Ligands of steroid/thyroid receptors induce cone photoreceptors in vertebrate retina

Thyroid Hormone Signaling Is Required for Dynamic Variation in Opsins in the Retina during Metamorphosis of the Japanese Flounder (Paralichthys olivaceus)

In the present study, we investigated the function of thyroid hormones (TH) in visual remodeling during Japanese flounder (Paralichthys olivaceus) metamorphosis through cellular molecular biology experiments. Our results showed that the expression of the five opsin genes of the flounder were highest in eye tissue and varied with the metamorphosis process. The expression of rh1, sws2aβ and lws was positively regulated by exogenous TH, but inhibited by thiourea (TU) compared to the control group. In addition, there was a significant increase in sws2aβ and lws in the rescue experiments performed with TU-treated larvae (p < 0.05). Meanwhile, T3 levels in flounder larvae were increased by TH and decreased by TU. Based on the differences in the expression of the three isoforms of the thyroid hormone receptor (TR) (Trαa, Trαb and Trβ), we further hypothesized that T3 may directly or indirectly regulate the expression of sws2aβ through Trαa. This study demonstrates the regulatory role of TH in opsins during flounder metamorphosis and provides a basis for further investigation on the molecular mechanisms underlying the development of the retinal photoreceptor system in flounders.

Bisphenol S disrupts opsins gene expression and impairs the light-sensing function via antagonizing TH-TRβ signaling pathway in zebrafish larvae

Bisphenol S (BPS) is extensively used in "bisphenol A-free" products such as baby bottles. Although the visual toxicity of BPS has been reported, the underlying mechanism was largely unknown. In the present study, zebrafish were exposed to 0, 4 and 400 nM BPS from 2 h post-fertilization (hpf) to 120 hpf to further explore the thyroid disruption mechanism underlying the BPS induced impairment of visual function. The results showed that BPS decreased T3 levels in larval eyes, induced retinal expression of thyroid hormone receptor β (TRβ), and thereby down-regulated the expression of TH-mediated opsin genes (opn1lw1, opn1lw2, opn1mw1, opn1mw2, opn1mw3, and opn1sw2) and impaired subsequent phototransduction pathways, leading to decreased visually mediated phototactic response and body color adaptation but stimulated visual motor response (VMR). Combining exposure of exogenous T3 or 1-850 (antagonist for TRβ) with BPS could partly compensate the inhibited expression of opsin genes (opn1mw2, opn1lw1, and opn1lw2) and alleviate the hyperactivity of larval VMR caused by BPS alone, suggesting that BPS disrupted the opsins expression and also light-sensing function via antagonizing TH-TRβ signaling pathway. This study underlined the importance of TH signaling in regulating the proper vision and proposed a novel mechanism for the visual toxicity of BPS.

Maternal hypothyroidism is associated with M-opsin developmental delay

Thyroid hormones are critical for the development of opsins involved in color vision. Hypothyroid mice show delayed M-opsin development and expanded distribution of S-opsin on the retina. However, the effects of maternal hypothyroidism on opsin development remain unknown. This study investigates the effects of congenital central hypothyroidism and maternal hypothyroidism on opsin development in thyrotropin-releasing hormone knockout (TRH-/-) mice. We examined the mRNA expression and protein distribution of S/M-opsin on postnatal days (P)12 and 17, as well as mRNA expression of type 2 and 3 iodothyronine deiodinase (DIO2 and DIO3, respectively) in the retina and type 1 iodothyronine deiodinase (DIO1) in the liver at P12 in TRH+/- mice born to TRH+/- or TRH-/- dams, and conducted S/M-opsin analysis in TRH+/+ or TRH-/- mice born to TRH+/- dams at P12, P17, and P30. M-opsin expression was lower in TRH+/- mice born to TRH-/- dams than in those born to TRH+/- dams, whereas S-opsin expression did not significantly differ between them. DIO1, DIO2, and DIO3 mRNA expression levels were not significantly different between the two groups; therefore, thyroid function in peripheral tissues in the pups was similar. S/M-opsin expression did not significantly differ between the TRH+/+ and TRH-/- mice born to TRH+/- dams on any postnatal day. These results demonstrate that maternal hypothyroidism causes M-opsin developmental delay during the early developmental stages of neonatal mice, and TRH-/- mice, a model of congenital central hypothyroidism, born to a euthyroid dam do not have delayed opsin development.

Endocrine regulation of multichromatic color vision

Vertebrate color vision requires spectrally selective opsin-based pigments, expressed in distinct cone photoreceptor populations. In primates and in fish, spectrally divergent opsin genes may reside in head-to-tail tandem arrays. Mechanisms underlying differential expression from such arrays have not been fully elucidated. Regulation of human red (LWS) vs. green (MWS) opsins is considered a stochastic event, whereby upstream enhancers associate randomly with promoters of the proximal or distal gene, and one of these associations becomes permanent. We demonstrate that, distinct from this stochastic model, the endocrine signal thyroid hormone (TH) regulates differential expression of the orthologous zebrafish lws1/lws2 array, and of the tandemly quadruplicated rh2-1/rh2-2/rh2-3/rh2-4 array. TH treatment caused dramatic, dose-dependent increases in abundance of lws1, the proximal member of the lws array, and reduced lws2 Fluorescent lws reporters permitted direct visualization of individual cones switching expression from lws2 to lws1 Athyroidism increased lws2 and reduced lws1, except within a small ventral domain of lws1 that was likely sustained by retinoic acid signaling. Changes in lws abundance and distribution in athyroid zebrafish were rescued by TH, demonstrating plasticity of cone phenotype in response to this signal. TH manipulations also regulated the rh2 array, with athyroidism reducing abundance of distal members. Interestingly, the opsins encoded by the proximal lws gene and distal rh2 genes are sensitive to longer wavelengths than other members of their respective arrays; therefore, endogenous TH acts upon each opsin array to shift overall spectral sensitivity toward longer wavelengths, underlying coordinated changes in visual system function during development and growth.

