Dio3 dosage is detrimental to development, suggesting that the level of thyroid hormone action needs to be exquisitely tailored in a timely fashion to the requirements of particular tissues. An appropriate Dio3 dosage is the result of the coordinated action of certain genomic elements and epigenetic marks in the Dlk1-Dio3 domain.

GENERAL SIGNIFICANCE

The imprinting of Dio3 prompts intriguing questions about why the level of thyroid hormone signaling should be regulated in this rare epigenetic manner, and to what extent altered Dio3 expression due to aberrant imprinting may be implicated in human conditions. This article is part of a Special Issue entitled Thyroid hormone signalling.