Photoreceptor Progenitors Depend Upon Coordination of gdf6a, thrβ, and tbx2b to Generate Precise Populations of Cone Photoreceptor Subtypes

Purpose

Replacing cone photoreceptors, the units of the retina necessary for daytime vision, depends upon the successful production of a full variety of new cones from, for example, stem cells. Using genetic experiments in a model organism with high cone diversity, zebrafish, we map the intersecting effects of cone development factors gdf6a, tbx2b, and thrβ.

Methods

We investigated these genes of interest by using genetic combinations of mutants, gene knockdown, and dominant negative gene expression, and then quantified cone subtype outcomes (which normally develop in tightly regulated ratios).

Results

Gdf6a mutants have reduced blue cones and, discovered here, reduced red cones. In combined gdf6a/tbx2b disruption, the loss of gdf6a in heterozygous tbx2b mutants reduced UV cones. Intriguingly, when we disrupted thrβ in gdf6a mutants by using a thrβ morpholino, their combined early disruption revealed a lamination phenotype. Disrupting thrβ activity via expression of a dominant negative thrβ (dnthrβ) at either early or late retinal development had differential outcomes on red cones (reduced abundance), versus UV and blue cones (increased abundance). By using dnthrβ in gdf6a mutants, we revealed that disrupting thrβ activity did not change gdf6a mutant cone phenotypes.

Conclusions

Gdf6a loss directly affects blue and red cones and indirectly affects UV cones by increasing sensitivity to additional disruption, such as reduced tbx2b, resulting in fewer UV cones. The effects of thrβ change through photoreceptor development, first promoting red cones and restricting UV cones, and later restricting UV and blue cones. The effects of gdf6a on UV, blue, and red cone development overlap with, but likely supersede, those of thrβ.

Expression of the inactivating deiodinase, Deiodinase 3, in the pre-metamorphic tadpole retina

Thyroid hormone (TH) orchestrates amphibian metamorphosis. Thus, this developmental phase is often used to study TH-dependent responses in specific tissues. However, TH signaling appears early in development raising the question of the control of TH availability in specific cell types prior to metamorphosis. TH availability is under strict temporal and tissue-specific control by deiodinases. We examined the expression of the TH-inactivating enzyme, deiodinase type 3 (D3), during early retinal development. To this end we created a Xenopus laevis transgenic line expressing GFP from the Xenopus dio3 promoter region (pdio3) and followed pdio3-GFP expression in pre-metamorphic tadpoles. To validate retinal GFP expression in the transgenic line as a function of dio3 promoter activity, we used in situ hybridization to compare endogenous dio3 expression to reporter-driven GFP activity. Retinal expression of dio3 increased during pre-metamorphosis through stages NF41, 45 and 48. Both sets of results show dio3 to have cell-specific, dynamic expression in the pre-metamorphic retina. At stage NF48, dio3 expression co-localised with markers for photoreceptors, rods, Opsin-S cones and bipolar neurons. In contrast, in post-metamorphic juveniles dio3 expression was reduced and spatially confined to certain photoreceptors and amacrine cells. We compared dio3 expression at stages NF41 and NF48 with TH-dependent transcriptional responses using another transgenic reporter line: THbZIP-GFP and by analyzing the expression of T3-regulated genes in distinct TH availability contexts. At stage NF48, the majority of retinal cells expressing dio3 were negative for T3 signaling. Notably, most ganglion cells were virtually both dio3-free and T3-responsive. The results show that dio3 can reduce TH availability at the cellular scale. Further, a reduction in dio3 expression can trigger fine-tuned T3 action in cell-type specific maturation at the right time, as exemplified here in photoreceptor survival in the pre-metamorphic retina.

Mechanisms of Photoreceptor Patterning in Vertebrates and Invertebrates

Across the animal kingdom, visual systems have evolved to be uniquely suited to the environments and behavioral patterns of different species. Visual acuity and color perception depend on the distribution of photoreceptor (PR) subtypes within the retina. Retinal mosaics can be organized into three broad categories: stochastic/regionalized, regionalized, and ordered. We describe here the retinal mosaics of flies, zebrafish, chickens, mice, and humans, and the gene regulatory networks controlling proper PR specification in each. By drawing parallels in eye development between these divergent species, we identify a set of conserved organizing principles and transcriptional networks that govern PR subtype differentiation.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

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.

Photoreceptor cell fate specification in vertebrates

Photoreceptors--the light-sensitive cells in the vertebrate retina--have been extremely well-characterized with regards to their biochemistry, cell biology and physiology. They therefore provide an excellent model for exploring the factors and mechanisms that drive neural progenitors into a differentiated cell fate in the nervous system. As a result, great progress in understanding the transcriptional network that controls photoreceptor specification and differentiation has been made over the last 20 years. This progress has also enabled the production of photoreceptors from pluripotent stem cells, thereby aiding the development of regenerative medical approaches to eye disease. In this Review, we outline the signaling and transcription factors that drive vertebrate photoreceptor development and discuss how these function together in gene regulatory networks to control photoreceptor cell fate specification.

Light, lipids and photoreceptor survival: live or let die?

Due to its constant exposure to light and its high oxygen consumption the retina is highly sensitive to oxidative damage, which is a common factor in inducing the death of photoreceptors after light damage or in inherited retinal degenerations. The high content of docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, has been suggested to contribute to this sensitivity. DHA is crucial for developing and preserving normal visual function. However, further roles of DHA in the retina are still controversial. Current data support that it can tilt the scale either towards degeneration or survival of retinal cells. DHA peroxidation products can be deleterious to the retina and might lead to retinal degeneration. However, DHA has also been shown to act as, or to be the source of, a survival molecule that protects photoreceptors and retinal pigment epithelium cells from oxidative damage. We have established that DHA protects photoreceptors from oxidative stress-induced apoptosis and promotes their differentiation in vitro. DHA activates the retinoid X receptor (RXR) and the ERK/MAPK pathway, thus regulating the expression of anti and pro-apoptotic proteins. It also orchestrates a diversity of signaling pathways, modulating enzymatic pathways that control the sphingolipid metabolism and activate antioxidant defense mechanisms to promote photoreceptor survival and development. A deeper comprehension of DHA signaling pathways and context-dependent behavior is required to understand its dual functions in retinal physiology.

Visual pigments and opsin expression in the juveniles of three species of fish (rainbow trout, zebrafish, and killifish) following prolonged exposure to thyroid hormone or retinoic acid

Thyroid hormone (TH) and retinoic acid (RA) are powerful modulators of photoreceptor differentiation during vertebrate retinal development. In the embryos and young juveniles of salmonid fishes and rodents, TH induces switches in opsin expression within individual cones, a phenomenon that also occurs in adult rodents following prolonged (12 week) hypothyroidism. Whether changes in TH levels also modulate opsin expression in the differentiated retina of fish is unknown. Like TH, RA is essential for retinal development, but its role in inducing opsin switches, if any, has not been studied. Here we investigate the action of TH and RA on single-cone opsin expression in juvenile rainbow trout, zebrafish, and killifish and on the absorbance of visual pigments in rainbow trout and zebrafish. Prolonged TH exposure increased the wavelength of maximum absorbance (λmax ) of the rod and the medium (M, green) and long (L, red) wavelength visual pigments in all fish species examined. However, unlike the opsin switch that occurred following TH exposure in the single cones of small juvenile rainbow trout (alevin), opsin expression in large juvenile rainbow trout (smolt), zebrafish, or killifish remained unchanged. RA did not induce any opsin switches or change the visual pigment absorbance of photoreceptors. Neither ligand altered cone photoreceptor densities. We conclude that RA has no effect on opsin expression or visual pigment properties in the differentiated retina of these fishes. In contrast, TH affected both single-cone opsin expression and visual pigment absorbance in the rainbow trout alevin but only visual pigment absorbance in the smolt and in zebrafish. The latter results could be explained by a combination of opsin switches and chromophore shifts from vitamin A1 to vitamin A2.

Thyroid hormone accelerates the differentiation of adult hippocampal progenitors

Disrupted thyroid hormone function evokes severe physiological consequences in the immature brain. In adulthood, although clinical reports document an effect of thyroid hormone status on mood and cognition, the molecular and cellular changes underlying these behavioural effects are poorly understood. More recently, the subtle effects of thyroid hormone on structural plasticity in the mature brain, in particular on adult hippocampal neurogenesis, have come to be appreciated. However, the specific stages of adult hippocampal progenitor development that are sensitive to thyroid hormone are not defined. Using nestin-green fluorescent protein reporter mice, we demonstrate that thyroid hormone mediates its effects on hippocampal neurogenesis by influencing Type 2b and Type 3 progenitors, although it does not alter proliferation of either the Type 1 quiescent progenitor or the Type 2a amplifying neural progenitor. Thyroid hormone increases the number of doublecortin (DCX)-positive Type 3 progenitors, and accelerates neuronal differentiation into both DCX-positive immature neurones and neuronal nuclei-positive granule cell neurones. Furthermore, we show that this increase in neuronal differentiation is accompanied by a significant induction of specific transcription factors involved in hippocampal progenitor differentiation. In vitro studies using the neurosphere assay support a direct effect of thyroid hormone on progenitor development because neurospheres treated with thyroid hormone are shifted to a more differentiated state. Taken together, our results indicate that thyroid hormone mediates its neurogenic effects via targeting Type 2b and Type 3 hippocampal progenitors, and suggests a role for proneural transcription factors in contributing to the effects of thyroid hormone on neuronal differentiation of adult hippocampal progenitors.

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.

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.

A component of retinal light adaptation mediated by the thyroid hormone cascade

Analysis with DNA-microrrays and real time PCR show that several genes involved in the thyroid hormone cascade, such as deiodinase 2 and 3 (Dio2 and Dio3) are differentially regulated by the circadian clock and by changes of the ambient light. The expression level of Dio2 in adult rats (2-3 months of age) kept continuously in darkness is modulated by the circadian clock and is up-regulated by 2 fold at midday. When the diurnal ambient light was on, the expression level of Dio2 increased by 4-8 fold and a consequent increase of the related protein was detected around the nuclei of retinal photoreceptors and of neurons in inner and outer nuclear layers. The expression level of Dio3 had a different temporal pattern and was down-regulated by diurnal light. Our results suggest that DIO2 and DIO3 have a role not only in the developing retina but also in the adult retina and are powerfully regulated by light. As the thyroid hormone is a ligand-inducible transcription factor controlling the expression of several target genes, the transcriptional activation of Dio2 could be a novel genomic component of light adaptation.

Plasticity of photoreceptor-generating retinal progenitors revealed by prolonged retinoic acid exposure

Background

Retinoic acid (RA) is important for vertebrate eye morphogenesis and is a regulator of photoreceptor development in the retina. In the zebrafish, RA treatment of postmitotic photoreceptor precursors has been shown to promote the differentiation of rods and red-sensitive cones while inhibiting the differentiation of blue- and UV-sensitive cones. The roles played by RA and its receptors in modifying photoreceptor fate remain to be determined.

Results

Treatment of zebrafish embryos with RA, beginning at the time of retinal progenitor cell proliferation and prior to photoreceptor terminal mitosis, resulted in a significant alteration of rod and cone mosaic patterns, suggesting an increase in the production of rods at the expense of red cones. Quantitative pattern analyses documented increased density of rod photoreceptors and reduced local spacing between rod cells, suggesting rods were appearing in locations normally occupied by cone photoreceptors. Cone densities were correspondingly reduced and cone photoreceptor mosaics displayed expanded and less regular spacing. These results were consistent with replacement of approximately 25% of positions normally occupied by red-sensitive cones, with additional rods. Analysis of embryos from a RA-signaling reporter line determined that multiple retinal cell types, including mitotic cells and differentiating rods and cones, are capable of directly responding to RA. The RA receptors RXRγ and RARαb are expressed in patterns consistent with mediating the effects of RA on photoreceptors. Selective knockdown of RARαb expression resulted in a reduction in endogenous RA signaling in the retina. Knockdown of RARαb also caused a reduced production of rods that was not restored by simultaneous treatments with RA.

Conclusions

These data suggest that developing retinal cells have a dynamic sensitivity to RA during retinal neurogenesis. In zebrafish RA may influence the rod vs. cone cell fate decision. The RARαb receptor mediates the effects of endogenous, as well as exogenous RA, on rod development.

Mouse retinal development: a dark horse model for systems biology research

The developing retina is an excellent model to study cellular fate determination and differentiation in the context of a complex tissue. Over the last decade, many basic principles and key genes that underlie these processes have been experimentally identified. In this review, we construct network models to summarize known gene interactions that underlie determination and fundamentally affect differentiation of each retinal cell type. These networks can act as a scaffold to assemble subsequent discoveries. In addition, these summary networks provide a rational segue to systems biology approaches necessary to understand the many events leading to appropriate cellular determination and differentiation in the developing retina and other complex tissues.

Thyroid hormone accelerates opsin expression during early photoreceptor differentiation and induces opsin switching in differentiated TRα-expressing cones of the salmonid retina

Thyroid hormone and its receptors (TRs) regulate photoreceptor differentiation and visual pigment protein (opsin) expression in the retinas of several vertebrates, including rodents and fish. In some of these animals, opsin expression can arise through switches within differentiated cone photoreceptors. In salmonid fishes, single cones express ultraviolet (SWS1) opsin during embryonic development and switch to blue (SWS2) opsin as the fishes grow. It is unknown whether thyroid hormone regulates opsin expression during early cone differentiation and acts through TRs to induce opsin switches in differentiated cones of the salmonid retina. Using in situ hybridization, we characterized the spatiotemporal dynamics of opsin expression and switching in embryos treated with exogenous thyroid hormone or propylthiouracil (PTU), a pharmacological inhibitor of thyroid hormone synthesis. We combined immunohistochemistry with in situ hybridization to map TRα expression with respect to cones undergoing the opsin switch in older juvenile fish. Thyroid hormone accelerated opsin expression in differentiating cones and induced the opsin switch in differentiated single cones, whereas PTU repressed the opsin switch. TRα was not detected in differentiating photoreceptors as opsin expression initiated, but was later expressed in differentiated single cones before the onset of the opsin switch. TRα expression exhibited a dynamic dorsoventral distribution that paralleled the progression of the opsin switch. Together, our results show that thyroid hormone is required for opsin switching in the retina of salmonid fishes and suggest that TRα may be involved in regulating this phenomenon.

Analysis of thyroid response element activity during retinal development

Thyroid hormone (TH) signaling components are expressed during retinal development in dynamic spatial and temporal patterns. To probe the competence of retinal cells to mount a transcriptional response to TH, reporters that included thyroid response elements (TREs) were introduced into developing retinal tissue. The TREs were placed upstream of a minimal TATA-box and two reporter genes, green fluorescent protein (GFP) and human placental alkaline phosphatase (PLAP). Six of the seven tested TREs were first tested in vitro where they were shown to drive TH-dependent expression. However, when introduced into the developing retina, the TREs reported in different cell types in both a TH-dependent and TH-independent manner, as well as revealed specific spatial patterns in their expression. The role of the known thyroid receptors (TR), TRα and TRβ, was probed using shRNAs, which were co-electroporated into the retina with the TREs. Some TREs were positively activated by TR+TH in the developing outer nuclear layer (ONL), where photoreceptors reside, as well as in the outer neuroblastic layer (ONBL) where cycling progenitor cells are located. Other TREs were actively repressed by TR+TH in cells of the ONBL. These data demonstrate that non-TRs can activate some TREs in a spatially regulated manner, whereas other TREs respond only to the known TRs, which also read out activity in a spatially regulated manner. The transcriptional response to even simple TREs provides a starting point for understanding the regulation of genes by TH, and highlights the complexity of transcriptional regulation within developing tissue.

Genetic basis of differential opsin gene expression in cichlid fishes

Visual sensitivity can be tuned by differential expression of opsin genes. Among African cichlid fishes, seven cone opsin genes are expressed in different combinations to produce diverse visual sensitivities. To determine the genetic architecture controlling these adaptive differences, we analysed genetic crosses between species expressing different complements of opsin genes. Quantitative genetic analyses suggest that expression is controlled by only a few loci with correlations among some genes. Genetic mapping identifies clear evidence of trans-acting factors in two chromosomal regions that contribute to differences in opsin expression as well as one cis-regulatory region. Therefore, both cis and trans regulation are important. The simple genetic architecture suggested by these results may explain why opsin gene expression is evolutionarily labile, and why similar patterns of expression have evolved repeatedly in different lineages.

Cyclic AMP-dependent activation of rhodopsin gene transcription in cultured retinal precursor cells of chicken embryo

The present study describes a robust 50-fold increase in rhodopsin gene transcription by cAMP in cultured retinal precursor cells of chicken embryo. Retinal cells isolated at embryonic day 8 (E8) and cultured for 3 days in serum-supplemented medium differentiated mostly into red-sensitive cones and to a lesser degree into green-sensitive cones, as indicated by real-time RT-PCR quantification of each specific opsin mRNA. In contrast, both rhodopsin mRNA concentration and rhodopsin gene promoter activity required the presence of cAMP-increasing agents [forskolin and 3-isobutyl-1-methylxanthine (IBMX)] to reach significant levels. This response was rod-specific and was sufficient to activate rhodopsin gene transcription in serum-free medium. The increase in rhodopsin mRNA levels evoked by a series of cAMP analogs suggested the response was mediated by protein kinase A, not by EPAC. Membrane depolarization by high KCl concentration also increased rhodopsin mRNA levels and this response was strongly potentiated by IBMX. The rhodopsin gene response to cAMP-increasing agents was developmentally gated between E6 and E7. Rod-specific transducin alpha subunit mRNA levels also increased up to 50-fold in response to forskolin and IBMX, while rod-specific phosphodiesterase-VI and rod arrestin transcripts increased 3- to 10-fold. These results suggest a cAMP-mediated signaling pathway may play a role in rod differentiation.

ERG findings in three hypothyroid adult dogs with and without levothyroxine treatment

PURPOSE

To evaluate the effects of levothyroxine (LTh) on the electroretinogram (ERG) of adult dogs.

MATERIAL AND METHODS

Binocular, full field photopic and scotopic ERGs were recorded from an anesthetized Maltese Bichon cross (MB), a Yorkshire Terrier (YT) and a Shetland Sheepdog (SS) affected with hypothyroidism and treated with a daily dose of LTh at 20 microg/kg. The photopic ERGs were evoked to 12 different intensities ranging from 0.81 to -2.19 log cd.s/m(2) and presented under photopic conditions in order to assess (from the derived luminance-response curves) Vmax and b : a amplitude ratio parameters. Photopic flicker ERGs were obtained at 30 Hz. The scotopic ERGs (intensity: -3.09 log cd.s/m(2)) were recorded while the retina was dark-adapting and after 32 min of dark adaptation. This procedure was performed on two separate sessions: following a 3-day interruption of LTh treatment (S1) and following 30 days without interruption of LTh treatment (S2).

RESULTS

The mean photopic a-wave peak times were 9.8 ms at S1 and 5.0 ms at S2, respectively. The mean photopic b-wave peak times were 23.3 ms at S1 and 11.5 ms at S2, respectively, and the mean scotopic b-wave peak times (after 32 min of dark adaptation) were 45.2 ms at S1 and 26.0 ms at S2, respectively. No other significant ERG changes were observed.

CONCLUSION

Our results indicate that a dose of 20 microg/kg of LTh given to adult dogs was accompanied by a marked peak time shortening of both photopic and scotopic ERGs, without affecting other ERG parameters.

Thyroid hormone components are expressed in three sequential waves during development of the chick retina

BACKGROUND

Thyroid hormone (TH) is an important developmental regulator in many tissues, including the retina. TH is activated locally via deiodinase 2 (Dio2), and it is destroyed by deiodinase 3 (Dio3). The TH receptors, TRa and TRb, mediate TH activity through hormone and DNA binding, and interactions with transcription regulators.

RESULTS

In the current work, the expression of these TH components was examined in the chick retina over time. Three waves of expression were characterized and found to be correlated with critical developmental events. The first wave occurred as progenitor cells began to make photoreceptors, the second as some cell types adopted a more mature location and differentiation state, and the third as Müller glia were generated. The cell types expressing the components, as well as the kinetics of expression within the cell cycle, were defined. TRb expression initiated during G2 in progenitor cells, concomitant with NeuroD and Otx2, which are expressed in early photoreceptor cells. TRb was expressed in photoreceptor cells for several days and then was reduced in expression level, as the expression of Crx, a later photoreceptor gene, became more evident. Dio3 was expressed throughout the cell cycle in progenitor cells. TRa was in most, if not all, retinal cells. Dio2 appeared transiently in a ventral (high) to dorsal gradient, likely in a subset of photoreceptor cells.

CONCLUSION

Multiple TH components were expressed in dynamic patterns in cycling progenitor cells and photoreceptors cells across the developing chick retina. These dynamic patterns suggest that TH is playing several roles in retinal development, both within the cycling progenitor cells and possibly with respect to the timing of differentiation of photoreceptor cells.

Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Although the neural retina appears as a relatively uniform tissue when viewed from its surface, it is in fact highly patterned along its anterior-posterior and dorso-ventral axes. The question of how and when such patterns arise has been the subject of intensive investigations over several decades. Most studies aimed at understanding retinal pattern formation have used the retinotectal map, the ordered projections of retinal ganglion cells to the brain, as a functional readout of the pattern. However, other cell types are also topographically organized in the retina. The most commonly recognized example of such a topographic cellular organization is the differential distribution of photoreceptor types across the retina. Photoreceptor patterns are highly species-specific and may represent an important adaptation to the visual niche a given species occupies. Nevertheless, few studies have addressed this functional readout of pattern to date and our understanding of its development has remained superficial. Here, we review recent advances in understanding the molecular cascades that control regionalization of the eye anlage, relate these findings to the development of photoreceptor patterns and discuss common and unique strategies involved in both aspects of retinal pattern formation.

Mechanisms controlling Pax6 isoform expression in the retina have been conserved between teleosts and mammals

The Pax6 gene plays several roles in retinal development, including control of cell proliferation, maintenance of the retinogenic potential of progenitor cells, and cell fate specification. Emerging evidence suggests that these different aspects of Pax6 gene function are mediated by different isoforms of the Pax6 protein; however, relatively little is known about the spatiotemporal expression of Pax6 isoforms in the vertebrate retina. Using bacterial artificial chromosome (BAC) technology, we modified a zebrafish Pax6a BAC such that we could distinguish paired-containing Pax6a transcripts from paired-less Pax6a transcripts. In the zebrafish, the spatial and temporal onset of expression of these transcripts suggests that the paired-less isoform is involved in the cell fate decision leading to the generation of amacrine cells; however, because of limitations associated with transient transgenic analysis, it was not feasible to establish whether this promoter was active in all amacrine cells or in a specific population of amacrine cells. By making mice transgenic for the zebrafish Pax6a BAC reporter transgene, we were able to show that paired-containing and paired-less Pax6a transcripts were differentially expressed in amacrine subpopulations. Our study also directly demonstrates the functional conservation of the regulatory mechanisms governing Pax6 transcription in teleosts and mammals.

Modeling the role of mid-wavelength cones in circadian responses to light

Nonvisual responses to light, such as photic entrainment of the circadian clock, involve intrinsically light-sensitive melanopsin-expressing ganglion cells as well as rod and cone photoreceptors. However, previous studies have been unable to demonstrate a specific contribution of cones in the photic control of circadian responses to light. Using a mouse model that specifically lacks mid-wavelength (MW) cones we show that these photoreceptors play a significant role in light entrainment and in phase shifting of the circadian oscillator. The contribution of MW cones is mainly observed for light exposures of short duration and toward the longer wavelength region of the spectrum, consistent with the known properties of this opsin. Modeling the contributions of the various photoreceptors stresses the importance of considering the particular spectral, temporal, and irradiance response domains of the photopigments when assessing their role and contribution in circadian responses to light.

Direct and indirect effects of hedgehog pathway activation in the mammalian retina

The morphogen Sonic hedgehog (Shh) is expressed by the projection neurons of the retina, retinal ganglion cells (RGCs) and promotes retinal precursor cell (RPC) proliferation. To distinguish between direct and indirect effects of Hedgehog (Hh) pathway activation in the perinatal mouse retina, we followed the fate of cells that expressed a constitutively active allele of Smoothened (SMO-M2), the signal transduction component of the Hh pathway. SMO-M2 expression promoted a cell-autonomous increase in CyclinD1 expression and RPC proliferation and promoted the development of cells with an inner nuclear layer identity. SMO-M2 expression also inhibited rhodopsin expression in uninfected cells, thus highlighting an unexpected non-cell autonomous effect of Hh pathway activation on photoreceptor development.

Making the gradient: thyroid hormone regulates cone opsin expression in the developing mouse retina

Most mammals have two types of cone photoreceptors, which contain either medium wavelength (M) or short wavelength (S) opsin. The number and spatial organization of cone types varies dramatically among species, presumably to fine-tune the retina for different visual environments. In the mouse, S- and M-opsin are expressed in an opposing dorsal-ventral gradient. We previously reported that cone opsin patterning requires thyroid hormone beta2, a nuclear hormone receptor that regulates transcription in conjunction with its ligand, thyroid hormone (TH). Here we show that exogenous TH inhibits S-opsin expression, but activates M-opsin expression. Binding of endogenous TH to TRbeta2 is required to inhibit S-opsin and to activate M-opsin. TH is symmetrically distributed in the retina at birth as S-opsin expression begins, but becomes elevated in the dorsal retina at the time of M-opsin onset (postnatal day 10). Our results show that TH is a critical regulator of both S-opsin and M-opsin, and suggest that a TH gradient may play a role in establishing the gradient of M-opsin. These results also suggest that the ratio and patterning of cone types may be determined by TH availability during retinal development.

Properties of growth and molecular profiles of rat progenitor cells from ciliary epithelium

Recent studies have demonstrated that multipotent retinal stem or progenitor cells can be isolated from the ciliary epithelium (CE) of the eye using a neurosphere culture. In this study, we investigated the properties of growth and differentiation, and molecular profiles of rat adult ciliary epithelium (CE)-derived retinal progenitors and forebrain (FB) derived neurospheres. Under clonogenic culture conditions, we found that the CE-derived neurospheres contained fewer undifferentiated cells compared with the FB-derived neurospheres, and that CE-derived neurospheres initially expressed the set of Notch pathway molecules genes including Notch 1 and Delta 1, HES-1 and HES-5, but partially lose their expression after passaging. Furthermore, we found that the CE-derived neurospheres did not express several markers for in vivo embryonic retinal progenitors. Additionally, when the eye was divided into four subregions along its dorsoventral and nasotemporal axes and progenitor cells were obtained from the subregions, the progenitor cells did not express the subregion specific transcription factors, suggesting that subregional specificity is not maintained in vitro. Together, our results demonstrate that CE-derived progenitor cells may have intrinsic limitations in the production of cell types.

Characterization of retinoid-X receptor genes rxra, rxrba, rxrbb and rxrg during zebrafish development

During development of vertebrate embryos, retinoic acid plays a variety of roles that are mediated by binding to retinoic acid receptors (Rars) and their heterodimerization partners, the retinoid receptors (Rxrs). Here, we characterize the expression patterns of four zebrafish rxr genes during development and provide an analysis of the phylogenetic relationships between zebrafish and tetrapod Rxr genes based on sequence similarities and conserved syntenies. This analysis prompted the renaming of several of the zebrafish rxr genes to match their tetrapod orthologs. Understanding phylogenetic relationships among Rxr genes and their expression patterns during development provides a foundation for future studies of Rxr functions.

Challenges in the study of neuronal differentiation: a view from the embryonic eye

Progress in the study of the molecular mechanisms that regulate neuronal differentiation has been quite impressive in recent years, and promises to continue to an equally fast pace. This should not lead us into a sense of complacency, however, because there are still significant barriers that cannot be overcome by simply conducting the same type of experiments that we have been performing thus far. This article will describe some of these challenges, while highlighting the conceptual and methodological breakthroughs that will be necessary to overcome them.

Roles of cell-intrinsic and microenvironmental factors in photoreceptor cell differentiation

Photoreceptor differentiation requires the coordinated expression of numerous genes. It is unknown whether those genes share common regulatory mechanisms or are independently regulated by distinct mechanisms. To distinguish between these scenarios, we have used in situ hybridization, RT-PCR, and real-time PCR to analyze the expression of visual pigments and other photoreceptor-specific genes during chick embryo retinal development in ovo, as well as in retinal cell cultures treated with molecules that regulate the expression of particular visual pigments. In ovo, onset of gene expression was asynchronous, becoming detectable at the time of photoreceptor generation (ED 5-8) for some photoreceptor genes, but only around the time of outer segment formation (ED 14-16) for others. Treatment of retinal cell cultures with activin, staurosporine, or CNTF selectively induced or down-regulated specific visual pigment genes, but many cognate rod- or cone-specific genes were not affected by the treatments. These results indicate that many photoreceptor genes are independently regulated during development, are consistent with the existence of at least two distinct stages of gene expression during photoreceptor differentiation, suggest that intrinsic, coordinated regulation of a cascade of gene expression triggered by a commitment to the photoreceptor fate is not a general mechanism of photoreceptor differentiation, and imply that using a single photoreceptor-specific "marker" as a proxy to identify photoreceptor cell fate is problematic.

Targeted effects of retinoic acid signaling upon photoreceptor development in zebrafish

Retinoic acid (RA) is a signaling molecule important for photoreceptor development in vertebrates. The purpose of this study was to examine the mechanisms of the effects of RA upon developing rod and cone photoreceptors in the embryonic zebrafish. Exposure to exogenous RA increased the number of photoreceptors expressing rod opsin and red cone opsin, and decreased the number of photoreceptors expressing the blue and UV cone opsins, suggesting targeted effects of RA on photoreceptor development. RA exposure also increased opsin expression in individual rods and red cones, but decreased opsin expression in individual blue and UV cones, as indicated by differences in the strength of opsin hybridization in identified photoreceptors. RA exposure did not, however, significantly alter quantitative measures of photoreceptor pattern in a manner expected for changes in photoreceptor fate. These observations collectively indicate that RA treatment does not affect photoreceptor fate, but rather differentially influences opsin transcription in determined photoreceptors. An enzyme involved in RA synthesis, RALDH2, was immunocytochemically localized to retinal progenitor cells and the retinal pigmented epithelium (RPE), suggesting the presence of RA in the vicinity of developing photoreceptors. However, expression of an RA response element-driven transgene was restricted to the RPE, retinal progenitors, and a small population of neurons in ventral retina, suggesting that the endogenous RA signaling system is spatially limited within the eye.

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

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

Development of contrast sensitivity in infants with prenatal and neonatal thyroid hormone insufficiencies

Thyroid hormone is essential for normal brain development including structures critical for visual processing. While chick and rodent models have demonstrated abnormal visual development following prenatal thyroid hormone loss, comparable data do not exist in the human. To determine whether human infants with intrauterine and early postnatal thyroid hormone insufficiencies have compromised visual abilities, we investigated contrast sensitivity and visual acuity development in 13 infant offspring of women with hypothyroidism during pregnancy (HYPO), 16 preterm infants born between 32 and 35 weeks gestation, 12 infants with congenital hypothyroidism (CH), and 20 typically developing infants. All were assessed with the sweep visual evoked potential technique at 3, 4.5, and 6 months (corrected) age. Results showed significantly reduced contrast sensitivity but normal visual acuity in HYPO and CH groups relative to controls (p < 0.003 and p < 0.05 respectively). Stratification of the HYPO group into subgroups based on maternal TSH levels during the first half of pregnancy revealed lower contrast sensitivities for infants whose mothers' TSH values were above than below the median (p < 0.05). In the CH group, those with an absent thyroid gland and/or a newborn TSH value above 200 mIU/L had lower contrast sensitivities than did those with other etiologies or TSH levels below 100 mIU/L (p < 0.05). There were no significant effects involving the preterm group. These results indicate that thyroid hormone is important for human visual development.

Early expression of thyroid hormone receptor beta and retinoid X receptor gamma in the Xenopus embryo

The role of thyroid hormone in Xenopus metamorphosis is particularly well understood as it plays an essential role in that process. However, recent evidence suggests that thyroid hormone may play an earlier role in amphibian embryogenesis. We demonstrate that Xenopus thyroid hormone receptor beta (XTR beta) is expressed shortly after neural fold closure, and that its expression is localized to the developing retina. Retinoid X receptor gamma (RXR gamma), a potential dimerization partner for XTR beta, was also found to be expressed in the retina at early stages, and at later stages RXR gamma was also expressed in the liver diverticulum. Addition of either thyroid hormone or 9-cis retinoic acid, the ligands for XTR beta and RXR gamma, respectively, did not alter the expression of their receptors. However, the addition of thyroid hormone and 9-cis retinoic acid did alter rhodopsin mRNA expression. Addition of thyroid hormone generates a small expansion of the rhodopsin expression domain. When 9-cis retinoic acid or a combination of thyroid hormone and 9-cis retinoic acid was administered, there was a decrease in the expression domain of rhodopsin in the developing retina. These results provide evidence for an early role for XTR beta and RXR gamma in the developing Xenopus retina.

Foxn4 controls the genesis of amacrine and horizontal cells by retinal progenitors

During vertebrate retinogenesis, seven classes of cells are specified from multipotent progenitors. To date, the mechanisms underlying multipotent cell fate determination by retinal progenitors remain poorly understood. Here, we show that the Foxn4 winged helix/forkhead transcription factor is expressed in a subset of mitotic progenitors during mouse retinogenesis. Targeted disruption of Foxn4 largely eliminates amacrine neurons and completely abolishes horizontal cells, while overexpression of Foxn4 strongly promotes an amacrine cell fate. These results indicate that Foxn4 is both necessary and sufficient for commitment to the amacrine cell fate and is nonredundantly required for the genesis of horizontal cells. Furthermore, we provide evidence that Foxn4 controls the formation of amacrine and horizontal cells by activating the expression of the retinogenic factors Math3, NeuroD1, and Prox1. Our data suggest a model in which Foxn4 cooperates with other key retinogenic factors to mediate the multipotent differentiation of retinal progenitors.

Chromatin immunoprecipitation assay on the rainbow trout opsin proximal promoters illustrates binding of NF-kappaB and c-jun to the SWS1 promoter in the retina

Misexpression of opsins has been linked to apoptosis of photoreceptor cells in the vertebrate retina. Salmonid fish lose their ultraviolet-sensitive (UVS) cones through post-natal developmental apoptosis mediated by thyroid hormone (TH). In order to identify genetic mechanisms that may play a role in the loss of UVS cones, the transcriptional regulation of the SWS1 opsin in the rainbow trout (Oncorhynchus mykiss) was investigated. The Transfac database was interrogated with promoter sequence acquired by genome-walking PCR using MatInspector V2.2 to identify putative transcription factor (TF) binding sites. Putative binding sites for AP-1 (c-jun) and NF-kappaB were found in the SWS1 opsin promoter and were chosen for further investigation due to their high MatInspector scores, their established role in photoreceptor apoptosis, and their relative exclusion from other opsin promoters. NF-kappaB and c-jun proteins were visualized in rainbow trout retinal tissue with immunohistochemistry and c-jun was identified in rainbow trout retinal protein homogenate by immunoblot. A chromatin immunoprecipitation-polymerase chain reaction technique was employed to examine the in vivo interaction of c-jun and NF-kappaB proteins with their proposed binding sites in the opsin promoters. This analysis demonstrated that NF-kappaB and c-jun bind to the SWS1 opsin promoter, but not to the other rod and cone opsin promoters tested. Given the role of NF-kappaB and c-jun during photoreceptor apoptosis, the influence of their activity through TH and their selective binding to the SWS1 opsin promoter in rainbow trout, these TFs represent good candidates of mechanisms underlying UVS cone degeneration in salmonids.

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

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

Thyroid hormone and retinal development: an emerging field

Thyroid hormone appears to play a critical, yet not fully understood, role in the development of the neuroretina. This review focuses on recent experiments in the rodent, chicken, and amphibian, with an emphasis on how the hormone and its receptor isoforms influence retinal cell proliferation and cell fate decisions. The initial results are fueling the next generation of experiments in the retina, which promise to provide insights into the mechanisms of thyroid hormone action in a wide variety of developing neural tissue.

Thyroid-beta2 and the retinoid RAR-alpha, RXR-gamma and ROR-beta2 receptor mRNAs; expression profiles in mouse retina, retinal explants and neocortex

In neonatal retinal explants cultured long-term green cones are missing. Recently it was reported that thyroid hormone beta2 receptors (TR-beta2) are essential for these green cones to differentiate. Therefore transcript level of these receptors was investigated in our mouse retinal explants. However, thyroid receptors function as heterodimers with retinoid receptors (RR); so the fate of selected RRs was similarly analyzed using semi-quantitative RT-PCR. Loss of TR-beta2 and RR (RXR-gamma and ROR-beta2) mRNAs was observed after culturing the neonatal retina for 12 days. This indicates that these proteins are involved in determination of green cone identity. In addition, levels of the selected RR transcripts are differentially affected by short- or long-term culture. In the latter case an attached retinal pigment epithelium seems to play a protective role. Furthermore, divergent diurnal peaks of RR mRNAs are present in young as well as aged mouse retina and neocortex. This data might be relevant in the context of human ageing disorders.

Neurodevelopmental control by thyroid hormone receptors

Recent studies have provided insights into the neurodevelopmental functions of thyroid hormone signaling. The nuclear thyroid hormone receptors (TRs) are ligand-activated transcription factors and a variety of TR isotypes, generated by two genes, mediate distinct processes. In addition, deiodinase enzymes that regulate levels of the main active form of thyroid hormone, T3, are likely to cooperate closely with TRs in specifying a localized and timely response to thyroid hormones in target tissues. Some of the most sensitive processes controlled by these pathways are in the auditory and visual sensory systems.

Photoreceptor subtype specification: from flies to humans

Multiple cell types often differentiate from a pluripotent cell. These cells may then further diversify as distinct subtypes. The visual system provides an ideal model for studying subtype specification as various photoreceptors acquire different functions based on the type of opsin they express. Opsin expression is mostly controlled through transcriptional mechanisms that are evolutionary conserved from Drosophila to humans. In addition, it appears that, from a "default" developmental state, distinct "acquired" photoreceptor states develop upon receiving intrinsic or extrinsic signals. This review discusses factors involved in opsin gene regulation and how their integration may explain how subtype specificity is achieved.

Cellular competence plays a role in photoreceptor differentiation in the developing Xenopus retina

Factors in the environment appear to be responsible for inducing many of the cell fates in the retina, including, for example, photoreceptors. Further, there is a conserved order of histogenesis in the vertebrate retina, suggesting that a temporal mechanism interacts in the control of cellular determination. The temporal mechanism involved could result from different inducing signals being released at different times. Alternatively, the inducing signals might be present at many stages, but an autonomous clock could regulate the competence of cells to respond to them. To differentiate between these mechanisms, cells from young embryonic retinas were dissociated and grown together with those from older embryos, and the timing of photoreceptor determination assayed. Young cells appeared uninfluenced by older cells, expressing photoreceptor markers on the same time schedule as when cultured alone. A similar result was obtained when the heterochronic mixing was done in vivo by grafting a small plug of optic vesicle from younger embryos into older hosts. Even the graft cells at the immediate margin of the transplant failed to express photoreceptor markers earlier than normal, despite their being in contact with older, strongly expressing host cells. We conclude that retinal progenitors intrinsically acquire the ability to respond to photoreceptor-inducing cues by a mechanism that runs on a cell autonomous schedule, and that the conserved order of histogenesis is based in part on this competence clock.